U.S. patent application number 15/778864 was filed with the patent office on 2019-09-05 for metal hollow fiber electrode.
This patent application is currently assigned to Universiteit Twente. The applicant listed for this patent is Universiteit Twente. Invention is credited to Nieck Edwin BENES, Patrick DE WIT, Recep KAS, Guido MUL.
Application Number | 20190271089 15/778864 |
Document ID | / |
Family ID | 57681708 |
Filed Date | 2019-09-05 |
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United States Patent
Application |
20190271089 |
Kind Code |
A1 |
KAS; Recep ; et al. |
September 5, 2019 |
METAL HOLLOW FIBER ELECTRODE
Abstract
The invention is directed to a metal hollow fiber electrode, to
a method of electrolyzing carbon dioxide in an aqueous
electrochemical cell, to a method of converting carbon dioxide, to
a method of preparing a metal hollow fiber, to a use of a metal
hollow fiber electrode. The metal hollow fiber electrode comprises
aggregated copper particles forming an interconnected
three-dimensional porous structure, wherein said metal comprises
copper.
Inventors: |
KAS; Recep; (Enschede,
NL) ; DE WIT; Patrick; (Enschede, NL) ; BENES;
Nieck Edwin; (Enschede, NL) ; MUL; Guido;
(Enschede, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Universiteit Twente |
Enschede |
|
NL |
|
|
Assignee: |
Universiteit Twente
Enschede
NL
|
Family ID: |
57681708 |
Appl. No.: |
15/778864 |
Filed: |
November 24, 2016 |
PCT Filed: |
November 24, 2016 |
PCT NO: |
PCT/NL2016/050826 |
371 Date: |
May 24, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62259089 |
Nov 24, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/035 20130101;
C25B 11/0415 20130101; C25B 3/04 20130101; C25B 1/00 20130101; C25B
11/02 20130101; C25B 11/0447 20130101 |
International
Class: |
C25B 11/02 20060101
C25B011/02; C25B 1/00 20060101 C25B001/00; C25B 11/04 20060101
C25B011/04; C25B 11/03 20060101 C25B011/03; C25B 3/04 20060101
C25B003/04 |
Claims
1. A metal hollow fiber electrode, comprising aggregated copper
particles forming an interconnected three-dimensional porous
structure, wherein said metal comprises copper.
2. The metal hollow fiber electrode according to claim 1, wherein
said metal is copper.
3. The metal hollow fiber electrode according to claim 1, wherein
said fibers have an inner diameter of 0.1-10 mm.
4-5. (canceled)
6. The metal hollow fiber electrode according to claim 1, wherein
said fibers have an outer diameter of 0.1-10 mm.
7-8. (canceled)
9. The metal hollow fiber electrode according to claim 1, wherein
said fibers comprises or is composed of sintered copper
particles.
10. The metal hollow fiber electrode according to claim 1, wherein
said copper particles have an average particle diameter of 0.1-10
.mu.m.
11-12. (canceled)
13. The metal hollow fiber electrode according to claim 1, wherein
a porous outer layer of the hollow fiber is more dense than a
porous inner layer of the hollow fiber.
14. The metal hollow fiber electrode according to claim 1, wherein
said outer layer has a thickness in the range of 5-20 .mu.m.
15-16. (canceled)
17. A method of electrolyzing carbon dioxide in an aqueous
electrochemical cell comprising an anode and a cathode, wherein the
cathode comprises one or more metal hollow fiber electrodes
according to claim 1, said method comprising applying a potential
between said anode and cathode, and purging CO.sub.2 or a gas
mixture comprising CO.sub.2 through the wall of the metal hollow
fiber electrode.
18. The method according to claim 17, wherein said method is
performed in an aqueous environment.
19. The method according to claim 17, wherein said method is
performed at a temperature in the range of 5-80.degree. C.
20-21. (canceled)
22. A method of converting carbon dioxide into one or more selected
from the group consisting of carbon monoxide, formic acid, a
formate, methanol, acetaldehyde, methane, ethylene and ethane,
comprising electrolyzing CO.sub.2 by a method according to claim
17.
23. The method according to claim 22, wherein carbon dioxide is
converted into carbon monoxide.
24. A method of preparing a metal hollow fiber electrode according
to claim 1, comprising spinning a mixture comprising copper
particles, polymer and solvent together with a bore liquid to
obtain hollow fibers; subjecting the hollow fibers to a thermal
treatment such that copper particles are sintered together, thereby
yielding hollow copper oxide fibers; hydrogenating the hollow
copper oxide fibers.
25. The method according to claim 24, wherein said thermal
treatment comprises subjecting the hollow fibers to a temperature
of 500-800.degree. C.
26. (canceled)
27. The method according to claim 24, wherein the hollow fibers are
subjected to said thermal treatment for a period of 1-6 hours.
28. (canceled)
29. The method according to claim 24, wherein said hydrogenation
comprises subjecting the hollow copper oxide fibers to a
temperature of 200-400.degree. C.
30. (canceled)
31. The method according to claim 24, wherein the hollow copper
oxide fibers are hydrogenated for a period of 30-120 minutes.
32. (canceled)
33. The method according to claim 24, wherein the hollow copper
oxide fibers are hydrogenated in a flow of hydrogen in the
concentration range of 0-100 vol. %.
34. The method according to claim 24, wherein the hollow copper
oxide fibers are hydrogenated in a flow of hydrogen in a
concentration of 5 vol. % in a balance gas.
35. (canceled)
Description
[0001] This application is a United States National Phase under 35
U.S.C. .sctn. 371 of International Application No.
PCT/NL2016/050826, filed on Nov. 24, 2016, which claims the benefit
of, and priority to, U.S. Provisional Application No. 62/259,089,
filed on Nov. 24, 2015, both of which are hereby incorporated by
reference in their entirety for all purposes.
[0002] The invention is directed to a metal hollow fiber electrode,
to a method of electrolyzing carbon dioxide in an aqueous
electrochemical cell, to a method of converting carbon dioxide, to
a method of preparing a metal hollow fiber, to a use of a metal
hollow fiber electrode.
[0003] The accumulation of carbon dioxide (CO.sub.2) in the
atmosphere has already large impact on local climate conditions,
and starts to affect human and natural life. Immediate measures
must be taken to minimize carbon emissions. A promising way is to
convert CO.sub.2 to useful chemicals by using electricity generated
by renewable energy sources. Nevertheless, the development of an
efficient and stable electrocatalyst that can reduce CO.sub.2 at
high current densities, remains a challenge. In recent years,
researchers achieved to produce CO at low potentials in aqueous
solutions albeit with limited current densities or using noble
metals [Lu et al, Nat. Commun. 2014, 5, 3242; Zhu et al, J. Am.
Chem. Soc. 2014, 136, 16132-16135; Chen et al, J. Am. Chem. Soc.
2012, 134, 19969-19972]. Reasonable current densities at low
overpotentials towards CO were achieved by using ionic liquids
[Medina-Ramos et al., J. Am. Chem. Soc. 2014, 136, 8361-8367; Rosen
et al., Science 2011, 334, 643-644]. However, considering ionic
liquids, several issues remain challenging such as cost, recycling,
stability and electrolyzer performance in practical applications.
Aqueous conversion of CO.sub.2 is still more attractive if high
selectivity towards CO.sub.2 over water splitting can be achieved
at low overpotentials and high current densities.
[0004] Copper electrodes are well known for producing hydrocarbons
from CO.sub.2 with variable onset potentials (.about.0.5-0.7 V)
depending on the preparation method. Generally, high potentials
(.about.0.8-1 V) are necessary to obtain reasonable faradaic
efficiency (FE). Although less expensive and much more abundant
than other CO evolving catalysts such as e.g. silver and gold, poor
activity, selectivity and stability towards CO and formic acid have
been reported for polycrystalline copper. Recently, Li et al.
reported production of CO and formic acid with reasonable faradaic
efficiency at low overpotentials on copper nanoparticles, when
formed by electrochemical reduction of cuprous oxides [Li et al, J.
Am. Chem. Soc. 2012, 134, 7231-7234]. At a potential of -0.5 V vs.
RHE, a copper surface covered with nanoparticles delivered a
CO.sub.2 reduction current density of 2.1 mA/cm.sup.2, with a
faradaic efficiency of 35% and 32% for CO and formic acid
respectively. Even though such a selectivity can be considered as
low, this was the first publication showing CO and formic acid
formation is feasible over copper electrodes. Additionally,
production of energy dense products such as ethylene, ethanol and
methane might require selective conversion to CO first, since CO is
a common intermediate for nearly all hydrocarbons and
oxygenates.
[0005] Inorganic hollow fibers are of potential significance for
solid oxide fuel cells due to their high surface area to volume
ratio, higher power outputs and lower fabrication costs, but
utilization in room temperature solution based electrochemistry is
quite rare. Several examples exist using nickel and carbon hollow
fibers with dual functionality, where they served as both membrane
for effluent purification and as cathode for proton and oxygen
reductions, respectively. Recently, microtubular gas diffusion
electrodes made of carbon nanotubes were proposed for tubular
electrochemical reactor design [Gendel et al., Electrochem. Commun.
2014, 46, 44-47].
[0006] An objective of the invention is to overcome one or more
disadvantages seen in the prior art.
[0007] A further objective of the invention is to provide an
electrode that allows low pressure and room temperature
electrolysis of CO.sub.2.
[0008] It was found that one or more of these objectives can be
met, at least in part, by a metal hollow fiber electrode with a
specific structure.
[0009] Accordingly, in a first aspect, the invention is directed to
a metal hollow fiber electrode, comprising aggregated cooper
particles forming an interconnected three-dimensional porous
structure, wherein said metal comprises copper.
[0010] It was surprisingly found that metal hollow fiber electrodes
can be a potential candidate for low pressure and room temperature
electrolysis of CO.sub.2, due to their excellent mass transport
capabilities when used as gas diffuser and cathode. Not only the
hydrogen evolution reaction is suppressed on these electrodes to
levels not reached previously on copper surfaces, but also the
overall CO.sub.2 reduction current density is unprecedentedly high
at low potentials.
[0011] In accordance with the invention, the metal comprises copper
and other metals may optionally be present. More preferably, the
metal is copper.
[0012] The metal hollow fibers can typically have an inner diameter
of 0.1-10 mm, such as 0.5-5 mm, or 0.7-3 mm. The outer diameter of
the metal hollow fibers can be 0.1-10 mm, such as 0.5-5 mm, or
0.7-3 mm.
[0013] The fibers preferably comprise, or are composed of, sintered
copper particles. Solid state sintering is the process of taking
metal in the form of a powder and placing it into a mold or die.
Once compacted into the mold the material is placed under a high
heat for a long period of time. Under heat, bonding takes place
between the porous aggregate particles and once cooled the powder
has bonded to form a solid piece.
[0014] The copper particles in the metal hollow fiber electrode
preferably have an average particle diameter of 0.1-10 .mu.m, such
as 0.3-5 .mu.m, or 0.5-3 .mu.m.
[0015] In a preferred embodiment, a porous outer layer of the
hollow fiber is more dense than a porous inner layer of the hollow
fiber, said outer layer preferably having a thickness in the range
of 5-20 .mu.m, such as 12-18 .mu.m, or 10-15 .mu.m.
[0016] The preparation of the metal hollow fibers i.e. nickel and
stainless steel has been described in the literature previously
[Meng et al, J. Alloy Compd. 2009, 470, 461-464; Luiten-Olieman et
al, Scripta Mater. 2011, 65, 25-28]. The preparation of Cu hollow
fiber, on the other hand, has not been reported to the best of our
knowledge. The inventors adapted the method and prepared Cu hollow
fibers by spinning a mixture containing copper particles, polymer
and solvent. The mixture is suitably pressed through a spinneret
into a coagulation bath. In this bath, non-solvent induced phase
separation arrests the copper particles in the polymer matrix. By
adding a bore-liquid during spinning, a hollow fiber is obtained.
After thermal treatment, the polymer is decomposed and the copper
particles are sintered together, resulting in hollow, porous copper
oxide fibers. Hydrogenation of these precursor fibers at elevated
temperatures was applied to obtain metallic copper fibers. Typical
scanning electron microscope (SEM) images of the Cu hollow fibers
are shown in FIG. 1. The low and higher magnification images of the
external surface of the fibers show that the fiber is composed of
aggregated copper particles forming an interconnected 3-D porous
structure (FIG. 1a and 1b). The cross-sectional images of the
deliberately broken fibers exhibit fingerlike voids perpendicular
to the surface which are terminated by a 10-15 .mu.m thick
sponge-like porous outer layer (FIGS. 1c and 1d). Cu hollow fibers
can have outer and inner diameters ranging from 1.55.+-.0.1 mm to
1.3.+-.0.05 mm respectively (FIG. 1e). CO.sub.2 was pushed from the
inside out of the fiber, creating an overpressure around
1.70.+-.0.1 bars due to the resistance of the porous structure. Gas
bubbles emerging out of the fiber can be clearly seen in FIG. 1f.
The pressure is considered to spread out the finger-like holes
without resistance and to drop across the porous outer layer to
1.05 bar. When a potential is applied, electrochemical reduction
likely takes place both at the interface of CO.sub.2 (g) and water
(1) in contact with copper (s), as well as near the electrode
surface with dissolved CO.sub.2.
[0017] For monitoring the electrochemical activity of the fibers,
linear sweep voltammetry experiments were performed in CO.sub.2
saturated electrolyte, while Ar or CO.sub.2 were purged through the
fibers (FIG. 2). The current densities recorded during Ar purge are
mostly due to hydrogen evolution, which has an onset potential of
around -0.25 V vs. RHE. Bubbling CO.sub.2 through the fiber leads
to about two-fold increase in cathodic current densities at
potentials between -0.2 and -0.4 V vs. RHE. On the contrary, on
both smooth or rough copper surfaces lower current densities are
under CO.sub.2 atmosphere as compared to Ar atmosphere. This was
attributed to co-adsorption of CO, weakening the binding energy of
hydrogen to the electrode surface and retarding the hydrogen
evolution reaction. In other words, most of the cathodic current
was considered to be a result of hydrogen production at low
potentials, even in the presence of CO.sub.2 or CO. So the high
cathodic currents achieved in the presence of CO.sub.2 on Cu hollow
fiber is already an indication of distinctive activity towards
CO.sub.2 reduction.
[0018] To confirm the catalytic activity, the faradaic efficiency
of the major products was measured by varying the applied potential
between -0.15 V and -0.55 V vs. RHE (FIG. 2b). The onset of CO
formation is located at -0.15 V vs. RHE, implying an overpotential
of just .about.40 mV above the equilibrium potential (-0.11 V vs.
RHE). The total faradaic efficiency of the CO.sub.2 reduction
products adds up to .about.85% at potentials between -0.3 V and
-0.5 V vs. RHE. Specifically, a peak faradaic efficiency of
.about.72% was obtained for CO at a potential of -0.4 V vs. RHE,
whereas the maximum faradaic efficiency for CO on polycrystalline
copper and copper nanoparticles is around 20% (-0.8 V vs. RHE) and
45% (J.sub.CO.about.300 .mu.A/cm.sup.2), respectively. The decrease
in faradaic efficiency of CO at higher potentials (<-0.5 V)
implies CO formation is now most likely limited by desorption.
Additionally, ethylene was detected at these potentials (see table
1), which is formed by coupling of two CO molecules.
TABLE-US-00001 TABLE 1 Chemical analysis of the precursor copper
powder as provided by Skyspring Nanomaterials Copper Powder Purity
99% Average Particle Size 1-2 .mu.m Contents C 5000 O 10000 Al 1000
Ni 500 Fe 500 Pb <100 Cd <100 Hg <10
[0019] On that account, the formation of hydrocarbons still
requires relatively large overpotentials on Cu hollow fiber, and
direct CO.sub.2 reduction to energy dense products remains a
challenge. Formation of CO at very low potentials implies a better
stabilization of the key CO.sub.2-- intermediate which is formed by
the first electron transfer to adsorbed CO.sub.2. The remarkable
activity of the rough copper electrodes prepared by oxidation of
copper and subsequent reduction, namely oxide-derived copper, can
be attributed to the metastable sites exist in grain boundaries.
However, reasonable activities also achieved towards formic acid
and CO with faradaic efficiency up to 45% at a potential of -0.5 V
vs. RHE on copper nanofoams prepared by electrodeposition.
Interestingly, the production of formic acid was controlled by
changing thickness of the electrodeposited layer similar to the
oxide-derived copper where CO.sub.2 reduction activity was a
function of the parent oxide layer thickness. In addition, Reske et
al. showed the reduction of CO.sub.2 can be significantly enhanced
by decreasing the size of the copper nanoparticles which is
correlated to the number of uncoordinated cites [Reske et al., J.
Am. Chem. Soc. 2014, 136, 6978-69861]. By considering all these
studies, the increase in the CO.sub.2 reduction activity on porous,
foamy and nanoparticulate copper structures might be associated
with the abundance of the low coordinated sites such as kinks,
steps etc. which influence the binding energy of the key
intermediate CO.sub.2--.
[0020] On the other hand, contrary to polycrystalline copper and
copper nanoparticles, formation of formic acid is relatively
suppressed. Mechanistic information can be deduced for the two
electron reduction of CO.sub.2 to CO by using a Tafel plot shown in
FIG. 2c. The first step involves an electron transfer to adsorbed
CO.sub.2 which is coupled to a proton transfer. In following steps,
COOH intermediate accepts an electron and proton to form CO and
water. A slope around 116 mVdec.sup.-1 was recorded for copper
different electrodes that suggests a mechanism in which initial
electron transfer to CO.sub.2 is rate determining. The lower slope
of 93 mVdec.sup.-1 associated with the lower potential region is
most likely due to non-uniform potential or current distribution
within the solid porous matrix of the hollow fiber. This might be
either caused by the ohmic drop within solid porous matrix or
inhomogeneous distribution of the reactants to the electrode
surface. More importantly, at higher potentials, an apparent change
in Tafel slope is observed, which suggests a change in rate
determining step or mechanism. By also considering the fact that
higher pressures can induce CO production on copper nanoparticles
and indium electrodes, the rate of CO production compared to formic
acid might be more sensitive to CO.sub.2 concentration as has been
suggested for CO.sub.2 reduction in media of low proton
availability. Likewise, studies on various metal electrodes in
water showed a higher order dependence on CO.sub.2 pressure in a
second Tafel region for CO. Still, CO production as a function of
CO.sub.2 concentration in the solution requires further
investigation.
[0021] To test the stability of the Cu hollow fibers, 24 hours of
continuous electrolysis was performed at an applied potential of
-0.4 V vs. RHE (FIG. 2d and FIG. 7). After a.about.10% drop in
activity in the first 7 hours, noticeable from the slight curvature
of the plot in FIG. 2d, stable performance was achieved in the next
17 hours of experiment. SEM images of the electrode after
electrolysis showed no apparent difference in the morphology of the
Cu hollow fiber (FIG. 8). The activity of polycrystalline Cu is
well known to degrade very quickly within an hour, unless very high
purity electrolytes and electrodes are employed. Cu nanoparticles
on the other hand exhibited quite stable performance when thick
oxide layers are used as copper precursor. However, it is important
to note that these studies usually employ ultra-high purity copper
plates (99.9999%) whereas the purity of the precursor copper
powders used in this study is relatively low (99%), significantly
reducing the price for commercial application. X-ray photoelectron
spectroscopy (XPS) analysis of the precursor copper powder and Cu
hollow fibers before the and after electrolysis experiments
indicated that there is no detectable amount (>% 0.1) of
transition metal impurity at the surface of the catalyst (FIG. 12).
The major impurity within the hollow fiber is carbon which is also
present in the precursor copper powder. XPS spectra of the fibers
indicated the surface is composed of mainly with Cu.sup.0 and
Cu.sub.2O, the latter associated with the exposure of the sample to
air (FIG. 13). More importantly, the absence of shift in binding
energy of Cu2p peaks before and after preparation and electrolysis
suggests the absence of any alloy or carbide formation upon
annealing or electrolysis.
[0022] FIGS. 3a and 3b show the effect of the CO.sub.2 flow rate on
overall current density and faradaic efficiency of CO at an applied
voltage of -0.4 V vs. RHE, respectively. The current density
undoubtedly is proportional to the CO.sub.2 flow rate above -0.35 V
vs. RHE, until a certain flow rate was reached. The change in
faradaic efficiency of CO is consistent with the increase in
current density. A maximum faradaic efficiency of 75% was recorded
for CO at a potential of -0.4 V vs. RHE at optimized flow rate
which is almost twice of what has been recently reported for copper
nanoparticles at the same potential. The experiments as a function
of flow rate indicate that the faradaic efficiency of CO strictly
depends on supply of CO.sub.2 to the electrode surface. Therefore,
the high faradaic efficiencies observed in this study is a result
of improved mass transfer of CO.sub.2 integrated with the defect
rich, active and porous copper electrode. The steady behavior above
the flow rate of 30 ml min.sup.-1 implies all the active sites are
involved in converting CO.sub.2 to CO, and the catalyst has reached
its intrinsic limit. Previously, the CO.sub.2 reduction rate in
aqueous conditions was shown to be proportional to the CO.sub.2
pressure on different metal electrodes, suggesting high degrees of
coverage were not achieved even at pressures as high as 25 atm. On
the other hand, the intermediate CO, was considered to have a high
degree of coverage during electrochemical CO.sub.2 reduction on
copper electrodes, supported by spectroscopic studies. By
considering these facts, the high current densities achieved on Cu
hollow fibers is attributed to better removal of CO from the
surface, induced by a very high local concentration of CO.sub.2
near the electrode. Besides an improved performance, this also
enables the evaluation of the intrinsic activity of the
electrocatalyst especially when a competing reaction, i.e. hydrogen
evolution, simultaneously takes place.
[0023] An overview of the performance of different catalysts' as
compared to our Cu hollow fiber (Cu hf) is shown in FIG. 4. The
partial current density of CO (J.sub.CO), representing the
formation rate of CO, is plotted against the applied potential.
Besides the fact that the faradaic efficiency towards CO is
significantly improved compared to polycrystalline Cu and Cu
nanoparticles, the partial current density for CO formation on Cu
hollow fiber is exceptionally high. Cu hollow fibers can reduce
CO.sub.2 to CO electrochemically at a potential of -0.4 V vs. RHE,
with over 15 to 400 times higher rate than polycrystalline Cu and
Cu nanoparticles, respectively at a potential of -0.4 V vs. RHE.
While outcompeting the currently best performing copper based
electrodes, Cu hollow fibers also show comparable activities at low
potentials (-0.2 V to -0.6 V vs. RHE) to that of noble metal
catalysts evaluated in aqueous solutions (Au nanoparticles,
nanoporous Ag). It should be recalled that noble metal electrodes
benefit from a high overpotential for hydrogen evolution, while Cu
hollow fibers perform so well on the basis of the improved mass
transfer of CO.sub.2.
[0024] In addition to the comparison in FIG. 4, a beneficial
comparison would be on the basis of the performance of gas
diffusion electrodes, where the reaction also takes place at
gas-liquid-solid interfaces. Unfortunately, most of the studies use
high overpotentials (>1 V) to evaluate hydrocarbon formation on
Cu gas diffusion electrodes. In general, much higher currents
densities (0.1-1 A/cm.sup.2) are reported on gas diffusion
electrodes which can be attributed to high applied potentials which
decreases the overall energy efficiency. For instance, no CO
formation was reported on Ag gas diffusion electrodes at low
potentials where recorded onset potential was around -0.6 V vs.
RHE. Nevertheless, the inventors believe using a defect rich, rough
and porous copper based catalyst on a conventional gas diffusion
electrode would also yield a remarkable performance. The thickness
of the porous catalyst layer used in gas diffusion electrodes is
typically in the range from 5-20 .mu.m, similarly for copper hollow
fibers, the electrode thickness that participates into the
electrolysis is around 15-20 .mu.m estimated from nickel
electrodeposition and subsequent energy dispersive X-ray analysis
(FIG. 10). This thickness is also comparable to oxide films used to
prepare rough electrodes or electrodeposited 3-D porous structures
employed as electrodes. The geometrical current density of the
fibers are calculated by normalizing the current to the outer
surface area of the cylindrical hollow fibers. In addition, it is a
common practice to use the projected area of the 3-D electrode, or
so called apparent area, in conventional gas diffusion electrodes
to report current densities.
[0025] Gas diffusion electrodes played an important role in
fundamental electrocatalysis, however their mass production delayed
due to economic and technical issues. The mature dry-wet spinning
process allows mass production of organic hollow fibers that are
already commercially available. Preparation of hollow metal fibers
with diameters in the range of 100-500 .mu.m, on the other hand,
was developed recently which adapts a very similar method, implying
a great potential of large scale production. Microtubular geometry
has been deployed and investigated in solid oxide fuel cells for
decades which could allow the adaptation of technologies developed
such as stack design, sealing, current collection etc. Metal hollow
fibers might provide cost effective and compact diffusion media
and/or catalyst layer for gas diffusion electrodes which might also
eliminate resistance associated with catalyst support interface.
Furthermore, we believe there is plenty of room to increase the
production rate by considering the controllability of the internal
and external structure of the hollow fibers. The thickness of the
active catalyst layer can be tuned by changing 3-D geometry,
support material, porosity and/or precursor particle size, to
further optimize the production rate.
[0026] The results reported herein highlight a new area to explore
for the development of robust electrodes that can efficiently
catalyze conversion of CO.sub.2 at high rates in aqueous media.
Employing a simple, compact Cu hollow fiber as both gas diffuser
and an electrode, leads to very high CO production rates which are
comparable to what has been achieved by using noble metals.
Selective formation of CO is observed with a maximum faradaic
efficiency of 75% when high flow rates are used. The porous nature
of the hollow fibers provides high surface area and correspondingly
high geometric current densities for CO.sub.2 reduction ranging
from 2 mA cm.sup.-2 to 17 mA cm.sup.-2 at moderate potentials (-0.3
to -0.5 V vs. RHE). The remarkable performance of the hollow fibers
are attributed to defect-rich porous structure in addition to the
excellent mass transport capabilities. In general, hollow fibers
provide new possibilities to design practically relevant
microtubular electrodes and electrochemical reactors where one or
more reactants are present in the gas phase.
[0027] In a further aspect, the invention is directed to a method
of electrolyzing carbon dioxide in an aqueous electrochemical cell
comprising an anode and a cathode, wherein the cathode comprises
one or more metal hollow fiber electrodes according to the
invention, said method comprising [0028] applying a potential
between said anode and cathode, and [0029] purging CO.sub.2 or a
gas mixture comprising CO.sub.2 through the wall of the metal
hollow fiber electrode.
[0030] Preferably, the method of the invention is performed in an
aqueous environment.
[0031] The method can suitably be performed at a temperature in the
range of 5-80.degree. C., such as in the range of 10-30.degree. C.,
more preferably in the range of 15-25.degree. C.
[0032] In a further aspect, the invention is directed to a method
of converting carbon dioxide into one or more selected from the
group consisting of carbon monoxide, formic acid, a formate,
methanol, acetaldehyde, methane, ethylene and ethane, comprising
electrolyzing CO.sub.2 by a method according to the invention of
electrolyzing carbon dioxide in an aqueous electrochemical cell
comprising an anode and a cathode as described herein.
[0033] In a preferred embodiment, the carbon dioxide is converted
into carbon monoxide.
[0034] In yet a further aspect, the invention is directed to a
method of preparing a metal hollow fiber electrode according to the
invention, comprising: [0035] spinning a mixture comprising copper
particles, polymer and solvent together with a bore liquid to
obtain hollow fibers; [0036] subjecting the hollow fibers to a
thermal treatment such that copper particles are sintered together,
thereby yielding hollow copper oxide fibers; [0037] hydrogenating
the hollow copper oxide fibers.
[0038] The thermal treatment in this method preferably comprises
subjecting the hollow fibers to a temperature in the range of
500-800.degree. C., such as in the range of 550-700.degree. C. This
thermal treatment is preferably performed for a period of 1-6
hours, such as a period of 2-5 hours.
[0039] The hydrogenation preferably comprises subjecting the hollow
copper oxide fibers to a temperature in the range of
200-400.degree. C., such as in the range of 250-350.degree. C. This
hydrogenation is preferably performed for a period of 30-120
minutes, such as 45-90 minutes. Preferably the hydrogenation
comprises subjecting the hollow copper oxide fibers to a flow of
hydrogen in the concentration range of 0.1-100 vol. %, such as 5
vol. % in a balance gas.
[0040] In yet a further aspect, the invention is directed to the
use of a metal hollow fiber electrode according to the invention as
cathode and/or gas diffuser.
Preparation of Cu Hollow Fibers
[0041] Commercial available copper powder (Skyspring nanomaterials,
99%) with particle size of 1-2 .mu.m was used as catalyst
precursor. N-methylpyrrolidone (NMP, 99.5 wt. %, Sigma Aldrich),
Polyetherimide (PEI, Ultem 1000, General Electric) were used as
solvent and polymer respectively. Copper powder (71.09 wt. %) was
added to the NMP (22.14 wt. %) followed by stirring and ultrasonic
treatment for 30 min. After addition of PEI (6.76 wt. %) this
mixture was heated and kept at 50.degree. C. and 60.degree. C. for
30 minutes and 2 hours, respectively. Next, the solution is allowed
to cool down by stirring overnight which is followed by degassing.
Vacuum was applied for 90 min and the mixture was left
overnight.
[0042] Spinning was carried out at room temperature
(21.+-.3.degree. C.) using a stainless steel vessel, that was
pressurized to 1 bar using nitrogen. The mixture was pressed
through a spinneret (inner and outer diameters of 0.8 mm and 2.0
mm, respectively) into a coagulation bath containing tap water.
Deionized water was pumped through the bore of the spinneret with a
speed of 30 ml min.sup.-1 and the air gap was set to 1 cm.
[0043] After spinning the fibers were kept in the coagulation bath
for 1 day to remove traces of NMP, followed by drying for 1 day.
The green copper hollow fibers were thermally treated at
600.degree. C. for 3 hours (heating rate and cooling rates:
1.degree. C. min.sup.-1) in air to remove the PEI and subsequent
sintering of the copper particles. The oxidized hollow fibers were
reduced by hydrogenation at 280.degree. C. for 1 hour (H.sub.2 in
Argon: 4%, heating rate and cooling rate: 100.degree. C./min).
X-ray diffraction patterns were collected by using a Bruker D2
Phaser x-ray diffractometer, equipped with a Cu-K.alpha. radiation
source and operated at 30 kV and 10 mA (FIG. 5). SEM images were
taken using a Phillips FEI XL30 FEG-ESEM or FEI Sirion HR-SEM.
X-ray photoelectron spectroscopy (XPS) spectrum was collected by
using Quantera SXM (Scanning XPS microprobe) spectrometer equipped
with Al K.alpha. (1486.6 eV) X-ray source. The source was operated
with a 25 W emission power, beam size of 200 .mu.m and pass energy
of 224 eV. The resolution of the spectrometer was 0.1 eV and 0.2 eV
for high resolution element scan and survey spectra,
respectively.
Electrochemical CO.sub.2 Reduction
[0044] All solutions were prepared and all glassware were cleaned
by using deionized water (Millipore MilliQ, 18.2 MQ).
Electrochemical CO.sub.2 reduction activity of Cu HF's was measured
by using three electrode assembly in a glass cell at room
temperature and pressures. A Princeton Applied Research versaSTAT 3
potentiostat was used to control the potentials. The counter
electrode, Pt mesh, was separated by using a Nafion 112 membrane
(Sigma Aldrich). An Ag/AgCl (3 M NaCl BAST) reference electrode was
placed near the working electrode by using a Luggin capillary and
all the potentials were converted to RHE scale afterwards. IR drops
were measured before the electrolysis and compensated manually
after the experiments. 4.+-.0.5 cm long Cu HF are used as both
working electrode and as gas diffuser. The fibers were sealed from
the bottom by using epoxy glue and connected to gas inlet of the
cell. The cathodic compartment is filled with 100 ml, 0.3 M
KHCO.sub.3 (99.95%, Sigma Aldrich) and purged with CO.sub.2 at
least 20 minutes before the experiments. During the electrolysis
the CO.sub.2 is purged continuously through the fiber with 20 ml
min.sup.-1 unless otherwise indicated, and sampled via gas
chromatogram (GC) each 6 minutes. CO, CO.sub.2, H.sub.2 and
hydrocarbons are separated with GC equipped with two different
columns (ShinCarbon 2 m micropacked column and Rtx-1). A thermal
conductivity detector (TCD) and flame ionization detector (FID)
were used to perform the quantitative analysis of the gas phase
products. The time needed to reach steady state concentration was
approximately 10 minutes so all the reaction times were kept at
least 20 minutes. A control experiment was conducted at -0.5 V vs.
RHE under Argon atmosphere. No CO was detected that might
associated with the organic residues remaining from polymers that
is used during preparation of the hollow fibers. Liquid products
formed during the electrolysis were analyzed by using High
Performance Liquid Chromatography (HPLC) (Prominence HPLC,
Shimadzu; Aminex HPX 87-H column, Biorad).
Characterization of the Cu hollow fibers
[0045] Certified chemical analysis of the precursor copper powder
is given in table 1 (taken from the datasheet provided by the
company which supplied the copper powder, Skyspring Nanomaterials).
The XRD pattern and SEM images of the copper powder are given in
FIGS. 5 and 6. Copper powder is composed of spherical particles
with a broad size distribution from 500 nm-2 .mu.m. The XRD pattern
after calcination shows that CuO was formed after sintering of the
copper particles. The hydrogenation procedure reduces the copper
oxide to the metallic form. These fibers were used as both cathode
and gas diffuser, purging CO.sub.2 through the side walls of the
fiber. A summary of product distribution and total current density
as a function of applied potential is provided in table 2. The SEM
image of the fibers after 24 hours of electrolysis (FIG. 7) showed
changes in the morphology of the fibers are negligible (FIG.
8).
TABLE-US-00002 TABLE 2 Summary of the faradaic efficiencies (FE) as
a function of applied potential. Faradaic Efficiency (%) E vs RHE
J.sub.total Formic (v) (mA/cm.sup.2) CO Acid Ethylene H.sub.2 Total
-0.16 0.24 14.8 0.0 0.0 -- 14.8* -0.24 0.66 42.2 0.0 0.0 -- 42.2*
-0.30 2.5 55.9 25.1 0.0 19.4 100.4 -0.34 4.2 65.2 20.1 0.0 12.1
97.4 -0.36 4.8 68.6 16.2 0.0 11.8 96.6 -0.41 8.4 72.4 9.8 0.0 13.9
96.1 -0.47 14.7 68.5 16.5 0.1 11.7 96.8 -0.52 21.3 65.4 18.5 0.15
13.6 97.7 -0.55 35.7 53.3 26.2 0.2 16.9 96.6 *Hydrogen
concentrations were not measurable at these low potentials due to
the relatively low detection limit of the applied Thermal
Conductivity Detector The remaining current is considered to be due
to H.sub.2, since no formic acid is detected in this
potentials.
[0046] An example for the reproducibility of the experiments is
given in FIG. 9. Four different experiments were performed at an
applied potential of -0.4 V vs. RHE by using four different Cu
hollow fibers. There are only slight variations in faradaic
efficiency of CO and geometrical current density when different
fibers are employed as electrodes.
[0047] Electrodeposition experiments were performed on Cu hollow
fibers to determine the reactive zones. Nickel deposition was
performed from solutions of nickel nitrate (50 mM
Ni(NO.sub.3).sub.2, 5 mA cm.sup.-2 for 900 s) while purging Ar at
20 ml min.sup.-1. The SEM images in FIG. 10 show that the nickel
deposition takes place mostly on the outer surface. Energy
dispersive X-ray analysis (EDX) showed that the penetration depth
of Nickel through the fiber is around 15-20 .mu.m which indicates
the thickness of the electrode utilized during the CO.sub.2
reduction. The noisy character of the line scan is due to porous
nature of the Cu hollow fiber. The current values obtained are
normalized by the external geometrical surface area of the
cylindrical Cu hollow fibers. This has been done by considering the
facts that when nanoparticles, rough electrodes or 3-D porous
structures are employed as electrodes, the current density is
calculated by using the geometrical area of the electrode. The
thickness of the electrode utilized during CO.sub.2 reduction in
this study is comparable to 3-D porous and nanostructured surfaces.
Additionally, for gas diffusion electrodes, it is common to use the
projected area of the 3-D electrode. The electrochemical active
surface area is important to interpret the catalytic activity of
the fibers, whereas the geometrical current density is important
for the practical applications. X-ray photoelectron spectroscopy
(XPS) was collected by using Quantera SXM (Scanning XPS microprobe)
spectrometer equipped with Al K.alpha. (1486.6 eV) X-ray source.
The source was operated with a 25 W emission power, beam size of
200 .mu.m and pass energy of 224 eV. The resolution of the
spectrometer was 0.1 eV and 0.2 eV for high resolution element scan
and survey spectra, respectively. High resolution elemental scans
are performed for Cu, C, O, Al, Ni, Fe, Pb, Cd, Hg. The minimal
detectable amount changes with the sensitivity factors for the
elements. As a rule of thumb: lighter elements have smaller
sensitivity factors and are less good detectable than heavy
elements. Exceptions to this rule of thumb are Al, SI, P, S, Cl,
As, Se and Br. For our survey scans, light elements 0.5% and heavy
elements<0.1%. On flat samples, like the pieces of copper foil,
it is normal that hydrocarbons from ambient air stick to almost all
surface. The layer can rapidly grow to more than 0.5 nm and will
show up as a big part in the calculated contents. The layer also
acts as a shield for all electrons coming from below. Sputtering a
flat surface for a short time with an argon ion beam can reduce the
amount of carbon dramatically. However this sputtering at the same
time destroys the chemistry at the surface. On the powder the
carbon concentration is often lower because there is too much
surface for all the hydrocarbons to stick too. The atomic
concentrations of the elements measured can be calculated and with
the formula
C x = I x / S x I i / S i ##EQU00001##
wherein I.sub.i is the peak area of a photoelectron peak and
S.sub.i is the relative sensitivity factor of the peak. The
calculated amounts for the detected elements are given in table 3.
The elemental scan Cu 2p XPS spectrum indicated the presence of
Cu.sup.0 and/or Cu.sup.1+, but the peaks associated with Cu.sup.2+
in all the spectra were absent. Cu.sup.0 and Cu.sup.1+ ratio can be
roughly estimated by using Cu LMM peaks (FIG. 14). There are minor
impurities at the surface of the Cu hollow fiber before
electrolysis which are most like associated with the polymers used
in spinning process. These impurities are removed upon
electrolysis.
TABLE-US-00003 TABLE 3 The atomic concentrations of the elements
calculated from the intensities of the peaks present in XPS spectra
Cu.sup.0/ C.sup.+ From Cu LMM Sample C N O Na S Cl Ca Cu fit Cu
9.39 -- 43.87 -- -- -- -- 46.74 21/79 Powder Cu 28.82 0.65 44.76
1.15 1.30 0.28 0.37 22.67 52/48 Fiber (before) Cu 7.53 -- 42.40 --
-- -- -- 50.06 73/27 Fiber (after)
BRIEF DESCRIPTION OF THE FIGURES
[0048] FIG. 1: Physical characterization of Cu hollow fibers. a)
Low and b) High magnification SEM images of the outer surface of
the Cu hollow fiber. c) Cross-sectional image of perpendicularly
broken Cu hollow fiber. d) Outer surface of parallel broken Cu
hollow fiber along with the cross-section. e) Image of Cu hollow
fiber taken by the electron microscope, and f) Cu hollow fiber
employed as an electrode (20 ml min.sup.-1 gas flow and no applied
potential.
[0049] FIG. 2: Electrocatalytic performance of Cu hollow fibers. a)
Linear polarization curves for Cu hollow fiber under CO.sub.2 and
Ar atmosphere in 0.3 M of KHCO.sub.3 (Scan rate 50 mV s.sup.-1). b)
Faradaic efficiency (FE) of CO, formic acid and H.sub.2 at
different potentials. c) Overpotential vs. partial current density
of CO for Cu hollow fiber. d) Total production of CO at an applied
potential of -0.4 V for 24 hours of continuous experiment (flow
rate of CO.sub.2: 20 ml min.sup.-1).
[0050] FIG. 3: Electrocatalytic performance as a function of flow
rate. a) Linear polarization curves for different flow rates of
CO.sub.2 (Scan rate 50 mV s.sup.-1). b) Faradaic efficiency (FE) of
CO for different flow rates of CO.sub.2 and corresponding current
densities (applied potential of -0.4 V vs. RHE, 0.3 M KHCO.sub.3. *
Experiments are performed in CO.sub.2 saturated solutions.
[0051] FIG. 4: Activity of various electrodes in water: Overview of
different catalysts' performance at different potentials with a
plot of partial current density of CO at different potentials.
[0052] FIG. 5: XRD patterns of the starting copper powder, Cu
hollow fiber after calcination at 600.degree. C., and the copper
fiber after hydrogenation.
[0053] FIG. 6: SEM image of "as received" copper powder.
[0054] FIG. 7: The faradaic efficiency (FE) of CO and total current
density at an applied potential of -0.4 V for 24 hours of
continuous experiment (flow rate of CO.sub.2: 20 ml
min.sup.-1).
[0055] FIG. 8: SEM images of the Cu hollow fibers after 24 hours of
electrolysis.
[0056] FIG. 9: Reproducibility tests with 4 different fibers at a
potential of -0.4 V vs. RHE. Flow rate of CO.sub.2: 20 ml
min.sup.-1.
[0057] FIG. 10: EDX analysis demonstrating the wt. % of
electrodeposited nickel as a function of location from the outer
surface. Deposition of Ni was achieved on the copper hollow fibers
feeding 20 ml min.sup.-1 of argon through the porous wall into the
Ni.sup.2+ solution.
[0058] FIG. 11: SEM image of Cu hollow fiber showing the locations
of the SEM images taken to construct FIG. 1.
[0059] FIG. 12: X-ray photoelectron spectroscopy survey for copper
hollow fibers before electrolysis (includes 5 repeated scans).
[0060] FIG. 13: High-resolution X-ray photoelectron spectra
demonstrating the Cu 2p peaks indicative of predominantly
Cu.sup.0.
[0061] FIG. 14: High-resolution X-ray photoelectron spectrum of the
Cu LMM region for Cu hollow fibers after electrolysis. Some Cu(I)
might be present in the material.
* * * * *