U.S. patent application number 16/629728 was filed with the patent office on 2021-03-18 for membrane-coupled cathode for the reduction of carbon dioxide in acid-based electrolytes without mobile cations.
This patent application is currently assigned to Siemens Aktiengesellschaft. The applicant listed for this patent is Siemens Aktiengesellschaft. Invention is credited to Christian Reller, Bernhard Schmid, Gunter Schmid.
Application Number | 20210079538 16/629728 |
Document ID | / |
Family ID | 1000005289159 |
Filed Date | 2021-03-18 |
![](/patent/app/20210079538/US20210079538A1-20210318-D00000.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00001.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00002.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00003.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00004.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00005.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00006.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00007.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00008.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00009.png)
![](/patent/app/20210079538/US20210079538A1-20210318-D00010.png)
View All Diagrams
United States Patent
Application |
20210079538 |
Kind Code |
A1 |
Schmid; Bernhard ; et
al. |
March 18, 2021 |
Membrane-Coupled Cathode for the Reduction of Carbon Dioxide in
Acid-Based Electrolytes Without Mobile Cations
Abstract
Various embodiments include an electrolysis cell comprising: a
cathode space housing a cathode for the reduction of CO.sub.2; a
first ion exchange membrane including an anion exchanger and/or an
anion transporter, the first ion exchange membrane adjoining the
cathode space and in direct contact with the cathode; an anode
space housing an anode; a first separator membrane; and a salt
bridge space housing an electrolyte disposed between the first ion
exchange membrane and the first separator membrane. The electrolyte
in the salt bridge space comprises a liquid acid and/or a solution
of an acid.
Inventors: |
Schmid; Bernhard; (Duren,
DE) ; Reller; Christian; (Minden, DE) ;
Schmid; Gunter; (Hemhofen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Aktiengesellschaft |
Munchen |
|
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
Munchen
DE
|
Family ID: |
1000005289159 |
Appl. No.: |
16/629728 |
Filed: |
June 14, 2018 |
PCT Filed: |
June 14, 2018 |
PCT NO: |
PCT/EP2018/065854 |
371 Date: |
January 9, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 9/63 20210101; C25B
9/23 20210101; C25B 1/00 20130101 |
International
Class: |
C25B 9/10 20060101
C25B009/10; C25B 9/02 20060101 C25B009/02; C25B 1/00 20060101
C25B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2017 |
DE |
10 2017 211 930.6 |
Claims
1. An electrolysis cell comprising: a cathode space housing a
cathode for the reduction of CO.sub.2; a first ion exchange
membrane including an anion exchanger and/or an anion transporter,
the first ion exchange membrane adjoining the cathode space and in
direct contact with the cathode; an anode space housing an anode; a
first separator membrane; and a salt bridge space housing an
electrolyte disposed between the first ion exchange membrane and
the first separator membrane; wherein the electrolyte in the salt
bridge space comprises a liquid acid and/or a solution of an
acid.
2. An electrolysis cell comprising: a cathode space housing a
cathode for reducing CO.sub.2; a first ion exchange membrane
including an anion exchanger and/or anion transporter, the first
ion exchange membrane adjoining the cathode space and in direct
contact with the cathode; and an anode space housing an anode and
containing an electrolyte, the anode space adjoining the first ion
exchange membrane; wherein the electrolyte in the anode space
comprises a liquid acid and/or a solution of an acid.
3. The electrolysis cell as claimed in claim 1, wherein the second
ion exchange membrane is selected from the group consisting of: an
ion exchange membrane containing a cation exchanger, a bipolar
membrane, and a diaphragm.
4. The electrolysis cell as claimed in claim 1, wherein the anode
space houses an anolyte comprising a liquid and/or dissolved
acid.
5. The electrolysis cell as claimed in claim 2, wherein the anode
is in direct contact with the first ion exchange membrane.
6. The electrolysis cell as claimed in claim 1, wherein the
electrolysis is conducted with a current density of more than 50
mAcm.sup.-2.
7. The electrolysis cell as claimed in claim 1, wherein: an acid of
the electrolyte in the salt bridge space has a pK.sub.A of 6 or
less; and the liquid and/or dissolved acid comprises at least one
acid selected from the group consisting of: dilute or neat
H.sub.2SO.sub.4, a solution of H.sub.2N--SO.sub.2--OH, dilute or
neat HClO.sub.4, a solution of H.sub.3PO.sub.4, dilute or neat
CF.sub.3--COOH, dilute or neat CF.sub.3--SO.sub.2--OH, a solution
of (CF.sub.3--SO.sub.2).sub.2--NH, a solution of HF, dilute or neat
HCOOH, dilute or neat CH.sub.3--COOH, a solution of HCl, a solution
of HBr, and a solution of HI.
8. An electrolysis cell comprising: a cathode space housing a
cathode; a first ion exchange membrane including an anion exchanger
and/or anion transporter, the first ion exchange membrane adjoining
the cathode space in direct contact with the cathode; an anode
space housing an anode; a diaphragm adjoining the anode space; and
a salt bridge space disposed between the first ion exchange
membrane and the diaphragm; wherein the diaphragm is non-ion
conductive.
9. The electrolysis cell as claimed in claim 8, wherein at least
one of: the anode is in contact with the diaphragm; the anode is in
contact with a conductive structure on the side remote from the
salt bridge space; or the cathode is in contact with a conductive
structure on the side remote from the salt bridge space.
10. The electrolysis cell as claimed in claim 8, wherein at least
one of the cathode or the anode comprises at least one structure
selected from the group consisting of: a gas diffusion electrode, a
porous bound catalyst structure, a particulate catalyst on a
support, a coating of a particulate catalyst on the first and/or
second ion exchange membrane, a porous conductive carrier
impregnated with a catalyst, and a noncontinuous sheetlike
structure.
11. The electrolysis cell as claimed in claim 10, wherein: the
cathode comprises at least one structure selected from the group
consisting of: a gas diffusion electrode, a porous bound catalyst
structure, a particulate catalyst on a support, a coating of a
particulate catalyst on the first and/or second ion exchange
membrane, a porous conductive carrier impregnated with a catalyst,
and of a noncontinuous sheetlike structure containing an anion
exchange material and/or anion transport material; and the anode
comprises at least one structure selected from the group consisting
of: a gas diffusion electrode, a porous bound catalyst structure, a
particulate catalyst on a support, a coating of a particulate
catalyst on the first and/or second ion exchange membrane, a porous
conductive carrier impregnated with a catalyst, and a noncontinuous
sheetlike structure containing a cation exchange material.
12. The electrolysis cell as claimed in claim 8, wherein at least
one of the first ion exchange membrane or the diaphragm is
hydrophilic.
13. The electrolysis cell as claimed in claim 8, wherein the
electrolyte in the salt bridge space comprises a liquid acid and/or
a solution of an acid.
14. An electrolysis system comprising an electrolysis cell
comprising: a cathode space housing a cathode; a first ion exchange
membrane including an anion exchanger and/or anion transporter, the
first ion exchange membrane adjoining the cathode space in direct
contact with the cathode; an anode space housing an anode; a
diaphragm adjoining the anode space; and a salt bridge space
disposed between the first ion exchange membrane and the diaphragm;
wherein the diaphragm is non-ion conductive.
15. The electrolysis system as claimed in claim 14, further
comprising a recycling device connected to an outlet from the salt
bridge space and an inlet to the cathode space, to return a
reactant from the cathode reaction that can be formed in the salt
bridge space to the cathode space.
16. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a U.S. National Stage Application of
International Application No. PCT/EP2018/065854 filed Jun. 14,
2018, which designates the United States of America, and claims
priority to DE Application No. 10 2017 211 930.6 filed Jul. 12,
2017, the contents of which are hereby incorporated by reference in
their entirety.
TECHNICAL FIELD
[0002] The present disclosure relates to electrolysis of CO.sub.2.
Various embodiments include methods of electrolysis, electrolysis
systems comprising an electrolysis cell.
BACKGROUND
[0003] The combustion of fossil fuels currently covers about 80% of
global energy demand. These combustion processes emitted about
34,032.7 million metric tons of carbon dioxide (CO.sub.2) globally
into the atmosphere in 2011. This release is the simplest way of
disposing of large volumes of CO.sub.2 as well (brown coal power
plants exceeding 50,000 t per day). 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.
[0004] 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 should preferably be accomplished
using metals as catalysts. 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, discloses, by way of example,
Faraday efficiencies (FE) at different metal cathodes, some of
which are shown by way of example in table 1.
TABLE-US-00001 TABLE 1 Faraday efficiencies for the conversion of
CO.sub.2 to various products at various metal electrodes Electrode
CH.sub.4 C.sub.2H.sub.4 C.sub.2H.sub.5OH C.sub.3H.sub.7OH CO
HCOO.sup.- H.sub.2 Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au
0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4
94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3
2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0
0.0 0.0 97.4 5.0 102.4 Hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0
0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6
100.1 Cd 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0
95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0
0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0
0.0 0.0 0.0 0.0 0.0 99.7 99.7
[0005] Table 1 states Faraday efficiencies (FE) (in [%]) of
products formed in carbon dioxide reduction at various metal
electrodes. The values reported are applicable to a 0.1 M potassium
hydrogencarbonate solution as electrolyte. As apparent from table
1, the electrochemical reduction of CO.sub.2 at solid-state
electrodes in aqueous electrolyte solutions offers a multitude of
possible products.
[0006] There are currently discussions about the electrification of
the chemical industry. This means that chemical commodities or
fuels are to be produced preferentially from CO.sub.2 and/or CO
and/or H.sub.2O with supply of surplus electrical energy,
preferably from renewable sources. In the phase of introduction of
such technology, the aim is for the economic value of a substance
to be significantly greater than its calorific value, in order to
achieve economic viability at an early stage.
[0007] To achieve acceptable conversion rates in CO.sub.2
electrolysis, it is preferable to ensure sufficient availability of
CO.sub.2 at the catalytically active sites of the cathode. At
current densities exceeding .about.50 mAcm.sup.-2, however, this
supply is difficult through the solubility of CO.sub.2 in an
electrolyte. Therefore, at high current densities, the CO.sub.2 is
typically supplied directly as gas. What is called a three-phase
zone is advantageous here, where the reaction gas CO.sub.2, the
catalytically active electrode and the electrolyte are available.
For this purpose, it is possible to use porous electrodes, called
gas diffusion electrodes, which can be implemented in various
ways.
[0008] For example, they may take the form of electrically
conductive catalyst particles bound to polymers, for example of an
extruded or calendered film, which corresponds to an
all-active-catalyst gas diffusion electrode, or of a porous,
catalytically inactive but conductive electrode, for example in the
form of carbon fiber gas diffusion layers impregnated with a small
amount of active catalyst particles.
[0009] As an alternative, it is also possible to bind a catalyst to
a solid-state electrolyte, which can also be referred to as a
catalyst-coated membrane. In this case too, it is possible for a
three-phase zone to form between the catalyst, the solid-state
electrolyte and the CO.sub.2. With appropriate structures, an
electrochemical reduction of CO.sub.2 to chemically utilizable
products is possible. For example, US 2017/0037522 A1 describes a
process for preparing formic acid in an electrochemical
apparatus.
[0010] In addition, acids in the anode space are also completely
standard practice, as described, for example, in J. Shi, F. Shi, N.
Song, J-X. Liu, X-K Yang, Y-J Jia, Z-W Xiao, P. Du, Journal of
Power Sources, 2014, 259, 50-53. However, there is a need for a
simple and effective electrolytic method of reducing CO.sub.2 using
high current densities with simultaneous avoidance of the formation
of CO.sub.2 at the anode, for example by protolysis of
carbonate-containing electrolytes in aqueous electrolytes.
Electrolysis cells in which the CO.sub.2/O.sub.2 mixtures form at
the anode are known, for example, from US 2016 0251755 A1 and U.S.
Pat. No. 9,481,939.
SUMMARY
[0011] As an example, some embodiments of the teachings herein
include a method of electrolysis of CO.sub.2, wherein an
electrolysis cell comprising a cathode space comprising a cathode;
a first ion exchange membrane which contains an anion exchanger
and/or anion transporter and adjoins the cathode space, where the
cathode forms direct contact with the first ion exchange membrane;
an anode space comprising an anode; a first separator membrane; and
a salt bridge space, where the salt bridge space is disposed
between the first ion exchange membrane and the first separator
membrane, is used; wherein CO.sub.2 is reduced at the cathode,
wherein the electrolyte in the salt bridge space consists of a
liquid acid and/or a solution of an acid.
[0012] In some embodiments, an electrolysis cell comprising a
cathode space comprising a cathode; a first ion exchange membrane
which contains an anion exchanger and/or anion transporter and
adjoins the cathode space, where the cathode forms direct contact
with the first ion exchange membrane; and an anode space comprising
an anode, where the anode space adjoins the first ion exchange
membrane is used; wherein CO.sub.2 is reduced at the cathode,
wherein the electrolyte in the anode space consists of a liquid
acid and/or a solution of an acid.
[0013] In some embodiments, the second ion exchange membrane is
selected from an ion exchange membrane containing a cation
exchanger, a bipolar membrane and a diaphragm.
[0014] In some embodiments, the anode space comprises an anolyte
comprising a liquid and/or dissolved acid, preferably wherein the
anolyte and/or the acid in the salt bridge space does not comprise
any mobile cations except for protons and/or deuterons, especially
any metal cations.
[0015] In some embodiments, the anode lies against the first ion
exchange membrane.
[0016] In some embodiments, the electrolysis is conducted with a
current density of more than 50 mAcm.sup.-2, more than 100
mAcm.sup.-2, of 150 mAcm.sup.-2 or more, 170 mAcm.sup.-2 or more,
250 mAcm.sup.-2 or more, 400 mAcm.sup.-2 or more, or 600
mAcm.sup.-2 or more.
[0017] In some embodiments, an acid of the electrolyte in the salt
bridge space has a pK.sub.A of 6 or less, 5 or less, 3 or less, 1
or less, or 0 or less, wherein the liquid and/or dissolved acid is
selected from dilute or neat H.sub.2SO.sub.4, a solution of
H.sub.2N--SO.sub.2--OH, dilute or neat HClO.sub.4, a solution of
H.sub.3PO.sub.4, dilute or neat CF.sub.3--COOH, dilute or neat
CF.sub.3--SO.sub.2--OH, a solution of
(CF.sub.3--SO.sub.2).sub.2--NH, a solution of HF, dilute or neat
HCOOH, dilute or neat CH.sub.3--COOH, a solution of HCl, a solution
of HBr, a solution of HI, and/or mixtures thereof.
[0018] As another example, some embodiments include an electrolysis
cell comprising: a cathode space comprising a cathode; a first ion
exchange membrane which contains an anion exchanger and/or anion
transporter and adjoins the cathode space, where the cathode forms
direct contact with the first ion exchange membrane; an anode space
comprising an anode; and a diaphragm that adjoins the anode space;
further comprising a salt bridge space, wherein the salt bridge
space is disposed between the first ion exchange membrane and the
diaphragm, wherein the diaphragm is non-ion-conductive.
[0019] In some embodiments, the anode is in contact with the
diaphragm, and/or wherein the anode and/or the cathode is in
contact with a conductive structure on the side remote from the
salt bridge space.
[0020] In some embodiments, the cathode and/or the anode takes the
form of a gas diffusion electrode, of a porous bound catalyst
structure, of a particulate catalyst on a support, of a coating of
a particulate catalyst on the first and/or second ion exchange
membrane, of a porous conductive carrier impregnated with a
catalyst, and/or of a noncontinuous sheetlike structure.
[0021] In some embodiments, the cathode takes the form of a gas
diffusion electrode, of a porous bound catalyst structure, of a
particulate catalyst on a support, of a coating of a particulate
catalyst on the first and/or second ion exchange membrane, of a
porous conductive carrier impregnated with a catalyst, and/or of a
noncontinuous sheetlike structure, containing an anion exchange
material and/or anion transport material, and/or wherein the anode
takes the form of a gas diffusion electrode, of a porous bound
catalyst structure, of a particulate catalyst on a support, of a
coating of a particulate catalyst on the first and/or second ion
exchange membrane, of a porous conductive carrier impregnated with
a catalyst, and/or of a noncontinuous sheetlike structure,
containing a cation exchange material.
[0022] In some embodiments, the first ion exchange membrane and/or
the diaphragm are hydrophilic.
[0023] In some embodiments, the electrolyte in the salt bridge
space consists of a liquid acid and/or a solution of an acid,
preferably wherein an acid in the electrolyte in the salt bridge
space has a pK.sub.A of 6 or less, preferably 5 or less, further
preferably 3 or less, even further preferably 1 or less, especially
preferably 0 or less, further preferably wherein the liquid and/or
dissolved acid is selected from dilute or neat H.sub.2SO.sub.4, a
solution of H.sub.2N--SO.sub.2--OH, dilute or neat HClO.sub.4, a
solution of H.sub.3PO.sub.4, dilute or neat CF.sub.3--COOH, dilute
or neat CF.sub.3--SO.sub.2--OH, a solution of
(CF.sub.3--SO.sub.2).sub.2--NH, a solution of HF, dilute or neat
HCOOH, dilute or neat CH.sub.3--COOH, a solution of HCl, a solution
of HBr, a solution of HI, and/or mixtures thereof.
[0024] As another example, some embodiments include an electrolysis
system comprising an electrolysis cell as described above.
[0025] In some embodiments, there is a recycling device connected
to an outlet from the salt bridge space and an inlet to the cathode
space, which is set up to return a reactant from the cathode
reaction that can be formed in the salt bridge space to the cathode
space.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The appended drawings are intended to illustrate embodiments
of the teachings of the present disclosure and impart further
understanding thereof. In association with the description, they
serve to explain concepts and principles herein. Other embodiments
and many of the advantages mentioned are also apparent with regard
to the drawings. The elements of the drawings are not necessarily
shown true to scale with respect to one another. Elements, features
and components that are the same, have the same function and the
same effect, unless stated otherwise, are each given the same
reference numerals.
[0027] FIGS. 1 and 2 show a graphic representation of the cathodic
half-cell of the above-described transport model of ions of salts
and acids in an AEM adjoining a cathode.
[0028] FIG. 3 shows a schematic of an example of an electrolysis
system with an electrolysis cell as employed in some methods
incorporating the teachings herein.
[0029] FIGS. 4 and 5 show schematics of further examples of
electrolysis cells with which various methods incorporating the
teachings herein may be executed.
[0030] FIGS. 6 and 7 show schematic graphic representations of the
different release of CO.sub.2 in the case of use of a salt
electrolyte (FIG. 6) and an acid electrolyte (FIG. 7).
[0031] FIG. 8 shows a schematic of an electrolysis system of the
invention with an AEM diaphragm cell incorporating the teachings
herein in which methods incorporating the teachings herein can be
conducted.
[0032] FIG. 9 shows a schematic diagram of an AEM bipolar double
membrane cell in which methods incorporating the teachings herein
can likewise be conducted.
[0033] FIG. 10 shows a schematic of the experimental setup in
example 1.
[0034] FIG. 11 shows experimental results of example 1, wherein the
Faraday efficiency has been plotted against the current density
applied.
[0035] FIG. 12 shows a schematic of the experimental setup in the
present comparative example 1.
[0036] FIG. 13 shows the experimental results obtained thereby,
again by a plot of the Faraday efficiency against the current
density applied.
[0037] FIG. 14 compares the experimental results from example 1
(solid lines) with those from comparative example 1 (dotted
lines).
[0038] FIG. 15 shows a schematic diagram of the experimental setup
in comparative example 2.
[0039] FIG. 16 shows the experimental results thus obtained, again
by a plot of the Faraday efficiency against the current density
applied.
[0040] FIG. 17 compares the experimental results from example 1
(solid lines) with those from comparative example 2 (dotted
lines).
[0041] FIG. 18 shows a comparison of the gas chromatograms obtained
in comparative example 2 (solid line; w/o AEM) and example 1
(dotted line; w/AEM) at a current density of 150 mAcm.sup.-2.
[0042] FIGS. 19 and 20 each show schematics of the experimental
setup in reference examples 1 and 2.
[0043] FIGS. 21 and 22 show the experimental results obtained
therein.
DETAILED DESCRIPTION
[0044] Given sufficiently high current densities, CO.sub.2 can be
converted effectively in the presence of a liquid and/or dissolved
acid at a membrane facing the cathode space and containing an anion
exchanger and/or anion transporter, for example in a salt bridge
space and/or in the anolyte, to products utilizable further in an
economically viable manner, and the formation of hydrogen can
surprisingly be suppressed. The elucidations that follow are
applicable to the above systems, for example. On the basis of these
considerations, it is possible to create a charge transport model
for CO.sub.2 electrolysis as follows:
[0045] 1) Considerations for Salt Electrolytes
[0046] The cathodic reduction of CO.sub.2 to CO in the presence of
water can be represented in a simplest approximation by the
equation that follows, and analogous equations may also be adduced
correspondingly in respect of the preparation of hydrocarbons from
CO.sub.2:
CO.sub.2+H.sub.2O+2e.sup.-.fwdarw.CO+2OH.sup.- (1)
[0047] Since CO.sub.2 is typically available in excess in the
3-phase zone (although it may also be present in deficiency since
the CO.sub.2 cannot be assigned to any particular catalyst site),
the OH.sup.- ions formed can react therewith to give
HCO.sub.3.sup.- ions.
OH.sup.-+CO.sub.2.fwdarw.HCO.sub.3.sup.- (2)
The result is:
3CO.sub.2+H.sub.2O+2e.sup.-.fwdarw.CO+2HCO.sub.3.sup.- (1 into
2)
[0048] This reaction has far-reaching consequences for charge
transport within the porous electrode. Since HCO.sub.3.sup.-, by
contrast with OH.sup.-, is not capable of charge transport via the
Grotthuss mechanism, its molar conductivity is several times lower.
Furthermore, it should be noted that the solubility of alkali metal
hydrogencarbonates MHCO.sub.3 (M=Li, Na, K, Rb, Cs) is lower than
that of the hydroxides of the alkali metals, which can result more
quickly in unwanted crystallization of salts.
[0049] In CO.sub.2 electrolysis, however, electrolytes used are
frequently solutions of alkali metal salts. The molar conductivity
of HCO.sub.3.sup.- is only about half that of the alkali metal ions
(M.sup.+ hereinafter), and therefore the majority of the charge
transport in a region of the electrode where both the M.sup.+ ions
of the electrolyte and the HCO.sub.3.sup.- ions generated at the
cathode are present will be borne by the M.sup.+ ions. Owing to
their low conductivity, in this case, the HCO.sub.3.sup.- ions
generated at the cathode do not exit from the electrode into the
electrolyte. Instead, the more mobile M.sup.+ ions penetrate into
the electrode and form a salt with the HCO.sub.3.sup.- ions. This
salt can then exit as a solution or permeate on the side of the
electrode remote from the electrolyte. If, however, efficient
removal of this MHCO.sub.3 solution is not insured, there can also
be crystallization of these salts.
[0050] Over a prolonged period, this phenomenon leads to increasing
penetration of the electrode by the electrolyte solution. This can
result in irreversible pore flooding, which can lead to collapse of
the CO.sub.2 supply of the electrode and hence to the failure
thereof.
[0051] 2) Considerations for Solid-State Electrolytes
[0052] Solid-state electrolytes in electrolysis cells are, for
example, membranes made from polymers modified with charged
functionalities. Especially the usability of anion exchange
membranes (AEMs) for CO.sub.2 electrolysis is known from the
literature, for example from US 2016 0251755 A1, U.S. Pat. No.
9,481,939 and US 2017/0037522 A1.
[0053] In an AEM, the cationic functional groups are at fixed
locations. In the absence of other ions, the charge transport in
this case can therefore typically only be by HCO.sub.3.sup.- ions.
However, this process can more particularly only be employed when
the anode is also directly connected to the membrane. However, the
supply of the HCO.sub.3.sup.- ions to the anode is undesirable
since the CO.sub.2 formed there is released again by
neutralization.
H.sub.2O-2e.sup.-.fwdarw.2H.sup.++1/2O.sub.2
HCO.sub.3.sup.-+H.sup.+.fwdarw.H.sub.2O+CO.sub.2
[0054] It is mixed here with the oxygen formed at the anode, and
the result is a CO.sub.2/O.sub.2 mixture having a CO.sub.2 content
of up to 80 mol % which is difficult to process or virtually
unusable. As a result, in the case of CO, up to 67 mol % of the
CO.sub.2 used may be lost unutilized. As described above, the
considerations made above are also similarly applicable to other
products from the CO.sub.2 reduction. In the case of products that
derive from CO.sub.2 by reduction with more than two electrons, it
is possible, for example, for a proportion of the CO.sub.2 used
which is converted to hydrogen-carbonate to be correspondingly
higher. For methane for example:
9CO.sub.2+8e.sup.-+6H.sub.2O.fwdarw.CH.sub.4+8HCO.sub.3.sup.-
14CO.sub.2+12e.sup.-+8H.sub.2O.fwdarw.C.sub.2H.sub.4+12HCO.sub.3.sup.-
[0055] In this case, for example, it is possible for up to 89 mol %
of the CO.sub.2 used in the case of CH.sub.4 or 86 mol % in the
case of C.sub.2H.sub.4 to be lost via the anode. If, on the other
hand, a cathode-AEM composite is to be coupled to an anodic
half-cell balanced with HCO.sub.3.sup.-, an electrolyte is again
required. The aforementioned condition of the absence of other ions
then no longer exists, and the charge transport is again also borne
by ions other than the HCO.sub.3.sup.- ions, for example M.sup.+
ions in particular. Although the fixed cationic functional groups
of the AEM repel the M.sup.+ ions, a counterion to M.sup.+ ions in
the AEM is available in the form of the HCO.sub.3.sup.- ions.
[0056] Therefore, even within an AEM, a formal double salt system
can exist in which, for example, the anionic part is taken entirely
by HCO.sub.3.sup.-, while the cationic part is taken partly by
M.sup.+ ions and partly by the cationic functional groups of the
polymer. It is thus also possible for the penetration of M.sup.+ to
be limited but not entirely prevented by an AEM in the presence of
a salt electrolyte. In corresponding laboratory studies--as
specified in comparative example 1 below--it was possible to
observe crystallization of MHCO.sub.3 on the reverse side (gas
side) of the electrode. However, the phenomenon is significantly
attenuated compared to direct contact between cathode and
electrolyte. The share of HCO.sub.3.sup.- in the charge transport
can be distinctly increased compared to a mode of operation without
AEM and can be determined, for example, to be .about.50 mol %, for
example by CO.sub.2 measurement by gas chromatography, but is still
limited. The cause of this is the low mobility of hydrogencarbonate
anions as stated above and apparent from table 2 below, taken from
Current Separations 18:3 (1999), Conductance Measurements, Part 1:
Theory, Lou Coury, p. 91-96.
TABLE-US-00002 TABLE 20 Charge mobility of various ions Cation
.lamda..sup.0.sub.+ (S cm.sup.2/mol) Anion .lamda..sup.0.sub.- (S
cm.sup.2/mol) H.sup.+ 349.6 OH.sup.- 199.1 Li.sup.+ 38.7 F.sup.-
55.4 Na.sup.+ 50.10 Cl.sup.- 76.35 K.sup.+ 73.50 Br.sup.- 78.1
Rb.sup.+ 77.8 I.sup.- 76.8 Cs.sup.+ 77.2 NO.sub.2.sup.- 71.8
Ag.sup.+ 61.9 NO.sub.3.sup.- 71.46 NH.sub.4.sup.+ 73.5
ClO.sub.3.sup.- 64.6 Ethylammonium 47.2 ClO.sub.4.sup.- 67.3
Diethylammonium 42.0 IO.sub.4.sup.- 54.5 Triethylammonium 34.3
HCO.sub.3.sup.- 44.5 Tetraethylammonium 32.6 H.sub.2PO.sub.4.sup.-
57 Tetra-n-butylammonium 19.5 HSO.sub.3.sup.- 50 Dimethylammonium
51.8 HSO.sub.4.sup.- 50 Trimethylammonium 47.2
HC.sub.2O.sub.4.sup.- 40.2 Tetramethylammonium 44.9 HCOO.sup.- 54.6
Piperidinium 37.2 CH.sub.3COO.sup.- 40.9 Be.sup.2+ 90
C.sub.6H.sub.5COO.sup.- 32.4 Mg.sup.2+ 106.0 CO.sub.3.sup.2- 138.6
Ca.sup.2+ 119.0 HPO.sub.4.sup.2- 66 Sr.sup.2+ 118.9 SO.sub.4.sup.2-
160.0 Ba.sup.2+ 127.2 C.sub.2O.sub.4.sup.2- 148.2 Fe.sup.2+ 108.0
PO.sub.4.sup.3- 207 Cu.sup.2+ 107.2 Fe(CN).sub.6.sup.3- 302.7
Zn.sup.2+ 105.6 Fe(CN).sub.6.sup.4- 442.0 Pb.sup.2+ 142.0
UO.sub.2.sup.2+ 64 Al.sup.3+ 183 Fe.sup.3+ 204 La.sup.3+ 209.1
Ce.sup.3+ 209.4
[0057] 3) Considerations for Acidic Electrolytes:
[0058] CO.sub.2 electrolysis in aqueous electrolytes should
actually not be thermodynamically possible since the breakdown
voltage for the reaction
CO.sub.2.fwdarw.CO+1/2O.sub.2 1.32 V (3)
is higher than for the reaction:
H.sub.2O.fwdarw.H.sub.2+1/2O.sub.2 1.23 V (4)
[0059] The process is nevertheless possible under suitable
conditions since, firstly, suitable catalysts have a high
overvoltage for the water breakdown and, secondly, high local pH
values can typically develop in the immediate proximity of the
electrode at relatively high current densities. The latter effect
typically requires a diffusion gradient where the OH.sup.-,
CO.sub.3.sup.2- and HCO.sub.3.sup.- ions formed at the electrode
displace the counterions of M.sup.+ ions from the electrolyte.
Moreover, the M.sup.+ concentration in the immediate proximity of
the electrode should be increased by attraction of the M.sup.+ ions
by the electrical field. This can lower the water reduction
potential, which suppresses the evolution of hydrogen. By contrast,
the initial step in the CO.sub.2 reduction is not pH-dependent,
which means that it dominates for longer.
[0060] However, if acids are added to the electrolyte, this
gradient cannot form in sufficient intensity. In acidified
electrolytes, therefore, typically only H.sub.2/CO mixtures with a
large excess of H.sub.2 are obtained. In pure acid electrolytes,
the CO.sub.2 reduction usually takes place only in the trace
region. However, it should be mentioned at this point that the
above-described passage of electrolytes through the electrode is
not observed when pure acid electrolytes are used, as is also
apparent from comparative example 2 below. The passage of
electrolyte is accordingly caused, as discussed above, by the
penetration of cations into the electrode. Since no cations are
present in the present comparative example 2 and in the present
working example, as is described further in detail below, there is
no longer any passage of electrolyte either. The models discussed
above can be confirmed thereby.
[0061] 4) Considerations for a Combination of an AEM and Pure Acid
Electrolyte
[0062] An entirely different situation arises when an AEM is
introduced between a pure acid electrolyte (e.g. H.sub.2SO.sub.4,
as in working example 1). In this case, for example, even in a pure
acid electrolyte, very good selectivities for CO>95% can be
achieved at high current densities of >100 mAcm.sup.-2. The
reason for this lies in a peculiarity of the carbonate-acid-base
system. By contrast with other systems such as the sulfate system,
there is no neutral acid in the carbonate equilibrium.
CO.sub.3.sup.2-HCO.sub.3.sup.-CO.sub.2+H.sub.2O
SO.sub.4.sup.-HSO.sub.4.sup.-H.sub.2SO.sub.4
[0063] Consequently, HCO.sub.3.sup.- cannot function as counterion
for "H.sup.+", the only cations present in the electrolyte. It is
thus not possible for a double salt situation to exist, as with
alkali metal salt electrolytes. Presence of "H.sup.+" ions in the
AEM is therefore possible only when the acid anions (e.g.
SO.sub.4.sup.2-) of the electrolyte are also present in the AEM. If
these are displaced from the AEM by a sufficiently high ion
current, a high pH can be established in the cathode-AEM composite
in spite of an acid electrolyte. The only other charge transport
pathway is the conduction of OH.sup.- via the Grotthuss mechanism
through the membrane swollen in H.sub.2O, or hopping transport of
HCO.sub.3.sup.- from localized polymer-bound cation to localized
cation.
[0064] Since, as already elucidated, the acid ions first have to be
displaced from the AEM, exploitation of this effect requires a
minimum current density. In the present working example this was
about 50 mAcm.sup.-2; below this current density, almost
exclusively H.sub.2 evolution was observed. At high current
densities, the selectivity for CO was >90% and increased
constantly with rising current density, as also shown in the
working example hereinafter.
[0065] FIGS. 1 and 2 illustrate this difference in the use of
various electrolytes 1 that adjoin the anion exchange membrane AEM,
and which pass ions to the cathode K. In this context, FIG. 1 shows
the variant with a salt M.sup.+X.sup.- as electrolyte 1 by way of
example, whereas FIG. 2 shows the variant with an acid H+X.sup.- as
electrolyte 1.
[0066] In some embodiments, the teachings of the present disclosure
include a method of electrolysis of CO.sub.2, wherein an
electrolysis cell comprising [0067] a cathode space comprising a
cathode; [0068] a first ion exchange membrane which contains an
anion exchanger and/or anion transporter and adjoins the cathode
space, where the cathode forms direct contact with the first ion
exchange membrane, the contact in particular embodiments
additionally being ionic in nature; [0069] an anode space
comprising an anode; [0070] a first separator membrane; and [0071]
a salt bridge space, where the salt bridge space is disposed
between the first ion exchange membrane and the first separator
membrane, is used, wherein CO.sub.2 is reduced at the cathode,
wherein the electrolyte in the salt bridge space consists of a
liquid acid and/or a solution of an acid.
[0072] In some embodiments, a method of electrolysis of CO.sub.2,
wherein an electrolysis cell comprising: [0073] a cathode space
comprising a cathode; [0074] a first ion exchange membrane which
contains an anion exchanger and/or anion transporter and adjoins
the cathode space, where the cathode forms direct contact with the
first ion exchange membrane, the contact in particular embodiments
additionally being ionic in nature; and [0075] an anode space
comprising an anode, where the anode space adjoins the first ion
exchange membrane; is used, wherein CO.sub.2 is reduced at the
cathode, wherein the electrolyte in the anode space consists of a
liquid acid and/or a solution of an acid.
[0076] In some embodiments, an electrolysis cell comprises: [0077]
a cathode space comprising a cathode; [0078] a first ion exchange
membrane which contains an anion exchanger and/or anion transporter
and adjoins the cathode space, where the cathode forms direct
contact with the first ion exchange membrane; [0079] an anode space
comprising an anode; and [0080] a diaphragm that adjoins the anode
space;
[0081] further comprising a salt bridge space, wherein the salt
bridge space is disposed between the first ion exchange membrane
and the diaphragm, wherein the diaphragm is nonconductive.
[0082] In some embodiments, there is an electrolysis system
comprising the electrolysis cell described above, and/or include
the use of the electrolysis cell or of the electrolysis system for
electrolysis of CO.sub.2.
Definitions
[0083] Unless defined differently, technical and scientific
expressions used herein have the same meaning as commonly
understood by a person skilled in the art in the technical field of
the disclosure. Gas diffusion electrodes (GDE) in general are
electrodes in which liquid, solid and gaseous phases are present,
and where, in particular, a conductive catalyst catalyzes an
electrochemical reaction between the liquid phase and the gaseous
phase. They can be constructed in different ways, for example as a
porous "all-active-material catalyst", optionally with auxiliary
layers for adjustment of hydrophobicity, in which case it is
possible to produce, for example, a membrane-GDE composite, e.g.
AEM-GDE composite; as a conductive porous carrier to which a
catalyst may be applied in a thin layer, in which case it is
likewise again possible to produce a membrane-GDE composite, e.g.
AEM-GDE composite; or as a catalyst which is porous in the
composite and which may, optionally with additive, be applied
directly to a membrane, e.g. an AEM, and in that case can form a
CCM in the composite.
[0084] In the context of the present disclosure, "hydrophobic" is
understood to mean water-repellent. Hydrophobic pores and/or
channels are those that repel water. In particular, hydrophobic
properties are associated in accordance with the invention with
substances or molecules having nonpolar groups.
[0085] By contrast, "hydrophilic" is understood to mean the ability
to interact with water and other polar substances.
[0086] In the application, figures given relate to % by weight,
unless stated otherwise or apparent from the context.
[0087] Standard pressure is 101 325 Pa=1.01325 bar.
[0088] Electro-osmosis includes an electrodynamic phenomenon in
which a force in the cathode direction acts on particles having a
positive zeta potential that are present in solution, and a force
in the anode direction on all particles having a negative zeta
potential. If a conversion takes place at the electrodes, i.e. if
there is galvanic current flow, there is also a stream of matter of
the particles having positive zeta potential toward the cathode,
irrespective of whether or not the species is involved in the
conversion. The same is also true of a negative zeta potential and
the anode. If the cathode is porous, the medium is also pumped
through the electrode. This is also referred to as an
electro-osmotic pump. The streams of matter that result from
electro-osmosis can also flow counter to concentration gradients.
Diffusion-related currents that compensate for the concentration
gradients can be overcompensated as a result.
[0089] In some embodiments, methods of electrolysis of CO.sub.2,
wherein an electrolysis cell comprising [0090] a cathode space
comprising a cathode; [0091] a first ion exchange membrane which
contains an anion exchanger and/or anion transporter and adjoins
the cathode space, where the cathode forms direct contact with the
first ion exchange membrane; [0092] an anode space comprising an
anode; [0093] a first separator membrane; and [0094] a salt bridge
space, where the salt bridge space is disposed between the first
ion exchange membrane and the first separator membrane, is used,
wherein CO.sub.2 is reduced at the cathode, wherein the salt bridge
space includes a liquid acid and/or a dissolved acid. In some
embodiments, the electrolyte in the salt bridge space consists of
the liquid acid and/or the solution of an acid--for example a solid
or gaseous acid, for example in water, e.g. double-distilled or
demineralized water.
[0095] In some embodiments, methods of electrolysis of CO.sub.2,
wherein an electrolysis cell comprising [0096] a cathode space
comprising a cathode; [0097] a first ion exchange membrane which
contains an anion exchanger and/or anion transporter and adjoins
the cathode space, where the cathode forms direct contact with the
first ion exchange membrane; and [0098] an anode space comprising
an anode, where the anode space adjoins the first ion exchange
membrane; is used, wherein CO.sub.2 is reduced at the cathode,
wherein the anode space includes a liquid acid and/or a dissolved
acid. In some embodiments, the electrolyte in the anode space
consists of the liquid acid and/or the solution of an acid--for
example a solid or gaseous acid, for example in water, e.g.
double-distilled or demineralized water.
[0099] In order to illustrate the similarities and differences in
the methods in advance, these are illustrated by figures
beforehand, although the methods are not limited to the embodiments
shown in these figures. The individual constituents of the cells
used in the methods described herein, and also of the cell in which
the methods can be conducted, are then disclosed in detail
thereafter.
[0100] Illustrative different modes of operation of a
double-membrane cell and a single-membrane cell with which the
methods of the invention can be conducted are shown in FIGS. 3 to
5--in FIG. 3 also in conjunction with further constituents of an
electrolysis system, also with regard to the method of the
invention. In the figures, by way of example, a reduction of
CO.sub.2 to CO is assumed. In principle, however, the method is not
limited to this reaction but can also be used for any other
products, such as hydrocarbons, etc., e.g. in gaseous and/or liquid
form.
[0101] FIG. 3 shows, by way of example, a 2-membrane setup for
CO.sub.2 electroreduction with an acidic anode reaction. In each
case here, the cathode K is provided in the cathode space I and the
anode A in the anode space III, with a salt bridge space II formed
between these spaces, which is divided from the cathode space I by
a first ion exchange membrane, here in the form of an AEM, and from
the anode space III by a first separator membrane, here in the form
of a CEM, for example in the form of a cation and/or proton
exchange membrane. Additionally shown are the feed of catholyte k
to supply the cathode with substrate, for example
H.sub.2O-saturated gaseous CO.sub.2, electrolyte s in the salt
bridge space comprising liquid and/or dissolved acid that couples
the half-cells to one another, and anolyte a for supply of the
anode with substrate, e.g. HCl and/or H.sub.2O, and also a recycle
conduit R for CO.sub.2. The further symbols in FIG. 3 and also in
the analogous FIGS. 8, 9, 10, 12, 15, 19 and 20 are customary
fluidic connection symbols.
[0102] By contrast with the use of a neutral to weakly basic salt
electrolyte as salt bridge s, it is possible by the present method
in the first aspect to neutralize cathodically generated
HCO.sub.3.sup.- at the interface between the anion exchange
membrane (AEM) and the salt bridge electrolyte. This can prevent
HCO.sub.3.sup.- from getting to the anode and subsequently being
lost as unusable CO.sub.2/O.sub.2 mixture. Thus, in particular
embodiments, virtually pure CO2 with just minimal traces of
cathodic products is released in the salt bridge space, and can be
sent directly back to the cathode space I.
[0103] FIGS. 4 and 5 additionally show further constructions of an
electrolysis cell as can be employed in a method incorporating the
teachings herein. No salt bridge space is provided in the
two-chamber setup, and so the anode space III directly adjoins the
AEM, and it is possible here for the anode, as shown in FIGS. 4 and
5, to be present anywhere in the anode space III. Corresponding
configurations of the anode space are also possible in a method
having a set up as shown in FIG. 3, where the anode A thus does not
adjoin the CEM. The electrolysis cells shown in FIGS. 4 and 5 can
likewise be used in the electrolysis system shown in FIG. 3. It is
also possible for the different half-cells from FIGS. 3 to 5 and
also the corresponding arranged constituents of the electrolysis
system to be combined as desired, and likewise also with other
electrolysis half-cells (not shown). As apparent from FIGS. 3 to 5,
it is a feature of the methods taught herein that the cathode K
forms direct, especially also ionic, contact with the first ion
exchange membrane containing an anion exchanger and/or anion
transporter. In addition, the space adjoining the first ion
exchange membrane--either the salt bridge space II in FIG. 3 or the
anode space III in FIGS. 4 and 5--contains a liquid and/or
dissolved acid.
[0104] The methods herein have the particular feature of the use of
a liquid and/or dissolved acid in the salt bridge space or in the
anode space, specifically by comparison with strongly acidic ion
exchanger packages or similar solid apparatuses:
[0105] Firstly, gas bubbles that form from the reaction in the salt
bridge space or anode space can be transported away unhindered
through the fluid medium, which enables a simple mode of
operation.
[0106] Moreover, it is possible here to choose higher flow rates in
order to be able to assure better cooling of the system.
[0107] Furthermore, in the case of use of liquid and/or dissolved
acids, simpler and less costly operation is of course also
possible, especially by comparison with ion exchangers.
[0108] In addition, in the case of use of liquid and/or dissolved
acids, accumulation of metal impurities in parts of the
electrolysis cell can be avoided in that they are washed out by the
liquid and/or dissolved acid.
[0109] Correspondingly, external electrolyte treatment, for example
with a cation exchanger, is subsequently possible. This is
especially a great difference from US 2017/0037522 A1, in which an
empty or ion exchanger-packed middle chamber is disclosed.
[0110] The salt bridge space or the anode space, depending on the
embodiment, are not particularly restricted provided that they
correspondingly adjoin the first ion exchange membrane. The term
"salt bridge space" is used here with regard to its function of
acting as a bridge between the anode arrangement and cathode
arrangement, and in that respect of including cations and anions
which, however, need not necessarily form salts in the present
context. Since a liquid or dissolved acid is present in the salt
bridge space in the present context, this could also be called acid
bridge space or ion bridge space. However, since this term is not
in common use, the space is referred to in accordance with the
disclosure as salt bridge space even if no salt need be present
therein in the conventional sense. In some embodiments, there is an
electrolyte in the salt bridge space--if present--that can assure
electrolytic ionic connection between cathode arrangement and anode
arrangement. This electrolyte is also referred to as salt bridge
and includes a liquid and/or dissolved acid.
[0111] The salt bridge thus serves here as electrolyte, preferably
with high ion conductivity, and serves to establish contact between
the anode and cathode. In some embodiments, the salt bridge also
enables the removal of waste heat. Moreover, the salt bridge can
serve as reaction medium for the anodically and cathodically
generated ions such as protons or hydroxide or hydrogencarbonate
ions.
[0112] The technical teaching consists in the construction and
operation of the cathodic half-cell. The latter consists of a
gas-permeable electrically connected catalyst layer in direct
contact with an AEM, the opposite face of which is adjoined by an
acid-based electrolyte, preferably without alkali metal cations,
especially without metal cations. The acid here is not particularly
restricted, provided that it is in the form of a liquid and/or in
solution, i.e. the acid is able to flow through the salt bridge
space and/or the anode space. In some embodiments, the acid is
water-soluble and/or is in the form of a solution in a suitable
solvent such as water, alcohols, aldehydes, esters, carbonates,
etc., and/or mixtures, especially water, e.g. double-distilled or
demineralized water.
[0113] In some embodiments, an acid in the electrolyte in the salt
bridge space has a pK.sub.A of 6 or less, 5 or less, 3 or less, 1
or less, or 0 or less, where the liquid and/or dissolved acid may
be selected from dilute or neat H.sub.2SO.sub.4, a solution of
H.sub.2N--SO.sub.2--OH, dilute or neat HClO.sub.4, a solution of
H.sub.3PO.sub.4, dilute or neat CF.sub.3--COOH, dilute or neat
CF.sub.3--SO.sub.2--OH, a solution of
(CF.sub.3--SO.sub.2).sub.2--NH, a solution of HF, dilute or neat
HCOOH, dilute or neat CH.sub.3--COOH, a solution of HCl, a solution
of HBr, a solution of HI, and/or mixtures thereof.
[0114] In some embodiments, the acid electrolyte is notable for the
absence of mobile cations--as will be defined further down,
especially metal cations, except for "H.sup.+" or "D.sup.+". Rather
than H.sup.+ or D.sup.+, reference is made hereinafter solely to
H.sup.+ or protons. The electrolyte thus may not contain any mobile
cations except for "H.sup.+", especially any metal cations. In the
working example of the disclosure, sulfuric acid, especially dilute
sulfuric acid (H.sub.2SO.sub.4), was used, which, owing to its low
cost and its high conductivity, works as a liquid and/or dissolved
acid. In some embodiments, it is also possible to use other acids,
as set out above, e.g. strong acids with nonoxidizing anions such
as H.sub.2N--SO.sub.2--OH, HClO.sub.4, H.sub.3PO.sub.4,
CF.sub.3--COOH, CF.sub.3--SO.sub.2--OH,
(CF.sub.3--SO.sub.2).sub.2--NH, etc. It is also possible to use
weak acids in relatively high concentrations, for example greater
than 10% or 20% by weight, for example greater than 30% by weight,
or at their respective conductivity maximum, e.g. HF, HCOOH,
CH.sub.3--COOH. In some embodiments, this acid is identical to the
cathodic product from the CO.sub.2 electrolysis, for example in the
case of formic acid or acetic acid. In some embodiments, the acids
may be present in a concentration up to 30% by weight, up to 50% by
weight, up to 70% by weight, or up to 100% by weight. It is also
possible to use other acids, especially in the case of demonstrable
compatibility with the electrode catalysts, for example dissolved
HCl, HBr, HI.
[0115] In some embodiments, a salt electrolyte typically adjoining
the first ion exchange membrane, for example in the salt bridge
space or in the anode space, may be replaced by an acid. In the
presence of a salt bridge space, the advantage of the CO.sub.2-free
anode and the partial removal of the CO.sub.2 excess in the salt
bridge space continues to exist, as shown in schematic form in
FIGS. 6 and 7. By contrast with the use of a salt electrolyte 2 in
the salt bridge space, as shown in FIG. 6, however, the CO.sub.2 is
released not at the interface between CEM and electrolyte but, as
shown in FIG. 7, at the interface between AEM and acid 3. In FIGS.
6 and 7, an acid is also present here in the anode space.
[0116] Since acids are used both for the anolyte and for the salt
bridge in the variant shown, it is also possible to choose these
with identical composition. Since no osmotic pressures occur in
this case and the release of the CO.sub.2 can also take place
before the salt bridge, especially in the region of the first ion
exchange membrane and hence away from the first separator membrane,
if present, in which case the HCO.sub.3.sup.- no longer reaches the
first separator membrane in particular embodiments, it is no longer
absolutely necessary to use an ion-selective membrane as first
separator membrane, and it is also possible, for example, to use a
diaphragm in order to separate CO.sub.2 and O.sub.2.
Correspondingly, a diaphragm is also possible as the first
separator membrane, as detailed further hereinafter, and is
consequently also possible to use a corresponding electrolysis cell
of the invention, for example an AEM diaphragm cell--as detailed
further hereinafter--in the method of the invention.
[0117] It should be noted that it is possible in principle to pump
anolyte and salt bridge out of a common reservoir, in which case
reliable degassing of the electrolytes may be ensured, in order not
to entrain any gases. This is possible particularly efficiently
owing to the low solubility of CO.sub.2 in the acid-based
electrolyte. In some embodiments, methods are therefore conducted
at relatively high temperatures in the range of 50-120.degree. C.,
or between 60-90.degree. C., in order to further minimize gas
solubility.
[0118] In some embodiments, the acid concentration may be chosen
such that it lies at the conductivity maximum of the acid. It is
possible here for the conductivity, especially for sulfuric acid (3
mol/1=.about.30%) to be almost an order of magnitude higher than
that which can be achieved by salt concentrations that are
similarly high but at the saturation limit (1-2 mol/1).
Illustrative conductivities are shown in tables 3 and 4 for
sulfuric acid and phosphoric acid.
TABLE-US-00003 TABLE 3 Electrical conductivity of sulfuric acid
(and oleum) at 25.degree. C. (from Konduktometrie -
Leitfahigkeitsmessung [Conductometry - Conductivity Measurement],
Peter Bruttel, revised by Dr. Christine Thielen, Dr. Anja Zimmer,
Metrohm AG, Switzerland, page 37) % Conductivity % Conductivity
H.sub.2SO.sub.4 [mS/cm] H.sub.2SO.sub.4 (SO.sub.3) [mS/cm] 3.93 177
53.5 555 7.00 308 58.4 471 10.0 426 63.1 380 14.6 586 72.3 223 19.8
717 85.9 124 25.3 796 95.4 124 29.4 825 98.0 94.7 34.3 819 100.0
10.46 39.1 781 101.5 32.05 43.9 714 103.8 34.50 48.7 640 105.1
28.84 M(H.sub.2SO.sub.4) = 98.07 g/mol M(SO.sub.3) = 80.06
g/mol
TABLE-US-00004 TABLE 4 Electrical conductivity of phosphoric acid
at 25.degree. C. (from Konduktometrie - Leitfahigkeitsmessung,
Peter Bruttel, revised by Dr. Christine Thielen, Dr. Anja Zimmer,
Metrohm AG, Switzerland, page 37) % H.sub.3PO.sub.4 Conductivity
[mS/cm] 5 31 10 61 15 91 20 722 25 152 30 180 35 204 40 222 45 232
50 233 55 224 60 210 70 169 80 98 M(H.sub.3PO.sub.4) = 97.995
g/mol
[0119] The individual constituents of an electrolysis cell used in
the methods and of the electrolysis cell will now be described and
disclosed further. In some embodiments, the cathode space, the
anode space and any salt bridge space present, in the methods and
also in the electrolysis cell discussed hereinafter, are not
particularly restricted in terms of shape, material, dimensions,
etc., provided that they can accommodate the cathode, the anode and
the first ion exchange membrane and any first separator membrane.
The two or three spaces may be formed, for example, within a common
cell, in which case they may be separated correspondingly by the
first ion exchange membrane and optionally the first separator
membrane.
[0120] For the individual spaces, it is possible here, according to
the electrolysis to be conducted, to correspondingly provide inlet
and outlet devices for reactants and products, for example in the
form of liquid, gas, solution, suspension, etc., where these may
optionally also each be recycled. Nor is there any restriction in
this regard, and the flow through the individual spaces may be in
parallel streams or in countercurrent. For example, in the case of
electrolysis of CO.sub.2-- where this may still contain CO, i.e.,
for example, contains at least 20% by volume of CO.sub.2-- this may
be supplied to the cathode in solution, as a gas, etc.--for example
in countercurrent to an electrolyte stream in the salt bridge space
in the three-chamber setup or in the anode space in a two-chamber
setup (without first separator membrane). There is no restriction
in this regard.
[0121] Corresponding feed options also exist for the anode space
and will also be set out in more detail hereinafter. The respective
feed may be provided either in continuous or discontinuous form,
for example in pulsed form, etc., for which pumps, valves, etc. may
correspondingly be provided in an electrolysis system of the
invention, and also cooling and/or heating devices, in order to be
able to correspondingly catalyze desired reactions at the anode
and/or cathode. The materials of the respective spaces or of the
electrolysis cell and/or of the further constituents of the
electrolysis system may also be suitably adapted here to desired
reactions, reactants, products, electrolytes, etc. Furthermore, at
least one power source per electrolysis cell is of course also
included. Further apparatus parts that may occur in electrolysis
cells or electrolysis systems may be provided in the electrolysis
system of the invention or the electrolysis cell. In some
embodiments, these individual cells are used to construct a stack
comprising 2-1000 or 2-200 cells and the operating voltage thereof
may be in the range of 3-1500 V or 200-600 V.
[0122] In some embodiments, a reactant gas formed in the salt
bridge space, for example CO.sub.2 that may also contain H.sub.2
and/or CO, is recycled back in the direction of the cathode
space.
[0123] In some embodiments, the cathode is not particularly
restricted and may be matched to a desired half-reaction, for
example with regard to the reaction products, in that it forms
direct contact with the first ion exchange membrane, i.e. is in
direct contact with the first ion exchange membrane at at least one
point, wherein the cathode is in direct contact essentially in two
dimensions with the first ion exchange membrane. The cathode thus
directly adjoins the first ion exchange membrane at least in one
region.
[0124] A cathode for reduction of CO.sub.2 and optionally CO may
include, for example, a metal such as Cu, Ag, Au, Zn, Pb, Sn, Bi,
Pt, Pd, Ir, Os, Fe, Ni, Co, W, Mo, etc., or mixtures and/or alloys
thereof, e.g. Cu, Ag, Au, Zn, Pb, Sn, or mixtures and/or alloys
thereof, and/or a salt thereof, where suitable materials may be
matched to a desired product. The catalyst may thus be chosen
according to the desired product. In the case of the reduction of
CO.sub.2 to CO, for example, the catalyst may be based on Ag, Au,
Zn and/or compounds thereof, such as Ag.sub.2O, AgO, Au.sub.2O,
Au.sub.2O.sub.3, ZnO. For preparation of hydrocarbons, Cu or
Cu-containing compounds such as Cu.sub.2O, CuO and/or
copper-containing mixed oxides with other metals, etc. may be used.
For a preparation of formic acid, for example, catalysts based on
Pb and/or Cu, especially Cu, are possible. Since the formation of
hydrogen can be entirely suppressed by the ion transport at high
current densities, it is also possible to use catalysts for
CO.sub.2 reduction that do not have a high overvoltage with respect
to hydrogen, for example reduction catalysts such as Pt, Pd, Ir, Os
or carbonyl-forming metals such as Fe, Ni, Co, W, Mo. Thus, the
mode of operation described in conjunction with the cell design
opens up new routes in CO.sub.2 reduction chemistry that do not
depend on the hydrogen overvoltage.
[0125] The cathode is the electrode at which the reductive
half-reaction takes place. It may be in single-part or multipart
form and take the form, for example, of a gas diffusion electrode,
porous electrode, or be directly in a composite with the AEM, etc.
At least the following embodiments, for example, are possible here:
[0126] gas diffusion electrode or porous bound catalyst structure
which, in particular embodiments, may be bonded to the first ion
exchange membrane, for example an anion exchange membrane (AEM),
for example in an ion-conducting and/or mechanical manner, by means
of a suitable ionomer, for example an anionic ionomer; [0127] gas
diffusion electrode or porous bound catalyst structure which, in
particular embodiments, may have been pressed partially into the
first ion exchange membrane, for example an AEM; [0128] porous,
conductive, catalytically inactive structure, e.g. carbon-paper GDL
(gas diffusion layer), carbon-cloth GDL and/or polymer-bound film
of granular glassy carbon impregnated with the catalyst for the
cathode and optionally an ionomer that enables the binding to the
first ion exchange membrane, for example an AEM, in which case the
electrode may have been pressed mechanically onto the first ion
exchange membrane, for example an AEM, or pressed beforehand with
the first ion exchange membrane, for example an AEM, in order to
form a composite; [0129] particulate catalyst that has been applied
by means of a suitable ionomer to a suitable carrier, for example a
porous conductive carrier, and in particular embodiments may adjoin
the first ion exchange membrane, for example an AEM; [0130]
particulate catalyst that has been pressed into or coated onto the
first ion exchange membrane, for example an AEM, and
correspondingly bonded in an electrically conductive manner, for
example, in which case this structure may be pressed, for example,
as what is called a CCM (catalyst-coated membrane) onto a
conductive porous electrode, where catalytic activity of this
electrode is not required in principle and, for example, it is
possible to use carbon-based GDLs or grids, for example of
titanium, and it is not ruled out here that this electrode contains
or consists in large portions of ionomers and/or the active
catalyst; [0131] noncontinuous sheetlike structure, for example a
mesh or an expanded metal, which, for example, consists of or
comprises or has been coated with a catalyst and, in particular
embodiments, adjoins the first ion exchange membrane, for example
an AEM; [0132] polymer-bound all-active catalyst structure composed
of particulate catalyst that contains or has subsequently been
impregnated with an ionomer that enables binding to the first ion
exchange membrane, for example an AEM, in which case the electrode
may be mechanically pressed onto the first ion exchange membrane,
for example an AEM, or have been pressed beforehand with the first
ion exchange membrane, for example an AEM, in order to form a
composite; [0133] porous conductive carrier that has been
impregnated with a suitable catalyst and optionally an ionomer and,
in particular embodiments, adjoins the first ion exchange membrane,
for example an AEM; [0134] non-ion-conductive gas diffusion
electrode that has been subsequently impregnated with a suitable
ionomer, for example an anion-conductive ionomer, and, in
particular embodiments, adjoins the first ion exchange membrane,
for example an AEM, or has been bonded thereto, for example via an
ionomer.
[0135] Various combinations of the above-described electrode
structures are also possible as cathode. The corresponding cathodes
here too may contain materials that are customary in cathodes, such
as binders, ionomers, for example anion-conductive ionomers,
fillers, hydrophilic additives, etc., which are not particularly
restricted. As well as the catalyst, the cathode may thus, in
particular embodiments, contain at least one ionomer, for example
an anion-conductive or anion-transporting ionomer (e.g. anion
exchange resin, anion transport resin) that may comprise, for
example, various functional groups for ion exchange that may be the
same or different, for example tertiary amine groups, alkyl
ammonium groups and/or phosphonium groups, a carrier material, for
example a conductive carrier material (e.g. a metal such as
titanium), and/or at least one nonmetal such as carbon, Si, boron
nitride (BN), boron-doped diamond, etc., and/or at least one
conductive oxide such as indium tin oxide (ITO), aluminum zinc
oxide (AZO) or fluorinated tin oxide (FTO)--for example as used for
production of photoelectrodes, and/or at least one polymer based on
polyacetylene, polyethoxythiophene, polyaniline or polypyrrole, as,
for example, in polymer-based electrodes; nonconductive carriers,
for example polymer meshes, are possible given adequate
conductivity of the catalyst layer, binders (e.g. hydrophilic
and/or hydrophobic polymers, e.g. organic binders, 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), conductive
fillers (e.g. carbon), nonconductive fillers (e.g. glass) and/or
hydrophilic additives (e.g. Al.sub.2O.sub.3, MgO.sub.2, hydrophilic
materials such as polysulfones, e.g. polyphenylsulfones,
polyimides, polybenzoxazoles or polyetherketones, or generally
polymers that are electrochemically stable in the electrolyte,
polymerized "ionic liquids", and or organic conductors such as
PEDOT:PSS or PANI (camphorsulfonic acid-doped polyaniline), which
are not particularly restricted.
[0136] The cathode, e.g. in the form of a gas diffusion electrode,
for example bonded to the first ion exchange membrane, or present
in the form of a CCM, in particular embodiments, contains
ion-conductive components, especially an anion-conductive
component. Other cathode forms are also possible, for example
cathode constructions as described in US2016 0251755-A1 and U.S.
Pat. No. 9,481,939.
[0137] The anode is not particularly restricted either and may be
matched to a desired half-reaction, for example with regard to the
reaction products. At the anode, which is electrically connected to
the cathode by means of a power source to provide the voltage for
the electrolysis, the oxidation of a substance takes place in the
anode space. Furthermore, the anode material is 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, iron, nickel
etc.
[0138] 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 may also have
been merely superficially applied by thin-film methodology, for
example on a titanium and/or carbon carrier. The anode catalyst is
not particularly restricted. Catalysts used for production of
O.sub.2 or Cl.sub.2 also include, for example, IrO.sub.x
(1.5<x<2) or RuO.sub.2. These may also be in the form of a
mixed oxide with other metals, e.g. TiO.sub.2, and/or have been
supported on a conductive material such as C (in the form of
conductive carbon black, activated carbon, graphite, etc.).
Alternatively, it is also possible to utilize catalysts based on
Fe--Ni or Co--Ni for production of O.sub.2. For this purpose, for
example, the construction described below with a bipolar membrane
is suitable.
[0139] The anode is the electrode at which the oxidative
half-reaction takes place. It may likewise take the form of a gas
diffusion electrode, porous electrode or all-active electrode or
solid electrode, etc. At least the following embodiments are
possible: [0140] gas diffusion electrode or porous bound catalyst
structure which, in particular embodiments, may be bonded to the
first separator membrane, if present, for example a cation exchange
membrane (CEM) or a diaphragm, for example in an ion-conducting
and/or mechanical manner, by means of a suitable ionomer, for
example a cationic ionomer; [0141] gas diffusion electrode or
porous bound catalyst structure which, in particular embodiments,
may have been pressed partially into the first separator membrane,
for example a CEM or a diaphragm; [0142] particulate catalyst that
has been applied by means of a suitable ionomer to a suitable
carrier, for example a porous conductive carrier, and in particular
embodiments may adjoin the first separator membrane, for example a
CEM or a diaphragm; [0143] particulate catalyst that has been
pressed into the first separator membrane, for example a CEM or a
diaphragm, and correspondingly bonded in an electrically conductive
manner, for example; [0144] noncontinuous sheetlike structure, for
example a mesh or an expanded metal, which, for example, consists
of or comprises or has been coated with a catalyst and, in
particular embodiments, adjoins the first separator membrane, for
example a CEM or a diaphragm; [0145] solid electrode, in which case
there may also be a gap between the first separator membrane, for
example a CEM or a diaphragm, and the anode, although this is not
preferred; [0146] porous conductive carrier that has been
impregnated with a suitable catalyst and optionally an ionomer and,
in particular embodiments, adjoins the first separator membrane,
for example a CEM or a diaphragm; [0147] non-ion-conductive gas
diffusion electrode that has been subsequently impregnated with a
suitable ionomer, for example a cation-conductive ionomer, and, in
particular embodiments, adjoins the first separator membrane, for
example a CEM or a diaphragm; [0148] any desired variants of the
embodiments discussed, where the electrode contains, for example,
an anodically stable anion-conductive material and directly adjoins
the anion-conductive layer of a bipolar membrane.
[0149] The anode here may follow on from the acid electrolyte or
else directly adjoin the first ion exchange membrane, for example
an AEM, for example in the form of a sheetlike structure (e.g.
fine-mesh coated grid), such that there is no salt bridge space.
Here too, various combinations of the different anode structures
are possible. In some embodiments, the cathode is coupled to the
anodic half-cell via the liquid acid, for example in the salt
bridge space or in the anode space, for example in the salt bridge
space.
[0150] The corresponding anodes may likewise contain materials that
are customary in anodes, such as binders, ionomers, for example
including cation-conducting ionomers, for example containing
sulfonic acid and/or phosphonic acid groups, fillers, hydrophilic
additives, etc., which are not particularly restricted, which have
also been described above, for example, with regard to the
cathodes.
[0151] In some embodiments, it is possible to combine the
electrodes mentioned by way of example above with one another as
desired. Furthermore, it is also possible for electrolyte to be
present in the anode space and cathode space, which are also
respectively referred to as anolyte and catholyte, but it is not
ruled out in that no electrolytes are present in the two spaces and
they are correspondingly supplied, for example, solely with gases
for conversion, for example CO.sub.2 only, optionally also as a
mixture with, for example, CO and/or H.sub.2O, which may optionally
also be in liquid form, for example as an aerosol, but with gaseous
H.sub.2O to the cathode and/or water or HCl to the anode. In some
embodiments, an anolyte is present, which may differ from or
correspond to the salt bridge, i.e. the electrolyte of the salt
bridge space, which includes a liquid and/or dissolved acid--if
present, for example with regard to solvents, acids present, etc.
If no salt bridge is present, the anolyte comprises a liquid and/or
dissolved acid.
[0152] A catholyte here is the electrolyte flow around the cathode
and, in particular embodiments, serves to supply the cathode with
substrate or reactant. The embodiments which follow are possible,
for example. The catholyte may take the form, for example, of a
solution of the substrate (CO.sub.2) in a liquid carrier phase
(e.g. water) and/or of a mixture of the substrate with other gases
(e.g. CO+CO.sub.2; water vapor+CO.sub.2). It is likewise possible
for recycled gases such as CO and/or H.sub.2 to be present as a
result of recycling. It is also possible as described above, for
the substrate to be present as a pure phase, e.g. CO.sub.2. If the
reaction gives rise to uncharged liquid products, these may be
washed out by the catholyte and may subsequently also optionally be
removed in a corresponding manner.
[0153] An anolyte is an electrolyte flow around the anode or at the
anode and, in particular embodiments, serves to supply the anode
with substrate or reactant and optionally to transport anode
products away. The embodiments that follow are possible, for
example. The anolyte may take the form of a solution of the
substrate (e.g. hydrochloric acid=HCl.sub.aq) in a liquid carrier
phase (e.g. water), optionally with conductive salts that are not
restricted--especially in the case of use of a bipolar membrane as
the first separator membrane, where the anolyte here may also be
basic and may also contain cations, as described above, or of a
mixture of the substrate with other gases (e.g. hydrogen
chloride=HCl.sub.g+H.sub.2O, SO.sub.2, etc.). As is also the case
for the catholyte, however, the substrate may also take the form of
a pure phase, for example in the form of hydrogen chloride
gas=HCl.sub.g.
[0154] In some embodiments, the anode space comprises an anolyte
comprising a liquid and/or dissolved acid, preferably wherein the
anolyte and/or the acid in the salt bridge space or the electrolyte
in the salt bridge space does not include any mobile cations except
for protons and/or deuterons, especially any metal cations. In some
embodiments, the acid in the salt bridge space does not comprise
any mobile cations except for protons and/or deuterons, especially
any metal cations. In some embodiments, the anolyte does not
comprise any mobile cations except for protons and/or deuterons,
especially any metal cations. Mobile cations here are cations that
are not bonded to a support by a chemical bond and/or especially
have an ion mobility of more than 110.sup.-8 m.sup.2/(sV),
especially of more than 110.sup.-10 m.sup.2/(sV). In some
embodiments, the anodic half-reaction does not release or produce
any mobile cations except for "D.sup.+", "H.sup.+", especially any
metal cations. In such a case, therefore, for the specific case of
the evolution of O.sub.2 at the anode, water (especially in the
case of the CCM anode) or acids with non-oxidizable anions are
possible anolytes or reagents. For the production of halogen at the
anode, especially for this case, the halogen-hydrogen acids HCl,
HBr or HI are correspondingly suitable, and halide salts may not be
suitable in the case of use of a diaphragm as the first separator
membrane, but may be used in the case of use of a bipolar membrane
as the first separator membrane. It is also possible to use
SO.sub.2 in the anolyte for preparation of sulfuric acid, or
H.sub.2O for preparation of H.sub.2O.sub.2, etc.
[0155] In some embodiments, the anolyte is an aqueous electrolyte,
where appropriate reactants that are converted at the anode may
optionally be added to the anolyte. The addition of reactants here
is not restricted. The addition of reactants on supply to the
cathode space is likewise not restricted. For example, CO.sub.2 can
be added to water outside the cathode space, or can also be added
via a gas diffusion electrode, or can also be supplied solely as a
gas to the cathode space. Corresponding considerations are
analogously possible for the anode space, according to the
reactants used, for example water, HCl, etc., and the desired
product.
[0156] The first ion exchange membrane which contains an anion
exchanger and/or anion transporter or an anion transport material
and which adjoins the cathode space is not particularly restricted
in accordance with the invention. In some embodiments, it separates
the cathode from the salt bridge space, or, in the method of the
second aspect, it separates the cathode from the anode space, so as
to result in, from the direction of the cathode space comprising
CO.sub.2 in the electrolyte direction, the sequence of
cathode/first ion exchange membrane/salt bridge space (first
aspect) or cathode/first ion exchange membrane/anode space. In some
embodiments, it contains or consists of an anion exchanger which,
in the zero-current state, is in the form of an acid anion salt,
preferably corresponding to the acid of the salt bridge, and
further may be converted to the hydrogencarbonate/carbonate form
over and above a minimum current density.
[0157] In some embodiments, the first ion exchange membrane is an
anion exchange membrane and/or anion transport membrane. In some
embodiments, the first ion exchange membrane may have a hydrophobic
layer, for example on the cathode side, for better contacting with
gas. In some embodiments, the anion exchange membrane and anion
transport membrane additionally functions as a cation blocker
(albeit in traces, for example), and especially as a proton
blocker. Specifically an anion exchanger and/or anion transporter
with cations bound in a fixed manner may constitute a blockage here
for mobile cations by coulombic repulsion, which can additionally
counteract separation of salts, especially within the cathode. The
cause of this is probably the above-described formation of
hydrogencarbonate ions during the electrolysis and the resulting
formation of hydrogencarbonate salts from the cations transported
through the membrane, if present. Without liquid electrolyte or
sufficiently active anion transport, these or their salts typically
cannot be removed.
[0158] Especially in the case of a membrane-electrode assembly
(MEA), the enrichment of the electrolyte cations in the region of
the interface is typically attributable to electroosmosis. In that
case, a concentration gradient cannot simply be dissipated here on
the electrode side since a catalyst-based cathode configured as set
out above, for example a gas diffusion electrode or a CCM, usually
has only very poor anion conductivity. The integration of
anion-conducting components here can distinctly improve the anion
conductivity. In the method described here, the electrolyte
contains solely protons. Like metal cations, these are likewise
pulled in the cathode direction by the electrical field, but they
cannot pass through the AEM as such since they react with
hydrogencarbonate ions present therein. The fundamental difference
from the use of salt electrolytes is that the charge transport at
the AEM-electrolyte boundary is not through migration of a charge
carrier but through destruction of two oppositely charged charge
carriers.
[0159] To improve operational stability, ion transporters,
especially anion transport resins, may be used as binder material
or additive in the electrode itself and/or in an anion exchanger
layer adjoining the cathode in order to rapidly lead off or partly
buffer OH.sup.- ions that form, for example, such that the reaction
with CO.sub.2 and the associated formation of hydrogen-carbonates
can be reduced or the anion transport resins conduct
HCO.sub.3.sup.- themselves. In principle, anion transport can be
effected by anion exchangers. In addition, an integrated anion
exchanger again specifically constitutes a blockage for cations,
for example including traces of metal cations, which can
additionally counteract separation of salts and contamination of
the electrode. In the case of protons, the formation of hydrogen
can be suppressed.
[0160] The first ion exchange membrane, for example from the
cathode side adjoining the salt bridge in the method of the first
aspect, may thus contain, for example, an anion exchanger and/or
anion transporter in the form of an anion exchanger and/or
transporter layer, in which case further layers such as
hydrophobizing layers may be present to improve contact with the
gas, for example CO.sub.2.
[0161] In some embodiments, the first ion exchange membrane is an
anion exchange membrane and/or anion transport membrane, i.e., for
example, an ion-conductive membrane (or else in the broader sense a
membrane having an anion exchange layer and/or anion transport
layer) with positively charged functionalizations, which is not
particularly restricted. In some embodiments, charge transport
takes place through anions in the anion exchange layer and/or anion
transport layer or an anion exchange membrane and/or anion
transport membrane.
[0162] In some embodiments, the first ion exchange membrane and
anion exchange layer and/or anion transport layer therein or an
anion exchange membrane and/or anion transport membrane serves to
provide anion transport along positive charges at fixed locations.
It is possible here to reduce or completely prevent the penetration
of a proton-containing electrolyte into the cathode, for example,
which is promoted by electro-osmotic forces. The ion exchanger
present in the membrane, in particular embodiments, especially in
operation, can be converted to the carbonate/hydrogencarbonate form
and hence suppress the passage of protons through the membrane to
the cathode.
[0163] A suitable first ion exchange membrane, for example anion
exchange membrane and/or an ion transport membrane, in particular
embodiments, shows good wettability by water and/or acids,
especially aqueous acids, high ion conductivity, and/or tolerance
of the functional groups present therein to high pH values,
especially does not show any Hoffmann elimination. An illustrative
AEM of the invention is the A201-CE membrane sold by Tokuyama which
is used in the example, the "Sustainion" sold by Dioxide Materials,
or an anion exchange membrane sold by Fumatech, for example Fumasep
FAS-PET or Fumasep FAD-PET.
[0164] Furthermore, the first separator membrane is not
particularly restricted, if present, i.e., for example, in the
methods described herein. In some embodiments, the first separator
membrane (adjoining the salt bridge, viewed from the anode side) is
selected from an ion exchange membrane containing a cation
exchanger, a bipolar membrane, where the cation-conducting layer in
the case of the bipolar membrane may be oriented toward the cathode
and the anion-conducting layer toward the anode, and a
diaphragm.
[0165] A suitable first separator membrane, for example a cation
exchange membrane or a bipolar membrane, contains, for example, a
cation exchanger that may be in contact with the electrolyte in the
salt bridge space. It may contain, for example, a cation exchanger
in the form of a cation exchanger layer, in which case further
layers such as hydrophobizing layers may be present. It may
likewise take the form of a bipolar membrane or of a cation
exchange membrane (CEM).
[0166] The cation exchange membrane or cation exchange layer may
be, for example, an ion-conductive membrane or ion-conductive layer
having negatively charged functionalizations. An illustrative mode
of charge transport into the salt bridge in such a first separator
membrane is through cations. For example, commercially available
Nafion.RTM. membranes are suitable as CEM, or else the Fumapem-F
membranes sold by Fumatech, Aciplex sold by Asahi Kasei, or the
Flemion membranes sold by AGC. In principle, however, it is also
possible to use other polymer membranes modified with strongly
acidic groups (groups such as sulfonic acid, phosphonic acid). In
particular embodiments, the first separator membrane prevents the
passage of anions, especially HCO.sub.3.sup.-, into the anode
space.
[0167] In some embodiments, the first separator membrane may take
the form of a diaphragm, which means that the cell can be
configured in a less complex and cheaper manner. In some
embodiments, the diaphragm essentially separates the anode space
and the salt bridge space, for example to an extent of more than
70%, 80% or 90%, based on the interface between anode space and
salt bridge space, or separates the anode space and the salt bridge
space, i.e. to an extent of 100%, based on the interface between
anode space and salt bridge space. In some embodiments, the use of
the liquid acid in the salt bridge space can prevent
HCO.sub.3.sup.- ions from getting into the anode space. In this
respect, it is thus possible to dispense with a cation exchange
layer in the first separator membrane.
[0168] The diaphragm here is not particularly restricted and may be
based, for example, on a ceramic (e.g. ZrO.sub.2 or
Zr.sub.3(PO.sub.4).sub.3) and/or a swellable functionalized
polymer, e.g. PTFE. It is also possible for binders (e.g.
hydrophilic and/or hydrophobic polymers, for example organic
binders, 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), conductive fillers (e.g. carbon), nonconductive fillers
(e.g. glass) and/or hydrophilic additives (e.g. Al.sub.2O.sub.3,
MgO.sub.2, hydrophilic materials such as polysulfones, e.g.
polyphenylsulfones (PPSU), polyimides, polybenzoxazoles or
polyetherketones, or polymers that are generally electrochemically
stable in the electrolyte to be present.
[0169] In some embodiments, the diaphragm is porous and/or
hydrophilic. Since it is itself non-ion-conductive, it should
preferably be capable of swelling in the acid electrolytes used.
Furthermore, it constitutes a physical barrier to gases and cannot
be penetrated by gas bubbles. For example, it is a porous polymer
structure, where the base polymer is hydrophilic (e.g. PPSU). By
contrast with CEM or bipolar membrane, the polymer does not
comprise any charged functionalizations. In addition, the diaphragm
may further preferably contain hydrophilic structure-imparting
components such as metal oxides (e.g. ZrO.sub.2) or ceramics, as
set out above.
[0170] A suitable first separator membrane, for example a cation
exchange membrane, a bipolar membrane and/or a diaphragm, in
particular embodiments, shows good wettability by water and/or
acids, high ion conductivity, stability to reactive species that
can be generated at the anode (which is the case for perfluorinated
polymers, for example), and/or stability in the pH regimes
required, especially to the liquid acid in the salt bridge space.
In some embodiments, the first ion exchange membrane and/or the
first separator membrane are hydrophobic, such that they form a CCM
with the electrodes, at least on the side facing the electrodes,
such that the reactants for the electrodes are in gaseous form. In
some embodiments, the anode and/or cathode are at least partly
hydrophilic. In some embodiments, the first ion exchange membrane
and/or the first separator membrane are wettable with water. In
order to assure good ion conductivity of the ionomers, swelling
with water may be used. It has been found in the experiment that
poorly wettable membranes can lead to distinct worsening of the
ionic binding of the electrodes.
[0171] For some of the electrochemical conversions at the catalyst
electrodes, the presence of water is also advantageous, e.g.:
3CO.sub.2+H.sub.2O+2e.sup.-.fwdarw.CO+2HCO.sub.3.sup.-
[0172] Therefore, the anode and/or cathode may also have sufficient
hydrophilicity. This can optionally be adjusted by hydrophilic
additives such as TiO.sub.2, Al.sub.2O.sub.3, or other
electrochemically inert metal oxides, etc.
[0173] In some embodiments, it is especially possible to use at
least one of the following first separator membranes: [0174] A
diaphragm may be used when the salt bridge (the electrolyte in the
salt bridge space) and the anolyte include or consist of an
identical, preferably inert, acid, in which case the diaphragm
serves to keep gases separate, such that carbon dioxide does not
pass into the anode space, and/or when O.sub.2 is produced at the
anode, especially in order to save costs.
[0175] A corresponding construction of an illustrative electrolysis
system with diaphragm DF is shown in FIG. 8, where the further
system constituents here correspond to those in FIG. 3. [0176] A
cation exchange membrane or a membrane with a cation exchange layer
may be used especially when the salt bridge and the anolyte are not
identical, and/or especially when the anolyte contains HCl, HBr
and/or HI, and/or when chlorine is produced at the anode. Since the
cation exchange membrane prevents the passage of anions from the
anolyte into the salt bridge and, by contrast with the diaphragm,
does not have open porosity, the anode can be configured more
freely. In principle, the anode reaction in such an embodiment is
limited only in that it does not release any mobile cations except
for protons that can pass into the salt bridge via the CEM. [0177]
A bipolar membrane, where an anion exchange layer and/or anion
transport layer of the bipolar membrane may be directed toward the
anode space and a cation exchange layer and/or cation transport
layer of the bipolar membrane toward the salt bridge space, is used
especially when the salt bridge and the anolyte are not identical,
and/or the anolyte especially contains bases and/or salts, and/or
in the case of use of aqueous electrolytes. Especially in the case
of use of bipolar membranes as the first separator membrane, the
anode space may be configured independently of the salt bridge and
the cathode space, which allows a multitude of anode reactions with
desired products, and, especially in the case of use of bases, it
is also possible to use cheaper anodes or anode catalysts, for
example nickel-based anode catalysts for evolution of oxygen.
[0178] An illustrative specific construction with a bipolar
membrane is shown in FIG. 9, which, by way of example, shows a
2-membrane setup for CO.sub.2 electroreduction with AEM on the
cathode side and bipolar membrane (CEM/AEM) on the anode side,
where, as in FIGS. 1 to 3 as well, the supply of catholyte k,
electrolyte s with liquid and/or dissolved acid (electrolyte for
the salt bridge space) and anolyte a, and also recycling R of
CO.sub.2, are shown here and, by way of example, water is oxidized
on the anode side. The further reference numerals correspond to
those in FIG. 3.
[0179] A bipolar membrane may be executed, for example, as a
sandwich of a CEM and of an AEM. In this membrane, however, there
are typically not two superposed membranes, but rather a membrane
having at least two layers. The diagram in FIG. 9 with AEM and CEM
serves here merely for illustration of the preferred orientation of
the layers. The AEM or anion exchange layer points toward the
anode, and the CEM or cation exchange layer toward the cathode.
These membranes are virtually impassable both to anions and
cations. The conductivity of a bipolar membrane is accordingly not
based on transport capacity for ions. The ions are instead
typically transported via acid-base dissociation of water in the
middle of the membrane. As a result, two oppositely charged charge
carriers are generated, which are transported away by the
electrical field.
[0180] The OH.sup.- ions thus generated can be guided to the anode
through the AEM part of the bipolar membrane, where they are
oxidized,
4OH.sup.-.fwdarw.O.sub.2+2H.sub.2O+4e.sup.-
[0181] and the "H.sup.+" ions through the CEM part of the bipolar
membrane into the salt bridge or salt bridge space II, where they
can be neutralized by the HCO.sub.3.sup.- ions generated at the
cathode.
HCO.sub.3.sup.-+H.sup.+.fwdarw.CO.sub.2+H.sub.2O
[0182] However, since the conductivity of the bipolar membrane is
based on the separation of charges in the membrane, a higher
voltage drop is typically to be expected. The advantage of such a
construction may lie in the decoupling of the electrolyte circuits
since, as already mentioned, the bipolar membrane is virtually
impermeable to all ions.
[0183] In this way, it is also possible to implement a setup for a
basic anode reaction that does not need constant replenishment and
removal of salts or anode products. This is otherwise possible only
in the case of use of anolytes based on acids with
electrochemically inactive anions, for example H.sub.2SO.sub.4. In
the case of use of a bipolar membrane, it is also possible to use
hydroxide electrolytes such as KOH or NaOH as anolyte. High pH
values thermodynamically promote the oxidation of water and allow
the use of significantly less costly anode catalysts, for example
based on nickel-iron, that would not be stable under acidic
conditions.
[0184] Some embodiments, in the case of use of a bipolar membrane
as the first separator membrane, also include the use of bases, for
example a hydroxide base, as anolyte when an acid is used in the
salt bridge. The advantage here is that significantly less costly
anode catalysts can be used in basic anolyte, for example based on
Ni/Fe.
[0185] In addition, the anode and/or cathode, in particular
embodiments, have sufficient hydrophilicity. This can optionally be
adjusted by hydrophilic additives such as TiO.sub.2,
Al.sub.2O.sub.3, or other electrochemically inert metal oxides,
etc.
[0186] In some embodiments, the cathode and/or anode takes the form
of a gas diffusion electrode, of a porous bound catalyst structure,
of a particulate catalyst on a carrier, of a coating of a
particulate catalyst on the first and/or second ion exchange
membrane, of a porous conductive carrier impregnated with a
catalyst, and/or of a noncontinuous sheetlike structure. In
particular embodiments, the cathode takes the form of a gas
diffusion electrode, of a porous bound catalyst structure, of a
particulate catalyst on a carrier, of a coating of a particulate
catalyst on the first and/or second ion exchange membrane, of a
porous conductive carrier impregnated with a catalyst, and/or of a
noncontinuous sheetlike structure containing an anion exchange
material and/or anion transport material. In particular
embodiments, the anode takes the form of a gas diffusion electrode,
of a porous bound catalyst structure, of a particulate catalyst on
a carrier, of a coating of a particulate catalyst on the first
and/or second ion exchange membrane, of a porous conductive carrier
impregnated with a catalyst, and/or of a noncontinuous sheetlike
structure, containing a cation exchange material and/or coupled
and/or bound to a bipolar membrane. The various embodiments of the
cathode and anode are combinable here with one another as
desired.
[0187] In some embodiments, the anode is in contact with the first
separator membrane as already described above by way of example.
This enables good binding to the salt bridge space. In this case,
in addition, no charge transport through the anolyte is necessary
and the charge transport pathway is shortened. It is thus also
possible to avoid electrical shadowing effects by support
structures between the anode and first separator membrane.
[0188] In some embodiments, the anode and/or the cathode is in
contact with a conductive structure on the side remote from the
salt bridge space. The conductive structure here is not
particularly restricted. The anode and/or the cathode, in
particular embodiments, is thus in contact via conductive
structures on the side remote from the salt bridge. These are not
particularly restricted. These may, for example, be carbon
nonwovens, metal foams, metal knits, expanded metals, graphite
structures or metal structures.
[0189] In some embodiments, the electrolysis is conducted with a
current density of more than 50 mAcm.sup.-2, more than 100
mAcm.sup.-2, of 150 mAcm.sup.-2 or more, 170 mAcm.sup.-2 or more,
200 mAcm.sup.-2 or more, 250 mAcm.sup.-2 or more, e.g. 300
mAcm.sup.-2 or more, 400 mAcm.sup.-2 or more, or 600 mAcm.sup.-2 or
more. As set out above, it is possible here--contrary to
expectations--to improve the Faraday yield.
[0190] In some embodiments, the methods of the invention provide
comparatively low demands on the chemical stability of the first
ion exchange membrane, for example an AEM. The stability and hence
the usability of AEMs in particular is currently limited mainly by
two degradation mechanisms, firstly by the often inadequate
stability of the functional groups to concentrated bases, e.g. KOH
(Hoffmann elimination of quaternary ammonium ions), and secondly by
the destruction of the polymer backbone by anodic oxidation. Since
only acid electrolytes are used in contact with the first ion
exchange membrane in the electrolysis systems introduced here, the
first ion exchange membrane, for example an AEM, is never exposed
to concentrated bases. Moreover, the anode preferably does not
directly adjoin the first ion exchange membrane, for example an
AEM, which also rules out anodic damage to this membrane.
[0191] It is possible by the present electrolysis method to obtain
various products from CO.sub.2, for instance CO and/or
hydrocarbons. It is also possible to electrochemically produce
formate from CO.sub.2.
2CO.sub.2+H.sub.2O+2e.sup.-.fwdarw.HCOO.sup.-+HCO.sub.3.sup.-
HCOO.sup.-+H.sub.2O.revreaction.OH.sup.-+HCOOH (5)
[0192] In customarily used carbonate-buffered salt electrolytes as
salt bridge, there is typically deprotonation of the formic acid.
The actual product is thus formate salts.
HCOOH+MHCO.sub.3.fwdarw.HCOOM+H.sub.2O+CO.sub.2 (6)
[0193] The cleavage of formate salts to formic acid is technically
difficult and costly, which has today limited the usability of
CO.sub.2 electrolysis to formic acid.
[0194] In the system described here, this deprotonation does not
take place since the electrolyte used is an acid-containing
electrolyte, especially pure acid. For further simplification, it
is also possible, for example, to use formic acid, e.g. dilute
formic acid, as electrolyte in the salt bridge, which can be
concentrated by the electrolysis, which is promoted by an
appropriate electrical conductivity of the formic acid as apparent
from table 5.
TABLE-US-00005 TABLE 5 Conductivity of organic acids at 25.degree.
C. Formic acid Conductivity Acetic acid Conductivity [% by wt.]
[mS/cm] [% by wt.] [mS/cm] 5 6.22 5 1.36 10 8.26 10 1.76 15 9.86 15
1.82 20 11.1 20 1.82 25 11.4 25 1.71 30 11.8 30 1.58 40 11.1 40
1.23 50 9.78 50 0.840 60 7.92 60 0.521 70 5.92 70 0.270 80 3.92 80
0.093 90 1.95 100 0.32 M(HCOOH) = 46.026 g/mol M(CH.sub.3COOH) =
60.052 g/mol
[0195] In operation, according to table 5, for example, a 10% by
weight formic acid is used as the initial charge, which is
concentrated in operation to 60-70% by weight, for example. Then
the electrolyte is drawn off down to a residue which is utilized to
re-establish the starting concentration of 10% by weight. Systems
that work continuously in a relatively narrow concentration range
are likewise conceivable. For formic acid it is possible with
preference to use electrodes such as those based on or composed of
tin or lead. The HCO.sub.3.sup.- transport that occurs demonstrates
that a high pH exists in the region of the cathode. Since formic
acid has a lower pK.sub.A than CO.sub.2, it is in the form of
formate in the region of the cathode. These anions are then, for
example, transported away through the first ion exchange membrane,
e.g. AEM, into the salt bridge (first aspect) or the anolyte
(second aspect) and reprotonated by the acid therein. This is
regenerated by the protons that pass over from the anodic half-cell
or are present in the anolyte. There is no likelihood of the formic
acid exiting on the side of the electrode remote from the salt
bridge space, if present.
[0196] For the CO.sub.2 to CO electrolysis, for example, a first
ion exchange membrane, e.g. AEM diaphragm cell, is advantageous
since the components are less costly and the electrical resistance
of the cell is lower.
[0197] A double-membrane cell with an acid salt bridge is also
advantageous for such applications in which exchange of anions
between the salt bridge and anolyte is to be avoided, for example
[0198] when anolyte and salt bridge are not identical; [0199] in
the co-electrolysis of CO.sub.2 and HCl in order to simultaneously
form a CO.sub.2 reduction product, e.g. CO, and CL.sub.2, [0200] in
the above-described preparation of formic acid in order to avoid
reoxidation of the formic acid at the anode; [0201] in the case of
use of copper-based cathodes that produce alkenes, alkanes,
alcohols and liquid oxygenates; [0202] in the case of any
combinations of these points.
[0203] CO.sub.2 is electrolyzed by the method of the invention,
although it is not ruled out that a further reactant such as CO is
present alongside CO.sub.2 on the cathode side, which can likewise
be electrolyzed, i.e. there is a mixture comprising CO.sub.2 and
also, for example, CO. For example, a reactant contains, on the
cathode side, at least 20% by volume of CO.sub.2, for example at
least 50% or at least 70% by volume of CO.sub.2, and the reactant
on the cathode side is especially 100% by volume of CO.sub.2.
[0204] In some embodiments, there is an electrolysis cell
comprising [0205] a cathode space comprising a cathode; [0206] a
first ion exchange membrane which contains an anion exchanger
and/or anion transporter and adjoins the cathode space, where the
cathode forms direct contact with the first ion exchange membrane;
[0207] an anode space comprising an anode; and [0208] a diaphragm
that adjoins the anode space;
[0209] further comprising a salt bridge space, wherein the salt
bridge space is disposed between the first ion exchange membrane
and the diaphragm.
[0210] This electrolysis cell can be used to perform the methods
described herein. Consequently, all the features discussed with
regard to the methods of the invention are also applicable in the
case of the electrolysis cell of the invention. Particularly the
cathode space, the cathode, the first ion exchange membrane, the
anode space, the anode, diaphragm and the salt bridge space have
already been discussed with regard to the methods of the invention.
The corresponding features may thus be detailed in accordance with
those discussed above in the electrolysis cell of the invention.
The electrolysis cell and the electrolysis system thus especially
find use in the methods described herein for electrolysis of
CO.sub.2, and therefore aspects that are discussed in connection
therewith above and hereinafter also relate to the electrolysis
cell and to the electrolysis system. Correspondingly, aspects
associated with the electrolysis cell and/or electrolysis system
may also relate to the methods described herein.
[0211] Also described is an electrolysis cell comprising: [0212] a
cathode space comprising a cathode; [0213] a first ion exchange
membrane which contains an anion exchanger and/or anion transporter
and adjoins the cathode space, where the cathode forms direct
contact with the first ion exchange membrane; [0214] an anode space
comprising an anode; and [0215] a first separator membrane that
adjoins the anode space; further comprising a salt bridge space,
wherein the salt bridge space is disposed between the first ion
exchange membrane and the first separator membrane, wherein the
salt bridge space comprises a liquid and/or dissolved acid. This
cell too can be used to conduct the methods, and so the features
described therein may be employed correspondingly here.
[0216] In some embodiments, the anode is in contact with the
diaphragm. In particular embodiments, the anode and/or cathode is
in contact with a conductive structure on the side remote from the
salt bridge space. In some embodiments, the cathode and/or the
anode takes the form of a gas diffusion electrode, of a porous
bound catalyst structure, of a particulate catalyst on a support,
of a coating of a particulate catalyst on the first and/or second
ion exchange membrane, of a porous conductive carrier impregnated
with a catalyst, and/or of a noncontinuous sheetlike structure.
[0217] In particular embodiments, the cathode takes the form of a
gas diffusion electrode, of a porous bound catalyst structure, of a
particulate catalyst on a support, of a coating of a particulate
catalyst on the first and/or second ion exchange membrane, of a
porous conductive carrier impregnated with a catalyst, and/or of a
noncontinuous sheetlike structure, containing an anion exchange
material and/or anion transport material, and/or the anode takes
the form of a gas diffusion electrode, of a porous bound catalyst
structure, of a particulate catalyst on a support, of a coating of
a particulate catalyst on the first and/or second ion exchange
membrane, of a porous conductive carrier impregnated with a
catalyst, and/or of a noncontinuous sheetlike structure, containing
a cation exchange material.
[0218] In some embodiments, the first ion exchange membrane and/or
the diaphragm is hydrophilic.
[0219] In some embodiments, the salt bridge space comprises a
liquid and/or dissolved acid, where an acid in the liquid and/or
dissolved acid in the salt bridge space has a pK.sub.A of 6 or
less, 5 or less, 3 or less, 1 or less, or 0 or less, where the
liquid and/or dissolved acid may be selected from dilute or neat
H.sub.2SO.sub.4, a solution of H.sub.2N--SO.sub.2--OH, dilute or
neat HClO.sub.4, a solution of H.sub.3PO.sub.4, dilute or neat
CF.sub.3--COOH, dilute or neat CF.sub.3--SO.sub.2--OH, a solution
of (CF.sub.3--SO.sub.2).sub.2--NH, a solution of HF, dilute or neat
HCOOH, dilute or neat CH.sub.3--COOH, a solution of HCl, a solution
of HBr, a solution of HI, and/or mixtures thereof. In particular
embodiments, the electrolyte in the salt bridge space consists of a
liquid and/or dissolved acid and any unavoidable impurities.
[0220] In some embodiments, the anode space contains an acid which
may be identical to the electrolyte in the salt bridge, especially
if the second membrane takes the form of a diaphragm.
[0221] In some embodiments, an electrolysis system comprises the
electrolysis cells described above. The corresponding embodiments
of the electrolysis cell and also further illustrative components
of an electrolysis system have already been discussed above and are
thus also applicable to the electrolysis system. In some
embodiments, an electrolysis system comprises a multitude of
electrolysis cells, although it is not ruled out that other
electrolysis cells are present in addition.
[0222] In some embodiments, the electrolysis system further
comprises a recycling device connected to an outlet from the salt
bridge space and an inlet to the cathode space, which is set up to
return a reactant from the cathode reaction that can be reformed in
the salt bridge space, especially a gaseous reactant or one
immiscible with the electrolyte, to the cathode space, such as
CO.sub.2, where this may also contain CO and/or H.sub.2.
[0223] In some embodiments, the electrolysis system further
comprises an external device for electrolyte treatment, especially
an apparatus for removal of dissolved gases from an acid which is
particularly used to treat the anolyte and/or the electrolyte in
the salt bridge space, in order to remove gases such as CO.sub.2 or
O.sub.2, for example, and hence to enable recycling of anolyte
and/or the electrolyte in the salt bridge space. In some
embodiments, both are pumped out of a common reservoir, i.e. there
is just one common anolyte/electrolyte for the salt bridge space
reservoir, i.e. the anolyte and the electrolyte in the salt bridge
space are identical.
[0224] In some embodiments, the electrolysis system comprises two
separate circuits for anolyte and electrolyte in the salt bridge
space, which may optionally have separate devices for electrolyte
treatment, especially apparatuses for removal of dissolved gases
from an acid, or where only the circuit for the electrolyte in the
salt bridge space has a corresponding device.
[0225] The above embodiments, configurations, and developments can,
if viable, be combined with one another as desired. Further
possible configurations, developments, and implementations of the
teachings herein also include combinations that have not been
mentioned explicitly of features 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 as improvements or
supplementations.
[0226] The teachings herein are elucidated further in detail
hereinafter with reference to various examples thereof. However,
the scope of the disclosure is not limited to these examples.
EXAMPLES
Example 1
[0227] The construction of the electrolysis apparatus in example 1
is based on the construction shown in FIG. 3 and is shown in
schematic form in FIG. 10. In this working example, a three-chamber
cell was used. The cathode used was a carbon GDL coated with silver
particles: Freudenberg HL 23. The particles were precipitated by
means of NaBH.sub.4 from AgNO.sub.3 in ethanol as follows:
AgNO.sub.3 (3.4 g, 20 mmol) was dissolved in ethanol (250 ml).
NaBH.sub.4 (3 g, 80 mmol) was dissolved in NaOH-saturated methanol
(100 ml), and this solution was added dropwise. Once all the silver
had been precipitated (no black color at the site of dropwise
addition), the addition was stopped. The precipitate was
transferred to a frit (P4) and washed 4.times. with 50 ml each time
of ethanol and 1.times. with 50 ml of diethyl ether. Subsequently,
the powder was dried under reduced pressure. Yield: 2.88 g of
borate-stabilized particles.
[0228] The particles (90 mg) were used to produce a dispersion
comprising the ionomer AS-4 (anion exchanger, Tokuyama) (180 mg of
5% solution in n-PrOH) (n-propanol)) in n-PrOH (2.8 g). Three
layers of this dispersion were applied to a 60 cm.sup.2 piece of
the GDL. A 10 cm.sup.2 piece of this cathode was pressed
mechanically onto an A201-CE AEM (Tokuyama) and the cathode was
contacted by a titanium frame.
[0229] The anode used was an IrO.sub.2-coated expanded Ti metal
with mesh size 1.times.2 mm. The CEM used was a Nafion N115
membrane that was pressed directly onto the expanded metal. In
order to assure sufficient mechanical contact pressure, five
polymer meshes with a mesh size of 0.5 mm were integrated into the
cell. The electrolyte used in the salt bridge space II and in the
anode space III was 0.1 M H.sub.2SO.sub.4. CO.sub.2 was supplied
via a gas moistener GH with water. The CO.sub.2 flow rate was
chosen for the current densities of 50, 100 & 150 mAcm.sup.-2
such that a threefold excess is available (.lamda.=4). For a first
measurement at 10 mAcm.sup.-2, for measurement-related reasons, the
same gas supply was chosen as for 50 mAcm.sup.-2 (.lamda.=20).
Oxygen was produced at the anode, and a product gas from the
cathode space K, after passing through a bubbler B, was analyzed by
a gas chromatograph GC. A gas separated out in the salt bridge
space was likewise analyzed by means of GC.
[0230] At the start of the experiment, the cell was run in at 4 V
for 20 minutes. Subsequently, the cell was run in at 10 mAcm.sup.-2
for a further 30 min. Thereafter, both the amount and the
composition of the gases in gap I and gap 2 were determined at 10,
50, 100 and 150 mAcm.sup.-2.
[0231] Observations:
[0232] At the lowest current density of 10 mAcm.sup.-2 used, no
evolution of gas was observed in the salt bridge space II. At the
higher current densities of 50, 100 and 150 mAcm.sup.-2, evolution
of gas was observed in the salt bridge space II. Gas chromatography
analysis of this gas showed that it is pure CO.sub.2>98% by
volume. The proportion of CO in this gas is below 1% by volume. The
highest H.sub.2 content was found to be about 1.5% by volume.
Direct recycling of this gas into the cathode feed is thus
possible.
[0233] No significant penetration of the electrolyte to the reverse
side of the cathode was observed throughout the duration of the
experiment. When the cell was dismantled, neither liquid nor salt
crystallites were found on the reverse side of the electrode. A
final pH measurement of the fill solution of the bubbler B gave a
pH .about.5, which finally rules out passage of the acid
electrolyte.
[0234] The experimental results of example 1 are shown in FIG. 11,
in which Faraday efficiency FE is plotted against the applied
current density J.
[0235] In this cell, the Faraday efficiency for CO rises constantly
with the current density. The reason for this is the
above-described transport model. Owing to the integrated spacers,
the electrolyte in the cell is heated to .about.60.degree. C., but
this did not have any adverse effect on selectivity.
Comparative Example 1
[0236] The experimental setup used in comparative example 1 is
shown in FIG. 12 and corresponds essentially to that of example 1
and is identical with regard to the apparatus constituents except
that the acid in the salt bridge space II has been replaced by a
KHCO.sub.3 salt electrolyte.
[0237] For current densities of 50 and 100 mAcm.sup.-2, the
electrolyte used in the salt bridge space was 1 M KHCO.sub.3. For
150 mAcm.sup.-2, for apparatus reasons (maximum potentiostat
voltage attained), it was necessary to switch to a 2 M KHCO.sub.3.
A threefold excess of CO.sub.2 was employed at all current
densities.
[0238] Observations:
[0239] Evolution of gas was observed in the salt bridge space II at
all current densities. No increased passage of electrolyte was
observed. However, when the cell was taken apart, liquid and salt
crystallites were found on the reverse side of the electrode on the
cathode.
[0240] The experimental results of comparative example 1 are shown
in FIG. 13, in which Faraday efficiency FE is again plotted against
the applied current density J.
[0241] As can be seen in FIG. 13, the selectivity falls with rising
current density. This is caused by the increased passage of alkali
metal cations through the electrode and the associated partial
flooding of the electrode.
[0242] A comparison between comparative example 1 (dotted lines)
and the working example (solid lines) in FIG. 14 shows the
advantages of the method of the invention at elevated current
densities by comparison with the conventional salt electrolyte.
Comparative Example 2
[0243] A schematic diagram of the experimental setup in comparative
example 2 is shown in FIG. 15. In this comparative example, by
comparison with example 1, the AEM was omitted in order to show
that it is essential, with the further experimental setup
corresponding to that of example 1. It should be noted that the
cathode still contains an anion exchange ionomer corresponding to
the polymer basis of the AEM.
[0244] Observations:
[0245] Evolution of gas was observed in the salt bridge space II at
all current densities measured. However, the analysis of this gas
shows that this gas, by contrast with the working example, is
mainly H.sub.2 (81% by volume of H.sub.2, 18% by volume of
CO.sub.2). Moreover, no passage of liquid through the cathode was
observed. 60% by volume of hydrogen was observed in the cathode
space I.
[0246] The experimental results of comparative example 2 are shown
in FIG. 16, in which Faraday efficiency FE is again plotted against
the applied current density J. The preferred preparation of
hydrogen is apparent therefrom.
[0247] The comparison shown in FIG. 17 between comparative example
2 (dotted lines) and the working example (solid lines) shows the
advantageous configuration in example 1 with elevated selectivity
for CO. This is also apparent from the comparison of the gas
chromatograms, shown in FIG. 18, of comparative example 2 (solid
line) and the working example (dotted line) at J=150 mAcm.sup.-2,
with the measurement without AEM (w/o AEM) shown here as a solid
line and that with AEM (w/AEM) as a dotted line.
[0248] What is described in example 1 and in comparative example 2
is that no liquid penetrates through the cathode to the side remote
from the electrolyte when acid electrolytes are used. However,
escape of liquid from the GDE over long periods of operation would
be conceivable in principle. As a result of the construction, the
liquid in that case is not a concentrated carbonate solution but
virtually pure water, and especially not a salt solution--as in the
case of metal cation-containing electrolytes. This circumstance
brings advantages in the construction of the cell and in the design
of the overall electrolyte system. It was observed that titanium
contacts, for example, can corrode on contact with salt solutions
that pass through the electrode as a result of the strongly
negative potential. As a consequence, the permeate turns blue
(Ti.sup.3+). Titanium corrosion is confirmed here as the cause of
the blue color by means of chronotropic acid, and cathodic
corrosion is detected in control experiments. The permeate liquids
(if present at all) have low or zero electrical conductivity in the
arrangement of the invention presented here or in the methods of
the invention. The contacts are nevertheless exposed to a strongly
negative potential, but not subjected to ionic contact.
Consequently, such corrosion phenomena occur to a significantly
limited degree, if at all. Since any liquid that occurs on the
reverse side of the electrode is water, this does not contain any
ions that have to be returned to the electrolyte. This liquid can
therefore simply be discarded. Any corrosion products of the
contacts that occur are correspondingly not washed into the
electrolytes.
Reference Examples 1 and 2
[0249] In reference examples 1 and 2, the effects of a low anode pH
on cell voltage were examined.
[0250] According to the Nernst equation, the oxidation potential of
water to oxygen is dependent on the pH of the electrolyte.
E = 1.2 V - 2.3 .times. RT F .times. pH ##EQU00001##
[0251] In order to minimize the cell voltage, a maximum pH in the
region of the anode is thus accordingly advisable. However, this
can be maintained in accordance with the invention under the
boundary condition of a CO.sub.2-free anode only with use of a
bipolar membrane.
[0252] With a cation exchange membrane or a diaphragm, the cations
would be transported out of the anode space, which would lead to
lowering of the pH. An anion exchange membrane at the anode would
lead to penetration of HCO.sub.3.sup.- into the anode space, which
would lead to unwanted mixing of the oxygen generated at the anode
with CO.sub.2.
[0253] In order to enable constant operation, an acid (except in
the case of use of a bipolar membrane) is chosen as anolyte. This
at first does not seem very advantageous from the point of view of
the cell voltage, since this course of action leads to a high water
oxidation potential. However, it has been shown experimentally that
the thermodynamic considerations (according to the Nernst equation)
are applicable only to the "onset" region, (i.e. the region of
minimum current densities). At high current densities, the same
cell voltage was observed for an acidic anode and a pH-neutral to
slightly basic anode.
[0254] For this purpose, a simple comparative experiment was
conducted. First of all, the U-I characteristic was recorded on a
simple construction with acid anolyte and neutral-buffered salt
bridge FIG. 19--with the corresponding constituents from example 1
and comparative example 1. Subsequently, the periphery was
reconstructed according to FIG. 20, such that the anode was now
supplied with neutral-buffered electrolyte. No changes were made to
the cell. In addition, the anode is a "zero-gap" anode directly
adjoining the membrane. The conductivity of the anolyte is thus of
no importance for the voltage. The electrolyte in the salt bridge
is identical in both cases. All changes to the voltage are
therefore attributable to the different pH of the anolyte.
Subsequently, a U-I characteristic was recorded once again. The
construction in FIG. 19 is an adaptation of an alkali electrolysis
cell for CO.sub.2 electrolysis. The replacement of the cation
exchange membrane by a diaphragm was dispensed with for reasons of
comparability.
[0255] It should be noted here that the anolyte in the construction
according to FIG. 20 does not contain any anions of stable acids.
Therefore, the imposition of a locally low pH, as would be
possible, for example, in the case of Na.sub.2SO.sub.4, is likewise
ruled out here.
[0256] FIGS. 21 and 22 show the comparison of the UI
characteristics with the measurements with the construction
according to FIG. 19 with filled squares and the measurements with
the construction according to FIG. 20 with open circles, with FIG.
21 showing the "onset" region of the characteristic (especially on
the left) and FIG. 22 showing the complete characteristic up to 200
mAcm.sup.-2.
[0257] As apparent from FIG. 21, the electrolysis in the case of
the acid anolyte sets in about 480 mV later. This fits well with
the expected value of 460 mV which is to be expected for a pH
difference of 7. However, it is apparent from FIG. 22 that this
effect is only applicable in the "onset" region. Above a current
density of 100 mAcm.sup.-2, the characteristics coincide.
[0258] This shows clearly that no disadvantages with regard to cell
voltage arise from the use of acids as anolyte for a productive
electrolysis system which is operated at high current
densities.
[0259] Effects of the Release of CO.sub.2 on Cell Voltage:
[0260] In the two constructions in the reference examples, CO.sub.2
is released from HCO.sub.3.sup.- in the cell. In the case of the
construction according to FIG. 19 this takes place in the salt
bridge, and in the case of the construction according to FIG. 20 in
the anode space. In both cases, four times the volume of CO.sub.2
is released compared to the oxygen generated at the anode.
[0261] In the case of the construction according to FIG. 19, this
takes place in front of the CEM in the salt bridge space. In the
case of FIG. 20, this takes place in the immediate proximity of the
anode. However, the gas bubbles that form there are transported
away behind the anode. They are thus not in the flow pathway, which
explains the smooth curve for this construction (filled squares) in
FIG. 22. However, it is also apparent from the figure that the
total voltage does not rise as a result of the load on the salt
bridge.
[0262] Also contemplated in the context of the present disclosure
is the use not of a straight cathode but of a cathode-AEM
composite. For these constructions, a transfer coefficient for
CO.sub.2 of .ltoreq.0.55 was observed experimentally. The gas load
on a salt bridge space is thus only about half as high as in the
present comparative example. Accordingly, up to a current density
of 400 mAcm.sup.-2, no significant increase in voltage as a result
of these gas bubbles is to be expected in comparable
constructions.
[0263] The situation is different for the anode. As a result of the
transition from an acid anolyte to a carbonate-containing,
neutral-buffered electrolyte, it is subjected to five times the
load of gases formed. As a result, parts of the anode can be
isolated and cut off from the electrolyte that simultaneously
constitutes the substrate. In the region of 150-200 mAcm.sup.-2,
the voltage for the acid anolyte is actually lower, which is
attributable not least to the high gas load on the anode (in both
cases a non-continuous sheetlike structure with catalyst
coating).
[0264] It is a feature of the present teachings that liquid and/or
dissolved acids, especially pure acids, can be used as electrolytes
for CO.sub.2 electrolysis at high current densities and
simultaneously high Faraday efficiencies. In addition, in the form
of a three-chamber construction with first ion exchange membrane
and diaphragm, for example an AEM diaphragm double-separator cell,
a new cell type has been introduced.
[0265] The following advantages are among those that arise over
existing embodiments: [0266] No release of CO.sub.2 at the anode,
only of O.sub.2 or other anodic products [0267] CO.sub.2 is
released in a separate chamber and can be recycled [0268] No
separation of salts [0269] Faraday efficiency of CO production
increases with rising current density [0270] Very little permeate,
if any, into the gas space of the cathode space [0271] When the
same acid is used in the anode space and in the salt bridge, a
diaphragm is sufficient for separation of anode gas and CO.sub.2
[0272] Also applicable to the production of other CO.sub.2
reduction products (e.g. formic acid)
[0273] In addition, a CO.sub.2-free anode is obtained not via the
construction of the anodic half-cell but via that of the cathodic
half-cell. This result is entirely unexpected and is based on the
mechanism of anion-based charge transport, compensated by fixed
positive charges.
* * * * *