U.S. patent application number 16/333696 was filed with the patent office on 2019-07-25 for device for continuous operation of an electrolysis cell having a gaseous substrate and gas diffusion electrode.
This patent application is currently assigned to Siemens Aktiengesellschaft. The applicant listed for this patent is SIEMENS AKTIENGESELLSCHAFT. Invention is credited to Helmut Eckert, Ralf Krause, Christian Reller, Bernhard Schmid, Gunter Schmid, Dan Taroata.
Application Number | 20190226105 16/333696 |
Document ID | / |
Family ID | 59745896 |
Filed Date | 2019-07-25 |
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United States Patent
Application |
20190226105 |
Kind Code |
A1 |
Eckert; Helmut ; et
al. |
July 25, 2019 |
Device for Continuous Operation of an Electrolysis Cell Having a
Gaseous Substrate and Gas Diffusion Electrode
Abstract
Various embodiments include a method for continuous operation of
an electrolysis cell with a gaseous substrate, the method
comprising: supplying an electrolyte to the electrolysis cell via
an electrolyte feed; flowing the electrolyte out of the
electrolysis cell into the gas space through a gas diffusion
electrode; collecting the electrolyte from the electrolyte flow
into the gas space in a collecting region in the gas space; and
sucking the collected electrolyte out of said gas space via a
connection between the gas space and electrolyte feed.
Inventors: |
Eckert; Helmut; (Rottenbach,
DE) ; Krause; Ralf; (Herzogenaurach, DE) ;
Reller; Christian; (Minden, DE) ; Schmid;
Bernhard; (Erlangen, DE) ; Schmid; Gunter;
(Hemhofen, DE) ; Taroata; Dan; (Erlangen,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIEMENS AKTIENGESELLSCHAFT |
Munchen |
|
DE |
|
|
Assignee: |
Siemens Aktiengesellschaft
Muenchen
DE
|
Family ID: |
59745896 |
Appl. No.: |
16/333696 |
Filed: |
August 24, 2017 |
PCT Filed: |
August 24, 2017 |
PCT NO: |
PCT/EP2017/071292 |
371 Date: |
March 15, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 1/04 20130101; C25B
1/46 20130101; C25B 9/18 20130101; C25B 9/00 20130101; Y02E 60/366
20130101; Y02E 60/36 20130101; C25B 1/10 20130101; C25B 11/035
20130101; C25B 1/00 20130101; C25B 15/08 20130101; C25B 3/04
20130101 |
International
Class: |
C25B 15/08 20060101
C25B015/08; C25B 11/03 20060101 C25B011/03; C25B 9/18 20060101
C25B009/18; C25B 1/10 20060101 C25B001/10; C25B 1/46 20060101
C25B001/46; C25B 3/04 20060101 C25B003/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 20, 2016 |
DE |
10 2016 217 989.6 |
Claims
1. A method for continuous operation of an electrolysis cell with a
gaseous substrate, the method comprising: supplying an electrolyte
to the electrolysis cell via an electrolyte feed; flowing the
electrolyte out of the electrolysis cell into the gas space through
a gas diffusion electrode; collecting the electrolyte from the
electrolyte flow into the gas space in a collecting region in the
gas space; and sucking the collected electrolyte out of said gas
space via a connection between the gas space and electrolyte
feed.
2. The method as claimed in claim 1, wherein the electrolyte feed
exerts a suction effect force on the gas space.
3. The process as claimed in claim 2, wherein the connection
between the gas space and the electrolyte feed comprises a Venturi
nozzle through which the electrolyte is fed to the electrolysis
cell.
4. The process as claimed in claim 1, wherein the electrolyte is
sucked out of the gas space periodically.
5. The process as claimed in claim 4, wherein the periodic suction
is triggered via a closed-loop control mechanism.
6. The process as claimed in claim 5, wherein the closed-loop
control mechanism is mechanical.
7. The process as claimed in claim 6, wherein: the closed-loop
control mechanism comprises a float in the collecting region in the
gas space; and the control mechanism enables outflow to the
connection between the gas space and the electrolyte feed depending
on the fill level of the electrolyte in the collecting region in
the gas space.
8. The process as claimed in claim 1, wherein the suction is
actuated by a valve controlling the connection between the gas
space and the electrolyte feed.
9. The process as claimed in claim 8, wherein: the valve is coupled
to a fill level sensor for electrolyte in the gas space via a
closed-loop controller; and the valve is controlled with reference
to a measurement by a fill level sensor.
10. An apparatus for continuous operation of an electrolysis cell
with a gaseous substrate, the apparatus comprising: an anode and a
cathode, wherein at least one of the anode and cathode takes the
form of a gas diffusion electrode; a cell space configured be
filled with an electrolyte and into which the anode and cathode are
at least partly disposed; an electrolyte feed into the cell space;
a gas space to feed the gas diffusion electrode with a gaseous
substrate, the gas space disposed on a side of the gas diffusion
electrode remote from the cell space; a feed to feed the gas space
with the gaseous substrate; a collecting region in the gas space
configured to collect electrolyte in the gas space; and a
connection between the gas space and the electrolyte feed to remove
electrolyte that has collected in the collecting region in the gas
space from said gas space.
11. The apparatus as claimed in claim 10, wherein the connection
between the gas space and the feed for electrolyte comprises a
Venturi nozzle.
12. The apparatus as claimed in claim 10, further comprising a
closed-loop control mechanism to control the removal of the
electrolyte from the collecting region in the gas space.
13. The apparatus as claimed in claim 12, wherein: the closed-loop
control mechanism comprises a float disposed in the collecting
region in the gas space; and the control mechanism periodically
breaks the connection between the gas space and the feed for
electrolyte.
14. The apparatus as claimed in claim 13, wherein the float
comprises a cone or frustocone, wherein the tip of the cone or the
circular face of the frustocone having the smaller size projects
into the connection between the gas space and the feed for
electrolyte.
15. The apparatus as claimed in claim 12, wherein: the closed-loop
control mechanism comprises a valve in the connection between the
gas space and the feed for electrolyte coupled to a fill level
sensor in the gas space and a closed-loop controller; the fill
level sensor and the closed-loop controller control the valve in
the connection between the gas space and the feed for electrolyte
depending on the fill level of the electrolyte in the gas space.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a U.S. National Stage Application of
International Application No. PCT/EP2017/071292 filed Aug. 24,
2017, which designates the United States of America, and claims
priority to DE Application No. 10 2016 217 989.6 filed Sep. 20,
2016, the contents of which are hereby incorporated by reference in
their entirety.
TECHNICAL FIELD
[0002] The present disclosure relates to electrolysis. Various
embodiments may include processes for continuous operation of an
electrolysis cell with a gaseous substrate and/or devices for
performing the process.
BACKGROUND
[0003] The combustion of fossil fuels currently covers about 80% of
global energy demand. These combustion processes emitted about 34
032.7 million metric tons of carbon dioxide (CO.sub.2) globally
into the atmosphere in 2011. This release is the simplest way of
disposing of large volumes of CO.sub.2 as well (brown coal power
plants exceeding 50 000 t per day). Discussion about the adverse
effects of the greenhouse gas CO.sub.2 on the climate has led to
consideration of reutilization of CO.sub.2. In thermodynamic terms,
CO.sub.2 is at a very low level and can therefore be reduced again
to usable products only with difficulty.
[0004] In nature, CO.sub.2 is converted to carbohydrates by
photosynthesis. This process, which is divided up into many
component steps over time and spatially at the molecular level, is
copiable on the industrial scale only with great difficulty. The
more efficient route at present compared to pure photocatalysis is
the electrochemical reduction of the CO.sub.2. A mixed form is
light-assisted electrolysis or electrically assisted
photocatalysis. The two terms can be used synonymously, according
to the viewpoint of the observer. As in the case of photosynthesis,
in this process, CO.sub.2 is converted to a higher-energy product
(such as CO, CH.sub.4, C.sub.2H.sub.4, etc.) with supply of
electrical energy (optionally in a photo-assisted manner) which is
obtained from renewable energy sources such as wind or sun. The
amount of energy required in this reduction corresponds ideally to
the combustion energy of the fuel and should only come from
renewable sources. However, overproduction of renewable energies is
not continuously available, but at present only at periods of
strong insolation and wind. However, this state of affairs will
further intensify in the near future with the further rollout of
sources of renewable energy.
[0005] The electrochemical reduction of CO.sub.2 over solid-state
electrodes in aqueous electrolyte solutions offers a multitude of
product options that are shown in table 1 below, taken from Y.
Hori, Electrochemical CO.sub.2 reduction on metal electrodes, in:
C. Vayenas, et al. (eds.), Modern Aspects of Electrochemistry,
Springer, New York, 2008, pp. 89-189.
TABLE-US-00001 TABLE 1 Faraday efficiencies for carbon dioxide over
various metal electrodes Electrode CH.sub.4 C.sub.2H.sub.4
C.sub.2H.sub.5OH C.sub.3H.sub.7OH CO HCOO.sup.- H.sub.2 Total Cu
33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7
10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0
79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0
0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4
Hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9
3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 Cd 1.3 0.0 0.0 0.0
13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8
0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8
Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0
99.7 99.7
[0006] In operation of such electrolysis cells for CO.sub.2
reduction, it has been found that electrolyte passes through a gas
diffusion electrode (GDE) and leads to an electrolyte accumulation
in the gas space. Both in flow-through and flow-by operation, the
gas flow causes the electrolyte to lose its solvent, especially
water, as a result of which depositions of salt occur in the gas
space and, particularly disadvantageously, in the GDE itself. These
lead to loss of selectivity and ultimately to the failure of the
electrode or the electrolyzer.
[0007] DE 10 2012 204 041 A1, e.g. paragraphs [0007], [0008],
[0041] and [0059], or DE 10 2013 011 298 A1, describes the mode of
operation of an "oxygen-depolarized cathode". Also described
therein is the passage of electrolyte through the GDE. DE 10 2012
204 041 A1 additionally describes the possibility of blockage of
the pores of the GDE.
[0008] The phenomenon of depositions of salt is particularly
dominant here in electrolyzers that convert a gaseous substrate
over a gas diffusion electrode back to gaseous substrates. There is
thus a need for a process and an apparatus by which these problems
caused by electrolyte passage can be reduced or remedied.
SUMMARY
[0009] The present disclosure describes a mode of operation in
which salt migration takes place in such a way that an electrolysis
cell nevertheless runs stably. More particularly, salting-out of
electrolyte can be avoided in spite of passage through the
electrode, and good electrolysis performance can be obtained over a
long period.
[0010] For example, some embodiments may include a process for
continuous operation of an electrolysis cell with a gaseous
substrate, wherein an electrolyte is supplied to the electrolysis
cell via an electrolyte feed, an electrolyte flow out of the
electrolysis cell into the gas space takes place through a gas
diffusion electrode, and the electrolyte from the electrolyte flow
into the gas space is collected in a collecting region in the gas
space, and the collected electrolyte is sucked out of said gas
space, wherein the suction is effected via a connection between the
gas space and electrolyte feed.
[0011] In some embodiments, the suction is effected in that the
electrolyte feed exerts a suction effect on the gas space.
[0012] In some embodiments, the suction effect arises in that the
connection between the gas space and the electrolyte feed comprises
a Venturi nozzle through which the electrolyte is fed to the
electrolysis cell.
[0013] In some embodiments, the electrolyte is sucked out of the
gas space periodically.
[0014] In some embodiments, the periodic suction is effected via a
closed-loop control mechanism that controls the periodic
suction.
[0015] In some embodiments, the closed-loop control mechanism is
mechanical.
[0016] In some embodiments, the closed-loop control mechanism
comprises a float which is present in the collecting region in the
gas space and enables outflow to the connection between the gas
space and the electrolyte feed depending on the fill level of the
electrolyte in the collecting region in the gas space, with
periodic opening of the float.
[0017] In some embodiments, the suction is controlled by a valve
that controls the connection between the gas space and the
electrolyte feed.
[0018] In some embodiments, the valve is coupled to a fill level
sensor for electrolyte in the gas space via a closed-loop
controller, wherein the valve is controlled with reference to a
measurement by a fill level sensor.
[0019] As another example, some embodiments include an apparatus
for continuous operation of an electrolysis cell with a gaseous
substrate, comprising an electrolysis cell comprising: an anode, a
cathode, wherein at least one of the anode and cathode takes the
form of a gas diffusion electrode, a cell space which is designed
to be filled with an electrolyte and into which the anode and
cathode are at least partly introduced; a feed for electrolyte
designed to feed the cell space with the electrolyte; a gas space
designed to feed the gas diffusion electrode with a gaseous
substrate, wherein the gas space is provided on a side of the gas
diffusion electrode remote from the cell space; a feed for a
gaseous substrate designed to feed the gas space with the gaseous
substrate; a collecting region in the gas space designed to collect
electrolyte in the gas space; and a connection between the gas
space and the feed for electrolyte, designed to remove electrolyte
that has collected in the collecting region in the gas space from
said gas space.
[0020] In some embodiments, the connection between the gas space
and the feed for electrolyte comprises a Venturi nozzle.
[0021] In some embodiments, there is a closed-loop control
mechanism designed to control the removal of the electrolyte from
the collecting region in the gas space.
[0022] In some embodiments, the closed-loop control mechanism
comprises a float which is provided in the collecting region in the
gas space and is designed to periodically break the connection
between the gas space and the feed for electrolyte.
[0023] In some embodiments, the float is designed as a cone or
frustocone, wherein the tip of the cone or the circular face of the
frustocone having the smaller size projects into the connection
between the gas space and the feed for electrolyte.
[0024] In some embodiments, the closed-loop control mechanism
comprises a valve in the connection between the gas space and the
feed for electrolyte which is coupled to a fill level sensor in the
gas space and a closed-loop controller, wherein the fill level
sensor and the closed-loop controller are designed to control the
valve in the connection between the gas space and the feed for
electrolyte depending on the fill level of the electrolyte in the
gas space.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The appended drawings are intended to illustrate embodiments
of the teachings of the present disclosure and impart further
understanding thereof. In connection with the description, they
serve to explain concepts and principles of the teachings. Other
embodiments and many of the advantages mentioned are apparent with
regard to the drawings. The elements of the drawings are not
necessarily shown to scale relative to one another. Elements,
features and components that are the same, have the same function
and the same effect are each given the same reference numerals in
the figures of the drawings unless stated otherwise.
[0026] FIGS. 1 to 5 show, in schematic form, illustrative diagrams
of a possible construction of an electrolysis cell;
[0027] FIG. 6 shows, in schematic form, a configuration of an
electrolysis plant for CO2 reduction without the configuration of
the connection between electrolyte feed and gas diffusion
electrode;
[0028] FIG. 7 shows, in schematic form, a configuration of an
electrolysis plant for CO2 reduction with a gas diffusion electrode
incorporating teachings of the present disclosure;
[0029] FIG. 8 shows, in schematic form, the construction of a
Venturi nozzle incorporating teachings of the present disclosure;
and
[0030] FIGS. 9 to 13 show, in schematic form, various embodiments
for closed-loop control of the suction of electrolyte out of a
collecting region in the gas space of an apparatus with a gas
diffusion electrode and electrolyte passage.
DETAILED DESCRIPTION
[0031] In some embodiments, a process for continuous operation of
an electrolysis cell with a gaseous substrate, includes an
electrolyte is supplied to the electrolysis cell via an electrolyte
feed, an electrolyte flow out of the electrolysis cell into the gas
space takes place through a gas diffusion electrode, and the
electrolyte from the electrolyte flow into the gas space is
collected in a collecting region in the gas space, and the
collected electrolyte is sucked out of said gas space, wherein the
suction is effected via a connection between the gas space and
electrolyte feed.
[0032] In some embodiments, an apparatus for continuous operation
of an electrolysis cell with a gaseous substrate, comprising an
electrolysis cell comprises: an anode, a cathode, wherein at least
one of the anode and cathode takes the form of a gas diffusion
electrode, and a cell space which is designed to be filled with an
electrolyte and into which the anode and cathode are at least
partly introduced; a feed for electrolyte designed to feed the cell
space with the electrolyte; a gas space designed to feed the gas
diffusion electrode with a gaseous substrate, wherein the gas space
is provided on a side of the gas diffusion electrode remote from
the cell space; a feed for a gaseous substrate designed to feed the
gas space with the gaseous substrate; a collecting region in the
gas space designed to collect electrolyte in the gas space; and a
connection between the gas space and the feed for electrolyte,
designed to remove electrolyte that has collected in the collecting
region in the gas space from said gas space.
[0033] In some embodiments, a process for continuous operation of
an electrolysis cell with a gaseous substrate includes an
electrolyte is supplied to the electrolysis cell via an electrolyte
feed, and an electrolyte flow out of the electrolysis cell into the
gas space takes place through a gas diffusion electrode, the
electrolyte from the electrolyte flow, especially unwanted
electrolyte flow, into the gas space is collected in a collecting
region in the gas space, and the collected electrolyte is sucked
out of said gas space, wherein the suction is effected via a
connection between the gas space and electrolyte feed. The
electrolyte flow is unavoidable particularly in specific
embodiments.
[0034] The processes described herein are suitable for all
electrolysis cells with a gaseous substrate and especially a gas
diffusion electrode, but may be used for electrolysis of CO.sub.2
or CO, for example to CO or hydrocarbons. It is therefore described
specifically in connection with CO.sub.2 electrolysis to CO or
hydrocarbons, but as stated is not restricted thereto. For carbon
dioxide electrolysis, it is possible here, in some embodiments, to
use gas diffusion electrodes as cathodes which comprise or consist
of precious metals such as silver or gold, e.g. silver, and/or
copper (for example for hydrocarbon formation in CO.sub.2
reduction) If an oxygen-depolarized electrode is provided as gas
diffusion electrode, this may consist of or at least comprise
silver, for example.
[0035] Useful gaseous substrates in general include any gaseous
substrates that can be employed in an electrolysis, such as carbon
dioxide, carbon monoxide, oxygen, etc., e.g. carbon dioxide or
carbon monoxide. The electrolyte in the process is likewise not
particularly restricted and may include, for example, those that
are usually used in electrolysis. In some embodiments, the
electrolyte comprises an aqueous electrolyte in which conductive
salts may be dissolved. Useful salts include, for example, those
with alkali metal cations such as Na.sup.+, K.sup.+, etc., and with
suitable anions, for example halogen anions such as Cl.sup.-,
Br.sup.-, etc., sulfate and/or sulfonate ions, carbonate and/or
hydrogencarbonate ions, etc., and/or mixtures thereof, and it is
also possible to use ionic liquids additionally or alternatively,
optionally also in solution.
[0036] A gas diffusion electrode is understood to mean an electrode
through which the gaseous substrate is introduced into the
electrolysis cell. In terms of its construction, the latter is not
particularly restricted and especially has a porous
configuration.
[0037] By a suitable adjustment of hydrophilicity and/or
hydrophobicity in the gas diffusion electrode (GDE), it is also
possible here, in some embodiments, to adjust the electrolyte flow
from the electrolysis cell into which the electrolyte is introduced
and in which the electrolysis takes place into the gas space
through which the gaseous substrate is fed in. The gas diffusion
electrode can thus, for example, be produced via adjustment of its
hydrophobicity/hydrophilicity such that a certain electrolyte flow
therethrough is enabled. The adjustment can be undertaken here in a
suitable manner and is not particularly restricted.
[0038] By virtue of the passage of the electrolyte through the gas
diffusion electrode, it was possible in processes known to date for
salt formation or salt precipitation to occur at the gas diffusion
electrode. For better understanding of the present teachings, this
phenomenon is detailed hereinafter by way of example with reference
to various processes in which the processes or the devices
described herein can be employed.
[0039] It should be noted that the reflux of the electrolyte can
simultaneously serve to eliminate or to avoid depositions of salt
in the GDE. The phenomenon of salt precipitation can occur here in
a wide variety of different modes of operation. For example, in the
case of a chloralkali electrolysis with an oxygen-depolarized
electrode, it can be described as follows:
[0040] According to prior art, in chloralkali electrolysis, the
anolyte space is supplied continuously with sodium chloride as an
aqueous solution. At the anode, chloride (Cl.sup.-) is oxidized to
chlorine (Cl.sub.2) which leaves the electrolysis cell.
2Cl.sup.-.fwdarw.Cl.sub.2+2e.sup.- (charge neutrality)
[0041] The electrodes released are transported to the cathode
through the potential source applied. In order to obtain electrical
neutrality of the overall system, a membrane results in a
corresponding stream of an equal number of cations.
[0042] At the cathode, water is reduced in the conventional
membrane process.
H.sub.2O+2e.sup.-.fwdarw.H.sub.2+2OH.sup.- (charge neutrality)
[0043] The negative charges can be compensated for by cations,
Na.sup.+ here for example, meaning that the OH.sup.- ions that form
at the cathode can leave the cathode space continuously, for
example as sodium hydroxide solution. In the case of use of an
oxygen-depolarized cathode as gas diffusion electrode, no hydrogen
is formed, but rather water. The enthalpy of water formation here
can lower the necessary system potential, such that less energy is
consumed.
H.sub.2O+1/2O.sub.2+2e.sup.-.fwdarw.H.sub.2O+2OH.sup.- (charge
neutrality)
[0044] The oxygen-depolarized cathode in chloralkali electrolysis
consists, for example, of silver, which can also be used for
CO.sub.2 reduction to CO. To balance the charge, cations, e.g.
Na.sup.+ ions, move in the cathode space direction, and these have
to be removed continuously from the electrolyte in the form of
sodium hydrogencarbonate. One possible anode reaction here with
oxygen, for example, is as follows:
H.sub.2O.fwdarw.O.sub.2+2H.sup.++2e.sup.-
[0045] Continuous operation is possible in an electrolysis, for
example, when the pH is balanced by continuous mixing of catholyte
and anolyte outside the electrolysis cell as well. However, this is
complex. In some embodiments, however, continuous mixing of anolyte
and catholyte is also possible outside the electrolysis cell. It is
of course also possible for anolyte and catholyte to be identical
as electrolyte, but they may also be different.
[0046] Another option is, for example, to make the anolyte acidic,
such that only protons pass through a membrane in the cathode
direction. It may be necessary here to introduce balancing of
concentration, in order that protons, like water molecules in the
case of other cations, are actively pushed through the membrane.
This measure too can of course also be taken additionally in the
process of the invention. Salt deposition in the gas space or gas
feed space can then arise as a result of the process which follows.
Hydroxide ions that form in the abovementioned examples can
penetrate through to the reverse side of the porous cathode
together with the corresponding cations (Na.sup.+, K.sup.+, etc.),
which can form salts and be deposited on the reverse side of the
electrode or else in the pores.
[0047] A further example is the electrolytic reduction of carbon
dioxide. At the cathode, in all modes of operation, according to
the electrode material, different products can form.
EXAMPLES
[0048] Carbon monoxide:
CO.sub.2+2e.sup.-+H.sub.2O.fwdarw.CO+2OH.sup.-
Ethylene:
2CO.sub.2+12e.sup.-+8H.sub.2O.fwdarw.C.sub.2H.sub.4+12OH.sup.-
Methane: CO.sub.2+8e.sup.-+4H.sub.2O.fwdarw.CH.sub.4+4OH.sup.-
Ethanol:
2CO.sub.2+12e.sup.-+9H.sub.2O.fwdarw.C.sub.2H.sub.5OH+12OH.sup.-
-
Monoethylene glycol:
2CO.sub.2+10e.sup.-+8H.sub.2O.fwdarw.HOC.sub.2H.sub.4OH+10OH.sup.-
[0049] Salt deposition in the gas space can then take place by the
process which follows. Hydroxide ions that form in the
abovementioned examples can penetrate through to the reverse side
of the porous cathode together with the corresponding cations
(Na.sup.+, K.sup.+, etc.). In conjunction with CO.sub.2, according
to the pH, the corresponding hydrogencarbonate or carbonate can
precipitate out (salt deposition, salting-out).
[0050] In some embodiments, the electrolyte is collected in a
collecting region in the gas space and the collected electrolyte is
sucked out of it in order that no electrolyte remains in the gas
space, and so no salt is deposited as a result of removal of
solvent. The collecting region is not particularly restricted here,
provided that it can collect the electrolyte, for example as a
liquid or solution, and provided that the collected electrolyte can
be sucked out of it, for example through an opening or outflow with
an appropriate removal device which is connected to the electrolyte
feed and hence forms a connection between the gas space and the
electrolyte feed. In some embodiments, the collecting region is at
a lower end of the gas space, e.g. below a level (viewed from the
base) of the gas diffusion electrode or below the lower end
thereof, such that the electrolyte, after passing into the gas
space, can flow downward by virtue of gravity, such that it does
not remain for too long at the gas diffusion electrode, for example
the reverse side and/or in pores thereof. The connection between
the gas space and the electrolyte feed is preferably made by means
of an opening or an outflow in the collecting region in the gas
space, preferably at a lower end of the collecting region.
[0051] The connection between the gas space and the electrolyte
feed is not particularly restricted and may be made, for example,
by means of suitable pipes, hoses, etc., e.g. pipes, where the
material may preferably be matched to the material of a recycled
electrolyte that has been collected in the collecting region, and,
for example, also correspond to the material for a feed or feed
device for the electrolyte, which is likewise not particularly
restricted and may take the form of a pipe, for example. In
specific embodiments, the collecting region in the gas space is not
in contact with the gas diffusion electrode. The pipes are not
particularly restricted with regard to their further form, but in
specific embodiments have a circular cross section in order to
enable good transport or flow of the electrolyte.
[0052] In some embodiments, the suction is effected in that the
electrolyte feed exerts a suction effect on the gas space. This can
ensure that not too much electrolyte collects in the collecting
region and hence comes into contact with the gas diffusion
electrode again; in this way, it is also possible to minimize or
prevent any influence on gas supply. In some embodiments, the gas
is not supplied from the collecting region in the gas space, such
that the gas does not flow or bubble through the electrolyte.
Nevertheless, the collecting region is in the gas space, i.e. is
also in contact with the feed for the gaseous substrate.
[0053] In some embodiments, an apparatus which can be used for the
processes described herein thus comprises a collecting region for
electrolyte, following gravity, at the lower end of the gas
diffusion electrode or below that. The gas space and electrolyte
feed may be connected in such a way that the electrolyte feed
exerts a suction effect on the gas space. This can be achieved, for
example, in that the connection is configured in the form of a
Venturi nozzle, Laval nozzle or the like, where the connection is
preferably made in the region in which the respective nozzle is
narrowed and the electrolyte thus has an elevated velocity in the
feed.
[0054] In some embodiments, the suction effect thus arises in that
the connection between the gas space and the electrolyte feed
comprises a Venturi nozzle through which the electrolyte is fed to
the electrolysis cell. In this case too, the connection to the gas
space may be located at a narrowed site in the Venturi nozzle. In
principle, the present approach is based on the principle of the
Venturi nozzle, which is shown in FIG. 8 in schematic form and by
way of example including the connection to the gas space L2. The
principle is based on the fact that the flow rate of a medium
flowing through a pipe is inversely proportional to a varying pipe
cross section. This means that the velocity is at its greatest
where the cross section of the pipe is at its smallest. According
to Bernoulli's law, moreover, in a flowing fluid (gas or liquid), a
rise in velocity is accompanied by a drop in pressure.
[0055] Accordingly, for a nozzle according to FIG. 8,
p.sub.1>p.sub.2 where p.sub.1 is the pressure of the electrolyte
supplied in flow direction upstream of the nozzle and p.sub.2 is
the pressure of the electrolyte in the smallest cross section of
the nozzle and also the connection to the gas space L2, and v.sub.1
is the velocity of the electrolyte in flow direction upstream of
the nozzle and v.sub.2 is the velocity of the electrolyte in the
smallest cross section of the nozzle. It is possible to make use of
this relationship for the suction of the electrolyte out of the
collecting region in the gas space.
[0056] The Venturi nozzle, like a Laval nozzle as well, is not
particularly restricted in its shape, provided that the cross
section of the nozzle at first decreases in flow direction of the
electrolyte supplied. The shape of the cross section is not
particularly restricted and may be round, elliptical, square,
rectangular, triangular, etc., but is round in specific
embodiments. A symmetrical nozzle shape may be useful. In addition,
the Venturi nozzle, like a Laval nozzle as well, is also not
subject to any further restriction in terms of its configuration,
where the connection to the gas space or collecting region in the
gas space is preferably at the narrowest point of the nozzle.
[0057] The conduit L2 (where the pressure p2 exists) is connected
in FIG. 8 to the gas space of the electrolysis cell in which the
electrolyte collects. In some embodiments, the electrolyte is
sucked out of the gas space periodically. This can ensure that the
electrolyte that has passed through the GDE is sucked out
regularly; on the other hand, a sufficiently long period is also
available for electrolysis without interference by the suction. In
specific embodiments, the suction is effected in such a way that
not the entire electrolyte is sucked out of the collecting region
in order to be able to better stabilize the pressure in the system
and to prevent gaseous substrate from passing into the connection.
In specific embodiments, passage of gaseous substrate into the
connection between the gas space and the feed for electrolyte is
inhibited or prevented.
[0058] In order to achieve periodic suction, the periodic suction,
in some embodiments, is effected via a closed-loop control
mechanism that controls the periodic suction. This closed-loop
control mechanism is not subject to any further restriction and is
further described in detail by way of example hereinafter with
reference to various embodiments. By virtue of the closed-loop
control mechanism, in some embodiments, it is especially possible
to seal off or close the connection between the gas space and the
feed for electrolyte from the feed for electrolyte, for example
periodically, for example by means of an occlusion, for instance a
liquid occlusion, that may be present at any point in the
connection between the gas space and the feed for electrolyte, for
example in the form of a valve, for example at a pipe connection
close to the electrolyte feed, for example at a T-piece, or as an
occlusion in an outflow out of the collecting region, for example,
in the gas space, for example even in the form of a float.
[0059] The closed-loop control mechanism may trigger, for example,
opening of the occlusion depending on measurements or sensor data,
for example with reference to a fill level of the electrolyte in
the collecting region, but may also be fully automatic, or else
mechanical or mechanically self-regulating without any need for a
measurement. Thus, the devices described herein, in some
embodiments, comprise at least one closed-loop control mechanism,
e.g. a liquid occlusion and a closed-loop control mechanism that
opens access to the electrolyte stream as soon as a certain fill
level in the collecting region has been attained. The closed-loop
control mechanism here may also be integrated with the liquid
occlusion, for example in the case of use of a float. In some
embodiments, the closed-loop control mechanism is mechanical. This
can minimize the complexity involved in the connection between gas
space and feed for electrolyte.
[0060] In some embodiments, the closed-loop control mechanism
comprises a float which is present in the collecting region in the
gas space and enables outflow to the connection between the gas
space and the electrolyte feed depending on the fill level of the
electrolyte in the collecting region in the gas space, with opening
of the float, for example periodically. It should be noted here
that the gas space must not only adjoin the GGE but may also
encompass another region of the gas feed, for example an
intermediate vessel which may be provided, for example, in the base
direction beneath the GDE and into which the electrolyte that has
passed through can flow.
[0061] In some embodiments, the float takes the form of a cone or
frustocone, for example of a stopper, where the tip of the cone or
the circular face of the frustocone having the smaller size
projects into the connection between the gas space and the feed for
electrolyte. The frustocone can achieve a kind of "wedge shape"
that closes the opening of the connection between the gas space and
the feed for electrolyte in the collecting region of the gas space.
In some embodiments, the float is made of a material which assures
a corresponding occlusion but on the other hand is not attacked by
the electrolyte and/or the gaseous substrate, for example based on
elastomer or thermoset. For adjustment of the density, it is
possible to use ceramic fillers. The density can also be adjusted,
for example, via fluorination of the plastic. Rather than floats,
it is alternatively possible to provide other closure devices such
as flaps etc.
[0062] The float 9 may close off a feed of electrolyte from the
collecting region 2 of the gas space 1 to the connection,
especially of a Venturi nozzle or Venturi unit, in an off state, as
shown in FIGS. 11 to 13, which show various embodiments with the
float 9. As shown in FIG. 11, the pressure difference between the
pressure in the gas chamber p.sub.G and the pressure p2 in the
connection to the Venturi nozzle in an off state exerts an
additional force Fi on the float, caused by the pressure
difference. In flow-through operation, the pressure in the gas
space as a whole (not at the nozzle) is higher than in the
electrolyte. With increasing amount of electrolyte in the
collecting region 2, an upward force F.sub.2 on the float 9 is
generated. The float 9, or another occlusion, is designed such that
the float clears the opening 3a of the connection between the gas
space and the feed of the electrolyte 3 to the Venturi nozzle over
and above a certain fill level. The nozzle sucks the electrolyte
out of the gas space 1 until the float 9 closes the opening again.
The density of the float here is less than that of the electrolyte.
By means of guides 9a that may be made of the same material as the
float, for example, it is possible to assure better occlusion of
the opening 3a.
[0063] The dimensions of the float 9 may be such that the valve
opens twice per minute or less, or even once per minute or less.
The utilization of a float 9 simultaneously assures hysteresis in
the overall system, such that vibrations can be avoided. The float
9 and the feed of the electrolyte in the Venturi nozzle may be
chosen such that the collecting region 2 is not completely emptied.
This is supposed to prevent a gaseous substrate such as CO.sub.2
from additionally being removed from the gas space 1 and the
pressure p.sub.G in the gas space 1 from falling significantly.
[0064] FIG. 12 shows a further embodiment of the gas space 1 with
collecting region 2 in which the float 9, however, has a different
stopper shape where there are adjoining cylinders on either side of
the frustocone in order firstly to achieve better occlusion, but
secondly also to control the buoyancy of the float by altering the
mass of the float, for example. In FIG. 13, the float 9 is in
conical form, which means that the electrolyte can also be recycled
gradually out of the collecting region 2 to the feed of the
electrolyte 3 and hence significant variations in the feed can be
avoided thereby.
[0065] The apparatus with a float 9 can be utilized either in
flow-through operation (with gas supply through the electrode) or
in flow-by operation (with gas supply along the electrode and
diffusion of the gas through the electrode). In flow-through
operation, approximately the integral pressure p.sub.G-p.sub.2 in
the gas space 1 is typically higher, which pushes the float 9, in
addition to its own weight, against the connection to the
electrolyte. It is therefore typically necessary to expend a
somewhat greater force F.sub.2 on the float 9 in order to open the
opening 3a and hence the connection. The force F.sub.2 to be
expended is also determined by the size of the opening 3a.
[0066] In flow-by operation, the integral gas space/electrolyte
pressure of .about.p.sub.G-p.sub.2 in the switched-off electrolysis
system may be about the same. When the system is switched on, the
potentials that occur result in electrolyte flow through the GDE
back into the gas space 1. The float 9 closes the connection by
virtue of the Venturi effect and by virtue of its own weight. Over
and above a certain fill level, the opening 3a is opened and
electrolyte that has passed through is returned back to the
electrolyte circuit.
[0067] In some embodiments, there is a further closed-loop control
mechanism, such as a further occlusion, e.g. a valve, which can be
closed by a closed-loop controller. It is also of course possible
to provide multiple valves. In some embodiments, the suction is
controlled by a valve that controls the connection between the gas
space and the electrolyte feed.
[0068] In some embodiments, the valve is coupled to a fill level
sensor for electrolyte in the gas space via a closed-loop
controller, wherein the valve is controlled with reference to a
measurement by a fill level sensor. The fill level can be measured
here, for example, electronically, optically, by a pressure
measurement, etc., and the fill level sensor is not particularly
restricted.
[0069] A corresponding system with valves is shown in schematic
form in FIGS. 9 and 10. As shown in FIG. 9, the closed-loop control
unit may be accomplished by means of sensors, e.g. electrical
sensors, and corresponding valves 4. The fill level in the
collecting region 2 may be measured here, for example, by means of
a pressure sensor 5. A correspondingly designed closed-loop
controller 6, provided, for example, in the form of a
p.sub.max/p.sub.min valve control unit, opens a valve 4 between the
conduit L2 (not shown) and the gas space with collecting region 2
on exceedance of a fixed upper pressure limit p.sub.max. As a
result of the pressure differential .DELTA.p=p.sub.max-p.sub.2, the
collected electrolyte is drawn back out of the gas space 1 and, for
example, fed back to the electrolyte circuit. If the pressure in
the gas space 1 goes below a pressure p.sub.min, the valve 4 is
closed again. For instance, the level of the electrolyte that
passes through the GDE into the gas chamber can be kept at a
predefined level and hence salt formation on the reverse side of
the GDE can be prevented. In some embodiments, the fill level can
alternatively be measured via a magnetic float 7 and reed switch 8,
as shown by way of example in FIG. 10.
[0070] In some embodiments, the salt concentration in the
electrolyte is chosen such that no salt deposition takes place
during operation, i.e. during electrolysis. This concentration can
be suitably determined, for example, in accordance with the
solubility of a conductive salt etc. in the electrolyte.
[0071] In some embodiments, an apparatus for continuous operation
of an electrolysis cell with a gaseous substrate, comprising an
electrolysis cell comprises: an anode, a cathode, wherein at least
one of the anode and cathode takes the form of a gas diffusion
electrode, a cell space which is designed to be filled with an
electrolyte and into which the anode and cathode are at least
partly introduced; a feed for electrolyte designed to feed the cell
space with the electrolyte; a gas space designed to feed the gas
diffusion electrode with a gaseous substrate, wherein the gas space
is provided on a side of the gas diffusion electrode remote from
the cell space; a feed for a gaseous substrate designed to feed the
gas space with the gaseous substrate; a collecting region in the
gas space designed to collect electrolyte in the gas space; and a
connection between the gas space and the feed for electrolyte,
designed to remove electrolyte that has collected in the collecting
region in the gas space from said gas space.
[0072] The apparatus described is suitable for all electrolysis
cells with a gaseous substrate (CO.sub.2, CO) and a gas diffusion
electrode. In specific embodiments, the apparatus serves for
CO.sub.2 electrolysis to CO or hydrocarbons. The apparatus can be
used to conduct the processes described herein. The feed for
electrolyte, the gas space, the collecting region in the gas space
and the connection between the gas space and the feed for
electrolyte have already been discussed in connection with the
process and hence correspond to those discussed above. Beyond that,
these are not particularly restricted.
[0073] The feed for the gaseous substrate designed to feed the gas
space with the gaseous substrate is not particularly restricted
either, provided that it is capable of supplying gas and is
preferably not impaired by the gas, and may be designed, for
example, as a pipe, hose or the like. Furthermore, the apparatus
may also have a removal device for electrolyte and/or a liquid or
dissolved product and/or a removal device for a gaseous product
and/or unconsumed gaseous substrate, which are not particularly
restricted.
[0074] In some embodiments, the electrolysis cell comprises at
least an anode and a cathode, at least one of which takes the form
of a gas diffusion electrode, and a cell space designed to be
filled with an electrolyte and into which the anode and the cathode
have been at least partly introduced. In some embodiments, both the
anode and the cathode take the form of a gas diffusion electrode.
In some embodiments, the anode takes the form of a gas diffusion
electrode. In some embodiments, the cathode takes the form of a gas
diffusion electrode. In some embodiments, carbon dioxide or carbon
monoxide is electrolytically converted at the cathode, i.e. the
cathode is designed such that it can convert carbon dioxide, for
example in the form of a copper-containing (CO.sub.2, CO) and/or
silver-containing (CO.sub.2) gas diffusion electrode.
[0075] The electrolysis cells used correspond, for example, to
those of the prior art that are shown in schematic form in FIGS. 1
to 5; the figures show cells with a membrane M, which may also be
absent in the apparatus of the invention, but is employed in
specific embodiments, and which can separate an anode space I and a
cathode space II. If such a membrane is present, this is not
particularly restricted and is matched, for example, to the
electrolysis, for example the electrolyte and/or the anode reaction
and/or cathode reaction.
[0076] The electrochemical reduction, for example of CO.sub.2,
takes place in an electrolysis cell which typically consists of an
anode and a cathode space. FIGS. 1 to 5 show examples of a possible
cell arrangement. For each of these cell arrangements, it is
possible to use a gas diffusion electrode described herein, for
example as cathode.
[0077] By way of example, the cathode space II in FIGS. 1 and 2 is
configured such that a catholyte is supplied from below and then
leaves the cathode space II in the upward direction. In some
embodiments, the catholyte can be supplied from above, as in the
case of falling-film electrodes for example. At the anode A, which
is electrically connected to the cathode K by means of a power
source for provision of the potential for the electrolysis, the
oxidation of a substance which is supplied from below, for example
with an anolyte, takes place in the anode space I, and the anolyte
with the product of the oxidation then leaves the anode space. In
the 3-chamber construction shown in FIGS. 1 and 2, a reaction gas,
for example carbon dioxide, can be conveyed into the cathode space
II for reduction through the gas diffusion electrode, here by way
of example the cathode K (not shown in detail as the GDE), by way
of example in flow-by operation as in FIG. 1 or in flow-through
operation in FIG. 2. In some embodiments, there is one or more
porous anodes A.
[0078] In FIGS. 1 and 2, the spaces I and II are separated by a
membrane M. By contrast, in the PEM (proton or ion exchange
membrane) construction of FIG. 3, the gas diffusion electrode as
cathode K (likewise not shown in detail as the GDE) and a porous
anode A directly adjoin the membrane M, which results in separation
of the anode space I from the cathode space II. The construction in
FIG. 4 corresponds to a mixed form of the construction from FIG. 2
and the construction from FIG. 3, with provision of a construction
with the gas diffusion electrode and gas feed G in flow-through
operation on the catholyte side, as shown in FIG. 2, whereas a
construction as in FIG. 3 is provided on the anolyte side.
[0079] In some embodiments, mixed forms or other configurations of
the electrode spaces described by way of example are also
conceivable. Embodiments without a membrane are also conceivable.
In some embodiments, the electrolyte on the cathode side and the
electrolyte on the anode side may thus be identical, and the
electrolysis cell/electrolysis unit may not need a membrane.
However, it is not ruled out that the electrolysis cell in such
embodiments has a membrane, although this is associated with
additional expenditure with regard to the membrane and also the
potential applied. Catholyte and anolyte may also optionally be
mixed again outside the electrolysis cell.
[0080] FIG. 5 corresponds to the construction of FIG. 4, except
that, again, as in FIG. 1, the gas supply G here takes place in
flow-by operation and the passage of reactants and products E and P
is shown.
[0081] FIGS. 1 to 5 are schematic diagrams. The electrolysis cells
from FIGS. 1 to 5 may also be combined to form mixed variants. For
example, the anode space may be designed as a PEM half-cell, as in
FIG. 3, while the cathode space consists of a half-cell that
includes a certain electrolyte volume between membrane and
electrode, as shown in FIG. 1. The membrane may also have a
multilayer design, such that separate feeds of anolyte or catholyte
are enabled. Separation effects are achieved in aqueous
electrolytes, for example, via the hydrophobicity of interlayers.
Conductivity can nevertheless be assured when conductive groups are
integrated into such separation layers. The membrane may be an
ion-conducting membrane, or a separator that brings about merely
mechanical separation and is permeable to cations and anions.
[0082] The use of the gas diffusion electrode described herein
makes it possible to construct a three-phase electrode. For
example, a gas can be guided from the rear to the electrically
active front side of the electrode in order to conduct an
electrochemical reaction there. In some embodiments, the gas may
also merely flow by the gas diffusion electrode, meaning that a gas
such as CO.sub.2 is guided past the rear of the gas diffusion
electrode in relation to the electrolyte, in which case the gas can
penetrate through the pores of the gas diffusion electrode and the
product can be removed at the back. In some embodiments, the gas
flow in the flow-by regime is in the reverse direction to the flow
of the electrolyte, in order that a liquid that has been forced
through, such as electrolyte, can be transported away.
[0083] In specific embodiments, the connection between the gas
space and the feed for electrolyte comprises a Venturi nozzle or
another nozzle, for instance a Laval nozzle, preferably a Venturi
nozzle.
[0084] In some embodiments, the apparatus may further comprise a
closed-loop control mechanism designed to control the removal of
the electrolyte from the collecting region in the gas space. This
closed-loop control mechanism is not subject to any further
restriction and corresponds, for example, to the descriptions given
in connection with the processes described herein. By virtue of the
closed-loop control mechanism, it is especially possible, in some
embodiments, to seal off or close the connection between the gas
space and the feed for electrolyte from the feed for electrolyte,
for example periodically, for example by means of an occlusion, for
instance a liquid occlusion, that may be present at any point in
the connection between the gas space and the feed for electrolyte,
for example in the form of a valve, for example at a pipe
connection close to the electrolyte feed, for example at a T-piece,
or as an occlusion in an outflow out of the collecting region, for
example, in the gas space, for example even in the form of a float.
The closed-loop control mechanism may trigger, for example, opening
of the occlusion depending on measurements or sensor data, for
example with reference to a fill level of the electrolyte in the
collecting region, but may also be fully automatic, or else
mechanical without any need for a measurement.
[0085] Thus, in some embodiments, there is at least one closed-loop
control mechanism, e.g. a liquid occlusion and a closed-loop
control mechanism that opens access to the electrolyte stream as
soon as a certain fill level in the collecting region has been
attained. The closed-loop control mechanism here may also be
integrated with the liquid occlusion, for example in the case of
use of a float. In some embodiments, the closed-loop control
mechanism comprises a float which is provided in the collecting
region in the gas space and is designed to periodically stop the
connection between the gas space and the feed for electrolyte. The
float here may be of any desired design provided that it can stop
the connection between the gas space and the feed for electrolyte.
In some embodiments, the float takes the form of a cone or
frustocone, where the tip of the cone or the circular face of the
frustocone having the smaller size projects into the connection
between the gas space and the feed for electrolyte.
[0086] In some embodiments, the closed-loop control mechanism
comprises a valve in the connection between the gas space and the
feed for electrolyte, coupled to a fill level sensor in the gas
space and a closed-loop controller, wherein the fill level sensor
and the closed-loop controller are designed to control the valve in
the connection between the gas space and the feed for electrolyte
depending on the fill level of the electrolyte in the gas
space.
[0087] In some embodiments, the apparatus comprises multiple
electrolysis cells or a stack of electrolysis cells, in each of
which at least one of the anode and cathode takes the form of a gas
diffusion electrode, in which case each of these electrolysis cells
has at least one gas space connected in each case either to one
feed for electrolyte for all cells or to multiple feeds for
electrolyte for all cells, for example including separate feeds for
electrolyte for each cell, via a connection between the gas space
and the corresponding feed for electrolyte. The multiple
electrolysis cells may then be combined to form a cell stack (e.g.
100 or more cells) in order to save space. In the case of such a
stack, especially for reasons of space, the use of floats as
control mechanism or self-regulating system is advantageous. In
specific embodiments, it is thus also possible to employ
apparatuses having multiple electrolysis cells or cell stacks
having, for example, 100 or more cells, where the use of floats is
likewise advantageous here for the control of the connection
between the gas space and the corresponding feed for
electrolyte.
[0088] In some embodiments, the apparatus may comprise further
constituents present in an electrolysis plant, i.e. not only the
power source for the electrolysis but also various cooling and/or
heating devices etc. These further constituents of the apparatus,
for example of an electrolysis plant, are not subject to any
further restriction and may be provided in a suitable manner.
[0089] The above embodiments, configurations, and developments can,
if viable, be combined with one another as desired. Further
possible configurations, developments, and implementations of the
teachings herein also include combinations that have not been
mentioned explicitly or features that have been described above or
are described hereinafter for the working examples. More
particularly, the person skilled in the art will also add
individual aspects as improvements or supplementations to the
respective basic form of the present teachings. The description
hereinafter makes reference to some illustrative embodiments, but
these do not restrict the scope of the teachings.
EXAMPLES
[0090] All experiments, and also the comparative examples and
examples, were conducted at room temperature of about 20.degree.
C.-25.degree. C., unless stated otherwise.
Comparative Example 1
[0091] An apparatus for CO.sub.2 electrolysis without connection of
electrolyte feed and gas feed is shown in FIG. 6.
[0092] An electrolysis is shown, in which carbon dioxide is reduced
on the cathode side and water is oxidized on the anode side A. On
the anode side, it would be possible, for example, for a reaction
of chloride to give chlorine, bromide to give bromine, sulfate to
give peroxodisulfate (with or without evolution of gas), etc. to
take place. Platinum is an example of a suitable anode A, and
copper of a suitable cathode K. The two electrode spaces of the
electrolysis cell are separated by a membrane M made of
Nafion.RTM.. The incorporation of the cell into a system with an
anolyte circuit 10 and catholyte circuit 20 is shown in FIG. 6
without a gas diffusion electrode (or FIG. 7 with the gas diffusion
electrode; see comparative example 2).
[0093] On the anode side, water with electrolyte additions is fed
into an electrolyte reservoir vessel 12 via an inlet 11. However,
it is not ruled out that water is supplied additionally or instead
of the inlet 11 at another point in the anolyte circuit 10, since,
according to FIG. 6, the electrolyte reservoir vessel 12 can also
be used for gas separation. The electrolyte is pumped out of the
electrolyte reservoir vessel 12 by means of the pump 13 into the
anode space, where it is oxidized. The product is then pumped back
into the electrolyte reservoir vessel 12, where it can be removed
to the product gas vessel 26. Via a product gas outlet 27, the
product gas can be withdrawn from the product gas vessel 26. The
product gas can of course also be removed elsewhere. The result is
thus an anolyte circuit 10 since the electrolyte is circulated on
the anode side.
[0094] On the cathode side, in the catholyte circuit 20, carbon
dioxide is introduced via a CO.sub.2 inlet 22 into an electrolyte
reservoir vessel 21, where it is physically dissolved, for example.
By means of a pump 23, this solution is brought into the cathode
space, where the carbon dioxide is reduced at the cathode K, for
example to CO at a silver cathode. An optional further pump 24 then
pumps the solution containing CO and unconverted CO.sub.2 obtained
at the cathode K further to a vessel for gas separation 25, where
the product gas comprising CO can be removed into a product gas
vessel 26. Via a product gas outlet 27, the product gas can be
removed from the product gas vessel 26. The electrolyte is in turn
pumped from the vessel for gas separation back to the electrolyte
reservoir vessel 21, where carbon dioxide can be added again. Here
too, merely an illustrative arrangement of a catholyte circuit 20
is specified, where the individual apparatus components of the
catholyte circuit 20 may also be arranged differently, for example
in that the gas separation is already effected in the cathode
space.
[0095] In some embodiments, the gas separation and the gas
saturation are effected separately; in other words, in one of the
vessels, the electrolyte is saturated with CO.sub.2 and then pumped
through the cathode space as a solution without gas bubbles. The
gas that exits from the cathode space then consists predominantly
of CO since CO.sub.2 itself remains dissolved since it has been
consumed and hence the concentration in the electrolyte is somewhat
lower. The electrolysis is effected in FIG. 6 by addition of
current via a power source which is not shown.
[0096] In order to feed the electrolyte and the CO.sub.2 dissolved
in the electrolyte to the electrolysis unit with variable pressure
over time, valves 30 are included in the anolyte circuit 10 and
catholyte circuit 20, which are controlled by a control device
which is not shown and hence control the feed of anolyte and
catholyte to the anode and cathode respectively, as a result of
which the feed is effected with variable pressure and product gas
can be purged out of the respective electrode cells.
[0097] The valves 30 are shown in the figure upstream of the inlet
into the electrolysis cell, but may also be provided, for example,
downstream of the outlet from the electrolysis cell and/or at other
points in the anolyte circuit 10 or catholyte circuit 20. It is
also possible, for example, for a valve 30 to be present in the
anolyte circuit upstream of the inlet into the electrolysis cell,
while the valve in the catholyte circuit 20 is beyond the
electrolysis cell, or vice versa.
[0098] In the operation of the cell, there was salt formation at
the cathode K.
Comparative Example 2
[0099] The apparatus shown in FIG. 7 corresponds to the apparatus
in comparative example 1, where the cathode K here takes the form
of a flow-through gas diffusion electrode. In the operation of the
cell, there was likewise salt formation at the cathode K.
Example 1
[0100] The construction in example 1 corresponds to that in
comparative example 2, except that the valves 30 are dispensed with
and a Venturi nozzle is provided in the catholyte circuit 20 in
place thereof, which is connected to the feed for CO.sub.2 in the
gas space in accordance with the construction in FIG. 11. Under
"normal" operating conditions, operability over 1000 h was
demonstrated.
[0101] In the operation of customary electrolysis cells, it has
been found that electrolyte passes through gas diffusion electrodes
(GDE) and leads to electrolyte collection in the gas space. Both in
the case of flow-through operation and flow-by operation, the
electrolyte loses solvent, especially water, as a result of the gas
flow, as a result of which depositions of salt occur in the gas
space and particularly disadvantageously in the GDE itself. These
lead to loss of selectivity and ultimately to failure of the
electrode or electrolyzer.
[0102] This problem can be solved by supplementing the electrolysis
cell with a stackable, e.g. purely mechanical connection between
gas space and electrolyte feed, which can suck out liquid that has
passed through into the gas space, preferably periodically. The
apparatus may comprise a suction unit that works by the "Venturi
principle" and a float that ensures the necessary hysteresis. The
amount of electrolyte that flows through the GDE can be achieved,
for example, via an adjustment of the hydrophilicity of the
GDE.
* * * * *