Method for operating an SOFC fuel cell or SOEC reactor in hot standby mode.

Abstract

The present invention relates to a method for operating a fuel cell (SOFC) or a high-temperature electrolysis or co-electrolysis reactor (1) having a stack of solid oxide elementary electrochemical cells (SOEC) in hot standby mode, comprising the step of supplying pulses of safety gas to the compartment on the hydrogen / water electrode (H2 / H2O) side at regular intervals during a given period to refresh the gas present in said compartment during a given period in the absence of current flowing from or applied to the stack, respectively, or when the temperature of the cell or reactor is required to increase or decrease, or when the cell voltage drops below a threshold value.

Description

[Technical Field]

[0001] This invention relates to the field of solid oxide fuel cells (SOFCs), high-temperature electrolysis of water or CO2 (HTE, or HTSE, an abbreviation for "High Temperature Steam Electrolysis"), co-electrolysis of water vapor and carbon dioxide (CO2), and further to the field of solid oxide electrolytic cells (SOECs).

[0002] More specifically, the present invention relates to the operation of an SOFC fuel cell or the electrolytic or co-electrolytic reactor of a unit in a mode referred to as standby, during production stoppage, i.e., when the output or input current is zero and / or when the cell is disconnected, when the level of available electricity is low, or when there is insufficient access to the reactants. [Background technology]

[0003] The electrolysis of water is an electrolytic reaction that uses electric current to decompose water into gaseous dihydrogen and dioxygen, following the reaction: H2O → H2 + 1 / 2O2.

[0004] For the electrolysis of water, it is advantageous to carry it out at high temperatures, typically 600°C to 1000°C, because some of the energy required for the reaction can be supplied by heat, which is cheaper than electricity, and because the activation of the reaction is more effective at high temperatures and does not require precious metal catalysts. To implement high-temperature electrolysis, it is known to use SOEC ("Solid Oxide Electrolyte Cell") type electrolytic cells, which consist of a stack of basic units having solid oxide electrolytic cells, each consisting of at least three layers of anode / electrolyte / cathode, superimposed on each other, as well as interconnecting plates made of metal alloy, also called bipolar plates or interconnectors. The function of the interconnector is to ensure both the passage of current and the circulation of gases near each cell (injected water vapor, extracted hydrogen and oxygen in HTE electrolytic cells, injected air and hydrogen, and extracted water in SOFC cells), and to separate the anode and cathode compartments, which are the sections in which the gases on the anode and cathode sides of the cell circulate, respectively. To perform high-temperature steam electrolysis (HTE), H2O steam is injected into the cathode compartment. Due to the influence of the current applied to the cell, dissociation of water molecules in the form of steam occurs at the interface between the hydrogen electrode (cathode) and the electrolyte. This dissociation generates dihydrogen gas, H2, and oxygen ions. The dihydrogen is collected and removed at the outlet of the hydrogen compartment. Oxygen ions (O) 2- It migrates through the electrolyte and recombines with dioxygen at the interface between the electrolyte and the oxygen electrode (anode).

[0005] As schematically shown in Figure 1, each basic electrolytic cell 1 is formed by a cathode 2 and an anode 4 positioned on either side of a solid electrolyte 3, generally in the form of a membrane. The two electrodes (cathode and anode) 2, 4 are electron conductors made of porous material, and the electrolyte 3 is an airtight electron insulator and ion conductor. The electrolyte is particularly an anion conductor, more specifically O 2- It may also be an anion conductor, in which case the electrolytic cell is called an anion electrolytic cell.

[0006] Electrochemical reactions occur at the interface between each electron conductor and an ionic conductor.

[0007] At the cathode 2, the half-reaction is as follows: 2H2O + 4e - → 2H2 + 2O 2- .

[0008] At the anode 4, the half-reaction is as follows: 2O 2- → O2 + 4e - .

[0009] The electrolyte 3 intervening between the two electrodes 2 and 4 is the place where O 2- ions migrate under the influence of the electric field generated by the potential difference imposed between the anode 4 and the cathode 2.

[0010] The electrolysis of CO2 operates on the same principle as the electrolysis of water, except that the half-reaction at the cathode is as follows: 2CO2 + 4e - → 2CO + 2O 2- .

[0011] In the battery mode, the half-reactions are reversed, but O 2- ions migrating in the electrolyte are always present.

[0012] As shown between the parentheses in FIG. 1, the water vapor at the cathode inlet may be accompanied by hydrogen, H2, and the hydrogen generated and recovered at the outlet may be accompanied by water vapor. Similarly, as shown by the dashed line, exhaust gas such as air may be optionally injected from the inlet to remove the generated oxygen. The injection of the exhaust gas has an additional function of acting as a heat regulator.

[0013] The basic electrolytic reactor consists of the above-described basic cell having a cathode 2, an electrolyte 3, and an anode 4, and two monopole connectors providing electrical, hydraulic, and heat distribution functions.

[0014] To increase the flow rates of hydrogen and oxygen produced, it is known that multiple basic electrolytic cells are stacked on top of each other and separated by interconnection devices, commonly referred to as bipolar interconnection plates or interconnectors. The assembly is located between two end interconnection plates that provide the electrical and gas supply means for the electrolytic cell (electrolytic reactor).

[0015] A high-temperature water electrolytic cell (HTE) includes at least one electrolytic cell, and generally multiple electrolytic cells are stacked on top of each other. Each basic cell is formed by an electrolyte, a cathode, and an anode, with the electrolyte interposed between the anode and the cathode.

[0016] Fluid and electrical interconnect devices that electrically contact one or more electrodes generally provide the function of introducing and collecting electric current and partitioning one or more chambers / compartments for gas circulation.

[0017] Therefore, the function of the "cathode" compartment chamber is to distribute electric current and water vapor, and to recover hydrogen at the cathode it is in contact with.

[0018] The function of the "anode" compartment chamber is to distribute the electric current and, if necessary, recover the oxygen generated at the anode it is in contact with using exhaust gas.

[0019] Figure 2 is an exploded view of a basic unit of a prior art high-temperature steam electrolytic cell. This HTE electrolytic cell has multiple basic solid oxide (SOEC) type electrolytic cells C1, C2, etc., stacked alternately by an interconnector 5. Each cell C1, C2, etc. consists of cathodes 2.1, 2.2, etc., with electrolytes 3.1, 3.2, etc., placed between them, and anodes 4.1, 4.2. Current is supplied in series and gas is supplied in parallel to the electrolytic cell assembly.

[0020] The interconnector 5 is a component made of a metal alloy, and provides separation between the cathode compartment 50 and the anode compartment 51, which are respectively 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. It also ensures the distribution of gas between cells. The injection of water vapor into each basic unit is carried out in the cathode compartment 50. The collection of hydrogen and residual water vapor generated at the cathodes 2.1, 2.2, etc. is carried out in the cathode compartment 50 downstream of the cells C1, C2, etc., after the dissociation of water vapor by the cells C1, C2, etc. The collection of oxygen generated at the anode 4.2 is carried out in the anode compartment 51 downstream of the cells C1, C2, etc., after the dissociation of water vapor by the cells C1, C2, etc.

[0021] The interconnector 5 ensures the passage of current between cells C1 and C2 by contact, preferably direct contact, between adjacent electrodes, namely the anode 4.2 and the cathode 2.1.

[0022] In a solid oxide fuel cell, SOFC, the cells C1, C2, etc. used and the interconnector 5 are the same components, but the operation involves the reverse current direction, as well as air or oxygen O2 supplied to the section that becomes the cathode compartment, and hydrogen and / or methane CH4 as fuel supplied to the section that becomes the anode compartment, which is opposite to that of the HTE electrolyzer described immediately above.

[0023] Regarding the material, the solid electrolyte is a material impermeable to gases, which should allow oxygen atoms to diffuse in the form of O 2- ions at temperatures above 500 °C.

[0024] Regarding each electrode of the SOEC / SOFC cell, generally, it consists of a porous cermet mainly composed of silica and nickel on the hydrogen / H2O side (the cathode in the (co)electrolysis mode and the anode in the SOFC cell mode).

[0025] For operation, the cermet on the hydrogen / H2O side needs to contain nickel, and the nickel contains a reduced form. This is because this reduced metal plays a role in breaking the H - O bond. However, O 2-Even without an electric current, ions can move from the air / O2 side cermet through the electrolyte to the H2 side cermet.

[0026] Furthermore, when an SOFC battery or an HTE / SOEC electrolytic reactor or co-electrolytic reactor is started, more specifically in the latter case, a lack of current may frequently occur, potentially leading to intermittent electricity generation.

[0027] It has been proven necessary to ensure that the temperature of the SOFC fuel cell or HTE / SOEC co-electrode or electrolytic reactor remains constant, while avoiding excessively rapid thermal cycling that could damage them, and on the other hand, to provide options in that the HTE / SOEC reactor can be quickly started up as soon as electricity becomes available again, or to utilize the current generated for the cell. Such operating modes are known by the terms "standby" or "hot standby" mode.

[0028] If the current is zero, then the above O 2- Although the ion flow is small, in cells that are typically kept at an operating temperature of 700°C to 800°C for extended periods, gradual oxidation of the H2-side cermet due to the aforementioned flow may be observed.

[0029] To limit the risk of such oxidation while keeping an SOEC reactor or SOFC fuel cell in hot standby mode, i.e., at a temperature high enough to start up virtually instantaneously, the most common method involves flushing the H2 / H2O side chamber with a continuous stream of pure hydrogen or hydrogen diluted with an inert gas.

[0030] For safety and cost reasons, safety gases consisting of nitrogen with approximately 5% H2 are preferred. These safety gases may be supplied from a container or generated in situ by a dedicated electrolytic reactor and / or air separation unit (ASU), which can produce particularly high-purity oxygen, nitrogen, and noble gases.

[0031] However, continuous flushing with safety gas is costly in the following ways: - Materials to be implemented: Safety gas can be reused and circulated in the loop, but it needs to be purged after each startup to avoid contamination of the generated hydrogen. - Electricity consumption: Gas must be moved, especially using a circulating fan. - Heat consumption: Safety gases must be preheated so as not to cool them before they arrive at the high-temperature chamber of the SOEC reactor or SOFC fuel cell.

[0032] Other solutions as alternatives to the flow of safety gases are publicly known from the literature.

[0033] For example, patent US9,005,827B2 describes a method for continuously operating each cell with a low current applied at a cell voltage in the range of 700 to 1500 mV in order to prevent the re-oxidation of nickel Ni to NiO.

[0034] Patent JP2626395B2 also proposes periodically using SOFC batteries in electrolytic mode to reduce the cermet, which can be partially oxidized during operation of the SOFC battery, thereby extending the battery life.

[0035] In contrast, it is also known that the operation of an SOEC or co-electrolytic reactor can be reversed, i.e., operated in SOFC fuel cell mode, to generate an electric current from hydrogen (H2), synthesis gas (a mixture of hydrogen (H2) and carbon monoxide (CO)), or methane, thereby enabling the reactor temperature to be maintained. This has a major drawback: the current generated is not necessarily recoverable because there is no electricity available from an external power source. Furthermore, another major drawback is that the fuel, i.e., H2, synthesis gas, or methane, is consumed in this way solely for the purpose of maintaining the reactor temperature, i.e., burned, without producing any other combustible products, and only electricity that is not necessarily recoverable at present.

[0036] Patent application US2003 / 0235752 proposes the placement of a getter material such as nickel, which can react with trace amounts of oxygen in the flow entering the hydrogen compartment so that it is oxidized instead of the cermet. This solution may allow flushing with virtually pure nitrogen (H2-free) as any remaining trace amounts of oxygen are captured by the added material. Such a flushing gas (pure nitrogen) has the advantage of being relatively inexpensive, but this implementation involves the introduction of O into the electrolyte. 2- This method does not solve the problem of energy consumption of flushing gas due to ion electrophoresis, the use of a compressor, or the need for preheating. [Prior art documents] [Patent Documents]

[0037] [Patent Document 1] U.S. Patent No. 9,005,827 [Patent Document 2] Patent No. 2626395 [Patent Document 3] U.S. Patent No. 2003 / 0235752 [Overview of the project] [Problems that the invention aims to solve]

[0038] Therefore, in particular to overcome the aforementioned drawbacks, there is a need to improve existing solutions for maintaining hot standby mode while limiting the risk of oxidation in SOEC reactors or SOFC fuel cells.

[0039] The objective of this invention is to satisfy this need at least partially. [Means for solving the problem]

[0040] To do this, the present invention relates to a method for operating a fuel cell (SOFC) or high-temperature co-electrolytic or electrolytic reactor having a stack of solid oxide type basic electrochemical cells (SOECs) in hot standby mode, comprising the step of supplying pulses of safety gas to a hydrogen / water (H2 / H2O) electrode side compartment at regular intervals over a given period of time, to refresh the gas present in the compartment, when no current is flowing out and / or applied to the stack, or when the temperature of the battery or reactor should rise or fall, or when the cell voltage drops below a threshold.

[0041] Herein, and in the context of the present invention, “hot standby mode” is understood to mean keeping the SOFC fuel cell or SOEC electrolytic reactor at its normal operating temperature, typically 700°C to 800°C, while it is stopped due to a lack of current at the outlet (cell) or inlet (SOEC reactor).

[0042] The safety gas is advantageously selected from pure hydrogen (H2) and hydrogen (H2) diluted with nitrogen, preferably to 1% to 5% by volume with nitrogen. Hydrogen (H2) diluted to approximately 3% by volume with nitrogen is optimal.

[0043] Advantageously, the stack voltage is monitored. When the cell voltage exceeds 0.8V or lower, a gas pulse is delivered. In other words, the process of supplying safety gas pulses is advantageously performed for cell voltage thresholds of 0.8V or less.

[0044] Another advantage is that the pulse flow rate of the safety gas is 10 NmL / min / cm³. 2 Less than or equal to, preferably 5 NmL / min / cm² 2 It is less than 6 NmL / min / cm³. Typically, the flow rate is about 6 NmL / min / cm³. 2 That is the case.

[0045] The pulse flow rate, interval, and duration depend on the installation configuration and the volume / distance ratio between the cell stack and the measurement and control unit. The pulse profile (ramp between zero and maximum flow rate) can also be advantageously varied by the stack model and supply line configuration to limit the effects of "water hammer," which can be detrimental to the electrochemical system.

[0046] According to a favorable modification, when a pulse of safety gas is not supplied to the hydrogen / water (H2 / H2O) electrode side compartment, all gas supply lines to the reactor or fuel cell are closed to limit the cooling of the reactor or fuel cell by gas movement.

[0047] According to an advantageous embodiment, simultaneously with or with a time delay from the pulsing of the safety gas, the compartment on the oxygen (O2) electrode side is purged with a neutral gas or a gas that is significantly oxygen-deficient. This reduces the partial pressure of oxygen, thereby reducing the amount of oxygen being delivered towards the electrolyte. 2- The flow of ions is reduced or eliminated.

[0048] In a favorable modification, the stack is heated to maintain its temperature simultaneously with a pulse of safety gas to compensate for heat loss due to convection through the chamber housing the SOEC reactor or SOFC fuel cell. In this modification, the stack is heated using a heated bottom plate in contact with the stack.

[0049] The method according to the present invention can be advantageously implemented in a unit referred to as a power-to-gas unit, which includes multiple reactors (SOECs).

[0050] As a result, the present invention essentially consists of intermittently delivering a safety gas to an H2 / H2O compartment / chamber at regular intervals in a solid oxide electrochemical cell system (SOEC reactor or SOFC fuel cell) in hot standby mode.

[0051] In this way, by regularly replacing the existing gas with an appropriate safe gas, the risk of oxidation of the hydrogen electrode cermet is eliminated.

[0052] Furthermore, the use of an intermittent flow of safety gas has the effect of eliminating cooling due to gas convection within the chamber where the reactor / SOFC fuel cell is located.

[0053] Heat loss due to the hot chamber can be compensated for by heating the chamber itself or by directly heating the cell stack, particularly by heating the bottom plate that is in contact with the stack.

[0054] The frequency and amount (flow rate, duration) of flushing with safety gas must be set as a function of the implemented electrochemical system. More specifically, the following settings can be made: - O that moves easily 2- Depending on the type and manufacturer of the cell, which directly affects the ion flow: this can vary because it depends on the varying thicknesses of the layers that make up the cell (cermet on the H2 and O2 sides, electrolyte). - Depending on the partial pressure of O2 on the O2 circulation compartment side: The higher the partial pressure of the compartment, the more easily the cermet on the O2 side is oxidized, which causes O to oxidize at the O2 cermet / electrolyte interface. 2- The driving force for ion production increases. - Volume of piping from safety gas reservoir / circulator: The longer the distance traveled, and the larger the volume it represents, the greater the need to inject gas to refresh the atmosphere in the electrochemical cell stack. -Concentration of reducing agent in safety gases, especially hydrogen: The greater the reducing effect of this gas, the smaller the volume required to refresh the reducing atmosphere in the stack.

[0055] Finally, the present invention offers many advantages, among which the following can be mentioned: - Reduce energy costs by consuming just enough safety gas to avoid oxidation of the cermet in SOFC fuel cells or SOEC reactors. - Minimizing the load on the hardware required to supply safety gases reduces investment costs and extends the lifespan.

[0056] Further advantages and features of the present invention will become clearer by reading the detailed description of implementation examples of the invention, which are given by non-limiting examples with reference to the following figures. [Brief explanation of the drawing]

[0057] [Figure 1] This is a schematic diagram illustrating the operating principle of a high-temperature water electrolytic cell. [Figure 2] This is a schematic exploded view of a portion of a high-temperature steam electrolytic cell, including the interconnector. [Modes for carrying out the invention]

[0058] Figures 1 and 2 have already been described in the preamble. Therefore, they will not be described below.

[0059] Furthermore, it is specified that the electrolytic cell or fuel cell described is a solid oxide type that operates at high temperatures (SOEC, an abbreviation for "Solid Oxide Electrolyte Cell," or SOFC, an abbreviation for "Solid Oxide Fuel Cell"). As a result, all components of the electrolytic cell or stack (anode / electrolyte / cathode) are ceramics. The high-temperature operating temperature of the electrolytic cell (electrolytic reactor) or cell is typically 600°C to 1000°C. Typically, the characteristics of the cathode-supported (CSC) type SOEC electrolytic cell according to the present invention may be as shown in Table 1 below.

[0060] [Table 1]

[0061] According to the present invention, when the SOEC reactor or SOFC fuel cell is in hot standby mode, the safety gas is delivered intermittently at regular intervals to the H2 / H2O compartment / chamber.

[0062] The safety gas is preferably hydrogen (H2) diluted with nitrogen to approximately 3% by volume.

[0063] Advantageously, the stack voltage is monitored. If the cell voltage exceeds 0.8V or lower, a pulse of safety gas is delivered.

[0064] Typically, the pulse flow rate of safety gas is approximately 6 NmL / min / cm³. 2 That is the case.

[0065] The present invention is not limited to the examples described immediately prior to this document, and the features of the examples shown can be combined within a range of variations not specifically indicated.

[0066] Further modifications and improvements may be envisioned without departing from the scope of the present invention. [Explanation of Symbols]

[0067] 1. Basic electrolytic cell / electrolytic reactor / co-electrolytic reactor 2 cathode 2.1 Cathode 2.2 Cathode 3 Solid electrolyte 3.1 Electrolytes 3.2 Electrolytes 4 Anode 4.1 Anode 4.2 Anode 5 Interconnectors 50 Cathode compartments 51 Anode compartment C1 Basic electrolytic cell C2 Basic electrolytic cell

Claims

1. A method for operating a fuel cell (SOFC) or high-temperature co-electrolytic or electrolytic reactor (1) having a stack of basic solid oxide type electrochemical cells (SOECs) in hot standby mode, wherein when no current flows out of and / or is applied to the stack for a given period of time, or when the temperature of the battery or reactor should rise or fall, or when the cell voltage drops below a threshold, hydrogen / water (H ２ / H ２ O) A method comprising the step of supplying pulses of safety gas to an electrode-side compartment at regular intervals during a given period of time, thereby refreshing the gas present in the compartment.

2. The aforementioned safety gas is pure hydrogen (H ２ ) and hydrogen (H) diluted with nitrogen, preferably diluted to 1% to 5% by volume with nitrogen. ２ The method according to claim 1, selected from among the following.

3. The method according to claim 1 or 2, wherein the step of supplying the pulse of the safety gas is performed for a cell voltage threshold of 0.8V or less.

4. The pulse flow rate of the safety gas is 10 N mL / min / cm². ２ Less than 5 N mL / min / cm², preferably 5 N mL / min / cm² ２ The method according to any one of claims 1 to 3, wherein the result is less than [amount missing].

5. The aforementioned hydrogen / water (H ２ / H ２ O) When no pulse of safety gas is supplied to the compartment on the electrode side, all gas supply lines of the reactor or fuel cell are closed to limit the cooling of the reactor or fuel cell by the movement of the gas, the method according to any one of claims 1 to 4.

6. Simultaneously with the purging of the safety gas or with a time shift from the purging of the safety gas, the compartment on the oxygen (O ２ ) electrode side is purged using a neutral gas or a gas that is significantly oxygen-deficient, according to any one of claims 1 to 4.

7. The method according to any one of claims 1 to 6, wherein the stack is heated to maintain the temperature of the safety gas at the same time as the safety gas is pulsing.

8. The method according to claim 7, wherein the stack is heated using a heating bottom plate that is in contact with the stack.

9. The method according to any one of claims 1 to 8, implemented in a unit called a power-to-gas unit, which includes multiple reactors (SOEC).

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