U.S. patent application number 12/649945 was filed with the patent office on 2010-05-13 for cathodes for microbial electrolysis cells and microbial fuel cells.
This patent application is currently assigned to The Penn State Research Foundation. Invention is credited to Douglas Call, Shaoan Cheng, Bruce Logan, Matthew Merrill.
Application Number | 20100119920 12/649945 |
Document ID | / |
Family ID | 42310600 |
Filed Date | 2010-05-13 |
United States Patent
Application |
20100119920 |
Kind Code |
A1 |
Logan; Bruce ; et
al. |
May 13, 2010 |
CATHODES FOR MICROBIAL ELECTROLYSIS CELLS AND MICROBIAL FUEL
CELLS
Abstract
An apparatus is provided according to embodiments of the present
invention which includes a reaction chamber having a wall defining
an interior of the reaction chamber and an exterior of the reaction
chamber; exoelectrogenic bacteria disposed in the interior of the
reaction chamber; an aqueous medium having a pH in the range of
3-9, inclusive, the aqueous medium including an organic substrate
oxidizable by exoelectrogenic bacteria and the medium disposed in
the interior of the reaction chamber. An inventive apparatus
further includes an anode at least partially contained within the
interior of the reaction chamber; and a brush or mesh cathode
including stainless steel, nickel or titanium, the cathode at least
partially contained within the interior of the reaction
chamber.
Inventors: |
Logan; Bruce; (State
College, PA) ; Call; Douglas; (Spring Mills, PA)
; Merrill; Matthew; (State College, PA) ; Cheng;
Shaoan; (State College, PA) |
Correspondence
Address: |
GIFFORD, KRASS, SPRINKLE,ANDERSON & CITKOWSKI, P.C
PO BOX 7021
TROY
MI
48007-7021
US
|
Assignee: |
The Penn State Research
Foundation
University Park
PA
|
Family ID: |
42310600 |
Appl. No.: |
12/649945 |
Filed: |
December 30, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11799149 |
May 1, 2007 |
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12649945 |
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12145722 |
Jun 25, 2008 |
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11799149 |
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11180454 |
Jul 13, 2005 |
7491453 |
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12145722 |
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11799194 |
May 1, 2007 |
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12145722 |
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12177962 |
Jul 23, 2008 |
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11799194 |
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11799194 |
May 1, 2007 |
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12177962 |
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60796761 |
May 2, 2006 |
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60588022 |
Jul 14, 2004 |
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60608703 |
Sep 10, 2004 |
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60796761 |
May 2, 2006 |
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60945991 |
Jun 25, 2007 |
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60951303 |
Jul 23, 2007 |
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61141511 |
Dec 30, 2008 |
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Current U.S.
Class: |
429/401 ;
204/290.13; 205/638; 427/58 |
Current CPC
Class: |
Y02E 60/36 20130101;
H01M 4/8605 20130101; H01M 4/92 20130101; H01M 2004/8689 20130101;
H01M 4/8657 20130101; Y02E 60/527 20130101; H01M 4/8853 20130101;
H01M 4/9016 20130101; H01M 4/9008 20130101; Y02E 60/50 20130101;
H01M 8/16 20130101; Y02E 60/366 20130101 |
Class at
Publication: |
429/46 ;
204/290.13; 205/638; 427/58 |
International
Class: |
C25B 1/04 20060101
C25B001/04; C25B 9/00 20060101 C25B009/00; C25B 11/04 20060101
C25B011/04; B05D 5/00 20060101 B05D005/00 |
Goverment Interests
GOVERNMENT SUPPORT
[0005] This invention was made with government support under
Contract No. CBET-0730359 awarded by the National Science
Foundation. The government has certain rights in this invention.
Claims
1. An apparatus, comprising: a reaction chamber having a wall
defining an interior of the reaction chamber and an exterior of the
reaction chamber; exoelectrogenic bacteria disposed in the interior
of the reaction chamber; an aqueous medium having a pH in the range
of 3-9, inclusive, the aqueous medium comprising an organic
substrate oxidizable by exoelectrogenic bacteria, the aqueous
medium disposed in the interior of the reaction chamber; an anode
at least partially contained within the interior of the reaction
chamber; and a brush or mesh cathode comprising stainless steel,
nickel or titanium, the cathode at least partially contained within
the interior of the reaction chamber.
2. The apparatus of claim 1, wherein the cathode has, in operation,
a solution facing portion and a gas facing portion, and further
wherein PTFE is excluded from the gas facing portion.
3. The apparatus of claim 1, wherein microbes are substantially
excluded from contact with the cathode.
4. The apparatus of claim 1, wherein no exogenous noble metal
catalyst is in contact with the cathode.
5. The apparatus of claim 1, wherein the cathode further comprises
a catalyst.
6. The apparatus of claim 5, wherein the catalyst is a nickel or
platinum catalyst.
7. The apparatus of claim 6, wherein the nickel catalyst is a
nickel oxide catalyst.
8. The apparatus of claim 5, wherein the catalyst is an
electrodeposited catalyst.
9. The apparatus of claim 1, wherein the cathode is generally
tubular in shape, having a wall defining an interior space, an
interior wall surface, an exterior, and an exterior wall surface,
wherein the wall comprises a stainless steel, nickel or titanium
mesh, the mesh having a first mesh surface disposed towards the
interior space and a second mesh surface disposed towards the
exterior.
10. The apparatus of claim 1, wherein the mesh has a first mesh
surface and a second mesh surface and wherein a coating is present
on the first mesh surface, the second mesh surface or both the
first mesh surface and the second mesh surface.
11. The apparatus of claim 9, wherein the coating on the second
mesh surface is a water impermeable coating.
12. The apparatus of claim 9, wherein the coating on the second
mesh surface is an oxygen impermeable coating.
13. The apparatus of claim 10, wherein the coating on the second
mesh surface is an oxygen permeable coating.
14. The apparatus of claim 10, wherein the coating on the first
mesh surface, second mesh surface or both the first mesh surface
and second mesh surface comprises an electron conductive
binder.
15. The apparatus of claim 10, wherein the coating on the first
mesh surface, second mesh surface or both the first mesh surface
and second mesh surface comprises a catalyst.
16. The apparatus of claim 9, wherein the generally tubular cathode
has a cross section selected from the group consisting of:
circular, oval, oblong, square and rectangular.
17. The apparatus of claim 1, further comprising a power source
operably connected to add voltage to enhance an electrical
potential between the anode and cathode.
18. A biological process for producing hydrogen or electric
current, comprising: providing an apparatus according to claim 1;
maintaining oxidizing reaction conditions such that electrons are
produced by oxidation of the organic substrate by the electrogenic
bacteria and the electrons are transferred to an anode.
19. The biological process of claim 18, further comprising
application of a voltage in the range of 25-1000 millivolts,
enhancing an electrical potential between the anode and
cathode.
20. A method for fabricating a cathode comprising: coating a
stainless steel, nickel or titanium mesh, the mesh having wires
defining pores, with an oxygen permeable layer comprising a
viscoelastic polymer, the coating adherent to the mesh and present
in the pores, forming a continuous coating on one or both sides of
the mesh, wherein the mesh and coating are not exposed to a
temperature above 100.degree. C. or a pressure above ambient
pressure.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Patent Application Ser. No. 61/141,511, filed Dec. 30, 2008. This
application is also a continuation-in-part of U.S. patent
application Ser. No. 12/145,722, filed Jun. 25, 2008 which claims
priority from U.S. Provisional Patent Application Ser. No.
60/945,991, filed Jun. 25, 2007. U.S. patent application Ser. No.
12/145,722 is also a continuation-in-part of U.S. patent
application Ser. No. 11/180,454, filed Jul. 13, 2005, now U.S. Pat.
No. 7,491,453, which claims priority from U.S. Provisional Patent
Application Ser. No. 60/588,022, filed Jul. 14, 2004 and
60/608,703, filed Sep. 10, 2004. U.S. patent application Ser. No.
12/145,722 is also a continuation-in-part of U.S. patent
application Ser. No. 11/799,194, filed May 1, 2007, which claims
priority from U.S. Provisional Patent Application Ser. No.
60/796,761, filed May 2, 2006.
[0002] This application is also a continuation-in-part of U.S.
patent application Ser. No. 12/177,962, filed Jul. 23, 2008, which
claims priority of U.S. Provisional Patent Application Ser. No.
60/951,303, filed Jul. 23, 2007. U.S. patent application Ser. No.
12/177,962 also a continuation-in-part of U.S. patent application
Ser. No. 11/799,194, filed May 1, 2007, which claims priority from
U.S. Provisional Patent Application Ser. No. 60/796,761, filed May
2, 2006.
[0003] This application is also a continuation-in-part of U.S.
patent application Ser. No. 11/799,149, filed May 1, 2007, which
claims priority of U.S. Provisional Patent Application 60/796,761,
filed May 2, 2006.
[0004] The entire content of each application is incorporated
herein by reference.
FIELD OF THE INVENTION
[0006] The invention relates to cathodes used in microbial fuel
cells (MFCs), which are used for producing electricity; and
microbial electrolysis cells (MECs), which are used to produce
hydrogen.
SUMMARY OF THE INVENTION
[0007] An apparatus is provided according to embodiments of the
present invention which includes a reaction chamber having a wall
defining an interior of the reaction chamber and an exterior of the
reaction chamber; exoelectrogenic bacteria disposed in the interior
of the reaction chamber; an aqueous medium having a pH in the range
of 3-9, inclusive, the aqueous medium including an organic
substrate oxidizable by exoelectrogenic bacteria and the medium
disposed in the interior of the reaction chamber. An inventive
apparatus further includes an anode at least partially contained
within the interior of the reaction chamber; and a brush or mesh
cathode including stainless steel, nickel or titanium, the cathode
at least partially contained within the interior of the reaction
chamber.
[0008] Optionally, an inventive apparatus further includes a brush
or mesh cathode consisting essentially of stainless steel, nickel
or titanium.
[0009] Stainless steels included in a cathode of the present
invention can be any stainless steel, such as Austenitic, Ferritic
or Martensitic stainless steel. Non-limiting examples of included
stainless steels are SS 304, SS 316, SS 420 and SS 286.
[0010] Nickel included in a cathode of the present invention can be
any nickel. Non-limiting examples of included nickels are Ni 201,
Ni 400, Ni 625 and Ni HX. Titanium included in a cathode of the
present invention can be any titanium.
[0011] In particular embodiments, a cathode included in an
inventive apparatus has, in operation, a solution facing portion
and a gas facing portion, and PTFE is excluded from the gas facing
portion.
[0012] In preferred embodiments, microbes are substantially
excluded from contact with the cathode.
[0013] In certain embodiments, no exogenous noble metal catalyst is
present in the cathode.
[0014] In further embodiments, a catalyst is present in the
cathode. A catalyst such as nickel, platinum, activated carbon, or
CoTMPP is present in particular embodiments of cathodes of the
present invention. A nickel oxide catalyst is present in particular
embodiments of cathodes of the present invention.
[0015] In still further embodiments, a nickel oxide catalyst
included in a cathode of the present invention is electrodeposited
on a stainless steel, nickel or titanium brush or mesh.
[0016] An apparatus according to embodiments of the present
invention includes a cathode which is generally tubular in shape,
having a wall defining an interior space, an interior wall surface,
an exterior, and an exterior wall surface, wherein the wall
comprises a stainless steel, nickel or titanium mesh, the mesh
having a first mesh surface disposed towards the interior space and
a second mesh surface disposed towards the exterior. A generally
tubular cathode can have a cross section of various shapes such as
circular, oval, oblong, square and rectangular.
[0017] Optionally, the mesh has a first mesh surface and a second
mesh surface and a coating is present on the first mesh surface,
the second mesh surface or both the first mesh surface and the
second mesh surface. For example, an included coating is a
diffusion layer or a cathode protection layer.
[0018] In particular embodiments, the second mesh surface is
disposed towards the exterior of the tubular cathode or is exposed
to the exterior of the reactor and the coating on the second mesh
surface is a water impermeable coating.
[0019] In particular embodiments, the second mesh surface is
disposed towards the exterior of the tubular cathode or the
exterior of the reactor and the coating on the second mesh surface
is an oxygen impermeable coating.
[0020] In particular embodiments, the second mesh surface is
disposed towards the exterior of the reactor and the coating on the
second mesh surface is an oxygen permeable coating.
[0021] Optionally, the coating on the first mesh surface, second
mesh surface or both the first mesh surface and second mesh surface
includes an electron conductive binder.
[0022] in a further option the coating on the first mesh surface,
second mesh surface or both the first mesh surface and second mesh
surface includes a catalyst.
[0023] A microbial electrolysis apparatus according to embodiments
of the present invention includes a power source operably connected
to add voltage to enhance an electrical potential between the anode
and cathode.
[0024] A microbial electrolysis apparatus according to embodiments
of the present invention includes a hydrogen fuel cell power source
operably connected to add voltage to enhance an electrical
potential between the anode and cathode, wherein the hydrogen fuel
cell power source is at least partially fuelled by the microbial
electrolysis apparatus.
[0025] Biological processes for producing hydrogen or electric
current are provided according to embodiments of the present
invention which include providing an apparatus which includes a
reaction chamber having a wall defining an interior of the reaction
chamber and an exterior of the reaction chamber; exoelectrogenic
bacteria disposed in the interior of the reaction chamber; an
aqueous medium having a pH in the range of 3-9, inclusive, the
aqueous medium including an organic substrate oxidizable by
exoelectrogenic bacteria and the medium disposed in the interior of
the reaction chamber, wherein the apparatus further includes an
anode at least partially contained within the interior of the
reaction chamber; and a brush or mesh cathode including stainless
steel, nickel or titanium, the cathode at least partially contained
within the interior of the reaction chamber; and maintaining
oxidizing reaction conditions such that electrons are produced by
oxidation of the organic substrate by the electrogenic bacteria and
the electrons are transferred to an anode.
[0026] Embodiments of a biological process for producing hydrogen
further include application of a voltage in the range of 25-1000
millivolts, enhancing an electrical potential between the anode and
cathode.
BACKGROUND OF THE INVENTION
[0027] Both electricity and hydrogen production result from the
degradation of organic matter by microbes, such as exoelectrogenic
bacteria. Microbes oxidize organic matter, releasing electrons to a
circuit and protons into solution. In an MFC at the cathode, the
electrons and protons combine with oxygen to form water. To make
hydrogen in an MEC, the MFC is modified by excluding oxygen and
adding a small additional voltage. Electrons and protons combine on
the cathode in the MEC to form hydrogen gas. MFCs and MECs can be
used in various applications, such as a method of wastewater
treatment, or as a method for renewable energy production, for
example. Examples of MFCs for making electricity are exemplified in
Liu and Logan (2004) and Liu et al. (2004). Examples of MECs are
given by Liu et al. (2005), Cheng and Logan (2007c), and Call and
Logan (2008).
[0028] Performance of current MECs and MFCs can be limited by the
cathode and current cathodes require expensive materials, such as
platinum. Thus, improved cathodes for MECs and MFCs are
required.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 illustrates an embodiment of an inventive MEC or MFC
system;
[0030] FIG. 2 illustrates a brush cathode having stainless steel,
nickel or titanium bristles included in embodiments of an MEC or
MFC;
[0031] FIG. 3 illustrates a cross sectional view of an embodiment
of a stainless steel, nickel or titanium mesh cathode;
[0032] FIG. 4 illustrates a tubular embodiment of a stainless
steel, nickel or titanium mesh cathode;
[0033] FIG. 5 illustrates reactor schematics: reactor VB (vertical
brush): half brush anode, HBA; half brush cathode, HBC; reactor HB
(horizontal brush): full brush anode, FBA; full brush cathode, FBC;
reactor FC (flat cathode): platinized carbon cloth cathode, Pt,
stainless steel cathode, SS. power supply, PS. 10.OMEGA. resistor,
R;
[0034] FIG. 6 is a graph showing current densities versus time for
SS brush cathodes with different bristle loadings of 100%, 50%,
25%, 10% or 0% (brush base core only) at E.sub.ap=0.6 V;
[0035] FIG. 7 is a graph showing cathode potentials (versus
Ag/AgCl) versus time for consecutive batch cycles using SS brush
cathodes with different bristle loadings at E.sub.ap=0.6 V;
[0036] FIG. 8 is a graph showing current densities versus time for
a 100% loaded SS brush cathode (SSB 100%), a flat SS cathode (SS
flat), a SS brush core (SS core), and a graphite brush cathode (GB)
at E.sub.ap=0.6 V;
[0037] FIG. 9 is a graph showing current density versus time for
both the platinized carbon cloth cathode (Pt) and the SS brush
cathode cut in half (Half SS) at E.sub.ap=0.6 V;
[0038] FIG. 10 is a graph showing cathode potentials (versus
Ag/AgCl) versus time for both the Pt/C cathode and the SS brush
cathode cut in half (Half SS) at E.sub.ap=0.6 V;
[0039] FIG. 11 is a graph showing LSV curves for the platinized
cathode (Pt), the 100% loaded SS brush cathode before (pre) and
after (post) accelerated use, and the flat SS cathode (SS
Flat);
[0040] FIG. 12 is a graph showing gas production of MECs with
different stainless steel and nickel cathodes at an applied voltage
or 0.9V;
[0041] FIG. 13 is a graph showing gas production of MECs with
different stainless steel and nickel cathodes at an applied voltage
or 0.6V;
[0042] FIG. 14 is a graph showing current densities for MECs with
platinum, Ni 625 or SS A286 cathodes at applied voltages of 0.6 and
0.9V;
[0043] FIG. 15A shows a Tafel plots for an MEC including a
stainless steel 286 alloy cathode;
[0044] FIG. 15B shows a Tafel plot for an MEC including a platinum
metal cathode;
[0045] FIG. 16 is a graph showing gas production of MECs including
cathodes with or without electrodeposited nickel oxide layers on SS
A286 and Ni 625, operated at an applied voltage of 0.6V;
[0046] FIG. 17A is a graph showing total gas and current production
versus time using a Ni 625+NiO.sub.x cathode;
[0047] FIG. 17B is a graph showing total gas and current production
versus time using a SS A286+NiO.sub.x cathode;
[0048] FIG. 18 shows Tafel plots for the indicated MEC cathodes in
2 mM phosphate buffer, scan rate 2 mV/s, third scan;
[0049] FIG. 19A is a graph showing total gas production for MECs
with Ni210, Ni210+CB, eNiOx or Pt catalyst cathodes, as a function
of cycle number at an applied voltage of 0.6 V;
[0050] FIG. 19B is a graph showing maximum current for MECs with
Ni210, Ni210+CB, eNiOx or Pt catalyst cathodes, as a function of
cycle number at an applied voltage of 0.6 V;
[0051] FIG. 20A is a graph showing hydrogen production rate in an
MEC using a Ni210 catalyst cathode at different applied
voltages;
[0052] FIG. 20B is a graph showing cathodic recovery and Coulombic
efficiency in an MEC using a Ni210 catalyst cathode at different
applied voltages;
[0053] FIG. 20C is a graph showing energy recovery based on
electrical input and overall energy recovery in an MEC using a
Ni210 catalyst cathode at different applied voltages;
[0054] FIG. 21 shows current density as a function of time for both
SS mesh and SS solid cathodes in an MEC at E.sub.AP=0.6 V;
[0055] FIG. 22 is a graph showing voltage generation in an MFC
using a SS mesh cathode and a Pt catalyst with 2 PDMS/carbon
diffusion layers (M2) compared to an MFC using carbon cloth
cathodes with 4 diffusion layers (CC4);
[0056] FIG. 23A is a graph showing power density in an MFC using a
cathode containing SS mesh with Pt catalyst and 1-5 layers of
PDMS/carbon DLs (M1-M5) as a function of current density
(normalized to cathode surface area) obtained by varying the
external circuit resistance (1000-50.OMEGA.);
[0057] FIG. 23B is a graph showing power density in an MFC using
carbon cloth cathodes with Pt and the same DLs (CC1-CC5) as a
function of current density (normalized to cathode surface area)
obtained by varying the external circuit resistance
(1000-50.OMEGA.);
[0058] FIG. 24A is a graph showing LSV of MFCs including SS mesh
cathodes with a Pt catalyst and 1-5 PDMS/carbon DLs (M1-M5);
[0059] FIG. 24B is a graph showing LSV of an MFC including cathode
M1 compared with MFCs including cathodes having additional PDMS
layers (MP2-MP5), each including Pt catalyst;
[0060] FIG. 24C is a graph showing LSV of an MFC including cathode
M2 compared with an MFC including a cathode having a
solution-facing side coating containing only carbon black (M2, no
Pt), and a cathode with no coating on the solution-facing side (M2,
no Pt, no CB);
[0061] FIG. 25A is a graph showing the CE of an MFC including a SS
mesh cathode with Pt catalyst and 1-5 layers of PDMS/carbon DLs
(M1-M5) as a function of current density (normalized to cathode
surface area) obtained by varying the external circuit resistance
(1000-50.OMEGA.);
[0062] FIG. 25B is a graph showing the CE of MFCs including carbon
cloth cathodes with Pt and 1-5 layers of PDMS/carbon DLs (CC1-CC5)
as a function of current density (normalized to cathode surface
area) obtained by varying the external circuit resistance
(1000-50.OMEGA.);
[0063] FIG. 26 is a graph showing oxygen permeability of SS mesh
cathodes including a Pt catalyst and PDMS/carbon DLs (M) or PDMS
(MP) DLs upon PDMS/carbon base layer; and
[0064] FIG. 27 is a graph showing voltage generation in an MFC
using different SS mesh cathodes.
DETAILED DESCRIPTION OF THE INVENTION
[0065] Microbial fuel cells according to the present invention are
provided which are configured to produce electricity (MFC) and/or
hydrogen (MEC) in particular embodiments. An MFC or MEC of the
present invention includes at least one anode, at least one
cathode, a reaction chamber in which an anode and cathode are at
least partially disposed, and a conductive conduit for electrons in
electrical communication with the anode and the cathode. In the
case of an MEC, a power source for enhancing an electrical
potential between the anode and cathode is further included.
[0066] A reaction chamber may have one or more compartments, such
as an anode compartment and a cathode compartment separated, for
instance, by a cation or anion exchange membrane or other
separator. Alternatively, a reaction chamber may be a single
compartment configuration with no separator present between the
anode and cathode. One or more channels may be included in a
reaction chamber for addition and removal of various substances
such as substrates for bacterial metabolism and products such as
hydrogen.
[0067] In an MFC, oxygen is present at the cathode to facilitate
the reaction of protons, electrons and oxygen to form water. In an
MEC, oxygen is substantially excluded from the cathode area and a
power source for enhancing an electrical potential between the
anode and cathode by application of a voltage is included.
[0068] FIG. 1 illustrates an embodiment of an inventive MEC or MFC
at 10. In this illustration, a reaction chamber is shown having a
wall 5 defining an interior and exterior of the reaction chamber,
and fluid, such as an aqueous solution containing a biodegradable
substrate, in the interior of the reaction chamber, the fluid level
shown at 6. An anode having bacteria disposed thereon is shown at
12 and a cathode is shown at 16. A space 8 between the electrodes
is further depicted. Space 8 is minimized to improve system
performance and is generally in the range of 0.1-100 cm, inclusive.
An optional separator, such as a proton exchange membrane (PEM) or
filter separator, is shown at 14 positioned between the anode 12
and cathode 16. A conduit for electrons 17 is shown along with a
connected power source (MEC) or load (MFC) shown at 18. Channels 20
and 22 are shown which can serve as flow paths for materials
entering or leaving the reaction chamber.
[0069] Cathodes
[0070] The present invention provides cathodes for MFCs and MECs
that provide good performance for these systems.
[0071] In embodiments of the present invention, cathodes are
characterized by high specific surface area.
[0072] In particular embodiments, an inventive cathode has a
specific surface area greater than 10 m.sup.2/m.sup.3. Specific
surface area is here described as the total surface area of the
cathode per unit of cathode volume. In further embodiments, a
cathode of the present invention has a specific surface area
greater than 1000 m.sup.2/m.sup.3. In still further embodiments, a
cathode of the present invention has a specific surface area
greater than 5,000 m.sup.2/m.sup.3. In yet further embodiments, a
cathode of the present invention has a specific surface area
greater than 10,000 m.sup.2/m.sup.3.
[0073] Exemplary high surface area cathodes of the present
invention include metal brush cathodes and metal mesh cathodes,
where the metal is stainless steel, nickel or titanium. A nickel
brush or mesh cathode can be nickel metal or a nickel alloy. The
term "nickel" is used herein to refer to nickel metal and nickel
alloys unless otherwise specified. A titanium brush or mesh cathode
can be titanium metal or a titanium alloy. The term "titanium" is
used herein to refer to titanium metal and titanium alloys unless
otherwise specified.
[0074] A metal brush cathode includes one or more conductive
fibers. In particular embodiments the one or more fibers are
attached to a support. A plurality of fibers is attached to the
support and the fibers extend generally radially from the support
in specific embodiments. A brush electrode optionally includes a
centrally disposed support having a longitudinal axis.
[0075] Brush electrodes include a variety of configurations
illustratively including various twisted wire brush configurations
and strip brush configurations. For example, a particular twisted
wire brush configuration includes a support formed from two or more
strands of wire and fibers attached between the wires. In a further
example, a strip brush configuration includes fibers attached to a
conductive backing strip, the strip attached to the support.
[0076] Fibers of a brush cathode are electrically conductive and
are in electrical communication with the support and with an
anode.
[0077] Metal brush cathodes according to embodiments of the present
invention include stainless steel, nickel or titanium fibers
attached to a stainless steel, nickel or titanium support.
[0078] FIG. 2 illustrates a high specific surface area stainless
steel, nickel or titanium brush cathode included in embodiments of
an MEC or MFC of the present invention. FIG. 2 shows a
configuration of a brush cathode 20 in which stainless steel,
nickel or titanium bristles 24 are placed substantially
perpendicular to and between two or more conductive, corrosion
resistant wires which form a support 22 such that the bristles 24
extend substantially radially from the support 22. A wire is
optionally twisted around the brushes to maintain good electrical
contact with the wire. A conductive connector is typically attached
to the support 22 to electrically connect the cathode to the
anode.
[0079] Brush cathode configurations can include multiple
discontinuous bristles and/or one or more continuous wires wound
about a central axis, forming looped bristles. Where no support is
included, a conductive connector is attached to the wire or wires
forming the bristles to electrically connect the cathode anode to
the anode. Where a support is included, a conductive connector is
typically attached to the support to electrically connect the
cathode and an anode. Bristles of a brush cathode can be randomly
or non-randomly oriented.
[0080] Optionally, a brush cathode includes bristles that extend
substantially radially from a central axis forming a cylindrical
brush. In a further option, bristles extend substantially radially
from a central axis forming a partial cylindrical shape, such as a
half cylinder or quarter cylinder. A half cylindrical brush cathode
is preferred in particular MEC and MFC embodiments.
[0081] A brush cathode optionally includes one or more
coatings.
[0082] Metal mesh cathodes according to embodiments of the present
invention include a stainless steel, nickel or titanium mesh.
[0083] Various U.S. standard mesh sizes having pore sizes of about
one centimeter or less, for example U.S. standard 7/16 inch mesh,
1/4 inch mesh, and U.S. standard mesh Nos. 4, 5, 6, 7, 8, 10, 12,
14, 16, 18, 20, 25, 30, 35, 40, 42, 44, 50, 54, 60, 70, 90, 120,
140, 165, 200, 325, 400 and 500 mesh, are included in cathodes for
use in MECs and MFCs according to particular embodiments. Mesh
sizes are known in the art and particular mesh dimensions are
illustrated below:
TABLE-US-00001 mesh # 42 44 50 60 80 80 90 120 165 500 wire 0.0055
0.0055 0.0055 0.0075 0.0037 0.0055 0.0055 0.004 0.0019 0.001
diameter (inch) pore size (inch) 0.018 0.0172 0.0145 0.009 0.0088
0.007 0.006 0.0043 0.0042 0.001 Calculated 11.20 11.90 13.75 17.63
22.21 23.60 27.90 35.87 46.42 151.15 Surface Area (cm.sup.2) area
per 1.60 1.70 1.96 2.52 3.17 3.37 3.99 5.12 6.63 21.59 area
(cm.sup.2/cm.sup.2) area per 114.53 121.69 140.61 132.21 337.62
241.33 285.31 504.36 1374.05 8501.15 volume (cm.sup.2/cm.sup.3)
area per reactor 0.37 0.40 0.46 0.59 0.74 0.79 0.93 1.20 1.55 5.04
volume (cm.sup.2/cm.sup.3) Measured 12.23 12.35 13.63 19.79 15.03
17.11 16.96 23.26 18.45 15.54 Surface Area (cm.sup.2) area per 1.75
1.76 1.95 2.83 2.15 2.44 2.42 3.32 2.64 2.22 area
(cm.sup.2/cm.sup.2) area per 125.06 126.32 139.37 148.37 228.46
174.95 173.48 327.12 546.06 874.25 volume (cm.sup.2/cm.sup.3) area
per reactor 0.41 0.41 0.45 0.66 0.50 0.57 0.57 0.78 0.61 0.52
volume (cm.sup.2/cm.sup.3) *projected electrode area = 7 cm.sup.2
*reactor volume for calculation = 30 cm.sup.3
[0084] For example, 42 mesh with a wire diameter of 0.0055 inches
or 13.97 mm has an open area of 59.1%, and an opening width of
0.018 inches. Specific surface area of the mesh is estimated for 42
mesh at about 11,000 m.sup.2/m.sup.3 based on the volume defined by
the thickness of the mesh and the geometric surface area of a
wire.
[0085] In preferred MEC embodiments, the pore size of the stainless
steel, nickel or titanium mesh is in the range of 0.005-0.02 inch,
inclusive. In preferred MFC embodiments, the pore size of the
stainless steel, nickel or titanium mesh is in the range of
0.005-0.4 inch, inclusive.
[0086] FIG. 3 illustrates a cross sectional view of an embodiment
of a stainless steel, nickel or titanium mesh cathode 30. Wires 32
of the mesh are shown along with an optional first coating 34 on
one side of the mesh and an optional second coating 36 on the
opposing side of the mesh.
[0087] A stainless steel, nickel or titanium mesh included in a
cathode according to embodiments of the present invention can be
shaped to increase surface area. For example, the mesh may be
pleated to achieve an accordion fold.
[0088] In preferred embodiments, the mesh forms a wall defining an
interior space. In further preferred embodiments, the interior
space is open to the exterior of the reactor or to a gas space in
the reactor at one or both ends. Thus, cathodes according to
embodiments of the present invention can be generally tubular in
shape, having a wall defining an interior space, an interior wall
surface, an exterior, and an exterior wall surface. Such generally
tubular cathodes have any of various cross sectional shapes,
including, but not limited to, circular, oblong, square or
rectangular. In an MFC, an inventive tubular cathode is configured
so that air is present inside the tube, and water outside the tube.
In an MEC, an inventive tubular cathode is configured to separate
hydrogen produced from liquid in the reactor where the tube may
contain only gas, or may contain an aqueous medium similar or
different from that in the reactor. For example, the interior space
defined by the wall of the tubular cathode may contain liquid
having a lower or higher pH than the solution containing the
bacteria in order to protect the bacteria from the extreme pH
environment of the tubular solution. The interior of tubular
cathodes of MFCs or MECs may be flushed with solutions or gases to
clean or maintain them.
[0089] FIG. 4 shows a tubular embodiment of a mesh cathode 50.
Illustrated is an optional coating 52 on the side of the mesh
disposed toward the exterior of the cathode and an optional coating
54 on the side of the mesh disposed toward the interior of the
cathode. The mesh is shown at 56.
[0090] Cathode Coatings
[0091] In a further option, a cathode of the present invention may
include one or more coatings on one or more cathode surfaces. In
particular embodiments, one or more coatings are included on an
inner cathode surface, that is, a cathode surface present in the
interior volume of the reaction chamber, and/or an outer surface,
that is, a cathode surface exterior to the reaction chamber. A
cathode surface exterior to the reaction chamber is likely to be
present where a gas cathode is used, where the exterior cathode
surface is in contact with a gas.
[0092] In further embodiments, one or more coatings are included on
an interior wall surface of a tubular cathode and/or an exterior
wall surface of a tubular cathode.
[0093] Exemplary coatings are functionalized to inhibit or allow
passage of a selected substance, such as water and/or oxygen,
through the wall.
[0094] A coating may include a binder, such as an electron or
proton conductive binder.
[0095] One or more coatings may be added to act as cathode
protection layers or diffusion layers, for example.
[0096] A cathode optionally contains one or more cathode shielding
materials. Such a shielding material may preferably include a layer
of a shielding material disposed on any cathode surface, including
an inner cathode surface, that is, a cathode surface present in the
interior volume of the reaction chamber, and an outer surface, that
is, a cathode surface exterior to the reaction chamber. A cathode
surface exterior to the reaction chamber is likely to be present
where a gas cathode is used, where the exterior cathode surface is
in contact with a gas.
[0097] A cathode protective layer, for instance, may be used to
prevent contact of microbes or other materials with the cathode
surface in both electrode assemblies for current producing systems
and for hydrogen gas generation systems. A cathode protection layer
for a current producing microbial fuel cell system can be used as a
support for microbes such as bacterial colonization wherein
bacteria scavenge oxygen in the vicinity of the cathode so it does
not leak into the reactor and it may not directly contact the
anode.
[0098] Thus, in particular embodiments, an inner cathode surface is
protected by a cathode protection layer (CPL). A function of the
CPL is to protect the cathode from biofouling of the catalyst.
Further, a CPL reduces diffusion of carbon dioxide to the cathode
so as to limit methane formation from both abiotic and biotic
sources, or from the action of bacteria, at the cathode. In
embodiments of an inventive system, a CPL is configured such that
it is in contact with an inner surface of a cathode. Thus, for
instance, a CPL may be configured to cover the inner surface of the
cathode partially or wholly, such as by bonding of the CPL to the
cathode.
[0099] The cathode protection layer may contain chemicals or metals
that interfere with bacterial adhesion to the cathode, for example
silver particles or cationic surfactants.
[0100] Optionally, in a further embodiment, a CPL is present in the
interior of an MFC or MEC reaction chamber but not in contact with
the cathode. The inclusion of such a CPL defines two or more
regions of such a reactor based on the presence of the CPL. The CPL
can be proton, liquid, and/or gas permeable barriers, such as a
filter. For example, a filter for inhibiting introduction of large
particulate matter into the reactor may be positioned between the
anode and cathode such that material flowing through the reaction
chamber between the anode and cathode passes through the filter.
Alternatively or in addition, a filter may be placed onto the
cathode, restricting the passage of bacteria-sized particles to the
cathode. Further, a filter may be positioned between an inlet
channel and/or outlet channel and the interior of the reaction
chamber or a portion thereof. Suitable filters may be configured to
exclude particles larger than 0.01 micron-1 micron for example. In
particular embodiments, a CPL includes a "proton diffusion layer"
for selectively allowing passage of material to the vicinity of a
cathode. In one embodiment, a diffusion layer includes an ion
exchange material. Any suitable ion conducting material which
conducts protons may be included in a proton exchange membrane. For
example, a perfluorinated sulfonic acid polymer membrane may be
used. In particular, a proton exchange membrane such as NAFION,
that conducts protons, may be used for this purpose. A further
example of an ion conducting material is polyphenyl sulfone,
available commercially as RADEL R.
[0101] In particular embodiments of the present invention, a
diffusion layer includes an anion exchange material. For example,
the diffusion layer includes an anion exchange material that
conducts anions, associated with protons produced by anodophilic
bacteria, to the cathode, such as a quaternary amine styrene
divinylbenzene copolymer. An included diffusion layer further
functions to inhibit diffusion of gas to or from a cathode relative
to the anode chamber. Without wishing to be bound by theory it is
believed that the protons associated with the negatively charged,
anionic, ion exchange groups, such as phosphate groups,
specifically allow passage of negatively charged anions that
contain positively charged protons but overall carry a net negative
charge, and not allowing passage of positively charged ions and
reducing the diffusion of hydrogen into the anode chamber. Such a
diffusion layer allows for efficient conduction of protons across
the barrier while inhibiting backpassage of hydrogen. An example of
such a diffusion layer material is the anion exchange membrane
AMI-7001, commercially supplied by Membranes International, Glen
Rock, N.J. In addition to membrane form, the diffusion layer can
also include an anion conducting material applied as a paste
directly to a cathode. For example, an anion exchange material can
be used to contain a catalyst applied to a cathode.
[0102] A diffusion layer for an electrode assembly for a current
producing microbial fuel cell system can be configured to allow
oxygen diffusion to the catalyst from the air-facing side into the
conductive electrode matrix, and to reduce oxygen diffusion into
the system.
[0103] An exemplary diffusion layer coated on the air-facing side
of a gas cathode is a carbon/PTFE layer or one or more additional
PTFE diffusion layers. The carbon/PTFE base layer can be prepared
by applying a mixture of carbon powder (Vulcan XC-72) and 30 wt %
PTFE solution (20 .mu.l/mg of carbon powder) onto one side of the
carbon cloth, air-drying at room temperature for 2 h, followed by
heating at 370.degree. C. for 0.5 h. The carbon loading in an
exemplary diffusion layer is 2.5 mg cm.sup.-2.
[0104] In certain MFC cathode embodiments, a oxygen permeable
cathode diffusion layer is included which contains a viscoelastic
polymer. In particular embodiments, the viscoelastic polymer is an
organosilicon compound, particularly a siloxane polymer.
Poly(dimethylsiloxane) (PDMS) is a preferred siloxane polymer
included in a diffusion layer of an inventive cathode according to
certain embodiments. Poly(1-trimethylsilyl-1-propyne) [PTMSP] is a
further example of a preferred siloxane polymer included in a
diffusion layer of an inventive cathode according to certain
embodiments.
[0105] In preferred embodiments, an included viscoelastic polymer
is cured at temperatures of 40.degree. C. or less.
[0106] Oxygen permeable thermoplastics, such as crosslinked
poly(butadiene) are included in an MFC cathode diffusion layer
according to particular embodiments of the present invention.
[0107] In further preferred MFC cathode embodiments, PTFE is
excluded from the cathode diffusion layer.
[0108] In preferred MFC cathode embodiments, an oxygen permeable
cathode diffusion layer includes conductive carbon and a
viscoelastic polymer. Conductive carbon includes in an oxygen
permeable cathode diffusion layer illustratively includes graphite,
carbon nanoparticles such as carbon nanotubes and carbon black.
[0109] The amount of each component and the thickness of the
cathode diffusion layer is adjusted for a particular cathode and
MFC configuration to achieve the desired oxygen diffusion under
given operating conditions. In particular embodiments, a cathode
diffusion layer includes viscoelestic polymer in amounts of
1.times.10.sup.-2-1.times.10.sup.-4 mg/cm.sup.2, inclusive, of mesh
and conductive carbon in amounts of 0.1-10 mg/cm.sup.2, inclusive,
of mesh, although more or less of each component can be used.
[0110] In preferred MFC and MEC cathode embodiments, microorganisms
are excluded from the cathode or are present only in amounts which
produce no detectable effect on MFC or MEC performance.
[0111] In particular embodiments, an outer surface of a cathode is
covered partially or preferably wholly by a cathode diffusion layer
(CDL). The CDL may be directly exposed to the gas phase and
inhibits water leakage through the cathode from the interior of the
reaction chamber.
[0112] Further, in MEC embodiments, a CDL is hydrogen permeable,
allowing hydrogen to freely diffuse from the catalyst in the
cathode into a gas collection chamber, gas conduit or other
component of a gas collection system, such as may be present in an
MEC. A CDL may further provide support for the cathode and may
further form a portion of a wall of a reaction chamber. A CDL can
also help to reduce bacteria from reaching the cathode and fouling
the surface. A CDL includes a hydrogen permeable hydrophobic
polymer material such as polytetrafluoroethylene (PTFE) or like
materials. The thickness of this material can be varied or multiple
layers can be applied depending on the need to reduce water
leakage.
[0113] Cathodes according to embodiments of the present invention
include a metal mesh and a conductive coating, for example carbon
black in a binder of Nafion or PTFE in contact with the metal mesh.
Additional layers can be placed onto this structure, for example, a
PTFE diffusion layer on the air side to inhibit water permeability
and to reduce oxygen diffusion through the cathode and into the
water.
[0114] Cathode Catalyst
[0115] In some MEC embodiments, stainless steel serves as the sole
cathode catalyst. In particular MEC embodiments, the cathode
consists essentially of stainless steel, nickel or titanium in
brush or mesh form. Combinations of stainless steel, nickel and
titanium can be used.
[0116] In some embodiments, stainless steel cathode catalysis is
enhanced through the use of steels that have a nickel content of at
least 5% by weight. In further embodiments, the performance of
stainless steel cathode catalysis is enhanced through the use of
steels that have a nickel content of at least 8% by weight. In
still further embodiments, the performance of stainless steel
cathode catalysis is enhanced through the use of steels that have a
nickel content of at least 15% by weight. In yet further
embodiments, the performance of stainless steel cathode catalysis
is enhanced through the use of steels that have a nickel content of
at least 20% by weight.
[0117] Optionally, a cathode described herein includes an added
catalyst, such as, but not limited to, a nickel or platinum
catalyst. A non-precious metal catalyst such as cobalt
tetramethoxyphenylporphyrin (CoTMPP) can be included.
[0118] In a preferred option, an added nickel catalyst is a nickel
oxide catalyst. For example, one or more nickel oxides is deposited
on a stainless steel and/or nickel cathode by electrochemical
deposition in order to increase catalytic efficiency.
[0119] Activated carbon is an included catalyst in preferred
embodiments of inventive cathodes.
[0120] An included catalyst can be integrated with a cathode by
methods including, but not limited to electrodeposition, a chemical
reaction, and chemical precipitation. A catalyst can be included in
a cathode coating.
[0121] In preferred embodiments, no noble metal catalyst is added
to a cathode of the present invention. While small amounts of noble
metals may be present as impurities in stainless steel, nickel or
titanium used, no noble metal exogenous to the stainless steel,
nickel or titanium is present in preferred embodiments of an
inventive cathode. Noble metals typically included as cathode
catalysts are platinum and palladium. Thus, in preferred
embodiments, no platinum or palladium is added to a cathode of the
present invention. In further preferred embodiments, substantially
no platinum or palladium is present in a cathode of the present
invention. The term "substantially no platinum or palladium" refers
to an undetectable or catalytically negligible amount of platinum
or palladium. For example, where platinum or palladium are
undetectable by multi-channel atomic emission spectrometry or is
present in amounts of 0.01% by weight or less, it is considered
that substantially no platinum or palladium is present in a cathode
of the present invention.
[0122] Anodes
[0123] An anode in embodiments of MFCs an MECs of the present
invention includes a conductive and corrosion-resistant or
non-corroding material, for example carbon paper or cloth, carbon
foam, graphite rods, blocks or fibers either in random bundles or
arranged in brush form (Logan, 2008; Logan, et al., 2007b). An
anode material can be treated to make bacteria more easily adhere
to the surface. In addition, an anode is optionally treated to
increase current densities, for example by using a high-temperature
ammonia gas treatment as described herein.
[0124] Optionally, an anode included in an MFC or MEC is
characterized by high specific surface area, for instance as
described in U.S. patent application Ser. Nos. 11,799,194 and
12/145,722.
[0125] In preferred embodiments, an anode included in embodiments
of MECs and MFCs of the present invention is a brush having
graphite fiber bristles in electrical contact with a conductive
core.
[0126] Electrode Assemblies
[0127] An anode and cathode may have any of various shapes and
dimensions and are positioned in various ways in relation to each
other. In one embodiment, the anode and the cathode each have a
longest dimension, and the anode and the cathode are positioned
such that the longest dimension of the anode is parallel to the
longest dimension of the cathode. In another option, the anode and
the cathode each have a longest dimension, and the anode and the
cathode are positioned such that the longest dimension of the anode
is perpendicular to the longest dimension of the cathode. Further
optionally, the anode and the cathode each have a longest
dimension, and the anode and the cathode are positioned such that
the longest dimension of the anode is perpendicular to the longest
dimension of the cathode. In addition, the anode and the cathode
may be positioned such that the longest dimension of the anode is
at an angle in the range between 0 and 180 degrees with respect to
the longest dimension of the cathode.
[0128] Space between an anode and cathode is minimized to improve
system performance and is generally in the range of 0.1-100 cm,
inclusive.
[0129] Optionally, an inventive system is provided which includes
more than one anode and/or more than one cathode. For example, from
1-100 additional anodes and/or cathodes may be provided. The number
and placement of one or more anodes and/or one or more electrodes
may be considered in the context of the particular application. For
example, in a particular embodiment where a large volume of
substrate is to be metabolized by microbial organisms in a reactor,
a larger area of anodic surface may be provided. Similarly, a
larger area of cathode surface may be appropriate. In one
embodiment, an electrode surface area is provided by configuring a
reactor to include one or more electrodes that project into the
reaction chamber. In a further embodiment, an electrode surface
area is provided by configuring the cathode as a wall of the
reactor, or a portion of the wall of the reactor. The ratio of the
total surface area of the one or more anodes to the total volume of
the interior of the reaction chamber is in the range of about
10000:1-1:1, inclusive, square meters per cubic meter in particular
embodiments. In further embodiments, the ratio is in the range of
about 5000:1-100:1.
[0130] In general, an anode has a surface having a surface area
present in the reaction chamber and the cathode has a surface
having a surface area in the reaction chamber. In one embodiment, a
ratio of the total surface area of anodes to surface area of
cathodes in an inventive system is about 1:1. In one embodiment,
the anode surface area in the reaction chamber is greater than the
cathode surface area in the reaction chamber. This arrangement has
numerous advantages such as lower cost where a cathode material is
expensive, such as where a platinum catalyst is included. In
addition, a larger anode surface is typically advantageous to
provide a growth surface for exoelectrogens to transfer electrons
to the anode. In a further preferred option a ratio of the anode
surface area in the reaction chamber to the cathode surface area in
the reaction chamber is in the range of 1.5:1-1000:1 and more
preferably 2:1-10:1.
[0131] The ratio of the total surface area of the one or more
cathodes to the total volume of the interior of the reaction
chamber is in the range of about 10000:1-1:1, inclusive, square
meters per cubic meter in particular embodiments. In further
embodiments, the ratio is in the range of about 1000:1-10:1. The
total surface area of the cathodes described here is exclusive of
the surface area of any catalyst included in the cathode.
[0132] System Configurations and Components
[0133] A power source for enhancing an electrical potential between
the anode and cathode is included in MECs of the present invention.
Power sources used for enhancing an electrical potential between
the anode and cathode are not limited and illustratively include
grid power, solar power sources, wind power sources. Further
examples of a power source suitable for use in an inventive system
illustratively include a DC power source and an electrochemical
cell such as a battery or capacitor.
[0134] In a particular embodiment, a power supply for an MEC is an
MFC.
[0135] In a particular embodiment, a portion of the hydrogen
generated in an MEC of the present invention is used to power a
hydrogen fuel cell, the hydrogen fuel cell serving as a power
source for the MEC.
[0136] An ion exchange membrane is optionally disposed between an
anode and a cathode in embodiments of the present invention.
[0137] An MEC or MFC according to the present invention may be
configured as a self-contained system in particular embodiments.
Thus, for example, a quantity of a biodegradable substrate is
included in the reactor and no additional substrate is added.sub.--
In further options, additional substrate is added at intervals or
continuously such that the system operates as a batch processor or
as a continuous flow system.
[0138] A hydrogen gas collection system is optionally included in
an inventive MEC such that the hydrogen gas generated is collected
and may be stored for use, or directed to a point of use, such as
to a hydrogen fuel powered device. For example, a hydrogen gas
collection unit may include one or more hydrogen gas conduits for
directing a flow of hydrogen gas from the cathode to a storage
container or directly to a point of use. A hydrogen gas conduit is
optionally connected to a source of a sweep gas. For instance, as
hydrogen gas is initially produced, a sweep gas may be introduced
into a hydrogen gas conduit, flowing in the direction of a storage
container or point of hydrogen gas use. For instance, a hydrogen
collection system may include a container for collection of
hydrogen from the cathode. A collection system may further include
a conduit for passage of hydrogen. The conduit and/or container may
be in gas flow communication with a channel provided for outflow of
hydrogen gas from the reaction chamber. Typically, the conduit
and/or container are in gas flow communication with the cathode,
particularly where the cathode is a gas cathode.
[0139] A channel is included defining a passage from the exterior
of the reaction chamber to the interior in particular embodiments.
More than one channel may be included to allow and/or regulate flow
of materials into and out of the reaction chamber. For example, a
channel may be included to allow for outflow of a gas generated at
the cathode. Further, a channel may be included to allow for
outflow of a gas generated at the anode.
[0140] In a particular embodiment of a continuous flow
configuration, a channel may be included to allow flow of a
substance into a reaction chamber and a separate channel may be
used to allow outflow of a substance from the reaction chamber.
More than one channel may be included for use in any inflow or
outflow function.
[0141] A regulator device, such as a valve, may be included to
further regulate flow of materials into and out of the reaction
chamber. Further, a cap or seal is optionally used to close a
channel. For example, where a fuel cell is operated remotely or as
a single use device such that no additional materials are added, a
cap or seal is optionally used to close a channel.
[0142] A pump may be provided for enhancing flow of liquid or gas
into and/or out of a reaction chamber.
[0143] Exoelectrogenic microbes included in an MFC or MEC
preferably include at least one or more species of exoelectrogenic
bacteria. The terms "exoelectrogenic bacteria" and "anodophilic
bacteria" are used interchangeably herein refer to bacteria that
transfer electrons to an electrode, either directly or indirectly.
In general, exoelectrogenic bacteria are obligate or facultative
anaerobes. Examples of exoelectrogenic bacteria include bacteria
selected from the families Aeromonadaceae, Alteromonadaceae,
Clostridiaceae, Comamonadaceae, Desulfuromonaceae,
Enterobacteriaceae, Geobacteraceae, Pasturellaceae, and
Pseudomonadaceae. These and other examples of bacteria suitable for
use in an inventive system are described in Bond, D. R., et al.,
Science 295, 483-485, 2002; Bond, D. R. et al., Appl. Environ.
Microbial. 69, 1548-1555, 2003; Rabaey, K., et al., Biotechnol.
Lett. 25, 1531-1535, 2003; U.S. Pat. No. 5,976,719; Kim, H. J., et
al., Enzyme Microbial. Tech. 30, 145-152, 2002; Park, H. S., et
al., Anaerobe 7, 297-306, 2001; Chauduri, S. K. et al., Nat.
Biotechnol., 21:1229-1232, 2003; Park, D. H. et al., Appl.
Microbiol. Biotechnol., 59:58-61, 2002; Kim, N. et al., Biotechnol.
Bioeng., 70:109-114, 2000; Park, D. H. et al., Appl. Environ.
Microbial., 66, 1292-1297, 2000; Pham, C. A. et al., Enzyme Microb.
Technol., 30: 145-152, 2003; and Logan, B. E., et al., Trends
Microbiol., 14(12):512-518.
[0144] Exoelectrogenic bacteria preferably are in contact with an
anode for direct transfer of electrons to the anode. However, in
the case of exoelectrogenic bacteria which transfer electrons
through a mediator, the bacteria may be present elsewhere in the
reactor and still function to produce electrons useful in an
inventive process.
[0145] Optionally, a mediator of electron transfer is included in a
fuel cell.sub.-- Such mediators are exemplified by ferric oxides,
neutral red, anthraquinone-1,6-disulfonic acid (ADQS) and
1,4-napthoquinone (NQ). Mediators are optionally chemically bound
to the anode, or the anode modified by various treatments, such as
coating, to contain one or more mediators.
[0146] Exoelectrogenic bacteria may be provided as a purified
culture, enriched in exoelectrogenic bacteria, or even enriched in
a specified species of bacteria, if desired. Pure culture tests
have reported Coulombic efficiencies as high as 98.6% in Bond, D.
R. et al., Appl. Environ. Microbiol. 69, 1548-1555, 2003. Thus, the
use of selected strains may increase overall electron recovery and
hydrogen production, especially where such systems can be used
under sterile conditions. Bacteria can be selected or genetically
engineered that can increase Coulombic efficiencies and potentials
generated at the anode.
[0147] Further, a mixed population of bacteria may be provided,
including exoelectrogenic anaerobes and other bacteria.
[0148] A biodegradable substrate included in a microbial fuel cell
according to embodiments of the present invention is oxidizable by
exoelectrogenic bacteria or biodegradable to produce a material
oxidizable by exoelectrogenic bacteria.
[0149] A biodegradable substrate is an organic material
biodegradable to produce an organic substrate oxidizable by
exoelectrogenic bacteria in preferred embodiments. Any of various
types of biodegradable organic matter may be used as "fuel" for
bacteria in an MEC or MFC, including carbohydrates, amino acids,
fats, lipids and proteins, as well as animal, human, municipal,
agricultural and industrial wastewaters. Naturally occurring and/or
synthetic polymers illustratively including carbohydrates such as
chitin and cellulose, and biodegradable plastics such as
biodegradable aliphatic polyesters, biodegradable
aliphatic-aromatic polyesters, biodegradable polyurethanes and
biodegradable polyvinyl alcohols. Specific examples of
biodegradable plastics include polyhydroxyalkanoates,
polyhydroxybutyrate, polyhydroxyhexanoate, polyhydroxyvalerate,
polyglycolic acid, polylactic acid, polycaprolactone, polybutylene
succinate, polybutylene succinate adipate, polyethylene succinate,
aliphatic-aromatic copolyesters, polyethylene terephthalate,
polybutylene adipate/terephthalate and polymethylene
adipate/terephthalate.
[0150] Organic substrates oxidizable by exoelectrogenic bacteria
are known in the art. Illustrative examples of an organic substrate
oxidizable by exoelectrogenic bacteria include, but are not limited
to, monosaccharides, disaccharides, amino acids, straight chain or
branched C.sub.1-C.sub.7 compounds including, but not limited to,
alcohols and volatile fatty acids. In addition, organic substrates
oxidizable by exoelectrogenic bacteria include aromatic compounds
such as toluene, phenol, cresol, benzoic acid, benzyl alcohol and
benzaldehyde. Further organic substrates oxidizable by
exoelectrogenic bacteria are described in Lovely, D. R. et al.,
Applied and Environmental Microbiology 56:1858-1864, 1990. In
addition, a provided substrate may be provided in a form which is
oxidizable by exoelectrogenic bacteria or biodegradable to produce
an organic substrate oxidizable by exoelectrogenic bacteria.
[0151] Specific examples of organic substrates oxidizable by
exoelectrogenic bacteria include glycerol, glucose, acetate,
butyrate, ethanol, cysteine and combinations of any of these or
other oxidizable organic substances.
[0152] The term "biodegradable" as used herein refers to an organic
material decomposed by biological mechanisms illustratively
including microbial action, heat and dissolution. Microbial action
includes hydrolysis, for example.
[0153] Methods
[0154] Methods of producing electricity or hydrogen using microbial
fuel cells or microbial electrolysis cells including an inventive
cathode are provided according to the present invention.
[0155] A biological process for producing hydrogen or electric
current according to embodiments of the present invention includes
providing an MEC or MFC, the MEC or MFC including a reactor having
an interior; providing exoelectrogenic bacteria disposed within the
interior of the reactor; introducing a biodegradable organic
material oxidizable by an oxidizing activity of the exoelectrogenic
bacteria; incubating the organic material oxidizable by the
exoelectrogenic bacteria under oxidizing reaction conditions such
that electrons are produced and transferred to an anode. In an MFC,
the electrons are transferred to the anode, and, through a load
such as a device to be powered, to a stainless steel, nickel or
titanium-containing cathode. Protons and electrons then react with
oxygen at the cathode, producing water. In an MEC, the electrons
are transferred to the anode and a power source is activated to
increase a potential between the anode and a stainless steel,
nickel or titanium-containing cathode, such that electrons and
protons combine to produce hydrogen gas. Preferably, the activation
of the power source includes application of a voltage in the range
of 25-1000 millivolts, preferably in the range of 50-900
millivolts.
[0156] In operation, reaction conditions include variable such as
pH, temperature, osmolarity, and ionic strength of the medium in
the reactor.
[0157] In highly preferred embodiments, alkaline reactor conditions
in an MEC or MFC reactor are avoided and the pH-1 of the medium in
the reactor is in the range of pH 3-pH 9, inclusive, and preferably
between pH 5-pH 8.5 inclusive. It is noted that conditions for use
of a cathode according to the present invention in an MEC are
significantly different compared to conditions of oxygen reduction
in seawater. Hydrogen evolution in an MEC takes place in neutral pH
solutions, such as pH 5-9, over a large range of salinities. In
contrast to previous methods, metals, such as stainless steel, are
used in methods of the present invention as catalysts for hydrogen
evolution at neutral pH. It is a further aspect of inventive
cathodes that nickel oxides work well for hydrogen evolution in
neutral pH conditions and in MECs.
[0158] An aqueous medium in a reaction chamber of an MEC or MFC of
the present invention is formulated to be non-toxic to
exoelectrogenic microbes in contact with the aqueous medium.
Further, the medium or solvent may be adjusted to a be compatible
with exoelectrogenic microbe metabolism, for instance by adjusting
pH to be in a desired range, by adding a buffer to the medium or
solvent if necessary, and by adjusting the osmolarity of the medium
or solvent by dilution or addition of a osmotically active
substance. Ionic strength may be adjusted by dilution or addition
of a salt for instance. Further, nutrients, cofactors, vitamins and
other such additives may be included to maintain a healthy
bacterial population, if desired, see for example examples of such
additives described in Lovley and Phillips, Appl. Environ.
Microbiol., 54(6):1472-1480.
[0159] Reaction temperatures are typically in the range of about
10-40.degree. C. for non-thermophilic bacteria, although the device
may be used at any temperature in the range of 0 to 100.degree. C.
by including suitable bacteria for growing at selected
temperatures. However, maintaining a reaction temperature above
ambient temperature may require energy input and it is preferred to
maintain the reactor temperature at about 15-25.degree. C. without
input of energy. Reaction temperatures in the range of
16-25.degree. C., inclusive or more preferably temperatures in the
range of 18-24.degree. C., inclusive and further preferably in the
range of 19-22.degree. C., inclusive, allow hydrogen generation,
electrode potentials, Coulombic efficiencies and energy recoveries
comparable to reactions run at 32.degree. C. which is generally
believed to be an optimal temperature for anaerobic growth and
metabolism, including oxidation of an organic material. In
particular embodiments, an MFC or MEC reactor is operated at
temperatures up to about 40.degree. C. at start-up and the
temperature is then allowed to operate at ambient temperatures in
the range of 10-40.degree. C.
[0160] Ionic strength of a medium in a reactor is preferably in the
range of 50-500 millimolar, more preferably in the range of 75-450
millimolar inclusive, and further preferably in the range of
100-400 millimolar, inclusive.
[0161] Methods for Fabricating Cathodes
[0162] Methods are provided according to embodiments of the present
invention which include fabricating a cathode for an MEC or MFC
without exposing the cathode to temperatures above 100.degree. C.
and/or pressures above ambient pressure. In particular embodiments,
a coating included in a cathode of the present invention is applied
to a stainless steel, nickel or titanium mesh without pressure
application, such as by painting the mesh with a desired coating so
that the coating adheres to the mesh and is present in the pores,
forming a continuous coating on one or both sides of the mesh. In
further particular embodiments, a coating included in a cathode of
the present invention is applied to a stainless steel, nickel or
titanium mesh and is not exposed to temperatures above 100.degree.
C. The term "ambient pressure" refers to air pressure of the
surrounding atmosphere, generally about 1 atmosphere. The described
preference against exposure to pressures above ambient pressure is
intended to exclude "hot-press" application of materials in
preferred embodiments.
[0163] Embodiments of inventive compositions and methods are
illustrated in the following examples. These examples are provided
for illustrative purposes and are not considered limitations on the
scope of inventive compositions and methods.
Examples
Example 1
[0164] Cathodes
[0165] SS brush cathodes (Gordon Brush Mfg Co., Inc., Commerce,
Calif.) were made of grade 304 SS, which has the composition: 0.08%
C, 2% Mn, 0.045% P, 0.03% S, 1% Si, 18-20% Cr, and 8-11% Ni
(balance Fe) (ASTM. Document number A 959-07. Standard guide for
specifying harmonized standard grade compositions for wrought
stainless steels. Table 1. Chemical Composition Limits, %., Oct. 4,
2008). The bristles (0.008 cm diameter) were wound into a twisted
SS core (0.20 cm diameter) using an industrial brush manufacturing
machine. The brushes were 2.5 cm long and 2.5 cm in diameter. On
the basis of the mass and estimated surface area of the bristles,
each brush (100% loading case) had 310 cm.sup.2 of surface area,
producing 2500 m.sup.2/m.sup.3-brush volume (95% porosity), for a
specific surface area of AS=650 m.sup.2/m.sup.3 of reactor volume.
In some tests, brushes with reduced bristle loadings of 50%, 25%,
and 10% were used, with surface areas of 160 cm.sup.2 (AS=340
m.sup.2/m.sup.3), 110 cm.sup.2 (AS=240 m.sup.2/m.sup.3), and
m.sup.2/m.sup.3), respectively. These areas include the surface
area of the SS core, which is estimated at 2.4 cm.sup.2 (5.1
m.sup.2/m.sup.3) based on the projected area of a cylinder. A flat
piece of grade 304 SS (McMaster-Carr, Cleveland, Ohio) was used in
some tests (surface area of 7 cm.sup.2). SS cathodes were cleaned
before use by sonication in an ultrasonic cleaner (model 1510,
Branson, Danbury, CT) for 10 min in 70% ethanol, followed by
rinsing with DI water, and sonication again for 5 min in fresh DI
water. In one test a graphite fiber brush electrode containing a
titanium wire core (surface area of 0.22 m.sup.2; AS=4600
m.sup.2/m.sup.3) (Logan, B et al Environ. Sci. Technol. 2007, 41
(9), 3341-3346) was used as the cathode.
[0166] Reactor Construction
[0167] FIG. 5 diagramatically shows three different MEC
architectures used to determine the effect of cathode brush
architecture, with all reactors containing an ammonia treated,
graphite fiber brush anode in which ammonia gas treatment of an
anode is accomplished using a thermogravimetric analyzer. For this
procedure, the furnace temperature was ramped up to 700.degree. C.
at 50.degree. C./min using nitrogen gas (70 mL/min) before
switching the gas feed to 5% NH.sub.3 in helium gas. The anode is
held at 700.degree. C. for 60 min. before being cooled to room
temperature under nitrogen gas (70 mL/min) over 120 min.
[0168] The first reactor (V=28 mL) contained a 100% loaded SS brush
oriented vertically above and parallel to the core of the anode
(Reactor VB). In order to reduce the spacing to 0.5 cm between the
electrodes, both brushes were cut in half using scissors, each one
forming a half-cylinder. The half SS brush had a reduced surface
area of A=230 cm.sup.2 but an increased specific surface area of
AS=810 m.sup.2/m.sup.3. An anaerobic gas collection tube was
installed above the brush cathode. A second reactor (V=48 mL) was
made by combining a cube-shaped MFC to a second cube-shaped reactor
that was 2.5 cm in length and had a gas collection tube attached on
top (Reactor HB). Reactor HB was used to examine SS brush cathodes
with different surface areas and the graphite brush cathode, with
each cathode brush inserted perpendicular to the core of the anode.
A third reactor contained either a Pt/C cathode (0.5
mg-Pt/cm.sup.2) or a flat SS cathode (Reactor FC). Both flat
cathodes had specific surface areas of AS=25 m.sup.2/m.sup.3. Prior
to starting a batch cycle the gas collection tubes were crimped
shut.
[0169] Startup and Operation
[0170] The brush anodes were first enriched in an MFC using the
effluent from an active MFC. The anodes were transferred to MECs
and fed sodium acetate (1 g/L; J. T. Baker) in a 50 mM phosphate
buffer medium (PBS; Na.sub.2HPO.sub.4, 4.58 g/L; and
NaH.sub.2PO.sub.4H.sub.2O, 2.45 g/L, pH=7.0) and nutrient solution
(NH.sub.4Cl, 0.31 g/L; KCl, 0.13 g/L; trace vitamins and minerals
having a final solution conductivity of 7.5 mS/cm. At the end of
each batch cycle, the crimp tops were removed, the contents
drained, and the reactors left exposed to air for 20 min to help
inhibit the growth of methanogens. After adding the medium and
recrimping the collection tubes, the reactors were sparged for 15
min with ultrahigh purity nitrogen (UHP) (99.998%), covered
wi.sub.th aluminum foil to prevent the growth of phototrophic
microorganisms, and placed in a constant temperature room
(30.degree. C.). Performance of the reactors was evaluated in terms
of current density and continuous gas production rate using a
respirometer. Gas analysis as previously described was performed
for the optimized reactor (Reactor VB) (Call, D. et al, Environ.
Sci. Technol. 2008, 42 (9), 3401-3406). Complete substrate removal
was assumed for each batch cycle, equivalent to a chemical oxygen
demand (COD) of 0.022 g-COD. A fixed voltage (E.sub.ap) of 0.6 V
was applied to the reactor circuit using a power source (model
3645A; Circuit Specialists, Inc., Mesa, Ariz.), and the current was
determined by measuring the voltage across a 10.OMEGA. resistor. An
Ag/AgCl reference electrode (RE-5B; BASi, West Lafayette, Ind.) was
placed in each reactor, with the cathode potential recorded using a
multimeter (Model 2700; Keithley Instruments, Inc., Cleveland,
Ohio).
[0171] Effect of Cathode Surface Area
[0172] The impact of cathode surface area was evaluated using MECs
with horizontally placed brush cathodes (Reactor HB). Varying the
SS brush bristle loadings did not substantially impact current
generation (FIG. 6). For brush bristle loadings of 50-100%, the,
current density remained around 90 A/m.sup.3. Lowering the bristle
loading below 50% resulted in a slight decrease in current density
to 85.+-.3 A/m.sup.3 for the 25% loaded brush and 78.+-.4 A/m.sup.3
for the 10% loaded brush. With no brush bristles (base core only),
the MEC generated 24.+-.0 A/m.sup.3, indicating there was a
significant level of activity due to the SS core on current
density. The cathodic overpotential decreased with the increasing
bristle loadings from no bristles up to 25% bristle loading (FIG.
7). The 50% loaded brush exhibited the lowest cathodic
overpotential of -0.968 (0.007 V, while the 100% loaded brush
reached -0.990.+-.0.002 V.sub.-- The brush core with no bristles
had the highest cathodic overpotential of -1.082.+-.0.005 V (vs
Ag/AgCl).
[0173] Current Densities Using Other Cathodes
[0174] To examine the impact of material composition on current
generation, a graphite fiber brush cathode containing a titanium
wire core was tested in Reactor FIB. Although the specific surface
area of the graphite brush was 7 times larger than the 100% SS
brush tested, current production was substantially lower. A current
density of 1.7.+-.0.0 A/m.sup.3 was achieved after three days (FIG.
8). The SS brush core with no bristles and identical electrode
spacing generated a current density 14 times larger than the
graphite brush. Thus, large surface area alone could not account
for the performance of the SS brushes. The importance of the SS as
a catalyst was further verified by using a flat SS cathode in
Reactor FC. Although the specific surface area of the flat SS
cathode was more than a hundred fold smaller than the graphite
brush cathode, current generation was greater (64.+-.2 A/m.sup.3).
The current density produced by the flat SS cathode (2.6 cm
electrode spacing) was also 2.7 times greater than the SS brush
core (24.+-.0 A/m.sup.3; 3.5 cm electrode spacing). Although the
flat SS cathode had a slightly larger surface area (A=7 cm.sup.2)
than the SS brush core (A=2.4 cm.sup.2), the higher current density
of the flat SS cathode suggests that the orientation and distance
of the cathode was more important for increased current density
than surface area.
[0175] Comparison to a Platinized Cathode
[0176] Because the brush bristle loadings did not have an
appreciable impact on current production, it was believed that the
main factor limiting power generation was electrode distance.
Therefore, a fully loaded SS brush was trimmed in half and placed
as close as possible above a similarly trimmed graphite brush anode
(Reactor VB, AS) 810 m.sup.2/m.sup.3) in order to create a
configuration capable of generating current densities similar to
Pt/C cathodes. During the first few cycles, the current density was
greater in the MEC using the Pt/C cathode (Reactor FC) than in the
MEC with the vertically aligned SS brush cathode (FIG. 9). Within
four cycles, however, Reactor VB was producing the highest current
density of 194.+-.1 A/m.sup.3, compared to 182.+-.2 A/m.sup.3 for
Reactor FC. For the final three batch cycles, both reactors
generated a similar average current density, with Reactor FC
reaching 188.+-.10 A/m.sup.3 and Reactor VB obtaining 186.+-.2
A/m.sup.3. The higher current density of Reactor VB with the SS
brush was a result of a lower cathodic overpotential than that of
Reactor FC with the Pt/C cathode (FIG. 10). During the first batch
cycle, the Pt/C cathode had a higher overpotential than that of the
SS brush, likely due to the higher current density. By the second
cycle, both the SS brush and Pt/C cathode exhibited roughly the
same overpotential, but several later cycles the Pt/C cathode
showed an increase in overpotential (cycles 3 and 4). This trend
may have been due to minor Pt catalyst inactivation in combination
with an activation of the SS for the hydrogen evolution reaction
(HER). After the first two cycles of reactor acclimation, the SS
cathode in Reactor VB produced a cathode potential of
-0.910.+-.0.002 V, whereas the Pt/C cathode exhibited a higher
overpotential with a value of -0.924.+-.0.003 V. These potentials
correspond to cathodic losses of about 0.29 V for the SS brush and
0.30 V for the Pt/C cathode relative to the equilibrium potential
of hydrogen formation (-0.62 V vs Ag/AgCl).
[0177] Energy Recoveries and Production Rates
[0178] Hydrogen production, energy recovery, and hydrogen recovery
results were calculated as described in Logan, B. E. et al,
Environ. Sci. Technol. 2008, 42 (23), 8630-8640; Call, D. et al,
Environ. Sci. Technol. 2008, 42 (9), 3401-3406). The recoveries and
production rates for the SS brush in Reactor VB were averaged over
the last three cycles in FIG. 9. Relative to only the electrical
energy input, the energy recovery reached .eta..sub.E=221.+-.8%.
When the substrate energy was also included, the overall energy
recovery was .eta..sub.E+s=78.+-.5%. The cathodic hydrogen recovery
was r.sub.CAT=83.+-.8%, and the average hydrogen production rate
was Q=1.7.+-.0.1 m.sup.3-H.sub.2/m.sup.3-d.
[0179] Linear Sweep Voltammetry
[0180] Linear sweep voltammetry (LSV) was performed on a
potentiostat (model PC4/750, Gamry Instruments, Warminster, Pa.)
with 1 mV/s rates on the cathodes (100% loaded SS brush, flat SS,
and Pt/C) at 30.degree. C. in a 28 mL reactor. The LSV reactor also
included an Ag/AgCl reference electrode (Princeton Applied
Research, Oak Ridge, Tenn.) and a 2 cm.sup.2 pure platinum foil
counter electrode. The reactor was filled with 50 mM PBS, pH 7.0,
without trace nutrients and sparged with UHP nitrogen.
Chronopotentiometry at 50 mA for 24 h in 50 mM PBS was performed to
simulate accelerated use of a 100% loaded SS brush. Stripping was
performed with cyclic voltammetry in 0.5 MH.sub.2SO.sub.4 from -0.5
to +1.5 V vs Ag/AgCl at 250 mV/s.
[0181] LSV scans performed only on the cathodes and not the
assembled MECs indicated that the Pt/C cathode could initially
operate at 0.1-0.2 V lower cathodic overpotentials than those of
the 100% loaded SS brush (FIG. 11). The activity of the SS brush
for hydrogen evolution improved after simulating accelerated use,
resulting in catalytic activity similar to the Pt/C cathode. In the
initial LSV, the SS brush had a resting potential of +0.06 V vs NHE
(where the current was zero) and small positive currents for more
positive potentials. After accelerated use, the resting potential
shifted to -0.08 V vs NHE. To remove any possible SS surface
corrosion products that may have accumulated during the accelerated
use, the SS brush was stripped using cyclic voltammetry until the
currents corresponding to hydrogen and oxygen evolution became
constant (about five cycles). A third LSV performed on the SS brush
after cyclic voltammetry produced results very similar those
obtained after the initial use LSV (data not shown), suggesting
that corrosion products on the surface of the SS that occur with
use cause the SS to become more active toward hydrogen evolution.
Compared to a flat SS cathode, the SS brush exhibited a lower
overpotential, particularly at lower currents, thus confirming the
effectiveness of the high surface area. Current generation occurred
below the standard state theoretical potential for hydrogen
production (-0.42 V vs NHE; P.sub.H2=1 atm) in FIG. 11 because the
LSV was performed under atmospheric conditions where the partial
pressure of hydrogen (P.sub.H2=5.times.10-5 atm) lowers the
theoretical potential to -0.29 V.
[0182] High current densities were achieved in MECs without a
precious metal catalyst by using high surface area SS cathodes.
Example 2
[0183] Hydrogen production in an MEC using a cathode made of
stainless steel, nickel, and stainless steel with a high nickel
content. Single-chamber MEC reactors were constructed from
polycarbonate cut to produce a cylindrical chamber 4 cm long by 3
cm in diameter (empty bed volume of 28 mL). The anodes were ammonia
treated graphite brushes, 25 mm diameter.times.25 mm length, 0.22
m.sup.2 surface area. Ammonia treatment of the graphite brushes was
accomplished as described in Example 1.
[0184] Reactors were inoculated with the anode solution from
another acetate-fed MEC reactor that had been running for over 1
year and acetate (1 g/L) in medium. The medium used was a 50 mM
phosphate buffer solution (4.58 g/L Na.sub.2HPO.sub.4 and 2.45 g/L
Na.sub.2HPO.sub.4H.sub.2O; pH=7.0), 0.31 g/L NH.sub.4Cl, 0.13 g/L
KCl, and trace vitamins and minerals.
[0185] Cathodes of stainless steel alloys 304, 316, 420 and A286 or
nickel alloys 201, 400, 625 and HX were made by cutting sheet metal
(McMaster-Carr, IL) into 3.8 cm diameter disks. Metal compositions
are listed in Table I. A platinum metal disk (99.9% purity) used
for comparison to these other metal materials was pre-cut by the
manufacturer (Hauser & Miller, MO). Metal cathodes were cleaned
with ethanol before placing them in the reactors. Carbon cloth
cathodes (projected surface area of 7 cm.sup.2) were made using a
platinum catalyst (0.5 mg cm.sup.-2)
TABLE-US-00002 TABLE I Stainless Steel and Nickel Alloys
Composition (% by weight) Alloy Fe C Mn P S Mo Si Cr Ni Cu Other
Other SS 304 0.08 2 0.45 0.03 0 1 18-20 8-10.5 1 SS 316 0.08 2 0.05
0.03 2-3 1 16-18 10-14 2-3 SS 420 0.15 1 0.04 0.03 0 1 13 SS A286
0.08 2 0.025 0.025 1-1.5 1 13.5-16 24-27 1.9-2.35 Ti Ni 201 0.4
0.02 0.35 99 0.25 .35 Si .01 S Ni 400 1.6 1.1 65.1 32 Ni 625 2.5 9
21.5 61 3.6 Nb Ni HX 18 0.1 9 22 47 0.6 W 1.5 Co
[0186] A power source (3645A; Circuit Specialists, Inc., AZ) was
used to apply either 0.6 or 0.9V to the reactors. After each cycle,
the reactors were drained, refilled with substrate solution, and
sparged with ultra high purity nitrogen gas for 5min. The reactors
were maintained in a 30.degree. C. constant temperature room. Once
reactors reached similar current (.about.0.57mA cm .sup.2) and gas
production volumes (.about.30 ml) for three consecutive cycles
using carbon cloth cathodes, the cathodes were replaced with sheet
metal cathodes. All reactors were run in duplicate, and tests with
new cathodes were run for at least three consecutive cycles.
[0187] Analysis
[0188] Gas production was measured using a respirometer (AER-200,
Challenge Technology, AZ). Gas flowing out of the respirometer was
collected in sampling gas bags (250 ml capacity, Cali-5 bond,
Calibrated Instruments Inc., NY). The composition of the MEC
headspace and the gas bags were analyzed using two gas
chromatographs (models 8610B and 310, SRI Instruments, CA) equipped
with Alltech Molesieve 5A 80/100 stainless steel-tubing columns and
thermal conductivity detectors (TCDs). Argon was used as the
carrier gas for H.sub.2, O.sub.2, N.sub.2 and CH.sub.4 analysis,
and helium was used as the carrier gas for CO.sub.2 analysis.
Voltage across an external resistor (R.sub.ex=10.OMEGA.) was
measured using a multimeter (2700, Keithley Instruments, Inc., OH)
to calculate current. Electrochemical experiments were conducted
with a potentiostat (PC4/750TM, Echem Analyst, v. 5.5, Gamry
Instruments, PA). CV scans were done over three cycles, from 0 to 1
V, at a scan rate of 1 mVs.sup.-1 on the MEC cells after use.
Scanning electron microscopy/energy dispersive X-ray spectroscopy
(SEM-EDS) analysis was done at 20 kV (Quanta 200, FEI, OR).
[0189] Calculations
[0190] Hydrogen recovery, energy recovery, volumetric density and
hydrogen production rates were used to evaluate reactor performance
(2). The theoretical number of hydrogen moles produced
(n.sub.H2,COD), based on COD removal is:
n H 2 , COD = b e 02 v L .DELTA. COD 2 M 02 ( 1 ) ##EQU00001##
where b.sub.eO2=4 is the number of electrons exchanged per mole of
oxygen, v.sub.L=32 ml the volume of liquid in the reactor,
M.sub.O2=32 g mol.sup.-1 the molecular weight of oxygen, 2 the
number of moles of electrons per mole of hydrogen gas, and
.DELTA.COD the change in substrate concentration (g L.sup.-1).
[0191] The theoretical number of hydrogen moles that can be
recovered based on the measured current (n.sub.H2,cat) is:
n H 2 , cat = .intg. t = 0 t I t 2 F ( 2 ) ##EQU00002##
where I=V/R.sub.ex is the current (A) calculated from the voltage
across the resistor (10.OMEGA.) and di is the time interval (1,200
s) for data collection.
[0192] The overall hydrogen recovery (r.sub.H2,COD) is the ratio of
hydrogen recovered compared to the maximum theoretical hydrogen
produced based on substrate utilization:
r H 2 , COD = n H 2 n H 2 , COD ( 3 ) ##EQU00003##
where n.sub.H2 is the actual number of hydrogen moles produced. The
cathodic hydrogen recovery (r.sub.H2,cat) is the fraction of
electrons that are recovered as hydrogen gas from the total number
of electrons that reach the cathode, or
r H 2 , cat = n H 2 n H 2 , cat ( 4 ) ##EQU00004##
[0193] The Coulombic efficiency (C.sub.E) is the ratio of electrons
recovered as hydrogen gas relative to the total electrons available
from substrate consumption, calculated as:
C E = n H 2 , cat n H 2 , COD = r H 2 , COD r H 2 , cat ( 5 )
##EQU00005##
[0194] The energy efficiency relative to electrical input
(.eta..sub.E) is the ratio of energy content of hydrogen produced
to the input electrical energy:
.eta. E = W H 2 W E = n H 2 .DELTA. H H 2 1 n ( IE ap .DELTA. t - I
2 R ex .DELTA. t ) ( 6 ) ##EQU00006##
where W.sub.H2(kJ) is the energy produced by hydrogen, W.sub.E(kJ)
the amount of energy added to the circuit by the power source minus
the losses across the resistor, .DELTA.H.sub.H2=285.83 kJ/mol the
energy content of hydrogen based on the heat of combustion and
E.sub.ap (V) the voltage applied by the power source. The number of
moles of substrate consumed during a batch cycle based on COD
removal (n.sub.s) is:
n s = .DELTA. COD v L M s ( 7 ) ##EQU00007##
where M.sub.S=82 g mol.sup.-1 is the substrate's molecular weight.
When using sodium acetate, the molecular weight needs to be
multiplied by a conversion factor of 0.78 g COD g.sup.-1 sodium
acetate. The energy efficiency relative to the substrate
(.eta..sub.S) is:
.eta. S = W H 2 W S = n H 2 .DELTA. H H 2 .DELTA. H S n S ( 8 )
##EQU00008##
where .DELTA.Hs=870.28 kJ/mol is the heat of combustion of the
substrate. The overall energy recovery based on both electric and
substrate inputs (.eta..sub.E+S) is:
.eta. E + S = W H 2 W E + W S ( 9 ) ##EQU00009##
[0195] The hydrogen production rate (Q) (m.sup.3
H.sub.2m.sup.-3d.sup.-1) was evaluated in terms of current produced
per volume of reactor and the gas rate per volume as:
Q=3.68.times.10.sup.-5I.sub.VTr.sub.H2,cat (10)
where 3.68.times.10.sup.-5 is a constant that includes Faraday's
constant, a pressure of 1 atm and unit conversions, I.sub.V (A
m.sup.-3) is the volumetric current density averaged over a 4 hour
period of maximum current production and divided by the liquid
volume, and T(K) is the temperature.
[0196] The Butler-Volmer reaction for hydrogen evolution was used
to determine the catalytic performance of the metals, where the
reverse current was considered negligible. CV scans for the
complete MEC's were converted to Tafel plots by plotting log I as a
function of voltage. The transformed Butler-Volmer equation was
used to obtain slopes and y-intercepts via linear regression of the
Tafel plots using:
log J = log J 0 + .alpha. c n e F 2.303 RT ( E - E 0 ) ( 11 )
##EQU00010##
where J (A cm.sup.-2) is the current density, J.sub.0 (A cm.sup.-2)
is the exchange current density, .alpha..sub.c is the cathodic
transfer coefficient, n.sub.e is the number of electrons per
reaction, E (V) is the working potential and E.sub.0 (V) is the
equilibrium potential. The equilibrium potential (E.sub.0) is equal
to the hydrogen potential (E.sub.H2):
E H 2 = 0 + 0.0602 log [ 1 / 2 H 2 H + ] = 0 - 0.0602 pH + 0.0301
log ( p H 2 ) ( 12 ) ##EQU00011##
[0197] The equilibrium potential E.sub.0=E.sub.H2=-0.4458V for the
experimental conditions presented: T=30.degree. C., pH=7 and a
partial pressure for hydrogen p.sub.H2=0.15 atm. The hydrogen
partial pressure value was the average hydrogen gas composition of
all MEC reactors over complete cycles.
[0198] SS alloys A286 (21.2.+-.2.2 ml) and 304 (19.1.+-.1.1 ml)
produced twice as much hydrogen as Ni 201 (9.5.+-.1.6 ml) or SS 316
(9.5.+-.2.6 ml) at an applied voltage of 0.9V (FIG. 12). Platinum
sheet metal produced slightly less hydrogen gas (18.9.+-.5.4 nil)
than SS A286 and SS 304. While gas production was consistent over
multiple cycles with the SS and Ni materials, gas production with
platinum sheet metal decreased with continued use. The total gas
production during the first cycle using platinum was 34.5.+-.2.6
ml, but only 19.2.+-.1.3 ml by the third cycle. This change in gas
production resulted in a higher variability of the gas produced
with platinum than with the other metals.
TABLE-US-00003 TABLE II MEC results for different metal cathodes
(stainless steel, nickel and platinum) at an applied voltage of 0.9
V Metal r.sub.H2,cat (%) r.sub.H2, COD (%) .eta..sub.E (%)
.eta..sub.E+S (%) I.sub..nu. (A/m.sup.3) Q (m.sup.3/m.sup.3 d)
H.sub.2 (%) SS 304 53 .+-. 1 49 .+-. 0 90 .+-. 2 38 .+-. 1 100 .+-.
4 0.59 .+-. 0.01 77 .+-. 1 SS 316 27 .+-. 6 25 .+-. 6 47 .+-. 10 19
.+-. 4 116 .+-. 1 0.35 .+-. 0.08 55 .+-. 10 SS 420 43 .+-. 2 38
.+-. 1 73 .+-. 3 30 .+-. 1 122 .+-. 10 0.58 .+-. 0.07 67 .+-. 2 SS
A286 61 .+-. 3 62 .+-. 6 107 .+-. 5 46 .+-. 3 222 .+-. 4 1.50 .+-.
0.04 80 .+-. 2 Ni 201 27 .+-. 4 26 .+-. 3 46 .+-. 7 20 .+-. 3 127
.+-. 8 0.38 .+-. 0.04 57 .+-. 3 Ni 400 31 .+-. 5 31 .+-. 8 53 .+-.
9 23 .+-. 5 116 .+-. 9 0.41 .+-. 0.10 62 .+-. 8 Ni 625 43 .+-. 9 41
.+-. 13 75 .+-. 16 31 .+-. 8 160 .+-. 22 0.79 .+-. 0.27 67 .+-. 9
Ni HX 40 .+-. 8 38 .+-. 7 68 .+-. 14 29 .+-. 5 124 .+-. 14 0.55
.+-. 0.11 69 .+-. 4 Pt 47 .+-. 2 46 .+-. 4 81 .+-. 3 35 .+-. 2 129
.+-. 7 0.68 .+-. 0.06 74 .+-. 2
[0199] Table II is a summary of MEC results for different metal
cathodes (stainless steel, nickel and platinum) at an applied
voltage of 0.9 V.
[0200] The best performing alloys based on MEC recoveries and
efficiencies were SS A286, SS 304 and Ni 625 (Table II) (Eap=0.9
V). Of these three materials, SS A286 consistently had the best
performance for all parameters used to evaluate the MECs (rH.sub.2,
cat, rH.sub.2, COD, .eta.E, .eta.E+S, IV, Q, and H.sub.2 content).
The hydrogen production rate was significantly higher for SS A286
(Q=1.5 m.sup.3 m.sup.-3 day.sup.-1) than for any of the other
metals, including platinum (Q=0.68 m.sup.3m.sup.-3 day.sup.-1). The
platinum sheet metal displayed only average performance compared to
the other metals, being surpassed by both SS 304 and SS A286 in
terms of hydrogen recoveries and energy efficiencies at an applied
voltage of 0.9 V. Overall gas production was reduced for all the
metals at a lower applied voltage of 0.6V (average=6.8.+-.3.9 ml
H.sub.2) compared to 0.9V (21.3.+-.3.8 ml H.sub.2) (FIG. 13).
Hydrogen concentrations at 0.6V were reduced to 17.2.+-.13.2%
H.sub.2 (vs. 67.5.+-.8.6% H.sub.2 at 0.9 V), and methane
concentrations increased (69.0.+-.13.3% at 0.6V vs. 23.9.+-.8.3% at
0.9 V). Ni 625 performed better than the other metals in terms of
total hydrogen gas production at this lower applied voltage (6.61
ml but the product gas was mainly methane (47.3% CH.sub.4, 40.8%
H.sub.2, 11.9% CO.sub.2). Platinum sheet metal produced only 11.2
ml H.sub.2, with a gas composition of 49.8% CH.sub.4, 35.0% H.sub.2
and 15.1% CO.sub.2. Maximum current densities at 0.9V were higher
for both SS A286 (1.01.+-.0.18 mAcm.sup.-2) and Ni 625
(0.73.+-.0.099 mAcm.sup.-2) than for the platinum sheet metal
(0.59.+-.0.03 mAcm.sup.2) (FIG. 14). At 0.6 V, the difference
between current densities of these metals was almost non-existent
(0.25.+-.0.014 to 0.39.+-.0.014 mAcm.sup.-2). Therefore, a higher
applied voltage was needed to properly differentiate these metal
surfaces. The performance of the metal alloys for use as cathodes
in MECs was evaluated on the basis of the slopes and y-intercepts
from Tafel plots (Table III).
TABLE-US-00004 TABLE III Low Current Density High Current Density
Slope Slope V- (decade A Y-intercept (decade A Y-intercept
intersect Metal cm.sup.-2 V.sup.-1) (A cm.sup.-2) cm.sup.-2
V.sup.-1) (A cm.sup.-2) (V) Ni 625 -3.68 -5.37 -0.98 -3.94 -0.54 Ni
HX -3.70 -5.25 -0.91 -3.87 -0.51 Ni 201 -2.38 -4.73 -0.75 -3.74
-0.61 Ni 400 -2.30 -4.84 -0.76 -3.82 -0.67 SS 286 -4.44 -5.34 -0.88
-3.76 -0.45 SS 304 -2.18 -4.53 -0.64 -3.66 -0.56 SS 420 -2.94 -4.85
-0.88 -3.82 -0.49 SS 316 -2.39 -4.61 -0.94 -3.84 -0.53 Pt -4.31
-5.45 -0.82 -3.75 -0.48
[0201] The Tafel plots for SS A286 and platinum are shown as
typical examples in FIG. 15, with two linear regions: one at high
current densities (solid line) and one at low current densities
(dashed line). The larger Tafel slopes and y-intercepts indicate
better catalytic performance. The Tafel slope is a function of the
transfer coefficient a.sub.c and the number of electrons n.sub.c
transferred during the reaction. The y-intercept is controlled by
the exchange current density J.sub.0. The best cathodes based on
Tafel slopes and y-intercepts were SS 286, Ni 625, Ni HX and
platinum sheet metal, with slopes ranging from 3.68 to 4.31 decade
A cm.sup.-2 V.sup.-1 and y-intercepts of 5.25-5.45A cm.sup.-2 at
low current densities. V-intersect is the voltage at which the
linear regressions intersect. Ideally, the MEC should operate at a
higher current density for a given overpotential. SS 286 has the
lowest V-intersect (0.45 V) of all the metals tested. The ranking
of the metal alloys based on electrochemical results thus confirms
the same relative performance of the materials observed in MEC
tests.
[0202] Particles on carbon cloth cathodes compared to metal sheet
cathodes. The performance of the platinum sheet metal was compared
to the higher surface area platinum particle catalyst bound on
carbon cloth usually used in MEC studies. Current densities
produced by the platinum sheet metal cathode at an applied voltage
of 0.9V (0.59.+-.0.03 mAcm.sup.-2) were similar to the current
densities achieved by the platinum particle bound on carbon cloth
at an applied voltage of 0.6V (0.56.+-.0.03 mAcm.sup.2).
[0203] Platinum has been assumed to be the most efficient catalyst
for electrohydrogenesis in MECs. The results obtained here,
however, show that the performance of platinum can be surpassed by
certain stainless steel and nickel alloys. In all cases, for
example, SS A286 showed better performance than platinum and the
other alloys evaluated in terms of hydrogen gas production, total
gas production, cathodic hydrogen recoveries (rH.sub.2, cat) and
energy recoveries (.eta..sub.E, .eta..sub.E+S). Furthermore, the
volumetric hydrogen production rate (Q) for SS A286 was 4.3 times
higher than the SS 316, and 2.2 times better than platinum sheet
metal disk. Tafel slopes and intercepts confirmed the superior
performance of SS A286 and the general ranking of the other alloys
evaluated in MEC tests.
Example 3
[0204] Hydrogen production in an MEC using a cathode with
electrochemically deposited nickel oxide. The same reactor and
conditions were examined as described in Example 2, except here a
nickel oxide catalyst was deposited through cathodic
electrodeposition onto a sheet metal support using a 12.9 cm.sup.2
nickel foam anode. Electrodeposition was achieved by applying 20V
at .about.2 A for 30 s (1696 power source, B&K Precision, CA)
in a solution containing 12 mM Ni SO.sub.4 and 20 mM
(NH.sub.4).sub.2SO.sub.4 at a pH=2.0 by adding H.sub.2SO.sub.4.
Cyclic voltammetry (CV) scans were performed on the
electrodeposited metal to ensure consistent electrodeposition.
Tests were conducted in a Lexan cell using a 50 mM phosphate
buffer, a Ag/AgCl reference electrode, and a platinum counter
electrode (3 cm.times.5 mm) with a scan range of 0.2 to -1.2V and a
scan rate of 3 mVs.sup.-1. Consistent electrodeposition was
confirmed as all nickel oxide cathodes had similar hydrogen
evolution potentials between -0.65 and -0.70V. The electrodes were
subsequently cut to size (3.8 cm diameter disks) and rinsed with
deionized water before placing them in the reactors.
[0205] Cathode performance was further improved by
electrodepositing a nickel oxide layer on the surface of the sheet
metal. For example, gas production increased from 9.4 to 25 ml for
SS A286 and from 16.2 to 25 ml for Ni 625 (FIG. 16) at an applied
voltage of 0.6 V. Methane gas production was reduced from 6.8 to
4.1 ml for SS A286 and from 7.7 to 4.2 ml for Ni 625. Hydrogen
production and recoveries were 4-40 times higher than the original
values without the metal oxide (Table IV).
TABLE-US-00005 TABLE IV Summary of MEC results for metal cathodes
with electrodeposited nickel oxide layer, compared to platinum, at
an applied voltage of 0.6 V. Metal r.sub.H2,cat (%) r.sub.H2, COD
(%) .eta..sub.E (%) .eta..sub.E+S (%) I.sub..nu. (A/m.sup.3) Q
(m.sup.3/m.sup.3 d) H.sub.2 (%) SS A286 1.2 .+-. 0.1 1.1 .+-. 0.1
3.1 .+-. 0.1 1.1 .+-. 0.1 71 .+-. 3 0.01 .+-. 0.001 6 .+-. 1 Ni 625
12 .+-. 5 11 .+-. 4 31 .+-. 13 10 .+-. 4 86 .+-. 3 0.1 .+-. 0.04 35
.+-. 2 Pt 12 .+-. 5 4 .+-. 1 31 .+-. 12 4 .+-. 2 55 .+-. 3 0.08
.+-. 0.03 36 .+-. 1 SSA286 + NiO.sub.x 52 .+-. 4 56 .+-. 2 137 .+-.
12 48 .+-. 3 130 .+-. 21 0.76 .+-. 0.16 76 .+-. 2 Ni625 + NiO.sub.x
52 .+-. 9 56 .+-. 10 137 .+-. 24 48 .+-. 9 131 .+-. 7 0.76 .+-.
0.15 76 .+-. 5
[0206] Both nickel oxide modified metals reached similar hydrogen
production and recovery values, suggesting the sheet metal was less
of a factor than the metal oxide surface for performance. For
example, energy recovery based on electrical input (.eta..sub.E)
increased from 3.1% (SS A286) and 31% (Ni 625) to 137% for both SS
A286 and Ni 625 plus nickel oxide. Volumetric hydrogen production
rates (Q) also improved from 0.01 (SS A286) and 0.1 (Ni 625) to
0.76 m3 H.sub.2m.sup.-3 day.sup.-1 for both nickel oxide modified
metals. In comparison, platinum sheet metal performance at applied
0.6V was similar to the performance of metals without the nickel
oxide layer (Table IV): low recoveries (.eta..sub.E=31%,
.eta..sub.E+S=4%), low gas production (Q=0.08 m3 H.sub.2m.sup.-3
day.sup.-1) and low hydrogen content (H2=36%). Stability of the
MECs with nickel oxide cathodes was examined by running the
reactors for 15 days (FIG. 17). The initial high gas production and
current densities decreased over the first few cycles. Current
appeared to stabilize after the first three cycles, while gas
production stabilized after seven cycles. The initial decrease in
performance was confirmed through changes in the Tafel plot
parameters (Table V).
TABLE-US-00006 TABLE V Tafel plots's slope and Y-intercepts for
MEC's with and without nickel oxide electrodeposited on Ni 625 and
SS 286 alloys Slope Y-intercept Metal Day # (decade A cm.sup.-2
V.sup.-1) (A cm.sup.-2) Ni 625 + NiO.sub.x 5 -1.87 -4.10 Ni 625 +
NiO.sub.x 15 -1.29 -4.06 SS 286 + NiO.sub.x 5 -1.54 -3.90 SS 286 +
NiO.sub.x 15 -1.04 -3.82
[0207] There was a 30% decrease in Tafel slope values between day 5
and day 15 (1.87 to 1.29 decade Acm.sup.-2 V.sup.-1 for Ni
625+NiOx; 1.54 to 1.04 decade Acm.sup.-2 V.sup.-1 for SS 286+NiOx),
and a slight decrease in the y-intercept values (4.10 to 4.06
Acm.sup.-2 for Ni 625+NiOx; 3.90 to 3.82 Acm.sup.-2 for SS
A286+NiOx).
[0208] When a nickel oxide layer was applied to the cathode by
electrodeposition, current densities, total hydrogen gas
production, cathodic recoveries, energy efficiencies, and hydrogen
production rates improved by a factor of four. It was also found
that the MEC provided good performance, even at the lower applied
voltage of 0.6 V. The use of a lower voltage significantly improved
the process energy efficiency based on energy input, for example,
from .eta..sub.E=3.1% (SS A286) and .eta..sub.E=31% (Ni 625) to
.eta..sub.E=137% (nickel oxide on either metal surface).
Example 4
[0209] Cathodes
[0210] Commercially-available metal powders of nickel (2-10 .mu.m),
nickel oxide (.ltoreq.74 .mu.m), and stainless steel catalysts
(.ltoreq.140 .mu.m) were obtained from Alfa-Aesar, MA. Filamentous
nickel powders with smaller particle sizes were obtained from INCO
specialty products, NJ (Ni 210: 0.5-1 .mu.m, Ni 110: 1-2 .mu.m and
Ni 255: 2.2-2.8 .mu.m; all >99% pure). Cathodes were made by
mixing the metal powder with Nafion.TM. binder (Sigma-Aldrich, MO),
and applying the mixture using a brush onto carbon cloth (surface
area 7 cm.sup.2, 30% wet proof, BASF Fuel Cell, NJ). Platinum
catalyst was used as a control (0.002 .mu.m) (10 wt % on Vulcan
XC-72; BASF Fuel Cell, NJ).
[0211] Nickel oxide was electrodeposited on carbon cloth by
applying 20 V at .about.1.5 A for 40s (1696 power source, B&K
Precision, USA) with an anode stainless steel brush (SS type
Cronifer 1925 HMo, made in house) in a solution containing 18 mM
NiSO.sub.4 and 35 mM (NH.sub.4).sub.2SO.sub.4 at a pH=2.0 (adjusted
by adding H.sub.2SO.sub.4). Carbon cloth cathodes were prepared
before electrodeposition by applying a base coat of carbon black
(CB, 5 mg/cm.sup.2) and NAFION (33 .mu.L/cm.sup.2).
[0212] Electrochemical Evaluation of Catalysts
[0213] Performance of the cathodes was evaluated by LSV using a
potentiostat (PC4/750TM, Echem Analyst, v. 5.5, Gamry Instruments,
PA). The cathodes were placed in electrochemical cells (4 cm long
by 3 cm diameter) with an Ag/AgCl reference electrode and platinum
wire counter electrode in 2 mM phosphate buffer solution (pH 7.0).
LSV scans from -0.4 to -1.4 V with IR compensation (to compensate
for the ohmic drop between the working and reference electrode)
were repeated three times, at a scan rate of 2 mV/s.
[0214] MEC Reactor Construction
[0215] Single-chamber MECs made of Lexan were 4 cm long containing
3 cm diameter cylindrical-shaped chambers. Anodes were
ammonia-treated graphite fiber brushes (25 mm diameter.times.25 mm
length, 0.22 m.sup.2 surface area) made with a titanium wire
twisted core. The anodes were first enriched with bacteria in
microbial fuel cells (MFCs) containing conventional Pt-catalyst air
cathodes that were inoculated using a solution from an acetate-fed
MFC reactor that had been running for over two years. Duplicate
reactors were operated in fed-batch mode using acetate (1 g/L) and
a 50 mM phosphate buffer nutrient medium (pH 7) in a 30.degree. C.
temperature room. After at least three repeatable cycles, the MFCs
were modified to function as MECs by replacing the cathodes and
sealing the end of the reactors from air, providing an oxygen-free
environment. The voltage needed for MECs was supplied via an
external power source (3645A; Circuit Specialists, Inc, Arizona).
After each fed batch cycle (when gas production stopped), the
reactors were drained, exposed to air for 15 minutes to minimize
growth of methanogens (except as noted), refilled with substrate
solution, and sparged with ultra high purity nitrogen gas for five
minutes. For tests done under complete anaerobic conditions, the
reactors were drained and refilled inside an anaerobic glove box
(N.sub.2/H.sub.2 volume ratio of 95/5). In this case, it was not
necessary to sparge the reactors with nitrogen.
[0216] Analysis After MEC Cycles
[0217] Continuous gas production was measured using a respirometer
(AER-200, Challenge Technology, AZ), with the gas collected in gas
bags (100 ml capacity, Cali-5 bond, Calibrated Instruments Inc.,
NY). The composition of the gas in the MEC headspace and gas bags
was analyzed using two gas chromatographs (models 8610B and 310,
SRI Instruments, CA) with molesieve columns (5A 80/100, Alltech,
IL) and thermal conductivity detectors. Argon was used as the
carrier gas for H.sub.2, O.sub.2, N.sub.2 and CH.sub.4 analysis,
and helium was used as the carrier gas for CO.sub.2 analysis.
[0218] Cathodes were examined using scanning electron
microscopy/energy dispersive X-ray spectroscopy (SEM-EDS) at 20 kV
(Quanta 200, FEI, OR). Soluble nickel was analyzed via inductively
coupled plasma atomic emission spectroscopy (ICP-AES; Optima
5300DV, Perkin-Elmer, MA) at a detection limit of 0.01 ppm. Surface
area was obtained by multipoint BET (Brunauer, Emmett, and Teller)
based on nitrogen adsorption (ASAP 2020, Micromeritics, GA).
[0219] Calculations
[0220] The calculated total geometric surface area of the catalyst
particles in the cathodes, A.sub.b,p (m.sup.2), is:
A b , p = A p m V p .rho. b , p = 4 .pi. r 2 m 4 / 3 .pi. r 3 .rho.
b , p = 3 m .rho. b , p r ( 13 ) ##EQU00012##
[0221] where A.sub.p is the surface area of a single particle;
V.sub.p the volume of particles calculated using the average
particle radius, r; .rho..sub.b,p the bulk density of the particle
(provided by the manufacturer); and m the mass of catalyst added to
the cathode.
[0222] The performance of the MEC reactors was evaluated as
described in Example 2: Coulombic efficiency (CE) (%) based on
total Coulombs recovered compared to the initial mass of substrate;
cathodic hydrogen recovery (r.sub.H2,cat) (%) or the recovered
electrons as hydrogen compared to the current transferred; overall
hydrogen recovery (r.sub.H2,COD) (%), defined as the percentage of
hydrogen recovered compared to the theoretical maximum based on
added substrate; volumetric current density (I.sub.V) (A/m.sup.3),
calculated from the maximum current production over a 4-hr period
normalized to the volume of solution; volumetric hydrogen
production rate (Q) (m.sup.3 H.sub.2/m.sup.3 d) based on hydrogen
gas produced normalized to the reactor volume; energy recovery
relative to electrical input (.eta..sub.E) (%); and overall energy
recovery (.eta..sub.E+S) (%) based on both electrical input and
heat of combustion of the substrate (.DELTA.H.sub.facetate=870.28
kJ/mol).
[0223] Cathode Selection by LSV
[0224] An MEC with a Pt catalyst typically produces 4-6 mA, or
0.6-0.9 mA/cm.sup.2 (7 cm.sup.2 cathode projected surface area).
Overpotentials for metal catalysts of different sizes and loadings,
and with different amounts of binder, were compared at a current
density in this range (-0.63 mA/cm.sup.2=-3.2 log A/cm.sup.2) to
better predict their performance relative to MEC conditions. Table
VI shows overpotentials vs SHE at current density of -3.2 log
A/cm.sup.2 for cathodes during third LSV scan at 2 mV/s. The
surface area indicated in Table VI was calculated using equation
(13).
TABLE-US-00007 TABLE VI Particle Surface size Area Qty CB Nafion
Potential Catalyst (.mu.m) (m.sup.2) (mg) (mg) (.mu.L) (V) None
(CB) NA 0.00 0 60 400 -0.970 Platinum 0.002 1.45 10 50 400 -0.500
Ni 210 0.5-1 0.60 60 0 267 -0.500 Ni 210 0.5-1 0.60 60 0 300 -0.583
Ni 210 0.5-1 0.60 60 0 325 -0.713 Ni 210 0.5-1 0.60 60 0 375 -0.713
Ni 210 0.5-1 0.60 60 0 400 -0.720 Ni 210 0.5-1 0.60 60 30 400
-0.668 Ni 110 1-2 0.17 60 30 400 -0.720 Ni 255 2.2-2.8 0.23 60 30
400 -0.721 Ni 10255 2.2-3 0.24 60 30 400 -0.760 Ni 10256 3-7 0.03
60 30 400 -0.739 Ni 210 0.5-1 0.45 30 0 400 -0.747 Ni 210 0.5-1
1.35 90 0 400 -0.727 Ni 210 0.5-1 0.45 30 30 400 -0.683 Ni 10255
2.2-3 0.23 60 0 400 -0.760 Ni 10256 3-7 0.03 60 0 400 -0.740 NiO
87302 74 0.001 60 0 400 -1.110 eNiOx 0.001 ND ND 60 400 -0.800 SS
316 16 0.01 60 0 400 -1.140 SS 316 150 0.002 120 0 400 -0.863 SS
410 150 0.002 120 0 400 -0.913 SS 304 150 0.002 120 0 400 -0.813 SS
303 150 0.002 120 0 400 -0.953 NA--not applicable, ND--not
determined.
[0225] Both Ni 210 on carbon cloth (60 mg Ni, 267 .mu.L Nafion) and
the standard Pt cathode (10 mg Pt, 400 .mu.L Nafion) had the same
low overpotential of -0.500 V at this current density. Current
densities produced with these two materials were also very similar
over the complete range of applied voltages as shown in FIG.
18.
[0226] MEC Performance
[0227] The two metal powder and binder combinations that produced
the lowest overpotentials in LSV scans (Ni 210 with 267 .mu.L
Nafion, and Ni 210+CB with 400 .mu.L Nafion) (Table VI) were used
as cathodes in MECs, and their performance was compared to the same
reactors with Pt cathodes. Electrodeposited nickel oxide (eNiOx)
was also used as an MEC cathode. The resulting BET total surface
areas were 4.31 m.sup.2/g (Ni 210), 11.8 m.sup.2/g (Ni210+CB), 17.3
m.sup.2/g (eNiOx), and 11.2 m.sup.2/g (Pt).
[0228] Volumetric Gas Production and Composition
[0229] Table VII and FIGS. 19A and 19B show volumetric gas
production, gas composition and current production for MECs using
nickel cathodes, Ni 210, Ni 210+CB or eNiOx compared with Pt
cathodes.
TABLE-US-00008 TABLE VII Summary of MEC results for Ni210, Ni210 +
CB, eNiOx and Pt catalyst cathodes at an applied voltage of 0.6 V,
eighth cycle of operation. H.sub.2 Iv Q CE r.sub.H2,cat
r.sub.H2,COD .eta..sub.E .eta..sub.E+S Metal (%) (A/m.sup.3)
(m.sup.3/m.sup.3d) (%) (%) (%) (%) (%) Ni210 92 .+-. 0 160 .+-. 31
1.3 .+-. 0.3 92.7 .+-. 15.8 79 .+-. 10 73 .+-. 3 210 .+-. 40 65
.+-. 2 Ni210 + CB 92 .+-. 1 139 .+-. 2 1.2 .+-. 0.1 83.8 .+-. 1.2
94 .+-. 5 79 .+-. 5 252 .+-. 12 73 .+-. 4 eNiOx 94 .+-. 0 103 .+-.
4 0.9 .+-. 0.1 87.1 .+-. 2.3 86 .+-. 1 75 .+-. 1 215 .+-. 8 67 .+-.
0 Pt 92 .+-. 2 186 .+-. 4 1.6 .+-. 0.0 85.0 .+-. 6.4 89 .+-. 7 75
.+-. 0 239 .+-. 21 70 .+-. 2
[0230] MEC Performance with Ni210 Cathodes as a Function of Applied
Voltage
[0231] Hydrogen production rates with the two Ni cathodes increased
with applied voltage and were not significantly different from each
other, with the largest rates produced at the highest applied
voltage of 0.8 V (Q=1.85 m.sup.3/m.sup.3/d, Ni 210) (FIG. 20A).
There was no hydrogen production with the Ni catalysts at an
applied voltage of 0.3 V. Coulombic efficiencies (CE) decreased
slightly with applied voltage (89% at 0.8 V, to 81% at 0.4 V) (FIG.
20B). Cathodic hydrogen recovery reached a maximum at 0.7 V (Ni
210=93%, Ni 210+CB=91%). Similarly, energy recovery based on
electrical input (.eta..sub.E) and overall energy recovery
(.eta..sub.E+S) increased with increasing applied voltage, with the
maximum values for .eta..sub.E at 0.8 V of 240%, and for
.eta..sub.E+S at 0.7 V of 74%.
Example 5
[0232] Hydrogen production in an MEC using a cathode with a mesh
structure. Hydrogen production in an MEC using a cathode with a
mesh structure. Preliminary tests conducted with SS mesh are shown
in FIG. 21. A single-chamber MEC made of Lexan was 4 cm long
containing a 3 cm diameter cylindrical-shaped chamber with a
graphite fiber brush anode, and using a 50 mM phosphate buffer
solution and a fuel of 1 g/L sodium acetate. The cathode was either
a flat sheet of SS305 (7 cm2) or SS mesh made of the same material.
The mesh cathode produced nearly 80 A/m.sup.3 compared to .about.65
A/m.sup.3 for the flat sheet. Thus, the use of the higher surface
area mesh produced more current than a flat sheet of the same
material in an MEC.
Example 6
[0233] The cathodes include a current collector (stainless steel
mesh, SS), catalyst (Pt), and diffusion layer
(poly(dimethylsiloxane), PDMS) in one single cathode structure. The
SS mesh used in this example (type 304 SS, 90.times.90, woven wire
diameter 0.0055 inches, McMaster-Carr, OFT) had 90.times.90
openings per square inch. PDMS was made using a 10:1 mixture of
SYLGARD 184 silicone elastomer base and SYLGARD 184 silicone
elastomer curing agent (Dow Corning, MI), that was further diluted
to 10 wt % with toluene to decrease the solution viscosity. The
PDMS (6.25.times.10.sup.-3 mg/cm.sup.2) was applied with carbon
black (1.56 mg/cm.sup.2) to the side of the SS that faced the air.
After applying this first PDMS/carbon black as a base layer, one to
four additional diffusion layers (DLs) containing PDMS/carbon black
or only PDMS were applied on top of this base diffusion layer at
the same mass loading as the base diffusion layer. After applying
each diffusion layer, cathodes were air dried for 2 hours, and then
heated at 80.degree. C. for 30 min to crosslink the PDMS oligomers.
After applying these DLs, a Pt catalyst layer (5 mg/cm.sup.2 10% Pt
on Vulcan XC-72 with 33.3 .mu.L/cm.sup.2 of 5 wt % Nafion as
binder) was applied to the SS mesh on the side facing the solution
and the coated cathode was dried for at least 1 day at room
temperature before being used. Cathodes were also prepared with no
coating on the solution-facing side of mesh, or with only a carbon
black layer (both with 2 PDMS/carbon DLs on the air side).
[0234] Carbon cloth (E-Tek, Type B, 30% wet proofing, BASF Fuel
Cell, Inc. NJ) was also tested as a cathode supporting material.
One or more DLs of PDMS/carbon and the Pt catalyst were applied as
described above for the metal mesh cathode.
[0235] MFC Construction and Operation.
[0236] MFCs were single-chamber cubic-shaped reactors constructed
as described in Example 2 with an anode chamber 4 cm long and 3 cm
in diameter. The anode was an ammonia gas treated graphite fiber
brush (25 mm diameter.times.25 mm length; fiber type PANEX 33 160K,
ZOLTEK (continuous carbon fiber manufactured from polyacrylonitrile
(PAN) precursor having fiber diameter 7.2 .mu.m (0.283 mil), no
twist, 117,472 Denier (g/9000 m), 77 m/kg (114 ft/lb) yield and
0.06493 cm.sup.2 (0.01006 in.sup.2) average tow cross sectional
area). All reactors were inoculated using a solution from an MFC
operated for over 1 year (initially inoculated from the effluent of
the primary clarifier of the local wastewater treatment plant). The
medium contained acetate as the fuel (0.5 g/L for fixed resistance
tests, and 1.0 g/L for polarization tests), and a phosphate buffer
nutrient solution (PBS; conductivity of 8.26 mS/cm) containing:
Na.sub.2HPO.sub.4, 4.58 g/L; NaH.sub.2PHO.sub.4.H.sub.2O 2.45 g/L;
NH.sub.4Cl 0.31 g/L; KCl 0.13 g/L; trace minerals (12.5 mL/L) and
vitamins (5 mL/L). Reactors were all operated in fed-batch mode at
30.degree. C. and were refilled each time when the voltage
decreased to less than 20 mV forming one complete cycle of
operation.
[0237] Calculations and Measurements
[0238] Voltage (E) across the external resistor (1 k.OMEGA., except
as noted) in the MFC circuit was measured at 20 min intervals using
a data acquisition system (2700, Keithley Instrument, OH) connected
to a personal computer. Current (I=E/R), power (P=IE) were
calculated as described in Logan, B. et al, Environmental Science
& Technology, 2006, 40:5181-5192, with the current density and
power density normalized by the projected surface area of the
cathode. To obtain the polarization and power density curves as a
function of current, external circuit resistances were varied from
1000 to 50.OMEGA. in decreasing order. Each resistor was used for a
full fed-batch cycle, and the COD of the solution at the end of the
cycle was measured using standard methods such as described in
Standard Methods for the Examination of Water and Wastewater, 21st.
ed.; American Public Health Association: New York, 2005. The CE was
calculated based on total COD removal over the cycle, as described
Logan, B. et al, Environmental Science & Technology, 2006,
40:5181-5192.
[0239] Linear sweep voltammetry (LSV) was used to assess
electrochemical performance of the cathodes. Current was measured
in 50 mM PBS in the absence of nutrients and exoelectrogens using a
potentiostat (PC4/750, Gamry Instruments). A two chamber
electrochemical cell with each chamber 2 cm in length and 3 cm in
diameter separated by an anion exchange membrane (AMI-7001,
Membrane International Inc., NJ) was used for measurements,
consisting of a working electrode (cathode with 7 cm.sup.2
projected surface area), counter electrode (Pt plate with a
projected surface area of 2 cm.sup.2), and an Ag/AgCl reference
electrode (RE-5B; BASi, West Lafayette, Ind.). The scan rate was 1
mV/s, and potential was scanned from +0.3 V to -02 V (vs.
Ag/AgCl).
[0240] Oxygen permeability was measured in terms of oxygen transfer
coefficient as described in Cheng, S. et al, Electrochemistry
Communications, 2006, 8:489-494. The 4-cm cubical reactor used in
MFC tests was used for oxygen transport measurements. Dissolved
oxygen concentrations were measured using a non-consumptive
dissolved oxygen probe (FOXY, Ocean Optics, Inc., Dunedin,
Fla.).
[0241] Performance of SS Mesh Cathodes in MFCs Compared with Carbon
Cloth Cathodes
[0242] MFCs with SS mesh or carbon cloth cathodes and a Pt catalyst
rapidly produced voltage after inoculation, and generated stable
voltages at a fixed resistance. Differences in voltage among these
reactors at a high external resistance of 1 k.OMEGA. were small,
although in general the SS mesh produced higher voltages than the
carbon cloth cathodes.
[0243] FIG. 22 is a graph showing voltage generation in an MFC
using a SS mesh cathode and a Pt catalyst with 2 PDMS/carbon
diffusion layers (M2) compared to an MFC using carbon cloth
cathodes with 4 diffusion layers (CC4); using 50 mM PBS buffer and
0.5 g/L sodium acetate. FIG. 22 shows that the largest maximum
voltage that was produced over a total of 14 batch cycles of
operation was of 602.+-.5 mV (.+-.S.D., n=14 cycles) obtained using
the SS mesh cathode with 2 PDMS/carbon layers. In contrast, the
highest value of carbon cloth cathodes was 585.+-.4 mV for MFCs
with 4 PDMS/carbon layers.
[0244] FIG. 23A is a graph showing power density in an MFC using a
cathode containing SS mesh with Pt catalyst and 1-5 layers of
PDMS/carbon DLs (M1-M5) as a function of current density
(normalized to cathode surface area) obtained by varying the
external circuit resistance (1000-50.OMEGA.). FIG. 23B is a graph
showing power density in an MFC using carbon cloth cathodes with Pt
and the same DLs (CC1-CC5) as a function of current density
(normalized to cathode surface area) obtained by varying the
external circuit resistance (1000-50.OMEGA.). Error bars.+-.SD in
FIGS. 23A and 23B are based on measurement of two duplicate
reactors. Large differences in power production were observed based
on polarization data. The largest maximum power density using a SS
mesh cathode of 1610.+-.56 mW/m.sup.2 (.+-.S.D. for duplicate
reactors) was achieved with 2 PDMS/carbon layers. This was similar
to that produced with a single layer (1592.+-.19 mW/m.sup.2), but
three or more layers decreased performance to as little as 1010
mW/m.sup.2 (FIG. 23A). Maximum power densities produced using
carbon cloth cathodes with PDMS/carbon layers varied over a smaller
range of 1553.+-.19 mW/m.sup.2 (1 DL) to 1635.+-.62 mW/m.sup.2 (3
DLs) (FIG. 23B). Thus, there was much less of an effect of the
number of layers on power generation with the carbon cloth material
than with the SS mesh.
[0245] Performance of SS Mesh Cathodes in Electrochemical
Tests.
[0246] LSV tests were conducted using SS mesh cathodes to evaluate
the electrochemical performance of the cathodes in the absence of
bacteria. FIG. 24A is a graph showing LSV of MFCs including SS mesh
cathodes with a Pt catalyst and 1-5 PDMS/carbon DLs (M1-M5). FIG.
24B is a graph showing LSV of an MFC including cathode M1 compared
with MFCs including cathodes having additional PDMS layers
(MP2-MP5), each including Pt catalyst. FIG. 24C is a graph showing
LSV of an MFC including cathode M2 compared with an MFC including a
cathode having a solution-facing side coating containing only
carbon black (M2, no Pt), and a cathode with no coating on the
solution-facing side (M2, no Pt, no CB).
[0247] All voltammograms with the SS mesh cathodes containing a Pt
catalyst and 1-5 PDMS/carbon layers had similar current densities
at a given applied voltage (FIG. 24A). The cathode which had the
largest current response had only 1 PDMS/carbon base layer. Current
densities with the SS mesh cathodes with 2-4 layers had only
slightly reduced activities compared to the single PDMS/carbon base
layer. This suggests that the different performance of the SS mesh
cathodes with a different number of DLs in MFC tests was not due to
their inherent electrochemical properties, but rather other effects
such as development of a cathode biofilm or oxygen intrusion
through the DLs and the effects on the bacteria in the anode
chamber.
[0248] Voltammograms were also conducted using the SS mesh
containing only PDMS (no carbon black) applied to the PDMS/carbon
base layer. These cathodes with one to four additional PDMS layers
showed poorer electrochemical performance, and had a much wider
range in electrochemical performance, than the SS cathodes with
both PDMS and carbon (FIG. 24B). With only PDMS, electrochemical
performance decreased with the additional layers. This indicates
that the carbon black material is needed with PDMS to ensure good
electrochemical performance. When cathodes with 2 PDMS/carbon DLs
were examined using LSV that had only carbon black on the side of
the SS mesh facing solution (no Pt), there was little current over
the range of voltages examined (FIG. 24C). In addition, cathodes
prepared without carbon black or Pt were similarly ineffective at
oxygen reduction. These results show that the SS and carbon black
did not detectably catalyze oxygen reduction.
[0249] MFC tests with SS mesh cathodes in general produced much
higher Coulombic efficiencies (CEs) than those with carbon cloth
cathodes. FIG. 25A is a graph showing the CE of an MFC including a
SS mesh cathode with Pt catalyst and 1-5 layers of PDMS/carbon DLs
(M1-M5) as a function of current density (normalized to cathode
surface area) obtained by varying the external circuit resistance
(1000-50.OMEGA.). FIG. 25B is a graph showing the CE of MFCs
including carbon cloth cathodes with Pt and 1-5 layers of
PDMS/carbon DLs (CC1-CC5) as a function of current density
(normalized to cathode surface area) obtained by varying the
external circuit resistance (1000-50.OMEGA.). Error bars.+-.SD in
FIGS. 25A and 25B are based on measurement of two duplicate
reactors. In each of these cases, the CE increased with current
density. CEs of the SS mesh cathode ranged from 15% to 64% with
single PDMS/carbon base layer, and only slightly increased when
adding the second layer. The highest CE of 80% was obtained when 3
DLs were applied to this cathode. In contrast, the carbon cloth
cathodes CEs ranged from 13 to 46% with the first DL, with the
highest value of 57% with 4 DLs. COD removals over a cycle of
operation ranged from 90% to 95%, and there was no effect of the
number of DLs or the type of material (SS or carbon cloth) on COD
removal.
[0250] Oxygen Permeability of the Cathodes.
[0251] PDMS is relatively permeable to oxygen, but increasing the
number of PDMS diffusion layers should reduce oxygen transfer due
to the increased thickness of the DL. FIG. 26 is a graph showing
oxygen permeability of SS mesh cathodes including a Pt catalyst and
PDMS/carbon DLs (M) or PDMS (MP) DLs upon PDMS/carbon base layer.
Error bars.+-.SD in FIG. 26 are based on two or more measurements.
With one base layer of PDMS/carbon on the SS mesh cathode, the
oxygen mass transfer coefficient was 1.2.+-.0.1.times.10.sup.-3
cm/s. Successive application of multiple PDMS/carbon DLs decreased
the oxygen mass transfer coefficient from
1.1.+-.0.1.times.10.sup.-3 cm/s (2 layers) to
0.7.+-.0.1.times.10.sup.-3 cm/s (5 layers) (FIG. 26). Addition of
only PDMS (no carbon) onto this base layer decreased the mass
transfer coefficient to 0.7.+-.0.1.times.10.sup.-3 cm/s, with the
lowest value of 0.2.+-.0.1.times.10.sup.-3 cm/s obtained with four
pure PDMS layers. Thus, the addition of carbon with PDMS created a
more oxygen permeable material than the PDMS alone. A carbon cloth
cathode with 4 PTFE layers obtained an oxygen transfer coefficient
of 1.1.+-.0.1.times.10.sup.-3 cm/s.
[0252] Water Losses
[0253] The addition of a PDMS layer was important for controlling
water losses from the cathode. SS mesh cathode with the base
PDMS/carbon layer had an initial water evaporation loss of 5% of
the water in the anode chamber each day. Water losses decreased
with additional DLs, and were not detectable for cathodes with five
DLs. For carbon cloth cathodes, the water losses were larger, with
10% to 5% per day with one to five DLs. However, as a biofilm
developed on the cathodes after several cycles, water loss
gradually decreased for both SS and carbon cloth cathodes by about
20-30%.
[0254] As shown herein. PDMS mixed with carbon black is effective
at reducing water losses and allowing oxygen transfer to the
cathode catalyst. Use of a SS mesh cathode with two PDMS/carbon
layers, resulted in a maximum power density of 1610.+-.56
mW/m.sup.2 (47.0.+-.1.6 W/m.sup.3). In comparison, the best
performance with a carbon cloth cathode was 1635.+-.62 mW/m.sup.2
with three PDMS/carbon layers. The recovery of the substrate as
current was also improved using SS mesh cathodes, with CEs ranging
from 15-67% for the SS cathodes, compared to 14-51% for the carbon
cloth cathodes for the above two cases.
[0255] The combination of SS mesh and PDMS/carbon DLs produced a
structure that had an improved CE compared to previously examined
materials, likely as a result of higher current densities and
reduced oxygen transfer coefficients. SS mesh cathodes had a CE as
high as 80% with 3 PDMS/carbon DLs, over a current density range of
0.8-6.6 A/m.sup.2. Carbon cloth cathodes with the same DL had CEs
that ranged from 13% to 57% over similar current densities. These
CEs can be compared with those of carbon cloth cathodes with 4 PTFE
DLs that had CEs ranging from 20% to 27% at current densities of
0.8-2.5 A/m.sup.2 using a flat carbon cloth anode, and from 40% to
60% at 0.8-11 A/m.sup.2 using a graphite fiber brush anode. A
comparison of these results suggests that high CEs achieved with
the SS mesh cathodes are partly due to high current densities. When
the current density is increased, the cycle time is reduced, and
thus the amount of oxygen that can diffuse into the reactor is
substantially reduced in proportion to the cycle time. However,
even in the high current density range of >5 A/m.sup.2, SS mesh
cathode had a higher maximum CE than other materials, likely due to
the lower oxygen permeability of the mesh DL.
Example 7
[0256] Electricity generation in an MFC using a tubular cathode
made of stainless steel mesh.
[0257] For this example, a cube-shaped MFC reactor with a
cylindrical tube center is used, with the electrodes placed on
either side of the reactor. The anode was an ammonia gas treated
graphite fiber brush (1.4 cm diameter.times.2.5 cm length, fiber
type: PANEX.RTM. 33 160K, ZOLTEK) with a surface area of 1300
cm.sup.2 (95% porosity) placed in the center of the reactor. The
cathode was SS mesh of mesh size 50 or 70, containing a catalyst
layer of Pt. For these experiments several diffusion layers (DLs),
in this case made of polytetrafluoroethylene (PTFE), were placed on
the air-facing side on a carbon/PTFE base layer. The voltage
produced was approximately 500 mV, which compared favorably to
carbon paper in this type of MFC design.
[0258] FIG. 27 is a graph showing voltage generation in an MFC
using cathodes containing SS U.S. standard mesh No. 50 or No.
70.
REFERENCES
[0259] ASTM. (2007) Document number A 959-07. Standard guide for
specifying harmonized standard grade compositions for wrought
stainless steels. Table 1. Chemical Composition Limits, %. [0260]
Call, D. and Logan, B. E. (2008) Hydrogen production in a single
chamber microbial electrolysis cell (MEC) lacking a membrane.
Environ. Sci. Technol. 42(9), 3401-3406. [0261] Cheng, S., Liu, H.
and Logan, B. E. (2006) Power densities using different cathode
catalysts (Pt and CoTMPP) and polymer binders (Nafion and PTFE) in
single chamber microbial fuel cells. Environ. Sci. Technol. 40,
364-369. [0262] Cheng, S. and Logan, B. E. (2007a) Ammonia
treatment of carbon cloth anodes to enhance power generation of
microbial fuel cells. Electrochem. Commun. 9(3), 492-496. [0263]
Cheng, S. and Logan, B. E. (2007b) Ammonia treatment of carbon
cloth anodes to enhance power generation of microbial fuel cells.
Electrotherm. Commun. 9(3), 492-496. [0264] Cheng, S. and Logan, B.
E. (2007c) Sustainable and efficient biohydrogen production via
electrohydrogenesis. Proc. Natl. Acad. Sci. USA 104(47),
18871-18873. [0265] Daniele, S., Baldo, M. A., Bragato, C. and
Lavagnini, I. (1998) Steady state voltammetry in the process of
hydrogen evolution in buffer solutions. Analytica Chimica Acta 361,
141-150. [0266] Daniele, S., Lavagnini, I., Baldo, M. A. and Magno,
F. (1996) Steady state voltammetry at microelectrodes for the
hydrogen evolution from strong and weak acids under pseudo-first
and second order kinetic conditions. J. Electroanal. Chem. 404,
105-111. [0267] Dougherty, R. C. and Merrill, M. D. (2008)
Composites and electrodes for electrochemical devices and processes
for producing the same, USA. [0268] Liu, H., Grot, S. and Logan, B.
E. (2005) Electrochemically assisted microbial production of
hydrogen from acetate. Environ. Sci. Technol. 39(11), 4317-4320.
[0269] Liu, H. and Logan, B. E. (2004) Electricity generation using
an air-cathode single chamber microbial fuel cell in the presence
and absence of a proton exchange membrane. Environ. Sci. Technol.
38(14), 4040-4046. [0270] Liu, H., Ramnarayanan, R. and Logan, B.
E. (2004) Production of electricity during wastewater treatment
using a single chamber microbial fuel cell. Environ. Sci. Technol.
38(7), 2281-2285. [0271] Logan, B., Cheng, S., Watson, V. and
Estadt, G. (2007a) Graphite Fiber Brush Anodes for Increased Power
Production in Air-Cathode Microbial Fuel Cells. Environ. Sci.
Technol. 41(9), 3341-3346. [0272] Logan, B. E. (2008) Microbial
fuel cells, John Wiley & Sons, Inc., Hoboken, N.J. [0273]
Logan, B. E., Cheng, S., Watson, V. and Estadt, G. (2007b) Graphite
fiber brush anodes for increased power production in air-cathode
microbial fuel cells. Environ. Sci. Technol. 41(9), 3341-3346.
[0274] Olivares-Ramirez, J. M., Campos-Cornelio, M. L., Godinez, J.
U., Borja-Arco, E. and Castellanos, R. H. (2007) Studies on the
hydrogen evolution reaction on different stainless steels. Int. J.
Hydrogen Energy 32, 3170-3173. [0275] Zuo, Y., Cheng, S., Call, D.
and Logan, B. E. (2007) Tubular membrane cathodes for scalable
power generation in microbial fuel cells. Environ. Sci. Technol.
41(9), 3347-3353.
[0276] Any patents or publications mentioned in this specification
are incorporated herein by reference to the same extent as if each
individual publication is specifically and individually indicated
to be incorporated by reference. U.S. patent application Ser. Nos.
11/799,194; 12/145,722; 12/177,962; 11/180,454; 11/799,149; and
U.S. Provisional Patent Application Ser. No. 61/141,511 are
incorporated herein by reference in their entirety.
[0277] The compositions and methods described herein are presently
representative of preferred embodiments, exemplary, and not
intended as limitations on the scope of the invention. Changes
therein and other uses will occur to those skilled in the art. Such
changes and other uses can be made without departing from the scope
of the invention as set forth in the claims.
* * * * *