U.S. patent application number 16/763157 was filed with the patent office on 2020-12-10 for ethylene-selective electrode with a mixed valence cu4o3 catalyst.
This patent application is currently assigned to Siemens Aktiengesellschaft. The applicant listed for this patent is Siemens Aktiengesellschaft. Invention is credited to Nemanja Martic, David Reinisch, Christian Reller, Bernhard Schmid, Gunter Schmid.
Application Number | 20200385877 16/763157 |
Document ID | / |
Family ID | 1000005092435 |
Filed Date | 2020-12-10 |
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United States Patent
Application |
20200385877 |
Kind Code |
A1 |
Martic; Nemanja ; et
al. |
December 10, 2020 |
ETHYLENE-SELECTIVE ELECTRODE WITH A MIXED VALENCE CU4O3
CATALYST
Abstract
An electrode including Cu.sub.4O.sub.3, in particular an
ethylene-selective electrode with a mixed valence Cu.sub.4O.sub.3
catalyst. A method for producing an electrode of this type, an
electrolytic cell having an electrode of this type, and a method
for electrochemically converting carbon dioxide using such an
electrode including Cu.sub.4O.sub.3.
Inventors: |
Martic; Nemanja; (Erlangen,
Bayern, DE) ; Reller; Christian; (Minden, DE)
; Schmid; Gunter; (Hemhofen, DE) ; Schmid;
Bernhard; (Duren, DE) ; Reinisch; David;
(Bamberg, Bayern, BA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Aktiengesellschaft |
Munich |
|
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
Munich
DE
|
Family ID: |
1000005092435 |
Appl. No.: |
16/763157 |
Filed: |
October 19, 2018 |
PCT Filed: |
October 19, 2018 |
PCT NO: |
PCT/EP2018/078704 |
371 Date: |
May 11, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 3/04 20130101; C23C
24/02 20130101; C25B 11/035 20130101; C25B 11/0452 20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 11/03 20060101 C25B011/03; C25B 3/04 20060101
C25B003/04; C23C 24/02 20060101 C23C024/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2017 |
DE |
10 2017 220 450.8 |
Claims
1. An electrode, comprising: Cu.sub.4O.sub.3.
2. The electrode as claimed in claim 1, wherein the Cu.sub.4O.sub.3
is present in an amount of 0.1-100% by weight based on the
electrode.
3. The electrode as claimed in claim 1, wherein the Cu.sub.4O.sub.3
has been applied to a support.
4. The electrode as claimed in claim 3, wherein the Cu.sub.4O.sub.3
has been applied with a mass coverage of at least 0.5
mg/cm.sup.2.
5. The electrode as claimed in claim 1, wherein the electrode is a
gas diffusion electrode.
6. An electrolysis cell, comprising: an electrode as claimed in
claim 1.
7. A process for producing an electrode comprising Cu.sub.4O.sub.3
on a substrate, the process comprising: preparing a mixture
comprising Cu.sub.4O.sub.3 and optionally at least one binder, or
providing a powder consisting of Cu.sub.4O.sub.3, applying the
mixture comprising Cu.sub.4O.sub.3 or the powder consisting of
Cu.sub.4O.sub.3 to a support, preferably in the form of a sheetlike
structure, and dry rolling the mixture comprising Cu.sub.4O.sub.3
or the powder consisting of Cu.sub.4O.sub.3 onto the support.
8. The process as claimed in claim 7, wherein the preparing of the
mixture comprises mixing for 60-200 s.
9. The process as claimed in claim 7, wherein the rolling
application is effected at a temperature of 25-100.degree. C.
10. A process for producing an electrode comprising Cu.sub.4O.sub.3
on a substrate, the process comprising: providing a support,
applying a suspension comprising Cu.sub.4O.sub.3 and optionally at
least one binder to the support, and drying the suspension; or
providing a support, and applying Cu.sub.4O.sub.3 or a mixture
comprising Cu.sub.4O.sub.3 from the gas phase.
11. The process as claimed in claim 10, wherein the support is a
gas diffusion electrode or gas diffusion layer.
12. The process as claimed in claim 7, wherein the at least one
binder is present in the mixture.
13. The process as claimed in claim 12, wherein the at least one
binder is present in the mixture in an amount of >0 to 30% by
weight, based on the total weight of Cu.sub.4O.sub.3 and the at
least one binder.
14. A process for producing the electrode of claim 1, the process
comprising: producing a powder comprising Cu.sub.4O.sub.3; and
rolling the powder out to give an electrode.
15. A process for electrochemical conversion of carbon dioxide
and/or carbon monoxide, the process comprising: introducing carbon
dioxide and/or carbon monoxide at the cathode into an electrolysis
cell comprising an electrode as claimed in claim 1 as cathode and
reduced.
16. A process for reduction of CO.sub.2 and/or CO, comprising:
using the electrode of claim 1.
17. A process for the electrolysis of CO.sub.2 and/or CO,
comprising: using the electrode of claim 1.
18. The electrode as claimed in claim 2, wherein the
Cu.sub.4O.sub.3 is present in an amount of 40-100% by weight based
on the electrode.
19. The electrode as claimed in claim 2, wherein the
Cu.sub.4O.sub.3 is present in an amount of 70-100% by weight based
on the electrode.
20. The process as claimed in claim 9, wherein the rolling
application is effected at a temperature of 60-80.degree. C.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2018/078704 filed 19 Oct. 2018, and claims
the benefit thereof. The International Application claims the
benefit of German Application No. DE 10 2017 220 450.8 filed 16
Nov. 2017. All of the applications are incorporated by reference
herein in their entirety.
FIELD OF INVENTION
[0002] The present invention relates to an electrode comprising
Cu.sub.4O.sub.3, to a process for producing such an electrode, to
an electrolysis cell comprising such an electrode, and to a process
for electrochemical conversion of carbon dioxide and/or carbon
monoxide using such an electrode.
BACKGROUND OF INVENTION
[0003] At present, about 80% of global energy demand is covered by
the combustion of fossil fuels, the combustion processes of which
cause global emission of about 34 000 million tonnes of carbon
dioxide into the atmosphere per annum. This release into the
atmosphere disposes of the majority of carbon dioxide, which can be
up to 50 000 tonnes per day in the case of a brown coal power
plant, for example. Carbon dioxide is one of the gases known as
greenhouse gases, the adverse effects of which on the atmosphere
and the climate are a matter of discussion. It is a technical
challenge to produce products of value from CO.sub.2. Since carbon
dioxide is at a very low thermodynamic level, it can be reduced to
reutilizable products only with difficulty, which has left the
actual reutilization of carbon dioxide in the realm of theory or in
the academic field to date.
[0004] Natural carbon dioxide degradation proceeds, for example,
via photosynthesis. This involves conversion of carbon dioxide to
carbohydrates in a process subdivided into many component steps
over time and, at the molecular level, in terms of space. As such,
this process cannot easily be adapted to the industrial scale. No
copy of the natural photosynthesis process with photocatalysis on
the industrial scale to date has had adequate efficiency.
[0005] An alternative is the electrochemical reduction of carbon
dioxide. Systematic studies of the electrochemical reduction of
carbon dioxide are still a relatively new field of development.
Only in the last few years have there been efforts to develop an
electrochemical system that can reduce an acceptable amount of
carbon dioxide. Research on the laboratory scale has shown that
electrolysis of carbon dioxide is advantageously accomplished using
metals as catalysts. For example, the publication "Electrochemical
CO.sub.2 reduction on metal electrodes by Y. Hori", published in:
C. Vayenas, et al. (eds.), Modern Aspects of Electrochemistry,
Springer, New York, 2008, p. 89-189, gives Faraday efficiencies at
different metal cathodes which are listed in table 1 below, taken
from this publication.
TABLE-US-00001 TABLE 1 Faraday efficiencies in the electrolysis of
CO.sub.2 over various electrode materials Elec- trode 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
[0006] The table reports Faraday efficiencies [%] of products that
form in the reduction of carbon dioxide at various metal
electrodes. The values reported are applicable here to a 0.1 M
potassium hydrogencarbonate solution as electrolyte and current
densities below 10 mA/cm.sup.2.
[0007] While carbon dioxide is reduced almost exclusively to carbon
monoxide at silver, gold, zinc, palladium and gallium cathodes, for
example, a multitude of hydrocarbons form as reaction products at a
copper cathode.
[0008] For example, in an aqueous system, predominantly carbon
monoxide and a little hydrogen would form at a silver cathode. The
reactions at anode and cathode in that case can be represented by
way of example by the following reaction equations:
Cathode: 2 CO.sub.2+4 e.sup.-+4 H.sup.+.fwdarw.2 CO+2 H.sub.2O
Anode: 2 H.sub.2O.fwdarw.O.sub.2+4 H.sup.++4 e.sup.-
[0009] Of particular economic interest, for example, is the
electrochemical production of carbon monoxide, ethylene or
alcohols.
[0010] Examples:
Carbon monoxide: CO.sub.2+2e.sup.-+H.sub.2O.fwdarw.CO+2OH.sup.-
Ethylene: 2 CO.sub.2+12 e.sup.-+8 H.sub.2O.fwdarw.C.sub.2H.sub.4+12
OH.sup.-
Methane: CO.sub.2+8 e.sup.-+6 H.sub.2O.fwdarw.CH.sub.4+8
OH.sup.-
Ethanol: 2 CO.sub.2+12 e.sup.-+9
H.sub.2O.fwdarw.C.sub.2H.sub.5OH+12 OH.sup.-
Monoethylene glycol: 2 CO.sub.2+10 e.sup.-+8
H.sub.2O.fwdarw.HOC.sub.2H.sub.4OH+10 OH.sup.-
[0011] The reaction equations show that, for the production of
ethylene from CO.sub.2, for example, 12 electrons have to be
transferred.
[0012] The stepwise reaction of CO.sub.2 proceeds via a multitude
of surface intermediates (--CO.sub.2.sup.-, --CO, .dbd.CH.sub.2,
--H, etc). For each of these intermediates, there should
advantageously be a strong interaction with the catalyst surface or
the active sites, such that a surface reaction (or further
reaction) between the corresponding adsorbates is enabled. Product
selectivity is thus directly dependent on the crystal surface or
interaction thereof with the surface species. For example, an
elevated ethylene selectivity has been shown by experiments on
monocrystalline high-index surfaces (Cu 711, 511) in Journal of
Molecular Catalysis A Chemical 199(1):39-47, 2003. Materials that
have a high number of crystallographic levels or have surface
defects likewise have elevated ethylene selectivities, as shown in
C. Reller, R. Krause, E. Volkova, B. Schmid, S. Neubauer, A. Rucki,
M. Schuster, G. Schmid, Adv. Energy Mater. 2017, 1602114 (DOI:
10.1002/aenm.201602114), and DE102015203245 A1.
[0013] There is thus a close relationship between the nanostructure
of the catalyst material and the ethylene selectivity. As well as
the property of selectively forming ethylene, the material should
retain its product selectivity even at high conversion rates
(current densities), i.e. the advantageous structure of the
catalyst centers should be conserved. However, owing to high
surface mobility of copper, the defects or nanostructures generated
typically do not have prolonged stability, and so, even after a
short time (60 min), degradation of the electrocatalyst can be
observed. As a result of the structural alteration, the material
loses the propensity to form ethylene. Moreover, with voltage
applied to structured surfaces, the potentials vary easily, such
that certain intermediates are formed preferentially in a small
area at certain points, and these can then react further at a
slightly different point. As in-house studies have shown, potential
variations well below 50 mV are significant.
[0014] On the basis of numerous studies, it is now generally
acknowledged that Cu.sub.2O increases the selectivity of copper for
C.sub.2H.sub.4 and hydrocarbons. On the other hand, copper(II)
oxide (CuO), owing to its poor properties as catalyst for CO.sub.2
electroreduction, has attracted little attention. Owing to its
morphology (nanowires, dendrites, needles, thin layers, particles,
etc.) and exposed crystal planes, Cu.sub.2O can be selective for
various liquid and gaseous C1 and C2 products. However, stability
is still one of the greatest disadvantages for the use of Cu.sub.2O
phases in prolonged CO.sub.2 electroreduction, since it is not
stable to reduction under operating conditions, as apparent from
the Pourbaix diagram for copper.
[0015] There are no known catalyst systems to date that have
prolonged stability and are capable of electrochemically reducing
CO.sub.2 to ethylene at high current density >100 mA/cm.sup.2.
Current densities of industrial relevance can be achieved using gas
diffusion electrodes (GDEs). This is known from the prior art, for
example, for chlor-alkali electrolyses implemented on the
industrial scale.
[0016] Cu-based gas diffusion electrodes for production of
hydrocarbons on the basis of CO.sub.2 are already known from the
literature. In the studies by R. Cook in J. Electrochem. Soc., Vol.
137, No. 2, 1990, for example, a wet-chemical process based on a
PTFE 30B (suspension)/Cu(OAc).sub.2/Vulkan XC 72 mixture is
mentioned. The method states how a hydrophobic conductive gas
transport layer is applied using three coating cycles, and a
catalyst-containing layer using three further coating operations.
Each layer 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 were reported. Reproduction
experiments demonstrate that the static pressing method described
does not lead to stable electrodes. A disadvantageous effect of the
added Vulkan XC 72 was likewise found, and so it was likewise the
case that no hydrocarbons were obtained.
[0017] The most efficient catalysts for reduction of CO.sub.2 to
higher hydrocarbons and ethylene have to date been copper (with
various morphologies) and copper(I) oxide (with various
morphologies). One example of a current Cu catalyst for CO.sub.2
reduction can be found, for example, in Ma, S. et al., One-step
electrosynthesis of ethylene and ethanol from CO.sub.2 in an
alkaline electrolyzer, J. Power Sources 301, 219-228 (2016).
[0018] However, there is still a need for highly efficient
electrodes and electrolysis systems having prolonged stability for
efficient preparation of ethylene from carbon dioxide.
SUMMARY OF INVENTION
[0019] The inventors have found that Cu.sub.4O.sub.3 is of
excellent suitability as a catalyst having prolonged stability for
the reduction of carbon dioxide to ethylene. Cu.sub.4O.sub.3 has to
date never been used or considered as catalyst for the
electrochemical reduction of CO.sub.2. In this respect, what is
disclosed in accordance with the invention is that Cu.sub.4O.sub.3
is used as catalyst for the electrochemical reduction of CO.sub.2.
In addition, Cu.sub.4O.sub.3 may also be just a catalyst
constituent. The Cu.sub.4O.sub.3 can also be used as pre-catalyst.
Furthermore, under acidic conditions, reduction with dendrite
formation is possible. More particularly, a gas diffusion electrode
comprising Cu.sub.4O.sub.3 is disclosed as electrocatalyst for
CO.sub.2 reduction that exhibits high activity (>400
mA/cm.sup.2) and selectivity for ethylene.
[0020] The inventors have found more particularly that
advantageously gas diffusion electrodes or layers, advantageously
with at least 0.5 mg/cm.sup.2 of Cu.sub.4O.sub.3 catalyst, have the
following advantages in the electrochemical reduction of CO.sub.2
to hydrocarbons:--higher selectivity for ethylene compared to Cu,
Cu.sub.2O and CuO;--higher stability at reduction potential against
reduction to Cu;--superior activity compared to Cu, Cu.sub.2O and
CuO; and--a lower overvoltage for the reduction of CO.sub.2 to
ethylene compared to Cu, Cu.sub.2O and CuO.
[0021] In a first aspect, the present invention relates to an
electrode, especially for an electrochemical conversion in liquid
electrolytes, comprising Cu.sub.4O.sub.3.
[0022] Also disclosed is an electrolysis cell comprising the
electrode of the invention, advantageously as cathode.
[0023] Likewise disclosed is a process for producing an electrode
comprising Cu.sub.4O.sub.3 on a support, comprising--preparing a
mixture comprising Cu.sub.4O.sub.3 and optionally at least one
binder, or providing a powder consisting of
Cu.sub.4O.sub.3,--applying the mixture comprising Cu.sub.4O.sub.3
or the powder consisting of Cu.sub.4O.sub.3 to a, for example
copper-containing, support, advantageously in the form of a
sheetlike structure, and--dry rolling the mixture comprising
Cu.sub.4O.sub.3 or the powder consisting of Cu.sub.4O.sub.3 onto
the support.
[0024] Also disclosed is a process for producing an electrode
comprising Cu.sub.4O.sub.3 on a substrate, comprising--providing a
support,--applying a suspension comprising Cu.sub.4O.sub.3 and
optionally at least one binder to the support, and--drying the
suspension.
[0025] The present invention additionally also relates to a process
for producing an electrode comprising Cu.sub.4O.sub.3,
comprising--preparing a powder comprising Cu.sub.4O.sub.3;
and--rolling the powder out to give an electrode.
[0026] These processes of the invention can especially be used to
produce an electrode of the invention.
[0027] The present invention still further comprises a process for
electrochemical conversion of carbon dioxide and/or carbon
monoxide, wherein carbon dioxide and/or carbon monoxide is
introduced at the cathode into an electrolysis cell comprising an
electrode of the invention as cathode and reduced.
[0028] The present invention additionally relates to the use of
Cu.sub.4O.sub.3 for reduction of CO.sub.2, and to the use of
Cu.sub.4O.sub.3 in the electrolysis of CO.sub.2.
[0029] Further aspect of the present invention can be taken from
the dependent claims and the detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The appended drawings are intended to illustrate embodiments
of the present invention and impart further understanding thereof.
In association 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. The elements of the drawings are not necessarily shown
true to scale relative to one another. Elements, features and
components that are the same, have the same function and the same
effect are each given the same reference signs in the figures of
the drawings, unless stated otherwise.
[0031] The electrochemical stability of paramelaconite,
Cu.sub.4O.sub.3, is shown in a Pourbaix diagram in FIG. 1.
[0032] FIGS. 2 to 19 (FIGS. 17-19 with feed and removal devices,
etc.) show electrolysis cells in schematic form, in which the
electrode of the invention, especially in the form of a gas
diffusion electrode or a gas diffusion layer, can be employed, and
which are thus possible embodiments of an electrolysis cell of the
invention.
[0033] FIGS. 20 and 21 show measurement results of data recorded
with a powder x-ray diffractometer (PXRD) of powder obtained in the
examples comprising Cu.sub.4O.sub.3, and FIGS. 22 and 23 show
images of the powder with a scanning electron microscope (SEM).
[0034] FIGS. 24 to 31 show measurement results that have been
obtained in inventive examples and comparative examples in
electrolysis cells in the reduction of CO.sub.2.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
[0035] Unless defined differently, technical and scientific terms
used herein have the same meaning as commonly understood by a
person skilled in the art in the specialist field of the
invention.
[0036] An electrode is an electrical conductor that can supply
electrical current to a liquid, gas, vacuum or a solid state body.
More particularly, an electrode is not a powder or a particle, but
may comprise particles and/or a powder or be produced from a
powder. A cathode here is an electrode at which an electrochemical
reduction can take place, and an anode an electrode at which an
electrochemical oxidation can take place. The electrochemical
conversion takes place here, in particular embodiments, in the
presence of advantageously aqueous electrolytes.
[0037] Stated amounts in the context of the present invention
relate to % by weight, unless stated otherwise or apparent from the
context. In the gas diffusion electrode of the invention, the
percentages by weight add up to 100% by weight.
[0038] "Hydrophobic" in the context of the present invention is
understood to mean water-repellent. According to the invention,
hydrophobic pores and/or channels are thus those that repel water.
More particularly, hydrophobic properties are associated according
to the invention with substances or molecules having nonpolar
groups.
[0039] "Hydrophilic", by contrast, is understood to mean the
ability to interact with water and other polar substances.
[0040] The present invention relates, in a first aspect, to an
electrode comprising Cu.sub.4O.sub.3. In particular embodiments,
the electrode of the invention is a cathode, i.e. can be connected
as cathode. In particular embodiments, Cu.sub.4O.sub.3 is used as
catalyst for the electrochemical reduction of CO.sub.2. In
addition, the Cu.sub.4O.sub.3, in particular embodiments, may also
be a catalyst constituent. The Cu.sub.4O.sub.3, in particular
embodiments, may also be used as pre-catalyst. Under acidic
conditions, in addition, reduction with dendrite formation is
possible, such that the electrode of the invention, in particular
embodiments, may comprise the Cu.sub.4O.sub.3 in the form of
dendrites, although these are not particularly restricted.
[0041] Paramelaconite (Cu.sub.4O.sub.3), together with copper(I)
oxide (Cu.sub.2O) and copper(II) oxide (CuO), forms part of the
copper oxide family. Although Cu.sub.2O and CuO have been studied
in detail, less is known about Cu.sub.4O.sub.3 since it is rare and
its synthesis is complex. Cu.sub.3O.sub.4 is a metastable phase
which is not directly obtainable by thermal oxidation of
oxygen-free copper.
[0042] The copper-oxygen system is an example of a simple eutectic
system. Oxygen-rich copper contains from 0.01% to 0.05% by weight
of oxygen, but may contain up to 0.1% by weight. The solidification
thereof commences with core formation on cooling below the liquidus
temperature. With falling temperature, these cores that are
essentially pure copper become ever larger and the liquid becomes
richer in oxygen. The remaining Cu-oxygen environment in the solid
phase can form a tetragonal structure comparable to the
Cu.sub.4O.sub.4 structure, or form Cu. In copper refining, for
example, air is injected into the melt in order to oxidize
impurities, and oxygen can be absorbed by the copper in this step.
In the refining of copper, oxygen and hydrogen counteract one
another. The oxygen content can often be removed by chamfering.
Oxygen-rich copper showed much higher Faraday efficiencies for
ethylene in the electroreduction of CO.sub.2. This effect could be
connected to remaining oxygen species beneath the surface, as
described by A. Eilert, J. Phys. Chem. 2017, 8 (1), pp. 285-290.
Nevertheless, Cu.sub.4O.sub.3 and similar coordination compounds
between copper and oxygen have not been taken into account to date
in this regard.
[0043] A phase diagram for copper-oxygen <55 at % can be found,
for example, in Landoll-Bornstein--Group IV Physical Chemistry
Volume 5D: in Springer Materials A Predel, B. E Madelung,
Springer-Verlag Berlin Heidelberg 1994, p. 1097, and an oxygen
pressure-temperature phase diagram, for example, in
Landolt-Bornstein--Group IV Physical Chemistry Volume 5D: in
Springer Materials A Predel, B. E Madelung, Springer-Verlag Berlin
Heidelberg 1994, p. 1097.
[0044] Cu.sub.4O.sub.3 is a mixed-valency oxide having equal
proportions of mono- and divalent Cu ions and is therefore
sometimes also formally written as Cu.sup.+2Cu.sup.2+.sub.2O.sub.3.
The crystal structure (l4.sub.1/amd space group) of paramelaconite
has been identified as tetragonal, consisting of interpenetrating
chains of Cu.sup.+--O and Cu.sup.2+--O. The Cu.sup.2+ ions are
coordinated to two O.sup.2- ions, while the Cu.sup.+ ions have
planar coordination to four O.sup.2- ions. Paramelaconite is
thermodynamically stable below 300.degree. C.; at temperatures
above 300.degree. C. it breaks down to CuO and Cu.sub.2O.
[0045] The electrochemical stability of paramelaconite is shown in
the Pourbaix diagram in FIG. 1. The diagram shows the higher
electrochemical stability of Cu.sub.4O.sub.3 to reduction by
comparison with Cu.sub.2O. As apparent from the diagram, a
advantageous operating range of electrodes comprising
paramelaconite is between pH=6-14 or better between 10 and 14.
[0046] Although paramelaconite was first discovered in the 1870s,
the controlled synthesis of Cu.sub.4O.sub.3 crystals with reliable
phase purity using conventional aqueous chemistry has been a
challenge to date, since it is difficult to stabilize Cu.sup.2+ and
Cu.sup.+ simultaneously. There have been multiple attempts to date
to synthesize paramelaconite from the liquid phase, but all these
methods suffered from a low paramelaconite yield and small crystal
size. It is obtainable by a solvothermal method by prolonged
thermal oxidation of copper under air in the presence of boiling
aqueous NH.sub.3, as described in P. Morgan, Journal of Solid State
Chemistry, 121, 1, 5 Jan. 1996, pages 33-37.
[0047] Furthermore, in such processes, only microscopically small
amounts of Cu.sub.4O.sub.3 are prepared, which are simultaneously
also highly contaminated with CuO and Cu.sub.2O. In 2012, Zhao et
al. in Zhao, L. et al., Facile Solvothermal Synthesis of Phase-Pure
Cu.sub.4O.sub.3 Microspheres and Their Lithium Storage Properties,
Chem. Mater. 2012, 24, pages 1136-1142, described the synthesis of
single-phase paramelaconite microspheres by a simple solvothermal
method. The Cu.sub.4O.sub.3 microspheres were obtained by reacting
the copper(II) nitrate trihydrate precursor
(Cu(NO.sub.3).sub.2.3H.sub.2O) in a mixed solvent composed of
ethanol and N,N-dimethylformamide (DMF). The reaction was conducted
in a 50 mL Teflon-lined stainless steel autoclave at 130.degree. C.
over several hours. As shown in the examples, the inventors were
able, by a synthesis along the route followed by Zhao et al., to
increase the reaction volume to 1.1 L and to increase the yield to
more than 10 g.
[0048] In the electrode of the invention, the amount of
Cu.sub.4O.sub.3 is not particularly restricted. In particular
embodiments, the Cu.sub.4O.sub.3 is present in an amount of
0.1-100% by weight, advantageously 40-100% by weight, more
advantageously 70-100% by weight based on the electrode. In
particular embodiments, the Cu.sub.4O.sub.3 is present in an amount
of 0.1-100% by weight, advantageously 40-100% by weight, more
advantageously 70-100% by weight, based on the catalytically active
part of the electrode, for example in a layer of the electrode of
the invention, for example when the electrode of the invention is
in multilayer form, for example with a gas diffusion layer, and/or
in the form of a gas diffusion electrode.
[0049] In particular embodiments, Cu.sub.4O.sub.3 has been applied
to a support which is not particularly restricted, either with
regard to the material or to the composition. A support here may,
for example, be a compact solid-state body, for example in the form
of a pin or strip, for example a metal strip, for example
comprising a metal such as Cu or an alloy thereof or consisting of
a metal such as Cu or an alloy thereof, or a porous structure, for
example a sheetlike structure such as a mesh, a knit, etc., or a
coated body. The support may also take the form, for example, of a
gas diffusion electrode, optionally also having multiple, e.g. 2,
3, 4, 5, 6 or more, layers of a suitable material or of a gas
diffusion layer on a suitable substrate, which is likewise not
particularly restricted and may likewise comprise multiple layers,
e.g. 2, 3, 4, 5, 6 or more. The gas diffusion electrode or gas
diffusion layer used may correspondingly also be a commercially
available electrode or layer. The support material is
advantageously conductive and comprises, for example, a metal
and/or an alloy thereof, a ceramic, for example ITO, an inorganic
nonmetallic conductor such as carbon and/or a conductive
polymer.
[0050] It is of course not impossible that the Cu.sub.4O.sub.3 is
also used in the production of gas diffusion layers or gas
diffusion electrodes. Thus, an electrode of the invention in
particular embodiments is a gas diffusion electrode or an electrode
comprising a gas diffusion layer, wherein the gas diffusion
electrode or gas diffusion layer contains or even consists of
Cu.sub.4O.sub.3. If a gas diffusion layer comprising
Cu.sub.4O.sub.3 is present, this may have been applied to a porous
or nonporous substrate.
[0051] When the Cu.sub.4O.sub.3 has been applied to a support, in
particular embodiments, it has been applied with a mass coverage of
at least 0.5 mg/cm.sup.2. The application here is advantageously
not two-dimensional, in order to be able to provide a greater
active surface area. Moreover, the application advantageously forms
pores, or pores of the support are essentially not closed, such
that a gas such as carbon dioxide can easily reach the
Cu.sub.4O.sub.3. In particular embodiments, the Cu.sub.4O.sub.3 has
been applied with a mass coverage between 0.5 and 20 mg/cm.sup.2,
advantageously between 0.8 and 15 mg/cm.sup.2, more advantageously
between 1 and 10 mg/cm.sup.2. Proceeding from these values, the
amount of Cu.sub.4O.sub.3 as catalyst can be suitably determined
for application to a particular support.
[0052] The inventors have more particularly found that gas
diffusion electrodes or layers in particular, advantageously with
at least 1 mg/cm.sup.2 of Cu.sub.4O.sub.3 catalyst, have the
following advantages in the electrochemical reduction of CO.sub.2
to hydrocarbons:--higher selectivity for ethylene compared to Cu,
Cu.sub.2O and CuO;--higher stability at reaction potential against
reduction to Cu;--superior activity compared to Cu, Cu.sub.2O and
CuO; and--a lower overvoltage for the reduction of CO.sub.2 to
ethylene compared to Cu, Cu.sub.2O and CuO.
[0053] In particular embodiments, the electrode of the invention is
a gas diffusion electrode which is not particularly restricted and
may be in single- or multilayer form, for example with 2, 3, 4, 5,
6 or more layers. In such a multilayer gas diffusion electrode, it
is then possible, for example, for the Cu.sub.4O.sub.3 also to be
present solely in one layer or not in all layers, i.e., for
example, to form one or more gas diffusion layers. Especially with
a gas diffusion electrode, good contacting with a gas comprising
CO.sub.2 or consisting essentially of CO.sub.2 is very efficiently
possible, such that efficient electrochemical preparation of
C.sub.2H.sub.4 can be achieved here. Furthermore, this can
alternatively be brought about with an electrode comprising a gas
diffusion layer comprising or consisting of Cu.sub.4O.sub.3, since
a large reaction area can also be offered here to such a gas.
[0054] In particular, the following important specific parameters
and properties of a hydrocarbon-selective gas diffusion electrode
or gas diffusion layer have been found: [0055] Good wettability of
the electrode surface in order that an aqueous electrolyte or H+
ions can come into contact with catalyst (H+ is required for
ethylene). [0056] High electrical conductivity of the electrode or
of the catalyst and a homogeneous potential distribution over the
entire electrode area (potential-dependent product selectivity).
[0057] High chemical and mechanical stability in electrolysis
operation (suppression of cracking and corrosion). [0058] The ratio
between hydrophilic and hydrophobic pore volume is advantageously
in the region of about (0.01-1):3, more advantageously
approximately in the region of (0.1-0.5):3 and advantageously about
0.2:3. [0059] Defined porosity with a suitable ratio between
hydrophilic and hydrophobic channels or pores (assurance of
CO.sub.2 availability in the simultaneous presence of H+ ions).
[0060] Average pore sizes in the range from 0.2 to 7 .mu.m,
advantageously in the range from 0.4 to 5 .mu.m and more
advantageously in the range from 0.5 to 2 .mu.m have also been
found to be advantageous in a gas diffusion electrode or a gas
diffusion layer.
[0061] In particular embodiments, catalyst particles comprising or
consisting of Cu.sub.4O.sub.3, for example Cu.sub.4O.sub.3
particles, that are used for production of the electrode of the
invention, especially a gas diffusion electrode, or a gas diffusion
layer have a uniform particle size, for example between 0.01 and
100 .mu.m, for example between 0.05 and 80 .mu.m, advantageously
0.08 to 10 .mu.m, more advantageously between 0.1 and 5 .mu.m, for
example between 0.5 and 1 .mu.m. Moreover, the catalyst particles,
in particular embodiments, also have a suitable electrical
conductivity, especially a high electrical conductivity .sigma. of
>10.sup.3 S/m, advantageously 10.sup.4 S/m or more, more
advantageously of 10.sup.5 S/m or more, especially 10.sup.6 S/m or
more, where suitable additives may optionally be added here to the
paramelaconite in order to increase the conductivity of the
particles, for example metal particles. In addition, the catalyst
particles, in particular embodiments, have a low overvoltage for
the electroreduction of CO.sub.2. 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 and/or additives, it is possible to
achieve high selectivity and prolonged stability.
[0062] For a particularly good catalytic activity, a gas diffusion
electrode or an electrode with a gas diffusion layer should have
hydrophilic and hydrophobic regions that enable a good relationship
between the three phases: liquid, solid, gaseous. Particularly
active catalyst sites are in the three-phase region of liquid,
solid, gaseous. An ideal gas diffusion electrode thus has
penetration of the bulk material with hydrophilic and hydrophobic
channels in order to obtain a maximum number of three-phase regions
for active catalyst sites. Similarly, a gas diffusion layer should
correspondingly also have hydrophilic and hydrophobic channels.
[0063] For hydrocarbon-selective gas diffusion electrodes and gas
diffusion layers, accordingly, multiple intrinsic properties are
needed. There is a close interplay between the electrocatalyst and
the electrode.
[0064] It is not ruled out in accordance with the invention that
the electrode of the invention, as well as Cu.sub.4O.sub.3, also
comprises further constituents such as promoters, conductivity
additives, co-catalysts and/or binding agents/binders (the terms
binding agent and binder are treated as synonymous words with the
same meaning in the context of the present invention). For example,
as specified above, it is possible to add additives to increase the
conductivity, in order to enable good electrical and/or ionic
contacting of the Cu.sub.4O.sub.3. Co-catalysts may, for example,
optionally catalyze the formation of further products from ethylene
and/or else the formation of intermediates in the electrochemical
reduction of CO.sub.2 to ethylene, but may also possibly catalyze
entirely different reactions, for example when a reactant other
than CO.sub.2 is used in an electrochemical reaction, for example
an electrolysis.
[0065] The electrode of the invention in particular, especially a
gas diffusion electrode or a gas diffusion layer, may include at
least one binder, which is not particularly restricted, and it is
also possible to use two or more different binders, including in
different layers of the electrode. The binding agent or binder for
the gas diffusion electrode of the invention, if present, is not
particularly restricted and includes, for example, a hydrophilic
and/or hydrophobic polymer, for example a hydrophobic polymer. This
can achieve a suitable adjustment of the predominantly hydrophobic
pores or channels. In particular embodiments, the at least one
binder is an organic binder, for example selected from PTFE
(polytetrafluoroethylene), PVDF (polyvinylidene difluoride), PFA
(perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene
copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures
thereof, especially PTFE. The hydrophilicity can also be adjusted
using hydrophilic materials such as polysulfones, i.e.
polyphenylsulfones, polyimides, polybenzoxazoles or
polyetherketones, or generally polymers that are electrochemically
stable in the electrolyte, for example including polymerized "ionic
liquids", or organic conductors such as PEDOT:PSS or PANI
(camphorsulfonic acid-doped polyaniline). This can achieve a
suitable adjustment of the hydrophobic pores or channels. More
particularly, the gas diffusion electrode can be produced using
PTFE particles having a particle diameter between 0.01 and 95
.mu.m, advantageously between 0.05 and 70 .mu.m, more
advantageously between 0.1 and 40 .mu.m, e.g. 0.3 to 20 .mu.m, e.g.
0.5 to 20 .mu.m, e.g. about 0.5 .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 approximately 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,
to ISO 13321 or D4894-98a and may correspond, for example, to
manufacturer data (e.g. TF 9205: average particle size 8 .mu.m to
ISO 13321; TF 1750: average particle size 25 .mu.m to ASTM
D4894-98a).
[0066] The binder may be present, for example, in a proportion of
0.1% to 50% by weight, for example when a hydrophilic ion transport
material is used, e.g. 0.1% to 30% by weight, advantageously from
0.1% to 25% by weight, e.g. 0.1% to 20% by weight, more
advantageously from 3% to 20% by weight, more advantageously 3% to
10% by weight, even more advantageously 5% to 10% by weight, based
on the gas diffusion electrode. In particular embodiments, the
binder has significant shear-thinning characteristics, such that
fiber formation takes place during the mixing process. Ion
transport materials may be mixed in, for example, with higher
contents when they contain hydrophobic or hydrophobizing structural
units especially containing F, or fluorinated alkyl or aryl units.
Fibers formed in the course of production should ideally wind
around the particles without completely surrounding the surface.
The optimal mixing time can be determined, for example, by direct
visualization of the fiber formation in a scanning electron
microscope.
[0067] It is also possible to employ an ion transport material in
the electrode of the invention, which is not particularly
restricted. The ion transport material, for example an ion exchange
material, is not particularly restricted in accordance with the
invention and may, for example, be an ion transport resin, for
example an ion exchange resin, or else a different ion transport
material, for example an ion exchange material, for example a
zeolite, etc. In particular embodiments, the ion transport material
is an ion exchange resin. This is not particularly restricted here.
In particular embodiments, the ion transport material is an anion
transport material, for example an anion exchange resin. In
particular embodiments, the anion transport material or anion
transporter is an anion exchange material, for example an anion
exchange resin. In particular embodiments, the ion transport
material also has a cation blocker function, i.e. can prevent or at
least reduce penetration of cations into the electrode, especially
a gas diffusion electrode or an electrode having a gas diffusion
layer. Specifically an integrated anion transporter or an anion
transport material with firmly bound cations can constitute a
barrier here to mobile cations through coulombic repulsion, which
can additionally counteract salt deposition, especially within a
gas diffusion electrode or a gas diffusion layer. It is unimportant
here whether the gas diffusion electrode is fully permeated by the
anion transporter. Anion-conducting additives which are not
particularly restricted can additionally increase the performance
of the electrode, especially in a reduction. For example, it is
possible here to use an ionomer, for example 20% by weight
alcoholic suspension or a 5% by weight suspension of an anion
exchanger monomer (e.g. AS 4 Tokuyama). It is also possible, for
example, to use type 1 (typically trialkylammonium-functionalized
resins) and type 2 (typically alkylhydroxyalkyl-functionalized
resins) anion exchange resins.
[0068] In a further aspect, the present invention relates to an
electrolysis cell comprising the electrode of the invention. The
electrode may take the form here of a compact solid-state body, of
a porous electrode, e.g. gas diffusion electrode, or of a coated
body, for example with a gas diffusion layer, preference being
given to executions as a gas diffusion electrode or electrode
having a gas diffusion layer comprising or consisting of
Cu.sub.4O.sub.3. In the electrolysis cell of the invention, the
electrode of the invention is advantageously the cathode, in order
to enable reduction, for example, of a gas comprising or consisting
of CO.sub.2 and/or CO.
[0069] The further constituents of the electrolysis cell are not
particularly restricted, and include those that are commonly used
in electrolysis cells, for example a counterelectrode.
[0070] In particular embodiments, the electrode of the invention in
the electrolysis cell is a cathode, i.e. connected as cathode. In
particular embodiments, the electrolysis cell of the invention
further comprises an anode and at least one membrane and/or at
least one diaphragm between the cathode and anode, for example at
least one anion exchange membrane.
[0071] The further constituents of the electrolysis cell, for
instance the counterelectrode, e.g. the anode, optionally a
membrane and/or a diaphragm, feed(s) and drain(s), the voltage
source, etc., and further optional apparatuses such as heating or
cooling devices, are not particularly restricted in accordance with
the invention, nor are anolytes and/or catholytes that are used in
such an electrolysis cell, with use of the electrolysis cell in
particular embodiments on the cathode side for reduction of carbon
dioxide and/or CO. In the context of the invention, the
configuration of the anode space and of the cathode space is
likewise not particularly restricted.
[0072] An electrolysis cell of the invention may likewise be
employed in an electrolysis system. An electrolysis system is thus
also specified, comprising the electrode of the invention or the
electrolysis cell of the invention.
[0073] A suitable electrolysis cell for the use of the electrode of
the invention, for example gas diffusion electrode, comprises, for
example, the electrode of the invention as cathode with an anode
that is not subject to any further restriction. The electrochemical
conversion at the anode/counterelectrode is likewise not
particularly restricted. The cell is advantageously divided by the
electrode according to the invention as gas diffusion electrode or
as electrode having a gas diffusion layer into at least two
chambers, of which the chamber remote from the counterelectrode
(behind the GDE) functions as gas chamber. One or more electrolytes
may flow through the remainder of the cell. The cell may also
comprise one or more separators, such that the cell may also
comprise, for example, 3 or 4 chambers. These separators may be
either gas separators (diaphragms) having no intrinsic ion
conductivity or else ion-selective membranes (anion exchange
membrane, cation exchange membrane, proton exchange membrane) or
bipolar membranes, which are not particularly restricted. It is
possible for one or more electrolytes to flow across these
separators from both sides, or else, if they are suitable for this
kind of operation, for the separators to directly adjoin one of the
electrodes. For example, both the cathode and the anode may be
executed as a half-membrane electrode composite, where, in the case
of the cathode, the electrode of the invention, especially as a gas
diffusion electrode or as an electrode with a gas diffusion layer,
is advantageously part of this composite. The counterelectrode may
also be executed, for example, as a catalyst-coated membrane. In a
two-chamber cell, it is also possible for both electrodes to
directly adjoin a common membrane. If the electrode of the
invention as a gas diffusion electrode does not directly adjoin a
separator membrane, either "flow-through" operation in which the
feed gas flows through the electrode or "flow-by" operation in
which the feed gas is guided past the side remote from the
electrolyte is possible. If the gas diffusion electrode directly
adjoins the separator or one of the separators, accordingly, only
"flow-by" operation is possible. Reference is made to "flow-by"
particularly when more than 95% by volume, advantageously more than
98% by volume, of the product gases is discharged via the gas side
of the electrode.
[0074] Illustrative configurations for a construction of general
electrolysis cells--including in accordance with the above
remarks--and of possible anode and cathode spaces are shown in
schematic form in FIGS. 2 to 19, with further constituents for the
purposes of an electrolysis system shown in schematic form in FIGS.
17 to 19. The part that follows especially shows illustration of
electrolysis cell concepts that are compatible with the process of
the invention for electrochemical conversion of carbon dioxide
and/or carbon monoxide.
[0075] The following abbreviations are used in FIGS. 1 to 19:
[0076] I-IV: spaces in the electrolysis cell, as respectively
described hereinafter
[0077] K: cathode
[0078] M: membrane
[0079] A: anode
[0080] AEM: anion exchange membrane
[0081] CEM: cation/proton exchange membrane
[0082] DF: diaphragm
[0083] k: catholyte
[0084] a: anolyte
[0085] GC: gas chromatograph
[0086] GH: gas humidification
[0087] P: permeate
[0088] The other symbols in the diagrams are standard fluidic
connection symbols.
[0089] The figures show illustrative constructions with different
membranes, but these are not intended to restrict the cells shown.
For instance, rather than a membrane, it is also possible to
provide a diaphragm. The figures also show, on the cathode side, a
reduction of a gas, for example comprising or essentially
consisting of CO.sub.2, where the electrolysis cells are also not
restricted thereto and, accordingly, reactions on the cathode side
in the liquid phase or solution, etc., are also possible. In this
regard too, the figures do not restrict the electrolysis cell of
the invention. It is likewise possible for anolytes, catholytes and
any electrolytes in an interspace in the various constructions to
be the same or different, and they are not particularly
restricted.
[0090] FIG. 2 shows an arrangement in which both the cathode K and
the anode A adjoin a membrane M, and a reaction gas flows past the
back of the cathode K in the cathode space I. On the anode side is
the anode space II. In FIG. 3, by comparison with FIG. 2, there is
no membrane, and cathode K and anode A are separated by the space
II. The construction in FIG. 4, in terms of its construction,
corresponds essentially to that of FIG. 3, except that the cathode
K here is in flow-through mode.
[0091] FIG. 5 shows a two-membrane arrangement, wherein a bridge
space II is provided between two membranes M, which
electrolytically couples the cathode K and the anode A. The cathode
space I corresponds to that of FIG. 1, and the anode space III to
the anode space II of FIG. 1. The arrangement in FIG. 6 differs
from that of FIG. 5 in that the anode A does not adjoin the second
membrane M on the right.
[0092] FIGS. 7 to 11 again show arrangements with just one
membrane. In FIG. 7, as in FIG. 1, the cathode K in space I is in
flow-by mode, while a cathode space II adjoins the membrane M on
the other side. The membrane M is in turn separated from the anode
A by the anode space III. The construction in FIG. 8 corresponds to
that in FIG. 7, except that the cathode K here is in flow-through
mode. In FIGS. 9 and 10, the membrane M directly adjoins the anode
A, such that the anode space III is on the side of the anode A
remote from the membrane M; otherwise, these respectively show the
flow-by and flow-through variant of FIGS. 7 and 8. FIG. 11 shows a
flow-by variant in which the membrane M adjoins the cathode, space
II establishes electrolytic contact with the anode A, and space III
is on the opposite side of the anode A.
[0093] FIGS. 12 to 16 show further variants of two-membrane
arrangements with flow-by variants of the cathode in FIGS. 12, 14
and 16, and flow-through variants in FIGS. 13 and 15. In FIGS. 12
and 13, a membrane (on the right) adjoins the anode, such that the
anode space IV adjoins the anode on the right and coupling to the
cathode space II takes place via the bridge space III. Such
coupling likewise takes place in FIGS. 14 and 15, where the anode
space IV here lies between membrane M and anode A. In FIG. 16, in
turn, a membrane M (on the left) adjoins the cathode K, such that
coupling to the anode space III via the bridge space II is
envisaged, with a further space IV provided to the right of the
anode A, in which, for example, a further reactant gas for
oxidation at the anode A can be supplied.
[0094] FIGS. 17 to 19 show cell variants in which, by way of
example, reduction of CO.sub.2 at the cathode K after supply to
space I and oxidation of water at the anode A--which is supplied to
the anode space III with the anolyte a--to oxygen is shown, where
these reactions do not restrict the electrolysis cells and
electrolysis systems shown. FIGS. 17 and 18 additionally show that
the CO.sub.2 can be humidified in a gas humidification GH, in order
to facilitate ionic contacting with the cathode K. In addition, as
shown in FIGS. 17 to 19, the product gas from the reduction can
additionally be analyzed with a gas chromatograph GC. The same
applies, as shown in FIGS. 17 and 18, after removal of a permeate p
for the reactant gas. In FIG. 17, a catholyte k is supplied to the
bridge space II, which enables electrolytic coupling between
cathode K and anode A, with the cathode K adjoining an anion
exchange membrane AEM and the anode A adjoining a cation exchange
membrane CEM. In FIG. 18, only a cation exchange membrane CEM is
present; otherwise, the construction corresponds to that of FIG.
17, except that the space II here is in direct contact with the
cathode K, i.e. does not constitute a bridge space. In the cell
construction of FIG. 19, by comparison with FIG. 18, the cation
exchange membrane CEM does not adjoin the anode.
[0095] In addition, there are also possible cell variants as
already described in DE 10 2015 209 509 A1, DE 10 2015 212 504 A1,
DE 10 2015 201 132 A1, DE 102017208610.6, DE 102017211930.6, US
2017037522 A1 or U.S. Pat. No. 9,481,939 B2, and in which an
electrode of the invention may likewise be employed.
[0096] As apparent from the above, the present electrode results in
a multitude of possible cell arrangements.
[0097] The aspects that follow relate to various production
processes for producing an electrode. The processes of the
invention can especially produce an electrode of the invention,
such that elucidations relating to particular constituents of the
electrode can also be applied to the processes.
[0098] A further aspect of the present invention relates to a
process for producing an electrode comprising Cu.sub.4O.sub.3 on a
support, comprising--preparing a mixture comprising Cu.sub.4O.sub.3
and optionally at least one binder, or providing a powder
consisting of Cu.sub.4O.sub.3,--applying the mixture comprising
Cu.sub.4O.sub.3 or the powder consisting of Cu.sub.4O.sub.3 to a
support, for example a copper-containing support, advantageously in
the form of a sheetlike structure, and--dry rolling the mixture
comprising Cu.sub.4O.sub.3 or the powder consisting of
Cu.sub.4O.sub.3 onto the support. More particularly, this process,
like the other processes of the invention, as indicated above, can
be used to produce an electrode of the invention.
[0099] For processing of a mixture, for example a powder mixture,
or of a powder to give an electrode, especially a gas diffusion
electrode or an electrode with a gas diffusion layer, it is
possible, for example, to employ the dry calendering process
described in DE 102015215309.6 or WO 2017/025285. In this respect,
with regard to the production process by dry calendering, reference
is also made to this application.
[0100] The preparation of the mixture comprising Cu.sub.4O.sub.3
and optionally at least one binder is not particularly restricted
here and can be effected in a suitable manner. For example, the
mixing can be effected with a knife mill, but is not limited
thereto. A advantageous mixing time in a knife mill is in the range
of 60-200 s, advantageously between 90-150 s. Other mixing times
may correspondingly also arise for other mixers. In particular
embodiments, however, the preparing of the mixture comprises mixing
for 60-200 s, advantageously 90-150 s.
[0101] The applying of the mixture or the powder to a support, for
example a copper-containing support, advantageously in the form of
a sheetlike structure, is likewise not particularly restricted and
can be effected, for example, by applying in powder form, etc. The
support here is not particularly restricted and may correspond to
the above descriptions with regard to the electrode, and it may be
executed here, for example, as a mesh, grid, etc.
[0102] The dry rolling of the mixture or the powder onto the
support is not particularly restricted either, and can be effected,
for example, with a roller. In particular embodiments, the rolling
application is effected at a temperature of 25-100.degree. C.,
advantageously 60-80.degree. C.
[0103] Nor is it ruled out in accordance with the invention that
multiple layers are collectively applied and rolled onto a support
by this process, for example a hydrophobic layer that can establish
better contact with a gas comprising CO.sub.2 and hence can improve
gas transport to the catalyst.
[0104] It is also possible for the catalyst, i.e. Cu.sub.4O.sub.3,
to be sieved onto an existing electrode without an additional
binder. The base layer may then also be produced, for example, from
powder mixtures of a Cu powder, for example with a grain size of
100-160 .mu.m, with a binder, e.g. 10-15% by weight of PTFE Dyneon
TF 1750 or 7-10% by weight of Dyneon TF 2021.
[0105] A further aspect of the present invention relates to a
process for producing an electrode comprising Cu.sub.4O.sub.3 on a
support, comprising--providing a support,--applying a suspension
comprising Cu.sub.4O.sub.3 and optionally at least one binder to
the support, and--drying the suspension; or--providing a support,
and--applying Cu.sub.4O.sub.3 or a mixture comprising
Cu.sub.4O.sub.3 from the gas phase. More particularly, it is also
possible by this process, like the other processes of the invention
too, to produce an electrode of the invention.
[0106] Here too, the providing of the support is not particularly
restricted, and it is possible to use, for example, the support
discussed in the context of the electrode, for example including a
gas diffusion electrode or gas diffusion layer, for example on a
suitable substrate. The applying of the suspension is likewise not
particularly restricted, and can be effected, for example, by
dropwise application, dipping, etc. The material may thus be
applied, for example, as a suspension to a commercially available
GDL (e.g. Freudenberg C2, Sigracet 35 BC). It is preferable when an
ionomer, for example 20% by weight alcoholic suspension or a 5% by
weight suspension of an anion exchange ionomer (e.g. AS 4
Tokuyama), is also used here, and/or other additives, binders,
etc., that have been discussed in the context of the electrode of
the invention. For example, it is also possible to use type 1
(typically trialkylammonium-functionalized resins) and type 2
(typically alkylhydroxyalkyl-functionalized resins) anion exchange
resins.
[0107] The drying of the suspension is likewise not restricted and
it is possible, for example, to effect solidification by
evaporation or precipitation with separation of the solvent or
solvent mixture from the suspension, which are not particularly
restricted.
[0108] In the alternative embodiment of the applying of
Cu.sub.4O.sub.3 or a mixture comprising Cu.sub.4O.sub.3 from the
gas phase, the providing of a support is likewise not particularly
restricted, and can be effected as above. The applying of
Cu.sub.4O.sub.3 or of a mixture comprising Cu.sub.4O.sub.3 from the
gas phase is likewise not particularly restricted and can be
effected, for example, based on physical gas phase deposition
methods such as laser ablation or chemical vapor deposition (CVD).
In this way, it is possible to obtain thin films comprising
paramelaconite.
[0109] In particular embodiments, the support is a gas diffusion
electrode or a gas diffusion layer.
[0110] In the processes specified above in which not only
paramelaconite but also other constituents may be present in a
mixture or suspension, in particular embodiments, the at least one
binder is present in the mixture or suspension, wherein the at
least one binder advantageously comprises an ionomer. In particular
embodiments, the at least one binder is present in the mixture or
suspension in an amount of >0% to 30% by weight, based on the
total weight of Cu.sub.4O.sub.3 and the at least one binder.
[0111] A further aspect relates to a process for producing an
electrode comprising Cu.sub.4O.sub.3, comprising a preparation of a
powder comprising Cu.sub.4O.sub.3; and rolling out the powder to
give an electrode. The production of the powder comprising
Cu.sub.4O.sub.3 is not particularly restricted here, nor is the
rolling-out to give a powder, for example with a roller. The
rolling-out can be effected, for example, at a temperature of 15 to
300.degree. C., e.g. 20 to 250.degree. C., e.g. 22 to 200.degree.
C., advantageously 25-150.degree. C., further advantageously
60-80.degree. C. With regard to the powder, it is again also
possible to make reference to the embodiments above relating to the
electrode of the invention. This process can especially, like the
other processes of the invention as well, be used to produce an
electrode of the invention.
[0112] A further aspect of the present invention is a process for
electrochemical conversion of carbon dioxide and/or carbon
monoxide, wherein carbon dioxide and/or carbon monoxide are
introduced at the cathode into an electrolysis cell--comprising an
electrode of the invention as cathode--and reduced.
[0113] The present invention thus also relates to a process and to
an electrolysis system for electrochemical utilization of carbon
dioxide. Carbon dioxide (CO.sub.2) is introduced into an
electrolysis cell and reduced at a cathode with the aid of an
electrode of the invention, for example a gas diffusion electrode
(GDE), on the cathode side. GDEs are electrodes in which liquid,
solid and gaseous phases are present and where the conductive
catalyst catalyzes the electrochemical reaction between the liquid
phase and the gaseous phase.
[0114] The introducing of the carbon dioxide and/or optionally also
carbon monoxide at the cathode is not particularly restricted here,
and can be effected from the gas phase, from solution, etc.
[0115] In order to assure sufficiently high conductivity in the
cathode space, an aqueous electrolyte in contact with the cathode,
in particular embodiments, contains a dissolved "conductive salt"
which not particularly restricted. The electrocatalyst used should
ideally enable a high Faraday efficiency at high current density
for a corresponding target product. Industrially relevant
electrocatalysts should additionally have prolonged stability. For
the selective production of the carbon monoxide product, pure
silver catalyst that meet industrial demands are already available.
For the selective electroreduction of CO.sub.2 to ethylene or
alcohols, there are currently no known catalysts available that
meet these demands. The synthesis concept described here enables
the production of electrocatalysts with a low overvoltage and an
elevated selectivity for ethylene and alcohols, for example ethanol
and/or propanol.
[0116] In particular embodiments, the electrochemical conversion,
for example an electrolysis, is effected at a current density of
200 mA/cm.sup.2 or more, advantageously 250 mA/cm.sup.2 or more,
more advantageously 300 mA/cm.sup.2 or more, even more
advantageously 350 mA/cm.sup.2 or more, especially at more than 400
mA/cm.sup.2. The electrochemical conversion is advantageously
effected at a pH of pH=6-14, advantageously at a pH between 10 and
14.
[0117] In the reduction at the cathode, it is especially also
possible to obtain ethylene. Thus, this process of the invention is
also a process for preparing ethylene.
[0118] Additionally disclosed is also use of Cu.sub.4O.sub.3 for
the reduction of CO.sub.2, and also the use of Cu.sub.4O.sub.3 in
the electrolysis of CO.sub.2.
[0119] The above embodiments, configurations 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 to the respective basic form of the
present invention as improvements or supplementations.
[0120] The invention is elucidated further in detail hereinafter
with reference to various examples thereof. However, the invention
is not limited to these examples.
EXAMPLES
Example 1
[0121] The synthesis of the Cu.sub.4O.sub.3 phase was inspired by a
synthesis route (mg range) described in the publication by Zhao et
al. (Zhao et al., Chem. Mater. 2012, 24, pages 1136-1142).
[0122] A typical synthesis comprises a dissolution of 50 mM
Cu(NO.sub.3).sub.2.3H.sub.2O in 1.1 L of mixed ethanol-DMF solvent
(the volume ratio of ethanol to DMF is 1:2). The solution was
stirred for 10 min and then transferred to a 1.5 L glass insert
that was then inserted into a stainless steel autoclave (BR-1500
high-pressure reactor, Berghof). The autoclave was closed and the
reaction mixture was held therein at 130.degree. C. for 24 h. After
24 h, the glass insert with the reaction mixture was removed from
the autoclave and cooled down to room temperature by means of an
ice bath. The reaction product precipitated out in the glass
insert. After cooling, the supernatant was removed from the glass
insert and the remaining solid-state product was collected by
centrifuging and washing three times with ethanol. The powder
obtained was first dried under an argon stream and then dried under
reduced pressure. Finally, the powder was stored in a glovebox
under inert atmosphere.
[0123] An x-ray diffractometry (XRD) analysis of the powder
prepared showed the presence of the following phases, as shown in
FIGS. 20 and 21: Cu.sub.4O.sub.3 (reference numeral 13), Cu.sub.2O
(reference numeral 11) and Cu (reference numeral 12). FIG. 20 is a
plot of the angle 20 (coupled 2 theta/theta, WL=1.54060) against
the number of pulses I. FIG. 21 is a plot of the angle 20 against
the root of the intensity (I.sup.1/2) in root counts (C.sup.1/2). A
quantitative phase analysis was conducted. About 40% by weight of
the powder obtained was Cu.sub.4O.sub.3; the remainder was
Cu.sub.2O with traces of copper. SEM images of the powder obtained
are shown in FIGS. 22 and 23.
[0124] A gas diffusion electrode (GDE) containing Cu.sub.4O.sub.3
as catalyst for CO.sub.2 electroreduction was prepared as follows.
The previously synthesized powder that contained Cu.sub.4O.sub.3
was cast onto a gas diffusion layer (GPL; Freudenberg H23C2 GDL)
from solution, as follows. The binder used was an ionomer (e.g. AS4
from Tokuyama). The ionomer solution is added to the powder
containing Cu.sub.4O.sub.3 catalyst particles that has been
dispersed in 1-propanol beforehand. The amount of the catalyst
powder used depends on the desired catalyst loading, but is
generally set for a mass coverage on the gas diffusion layer of
between 1 mg/cm.sup.2 and 10 mg/cm.sup.2, e.g. here by way of
example 4.5 mg/cm.sup.2. The dispersion was then left in an
ultrasound bath for 30 min, whereupon a homogeneous catalyst ink
was formed. After the ultrasound treatment, the catalyst ink was
poured on and dried in an inert atmosphere (argon).
[0125] The electrochemical performance of the GDE containing
Cu.sub.4O.sub.3 as catalyst was tested in the electrolysis setup
described hereinafter. For this purpose, a stacked three-chamber
flow cell was used. The first chamber, which was used as gas supply
chamber, was separated from the second chamber by the GDE. The
second and third chambers respectively contained a catholyte and an
anolyte and were separated by a Nafion 117 membrane. The
electrolytes were pumped through the cell in two separate cycles.
The anode space was filled with 2.5 M KOH and had an
IrO.sub.2-containing anode. For the cathode space, the GDE was used
as cathode and 0.5 M K.sub.2SO.sub.4 as electrolyte. The
counterelectrode used was a solid, IrO.sub.2-coated Ti plate. The
cell was equipped with an Ag/AgCl/3M KCl reference electrode. For
potentiostatic measurements, the cathode was connected as working
electrode.
[0126] In order to demonstrate the superior activity and
selectivity for ethylene, the GDE comprising Cu.sub.4O.sub.3 that
was produced above was compared with two other GDEs that contained
copper particles (Roth) and Cu.sub.2O particles as catalysts. All
three GDLs were prepared by the same method as described above.
Copper and Cu.sub.2O were selected because they currently represent
the state of the art for the reduction of CO.sub.2 to higher
hydrocarbons, including ethylene. The experiments were conducted in
potentiostatic electrolysis mode, meaning that the cell potential
was kept constant during the experiment. Gaseous products were
analyzed with a Thermo Scientific Trace 1310 gas chromatograph.
[0127] The results of the electrochemical measurements are shown in
FIGS. 24 and 25.
[0128] As apparent from FIGS. 24 and 25, the
Cu.sub.4O.sub.3-containing GDE showed a maximum selectivity of 27%
Faraday efficiency (FE) for ethylene at 1.05 V (versus Ag/AgCl) and
a current density J of 420 mA/cm.sup.2. At the same potential, the
Cu-- and Cu.sub.2O-containing GDEs showed less than 2.5% FE for
ethylene at an overall current density of 20 mA/cm2. This shows
that a GDE containing Cu.sub.4O.sub.3 shows a 10-fold increase in
the ethylene FE at distinctly higher possible overall current
densities (more than 20-fold) compared to GDEs comprising Cu and
Cu.sub.2O. In order to further illustrate the distinct improvement
in activity of the Cu.sub.4O.sub.3 phase by comparison with Cu and
Cu.sub.2O, the specific current density for ethylene formation for
these three copper phases was plotted against the cathode
potentials tested, as shown in FIG. 26. At a cathode potential of
-1.05 V (versus Ag/AgCl), GDE with Cu.sub.4O.sub.3 shows a
1000-fold increase compared to the Cu.sub.2O GDE and a 100-fold
increase compared to the Cu GDE.
[0129] Further gaseous products detected are: CO, CH.sub.4,
C.sub.2H.sub.6 and H.sub.2. The FE values for these products are
shown in FIGS. 27 to 29, in which the FE values are plotted against
the cathode potential for all gaseous products detected. It is of
interest that the GDE with Cu.sub.4O.sub.3 was the only one capable
of reducing CO.sub.2 to C.sub.2H.sub.6 (albeit only in small
amounts, less than 0.5% FE).
Example 2
[0130] A GDE containing Cu.sub.4O.sub.3 as catalyst was likewise
tested in a double-membrane test setup according to FIG. 17. The
GDE was produced as described above. 0.5 M H.sub.2SO.sub.4 was used
as electrolyte between the anion exchange membrane AEM (Sustainion
x37-50 membrane) and the cathode exchange membrane CEM (Nafion 117
membrane), and as the electrolyte that circulated in the chamber
behind the anode. The measurements were conducted in galvanostatic
mode, meaning that the GDE was tested at various constant current
values. The counterelectrode used was a solid IrO2-coated Ti plate.
The cell was equipped with an Ag/AgCl/3M KCl reference electrode.
For the galvanostatic measurements, the cathode was connected as
working electrode. Since H.sub.2SO.sub.4 was used as electrolyte
overall, the pH during the experiment was close to zero. This
experiment shows the stability of the Cu.sub.4O.sub.3 phase under
extreme acidic conditions (relatively high current densities) in a
novel double-membrane setup with a zero-gap anode (CEM directly on
the anode surface) and a zero-gap cathode (AEM directly on the
cathode surface). The results of the measurements are shown in
FIGS. 30 and 31, with FIG. 30 showing the FE values for all
products depending on the current density, and FIG. 31 the FE for
ethylene. In addition, it was observed that alcohols such as
ethanol and propanol were also obtainable.
[0131] The present disclosure is of the use of the comparatively
rare Cu.sub.4O.sub.3 phase as catalyst for CO.sub.2 reduction,
where it is possible to increase the activity and selectivity of a
gas diffusion electrode (GDE) containing Cu.sub.4O.sub.3 by
comparison with a GDE containing a Cu.sub.2O phase. To date, the
Cu.sub.4O.sub.3 copper oxide phase has never been studied as a
possible catalyst for CO.sub.2 reduction. The Cu.sub.4O.sub.3 phase
shows relatively high stability and can, as shown by x-ray
diffractometry, be clearly distinguished from Cu.sub.2O and CuO on
account of the different crystal structure. In a formal sense, the
oxidation state of copper in this structure is 1.5. The synthesis
of Cu.sub.4O.sub.3 was scaled up in the present context, by
comparison with literature methods, from a few milligrams to a 10 g
scale. A gas diffusion electrode containing Cu.sub.4O.sub.3 as
electrocatalyst for CO.sub.2 reduction was studied for the first
time and showed high activity and selectivity for ethylene.
* * * * *