Method for operating a fuel cell (SOFC) or a reactor (SOEC) in hot standby mode

Description

Technical field

[0001] The present invention relates to the field of solid oxide fuel cells (SOFC, English acronym for "solid oxide fuel cells"). Solid Oxide Fuel Cell ") and that of high temperature water electrolysis (EHT, or EVHT for high temperature water vapor electrolysis, or HTE English acronym for " High Temperature Electrolysis ", or HTSE, the English acronym for " High Temperature Steam Electrolysis ") or CO 2 , or co-electrolysis of water vapor and carbon dioxide CO 2 also with solid oxides (SOEC, English acronym for " Solid Oxide Electrolysis Cell ”) .

[0002] The invention relates more particularly to the operation of SOFC fuel cells or electrolysis or co-electrolysis reactors of the unit, during a production shutdown, i.e. with zero output or input current, respectively in the event of a cut-off of the cell, in the event of a low level of available electricity, or in the event of insufficient access to the reagents, according to a so-called stand-by mode. Prior art

[0003] Electrolysis of water is an electrolytic reaction that decomposes water into oxygen and hydrogen gas with the help of an electric current according to the reaction: H 2 O→H 2 + 1 / 2O 2 .

[0004] To carry out the electrolysis of water, it is advantageous to carry it out at high temperature typically between 600 and 1000°C, because part of the energy necessary for the reaction can be provided by heat which is cheaper than electricity and the activation of the reaction is more efficient at high temperature and does not require a noble catalyst. To implement electrolysis at high temperature, it is known to use an SOEC type electrolyser (acronym for " Solid Oxide Electrolyte Cell"), consisting of a stack of elementary patterns, each comprising a solid oxide electrolysis cell, consisting of at least three anode / electrolyte / cathode layers superimposed on each other, and interconnection plates made of metal alloys, also called bipolar plates, or interconnectors. The interconnectors have the function of ensuring both the passage of electric current and the circulation of gases in the vicinity of each cell (injected water vapor, hydrogen and oxygen extracted in an EHT electrolyzer; injected air and hydrogen and extracted water in an SOFC stack) and of separating the anode and cathode compartments, which are the gas circulation compartments on the anode and cathode sides of the cells, respectively. To carry out the electrolysis of water vapor at high temperature EHT, water vapor H 2 O is injected into the cathode compartment.Under the effect of the current applied to the cell, the dissociation of water molecules in vapor form is carried out at the interface between the hydrogen electrode (cathode) and the electrolyte: this dissociation produces dihydrogen gas H 2 and oxygen ions. The dihydrogen is collected and evacuated at the outlet of the hydrogen compartment. The oxygen ions O 2-< migrate through the electrolyte and recombine into dioxygen at the interface between the electrolyte and the oxygen electrode (anode).

[0005] As schematized in Figure 1, each elementary electrolysis cell 1 is formed of a cathode 2 and an anode 4, placed on either side of a solid electrolyte 3 generally in the form of a membrane. The two electrodes (cathode and anode) 2,4 are electronic conductors, made of porous material, and the electrolyte 3 is gas-tight, electronically insulating and ionically conductive. The electrolyte may in particular be an anionic conductor, more precisely an anionic conductor of O 2-< ions and the electrolyser is then called an anionic electrolyser.

[0006] Electrochemical reactions take place at the interface between each of the electronic conductors and the ionic conductor.

[0007] At cathode 2, the half-reaction is as follows: 2 H 2 O + 4 e -< → 2 H 2 + 2 O 2-< .

[0008] At anode 4, the half-reaction is: 2O 2-< → O 2 + 4 e -< .

[0009] The electrolyte 3 intercalated between the two electrodes 2, 4 is the place of migration of the O 2-,< ions under the effect of the electric field created by the potential difference imposed between the anode 4 and the cathode 2.

[0010] The electrolysis of CO2 acts on the same principle as that of water, except that the half-reaction at the cathode becomes: 2 CO2 + 4 e-< → 2 CO + 2 O2-< .

[0011] In battery mode, the half-reactions are reversed, but we still have the O 2-< ions migrating through the electrolyte.

[0012] As shown in parentheses in Figure 1, the water vapor at the cathode inlet can be accompanied by hydrogen H 2 and the hydrogen produced and recovered at the outlet can be accompanied by water vapor. Similarly, as illustrated in dotted lines, a draining gas, such as air, can also be injected at the inlet to evacuate the oxygen produced. The injection of a draining gas has the additional function of acting as a thermal regulator.

[0013] An elementary electrolysis reactor consists of an elementary cell as described above, with a cathode 2, an electrolyte 3, and an anode 4 and two monopolar connectors which provide the electrical, hydraulic and thermal distribution functions. To increase the flow rates of hydrogen and oxygen produced, it is known to stack several elementary electrolysis cells on top of each other by separating them by interconnection devices, usually called interconnectors or bipolar interconnection plates. The assembly is positioned between two end interconnection plates which support the electrical and gas supplies of the electrolyzer (electrolysis reactor).

[0014] A high temperature water electrolyser (HTE) thus comprises at least one, generally a plurality of electrolysis cells stacked on top of each other, each elementary cell being formed of an electrolyte, a cathode and an anode, the electrolyte being intercalated between the anode and the cathode.

[0015] The fluidic and electrical interconnection devices which are in electrical contact with one or more electrodes generally provide the functions of supplying and collecting electric current and delimit one or more gas circulation chambers / compartments.

[0016] Thus, a so-called cathode compartment chamber has the function of distributing electric current and water vapor as well as recovering hydrogen at the cathode in contact.

[0017] A so-called anode compartment chamber has the function of distributing the electric current as well as recovering the oxygen produced at the anode in contact, possibly using a draining gas.

[0018] There Figure 2 represents an exploded view of elementary patterns of a high-temperature water vapor electrolyzer according to the state of the art. This EHT electrolyzer comprises a plurality of elementary electrolysis cells Cl, C2... of the solid oxide type (SOEC) stacked alternately with interconnectors 5. Each cell C1, C2... consists of a cathode 2.1, 2.2,... and an anode 4.1, 4.2, between which an electrolyte 3.1, 3.2... is arranged. All of the electrolysis cells are supplied in series by the electric current and in parallel by the gases.

[0019] The interconnector 5 is a metal alloy component that ensures the separation between the cathode 50 and anode 51 compartments, defined by the volumes between the interconnector 5 and the adjacent cathode 2.1 and between the interconnector 5 and the adjacent anode 4.2 respectively. It also ensures the distribution of gases to the cells. The injection of water vapor into each elementary pattern is done in the cathode compartment 50. The collection of the hydrogen produced and the residual water vapor at the cathode 2.1, 2.2 ... is carried out in the cathode compartment 50 downstream of the cell C1, C2 ... after dissociation of the water vapor by the latter. The collection of the oxygen produced at the anode 4.2 is carried out in the anode compartment 51 downstream of the cell C1, C2 ... after dissociation of the water vapor by the latter.

[0020] The interconnector 5 ensures the passage of current between the cells C1 and C2 by contact, preferably direct, with the adjacent electrodes, i.e. between the anode 4.2 and the cathode 2.1.

[0021] In a SOFC solid oxide fuel cell, the cells C1, C2... and interconnectors 5 used are the same components, but the operation is the opposite of that of an EHT electrolyser as just explained with a reversed direction of current, with air or oxygen O 2 which feeds the compartments which have become cathodic and hydrogen and / or methane CH 4 as fuel which feeds the compartments which have become anodic.

[0022] As for materials, the solid electrolyte is a gas-impermeable material, which must allow the diffusion of oxygen atoms in the form of O 2-< ions above 500°C.

[0023] Each electrode of a SOEC / SOFC cell is made of a usually porous cermet, composed mainly of silica and nickel on the hydrogen / H 2 O side (cathode in (co-)electrolysis, anode in SOFC cell).

[0024] To function, a hydrogen / H 2 O side cermet must have the nickel it includes in reduced form: in fact, this reduced metal has the role of breaking the HO bonds. However, the O 2-< ions are capable of migrating from the air / O 2 side cermet to the H 2 side cermet through the electrolyte, even in the absence of current.

[0025] And the absence of current can often occur once the SOFC batteries or EHT / SOEC electrolysis or co-electrolysis reactors have been put into operation, especially for the latter in the event of possible intermittence of electricity production.

[0026] It is necessary to ensure that SOFC fuel cells or EHT / SOEC electrolysis or co-electrolysis reactors are kept at the correct temperature, on the one hand, to avoid excessively rapid thermal cycling that could damage them and, on the other hand, to offer the possibility of rapid start-up as soon as electricity becomes available again for the EHT / SOEC reactors or to use the current produced for the batteries. Such an operating mode is known as "stand-by" or "hot stand-by" mode. Although the O 2-< ion flux mentioned above is low at zero current, a cell maintained at working temperature, typically between 700 and 800°C, for an extended period of time may gradually see its H 2-side cermet oxidized by said flux.

[0027] To limit these oxidation risks while maintaining a SOEC reactor or SOFC fuel cell in hot standby mode, i.e. maintained at a temperature high enough to start almost instantly, the most widespread method consists of sweeping the H2 / H2O side chambers with a continuous flow of hydrogen, either pure or diluted in an inert gas.

[0028] For safety and cost reasons, safety gases with a concentration of around 5% H2 in nitrogen tend to be preferred. The safety gas can either be supplied from a container or produced on-site from a dedicated electrolysis reactor and / or an Air Separation Unit (ASU) which produces, among other things, high-purity oxygen, nitrogen and rare gases.

[0029] However, this continuous safety gas sweep has costs in terms of: materials used: a safety gas can be recycled and circulate in a loop, but it must be purged at each start-up otherwise the hydrogen produced will be contaminated; electrical consumption: the gas must be set in motion, in particular by means of a circulator; thermal consumption: the safety gas must be preheated before arriving in the high-temperature enclosure that constitutes a SOEC reactor or SOFC fuel cell, so as not to cool the latter.

[0030] Other solutions, alternatives to the safety gas flow, are known from the literature.

[0031] For example, US Patent 9,005,827B2 describes a method in which each cell is kept operating under a low applied current with a cell voltage ranging from 700 to 1500 mV, in order to prevent reoxidation of nickel Ni to NiO.

[0032] Patent JP2626395B2 also proposes using an SOFC battery in electrolysis mode periodically, in order to reduce the cermets that can partially oxidize during its operation, which extends the battery's lifespan.

[0033] On the contrary, it is also known to reverse the operation of a SOEC or co-electrolysis reactor, that is to say to operate them in SOFC fuel cell to produce current from hydrogen H 2 , syngas (mixture of hydrogen H 2 and carbon monoxide CO), or methane, which makes it possible to maintain the reactor temperature. This has the major disadvantage of producing electric current that is not necessarily usable, as soon as there is no more electricity available from external sources. In addition, another major disadvantage is that we consume, that is to say we burn, fuel, i.e. H 2 , syngas or methane, only for the purpose of maintaining the reactor temperature and without obtaining any other combustible product but only electricity that is not necessarily usable at that precise moment.

[0034] Patent application US2003 / 0235752 proposes the arrangement of capture materials, such as nickel, capable of reacting with traces of oxygen in the flow entering the hydrogen compartment, so that these materials are oxidized rather than the cermets. This solution can make it possible to carry out a sweep with almost pure nitrogen (without H 2 ), the traces of oxygen still present being captured by the added material(s). Such a sweep gas (pure nitrogen) has the advantage of being less expensive, but its implementation would not solve the problem of migration of O 2-< ions in the electrolyte, nor the energy consumption of the sweep gas due to the use of a compressor and the need for preheating.

[0035] There is therefore a need to improve existing solutions for maintaining hot standby mode while limiting the risks of oxidation of an SOEC reactor or an SOFC fuel cell, in particular in order to overcome the aforementioned drawbacks.

[0036] The aim of the invention is to meet at least part of this need. Statement of the invention

[0037] To do this, the invention relates to a method for operating in hot standby mode a fuel cell (SOFC) or a high-temperature electrolysis or co-electrolysis reactor, with a stack of elementary electrochemical cells of the solid oxide type (SOEC), the method comprising, during a given period of absence of an electric current respectively outgoing or applied to the stack or when it is desired to achieve a rise or fall in temperature of the cell or the reactor, a step of supplying the compartments on the side of the hydrogen / water electrodes (H 2 / H 2 O), by pulses of a safety gas at regular time intervals during the given period or when the cell voltage becomes lower than a threshold value, so as to renew the gas(es) present in said compartments.

[0038] By "hot standby mode" is meant here and within the scope of the invention, the maintenance of a SOFC fuel cell or a SOEC electrolysis reactor at a normal operating temperature, typically 700 to 800°C, when operation is stopped due to the absence of current at the output (cell) or at the input (SOEC reactor).

[0039] The safety gas is advantageously chosen from pure hydrogen (H2), hydrogen (H2) diluted in nitrogen, preferably diluted from 1 to 5 vol% in nitrogen. An optimum is hydrogen (H2) diluted to approximately 3 vol% in nitrogen.

[0040] Advantageously, the voltage of the stack(s) is monitored. If the cell voltage falls below 0.8V or a lower value, a gas pulse will be delivered. In other words, the step of supplying with safety gas pulses is advantageously carried out for a cell voltage threshold value less than or equal to 0.8V. Advantageously again, the flow rate of safety gas per pulse is less than or equal to less than 10 NmL / min / cm 2< , preferably less than 5 NmL / min / cm 2< . Typically, it is of the order of 6 NmL / min / cm 2< .

[0041] The flow rate, interval and duration of the pulses will depend on the configuration of the installation, and the volume / distance ratio between the cell stacks and the measurement and control devices. The pulse profile (ramps between zero flow rate and maximum flow rate) can also advantageously vary depending on the stack model, and the configuration of the supply lines, in order to limit the effects of "water hammer" which can be harmful to the electrochemical system.

[0042] According to an advantageous variant, when no safety gas pulse supplies the compartments on the side of the hydrogen / water electrodes (H 2 / H 2 O), all the gas supply lines of the reactor or the fuel cell are closed, so as to limit the cooling thereof by gas displacement.

[0043] According to an advantageous embodiment, the compartments on the side of the oxygen (O 2 ) electrodes are purged simultaneously or with a time shift from the safety gas pulses, using a neutral or highly oxygen-depleted gas. This reduces or even eliminates the flow of O 2-< ions sent to the electrolyte by reducing the partial pressure of oxygen.

[0044] According to an advantageous variant, to compensate for thermal losses by convection through the enclosure which houses the SOEC reactor or the SOFC fuel cell, the stack is heated simultaneously with the safety gas pulses to maintain its temperature.

[0045] According to this variant, the stack is preferably heated by means of a heating soleplate in contact with the stack.

[0046] The method according to the invention can advantageously be implemented in a unit, called power-to-gas, comprising a plurality of reactors (SOEC).

[0047] Thus, the invention essentially consists of sending into a solid oxide electrochemical cell system (SOEC reactor or SOFC fuel cell), which is in hot standby mode, a safety gas into the H 2 / H 2 O compartments / chambers, intermittently at regular time intervals.

[0048] By regularly renewing the gas present with a suitable safety gas, the risk of oxidation of the hydrogen electrode cermets is eliminated.

[0049] Using an intermittent flow of safety gas also eliminates convection cooling of the gas in the enclosure in which the SOFC reactor / fuel cell is placed.

[0050] Heat losses through the hot enclosure can be compensated by heating the enclosure itself or directly by heating the stack of cells, in particular by means of a heating sole in contact with the stack.

[0051] The frequency of the safety gas sweep, and its quantity (flow rate, duration) must be adjusted according to the electrochemical system being implemented. More specifically, their adjustment can be made: depending on the type and manufacturer of the cells which directly impacts the flow of O 2-< ions likely to migrate: this can be variable, as it depends on the different thicknesses of the constituent layers of a cell (cermets on the H 2 and O 2 sides, electrolyte); depending on the partial pressure of O 2 on the side of the O 2 circulation compartments: the higher the partial pressure of the compartment, the more easily the cermet on the O 2 side is oxidized, which increases the driving force for the creation of O 2-< ions at the O 2 cermet / electrolyte interface; the volume of piping from the safety gas tank / circulator: the greater the distance to be covered and the volume that this represents, the more gas needs to be injected to renew the atmosphere of a stack of electrochemical cells;the concentration of reducing agent, in particular hydrogen in the safety gas: the more reducing this gas, the lower the volume required to renew the reducing atmosphere of a stack can be.;

[0052] Ultimately, the invention brings many advantages, including: reduction of energy costs by consuming only the necessary safety gas to avoid any oxidation of the cermets in a SOFC fuel cell or SOEC reactor; less stress on the equipment required to supply safety gas and therefore a longer service life at a lower investment cost.

[0053] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given for illustrative and non-limiting purposes with reference to the following figures. Brief description of the drawings

[0054] [ Fig 1 ] there Figure 1 is a schematic view showing the operating principle of a high-temperature water electrolyzer. Fig 2 ] there Figure 2 is an exploded schematic view of a portion of a high temperature water vapor electrolyzer including interconnectors. Detailed description

[0055] THE Figures 1 and 2 have already been commented on in the preamble. They will therefore not be detailed below.

[0056] It is also specified that the electrolysers or fuel cells described are of the solid oxide type (SOEC, English acronym for " Solid Oxide Electrolyte Cell » or SOFC, English acronym for “ Solid Oxide Fuel Cell») operating at high temperature. Thus, all the components (anode / electrolyte / cathode) of an electrolysis cell or battery are ceramics. The high operating temperature of an electrolyzer (electrolysis reactor) or battery is typically between 600°C and 1000°C. Typically, the characteristics of a SOEC electrolysis cell suitable for the invention, of the cathode support type (CSC), may be those indicated as follows in Table 1 below.

[0057] According to the invention, when a SOEC reactor or a SOFC fuel cell is in hot standby mode, a safety gas in the H 2 / H 2 O compartments / chambers, intermittently at regular time intervals.

[0058] The safety gas is advantageously hydrogen (H2) diluted to approximately 3 vol% in nitrogen.

[0059] Advantageously, the voltage of the stack(s) is monitored. If the cell voltage falls below 0.8V or a lower value, a safety gas pulse is delivered.

[0060] Typically, the safety gas flow rate per pulse is in the order of 6 NmL / min / cm 2< .

[0061] The invention is not limited to the examples which have just been described; in particular, it is possible to combine characteristics of the examples illustrated within non-illustrated variants.

Claims

1. Method for operating, in hot stand-by mode, a fuel cell (SOFC) or a high temperature co-electrolysis or electrolysis reactor (1) having a stack of elementary electrochemical cells of the solid oxide type (SOEC), the method comprising, for a given period of time in which there is no electrical current exiting and / or applied to the stack, or when the temperature of the cell or the reactor is to be raised or lowered, a step of supplying pulses of a safety gas to the compartments on the side of the hydrogen / water (H2 / H2O) electrodes, at regular intervals for the given period of time, or when the cell voltage drops below a threshold value, so as to renew the gas(es) present in said compartments.

2. Method according to Claim 1, wherein the safety gas is selected from among pure hydrogen (H2), and hydrogen (H2) diluted in nitrogen, preferably diluted from 1% to 5% by volume in nitrogen.

3. Method according to Claim 1 or 2, wherein the step of supplying pulses of safety gas is performed for a cell voltage threshold value less than or equal to 0.8 V.

4. Method according to one of the preceding claims, wherein the flow rate of pulses of safety gas is less than 10 NmL / min / cm2, preferably less than 5 NmL / min / cm2.

5. Method according to one of the preceding claims, in which, when no pulses of safety gas are supplied to the compartments on the side of the hydrogen / water (H2 / H2O) electrodes, all the gas supply lines of the reactor or the fuel cell are closed so as to limit the cooling of the reactor or fuel cell via the movement of gas.

6. Method according to one of the preceding claims, in which, at the same time as or with a temporal shift from the pulsing of safety gas, the compartments on the side of the oxygen (O2) electrodes are purged using a neutral gas or a greatly oxygen-depleted gas.

7. Method according to one of the preceding claims, in which, at the same time as the pulsing of safety gas, the stack is heated to maintain its temperature.

8. Method according to Claim 7, in which the stack is heated using a heating baseplate in contact with the stack.

9. Method according to one of the preceding claims implemented in a unit, referred to as power-to-gas unit, comprising a plurality of reactors (SOEC).