FIELD
[0001] Arrangements disclosed herein relate to an electrolysis device and an electrolysis
method.
BACKGROUND
[0002] There has been a concern about the depletion of fossil fuels such as petroleum and
coal, and expectations are increasing for sustainable renewable energy. Examples of
the renewable energy include those by solar power generation, hydroelectric power
generation, wind power generation, and geothermal power generation. The amount of
powers generated by these depends on weather, nature conditions, and so on and thus
they are power sources whose outputs vary (variable power sources) and have a problem
of difficulty in stably supplying the power. In light of this, it has been attempted
to adjust power by combining a variable power source and a storage battery. Storing
the power, however, has problems of the cost of the storage battery and loss during
the power storage.
[0003] Also drawing attention as decarbonization attempts are: water electrolysis technology
that electrolyzes water (H
2O) to produce hydrogen (H
2); and carbon dioxide electrolysis technology that electrolyzes carbon dioxide (CO
2) and electrochemically reduces it to convert it to a chemical substance (chemical
energy) such as a carbon compound such as carbon monoxide (CO), formic acid (HCOOH),
methanol (CH
3OH), methane (CH
4), acetic acid (CH
3COOH), ethanol (C
2H
5OH), ethane (C
2H
6), or ethylene (C
2H
4). Connecting a variable power source that uses renewable energy to such an electrolysis
device is advantageous in that power adjustment and hydrogen production, and carbon
dioxide recycling are achieved at the same time.
[0004] As a carbon dioxide electrolysis device, a structure is under consideration, for
example, in which a catholyte and a CO
2 gas are in contact with a cathode and an anolyte is in contact with an anode. Such
a structure will be called a carbon dioxide electrolysis cell here. For example, if
a reaction of producing, for example, CO from CO
2 is caused for a long time using such an electrolysis cell by passing a constant current
to the cathode and the anode, there arise problems of time-dependent deterioration
in cell output such as a decrease in the amount of CO produced and an increase in
cell voltage. One example of the deterioration is a phenomenon that salt originating
in an electrolyte of the solution precipitates in a gas channel to obstruct the flow
of the gas or the like, against which the introduction of a refresh operation to dissolve
the salt is under consideration.
[0005] However, it is becoming clear that the electrolysis device of carbon dioxide (CO
2) or the like undergoes various deterioration phenomena in addition to the phenomenon
of the precipitation of salt in the channel. Further, an electrolysis device of, for
example, nitrogen (N
2) undergoes a deterioration phenomenon unique to N
2 electrolysis, in addition to the same deterioration phenomena as that in the CO
2 electrolysis. This necessitates appropriately setting a determination standard according
to the types of an electrolyte and deterioration, such as continuing the operation
or executing a work of stopping the operation and performing the maintenance of the
electrolysis cell according to an electrolyte or a deteriorated place. Therefore,
it is required to determine cell states such as the state and type of the deterioration
of the electrolysis cell.
SUMMARY
[0006] A problem to be solved by arrangements of the present invention is to provide an
electrolysis device and an electrolysis method that enable the determination of the
state of an electrolysis cell.
[0007] According to arrangements of the present invention, an electrolysis device and an
electrolysis method that enable the determination of the state of an electrolysis
cell are provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008]
FIG. 1 is a diagram illustrating an electrolysis device of an arrangement.
FIG. 2 is a view illustrating an electrolysis cell in a carbon dioxide electrolysis
device of a first arrangement.
FIG. 3 is a chart illustrating a deterioration detection process by the carbon dioxide
electrolysis device of the first arrangement.
FIG. 4 is a diagram illustrating an equivalent circuit model of the electrolysis cell
in the carbon dioxide electrolysis device of the first arrangement.
FIG. 5 is a table illustrating equivalent circuit parameters of the equivalent circuit
model illustrated in FIG. 4.
FIG. 6 is a view illustrating an electrolysis cell in a carbon dioxide electrolysis
device of a second arrangement.
FIG. 7 is a diagram illustrating an equivalent circuit model of an electrolysis cell
in a carbon dioxide electrolysis device of a third arrangement.
FIG. 8 is a table illustrating equivalent circuit parameters of the equivalent circuit
model illustrated in FIG. 7.
FIG. 9 is a diagram illustrating an equivalent circuit model of an electrolysis cell
in a nitrogen electrolysis device of a fourth arrangement.
FIG. 10 is a chart illustrating a design process of an electrolysis device according
to a fifth arrangement.
FIG. 11 is a chart illustrating measurement data and simulation data of CO part current
density Jco and H2 part current density JH2 according to Example 1.
FIG. 12 is a chart illustrating measurement data and simulation data of cell voltage
Vcell, cathode potential Vcm, and anode potential Vam according to Example 1.
FIG. 13 is a chart illustrating measurement data and simulation data of CO Faraday
efficiency FEco and H2 Faraday efficiency H2 according to Example 1.
FIG. 14 is a chart illustrating measurement data and simulation data of cathode output
gases (CO, H2, and CO2) according to Example 1.
FIG. 15 is a chart illustrating measurement data and simulation data of anode output
gases (O2 and CO2) according to Example 1.
DETAILED DESCRIPTION
[0009] An electrolysis device of an arrangement include: an electrolysis cell including
a cathode part to be supplied with a gas or a liquid containing a substance to be
reduced and in which a reduction electrode is disposed, an anode part to be supplied
with a liquid containing a substance to be oxidized and in which an oxidation electrode
is disposed, and a diaphragm provided between the cathode part and the anode part;
a supply power property obtaining unit that obtains a property of power that is to
be supplied to the electrolysis cell; an input gas property obtaining unit that obtains
a property of a gas that is to be input to the electrolysis cell; an electric property
obtaining unit that obtains an electric property of the electrolysis cell; an output
gas property obtaining unit that obtains a property of an output gas of the electrolysis
cell; a temperature control unit that controls a temperature of the electrolysis cell;
a temperature obtaining unit that obtains the temperature of the electrolysis cell;
a data storage unit that stores data from the supply power property obtaining unit,
the input gas property obtaining unit, the electric property obtaining unit, the output
gas property obtaining unit, and the temperature obtaining unit; and a data processing
unit to which the data is sent from the data storage unit and that processes the data
to determine a state of the electrolysis cell.
[0010] Electrolysis devices and electrolysis methods of arrangements will be hereinafter
described with reference to the drawings. In the arrangements below, substantially
the same constituent parts are denoted by the same reference signs and a description
thereof may be partly omitted. The drawings are schematic, and the relation of thickness
and planar dimension, a thickness ratio among parts, and so on may be different from
actual ones.
[0011] FIG. 1 is a diagram illustrating an electrolysis device 1 of an arrangement. The
electrolysis device 1 illustrated in FIG. 1 includes an electrolysis cell 2; a supply
power control unit 3 that controls power that is to be supplied to the electrolysis
cell 2; a supply power property obtaining unit 4 that obtains the properties of the
supply power; a gas/electrolysis solution control unit 5 that controls a gas and an
electrolysis solution that are to be supplied to the electrolysis cell 2; an input
gas property obtaining unit 6 that obtains the properties of the input gas that is
to be supplied; an electric property obtaining unit 7 that obtains the electric properties
of the electrolysis cell 2; an output gas property obtaining unit 8 that obtains the
properties of an output gas of the electrolysis cell 2; a temperature control unit
9 that controls the temperature of the electrolysis cell 2; a temperature obtaining
unit 10 that obtains the temperature of the electrolysis cell 2; a data storage unit
11 that stores data from the supply power property obtaining unit 4, the input gas
property obtaining unit 6, the electric property obtaining unit 7, the output gas
property obtaining unit 8, and the temperature obtaining unit 10; a data processing
unit 12 to which the data is transmitted from the data storage unit 11 and that processes
the transmitted data; and a display unit 13. These parts will be described in detail
below.
[0012] The electrolysis cell 2, which has a structure appropriate for a substance that is
to be electrolyzed by the electrolysis device 1, includes at least a reduction electrode
chamber that is supplied with a gas or a liquid containing a substance to be reduced
and in which a reduction electrode is disposed, an oxidation electrode chamber that
is supplied with a liquid containing a substance to be oxidized and in which an oxidation
electrode is disposed, and a diaphragm provided between the reduction electrode chamber
and the oxidation electrode chamber. Examples of the substance that is to be electrolyzed
by the electrolysis device 1 include carbon dioxide (CO
2), nitrogen (N
2), and water (H
2O). Through the electrolysis and reduction of CO
2, a carbon compound such as carbon monoxide (CO), formic acid (HCOOH), methane (CH
4), methanol (CH
3OH), ethane (C
2H
6), ethylene (C
2H
4), ethanol (C
2H
5OH), formaldehyde (HCHO), or ethylene glycol (C
2H
6O
2) is produced. At the same time with the reduction reaction of CO
2, hydrogen (H
2) is sometimes produced through a reduction reaction of H
2O. In the case where N
2 is electrolyzed to be reduced, ammonia (NH
3) is produced.
(First Arrangement)
[0013] As a first arrangement, an electrolysis device 1 of carbon dioxide (CO
2) will be described with reference to FIG. 1 and FIG. 2. As illustrated in FIG. 2,
out of the electrolysis cell 2 illustrated in FIG. 1, an electrolysis cell 2 (2A)
for electrolyzing CO
2 includes a cathode part (reduction electrode chamber) 24 having a first storage part
(storage tank) 22 for storing a first electrolysis solution 21 containing CO
2 and a reduction electrode (cathode) 23 disposed in the first storage part 22, an
anode part (oxidation electrode chamber) 28 having a second storage part (storage
tank) 26 for storing a second electrolysis solution 25 containing water and an oxidation
electrode (anode) 27 disposed in the second storage part 26, and a diaphragm 29 disposed
between the first storage part 22 and the second storage part 26. The first storage
part 22, the second storage part 26, and the diaphragm 29 form a reaction tank 30.
[0014] The reaction tank 30 is divided into two chambers, the first storage part 22 and
the second storage part 26, by the diaphragm 29 allowing ions such as hydrogen ions
(H
+), hydroxide ions (OH
-), hydrogen carbonate ions (HCO
3-), and carbonate ions (CO
3-) to move therethrough. The reaction tank 30 may be formed of, for example, a white
quartz glass plate, an acrylic resin (PMMA), polystyrene (PS), or the like. The reaction
tank 30 may be partly formed of a light-transmitting material and the other part thereof
may be formed of a resin material. Examples of the resin material include polyether
ether ketone (PEEK), polyamide (PA), polyvinylidene fluoride (PVDF), polyacetal (POM)
(copolymer), polyphenylene ether (PPE), an acrylonitrile-butadiene-styrene copolymer
(ABS), polypropylene (PP), and polyethylene (PE).
[0015] In the first storage part 22, the reduction electrode 23 is disposed and CO
2 is further stored. In the first storage part 22, CO
2 is stored, for example, as the first electrolysis solution 21 containing the same.
The first electrolysis solution 21 functions as a reduction electrode solution (catholyte)
and contains carbon dioxide (CO
2) as the substance to be reduced. Here, CO
2 present in the first electrolysis solution 21 need not be gaseous and may be in a
dissolved form or may be in the form of carbonate ions (CO
32-), hydrogen carbonate ions (HCO
3-), or the like. The first electrolysis solution 21 may contain hydrogen ions, and
it is preferably an aqueous solution. In the second storage part 26, the oxidation
electrode 27 is disposed and the second electrolysis solution 25 containing water
is further stored. The second electrolysis solution 25 functions as an oxidation electrode
solution (anolyte) and contains, as the substance to be oxidized, water (H
2O), chloride ions (Cl
-), carbonate ions (CO
32-), hydrogen carbonate ions (HCO
3-), or the like, for instance. The second electrolysis solution 25 may be an alcohol
aqueous solution or an aqueous solution of an organic substance such as amine.
[0016] Varying the amounts of water contained in the first and second electrolysis solutions
21, 25 or changing the electrolysis solution components can change reactivity to change
the selectivity of the substance to be reduced or a ratio of produced chemical substances.
The first and second electrolysis solutions 21, 25 may contain a redox couple as required.
Examples of the redox couple include Fe
3+/Fe
2+ and IO
3-/I
-. The first storage part 22 is connected to a gas supply channel 31 that supplies
a source gas containing CO
2 and a first solution supply channel 32 that supplies the first electrolysis solution
21 and is further connected to a first gas and solution discharge channel 33 that
discharges a reaction gas and the first electrolysis solution 21. The second storage
part 26 is connected to a second solution supply channel 34 that supplies the second
electrolysis solution 25 and is further connected to a second gas and solution discharge
channel 35. The first and second storage parts 22, 26 may each have a space for storing
gases contained in a reactant and a product.
[0017] The pressures in the first and second storage parts 22, 26 are preferably set to
pressures at which CO
2 does not liquefy. Specifically, their pressures are preferably adjusted to, for example,
a range of not lower than 0.1 MPa nor higher than 6.4 MPa. If the pressures in the
storage parts 22, 26 are lower than 0.1 MPa, the efficiency of the CO
2 reduction reaction may decrease. If the pressures in the storage parts 22, 26 exceed
6.4 MPa, CO
2 liquefies, and the efficiency of the CO
2 reduction reaction may decrease. A differential pressure between the first storage
part 22 and the second storage part 26 may cause the breakage or the like of the diaphragm
29. Therefore, the difference between the pressures of the first storage part 22 and
the second storage part 26 (differential pressure) is preferably 1 MPa or lower.
[0018] The lower the temperatures of the electrolysis solutions 21, 25, the larger the dissolution
amount of CO
2, but low temperatures result in high solution resistance and a high theoretical voltage
of the reaction and thus are disadvantageous from a viewpoint of the CO
2 electrolysis. On the other hand, high temperatures of the electrolysis solutions
21, 25 result in a small dissolution amount of CO
2 but are advantageous from a viewpoint of the CO
2 electrolysis. Therefore, the operating temperature condition of the electrolysis
cell 2Ais preferably in a mid-temperature range, for example, in a range of not lower
than the atmospheric temperature nor higher than the boiling points of the electrolysis
solutions 21, 25. In the case where the electrolysis solutions 21, 25 are aqueous
solutions, a temperature of not lower than 10°C nor higher than 100°C is preferable
and a temperature of not lower than 25°C nor higher than 80°C is more preferable.
The operation under higher temperatures is allowed in the case where the source gas
containing CO
2 is filled in the first storage part 22 and water vapor is filled in the second storage
part 26. In this case, the operating temperature is decided in consideration of the
heat resistance of members such as the diaphragm 29. In the case where the diaphragm
29 is an ion exchange membrane or the like, the maximum operating temperature is 180°C,
and in the case where the diaphragm 29 is a polymeric porous membrane such as Teflon
(registered trademark), the maximum temperature is 300°C.
[0019] The first electrolysis solution 21 and the second electrolysis solution 25 may be
electrolysis solutions containing different substances or may be the same electrolysis
solutions containing the same substance. In the case where the first electrolysis
solution 21 and the second electrolysis solution 25 contain the same substance and
the same solvent, the first electrolysis solution 21 and the second electrolysis solution
25 may be regarded as one electrolysis solution. Further, pH of the second electrolysis
solution 25 may be higher than pH of the first electrolysis solution 21. This facilitates
the movement of ions such as hydrogen ions or hydroxide ions through the diaphragm
29. This also achieves the effective progress of the redox reaction owing to a liquid
junction potential due to the pH difference.
[0020] The first electrolysis solution 21 is preferably a solution high in CO
2 absorptance. CO
2 does not necessarily have to be in a dissolved form in the first electrolysis solution
21, and CO
2 in a bubble form may be mixed and present in the first electrolysis solution 21.
Examples of the electrolysis solution containing CO
2 include aqueous solutions containing hydrogen carbonate or carbonate such as lithium
hydrogen carbonate (LiHCO
3), sodium hydrogen carbonate (NaHCO
3), potassium hydrogen carbonate (KHCO
3), cesium hydrogen carbonate (CsHCO
3), sodium carbonate (Na
2CO
3), and potassium carbonate (K
2CO
3), phosphoric acid, boric acid, or the like. The electrolysis solution containing
CO
2 may contain any of alcohols such as methanol, ethanol, and acetone or may be an alcohol
solution. The first electrolysis solution 21 may be an electrolysis solution containing
a CO
2 absorbent that lowers the reduction potential of CO
2, has high ion conductivity, and absorbs CO
2.
[0021] The second electrolysis solution 25 may be a solution containing water (H
2O), for example, an aqueous solution containing a desired electrolyte. This solution
is preferably an aqueous solution that promotes an oxidation reaction of water. Examples
of the aqueous solution containing the electrolyte include aqueous solutions containing
phosphate ions (PO
43-), borate ions (BO
33-), sodium ions (Na
+), potassium ions (K
+), calcium ions (Ca
2+), lithium ions (Li
+), cesium ions (Cs
+), magnesium ions (Mg
2+), chloride ions (Cl
-), hydrogen carbonate ions (HCO
3-), carbonate ions (CO
32-), hydroxide ions (OH
-), or the like.
[0022] As the aforesaid electrolysis solutions 21, 25, an ionic liquid that is composed
of salt of cations such as imidazolium ions or pyridinium ions and anions such as
BF
4- or PF
6- and that is in a liquid form in a wide temperature range, or an aqueous solution
thereof is usable, for instance. Other examples of the electrolysis solutions include
solutions of amine such as ethanolamine, imidazole, or pyridine and aqueous solutions
thereof. Examples of the amine include primary amine, secondary amine, and tertiary
amine. These electrolysis solutions may be high in ion conductivity, have properties
of absorbing carbon dioxide, and have characteristics of decreasing reduction energy.
[0023] Examples of the primary amine include methylamine, ethylamine, propylamine, butylamine,
pentylamine, and hexylamine. A hydrocarbon of the amine may be replaced by alcohol,
halogen, or the like. Examples of the amine whose hydrocarbon is replaced include
methanolamine, ethanolamine, and chloromethylamine. Further, an unsaturated bond may
be present therein. The same applies to hydrocarbons of the secondary amine and the
tertiary amine.
[0024] Examples of the secondary amine include dimethylamine, diethylamine, dipropylamine,
dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine, and dipropanolamine.
The replaced hydrocarbons may be different. This also applies to the tertiary amine.
Examples of one whose hydrocarbons are different include methylethylamine and methylpropylamine.
[0025] Examples of the tertiary amine include trimethylamine, triethylamine, tripropylamine,
tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine,
tributanolamine, triexanolamine, methyldiethylamine, and methyldipropylamine.
[0026] Examples of the cations of the ionic liquid include 1-ethyl-3-methylimidazolium ions,
1-methyl-3-propylimidazolium ions, 1-butyl-3-methylimidazolium ions, 1-methyl-3-pentylimidazolium
ions, and 1-hexyl-3-methylimidazolium ions.
[0027] The second position of the imidazolium ion may be replaced. Examples of the cation
resulting from the replacement of the second position of the imidazolium ion include
a 1-ethyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-propylimidazolium ion, a 1-butyl-2,3-dimethylimidazolium
ion, a 1,2-dimethyl-3-pentylimidazolium ion, and a 1-hexyl-2,3-dimethylimidazolium
ion.
[0028] Examples of the pyridinium ions include methylpyridinium, ethylpyridinium, propylpyridinium,
butylpyridinium, pentylpyridinium, and hexylpyridinium. In the imidazolium ion and
the pyridinium ion, an alkyl group may be replaced, or an unsaturated bond may be
present.
[0029] Examples of the anions include fluoride ions (F
-), chloride ions (Cl
-), bromide ions (Br
-), iodide ions (I
-), BF
4-, PF
6-, CF
3COO
-, CF
3SO
3-, NO
3-, SCN
-, (CF
3SO
2)
3C
-, bis(trifluoromethoxysulfonyl)imide, bis(trifluoroethoxysulfonyl)imide, and bis(perfluoroethylsulfonyl)imide.
The ionic liquid may be composed of dipolar ions formed of the cations and the anions
that are connected by hydrocarbons. Note that a buffer solution such as a potassium
phosphate solution may be supplied to the storage parts 22, 26.
[0030] As the diaphragm 29, a membrane selectively allowing the flow of anions or cations
is used. Consequently, the electrolysis solutions 21, 25 in contact with the reduction
electrode 23 and the oxidation electrode 27 respectively can be electrolysis solutions
containing different substances. Further, it is possible to promote a reduction reaction
and an oxidation reaction owing to a difference in ionic strength, a difference in
pH, and so on. The use of the diaphragm 29 can separate the first electrolysis solution
21 and the second electrolysis solution 25 from each other. The diaphragm 29 may have
a function of allowing the permeation of part of the ions contained in the electrolysis
solutions 21, 25 in which these electrodes 23, 27 are immersed, that is, a function
of shutting off one kind of ions or more contained in the electrolysis solutions 21,
25. As a result, the two electrolysis solutions 21, 25 can be solutions different
in pH, for instance.
[0031] Examples usable as the diaphragm 29 include ion exchange membranes such as NEOSEPTA
(registered trademark) of ASTOM Corporation, SELEMION (registered trademark) of AGC
Inc., Aciplex (registered trademark) of AGC Inc., Fumasep (registered trademark) and
fumapem (registered trademark) of Fumatech BWT GmbH, Nafion (registered trademark)
of DuPont, which is a fluorocarbon resin formed of sulfonated and polymerized tetrafluoroethylene,
lewabrane (registered trademark) of LANXESS AG, IONSEP (registered trademark) of IONTECH
Inc., Mustang (registered trademark) of PALL Corporation, relax (registered trademark)
of MEGA Co., Ltd., and GORE-TEX (registered trademark) of W.L. Gore & Associates GmbH.
The ion exchange membrane may be formed using a membrane whose basic structure is
hydrocarbons, or in the case of anion exchange, it may be a membrane having an amine
group. In the case where the first electrolysis solution 21 and the second electrolysis
solution 25 are different in pH, the use of a bipolar membrane in which a cation exchange
membrane and an anion exchange membrane are stacked makes it possible to keep pH of
the electrolysis solutions stable when they are used.
[0032] Examples usable as the diaphragm 29 other than the ion exchange membrane include:
porous membranes of a silicone resin, a fluorine-based resin such as perfluoroalkoxyalkane
(PFA), a perfluoroethylene-propane copolymer (FEP), polytetrafluoroethylene (PTFE),
an ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF),
polychlorotrifluoroethylene (PCTFE), and an ethylene-chlorotrifluoroethylene copolymer
(ECTFE), polyethersulfone (PES), or ceramic; and insulating porous bodies such as
a glass filter, a filling filled with agar, zeolite, and oxide. A hydrophilic porous
membrane is preferable as the diaphragm 29 because it is not clogged with bubbles.
[0033] The reduction electrode 23 is an electrode (cathode) that reduces carbon dioxide
(CO
2) to produce a carbon compound. The reduction electrode 23 is disposed in the first
storage part 22 to be immersed in the first electrolysis solution 21. The reduction
electrode 23 contains a reduction catalyst for producing the carbon compound through
the reduction reaction of CO
2, for instance. Examples of the reduction catalyst include a material that lowers
activation energy for reducing CO
2. In other words, a material that lowers overvoltage when the carbon compound is produced
through the reduction reaction of CO
2 is usable.
[0034] As the reduction electrode 23, a metal material or a carbon material is usable, for
instance. As the metal material, metal such as gold, aluminum, copper, silver, platinum,
palladium, zinc, mercury, indium, nickel, or titanium, or an alloy containing the
aforesaid metal is usable, for instance. As the carbon material, graphene, carbon
nanotube (CNT), fullerene, ketjen black, or the like is usable, for instance. The
reduction catalyst is not limited to these, and a metal complex such as a Ru complex
or a Re complex, or an organic molecule having an imidazole skeleton or a pyridine
skeleton may be used, for instance. The reduction catalyst may be a mixture of a plurality
of materials. The reduction electrode 23 may have a structure in which the reduction
catalyst in a thin film form, a lattice form, a granular form, a wire form, or the
like is provided on a conductive base material, for instance.
[0035] The carbon compound produced through the reduction reaction in the reduction electrode
23 differs depending on the kind of the reduction catalyst and so on, and examples
thereof include carbon monoxide (CO), formic acid (HCOOH), methane (CH
4), methanol (CH
3OH), ethane (C
2H
6), ethylene (C
2H
4), ethanol (C
2H
5OH), formaldehyde (HCHO), and ethylene glycol (C
2H
6O
2). The reduction electrode 23 sometimes causes a side reaction to produce hydrogen
(H
2) through a reduction reaction of water (H
2O) simultaneously with the reduction reaction of carbon dioxide (CO
2).
[0036] The oxidation electrode 27 is an electrode (anode) that oxidizes the substance to
be oxidized such as the substance or ions contained in the second electrolysis solution
25. For example, it oxidizes water (H
2O) to produce oxygen or a hydrogen peroxide solution or oxidizes chloride ions (Cl
-) to produce chlorine. The oxidation electrode 27 is disposed in the second storage
part 26 to be immersed in the second electrolysis solution 25. The oxidation electrode
27 contains an oxidation catalyst for the substance to be oxidized. As the oxidation
catalyst, used is a material that decreases activation energy for the oxidation of
the substance to be oxidized, in other words, a material that lowers reaction overpotential.
[0037] Examples of such an oxidation catalyst material include metals such as ruthenium,
iridium, platinum, cobalt, nickel, iron, and manganese. Further, binary metal oxide,
ternary metal oxide, quaternary metal oxide, or the like is usable. Examples of the
binary metal oxide include manganese oxide (Mn-O), iridium oxide (Ir-O), nickel oxide
(Ni-O), cobalt oxide (Co-O), iron oxide (Fe-O), tin oxide (Sn-O), indium oxide (In-O),
and ruthenium oxide (Ru-O). Examples of the ternary metal oxide include Ni-Fe-O, Ni-CoO,
La-Co-O, Ni-La-O, and Sr-Fe-O. Examples of the quaternary metal oxide include Pb-Ru-Ir-O
and La-Sr-Co-O. The oxidation catalyst is not limited to these and may be metal hydroxide
containing cobalt, nickel, iron, manganese, or the like or a metal complex such as
a Ru complex or an Fe complex. A mixture of a plurality of materials may also be used.
[0038] The oxidation electrode 27 may be formed of a composite material containing both
the oxidation catalyst and a conductive material. Examples of the conductive material
include carbon materials such as carbon black, activated carbon, fullerene, carbon
nanotube, graphene, ketjen black, and diamond, transparent conductive oxides such
as indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped
zinc oxide (AZO), and antimony-doped tin oxide (ATO), metals such as Cu, Al, Ti, Ni,
Ag, W, Co, and Au, and an alloy containing at least one of these metals. For example,
the oxidation electrode 27 may have a structure in which the oxidation catalyst in
a thin film form, a lattice form, a granular form, a wire form, or the like is provided
on a conductive base material. As the conductive base material, a metal material containing
titanium, a titanium alloy, or stainless steel is used, for instance.
[0039] The configurations and operations of the data processing unit 12 and so on in the
electrolysis device 1 illustrated in FIG. 1 will be described. The supply power control
unit 3 controls power for causing a redox reaction in the electrolysis cell 2A and
is electrically connected to the reduction electrode 23 and the oxidation electrode
27 of the electrolysis cell 2A. A not-illustrated power source is connected to the
supply power control unit 3. In the supply power control unit 3, electric devices
such as a DC-AC converter, a DC-DC converter, an AC-DC converter, an inverter, a converter,
and switches are installed. The power source connected to the supply power control
unit 3 may be a power source that converts renewable energy to electric energy to
supply it or may be a typical commercial power source, battery, or the like. Examples
of the power source using the renewable energy include a power source that converts
kinetic energy or potential energy such as wind power, water power, geothermal energy,
or tidal power to electric energy, a power source such as a solar cell having a photoelectric
conversion element that converts light energy to electric energy, a power source such
as a fuel cell or a storage battery that converts chemical energy to electric energy,
and a power source that converts vibrational energy such as sound to electric energy.
[0040] The supply power property obtaining unit 4 obtains the properties of the power, that
is, a voltage and a current, that is to be supplied to the electrolysis cell 2A. The
supply power properties obtained by the supply power property obtaining unit 4 are
transmitted to the data storage unit 11 through a signal line. The supply power control
unit 3 and the supply power property obtaining unit 4 may be independent structures
or may be an integrated structure.
[0041] The gas/electrolysis solution control unit 5 controls the flow rates of the CO
2-containing gas and the electrolysis solutions that are to be input to the electrolysis
cell 2A. It may further control the dew point, temperature, pressure, and so on of
the gas and the pressure, temperature, composition, pH, and so on of the electrolysis
solutions. The input gas property obtaining unit 6 obtains the flow rate and composition
of the CO
2-containing gas that is to be input to the electrolysis cell 2A. It may have a function
of obtaining the properties such as the dew point, temperature, pressure, and so on
of the CO
2-containing gas. The obtained input gas properties are transmitted to the data storage
unit 11 through a signal line. The gas/electrolysis solution control unit 5 and the
input gas property obtaining unit 6 may be independent structures or may be an integrated
structure.
[0042] The electric property obtaining unit 7 obtains the electric properties such as cell
voltage and cell current of the electrolysis cell 2A. To improve the accuracy of equivalent
circuit parameters, it is preferable to assemble a reference electrode in the electrolysis
cell 2A to obtain the potentials of the cathode 23 and the anode 27 relative to the
reference electrode. The electric property obtaining unit 7 may have a function of
obtaining the impedance of the electrolysis cell 2A. The electric properties are transmitted
to the data storage unit 11 through a signal line.
[0043] The output gas property obtaining unit 8 obtains the flow rate of the gas output
from the cathode 23 of the electrolysis cell 2A and the concentrations of CO
2 and various gases produced through the CO
2 reduction reaction. It may further have a function of obtaining the concentrations
of H
2 and other gases produced through a side reaction. It may also have a function of
obtaining the flow rates of O
2 and CO
2 which are output from the anode 27, the concentrations of the gases, and so on. The
output gas properties are transmitted to the data storage unit 11 through a signal
line.
[0044] The temperature control unit 9 controls the temperature of the electrolysis cell
2A to a predetermined value and has a function of controlling the heating by a heater
assembled in the electrolysis cell 2A and the flow of a refrigerant to a cooling water
channel. The temperature obtaining unit 10 obtains the temperature of the electrolysis
cell 2A. The obtained temperature is transmitted to the data storage unit 11 through
a signal line. The temperature control unit 9 and the temperature obtaining unit 10
may be independent structures or may be an integrated structure.
[0045] The data storage unit 11 includes a control device such as a computer and has a function
of storing data in a recording medium such as a memory, a hard disk, or SSD and a
data transceiving function. The display unit 13 is a display and has a function of
displaying information sent from the data storage unit 11 and a deterioration detecting
unit. The data storage unit 11 and the display unit 13 may be independent structures
or may be an integrated structure such as a computer.
[0046] The data processing unit 12 includes a computer such as PC or a microcomputer, for
instance, and based on the data transmitted from the data storage unit 11, calculates
an equivalent circuit model and equivalent circuit parameters. The data processing
unit 12 performs the inference of a deteriorated place, the calculation of a deterioration
degree, and so on using the equivalent circuit parameters. It further determines whether
to continue the operation of the CO
2 electrolysis cell 2A, whether to execute its refresh operation, or whether to stop
its operation, based on the information of the deteriorated place and the deterioration
degree, and transmits commands to the supply power control unit 3, the gas/electrolysis
solution control unit 5, and the temperature control unit 9. The data processing unit
12 may be installed near the electrolysis cell 2A or may be installed in the cloud
to perform remote diagnosis. Installing the data processing unit 12 in the cloud enables
the integrated management of stored data of the electrolysis cells 2A installed at
various places to improve the accuracy of deterioration detection. Because of this,
the data processing unit 12 is preferably installed in the cloud.
[0047] Next, a deterioration detecting method of the electrolysis device 1 will be described
with reference to FIG. 3. First, database of an equivalent circuit model and equivalent
circuit parameters of the electrolysis cell 2A at design time is created and stored
in the data processing unit 12. Another method to create the database is to test-operate
the electrolysis cell 2A using the supply power control unit 3 and the gas/electrolysis
solution control unit 5, and calculate an equivalent circuit model before real operation,
obtain its circuit parameters, and create their database (S 1). The real operation
of the electrolysis cell (or the electrolysis cell stack) 2 is started, the supply
power properties, the input gas properties, and the electric properties, output gas
properties, and temperature of the electrolysis cell 2A are obtained, and these data
are stored in the data storage unit 11 (S2). These data are transmitted to the data
processing unit 12, and the data processing unit 12 performs an arithmetic operation
to calculate an equivalent circuit model and equivalent circuit parameters (S3).
[0048] The data processing unit 12 compares the equivalent circuit model and its equivalent
circuit parameters at the design time or before the real operation which are obtained
at S 1 with the equivalent circuit model and the equivalent circuit parameters during
the real operation to infer a deteriorated place and calculate a deterioration degree.
From the development of the deterioration degree, it further predicts a life span
up to the stop of the operation (S4). It is determined whether or not the deterioration
degree exceeds an operation stop standard (S5), and in the case where the deterioration
degree exceeds the operation stop standard, the real operation of the electrolysis
cell (or the electrolysis cell stack) 2 is stopped (S6). The maintenance appropriate
for the deteriorated place is performed or the electrolysis cell 2A is changed when
the operation is stopped (S7). The various property data stored in the data storage
unit 11 at S2, the equivalent circuit model and the equivalent circuit parameters
which are calculated at S3, and the deteriorated place, the deterioration degree,
and the life span up to the operation stop which are calculated at S4 are transmitted
to the display unit 13, and are displayed on the display unit 13 (S8).
[0049] Next, the operation of the electrolysis device 1 of CO
2 will be described. The description here is about the case where using an aqueous
solution containing CO
2 and an aqueous potassium hydrogen carbonate (KHCO
3) solution as the electrolysis solutions 21, 25, mainly carbon monoxide (CO) is produced
through the reduction of CO
2 and oxygen is produced through the oxidation of water (H
2O) or hydroxide ions (OH
-). The reduction reaction of CO
2 is not limited to the reaction of producing CO and may be a reaction of producing
C
xH
yO
z, specifically, a carbon compound such as formic acid (HCOOH), methane (CH
4), methanol (CH
3OH), ethane (C
2H
6), ethylene (C
2H
4), ethanol (C
2H
5OH), formaldehyde (HCHO), or ethylene glycol (C
2H
6O
2).
[0050] When a voltage equal to or higher than an electrolysis voltage is applied across
the reduction electrode (cathode) 23 and the oxidation electrode (anode) 27, a reduction
reaction of CO
2 occurs near the reduction electrode 23 in contact with the first electrolysis solution
21. As shown by the following equation (1), electrons (e-) supplied from the power
source reduce CO
2 contained in the first electrolysis solution 21 to produce CO and OH
-. As shown by the equation (2) and the equation (3), part of the produced OH
- reacts with CO
2, resulting in the production of hydrogen carbonate ions (HCO
3-) or carbonate ions (CO
32-). The voltage across the reduction electrode 23 and the oxidation electrode 27 causes
part of OH
-, HCO
3-, and CO
32- to move into the second electrolysis solution 25 through the diaphragm 29.
2CO
2 + 2H
2O + 4e
- →2CO + 4OH
- ... (1)
2CO
2 + 2OH
- → 2HCO
3- ... (2)
2HCO
3- + 2OH
- → CO
32- + H
2O ... (3)
[0051] Near the oxidation electrode 27 in contact with the second electrolysis solution
25, an oxidation reaction of water (H
2O) occurs. As shown by the following equation (4), the oxidation reaction of H
2O contained in the second electrolysis solution 25 occurs, electrons are lost, and
oxygen (O
2) and hydrogen ions (H
+) are produced.
2H
2O → 4H + O
2 + 4e
- ... (4)
[0052] As shown by the following equation (5) to equation (7), part of the produced hydrogen
ions (H
+) reacts with part of hydroxide ions (OH
-), hydrogen carbonate ions (HCO
3-), or carbonate ions (CO
32-) which have moved through the diaphragm 29, resulting in the production of H
2O and CO
2.
2H
+ + CO
32- → H
2O + CO
2 (5)
2H
+ + 2HCO
3- → 2H
2O + 2CO
2 (6)
H
+ + OH
- → H
2O (7)
[0053] The above describes the operation based on the production of OH
- in the reduction electrode 23, but the operation may be based on the production and
movement of H
+ in the oxidation electrode 27. When a voltage equal to or higher than the electrolysis
voltage is applied across the reduction electrode 23 and the oxidation electrode 27,
an oxidation reaction of water (H
2O) occurs near the oxidation electrode 27 in contact with the second electrolysis
solution 25. As shown by the following equation (8), the oxidation reaction of H
2O contained in the second electrolysis solution 25 occurs, electrons are lost, and
oxygen (O
2) and hydrogen ions (H
+) are produced. The produced hydrogen ions (H
+) partly move into the first electrolysis solution 21 through the diaphragm 29.
2H
2O → 4H
+ + O
2 + 4e
- (8)
[0054] When the hydrogen ions (H
+) produced in the oxidation electrode 27 side reach the vicinity of the reduction
electrode 23 and electrons (e
-) are supplied to the reduction electrode 23 from the power source, a reduction reaction
of carbon dioxide (CO
2) occurs. As shown by the following equation (9), the hydrogen ions (H
+) having moved to the vicinity of the reduction electrode 23 and the electrons (e-)
supplied from the power source reduce CO
2 contained in the first electrolysis solution 21 to produce carbon monoxide (CO).
2CO
2 + 4H
+ + 4e
- → 2CO + 2H
2O (9)
[0055] The data storage unit 11 illustrated in FIG. 1 is configured to store data from the
supply power property obtaining unit 4, the input gas property obtaining unit 6, the
electric property obtaining unit 7, the output gas property obtaining unit 8, and
the temperature obtaining unit 10. The data processing unit 12 is configured to receive
the aforesaid data from the data storage unit 11 and process the data to determine
the state of the electrolysis cell 2A. Specifically, the data processing unit 12 calculates
the equivalent circuit model and the equivalent circuit parameters of the electrolysis
cell 2A based on the processing results of the data and determines the state of the
electrolysis cell 2A based on the calculation results of the equivalent circuit model
and the equivalent circuit parameters. One specific example of the determination of
the state of the electrolysis cell 2A is to infer a deteriorated place or the like
of the electrolysis cell 2A, and another specific example thereof is to calculate
a deterioration degree of the deteriorated place.
[0056] Next, a method of calculating the equivalent circuit model and the equivalent circuit
parameters, a method of inferring the deteriorated place, and a method of calculating
the deterioration degree by the data processing unit 12 will be described with reference
to FIG. 4. FIG. 4 illustrates an example of the equivalent circuit model of the CO
2 electrolysis cell 2A. In the description here, the case where a CO
2 reduction product (C
xH
yO
z) is produced through the reduction reaction of CO
2 will be taken as an example. In the case where CO is produced, C
xH
yO
z can be replaced by CO. In a cathode part 24 in the equivalent circuit model, a C
xH
yO
z producing part and a side reaction H
2 producing part are connected in parallel, and cathode resistance is connected in
series to them. A diaphragm part 29 has diaphragm resistance. In an anode part 28,
an O
2 producing part and anode resistance are connected in series. The cathode part 24,
the diaphragm part 29, and the anode part 29 are connected in series.
[0057] The current densities J
A of the C
xH
yO
z producing part, the H
2 producing part, and the O
2 producing part are represented by the Tafel equation in the following formula (10),
for instance.
[Math. 1]
[0058] In the subscript A, C
xH
yO
z ER (C
xH
yO
z Evolution Reaction) is entered in the case of C
xH
yO
z production, HER (Hydrogen Evolution Reaction) is entered in the case of H
2 production, and OER (Oxygen Evolution Reaction) is entered in the case of O
2 production. J
0, A represents exchange current density, B
A represents Tafel slope, and η
A represents overvoltage. Note that J
0, A and B
A are parameters that vary with temperature T. The cathode resistance is represented
by R
cathode, the diaphragm resistance is represented by R
membrane, the anode resistance is represented by R
anode. These are parameters that vary with temperature.
[0059] In the case where the flow rate of CO
2 introduced to the CO
2 electrolysis cell 2A is high enough, the formula (10) is usable. In the case where
the CO
2 flow rate is low and consideration is given to that the C
xH
yO
z production current density is restricted by the CO
2 flow rate, that is, has a limit, the current density J
CxHyOz ER include low CO2 of the C
xH
yO
z producing part is represented by a relational formula including, as variables, the
Tafel equation in the formula (10) and the C
xH
yO
z production limit current density J
CxHyOz ER, L as shown by the following formula (11). Here, f1 indicates a function.
[Math. 2]
[0060] As shown by the equation (2), the equation (3), the equation (5), and the equation
(6), CO
2 in the cathode side is converted to HCO
3- and CO
32- and they move to the anode side and are converted again to CO
2. Here, the flow rate of CO
2 moving from the cathode side to the anode side as a result of the aforesaid ion movement
is represented by flow
CO2 from cathode to anode. Since the flow rate of CO
2 usable for the production of C
xH
yO
z decreases by an amount corresponding to the flow rate of CO
2 moving from the cathode side to the anode side, the C
xH
yO
z production limit current density J
CxHyOz ER, L is represented by a relational formula including, as variables, the flow rate flow
CO2 cathode, input of CO
2 introduced to the cathode part and the flow rate flow
CO2 from cathode to anode of CO
2 moving from the cathode side to the anode side, as shown by the following formula
(12). Here, f2 indicates a function.
[Math. 3]
[0061] The current I
cell flowing in the electrolysis cell illustrated in FIG. 4 has the following relation
with the current J
CxHyOz ER include low CO2 (this may be J
CxHyOz ER) used for the production of C
xH
yO
z and the current J
HER used for the production of H
2.
[Math. 4]
[0062] "Aelectrode" is an electrode area of the cathode or the anode, and tyically, the
cathode and the anode have the same electrode area in many cases. Further, the cell
voltage V
cell is represented as follows.
[Math. 5]
[0063] E
0OER and E
0CxHyOz are theoretical potentials of O
2 production and C
xH
yO
z production and vary with temperature. R
cathode is the cathode resistance, R
membrane is the diaphragm resistance, and R
anode is the anode resistance.
[0064] Next, FIG. 5 illustrates the equivalent circuit parameters of the equivalent circuit
model illustrated in FIG. 4. Using one of or two or more of the supply power properties,
the input gas properties, and the electric properties, output gas properties, and
temperature of the CO
2 electrolysis cell which are collected before the real operation, the equivalent circuit
parameters of these are calculated by fitting. For the fitting, spreadsheet software
or a circuit simulator is usable. Using the data sent from the data storage unit 11
to the data processing unit 12 during the real operation, the equivalent circuit parameters
are periodically calculated, and the equivalent circuit parameters before the real
operation or at the design time and the equivalent circuit parameters during the real
operation are compared, whereby it is possible to infer a deteriorated place.
[0065] Further, for each of the equivalent circuit parameters, the deterioration degree
D can be calculated based on the following formula (15).
[Math. 6]
[0066] Setting a determination standard for the aforesaid deterioration degree D enables
the determination on operation stop, refresh operation, or maintenance. Further, the
development over time of the deterioration degree D can be represented by a regression
formula which is a linear function shown in the following formula (16), for instance.
[Math. 7]
[0067] In the math expression, "a" is a variation of D per unit time and "b" is an intercept.
The use of the formula (16) enables the estimation of the remaining time up to the
determination standard. An approximate formula of the development over time of the
deterioration degree D is not limited to the formula (16) and may be a quadratic formula
or a polynomial. Further, the remaining time up to the determination standard may
be estimated using machine learning based on database of other electrolysis cell's
equivalent circuit parameters and deterioration degrees stored in the data processing
unit 12.
(Second Arrangement)
[0068] Next, the configuration and a deterioration detecting system of a carbon dioxide
electrolysis device of a second arrangement will be described with reference to FIG.
1 and FIG. 6. The deterioration detecting system of the carbon dioxide electrolysis
device 1 of the second arrangement is the same as the deterioration detecting system
of the first arrangement. In the carbon dioxide electrolysis device 1 of the second
arrangement, a contact form of a gas containing CO
2 (sometimes referred to as a CO
2 gas) with a reduction electrode 23 and a contact form of a second electrolysis solution
(anolyte) containing water with an oxidation electrode 27 in an electrolysis cell
2B are different from those in the electrolysis cell 2A of the first arrangement.
The electrolysis cell 2B of the carbon dioxide electrolysis device 1 of the second
arrangement differs in the configuration from the electrolysis cell 2A according to
the first arrangement. Except for the above, the configurations of its parts, for
example, the specific configurations of the reduction electrode 23, the oxidation
electrode 27, a diaphragm 29, the second electrolysis solution, and so on are the
same as those of the first arrangement.
[0069] The electrolysis cell 2B according to the second arrangement includes the reduction
electrode 23, the oxidation electrode 27, the diaphragm 29, a first channel 36 in
which the gas containing CO
2 flows, a second channel 37 in which the second electrolysis solution (anolyte) containing
water flows, a first current collector plate 38 electrically connected to the reduction
electrode 23, and a second current collector plate 39 electrically connected to the
oxidation electrode 27. The reduction electrode 23 and the first channel 36 facing
it form a cathode part (reduction electrode chamber) 24. The oxidation electrode 27
and the second channel 37 facing it form an anode part (oxidation electrode chamber)
28.
[0070] In the second arrangement, a first electrolysis solution containing CO
2 instead of the gas containing CO
2 may flow in the first channel 36. Another adoptable configuration is to provide a
not-illustrated channel between the reduction electrode 23 and the diaphragm 29, have
the gas containing CO
2 flow in the first channel 36, and have the first electrolysis solution flow in the
channel between the reduction electrode 23 and the diaphragm 29. The first electrolysis
solution used in this case may contain CO
2 or may be one not containing CO
2. Further, instead of the second electrolysis solution containing water, a gas containing
water vapor is also usable.
[0071] During the operation of the electrolysis cell 2B, the supply of the gas containing
CO
2 is sometimes stopped because the first channel 36 is clogged when a reduction product
of CO
2 or a component of the second electrolysis solution having moved to the reduction
electrode 23 side solidifies to precipitate in the first channel 36. Therefore, in
order to inhibit the formation of the precipitates, the gas containing CO
2 preferably contains moisture. However, too large a moisture content in the gas containing
CO
2 is not preferable because this results in the supply of a large amount of moisture
to the surface of a catalyst in the reduction electrode 23 to easily cause the production
of hydrogen. Therefore, the moisture content in the gas containing CO
2 is preferably 20% to 90% and more preferably 30% to 70% in terms of relative humidity.
[0072] A first supply channel 31 that supplies the gas containing CO
2 and a first discharge channel 33 that discharges a produced gas are connected to
the first channel 36. A second supply channel 34 that supplies the electrolysis solution
containing water and a second discharge channel 35 are connected to the second channel
37. The first channel 36 is disposed to face the reduction electrode 23. The first
channel 36 is connected to the first supply channel 31 and is supplied with the gas
containing CO
2 from the first supply channel 31. The CO
2 gas or the first electrolysis solution (catholyte) comes into contact with the reduction
electrode 23 when it flows in the first channel 36. CO
2 in the CO
2 gas or the catholyte passing through the reduction electrode 23 is reduced by the
reduction electrode 23. A gas or solution containing a reduction reaction product
of CO
2 is discharged from the first discharge channel 33.
[0073] The second channel 37 is disposed to face the oxidation electrode 27. A not-illustrated
solution tank or the like is connected to the second channel 37, and the anolyte comes
into contact with the oxidation electrode 27 when it flows in the second channel 37.
H
2O in the anolyte passing through the oxidation electrode 27 is oxidized by the oxidation
electrode 27.
[0074] In the deterioration detecting system of the carbon dioxide electrolysis device of
the second arrangement, the equivalent circuit model illustrated in FIG. 4 can be
employed as in the first arrangement, and the equivalent circuit parameters illustrated
in FIG. 5 are calculated by fitting. Using data sent from the data storage unit 11
to the data processing unit 12 during operation, the equivalent circuit parameters
are periodically calculated, and equivalent circuit parameters before the real operation
or at design time are compared with the equivalent circuit parameters during the real
operation, whereby a deteriorated place can be inferred. Further, by setting a determination
standard for a deterioration degree D, it is possible to determine whether to stop
the operation, whether to execute a refresh operation, and whether to perform maintenance.
(Third Arrangement)
[0075] The configuration and a deterioration detecting system of a carbon dioxide electrolysis
device of a third arrangement will be described with reference to FIG. 1, FIG. 6,
and FIG. 7. The carbon dioxide electrolysis device of the third arrangement includes
two H
2 production equivalent circuits. The electrolysis device of the third arrangement
has the same configuration and deterioration detecting system as those of the electrolysis
device of the first arrangement or the second arrangement. An electrolysis cell according
to the third arrangement has the same configuration as that of the electrolysis cell
according to the second arrangement, for instance. However, an equivalent circuit
model used in the data processing unit according to the third arrangement is different
from the equivalent circuit model according to the first arrangement. The equivalent
circuit model used in the data processing unit 12 according to the third arrangement
will be described with reference to FIG. 7.
[0076] FIG. 7 illustrates an example of the equivalent circuit model of the carbon dioxide
electrolysis cell. In the description here, the case where C
xH
yO
z is produced through a CO
2 reduction reaction is taken as an example. In the case of CO production, C
xH
yO
z can be replaced by CO. In a cathode part of the equivalent circuit model, a C
xH
yO
z producing part and two side reaction H
2 producing parts are connected in parallel, and cathode resistance is connected thereto
in series. The H
2 producing parts are a H
2 producing part employed for a low current density region (low current density) and
a H
2 producing part employed for a high current density region (high current density),
and they are connected in parallel. A diaphragm part is diaphragm resistance. In an
anode part, an O
2 producing part and anode resistance are connected in series. The cathode part, the
diaphragm part, and the anode part are connected in series.
[0077] In the parameter of the Tafel equation in the formula (10), in the case where the
subscript A is the H
2 producing part (low current density), "HER low" is used, and in the case where it
is the H
2 producing part (high current density), "HER high" is used. FIG. 8 illustrates equivalent
circuit parameters of the equivalent circuit model according to the third arrangement.
Using the supply power properties, the input gas properties, and the electric properties,
output gas properties, and temperature of the electrolysis cell which are collected
before real operation, these equivalent circuit parameters are calculated by fitting.
For the fitting, spreadsheet software or a circuit simulator is usable. Using data
sent from the data storage unit 11 to the data processing unit 12 during the real
operation, the equivalent circuit parameters are periodically calculated, and the
equivalent circuit parameters before the real operation or at design time are compared
with the equivalent circuit parameters during the real operation, whereby a deteriorated
place can be inferred. Further, by setting a determination standard for a deterioration
degree D, it is possible to determine whether to stop the operation, whether to execute
a refresh operation, or whether to perform maintenance.
(Fourth Arrangement)
[0078] The configuration and a deterioration detecting system of an electrolysis device
of a fourth arrangement will be described with reference to FIG. 1, FIG. 2, FIG. 6,
and FIG. 9. The electrolysis device of the fourth arrangement is a device that electrolyzes
and reduces nitrogen (N
2) to produce ammonia (NH
3). The electrolysis device of the fourth arrangement is the same in the device configuration
itself as the electrolysis device 1 of the first arrangement illustrated in FIG. 1
though being different in an electrolyte and an electrolysis product. Further, in
the electrolysis device of the fourth arrangement, an electrolysis cell is also the
same as the electrolysis cell 2A illustrated in FIG. 2. In the electrolysis device
of the fourth arrangement, an electrolysis cell having the same configuration as that
of the electrolysis cell 2B illustrated in FIG. 6 may also be used.
[0079] In the fourth arrangement, a substance to be reduced in the cathode part of the electrolysis
cell illustrated in FIG. 2, which is nitrogen (N
2), and an equivalent circuit model are different from those in the first arrangement.
The first electrolysis solution stored in the cathode part contains N
2 as the substance to be electrolyzed. Alternatively, in the electrolysis cell illustrated
in FIG. 6, a N
2 gas may be supplied as the substance to be electrolyzed to the cathode part.
[0080] In the case where nitrogen (N
2) is reduced, the first electrolysis solution preferably contains an ammonia production
catalyst and a reducing agent for the production of ammonia through the N
2 reduction, separately from an electrochemical reaction. As the reducing agent, a
halide (II) or the like of a lanthanoid metal is used. Examples of the lanthanoid
metal include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium
(Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),
holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), among
which Sm is preferable. Examples of halogen include chlorine (Cl), bromide (Br), and
iodine (I), among which iodine is preferable. As the halide (II) of the lanthanoid
metal, samarium(II) iodide (SmI
2) is more preferable.
[0081] The ammonia production catalyst promotes the production of ammonia from nitrogen
under the presence of the reducing agent, and is, but not limited to, a molybdenum
complex, for instance. Examples of the ammonia production catalyst include molybdenum
complexes (A) to (D) listed below, for instance.
[0082] A first example is (A) a molybdenum complex having, as a PCP ligand, N,N-bis(dialkyl-phosphinomethyl)dihydrobenzo
imidazolidine (where the two alkyl groups may be identical or may be different, and
at least one hydrogen atom of the benzene ring may be replaced by an alkyl group,
an alkoxy group, or a halogen atom).
[0083] A second example is (B) a molybdenum complex having, as a PNP ligand, 2-6-bis(dialkyl-phosphinomethyl)pyridine
(where the two alkyl groups may be identical or may be different, and at least one
hydrogen atom of the pyridine ring may be replaced by an alkyl group, an alkoxy group,
or a halogen atom).
[0084] A third example is (C) a molybdenum complex having, as a PPP ligand, bis(dialkyl-phosphinomethyl)arylphosphine
(where the two alkyl groups may be identical or may be different).
[0085] A fourth example is (D) a molybdenum complex represented by trans-Mo(N
2)
2(R1R2R3P)
4 (where R1, R2, and R3 are alkyl groups or aryl groups that may be identical or may
be different, and two R3's may be linked to form an alkylene chain).
[0086] In the above molybdenum complexes, the alkyl group may be, for example, a methyl
group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group,
or a straight-chain or branched alkyl group such as a structural isomer of any of
these, or may be a cyclic alkyl group such as a cyclopropyl group, a cyclobutyl group,
a cyclopentyl group, or a cyclohexyl group. The carbon number of the alkyl group is
preferably 1 to 12, and more preferably 1 to 6. The alkoxy group may be, for example,
a methoxy group, an ethoxy group, a propoxy group, a butoxy group, a pentoxy group,
a hexyloxy group, or a straight-chain or branched alkoxy group such as a structural
isomer of any of these, or may be a cyclic alkoxy group such as a cyclopropoxy group,
a cyclobutoxy group, a cyclopentoxy group, or a cyclohexyloxy group. The carbon number
of the alkoxy group is preferably 1 to 12, and more preferably 1 to 6. Examples of
the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an
iodine atom.
[0087] The amount of the ammonia production catalyst used may be appropriately selected
within a range of 0.00001 to 0.1 mol/L equivalent weight, and is preferably 0.0001
to 0.05 mol/L equivalent weight, and more preferably 0.0005 to 0.01 mol/L equivalent
weight, relative to the electrolysis solution.
[0088] Next, the operation of the electrolysis device to produce ammonia through the N
2 reduction reaction will be described. When a voltage equal to or higher than an electrolysis
voltage is applied across the reduction electrode (cathode) and the oxidation electrode
(anode), an oxidation reaction of water (H
2O) or hydroxide ions (OH
-) in the second electrolysis solution occurs electrochemically in the oxidation electrode.
For example, in the case where the second electrolysis solution has a hydrogen ion
concentration of 7 or less (pH ≤ 7), H
2O is oxidized and O
2 and H
+ are produced based on the following equation (17). In the case where the hydrogen
ion concentration of the second electrolysis solution is larger than 7 (pH > 7), OH
- is oxidized and O
2 and H
2O are produced based on the following equation (18).
3H
2O → 3/2O
2 + 6H
+ + 6e
- ... (17)
6OH
- → 3/2O
2 + 3H
2O + 3H
2O + 6e
- ... (18)
[0089] In the first storage part (first electrolysis tank) 22, separately from the electrochemical
reaction, nitrogen (N
2) in the first electrolysis solution is reduced by the ammonia production catalyst
and the reducing agent, resulting in the production of ammonia (NH
3). In the case where, for example, SmI
2 is used as the reducing agent, N
2 in the first electrolysis solution is reduced, resulting in the production of ammonia
(NH
3) based on the following equation (19).
N
2 + 6SmI
2 + 6H
2O → 2NH
3 + 6SmI
2 (OH) ... (19)
[0090] As shown by the above equation (19), as a result of the production of NH
3, the reducing agent SmI
2 is oxidized, and if this state is left as it is, SmI
2 loses the function as the reducing agent. That is, in the case where the reduction
reaction of N
2 in the first electrolysis solution is caused in a first electrolysis tank not having
a reduction electrode that electrochemically causes the reduction reaction, the reduction
reaction of N
2 stops and the production of NH
3 finishes at an instant when the reducing agent in the amount put into the first electrolysis
tank in the initial state is consumed by the reduction reaction of N
2. Regarding this point, in the electrolysis device of the arrangement, since the reduction
electrode that causes the electrochemical reduction reaction is disposed in the first
electrolysis tank, the reducing agent resulting from the oxidation by the reduction
electrode, that is, SmI
2(OH), can be reduced to be regenerated based on the following equation (20). This
enables the reduction reaction of N
2 to continuously last. The amount of the reducing agent used is preferably 0.01 to
2 mol/L, more preferably 0.1 to 1 mol/L relative to the first electrolysis solution
in order to promote its reaction with the ammonia production catalyst.
6SmI
2(OH) + 6e
- → 6SmI
2 + 6OH
- (20)
[0091] FIG. 9 illustrates an equivalent circuit model of the electrolysis cell that produces
NH
3 using SmI
2 as the reducing agent. As shown by the aforesaid equation (20), SmI
2(OH) is electrochemically regenerated into SmI
2. For this purpose, in a cathode part of the equivalent circuit model, a SmI
2 regenerating part and a side reaction H
2 producing part are connected in parallel. The current density of the SmI
2 regenerating part is represented by the Tafel equation in the aforesaid formula (10).
In the subscript A in the formula (10), SmI
2 RR (SmI
2 regeneration reaction) is entered.
(Fifth Arrangement)
[0092] A method of designing an electrolysis device and an electrolysis system of a fifth
arrangement will be described with reference to FIG. 10. The electrolysis system of
the fifth arrangement is the same as the electrolysis system of the first and second
arrangement. In the method of designing the electrolysis system of the fifth arrangement,
the system is designed using the equivalent circuit model and the equivalent circuit
parameters of any of the first to fourth arrangements.
[0093] First, an electrolysis cell serving as a reference (reference electrolysis cell)
is operated, the supply power properties, the input gas properties, the electric properties,
the output gas properties, and the temperature properties are obtained, and their
measurement data are stored in the data storage unit (S1). Before the measurement
data are obtained, it is preferable to execute an aging operation of passing a current
in advance to stabilize cell properties. Since the cell properties are more stabilized
as the time of the aging operation is longer, the time of the aging operation is preferably
one hour or longer, and more preferably two hours or longer. The data processing unit
selects a candidate for the equivalent circuit model of the electrolysis cell (S2).
[0094] The data processing unit calculates parameters of the equivalent circuit model by
fitting such that a square error between the measurement data of the reference electrolysis
cell obtained at S 1 and simulation data of the equivalent circuit model becomes small
(S3). A determination standard for the square error between the measurement data and
the simulation data of the equivalent circuit model is set in advance, and when the
square error is larger than the determination standard, the candidate for the equivalent
circuit model at S2 is changed. In the case where the square error is smaller than
the determination standard, it is determined that the equivalent circuit model selected
at S2 is valid (S4). The electrolysis system is designed using the equivalent circuit
model determined as valid and the parameters of the equivalent circuit model (S5).
EXAMPLE
[0095] Next, an example and its evaluation results will be described.
(Example 1)
[0096] The carbon dioxide electrolysis cell whose configuration is illustrated in FIG. 6
was manufactured. The carbon dioxide electrolysis cell was operated by the deterioration
detecting system of the electrolysis device illustrated in FIG. 1. As the reduction
electrode in the carbon dioxide electrolysis cell, an electrode in which gold nanoparticle-carrying
carbon particles were applied on carbon paper was used. The average particle size
of the gold nanoparticles was 2 nm, and their carried amount was 10% by mass. As the
oxidation electrode, an electrode in which IrO
2 nanoparticles were applied on a Ti mesh was used. As the diaphragm, an anion exchange
membrane was used. The reduction electrode and the oxidation electrode cut to have
a 16 cm
2 electrode area were used. As in the carbon dioxide electrolysis cell whose structure
is illustrated in FIG. 6, the first current collector plate, the first channel, the
reduction electrode, the diaphragm, the oxidation electrode, the second channel, and
the second current collector plate were stacked in order from the left, and the resultant
was sandwiched by an insulating plate, a cooling water channel, and a support plate,
which are not illustrated, to form the carbon dioxide electrolysis cell. Further,
to simply monitor a reduction electrode potential and an oxidation electrode potential,
a not-illustrated Pt foil as a reference electrode was brought into contact with a
reduction electrode side of the diaphragm.
[0097] Using the gas/electrolysis solution control unit, CO
2 was introduced to the first channel of the carbon dioxide electrolysis cell at a
flow rate of 80 sccm, and a 0.1 M KHCO
3 electrolysis solution was introduced to the second channel at a flow rate of 10 mL/min.
Further, using the temperature control unit and the temperature obtaining unit, the
carbon dioxide electrolysis cell was temperature-controlled to 40°C while a heater
and a cooling water channel, which are not illustrated, were in close contact with
the carbon dioxide electrolysis cell. As the supply power control unit, the supply
power property obtaining unit, and the electric property obtaining unit, used was
a potentiostat/galvanostat in which their functions are integrated. As the output
gas property obtaining unit, a volumetricflow meter or a gas chromatograph was used.
[0098] A current was passed to the carbon dioxide electrolysis cell, and current density
dependences of the supply power properties (supplied current and voltage), the input
gas properties (the flow rate of a gas input to the cathode), the cell temperature,
the electric properties (cell current, cell voltage, cathode potential, anode potential,
cell resistance), and the output gas properties (the flow rates of gases output from
the cathode and the anode, the concentrations of various gases) were obtained. In
the cathode, Co was produced through a CO
2 reduction reaction and H
2 was produced through a side reaction. In the anode, O
2 was produced through an oxidation reaction of water. Since the behavior of a H
2 production reaction in a low current density region and that in a high current density
region were different, the circuit illustrated in FIG. 7 in which the CO producing
part and the two H
2 producing parts are connected in parallel was used as the equivalent circuit model.
[0099] A CO part current density (current density contributing to CO production) and a H
2 part current density (current density contributing to H
2 production) were calculated from the flow rate of the gas output from the cathode
and the gas concentrations of CO and H
2, and equivalent circuit parameters were decided such that errors between measurement
data and simulation data of the CO part current density Jco, the H
2 part current density J
H2, the cell voltage V
cell, the cathode potential V
cm, and the anode potential V
am became small as illustrated in FIG. 11 and FIG. 12. As illustrated in FIG. 13, FIG.
14, and FIG. 15, the simulation data of CO Faraday efficiency (FEco), H
2 Faraday efficiency (FE
H2), cathode output gases (CO, H
2, and CO
2), and anode output gases (O
2 and CO
2) well reproduce the measurement data. Therefore, using the equivalent circuit model
in FIG. 7 to study changes in the equivalent circuit parameters during operation enables
the detection of deterioration.
[0100] It should be noted that the configurations of the above-described arrangements may
be employed in combination, and they may be partly replaced. While certain arrangements
of the present invention have been described here, these arrangements have been presented
by way of example only, and are not intended to limit the scope of the inventions.
Indeed, the novel arrangements described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in the form of the
arrangements described herein may be made without departing from the spirit of the
inventions. The accompanying claims and their equivalents are intended to cover such
forms or modifications as would fall within the scope and spirit of the inventions.
(Numbered Clauses relating to the arrangements)
[0101]
- 1. An electrolysis device comprising:
an electrolysis cell including a cathode part to be supplied with a gas or a liquid
containing a substance to be reduced and in which a reduction electrode is disposed,
an anode part to be supplied with a liquid containing a substance to be oxidized and
in which an oxidation electrode is disposed, and a diaphragm provided between the
cathode part and the anode part;
a supply power property obtaining unit that obtains a property of power that is to
be supplied to the electrolysis cell;
an input gas property obtaining unit that obtains a property of a gas that is to be
input to the electrolysis cell;
an electric property obtaining unit that obtains an electric property of the electrolysis
cell;
an output gas property obtaining unit that obtains a property of an output gas of
the electrolysis cell;
a temperature control unit that controls a temperature of the electrolysis cell;
a temperature obtaining unit that obtains the temperature of the electrolysis cell;
a data storage unit that stores data from the supply power property obtaining unit,
the input gas property obtaining unit, the electric property obtaining unit, the output
gas property obtaining unit, and the temperature obtaining unit; and
a data processing unit to which the data is sent from the data storage unit and that
processes the data to determine a state of the electrolysis cell.
- 2. The electrolysis device according to clause 1,
wherein the data processing unit is configured to calculate an equivalent circuit
parameter by fitting using measurement data from at least one of the supply power
property obtaining unit, the input gas property obtaining unit, the electric property
obtaining unit, the output gas property obtaining unit, and the temperature obtaining
unit and simulation data of an equivalent circuit model of the electrolysis cell,
and to detect deterioration based on information of the equivalent circuit parameter.
- 3. The electrolysis device according to clause 2,
wherein the data processing unit is configured to infer a deteriorated place by comparing
the equivalent circuit parameter before real operation or at design time with the
equivalent circuit parameter during the real operation.
- 4. The electrolysis device according to clause 3,
wherein the data processing unit is configured to find a deterioration degree of the
equivalent circuit parameter, and to determine whether to stop the operation of the
electrolysis cell by setting a determination standard for the deterioration degree,
the deterioration degree being represented by [(the equivalent circuit parameter during
the real operation) - (the equivalent circuit parameter before the real operation
or at the design time)] / (the equivalent circuit parameter before the real operation
or at the design time).
- 5. The electrolysis device according to any one of clause 2 to clause 4,
wherein the data processing unit is configured to calculate an equivalent circuit
as the equivalent circuit model, the equivalent circuit including: a cathode part
having a carbon dioxide reduction substance producing part and a hydrogen producing
part that are connected in parallel and to which a series resistance is connected
in series; a diaphragm part; and an anode part having an oxygen producing part and
a series resistance that are connected in series, the cathode part, the diaphragm
part, and the anode part being connected in series.
- 6. The electrolysis device according to clause 5,
wherein the data processing unit is configured to calculate a current density of the
carbon dioxide reduction substance producing part of the equivalent circuit model
based on a relational formula including, as variables, a current density represented
by a Tafel equation and a production limit current density of the carbon dioxide reduction
substance.
- 7. The electrolysis device according to a clause 6,
wherein the data processing unit is configured to calculate the production limit current
density of the carbon dioxide reduction substance based on a relational formula including,
as variables, a flow rate of the substance to be reduced introduced to the cathode
part and a flow rate of the substance to be reduced that has moved to the anode part
from the cathode part.
- 8. The electrolysis device according to any one of clause 1 to clause 7,
wherein the data processing unit is installed in a cloud and is configured to determine
the state of the electrolysis cell remotely.
- 9. The electrolysis device according to any one of clause 1 to clause 8,
wherein the electrolysis cell is configured to produce a carbon compound by supplying
carbon dioxide as the substance to be reduced, or to produce ammonia by supplying
nitrogen as the substance to be reduced.
- 10. An electrolysis method comprising:
supplying a gas or a liquid containing a substance to be reduced to a cathode part
of an electrolysis cell, supplying a liquid containing a substance to be oxidized
to an anode part of the electrolysis cell, and operating the electrolysis cell, the
electrolysis cell including the cathode part in which a reduction electrode is disposed,
the anode part in which an oxidation electrode is disposed, and a diaphragm provided
between the cathode part and the anode part;
obtaining property data of power that is to be supplied to the electrolysis cell,
property data of a gas that is to be input to the electrolysis cell, electric property
data of the electrolysis cell, property data of an output gas of the electrolysis
cell, and temperature data of the electrolysis cell, all the data being obtained during
the operation of the electrolysis cell; and
processing the property data of the power, the property data of the gas, the electric
property data, the property data of the output gas, and the temperature data to obtain
an equivalent circuit model and an equivalent circuit parameter of the electrolysis
cell, and determining a state of the electrolysis cell, using the equivalent circuit
model and the equivalent circuit parameter of the electrolysis cell.
- 11. The electrolysis method according to clause 10,
wherein the determining the state of the electrolysis cell comprises determining a
deterioration state of the electrolysis cell, using the equivalent circuit model and
the equivalent circuit parameter.
- 12. The electrolysis method according to clause 11,
wherein the determining the state of the electrolysis cell comprises calculating the
equivalent circuit parameter by fitting, using the obtained data and simulation data
of the equivalent circuit model, and detecting a deterioration of the electrolysis
cell based on information of the equivalent circuit parameter.
- 13. The electrolysis method according to clause 11 or clause 12,
wherein the determining the state of the electrolysis cell comprises finding a deterioration
degree of the equivalent circuit parameter, and determining whether to stop the operation
of the electrolysis cell by setting a determination standard for the deterioration
degree, the deterioration degree being represented by [(the equivalent circuit parameter
during the real operation) - (the equivalent circuit parameter before the real operation
or at the design time)]/(the equivalent circuit parameter before the real operation
or at the design time).
- 14. The electrolysis method according to clause 10,
wherein the obtaining the data of the electrolysis cell comprises obtaining data of
a reference electrolysis cell, and
wherein the electrolysis cell is designed based on the equivalent circuit model and
the equivalent circuit parameter that are derived from the data of the reference electrolysis
cell.
- 15. The electrolysis method according to clause 14, further comprising:
selecting a candidate for the equivalent circuit model;
calculating the equivalent circuit parameter by fitting such that a square error between
the data of the reference electrolysis cell and simulation data of the selected equivalent
circuit model becomes small; and
determining whether the equivalent circuit parameter is valid, using the square error
between the data of the reference electrolysis cell and the simulation data of the
equivalent circuit parameter; and
designing the electrolysis cell using the equivalent circuit model and the equivalent
circuit parameter determined as valid.