U.S. patent application number 17/251787 was filed with the patent office on 2021-07-08 for gas diffusion electrode for carbon dioxide treatment, method for production thereof, and electrolysis cell having a gas diffusion electrode.
This patent application is currently assigned to Siemens Aktiengesellschaft. The applicant listed for this patent is Siemens Aktiengesellschaft. Invention is credited to Martin Kalmar Hansen, Christian Reller, Kasper Tipsmark Therkildsen.
Application Number | 20210207277 17/251787 |
Document ID | / |
Family ID | 1000005519558 |
Filed Date | 2021-07-08 |
United States Patent
Application |
20210207277 |
Kind Code |
A1 |
Hansen; Martin Kalmar ; et
al. |
July 8, 2021 |
GAS DIFFUSION ELECTRODE FOR CARBON DIOXIDE TREATMENT, METHOD FOR
PRODUCTION THEREOF, AND ELECTROLYSIS CELL HAVING A GAS DIFFUSION
ELECTRODE
Abstract
A gas diffusion electrode for carbon dioxide treatment includes
a metal substrate and an electrically conductive catalyst layer
which is applied thereto and has hydrophilic pores and/or channels
and hydrophobic pores and/or channels, the catalyst layer including
metal particles which are coated at least in regions with a
polymeric binder material. A method produces a gas diffusion
electrode for CO2 treatment, and an electrolysis cell has a
corresponding gas diffusion electrode.
Inventors: |
Hansen; Martin Kalmar;
(Vanlose, DK) ; Reller; Christian; (Minden,
DE) ; Therkildsen; Kasper Tipsmark; (Lille-Skensved,
DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Aktiengesellschaft |
Munich |
|
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
Munich
DE
|
Family ID: |
1000005519558 |
Appl. No.: |
17/251787 |
Filed: |
May 16, 2019 |
PCT Filed: |
May 16, 2019 |
PCT NO: |
PCT/EP2019/062595 |
371 Date: |
December 12, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 1/23 20210101; C25B
3/07 20210101; C25B 3/26 20210101; C25B 11/081 20210101; C25B 3/03
20210101; C25B 11/032 20210101 |
International
Class: |
C25B 11/032 20060101
C25B011/032; C25B 11/081 20060101 C25B011/081; C25B 3/26 20060101
C25B003/26; C25B 3/03 20060101 C25B003/03; C25B 3/07 20060101
C25B003/07; C25B 1/23 20060101 C25B001/23 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 27, 2018 |
DE |
10 2018 210 458.1 |
Claims
1-16. (canceled)
17. A gas diffusion electrode for utilization of carbon dioxide,
comprising: a metallic support, and an electrically conductive
catalyst layer which has been applied to this metallic support and
has hydrophilic pores and/or channels and hydrophobic pores and/or
channels, wherein the catalyst layer comprises metallic particles
which are coated at least in subregions with a polymeric binder,
wherein, in the production of the gas diffusion electrode, the
metallic particles are coated with fibers of the binder material
before application to the support.
18. The gas diffusion electrode as claimed in claim 17, wherein the
catalyst layer has a bubble formation point above 40 mbar.
19. The gas diffusion electrode as claimed in claim 17, wherein the
flooding pressure of the catalyst layer is above 150 mbar.
20. The gas diffusion electrode as claimed in claim 17, wherein
silver particles have been used as metallic particles.
21. The gas diffusion electrode as claimed in claim 17, wherein the
average particle diameter of the metallic particles is in the range
from 1 .mu.m to 10 .mu.m.
22. The gas diffusion electrode as claimed in claim 17, wherein the
metallic particles have a specific BET surface area in the range
from 0.1 m2/g to 10 m2/g.
23. The gas diffusion electrode as claimed in claim 17, wherein
from 0.1 to 30% by weight of the polymeric binder in each case
based on a catalyst/binder mixture from which the catalyst layer is
formed, have been used.
24. The gas diffusion electrode as claimed in claim 17, wherein the
average particle diameter of the polymeric binder is in the range
from 0.5 .mu.m to 20 .mu.m.
25. The gas diffusion electrode as claimed in claim 17, wherein the
porosity of the catalyst layer is in the range from 60% to 80%.
26. The gas diffusion electrode as claimed in claim 17, wherein the
ratio of hydrophilic pores and/or channels and hydrophobic pores
and/or channels in the catalyst layer is in the range from 50:50 to
20:80.
27. The gas diffusion electrode as claimed in claim 17, wherein a
gauze, preferably a silver gauze having a mesh opening in the range
from 0.3 mm to 1.4 mm, has been used as metallic support.
28. The gas diffusion electrode as claimed in claim 17, wherein the
gauze has a wire diameter in the range from 0.1 mm to 0.25 mm.
29. A process for producing a gas diffusion electrode for
utilization of CO.sub.2, comprising: producing a mixture of
metallic particles and at least one binder material to form a
mixture, applying the mixture to a metallic support, and embedding
the applied mixture into the metallic support, wherein the metallic
particles are coated at least in subregions with the polymeric
binder material during the production of the mixture and the
metallic particles are coated with fibers of the binder material
before application to the support and wherein an electrically
conductive catalyst layer having hydrophilic pores and/or channels
and hydrophobic pores and/or channels is produced by: embedding the
mixture by means of an extraction process into the metallic
support, or embedding the mixture by dry rolling-on into the
metallic support.
30. An electrolysis cell, comprising: a gas diffusion electrode as
claimed in claim 17.
31. The gas diffusion electrode as claimed in claim 18, wherein the
catalyst layer has a bubble formation point is in the range from 80
mbar to 150 mbar.
32. The gas diffusion electrode as claimed in claim 19, wherein the
flooding pressure of the catalyst layer is in the range from 200
mbar to 1000 mbar.
33. The gas diffusion electrode as claimed in claim 21, wherein the
average particle diameter of the metallic particles is in the range
from 2 .mu.m to 5 .mu.m.
34. The gas diffusion electrode as claimed in claim 23, wherein
from 5 to 25% by weight of the polymeric binder, in each case based
on a catalyst/binder mixture from which the catalyst layer is
formed, have been used.
35. The gas diffusion electrode as claimed in claim 23, wherein
from 15 to 20% by weight of the polymeric binder, in each case
based on a catalyst/binder mixture from which the catalyst layer is
formed, have been used.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2019/062595 filed 16 May 2019, and claims the
benefit thereof. The International Application claims the benefit
of German Application No. DE 10 2018 210 458.1 filed 27 Jun. 2018.
All of the applications are incorporated by reference herein in
their entirety.
FIELD OF INVENTION
[0002] The present invention relates to a gas diffusion electrode
for utilization of carbon dioxide and also a process for producing
a gas diffusion electrode. The invention further relates to an
electrolysis system comprising a corresponding gas diffusion
electrode.
BACKGROUND OF INVENTION
[0003] At present, about 80% of worldwide energy requirements are
covered by the combustion of fossil fuels. As a result of these
combustion processes, about 34 000 million metric tons of the
greenhouse gas carbon dioxide (CO.sub.2) are emitted into the
atmosphere every year worldwide. The major part of the carbon
dioxide is disposed of by this liberation into the atmosphere (in
the case of large brown coal power stations, more than 50 000
metric tons per day).
[0004] Owing to the increasing scarcity of fossil fuel resources
and the volatile availability of renewable energy sources, research
into the reduction of CO.sub.2 is of ever increasing interest. In
this way CO.sub.2 emissions would be decreased and the CO.sub.2
could be utilized as inexpensive carbon source.
[0005] The discussion about the adverse effects of CO.sub.2 on the
climate has led to reutilization of CO.sub.2 being considered.
However, CO.sub.2 is thermodynamically in a very low position and
can therefore be reduced again to give usable products only with
difficulty.
[0006] A natural degradation of carbon dioxide occurs, for example,
by means of photosynthesis. Here, carbon dioxide is converted into
carbohydrates in a process divided into many substeps over time and
spatially on a molecular level. However, this process cannot
readily be carried over to an industrial scale. A copy of the
natural photosynthesis process using industrial photocatalysis has
hitherto not been sufficiently efficient.
[0007] A further method is the electrochemical reduction of carbon
dioxide. Systematic studies on the electrochemical reduction of
carbon dioxide are still a relatively young field of development.
Only since a few years ago have efforts been made to develop an
electrochemical system which can reduce an acceptable amount of
carbon dioxide.
[0008] Research studies on a laboratory scale have shown that
metals are preferably to be used as catalysts for electrolysis of
carbon dioxide. Faraday efficiencies at various metal cathodes are
known from the publication Electrochemical CO.sub.2 reduction on
metal electrodes by Y. Hori, published in: C. Vayenas, et al.
(Eds.), Modern Aspects of Electrochemistry, Springer, N.Y., 2008,
pp. 89-189.
[0009] The Faraday efficiencies (FE [%]) reported in table 1 below
apply to products which are formed in the reduction of carbon
dioxide at various metal electrodes. The values indicated are for a
0.1 M potassium hydrogencarbonate solution as electrolyte.
TABLE-US-00001 TABLE 1 Faraday efficiencies for the conversion of
CO.sub.2 into products at various metal electrodes Electrode
CH.sub.4 C.sub.2H.sub.4 C.sub.2H.sub.5OH C.sub.3H.sub.7OH CO
HCOO.sup.- H.sub.2 Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au
0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4
94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3
2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0
0.0 0.0 97.4 5.0 102.4 Hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0
0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6
100.1 Cd 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0
95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0
0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0
0.0 0.0 0.0 0.0 0.0 99.7 99.7
[0010] While carbon dioxide is, for example, reduced virtually
exclusively to carbon monoxide at silver, gold, zinc, palladium and
gallium cathodes, many hydrocarbons are formed as reaction products
at a copper cathode. Thus, for example, predominantly carbon
monoxide and little hydrogen would be formed at a silver cathode.
The reactions at anode and cathode can be represented by the
following reaction equations:
[0011] Cathode: 2 CO.sub.2+4 e.sup.-+4 H.sup.+.fwdarw.2 CO+2
H.sub.2O
[0012] Anode: 2 H.sub.2O.fwdarw.O.sub.2+4 H.sup.++4 e.sup.-
[0013] The electrochemical production of, for example, carbon
monoxide, methane, ethene and also ethylene is of particular
economic interest. The corresponding overall reaction equations are
shown below:
Carbon monoxide: CO.sub.2+2e.sup.-+H.sub.2O.fwdarw.CO+2 OH.sup.-
Ethylene: 2 CO.sub.2+12 e.sup.-+8 H.sub.2O.fwdarw.C.sub.2H.sub.4+12
OH.sup.- Methane: CO.sub.2+8 e.sup.-+6 H.sub.2O.fwdarw.CH.sub.4+8
OH.sup.- Ethanol: 2 CO.sub.2+12 e.sup.-+9
H.sub.2O.fwdarw.C.sub.2H.sub.5OH+12 OH.sup.- Monoethylene glycol: 2
CO.sub.2+10 e.sup.-+8 H.sub.2O.fwdarw.HOC.sub.2H.sub.4OH+10
OH.sup.-
[0014] Only in recent years have there been an increased number of
systematic studies on the electrochemical reduction of CO.sub.2.
Despite many efforts, there has hitherto not been any success in
developing an electrochemical system by means of which CO.sub.2
could be reduced to competitive energy carriers in a long-term
stable manner and energetically favorably at a sufficiently high
current density and acceptable yield.
[0015] Owing to the increasing scarcity of fossil fuel resources
and the volatile availability of renewable energy sources, research
into the reduction of CO.sub.2 is of ever increasing interest. In
this way CO.sub.2 emissions would be decreased and the CO.sub.2
could be utilized as inexpensive carbon source. However, to ensure
a high current density or in attempts to increase this further,
only the carbon dioxide reduction occurring at the catalytically
active cathode surface of the electrolysis cell has hitherto been
examined.
[0016] Electrolysis cells which are suitable for the
electrochemical reduction of carbon dioxide usually consist of a
cathode space and an anode space. To achieve effective reaction of
the CO.sub.2 introduced, the cathode is ideally configured as a
porous gas diffusion electrode. Gas diffusion electrodes (GDEs) are
porous electrodes in which liquid, solid and gaseous phases are
present and the electrically conductive catalyst catalyzes the
electrochemical reaction between the liquid phase and the gaseous
phase.
[0017] Catalyst-based gas diffusion electrodes which are known in a
similar way from industrial chloralkali electrolysis are preferably
used for the electrochemical utilization of carbon dioxide. The
catalyst-based gas diffusion electrode can either be in contact
with a liquid, salt-containing electrolyte or in a special case can
rest directly against the separator membrane. In the latter case,
ionic attachment of the catalyst particles to the membrane is
necessary since the membrane is used as solid electrolyte in this
mode of operation.
[0018] The gas diffusion electrodes used in CO.sub.2 reduction
usually consist of a mixture of an inorganic metal catalyst (Ag,
Au, Cu, Pb, etc.) and an organic binder (PTFE, PVDF, PFA, FEP,
PFSA). The prepared electrodes are characterized by a high
connectivity of the pores and a broad pore radius distribution. The
use of gas diffusion electrodes in the electroreduction of CO.sub.2
in aqueous electrolyte solutions is possible within a relatively
narrow process window over a time of >1000 hours.
[0019] Anode space and cathode space of electrolysis cells suitable
for the electrochemical reduction of carbon dioxide are typically
kept separate from one another in a CO.sub.2 electrolyzer by means
of a cation-selective membrane, an anion-selective membrane or a
diaphragm. This prevents undesirable mixing of the gaseous
materials of value formed at the cathode and at the anode.
[0020] Passage of the electrolyte used through the cathode occurs
as a result of two driving forces. One driving force is the
electrostatic attraction of the electrolyte cations. Secondly,
anionic species (generally hydrogencarbonate ions) are generated at
the cathode and require a cation for balancing the charge. This
results in a concentration gradient which leads to penetration of
cations into the electrode.
[0021] However, the extent of this passage frequently goes beyond
the degree necessary for ionic attachment. Owing to electroosmosis,
the electrolyte also gets to the side of the electrode facing away
from the electrolyte chamber. This leads in a limiting case to
blockage of the pores of the electrode, which results in an
undesirable undersupply of CO.sub.2 to the catalyst.
[0022] As a further limiting case, substantial passage of the
aqueous medium through the pores, which contributes to flooding of
the pore system and likewise to an undersupply of CO.sub.2 to the
catalyst, can be observed. These problems are frequently observed
in the case of electrolyzer constructions in which the cathode is
in direct contact with a liquid, salt-containing electrolyte.
[0023] A further problem associated with this variant is flooding
of the pores with electrolyte. A known cause of the penetration of
electrolyte into the pores of the electrode is the hydrostatic
pressure of the water column in the electrolyte gap, which limits
the industrial construction height of the electrolysis cells.
Furthermore, increasing salt crystallization can be observed in the
region of the side facing away from the electrolyte during
operation, and this leads firstly to blockage of the pores of the
electrode, so that an undersupply of CO.sub.2 to the catalyst is
also the consequence here. Furthermore, substantial passage of the
aqueous medium through the pores, which contributes to flooding of
the pore system and likewise to an undersupply of CO.sub.2 to the
catalyst, is observed.
[0024] A stable operating state is achieved when the limiting cases
mentioned are avoided. Consequently, it is technically necessary to
widen the stable operating window for industrial use of the
technology in order to ensure more efficient conversion of the
CO.sub.2 in long-term operation of large cells and to avoid the
above-described problems.
[0025] In the case of cell constructions without an electrolyte gap
or when using an MEA (membrane electrode assembly) having a gas
diffusion electrode as cathode, severe salt formation in the region
of the interface between gas diffusion electrode (cathode) and the
separator membrane can occur during electrolysis operation, so that
stable electrolysis operation is not ensured.
[0026] A cause of this is the above-described formation of
hydrogencarbonate salts from the cations transported through the
membrane and the hydrogencarbonate ions formed at the cathode.
Without liquid electrolytes, these salts cannot be removed. The
accumulation of the electrolyte cations in the region of the
interface is attributable to electroosmosis. The concentration
gradient cannot be dissipated on the electrode side since a
catalyst-based gas diffusion electrode has only a very poor ionic
conductivity.
[0027] In summary, the current densities of previously known
methods without gas diffusion electrodes are far below the values
of <100 mA/cm.sup.2 relevant for economical use. Industrially
relevant current densities can be achieved using gas diffusion
electrodes. This is known from the existing prior art, for example
for industrially operated chloralkali electrolyses.
[0028] Thus, silver/silver oxide/PTFE-based gas diffusion
electrodes have been used industrially in recent times for the
production of sodium hydroxide in the existing chloralkali
electrolysis process (oxygen-depolarized electrodes). The
efficiency of the chloralkali electrolysis process was able to be
increased by 30-40% compared to a conventional electrode. The
methodology of embedding catalyst in PTFE is known from many
publications and patents.
[0029] The method of embedding catalyst in PTFE is known from many
publications. The methodology of the "dry process" is based on a
roller calendering process of PTFE/catalyst powders. The technique
underlying the method is attributable to EP 0 297 377 A2, in which
Mn.sub.2O.sub.3-based electrodes for batteries were produced.
[0030] In DE 3 710 168 A1, reference is made for the first time to
the use of the dry process in the preparation of metallic
electrocatalyst electrodes. The technique was also used in patents
for producing silver-based (silver(I) oxide or silver(II) oxide)
gas diffusion electrodes (oxygen-depolarized electrodes).
[0031] EP 2 444 526 A2 and DE 10 2005 023 615 A1 disclose mixtures
having a binder content of 0.5-7%. As carrier, mention was made of
Ag or nickel gauzes having a wire diameter of 0.1-0.3 mm and a mesh
opening of 0.2-1.2 mm. The powder is applied directly to the gauze
before it is fed to the roller calender.
[0032] DE 10 148 599 A1 and EP 0 115 845 B1 describe a similar
process in which the powder mixture is firstly extruded to give a
sheet or film which is pressed onto the gauze in a further
step.
[0033] DE 10 2015 215 309 A1 relates to a gas diffusion electrode
comprising a copper-containing support and a first layer comprising
at least copper and at least one binder, wherein the layer
comprises hydrophilic and hydrophobic pores and/or channels. Here,
a plurality of layers are applied individually in the form of
mixtures to the support and then rolled on together. The particle
sizes of copper and binder are set suitably. The mechanical
stressing of the binder by the rolling process leads to cros
slinking of the powder by formation of binder channels, for example
PTFE fibrils. The degree of fibrillation of the binder correlates
directly with an applied shear rate.
[0034] Owing to the lower mechanical stability, the latter method
is less suitable than the above-described single-stage process. EP
2 410 079 A2 describes the single-stage process for producing a
silver-based oxygen-depolarized electrode with addition of metal
oxide additives such as TiO.sub.2, Fe.sub.3O.sub.4,
Fe.sub.2O.sub.3, NiO.sub.2, Y.sub.2O.sub.3, Mn.sub.2O.sub.3,
Mn.sub.5O.sub.8, W0.sub.3, CeO.sub.2 and also spinels such as
CoAl.sub.2O.sub.4, Co(AlCr).sub.2O.sub.4 and also inverse spinels
such as (Co,Ni,Zn).sub.2 (Ti,Al)O.sub.4, perovskites such as
LaNiO.sub.3, ZnFe.sub.2O.sub.4.
[0035] Additions of silicon nitride, boron nitride, TiN, AN, SiC,
TiC, CrC, WC, Cr.sub.3C.sub.2, TiCN have likewise been found to be
suitable, and oxides of the type ZrO.sub.2, WO.sub.3 were
identified as being particularly suitable. The materials are
expressly said to be fillers without catalytic activity. The
objective here is expressly the reduction of the hydrophobic
character of the electrode.
[0036] DE 10 335 184 A1 discloses further catalysts which can
alternatively be used for oxygen-depolarized electrodes: noble
metals, e.g. Pt, Rh, Ir, Re, Pd, noble metal alloys, e.g. Pt-Ru,
noble metal-containing compounds, e.g. noble metal-containing
sulfides and oxides, and also Chevrel phases, e.g.
Mo.sub.4Ru.sub.2Se.sub.8 or Mo.sub.4Ru.sub.2S.sub.8, with these
also being able to contain Pt, Rh, Re, Pd, etc.
[0037] Known Cu-based gas diffusion electrodes for producing
hydrocarbons on the basis of CO.sub.2 are mentioned, for example,
in the studies by R. Cook [J. Electrochem. Soc., Vol. 137, No. 2,
1990]. There, mention is made of a wet chemical process based on a
PTFE 30B (suspension)/Cu(OAc).sub.2/Vulkan XC 72 mixture. The
method describes how a hydrophobic gas transport layer is applied
by means of three coating cycles and a catalyst-comprising layer is
applied by means of three further coating operations.
[0038] After each layer, a drying phase (325.degree. C.) with a
subsequent static pressing operation (1000-5000 Psi) is carried
out. A Faraday efficiency of >60% and a current density of
>400 mA/cm.sup.2 were reported for the electrode obtained.
However, reproduction experiments indicated subsequently as
comparative examples demonstrate that the static pressing process
described does not lead to stable electrodes. Likewise, an adverse
influence of the added Vulcan XC 72 was likewise established, so
that once again no hydrocarbons were obtained.
[0039] The calendering methods described lead to highly porous
single-layer electrodes which are characterized by a low flow
resistance. As a result of the high porosity (50-70%) and the large
pore opening radii brought about by such a method of production,
the gas diffusion electrodes prepared in this way have a very
narrow operating window when used for CO.sub.2 electrolysis in
aqueous electrolytes. This is usually characterized by cations such
as Li+, K+, Na+ and Cs+ of the electrolyte having penetrated into
the porous structure due to the electric attraction of the cathode
and there forming hydrogen carbonates with OH ions formed and
absorbed CO.sub.2 according to the following reaction equation,
which hydrogen carbonates usually precipitate because of the high
salt content of the electrolyte:
M.sup.++CO.sub.2+OH.sup.-.fwdarw.MHCO.sub.3.dwnarw.
[0040] As a further undesirable consequence, passive penetration of
water as a result of diffusion along the concentration gradient
(osmosis) is observed. This effect is also known as the "water
entry pressure effect". As a result of the salt formation
described, complete blockage of the pore structure can occur,
depending on the amount of salt and moisture content. Apart from
complete blockage by salt crystals, there is the possibility of
complete flooding with electrolyte, so that this exits continuously
at the rear side.
[0041] Both limiting states lead to breakdown of stable operation
and therefore have a direct effect on the product Faraday
efficiencies determined and on the achievable current density. The
latter phenomenon is also known from the field of chloralkali
electrolysis and has been considered to be critical in DE 10 2010
054 643 A1 and EP 2 398 101 A1 since the liquid passing through can
form a continuous film on the rear side, which film prevents
further passage of gas into the pore system.
[0042] In order to ensure an ideal operating state of a gas
diffusion electrode, stable formation of the three-phase boundary
(catalyst/electrolyte/gas) has to be ensured. In the case of
complete flooding with electrolyte, CO.sub.2 mass transfer is
virtually impossible, so that the formation of hydrogen
predominates in this operating state. In the case of partial
flooding with electrolyte, only a partial undersupply with CO.sub.2
is present. The two states can be converted into one another by
pressure influences (differential pressure between the electrolyte
and the gas space). Complete suppression of flooding of the
electrode, i.e. passage of electrolyte, cannot be prevented by
means of the two above-described systems.
[0043] A further criterion for operation of a gas diffusion
electrode is the bubble formation point which because of the high
porosity is very low with values in the range from 5 mbar to 20
mbar in the case of calendered electrodes. Electrodes having low
bubble formation points react relatively strongly to pressure
fluctuations, so that regulation of the differential pressure
(buildup pressure of the CO.sub.2 behind the electrode) by means of
a differential pressure regulator is complicated in an industrial
application. As in DE 10 2013 011 298 A1, a complicated regulating
loop with the regulation parameters gas composition, pressure and
volume flow becomes necessary.
SUMMARY OF INVENTION
[0044] It is therefore an object of the invention to provide a
possibility by means of which a targeted electrochemical
utilization of CO.sub.2 can be implemented in a widened operating
window.
[0045] This object is achieved according to the invention by the
features of the independent claims. Advantageous embodiments of the
invention are set forth in the dependent claims and the following
description.
[0046] The gas diffusion electrode of the invention is used for
utilization of carbon dioxide and comprises a metallic support and
an electrically conductive catalyst layer which has been applied to
this metallic support and has hydrophilic pores and/or channels and
hydrophobic pores and/or channels, wherein the catalyst layer
comprises metallic particles which are coated at least in
subregions with a polymeric binder.
[0047] The gas diffusion electrode of the invention is both
CO-selective and C.sub.2H.sub.4-selective. Due to the at least
partially precoated metallic particles, the penetration of the
electrolyte is prevented by a very strongly pronounced hydrophobic
character of the electrode. The hydrophobicization of the catalyst
itself is critical to this.
[0048] The metallic particles are for this purpose coated with
binder fibers or binder material fibrils before application to the
support, as a result of which the hydrophobicity of the pores in
the catalyst layer is increased. The penetration of the electrolyte
into the electrode is prevented in this way and stable formation of
the three-phase boundary between the catalyst layer, the
electrolyte and the respective gas is ensured.
[0049] Overall, coating of the metallic particles (advantageously
silver particles) with the binder material (advantageously PTFE)
offers the following advantages:--Hydrophobicization of the
catalyst and thus suppression of electroosmosis.--The hydrophobic
and hydrophilic regions make the presence of a three-phase boundary
at each metallic particle possible, as a result of which precise
localization of the reaction zone is not absolutely necessary.--The
long-term stability of the gas diffusion electrode is significantly
improved by the inert binder polymer.--The fibrillation allows a
high loading with the polymeric binder material above 20% by
weight. This is not possible in the case of purely random mixing of
metallic particles and binder material since the particles are
insulated here.
[0050] In other words, the binder material in the present case
functions not only as an "adhesive" but, as a result of the at
least partial coating of the metallic particles (catalyst
particles), also prevents undesirable electroosmosis in a targeted
manner, so that no electrolyte gets to the side of the gas
diffusion electrode facing away from the electrolyte chamber. The
at least partially coated catalyst particles are advantageously
part of a mixture which together with the binder material not
adhering to catalyst particles form the catalyst layer of the gas
diffusion electrode. The coating on the metallic particles is
advantageously formed by fibrils formed as a result of the
process.
[0051] The CO- and C.sub.2H.sub.4-selective gas diffusion electrode
also satisfies the following requirements which block the passage
of electrolyte through the gas diffusion electrode and are
accordingly necessary for the selective product formation according
to the invention.--Accessibility of the catalyst particles for the
feedgas CO.sub.2 via the hydrophobic pores.--Hydrophilic regions in
the catalyst layer which allow contact between the electrolyte and
the catalyst particles.--High electrical conductivity of the gas
diffusion electrode or the catalyst layer formed, and also
homogeneous distribution of the potential over the entire electrode
area (potential-dependent product selectivity).--High chemical and
mechanical stability in electrolysis operation (suppression of
crack formation and corrosion).--Defined porosity with a suitable
ratio between hydrophilic and hydrophobic channels and pores in the
direct vicinity of one another to ensure CO.sub.2 availability in
the simultaneous presence of H.sup.+ ions.
[0052] Preference is given to all particles present being part of
the three-phase boundary, so as to be able to achieve high current
densities. Furthermore, the pore system in the catalyst layer
displays sufficient absorption of intermediates to ensure further
reaction or dimerization/oligomerization. The reaction zone is
advantageously located directly on the side of the gas diffusion
electrode facing the electrolyte.
[0053] Furthermore, the catalyst layer satisfies, in particular,
the following requirements in order to ensure the electrochemical
reduction of CO.sub.2 to ethylene:--Uniform particle size with high
specific surface area--Dendritic morphology without isolated
centers or clusters--High purity without traces of foreign
transition metals or carbon constituents such as soot or carbonized
material--Use of electrochemically stable oxides for stabilizing
the structural defects and high selectivity and long-term
stability--Low overvoltage for the reduction of CO.sub.2.
[0054] In a particularly advantageous embodiment of the invention,
the catalyst layer has a bubble formation point ("bubble point")
above 40 mbar, in particular in the range from 80 mbar to 150 mbar.
The value of the bubble formation point indicates the pressure
which is necessary to push liquid out of the pores of the catalyst
layer. The larger the pore, the smaller the pressure required to
make it free. Air which travels through the empty pore is detected
as bubbles. The differential pressure necessary to push out the
first bubble is defined as the bubble formation point.
[0055] The flooding pressure ("wetting point") of the catalyst
layer is advantageously above 150 mbar, advantageously in the range
from 200 mbar to 1000 mbar. The penetration of fluids into the
catalyst layer can thus occur only at pressures which are
significantly higher compared to the prior art because of the
precoated metallic particles. Flooding of the gas diffusion
electrode is effectively prevented in this way.
[0056] Preference is given to using silver particles as metallic
particles. As an alternative, copper particles are advantageous as
metallic particles. Regardless of the type of metallic particles
used, these have such a nature that the binder material used, in
particular in the form of fibers or fibrils, at least partially
wraps around the particles during production of a particle/binder
mixture (for formation of the catalyst layer).
[0057] The average particle diameter d.sub.50 of the metallic
particles is advantageously in the range from 1 .mu.m to 10 .mu.m
and more advantageously from 2 .mu.m to 5 .mu.m. Spherical
particles are advantageously used as metallic particles. The
metallic particles advantageously have a BET surface area in the
range from 0.1 m.sup.2/g to 10 m.sup.2/g. The BET surface area is
calculated according to BET=6/(.delta.(Ag)/d(metallic
particles).
[0058] The catalyst layer advantageously comprises promoters which
interact with the metallic particles to improve the catalytic
activity of the gas diffusion electrode. The catalyst layer
advantageously contains at least one metal oxide which
advantageously has a lower reduction potential than the evolution
of ethylene, so that the formation of ethylene from CO.sub.2 by
means of the gas diffusion electrode of the invention is made
possible. Furthermore, the metal oxides are advantageously not
inert but instead should advantageously represent hydrophilic
reaction sites which can serve to provide protons.
[0059] The polymeric binder material advantageously has a strongly
pronounced shear-thinning behavior, so that fiber formation takes
place during the mixing process. The fibers or fibrils of the
polymeric binder material which are formed during the mixing
process wrap around or become laid around the metallic particles
without completely enclosing the surface. The binder polymer is
advantageously stable in a strongly alkaline environment.
[0060] PTFE (polytetrafluoroethylene) is advantageously used as
polymeric binder (binder polymer). As powders, Dyneon.RTM. TF 9205
and Dyneon TF 1750 have been found to be particularly useful.
Particular advantage is given to using from 0.1 to 30% by weight of
the polymeric binder material, advantageously from 5 to 25% by
weight and more advantageously from 15 to 20% by weight. The
average particle diameter (d.sub.50) of the polymeric binder
material is advantageously in the range from 0.5 .mu.m to 20
.mu.m.
[0061] After production of the appropriate catalyst/binder mixture,
this is applied to the metallic support and subsequently
consolidated. The catalyst layer having the appropriate pores or
channels is formed here. The porosity of the catalyst layer here is
advantageously in the range from 60% to 80%. The values of the
porosity of the catalyst layer here relate to the proportion of the
free spaces (pores and/or channels) within the catalyst layer
relative to the volume of the catalyst layer.
[0062] The ratio of hydrophilic pores and/or channels and
hydrophobic pores and/or channels in the catalyst layer is
advantageously in the range from 50:50 to 20:80. Here, the ratio
within the catalyst layer can have been shifted in favor of the
hydrophilic pores. The base layer advantageously has exclusively
hydrophobic regions.
[0063] As metallic support, advantage is given to using a gauze
having a mesh opening in the range from 0.3 mm to 1.4 mm. This
makes it possible to ensure both a satisfactory mechanical
stability and also the functionality as gas diffusion electrode,
for example in respect of a high electrical conductivity. As an
alternative, the support can also be in the form of parallel wires
for the purposes of the invention.
[0064] Depending on the metallic particles used, a
silver-containing or copper-containing gauze (or a corresponding
sheet-like structure made of wire) is advantageously used as
support. The gauze used as metallic support, in particular the
silver gauze, advantageously has a wire diameter in the range from
0.1 mm to 0.25 mm.
[0065] The process of the invention serves to produce a gas
diffusion electrode for utilization of CO.sub.2. The process
comprises production of a mixture of metallic particles and at
least one binder to form a mixture, application of the mixture to a
metallic support and embedding of the applied mixture into a
metallic support. According to the invention, the metallic
particles are coated at least in subregions with the polymeric
binder during the production of the mixture.
[0066] As a result of the production of a mixture of metallic
particles and binder material before application of the mixture,
the desired partial coating of the metallic particles with fibers
or fibrils is achieved, so that an increased hydrophobicity of the
boundary layer of the gas diffusion electrode is achieved. The
mixing procedure and the at least partial coating of the metallic
particles associated therewith is the property-dominating process
step for the gas diffusion electrode.
[0067] The mixing time depends critically on the properties of the
catalyst powder, i.e. the metallic particles. The hardness and the
specific surface area of the metallic particles are of particular
significance here. The catalyst powder and the binder material are
advantageously present in a homogeneous mixture before application
of high shear forces.
[0068] The catalyst layer is advantageously produced with a bubble
formation point above 40 mbar and in particular in the range from
80 mbar to 150 mbar. Further advantage is given to the catalyst
layer being produced in such a way that the flooding pressure of
the catalyst layer is above 150 mbar, advantageously in the range
from 200 mbar to 1000 mbar.
[0069] For the purposes of the invention, the application of the
mixture can be carried out in various ways. Suitable methods are,
for example, sprinkling on, sieving on, doctor blade coating or the
like.
[0070] In an advantageous embodiment, the mixture is embedded by
means of an extraction process into the support. The extraction
process leads to electrodes having a bubble point of 100-250 mbar.
In the case of calendered (rolled) electrodes, the bubble point of
the hydrophobic base layer is in the range from 10 mbar to 20 mbar.
Application of a hydrophilic catalyst layer makes it possible to
increase the bubble point by up to 200 mbar.
[0071] In the extraction process, a suspension is produced from the
metallic, at least partially coated particles, the polymeric binder
and a solvent and this suspension is applied to the support.
Preference is given to using a fine-meshed polymer gauze composed
of PP as support. The use of metal gauzes is also possible as an
alternative. Suitable solvents have been found to be, in
particular, N-methyl-2-pyrrolidone, dimethyl sulfoxide and
dimethylformamide. As an alternative, the use of
.gamma.-butyrolactone is also possible. The support laden with the
suspension is then dipped into a precipitation bath for the
polymeric binder, filled with a nonsolvent for the polymeric
binder. There, replacement of the solvent of the suspension by
nonsolvent occurs as a result of diffusion and phase separation
thus occurs. The polymeric binder solidifies and forms a porous
matrix.
[0072] As an alternative, advantage is also given to the mixture
being embedded by dry rolling-on into the metallic support. The
mechanical stressing of the binder material by the rolling process
leads to cros slinking of the mixture as a result of formation of
binder channels, for example PTFE fibrils. The attainment of this
state is particularly important in order to ensure a suitable
porosity and mechanical stability of the electrode.
[0073] The mixture is advantageously rolled on with a ratio between
the exit thickness H and gap width H.sub.0 in the range from 1 to
1.5, with the rate of rotation of the roller being in the range
from 1.2 to 2. The roller advantageously rotates at a rate of
rotation in the range from 0.5 rpm to 2 rpm. Application is
advantageously carried out at a flow rate Q in the range from 0.07
m/min to 0.3 m/min. In addition, heating of the rollers can assist
the flow process. The advantageous temperature range is from room
temperature to 200.degree. C. and more advantageously
40-100.degree. C.
[0074] The degree of fibrillation of the binder material
(structural parameter) correlates directly with the applied shear
rate since the binder material, in particular a polymer, behaves as
shear-thinning (non-Newtonian) fluid on rolling out during
application. After application, the layer obtained has an elastic
character due to the fibrillation. This structural change is
irreversible, so that this effect can no longer be increased
subsequently by further rolling-out, but instead the layer is
damaged on further action of shear forces due to the elastic
behavior. A particularly high degree of fibrillation can
disadvantageously lead to a layer-side rolling together of the
electrode, so that excessively high contents of binder should be
avoided.
[0075] The advantages and advantageous embodiments described for
the gas diffusion electrode of the invention apply equally to the
process of the invention and can accordingly be carried over
analogously to this.
[0076] The electrolysis cell of the invention comprises a gas
diffusion electrode as per one of the above-described embodiments.
The gas diffusion electrode is advantageously used as cathode here.
The gas diffusion electrode is advantageously configured
specifically for operation in plate electrolyzers. The electrolysis
cell is advantageously configured on the cathode side for the
reduction of carbon dioxide.
[0077] The further constituents of the electrolysis cell, for
instance the anode, optionally one or more membranes, feed
conduit(s) and discharge conduit(s), the voltage source and also
further optional facilities such as cooling or heating devices, are
basically variable according to the invention. The same applies to
the anolytes and/or catholytes which are used in such an
electrolysis cell.
[0078] The advantages and advantageous embodiments described for
the gas diffusion electrode of the invention and the process of the
invention apply equally to the electrolysis cell of the invention
and can accordingly be carried over analogously to this.
[0079] In the following, the production of a CO-selective gas
diffusion electrode is explained in more detail.
[0080] Firstly, the catalyst precursor silver(I) oxide Ag.sub.2O
was produced by precipitation (2 AgNO.sub.3+2
NaOH.fwdarw.Ag.sub.2O+2 NaNO.sub.3+H.sub.2O). For this purpose, 200
g (1.177 mol) of silver nitrate dissolved in water were heated to
60.degree. C. A 4 M sodium hydroxide solution (160 g/l) was added
dropwise while stirring. The precipitated silver(I) oxide was
centrifuged off as product, washed until neutral and subsequently
dried.
[0081] Mixing of the catalyst particles (metallic particle) and the
binder and thus the at least partial coating of the particles were
carried out both in an Eirich mixer EL1 and also in an IKA-A10
cutter mill using cemented carbide (WC) cutters. 35 g of dry
silver(I) oxide which had been precipitated by the above-described
procedure, 10 g of purified silver powder (d.sub.50=2.mu.m-3 .mu.m)
and then 15 g of PTFE (Dyneon TF 2021) were introduced in each
case. The total loading was 60 g.
[0082] In the Eirich mixer, mixing was carried out at a speed of
rotation of the star swirler of from 2 to 7 m/sec. for 5 minutes
with the same direction of rotation of mixing vessel and swirler. A
mixing operation in the IKA cutter mill took 15 seconds and was
repeated 7 times with a pause of 15 seconds each time. After each
passage, manual stirring was carried out. The resulting powder
mixtures were stored airtight in closed containers.
[0083] Table 2 shows the relationships between various mixing times
and the respective Vickers hardness for the Eirich mixer and the
IKA cutter mill.
TABLE-US-00002 TABLE 2 Mixing time for PTFE (Dyneon TF 2021) as a
function of the Vickers hardness of the metallic particles used
Vickers hardness Mixing time in IKA A10 Mixing time in EL1 Eirich
(kp/mm.sup.2) (widia) (widia) 5-30 7 * 15 sec. 30 * 15 sec. 50-100
6 * 15 sec. 20 * 15 sec. 1000-2000 3 * 15 sec. 10 * 15 sec.
[0084] The dry calendering process was used here for the subsequent
production of the gas diffusion electrode. The premixed power
mixture obtained (composed of the at least partially coated
metallic particles and the binder material) was applied to a metal
gauze. A woven silver wire mesh (mesh: 60.times.60) having a mesh
opening of 0.296 mm and a wire diameter of 0.127 mm was used for
this purpose. The weight of the gauze was 0.48 kg/m.sup.2 and it
had a size of 2.13 m.times.305 mm.
[0085] The woven silver wire mesh was firstly degreased by means of
acetone and the surface was pickled by means of 2N HNO.sub.3 (30-60
min). The woven silver wire mesh was blown dry with compressed air
and provided on one side with a stuck-on Capton adhesive tape. The
woven wire mesh provided with the adhesive tape was fixed flat by
means of a vacuum plate and a stainless steel frame having a
thickness of 0.5 mm and an open area of L.sub.x=120 mm*60 mm was
placed on top.
[0086] The application of the premixed powder was carried out by
sieving-on through a PP sieve having a mesh opening of w=1 mm, so
that only a thin uniform covering was firstly achieved. The powder
was then stirred up in the sieve using a plastic spatula and
applied continuously until the height of the frame had been
reached. The excess amount of powder on the frame periphery is
removed completely.
[0087] A striking tool having a rounded striking edge was used for
this purpose. This tool was held at an angle of 10.degree. to the
substrate and quickly moved back and forth during the striking
operation. After striking, further sieving with repetition of the
striking operation was carried out in loosely packed regions until
a uniform surface had been obtained.
[0088] To embed the mixture and thus form a corresponding gas
diffusion electrode, both the extraction process (a) and also the
dry calendering process (b) were used. The two processes are
explained separately below.
[0089] (a) Production of a Gas Diffusion Electrode by Means of the
Extraction Process
[0090] To produce a gas diffusion electrode by means of the
extraction process, 48 g of silver powder (2-3 .mu.m) were mixed
with 2 g of PTFE Dyneon TF 2021 7 times for 15 seconds each time in
an IKA A10 cutter mill. Furthermore, 14 g of PVDF were dissolved in
68 ml of N-methyl-2-pyrrolidone (NMP) at 80.degree. C. with
stirring (20% by weight). The mixture of 48 g of silver power and 2
g of PTFE was added to this solution and dispersed twice for 30
seconds using an Ultraturrax. The resulting dispersion was applied
by means of doctor blade coating to a PP gauze and the NMP was
extracted in a water/isopropanol bath (50% by volume of
isopropanol). Curing was then carried out in a second water bath,
with the oligomers finally being linked together.
[0091] (b) Production of a Gas Diffusion Electrode by Means of the
Dry Calendering Process
[0092] To embed the mixture by means of the dry calendering
process, the mixture was rolled into the gauze structure. In a
two-roll calender model Dima B64E having a roll width of 130 mm and
a roll diameter of 64 mm, film extrusion is carried out at a tape
speed of 30 cm/min at a gap thickness of 0.6 mm. The Capton film is
then removed and the roller gap is reduced to 0.3 mm and the
electrode is rolled again.
[0093] Rolling of the coated particles into the gauze structure is
expressly desired in order to ensure a high mechanical stability of
the electrode, which is not the case in the abovementioned
two-stage process where the pre-extruded film only rests on the
gauze. The mechanical stressing of the polymer particles by the
rolling process leads to cros slinking of the powder by formation
of PTFE fibrils. This ensures the suitable porosity and mechanical
stability of the electrode.
[0094] The electrochemical activation of the electrode was carried
out in 2.5 M KOH at a current density of 200 mA/cm.sup.2 using a
platinum counterelectrode. The electrode spacing was 2 cm. After
switching on the current, an immediate reduction of the silver(I)
oxide over the full area occurred. The clamping voltage rose from
3.8 V to 5.8 V and remained constant at this value. The electrode
was taken out after an activation time of 30 minutes and the
surface was rinsed under flowing deionized water with rubbing, so
that the dark veil formed could be removed. The electrode was
washed with isopropanol and ether to simplify drying.
BRIEF DESCRIPTION OF THE DRAWINGS
[0095] Working examples of the invention will be described in more
detail below with the aid of a drawing. The drawing shows:
[0096] FIG. 1 a schematic depiction of a section of a catalyst
layer as per the prior art,
[0097] FIG. 2 a schematic depiction of a section of a further
catalyst layer as per the prior art,
[0098] FIG. 3 a schematic depiction of a section of a catalyst
layer according to the invention as per the prior art,
[0099] FIG. 4 a representation of the course over time of the
Faraday efficiency of a gas diffusion electrode,
[0100] FIG. 5 a representation of the course over time of the
Faraday efficiency of a further gas diffusion electrode,
[0101] FIG. 6 a schematic depiction of an electrolysis cell having
a gas diffusion electrode,
[0102] FIG. 7 a schematic depiction of an alternative electrolysis
cell having a gas diffusion electrode, and
[0103] FIG. 8 a schematic depiction of a further electrolysis cell
having a gas diffusion electrode.
DETAILED DESCRIPTION OF INVENTION
[0104] FIGS. 1 to 3 each schematically show sections of gas
diffusion electrodes 1, 3, 5 with corresponding catalyst layers 7,
9, 11. Each of the catalyst layers has been applied to a metallic
support 12 (in the present case merely indicated by an arrow) and
embedded therein.
[0105] The catalyst layers 7 and 9 of the gas diffusion electrodes
1, 3 as per FIGS. 1 and 2 show examples of the prior art; the
section as per FIG. 3 concerns a gas diffusion electrode 5
according to the invention.
[0106] It can be seen from FIG. 3 that the gas diffusion electrode
5 produced by the process of the invention comprises a catalyst
layer 11 with at least partially coated metallic particles 13. The
particles are coated in subregion 16 with PTFE as polymeric binder
material 15 which partially encloses the particles 13 in the form
of fibrils 17.
[0107] In contrast thereto, the fibrils 17 of FIG. 1 are arranged
between mixed molecules of the binder material 15 and the metallic
particles 13. The metallic particles are not fibrilated. The
depiction here is of a single-layer gas diffusion electrode 1.
[0108] In FIG. 2, the gas diffusion electrode 3 has two separate
layers 19, 21. Fibrils 17 are in each case likewise arranged
between the metallic particles 13 in the first layer 19 and between
the binder material molecules 15 in the second layer 21.
[0109] FIGS. 4 and 5 show representations 23, 26 of the course over
time of the Faraday efficiencies which were obtained in the
electrochemical characterization of a gas diffusion electrode 5
according to the invention. The gas diffusion electrode 5 was
produced by means of the dry calendering process.
[0110] In FIGS. 4 and 5, the Faraday efficiency [%] is in each case
shown as a function of the current density [J/mA*cm.sup.-2]. A
Faraday efficiency for CO.sub.2 of 100% (curve 24) and for H.sub.2
of 0% (curve 25) is obtained at 30.degree. C. in 0.5 M
K.sub.2SO.sub.4 and 1 M KHCO.sub.3 at 250 mA/cm.sup.2 (FIG. 4). At
30.degree. C. in 0.5 M K.sub.2SO.sub.4 and 1 M KHCO.sub.3 at 300
mA/cm.sup.2, a Faraday efficiency for CO.sub.2 of 80% (curve 27)
and for H.sub.2 of about 20% (curve 28) is obtained (FIG. 5).
[0111] Furthermore, the following was observed: the production of
the powder mixture containing 25% by weight of PTFE required a
stronger mixing apparatus than the IKA A10 because of the high
viscosity. Sieving-on of the powder mixture was associated with
greater difficulty than in the case of comparable mixtures
containing 5% by weight to 10% by weight of PTFE. The total
porosity of the electrode did not increase significantly at a
higher PTFE content.
[0112] The following relationships were able to be identified: at a
constant roller gap, the total porosity of the gas diffusion
electrode 5 could be influenced by the activation so that when the
current density was halved in the range from 50 to 400 mA/cm.sup.2
the porosity increased by about 50%. PTFE contents of 5% by weight
and 10% by weight were not sufficiently high to prevent permeation
of the electrolyte. At a PTFE content of 25%, flooding of the gas
diffusion electrode 11 in electrolysis operation could be
prevented.
[0113] FIGS. 6 to 8 schematically show various electrolysis cells
31, 33, 35 which are suitable in principle for electrochemical
reduction of CO.sub.2 using in each case a gas diffusion electrode
5 according to the invention.
[0114] The electrolysis cell 31 of FIG. 6 displays a 3-chamber
structure having an anode space I and a cathode space II which are
separated from one another by a membrane M. The cathode space II of
FIG. 6 is, by way of example, configured so that a catholyte is fed
in from below and then leaves the cathode space II in an upward
direction. As an alternative, the catholyte can also be fed in from
the top, as in the case of, for example, falling film electrodes.
At the anode A which is electrically connected to the cathode K
(gas diffusion electrode 5) by means of a current source for
providing the voltage for the electrolysis, the oxidation of a
material which is fed in from the bottom, for example with an
anolyte, takes place in the anode space I. The anolyte leaves the
anode space together with the oxidation product.
[0115] In addition, a reaction gas such as, in particular, carbon
dioxide can be conveyed through the gas diffusion electrode into
the cathode space II for reduction in the electrolysis cell 31 of
FIG. 6. Although not shown, embodiments having a porous anode are
also conceivable.
[0116] In contrast thereto, the cathode K (gas diffusion electrode
5) and a porous anode A lie directly against the membrane M in the
PEM (proton- or ion-exchange membrane) structure of the
electrolysis cell 33 shown in FIG. 7, so that the anode space I is
separated from the cathode space II.
[0117] The structure of the electrolysis cell 35 of FIG. 8
corresponds to a mixed form of the structures of FIGS. 6 and 7,
with a structure having the gas diffusion electrode 5 (as per FIG.
6) being provided on the catholyte side and a structure as per FIG.
7 being provided on the anolyte side. Of course, mixed forms or
other variants of the electrode spaces presented by way of example
are also conceivable. Embodiments without a membrane are also
conceivable. In particular embodiments, the cathode-side
electrolyte and the anode-side electrolyte can thus be identical,
so that the respective electrolysis cell/electrolysis unit can be
constructed without a membrane M. It is equally possible for the
respective electrolysis cell in such embodiments to have a membrane
M.
[0118] In particular embodiments, the distance between electrode
and membrane is very small or 0 when the membrane has a porous
configuration and includes an introduction facility for the
electrolyte. The membrane can also have a multilayer configuration
so that separate introductions of anolyte and catholyte are made
possible. Separation effects are achieved in the case of aqueous
electrolytes by, for example, the hydrophobicity of intermediate
layers. Conductivity can nevertheless be ensured when conductive
groups are integrated into such separation layers. The membrane can
be an ion-conducting membrane or a separator which effects only
mechanical separation and is permeable to cations and anions.
[0119] As a result of the use of the gas diffusion electrode 5
accordig to the invention, it is possible to construct a
three-phase electrode. For example, a gas can be supplied from
behind to the electrically active front side of the gas diffusion
electrode 5 in order to carry out an electrochemical reaction
there. As an alternative, flow can also occur only to the rear side
of the gas diffusion electrode 5, with a gas such as, in particular
CO.sub.2 being conveyed past the rear side of the gas diffusion
electrode 5 relative to the electrolyte. The gas then penetrates
through the pores of the gas diffusion electrode 5 and the product
is discharged at the rear side.
[0120] The gas flow in the case of flow on the rear side is
advantageously in the direction opposite to the flow of the
electrolyte, so that any liquid which has been pushed through can
be transported away. Here too, a gap between the gas diffusion
electrode and the membrane is advantageous as electrolyte
reservoir.
[0121] Further details regarding particular embodiments of
electrolysis cells according to the invention having gas diffusion
electrodes for utilization of carbon dioxide may also be found in
DE 10 2015 215 309 A1.
[0122] The above embodiments, configurations and further
developments can, if it serves a purpose, be combined with one
another in any way. Further possible configurations, further
developments and implementations of the invention also encompass
combinations not explicitly mentioned of features of the invention
described above or in the following in respect of the working
examples. In particular, a person skilled in the art will also add
individual aspects as improvements or supplementations to the
respective basic form of the present invention.
LIST OF REFERENCE NUMERALS
[0123] 1 Gas diffusion electrode (as per the prior art)
[0124] 3 Gas diffusion electrode (as per the prior art)
[0125] 5 Gas diffusion electrode
[0126] 7 Catalyst layer
[0127] 9 Catalyst layer
[0128] 11 Catalyst layer
[0129] 12 Metallic support
[0130] 13 Metallic particles
[0131] 15 Binder material
[0132] 16 Coated subregions
[0133] 17 Fibrils
[0134] 19 First layer of gas diffusion electrode
[0135] 21 Second layer of gas diffusion electrode
[0136] 23 Representation of Faraday efficiency
[0137] 24 Faraday efficiency for CO.sub.2
[0138] 25 Faraday efficiency for H.sub.2
[0139] 26 Representation of Faraday efficiency
[0140] 27 Faraday efficiency for CO.sub.2
[0141] 28 Faraday efficiency for H.sub.2
[0142] 31 Electrolysis cell
[0143] 33 Electrolysis cell
[0144] 35 Electrolysis cell
* * * * *