U.S. patent application number 16/771065 was filed with the patent office on 2021-06-17 for anion exchanger fillings through which flow can occur for electrolyte splitting in co2 electrolysis for better spatial distribution of gassing.
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, Dan Taroata.
Application Number | 20210180196 16/771065 |
Document ID | / |
Family ID | 1000005493421 |
Filed Date | 2021-06-17 |
United States Patent
Application |
20210180196 |
Kind Code |
A1 |
Schmid; Bernhard ; et
al. |
June 17, 2021 |
ANION EXCHANGER FILLINGS THROUGH WHICH FLOW CAN OCCUR FOR
ELECTROLYTE SPLITTING IN CO2 ELECTROLYSIS FOR BETTER SPATIAL
DISTRIBUTION OF GASSING
Abstract
An electrolysis cell having a multi-chamber structure, wherein
an anion exchanger with a first ion exchanger membrane connects to
a cathode chamber, wherein a salt bridge chamber connects to the
first ion exchanger membrane, the salt bridge chamber with a fixed
anion exchanger. An electrolysis system has such an electrolysis
cell and a method for electrolysis of CO.sub.2 uses such an
electrolysis cell or electrolysis system.
Inventors: |
Schmid; Bernhard; (Duren,
DE) ; Schmid; Gunter; (Hemhofen, DE) ; Reller;
Christian; (Minden, DE) ; Taroata; Dan;
(Erlangen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Aktiengesellschaft |
Munich |
|
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
Munich
DE
|
Family ID: |
1000005493421 |
Appl. No.: |
16/771065 |
Filed: |
November 19, 2018 |
PCT Filed: |
November 19, 2018 |
PCT NO: |
PCT/EP2018/081741 |
371 Date: |
June 9, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 1/00 20130101; C25B
9/23 20210101; C25B 3/26 20210101 |
International
Class: |
C25B 9/23 20060101
C25B009/23; C25B 1/00 20060101 C25B001/00; C25B 3/26 20060101
C25B003/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2017 |
DE |
10 2017 223 521.7 |
Claims
1. An electrolysis cell, comprising: a cathode compartment
comprising a cathode; a first ion exchange membrane, which contains
an anion exchanger and which is adjacent to the cathode
compartment, wherein the cathode comes into contact with the first
ion exchange membrane; an anode compartment comprising an anode;
and a first separator, which is adjacent to the anode compartment;
a salt bridge compartment, wherein the salt bridge compartment is
arranged between the first ion exchange membrane and the first
separator, wherein the salt bridge compartment comprises a solid
anion exchanger, which is at least partially in contact with the
first ion exchange membrane.
2. The electrolysis cell as claimed in claim 1, wherein the solid
anion exchanger comprises in the salt bridge compartment cations,
which are immobilized in a polymeric backbone.
3. The electrolysis cell as claimed in claim 1, wherein the solid
anion exchanger is present as a bed and/or a porous structure.
4. The electrolysis cell as claimed in claim 1, wherein the solid
salt bridge compartment further comprises uncharged particles,
nonionic ion exchangers and/or cation exchangers.
5. The electrolysis cell as claimed in claim 1, wherein the first
separator is a cation exchange membrane, a bipolar membrane or a
diaphragm.
6. The electrolysis cell as claimed in claim 1, wherein the solid
anion exchanger is basic, or strongly basic.
7. The electrolysis cell as claimed in claim 1, wherein the solid
anion exchanger is hydrophilic.
8. The electrolysis cell as claimed in claim 1, wherein the salt
bridge compartment further comprises a solid cation exchanger,
which is at least partially in contact with the first
separator.
9. An electrolysis system, comprising: an electrolysis cell as
claimed in claim 1.
10. The electrolysis system as claimed in claim 9, further
comprising: a return device that is connected to an outlet of the
salt bridge compartment and an inlet of the cathode compartment,
which is configured to recycle a reactant of the cathode reaction,
which can be formed in the salt bridge compartment, back into the
cathode compartment.
11. A method for the electrolysis of CO.sub.2 with an electrolysis
cell as claimed in claim 1, the method comprising: reducing
CO.sub.2 at the cathode, wherein hydrogencarbonate and/or carbonate
generated at the cathode by the first ion exchange membrane
migrates to an electrolyte in the salt bridge compartment, wherein
the hydrogencarbonate and/or carbonate is also transported through
the solid anion exchanger in the salt bridge compartment away from
the first ion exchange membrane.
12. The method as claimed in claim 11, wherein the salt bridge
compartment comprises an aqueous electrolyte.
13. The method as claimed in claim 11, wherein the electrolyte of
the salt bridge compartment comprises an acid, or a water-soluble
acid, or a water-miscible acid.
14. The method as claimed in claim 11, wherein the electrolyte of
the salt bridge compartment essentially comprises no mobile cations
other than H.sup.+ and/or hydrated variants thereof.
15. A method for the electrolysis of CO.sub.2 and/or CO, the method
comprising: performing electrolysis using the electrolysis cell as
claimed in claim 1.
16. The electrolysis cell as claimed in claim 2, wherein the
cations exchange hydrogencarbonate and/or carbonate ions.
17. The electrolysis cell as claimed in claim 4, wherein the
uncharged particles, nonionic ion exchangers and/or cation
exchangers are contained in an area adjacent to the first ion
exchange membrane in an amount of up to 20 vol. %, based on the
total amount of the solid anion exchanger and the uncharged
particles, nonionic ion exchangers and/or cation exchangers.
18. The electrolysis cell as claimed in claim 4, wherein the
uncharged particles and/or nonionic ion exchangers are contained in
an area adjacent to the first ion exchange membrane in an amount of
up to 20 vol. %, based on the total amount of the solid anion
exchanger and the uncharged particles, nonionic ion exchangers
and/or cation exchangers.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2018/081741 filed 19 Nov. 2018, and claims
the benefit thereof. The International Application claims the
benefit of German Application No. DE 10 2017 223 521.7 filed 21
Dec. 2017. All of the applications are incorporated by reference
herein in their entirety.
FIELD OF INVENTION
[0002] The present invention relates to an electrolysis cell with a
multi-compartment structure, wherein a first ion exchange membrane
comprising an anion exchanger is adjacent to a cathode compartment,
wherein a salt bridge compartment which comprises a solid anion
exchanger is adjacent to this first ion exchange membrane; an
electrolysis system with such an electrolysis cell; and a method
for the electrolysis of CO.sub.2 using such an electrolysis cell or
electrolysis system.
BACKGROUND OF INVENTION
[0003] At present, approximately 80% of world energy demand is met
by the combustion of fossil fuels. Because of these combustion
processes, approximately 34,032.7 million tons of carbon dioxide
(CO.sub.2) were released into the atmosphere worldwide in 2011.
This release is the simplest way of disposing of even large amounts
of CO.sub.2 (with lignite-fired power plants accounting for over
50,000 t per day).
[0004] The discussion on the negative effects of the greenhouse gas
CO.sub.2 on climate has led to consideration of how to recycle
CO.sub.2. From a thermodynamic standpoint, CO.sub.2 shows an
extremely low value and therefore cannot readily be reduced to
usable products.
[0005] In nature, CO.sub.2 is converted by photosynthesis to
carbohydrates. This process, which is divided into many partial
steps both temporally, and on a molecular level, spatially, can be
reproduced on an industrial scale only with great difficulty. The
method that is currently more efficient compared to pure
photocatalysis is the electrochemical reduction of CO.sub.2. A
mixed form is light-assisted electrolysis or electrically assisted
photocatalysis. The two terms are to be used as synonyms, depending
of the viewpoint of the observer.
[0006] In this process, as is the case in photosynthesis, CO.sub.2
is converted to an energetically higher value product (such as CO,
CH.sub.4, C.sub.2H.sub.4, etc.) under supply of electrical energy
(optionally photo-assisted), which can be obtained from renewable
energy sources such as wind or sun. The amount of energy required
in this reduction ideally corresponds to the combustion energy of
the fuel and should only be derived from renewable sources.
However, surplus production of renewable energy is not continually
available, but at the moment only during times of strong solar
radiation and heavy wind. However, this will be further increased
in the near future with further development of renewable
energy.
[0007] The electrochemical reduction of CO.sub.2 on solid-state
electrodes in aqueous electrolyte solutions provides a wide variety
of product possibilities, with Faraday efficiencies on various
metal cathodes being shown by way of example in Table 1, taken from
"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, pp. 89-189.
TABLE-US-00001 TABLE 1 Faraday efficiencies in the electrolysis of
CO.sub.2 on various electrode materials. 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
[0008] The electrification of the chemical industry is currently
being discussed. This means that preferably, chemical raw materials
or fuels are intended to be produced from CO.sub.2 (CO) and
H.sub.2O while preferably supplying surplus electrical energy from
renewable sources. In the introductory phase of such a technology,
efforts are being made to ensure that the economic value of a
substance is significantly greater than its heating value
(combustion value).
[0009] Electrolysis methods have undergone considerable further
development over the past few decades. For example, it has been
possible to optimize PEM water electrolysis with respect to high
current densities. Large-scale electrolyzers showing performance in
the megawatt range are being introduced onto the market.
[0010] It has been found in CO.sub.2 electrolysis that coupling of
the cathode to an anion exchange membrane (AEM) provides
significant advantages with respect to selectivity, stability, and
technical feasibility.
[0011] Moreover, it is known that by means of the AEM on the
cathode, HCO.sub.3.sup.- produced as a byproduct can be further
transported in the direction of the anode and decomposed into
CO.sub.2 at a site depending on the cell design, for example by
protons formed on the anode side. It has been shown that it can be
advantageous to use three-compartment cells in which the CO.sub.2
is generated separately from the product of the electrodes, as this
makes recycling easier. Corresponding cell designs can be found for
example in US 2017037522 A1, DE 102017208610.6, and DE
102017211930.6.
[0012] In these cells, the cathode compartment is ordinarily
delimited by an AEM. This allows cathodically produced anions such
as HCO.sub.3.sup.-, CO.sub.3.sup.2-, and OH.sup.- to be transported
away in the direction of the anode. With respect to the
configuration of the anode compartment and the gap between the
anode and cathode compartments, these sources vary to a great
degree. However, they have in common an area between the AEM, which
delimits the cathode compartment, and the anode, said area
containing a strongly acidic medium or producing protons, in which
the HCO.sub.3.sup.- and CO.sub.3.sup.2- are decomposed by
protonation into CO.sub.2. Moreover, the charge transport in all of
these cells can be carried in sections by various charge carriers.
In contrast to other electrochemical arrangements, in this case the
charge carriers are ordinarily not exchanged between the half
cells, but are destroyed in the additional gap between them.
[0013] In US 2017037522 A1 and DE 102017211930.6, the central gap
exclusively contains strongly acidic media. The generation of
CO.sub.2 therefore ordinarily takes place directly on the surface
of the AEM, wherein in US 2017037522 A1, the medium of the gap is
solid, while it is liquid in DE 102017211930.6. The surface of the
AEM can be strongly impacted with gas bubbles, which can lead to a
partial insulation of the membrane and thus to greater electrical
losses in the cell. In addition, direct contact between strongly
acidic solid media and the AEM should be avoided, as the solid
media cannot avoid the CO.sub.2 generated at this pH limit.
[0014] In DE 102017208610.6, the gap contains a neutral to weakly
basic electrolyte, which as a rule contains carbonates. Therefore,
the CO.sub.2 generation ordinarily takes place on the surface of
the second separator membrane, which can be just as problematic. In
addition, it has been found that the use of salts, in particular
with metal cations, in electrolyte gaps can be disadvantageous.
[0015] At present, there are no known solutions for the problems
described above. The extent thereof, as described in DE
102017208610.6, can be reduced by increasing the system pressure.
However, excessively increasing the pressure is not desirable
because of the increased solubility of gases in water resulting
therefrom.
[0016] There is thus a need for an improved electrolysis cell or an
improved electrolysis system in which one can effectively prevent
the binding of gas bubbles to a membrane in a multi-compartment
system.
SUMMARY OF INVENTION
[0017] The inventors found that by using an additional electrolyzer
component, it was possible in particular to improve cell voltage,
operating stability, and energy efficiency. In this case, this
component is preferably integrated such that in the resulting cell
as a whole, neither salt encrustation of the electrodes nor
CO.sub.2 generation in the anode compartment are possible. The
present invention thus constitutes a significant improvement over
previously disclosed cell designs.
[0018] This is achieved in particular in that a salt bridge
compartment in an electrolysis cell is filled with a solid anion
exchanger that comprises, at least in the vicinity of the
cathode/AEM, e.g. a hydrogencarbonate-, carbonate- and/or
hydroxide-conductive, for example strongly basic anion exchanger.
The anion exchanger makes it possible for gassing, for example the
release of CO.sub.2 in CO.sub.2 electrolysis, to be distributed
into the volume of the salt bridge compartment rather than taking
place only at the AEM-salt bridge compartment interface.
[0019] In the following, the terms anion exchanger and anion
transporter are used as synonyms. The transport function is
characterized in that the anion exchange/anion transport material
provides cations that compensate for the charge of the anions.
According to certain embodiments, the anion itself is bound only so
lightly that dynamic exchange is possible, thus providing a
transport path for the anion in the electrolyte. At the same time,
the cation is immobilized on the polymer backbone of the anion
exchange material so that it cannot participate in charge transport
processes.
[0020] In a first aspect, the present invention relates to an
electrolysis cell, comprising --a cathode compartment comprising a
cathode; --a first ion exchange membrane, which contains an anion
exchanger and which is adjacent to the cathode compartment, wherein
the cathode comes into contact with the first ion exchange
membrane; --an anode compartment comprising an anode; and --a first
separator, which is adjacent to the anode compartment; further
comprising a salt bridge compartment, wherein the salt bridge
compartment is arranged between the first ion exchange membrane and
the first separator, wherein the salt bridge compartment comprises
a solid anion exchanger, which is at least partially in contact
with the first ion exchange membrane.
[0021] Further disclosed is an electrolysis system comprising an
electrolysis cell according to the invention.
[0022] In addition, the present invention relates to a method for
the electrolysis of CO.sub.2, wherein an electrolysis cell
according to the invention or an electrolysis system according to
the invention is used, wherein CO.sub.2 is reduced at the cathode
and hydrogencarbonate and/or carbonate generated at the cathode
migrates through the first ion exchange membrane to an electrolyte
in the salt bridge compartment, wherein the hydrogencarbonate
and/or carbonate is also transported through the solid anion
exchanger in the salt bridge compartment away from the first ion
exchange membrane.
[0023] Yet a further aspect of the present invention relates to the
use of an electrolysis cell according to the invention or an
electrolysis system according to the invention for the electrolysis
of CO.sub.2 and/or CO.
[0024] Further aspects of the present invention are to be found in
the dependent claims and the detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The attached drawings are intended to illustrate embodiments
of the present invention and to facilitate further understanding
thereof. In combination with the description, they serve to explain
concepts and principles of the invention. Other embodiments and
many of the advantages mentioned can be derived from the
drawings.
[0026] The elements in the drawings are not necessarily shown to
scale with respect to one another. Elements, features, and
components that are identical, have the same function, or have the
same action are indicated in the figures of the drawings by the
same respective reference numbers unless otherwise specified.
[0027] FIGS. 1 to 9 are schematic diagrams of possible
configurations of an electrolysis cell according to the
invention.
[0028] In FIG. 10, a schematic diagram of an electrolysis system
according to the invention is shown by way of example.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
[0029] Unless otherwise specified, the technical and scientific
terms used herein have the same meaning as that which would be
understood by the person skilled in the art in the field of the
invention.
[0030] Quantities given in the context of the present invention
refer to wt. % unless otherwise specified or unless the context
clearly indicates otherwise. In the gas diffusion electrode
according to the invention, the total of components in wt. %
amounts to 100 wt. %.
[0031] In the context of the present invention, the term
hydrophobic is understood to mean water-repellent. According to the
invention, therefore, hydrophobic pores and/or channels are those
which repel water. In particular, hydrophobic properties according
to the invention are associated with substances or molecules having
nonpolar groups.
[0032] In contrast, the term hydrophilic is understood to refer to
the capacity to interact with water and other polar substances.
[0033] In general, gas diffusion electrodes (GDE) 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 and the gaseous phase.
[0034] The configuration can be of various types, for example a
porous "solid material catalyst," optionally with auxiliary layers
for adjusting the hydrophobicity, wherein for example a
membrane-GDE composite, e.g. an AEM-GDE composite, can then be
produced; a conductive porous carrier to which a catalyst can be
applied in a thin layer, wherein again a membrane-GDE composite,
e.g. an AEM-GDE composite, can then likewise be produced; or a
porous composite in the catalyst that can optionally be applied
with an additive directly to a membrane, e.g. an AEM, and can then
form in the composite a membrane coated with a catalyst (CCM;
catalyst coated membrane).
[0035] The normal pressure is 101325 Pa=1.01325 bar.
[0036] Electro-osmosis: Electro-osmosis is understood to refer to
an electrodynamic phenomenon in which a force is exerted toward the
cathode on particles in solution with a positive zeta potential,
and a force is exerted toward the anode on all particles having a
negative zeta potential. If conversion occurs on the electrode,
i.e. if a galvanic current flows, a material flow of the particles
with a positive zeta potential to the cathode also takes place,
regardless of whether the species is involved in the conversion or
not. The same applies for 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.
[0037] The material flows caused by electro-osmosis can also flow
against the concentration gradients. In this manner, flows caused
by diffusion that can offset the concentration gradients can be
overcompensated for.
[0038] In a first aspect, the present invention relates to an
electrolysis cell, comprising --a cathode compartment comprising a
cathode; --a first ion exchange membrane, which contains an anion
exchanger and which is adjacent to the cathode compartment, wherein
the cathode comes into contact with the first ion exchange
membrane; --an anode compartment comprising an anode; and --a first
separator, which is adjacent to the anode compartment; further
comprising a salt bridge compartment, wherein the salt bridge
compartment is arranged between the first ion exchange membrane and
the first separator, wherein the salt bridge compartment comprises
a solid anion exchanger, which is at least partially in contact
with the first ion exchange membrane.
[0039] The salt bridge compartment is not particularly limited,
provided that it is correspondingly connected to the first ion
exchange membrane at least partially, in particular mechanically or
ionically, so that the solid anion exchanger can be at least
partially in contact with the first ion exchange membrane therein.
According to certain embodiments, the solid anion exchanger is in
contact with the first ion exchange membrane at least essentially
in an area in which the cathode is in contact with the first ion
exchange membrane on an opposite side of this membrane, or in an
area that is larger. This allows a favorable transfer of anions to
be ensured that are generated in the cathode and fed through the
first ion exchange membrane. Here, the term "in contact with the
first ion exchange membrane" does not exclude the possibility that
the contact does not take place over the entire surface, but is
such, according to certain embodiments, that a material flow of
fluids, i.e. liquids and/or gases, through the solid anion
exchanger is also possible.
[0040] The term salt bridge compartment is used with respect to its
function of acting as a "bridge" between the anode arrangement and
cathode arrangement and comprising cations and anions which,
however, in the present case do not have to form salts. In the
present case, as at least one ion exchanger is present in the salt
bridge compartment, one could also refer to said compartment as an
ion bridge compartment. However, as this term is not commonly used,
the compartment according to the invention will be referred to as a
salt bridge compartment, even though it is not necessary for any
salt to be present therein in the classical sense.
[0041] The dimensions of the salt bridge compartment are also not
particularly limited, and it can be configured for example as a
compartment or gap, e.g. between the first ion exchange membrane
and the first separator, which for example are arranged parallel to
each other.
[0042] The salt bridge compartment need not necessarily be in
contact with the first separator that is adjacent to the anode
compartment, i.e., more than three compartments may also be present
in an electrolysis cell according to the present invention. The
expression "arranged between the first ion exchange membrane and
the first separator" thus means that the salt bridge compartment
can be located at any desired position between the first ion
exchange membrane and the first separator, provided that it
comprises a solid anion exchanger that is at least partially in
contact with the first ion exchange membrane. The salt bridge
compartment is thus adjacent to the first ion exchange membrane,
which however does not rule out the possibility that even a second
separator or even further separators and/or further cell
compartments are present and oriented toward the first separator.
According to certain embodiments, the salt bridge compartment is in
contact with the first separator. Therefore, the electrolysis cell
according to the invention can be configured for example as a
multi-compartment cell, e.g. a three-compartment cell, as described
in US 2017037522 A1, DE 102017208610.6, and DE 102017211930.6, and
reference is made thereto with respect to such cells. For example,
therefore, a three-compartment cell may be present having three
compartments (I, II, III). With the salt bridge compartment,
electrolytic contact between the cathode compartment and the anode
compartment can thus be achieved and/or facilitated.
[0043] The cathode compartment, anode compartment and salt bridge
compartment are not particularly limited in the electrolysis cell
according to the invention with respect to form, material,
dimensions, etc., provided that they can accommodate the cathode,
the anode and the first ion exchange membrane and the first
separator. The three compartments are formed in the electrolysis
cell according to the invention, wherein they can then be
correspondingly separated, for example by the first ion exchange
membrane and the first separator, for example with the first
separator arranged between the salt bridge compartment and the
anode compartment.
[0044] For the individual compartments, depending on the
electrolysis to be carried out, inlet and outlet devices for
reactants and products, for example in the form of a liquid, gas,
solution, suspension, etc., can be correspondingly provided,
wherein these can also optionally be recycled respectively. There
is also no limitation in this respect, and flow can occur through
the individual compartments in parallel flows or in counterflow.
For example, in electrolysis of CO.sub.2--which can further
comprise CO, i.e. for example containing at least 20 vol. %
CO.sub.2--the CO.sub.2 can be supplied to the cathode in solution,
as a gas, etc.--there can for example be a counterflow to an
electrolyte flow in the salt bridge compartment with a
three-compartment configuration. There is no limitation in this
respect.
[0045] There are corresponding possibilities for the inlet in the
anode compartment as well, and these will also be discussed in
further detail below. The respective inlet can be configured either
continuously or discontinuously, for example in a pulsed
configuration, etc. for which purpose corresponding pumps, valves,
etc. can be provided in an electrolysis system according to the
invention--which will also be further discussed below--as well as
cooling and/or heating devices in order to allow corresponding
catalysis of desired reactions at the anode and/or cathode.
[0046] The materials of the respective compartments or of the
electrolysis cell and/or the further components of the electrolysis
system can also be correspondingly adapted in a suitable manner to
the desired reactions, reactants, products, electrolytes, etc. In
addition, each electrolysis cell of course also comprises at least
one power source. Further device components that occur in
electrolysis cells or electrolysis systems can also be provided in
the electrolysis system or electrolysis cell according to the
invention. According to certain embodiments, these individual cells
are combined into a stack that comprises 2-1000, preferably 2-200
cells, and the operating voltage of which is preferably in the
range of 3-1500 V, particularly preferably 200-600 V.
[0047] According to certain embodiments, a gas formed in the salt
bridge compartment, which e.g. corresponds to the reactant gas,
e.g. CO.sub.2, which may also optionally contain trace amounts of
H.sub.2 and/or CO, may be recycled back in the direction of the
cathode compartment, where a corresponding return device may be
provided in an electrolysis system according to the invention.
[0048] The cathode is not particularly limited according to the
invention and can be adapted to a desired half reaction, for
example with respect to the reaction products, provided that it is
in direct contact with the first ion exchange membrane, i.e. is
directly in contact with the first ion exchange membrane at at
least one site, preferably wherein the cathode is essentially in
direct planar contact with the first ion exchange membrane. The
cathode is thus directly adjacent, at least in one area, to the
first ion exchange membrane. A cathode for the reduction of
CO.sub.2 and optionally CO can for example comprise 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, preferably Cu, Ag, Au, Zn,
Pb, Sn, or mixtures and/or alloys thereof, and/or a salt thereof,
wherein suitable materials can be adapted to a desired product. The
catalyst can thus be selected according to the desired product. In
the case of reduction of CO.sub.2 to CO, for example, the catalyst
is preferably 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 the production
of hydrocarbons, Cu or Cu-containing compounds such as CU.sub.2O,
CuO and/or copper-containing mixed oxides with other metals, etc.,
are preferred. For a production of formic acid, for example,
catalysts based on Pb, Sn and/or Cu, in particular Pb, Sn, may be
used. As according to certain embodiments, hydrogen formation at
high current densities may be completely inhibited by anion
transport, catalysts for CO.sub.2 reduction that do not possess
high overvoltage with respect to hydrogen can be used, e.g.
reduction catalysts such as Pt, Pd, Ir, Os or carbonyl-forming
metals such as Fe, Ni, Co, W, Mo. Thus the described operating
method, in combination with the cell design, opens up new pathways
in CO.sub.2 reduction chemistry that are not dependent on hydrogen
overvoltage.
[0049] The cathode is the electrode on which the reductive half
reaction takes place. It can have a single or multiple component(s)
and can be configured for example as a gas diffusion electrode, a
porous electrode or directly with the AEM in the composite,
etc.
[0050] The following embodiments are possible, for example: --a gas
diffusion electrode or porous bound catalyst structure, which
according to certain embodiments can e.g. be ion-conductively
and/or mechanically bonded by means of a suitable ionomer, for
example a anionic ionomer, to the first ion exchange membrane, for
example an anion exchange membrane (AEM); --a gas diffusion
electrode or porous bound catalyst structure, which according to
certain embodiments can be partially pressed onto the first ion
exchange membrane, for example an AEM; --a porous, conductive,
catalytically inactive structure, e.g. a carbon-paper GDL (gas
diffusion layer), a carbon-cloth GDL, and/or a polymer-bound film
of granular vitreous carbon, which is impregnated with the catalyst
of the cathode and optionally an ionomer that allows the binding to
the first ion exchange membrane, for example an AEM, wherein the
electrode can then be mechanically pressed onto the first ion
exchange membrane, for example an AEM, or can be pre-pressed
together with the first ion exchange membrane, for example an AEM,
in order to form a composite; --a particulate catalyst, which is
applied by means of a suitable ionomer to a suitable carrier, for
example a porous conductive carrier, and according to certain
embodiments can be adjacent to the first ion exchange membrane, for
example an AEM; --a particulate catalyst, which is pressed into the
first ion exchange membrane, for example an AEM, or is coated
thereon and for example is correspondingly conductively bound,
wherein this structure can then be pressed for example as a
so-called CCM (catalyst-coated membrane) onto a conductive, porous
electrode, wherein a catalytic activity of this electrode is
generally not necessary and for example carbon-based GDLs or
gratings, for example of titanium, can be used, wherein it is not
excluded for this electrode to contain ionomers and/or the active
catalyst or to consist in large part thereof; --a non-closed flat
structure, e.g. a mesh or a metal mesh, which for example consists
of or comprises a catalyst or is coated therewith and according to
certain embodiments is adjacent to the first ion exchange membrane,
for example an AEM; --a polymer-bound solid catalyst structure of a
particulate catalyst, which comprises an ionomer that allows
binding to the first ion exchange membrane, for example an AEM, or
has been subsequently impregnated therewith, wherein the electrode
is then mechanically pressed onto the first ion exchange membrane,
for example an AEM, or can be pre-pressed together with the first
ion exchange membrane, for example an AEM, in order to form a
composite; --a porous, conductive carrier, which is impregnated
with a suitable catalyst and optionally an ionomer and according to
certain embodiments is adjacent to the first ion exchange membrane,
for example an AEM; --a non-ion-conductive gas diffusion electrode,
which has subsequently been impregnated with a suitable Ionomer,
for example an anion-conductive Ionomer, and according to certain
embodiments is adjacent to the first ion exchange membrane, for
example an AEM, or is bound thereto, e.g. via an ionomer.
[0051] Different combinations of the above-described electrode
structures are also possible for use as a cathode.
[0052] The corresponding cathodes can also contain materials
commonly used in cathodes, such as binders, ionomers, for example
anion-conductive ionomers, fillers, hydrophilic additives, etc.,
which are not particularly limited. In addition to the catalyst,
the cathode can also, according to certain embodiments, comprise at
least one ionomer, for example an anion-conductive or
anion-transporting ionomer (e.g. an anion exchange resin, an anion
transport resin) which e.g. can comprise various functional groups
for ion exchange that can be the same or different, for example
tertiary amine groups, alkylammonium groups and/or phosphonium
groups), an e.g. conductive carrier material (e.g. a metal such as
titanium), and/or at least one non-metal 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 is used
for the production of photoelectrodes, and/or at least one polymer
based on polyacetylene, polyethoxythiophene, polyaniline or
polypyrrole, such as for example in polymer-based electrodes;
non-conductive carriers such as e.g. polymer networks are possible
for example if the catalyst layer has sufficient conductivity),
binders (e.g. hydrophilic and/or hydrophobic polymers, e.g. organic
binders, e.g. selected from PTFE (polytetrafluorethylene), PVDF
(polyvinylidene difluoride), PFA (perfluoroalkoxy polymers), FEP
(fluorinated ethylene-propylene copolymers), PFSA
(perfluorosulfonic acid polymers), and mixtures thereof, in
particular PTFE), conductive fillers (e.g. carbon), non-conductive
fillers (e.g. glass) and/or hydrophilic additives (e.g.
Al.sub.2O.sub.3, MgO.sub.2, hydrophilic materials such as
polysulfone, e.g. polyphenylsulfone, polyimide, polybenzoxazole or
polyether ketone or polymers that are generally electrochemically
stable in the electrolyte, polymerized "ionic liquids," and/or
organic conductors such as PEDOT:PSS or PANI (camphor sulfonic acid
doped polyaniline), which are not particularly limited.
[0053] The cathode, in particular in the form of a gas diffusion
electrode, e.g. connected to the first ion exchange membrane, or
contained in the form of a CCM, comprises according to certain
embodiments ion-conductive components, in particular an
anion-conductive component.
[0054] Other cathode forms are also possible, for example cathode
structures such as those described in US 20160251755 A1 and U.S.
Pat. No. 9,481,939.
[0055] The anode is also not particularly limited according to the
invention and can be adapted to a desired half reaction, for
example with respect to the reaction products. The oxidation of a
substance takes place in the anode compartment on the anode, which
is electrically connected to the cathode by means of a power source
for supplying the voltage for the electrolysis. In addition, the
material of the anode is not particularly limited and depends
primarily on the desired reaction. Examples of anode materials
include platinum or platinum alloys, palladium or palladium alloys
and vitreous carbon, iron, nickel, etc. Further anode materials are
also conductive oxides such as doped or undoped TiO.sub.2, indium
tin oxide (ITO), fluorine-doped tin oxide (FTO), aluminum-doped
zinc oxide (AZO), iridium oxide, etc. Optionally, these
catalytically active compounds can also be superficially applied
only in thin-film technology, for example to a titanium and/or
carbon carrier. The anode catalyst is not particularly limited. For
example, as a catalyst for O.sub.2 or Cl.sub.2 production, one also
uses IrO.sub.x (1.5<x<2) or RuO.sub.2. These can also be
present as mixed oxides with other metals, e.g. TiO.sub.2, and/or
be supported on a conductive material such as C (in the form of
conductive carbon black, activated carbon, graphite, etc.).
Alternatively, catalysts based on Fe--Ni or Co--Ni can also be used
for O.sub.2 generation. For example, the structure described below
with a bipolar membrane or a bipolar membrane is suitable for this
purpose.
[0056] The anode is the electrode on which the oxidative half
reaction takes place. It can also be configured as a gas diffusion
electrode, a porous electrode, or a full electrode or solid
electrode, etc.
[0057] The following embodiments are possible: --a gas diffusion
electrode or porous bound catalyst structure, which according to
certain embodiments can be e.g. ion-conductively and/or
mechanically bonded by means of a suitable ionomer, for example a
cationic ionomer, to the first separator, for example a cation
exchange membrane (CEM) or a diaphragm; --a gas diffusion electrode
or porous bound catalyst structure, which according to certain
embodiments can be partially pressed into the first separator, for
example a CEM or a diaphragm; --a particulate catalyst, which is
applied by means of a suitable ionomer onto a suitable carrier, for
example a porous conductive carrier, and according to certain
embodiments can be adjacent to the first separator, for example a
CEM or a diaphragm; --a particulate catalyst, which is pressed into
the first separator, for example a CEM or a diaphragm, and for
example is correspondingly conductively bound; --a non-closed flat
structure, e.g. a mesh or a metal mesh, which for example consists
of or comprises a catalyst or is coated therewith and according to
certain embodiments is adjacent to the first separator, for example
a CEM or a diaphragm; --a solid electrode, wherein in this case
there can also be a gap between the first separator, for example a
CEM or a diaphragm, and the anode; --a porous, conductive carrier,
which is impregnated with a suitable catalyst and optionally an
ionomer and according to certain embodiments is adjacent to the
first separator, for example a CEM or a diaphragm; --a
non-ion-conductive gas diffusion electrode, which has been
subsequently impregnated with a suitable Ionomer, for example a
cation-conductive ionomer, and according to certain embodiments is
adjacent to the first separator, for example a CEM or a diaphragm;
--any desired variants of the discussed embodiments, wherein the
electrode e.g. contains an anodically stable anion-conductive
material and is directly adjacent to the anion-conductive layer of
a bipolar membrane.
[0058] In this case as well, various combinations of the different
anode structures are possible.
[0059] The corresponding anodes can also contain materials commonly
used in anodes, such as binders, ionomers, e.g. also
cation-conductive ionomers, for example containing sulfonic acid
and/or phosphonic acid groups, fillers, hydrophilic additives,
etc., which are not particularly limited, and which for example are
also described above with respect to the cathode.
[0060] According to certain embodiments, the cathode and/or the
anode is/are configured as a gas diffusion electrode, as a porous
bound catalyst structure, as a particulate catalyst on a carrier,
as a coating of a particulate catalyst on the first and/or second
ion exchange membrane, as a porous conductive carrier in which a
catalyst is impregnated, and/or as a non-closed flat structure.
According to certain embodiments, the cathode is configured as a
gas diffusion electrode, as a porous bound catalyst structure, as a
particulate catalyst on a carrier, as a coating of a particulate
catalyst on the first and/or second ion exchange membrane, as a
porous conductive carrier in which a catalyst is impregnated,
and/or as a non-closed flat structure, which contain(s) an anion
exchange material and/or an anion transport material. According to
certain embodiments, the anode is configured as a gas diffusion
electrode, as a porous bound catalyst structure, as a particulate
catalyst on a carrier, as a coating of a particulate catalyst on
the first and/or second ion exchange membrane, as a porous
conductive carrier in which a catalyst is impregnated, and/or as a
non-closed flat structure, which contain(s) a cation exchange
material and/or is/are coupled and/or bound to a bipolar
membrane.
[0061] According to certain embodiments, the anode and/or the
cathode is/are brought into contact on the side opposite the salt
bridge compartment with a conductive structure. The conductive
structure is not particularly limited here. According to certain
embodiments, the anode and/or the cathode are thus brought into
contact with side facing away from the salt bridge with conductive
structures. These are not particularly limited. For example here,
these can be carbon flows, metal foams, metal knit fabrics, metal
meshes, graphite structures, or metal structures.
[0062] In an electrolysis cell according to the invention as well
as in the methods according to the invention, the above electrodes
mentioned by way of example can be combined with one another in any
desired manner.
[0063] In addition, electrolytes can also be present in the anode
compartment and/or cathode compartment, and these are also referred
to as the anolyte or catholyte, but it is not excluded according to
the invention for no electrolytes to be present in the two
compartments, and correspondingly, for example, for only gases to
be fed to said compartments for conversion, for example only
CO.sub.2, optionally also as a mixture with e.g. CO and/or
H.sub.2O, which can optionally also be a fluid, e.g. an aerosol,
but preferably gaseous H.sub.2O to the cathode and/or water or HCl
to the anode. According to certain embodiments, an anolyte is
present, which can be different from an electrolyte of the salt
bridge compartment or can correspond thereto, for example with
respect to solvents, acids etc. contained therein.
[0064] Here, a catholyte is the electrolyte flow around the cathode
or on the cathode and serves according to certain embodiments to
provide the cathode with substrate or reactant.
[0065] For example, the following embodiments are possible. The
catholyte can be present e.g. as a solution of substrate (CO.sub.2)
in a liquid carrier phase (e.g. water) and/or as a mixture of the
substrate with other gases (e.g. CO+CO.sub.2; water vapor+CO.sub.2,
N.sub.2 and/or also certain proportions of O.sub.2, SO.sub.2,
SO.sub.3; etc.). Gases recycled through a return line, such as CO
and/or H.sub.2, can also be present. The substrate can also be in
the form of a pure phase, e.g. CO.sub.2. If uncharged liquid
products are generated in the reaction, they can be washed out by
the catholyte and can then optionally also be correspondingly
separated out.
[0066] An anolyte is an electrolyte flow around the anode or at the
anode and serves according to certain embodiments to provide the
anode with substrate or reactant, and optionally to transport anode
products away. For example, the following embodiments are possible.
The anolyte can be present as a solution of the substrate (e.g.
sulfuric acid=HCl.sub.aq) in a liquid carrier phase (e.g. water),
optionally with conductive salts, which are not limited--in
particular in the use of a bipolar membrane as a first separator
membrane, wherein the anolyte can also become basic and can also
contain cations, as described below, or as 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 can also be in the form
of a pure phase, e.g. in the form of hydrogen chloride
gas=HCl.sub.g.
[0067] According to certain embodiments, the anolyte is an aqueous
electrolyte, wherein corresponding reactants, which are converted
at the anode, can optionally be added to the anolyte. Addition of
the reactant in this case is not particularly limited. Moreover,
reactant addition at the inlet to the cathode compartment is also
not limited. For example, CO.sub.2 can thus be added outside the
cathode compartment to water, or can also be added via a gas
diffusion electrode, or can also be supplied only as a gas to the
cathode compartment. Analogously, corresponding configurations are
possible for the anode compartment, depending on the reactant used,
e.g. water, HCl, NaCl, KCl, etc. and the desired product.
[0068] The first ion exchange membrane, which contains an anion
exchanger and/or anion transporter or an anion transport material
and which is adjacent to the cathode compartment, is not
particularly limited according to the invention. In the
electrolysis cell according to the invention and the method
according to the invention, it separates the cathode from the salt
bridge compartment, such that the sequence from the direction of
the cathode compartment in the direction of the electrolyte is
cathode/first ion exchange membrane/salt bridge compartment. In
particular, according to certain embodiments, it contains an anion
exchanger or is composed thereof, which is present in the
currentless state in the form of an acid-anion salt, preferably
corresponding to an acid that is present in the salt bridge
compartment, and is further preferably converted to the
hydrogencarbonate/carbonate form as of a minimum current density.
According to certain embodiments, the first ion exchange membrane
is an anion exchange membrane and/or an anion transport membrane.
According to certain embodiments, the first ion exchange membrane
can comprise a hydrophobic layer, for example on the cathode side,
for better gas contact. Preferably, the ion exchange membrane
and/or anion transport membrane also functions as a cation blocker
(even if only in trace amounts, for example), in particular a
proton blocker. In particular, an anion exchanger and/or anion
transporter with solidly bound cations can constitute a blockade
here for mobile cations through Coulomb repulsion, which can
additionally counteract salt precipitation, in particular within
the cathode.
[0069] In the case of a membrane-electrode arrangement (MEA) in
particular, the accumulation of the electrolyte cations in the area
of the interface is ordinarily attributable to electro-osmosis. In
this case, it can be difficult to reduce a concentration gradient
on the electrode side, as a catalyst-based cathode that is
configured as discussed above, e.g. a gas diffusion electrode or a
CCM, may show only extremely poor anion conductivity depending on
the anion and a selected electrolyte. In this case, the anion
conductivity can be significantly improved by the integration of
anion-conductive components.
[0070] In order to improve operating stability, ion transporters,
in particular anion transport resins, can be used as a binding
material or an additive in the electrode itself and/or in an anion
exchange layer adjacent to the cathode, in order for example to
quickly dissipate or partially buffer any Off ions generated, such
that the reaction with CO.sub.2 and the accompanying formation of
hydrogencarbonates and/or carbonates can be reduced or the anion
transport resins themselves conduct HCO.sub.2 or CO.sub.3.sup.2-.
In principle, anion transport can take place by means of anion
exchangers. Moreover, an integrated anion exchanger in particular
also constitutes a blockade for cations, e.g. metal cation traces
as well, which can additionally counteract salt precipitation and
contamination of the electrode. In the case of protons, for
example, hydrogen formation can also be inhibited.
[0071] The first ion exchange membrane can thus for example contain
an anion exchanger and/or an anion transporter in the form of an
anion exchange and/or transport layer, wherein further layers can
then be included, such as hydrophobicity-imparting layers in order
to improve the contact with a gas such as CO.sub.2. According to
certain embodiments, the first ion exchange membrane is an anion
exchange membrane and/or anion transport membrane, e.g. for example
an ion-conductive membrane (or also in the broader sense a membrane
with an anion exchange layer and/or an anion transport layer) with
positively charged functional groups, with this not being
particularly limited. A preferred charge transport takes place in
the anion exchange layer and/or the anion transport layer or an ion
exchange membrane and/or an anion transport membrane by means of
anions. In particular, the first ion exchange membrane and/or in
particular an anion exchange layer and/or an anion transport layer
or an anion exchange membrane and/or an anion transport membrane
therein serve(s) to provide an anion transport along stationary
fixed positive charges. In this manner, in particular, penetration
of an e.g. proton-containing electrolyte into the cathode due to
electro-osmotic forces can be reduced or completely prevented. In
particular, according to certain embodiments, the ion exchanger
contained in the membrane can be converted in operation to the
carbonate/hydrogencarbonate form and the passage of protons through
the membrane to the cathode can thus be prevented.
[0072] According to certain embodiments, a suitable first ion
exchange membrane, for example an anion exchange membrane and/or an
anion transport membrane, shows favorable wettability by water
and/or acids, in particular aqueous acids, high ion conductivity,
and/or a tolerance of the functional groups contained therein with
respect to high pH values, and in particular shows no Hoffmann
elimination. An exemplary AEM according to the invention is the
A201-CE membrane produced by Tokuyama used in the example, the
"Sustainion" produced by Dioxide Materials, or an anion exchange
membrane produced by Fumatech such as e.g. the Fumasep FAS-PET or
the Fumasep FAD-PET.
[0073] Otherwise, the first separator is not particularly
limited.
[0074] According to certain embodiments, the first separator, which
for example according to certain embodiments is adjacent to the
salt bridge compartment seen from the anode side, is selected from
an ion exchange membrane containing a cation exchanger, a bipolar
membrane, wherein preferably, in said bipolar membrane, the
cation-conductive layer is oriented toward the cathode and the
anion-conductive layer is oriented toward the anode, and a
diaphragm. According to certain embodiments, the first separator is
a cation exchange membrane, a bipolar membrane or a diaphragm.
[0075] A suitable first separator, for example a cation exchange
membrane or a bipolar membrane, contains for example a cation
exchanger, which can be in contact with the salt bridge
compartment. For example, it can contain a cation exchanger in the
form of a cation exchange layer, wherein further layers such as
hydrophobicity-imparting layers can then be included. It can also
be configured as a bipolar membrane or as a cation exchange
membrane (CEM). The cation exchange membrane or cation exchange
layer is e.g. an ion-conductive membrane or ion-conductive layer
with negatively charged functional groups. An exemplary charge
transport into the salt bridge compartment takes place in such a
first separator by means of cations. For example, commercially
available Nafion.RTM. membranes are suitable as a CEM, or also the
Fumapem-F membranes produced by Fumatech, Aciplex produced by Asahi
Kasei, or the Flemion membranes produced by AGCs. In principle,
however, other modified polymer membranes with strongly acidic
groups (groups such as sulfonic acid or phosphonic acid) can also
be used. According to certain embodiments, the first separator
prevents the movement of anions, in particular HCO.sub.3.sup.-,
into the anode compartment.
[0076] In addition, in the electrolysis cell according to the
invention, as well as in the method according to the invention, the
first separator can be configured as a diaphragm, which allows the
cell to be configured in a less complex and expensive manner.
According to certain embodiments, the diaphragm essentially
separates the anode compartment and the salt bridge compartment,
for example to more than 70%, 80%, or 90%, based on the interface
between the anode compartment and the salt bridge compartment, or
separates the anode compartment and the salt bridge compartment,
i.e. to 100%, based on the interface between the anode compartment
and the salt bridge compartment. The same also applies to other
first separators. Particularly preferred are embodiments that
produce gas separation, e.g. of the CO.sub.2 in the salt bridge
compartment and the O.sub.2 in the anode compartment.
[0077] The diaphragm is not particularly limited and can for
example be based 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. Binders (e.g. hydrophilic and/or hydrophobic
polymers, e.g. organic binders, e.g. selected from PTFE
(polytetrafluorethylene), PVDF (polyvinylidene difluoride), PFA
(perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene
copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures
thereof, in particular PTFE)), conductive fillers (e.g. carbon),
non-conductive fillers (e.g. glass) and/or hydrophilic additives
(e.g. Al.sub.2O.sub.3, MgO.sub.2, hydrophilic materials such as
polysulfone, e.g. polyphenylsulfone (PPSU), polyimide,
polybenzoxazole or polyether ketone or polymers that are generally
electrochemically stable in the electrolyte can also be
present.
[0078] According to certain embodiments, the diaphragm is porous
and/or hydrophilic. As it is not ion-conductive per se, it should
preferably be capable of swelling in an electrolyte, for example an
acid. Moreover, it constitutes a physical barrier for gases and
cannot be penetrated by gas bubbles. For example, it is a porous
polymer structure, wherein the base polymer is hydrophilic (e.g.
PPSU). In contrast to the CEM or bipolar membrane, the polymer does
not comprise any charged functional groups. Further preferably,
moreover, the diaphragm can contain hydrophilic structuring
components such as metal oxides (e.g. ZrO.sub.2 and/or other
materials such as particles over the surface thereof) or ceramics,
as mentioned above.
[0079] According to certain embodiments, a suitable first
separator, for example a cation exchange membrane, a bipolar
membrane and/or a diaphragm, shows favorable wettability by water
and/or acids, a high ion conductivity, stability with respect to
reactive species that can be generated at the anode (given for
example for perfluorinated polymers), and/or stability in the
required pH range, for example with respect to an acid in the salt
bridge compartment.
[0080] According to certain embodiments, the first ion exchange
membrane and/or the first separator are hydrophobic, in particular
such that they form a CCM with the electrodes, at least on the side
facing the electrodes, so that the reactants of the electrodes are
in gaseous form. According to certain embodiments, the anode and/or
cathode are at least partially hydrophilic. According to certain
embodiments, the first ion exchange membrane and/or the first
separator are wettable with water. In order to ensure favorable ion
conductivity of ionomers, swelling with water is preferred. In the
experiment, it was found that poorly wettable membranes or
separators can cause significant deterioration in the ionic binding
of the electrodes.
[0081] The presence of water is also advantageous for some of the
electrochemical conversions at the catalyst electrodes. [0082] e.g.
3 CO.sub.2+H.sub.2O+2e.sup.-->CO+2 HCO.sub.3.sup.- [0083] e.g.
depending on the pH: 2CO.sub.2+2e.sup.-->CO+CO.sub.3.sup.2-
[0084] For this reason, according to certain embodiments, the anode
and/or cathode also show sufficient hydrophilicity. Optionally,
this can be adjusted by means of hydrophilic additives such as
TiO.sub.2, Al.sub.2O.sub.3, or other electrochemically inert metal
oxides, etc.
[0085] In particular, according to certain embodiments, at least
one of the following first separators is used: [0086] A diaphragm
is preferably used if the salt bridge (the electrolyte in the salt
bridge compartment) and the anolyte comprise or are composed of an
identical, preferably inert, acid, wherein the diaphragm then
serves here to keep gases separated, such that the carbon dioxide
does pass into the anode compartment, and/or if O.sub.2 is produced
at the anode, in particular in order to reduce costs. [0087] A
cation exchange membrane or a membrane with a cation exchange layer
are used in particular if an electrolyte in the salt bridge
compartment--also referred to in the context of the invention as a
"salt bridge"--and the anolyte are not identical, and/or in
particular if the anolyte contains HCl, HBr and/or HI, and/or if
chlorine production occurs at the anode. As the cation exchange
membrane prevents anions from passing from the anolyte into the
salt bridge, and unlike the diaphragm does not show open porosity,
the anode can be configured more freely. In principle, in such an
embodiment, it is only preferable for the anode reaction that it
does not release any mobile cations other than protons, which can
pass through the CEM into the salt bridge. [0088] A bipolar
membrane or bipolar membrane, wherein preferably an anion exchange
layer and/or an anion transport layer of the bipolar membrane is
oriented toward the anode compartment and a cation exchange layer
and/or cation transport layer of the bipolar membrane is oriented
toward the salt bridge compartment, is used in particular if the
salt bridge, i.e. the electrolyte in the salt bridge compartment,
and the anolyte are not identical, and/or if the anolyte in
particular contains bases and or salts, and/or in the use of
aqueous electrolytes. In the use of bipolar membranes as a first
separator in particular, the anode compartment can be configured
independently of the salt bridge and the cathode compartment, which
allows multiple anode reactions with desired products, and in the
use of bases in particular, cheaper anodes or anode catalysts, for
example nickel- or iron-based anode catalysts for oxygen
production, can also be used.
[0089] A bipolar membrane can for example be configured as a
sandwich of a CEM and an AEM. In this case, however, the membrane
ordinarily comprises at least two layers rather than two membranes
placed atop one another. These membranes are virtually impenetrable
to both anions and cations. Accordingly, the conductivity of a
bipolar membrane is not based on the transport capacity for ions.
Instead, the ion transport ordinarily takes place by means of
acid-base dissociation of water in the middle of the membrane. In
this manner, two oppositely charged charge carriers are generated
and transported away by the E field.
[0090] As the conductivity of the bipolar membrane is based on the
separation of charges in the membrane, however, one must ordinarily
expect higher voltage drop.
[0091] The advantage of such a structure lies in the decoupling of
the electrolyte circuits, because as mentioned above, the bipolar
membrane is virtually impenetrable to all ions.
[0092] In this manner, a structure can also be realized for a basic
anode reaction that obviates the need for constant addition and
removal of salts or anode products. This is otherwise possible only
with the use of anolytes based on acids with electrochemically
inactive anions such as e.g. H.sub.2SO.sub.4. In use of a bipolar
membrane, hydroxide electrolytes such as KOH or NaOH can also be
used as an anolyte. High pH values thermodynamically favor water
oxidation and allow the use of significantly cheaper anode
catalysts, e.g. those based on iron-nickel, which would not be
stabile in an acidic or neutral environment.
[0093] Within the meaning of the invention, therefore, in use of a
bipolar membrane as a first separator membrane, the use of bases,
e.g. a hydroxide base, as an anolyte is also possible if an acid is
used in the salt bridge.
[0094] According to the invention, it is not excluded for further
membranes and/or diaphragms to be provided in addition to the first
ion exchange membrane and the first separator.
[0095] According to certain embodiments, the anode comes into
contact with the first separator, as described above by way of
example. This makes favorable binding to the salt bridge
compartment possible. In this case, moreover, no charge transport
by the anolyte is needed, and the charge transport path is
shortened. Electric shading effects due to supporting structures
between the anode and the first separator can therefore also be
avoided.
[0096] The solid anion exchanger, which is at least partially in
contact with the first ion exchange membrane and is contained in
the salt bridge compartment, is not particularly limited according
to the invention, provided that it is present in the form of a
solid--i.e. not in solution--, can exchange anions, and is at least
partially in contact with the first ion exchange membrane.
Preferably, the solid anion exchanger is hydrophilic.
[0097] As discussed above, it is preferable for the solid anion
exchanger, at least in the area of the cathode on the opposite side
of the first ion exchange membrane, to be substantially in contact
therewith--that is, touching it--, i.e. for example for contact
with the solid anion exchanger to be more than 50% of the area of
the first ion exchange membrane, preferably more than 60%, further
preferably more than 70%, in particular more than 80% based on the
area of the first ion exchange membrane which is in contact with
the cathode.
[0098] According to certain embodiments, the solid anion exchanger
is not in contact with the first ion exchange membrane over its
entire surface, in particular not in the area in which the cathode
on the opposite side of the first ion exchange membrane comes into
contact therewith, in order to allow fluid transport between the
first anion exchange membrane and the solid anion exchanger to be
ensured. It is therefore also preferable according to certain
embodiments for the solid anion exchanger, at least in the area of
the cathode on the opposite side of the first ion exchange
membrane, to be in contact therewith, such that contact with the
solid anion exchanger is 99% or less of the area of the first ion
exchange membrane, preferably 97% or less, further preferably 95%
or less, in particular 92% or less, based on the area of the first
ion exchange membrane which is in contact with the cathode.
[0099] The further mechanical configuration of the solid anion
exchanger, which can also be understood as a filling medium, is not
particularly limited, and it can be configured for example as a bed
of solid anion exchange particles, which are not particularly
limited, as a porous structure, for example a spongelike structure,
and/or as an e.g. regular porous self-supporting structure. If the
solid anion exchanger is in the form of a bed of solid anion
exchange particles, the particles should preferably have a particle
size of between 5 .mu.m and 2 mm, further preferably between 100
.mu.m and 1 mm, wherein the particle size can be determined for
example by sieve analysis. According to certain embodiments, the
particles are adapted to the size of the cell and/or to the
corresponding flow regime. According to certain embodiments, the
solid anion exchanger is present as a bed and/or a porous
structure. According to certain embodiments, the solid anion
exchanger, optionally with further solid components, e.g. neutral
particles or cation exchangers, forms a filling in the salt bridge
compartment.
[0100] Examples of the mechanical configuration of the solid anion
exchanger include: --a compressed bed of e.g. pelletized anion
exchange particles; --a spongelike porous structure; --a regular
porous self-supporting structure such as can be obtained for
example by overmolding of polymer-beads with a solution of the
anion exchange material of the solid anion exchanger and subsequent
dissolution of the template beads. However, the structure should be
at least partially open or completely open in order to allow an
electrolyte and gas flow to be ensured.
[0101] In addition, porous carrier beads (latex beads) can also be
impregnated with an anion exchange ionomer and thus function as ion
exchange particles. Here, according to certain embodiments, the
bonding of a particle bed of any desired e.g. neutral and/or
uncharged particles, such as polymeric particles, to an anion
exchange ionomer is preferred. The advantage of this method lies in
a greater number of available exchange groups for the transport of
anions.
[0102] The solid anion exchanger serves as an open extension of the
first ion exchange membrane, for example an AEM, into the volume of
the salt bridge compartment (e.g. referred to as a gap volume if
the salt bridge compartment is configured as a gap). This allows
the surface area of anion-conductive components of the electrolysis
cell, for example hydrogencarbonate- and/or carbonate-conductive
components of the electrolysis cell in CO.sub.2 electrolysis, to be
sharply increased. A gas, such as e.g. the CO.sub.2 released by
protons from the anode, thus covers a smaller portion of the
surface area of the first ion exchange membrane and thus does not
cause ionic insulation. Moreover, the solid anion exchanger
possesses intrinsic ion conductivity, which can result in the
production of an additional conduction path through the salt bridge
compartment exclusively by means of solid electrolytes. In the area
of the first separator, for example not only HCO.sub.3.sup.- and/or
CO.sub.3.sup.2-, but additionally or solely one anion of the
electrolyte used in the salt bridge compartment, for example an
acid used, can serve as a mobile charge carrier in the solid ion
exchanger. If for example H.sub.2SO.sub.4 diluted in the salt
bridge compartment is used as an electrolyte, the solid anion
exchanger should preferably be selected such that in addition to
favorable HCO.sub.3.sup.- and/or CO.sub.3.sup.2- conductivity, it
also shows favorable SO.sub.4.sup.2- conductivity. Accordingly, the
material of the solid anion exchanger can be adapted not only to an
anion such as HCO.sub.3.sup.- and/or CO.sub.3.sup.2- that is
cathodically produced, but also to further anions, e.g. in the salt
bridge compartment.
[0103] The material of the solid anion exchanger is not limited,
provided that it is correspondingly adapted to the first ion
exchange membrane and/or an electrolyte in the salt bridge
compartment, except for the fact that it must be capable of anion
exchange and/or anion transport. For example, the solid anion
exchanger can comprise an anion exchange resin in which cations are
immobilized, preferably alkali metal or alkaline earth metal
cations, e.g. by means of complexation, and/or ammonium ions and/or
derivatized ammonium ions such as quaternary ammonium ions, further
preferably alkali metal cations and/or ammonium ions and/or
derivatized ammonium ions such as quaternary ammonium ions.
Moreover, for example phosphonium, pyridinium, piperidinium,
guanidinium, imidazolium, pyrazolium and/or sulfonium ions can be
bound to a carrier of the solid anion exchanger.
[0104] According to certain embodiments, the solid anion exchanger
comprises in the salt bridge compartment cations, which are
immobilized in a polymeric backbone, wherein the cations in
particular can exchange hydrogencarbonate ions and/or carbonate
ions, wherein these hydrogencarbonate ions and/or carbonate ions
should preferably be transportable by the solid anion exchanger in
order to provide suitable conductivity in the salt bridge
compartment.
[0105] Anion exchangers are ordinarily available in solid acidic
(e.g. in HSO.sub.4.sup.- form, where they can also be present as
solid acids), neutral (e.g. as a TFO or Cl salt) or basic form
(e.g. HCO.sub.3.sup.- form, weakly basic, or OH.sup.- form,
strongly basic). In the present invention, depending on the
operating method, various such anion exchangers can be present next
to one another in a cell according to the invention, wherein a
basic anion exchange is preferably used near to and/or on the first
ion exchange membrane.
[0106] According to certain embodiments, the solid anion exchanger
is basic, preferably strongly basic. According to certain
embodiments, the immobilized cations of the anion exchanger are
configured such that an ion pair formed by them is always present
in completely dissociated form, which can be controlled for example
by means of pH. According to certain embodiments, the solid anion
exchanger comprises hydrogencarbonate, carbonate and/or OH ions
and/or anions of the electrolyte used in the electrolyte of the
salt bridge compartment, e.g. an acid, as counterions. This allows
the transport of hydrogencarbonate and/or carbonate of the first
ion exchange membrane to be improved by means of ion hopping (in
contrast to the "tunneling" in the Grotthuss mechanism).
[0107] Theoretically, it would also generally be possible to
correspondingly continue the action of the first ion exchange
membrane with a solid, e.g. acidic ion exchanger and a basic liquid
electrolyte in the salt bridge compartment. In the special case of
CO.sub.2 electrolysis, however, this is not possible because basic
solutions are converted by the CO.sub.2 present into neutral
carbonate solutions. Within the meaning of the present invention,
basic media are also considered to be anion conductors, and acidic
media are considered to be proton conductors. As cation transport
predominates in neutral (e.g. alkaline) carbonates, a corresponding
carbonate solution produced does not correspond to an extension of
the first ion exchange membrane into the salt bridge
compartment.
[0108] According to certain embodiments, the solid anion exchanger
is hydrophilic. This means that it is lightly wettable with an
aqueous medium, with the result that an aqueous medium such as an
aqueous acid can be used in the salt bridge compartment. In
addition, the water can also serve as an additional reactant in
carbon dioxide reduction, as discussed above, thus allowing
favorable conductivity.
[0109] In addition, the first anion exchanger and/or a filling
comprising the solid anion exchanger, optionally with further solid
components, e.g. neutral particles or cation exchangers, also
serves to support the separators and/or membranes, e.g. the first
ion exchange membrane and the first separator, against one another
in the salt bridge compartment. As this filling possesses its own
ion conductivity, this type of support does not lead to insulation
of electrode areas. In assembling a cell stack, the bed can also be
used for force transfer (non-positive locking) via the entire
stack.
[0110] As discussed above, the filling contains, at least in the
area of the cathode/AEM, a solid e.g. strongly basic anion
exchanger. The filling can also consist entirely of a solid e.g.
strongly basic anion exchanger.
[0111] It is advantageous if the chemical nature of the solid anion
exchanger is as similar as possible to that of the first ion
exchange membrane, e.g. an AEM. Both components can also, for
example, be constructed based on the same polymer, wherein,
however, e.g. the chain length and/or the degree of crosslinking
can be different.
[0112] Examples of embodiments in which the salt bridge compartment
comprises only a solid anion exchanger are shown schematically in
FIGS. 1 to 3. In the figures, the first separator is shown such
that it is in contact with the anode. However, this is not
absolutely necessary according to the invention, and the separator
can also be separate from the anode, such that an anode compartment
can also be formed for example between the separator and the anode,
and the anode can optionally also comprise on an opposite side of
such an anode compartment a compartment for the supply of a
substrate, e.g. a gas.
[0113] In FIG. 1, the solid anion exchanger 4, for example a
strongly basic anion exchanger, is arranged in the salt bridge
compartment II, which is located between an anion exchange membrane
AEM as a first ion exchange membrane based on an e.g. strongly
basic anion exchange material 1 and a cation exchange membrane CEM
as a first separator based on an e.g. strongly acidic cation
exchange material 3. The cathode K and the cathode compartment I
are adjacent to the AEM, and the anode A and the anode compartment
III are adjacent to the CEM. As shown in the figure, the solid
anion exchanger 4 can be penetrated by fluids such as gases and/or
electrolytes. An inlet and an outlet are provided for each of the
three compartments I, II, and III.
[0114] An alternative embodiment is shown in FIG. 2, wherein the
electrolysis cell largely corresponds to that of FIG. 1, except
that the CEM has been replaced by a diaphragm D, for example in the
form of a hydrophilic gas separator 5. A further alternative
embodiment is found in FIG. 3, which also largely corresponds to
the embodiment of FIG. 1, wherein the CEM has been replaced by a
bipolar membrane BPM in which a cation exchange layer based on an
e.g. strongly acidic cation exchange material 3 is oriented toward
the salt bridge compartment II, while an anion exchange layer based
on an e.g. strongly basic anion exchange material 1 is oriented
toward the anode.
[0115] In addition to the solid anion exchanger, however, other
components of the filling in the salt bridge compartment that can
support the release of the hydrogencarbonate taken up in the solid
anion exchanger and/or transported therein are also possible.
[0116] The filling can thus comprise, e.g. in addition to an e.g.
strongly basic and/or weakly basic anion exchanger, nonionic ion
exchangers, e.g. polyalcohols, and/or cation exchangers, e.g.
weakly and/or strongly acidic cation exchangers, which are not
particularly limited. In experiments, for example in the examples
of DE 102017211930.6, it was found that less than one equivalent of
CO.sub.2 per flowing electron passes through the first ion exchange
membrane, e.g. an AEM, into a central salt bridge compartment. This
indicates a transfer of CO.sub.3.sup.2- and/or OH.sup.- in addition
to HCO.sub.3.sup.-. Acidic additives in the solid anion exchanger,
such as cation exchangers, can be used for example for converting
CO.sub.3.sup.2- to HCO.sub.3.sup.-. As a rule, monovalent ions are
mobile in ion exchange media, while polyvalent ions are mobile in
solution. By means of corresponding addition of this type, an
optimum highly conductive conduction path is produced for each type
of ion.
[0117] FIG. 4 shows a schematic view of an exemplary embodiment in
which such a filling is provided in the salt bridge compartment II
with a mixed ion exchange material 2 containing an e.g. strongly
basic anion exchange material, which for example can be
homogeneously mixed. The further configuration of the cell in FIG.
4 corresponds to that of FIG. 1.
[0118] A comparison of these two cell designs, with an e.g.
strongly basic anion exchange material 4 (FIG. 5) and a mixed ion
exchange material 2 containing an e.g. strongly basic anion
exchange material (FIG. 6), is also shown in FIGS. 5 and 6 with a
generic separator S composed of a generic material 6, which can
have a single or multi-layer structure. The further structure
corresponds to that shown in FIG. 1.
[0119] According to certain embodiments, the solid salt bridge
compartment further comprises non-ion-conductive and/or
unfunctionalized particulate matter or particles, nonionic ion
exchangers and/or cation exchangers, wherein preferably the
non-ion-conductive and/or unfunctionalized particles, nonionic ion
exchangers and/or cation exchangers, further preferably the
non-ion-conductive and/or unfunctionalized particles and/or
nonionic ion exchangers, are contained in an area not adjacent to
the first ion exchange membrane in an amount of up to 20 vol. %,
preferably up to 17 vol. %, further preferably up to 14 vol. %,
even further preferably up to 10 vol. % or up to 5 vol. %, based on
the total amount of the solid anion exchangers and uncharged
particles, nonionic ion exchangers and/or cation exchangers. The
mixture of the solid anion exchangers with the uncharged particles,
the nonionic ion exchangers and/or the cation exchangers is not
particularly limited, and can be homogeneous or heterogeneous, e.g.
in the form of layers, etc. The uncharged particles, nonionic ion
exchangers and/or cation exchangers are not particularly limited.
As the filling is configured in the form of layers, these layers
are preferably parallel to the first ion exchange membrane and/or
the first separator, wherein the layer adjacent to the first ion
exchange membrane comprises the uncharged particles, nonionic ion
exchangers and/or cation exchangers in an amount of up to 20 vol.
%, preferably up to 17 vol. %, further preferably up to 14 vol. %,
even further preferably up to 10 vol. % or up to 5 vol. %, based on
the layer, or comprises or contains only the solid anion
exchanger.
[0120] In contrast, a layer adjacent to the first separator can for
example comprise the solid anion exchanger in an amount of up to 20
vol. %, preferably up to 17 vol. %, further preferably up to 14
vol. %, even further preferably up to 10 vol. % or up to 5 vol. %,
based on the layer, wherein according to certain embodiments, the
remainder can be a solid cation exchanger. A layer adjacent to the
first separator can also comprise or contain only the solid cation
exchanger. According to certain embodiments, the salt bridge
compartment further comprises a solid cation exchanger, which is at
least partially in contact with the first separator.
[0121] As discussed above with respect to the solid anion
exchanger, it is preferable for the solid cation exchanger, at
least in the area of the anode on the opposite side of the first
separator, to be substantially in contact therewith, i.e. for
example in contact with, i.e. touching, more than 50% of the area
of the first separator, preferably more than 60%, further
preferably more than 70%, and in particular more than 80% based on
the area of the first separator, which is in contact with the
anode. According to certain embodiments, the solid cation exchanger
is not in contact with the first separator over its entire area, in
particular not in the area in which the anode on the opposite side
of the first separator is in contact therewith, in order to allow
fluid transport between the first separator and the solid cation
exchanger to be ensured. It is therefore also preferable according
to certain embodiments for the solid cation exchanger, at least in
the area of the anode on the opposite side of the first separator,
to be in contact with 99% or less of the area of the first
separator, preferably 97% or less, further preferably 95% or less,
in particular 92% or less, based on the area of the first
separator, which is in contact with the anode.
[0122] It is also applicable for the multi-layer configurations of
the salt bridge compartment that solid ion exchangers, which
contain no e.g. strongly basic anion exchanger materials and/or are
not solid anion exchangers, preferably are not in contact with the
first ion exchange membrane, e.g. an AEM, in order to prevent gas
release at the contact point.
[0123] According to certain embodiments, the composition of the
filling, in particular e.g. along the cathode-anode connection
line, need not be homogenous. The filling can thus also be coated,
for example with an e.g. strongly basic solid anion exchanger or a
mixture comprising the solid anion exchanger in the area of the
cathode and the first ion exchange membrane, e.g. an AEM, and an
e.g. strongly acidic solid cation exchanger or a mixture comprising
the solid cation exchanger in the area of the anode and the first
separator.
[0124] The number of different layers that can be used is not
specified, nor is the order thereof, provided that they meet the
requirement that the material adjacent to the first ion exchange
membrane, e.g. an AEM, contains an e.g. strongly basic anion
exchanger.
[0125] For example, in such a multi-layer structure, two or more
layers of the filling may be present, as shown by way of example in
FIGS. 7 to 9 for two layers.
[0126] The cell structure with cathode K, anode A, AEM and
separator S as well as the cathode compartment I and anode
compartment III corresponds to that shown in FIG. 5, and only the
structure of the filling in the salt bridge compartment II differs.
In FIG. 7, adjacent to the AEM is a layer with an e.g. strongly
basic anion exchange material 4, while adjacent to the separator S
is a layer with a mixed ion exchange material 2 containing an e.g.
strongly basic anion exchange material. In FIG. 8, this mixed ion
exchange material 2 containing an e.g. strongly basic anion
exchange material of FIG. 7 has been replaced by an e.g. acidic or
strongly acidic cation exchange material 3. In FIG. 9, however, in
comparison to FIG. 8, the material adjacent to the AEM has been
replaced by a mixed ion exchange material 2 containing an e.g.
strongly basic anion exchange material.
[0127] According to certain embodiments, the filling, even when
composed only of the solid anion exchanger, is not closed, such
that an amount of an electrolyte and/or a liquid-gas bubble gas can
flow through it, i.e. the filling does not comprise any pores or
structured free compartments.
[0128] A further aspect of the present invention relates to an
electrolysis system comprising an electrolysis cell according to
the invention. The corresponding embodiments of the electrolysis
cell as well as further exemplary components of an electrolysis
system according to the invention have already been discussed above
and are thus also applicable to the electrolysis system according
to the invention. According to certain embodiments, an electrolysis
system according to the invention comprises multiple electrolysis
cells according to the invention, wherein it is not excluded for
other additional electrolysis cells also to be present.
[0129] According to certain embodiments, the electrolysis system
according to the invention further comprises a return device, which
is connected to an outlet of the salt bridge compartment and an
inlet of the cathode compartment, which is configured to recycle a
reactant of the cathode reaction which can be formed in the salt
bridge compartment, back into the cathode compartment.
[0130] An electrolysis cell according to the invention is shown by
way of example in FIG. 10, wherein the electrolysis cell can be
configured with the cathode compartment I, the salt bridge
compartment II and the anode compartment III, the anode A, the
separator S, the cathode K and the first ion exchange membrane as
an AEM, for example according to the structure shown in FIG. 5 or
FIG. 6. CO.sub.2 is supplied to the cathode compartment, and the
remaining CO.sub.2, product P and optionally water are discharged
from the cathode compartment, wherein the water is separated off.
CO.sub.2 generated in the salt bridge compartment that may have
migrated into the salt bridge compartment can be recycled via a
return line to the inlet of the cathode compartment after
electrolyte j has been separated from the salt bridge compartment,
which can also be recycled. On the anode side, an anolyte A is
recycled to the anode compartment III, wherein anodic conversion of
H.sub.2O and/or HCl to O.sub.2 and/or Cl.sub.2 is shown here by way
of example, wherein the half cell reaction does not limit the
invention. The further symbols in FIG. 10 are common fluidic
circuit symbols.
[0131] According to certain embodiments, the electrolysis system
according to the invention further comprises an external device for
electrolyte treatment, in particular a device for the removal of
dissolved gases from an acid, with the anolyte and/or the
electrolyte in particular being treated in the salt bridge
compartment, in order for example to remove gases such as CO.sub.2
or O.sub.2 and thus allow recycling of the anolyte and/or the
electrolyte to the salt bridge compartment. This is particularly
advantageous in cases where both are pumped from a common
reservoir, i.e. there is only one common anolyte/electrolyte
available for the salt bridge compartment reservoir, which means
that the anolyte and the electrolyte in the salt bridge compartment
are identical.
[0132] According to certain embodiments, the electrolysis system
according to the invention comprises two separate circuits for the
anolyte and electrolyte in the salt bridge compartment, which can
optionally comprise separate devices for electrolyte treatment, in
particular devices for the removal of dissolved gases from an acid,
or wherein only the circuit for the electrolyte in the salt bridge
compartment comprises a corresponding device.
[0133] In yet a further aspect, the present invention relates to
the use of an electrolysis cell according to the invention or an
electrolysis system according to the invention, which can also
comprise multiple electrolysis cells according to the invention,
for the electrolysis of CO.sub.2 and/or
[0134] CO.
[0135] In addition, a method is disclosed for the electrolysis of
CO.sub.2, wherein an electrolysis cell according to the invention
or an electrolysis system according to the invention is used,
wherein CO.sub.2 is reduced at the cathode and hydrogencarbonate
and/or carbonate generated at the cathode by the first ion exchange
membrane migrates to an electrolyte in the salt bridge, wherein the
hydrogencarbonate and/or carbonate is also transported through the
solid anion exchanger in the salt bridge compartment away from the
first ion exchange membrane.
[0136] In addition to hydrogencarbonate and/or carbonate, it is not
also excluded for formate and/or acetate and/or further generated
anions to migrate through the first ion exchange membrane into the
electrolyte of the salt bridge compartment. The method according to
the invention is carried out with the electrolysis cell according
to the invention or the electrolysis system according to the
invention. Accordingly, all of the features discussed with respect
to the electrolysis cell according to the invention and the
electrolysis system according to the invention are also applicable
to the method according to the invention. In particular, the
cathode compartment, the cathode, the first ion exchange membrane,
the anode compartment, the anode, the separator, the salt bridge
compartment and the solid anion exchanger, as well as further
components, have already been discussed with respect to the
electrolysis cell according to the invention and the electrolysis
system according to the invention. Therefore, the corresponding
features can thus be implemented correspondingly in the method
according to the invention. Conversely, the method according to the
invention can also be implemented with the electrolysis cell
according to the invention or the electrolysis system according to
the invention, so that comments or aspects with respect to the
method for the electrolysis of CO.sub.2 according to the invention
can also be applied thereto, for example with respect to an
electrolyte in the salt bridge compartment and accompanying
configurations of the components of the electrolysis cell, such as
e.g. the first separator.
[0137] With the methods according to the invention, CO.sub.2 is
electrolyzed, wherein, however, it is not excluded, in addition to
CO.sub.2, for a further reactant such as CO to also be present on
the cathode side, which can also be electrolyzed, i.e. for a
mixture to be present that comprises CO.sub.2, as well as e.g. CO.
For example, a reactant on the cathode side comprises at least 20
vol. % of CO.sub.2, e.g. at least 50 or at least 70 vol. % of
CO.sub.2, and in particular, the reactant on the cathode side can
comprise up to 100 vol. % of CO.sub.2. In principle, the
electrolysis cell according to the invention can also convert pure
CO, wherein in this case, of course, no CO.sub.2 is then released
in the salt bridge compartment.
[0138] In the method according to the invention, an electrolyte,
i.e. a liquid medium, flows through the filling comprising the
solid anion exchanger or composed of the solid anion exchanger. The
electrolyte is not particularly limited, but according to certain
embodiments may also be aqueous. According to certain embodiments,
the salt bridge compartment thus comprises an aqueous electrolyte.
It may correspond to the anolyte and/or catholyte, as appropriate,
or may be different therefrom.
[0139] According to certain embodiments, the electrolyte of the
salt bridge compartment comprises an acid, preferably a
water-soluble or water-miscible acid. According to certain
embodiments, the electrolyte contains at least 10.sup.-6 mol/l of
H.sup.+ and/or hydrated variants thereof, preferably at least
10.sup.-4 mol/l, further preferably at least 10.sup.-3 mol/l, and
even further preferably at least 10.sup.-2 mol/l. According to
certain embodiments, the electrolyte of the salt bridge compartment
comprises essentially no mobile cations other than H.sup.+ and/or
hydrated variants thereof. Preferably, according to certain
embodiments, the electrolyte comprises no mobile cations other than
protons, with the exception of mobile cations in a number of common
contaminants. The electrolyte serves to discharge the CO.sub.2 and
keep the filling moist.
[0140] The at least one acid in the electrolyte in the salt bridge
compartment is not particularly limited, but is preferably a
water-soluble and/or water-miscible acid, such as for example HCl,
HBr, HI, H.sub.2SO.sub.4, H.sub.3PO.sub.4, HTfO (trifluoromethane
sulfonic acid), etc. The use of at least one acid in the
electrolyte promotes the CO.sub.2 release from hydrogencarbonate
and/or carbonate in the solid anion exchanger, for example a basic
or strongly basic ion exchanger. The release of the CO.sub.2
preferably takes place in the volume of the salt bridge compartment
and not at the contact surface between the filling comprising the
solid anion exchanger or the solid anion exchanger and the
separator, as this would also lead to considerable voltage
losses.
[0141] According to certain embodiments, an improvement in gas
release in the salt bridge compartment can be achieved using
multi-layer fillings, as described above. Of course, it is also
possible to build up an electrolyte gradient in the salt bridge
compartment in order to achieve a preferred release in the salt
bridge compartment and not on separators such as the first ion
exchange membrane, the first separator and optionally further
contained separators and/or ion exchange membranes, for example
with multiple electrolyte inlets to the salt bridge compartment or
layers of fillings, wherein in this case laminar flows are also
optionally possible in order to produce such electrolyte
gradients.
[0142] According to certain embodiments, the filling comprising the
solid anion exchanger or composed of the solid anion exchanger is
preferably ion-conductive in order to improve charge transport by
the electrolyte.
[0143] It is not excluded in the method according to the invention
to use only water as an electrolyte in the salt bridge compartment.
For this purpose, however, the first separator is preferably
configured as an ion exchange membrane comprising a cation
exchanger, for example as a cation exchange membrane (CEM), or as a
bipolar membrane (BPM). Preferably, in addition to the solid anion
exchanger, the solid filling also contains acidic components, e.g.
cation exchangers. Because of the conductivity, however, the use of
an acidic solution is preferred.
[0144] If the first separator is configured as a diaphragm, the
electrolyte comprises in the salt bridge compartment at least one
acid, as the diaphragm is not intrinsically ion-conductive. The
electrolyte of the salt bridge compartment can for example
correspond to the anolyte, but can also be different therefrom.
[0145] A particularly preferred embodiment of the method according
to the invention lies in the use of the solid anion exchanger,
optionally in a mixture with further components in the filling of
the salt bridge compartment, in combination with an acidic
electrolyte. In this manner, compared to the prior art, the contact
surface between the anion exchanger of the first ion exchange
membrane and the acidic media can be sharply increased. In
solutions according to the prior art, e.g. in US 2017037522 A1 and
DE 102017208610.6, the surface area of the first ion exchange
membrane is also the transition to the acidic medium in all cases.
According to the invention, this transition is moved into the
volume of the salt bridge compartment, thus massively increasing
the surface area. As a result, the insulating action of the gas
bubbles generated in CO.sub.2 electrolysis less adversely affects
the cell voltage. The action of the anion exchanger/transporter
contained in the first ion exchange membrane as a transporter for
anions can be continued by the filling in the salt bridge
compartment.
[0146] According to certain embodiments, the anode compartment
comprises an anolyte, which comprises a liquid and/or a dissolved
acid, preferably wherein the anolyte and/or the acid in the salt
bridge compartment or the electrolyte in the salt bridge
compartment comprise no mobile cations other than protons and/or
deuterons, in particular no metal cations. According to certain
embodiments, an acid in the salt bridge compartment comprises no
mobile cations other than protons and/or deuterons, in particular
no metal cations. According to certain embodiments, the anolyte
comprises no mobile cations other than protons and/or deuterons, in
particular no metal cations. Mobile cations are cations that are
not bound by a chemical bond to a carrier and/or in particular have
an ion mobility of more than 1.10.sup.-8 m.sup.2/(sV), in
particular more than 1.10.sup.-1.degree. m.sup.2/(sV). According to
certain embodiments, during the anodic half reaction, no mobile
cations other than "D.sup.+" and H.sup.+'', in particular no metal
cations, are released or produced. In such a case, therefore, for
the special case of O.sub.2 generation at the anode, for example,
water (in particular in the case of a CCM anode) or acids with
non-oxidizable anions may be used as an anolyte or reagent.
Accordingly, for halogenation at the anode, particularly in this
case, the halogen-hydrogen acids HCl, HBr and/or HI are suitable,
wherein for example halide salts are not suitable in use of a
diaphragm as a first separator membrane, but can be used in use of
a bipolar membrane as a first separator membrane. The use of
SO.sub.2 in the anolyte for the production of sulfuric acid or
H.sub.2O for the production of H.sub.2O.sub.2, etc. is also.
[0147] An exemplary method according to the invention will now be
discussed with respect to a special configuration of the
electrolysis cell of FIG. 5, which in the following will be further
implemented. The comments refer to a special configuration of the
electrolysis cell of FIG. 5, such that the following explanations
do not limit the embodiment shown in FIG. 5, which is described
above. In such an exemplary method, the cathode compartment I of
the salt bridge compartment II is separated by a composite of a
CO.sub.2 reducing cathode K and an AEM. CO.sub.2, for example
moistened CO.sub.2, flows through the cathode compartment I, where
it is reduced for example to CO and C.sub.2H.sub.4. The moistened
CO.sub.2 flow constitutes the substrate supply to the cathode. It
then constitutes the catholyte within the meaning of a classical
three-compartment cell.
[0148] The salt bridge compartment II is separated from the anode
compartment III by the first separator S (e.g. a diaphragm, a
bipolar membrane, a cation-conductive membrane) in conjunction with
the anode A, wherein--as discussed above--it is also possible for
the anode compartment III to be directly adjacent to the first
separator. The salt bridge compartment II is packed with a solid
filling through which substances can flow that contains an e.g.
strongly basic anion exchanger, and is flowed through by an
electrolyte flow, which in addition to water can also comprise an
acid.
[0149] The first separator can be freely selected e.g. from a
cation exchange membrane (CEM), a not intrinsically ion-conductive
hydrophilic gas separator (diaphragm), or a bipolar membrane (BPM),
in which the anion-conductive layer is preferably oriented toward
the anode. In use of a diaphragm, the electrolyte in anode
compartment III and the liquid electrolyte in the salt bridge
compartment II are preferably identical and conductive.
[0150] The anolyte, e.g. aqueous HCl, aqueous H.sub.2SO.sub.4,
H.sub.2O, etc., flows through the anode compartment III, which can
provide the anode A with substrate. In cases where the selected
electrolyte of the salt bridge compartment and the anolyte are
identical, they can also be obtained from a common reservoir,
wherein in particular, however, suitable devices are present in
order to prevent the discharge of dissolved gases (degassing), e.g.
in a return line of the electrolyte.
[0151] As mentioned above, in a departure from FIG. 5, the anode
compartment III can also be located between the anode A and the
first separator S. In such a case, however, the anolyte must be
conductive.
[0152] The cathode compartment I and the anode compartment III can
additionally comprise e.g. electrically conductive, e.g.
non-closed, structures, which are used for contacting of the
electrodes. If the anode is not adjacent to the first separator,
the requirement of conductivity can be dispensed with. Preferably,
the anolyte contains only salts and thus mobile "non-H.sup.+"
cations if the first separator is a bipolar membrane.
[0153] The electrochemical conversion at the anode is not further
limited, wherein it preferably leads to the transition of H.sup.+
from (bipolar membrane) or through (diaphragm or CEM) the first
separator into the electrolyte of the salt bridge compartment.
[0154] Provided this is useful, the embodiments, configurations and
improvements above can be combined with one another as desired.
Further possible configurations, improvements and implementations
of the invention comprise combinations of features of the invention
described above or below in connection with the examples, even if
they are not explicitly mentioned. In particular, the person
skilled in the art may also add individual aspects as improvements
on or supplements to the respective basic form of the present
invention.
* * * * *