U.S. patent application number 14/575593 was filed with the patent office on 2015-04-16 for materials and configurations for scalable microbial fuel cells.
The applicant listed for this patent is The Penn State Research Foundation. Invention is credited to Bruce E. Logan.
Application Number | 20150104670 14/575593 |
Document ID | / |
Family ID | 38661537 |
Filed Date | 2015-04-16 |
United States Patent
Application |
20150104670 |
Kind Code |
A1 |
Logan; Bruce E. |
April 16, 2015 |
MATERIALS AND CONFIGURATIONS FOR SCALABLE MICROBIAL FUEL CELLS
Abstract
Devices for production of electricity and/or hydrogen gas are
provided by the present invention. In particular, microbial fuel
cells for production of electricity and modified microbial fuel
cells for production of hydrogen are detailed. A tube cathode is
provided which includes a membrane forming a general tube shape. An
anode is provided which has a specific surface area greater than
100 m.sup.2/m.sup.3. In addition, the anode is substantially
non-toxic to anodophilic bacteria. Combinations of particular
anodes and cathodes are included in microbial fuel cells and
modified microbial fuel cells.
Inventors: |
Logan; Bruce E.; (State
College, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Penn State Research Foundation |
University Park |
PA |
US |
|
|
Family ID: |
38661537 |
Appl. No.: |
14/575593 |
Filed: |
December 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11799194 |
May 1, 2007 |
8962165 |
|
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14575593 |
|
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|
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60796761 |
May 2, 2006 |
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Current U.S.
Class: |
429/2 |
Current CPC
Class: |
H01M 4/8657 20130101;
Y02W 10/37 20150501; H01M 4/90 20130101; H01M 4/9008 20130101; H01M
8/16 20130101; Y02E 60/50 20130101; C02F 3/005 20130101; Y02E
60/527 20130101 |
Class at
Publication: |
429/2 |
International
Class: |
H01M 8/16 20060101
H01M008/16; C02F 3/00 20060101 C02F003/00; H01M 4/90 20060101
H01M004/90 |
Goverment Interests
GOVERNMENT SPONSORSHIP
[0002] This invention was made with government support under Grant
No. BES-0401885, awarded by the National Science Foundation and
Grant No. 68-3A75-3-150, awarded by the United States Department of
Agriculture. The Government has certain rights in the invention
Claims
1. A microbial fuel cell, comprising: a cathode comprising a
membrane, the membrane forming a cathode wall generally enclosing
and defining an interior space, the cathode wall having an internal
surface adjacent the interior space and an opposed external
surface, the wall extending between a first end and a second end;
an anode, the anode substantially non-toxic to anodophilic
bacteria; and an electrically conductive connector connecting the
anode and the cathode.
2. The microbial fuel cell of claim 1 wherein the membrane is
selected from the group consisting of: a nanofiltration membrane,
an ultrafiltration membrane, and an ion exchange membrane.
3. The microbial fuel cell of claim 1 further comprising a
conductive material in contact with the internal surface or the
external surface of the membrane, the conductive material in
electrically conductive connection with the electrically conductive
connector.
4. The microbial fuel cell of claim 3 wherein the conductive
material is a carbon-based material.
5. The microbial fuel cell of claim 3 wherein the conductive
material is graphite.
6. The microbial fuel cell of claim 3 wherein the conductive
material is a carbon-based coating, the carbon-based coating
present on at least about 50% of the internal surface or the
external surface of the membrane.
7. The microbial fuel cell of claim 1, further comprising a
catalyst.
8. The microbial fuel cell of claim 7, wherein the catalyst is
selected from the group consisting of: a metal-containing catalyst,
a non-metal containing catalyst, and a combination thereof.
9. The microbial fuel cell of claim 1, wherein the anode has a
specific surface area greater than 100 m.sup.2/ m.sup.3.
10. The microbial fuel cell of claim 1, wherein the anode is a
brush anode.
11. The microbial fuel cell of claim 1, comprising two or more
anodes.
12. The microbial fuel cell of claim 1, comprising two or more
cathodes.
13. The microbial fuel cell of claim 1, further comprising a power
source for enhancing an electrical potential between the anode and
the cathode in order to produce hydrogen from the microbial fuel
cell, the power source in electrical communication with the anode
and the cathode.
14. The microbial fuel cell of claim 13 wherein the power source is
a second microbial fuel cell, the second microbial fuel cell
configured to produce electricity.
15. A microbial fuel cell, comprising: an anode having a specific
surface area greater than 100 m.sup.2/m.sup.3, the anode
substantially non-toxic to anodophilic bacteria; a cathode; and an
electrically conductive connector connecting the anode and the
cathode.
16. The microbial fuel cell of claim 15, wherein the anode
comprises one or more conductive fibers.
17. The microbial fuel cell of claim 16, wherein the one or more
conductive fibers is attached to a conductive core support.
18. The microbial fuel cell of claim 17, wherein each individual
fiber of the one or more conductive fibers is attached to the
conductive core support.
19. The microbial fuel cell of claim 17, wherein at least a portion
of the one or more fibers comprises one or more carbon fibers.
20. The microbial fuel cell of claim 15, comprising two or more
anodes.
21. The microbial fuel cell of claim 15, comprising two or more
cathodes.
22. The microbial fuel cell of claim 15, further comprising a power
source for enhancing an electrical potential between the anode and
the cathode in order to produce hydrogen from the microbial fuel
cell, the power source in electrical communication with the anode
and the cathode.
23. The microbial fuel cell of claim 23 wherein the power source is
a second microbial fuel cell, the second microbial fuel cell
configured to produce electricity.
24. A cathode for a microbial fuel cell, comprising: a membrane,
the membrane forming a cathode wall generally enclosing and
defining an interior space, the cathode wall having an internal
surface adjacent the interior space and an opposed external
surface, the wall extending between a first end and a second end,
the wall having a general shape.
25. The cathode for a microbial fuel cell of claim 24 wherein the
membrane is selected from the group consisting of: a nanofiltration
membrane, an ultrafiltration membrane, and an ion exchange
membrane.
26. The cathode for a microbial fuel cell of claim 24 further
comprising a conductive material in contact with the internal
surface or the external surface of the membrane, the conductive
material in electrically conductive connection with the
electrically conductive connector.
27. The cathode for a microbial fuel cell of claim 26 wherein the
conductive material is a carbon-based material.
28. The cathode for a microbial fuel cell of claim 26 wherein the
conductive material is graphite.
29. The cathode for a microbial fuel cell of claim 26 wherein the
conductive material is present on at least about 50% of the
internal surface or the external surface of the membrane.
30. The cathode for a microbial fuel cell of claim 24, further
comprising a catalyst selected from the group consisting of: a
catalyst for enhancement of oxygen reduction and a catalyst for
enhancement of proton reduction.
31. The cathode of claim 24, wherein at least one of the first or
second ends is closed.
32. The cathode of claim 24, wherein the interior space is at least
partially filled with a liquid.
33. The cathode of claim 24, wherein the wall has a shape selected
from the group consisting of: generally cylindrical and generally
hollow slab-shaped.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/799,194, filed May 1, 2007, which claims
priority to U.S. Provisional Patent Application Ser. No.
60/796,761, filed May 2, 2006, the entire content of both of which
is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates generally to microbial fuel
cells. In particular, the present invention relates to scalable
configurations of electrodes and microbial fuel cells.
BACKGROUND OF THE INVENTION
[0004] Recent research advances have led to the development of fuel
cell devices which utilize bacteria as catalysts to create useful
products, such as electricity and hydrogen. The bacteria oxidize a
substrate, electrons produced are transferred to an anode and flow
to a cathode through a conductive connection which may be further
connected to a load, such as a device powered by electricity and/or
hydrogen produced by the fuel cell.
[0005] However, electrode configurations for microbial fuel cells
often limit power production and figure prominently in space
constraints associated with fuel cells.
[0006] Thus, there is a continuing need for scalable electrodes and
scalable electrode assembly configurations for microbial fuel
cells.
SUMMARY OF THE INVENTION
[0007] A microbial fuel cell is provided according to the present
invention which includes a cathode, the cathode including a
membrane, the membrane forming a cathode wall generally enclosing
and defining an interior space, the cathode wall having an internal
surface adjacent the interior space and an opposed external
surface, the wall extending between a first end and a second end.
The shape formed by the cathode wall is generally cylindrical in
particular embodiments. In further particular embodiments, the
shape formed by the cathode wall is generally slab or brick-shaped.
An anode is included in a microbial fuel cell which is
substantially non-toxic to anodophilic bacteria. An electrically
conductive connector connects the anode and the cathode.
[0008] A membrane included in the cathode is a nanofiltration
membrane, an ultrafiltration membrane, or an ion exchange membrane
in particular embodiments of a microbial fuel cell according to the
present invention.
[0009] An included membrane is optionally an electrically
conductive membrane and the membrane is in electrically conductive
connection with the electrically conductive connector.
[0010] In further embodiments, a conductive material is present in
contact with the internal surface or the external surface of the
membrane and the conductive material is in electrically conductive
connection with the electrically conductive connector.
[0011] A conductive material is optionally a carbon-based material.
Graphite is a particular carbon-based conductive material in
contact with the membrane in certain configurations.
[0012] Optionally, the conductive material is a carbon-based
coating. In specific microbial fuel cell configuration according to
the present invention, the carbon-based coating is present on at
least about 50% of the internal surface or the external surface of
the membrane.
[0013] A catalyst for enhancing reduction of an oxidant,
particularly, oxygen, is optionally present on the internal surface
or the external surface of the membrane in electricity generation
configurations of microbial fuel cells according to the present
invention. Suitable catalysts include metal-containing catalysts
such as Pt and non-metal containing catalysts, such as CoTMPP.
Combinations of catalysts are optionally included. Further, in
hydrogen generation configurations of microbial fuel cells
according to the present invention, a catalyst for catalyzing a
hydrogen evolution reaction is included. Suitable catalysts include
metal-containing catalysts such as Pt.
[0014] An included anode has a specific surface area greater than
100 m.sup.2/m.sup.3 in particular embodiments. A particular anode
type included in certain embodiments is a brush anode.
[0015] More than one anode and/or more than one cathode is included
in embodiments of a microbial fuel cell according to the present
invention.
[0016] A microbial fuel cell provided according to the present
invention is configured to produce hydrogen and/or electricity.
Where hydrogen is the desired product, a power source for enhancing
an electrical potential between the anode and the cathode is
included. An included power source may be any of various power
sources. In a particular embodiment, a microbial fuel cell
configured to produce electricity is included as a power source for
hydrogen production.
[0017] In particular embodiments, a microbial fuel cell is provided
which includes an anode having a specific surface area greater than
100 m.sup.2/m.sup.3. The anode is substantially non-toxic to
anodophilic bacteria. A cathode is also included in the microbial
fuel cell and the anode and the cathode are connected by an
electrically conductive connector.
[0018] An anode included in an embodiment of a microbial fuel cell
according to the present invention includes one or more
electrically conductive fibers. The one or more electrically
conductive fibers is attached to a conductive core support in one
configuration of an anode. In particular embodiments, each
individual fiber of the one or more conductive fibers is attached
to the conductive core support. Alternatively, a first portion of
the conductive fibers is attached to the conductive core support
and a second portion of the conductive fibers is attached to the
first portion of the conductive fibers and in electrical
communication therewith.
[0019] In particular embodiments, at least some of the conductive
fibers are carbon fibers.
[0020] More than one anode and/or more than one cathode is included
in embodiments of a microbial fuel cell according to the present
invention.
[0021] A power source for enhancing an electrical potential between
the anode and the cathode is included in particular embodiments in
order to produce hydrogen from the microbial fuel cell. In further
particular embodiments, the power source is in electrical
communication with the anode and the cathode. For example, an
included power source is a second microbial fuel cell, the second
microbial fuel cell configured to produce electricity.
[0022] A cathode for a microbial fuel cell is provided which
includes a membrane, the membrane forming a cathode wall having a
shape, the wall having an external surface and an internal surface,
the wall having the wall defining an interior space adjacent the
internal surface and an exterior adjacent the external surface, the
wall extending between a first end and a second end. The membrane
forming the wall is a nanofiltration membrane, an ultrafiltration
membrane, or an ion exchange membrane. The membrane forming the
wall is optionally an electrically conductive membrane in
electrically conductive connection with the electrically conductive
connector. In particular embodiments, a conductive material is in
contact with the internal surface or the external surface of the
membrane, the conductive material in electrically conductive
connection with the electrically conductive connector. A conductive
material is optionally a carbon-based material, such as graphite in
particular embodiments.
[0023] Where a conductive material is present on the membrane, the
conductive material is present on at least about 50% of the
internal surface or the external surface of the membrane.
[0024] In particular embodiments, a catalyst for enhancement of
oxygen reduction or a catalyst for enhancement of proton reduction
is in direct or indirect contact with the cathode membrane.
Optionally, at least one of the first or second ends of the wall is
closed.
[0025] In a particular embodiment of a hydrogen producing modified
microbial fuel cell, the interior space of the tube cathode is at
least partially filled with a liquid.
[0026] In further embodiments, the wall of the cathode is generally
cylindrical or generally slab-shaped.
[0027] An anode for a microbial fuel cell according to the present
invention includes an electrically conductive material having a
specific surface area greater than 100 m.sup.2/m.sup.3, the anode
substantially non-toxic to anodophilic bacteria. In particular
embodiments, the anode includes one or more conductive fibers.
Optionally, the one or more conductive fibers is attached to a
conductive core support. In particular embodiments, at least some
of the conductive fibers are directly attached to the support. In
further embodiments, each individual fiber of the one or more
conductive fibers is directly attached to the conductive core
support. Optionally, the electrically conductive material having a
specific surface area greater than 100 m.sup.2/m.sup.3 includes a
coating.
[0028] In a particular embodiment, the one or more fibers included
in an anode according to the present invention are treated with an
ammonia gas.
[0029] A system according to the present invention may be used as a
method of wastewater treatment coupled to electricity generation,
or as a method of renewable energy generation from non-waste
products, for example. Additionally, a system according to the
present invention may be used as a method of wastewater treatment
coupled to hydrogen generation. Thus, wastewater is provided as a
biodegradable fuel which is oxidized by bacteria in a microbial
fuel cell directly or which is biodegradable to produce products
oxidizable by bacteria in a microbial fuel cell.
[0030] A method for production of electricity is described
according to the present invention which includes providing a
microbial fuel cell including a tube cathode and/or brush anode,
inoculating the microbial fuel cell with bacteria, and supplying a
substrate oxidizable by bacteria; thereby producing
electricity.
[0031] A method for production of electricity is described
according to the present invention which includes providing a
microbial fuel cell including a tube cathode and/or brush anode,
inoculating the microbial fuel cell with bacteria, and supplying a
substrate oxidizable by bacteria and applying an additional
voltage, enhancing a potential between the anode and the cathode,
thereby producing hydrogen gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a schematic drawing of a brush anode;
[0033] FIG. 2 is a schematic drawing of a brush anode;
[0034] FIG. 3 is a schematic drawing of a brush anode;
[0035] FIG. 4 is a schematic drawing of a hollow generally
cylindrical membrane cathode;
[0036] FIG. 5 is a schematic drawing of a hollow generally
slab-shaped membrane cathode;
[0037] FIG. 6 is a schematic drawing of an electrode assembly for a
microbial fuel cell including a brush anode disposed in a tubular
cathode;
[0038] FIG. 7 is a schematic drawing of an electrode assembly for a
microbial fuel cell including multiple brush anodes connected to
multiple tubular cathodes;
[0039] FIG. 8 is a schematic drawing of an electrode assembly for a
microbial fuel cell including two brush anodes connected to a
tubular cathodes;
[0040] FIG. 9 is a schematic drawing of an electrode assembly for a
microbial fuel cell including multiple brush anodes connected to
multiple tubular cathodes;
[0041] FIG. 10 is a schematic drawing of an electrode assembly for
a microbial fuel cell including a brush anode and a hollow
cylindrical cathode;
[0042] FIG. 11 is a schematic drawing of an electrode assembly for
a microbial fuel cell including a brush anode and a hollow
slab-shaped cathode;
[0043] FIG. 12 is a schematic drawing of an electrode assembly for
a microbial fuel cell including an electricity generating module
including a brush anode and a tubular cathode powering a hydrogen
generating module including a brush anode and a tubular
cathode;
[0044] FIG. 13 is a schematic drawing of an electrode assembly for
a hydrogen generating microbial fuel cell including multiple brush
anodes connected to multiple cylindrical cathodes;
[0045] FIG. 14 is a schematic drawing of an electrode assembly for
a hydrogen generating microbial fuel cell including multiple brush
anodes connected to multiple slab-shaped cathodes;
[0046] FIG. 15 is a graph showing the initial four cycles of power
production in a microbial fuel cell including a brush anode;
[0047] FIG. 16A is a graph showing power density and cell
potentials in a microbial fuel cell including a brush anode;
[0048] FIG. 16B is a graph showing coulombic efficiency in a
microbial fuel cell including a brush anode;
[0049] FIG. 17 is a graph showing Nyquist plots corresponding to
the impedance spectra of microbial fuel cells including either a
cloth or brush anode, measured between the cathode and anode;
[0050] FIG. 18A is a graph showing power density curves for
microbial fuel cells containing various types of anodes in 200 mM
PBS;
[0051] FIG. 18B is a graph showing power density curves for
microbial fuel cells containing various types of anodes in 50 mM
PBS;
[0052] FIG. 19A is a graph showing power density curves using
varied loadings of randomly distributed 10 micron diameter graphite
fibers as the anode material;
[0053] FIG. 19B is a graph showing power density curves using
varied loadings of randomly distributed 6 micron diameter graphite
fibers as the anode material;
[0054] FIG. 20A is a graph showing power density, open symbols,
voltage, filled symbols as a function of current density normalized
to total reactor volume, obtained by varying the external circuit
resistance (40-3000.OMEGA.) for carbon paper anode microbial fuel
cells.
[0055] FIG. 20B is a graph showing electrode potentials, cathode
open symbols, anode filled symbols, as a function of current
density normalized to total reactor volume, obtained by varying the
external circuit resistance (40-30000.OMEGA.) for carbon paper
anode microbial fuel cells.
[0056] FIG. 21A is a graph showing power density (open symbols),
voltage (filled symbols) as a function of current density based on
reactor volume obtained by varying the external circuit resistance
(40-30000.OMEGA.) for brush anode microbial fuel cells.
[0057] FIG. 21B is a graph showing electrode potentials (cathode
open symbols, anode filled symbols) as a function of current
density based on reactor volume obtained by varying the external
circuit resistance (40-3000.OMEGA.) for brush anode microbial fuel
cells.
[0058] FIG. 22A is a graph showing power as a function of the
cathode surface area of tube-cathode microbial fuel cells with
brush anodes;
[0059] FIG. 22B is a graph showing volumetric power density as a
function of the cathode surface area of tube-cathode microbial fuel
cells with brush anodes;
[0060] FIG. 23A is a graph showing Figures voltage as a function of
time at a fixed resistance of 1000.OMEGA. (except as noted) for
brush anode microbial fuel cells operated in continuous or batch
mode;
[0061] FIG. 23B is a graph showing volumetric power density as a
function of current normalized to volume obtained by varying the
external circuit resistance (40-3000.OMEGA.) for brush anode
microbial fuel cells operated in continuous or batch mode;
[0062] FIG. 24 is a table showing electrode types and surface areas
used in Example 2 as well as ratios of electrode area to volume,
volumes, internal resistances, maximum power density normalized to
anode surface area or total reactor volume, and CEs for carbon
paper and brush anode MFC batch tests.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0063] Microbial fuel cells are provided according to the present
invention which include scalable electrodes and scalable electrode
assembly configurations for microbial fuel cells. The term
"microbial fuel cell" as used herein refers to a device using
bacteria as catalysts to oxidize a fuel and generate electrons
which are transferred to an anode. A microbial fuel cell typically
generates electricity. The term "microbial fuel cell" is also used
herein to refer to modified microbial fuel cells configured to
produce hydrogen. A microbial fuel cell modified to produce
hydrogen includes a power source for addition of a voltage and is
distinct from a water electrolyzer. A microbial fuel cell is also
known as a bio-electrochemically assisted microbial reactor
(BEAMR). Broad aspects of a hydrogen generation microbial fuel cell
(BEAMR) are described in U.S. patent application Ser. No.
11/180,454.
[0064] A microbial fuel cell is useful in various applications,
such as in wastewater treatment, or in renewable energy production,
for example. A microbial fuel cell according to the present
invention may be used to power a device, such as a portable
electronic device. A microbial fuel cell according to the present
invention is advantageously used in a remote device, such as a
marine sensor.
[0065] Broadly described, a microbial fuel cell includes bacteria
as a catalyst for generation of electrons for production of
electricity and/or hydrogen. A microbial fuel cell generally
includes an anode, a cathode and an electron conductor connecting
the anode and cathode. Bacteria capable of oxidizing a substrate to
produce electrons are included in a microbial fuel cell. A cation
exchange, anion exchange or neutral charge membrane is optionally
included in particular configurations of a microbial fuel cell.
[0066] Broadly describing operation of a microbial fuel cell
configured to produce electricity, a provided oxidizable substrate
is oxidized by bacteria which generate electrons and protons. Where
the substrate is an organic substrate carbon dioxide is also
produced. The electrons are transferred to the anode, and, through
a load such as a device to be powered, to the cathode. Protons and
electrons react with oxygen at the cathode, producing water.
[0067] Broadly describing operation of a microbial fuel cell
configured to produce hydrogen, a provided oxidizable substrate is
oxidized by bacteria which generate electrons and protons. Where
the substrate is an organic substrate carbon dioxide is also
produced. A power source is connected to the microbial fuel cell
and an additional voltage is applied. The electrons generated by
the bacteria are transferred to the anode, and, through a
conductive connector, to the cathode. Oxygen is substantially
excluded from the cathode area such that protons and electrons
combine at the cathode, producing hydrogen.
[0068] Electrodes included in a microbial fuel cell according to
the present invention are electrically conductive. Exemplary
conductive electrode materials include, but are not limited to,
carbon paper, carbon cloth, carbon felt, carbon wool, carbon foam,
graphite, porous graphite, graphite powder, graphite granules,
graphite fiber, a conductive polymer, a conductive metal, and
combinations of any of these.
[0069] Typically, an anode provides a surface for attachment and
growth of anodophilic bacteria and therefore an anode is made of
material compatible with bacterial growth and maintenance.
Compatibility of a material with bacterial growth and maintenance
in a microbial fuel cell may be assessed using standard techniques
such as assay with a viability marker such as Rhodamine 123,
propidium iodide, SYTO 9 and combinations of these or other
bacteria viability markers.
[0070] An anode included in embodiments of a microbial fuel cell
according to the present invention includes fibers of a conductive
anode material, providing a large surface area for contact with
bacteria in a microbial fuel cell.
[0071] Specific surface area of an anode included in embodiments of
a fuel cell according to the present invention is greater than 100
m.sup.2/m.sup.3. Specific surface area is here described as the
total surface area of the anode per unit of anode volume. Specific
surface area greater than 100 m.sup.2/m.sup.3 contributes to power
generation in microbial fuel cells according to embodiments of the
present invention. In further embodiments, fuel cells according to
the present invention include an anode having a specific surface
area greater than 1000 m.sup.2/m.sup.3. In still further
embodiments, fuel cells according to the present invention include
an anode having a specific surface area greater than 5,000
m.sup.2/m.sup.3. In yet further embodiments fuel cells according to
the present invention include an anode having a specific surface
area greater than 10,000 m.sup.2/m.sup.3. An anode configured to
have a high specific surface area allows for scaling of a microbial
fuel cell according to the present invention.
[0072] A brush anode is provided in particular embodiments which
has a specific surface area greater than 100 m.sup.2/m.sup.3. A
brush anode includes one or more conductive fibers. In particular
embodiments the one or more fibers are attached to a support.
[0073] A plurality of fibers is attached to the support and the
fibers extend generally radially from the support in specific
embodiments. A brush anode optionally includes a centrally disposed
support having a longitudinal axis.
[0074] Brush anodes 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.
[0075] Fibers of a brush anode are electrically conductive and are
in electrical communication with the support and with a cathode. In
particular embodiments, fibers and/or support of a brush anode
provide a support for colonization by anodophilic bacteria, such
that the brush anode is preferably substantially non-toxic to
anodophilic bacteria.
[0076] In particular embodiments, fibers of a brush anode include a
metallic and/or non-metallic conductive material which is
substantially non-toxic to anodophilic bacteria. In a specific
example, fibers include carbon fibers. Carbon fibers are optionally
substantially composed of graphite. In a further option, a carbon
material is mixed with a conductive polymer to form a fiber. In
still further embodiments, a polymer fiber is coated with a
conductive carbon material.
[0077] In one configuration, graphite fibers 112 of a brush anode
100 are placed substantially perpendicular to and between two or
more conductive, corrosion resistant wires which form a support 110
such that the carbon fibers 112 extend substantially radially from
the support 110 as shown in FIG. 1. A wire is optionally twisted
around the brushes to maintain good electrical contact with the
wire, forming an anode electrode. A conductive connector is
typically attached to the support 110 to connect the anode to the
cathode.
[0078] The graphite fibers included in a brush anode may be cut at
the ends as in FIG. 1 such that multiple discontinuous fibers 112
are present in the brush anode. In further embodiments, as
illustrated in FIG. 2, an anode 200 optionally includes one or more
fibers in a continuous ordered configuration, for instance to help
maintain fiber extension into an aqueous medium in a microbial fuel
cell. In the illustrated configuration, at least one continuous
fiber is wound about a central axis, forming looped fiber
extensions 212. An optional support 210 is shown in FIG. 2. Where
no support is included, a conductive connector is attached to the
fiber or fibers to connect the anode to the cathode. Where a
support is included, a conductive connector is typically attached
to the support to electrically connect the anode and a cathode.
[0079] In a further configuration, a brush anode 300 includes
randomly oriented graphite fibers 312 without a support forming a
type of continuous pad structure in electrical conduction
connection with a connector 310, shown in FIG. 3.
[0080] A brush anode electrode may include any of various coatings.
In particular embodiments a coating is included on a brush anode to
increase the efficiency of power production by bacteria on the
anode. For example, a brush anode electrode may be coated with a
material which increases the conductivity of electrons from
bacteria to a surface. Examples of materials which increase the
conductivity of electrons from bacteria to a surface include, but
are not limited to, neutral red, Mn.sup.4+, Fe.sub.3O.sub.4,
Ni2.sup.+, fluorinated polyanilines, such as poly(2-fluoroaniline)
and poly(2,3,5,6-tetrafluoroaniline) for example,
anthraquinone-1,6-disolfonic acid (AQDS), 1,4-naphthoquinone (NQ),
and combinations of any of these.
[0081] In a particular embodiment, an anode is treated with an
ammonia gas process to increase power production and reduce the
time needed to generate substantial power once the reactor is
inoculated. For example, a brush anode is treated with a heated
ammonia gas, such as NH.sub.3 gas. In a specific embodiment, a
brush anode is heated to 700.degree. C. and incubated with NH.sub.3
gas for about one hour.
[0082] Additional materials are optionally included in a brush
anode, for example to strengthen and support the graphite fibers or
to help clean the system by removing biofilm in cases where the
brushes can be moved around or swirled to clean the adjoining
surfaces, cathodes or other materials.
[0083] A cathode included in an inventive system may be configured
to be immersed in liquid or as a gas cathode, having a surface
exposed to a gas. A cathode preferably includes an electron
conductive material. Materials included in a cathode included in an
inventive system illustratively include, but are not limited to,
carbon paper, carbon cloth, carbon felt, carbon wool, carbon foam,
graphite, porous graphite, graphite powder, a conductive polymer, a
conductive metal, and combinations of any of these.
[0084] In particular embodiments, a microbial fuel cell provided
according to the present invention includes a cathode wherein the
cathode includes a membrane and the membrane forms a cathode wall.
The cathode wall has an external surface and an internal surface
and the wall defines an interior space adjacent to the internal
surface and an exterior adjacent to the external surface. The
cathode wall forms a shape which is generally cylindrical in
particular embodiments. In further particular embodiments, the
shape formed by the cathode wall is generally slab or brick-shaped,
having a hollow interior. Other hollow shapes are also possible,
illustratively including hollow disc-shaped.
[0085] A membrane forming a cathode wall is a porous membrane. The
membrane is sufficiently porous to allow diffusion of a desired
material through the membrane. For example, an included membrane is
porous to oxygen, protons and/or hydrogen gas in particular
embodiments of an inventive microbial fuel cell. In specific
embodiments of an electricity generating configuration of a
microbial fuel cell, an included membrane is porous to oxygen and
protons. In specific embodiments of a hydrogen generating modified
microbial fuel cell, an included membrane is porous to protons
where a catalyst is present on or adjacent to the internal surface
of the membrane. In further specific embodiments of a hydrogen gas
generating modified microbial fuel cell, an included membrane is
porous to protons and hydrogen gas where a catalyst is present on
or adjacent to the external surface of the membrane. In preferred
embodiments, the effective pores of an included membrane are
smaller than the size of a typical bacterium, about 1000
nanometers. Thus, the flow of water and/or bacteria through the
membrane and any included membrane coatings is restricted.
[0086] A membrane included in a cathode of the present invention is
not limited as to the material included in the membrane.
Microfiltration, nanofiltration and ion exchange membrane
compositions are known in the art and any of various membranes may
be used which exclude bacteria and allow diffusion of a desired gas
through the membrane. Illustrative examples of microfiltration,
nanofiltration and/or ion exchange membrane compositions include,
but are not limited to, halogenated compounds such as
tetrafluoroethylene, tetrafluoroethylene copolymers,
tetrafluoroethylene-perfluoroalkylvinylether copolymers,
polyvinylidene fluoride, polyvinylidene fluoride copolymers,
polyvinyl chloride, polyvinyl chloride copolymers; polyolefins such
as polyethylene, polypropylene and polybutene; polyamides such as
nylons; sulfones such as polysulfones and polyether sulfones;
nitrile-based polymers such as acrylonitriles; and styrene-based
polymers such as polystyrenes.
[0087] A membrane optionally include a structural support layer
such as a porous plastic backing layer. For example, a membrane is
optionally supported on a polyester layer. A support layer is
flexible in preferred embodiments.
[0088] Examples of suitable membrane materials are ultrafiltration
and nanofiltration membranes commonly employed in the water
treatment industry to filter water while excluding bacteria. For
example, a suitable membrane is ultrafiltration membrane B 0125
made by X-Flow, The Netherlands. Additional examples include CMI
and AMI ion exchange membranes made by Membranes International,
Inc. New Jersey, USA.
[0089] A membrane included in an inventive cathode includes a
conductive material such that the membrane is electrically
conductive and/or the membrane is coated on one side with a
conductive material.
[0090] In particular configurations, one or more coatings are
applied to the membrane in order to allow the material to become
electrically conductive. For example, a metal or carbon containing
coating is optionally applied to at least a portion of one side of
the membrane. In a particular embodiment, a graphite coating is
applied. An exemplary formulation of a graphite coating includes
products of Superior Graphite, formulations ELC E34, Surecoat
1530.
[0091] Optionally, a membrane material is fabricated to include an
electrically conductive material in the membrane, rendering a
membrane made from the material electrically conductive. For
example, carbon fibers may be mixed with a polymer typically used
in an ultrafiltration, nanofiltration and/or ion exchange
membrane.
[0092] Optionally, a catalyst for enhancing a desired reaction at
the cathode is included in a cathode according to the present
invention. Thus, a catalyst for enhancing reduction of oxygen is
included in an electricity producing configuration of a microbial
fuel cell. Further, a catalyst for enhancing reduction of protons
to hydrogen gas, that is enhancing a hydrogen evolution reaction,
is included in a hydrogen gas producing configuration of a
microbial fuel cell. An included catalyst typically enhances the
reaction kinetics, e.g. increases the rate of oxygen and/or proton
reduction. In addition, a catalyst reduces a need for applied
potential, the overpotential, for initiating oxygen and/or hydrogen
reduction.
[0093] A catalyst is optionally applied to a conductive membrane.
In a further option, a catalyst is mixed with a conductive material
to form a mixture which is applied to a membrane. In a further
option, a catalyst is applied to the membrane before or after
application of a conductive material.
[0094] In particular embodiments, a catalyst is optionally mixed
with a polymer and a conductive material such that a membrane
includes a conductive catalyst material integral with the membrane.
For example, a catalyst, is mixed with a graphite coating material
and the mixture is applied to a cathode membrane.
[0095] Suitable catalysts are known in the art and include metal
catalysts, such as a noble metal. Suitable catalyst metals
illustratively include platinum, nickel, copper, tin, iron,
palladium, cobalt, tungsten, and alloys of such metals. While a
catalyst metal such as platinum is included in a cathode in one
embodiment of an inventive system, the platinum content may be
reduced, for example to as little as 0.1 mg/cm.sup.2 without
affecting energy production. In further embodiments, an included
catalyst includes a non-noble metal containing catalyst such as
CoTMPP.
[0096] One or more additional coatings may be placed on one or more
electrode surfaces. Such additional coatings may be added to act as
diffusion layers, for example. A cathode protective layer, for
instance, may be added to prevent contact of bacteria or other
materials with the cathode surface while allowing oxygen diffusion
to the catalyst and conductive matrix. In further embodiments, a
cathode protective layer is included as a support for bacterial
colonization such that bacteria scavenge oxygen in the vicinity of
the cathode but do not directly contact the cathode.
[0097] FIG. 4 illustrates a generally cylindrical "tube" cathode
400 according to the present invention having a cathode wall which
has an external surface 414 and an internal surface 416 and the
wall defines an interior space 418 adjacent to the internal surface
416.
[0098] FIG. 5 illustrates a generally slab-shaped "tube" cathode
500 according to the present invention having a cathode wall which
has an external surface 514 and an internal surface 516 and the
wall defines an interior space 518 adjacent to the internal surface
516.
[0099] A tube cathode included in a microbial fuel cell configured
for electricity generation is open at one or both ends of its
length to an oxygen-containing medium. In particular embodiments, a
tube cathode included in a microbial fuel cell configured for
electricity generation is open at one or both ends to ambient
air.
[0100] A tube cathode included in a microbial fuel cell configured
for hydrogen generation according to embodiments of the present
invention is open at one end of its length to a receptacle or
conduit for collection or passage of generated hydrogen gas.
[0101] As described above, a tube cathode according to the present
invention has an interior space. The interior space of a tube
cathode included in a microbial fuel cell configured for hydrogen
generation according to embodiments of the present invention may be
gas filled in one option. Thus, for example, the interior space of
a tube cathode may initially contain ambient air at start-up and
contain increased amounts of hydrogen as hydrogen generation
proceeds during operation of the hydrogen generating microbial fuel
cell. The generated hydrogen flows from the interior space of the
tube cathode, for instance to a gas collection unit or device. In a
further embodiment, the interior space is filled or partially
filled with a liquid. Hydrogen generated during operation of the
hydrogen generating microbial fuel cell moves from the liquid
containing interior space, for instance to a gas collection unit or
device, efficiently with little back pressure into the liquid in
the interior space. The inclusion of a liquid in a tube cathode
aids in hydrogen evolution since it results in phase separation of
the hydrogen gas and liquid, reducing back diffusion into the anode
chamber. Larger amounts of hydrogen are recovered using a liquid in
the cathode interior space. A liquid included in the interior space
may be any of various liquids compatible with the cathode materials
and with hydrogen gas. Suitable liquids include aqueous liquids,
such as water, which may contain one or more salts, buffers, or
other additives.
[0102] In some embodiments, the cathode is operated so that water
is pulled through the porous membrane material of the cathode,
allowing contact of the water with the conductive coating or
conductive matrix of the membrane. The membrane material can be
enriched with carbon black to make it conductive, made with
graphite fibers, or coated in a way that still permits water flow
through the device.
[0103] Optionally, and preferably in some embodiments, the cathode
is a gas cathode. In particular embodiments, an included cathode
has a planar morphology, such as when used with a brush anode
electrode. In this configuration, the cathode is preferably a gas
diffusion electrode.
[0104] Optionally, an included cathode is disposed in an aqueous
medium, with dissolved oxygen in the medium serving to react at the
cathode.
[0105] In one embodiment of the invention a cathode membrane is
substantially impermeable to water.
[0106] In particular embodiments, the cathode 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. Thus, in one
embodiment 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 is preferably bonded to
the cathode to prevent water leakage through the cathode from the
interior of the reaction chamber. Further, in hydrogen generation
configurations, the 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. 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.
[0107] In a further embodiment, 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. A CPL
further acts to provide a support for bacterial colonization in the
vicinity of the cathode, allowing for scavenging of oxygen in the
cathode area without biofouling.
[0108] In one embodiment, 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 or surround the inner surface of the
cathode partially or wholly, such as by bonding of the CPL to the
cathode.
[0109] In a further embodiment, a CPL is present in the interior of
the 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 and the catalyst. 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. A CPL may also include material that aids bacterial
attachment, so that bacteria can scavenge dissolved oxygen that can
leak into the system.
[0110] In one embodiment, 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.
[0111] In one embodiment, a diffusion layer includes an anion
exchange material. In a preferred embodiment 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 the 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 passage 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 the cathode. In a preferred embodiment, an anion
exchange material can be used to contain the catalyst applied to
the cathode.
[0112] Fuel Cell Configurations
[0113] Broadly described, a microbial fuel cell includes an
electrode assembly including an anode, a cathode and an
electrically conductive connector connecting the anode and the
cathode. Further components of a microbial fuel cell may include a
reaction chamber in which an anode and cathode are at least
partially disposed. A reaction chamber may have one or more
compartments, such as an anode compartment and a cathode
compartment separated, for instance, by a cation exchange membrane.
Alternatively, a reaction chamber may be a single compartment
configuration. 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.
[0114] The electrodes of an electrode assembly can be placed in
various configurations relative to each other depending on the
desired application.
[0115] In general, an anode and a cathode are place in proximity.
In particular embodiments, an anode may contact a cathode, such as
where one or more fibers of a brush anode contact a tube cathode
having a catalyst on the inside of the tube.
[0116] In one configuration, the "brush" anode electrode is placed
inside the "tubular" cathode, with continuous water flow through
the interior of the tube and over the brush anode, with the cathode
catalyst applied on the outside of the tube.
[0117] In an example of such an arrangement, one or more brush
anode electrodes are placed inside of a tube cathode tube as shown
in FIG. 6. FIG. 6 shows an embodiment of an electrode assembly 600
for a microbial fuel cell having a brush anode 620 on the inside of
a tube cathode 630. The tube cathode 630 has a wall formed by a
membrane having an external surface 614 and an internal surface
616. External surface 614 is coated with a conductive catalyst
material (CSM). The tube cathode has an internal space defined by
the membrane and adjacent to the internal surface 616 which is open
to allow entry and/or directed flow of an aqueous medium. For
example, flow is directed through the tube so that it flows over
and around the highly conductive carbon fibers of the anode 620 to
which anodophilic bacteria attach. The bacteria oxidize organic
matter, releasing electrons to the anode fibers. These electrons
travel through the circuit 650 placed under a load 660 such that
the current can do work or be transferred for distant use as a
source of power. Protons produced from the oxidation of the organic
matter move in the water towards the cathode where they diffuse to
the site of the conductive material on the external surface 614 of
the tube cathode and if a catalyst is present, form water when
combined with oxygen and electrons from the circuit. In the
illustrated embodiment, electrons travel through connector 650 and
cathode connection 670 to the CSM on surface 614.
[0118] In a second configuration, one or more brush electrodes are
placed outside the tubular cathode. Optionally, flow of an aqueous
medium is directed through a reaction chamber containing one or
more brush electrodes and then over a surface of a cathode tube. A
tubular cathode in such a configuration can include a catalyst
layer on an outside or inside surface of the tube.
[0119] An example of such a configuration of an electrode assembly
for a microbial fuel cell in which the brush anodes are outside of
the cathode in the medium is shown in FIG. 7 at 700. FIG. 7 shows
multiple anodes 720 arranged in series with two tubular cathodes
730, the cathodes having a conductive catalyst material on the
outside of the tube 714. Medium flow is directed through the brush
electrodes and then flows on to the cathode, flowing over the
cathodes allowing good transfer of protons to the cathode surface.
The anode and cathode are electrically connected by an electrical
connector 750 through a load 760. The electrical connector further
includes a cathode connection 770 in contact with the CSM on
surface 714.
[0120] An embodiment including a conductive material on the outside
of the tube cathode provides good contact of the conductive cathode
surface with an aqueous medium as shown in FIG. 7.
[0121] In a further embodiment, a conductive material and catalyst
is disposed on the internal surface of the tube cathode. This
configuration has the benefit of keeping the conductive material
away from bacteria and potential chemicals in the aqueous medium
that might inactivate the catalyst or reduce its efficiency as
shown in FIG. 8. FIG. 8 shows an electrode assembly 800 for a
microbial fuel cell including multiple anodes 820 flanking a tube
cathode 830 with a conductive catalyst material on the inside of
the cathode tube, on the internal wall 816 defined by the membrane.
Also shown is the external surface of the tube cathode 814, a
connector 850 in electrical conduction contact with the anodes,
cathode and a load 860.
[0122] Alternatively in such a configuration, the cathode tube is
made of a conductive catalyst material and the outside of the tube
is non-conductive, such as by coating with a non-conductive
material. A cathode conductive layer can be coated with a
protective layer as noted above. If the tube cathode is coated with
conductive catalyst material on the internal wall, the cathode
protective layer must be oxygen permeable. If the tube cathode is
coated with conductive catalyst material on the external wall, the
cathode protective layer must be able to pass protons from the
water to the cathode surface; coatings that restrict oxygen
diffusion to the water are preferred in this arrangement.
[0123] In a further configuration, a brush anode electrode is
disposed external to a tube cathode lumen and the water is moved,
such as by suction, into the interior of the cathode tube membrane.
Optionally, the conductive catalyst material is disposed on the
outside, inside, or may be integral with the membrane material. In
such an arrangement, the water pulled through the cathode is
filtered, as through an ultrafiltration or nanofiltration
membrane.
[0124] FIG. 9 illustrates an electrode assembly 900 for a microbial
fuel cell including multiple anodes 920 and cathodes 930. Shown in
FIG. 9 are four anode-cathode modules such as shown in FIG. 8.
[0125] In the embodiment illustrated in FIG. 9, the electrode
assembly 900 is present in a microbial fuel cell reaction chamber
902 in an aqueous medium 904 for generation of electricity.
Channels 922 and 924 are illustrated, which may be used for
introduction and removal of one or more substances from the
reaction chamber 902. Tube cathodes 930 extend through the reaction
chamber such that the interior of the tube cathodes 930 is open to
the ambient atmosphere 926 and/or to a directed flow through the
tubes 930. Optionally, one end of a cathode tube 930 is closed or
reversibly capped. Anodes and cathodes included in the electrode
assembly 900 are electrically connected by an electrical connector
950. Generated electricity may be used to power a device,
illustrated as a load 960. Anode-cathode modules of an electrode
assembly may be linked in series to increase voltage or in parallel
to increase current. Where anode-cathode modules are linked in
series, the modules are substantially separated by a baffle as
shown at 928 in FIG. 9 such that the anodes are substantially
electrically isolated. The illustrated baffle 928 includes a pore
communicating with other reactor sections including other
anode-cathode modules. Combinations of anode-cathode assemblies
linked in series and in parallel may be used to increase both
voltage and current.
[0126] When electricity is the main product of an inventive system,
oxygen is present at the cathode to facilitate the reaction of
protons, electrons and oxygen to form water. A microbial fuel cell
according to the present invention may also be modified to generate
hydrogen. In a hydrogen generation embodiment of a microbial fuel
cell of the present invention, 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 in addition to that generated by the microbial fuel
cell without the supplementary power source is included.
[0127] A system according to the present invention may be adapted
to produce hydrogen gas by removing oxygen from the cathode area
and by applying a small voltage of sufficient magnitude to generate
hydrogen gas at the cathode surface that can be collected either
inside the tube or on the outside of the tube depending on the
configuration used. Broad aspects of a hydrogen generation
microbial fuel cell are described in U.S. patent application Ser.
No. 11/180,454.
[0128] In a hydrogen generation embodiment, an anode electrode may
be constructed and placed as described. However, for the cathode no
oxygen is needed and its presence is to be avoided. When oxygen is
removed, a slight voltage is added to that generated at the anode.
In general, the added amount is in the range between about 10-1000
millivolts. Hydrogen generated at the cathode is captured by
collecting the gas produced outside the tube when an anode is
placed inside the tube cathode, or by collecting the gas inside the
cathode tube when an anode is placed outside the tube cathode.
[0129] A brush or planar cathode can also be used in conjunction
with a brush anode for hydrogen generation. Similarly, a brush or
planar anode can be used in conjunction with a tube anode for
hydrogen generation. Furthermore, combinations of one or more brush
and/or planar anodes may be used with one or more brush, planar
and/or tube cathodes in embodiments of an inventive electrode
assembly for a microbial fuel cell.
[0130] A particular example of a hydrogen generation anode-cathode
assembly 1000 for a microbial fuel cell is shown in FIG. 10 which
shows a brush anode 1020 and a cylindrical tube cathode 1030
electrically connected by a connector 1050 through a load 1090. An
optional resistor 1090 is shown as the load in this figure. A power
source is included in a hydrogen generation fuel cell, not shown in
this figure. The tube cathode 1030 includes an external surface
1014 and an internal surface 1016.
[0131] A particular example of a hydrogen generation anode-cathode
assembly 1110 for a microbial fuel cell is shown in FIG. 11 which
shows a brush anode 1120 and a tube cathode having a slab-shape
1130 electrically connected by a connector 1150 through a load
1190. A power source is included in a hydrogen generation fuel
cell, not shown in this figure, is connected to the electrode
assembly. The tube cathode 1130 includes an external surface 1114
and an internal surface 1116.
[0132] FIG. 12 illustrates a schematic of an electrode assembly
1200 for a microbial fuel cell in which a first electrode assembly
configured to generate electricity is coupled to a second electrode
assembly configured to generate hydrogen. In one such embodiment, a
brush anode 1220 is electrically connected to tube cathode 1230,
optionally through a load 1290. A second brush anode 1222 is
electrically connected by connector 1252 to a second tube cathode
1232. The first electrode assembly is connected to the second
electrode assembly by electrical connector 1254 such that the
electricity produced by the first electrode assembly enhances an
electrical potential between the anode 1222 and cathode 1232 by
application of a voltage.
[0133] FIG. 13 illustrates a schematic of a microbial fuel cell
1300 for hydrogen generation including an electrode assembly having
a series of electrode modules. The electrode assembly is present in
a single tank reaction chamber 1302 in an aqueous medium 1304.
Channels 1322 and 1324 are optionally included for ingress and
egress of substances such as an aqueous medium into and out of the
reaction chamber. Channels 1327 and 1329 are optionally included
for ingress and egress of substances such as a sweep gas or
hydrogen gas into and out of a reaction chamber and/or hydrogen
collection vessel. Multiple anodes 1320 and tube cathodes 1330 are
depicted and are electrically connected by connector 1350. A power
source included in a hydrogen generation fuel cell, not shown in
this figure, is connected to the electrode assembly. Hydrogen gas
collected in the tube cathodes 1330 flows to a chamber 1380. The
gas may be collected from the chamber or may be directed out of the
chamber 1380 to a collection vessel or directly to a device to be
hydrogen powered, for example. Tube cathodes 1330 have an interior
space 1325 which is open at one end into chamber 1380. The interior
space 1325 may be gas filled in one option. Thus, for example, the
interior space 1325 of a tube cathode may initially contain ambient
air at start-up and contain increased amounts of hydrogen as
hydrogen generation proceeds during operation of the hydrogen
generating microbial fuel cell 1300. In a further embodiment, the
interior space 1325 is filled or partially filled with a liquid.
Hydrogen generated during operation of the hydrogen generating
microbial fuel cell 1300 moves from the liquid containing interior
space 1325 to chamber 1380 efficiently with little back pressure
into the liquid in the interior space 1325.
[0134] FIG. 14 illustrates a schematic of a series of electrode
assemblies 1400 for a microbial fuel cell for hydrogen generation.
The electrodes are present in a single tank reaction chamber 1402
in an aqueous medium 1404. Channels 1422 and 1424 are optionally
included for ingress and egress of substances such as an aqueous
medium into and out of the reaction chamber. Channels 1427 and 1429
are optionally included for ingress and egress of substances such
as a sweep gas or hydrogen gas into and out of a reaction chamber
and/or hydrogen collection vessel. Multiple anodes 1420 and tube
cathodes 1430 are depicted and are electrically connected by
connector 1450. A power source included in a hydrogen generation
fuel cell, not shown in this figure, is connected to the electrode
assembly. Hydrogen gas collected in the tube cathodes 1430 flows to
a chamber 1480. The gas may be collected from the chamber or may be
directed out of the chamber 1480 to a collection vessel or directly
to a device to be hydrogen powered, for example. The slab-shaped
cathode tubes shown span the reactor depth.
[0135] 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.
[0136] Electrodes of various sizes and shapes may be included in an
inventive system. 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 anodophiles 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.
[0137] Electrodes may be positioned in various ways to achieve a
desired spacing between the electrodes. For example, a first
electrode may be positioned such that its longest dimension is
substantially parallel to the longest dimension of a second
electrode. In a further embodiment, a first electrode may be
positioned such that its longest dimension is substantially
perpendicular with respect to the longest dimension of a second
electrode. Additionally, a first electrode may be positioned such
that its longest dimension is at an angle between 0 and 90 degrees
with respect to the longest dimension of a second electrode.
[0138] A cation exchange membrane is optionally disposed between an
anode and a cathode in embodiments of a microbial fuel cell
according to the present invention. A cation exchange membrane is
permeable to one or more selected cations. Particularly preferred
is a cation exchange membrane permeable to protons, a proton
exchange membrane. Suitable proton exchange membrane materials
include perfluorinated sulfonic acid polymers such as
tetrafluoroethylene and perfluorovinylether sulfonic acid
copolymers, and derivatives thereof. Specific examples include
NAFION, such as NAFION 117, and derivatives produced by E.I. DuPont
de Nemours & Co., Wilmington, Del.
[0139] A microbial fuel cell according to the present invention may
be configured as a self-contained fuel cell in particular
embodiments. Thus, for example, a quantity of a biodegradable
substrate is included in the fuel cell and no additional substrate
is added. In further options, additional substrate is added at
intervals or continuously such that the fuel cell operates as a
batch processor or as a continuous flow system.
[0140] 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.
[0141] Bacteria in a microbial fuel cell include at least one or
more species of anodophilic bacteria. The terms "anodophiles" and
"anodophilic bacteria" as used herein refer to bacteria that
transfer electrons to an electrode, either directly or by
endogenously produced mediators. In general, anodophiles are
obligate or facultative anaerobes. The term "exoelectrogens" is
also used to describe suitable bacteria. Examples of anodophilic
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. Microbiol. 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 Microbiol. 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. Microbiol., 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.
[0142] Anodophilic bacteria preferably are in contact with an anode
for direct transfer of electrons to the anode. However, in the case
of anodophilic 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.
[0143] Optionally, a mediator of electron transfer is included in a
fuel cell. 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.
[0144] Anodophilic bacteria may be provided as a purified culture,
enriched in anodophilic 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.
[0145] Further, a mixed population of bacteria may be provided,
including anodophilic anaerobes and other bacteria.
[0146] A biodegradable substrate included in a microbial fuel cell
according to embodiments of the present invention is oxidizable by
anodophilic bacteria or biodegradable to produce a material
oxidizable by anodophilic bacteria.
[0147] A biodegradable substrate is an organic material
biodegradable to produce an organic substrate oxidizable by
anodophilic bacteria in preferred embodiments. Any of various types
of biodegradable organic matter may be used as "fuel" for bacteria
in a 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.
[0148] Organic substrates oxidizable by anodophilic bacteria are
known in the art. Illustrative examples of an organic substrate
oxidizable by anodophilic 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 anodophilic bacteria include aromatic compounds such
as toluene, phenol, cresol, benzoic acid, benzyl alcohol and
benzaldehyde. Further organic substrates oxidizable by anodophilic
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
anodophilic bacteria or biodegradable to produce an organic
substrate oxidizable by anodophilic bacteria.
[0149] Specific examples of organic substrates oxidizable by
anodophilic bacteria include glycerol, glucose, acetate, butyrate,
ethanol, cysteine and combinations of any of these or other
oxidizable organic substances.
[0150] 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.
[0151] A microbial fuel cell according to the present invention may
be configured to produce electricity and/or hydrogen in particular
embodiments.
[0152] An embodiment of an inventive system is a completely
anaerobic system to generate hydrogen at the cathode by providing a
small added voltage to the circuit. This approach to
electrochemically assist hydrogen production is based on separating
the two electrodes into half cell reactions. The potential of the
anode is set by the oxidation of a substrate. Thus, the anode side
of an embodiment of an inventive system operates similarly to that
in a microbial fuel cell (MFC): bacteria oxidize an organic
compound completely to CO.sub.2 and transfer electrons to the
anode. The half reaction potential measured at the anode in an
embodiment of an inventive system tests as -480 mV (Ag/AgCl) or
-285 mV (NHE) (reduction).
[0153] In contrast, cathode operation in an embodiment of an
inventive anaerobic hydrogen generation system is significantly
altered from that in a standard MFC. By electrochemically
augmenting the cathode potential in a MFC circuit it is possible to
directly produce hydrogen from protons and electrons produced by
the bacteria. This approach greatly reduces the energy needed to
make hydrogen directly from organic matter compared to that
required for hydrogen production from water via electrolysis. In a
typical MFC, the open circuit potential of the anode is .about.-300
mV. Where hydrogen is produced at the cathode, the half reactions
occurring at the anode and cathode, with acetate oxidized at the
anode, are:
[0154] Anode: C.sub.2H.sub.4O.sub.2+2H.sub.2O.fwdarw.2CO.sub.2+8
e.sup.-+8H.sup.+
[0155] Cathode: 8H.sup.++8 e.sup.-.fwdarw.4H.sub.2
[0156] A power source for enhancing an electrical potential between
the anode and cathode is included. 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.
[0157] In a particular embodiment, a power supply for a hydrogen
producing microbial fuel cell is an electricity producing microbial
fuel cell.
[0158] In a further embodiment, a wall of the reaction chamber
includes two or more portions such as a structural portion and an
electrode portion. A structural portion provides structural support
for forming and maintaining the shape of the reaction chamber, as
in a conventional wall. An electrode portion of a wall may provide
structural support for the reaction chamber and in addition has a
functional role in a process carried out in an inventive system. In
such an embodiment, the structural portion and electrode portion
combine to form a wall defining the interior of the reaction
chamber. In a specific embodiment, the electrode portion of the
wall includes the cathode. Further, a support structure for
supporting an anode or cathode may be included in an electrode
portion of the wall. Such a support structure may further provide
structural support for forming and maintaining the shape of the
reaction chamber
[0159] A hydrogen gas collection system is optionally included in
an inventive microbial fuel cell configured to produce hydrogen
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.
[0160] 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.
[0161] An aqueous medium in a reaction chamber of a microbial fuel
cell is formulated to be non-toxic to bacteria in contact with the
aqueous medium in the fuel cell. Further, the medium or solvent may
be adjusted to a be compatible with bacterial metabolism, for
instance by adjusting pH to be in the range between about pH 3-9,
preferably about 5-8.5, inclusive, 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. Optionally, an aqueous medium in
contact with anodophilic bacteria contains a dissolved substrate
oxidizable by the bacteria.
[0162] In operation, reaction conditions include variable such as
pH, temperature, osmolarity, and ionic strength of the medium in
the reactor. In general, the pH of the medium in the reactor is
between 3-9, inclusive, and preferably between 5-8.5 inclusive.
[0163] 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 deg 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. A surprising finding of the present invention is
that 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.
[0164] 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.
[0165] 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.
[0166] 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.
[0167] 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.
[0168] A pump may be provided for enhancing flow of liquid or gas
into and/or out of a reaction chamber.
[0169] 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
Electrode Materials
[0170] In this example brush anodes are made of carbon fibers
(PANEX.RTM.33 160K, ZOLTEK) cut to a set length and wound using an
industrial brush manufacturing system into a twisted core
consisting of two titanium wires. Two brush sizes are used in this
example: a small brush 2.5 cm in outer diameter and 2.5 cm in
length; and a larger brush 5 cm in diameter and 7 cm in length.
Based on mass of fibers used in a single brush, and an average
fiber diameter of 7.2 microns, these anodes are estimated to have a
surface area of 0.22 m.sup.2 or 18,200 m.sup.2/m.sup.3-brush volume
for the small brush (95% porosity), and 1.06 m.sup.2 or 7170
m.sup.2/m.sup.3-brush volume for the larger brush (98%
porosity).
[0171] Except as noted, brush anodes are treated using ammonia gas
as described in Cheng, S.; Logan, B. E. Ammonia treatment of carbon
cloth anodes to enhance power generation of microbial fuel cells.
Electrochem. Commun. 2007, 9, 492-496. Briefly described, ammonia
gas treatment of an anode is accomplished using a thermogravimetric
analyzer in this example. Any furnace that allows for temperature
control may be used for ammonia gas treatment of an anode. The
furnace temperature is 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.
[0172] In some tests plain Toray carbon paper anodes, untreated and
non-wet proofed, E-TEK, having a projected area of 23 cm.sup.2,
both sides, are used for comparisons to brush anodes.
[0173] Random bundles of ammonia-treated graphite fibers are also
used in some tests, consisting of one to four tows of fibers with
each cut to a fixed length of 10 cm. The mass of each tow was
.about.0.1 g, with a projected surface area calculated as 0.020
m.sup.2 per tow for 10 micron diameter fibers (Granoc-Nippon) and
0.035 m.sup.2 per tow for the 6 micron diameter (#292 Carbon Fiber
Tow, Fibre Glast, Ohio).
[0174] The cathodes are made by applying platinum (0.5 mg/cm.sup.2
Pt) and four diffusion layers on a 30 wt % wet-proofed carbon cloth
(type B-1B, E-TEK) as described in Cheng, S. et al., Electrochem.
Commun. 2006, 8, 489-494. In some experiments, the cathodes are
prepared using the same method and additionally containing 40%
cobalt tetramethylphenylporphyrin (CoTMPP, 1.2 mg/cm.sup.2) as the
catalyst instead of Pt.
[0175] MFC Reactors
[0176] Two types of single-chambered MFCs are used to examine power
production using brush electrodes in this example: cube-shaped MFCs
(C-MFCs) which are designed to maximize power production; and
bottle-type MFCs containing a single side port (B-MFC) that are
created for examining power production by pure and mixed cultures
in an easily produced and inexpensive system. C-MFCs are
constructed as described in Liu, H.; Logan, B. E. Electricity
generation using an air-cathode single chamber microbial fuel cell
in the presence and absence of a proton exchange membrane. Environ.
Sci. Technol. 2004, 38, 4040-4046 except the anode that normally
rested against the closed end of the reactor is replaced by a small
brush electrode positioned in a concentric manner the core of the
cylindrical anode chamber. The brush end is fixed in the chamber (4
cm long by 3 cm in diameter; liquid volume of 26 ml) so that the
end is 1 cm from the cathode (3.8 cm diameter, 7 cm.sup.2 total
exposed surface area). The metal end of the brush protruds through
a hole drilled in the reactor that is sealed with epoxy (Quick
Set.TM. Epoxy, LOCTITE). CoTMPP is used as the catalyst in all
C-MFC tests in this example.
[0177] B-MFCs are made from common laboratory media bottles (320 mL
capacity, Corning Inc. NY), and are autoclavable even when fully
assembled. A large brush electrode is suspended in the middle of
the bottle containing 300 mL of medium, with the top of the brush
.about.6 cm from the bottle lid. The wire from the bush is placed
through the lid hole and sealed with epoxy. In tests using carbon
paper anodes (2.5 cm by 4.5 cm, 22.5 cm.sup.2 total), the
electrodes are placed .about.6 cm from the bottle lid and connected
to a titanium (99.8% pure) wire through a hole in the lid that is
sealed with epoxy. The 4-cm long side tube is set 5 cm from the
reactor bottom, with a 3.8 cm-diameter cathode held in place at the
end by a clamp between the tube and a separate single tube 4 cm
long, providing a total projected cathode surface area of 4.9
cm.sup.2 (one side of the cathode). In tests using random bundles
of fibers as the anode, the fibers are held by a pinch clamp
connected to a wire that is passed through a hole in the lid and
sealed with epoxy.
[0178] Reactor Inoculation.
[0179] C-MFCs are inoculated using pre-acclimated bacteria from
another MFC (originally inoculated with primary clarifier overflow)
that had been running in fed batch mode for over 6 months. The
reactor is fed a medium containing 1 g/L of acetate in 50 mM
phosphate buffer solution (PBS; Na.sub.2HPO.sub.4, 4.09 g/L and
NaH.sub.2PO.sub.4.H.sub.2O, 2.93 g/L) or 200 mM PBS, NH.sub.4Cl
(0.31 g/L) and KCl (0.13 g/L), and metal salt (12.5 mL/L) and
vitamin (5 mL) solutions as described in Lovley, D. R.; Phillips,
E. J. P. Novel mode of microbial energy metabolism: organic carbon
oxidation coupled to dissimilatory reduction of iron or manganese.
Appl. Environ. Microbiol. 1988, 54, 1472-1480. Feed solutions are
replaced when the voltage dropped below 20 mV, forming one complete
cycle of operation. C-MFCs are operated in a temperature controlled
room at 30.degree. C.
[0180] B-MFCs are inoculated using fresh primary clarifier overflow
(unless stated otherwise) in a 1 g/L glucose medium prepared as
described above with 50 or 200 mM PBS. In one separate set of tests
the reactor is inoculated with the same pre-acclimated bacterial
solution used to inoculate the C-MFCs. All B-MFCs are operated on
laboratory bench tops at ambient temperatures of 23.+-.3.degree.
C.
[0181] Analyses.
[0182] The voltage (V) across ad external resistor (1000.OMEGA.
except as noted) in the MFC circuit is monitored at 30 min
intervals using a multimeter (Keithley Instruments, OH) connected
to a personal computer. Current (I), power (P=IV) and coulombic
efficiency (CE) are calculated as described in Kim, J. R. et al.,
Appl. Microbiol. Biotechnol. 2005, 68, 23-30, with the power
density normalized by the projected surface area of one side of the
cathode, and volumetric power density normalized by the volume of
the liquid media. Internal resistance, R.sub.int, is measured using
electrochemical impedance spectroscopy (EIS) with a potentiostat
(PC 4/750, Gamry Instrument Inc., PA), with the anode chamber
filled with PBS and substrate. Impedance measurements were
conducted at the open circuit voltage (OCV) over a frequency range
of 10.sup.5 to 0.005 Hz with sinusoidal perturbation of 10 mV
amplitude as described in Cheng, S. et al., Environ. Sci. Technol.
2006, 40, 2426-2432. Polarization curves are obtained by measuring
the stable voltage generated at various external resistances and
then used to evaluate the maximum power density as described in
Logan, B. E. et al., Environ. Sci. Technol. 2006, 40, 5181-5192.
The C-MFCs are run for at least two complete operation cycles at
each external resistance, where each cycle takes .about.2 days. The
B-MFCs require much longer cycle times (.about.21 days), and
therefore polarization data are taken after 15 min at each external
resistance at the beginning of a single operation cycle. The
internal resistance, defined as the sum of all ohmic resistances
including electrolyte and contact resistances, for both C- and
B-MFCs was determined using Nyquist plots of the impedance spectra
from the real impedance Z.sub.re where it intersects the X-axis
(imaginary impedance Z.sub.im=0) as described in He, Z. et al.,
Environ. Sci. Technol. 2006, 40, 5212-5217; Cai, M. et al.,
Environ. Sci. Technol. 2004, 38, 3195-3202; Raz, S. et al., Solid
State Ionics 2002, 149, 335-341; and Cooper, K. R. et al., J. Power
Sources 2006, 160, 1088-1095.
[0183] Power Production Using C-MFCs.
[0184] Voltage generation cycles of C-MFCs with brush anodes were
reproducible after 4 feeding cycles with fresh media, producing a
maximum voltage of 0.57 V and a CE-41% with the 1000.OMEGA.
resistor. FIG. 15 shows the initial four cycles of power production
in a C-MFC with a brush anode, including 50 mM PBS and a
1000.OMEGA. resistor; arrows in the figure indicate when the
reactor was fed fresh medium.
[0185] Based on polarization data, the maximum power produced in
this fuel cell was 2400 mW/m.sup.2 at a current density of 0.82
mA/cm.sup.2 (R.sub.ext=50.OMEGA.), or 73 W/m.sup.3 when power was
normalized by the reactor liquid volume, illustrated in FIG. 16A.
CEs ranged from 40-60% depending on the current density as shown in
FIG. 16.
[0186] The internal resistance was R.sub.int=8.OMEGA. for the brush
C-MFC (200 mM PBS), versus R.sub.int=31.OMEGA. for a carbon cloth
C-MFC (200 mM PBS, 4 cm electrode spacing) as shown in FIG. 17 and
Table 1. FIG. 17 shows Nyquist plots corresponding to the impedance
spectra of the C-MFCs measured between the cathode and anode
(two-electrode mode) in 200 mM PBS. The MFC was discharged to 0.57
V at 1000 f and the external circuit had been disconnected for 2
hours. The internal resistance is obtained as the value of the
x-intercept.
[0187] Power Production Using B-MFCs.
[0188] Brush electrodes used in B-MFCs produced up to 1430
mW/m.sup.2 (2.3 W/m.sup.3), compared to 600 mW/m.sup.2 (0.98
W/m.sup.3) using carbon paper electrodes in a 200 mM PBS solution
as shown in FIG. 18. Using a lower ionic strength solution reduced
power production to 570 mW/m.sup.2 (0.93 W/m.sup.3) with a brush
anode, and 300 mW/m.sup.2 (0.50 W/m.sup.3) with a carbon paper
anode. This effect of solution conductivity shows that power
increases with ionic strength (up to the tolerance of the bacteria)
due to a reduction in ohmic resistance. The internal resistance of
the brush B-MFC was 50.OMEGA., with values for the other reactor
conditions summarized in Table 1.
TABLE-US-00001 TABLE 1 Power production and internal resistances of
MFCs containing various components (200 mM PBS). Internal Maximum
Power Reactor type Anode Resistance (.OMEGA.) (mW/m.sup.2)
(W/m.sup.3) C-MFC Small brush 8 2400 73 C-MFC.sup.a Carbon cloth 31
1070 29 B-MFC Large brush 50 1200 2.0 B-MFC Large brush.sup.b 49
1430 2.3 B-MFC Large brush, 58 750 1.2 untreated B-MFC Carbon paper
65 600 0.98 .sup.a4 cm electrode spacing. .sup.busing an inoculum
from a previously acclimated MFC
[0189] To confirm that treatment of the brush electrodes with
ammonia gas was an effective method of reducing the acclimation
time and increasing power, additional tests were conducted using
untreated brush anodes. Power production reached a maximum of 750
mW/m.sup.2 with the untreated anode, which is 37% less than that
obtained with ammonia treatment as shown in FIG. 18. Peak power
production for the first cycle took 330 hours, compared to 136
hours with the treated electrodes, illustrating that the ammonia
treatment reduces acclimation time. Power production with the brush
electrodes was also substantially higher than that produced with an
untreated carbon paper electrode, which produced a maximum of 600
mW/m.sup.2.
[0190] Power Production Using Random Fibers.
[0191] The use of random or unstructured configurations of graphite
fibers is examined using B-MFC reactors in this example. The
maximum power production using a random or unstructured graphite
fiber anode configuration was 1100 mW/m.sup.2 using 0.11 g of 6
micron-diameter fibers, as shown in FIG. 19. Using 10-micron
diameter fiber, power ranged from 690 mW/m.sup.2 to 850 mW/m.sup.2
for mass loadings of 0.09 g to 0.35 g. Power production using the 6
micron diameter fibers ranged from 770 to 1100 mW/m.sup.2 as shown
in FIG. 19.
Example 2
Cathode Preparation
[0192] An ultrafiltration hydrophilic tubular membrane (a
polysulfone membrane on a composite polyester carrier) with an
inner diameter of 14.4 mm (B0125, X-FLOW) and wall thickness of 0.6
mm is used as the tube-cathode. The tubes are cut to a length of 3,
6 or 12 cm (equal to a surface area of 13.5, 27 and 54 cm.sup.2)
and then are coated with two coats of a commercially available
graphite paint, ELC E34 Semi-Colloidal, Superior Graphite Co.
Co-tetra-methyl phenylporphyrin (CoTMPP) is used as the cathode
catalyst unless indicated otherwise. A CoTMPP/carbon mixture (20%
CoTMPP) is prepared as described in Cheng, S. et al., Environ. Sci.
Technol. 2006, 40, 364-369, and mixed with a 5% Nafion solution to
form a paste using 7 microliters of Nafion per mg of CoTMPP/C
catalyst. The paste is then applied to the air-facing surfaces of
all tube-cathodes to achieve .about.0.5 mg/cm.sup.2 CoTMPP loading.
In some tests a commercial carbon paper cathode containing Pt, 0.35
mg/cm.sup.2 of Pt catalyst, water proofed paper, E-Tek; A.sub.cat32
7 cm.sup.2, is used with the catalyst facing the water solution. A
3-cm tube-cathode containing only graphite paint is prepared as a
non-catalyst control.
[0193] Anode Preparation
[0194] The anode electrode is either a piece of plain. Toray carbon
paper, without wet proofing; E-Tek; A.sub.an=7 cm.sup.2, or a plain
graphite fiber brush, 25 mm diameter.times.25 mm length; fiber
type: PANEX.RTM. 33 160K, ZOLTEK, with an estimated surface area of
2235 cm.sup.2 (95% porosity).
[0195] Tube-Cathode Reactors with Carbon Paper Anodes
[0196] Each reactor configuration is referred to in this example
using the notation of X-YZ-J, where: X=anode material (C=carbon
paper, B=graphite brush); Y=cathode material (C=carbon paper,
T.sub.n=number of 3-cm lengths of tube cathodes, where n=1 to 4);
Z=catalyst (Pt=platinum; Co=CoTMPP; C=carbon without catalysts);
and J=cathode configuration (I=inside reactor, O=outside
reactor).
[0197] Three single-chamber carbon paper anode (C) MFCs are
constructed with the tube-cathodes located inside (I) cylindrical
chambered reactors, 4 or 6 cm length.times.3 cm diameter, as noted
in Table 2, FIG. 24. Table 2 shows electrode types and surface
areas used in this example, as well as ratios of electrode area to
volume, volumes, internal resistances, maximum power density
normalized to anode surface area or total reactor volume, and CEs
for all carbon paper and brush anode MFC batch tests in this
example.
[0198] Two reactors are constructed with CoTMPP coated
tube-cathodes (TCo). One has a single 3-cm tube (C-T.sub.1Co-I;
4-cm chamber), for a total cathode surface area of A.sub.cat=13.5
cm.sup.2, and a surface area normalized to the reactor volume of
A.sub.cat,s=59 m.sup.2/m.sup.3, while the other has two 3-cm tubes
connected by a wire (C-T.sub.2Co-I; 6-cm chamber; A.sub.cat=27
cm.sup.2, A.sub.cat,s=84 m.sup.2/m.sup.3).
[0199] A third reactor system is constructed containing a single
3-cm tube-cathode without any catalyst, C-T.sub.1C-I; 4-cm chamber;
A.sub.cat=13.5 cm.sup.2, A.sub.cat,s=59 m.sup.2/m.sup.3.
[0200] Each cathode tube is inserted through the center of a single
2 cm-long slice of the chamber, with the carbon paper anode placed
at an opposite side of another 2 cm-long slice. The CoTMPP catalyst
layer is coated on the inside of these tubes (membrane side) and
faced air.
[0201] A single-chamber cube MFC of same type as described in Liu,
H. et al., Environ. Sci. Technol. 2004, 38, 4040-4046, is also
tested by using a carbon paper anode and a carbon paper cathode
with a Pt catalyst (C-CPt-I; A.sub.cat=7 cm.sup.2, A.sub.cat,s=25
m.sup.2/m.sup.3), with the electrodes placed at opposite sides of
the chamber (4 cm length.times.3 cm diameter).
[0202] Tube-Cathode Reactors with Brush Anodes
[0203] Two different brush anode (B) MFC configurations are tested
with tube cathodes containing a CoTMPP catalyst (TCo): a
cylindrical chambered MFC (6 cm long.times.3 cm diameter) with
tubes inside (I) the reactor (B-T.sub.2Co-I); and the same type of
reactor (4 cm.times.3 cm diameter), but with the tube-cathode
placed outside (O) the reactor (B-T.sub.2Co-O) as noted in Table
2.
[0204] For the inside tube reactor, a graphite brush anode is
placed vertically in a 2-cm long reactor slide, and two
wire-connected tube cathodes each 3-cm long are inserted through
adjacent 2 cm slices producing a 6-cm long reactor (B-T.sub.2Co-I;
A.sub.cat=27 cm.sup.2, A.sub.cat,s=93 m.sup.2/m.sup.3). The
catalyst is coated on the inside of the tube (membrane side) and
faced the air. The MFC with the cathode tube placed outside of the
cube reactor are constructed using a brush anode placed
horizontally in the center of a 4-cm long chamber, with a single
6-cm long (2.times.3 cm) cathode tube extending from one side of
the chamber (B-T.sub.2Co-O; A.sub.cat=27 cm.sup.2, A.sub.cat,s=75
m.sup.21 m.sup.3). In this case, the catalyst is coated on the
outside of the tube (supporting side of membrane) and faced the
air.
[0205] To further investigate the effect of cathode surface area,
additional 3-cm tube-cathodes are added to the inside of the MFCs,
with external wires connecting the tubes (B-T.sub.3Co- and
B-T.sub.4Co-I). For reactors with tubes outside the reactor, the
tube length is increased to 12 (4.times.3) cm (B-T.sub.4Co-O),
producing a cathode surface area of 54 cm.sup.2.
[0206] Start Up and Operation
[0207] All MFCs in this example are inoculated with a 50:50 mixture
of domestic wastewater (.about.300 mg-COD/L) and glucose (0.8 g/L)
in phosphate buffer solution (PBS, 50 mM; pH=7.0) in a nutrient
medium as described in Liu, H. et al., Environ. Sci. Technol. 2004,
38, 4040-4046.
[0208] After 2-3 repeated feeding cycles, only media (no
wastewater) is added. Reactors are considered to be acclimated if
the maximum voltage produced is repeatable for at least three batch
cycles. Following these tests, brush anode reactors are switched to
200 mM PBS as solution conductivity increases power generation. The
medium in the reactor is refilled when the voltage dropped below
.about.20 mV (resistances of 40 to 500.OMEGA.) or .about.40 mV
(1000 to 3000.OMEGA.).
[0209] Reactors with brush anodes and tube cathodes placed inside
or outside the reactor are also operated in continuous flow mode
with a hydraulic retention time (HRT) of 24 hours (total volume of
reactor. The influent is fed from the anode side by using a
micro-infusion pump (AVI micro 210A infusion pump, 3M), with the
flow discharged from the cathode side. These experiments are
performed at 30.degree. C.
[0210] Calculations and Measurements
[0211] The Voltage (V) output of all reactors are measured across a
fixed external resistance (1000.OMEGA. except as noted) using a
data acquisition system (2700, Keithly, USA). Electrode potentials
are measured using a multimeter (83 III, Fluke, UAS) and a
reference electrode (Ag/AgCl; RE-5B, Bioanalytical systems, USA).
Current (I=V/R), power (P=IV), and CE (based on the input glucose)
are calculated as described in Zuo, Y.; et al., Energy & Fuels.
2006, 20(4), 1716-1721. Power and current density are either
normalized to the projected area of carbon paper anodes (m.sup.2)
or the total reactor volume (m.sup.3).
[0212] To obtain the polarization curve and power density curve as
a function of current, external circuit resistances are varied from
40-3000.OMEGA.. For batch tests, one resistor is used for a full
cycle (at least 24 hours) for at least two separate cycles, while
for continuous flow tests at least 24 hours is used for each
resistor.
[0213] Internal resistance, R.sub.int, is measured by
electrochemical impedance spectroscopy (EIS) over a frequency range
of 10.sup.5 to 0.005 Hz with sinusoidal perturbation of 10 mV
amplitude using a potentiostat (PC 4/750 potentiostat, Gamry
Instrument Inc.) for carbon paper anode MFCs filled with a nutrient
media containing 50 mM PBS and brush anode reactors using 200 mM
PBS. The anode is used as the working electrode and the cathode as
the counter and reference electrode as described in Cheng, S. et
al., Environ. Sci. Technol. 2006. 40, 2426-2432.
[0214] The maximum rate of oxygen transfer through a tube-cathode
is determined by measuring oxygen accumulation in an uninoculated
carbon paper anode MFC reactor containing a clean 3-cm tubular
membrane (without any graphite/catalysts) and de-oxygenated
deionized water. The effective oxygen mass transfer coefficient of
k is determined as described in Cheng, S. et al., Electrochem
Commun. 2006, 8, 489-494, with a dissolved oxygen probe (Foxy-21 G,
Ocean Optics Inc., Fl) placed at the centre of the stirred reactor.
The resistance of proton transport through the tubular membrane
cathode is determined by measuring the internal resistance increase
when adding this membrane material between two carbon electrodes in
a two-chamber cube reactor as described in Kim, J. et al., Environ.
Sci. Technol. 2007. 41(3), 1004-1009. The membrane tube is sliced
open, cut into a circular shape to produce a flat surface of 7
cm.sup.2, and then placed in the middle of the reactor with carbon
electrodes each spaced 2 cm from the membrane. The internal
resistances of the reactor with the membrane (R.sub.int,m+) and
without any membrane (R.sub.int,m-) are measured by EIS using a
potentiostat. The proton transport resistivity (.OMEGA.cm.sup.2) of
the tubular membrane is calculated as
(R.sub.int,m+-R.sub.int,m-).times.A.sub.mem.
[0215] COD concentrations of the reactor effluent are measured
using standard methods such as described in American Public Health
Association; American Water works association; Water Pollution
Control Federation. Standard Methods for the Examination of Water
and Wastewater, 19th ed.; Washington D.C. 1995.
[0216] Power Production from Tube Reactors with Carbon Paper
Anodes
[0217] Repeatable cycles of power production are rapidly generated
after acclimation of all four carbon paper anode MFC reactors. FIG.
20A shows power density, open symbols, voltage, filled symbols as a
function of current density normalized to total reactor volume,
obtained by varying the external circuit resistance
(40-3000.OMEGA.) for carbon paper anode MFCs. Error bars are
.+-.S.D. based on averages measured during stable power output in
two or more separate batch experiments.
[0218] Power density curves and polarization curves obtained by
varying the external circuit resistances from 40-3000.OMEGA. show
that the tube-cathode MFC with two CoTMPP coated tubes
(C-T.sub.2Co-I; A.sub.cat=27 cm.sup.2) produced power only somewhat
less than that achieved with a carbon paper cathode with Pt
catalyst (C-CPt-I; A.sub.cat=7 cm.sup.2), with a maximum power
density of 8.8.+-.1.0 W/m.sup.3 (403.+-.33 mW/m.sup.2, anode
surface area) for the tube-cathode system and 9.9.+-.0.1 W/m.sup.3
(394.+-.3 mW/m.sup.2) for the carbon paper cathode, both at
R.sub.ext=250.OMEGA.; shown in FIG. 20A. Decreasing the
tube-cathode area by 50% (C-T.sub.1Co-I, cm.sup.2) slightly
affected the volumetric power density (9.3.+-.0.3 W/m.sup.3;
R.sub.ext=250.OMEGA.) due to the reduced volume without the
cathode, but reduced power by 24% on the basis of the anode surface
area (306.+-.8 mW/m.sup.2). In the absence of a catalyst, the tube
reactor (C-T.sub.1C-I, A.sub.cat=13.5 cm.sup.2) produced much less
power, or 3.1.+-.0.1 W/m.sup.3 (R.sub.ext=250.OMEGA.), shown in
FIG. 20A. The internal resistances of these four MFCs ranged from
84 to 131.OMEGA. (Table 2).
[0219] FIG. 20B shows electrode potentials, cathode open symbols,
anode filled symbols, as a function of current density normalized
to total reactor volume, obtained by varying the external circuit
resistance (40-3000.OMEGA.) for carbon paper anode MFCs. Error bars
are .+-.S.D. based on averages measured during stable power output
in two or more separate batch experiments. FIG. 20B shows that
these carbon paper anode MFCs each had similar anode potentials at
the same current. The differences in power productions from these
four MFC reactors are a result of the differences in cathode
potentials. Tube-cathode potentials are improved by adding CoTMPP
as the catalyst and/or increasing the cathode surface area. With
13.5 or 27 cm.sup.2 of surface area, the CoTMPP coated
tube-cathodes (C-T.sub.1Co-I and C-T.sub.2Co-I) achieved almost
same potentials as the carbon paper Pt cathode (C-CPt-I) over the
current density range of 0-60 A/m.sup.3
[0220] Power Production from Tube Reactors with Brush Anodes.
[0221] All of the tube-reactors with brush anodes used in this
example generated repeatable power cycles after .about.14 batch
cycles (50 mM PBS). FIG. 21A shows power density (open symbols),
voltage (filled symbols) as a function of current density based on
reactor volume obtained by varying the external circuit resistance
(40-3000.OMEGA.) for these brush anode MFCs. Error bars are
.+-.S.D. based on averages measured during stable power output in
two or more separate batch experiments separate batch
experiments.
[0222] After the buffer concentration is increased to 200 mM, a
maximum volumetric power density of 17.7.+-.0.2 W/m.sup.3
(R.sub.ext=250.OMEGA.) is produced with two 3-cm tube cathodes
inside the reactor (B-T.sub.2Co-I, A.sub.cat=27 cm.sup.2) as shown
in FIG. 21A. The 200% increased power produced with the brush
versus the carbon paper anode in the same type of tube-cathode
reactor (C-T.sub.2Co-I, 8.8.+-.1.0 W/m.sup.3) is consistent with an
overall reduction in internal resistance (from 89 to 66.OMEGA.) and
a significant increase of the anode area (from 7 to 2235 cm.sup.2).
The power produced with brush anode and tube-cathodes inside the
reactor is also double the maximum power of 8.2.+-.0.2 W/m.sup.3
(R.sub.ext=250.OMEGA.) from the brush reactor with a single 6-cm
tube placed outside (B-T.sub.2Co-O, A.sub.cat=27 cm.sup.2) shown in
FIG. 21A.
[0223] FIG. 21B shows electrode potentials (cathode open symbols,
anode filled symbols) as a function of current density based on
reactor volume obtained by varying the external circuit resistance
(40-3000.OMEGA.) for brush anode MFCs. Error bars are .+-.S.D.
based on averages measured during stable power output in two or
more separate batch experiments separate batch experiments. The
increase in power output with the tubes inside the reactor is
caused by the higher cathode potentials as the brush anode
potentials remained unchanged over a current range of 0-58
A/m.sup.3, see FIG. 21B. The OCP of the cathode when inside the
reactor (250.+-.8 mV, vs Ag/AgCl) is 112 mV higher than when it is
placed outside the reactor (138.+-.16 mV). As the current
increased, the potential difference further increased to 240 mV at
58 A/m.sup.3 as shown in FIG. 21B.
[0224] Coulombic Efficiencies Using Tube-Cathodes.
[0225] The CEs of all reactors are a function of current densities
(Table 1; additional information in supporting information). With
carbon paper anodes, the tube-cathodes with a CoTMPP catalyst
achieved CEs as high as 40%, while carbon paper cathodes with Pt
(C-CPt-I) had CEs of 7-19%. Without a catalyst (C-T.sub.1C-I), the
CEs for the tube-cathode reactor ranged from 18 to 22%. By using a
graphite brush anode, and increasing the solution ionic strength
using 200 mM PBS further increased the CE to 52-58% when the tube
is placed outside the reactor (B-T.sub.2Co-0), and 70-74% for the
tube inside one (B-T.sub.2Co-I).
[0226] The higher CEs obtained with tube-cathode reactors are
thought to be due to lower O.sub.2 diffusion rates through the
tubular ultrafiltration membrane than through the carbon paper
cathode. For a clean tubular membrane, we measured an O.sub.2 mass
transfer coefficient k=7.8.times.10.sup.-5 cm/s, which could result
in as much oxygen transfer as 0.03 mgO.sub.2/h into an MFC system
with a tube-cathode surface area of 13.5 cm.sup.2 (C-T.sub.1Co-I
and C-T.sub.1C-I), or 0.06 mgO.sub.2/h for a surface area of 27
cm.sup.2 (C-T.sub.2Co-I, B-T.sub.2Co-I and B-T.sub.2Co-O). In
contrast, a carbon paper cathode of 7 cm.sup.2 (C-CPt-I) produced
an oxygen rate of 0.187 mg/h, Liu, H. et al., Environ. Sci.
Technol. 2004, 38, 4040-4046. It therefore seems likely that the
higher CEs of the tube cathode system are due to the reduction in
substrate lost to aerobic oxidation supported by oxygen diffusion
through the cathode.
[0227] Effect of Tube-Cathode Surface Area.
[0228] The effect of tube-cathode surface area is investigated for
brush anode reactors with the tube-cathodes placed inside or
outside the reactor. FIGS. 22A and 22B show power (A) and
volumetric power density (B) as a function of the cathode surface
area of tube-cathode MFCs with brush anodes. Error bars in these
figures are .+-.S.D. based on averages measured during stable power
output in two or more separate batch experiments. The cathode
surface areas for both configurations are increased from 27
(T.sub.2) to 40.5 (T.sub.3) or 54 cm.sup.2 (T.sub.4), by adding
more 3-cm tubes inside the reactor (B-T.sub.3Co-I and
B-T.sub.4Co-I) or extending the length of the outside tube up to 12
cm (B-T.sub.4Co-O). For the tubes inside the reactor, the maximum
power output increased with cathode surface area, producing 0.51 mW
(B-T.sub.2Co-I), 0.66 mW (B-T.sub.3Co-I) and 0.83 mW
(B-T.sub.4Co-I) (FIG. 22A). Since the reactor volume also increased
by 8 ml when adding each 3-cm tube, however, the volumetric power
densities produced by these different reactors with the tubes
inside the reactor are similar when normalized to volume, producing
for all cases a maximum of .about.18 W/m.sup.3 (FIG. 22B). When the
tube is placed outside the reactor, the maximum power output is not
improved with increased tube length (FIG. 22A). Although both
reactors produced .about.0.3 mW, the longer tube-cathode added 10
ml more volume than the shorter one, resulting in a decrease in
volumetric power from 8.2 (B-T.sub.2Co-O) to 6.5 W/m.sup.3
(B-T.sub.4Co-O) (FIG. 22B).
[0229] Continuous Flow Performance of Tube-Cathode Reactors.
[0230] Two brush anode MFCs with the tube cathodes inside or
outside the reactor are operated in continuous flow mode. FIGS. 23A
and 23B show voltage as a function of time at a fixed resistance of
1000.OMEGA. (except as noted) (A) and volumetric power density as a
function of current normalized to volume (B) obtained by varying
the external circuit resistance (40-3000.OMEGA.) for brush anode
MFCs operated in continuous or batch mode. Vertical lines indicate
where the external resistance was changed for polarization curve
measurements. Arrows indicate the replacement of the tube cathode
outside the reactor. With the tubes inside the reactor
(B-T.sub.2Co-I; A.sub.cat=27 cm.sup.2), the voltage output (520 mV
at 1000.OMEGA.) is immediately produced and is stable for more than
10 HRTs (FIG. 23A). Power density curves showed that the
performance is identical to that produced in fed-batch tests,
resulting in a maximum power density of .about.18 W/m.sup.3 (FIG.
23B).
[0231] Power density curves measured for the MFC with the tube
outside the reactor are also similar for continuous and fed batch
operation (FIG. 23B). However, the voltage produced by this reactor
(B-T.sub.2Co-O, A.sub.cat=27 cm.sup.2) is unstable over time, and
decreased from 500 to 380 mV (1000.OMEGA.) (FIG. 23A).
[0232] The effluents from both reactors operated in continuous flow
mode are analyzed with a fixed external resistor of 1000.OMEGA..
The reactor with the tube outside the MFC produced a COD removal of
53.+-.5%, compared to 37.+-.5% when the tubes are inside the
reactor.
[0233] Internal Resistance Contributed by Tube-Cathodes.
[0234] The internal resistance with a flat piece of tubular
membrane material (7 cm.sup.2) placed between two carbon electrodes
in a two-chamber cube reactor, is measured as
R.sub.int,m+=247.+-.6.OMEGA.. When the membrane is removed, the
internal resistance is R.sub.int,m-=84.+-.1.OMEGA.. These
resistances indicate that the proton transport resistivity of the
membrane is 1141 .OMEGA.cm.sup.2, resulting in internal resistances
of 84.OMEGA. or 42.OMEGA. for the 13.5 cm.sup.2 or 27 cm.sup.2
tubular membrane cathodes. This indicates that the membrane
accounted for up to 64% of the total internal resistances of the
tube-cathode reactors.
[0235] 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. Provisional Patent
Application 60/796,761, filed May 2, 2006, is incorporated herein
by reference in its entirety.
[0236] 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.
* * * * *