U.S. patent application number 15/751216 was filed with the patent office on 2018-08-16 for method for monitoring a process for powder-bed based additive manufacturing of a component and such a system.
This patent application is currently assigned to Siemens Aktiengesellschaft. The applicant listed for this patent is Siemens Aktiengesellschaft. Invention is credited to Ralf Krause, Anna Maltenberger, Christian Reller, Bernhard Schmid, Gunter Schmid.
Application Number | 20180230612 15/751216 |
Document ID | / |
Family ID | 56511563 |
Filed Date | 2018-08-16 |
United States Patent
Application |
20180230612 |
Kind Code |
A1 |
Krause; Ralf ; et
al. |
August 16, 2018 |
Method For Monitoring A Process For Powder-Bed Based Additive
Manufacturing Of A Component And Such A System
Abstract
A gas diffusion electrode and electrolysis cells containing gas
diffusion electrodes are provided. The gas diffusion electrodes
include a copper-containing carrier, and first and second layers.
The first layer comprising at least copper and at least one binder
having hydrophilic and hydrophobic pores. The second layer
comprising copper and at least one binder. The second layer present
atop the carrier and the first layer atop the second layer, wherein
the content of binder in the first layer is less than the binder in
the second layer.
Inventors: |
Krause; Ralf;
(Herzogenaurach, DE) ; Maltenberger; Anna;
(Leutenbach, DE) ; Reller; Christian; (Minden,
DE) ; Schmid; Bernhard; (Erlangen, DE) ;
Schmid; Gunter; (Hemhofen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Aktiengesellschaft |
Muenchen |
|
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
Muenchen
DE
|
Family ID: |
56511563 |
Appl. No.: |
15/751216 |
Filed: |
July 19, 2016 |
PCT Filed: |
July 19, 2016 |
PCT NO: |
PCT/EP2016/067165 |
371 Date: |
February 8, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/0405 20130101;
C25B 3/04 20130101; C25B 11/0489 20130101; C25B 11/0415 20130101;
C25B 11/035 20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 11/03 20060101 C25B011/03; C25B 3/04 20060101
C25B003/04 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 11, 2015 |
DE |
10 2015 215 309.6 |
Claims
1. A gas diffusion electrode comprising: a copper-containing
carrier, and a first layer comprising at least copper and at least
one binder, the first layer comprising hydrophilic and hydrophobic
pores, a second layer comprising copper and at least one binder,
the second layer present atop the carrier and the first layer atop
the second layer, wherein the content of binder in the first layer
is less than the binder in the second layer, and wherein the second
layer comprises 3-30% by weight of binder, and the first layer
comprises 0-10% by weight of binder.
2. The gas diffusion electrode as claimed in claim 1, wherein the
first layer does not comprise charcoal-based and/or carbon
black-based fillers.
3. The gas diffusion electrode as claimed in claim 1, wherein the
first layer does not comprise surface-active substances.
4. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises at least 40 at % of copper, based on the
layer.
5. The gas diffusion electrode as claimed in claim 1, wherein the
copper-containing carrier is a copper mesh.
6. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises at least one metal oxide having a lower
reduction potential than the evolution of ethylene selected from
the group consisting of ZrO.sub.2, Al.sub.2O.sub.3, CeO.sub.2,
Ce.sub.2O.sub.3, ZnO.sub.2, and MgO.
7. The gas diffusion electrode as claimed in claim 1, wherein the
second layer partly penetrates the first layer.
8. A process for producing a gas diffusion electrode, comprising:
producing a first mixture comprising at least copper and optionally
at least one binder, producing a second mixture comprising at least
copper and at least one binder, applying the second mixture
comprising at least copper and at least one binder to a
copper-containing carrier, in the form of a sheetlike structure,
applying the first mixture comprising at least copper and
optionally at least one binder to the second mixture, and dry
rolling the first and second mixtures onto the carrier to form at
least first and second layers, wherein the proportion of binder in
the second mixture is 3-30% by weight of binder, based on the
second mixture, and wherein the proportion of binder in the first
mixture is 0-10% by weight, based on the first mixture, where the
content of binder in the first mixture is smaller than in the
second mixture.
9. The process as claimed in claim 8, wherein the copper-containing
carrier comprises a copper mesh having a mesh size w of 0.3
mm<w<2.0 mm and a wire diameter x of 0.05 mm<x<0.5
mm.
10. The process as claimed in claim 8, wherein the bed height y of
the first mixture on the carrier is in the range of 0.3
mm<y<2.0 mm.
11. The process as claimed in claim 8, wherein the gap width in the
rolling application H.sub.0 is the height of the carrier +40% to
50% of the total bed height Hf of the first mixture.
12. The process as claimed in claim 8, wherein the rolling is
effected by a calender.
13. The process as claimed in claim 8, wherein the copper content
in the mixture is at least 40 at % of copper, based on the
mixture.
14. The process as claimed in claim 8, wherein the mixture further
comprises at least one metal oxide having a lower reduction
potential than the evolution of ethylene including at least one of
ZrO.sub.2, Al.sub.2O.sub.3, CeO.sub.2, Ce.sub.2O.sub.3, ZnO.sub.2,
and MgO.
15. An electrolysis cell comprising a gas diffusion electrode as
claimed in claim 1.
16. The gas diffusion electrode as claimed in claim 1, wherein the
second layer comprises 10-30% by weight binder and the first layer
comprises 0.1-10% by weight binder.
17. The gas diffusion electrode as claimed in claim 1, wherein the
second layer comprises 10-20% by weight binder and the first layer
comprises 1-10% by weight binder.
18. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises 1-7% by weight binder.
19. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises 3-7% by weight binder.
20. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises at least one copper-rich intermetallic phase
selected from the group of systems consisting of Cu--Al, Cu--Zr,
Cu--Y, Cu--Hf, CuCe, Cu--Mg, Cu--Y--Al, Cu--Hf--Al, Cu--Zr--Al,
Cu--Al--Mg, Cu--Al--Ce with copper contents >60 at %.
21. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises one or more of copper-containing perovskites,
defect perovskites, and perovskite-related compounds.
22. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises YBa.sub.2Cu.sub.3O.sub.7 where
0.ltoreq..delta..ltoreq.1.
23. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises a compound selected from the group consisting
of CaCu.sub.3Ti.sub.4O.sub.12,
La.sub.1.85Sr.sub.0.15CuO.sub.3.930Cl.sub.0.053, and
(La,Sr).sub.2CuO.sub.4.
24. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises at least 50 at % of copper, based on the
layer.
25. The gas diffusion electrode as claimed in claim 1, wherein the
first layer comprises at least 60 at % of copper, based on the
layer.
26. The process as claimed in claim 8, wherein the
copper-containing carrier comprises a copper mesh having a mesh
size w of 0.5 mm<w<1.0 mm and a wire diameter x of 0.1
mm.times.x.ltoreq.0.25 mm.
27. The process as claimed in claim 8, wherein the bed height y of
the first mixture on the carrier is in the range of 0.5
mm.ltoreq.y.ltoreq.1.0 mm.
28. The process as claimed in claim 8, wherein the mixture further
comprises at least one copper-rich intermetallic phase selected
from the group of the systems consisting of Cu--Al, Cu--Zr, Cu--Y,
Cu--Hf, CuCe, Cu--Mg, Cu--Y--Al, Cu--Hf--Al, Cu--Zr--Al,
Cu--Al--Mg, and Cu--Al--Ce with copper contents >60 at %.
29. The process as claimed in claim 8, wherein the mixture further
comprises at least one metal for formation of a copper-rich
metallic phase selected from the group consisting of Al, Zr, Y, Hf,
Ce, Mg, Y--Al, Hf--Al, Zr--Al, Al--Mg, and Al--Ce, such that the
copper content is >60 at %.
30. The process as claimed in claim 8, wherein the mixture further
comprises at least one of copper-containing perovskites, defect
perovskites, and perovskite-related compounds.
31. The process as claimed in claim 8, wherein the mixture further
comprises YBa.sub.2Cu.sub.3O.sub.7-.delta. where
0.ltoreq..delta..ltoreq.1.
32. The process as claimed in claim 8, wherein the mixture further
comprises at least one of CaCu.sub.3Ti.sub.4O.sub.12,
La.sub.1.85Sr.sub.0.15CuO.sub.3.930Cl.sub.0.053, and
(La,Sr).sub.2CuO.sub.4.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a U.S. National Stage Application of
International Application No. PCT/EP2016/067165 filed Jul. 19,
2016, which designates the United States of America, and claims
priority to DE Application No. 10 2015 215 309.6 filed Aug. 11,
2015, the contents of which are hereby incorporated by reference in
their entirety.
FIELD OF INVENTION
[0002] The present invention relates to a gas diffusion electrode
preferably comprising a copper-containing carrier and a first layer
comprising at least copper and at least one binder and a second
layer. The (first) layer comprises hydrophilic and hydrophobic
pores and/or channels. The second layer comprising copper and at
least one binder, wherein the second layer is present atop the
carrier and the first layer atop the second layer, wherein the
content of binder in the first layer is less than in the second
layer. The present invention also relates to a process for
producing a gas diffusion electrode and to an electrolysis cell
comprising a gas diffusion electrode.
BACKGROUND OF THE INVENTION
[0003] The combustion of fossil fuels currently supplies about 80%
of global energy demand. In 2011, these combustion processes emit
about 34 032.7 million metric tons of carbon dioxide (CO.sub.2)
globally into the atmosphere. This release is the simplest way of
disposing of large volumes of CO.sub.2 as well (large brown coal
power plants exceeding 50 000 t per day).
[0004] Discussion about the adverse effects of the greenhouse gas
CO.sub.2 on the climate has led to consideration of reutilization
of CO.sub.2. In thermodynamic terms, CO.sub.2 is at a very low
level and can therefore be reduced again to usable products only
with difficulty.
[0005] In nature, CO.sub.2 is converted to carbohydrates by
photosynthesis. This process, which is divided up into many
component steps over time and spatially at the molecular level, is
reproducible on the industrial scale only with difficulty. The more
efficient route at present compared to pure photocatalysis is the
electrochemical reduction of the CO.sub.2. As in the case of
photosynthesis, in this process, CO.sub.2 is converted to a
higher-energy product (such as CO, CH.sub.4, C.sub.2H.sub.4, C1-C4
alcohols etc.) with supply of electrical energy which is preferably
obtained from renewable energy sources such as wind or sun. The
amount of energy required in this reduction corresponds ideally to
the combustion energy of the fuel and should only come from
renewable sources or utilize electricity that cannot be accepted
from the grid at that moment. However, overproduction of renewable
energies is not continuously available, but at present only at
periods of strong insolation and/or wind. However, this state of
affairs will further intensify in the near future with the further
rollout of renewable energy or will level out since the
installations will be at different sites.
[0006] Not until the 1970s was there an increased level of
systematic studies of the electrochemical reduction of CO.sub.2. In
spite of many efforts, it has not been possible to date to develop
an electrochemical system with which CO.sub.2 could be reduced with
long-term stability and in an energetically favorable manner to
competitive energy sources with sufficiently high current density
and acceptable yield. Owing to the increasing scarcity of fossil
raw material and fuel resources and the volatile availability of
renewable energy sources, research in CO.sub.2 reduction is moving
into the focus of interest to an ever greater degree.
[0007] The electrochemical reduction of CO.sub.2 to hydrocarbons,
especially to the valuable chemical raw material C.sub.2H.sub.4
(.about. 1000/t) has been described in the literature since the
1990s. There has been a significant rise in research activities
over the last few years because the availability of excess
electrical energy from non-fossil generation sources such as solar
or wind is making the storage/utilization of this energy seem
viable from an economic point of view.
[0008] For electrolysis of CO.sub.2, in general, metals are used as
catalysts, some of which are shown by way of example in table 1,
taken from Y. Hori, Electrochemical CO.sub.2 reduction on metal
electrodes, in: C. Vayenas, et al. (eds.), Modern Aspects of
Electrochemistry, Springer, New York, 2008, pp. 89-189.
[0009] Table 1 shows the typical Faraday efficiencies (FE) over
various metal cathodes. For example, CO.sub.2 is reduced virtually
exclusively to CO over Ag, Au, Zn, and to some degree over Pd, Ga,
whereas a multitude of hydrocarbons are observed as reduction
products over copper. As well as pure metals, metal alloys and also
mixtures of metal and co-catalytically active metal oxide are also
of interest, since these can increase the selectivity for a
particular hydrocarbon. However, the prior art in this regard is
not yet very developed.
TABLE-US-00001 TABLE 1 Faraday efficiencies for carbon dioxide over
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] The following reaction equations show, by way of example,
reactions at an anode and at a cathode for reduction over a copper
cathode.
[0011] Of particular interest here is the formation of valuable
ethylene. Reductions over other metals are analogous to these.
2CO.sub.2+12e.sup.-+12H.sup.+.fwdarw.C.sub.2H.sub.4+4H.sub.2O
Cathode:
6H.sub.2O.fwdarw.3O.sub.2+12H.sup.++12e.sup.- Anode:
2CO.sub.2+2H.sub.2O.fwdarw.C.sub.2H.sub.4+3O.sub.2 Overall
equation:
[0012] The individual electrode equations show that very complex
processes that have not been elucidated in detail to date are
proceeding here with, for example, CO or formate intermediates. For
each of these intermediates, a particularly preferred position at
and/or on the copper cathodes should be necessary. This means that
the catalytic activity changes according to the crystallographic
orientation of the copper surface, as shown in, for example, Y.
Hori, I. Takahashi, O. Koga, N. Hoshi, "Electrochemical reduction
of carbon dioxide at various series of copper single crystal
electrodes"; Journal of Molecular Catalysis A: Chemical 199 (2003)
39-47; or M. Gattrell, N. Gupta, A. Co, "A review of the aqueous
electrochemical reduction of CO.sub.2 to hydrocarbons at copper";
Journal of Electroanalytical Chemistry 594 (2006) 1-19.
[0013] In order to be able to provide all these crystallographic
surfaces for a high efficiency of ethylene formation at high
current density, the electrode must not consist of a smooth sheet,
but should be micro- to nanostructured.
[0014] The accessibility of such catalytically active sites limits
the formation of ethylene to a Faraday efficiency of about 20%, or
restricts the achievable current density to +/-10 mA/cm.sup.2, as
described in K. P. Kuhl, E. R. Cave, D. N. Abram, and T. F.
Jaramillo, Energy and Environmental Science 5, 7050-7059
(2012).
[0015] Furthermore, H. Yano, T. Tanaka, M. Nakayama, K. Ogura,
"Selective electrochemical reduction of CO.sub.2 to ethylene at a
three-phase interface on copper(I) halide-confined Cu-mesh
electrodes in acidic solutions of potassium halides"; Journal of
Electroanalytical Chemistry 565 (2004) 287-293, achieved current
densities in the region of 100 mA/cm.sup.2, but ethylene here was
enriched to a Faraday efficiency (FE) of about 80% in the method of
circulating the gaseous substances, and so the "intrinsic" Faraday
efficiency of the electrode cannot be determined.
[0016] In summary, the current densities of methods known from the
prior art are well below the values of relevance for economic
utilization.
[0017] For the electrochemical reduction of CO.sub.2 to ethylene,
it was possible with the aid of copper catalysts deposited in situ
to achieve current densities of 170 mA/cm.sup.2 with a Faraday
efficiency of >55% over an electrolysis time of 60 min, as shown
in in-house studies. However, in the case of electrodes produced in
this way, the selectivity of the electrode can decrease with time,
which can lead to an increase in hydrogen production. A change in
the selectivity with time can be correlated to structural
coarsening of the material, which was also observable, for example,
from microscope images. Nano-dendritic copper structures containing
both Cu.sup.0 and Cu.sup.I in the form of Cu.sub.2O were identified
as a selective catalyst.
[0018] Current densities of industrial relevance can be achieved
using gas diffusion electrodes (GDE). This is known from the
existing prior art, for example, for chloralkali electrolyses
operated on the industrial scale. The use of copper-based gas
diffusion electrodes in electrolysis cells seems to be advantageous
for an energy-efficient conversion of matter from CO.sub.2 to
hydrocarbons. One electrode-specific feature of particular interest
is the selectivity (Faraday efficiency in %) and the conversion of
matter (current density in mA/cm.sup.2).
[0019] Silver/silver oxide/PTFE (polytetrafluoroethylene)-based gas
diffusion electrodes have been used on the industrial scale in
recent times for the production of sodium hydroxide solution in
existing chloralkali electrolysis processes (oxygen-depolarized
electrodes). It was possible to increase the efficiency of the
chloralkali electrolysis process by 30-40% by comparison with
conventional electrodes. The methodology of catalyst embedding with
PTFE is known from a multitude of publications and patterns.
[0020] The known embedding methods are divided into three different
process routes: [0021] 1. Wet methods using a surfactant-stabilized
PTFE microemulsion. [0022] 2. Wet methods using a
surfactant-stabilized Nafion.RTM. microemulsion. [0023] 3. Dry
methods by calendering of premixed catalyst/PTFE mixtures.
[0024] In this context, said wet method 1. can have the
disadvantages mentioned hereinafter, aside from the fact that
examples of gas diffusion electrodes known from the literature
contain the catalyst only as an additive and consist mainly of
bound conductive charcoal (for high conversions the catalyst
loading should be high):
The suspensions or pastes that are usually applied by spraying or
bar coating generally have long drying times, which means that
continuous production with relatively large electrode areas (of
industrial relevance) is not economically possible. Excessively
rapid drying leads to cracking, called "mud cracking", within the
layers applied, which makes the electrode unusable.
[0025] The porosity of the layer applied is determined (generated)
in the wet-chemical method virtually exclusively by the evaporation
of the solvent. This process is highly solvent- or boiling
point-dependent and can lead to a high reject rate of the
electrodes produced, since the evaporation cannot be assured in a
homogeneous manner over the entire area. A further central
disadvantage is the use of surface-active substances (surfactants)
or thickeners, plasticizers, which are used for stabilization of
the particle suspensions since they cannot be removed without
residue by the corresponding drying phases or the thermal
crosslinking process.
[0026] The embedding process 2., wherein Nafion.RTM.
(perfluorosulfonic acid, PFSA) is used as binder rather than PTFE,
likewise has corresponding disadvantages, since a wet-chemical
method using appropriate surfactants is being employed here too.
Nafion.RTM. itself is a hydrophilic ionomer having highly acidic
R--HSO.sub.3 groups which can lead to unwanted acid corrosion or
partial dissolution of the metal in the case of some catalysts.
Nafion.RTM.-bound layers additionally have much lower porosity than
PTFE-bound layers. The purely hydrophilic properties of Nafion.RTM.
can likewise be disadvantageous, since Nafion.RTM., owing to its
hydrophilic properties, is unsuitable for formation of hydrophobic
channels that are advantageous for gas transport within a gas
diffusion electrode. Usable electrodes comprising Nafion.RTM.
should therefore consist of multiple layers in order to be able to
implement the essential properties of a GDE. However, multilayer
coating processes are not very attractive for economic reasons.
Nafion.RTM.-based coating processes can additionally lead to
unwanted formation of hydrogen.
[0027] The drying method 3. is based on a roll calendering process,
for example of PTFE/catalyst powder. The corresponding technique
can be traced back to EP 0297377 A2, according to which electrodes
based on Mn.sub.2O.sub.3 were produced for batteries. DE 3710168A1
makes the first reference to the employment of the drying process
with regard to the preparation of metallic electrocatalyst
electrodes. The technique was additionally used in patents relating
to the production of silver-based (silver(I) or silver(II) oxide)
gas diffusion electrodes (oxygen-depolarized electrodes). The
patents EP 2444526 A2 and DE 10 2005 023615 A1 mention mixtures
having a binder content of 0.5-7%. The carrier used was Ag or
nickel meshes having a wire diameter of 0.1-0.3 mm and a mesh size
of 0.2-1.2 mm. The powder is applied directly to the mesh before it
is supplied to the roll calender. DE 10148599 A1 or EP 0115845 B1
described a similar process in which the powder mixture is first
extruded to give a sheet or film which is pressed onto the mesh in
a further step.
[0028] Owing to the low mechanical stability, the latter method is
less suitable than the above-specified one-step process. EP 2410079
A2 describes the one-stage process for production of a silver-based
oxygen-depolarized electrode with the addition of metal oxide
supplements 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,
WO.sub.3, CeO.sub.2 and spinels such as CoAl.sub.2O.sub.4,
Co(AlCr).sub.2O.sub.4 and 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. Supplements of silicon nitride, boron nitride,
TiN, AlN, SiC, TiC, CrC, WC, Cr.sub.3C.sub.2, TiCN have likewise
been found to be suitable, and oxides of the ZrO.sub.2, WO.sub.3
type have been identified as being particularly suitable. The
materials are explicitly declared as fillers having no catalytic
effect. The aim here is explicitly the reduction of the hydrophobic
character of the electrode.
[0029] DE 10335184 A1 discloses catalysts which can be used as an
alternative for oxygen-depolarized electrodes: precious metals,
e.g. Pt, Rh, Ir, Re, Pd, precious metal alloys, e.g. Pt--Ru,
precious metal compounds, e.g. precious metal sulfides and oxides,
and Chevrel phases, e.g. Mo.sub.4Ru.sub.2Se.sub.8 or
Mo.sub.4Ru.sub.2S.sub.8, where these may also contain Pt, Rh, Re,
Pd etc.
[0030] Known Cu-based gas diffusion electrodes for generation of
hydrocarbons on the basis of CO.sub.2 are mentioned, for example,
in the papers by R. Cook [J. Electrochem. Soc., vol. 137, no. 2,
1990]. This mentions a wet-chemical method 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 using
three coating cycles, and a catalyst-containing layer using three
further coatings. Each step is followed by a drying phase
(325.degree. C.) with a subsequent static pressing operation
(1000-5000 psi). For the electrode obtained, a Faraday efficiency
of >60% and a current density of >400 mA/cm.sup.2 are
reported. However, reproduction experiments which are cited
hereinafter as comparative examples demonstrate that the static
pressing method described does not lead to stable electrodes. The
added Vulkan XC 72 was likewise found to have an adverse effect,
and so it was likewise not possible to obtain any hydrocarbons.
[0031] DE 101 30 441 A1 discloses a biporous pore system in a gas
diffusion electrode, but no two-layer structure. For such a
one-layer structure, flooding of the electrode was observed in
in-house preliminary tests. A one-layer structure can also be
found, for example, in DE 10 2010 031 571 A1. According to DE 101
30 441 A1, a metallic support skeleton is rolled into a catalyst
film produced in that document.
[0032] US 2013/0280625 A1 discloses a two-layer structure of a gas
diffusion electrode, but does not disclose any hydrophobic pores,
and discloses only pores in the diffusion layer as hydrophilic
layer. A sacrificial material is used in an obligatory manner
therein, and is required for formation of pores. However, in-house
preliminary tests have shown that this is not appropriate to the
aim.
SUMMARY OF THE INVENTION
[0033] There is thus a need for cathodes for carbon dioxide
electrolysis, in which carbon dioxide can be converted effectively
to hydrocarbons. In addition, it is an object of the invention to
provide a catalyst concept that is not based on in situ copper
deposition but provides a copper gas diffusion electrode which can
be processed to give an electrode. Moreover, it is an object of the
present invention to develop selective electro-catalysts having
long-term stability and to embed them into gas diffusion electrodes
that can be connected to electrical contacts.
[0034] The inventors have found that particularly active and
C.sub.2H.sub.4-selective gas diffusion electrodes should satisfy a
multitude of parameters required for ethylene formation. There
follows a discussion of properties specific to the invention of an
electrode of the invention. Furthermore, the inventors have found
that specific demands on the catalyst are required in order that
the electrode can form ethylene. These criteria are not apparent
from the prior art and constitute the basis for the development of
hydrocarbon-selective electrodes of this kind.
[0035] The following important specific parameters and requirements
for a hydrocarbon-selective gas diffusion electrode have been
found: [0036] Good wettability of the electrode surface in order
that the aqueous electrolyte or H.sup.+ ions can come into catalyst
contact. (H.sup.+ is required for ethylene or alcohols such as
ethanol, propanol or glycol.) [0037] High electrical conductivity
of the electrode or of the catalyst and a homogeneous potential
distribution across the entire electrode area (potential-dependent
product selectivity). [0038] High chemical and mechanical stability
in electrolysis operation (suppression of cracking and corrosion).
[0039] Defined porosity with a suitable ratio between hydrophilic
and hydrophobic channels or pores (assurance of availability of
CO.sub.2 with simultaneous presence of H.sup.+ ions).
[0040] These can be achieved or fulfilled in accordance with the
invention.
[0041] In a first aspect, the present invention relates to a gas
diffusion electrode comprising a preferably copper-containing
carrier, preferably in the form of a sheetlike structure, and a
first layer comprising at least copper and at least one binder,
wherein the first layer comprises hydrophilic and hydrophobic pores
and/or channels, further comprising a second layer comprising
copper and at least one binder, wherein the second layer is present
atop the carrier and the first layer atop the second layer, wherein
the content of binder in the first layer is less than in the second
layer.
[0042] In a further aspect, the present invention relates to a
process for producing a gas diffusion electrode, comprising [0043]
producing a first mixture comprising at least copper and optionally
at least one binder, [0044] producing a second mixture comprising
at least copper and at least one binder, [0045] applying the second
mixture comprising at least copper and at least one binder to a
preferably copper-containing carrier, preferably in the form of a
sheetlike structure, [0046] applying the first mixture comprising
at least copper and optionally at least one binder to the second
mixture, [0047] optionally applying further mixtures to the first
mixture, and [0048] dry rolling the first and second mixtures and
optionally further mixtures onto the carrier to form a second layer
and a first layer and optionally further layers, wherein the
proportion of binder in the second mixture is 3-30% by weight,
preferably 10-30% by weight, further preferably 10-20% by weight,
based on the second mixture, and wherein the proportion of binder
in the first mixture is 0-10% by weight, preferably 0.1-10% by
weight, further preferably 1-10% by weight, even further preferably
1-7% by weight, even further preferably 3-7% by weight, based on
the first mixture, wherein the content of binder in the first
mixture is smaller than in the second mixture; or comprising [0049]
producing a first mixture comprising at least copper and at least
one binder, [0050] applying the first mixture comprising at least
copper and optionally at least one binder to a preferably
copper-containing carrier, preferably in the form of a sheetlike
structure, and [0051] dry rolling the first mixture onto the
carrier to form a first layer, wherein the proportion of binder in
the mixture is 3-30% by weight, preferably 3-20% by weight, further
preferably 3-10% by weight, based on the first mixture.
[0052] The present invention additionally relates, in yet a further
aspect, to an electrolysis cell comprising the gas diffusion
electrode of the invention.
[0053] Further aspects of the present invention can be taken from
the dependent claims and the detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] The appended drawings are intended to illustrate embodiments
of the present invention and impart further understanding thereof.
In conjunction with the description, they serve to elucidate
concepts and principles of the invention. Other embodiments and
many of the advantages mentioned are apparent with regard to the
drawings.
[0055] The elements of the drawings are not necessarily shown to
scale with respect to one another. Elements, features and
components that are identical, have the same function and the same
effect are each given the same reference numeral in the figures of
the drawings, unless stated otherwise.
[0056] FIG. 1 shows a schematic diagram of a gas diffusion
electrode of the invention with hydrophobic and hydrophilic regions
or channels.
[0057] FIG. 2 shows a schematic diagram of production of a gas
diffusion electrode of the invention based on an illustrative
PTFE-bound catalyst.
[0058] FIG. 3 shows a schematic of a further embodiment of a gas
diffusion electrode of the invention in the form of a multilayer
preparation.
[0059] FIGS. 4 to 6 show, in schematic form, illustrative diagrams
of a possible construction of an electrolysis cell in one
embodiment of the present invention.
[0060] FIGS. 7 and 8 show illustrative configuration forms for a
gas distribution chamber downstream of a gas diffusion electrode of
the invention in an electrolysis cell of the invention.
[0061] FIG. 9 shows the results of Faraday efficiencies of the
electrolysis cell from comparative example 3.
[0062] FIGS. 10 and 11 show the results of Faraday efficiencies of
the electrolysis cell from comparative example 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
Definitions
[0063] "Hydrophobic" in the context of the present invention is
understood to mean water-repellent. According to the invention,
hydrophobic pores and/or channels are those that repel water. More
particularly, hydrophobic properties are associated in accordance
with the invention with substances or molecules having nonpolar
groups.
[0064] "Hydrophilic", by contrast, is understood to mean the
ability to interact with water and other polar substances.
[0065] In the application, figures are reported in % by weight,
unless stated otherwise or apparent from the description.
[0066] In a first aspect, the present invention relates to
a gas diffusion electrode comprising a preferably copper-containing
carrier, preferably in the form of a sheetlike structure, and a
first layer comprising at least copper and at least one binder,
wherein the (first) layer comprises hydrophilic and hydrophobic
pores and/or channels, further comprising a second layer comprising
copper and at least one binder, wherein the second layer is present
atop the carrier and the first layer atop the second layer, wherein
the content of binder in the first layer is less than in the second
layer.
[0067] The second layer, just like the first layer, may comprise
hydrophilic and/or hydrophobic pores and/or channels.
[0068] What is also described is a gas diffusion electrode
comprising a preferably copper-containing carrier, preferably in
the form of a sheetlike structure, and
a first layer comprising at least copper and at least one binder,
wherein the layer comprises hydrophilic and hydrophobic pores
and/or channels.
[0069] FIG. 1 illustrates the relations between hydrophilic and
hydrophobic regions of a GDE, which can achieve a good triphasic
liquid/solid/gaseous relationship. In this case, in the electrode,
there are hydrophobic channels or regions 1 and hydrophilic
channels or regions 2 on the electrolyte side, with catalyst sites
3 of low activity present in the hydrophilic regions 2. In
addition, there are inactive catalyst sites 5 on the gas side.
[0070] Particularly active catalyst sites 4 are in the triphasic
liquid/solid/gaseous region. An ideal GDE thus has maximum
penetration of the bulk material by hydrophilic and hydrophobic
channels in order to obtain a maximum number of triphasic regions
for active catalyst sites. In this respect, it should be ensured in
accordance with the invention that the first layer comprises
hydrophilic and hydrophobic pores and/or channels. By suitable
adjustment of the first layer, it is possible to achieve the effect
that a maximum number of active catalyst sites are present in the
gas diffusion electrode, which is explained further in the further,
especially preferred embodiments and/or the dependent claims.
[0071] For hydrocarbon-selective gas diffusion electrodes for
carbon dioxide reduction, accordingly, more intrinsic properties
are needed than are offered by the known systems. The
electrocatalyst and the electrode are accordingly in a close
relationship.
[0072] The carrier here is not particularly restricted, provided
that it is suitable for a gas diffusion electrode and preferably
contains copper. For example, it is also possible for parallel
wires to form a carrier in the extreme case. In particular
embodiments, the carrier is a sheetlike structure, further
preferably a mesh, very preferably a copper mesh. This can assure
both adequate mechanical stability and functionality as a gas
diffusion electrode, for example with regard to a high electrical
conductivity. In particular embodiments, the carrier may also be
suitable with regard to the electrical conductivity of the first
layer. Through the use of copper in the carrier, it is possible to
provide a suitable conductivity and to reduce the risk of inward
entrainment of unwanted extraneous metals. In preferred
embodiments, the carrier therefore consists of copper. A preferred
copper-containing carrier, in particular embodiments, is a copper
mesh having a mesh size w of 0.3 mm<w<2.0 mm, preferably 0.5
mm<w<1.4 mm, and a wire diameter x of 0.05 mm<x<0.5 mm,
preferably 0.1 mm.ltoreq.x.ltoreq.0.25 mm.
[0073] In addition, by virtue of the fact that the first layer
comprises copper, it is also possible to assure a high electrical
conductivity of the catalyst and, especially in conjunction with a
copper mesh, a homogeneous potential distribution across the entire
electrode area (potential-dependent product selectivity).
[0074] In preferred embodiments, a preferably copper-containing
mesh, preferably the copper mesh which is used as carrier, has a
mesh size of the carrier between 0.3 and 2.0 mm, preferably between
0.5-1.4 mm, in order to achieve good conductivity and
stability.
[0075] In particular embodiments, the binder comprises a polymer,
for example a hydrophilic and/or hydrophobic polymer, for example a
hydrophobic polymer, especially PTFE. This can achieve suitable
adjustment of the hydrophobic pores or channels. More particularly,
the first layer is produced using PTFE particles having a particle
diameter between 5 and 95 .mu.m, preferably between 8 and 70 .mu.m.
Suitable PTFE powders include, for example, Dyneon.RTM. TF 9205 and
Dyneon TF 1750. Suitable binder particles, for example PTFE
particles, may, for example, be virtually spherical, for example
spherical, and may be produced, for example, by emulsion
polymerization. In particular embodiments, the binder particles are
free of surface-active substances. The particle size can be
determined here, for example, according to ISO 13321 or D4894-98a
and may correspond, for example, to the manufacturer data (e.g. TF
9205: mean particle size 8 .mu.m according to ISO 13321; TF 1750:
mean particle size 25 .mu.m according to ASTM D4894-98a).
[0076] In addition, the first layer comprises at least copper which
may, for example, be in the form of metallic copper and/or copper
oxide and which functions as catalyst site.
[0077] In particular embodiments, the first layer comprises
metallic copper in the 0 oxidation state.
[0078] In particular embodiments, the first layer comprises copper
oxide, especially Cu.sub.2O. The oxide here may contribute to
stabilizing the +1 oxidation states of copper and hence to
maintaining the selectivity for ethylene with long-term stability.
Under electrolysis conditions, it can be reduced to copper.
[0079] In particular embodiments, the first layer comprises at
least 40 at % (atom percent), preferably at least 50 at % and
further preferably at least 60 at % of copper, based on the layer.
This can assure both suitable mechanical stability and suitable
catalytic activity of this first layer that serves as catalyst
layer (CL). In particular embodiments, the copper for production of
the gas diffusion electrode of the invention is provided as
particles, which are defined further hereinafter.
[0080] In addition, the first layer may also comprise further
promoters which improve the catalytic activity of the GDE in
association with the copper. In particular embodiments, the first
layer comprises at least one metal oxide preferably having a lower
reduction potential than the evolution of ethylene, preferably
ZrO.sub.2, Al.sub.2O.sub.3, CeO.sub.2, Ce.sub.2O.sub.3, ZnO.sub.2,
MgO; and/or at least one copper-rich intermetallic phase,
preferably at least one Cu-rich phase selected from the group of
the binary systems Cu--Al, Cu--Zr, Cu--Y, Cu--Hf, CuCe, Cu--Mg and
the ternary systems Cu--Y--Al, Cu--Hf--Al, Cu--Zr--Al, Cu--Al--Mg,
Cu--Al--Ce with copper contents >60 at %; and/or
copper-containing perovskites and/or defect perovskites and/or
perovskite-related compounds, preferably
YBa.sub.2Cu.sub.3O.sub.7-.delta. where 0.ltoreq..delta..ltoreq.1
(corresponding to YBa.sub.2Cu.sub.3O.sub.7-.delta.X.sub..delta.),
CaCu.sub.3Ti.sub.4O.sub.12,
La.sub.1.85Sr.sub.0.15CuO.sub.3.930Cl.sub.0.053,
(La,Sr).sub.2CuO.sub.4.
[0081] Preferred promoters here are the metal oxides.
[0082] In particular embodiments, the metal oxide used is
water-insoluble, in order that aqueous electrolytes can be used in
an electrolysis using the gas diffusion electrode of the invention.
Moreover, by virtue of the redox potential of the metal oxide being
lower than that of the evolution of ethylene, it is possible to
ensure that ethylene can be prepared from CO.sub.2 by means of the
GDE of the invention. In particular embodiments, the oxides are not
to be reduced either in a carbon dioxide reduction. Nickel and
iron, for example, are unsuitable since hydrogen forms here.
Moreover, the metal oxides are preferably not inert, but should
preferably constitute hydrophilic reaction sites that can serve for
the provision of protons.
[0083] The promoters, especially the metal oxide, are able here to
promote the function and production of electro-catalysts having
long-term stability, in that they stabilize catalytically active
copper nanostructures. The structural promoters here can reduce the
high surface mobilities of the copper nanostructures and hence
reduce their tendency to sinter. The concept originates from
heterogeneous catalysis and is used successfully within
high-temperature processes.
[0084] Promoters used for the electrochemical reduction of CO.sub.2
may especially be the following metal oxides that cannot be reduced
to metals within the electrochemical window: ZrO.sub.2 (E=-2.3 V),
Al.sub.2O.sub.3 (E=-2.4 V), CeO.sub.2 (E=-2.3 V), MgO (E=-2.5). It
should be noted here that the oxides mentioned are not added as
additives but are part of the catalyst itself. The oxide, as well
as its function as a promoter, also fulfills the feature of
stabilizing copper in the I oxidation state and additionally also
intermediates in the reduction of carbon dioxide, such as CO,
C.sub.2H.sub.4 (or OH). There exist many Cu(I) complexes of CO and
C.sub.2H.sub.4, which suggests stability of these postulated
intermediates (see, for example, H. Tropsch, W. J. Mattox, J. Am.
Chem Soc. 1935, 57, 1102-1103; T. Ogura, Inorg. Chem., 1976, 15
(9), 2301-2303; J. S. Thompson, R. L. Harlow, J. F. Whitney, J. Am.
Chem. Soc., 1983, 105 3522-3527; and V. A. K. Adiraju, J. A.
Flores, M. Yousufuddin, H. V. Rasika Dias, Organometallics, 2012,
31, 7926-7932).
[0085] In particular embodiments, the catalyst has the following
inventive features: by contrast with the known heterogeneous
Cu/Al.sub.2O.sub.3, Cu/ZrO.sub.2, Cu/MgO/Al.sub.2O.sub.3 catalysts
used in industry, in particular embodiments, preferably only very
copper-rich catalysts having a molar proportion of >60 at % Cu
are used for the electrochemical reduction of CO.sub.2 owing to the
electrical conductivity required.
[0086] Especially preferred in gas diffusion electrodes of the
invention are metal oxide/copper catalyst structures that are
produced as follows:
[0087] For the production of the metal oxides, the precipitation,
in particular embodiments, cannot be effected as frequently
described in a pH regime between pH=5.5-6.5, but can be effected
within a range between 8.0-8.5, such that the precursors formed are
not hydroxide carbonates similar to malachite
(Cu.sub.2[(OH).sub.2|CO.sub.3]), azurite
(Cu.sub.3(CO.sub.3).sub.2(OH).sub.2) or aurichalcite
(Zn,Cu).sub.5[(OH).sub.6|(CO.sub.3).sub.2)], but are hydrotalcites
(Cu.sub.6Al.sub.2CO.sub.3(OH).sub.16.4(H.sub.2O)), which can be
obtained in a greater yield. Likewise suitable are layered double
hydroxides (LDHs) having a composition
[M.sup.z+.sub.1-xM.sup.3+.sub.x
(OH).sub.2].sup.q+(X.sup.n-).sub.q/n.yH.sub.2O where
M.sup.l+=Li.sup.+, Na.sup.+, K.sup.+, M.sup.2+=Ca.sup.2+,
Mg.sup.2+, Cu.sup.2+ and M.sup.3+=Al, Y, Ti, Hf, Ga. The
corresponding precursors can be precipitated under pH control by
co-dosage of a metal salt solution and a basic carbonate solution.
A particular feature of these materials is the presence of
particularly fine copper crystallites having a size of 4-10 nm,
which are structurally stabilized by the oxide present.
[0088] It is possible to achieve the following effects: the metal
oxide, owing to its high specific surface area, can lead to better
distribution of the catalyst metal; highly dispersed metal sites
can be stabilized by the metal oxide; CO.sub.2 chemisorption can be
improved by the metal oxide; copper oxides can be stabilized.
[0089] The precipitation can be followed by drying with subsequent
calcination in an O.sub.2/Ar gas stream. The oxide precursors
produced, according to the method, can also subsequently be reduced
directly in an H.sub.2/Ar gas stream, reducing solely the Cu.sub.2O
or CuO to Cu and conserving the oxide promoter. The activation step
can also be effected by electrochemical means subsequently. In
order to improve the electrical conductivity of the layer applied
prior to the electrochemical activation, it is also possible to
partly mix oxide precursors and activated precursors. In order to
be able to increase the underlying conductivity, it is also
possible to mix in 0-10% by weight of copper powder in a similar
particle size.
[0090] It is likewise not ruled out in accordance with the
invention that the ready-calendered electrode is subjected to a
subsequent calcination/thermal treatment before the electrochemical
activation is conducted.
[0091] A further means of production of suitable electro-catalysts
is based on the approach of the production of copper-rich
intermetallic phases, for example Cu.sub.5Zr, Cu.sub.10Zr.sub.7,
Cu.sub.51Zr.sub.14, which can be prepared from the melt.
Corresponding ingots can subsequently be ground and fully or partly
calcined in an O.sub.2/argon gas stream and converted to the oxide
form. Of particular interest are the Cu-rich phases of the binary
systems Cu--Al, Cu--Zr, Cu--Y, Cu--Hf, CuCe, Cu--Mg and the
corresponding ternary systems having Cu contents >60 at %:
CuYAl, CuHfAl, CuZrAl, CuAlMg, CuAlCe.
[0092] Copper-rich phases are known, for example, from E. Kneller,
Y. Khan, U. Gorres, The Alloy System Copper-Zirconium, Part I.
Phase Diagram and Structural Relations, Zeitschrift fur Metallkunde
77 (1), p. 43-48, 1986 for Cu--Zr phases, from Braunovic, M.;
Konchits, V. V.; Myshkin, N. K.: Electrical contacts, fundamentals,
applications and technology; CRC Press 2007 for Cu--Al phases, from
Petzoldt, F.; Bergmann, J. P.; Schurer, R.; Schneider, 2013, 67
Metall, 504-507 (see, for example, table 2) for Cu--Al phases, from
Landolt-Bornstein--Group IV Physical Chemistry Volume 5d, 1994, p.
1-8 for Cu--Ga phases, and from P. R. Subramanian, D. E. Laughlin,
Bulletin of Alloy Phase Diagrams, 1988, 9, 1, 51-56 for Cu--Hf
phases.
TABLE-US-00002 TABLE 2 Copper-aluminum phases (taken from Petzoldt,
F.; Bergmann, J. P.; Schurer, R.; Schneider, 2013, 67 Metall,
504-507) Cu Al Hardness Spec. el. Phase [% by wt.] [% by wt.] [HV]
resistance [.mu..OMEGA.cm] Cu 100 0 100 1.75 .GAMMA.
Cu.sub.9Al.sub.4 80 20 1050 14.2 .DELTA. Cu.sub.3Al.sub.2 78 22 180
13.4 .zeta..sub.2 Cu.sub.4Al.sub.3 75 25 624 12.2 .eta..sub.2 CuAl
70 30 648 11.4 .THETA. CuAl.sub.2 55 45 413 8.0 Al 0 100 60 2.9
[0093] In the case of these copper-rich intermetallic phases too,
the proportion of copper is preferably greater than 40 at %,
further preferably greater than 50 at %, more preferably greater
than 60 at %.
[0094] However, it is not ruled out here that the intermetallic
phases also contain nonmetal elements such as oxygen, nitrogen,
sulfur, selenium and/or phosphorus, i.e. oxides, sulfides,
selenides, nitrides and/or phosphides for example are present. In
particular embodiments, the intermetallic phases have been partly
oxidized.
[0095] In addition, it is possible to use the following
copper-containing perovskite structures and/or defect perovskites
and/or perovskite-related compounds for electro-catalysts,
especially for the formation of hydrocarbons:
YBa.sub.2Cu.sub.3O.sub.7-.delta. where 0.ltoreq..delta..ltoreq.1,
CaCu.sub.3Ti.sub.4O.sub.12,
La.sub.1.85Sr.sub.0.15,CuO.sub.3.930Cl.sub.0.053,
(La,Sr).sub.2CuO.sub.4. In addition, it is not ruled out that
mixtures of these materials can be used for electrode preparation
or, as required, subsequent calcination or activation steps are
conducted.
[0096] In particular embodiments, the catalyst particles comprising
or consisting of copper, for example copper particles, which are
used for production of the GDE of the invention, have a homogeneous
particle size between 5 and 80 .mu.m, preferably 10 to 50 .mu.m,
further preferably between 30 and 50 .mu.m. In addition, the
catalyst particles, in particular embodiments, have a high purity
without traces of extraneous metal. By suitable structuring,
optionally with the aid of promoters, it is possible to achieve
high selectivity and long-term stability.
[0097] It is likewise possible for the promoters, for example the
metal oxides, to have a corresponding particle size in the
production.
[0098] The above promoters can additionally achieve or improve the
following properties: [0099] Good wettability of the electrode
surface in order that the aqueous electrolyte or H.sup.+ ions can
come into catalyst contact. (H.sup.+ is required for ethylene or
alcohols such as ethanol, propanol or glycol.) [0100] High chemical
and mechanical stability in electrolysis operation (suppression of
cracking and corrosion). [0101] Defined porosity with a suitable
ratio between hydrophilic and hydrophobic channels or pores
(assurance of availability of CO.sub.2 with simultaneous presence
of H.sup.+ ions).
[0102] In order to further adjust the porosity of the electrode, in
particular embodiments, it is possible to add copper powder
supplements having a particle diameter of 50 to 600 .mu.m,
preferably 100 to 450 .mu.m, preferably 100-200 .mu.m. The particle
diameter of these supplements, in particular embodiments, is 1/3-
1/10 of the total layer thickness of the layer. Rather than Cu, the
supplement may also be an inert material such as a metal oxide.
This can achieve improved formation of pores or channels.
[0103] A gas diffusion electrode of the invention can especially be
produced by the production process of the invention as described
further down.
[0104] In particular embodiments, the first layer comprises less
than 5% by weight of, further preferably less than 1% by weight of
and even further preferably no charcoal- and/or carbon black-based
or -like fillers, for example conductive fillers, based on the
layer. It should be noted here that methods known from the
literature for GDE production generally refer, both for dry and wet
application, to the addition of activated carbons, conductive
blacks (such as Vulkan XC72), acetylene black or other charcoals.
However, it has been found in accordance with the invention that
even traces of charcoals and/or carbon black can distinctly reduce
the selectivity of the catalyst with respect to hydrocarbons and
promote the unwanted formation of hydrogen.
[0105] Moreover, the first layer, in particular embodiments, does
not contain any surface-active substances. In particular
embodiments, the first and/or second layer additionally do not
contain any sacrificial material, for example a sacrificial
material having a release temperature of roughly below 275.degree.
C., for example below 300.degree. C. or below 350.degree. C., and
especially any pore former(s) which can typically remain at least
partly in the electrode in the case of production of electrodes
using such a material.
[0106] It has been found by in-house experiments with electrodes
produced by wet-chemical means that these residues irreversibly
poison a Cu-based catalyst, and so electrodes thus produced did not
show any CO.sub.2 reduction to hydrocarbons. The use of
surface-active substances or surfactants, for example Triton X,
should therefore be avoided in particular embodiments, and so a
wet-chemical procedure for the embedding of Cu-based catalysts is
unsuitable.
[0107] If just one (first) layer is present in the GDE, the content
or proportion of binder, for example PTFE, in particular
embodiments, may be 3-30% by weight, preferably 3-20% by weight,
further preferably 3-10% by weight, even further preferably 3-7% by
weight, based on the one (first) layer.
[0108] The GDE of the invention further comprises a second layer
comprising copper and at least one binder, wherein the second layer
is present atop the carrier and the first layer atop the second
layer, wherein the content of binder in the first layer is smaller
than in the second layer. In addition, the second layer may
comprise coarser copper or inert material particles, for example
having particle diameters of 50 to 700 .mu.m, preferably 100-450
.mu.m, in order to provide a suitable channel or pore
structure.
[0109] In preferred embodiments, in this context, the second layer
comprises 3-30% by weight of binder, preferably 10-30% by weight of
binder, further preferably 10-20% by weight of binder, preferably
>10% by weight of binder, further preferably >10% by weight
and up to 20% by weight of binder, based on the second layer, and
the first layer preferably comprises 0-10% by weight of binder, for
example 0.1-10% by weight of binder, preferably 1-10% by weight of
binder, further preferably 1-7% by weight of binder, even further
preferably 3-7% by weight of binder, based on the first layer. The
binder here may be the same binder as in the first layer, for
example PTFE. In addition, the particles for production of the
second layer, in particular embodiments, may correspond to those in
the first layer, but may also be different therefrom. The second
layer here is a metal particle layer (MPL) beneath the catalyst
layer (CL). Through layering of this kind, it is possible to
specifically create highly hydrophobic regions in the MPL and
generate a catalyst layer having hydrophilic properties. By virtue
of the strongly hydrophobic character of the MPL, it is likewise
possible to prevent unwanted penetration of the electrolyte into
the gas transport channels, i.e. flooding thereof. Moreover, the
second layer forms the contact with the CO.sub.2 and should
therefore also be hydrophobic.
[0110] In particular embodiments, the second layer partly
penetrates the first layer. This can be achieved, for example, by
virtue of the process of the invention and enables a good
transition between the layers with regard to diffusion.
[0111] As well as the second layer, the GDE of the invention may
also have further layers, for example atop the first layer and/or
on the other side of the carrier.
[0112] For production of such a multilayer GDE, for example, it is
first possible to apply, by sieving application, a mixture for an
MPL based on a highly conductive copper mixture of dendritic copper
having particle sizes between 5-100 .mu.m, preferably less than 50
.mu.m, and coarser copper or inert material particles having
particle sizes of 100-450 .mu.m, preferably 100-200 .mu.m, having a
PTFE content of 3-30% by weight, preferably 20% by weight, in a
layer thickness of 0.5 mm for example, to a copper mesh having a
mesh size of 1 mm for example (thickness, for example, 0.2-0.6 mm,
e.g. 0.4 mm), and to draw it down by means of a frame or coating
bar. Corresponding dendritic copper may also be present in the
first layer. This may then be followed by further sieving
application of the catalyst/PTFE mixture (CL), for example with a
PTFE content of 0.1-10% by weight, and smoothing or drawdown, for
example by means of a frame of thickness 1 mm, so as to obtain a
total layer thickness (Hf) of 1 mm. The layer pre-prepared in this
way can then be fed to a calender having a gap width
H.sub.0=0.4-0.7 mm, preferably 0.5-0.6 mm, and rolled out, so as to
obtain a multilayer gas diffusion electrode as shown schematically
in FIG. 3, comprising a copper mesh 8, an MPL 9 and a CL 10. The
MPL can achieve better mechanical stability, a further reduction in
the penetration of the electrolyte and better conductivity,
especially when meshes are used as carriers.
[0113] Stepwise production of the GDE by respective sieving
application and rolling of each individual layer can lead to lower
adhesion between the layers and is therefore less preferred.
[0114] In a further aspect, the present invention relates to a
process for producing a gas diffusion electrode, comprising [0115]
producing a first mixture comprising at least copper and optionally
at least one binder, [0116] producing a second mixture comprising
at least copper and at least one binder, [0117] applying the second
mixture comprising at least copper and at least one binder to a
preferably copper-containing carrier, preferably in the form of a
sheetlike structure, [0118] applying the first mixture comprising
at least copper and optionally at least one binder to the second
mixture, [0119] optionally applying further mixtures to the first
mixture, and [0120] dry rolling the first and second mixtures and
optionally further mixtures onto the carrier to form a second layer
and a first layer and optionally further layers, wherein the
proportion of binder in the second mixture is 10-30% by weight,
preferably 10-20% by weight, based on the second mixture, and
wherein the proportion of binder in the first mixture is 0-10% by
weight, preferably 0.1-10% by weight, further preferably 1-10% by
weight, even further preferably 1-7% by weight, even further
preferably 3-7% by weight, based on the first mixture, wherein the
content of binder in the first mixture is smaller than in the
second mixture.
[0121] Also described is a process for producing a gas diffusion
electrode comprising [0122] producing a first mixture comprising at
least copper and at least one binder, [0123] applying the first
mixture comprising at least copper and optionally at least one
binder to a preferably copper-containing carrier, preferably in the
form of a sheetlike structure, and [0124] dry rolling the first
mixture onto the carrier to form a first layer, wherein the
proportion of binder in the mixture is 3-30% by weight, preferably
3-20% by weight, further preferably 3-10% by weight, even further
preferably 3-7% by weight, based on the first mixture.
[0125] The production of the first and second mixtures or of the
first mixture is not particularly restricted here and can be
effected in a suitable manner, for example by stirring, dispersing,
etc.
[0126] When the second mixture is applied, the first mixture may
also comprise 0% by weight of binder, i.e. no binder, since binder
from the second mixture can diffuse into the first layer that forms
from the first mixture in the course of rolling and hence the first
layer can also have a content of binder of, for example, at least
0.1% by weight, for example 0.5% by weight, as established in
preliminary experiments. In particular embodiments, however, the
first mixture in the case of application of 2 or more mixtures
comprises binder.
[0127] In particular embodiments, the binder comprises a polymer,
for example a hydrophilic and/or hydrophobic polymer, for example a
hydrophobic polymer, especially PTFE. This can achieve a suitable
adjustment of the hydrophobic pores or channels. More particularly,
for production of the first layer, PTFE particles having a particle
diameter between 5 and 95 .mu.m, preferably between 8 and 70 .mu.m,
are used. Suitable PTFE powders include, for example, Dyneon.RTM.
TF 9205 and Dyneon.RTM. TF 1750.
[0128] In particular embodiments, the copper for the production of
the mixture is in the form of particles or catalyst particles, for
example including dendritic copper, having a homogeneous particle
size between 5 and 80 .mu.m, preferably 10 to 50 .mu.m, further
preferably between 30 and 50 .mu.m. In addition, the catalyst
particles, in particular embodiments, have a high purity without
traces of extraneous metal. By suitable structuring, optionally
with the aid of the promoters, as described above, it is possible
to achieve high selectivity and long-term stability.
[0129] By suitable adjustment of the particle sizes of copper and
binder and any further additions such as promoters, it is possible
to control the pores and/or channels, i.e. the hydrophobic and
hydrophilic pores and/or channels, of the GDE for the passage of
gas and/or electrolyte and hence for the catalytic reaction.
[0130] In particular embodiments, the first and/or second mixtures
do not contain any sacrificial material, for example a sacrificial
material having a release temperature of about below 275.degree.
C., for example below 300.degree. C. or below 350.degree. C., and
especially no pore former(s) which can typically remain at least
partly in the electrode in the case of production of electrodes
using such a material.
[0131] In particular embodiments, the first and/or second mixtures
are not pasty, for example in the form of inks or pastes, but are
in the form of powder mixtures.
[0132] The application of a first, second and further mixture(s) is
not particularly restricted and can be effected, for example, by
scattering application, sieving application, bar coating, etc.
[0133] The rolling application is likewise not particularly
restricted and can be effected in a suitable manner. Rolling of the
mixture or mass (particles) into the structure of the carrier, for
example a mesh structure, is explicitly desirable in particular
embodiments in order to assure a high mechanical stability of the
electrode.
[0134] By virtue of the aforementioned two-stage process with
formation of a film, this is not the case; the pre-extruded film
here lies only on the mesh and has lower adhesion, and also
mechanical stability.
[0135] As a result, in the case of application of multiple layers
too, it is preferable that the mixtures for the layers are applied
individually to the carrier and are then rolled collectively, in
order to achieve better adhesion between the layers. In this way,
the layers may at least partly penetrate one another, for example
in a thickness of 1-20 .mu.m.
[0136] The mechanical stress on the binder, for example of polymer
particles, by the rolling process leads to crosslinking of the
powder through the formation of binder channels, for example PTFE
fibrils. The attainment of this state is particularly important in
order to guarantee suitable porosity or mechanical stability of the
electrode. The hydrophobicity can be adjusted via the respective
content of polymer or via the physical properties of the catalyst
powder. In the case of application of two (or more layers), a
suitable binder content in the second mixture has been found to be
10-30% by weight, preferably 10-20% by weight, based on the second
mixture, and a suitable proportion of binder in the first mixture
to be 0-10% by weight, 0.1-10% by weight, further preferably 1-10%
by weight, even further preferably 1-7% by weight, even further
preferably 3-7% by weight. In the case of application of just one
mixture, a particularly suitable binder content, for example PTFE
content, has been found to be 3-30% by weight, preferably 3-20% by
weight, further preferably 3-10% by weight and even further
preferably 3-7% by weight of binder, based on the first
mixture.
[0137] The degree of fibrillation of the binder, for example PTFE
(structure parameter correlates directly with the shear rate
applied, since the binder, for example a polymer, behaves as a
shear-thinning (pseudo-plastic) fluid in the rolling application.
After the extrusion, the layer obtained, by virtue of the
fibrillation, has an elastic character. This change in structure is
irreversible, and so this effect cannot be subsequently enhanced by
further rolling; instead, the layer, by virtue of the elastic
characteristics, is damaged with further action of shear forces.
Particularly significant fibrillation can disadvantageously lead to
the electrode rolling up on the layer side, and so excessively high
contents of binder should be avoided.
[0138] For dry rolling application, it is preferable that the water
content in the rolling operation corresponds, for example, to the
ambient humidity at most. For example, the content of water and
solvents in the rolling application is less than 5% by weight,
preferably less than 1% by weight and, for example, even 0% by
weight.
[0139] In particular embodiments, the copper-containing carrier is
a copper mesh having a mesh size w of 0.3 mm<w<2.0 mm,
preferably 0.5 mm<w<1.4 mm, and a wire diameter x of 0.05
mm<x<0.5 mm, preferably 0.1 mm.ltoreq.x.ltoreq.0.25 mm. The
rolling into a mesh, for example a copper mesh, allows the
interstices in the mesh, for example copper mesh, to be effectively
bridged by the overlying (for example highly conductive) layer and
enables complete 3D contact connection with the electrode. As a
result, higher oxide contents are possible.
[0140] In particular embodiments, the production of the gas
diffusion electrode of the invention is additionally based on the
exclusion of charcoal- and/or carbon black-based or -like fillers,
for example conductive fillers. The catalyst itself or dendritic
copper (formed, for example, through activation of the catalyst) or
mixtures of the two serve here as charcoal replacement.
[0141] In addition, the method of the invention, in particular
embodiments, does not need any surface-active
substances/surfactants or thickeners and additives (such as flow
improvers) that have been identified as catalyst poisons.
[0142] In particular embodiments, the bed height y of the first
mixture on the carrier in the application is in the range of 0.3
mm<y<2.0 mm, preferably 0.5 mm<y<1.0 mm. In the case of
multiple layers, each layer may have a corresponding bed height y,
but the bed heights of all layers preferably do not add up to more
than 2.0 mm, preferably to more than 1.5 mm, more preferably to
more than 1 mm.
[0143] In particular embodiments, the gap width in the rolling
application H.sub.0 is the height of the carrier +40% to 50% of the
total bed height Hf of the mixtures of the various layers, for
example of the bed height y of the first mixture if it is the only
one used.
[0144] In particular embodiments, the rolling application is
effected by means of a calender.
[0145] In particular embodiments, the copper content in the mixture
is at least 40 at %, preferably at least 50 at % and further
preferably at least 60 at % of copper, based on the mixture.
[0146] In particular embodiments, further additions to the mixture
include:
at least one metal oxide having a lower reduction potential than
the evolution of ethylene, preferably ZrO.sub.2, Al.sub.2O.sub.3,
CeO.sub.2, Ce.sub.2O.sub.3, ZnO.sub.2, MgO; and/or at least one
copper-rich intermetallic phase, preferably at least one Cu-rich
phase selected from the group of the binary systems Cu--Al, Cu--Zr,
Cu--Y, Cu--Hf, CuCe, Cu--Mg and/or the ternary systems Cu--Y--Al,
Cu--Hf--Al, Cu--Zr--Al, Cu--Al--Mg, Cu--Al--Ce with copper contents
>60 at %; and/or at least one metal for formation of a
copper-rich metallic phase, preferably Al, Zr, Y, Hf, Ce, Mg, or at
least two metals for formation of ternary phases, preferably Y--Al,
Hf--Al, Zr--Al, Al--Mg, Al--Ce, such that the copper content is
>60 at %; and/or copper-containing perovskites and/or defect
perovskites and/or perovskite-related compounds, preferably
YBa.sub.2Cu.sub.3O.sub.7-.delta.X.sub..sigma.,
CaCu.sub.3Ti.sub.4O.sub.12,
La.sub.1.85Sr.sub.0.15CuO.sub.3.930Cl.sub.0.053,
(La,Sr).sub.2CuO.sub.4.
[0147] The addition of the metal for formation of a copper-rich
metallic phase, preferably Al, Zr, Y, Hf, Ce, Mg, or at least two
metals for formation of ternary phases, preferably Y--Al, Hf--Al,
Zr--Al, Al--Mg, Al--Ce, such that the copper content is >60 at
%, can be effected, for example, in such a way that, in the
production of the gas diffusion electrode, intermetallic phases are
formed, for example through co-melting and thermal oxidation, and
can then be selectively reduced, for example by electrochemical
means. However, such co-melting in the mixture is effected here
before the binder is added. In such a case, there is thus a
sequence in that the metal is first added and fused with copper
before the binder and any further substances are added to the
mixture.
[0148] In particular embodiments, the process of the invention can
thus be effected by a calendering process as shown schematically in
FIG. 2. In this case, the catalyst particles 6 and the binder
particles 7, for example PTFE particles, are rolled onto the
carrier 8, here in the form of a copper mesh, with the aid of a
calender 11.
[0149] In particular embodiments, the rolling or calendering is
conducted at a roller speed between 0.3 and 3 rpm, preferably 0.5-2
rpm. In particular embodiments, the flow rate or an advance rate
(of the GDE in length per unit time, for example in the case of
calendering) Q is in the range from 0.04 to 0.4 m/min, preferably
0.07 to 0.3 m/min.
[0150] In order to further adjust the porosity of the electrode, in
particular embodiments, it is possible to add copper powder
supplements having a particle diameter of 50 to 600 .mu.m,
preferably 100 to 450 .mu.m, further preferably 100 to 200 .mu.m,
especially to the second mixture in the case of application of
multiple layers. The particle diameter of these supplements, in
particular embodiments, is 1/3- 1/10 of the total layer thickness
of the layer. Rather than copper, the supplement may also be an
inert material such as a metal oxide. In this way, it is possible
to achieve improved formation of pores or channels.
[0151] An illustrative process for producing a gas diffusion
electrode may thus proceed, for example, as follows: the GDE can be
produced using a dry calendering method in which a mixture of a
cold-flowing polymer (preferably PTFE) and the respective
pre-calcined catalyst powder comprising copper and optionally a
promoter is produced in an intensive mixing apparatus or laboratory
scale with a knife mill (IKA). The mixing procedure may, for
example, follow the following procedure, but is not restricted
thereto: grinding/mixing for 30 sec and pause for 15 sec for a
total of 6 min, these figures being based, for example, on the
knife mill with a total loading of 50 g. After the mixing
operation, the mixed powder attains a slightly tacky consistency,
with fibrillation here, for example, of the binder, for example
PTFE. According to the amount of powder or polymer/chain length
chosen, there may also be variation in the mixing time before this
state is attained.
[0152] The powder mixture obtained is subsequently scattered or
sieved onto a copper mesh having a mesh size of >0.5 mm and
<1.0 mm and a wire diameter of 0.1-0.25 mm in a bed thickness of
1 mm. The powder mixture applied is then drawn down, for example,
with a coating bar. This operation can be repeated more than once
until a homogeneous layer is obtained. Alternatively, the powder
mixture can be pelletized during or after the mixing operation in
order to obtain a pourable material, for example having an
agglomerate diameter of 0.05 to 0.2 mm.
[0153] In order that the powder does not trickle through the mesh,
the reverse side of the copper mesh can be sealed with a film
subject to no further restriction. The prepared layer is compacted
with the aid of a two-roll rolling device (calender). The rolling
process itself is characterized in that a reservoir of material
forms upstream of the roll. The speed of the roll is between 0.5-2
rpm and the gap width was adjusted to the height of the carrier
+40% to 50% of the bed height Hf of the powder, or corresponds
virtually to the thickness of the mesh +0.1-0.2 mm infeed.
[0154] In addition, the calender can also be heated. Preference is
given to temperatures in the range of 20-200.degree. C., preferably
20-50.degree. C.
[0155] The catalyst itself can be processed prior to the
application in the calcined state, for example also as a metal
oxide precursor, or already in the reduced state. Mixtures of the
two forms are possible. This is also true in the case of the
intermetallic phases or alloys described, and so these can likewise
be used in the oxide form or in the metallic state. Furthermore, it
is not ruled out that the calendered electrode can be calcined
subsequently, for example at 300-360.degree. C. for 5 to 15
min.
[0156] It is advantageous for the gas diffusion electrode of the
invention, especially in the case of hydrocarbon-selective copper
catalyst electrodes, to apply a copper-PTFE base layer as a second
layer for better contact connection with nanoscale materials, while
simultaneously maintaining a high porosity. The base layer may be
characterized by a very high conductivity, for example 7 mohm/cm or
more, and preferably has a high porosity, for example of 50-70%,
and a hydrophobic character. The binder content, for example PTFE,
may be chosen, for example, between 3-30% by weight, for example
10-30% by weight. The intermediate copper layer as the second layer
may itself be catalytically active in the region of the overlap
zone with the catalyst layer as the first layer, and especially
serves for better areal electrical connection of the
electrocatalyst and can improve the availability of CO.sub.2 owing
to the high porosity. With the aid of this method, the required
amount of catalyst can be reduced by a factor of 20-30. The
corresponding electrocatalyst/binder (e.g. PTFE) mixture can, in a
first step, be sieved out onto the reverse side of the current
distributor and calendered. It is additionally also possible to
apply the 2-layer variant described as a double layer. The binder
used, especially PTFE, in particular embodiments, should be treated
beforehand in a knife mill in order to achieve fiber formation.
Particularly suitable PTFE powders have been found to be, for
example, Dyneon.RTM. TF 9205 and Dyneon.RTM. TF 1750. In order to
promote this effect, abrasive hard materials may be mixed in the
range between 0-50% by weight. The following are examples of
suitable materials: SiC, B.sub.4C, Al.sub.2O.sub.3 (high-grade
corundum), SiO.sub.2 (crushed glass), preferably in a grain size of
50-150 .mu.m. The production of the gas diffusion electrode with a
binder-based (e.g. PTFE-based) diffusion barrier is based on
multiple layers that cannot be considered in isolation from one
another, but preferably have an overlap zone of maximum breadth in
the boundary regions, for example of 1-20 .mu.m.
[0157] The method of two-layer construction additionally includes
the option of dispensing with binder materials as the first layer
within the catalyst layer, which means that it is possible to
achieve better electrical conductivity. It is likewise possible to
process very ductile or brittle powder particles. This is not
possible in a single-layer construction. In the case of
mechanically sensitive catalysts, it is possible to dispense with
the process step of the knife mill, which means that the catalyst
remains unchanged since mechanical stress resulting from the mixing
operation can be avoided.
[0158] Subsequent electrochemical activation of the electrode
obtained, in particular embodiments, can optionally be conducted,
for example by chemical or electrochemical activation, and is not
particularly restricted. An electrochemical activation procedure
may lead to penetration of cations of the conductive salt of the
electrolyte (e.g. KHCO.sub.3, K.sub.2SO.sub.4, NaHCO.sub.3, KBr,
NaBr) into the hydrophobic GDE channels, thus creating hydrophilic
regions. This effect is particularly advantageous and has not been
described to date in the literature.
[0159] In yet a further aspect, the present invention relates to an
electrolysis cell comprising a gas diffusion electrode of the
invention, which is preferably used as cathode. In particular
embodiments, the gas diffusion electrodes of the invention can be
operated specifically in plate electrolyzers.
[0160] The further constituents of the electrolysis cell, for
instance the anode, optionally one or more membranes, inlet(s) and
outlet(s), the voltage source etc., and further optional devices
such as cooling or heating units, are not particularly restricted
in accordance with the invention, nor are anolytes and/or
catholytes that are used in such an electrolysis cell, and the
electrolysis cell, in particular embodiments, is used on the
cathode side for reduction of carbon dioxide.
[0161] In the context of the invention, the configuration of the
anode space and the cathode space is likewise not particularly
restricted.
[0162] Illustrative configurations for an exemplary construction of
a typical electrolysis cell and possible anode and cathode spaces
are shown in FIGS. 4 to 6.
[0163] Electrochemical reduction of CO.sub.2, for example, takes
place in an electrolysis cell typically consisting of an anode and
a cathode space. FIGS. 4 to 6 which follow show examples of a
possible cell arrangement. For each of these cell arrangements, it
is possible to use a gas diffusion electrode of the invention, for
example as cathode.
[0164] By way of example, the cathode space II in FIG. 4 is
configured such that a catholyte is supplied from the bottom and
then leaves the cathode space II at the top. Alternatively, the
catholyte can also be supplied from the top, as in the case, for
example, of falling-film electrodes. At the anode A which is
electrically connected to the cathode K by means of a power source
for provision of the voltage for the electrolysis, the oxidation of
a substance which is supplied from the bottom, for example, with an
anolyte takes place in the anode space I, and the anolyte together
with the oxidation product then leaves the anode space. In the
3-chamber construction shown in FIG. 4, it is additionally possible
to convey a reaction gas, for example carbon dioxide, through the
gas diffusion electrode into the cathode space II for reduction.
Although they are not shown, embodiments with a porous anode are
alternatively conceivable. In FIG. 4, the spaces I and II are
separated by a membrane M. By contrast, in the PEM (proton or ion
exchange membrane) construction of FIG. 5, the gas diffusion
electrode K and a porous anode A directly adjoin the membrane M, by
means of which the anode space I is separated from the cathode
space II. The construction in FIG. 6 corresponds to a mixed form of
the construction from FIG. 4 and the construction from FIG. 5,
wherein a construction with the gas diffusion electrode as shown in
FIG. 4 is provided on the catholyte side, whereas a construction as
in FIG. 5 is provided on the anolyte side. It will be appreciated
that mixed forms or other configurations of the electrode spaces
shown by way of example are also conceivable. Embodiments without a
membrane are additionally conceivable. In particular embodiments,
the electrolyte on the cathode side and the electrolyte on the
anode side may thus be identical, and the electrolysis
cell/electrolysis unit may not need a membrane. However, it is not
ruled out that the electrolysis cell in such embodiments has a
membrane, although this is associated with additional cost and
inconvenience with regard to the membrane and also to the voltage
applied. Catholyte and anolyte may also optionally be mixed again
outside the electrolysis cell.
[0165] FIGS. 4 to 6 are schematic diagrams. The electrolysis cells
from FIGS. 4 to 6 may also be combined to form mixed variants. For
example, the anode space may be configured as a PEM half-cell, as
in FIG. 5, while the cathode space consists of a half-cell
containing a certain electrolyte volume between membrane and
electrode, as shown in FIG. 4. In particular embodiments, the
distance between electrode and membrane is very small or 0 when the
membrane is porous and includes a feed of the electrolyte. The
membrane may also have a multilayer configuration, such that
separate feeds of anolyte and catholyte are enabled. Separation
effects are achieved in the case of aqueous electrolytes, for
example, through the hydrophobicity of interlayers. Conductivity
can nevertheless be assured if conductive groups are integrated
into separation layers of this kind. The membrane may be an
ion-conducting membrane, or a separator that brings about
mechanical separation only and is permeable to cations and
anions.
[0166] The use of the gas diffusion electrode of the invention
makes it possible to construct a three-phase electrode. For
example, a gas can be guided from behind toward the electrically
active front side of the electrode in order to conduct an
electrochemical reaction there. In particular embodiments, there
may also merely be flow along the back of the gas diffusion
electrode, meaning that a gas such as CO.sub.2 is guided along the
back side of the gas diffusion electrode relative to the
electrolyte, in which case the gas can penetrate through the pores
of the gas diffusion electrode and the product can be removed at
the back. Preferably, the gas flow in the case of backflow is the
reverse of the electrolyte flow, in order that any liquid forced
through can be transported away. In this case too, a gap between
the gas diffusion electrode and the membrane as electrolyte
reservoir is advantageous.
[0167] By virtue of the sufficient porosity of the gas diffusion
electrode, two modes of operation are thus possible: one cell
variant (a) enables direct active flow of a gas such as CO.sub.2
through the GDE. The products formed are removed from the
electrolysis cell through the catholyte outlet and separated from
the liquid electrolyte in a downstream phase separator. A
disadvantage of this method is the elevated mechanical stress on
the GDE and partial or complete forcing of the electrolyte out of
the pores. Disadvantages are likewise found to be the elevated
occurrence of gas in the electrolyte space and displacement of the
electrolyte. For the mode of operation, in addition, a high excess
of CO.sub.2 is required. In particular embodiments, only gas
diffusion electrodes having a porosity >70% and elevated
mechanical stability are suitable for this mode of operation. The
second cell variant describes a mode of operation in which the
CO.sub.2 flows within the rear region of the GDE by virtue of an
adjusted gas pressure. The gas pressure here should be chosen such
that it is equal to the hydrostatic pressure of the electrolyte in
the cell, such that no electrolyte is forced through. An essential
advantage of the cell variant is a higher conversion of the
reaction gas used, for example CO.sub.2, compared to the flow
variant.
[0168] In order still to prevent passage of electrolyte through the
gas diffusion electrode, it is possible to apply a film on the side
of the gas diffusion electrode remote from the electrolyte, i.e. on
the carrier, for example a mesh, in order to prevent the
electrolyte from passing through to the gas. The film here may be
provided suitably and is hydrophobic for example.
[0169] In particular embodiments, the electrolysis cell has a
membrane which separates the cathode space and the anode space of
the electrolysis cell in order to prevent mixing of the
electrolytes. The membrane here is not particularly restricted,
provided that it separates the cathode space and the anode space.
More particularly, it essentially prevents passage of the gases
that form at the cathode and/or anode through to the anode or
cathode space. A preferred membrane is an ion exchange membrane,
for example in polymer-based form. A preferred material for an ion
exchange membrane is a sulfonated tetrafluoroethylene polymer such
as Nafion.RTM., for example Nafion.RTM. 115. As well as polymer
membranes, it is also possible to use ceramic membranes, for
example the polymers that are mentioned in EP 1685892 A1 and/or are
laden with zirconia, for example polysulfones.
[0170] The material for the anode is likewise not particularly
restricted and depends primarily on the reaction desired.
Illustrative anode materials include platinum or platinum alloys,
palladium or palladium alloys and glassy carbon. Further anode
materials are also conductive oxides such as doped or undoped
TiO.sub.2, indium tin oxide (ITO), fluorine-doped tin oxide (FTO),
aluminum-doped zinc oxide (AZO), iridium oxide, etc. Optionally,
these catalytically active compounds can also be applied merely
superficially in thin-film methodology, for example on a titanium
carrier.
[0171] The electrolysis cells from FIGS. 4 to 6 can also be
combined to form mixed variants. For example, the anode space can
be configured as a proton exchange membrane (PEM) half-cell, while
the cathode space consists of a half-cell containing a certain
electrolyte volume between membrane and electrode. In the ideal
case, the distance between electrode and membrane is very small or
0 when the membrane is porous and includes a feed of the
electrolyte. The membrane may also have a multilayer configuration,
such that separate feeds of anolyte and catholyte are enabled.
Separation effects are achieved in the case of aqueous
electrolytes, for example, through the hydrophobicity of
interlayers. Conductivity can nevertheless be assured if conductive
groups are integrated into separation layers of this kind. The
membrane may be an ion-conducting membrane, or a separator that
brings about mechanical separation only.
[0172] For the distribution of a reaction gas, for example
CO.sub.2, behind a gas diffusion electrode of the invention, i.e.
on the carrier side, various gas distribution chambers may be
provided, of which two illustrative gas distribution chambers are
shown in FIGS. 7 and 8. These may be provided in order to further
increase the residence time of a reaction gas such as CO.sub.2 and
the associated conversion. The gas distributors, especially in the
case of a gas diffusion electrode with backflow, can contribute to
enhanced mass transfer across the entire electrode area.
[0173] Further aspects of the present invention relate to an
electrolysis system comprising an electrode of the invention or an
electrolysis cell of the invention, and the use of the gas
diffusion electrode of the invention in an electrolysis cell or
electrolysis system.
[0174] The further constituents of the electrolysis system are not
restricted any further and can be provided suitably.
[0175] The above embodiments, executions and developments can, if
viable, be combined with one another as desired. Further possible
configurations, developments and implementations of the invention
also include combinations that have not been mentioned explicitly
of features of the invention that have been described above or are
described hereinafter with regard to the working examples. More
particularly, the person skilled in the art will also add
individual aspects as improvements or supplementations to the
respective basic form of the present invention.
[0176] The invention is described hereinafter by some illustrative
embodiments, but these do not restrict the invention.
EXAMPLES
[0177] All experiments and also the comparative examples and
examples were conducted at a room temperature of about 20.degree.
C.-25.degree. C., unless stated otherwise.
[0178] The pressure in the comparative examples and examples was
likewise not varied, but left at room pressure (about 1.013
bar).
[0179] The further detailed data are reported for their respective
comparative examples or examples.
COMPARATIVE EXAMPLES (NEGATIVE EXPERIMENTS)
Comparative Example 1
[0180] In comparative example 1, a multilayer gas diffusion
electrode was produced according to the instructions of R. Cook (J.
Electrochem. Soc. 1990, 137, 2).
[0181] The hydrophobic gas transport layer was produced according
to the publication:
[0182] 2.5 g of Vulkan XC 72 and 2.8 g of Teflon 30B (DuPont) were
dispersed in 25 mL of water and applied to a dense copper mesh (100
mesh). The layer applied was dried under air and compressed at 344
bar for 2 min. This procedure was used to produce a total of three
layers. This was followed by the compression application of three
further catalyst-containing layers having the following mixing
ratio: 2.5 g of Vulkan XC 72, 2.61 g of Cu(OAc).sub.2*H.sub.2O,
0.83 g of Teflon 30B, dispersed in 25 mL of H.sub.2O. Each layer
applied was dried under air and then compressed at 69 bar. The
finished GDE was activated at 324.degree. C. in a 10% by volume
H.sub.2/Ar gas mixture for 3-4 h and finally compressed once again
at 69 bar for 30 sec.
[0183] Result: No mechanically stable GDE was obtained over an area
of 3.3 cm.sup.2. The drying procedure led to unwanted
"mud-cracking" of the layer.
[0184] Electrochemical characterization was accomplished using a
test setup that corresponds essentially to that of the
above-described electrolysis system from FIG. 6 with flow cells for
electrolysis.
[0185] In the flow cell, the cathode used was the particular gas
diffusion electrode (GDE) with an active area of 3.3 cm.sup.2, the
gas feed rate of carbon dioxide on the cathode side was 50 mL/min,
and the electrolyte flow rate on both sides was 130 mL/min. The
anode was iridium oxide on a titanium carrier with an active area
of 10 cm.sup.2. The catholyte was a 1 M KHCO.sub.3 solution with
KHCO.sub.3 in a 1 M concentration, and the anolyte was 1 M
KHCO.sub.3, each in deionized water (18 MS)), each in an amount of
100 mL, and the temperature was 25.degree. C. In addition, 0.5 M
K.sub.2SO.sub.4 was also tried as catholyte, and 2.5 M KOH as
anolyte.
[0186] In the electrochemical characterization of the GDE, it was
not possible to detect any ethylene, but exclusively hydrogen along
with small proportions of CO.
Comparative Example 2
[0187] In a further experiment, the water dispersant was exchanged
for ethylene glycol, and comparative example 2 otherwise
corresponds to comparative example 1 unless stated otherwise. The
use of the higher-boiling dispersant prevented cracking, but it was
again not possible to detect any ethylene selectivity.
[0188] The following method was used for this purpose:
[0189] 1.440 g of Vulkan XC 72 (49.5% by weight, 3.2 mg/cm.sup.2)
were mixed vigorously with 15 mL of ethylene glycol with a
disperser within 1 h. Then 2.44 g of a PTFE suspension (Teflon 30B,
50.41% by weight, 3.25 mg/cm.sup.2) were added while stirring. The
mixture was applied to a copper mesh that corresponded to the one
used in comparative example 1 with a coating bar with a thickness
of 100 .mu.m and dried under air for at least 24 h. Then the three
further catalyst-containing layers were applied as in comparative
example 1. Subsequently, the solvent was removed in a drying
cabinet at 270.degree. C. with a ramp of 10 K/min and isothermal
conditions for 1 h. Thereafter, a layer corresponding to the first
layer was applied (thickness 100 .mu.m) and the solvent was again
removed as above and left to dry under air for 24 h. The electrode
was then calcined in an oven at 350.degree. C. with a ramp of 10
K/min and isothermal conditions for 2 h and compressed at 5 bar and
160.degree. C. for 2 min.
Comparative Examples 3.1-3.5
[0190] The substrate used in comparative example 3.1 was a
commercially available carbon cloth for gas diffusion electrodes
(Flat.RTM. LT1400W, NuVant) in the form of a microporous layer.
[0191] A Nafion.RTM. D521 dispersion was applied to this gas
diffusion layer as electrocatalyst, which was produced as follows:
0.87 g of Cu(OAc).sub.2.H.sub.2O was dissolved in about 1 mL of
H.sub.2O. In addition, 1.36 g of Vulkan XC 72 were mixed with 15 mL
of ethylene glycol and the dissolved Cu(OAc).sub.2 was added and
dispersed for 1 h. Thereafter, 1.5 g of the Nafion.RTM. D521
suspension were added and stirred with a glass rod. Thereafter, the
mixture was applied to the hydrophobic gas diffusion layer, and
dried under air and then in a drying cabinet at 120.degree. C. for
2 h. This was followed by calcining in an oven at 250.degree. C.
with a slope of 10 K/min in an atmosphere of 10% by volume of
H.sub.2 in argon, and the calcining was continued under isothermal
conditions for a total of 240 min.
[0192] The electrode thus obtained was subsequently characterized
in terms of its electrochemical properties with a test setup that,
apart from the GDE, corresponded to the one from comparative
example 1.
[0193] In this case, the copper catalyst was provided by reduction
of Cu(OAc).sub.2.H.sub.2O.
[0194] In the electrochemical characterization, the results shown
in FIG. 9 were achieved, which shows the Faraday efficiency as a
function of current density. A Faraday efficiency of 10% is found
for ethylene, but this is not stable for a long period.
[0195] According to comparative example 3.1, the results shown in
table 3 were achieved by variation of the carrier (copper mesh with
a mesh size of 0.25 and a wire diameter of 0.14 mm) and of the
mixture applied. In comparative example 3.2, in addition, PTFE was
used rather than Nafion.RTM..
TABLE-US-00003 TABLE 3 Amounts and results in comparative examples
3.2-3.5 Carbon Amount of Max. FE binder Nafion .RTM. Catalyst
catalyst Catalyst for C.sub.2H.sub.4 Carrier [% by wt.] [% by wt.]
precursor [mg/cm.sup.2 ] [% by wt.] [%] Cu mesh 44.5 8.94 Cu
(OAc).sub.2 8.7 46.56 0.8% d = 0.14 (PTFE) [500 mA/cm.sup.2] Cu
mesh 44.2 16.8 Cu (OAc).sub.2 14.2 39 1.6% d = 0.14 [400
mA/cm.sup.2] Elat .RTM. 44.2 23.3 Cu (OAc).sub.2 14.2 39 3.8%
LT1400W [400 mA/cm.sup.2] Elat .RTM. 44 17.4 Cu (OAc).sub.2 17.2
38.6 0.2% LT1400W [600 mA/cm.sup.2]
Comparative Examples 4.1-4.4
[0196] A multilayer gas diffusion electrode was produced as in
comparative example 3.1, using a Cu/ZrO.sub.2 catalyst that had
been obtained from Cu.sub.8Zr.sub.3 as catalyst. In comparative
examples 4.2 and 4.4, the GDE was additionally reduced prior to the
measurement, 4.3 relates to an electrochemically activated
electrode and 4.4 relates to a hydrogen-activated electrode. The
amounts used and results obtained in comparative examples 4.1-4.4
are shown in table 4, with the results additionally shown in FIGS.
10 and 11 for comparative example 4.3. In this context, FIG. 10
shows a current series, and FIG. 11 a measurement at constant
current.
TABLE-US-00004 TABLE 4 Amounts and results in comparative examples
4.1-4.4 Carbon Amount of Max. FE binder Nafion .RTM. Catalyst
catalyst Catalyst for C.sub.2H.sub.4 Carrier [% by wt.] [% by wt.]
precursor [mg/cm.sup.2] [% by wt.] [%] Elat .RTM. 29.5 1.8
CuO/ZrO.sub.2 68.7 35.5 0.5% LT1400W [300 mA/cm.sup.2] Elat .RTM.
29.5 1.8 CuO/ZrO.sub.2 68.7 35.5 0.2% LT1400W [400 mA/cm.sup.2]
Elat .RTM. -- 2.4 CuO/ZrO.sub.2 97.6 35.5 7.3% LT1400W [300
mA/cm.sup.2] Elat .RTM. -- 2.4 CuO/ZrO.sub.2 97.6 35.5 3.3% LT1400W
[300 mA/cm.sup.2]
[0197] In the case of use of Cu/ZrO.sub.2 as catalyst, a stable
product spectrum was obtained over an electrolysis time of 150
min.
[0198] In general, the charcoal-based GDEs in comparative examples
1-4 showed elevated Faraday efficiencies for hydrogen. It was
concluded from this that carbon in the form of conductive blacks or
activated carbons is less suitable for the production of
ethylene-selective gas diffusion electrodes.
Comparative Example 5
[0199] Subsequently, therefore, a GDE based on an aqueous PTFE
dispersion with pure copper powder having a grain size of <45
.mu.m was produced in accordance with the method in Chemical
Engineering and Processing 52 (2012) 125-131. In this method, there
was absolutely no use of carbon in the form of conductive blacks or
activated carbons.
[0200] The material used was the following:
[0201] PTFE suspension: TF5035R, 58% by weight (Dyneon.TM.),
Surfactant: Triton-100 (Fluka Chemie AG)
[0202] Thickener: hydroxyethyl methylcellulose (WalocelMKX 70000 PP
01, Wolff Cellulosics GmbH & Co. KG).
[0203] As the starting mixture, a solution that contained 97% by
weight of Cu and 3% by weight of PTFE was produced as follows: 150
g of thickener solution (1% by weight of methylcellulose in
H.sub.2O), 90.0 g of copper powder, 53.7 g of H.sub.2O and 1.5 g of
surfactant were dispersed with an Ultra-Turrax T25 disperser at 13
500 rpm for 5 min (wait for 2 min after dispersing for 1 min).
[0204] Thereafter, 4.8 g of PTFE suspension were stirred in with a
glass rod and the suspension obtained was applied to a copper mesh
as used in comparative example 3.2 at 100.degree. C.
[0205] A further GDE was produced on the basis of 0.5% by weight of
PTFE by the same procedure. The gas diffusion electrodes produced
had very poor wettabilities and, in the case of the 0.5% PTFE
content, poor porosities, as determined visually and by microscopy.
In addition, it was found that the GDEs contained considerable
proportions of the surfactant used, which was identified as a
catalyst poison in a controlled experiment. It was likewise not
possible to drive out the corresponding catalyst poison Triton X
100 ((p-tert-octyl-phenoxy)polyethoxyethanol) without residue at
temperatures of >340.degree. C., as confirmed by scanning
electron microscopy.
[0206] Exclusively hydrogen was obtained with the electrodes
produced by this procedure. The experiments made it clear that the
use of surfactants is disadvantageous for the formation of
ethylene. The method likewise did not lead to homogeneous
porosities and, in the case of 3% by weight of PTFE, led to very
poor wettability.
Reference Example 1
Production of a Mixed Metal Oxide Catalyst by Coprecipitation:
[0207] Illustrative Method for Cu/Al.sub.2O.sub.3
[0208] An appropriate hydrotalcite precursor of the composition
[Cu.sub.0.6Al.sub.0.4(OH).sub.2](CO.sub.3).sub.0.4.mH.sub.2O
(unknown water content for the freshly precipitated hydrotalcite)
is prepared by a coprecipitation. Simultaneously added are a 0.41 M
metal salt solution (A) composed of Cu(NO.sub.3).sub.2.3H.sub.2O
(0.246 mol) and Al(NO.sub.3).sub.3.9H.sub.2O (0.164 M) and a
hydroxide/carbonate solution (B) composed of 0.3 M NaOH (12 g),
0.045 M (NH.sub.4).sub.2CO.sub.3 (4.32 g), such that the pH is
between pH 8 and 8.5.
[0209] The addition rate of the metal salt solution was chosen as
120 mL/h. Oswalt ripening was effected for 30 min. Thereafter, the
solids were filtered off and washed to neutrality. Thereafter, the
precursor was dried at 80.degree. C. for 12 h, pulverized and
calcined. The calcination step is effected in a tubular furnace
having a temperature ramp of .beta.=2 K/min up to 300.degree. C.
with isothermal conditions for 4 h in an argon/oxygen mixture: 20%
by volume of O.sub.2/Ar with a flow rate of 200 sccm. The precursor
prepared was sieved before use.
Examples: Production of Powder-Based GDEs
Comparative Example 6
[0210] A catalyst powder is prepared by coprecipitation of
Cu(NO.sub.3).sub.2.3H.sub.2O and ZrO(NO.sub.3).sub.2.xH.sub.2O
according to reference example 1 with the respective molar amounts
(mol). The pre-calcined catalyst powder (weight 45 g; particle size
<75 .mu.m by sieve analysis) is mixed on the laboratory scale
with a knife mill (IKA) (on a large scale, for example, with an
intensive mixing apparatus) with PTFE particles (weight 5 g;
Dyneon.RTM. TF 1750; particle size (d50)=8 .mu.m according to
manufacturer). The mixing procedure follows the following
procedure: grinding/mixing for 30 sec and wait for 15 sec for a
total of 6 min. This statement is based on the knife mill with
total loading 50 g. The mixed powder attains a slightly tacky
consistency after the mixing operation. The mixing time before this
state is attained may also vary according to the amount of powder
or the polymer chosen or the chain length. The powder mixture
obtained is subsequently applied by scattering or sieving to a
copper mesh having a mesh size of >0.5 mm and <1.0 mm and a
wire diameter of 0.1-0.25 mm in a bed thickness of 1 mm.
[0211] In order that the powder does not trickle through the mesh,
the reverse side of the copper mesh can be sealed with a film
subject to no further restriction. The prepared layer is compacted
with the aid of a two-roll rolling device (calender). The rolling
process itself is characterized in that a reservoir of material
forms upstream of the roll. The speed of the roll is between 0.5-2
rpm and the gap width was adjusted to the height of the carrier
+40% to 50% of the bed height Hf of the powder, or corresponds
virtually to the thickness of the mesh +0.1-0.2 mm infeed.
[0212] The gas diffusion electrode obtained is activated in an
electrolysis bath in a 1 M KHCO.sub.3 solution at a current density
of 15 mA/cm.sup.2 for 6 h.
Comparative Example 7
[0213] Dendritic copper powder (45 g; particle size <45 .mu.m,
determined by sieving with appropriate mesh size (45 .mu.m)) is
mixed with 5 g of PTFE in an IKA knife mill by the procedure
described in comparative example 6, and processed under the same
conditions to give a GDE. After activation, the GDE described gave
a Faraday efficiency of 16% at 170 mA/cm.sup.2, which remained
constant over the measurement time of about 90 min.
Comparative Example 8
[0214] Cu.sub.10Zr.sub.7 is calcined in a tubular furnace with a
temperature ramp of .beta.=2 K/min up to 600.degree. C. with
isothermal conditions for 4 h in an argon/oxygen mixture (20% by
volume of O.sub.2/Ar with a flow rate of 200 sccm). The oxide
precursor prepared, prior to use, is ground in a planetary ball
mill (Pulverisette) for 3 min and subsequently sieved (particle
size <75 .mu.m). 45 g of the catalyst obtained are mixed with 5
g of PTFE in an IKA knife mill by the procedure described in
comparative example 6 and processed under the same conditions to
give a GDE.
[0215] The GDEs from comparative examples 6 to 8 can be used in an
electrolysis cell as described above or hereinafter, for example as
cathode with which CO.sub.2 can be reduced.
Example 1
Production of a 2-Layer Electrode
[0216] Copper powder with a particle diameter of 100-200 .mu.m and
PTFE TF 1750 Dyneon were mixed in an IKA A10 knife mill for 6 min
(grinding for 15 sec, wait for 30 sec). The powder layer was then
sieved off and graded by means of a template of thickness 0.5 mm to
form a base layer. This was followed by extrusion with a 2-roll
calender with a roll separation of 0.5 mm. Thereafter, a catalyst
layer was applied by sieving application, for example in each case
analogously to comparative examples 6 to 8, through a 0.2 mm frame,
and extrusion was again effected with a 2-roll calender with a roll
separation of 0.35 mm. The result was a highly porous base layer
with a porosity of >70%, good mechanical stability and very good
conductivity at 5 mohm/cm. It was possible to use catalysts with a
copper content of 40% by weight.
[0217] Preferably, the catalysts had a purity above the
commercially available materials or quality standards, as in the
example as well. This was detectable by means of
(surface-sensitive) XPS. SEM/EDX mapping analyses likewise did not
indicate any impurities at all in the hydrophobic base layer.
[0218] It was additionally found that a copper content of >70%
is advantageous in order to enable a low electrical resistance of
the catalyst. The effect of the binder (PTFE) content with respect
to the carrier oxide is much smaller in terms of the effect on
conductivity.
[0219] Illustrative construction of a typical electrolysis cell:
The electrochemical reduction of the CO.sub.2 takes place in an
electrolysis cell which typically consists of an anode space and a
cathode space. FIGS. 4 to 6 show examples of a possible cell
arrangement. The concept presented hereinafter is applicable to
each of these cell arrangements.
[0220] The electrolysis cells from FIGS. 4 to 6 can also be joined
to form mixed variants. For example, the anode space can be
executed as a proton exchange membrane (PEM) half-cell, while the
cathode space consists of a half-cell containing a certain
electrolyte volume between membrane and electrode. In the ideal
case, the distance between electrode and membrane is very small or
0 when the membrane is porous and includes a feed of the
electrolyte. The membrane may also have a multilayer configuration,
such that separate feeds of anolyte and catholyte are enabled.
Separation effects are achieved in the case of aqueous
electrolytes, for example, through the hydrophobicity of
interlayers. Conductivity can nevertheless be assured if conductive
groups are integrated into separation layers of this kind. The
membrane may be an ion-conducting membrane, or a separator that
brings about mechanical separation only.
[0221] The present invention provides the possibility of producing
ethylene-selective, dimensionally stable gas diffusion electrodes
based on catalyst powder. This technique constitutes the basis for
the production of electrodes on a larger scale, which can achieve
current densities of >170 mA/cm.sup.2 according to the mode of
operation. All the methods known to date for production of
ethylene-selective copper electrodes are unsuitable for scaleup or
are not dimensionally stable. Gas diffusion electrodes of the
invention, by contrast, can be obtained by suitable adjustment of a
rolling process, especially a calendering process.
[0222] It is possible in accordance with the invention to obtain
highly electrically conductive, especially metal oxide-stabilized,
copper catalysts with copper nanostructures that enable oxidation
cycling between Cu(I)/Cu(0).
[0223] In particular embodiments, the production of the gas
diffusion electrode of the invention is additionally based on the
exclusion of conductive fillers based on charcoals or carbon
blacks. The charcoal substitute used here is the catalyst itself or
dendritic copper or mixtures of the two. Moreover, the method of
the invention, in particular embodiments, does not need
surface-active substances/surfactants or thickeners and additives
(such as flow improvers) which have been identified as catalyst
poisons.
* * * * *