Note: Descriptions are shown in the official language in which they were submitted.
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CATALYST SYSTEM FOR CATALYZED ELECTROCHEMICAL REACTIONS AND
PREPARATION THEREOF, APPLICATIONS AND USES THEREOF
The present invention generally relates to a catalyst system for catalyzed
electrochemical
reactions, comprising a conductive support and a catalyst, in particular for
reducing carbon
dioxide in order to prepare products or intermediates thereof like
carboxylates and/or carboxylic
acids.
The electrochemical conversion of carbon dioxide into economically valuable
materials such as
fuels and industrial chemicals or intermediate products thereof is gaining
interest in view of
mitigating the emission of carbon dioxide into the atmosphere, which is
responsible for climate
alterations, changes in pH of seawater and other potentially damaging effects
like melting of
polar ice and sea level rise.
Catalyzed electrochemical reduction of carbon dioxide for preparing
economically valuable
products is known in the art.
E.g. W02013/006711 discloses methods and systems for the electrochemical
conversion of
carbon dioxide to products like carboxylic acids, glycols and carboxylates in
the presence of a
homogeneous heterocyclic amine catalyst. In an embodiment the cathode of the
electrochemical
cell wherein the conversion is performed, comprises a material suitable for
the reduction of
carbon dioxide. Examples of the cathode materials include metal and metal
alloys, amongst
others indium and indium alloys.
W02014/032000 discloses a method of reducing carbon dioxide into one or more
organic
products in an electrochemical cell, wherein the cathode is an oxidized indium
electrode, in
particular an anodized indium electrode.
W02014/042781 discloses the electrochemical conversion of carbon dioxide into
products using
a high surface area cathode, wherein the cathode includes an indium coating
and has a void
volume of between about 30% to 98%. The cathode may also include indium
coatings and/or
metal structures further containing Pb, Sn, Hg, TI, In, Bi, and Cd, their
alloys, and combinations
thereof. Metals including Ti, Nb, Cr, Mo, Ag, Cd, Hg, TI, An, and Pb as well
as Cr-Ni-Mo steel
alloys among many others may be incorporated. The alloys of indium with other
metals, including
Sn, Pb, Hg, TI, Bi, Cu, and Cd and their mixed alloys and combinations thereof
on the exposed
catalytic surfaces of the electrode preferably comprise 5% to 99% indium.
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Preliminary research has indicated that not all the potential catalysts as
disclosed in the above
prior art documents function as desired in terms of selectivity, activity and
Faradaic efficiency.
Therefore there is an ongoing need to develop catalyst systems, which show an
improvement of
one or more of these catalyst properties.
In particular the present invention aims at providing a catalyst system having
a high Faradaic
efficiency for the electrochemical reduction of carbon dioxide and a high
selectivity towards
valuable reduction products, in particular carboxylic acids or intermediates
thereof, such as
carboxylate salts.
This invention provides a catalyst system for catalyzed electrochemical
reactions, comprising a
catalyst, wherein the catalyst comprises 5-94 wt.% bismuth and 6-95 wt.%
indium, based on the
total amount of bismuth and indium. This catalyst will herein below be
referred to as an indium
bismuth catalyst.
The catalyst system according to the invention for catalyzed electrochemical
reactions, in
particular reduction of carbon dioxide, preferably comprises an electrically
conductive support
and a catalyst, wherein the catalyst comprises 5-94 wt.% bismuth and 6-95 wt.%
indium, based
on the total amount of bismuth and indium.
Surprisingly it has been found that the binary metal combination of bismuth
and indium as
catalyst for the electrochemical conversion of carbon dioxide shows a good
selectivity for the
reduction of carbon dioxide into carboxylic acids and carboxylates, as well as
a good Faradaic
yield, in particular for the aqueous conversion of carbon dioxide to formate
salt.
Compared to other indium based binary metal catalysts the indium bismuth
catalyst shows an
improved Faradaic yield. The amount of bismuth is in the range of 5-94 wt.%
based on the total
amount of bismuth and indium, preferably in the range of 10-90 wt.%, more
preferably 30-90
wt.%, such as 35-90 wt.%. Experimental results have indicated that an amount
of bismuth in the
range of 40-60 wt.%, such as 45-55 wt.%, e.g. about 1:1 weight ratio of
bismuth to indium, offers
improved catalytic properties regarding carbon dioxide to formate conversion.
The catalyst can comprise a combination of bismuth and indium in different
thermodynamic
phases. Preferably an amorphous combination of bismuth and indium is used.
That is, preferably
the catalyst system according to the invention comprises a catalyst, wherein
the catalyst
comprises an amorphous combination of 5-94 wt.% bismuth and 6-95 wt.% indium,
based on the
total amount of bismuth and indium.
The indium bismuth catalyst according to the invention can be applied without
or in combination
with an electrically conductive support. It can, for example, be applied
without or in combination
with a carbon containing support. Preferably the indium bismuth catalyst is
applied in
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combination with an electrically conductive support. Therefore, the catalyst
system is preferably
a catalyst system for catalyzed electrochemical reactions, comprising an
electrically conductive
support and a catalyst, wherein the catalyst comprises 5-94 wt.% bismuth and 6-
95 wt.% indium,
based on the total amount of bismuth and indium.
As a conductive support a particulate material, in particular carbon
particles, is used. Preferably
the conductive support comprises a porous structure of carbon particles bonded
together. A
preferred binding material is a hydrophobic binder, such as a fluorinated
binder. The catalyst is
deposited onto or adhered to the conductive material. The weight ratio of
indium and bismuth to
carbon can advantageously be in the range of 0.10-1.50, e.g. about 30 wt.%.
Typically, the electrochemical reduction of carbon dioxide into chemical
reduction products is
performed in an electrochemical cell or photochemical cell having at least two
cell compartments
containing the respective electrodes. Carbon dioxide is supplied to the
cathode. The cathode is
preferably a gas-diffusion electrode providing a high surface area or
interface for solid-liquid-gas
contact. Such a gas-diffusion electrode comprises an electrically conductive
substrate, which
may serve as a supporting structure for a gas-diffusion layer. The gas-
diffusion layer provides a
thin porous structure or network e.g. made from carbon, for passing a gas like
carbon dioxide
from one side to the other. Typically the structure is hydrophobic to distract
water. The gas-
diffusion layer may comprise a catalytically active material.
Therefor a further aspect of the invention relates to a gas-diffusion
electrode, comprising a gas-
diffusion layer on an electrically conductive substrate, wherein the gas-
diffusion layer comprises
the catalyst system according to the invention as outlined above. The binary
metal catalyst
system of bismuth and indium according to the invention may be embedded in the
gas-diffusion
layer structure or provided as one or more additional separate layers thereof.
As explained
above, a particulate carbon is a preferred example of the conductive support
for the catalyst.
The catalyst system is preferably bonded to the electrically conductive
substrate using a
hydrophobic binder such as PTFE. Examples of suitable substrates include metal
structures like
expanded or woven metals, metal foams, and carbon structures including wovens,
cloth and
paper.
Yet another aspect of the invention is an electrochemical cell comprising at
least one gas
chamber and at least one liquid chamber, which chambers are separated by a gas-
diffusion
electrode according to the invention.
Generally the reduction of carbon dioxide is performed in an electrochemical
cell, typically a
divided cell having two cell compartments. One cell compartment contains the
anode, and the
other cell compartment contains a gas-diffusion cathode electrode according to
the invention,
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comprising the binary metal electrocatalyst of bismuth and indium. The two
cell compartments
may be separated by a suitable membrane, e.g. made from porous glass frit,
microporous
material, ion exchanging membrane or ion conducting bridge, allowing ionic
species to travel
from one compartment to the other, such as protons generated at the anode to
the cathode
compartment.
A further aspect of the invention concerns a method of preparing a gas-
diffusion electrode as
defined above, comprising the binary metal electrocatalyst system according to
the invention.
This manufacturing method comprises a step of providing an electrically
conductive substrate
and a step of applying the catalyst system according to the invention,
comprising indium and
bismuth and an electrically conductive support, in particular particulate
support material, and a
binder to the gas-diffusion layer of the gas-diffusion electrode.
The electrocatalyst loaded gas-diffusion electrode can be manufactured in
various ways
including spraying, casting and sintering, often using one or more suitable
binders.
The invention also relates to a method of electrocatalytically converting
carbon dioxide into
valuable products or product intermediates. This method comprises:
introducing an anolyte to a first cell compartment of an electrochemical cell,
the first cell
compartment comprising an anode;
introducing a catholyte and carbon dioxide to a second cell compartment of the
electrochemical
cell, the second cell compartment comprising a cathode, and
applying an electrical potential between the anode and the cathode sufficient
to reduce carbon
dioxide to a reduced reaction product,
wherein the cathode comprises a catalyst system according to the invention, in
particular the
cathode is a gas-diffusion electrode according to the invention.
The method according to the invention allows to reduce carbon dioxide to
carboxylic acid and
intermediates, including salts such as formate, glycolate, glyoxylate, oxalate
and lactate,
carboxylic acids, and glycols. The production of a carboxylic acid or
carboxylic acid intermediate
may be dependent on the pH of the electrolyte solution in the cell, with lower
pH ranges favoring
carboxylic acid production. The pH of the cathode compartment may be adjusted
to favor
production of one of a carboxylic acid or carboxylic acid intermediate over
production of the
other, such as by introducing an acid (e.g., HCI or H2SO4) to the cathode
compartment. The pH
of the catholyte is preferably between about 1 and 8. A pH range of 1-4 is
preferable for
production of carboxylic acids from carbon dioxide. A pH range of 4-8 is
preferable for
production of carboxylic acid intermediates from carbon dioxide.
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The electrical potential may be a DC voltage. In preferred embodiments, the
applied electrical
potential is generally between about -1.5V vs. SCE and about -6V vs. SCE,
preferably from
about -1.5V vs. SCE to about -5V vs. SCE, such as in the range of -3V vs. SCE
to -5V vs SCE
and more preferably from about -1.5V vs. SCE to about -4V vs. SCE.
5 High Faradaic yield and selectivity of the catalyst system according to
the invention for
conversion of carbon dioxide into formate/formic acid have been shown at the
cathode
according to the reaction CO2 + 2H+ + 2e-4 HCOOH, while at the anode water may
be oxidized
into oxygen and hydrogen ions according to 2H20 4 4H+ + 02 + 4e-.
The hydrogen ions pass through the ion exchange membrane from the anolyte
compartment to
the catholyte compartment in the electrochemical cell.
The carbon dioxide conversion to formate/formic acid is typically performed in
an aqueous
medium, wherein the CO2 is bubbled through the aqueous medium or distributed
to the gas-
diffusion electrode, e.g. using perculator systems.
Non-aqueous media may also be used, e.g. in the direct conversion of carbon
dioxide to oxalic
acid or oxalate.
A homogeneous heterocyclic catalyst may be added to the cathode compartment of
the cell
containing the cathode. The homogeneous heterocyclic catalyst may include, for
example, one
or more of 4-hydroxy pyridine, adenine, a heterocyclic amine containing
sulfur, a heterocyclic
amine containing oxygen, an azole, a benzinnidazole, a bipyridine, furan, an
innidazole, an
imidazole related species with at least one five-member ring, an indole, a
lutidine,
nnethylinnidazole, an oxazole, phenanthroline, pterin, pteridine, a pyridine,
a pyridine related
species with at least one six-member ring, pyrrole, quinoline, or a thiazole,
and mixtures thereof.
If present, the homogeneous heterocyclic catalyst is preferably present at a
concentration of
between about 0.001M and about 1M, and more preferably between about 0.01M and
0.5M.
The chemicals derived as reaction products from the direct electrochemical
conversion
according to the invention can be processed further into industrial products.
E.g. oxalic acid can
be used as a starting material for the production of ethylene glycol and/or
glycine. See e.g.
U52016/0017503. Hydrogen may be introduced to the carboxylic acid or
carboxylic acid
intermediate to produce a glycol or a carboxylic acid, respectively. Hydrogen
may be derived
from natural gas or water.
The invention is further illustrated by the attached drawings and examples.
In the drawings
Fig. 1 shows an embodiment of an electrochemical cell according to the
invention; and
Fig. 2 is an embodiment of a gas-diffusion electrode according to the
invention.
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In FIG. 1 a block diagram of a system 100 is shown in accordance with an
embodiment of the
present invention. System 100 may be utilized for electrochemical production
of carboxylic acid
intermediates, carboxylic acids, and glycols from carbon dioxide and water
(and hydrogen for
glycol production). The system 100 generally comprises an electrochemical cell
102, a liquid
source 104, an energy source 106, a carbon dioxide source 108, a product
extractor 110 and
an extractor 112, the latter in this embodiment for the recovery of oxygen
produced at the
anode. In an embodiment the liquid source 104 is a water source. In another
embodiment the
liquid source is an organic solvent source. A product or product mixture may
be obtained from
the product extractor 110 after extraction. An output gas containing oxygen
may be output from
the oxygen extractor 112 after extraction.
In the embodiment shown the cell 102 is a divided electrochemical cell. The
cell 102 reduces
carbon dioxide into products or product intermediates. The reduction may take
place by
introducing such as bubbling carbon dioxide into an electrolyte solution in
the cell 102. At the
cathode 120 comprising the catalyst system according to the invention carbon
dioxide is
reduced into a carboxylic acid or a carboxylic acid intermediate.
The cell 102 generally comprises two or more or cell compartments 114a, 114b,
a separator 116
e.g. a ion exchange membrane, an anode 118 in anode cell compartment 114a, and
a cathode
120 in cathode cell compartment 114b on an opposite side of the separator 116.
The cathode
120 includes a catalyst system according to the invention suitable for the
reduction of carbon
dioxide. An electrolyte solution e.g., anolyte 122a and catholyte 122b may
fill the respective cell
compartments 114a and 114b.
The liquid source 104 preferably includes a water source, such that the liquid
source 104 may
provide pure water to the cell 102. The liquid source 104 may provide other
fluids to the cell 102,
including an organic solvent, such as methanol, acetonitrile, and
dinnethylfuran. The liquid
source 104 may also provide a mixture of an organic solvent and water to the
cell 102.
The catholyte 122 may include an aromatic heterocyclic catalyst, e.g. in a
concentration of about
10 nnM to 1 M. The electrolyte may also include one or more suitable salts,
such as KCI, NaNO3,
Na2SO4, NaCL, NaF, NaCI04, KCI04, K2SiO3 or CaCl2, e.g. at a concentration of
about 0.5 M.
Other additives may include Group I cations (H, Ii, Na, K, Rb and Cs except
Fr), divalent cations
(e.g., Ca', Mg', Zn') ammonium, alkylannnnoniunn cations and alkyl amines.
Examples of
anions comprise halides, carbonates, bicarbonates, nitrates, nitrites,
perchlorates, phosphates,
polyphosphates, silicates and sulfates. .Bicarbonate is a preferred anion.
The pH of the cathode compartment 114b is preferably between about 1 and 8.
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The energy source 106 may include a variable voltage source. The energy source
106 may be
operational to generate an electrical potential between the anode 118 and the
cathode 120.
The gas source 108 preferably includes a carbon dioxide source, such that the
gas source 108
may provide carbon dioxide to the cell 102. E.g. the carbon dioxide is bubbled
directly into the
compartment 114b containing the cathode 120. For instance, the compartment
114b may
include a carbon dioxide input, such as a port 126a configured to be coupled
between the
carbon dioxide source and the cathode 120.
The carbon dioxide may be obtained from any source, preferably a renewable
source. The
product extractor 110 may include an organic product and/or inorganic product
extractor. The
product extractor 110 generally facilitates extraction of one or more products
e.g., carboxylic
acid, and /or carboxylic acid intermediate from the electrolyte 122. The
extraction may occur via
one or more of a solid sorbent, carbon dioxide- assisted solid sorbent, liquid-
liquid extraction,
nanofiltration, crystallization and electrodialysis. The extracted products
may be presented
through a port 126b of the system 100 for subsequent storage, consumption,
and/or processing
by other devices and /or processes at A. In an embodiment the carboxylic acid
or carboxylic acid
intermediate is continuously removed from the cell 102, where cell 102
operates on a continuous
basis, such as through a continuous flow-single pass reactor where fresh
catholyte and carbon
dioxide is fed continuously as the input, and where the output from the
reactor is continuously
removed. In other embodiments, the carboxylic acid or carboxylic acid
intermediate is
continuously removed from the catholyte 122 via one or more of adsorbing with
a solid sorbent,
liquid-liquid extraction, and electrodialysis.
The separated carboxylic acid or carboxylic acid intermediate may be placed in
contact with a
hydrogen stream at A, e.g. in an additional reactor, to produce a glycol or
carboxylic acid,
respectively.
Oxygen may be discharged from extractor 112 through port 128.
An embodiment of a gas-diffusion electrode according to the invention is shown
in Fig. 2.
Fig. 2 represents a schematic illustration of an electrochemical cell 200
utilizing an anode
electrode 202 for the anode reaction, in this specific embodiment a hydrogen
gas-diffusion
electrode, and a carbon dioxide gas-diffusion electrode 204 for the cathode
reaction of reducing
carbon dioxide e.g. to formate. The cathode 204 may have a carbon dioxide
internal gas plenum
206 in the current collector 208 of the electrode 204 to distribute carbon
dioxide evenly into the
gas-diffusion electrode. A cathode trickle bed solution distributor or
percolator 210 is present in
the catholyte cell compartment 212. The catholyte solution may be introduced
at the top entry
214 of the catholyte compartment 212 and the catholyte solution is distributed
evenly down the
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cell and is discharged via exit 216 at the bottom of the catholyte compartment
212.
Alternatively, the flow may be reversed, so that the flow is in the upward
vertical direction. The
solution may be fed at specific rates, such as in the range of 0.001 to 10
liters per minute or
more depending on the electrochemical cell dimensions, so that the cathode gas
diffusion
electrode 204 may not be flooded with the catholyte solution due to excessive
pressure, and so
as to maintain good ionic contact with the cathode gas diffusion electrode 204
for the transfer of
electrons into the solution in the reduction of carbon dioxide. The flow and
pressure of the
catholyte flow are such that minimal amounts of catholyte solution pass
through the gas diffusion
electrode 204 into the carbon dioxide gas plenum 206 inside the cathode
current collector 208,
and that the carbon dioxide gas reduction within the gas diffusion electrode
is sufficient, so as to
obtain a reasonable cathode current density, e.g. in the range of 10 mA/cm2 to
1000 mA/cm2, or
more preferably in a range of about 50 to 500 mA/cm2. An energy source (not
shown) is
operably coupled with the electrodes 202 and 204 to reduce carbon dioxide at
the cathode 204.
Carbon dioxide is fed to the gas-diffusion electrode 204 via entry 218 into
the gas plenum 206.
Micro-channels 220 may be provided to pass carbon dioxide from the plenum 206
to the gas-
diffusion electrode 204 that comprises the bismuth indium catalyst system.
Carbon dioxide
leaves the cell through exit 222.
The anode side of the cell is similarly constructed. In this embodiment
hydrogen gas is fed via
entry 224 to gas plenum 226 provided with nnicrochannels 228 and leaves the
cell via exit 230.
Anolyte is introduced at entry 232, flows through a distributor 234 down to
the exit 236. A ion
exchange membrane 238 is arranged between the anolyte and catholyte
distributors 234 and
210.
The cathode trickle bed 210 may include a thin construction, e.g. made from
non-conductive
corrosion resistant polymer plastics, such as PTFE, polypropylene and the
like, in the form of
.. screen-like or convoluted forms so to distribute the catholyte solution
evenly as it passes down
the gas-diffusion electrode 204. Alternatively, the trickle bed material may
include conductive
carbon and graphite, or potentially be manufactured from metal. The entry and
exit ports of the
catholyte compartment are designed such that the flow distribution of liquid
is uniform along the
cross section of the trickle bed at the top and bottom. In another embodiment
the GDE cathode
.. may be able to be operated in a partially flooded or possibly fully flooded
condition, and the flow
conditions and electrolyte may be adjusted to operate the cathode in this
mode.
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Example 1 Screening catalyst
Various binary metal catalysts were screened for their formate Faradaic yield
in a test set up.
The test set up comprised a 3 chambered glass cell wherein the electrodes were
positioned.
0.75M KHCO3 was used as elektrolyt. Potentiostatic (xV vs SCE) electrolysis
for the
electrochemical reduction of CO2 to formate was performed during 3.5-5 hrs.
Tables 1 and 2
show the results. It has appeared that a 50 wt.% Bi sample showed the best
results in this
screening test, while a 10 wt.% Bi sample outperformed a 90 wt.% Bi sample.
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Table 1. Screening test results
Catalyst E1/2 (V) vs SCE) Formate Faradaic Yield (cY0)
In/Bi 50/50 -1.90 79.88
In/Bi 90/10 -1.90 76.81
Anodized In -1.75 75.73
In/Bi 10/90 -1.90 71.97
Bi/Pb 55.5/44.5 -1.90 70.87
Sn/Zn 60/40 -1.90 57.81
In/Sn 70/30 -1.90 53.64
In/Zn 90/10 -1.90 51.81
Sn/Pb 50/50 -1.90 48.42
In/Sn 30/70 -1.90 45.00
In/Sn 50/50 -1.90 41.46
In/Sn 30/70 -1.60 30.24
In/Sn 96/4 -1.75 28.79
Au/Ni (82/18) -1.90 3.35
In -1.9 63.47
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Table 2. Screening Test Results
Alloy E1/2 (V) vs SCE Formate Faradaic Yield (%)
In/Sn 50:50 rod -1.46 7.69
-1.60 16.91
-1.90 54.21
Sn/Zn 60/40 -1.90 57.81
Bi:Pb -1.60 18.25
-1.75 68.78
-1.90 70.87
Sn:Pb -1.75 41.97
-1.90 48.42
In/Sn 70/30 -1.60 19.39
-1.75 46.87
-1.90 53.64
In/Sn 30/70 -1.60 30.24
-1.75 51.88
-1.90 45.00
In/Sn 96/4 -1.60 27.28
-1.75 28.79
In/Sn 50/50 -1.90 41.46
In/Bi 90/10 -1.75 82.26
-1.80 68.83
-1.90 76.82
In/Bi 10/90 -1.75 57.53
-1.80 65.57
-1.90 71.97
In/Bi 50/50 -1.75 73.70
-1.80 82.13
-1.90 79.88
In/Zn 90/10 -1.70 52.30
-1.80 59.64
-1.90 51.81
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Example 2. Preparation of binary metal catalyst system In/Bi on C
InCI3, Bi(NO3)3*5H20 and tri-sodium citrate dehydrate were weighted as shown
in Table 3 and
put inside a two-neck round bottom flask containing 100 mL of tri-ethylene
glycol and Vulcan
carbon (available from Cabot). The round bottom flask was placed in an oil
bath and fitted with a
condenser. The system was continuously purged with N2 gas. The oil bath
temperature was set
to 100 C. The content of the flask was stirred. After the system reached the
desired value of the
temperature, it was allowed to stabilize for about 10 minutes, before rapidly
injecting a water
solution of NaBH4 using a syringe and needle. The NaBH4 was freshly prepared
and sonicated
in order to speed up the solubilization process. As soon as the NaBH4 was
injected, a vigorous
bubbling was observed in the mixture. The color of the suspension was black
and no change in
it was observed throughout the course of the reaction. After injecting NaBH4,
the system was
maintained at 100 C under stirring for 15 minutes. Then the heater was turned
off and the
suspension was allowed to cool slowly. At room temperature the suspension was
transferred
into 4 centrifuge tubes and centrifuged at 8000 rpm for 30 min. The
supernatant was poured out
and ethanol was added into the tubes, followed by a thorough washing. The
washing was
performed by sonicating the suspension for 10 min. Then centrifugation at 8000
rpm for 30
minutes was performed. This process was repeated 3 times. At the end ethanol
(90 mL) was
added into the tubes and the overall content was transferred in a 100 mL glass
jar. The resulting
mixture was sonicated for 40 minutes at room temperature and then magnetically
stirred for 15
minutes. The thus obtained emulsion (catalyst ink) was ready for spray
application.
The In:Bi weight ratio in the thus prepared catalyst is 52.3:47.6.
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Table 3
Material Mass (mg)
InCI3 310
BiNO3 x H20 340
Na3Citrate 441
Carbon 716
NaBH4 946
Example 3. Preparation of Gas Diffusion Electrode (GDE)
A gas-diffusion electrode with a geometric surface area of about 172 cm2 was
cut using a
metallic blade. The GDE thus prepared was fixed on an aluminum panel using
magnets and
positioned at an angle of about 60 from the horizontal planed inside a
ventilated fume hood.
The catalyst ink was sprayed on the GDE using a manual air brusher at room
temperature under
atmospheric conditions.
1.13
