Method for operating an electrolyser system and electrolyser systems

Abstract

A system comprising an electrolyser cell stack of electrolyser cell units, a separation unit and a Fischer-Tropsch reactor unit and a method for operating the system. The method comprising providing fuel to a fuel volume of the electrolyser cell stack, wherein the fuel comprises steam and carbon dioxide and is provided to the fuel volume at a first temperature. The method comprises powering the electrolyser cell stack with electrical energy thereby converting, at least partially, the steam into hydrogen and oxygen, wherein hydrogen is released into the fuel volume and oxygen is released into an oxygen volume. Carbon dioxide and hydrogen are at least partially converted into carbon monoxide and water in the fuel volume. Powering the electrolyser cell stack comprises controlling a voltage supplied at an endothermic value.

Description

METHOD FOR OPERATING AN ELECTROLYSER SYSTEM AND ELECTROLYSER SYSTEMSField of the Disclosure

[0001] The present disclosure relates to a method for operating a system comprising an electrolyser cell stack of electrolyser cell units and systems comprising an electrolyser cell stack of electrolyser cell units.Background to the Disclosure

[0002] Electrochemical cell units are commonly referred to as fuel cell units or electrolyser cell units, and in some instances their names are interchangeable as some fuel cell units can work as electrolyser cell units and some electrolyser cell units can operate as fuel cell units, each either as a producer of electricity or in a regenerative mode - electrolyzing a fluid to electrochemically split it into two or more component parts. For example, some fuel cell units can produce electricity by using an electrochemical conversion process that oxidises fuel to produce electricity. Some fuel cell units can also, or instead, operate as regenerative fuel cells (or reverse fuel cells) units, often known as electrolyser cell units, for example to separate hydrogen and oxygen from water, carbon monoxide and oxygen from carbon dioxide, or nitrogen monoxide and oxygen from nitrogen dioxide. They may be tubular or planar in configuration. Planar cell units may be arranged overlying one another in a stack arrangement, for example 100-500 cell units in a stack, with the individual cell units arranged, for example, electrically in series. Tubular cell units may be arranged in groups or stacks thereof.

[0003] A solid oxide fuel cell (SOFC) unit that produces electricity is based upon a solid oxide electrolyte that conducts negative oxygen ions from an oxygen electrode to a fuel electrode located on opposite sides of the electrolyte. For this, a fuel, or reformed fuel, contacts the fuel electrode and an oxidant, such as air or an oxygen rich fluid, contacts the air electrode.

[0004] A solid oxide electrolyser cell (SOEC) sometimes has the same structure as an SOFC but is essentially a SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of fuel, for example water, by input of electrical energy and using the solid oxide electrolyte to produce hydrogen gas and oxygen. Conventional ceramic-supported (e.g. fuel electrode-supported) SOFCs and SOECs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOFCs and SOECs have been developed which have the active cell component layer supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported cell units are more robust, lower cost, have better thermal properties than ceramic-supported cell units and can be manufactured using conventional metal welding techniques.

[0005] Electrolyser cell units (and stacks thereof) operate at elevated temperatures. Intermediate or high temperature electrolysers (such as SOEC and molten carbonate electrolyte) have operational temperatures in excess of 670 K (approx. 400 C), typically 720 K to 970 K (approx. 450 to 700C) for an intermediate temperature electrolyser such as one based on an MS-SOEC. Electrolyser cell stack(s) are placed in electrolyser systems and the electrolyser system supplies input gas to and exhausts off-gas (product and unspent fuel) from the electrolyser cell stack(s) at the required operational temperatures.

[0006] Some electrolyser cell units and stacks thereof can operate in a co-electrolysis mode wherein carbon dioxide and steam are input as fuel and are both electrolysed, thereby producing carbon monoxide, hydrogen and oxygen (CO2 + H2O + 4e- -> CO + H2+2O2. The conversion of carbon dioxide and hydrogen to carbon monoxide may be a catalytic, endothermic reaction (AH = 41 kJ / mol) and is thus favoured at high temperatures.

[0007] When carbon dioxide and hydrogen are present in the fuel volume of an electrolyser cell stack, they can combine to form methane in an exothermic reaction. Accordingly, at lower operating temperatures, methane production is favoured over carbon monoxide production. Relatively high operating temperatures (e.g. 980 to 1120 K) of electrolyser cell units and electrolyser cell stacks thereof are preferred in order to selectively favour production of carbon monoxide over methane when steam and carbon dioxide are supplied. At such temperatures, the resistance of the electrolyser cell units and stacks thereof is low, leading to relatively high electrical current densities for a given voltage. A method of operation of an electrolyser cell stack is thus required which favours production of carbon monoxide and hydrogen at a current density suitable for present stack designs.

[0008] The present disclosure seeks to address, overcome or mitigate at least one of the prior art disadvantages.Summary of the Disclosure

[0009] In accordance with aspects herein, there is provided a method for operating a system comprising an electrolyser cell stack of electrolyser cell units; a controller configured to perform the method; and systems comprising an electrolyser cell stack of electrolyser cell units.

[0010] In a first aspect, there is provided a method for operating a system comprising an electrolyser cell stack of electrolyser cell units, the electrolyser cell stack comprising a fuel volume and an oxygen volume, the method comprising: providing fuel to the fuel volume of the electrolyser cell stack, wherein the fuel comprises steam and carbon dioxide, wherein the fuel is provided to the fuel volume at a first temperature; powering the electrolyser cell stack with electrical energy thereby converting,at least partially, the steam into hydrogen and oxygen, wherein hydrogen is released into the fuel volume and oxygen is released into the oxygen volume; converting, at least partially, the carbon dioxide and hydrogen into carbon monoxide and water in the fuel volume; wherein powering the electrolyser cell stack comprises controlling a voltage supplied at an endothermic value.

[0011] When steam and carbon dioxide are provided as fuel to an electrolyser cell stack, two reactions occur in parallel to electrolysis of steam:• Reaction 1: H2+ CO2- CO + H2O AH = 41 kJ / mol• Reaction 2: 3H2+ CO ^ CH4+ H2O AH = -206 kJ / molReaction 1 may be referred to as the reverse water-gas shift reaction, which converts carbon dioxide and hydrogen into carbon monoxide and water and is endothermic. Thus, Reaction 1 is favoured at relatively high temperatures. Reaction 2 may be referred to as methanation, which is the conversion of carbon monoxide to methane through hydrogenation and is exothermic. Relatively lower temperatures therefore favour Reaction 2.

[0012] The present method achieves relatively high selectivity toward carbon monoxide production (Reaction 1), and therefore low selectivity toward methane production. This is achieved by controlling a voltage supplied to the electrolyser cell stack at an endothermic value. An endothermic voltage is one which is lower than a thermoneutral voltage (thermoneutral voltage or V=VTN). The thermoneutral voltage is dependent on a given electrochemical cell (e.g., on technology and efficiency of a cell), but for some cell units, including those herein, may be ~1.28 V for steam electrolysis.

[0013] Relatively high temperatures are desired to favour Reaction 1. A problem with running an electrolyser cell stack at relatively high temperatures is that resistance will be relatively low. If operation of the electrolyser cell stack was performed at a thermoneutral voltage at a relatively high temperature, the current density would be high, potentially exceeding the design limits of current electrolyser cell stacks, thus leading to damage thereof. The present method advantageously includes controlling a voltage supplied to the stack at an endothermic value, meaning that the voltage is lowered compared to a thermoneutral voltage. In turn, the current density is controlled, i.e., lowered compared to the current density achieved while operating at the thermoneutral voltage, thereby enabling a relatively high temperature while keeping within acceptable current density ranges. The present method enables operation at a higher temperature (than if at a thermoneutral voltage or an exothermic voltage (V>VTN)) and thus a relatively greater conversion of carbon dioxide and hydrogen to carbon monoxide and water is achieved. The present method thus selectively produces carbon monoxide at a current density which electrolyser stacks can safely and efficiently withstand. Ultimately, carbon monoxide and hydrogen (also termed synthetic gas or syngas) can be transformed into hydrocarbons, for example liquid fuels (e.g. kerosene for aviation fuel, which may be termedsustainable aviation fuel, e.g., when power for the electrolysis reaction is provided from renewable sources), and polymers via the Fischer-Tropsch process.

[0014] Preferably, the first temperature is at most 970 K, preferably between 870 and 970 K, preferably between 870 and 950 K, and more preferably between 870 and 920 K. The conversion of carbon dioxide and hydrogen to carbon monoxide and hydrogen is endothermic. Production of carbon monoxide and hydrogen is therefore favoured at relatively high operating temperatures. In preferred arrangements, the first temperature is at least 830 K, or at least 850 K, or at least 870 K.

[0015] Preferably, the first temperature is within 15% of a maximum rated steady state temperature.Optionally, the first temperature is within 10% or 5% of a maximum rated steady state temperature, calculated on a Kelvin scale. A maximum rated steady state temperature may be between 870 and 1020 K, more preferably between 900 and 970 K, more preferably between 920 and 950 K.

[0016] Preferably, each electrolyser cell unit comprises a fuel electrode in fluidic communication with the fuel volume, a counter-electrode in fluidic communication with the oxygen volume, and an electrolyte separating the fuel electrode and the counter-electrode, the electrolyte comprising ceria, preferably doped ceria. In some examples, the doped ceria is gadolinium doped ceria. A ceria-based electrolyte allows operation at relatively low temperature for a solid oxide electrolyser cell stack (e.g. an SOEC), which improves efficiency and reduces system cost. A ceria-based electrolyte also allows efficient operation of an electrolyser cell stack at relatively higher temperatures, wherein the production of carbon monoxide is favoured over methane production, e.g., between 870 and 970 K.

[0017] Preferably, a steam utilisation (i.e., by the stack) is between 50% and 80%. Preferably, a steam utilisation is at most 75%, wherein the method further comprises transferring heat from the steam to the electrolyser cell stack. Optionally, the steam utilisation is at most 65%, at most 55%, at least 40%, or at least 50%. In some examples, steam utilisation may be at most 80%, for example where steam is relatively costly. In some examples, steam utilisation may be between 50% and 80%, preferably between 55% and 65%. The term "steam utilisation" may be a percentage of input steam electrochemically consumed by the electrolysis reaction in the electrolyser cell stack. In effect, the current provided to the electrolyser cell stack is sufficient to reduce only a part of the steam provided, hence steam utilisation is as described herein. In effect, the steam cycled through the stack is more than is needed for the reactions. Thus, a lower steam utilisation means there is more / excess steam (for a given current) than is needed electrochemically. Lower steam utilisation / excess of steam reduces risk of deposition of elemental carbon (from cracking methane or reduction of carbon dioxide with hydrogen). In such scenarios, the excess steam acts as a heat source and can maintain the stack (including the fuel volume) at a relatively high temperature despite the endothermic voltage and therefore improve the selectivity towards carbon monoxide production. Steam utilisation also affectscarbon monoxide production: Increasing concentration of steam in the fuel volume (i.e., lower steam utilisation) suppresses the methanation reaction by shifting its equilibrium toward reactants (carbon monoxide and hydrogen), thereby improving carbon monoxide selectivity. However, the water-gas shift reaction drives carbon monoxide toward carbon dioxide at high concentrations of steam, reducing net carbon dioxide conversion. Tests show that these competing effects produce a broad 'sweet spot' for carbon monoxide production. Therefore, preferably steam utilisation is between 50 and 75%, preferably between 55 and 70%, more preferably between 55 and 70%.

[0018] Preferably, the method further comprises providing a sweep gas to the oxygen volume of the electrolyser cell stack, wherein the sweep gas is provided at a third temperature and the third temperature is substantially equal to the first temperature. In this context, the term "substantially equal" may mean that the first and third temperatures are within 15 K of each other. The sweep gas advantageously assists in exhaust of oxygen from the oxygen volume. Providing the sweep gas at a temperature substantially equal to the first temperature maintains the relatively high temperature of the electrolyser cell units in order to keep the selectivity of endothermic carbon monoxide production relatively high (versus exothermic production of methane).

[0019] Preferably, providing the sweep gas comprises flowing the sweep gas through the oxygen volume in an opposite direction to the direction of flow of fluid through the fuel volume. This is a counterflow arrangement. Fluid flowing through the fuel volume comprises fuel and products of the reactions in the stack (e.g., hydrogen from electrolysis and carbon monoxide from reverse water gas shift reactions). Heat is transferred from the sweep gas to the cell units at least proximal to an inlet end (from the perspective of sweep gas flow) of the cell units. A counterflow arrangement is advantageous because the sweep gas is used to heat the cell units at an inlet of the oxygen volume (which is in the vicinity of the outlet of the fuel volume due to the counterflow arrangement). This enables the temperature of an outlet of the fuel volume to be heated, thereby improving selectivity for the endothermic conversion of hydrogen and carbon dioxide to carbon monoxide (which is promoted by relatively higher temperatures). Simultaneously, the peak temperature experienced by the electrolyser cell stack is minimised compared to running the stack in a co-flow arrangement with an elevated fuel inlet temperature to achieve the same fuel outlet temperature in the fuel volume. Further, due to the counterflow arrangement the cell units experience a more consistent temperature across their extent than for a co-flow arrangement, despite use of the endothermic voltage. A co-flow arrangement is where the fuel in the fuel volume flows in the same direction as the fluid (oxygen and sweep gas) in the oxygen volume.

[0020] Preferably, off-gas is exhausted from the oxygen volume, the off-gas comprising the sweep gas and oxygen as a product of the electrolyser cell units, wherein at least 30% by volume of the off-gas issweep gas. The sweep gas is used as a heat source to improve selectivity for carbon monoxide production, as more sweep gas than necessary for sweeping out the product is supplied to the oxygen volume. Preferably, the volume of the off-gas which is sweep gas is at least 40% or, at least 50%, at most 80% or at most 60%.

[0021] Preferably, off-gas is exhausted from the fuel volume at a second temperature lower than the first temperature. In some examples, the second temperature is between 5 and 60 K lower than the first temperature. Preferably, the second temperature is between 5 and 45 K, preferably between 5 and 30 K, preferably between 5 and 20 K. The second temperature is lower than the first temperature due to the endothermic voltage, but the difference between the first and second temperatures is reduced relative to that expected for the endothermic voltage, for example by counterflow of sweep gas through the oxygen volume and / or supply of excess steam to the fuel volume. Off-gas exhausted from the fuel volume comprises products of the reactions in the stack (e.g., hydrogen from electrolysis and carbon monoxide from reverse water gas shift reactions) and unspent fuel (i.e., unreacted steam and carbon dioxide).

[0022] Preferably, off-gas is exhausted from the fuel volume, wherein the off-gas comprises hydrogen and carbon monoxide produced in the electrolyser cell stack and unconsumed carbon dioxide and steam, the method further comprising separating the carbon dioxide and steam from the off-gas and recirculating the carbon dioxide and steam to the fuel volume of the electrolyser cell stack. Advantageously, unconsumed steam and carbon dioxide can be recirculated for use (as fuel) in the electrolyser cell stack. Steam may be converted from gaseous water to liquid water as part of the separation and / or recirculation processes.

[0023] Preferably, separating the steam from the off-gas comprises condensing the steam to liquid water. Preferably, a condenser is used for condensing the steam to liquid water.

[0024] Preferably, separating the carbon dioxide from the off-gas is performed by an amine absorption unit. Separation of carbon dioxide is preferably subsequent to separating steam from the off-gas by condensation, such that the amine absorption unit separates carbon dioxide from the remaining mixed off-gas of a mixture of carbon monoxide, hydrogen and carbon dioxide.

[0025] Preferably, the method further comprises compressing the off-gas from the fuel volume. In some examples, compressing the off-gas from the fuel volume is performed prior to separating the carbon dioxide and steam from the off-gas.

[0026] Preferably, the method further comprises providing hydrogen and carbon monoxide from the electrolyser cell stack to a Fischer-Tropsch reactor unit for performing a Fischer-Tropsch reaction, thereby producing a mixture of hydrocarbons as a product of the Fischer-Tropsch reaction. The Fischer-Tropsch reactor unit facilitates a series of chemical reactions which convert carbon monoxideand hydrogen into a product comprising a mixture of hydrocarbons of varying molecular weights. The hydrocarbons in the product typically have the formula CnH2n+2. A particularly desired hydrocarbon product is kerosene (C12H26), due to its use as aviation fuel, and may be referred to as sustainable aviation fuel due to use of the electrolyser in production of the carbon monoxide and hydrogen for the Fischer-Tropsch reaction.

[0027] Preferably, the method comprises transferring heat produced by the Fisher-Tropsch reactor unit to the fuel, prior to providing the fuel to the fuel volume of the electrolyser cell stack. Heat energy can thus be recycled within the system. Preferably, the method comprises transferring heat to liquid water to raise steam.

[0028] Preferably, the method comprises transferring heat to the fuel to at least partially heat the fuel to the first temperature.

[0029] Preferably, the method comprises using the mixture of hydrocarbons as a fuel. For example, kerosene produced in the method may be used as a sustainable aviation fuel. Preferably, power for the electrolyser is from a renewable source.

[0030] Preferably, the method comprises controlling the voltage supplied to be between 60 and 95% of a thermoneutral voltage. More preferably, the method comprises controlling the voltage supplied to be between 65 and 95%, more preferably between 65 and 90%, more preferably between 70 and 85% of a thermoneutral voltage (VTN). The VTN for steam at 870 K may be around 1.28V for the cell units described herein and thus the voltage may be controlled at, for example, 1.15V (i.e. at an endothermic value). VTN is a weak function of temperature and may also depend on losses within the system, however it is known within a system as the voltage at which, during operation, the temperature of off-gas exhausted from the cell units (and stack(s)) substantially equals the temperature of fluid supplied to the cell units (and stack(s)), for example a difference between said temperatures is less than 5 K.

[0031] Preferably, the method further comprises controlling the voltage supplied at a first voltage for a first time period and subsequently controlling the voltage supplied at a second voltage for a second time period, wherein the first voltage is lower than the second voltage and is lower than a or the thermoneutral voltage. Thus, the method preferably includes controlling the voltage supplied to increase toward the thermoneutral voltage as the electrolyser cell stack ages. The time periods may each be at least 100 hours - i.e., they are not accounting for transient behaviours or demands made of the system, but accounting for changes through life of the system. The second time period is subsequent to the first time period. By increasing the voltage through the life of the electrolyser cell stack, an increase in resistance due to stack aging is off-set and current is maintained. This allows thestack to age towards a thermoneutral voltage and an isothermal state, and may result in the stack operating at the thermoneutral voltage later in its life.

[0032] Preferably, the method further comprises one or more of: reducing the temperature of fuel provided to the fuel volume for the second time period relative to the first time period; increasing a or the steam utilisation for the second time period relative to the first time period; and decreasing a flow rate of a or the sweep gas to the oxygen volume for the second time period relative to the first time period. As the voltage is increased towards the thermoneutral voltage, the voltage becomes 'less' endothermicso that the temperature across the length of the cell unit becomes more uniform or even. As the voltage becomes closer to thermoneutral (i.e., less endothermic), the temperature-decreasing effect of the endothermic voltage and variance in temperature across the length of the cell units decreases, and so the abovementioned balancing factors can be adjusted to reduce parasitic loss from these factors.

[0033] Preferably, the method further comprises controlling the voltage supplied at a third voltage for a third time period, wherein the third voltage substantially equals the thermoneutral voltage. The third time period may be subsequent to the second time period. In this context, "substantially equals" may mean that the voltage is within 0.05V of the thermoneutral voltage. In effect, as the electrolyser cell stack ages, the method preferably includes controlling the voltage supplied to increase to equal the thermoneutral voltage.

[0034] Preferably, the electrolyser cell stack comprises a reverse water-gas shift catalyst in the fuel volume to convert, at least partially, the carbon dioxide and hydrogen into carbon monoxide and water. Preferably, the reverse water-gas shift catalyst may comprise one or more of Pt, Ru, Rb, Ni, or Fe. The reverse water-gas shift catalyst may be coated or deposited on an interconnect of a cell unit, e.g., on at least part of a first side of the interconnect adapted to face (and bound) the fuel volume. A second side of the interconnect may be adapted to face (and bound) the oxygen volume. Alternatively or additionally, the reverse water-gas shift catalyst may be in the material content of the interconnect. Preferably, the reverse water-gas shift catalyst is located along the length of the fuel volume. For example, the reverse water-gas shift catalyst may extend substantially along the entire length of the fuel volume.

[0035] Preferably, the electrolyser cell stack is a solid oxide electrolyser cell stack. Preferably, the system comprises a plurality of electrolyser cell stacks. More preferably, the system comprises a plurality of solid oxide electrolyser cell stacks. The cells in a stack are preferably in series. The plurality of stacks may be connected in series or in parallel.

[0036] In a second aspect, there is provided a controller configured to perform the method of the first aspect, or an electrolyser system comprising a controller configured to perform the method of the first aspect.

[0037] In an example, there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of the first aspect.

[0038] In another example, there is provided a non-transitory computer-readable medium with instructions stored thereon, that when executed by a processor, perform the steps of the method of the first aspect.

[0039] In a third aspect, there is provided a system comprising:an electrolyser cell stack of electrolyser cell units, the electrolyser cell stack comprising a fuel volume and an oxygen volume, a fuel volume inlet and a fuel volume outlet in fluidic communication with the fuel volume, and an oxygen volume inlet and an oxygen volume outlet in fluidic communication with the oxygen volume;a fuel supply line in fluidic communication with the fuel volume inlet for providing fuel to the fuel volume of the electrolyser cell stack at a first temperature, wherein the fuel is configured to comprise steam and carbon dioxide; an off-gas line in fluidic communication with the fuel volume outlet for exhausting off-gas from the fuel volume;a power supply unit configured to provide the electrolyser cell stack with electrical energy to convert, at least partially, steam into hydrogen and oxygen, wherein hydrogen is configured to be released into the fuel volume and oxygen is configured to be released into the oxygen volume;the electrolyser cell stack configured to convert, at least partially, carbon dioxide and hydrogen into carbon monoxide and water in the fuel volume; andthe power supply unit configured to power the electrolyser cell stack by controlling a voltage supplied at an endothermic value.

[0040] Conversion of carbon dioxide and hydrogen to carbon monoxide is endothermic and requires relatively high temperatures to ensure selectivity towards carbon monoxide production. The present method includes controlling a voltage supplied to the electrolyser cell stack at an endothermic value. An endothermic voltage is one which is lower than a thermoneutral voltage (VTN). The thermoneutral voltage may be dependent on a given electrochemical cell (e.g., on technology and efficiency of a cell), but for some cell units may be ~1.28 V for steam electrolysis. The thermoneutral voltage is one where heat and energy required for electrolysis balance with heat generated by flow of current through the cell.

[0041] In a fourth aspect, there is provided a system comprising:an electrolyser cell stack of electrolyser cell units, the electrolyser cell stack comprising a fuel volume and an oxygen volume, a fuel volume inlet and a fuel volume outlet in fluidic communication with the fuel volume, and an oxygen volume inlet and an oxygen volume outlet in fluidic communication with the oxygen volume, the electrolyser cell stack configured to be provided with fuel comprising steam and carbon dioxide;a separation unit;a Fischer-Tropsch reactor unit for performing a Fischer-Tropsch reaction;an off-gas line in fluidic communication with the fuel volume outlet of the electrolyser cell stack and configured to route off-gas from the fuel volume of the electrolyser stack to the separation unit;wherein the separation unit is configured to separate carbon dioxide from the off-gas from the fuel volume, and the system is configured to route carbon dioxide from the separation unit to the fuel volume inlet of the electrolyser cell stack;the system further comprising a purified off-gas line in fluidic communication with the separation unit and the Fischer-Tropsch reactor unit, the purified off-gas line configured to route hydrogen and carbon monoxide from the separation unit to the Fischer-Tropsch reactor unit.

[0042] Preferably, the system comprises a power supply unit configured to power the stack of electrolyser cell units (e.g. at an endothermic voltage). Preferably, the electrolyser cell stack is configured to be powered with electrical energy to convert, at least partially, the steam in to hydrogen and oxygen and configured to convert, at least partially, the carbon dioxide and hydrogen into carbon dioxide and water.

[0043] The system advantageously enables carbon monoxide and hydrogen produced in an electrolyser cell stack to be used in a Fischer-Tropsch reactor unit. The Fischer-Tropsch process converts carbon monoxide and hydrogen into hydrocarbons. The hydrocarbons in the product typically have the formula CnH2n+2. A particularly desired hydrocarbon product is kerosene (CuF s), due to its use as aviation fuel.

[0044] Preferably, the system comprises a heat exchanger configured to transfer heat from off-gas from the fuel volume to fuel upstream of the fuel volume inlet. This improves efficiency of the system.

[0045] Preferably, the system comprises a fuel supply line in fluidic communication with the fuel volume inlet for providing fuel to the fuel volume of the electrolyser stack, wherein the fuel comprises steam and carbon dioxide. Preferably, the fuel supply comprises a steam supply line for providing steam to the fuel volume of the electrolyser; and a carbon dioxide supply line for providing carbon dioxide to the fuel volume of the electrolyser. Typically, the steam supply line and carbon dioxide supply line meet at a mixing point and are mixed upstream of the fuel volume inlet.

[0046] Preferably, the system comprises a heat integration system, configured to transfer heat from the Fischer-Tropsch reactor unit to the fuel supply line to the electrolyser cell stack. This improves efficiency of the system. Transfer of heat from the Fischer-Tropsch reactor unit to fuel in the fuel supply line uses that heat to raise the temperature of inlet fuel, for example towards the first temperature. Additional heating means, for example a trim heater may be configured to further raise the temperature to the first temperature.

[0047] Preferably, the system comprises a condenser in the off-gas line downstream of the fuel volume outlet, configured to condense water out of the off-gas from the fuel volume and route condensed water to the fuel supply line. This enhances efficiency of the system by reducing water wastage.

[0048] Preferably, the system comprises a compressor in the off-gas line downstream of the fuel volume outlet, configured to compress off-gas to a pressure of between 2,000 and 3,000 kPa. The electrolyser may be configured for operation at elevated pressure, for example between 150 and 1000 kPa, and so the compressor may be configured to raise the pressure of the off-gas from between 150 and 1000 kPa to between 2,000 and 3,000 kPa. Operation of the electrolyser at elevated pressures reduces demand on the compressor.

[0049] Preferably, the system is configured to route heat produced from compression of off-gas to the fuel supply line. This enhances efficiency of the system.

[0050] Preferably, the system is configured to raise steam for the electrolyser cell stack using heat from the Fischer-Tropsch reactor unit.

[0051] Preferably, the system is configured to route liquid water condensed in compression of off-gas to the fuel supply line. This enhances efficiency of the system by reducing water wastage.

[0052] Preferably, the separation unit is an amine absorption unit. In some examples, the amine absorption unit is a component part of the heat integration system. This enhances efficiency of the system. The amine absorption unit may operate by cyclically heating and cooling an absorption medium, which may involve heating to between 420 and 520 K (to desorb carbon dioxide) and cooling to 290 to 340 K (to absorb carbon dioxide). Thus, heat integration of the amine absorption unit may be configured to transfer heat to the amine absorption unit (medium) from the oxygen volume offgas line (preferably downstream of the oxygen volume heat recovery heat exchanger and / or downstream of the carbon dioxide heat exchanger) or to transfer heat to the amine absorption unit (medium) from the Fischer-Tropsch reactor unit (e.g., from the outlet product stream thereof). Heat integration of the amine absorption unit may additionally or alternatively be configured to transfer heat from the amine absorption to the fuel volume off-gas line downstream of the condenser and / or compressor (preferably downstream of an intercooler subsequent to the compressor on the fuelvolume off-gas line). Alternatively, heat may be transferred from the amine absorption unit by heat exchange with ambient air or coolant.

[0053] Preferably, the fuel and oxygen volumes of the electrolyser cell units are arranged in counterflow.The fuel volume inlet and the oxygen volume outlet may be arranged at or proximal to at a first end of the electrolyser cell units and the fuel volume outlet and the oxygen volume inlet may be arranged at or proximal to a second end of the electrolyser cell units. The second end is opposed to the first end, with an electrochemically active cell region therebetween. In other words, flow in the fuel and oxygen volumes is configured to be in opposite directions.

[0054] Preferably, the system comprises a reverse water-gas shift catalyst in the fuel volume of the electrolyser cell stack. Preferably, the reverse water-gas shift catalyst may comprise one or more of Pt, Ru, Rb, Ni, or Fe. The reverse water-gas shift catalyst may be coated or deposited on an interconnect of a cell unit, e.g., on at least part of a first side of the interconnect adapted to face (and bound) the fuel volume. A second side of the interconnect may be adapted to face (and bound) the oxygen volume. Alternatively or additionally, the reverse water-gas shift catalyst may be in the material content of the interconnect. Preferably, the reverse water-gas shift catalyst is located along the length of the fuel volume. For example, the reverse water-gas shift catalyst may extend substantially along the entire length of the fuel volume.

[0055] Preferably, the system comprises a plurality of electrolyser cell stacks, wherein the fuel volume outlets of respective electrolyser cell stacks are fluidical ly connected to one another to form the offgas line. As a result, the other components of the system are common to the plurality of stacks (e.g., one separation unit serves the plurality of stacks). The fuel volume outlets may be off-gas outlets. The fuel supply line for the plurality of stacks may be common to the plurality of stacks, and splits to supply the fuel volume inlets of respective stacks.

[0056] Preferably, the system comprises at least one trim heater. The at least one trim heater may be upstream of the fuel volume inlet to the electrolyser cell stack. If a heat exchanger is present, the at least one trim heater may be downstream of the heat exchanger. The trim heater is configured to provide heat to the fuel upstream of the fuel volume inlet of the stack(s) (i.e., to the fuel supply line) to provide temperature control thereof and raise the fuel to the first temperature.

[0057] Preferably, the system comprises a controller configured to perform the method steps of the first aspect.

[0058] In order that the present disclosure be more readily understood, various aspects of specific embodiments will now be described in conjunction with the attached drawings.Brief Description of the Drawings

[0059] Figs. 1 and 2 are schematic views of electrolyser systems.

[0060] Fig. 3 is a method of operating an electrolyser system.

[0061] Fig. 4 is a control device for controlling an electrolyser system.

[0062] Fig. 5 is a schematic of a system comprising an electrolyser cell stack and a Fischer-Tropsch reactor unit.

[0063] Fig. 6 is a cross-sectional view of an electrolyser cell unit.

[0064] Fig. 7 is a plot which illustrates the effect of steam utilisation on carbon monoxide production.Detailed Description

[0065] The drawings are included for illustrative purposes only. Some of the figures indicate only one electrolyser cell unit (hereafter referred to simply as a "cell unit") in an electrolyser cell stack (also referred herein simply as a "stack"), however, it will be readily apparent that a stack may include one or more electrolyser cell units. Although not illustrated in the figures, a person of ordinary skill in the art would readily understand that multiple electrolyser cell stacks can be provided by repeating a stack that is described herein. In some examples, multiple electrolyser cell stacks, each comprising multiple electrolyser cell units, are provided. It will be appreciated that the fuel volume and oxygen volume inlets, outlets (off-gas), ducting, and manifolding, and their configuration are modified as appropriate for such embodiments, and will be readily apparent to a person of ordinary skill in the art.

[0066] In general, the present disclosure relates to a method for operating a system comprising an electrolyser cell stack of electrolyser cell units, wherein the fuel provided to the stack comprises steam and carbon dioxide, the method further comprising powering the electrolyser cell stack, including controlling a voltage supplied at an endothermic value. This method favours production of carbon monoxide over methane production. The present disclosure also generally relates to a system for performing this method, and a system including an electrolyser cell stack of electrolyser cell units and a Fischer-Tropsch reaction unit, for synthesising a mixture of hydrocarbons via the Fischer-Tropsch reaction for use, e.g., as sustainable fuel, particularly sustainable aviation fuel.

[0067] Figure 1 is a simplified schematic of an electrolyser system 100 including a plurality of electrolyser stacks 110 in an enclosure 105. The enclosure 105 may be a pressure vessel 105 configured to withstand a pressure difference between its interior and exterior.

[0068] An array 110 of four stacks 110 is shown in Fig. 1, but it will be understood that any number of stacks 110 may be present, including fewer and greater than four, for example 1, 2, 10, 12 and so forth.

[0069] Each stack 110 comprises a stack of electrolyser cell units. A typical stack may have 100 to 500 electrolyser cell units, although the number of cell units can vary to account for different requirements.

[0070] The electrolyser cell units each comprise a first fluid volume (for a first fluid - typically fuel for the electrolysis process - e.g. steam and carbon dioxide) and a second fluid volume (for a second fluid - typically oxygen as a product of the electrolysis process, and optionally a sweep gas), which fluid volumes are fluidically separated from one another such that the first and second fluids therein cannot mix.

[0071] Each stack 110 has a first fluid inlet 115 and a first fluid outlet 116, each of which is in fluidic communication with the first fluid volumes of each electrolyser cell unit. Supply of the first fluid to the first fluid volume is via the first fluid inlet 115 and exhaust from the first fluid volume is by the first fluid outlet 116. The first fluid is provided to the first fluid volume of the stacks 110 from a first fluid source 143 via the respective first fluid inlets 115 of the stacks 110.

[0072] In the example of Fig. 1, the first fluid is fully manifolded within the system. That is, there is a manifold enclosing the fluid within the system and providing fluidic communication between the first fluid source 143, external to the enclosure 105, and the first fluid inlets 115 of the stacks 110. Likewise, there is a manifold providing fluidic communication between the first fluid outlets 116 of the stacks 110 and a first fluid volume off-gas collection 163, external to the enclosure 105.

[0073] Each stack 110 may have a second fluid inlet 122 and a second fluid outlet 123, which, where present, is in fluidic communication with the second fluid volume of each cell unit 10. Exhaust from the second fluid volume is by the second fluid outlet 123, and an optional supply of a second fluid to the second fluid volume is via the optional second fluid inlet 122. The optional second fluid is provided, in this example, as a sweep flow gas to the second fluid volume of the stacks 110 from a second fluid source 144 via respective second fluid inlets 122 to assist in sweeping product (e.g. oxygen) from the cell units.

[0074] In the example of Fig. 1, the second fluid is open manifolded within the system. That is, there is a vessel inlet 120 to the enclosure 105 for delivery of the second fluid to the interior of the enclosure 105 (the interior of the vessel may be referred to as a vessel volume or common volume) from the second fluid source 144, but there is no branched manifold to deliver the second fluid to respective second fluid inlets 122 of each stack 110.

[0075] Exhaust from the second fluid volume of the stacks 110 is also open manifolded. That is, there is a vessel exhaust 121 from the enclosure 105 for exhaust of second fluid volume off-gas from the interior of the enclosure 105 to the second fluid off-gas collection 164 - in this example the off-gas orproduct, mixed with the second fluid, but there is no branched manifold between the respective second fluid outlets 123 of each stack 110 and the vessel exhaust 121.

[0076] In some cases the second fluid may be partially manifolded within the enclosure - i.e. there may exist a manifold between the vessel inlet 120 and the respective second fluid inlets 122 of the stacks 110, or there may exist a manifold between the respective second fluid outlets 123 of the stacks 110 and the vessel exhaust 121.

[0077] In some cases both the inlet and exhaust of the second fluid may be manifolded, as depicted in Fig. 2, to ensure that any second fluid (herein in this example a sweep gas) that is supplied to the second fluid volume is more effective at sweeping through the cell units, such that the product / off- gas released into the second fluid volume can then be removed therefrom.

[0078] Figure 2 is a simplified schematic of an electrolyser system 101, which is otherwise similar to the electrolyser system 100 of Fig. 1, includes inlet and exhaust manifolds for the second fluid which communicate with the second fluid source 144 and second fluid collection 164, respectively. Further, the electrolyser system 101 has a vessel inlet 120 and a vessel exhaust 121 for a vessel fluid supplied to a common volume 124 of the enclosure 105 that contains the stacks 110, from a common volume supply 165. Optionally the vessel fluid can be supplied via a heater 152. Heaters may optionally also be provided for the first and second fluids supplied from the first and second fluid sources 143, 144.

[0079] The vessel fluid may thereafter be exhausted from the common volume 124 to a common volume collection 166 through the vessel exhaust 121. As such, in this example the common volume of the enclosure 105 is in fluid communication with neither the first nor the second fluid volumes.

[0080] A system may comprise one or more vessels or enclosures, for example a system may include two, three or more vessels, each vessel housing plural stacks and the method herein being applied to all stacks in the system.

[0081] When the stacks are operated at elevated pressures, the vessel fluid in the common volume 124 may be regulated to balance the pressure in the common volume with the pressure in the first and / or second fluid volumes. Similarly, the pressures may be balanced between the first and second fluid volumes.

[0082] The vessel fluid may be a relatively inert gas to avoid corrosion of the components within the enclosure, and to minimize the possibility of a reaction between the vessel fluid and any leakage of product (e.g. hydrogen, carbon monoxide, oxygen or water) or first and second fluids from the manifolded passages elsewhere within the enclosure. For example, the vessel fluid may be air or nitrogen.

[0083] It will be appreciated that it is sometimes preferable for the second fluid to be open manifolded, as in Fig. 1, or partially manifolded as discussed above, such that there exists fluidic communicationbetween the second fluid volume of the stacks and the interior of the enclosure 105 for pressure equalisation therebetween - in turn simplifying control strategies and pipework required for the respective fluids.

[0084] It will be understood that similar partial or fully manifolded arrangements to those shown schematically in Figs. 1 and 2 are known in the art, albeit not with the stacks being provided in an enclosure.

[0085] In the examples of Figs. 1 and 1, the first fluid volume is preferably a fuel volume and is for fuel.The fuel comprises steam and carbon dioxide. In cases where the electrolyser cell units in the stacks 110 comprise an oxygen ion conducting electrolyte (e.g., solid oxide electrolyte cell units), the first fluid volume off-gas exhausted to the first fluid off-gas collection 163 comprises hydrogen and carbon monoxide produced in stacks 100 and unconsumed carbon dioxide and steam. The hydrogen in the off-gas is generated in the electrolysis reaction, and is mixed with unspent fuel (unconsumed carbon dioxide and steam) in the off-gas. The carbon monoxide in the off-gas is generated catalytically. The off gas is a product (i.e. hydrogen from steam) produced at a production rate by a given stack at a particular electrical current (the current resulting from a given voltage varying with temperature). The production rate is directly proportional to the current - and derivable therefrom. The second fluid volume is preferably an oxygen volume and is for oxygen produced by the electrolysis reaction and for any sweep gas supplied to that volume to assist in exhaust of oxygen released into said volume by the oxygen electrode. The second fluid volume off-gas exhausted to the second fluid off-gas collection 164 instead comprises oxygen that is generated in the electrolysis reaction. The second fluid off-gas may be pure oxygen or oxygen enriched sweep gas if mixed with a sweep gas. Likewise, the second fluid off-gas may be a product (additionally or alternatively to the abovementioned off-gas from the fuel volume) produced at a production rate by a given stack at a particular electrical current.

[0086] The second fluid volume off-gas is preferably controlled through selective use of a sweep gas such that it may comprise at least 30% by volume of sweep gas, with the remaining volume being oxygen generated by the cell units. In some examples, the second fluid volume off-gas 164 may be substantially pure oxygen, if the sweep gas is oxygen supplanted by the product of the electrolysis process (i.e., oxygen), although it might more normally be oxygen enriched air or nitrogen if air or nitrogen, respectively, are the sweep gas

[0087] Other than the fluid inlet(s) and outlets, the stack / electrolyser / vessel will have inputs (terminals, not shown) for power (for applying a current across the electrolyser cell units in the stacks 110).

[0088] Operationally, a stack will usually want to avoid large pressure differentials across the cell units, so a threshold pressure differential between the first and second fluid volumes can be set appropriately, or the first fluid volume's pressure (on a first side of each cell unit) can be controlled tobalance the pressures across the cell units. In one example, the pressure differential can be controlled (and is kept as small as possible) by use of a common regulator system across or between the two sides of the cell units (the fuel side and the oxygen side). However, in operation it is sometimes advantageous to control the pressure differential to be positive on the second fluid volume's side to ensure that any oxygen leaks leak into the fuel side, rather than the other way around, which could instead be hydrogen leaking to the oxygen side (e.g., where the fuel is steam and carbon dioxide). This is better for the stack's operational lifetime as there will be less degradation of the electrochemically active layers of the cell units, and less opportunity for an abrupt failure of the cell unit due to combustion of the hydrogen with the oxygen.

[0089] The electrolyser system typically operates at an elevated temperature, for example 870 to 970 K for cell units based on a solid oxide electrolyte. It will be appreciated that heat in the first fluid off-gas and the second fluid off-gas will typically be exchanged with (transferred to) the first fluid and, if present, second fluid prior to their delivery to the stacks 110, typically prior to their entry into the enclosure 205 using one or more heat exchangers (but typically two or more heat exchangers such that heat is recovered from the first fluid volume off-gas and the second fluid volume off-gas). Heaters 150 and / or 151 (e.g., an electric heater and / or a trim heater) may be provided in an input stream of the first and / or second fluid, respectively, to provide additional heat to said fluids and for providing additional heat control flexibility within the system, and may be used to raise the temperature of inlet fluid relative to off-gas so that endothermic voltages may be used. In some examples, a heater may be provided on each respective branch of the inlet manifold for one or both of the first and second fluid volumes, additional to or replacing the heaters 150 and / or 151. The heaters on each respective branch being individually controllable to control an inlet temperature to each stack. In some cases, these heaters may be positioned such that a heater is configured to heat fluid delivered to the inlet of two or more stacks. Said heaters may be sized for steady state requirements only, or may also be used for other operational modes, e.g. warm-up or standby. Alternative means to trim heaters, for supplying additional heat to the inlet fluid(s) may be used, such as heat exchange with a heat source.

[0090] In operation at a thermoneutral voltage, the first and second fluid volume off-gases will usually be at a similar temperature to the operational temperature of the electrolyser cell units, and at a similar (substantially the same) temperature to the input temperature of fluid(s) supplied to the first and second volumes. However, a specific difference between the off-gas and the input temperature will depend upon the amount of electrical power supplied to the electrolyser system / stacks / cell units, and the internal electrical resistance of the cell units. In steady state endothermic operation with a co-flow arrangement (flow in the same direction in the first and second fluid volumes), the first andsecond fluid volume off-gases will usually be at a lower temperature than the input temperature of fluid(s) supplied to the first and second volumes. The temperature of the cell units will be graded along their length from a temperature similar to that of the input temperature near an inlet to the fluid volume(s) to a temperature similar to that of the off-gases near an exhaust from the fluid volume(s).

[0091] The electrical power is generally supplied to the stacks with a constant current (with a voltage target). The stack is thus operated in galvanostatic conditions. The electrical resistance of the stack thus controls the voltage applied across the stack, and there is thus a variable power draw from each stack and cell unit as the electrical resistance changes. Alternatively, the power supplied to the stacks is controlled potentiostatically - with a constant voltage (and a current target).

[0092] In the prior art, it is known to use either constant voltages or constant currents on a stack, and then to control the current or the voltage, respectively, to maintain the stack at a substantially thermoneutral condition. This then allows the stack to avoid overcooling or overheating, as when at under-voltage (for a galvanostatic stack - i.e. constant current), the stack shows endothermic characteristics, and it thus cools the fluid (and likewise the operational temperature of the stack), whereby the fluid output temperature is lower than the fluid input temperature, whereas when at over-voltage (for a galvanostatic stack - i.e. constant current), the stack shows exothermic characteristics, and it thus heats the fluid, whereby the fluid output temperature is higher than the fluid input temperature.

[0093] At steady state, electrolyser systems may use different temperature and / or voltage conditions for different stacks. For example, thermoneutral conditions (thermoneutral voltage or V=VTN), overvoltage conditions (exothermic voltage or V>VTN) and under-voltage conditions (endothermic or V<VTN) may be used in order to achieve a target production rate of a product of the electrolysis reaction (e.g., rate of hydrogen output from the system). In the present disclosure, powering an electrolyser cell stack comprises controlling a voltage supplied at an endothermic value. In other examples, stacks may also be operated under galvanostatic conditions.

[0094] It will be appreciated that for an electrochemical cell unit or stack, current, voltage, and temperature are related to one another because the resistance of the cell units decreases with increasing temperature. Thus, for a first temperature, a voltage results in a first current, but at a higher temperature the same voltage results in a higher current. Meanwhile, production rate is proportional to current. This means that for a relatively low production rate stack there may be a relatively low current at a nominal temperature to due a relatively high resistance (which may be relatively high resistance due to low temperature and / or being a relatively old stack), accordingly a greater voltage may be needed to achieve a target current or production rate at a given temperature and / or a greatertemperature needed (to reduce resistance) to achieve the target current or production rate at a given voltage (e.g. an endothermic voltage).

[0095] Figure 3 is a method 300 for operating a system comprising an electrolyser cell stack of electrolyser cell units, the electrolyser cell stack comprising a fuel volume and an oxygen volume.

[0096] At step 305, the method comprises providing fuel to the fuel volume of the electrolyser cell stack, wherein the fuel comprises steam and carbon dioxide, wherein the fuel is provided to the fuel volume at a first temperature.

[0097] At step 310, the method comprises powering the electrolyser cell stack with electrical energy thereby converting, at least partially, the steam into hydrogen and oxygen, wherein hydrogen is released into the fuel volume and oxygen is released into the oxygen volume.

[0098] At step 315, the method comprises converting, at least partially, the carbon dioxide and hydrogen into carbon monoxide and water in the fuel volume.

[0099] At step 320, powering the electrolyser cell stack comprises controlling a voltage supplied at an endothermic value.

[0100] Accordingly, the method comprises controlling a voltage supplied at a value less than a thermoneutral voltage (i.e. an endothermic voltage). The thermoneutral voltage is dependent on a given electrochemical cell (e.g., on technology and efficiency of a cell), but for solid oxide cell units referred to herein utilising steam operating at around 770-970 K, the thermoneutral voltage may be around 1.28 V. Power supplied to the electrolyser stack leads to conversion of steam, supplied to the fuel volume, to oxygen and hydrogen by electrolysis. The oxygen is released into the oxygen volume and the hydrogen is released into the fuel volume.

[0101] Catalytic conversion of carbon dioxide and hydrogen to carbon monoxide (Reaction 1) takes place in the fuel volume, using hydrogen produced by electrolysis. The catalytic conversion is endothermic and thus relatively high temperatures favour carbon monoxide production (Reaction 1) over methanation (Reaction 2). Operating an electrolyser cell stack at relatively high temperatures leads to a relatively low resistance. If operation of the electrolyser cell stack was performed at a thermoneutral voltage at a relatively high temperature, the current density would be relatively high, which may damage cell units or potentially exceed design limits of current electrolyser cell stacks. In turn, the current density is controlled, i.e., lowered compared to the current density achieved while operating at the thermoneutral voltage, thereby enabling a relatively high temperature while keeping within acceptable current density ranges. The present method enables operation at a higher temperature (than if at a thermoneutral voltage or an exothermic voltage (V>VTN)) and thus a relatively greater conversion of carbon dioxide and hydrogen to carbon monoxide and water (Reaction 1, favoured over Reaction 2 by using higher temperatures) is achieved. The present method thus selectively producescarbon monoxide at a current density which electrolyser stacks can safely and efficiently withstand. Ultimately, carbon monoxide and hydrogen (also termed synthetic gas or syngas) can be transformed into hydrocarbons, for example liquid fuels (e.g. kerosene for aviation fuel) and polymers via the Fischer-Tropsch process.

[0102] The method may include further steps or defining features, as discussed below and / or described in the first aspect. For example, the first temperature is at most 970 K, preferably between 870 and 970 K, preferably between 870 and 950 K, and more preferably between 870 and 920 K. The first temperature may be within 15% of a maximum rated steady state temperature.

[0103] The method may comprise controlling the voltage supplied to be between 60 and 95% of a thermoneutral voltage.

[0104] Optionally, the method comprises controlling the voltage supplied at a first voltage for a first time period and subsequently controlling the voltage supplied at a second voltage for a second time period, wherein the first voltage is lower than the second voltage and is lower than a or the thermoneutral voltage. The method may further comprise one or more of: reducing the temperature of fuel provided to the fuel volume for the second time period relative to the first time period; increasing a or the steam utilisation for the second time period relative to the first time period; and decreasing a flow rate of a or the sweep gas to the oxygen volume for the second time period relative to the first time period.

[0105] Each electrolyser cell unit may comprise a fuel electrode in fluidic communication with the fuel volume, a counter-electrode in fluidic communication with the oxygen volume, and an electrolyte separating the fuel electrode and the counter-electrode, the electrolyte comprising ceria, preferably doped ceria.

[0106] Steam utilisation may be at most 75%, and the method may further comprise transferring heat from the steam to the electrolyser cell stack. Steam utilisation may be at least 40%. Steam utilisation also affects carbon monoxide production: Increasing concentration of steam in the fuel volume (i.e., lower steam utilisation) suppresses the methanation reaction by shifting its equilibrium toward reactants (carbon monoxide and hydrogen), thereby improving carbon monoxide selectivity within the stack. However, the water-gas shift reaction drives carbon monoxide toward carbon dioxide at high concentrations of steam, reducing net carbon dioxide conversion. Tests show that these competing effects produce a broad 'sweet spot' for carbon monoxide production, as shown in Fig. 7. Therefore, preferably steam utilisation is between 50 and 75%, preferably between 55 and 70%, more preferably between 55 and 70%.

[0107] The method may comprise providing a sweep gas to the oxygen volume of the electrolyser cell stack, wherein the sweep gas is provided at a third temperature and the third temperature issubstantially equal to the first temperature. Providing the sweep gas may comprise flowing the sweep gas through the oxygen volume in an opposite direction to the direction of flow of fluid through the fuel volume so that the sweep gas acts as a heat source for the outlet end (from the perspective of the fuel volume) of the cell unit.

[0108] When off-gas is exhausted from the oxygen volume, the off-gas may comprise the sweep gas and oxygen as a product of the electrolyser cell units, wherein at least 30% by volume of the off-gas is sweep gas.

[0109] Figure 4 shows a control device 400 for controlling an electrolyser system comprising electrolyser cell units arranged in one or more stacks. The control device 400 comprises an input device 402 for receiving input from sensors 404 so as to determine, for one, each, or a plurality of stacks in a system, at least one of: a stack operating voltage across the electrolyser stack(s), a stack operating current through the electrolyser stack(s), an inlet temperature at a fluid inlet, an outlet temperature at a fluid outlet and a common volume temperature. The control device 400 thus comprises a voltage monitoring system 406 for determining a stack operating voltage across the electrolyser stack(s), a current monitoring system for determining a stack operating current through the electrolyser stack(s), an inlet temperature monitoring and / or control system 408 for determining an inlet temperature at the fluid inlet and an outlet temperature monitoring and / or control system 410 for determining an outlet temperature at the at least one fluid outlet of the electrolyser stack(s). These systems may utilize sensors and data transmission devices or wiring. The control device receives sensor data relating to each of these measurements. The control device may control the system by controlling valves for supply to and exhaust from each fluid volume and power supplies for non-fluid heating. A suitably programmed processor 412 and associated memory 414 is provided for processing such inputs.

[0110] The control device's inlet temperature monitoring and / or control system 408 may comprise an output device for controlling the identity and temperature of a fluid entering the electrolyser stack(s) at a fluid inlet. A current control system 416 is also provided for controlling a current supply to (and in some cases from) the electrolyser stack.

[0111] During normal operation the control device is adapted to provide a constant current and / or voltage to the electrolyser cell stack(s), but the current / voltage (and / or temperature) may be varied according to the methods herein.

[0112] The control device 400 may also control non-fluid heat sources (heaters and such like described above) and so forth.

[0113] In such a way, the control device 400 is adapted to control an electrolyser stack(s) 110 and an electrolyser system 100, 101 or 500 - for example that shown in any of Figures 1, 2 and 5 as appended hereto.

[0114] Figure 5 shows a system 500 including an electrolyser cell stack 504 of electrolyser cell units 502.The electrolyser cell stack 504 may be a solid oxide electrolyser cell stack. The electrolyser cell stack 504 is shown to have four electrolyser cell units 502, although generally a stack 504 will have, e.g., 100-500 cell units. The electrolyser cell units 502 function as electrochemical cells for electrolysing steam.

[0115] Although not shown in Fig. 5, the electrolyser cell stack 504 comprises a fuel volume and an oxygen volume. The electrolyser cell stack 504 further comprises a fuel volume inlet 529 and a fuel volume outlet 527 in fluidic communication with the fuel volume; the fuel volume inlet 529 for delivery to and the fuel volume outlet 527 for exhaust from the fuel volume. The electrolyser cell stack 504 comprises an oxygen volume inlet 538 and an oxygen volume outlet 526 in fluidic communication with the oxygen volume; the oxygen volume inlet for delivery to and the oxygen volume outlet 526 for exhaust from the oxygen volume. The electrolyser cell stack 504, specifically the fuel volume, is configured to be provided with fuel comprising steam and carbon dioxide. A reverse-water gas shift catalyst may be present in the fuel volume of the stack 504.

[0116] The system 500 comprises a carbon dioxide supply 508 (e.g., a tank, or connection point to a CO2- producing subsystem) in fluidic communication with a carbon dioxide supply line 506. The system comprises a steam supply, in this case provided by a steam boiler and heat integration system 501 (which may include one or more heat exchangers to facilitate heat transfer from a water condenser 510, compressor 513, and Fischer-Tropsch reactor unit 519, heat transfer represented by 539, 514, and 525, respectively), in fluidic communication with a steam supply line 503. The system 500 further includes a water supply 512 in fluidic communication with the steam boiler and heat integration system 501, for supplying the steam boiler and heat integration system 501 with water. The steam boiler and heat integration system 501 is configured to heat water (which may be in a liquid or gaseous state) to supply steam at an appropriate temperature and route it to the stack 504 via the steam supply line 503.

[0117] Steam supply line 503 and carbon dioxide supply line 506 join at mixing point 530 to form a fuel supply line 531. Thus, the fuel volume inlet 529 of stack 504 is in fluidic communication with the carbon dioxide supply 508 and the steam boiler and heat integration system 501.

[0118] There is a trim heater 528 upstream of the fuel volume inlet 529 to the electrolyser cell stack 504 on the fuel supply line 531, i.e., the trim heater 528 is located between the mixing point 530 and the fuel volume inlet 529 . The trim heater 528 is configured to heat the fuel for delivery to the stack 504at a first temperature, e.g., to a desired temperature for favouring carbon monoxide production, e.g., between 870 and 970 K.

[0119] The fuel volume outlet 527 is in fluidic communication with a fuel volume off-gas line 505. The fuel volume off-gas line 505 is also in fluidic communication with a fuel volume heat recovery heat exchanger 507. The fuel volume heat recovery heat exchanger 507 is located between the water supply 512 and the steam boiler and heat integration system 501. Line 509 is configured to enable fluidic communication between the water supply 512 and the fuel volume heat recovery heat exchanger 507. The fuel volume heat recovery heat exchanger 507 is configured to exchange heat between (from) the fuel volume off-gas line 505 and (to) the water supply line 509. The fuel volume heat recovery heat exchanger 507 may be a component part of the heat integration system 501.

[0120] The fuel volume heat recovery heat exchanger 507 is upstream of and in fluidic connection with a water condenser 510 via the fuel volume off-gas line 505. The water condenser 510 is configured to condense at least a part of the water out of the fuel volume off-gas. Condensed water is configured to be recirculated via line 511 to the water supply 512. Heat generated at the water condenser 510 may be transferred to the water (steam) supply line 509 via the steam boiler and heat integration system 501, wherein the direction of heat transfer is indicated by dashed arrow 539. The water condenser 510 may be a component part of the heat integration system 501.

[0121] The water condenser 510 is in fluidic communication with a compressor 513, via the fuel volume off-gas line 505. The compressor 513 is downstream (along line 505) of the water condenser 510. The compressor 513 is configured to compress the off-gas from the fuel volume to a pressure of between 2,000 and 3,000 kPa. Line 515 is in fluidic communication with the compressor 513 and the water supply 512 for recirculation of water from the compressor 513 to the water supply 512 and subsequently to the fuel supply line 531. Heat from the compressor 513 is configured to be routed to the steam boiler and heat integration system 501, as indicated by arrow 514 (indicating the direction of heat transfer). The compressor 513 may be a component part of the heat integration system 501.

[0122] Downstream, on the fuel volume off-gas line 505, of the compressor 513 is a separation unit 516.The separation unit 516 is configured to separate carbon dioxide from the fuel volume off-gas and may form part of the heat integration system 501. The separation unit 516 may be an amine absorption unit comprising an amine aqueous solution solvent configured to absorb carbon dioxide. Carbon dioxide is configured to be recirculated from the separation unit 516 via line 517 to the carbon dioxide supply 508. The system 500 is thus configured to route carbon dioxide from the separation unit 516 to the fuel volume inlet 529. The system 500 is configured to recirculate water from the separation unit 516 via line 524 to the water supply 512. Although not shown in Figure 5, the system500 may include a methane separation unit on fuel volume off-gas line 505 configured to remove unwanted methane from the fuel volume off-gas.

[0123] A purified off-gas line 520 is configured to route hydrogen and carbon monoxide, also termed syngas, from the separation unit 516 to the heater 518. The heater 518 is configured to heat the purified off-gas to a temperature suitable for the Fischer-Tropsch reaction, e.g., to approximately 450- 470 K. The heater 518 is in fluidic communication, via purified off-gas line 520, with a Fischer-Tropsch reactor unit 519 configured to perform a Fischer-Tropsch reaction. The Fischer-Tropsch reactor unit 519 is therefore configured to facilitate a series of highly exothermic chemical reactions for converting carbon monoxide and hydrogen into a product 522 comprising hydrocarbons of varying molecular weights. The purified off-gas line 520 is thus configured to route carbon monoxide and hydrogen from the separation unit 516 to the Fischer-Tropsch reactor unit 519.

[0124] Methane is an undesired product of the Fischer-Tropsch reaction and of Reaction 2 which may occur in the electrolyser stack(s). A methane separator (not shown) may be configured to remove methane from the Fischer-Tropsch reactor unit 519. Line 521 is configured to be in fluidic communication with the Fischer-Tropsch reactor unit 519 and a methane store 523 and is configured to route methane to the methane store 523.

[0125] The system includes an oxygen volume off-gas line 532 in fluidic communication with the oxygen volume outlet 526 for exhaust of off-gas from the oxygen volume. The oxygen volume off-gas line 532 is in fluidic communication with an oxygen volume heat recovery heat exchanger 533, configured to transfer heat from the oxygen volume off-gas line 532 to a sweep supply line 534. The sweep supply line 534 is in fluidic communication with a sweep supply 535. The sweep supply 535 is upstream, on the sweep supply line 534, of the oxygen volume heat recovery heat exchanger 533. The oxygen volume off-gas line 532 is further routed to a carbon dioxide heat exchanger 536, configured to exchange heat between (from) the oxygen volume off gas line 532 and (to) the carbon dioxide supply line 506. Subsequent to the carbon dioxide heat exchanger 536, the oxygen volume off-gas line 532 is configured to route oxygen volume off-gas out of the system 500, for example to a flue / vent to the atmosphere, or to another subsystem.

[0126] A trim heater 537 is downstream, on the sweep supply line 534, of the oxygen volume heat recovery heat exchanger 533 and is configured to heat the sweep gas before entry of sweep gas into the stack 504 via sweep gas inlet 538.

[0127] In an alternative arrangement, the system 500 may include a plurality of stacks 504, wherein the fuel volume outlets 527 of respective stacks 504 are fluidically connected to one another to form the fuel volume off-gas line 505. Sweep and fuel supply lines may be similarly arranged, for example the fuel supply line 531 may branch into a plurality of supply lines to supply the plurality of stacks.

[0128] In use, fuel (carbon dioxide and steam) is provided to the mixing point 530. Specifically, carbon dioxide is supplied from carbon dioxide supply 508 and flows along the carbon dioxide supply line 506, which is fluidically connected to the carbon dioxide heat exchanger 536. Carbon dioxide passes through the carbon dioxide heat exchanger 536 whereby heat is exchanged between the oxygen volume off-gas line 532 and the carbon dioxide in the carbon dioxide supply line 506 to heat the carbon dioxide. Carbon dioxide subsequently arrives at the mixing point 530. Steam is generated by the steam boiler and heat integration system 501. Steam flows from the steam boiler and heat integration system 501 along steam supply line 503 to the mixing point 530. The steam and carbon dioxide (i.e. fuel) are mixed at mixing point 530 and subsequently flow along fuel supply line 531. The mixed fuel is heated by trim heater 528 to a first temperature, e.g., to a desired temperature for favouring carbon monoxide production, e.g., between 870 and 1070 K, preferably between 920 and 1020 K. Higher temperatures favour carbon monoxide production rather than methanation. However, relatively higher temperatures (e.g. above 970 K) presents a risk of cracking methane or carbon monoxide to produce carbon and hydrogen or water and therefore depositing elemental carbon within cell units and / or pipework of the system. The heated fuel mixture enters the fuel volume of stack 504 via the fuel volume inlet 529.

[0129] A power supply (not shown) powers the stack 504 with electrical energy at an endothermic voltage which enables a relatively higher first temperature (of fuel supplied to the stack(s)). Powering the stack 504 with electrical energy at least partially converts the steam (by electrolysis) into hydrogen and oxygen. Hydrogen is released into the fuel volume and oxygen is released into the oxygen volume. In addition to, and using the hydrogen product of electrolysis of steam, two reactions occur in parallel in the fuel volume:• Reaction 1: H2 + CO2 -> CO + H2O• Reaction 2: 3H2+ CO -> CH4+ H2OReaction 1 is the reverse water-gas shift reaction, which converts carbon dioxide and hydrogen into carbon monoxide and water in the fuel volume. Reaction 2 is methanation, which is the conversion of carbon monoxide to methane through hydrogenation. Reaction 2 may be referred to as CO methanation. CO2 methanation, CO2 + 4H2 -> CH4+ 2H2O may similarly occur, but may be less favoured and conversion rate may be reduced by the methods described herein. Reaction 1 is favoured over Reaction 2 by higher temperatures in the fuel volume, which are enabled by controlling a voltage supplied by the power supply at an endothermic value. Controlling the voltage supplied at an endothermic value maintains a current density which is safely tolerated by stack 504.

[0130] Off-gas is exhausted from the fuel volume via the fuel volume outlet 527, wherein the off-gas comprises hydrogen and carbon monoxide, unconsumed carbon dioxide and steam, and maycomprise methane as an unwanted product of reactions in the fuel volume (via Reaction 2). To utilise and recycle heat and excess water from the fuel volume off-gas for heating water, the fuel volume offgas passes through fuel volume off-gas line 505 to the fuel volume heat recovery heat exchanger 507. The fuel volume heat recovery heat exchanger 507 is downstream of and in fluid communication with the water supply 512 via line 509. At the fuel volume heat recovery heat exchanger 507, heat is transferred from fuel volume off-gas (line 505) to water (line 509) to heat said water. Water passes through line 509 to the steam boiler and heat integration system 501, which further heats the water (steam). Thus, 'line' 509 also represents a heat transfer pathway. Heated steam subsequently passes via steam supply line 503 to the mixing point 530 for re-use in the stack 504.

[0131] After the fuel volume off-gas leaves the fuel volume heat recovery heat exchanger 507, the fuel volume off-gas continues to flow along fuel volume off-gas line 505 and enters the water condenser 510, downstream of the fuel volume outlet 527. The water condenser 510 condenses at least part of the water out of the fuel volume off-gas and routes the condensed water to the water supply 512 via line 511 for use as fuel in the electrolyser. Heat generated at the condenser 510 is recovered by transfer to water (steam) by the boiler and heat integration system 501, said heat transfer is shown by arrow 539 (indicating the direction of heat transfer), so that the heat can be used to raise steam and / or to increase the temperature of steam for supply as fuel to the electrolyser cell stack 504.

[0132] Subsequent to the condenser 510, the fuel volume off-gas continues along fuel volume off-gas line 505 and through a compressor 513, which compresses the off-gas to 2,000-3,000 kPa, causing the fuel volume off-gas to increase in temperature. Heat generated at the compressor 513 is recovered by transfer to water (steam) by the boiler and heat integration system 501, said heat transfer is shown by arrow 514, so that it can be used to raise steam and / or to increase the temperature of steam for supply (as fuel) to the electrolyser cell stack 504.

[0133] An intercooler (not shown) may be present, which cools the fuel volume off-gas after compression by the compressor 513. Condensation produced by cooling the fuel volume off-gas can be routed via line 515 to the water supply 512, for use as fuel by the electrolyser.

[0134] The fuel volume off-gas leaving the compressor 513 enters the separation unit 516 which separates carbon dioxide from the fuel volume off-gas. The separated carbon dioxide exits the separation unit 516 and is routed to the carbon dioxide supply 508 via line 517 for use in the electrolyser cell stack 504 as fuel. Liquid water condensed out of off-gas by the separation unit 516 is routed to the water supply 512 via line 524. The separation unit 516 has a primary outlet path comprising hydrogen and carbon monoxide. The separation unit 516 may be an amine absorption unit which operates by cyclically heating and cooling an absorption medium, which may involve heating to between 420 and 520 K (to desorb carbon dioxide) and cooling to 290 to 340 K (to absorb carbondioxide). For this reason, the amine absorption unit may be heat integrated into the system. The heat integration may be configured to transfer heat to the amine absorption unit (medium) from the oxygen volume off-gas line (preferably downstream of the oxygen volume heat recovery heat exchanger 533 and / or downstream of the carbon dioxide heat exchanger 536) or to transfer heat to the amine absorption unit (medium) from the Fischer-Tropsch reactor unit 519 (e.g., from the outlet product stream thereof). Heat integration of the amine absorption unit may additionally or alternatively be configured to transfer heat from the amine absorption unit to the fuel volume off-gas line downstream of the condenser 510 and / or compressor 513 (preferably downstream of an intercooler subsequent to the compressor on the fuel volume off-gas line) and upstream of the heater 518. Alternatively, heat may be transferred from the amine absorption unit by heat exchange with ambient air or coolant.

[0135] At the point of separating the carbon dioxide from the fuel volume off-gas, methane may be separated from the fuel volume off-gas. A methane separation unit is not shown but known techniques may be used to carry this out, including membrane separation techniques, air separation unit (ASU) separation techniques, and / or pressure swing adsorption (PSA) separation techniques. For example, cryogenic distillation may be used.

[0136] Following separation of carbon dioxide and (optionally) methane from the fuel volume off-gas, the off-gas comprises hydrogen and carbon monoxide (also known as syngas) and may be referred to herein as purified off-gas. The purified off-gas is routed from the separation unit 516, through purified off-gas line 520 to heater 518. The heater 518 heats the purified off-gas to a temperature suitable for the Fischer-Tropsch reaction, e.g., approximately 450-470 K, before the purified off-gas is routed through a branch of the purified off-gas line 520 to the Fischer-Tropsch reactor unit 519.

[0137] The Fischer-Tropsch reactor unit 519 facilitates a series of highly exothermic chemical reactions which convert the purified off-gas into a product 522 comprising hydrocarbons of varying molecular weights. The hydrocarbons in the product 522 typically have the formula CnH2n+2. A particularly desired hydrocarbon product is kerosene (C12H26), due to its use as aviation fuel.

[0138] The heat released by these exothermic chemical reactions is used to heat water (steam) as fuel for the electrolyser, as shown by arrow 525 routing said heat from the Fischer-Tropsch reactor unit 519 to the steam boiler and heat integration system 501. The heat transfer pathway 525 may be a component part of the heat integration system 501. The heat is used to raise steam and / or increase the temperature of steam for supply as fuel to the stack 504 via steam supply line 503.

[0139] Methane is sometimes an undesired product of the reactions performed by the Fischer-Tropsch reactor unit 519. Methane can be removed using a methane separator (not shown) and routed along line 521 to methane store 523.

[0140] In use, the oxygen volume off-gas follows a different path to the fuel volume off-gas. Oxygen volume off-gas exits the oxygen volume of the stack 504 via the oxygen volume outlet 526 and is exhausted through oxygen volume off-gas line 532. The oxygen volume off-gas subsequently enters the oxygen volume heat recovery heat exchanger 533, which transfers heat from the oxygen volume off-gas line 532 to the sweep supply line 534. The oxygen volume off-gas leaving the oxygen volume heat recovery heat exchanger 533 enters the carbon dioxide heat exchanger 536 via the oxygen volume off-gas line 532. Heat is exchanged between (from) the oxygen volume off gas line 532 and (to) the carbon dioxide supply line 506. Subsequently, the oxygen volume off-gas is subsequently routed out of the system 500 or to another subsystem.

[0141] A sweep gas may be provided to the oxygen volume of the stack 504 via the sweep supply 535, which is in fluidic communication with the oxygen volume heat recovery heat exchanger 533. The sweep gas enters the oxygen volume heat recovery heat exchanger 533 via the sweep supply line 534. On exit of the sweep gas from the oxygen volume heat recovery heat exchanger 533, the sweep gas is heated by the trim heater 537 and enters the oxygen volume of the stack 504. The sweep gas may be provided to the oxygen volume at a temperature substantially equal to the temperature of the fuel (steam and carbon dioxide) provided to the fuel volume. Alternatively, or additionally, the sweep gas may flow through the oxygen volume in an opposite direction of flow of fuel through the fuel volume (i.e. the sweep gas and fuel may be in counterflow).

[0142] When a sweep gas is provided to the oxygen volume, the off-gas exhausted from the oxygen volume (via oxygen volume outlet 526 and through oxygen volume off-gas line 532) comprises sweep gas and oxygen as a product of the cell units 502, wherein at least 30% by volume of the off-gas is sweep gas. When excess steam is present and / or when the fuel and sweep gas are in counterflow, the fuel volume off-gas exhausted from fuel volume off-gas outlet 527 is lower than the temperature at which the fuel (steam and carbon dioxide) is provided, preferably between 5 and 60 K lower.

[0143] There are various ways in which the system 500 can be operated to provide and maintain a relatively high temperature in the fuel volume to favour production of carbon monoxide, without exceeding the maximum temperature safely tolerated by the stack 504. For example, fuel in the fuel volume and sweep gas in the oxygen volume may be ran in counterflow (as described in detail in relation to Figure 6), where the sweep gas acts as a heat source for the cell units, supplied to the cell units at an end thereof (because of the counterflow arrangement) which is cooled by the endothermic operation. Additionally or alternatively, the fuel (e.g. steam) may be provided in an excess, i.e., more fuel than is required for the electrochemical (electrolysis) reactions. The excess fuel acts as a heat source and can thus maintain the cell units and stack 504 at a relatively elevated temperature - and arelatively consistent temperature - along the length of the cell units for a given endothermic voltage. Operation at the elevated temperature favours the endothermic production of carbon monoxide.

[0144] During operation of system 500, the voltage may be controlled to be supplied at a first voltage for a first time period and subsequently the voltage is controlled supplied at a second voltage for a second time period, wherein the first voltage is lower than the second voltage and is lower than the thermoneutral voltage (the first and second voltages being lower than the thermoneutral voltage). Thus, the voltage may be controlled to increase toward the thermoneutral voltage as the electrolyser cell stack ages. The time periods may each be at least 100 hours - i.e., the time periods are not a result of transient behaviours or demands made of the system, but account for changes through life of the system. The second time period is subsequent to the first time period. By increasing the voltage through the life of the electrolyser cell stack 504, an increase in resistance due to stack aging is off-set and current is maintained. This allows the stack 504 to age towards a thermoneutral voltage and an isothermal state, and may result in the stack 504 operating at the thermoneutral voltage later in its life, thereby improving system efficiency and consistency of temperature long the cell unit. The flow rate of excess steam or sweep flow may be reduced in the second time period relative to the first time period, as the voltage becomes less endothermic.

[0145] Figure 6 shows a cross-sectional view of an electrolyser cell unit 600, which is a component part of an electrolyser cell stack (i.e. an electrolyser cell stack includes a plurality of electrolyser cell units 600). The cell unit 600 includes a support structure 601 (e.g. a metal support plate) and an interconnect 602 which are spaced apart from one another and are each self-supporting plates. The support structure 601 includes an electrochemically active cell region 606 comprising a first electrode (e.g., fuel electrode) 603, electrolyte 604 and counter-electrode (e.g., oxygen electrode) 605 which are respectively deposited in layers (e.g. as thin coatings / films) on and supported by the support structure 601. When arranged in an electrolyser cell stack, the electrochemically active cell region 606 faces the interconnect 602 of an adjacent cell unit (not shown).

[0146] The support structure 601 has a porous region 607 surrounded by a non-porous region with the overlaid layers of the electrochemically active cell region 606 being deposited on the porous region 607 so that gases may pass through the pores from one side of the support structure 601 to the opposite to access the electrodes coated thereon. The porous region 607 comprises small apertures (formed, e.g., by holes drilled through the support structure 601) extending through the support structure 601, in a location to overlie the first electrode (fuel electrode) 603, which is positioned upon the support structure 601. The first electrode (fuel electrode) 603 is located adjacent the porous region 607.

[0147] A first fluid volume (fuel volume) 608 is provided between the support structure 601 and the interconnect 602. The first fluid volume 608 is typically in fluidic communication with fluid ports (not shown), which extend though the support structure 601 outside of the electrochemically active cell region 606. The first fluid volume (fuel volume) 608 is for steam and carbon dioxide. In use, the steam and carbon dioxide are converted, at least partially, into carbon monoxide and hydrogen and released into the first fluid volume (fuel volume) 608. The electrochemically active cell region 606 converts steam into hydrogen, releasing hydrogen in to the first fluid volume. A catalyst may be present in or bounding the first fluid volume to promote the reverse water-gas shift reaction for producing carbon monoxide (Reaction 1). The catalyst may be a coating on one of the components, for example on the interconnect 602 or the support structure 601. The reverse water-gas shift catalyst may comprise one or more of Pt, Ru, Rb, Ni, or Fe. The flow direction of fuel and products in the first fluid volume 608 is indicated by arrow A.

[0148] A second fluid volume (oxygen volume) 609 is defined between the interconnect 602 of an adjacent cell unit (not shown) and the electrochemically active cell region 606 of the cell unit 600. A plurality of cell units 600 stacked upon each other form an electrolyser cell stack and therefore enclose the second fluid volume 609. The second fluid volume (oxygen volume) 609 is for oxygen which is released by electrolysing steam, and for sweep gas. The second fluid volume may be externally manifolded. The flow direction of oxygen and sweep gas in the second fluid volume 609 is indicated by arrow B.

[0149] Arrow A, which indicates the flow direction of fuel and products in the first fluid volume 608 and arrow B point in opposite directions. Thus, the arrangement of cell unit 600 is a counterflow arrangement (also known as contraflow) in which a first fluid flow path in the first fluid volume 608 is in an opposite direction to a second fluid flow path direction in the second fluid volume 609. The directions of arrows A and B may be reversed, as long as the fuel in the first fluid volume 608 flows in the opposite direction to the oxygen and sweep gas in the second fluid volume 609. Accordingly, the inlet for fuel supplied to the first fluid volume 608 is at the opposite end of the cell unit 600 to the inlet for sweep gas supplied to the second fluid volume 609. It follows that the outlet from the fuel volume (for hydrogen, unspent fuel, carbon monoxide, unspent carbon dioxide, and methane) is at the opposite end of the cell unit 600 to the outlet from the oxygen volume (for sweep gas and oxygen).

[0150] The counterflow arrangement is advantageous because the sweep gas in the second fluid volume 609 can be used to supply heat to (or remove heat from) cell units at the inlet of the second fluid volume 609, which is in the vicinity of the outlet of the first fluid volume 608. As described herein, sweep gas may be supplied to the second fluid volume 609 at a relatively high temperature, such that the sweep gas transfers heat to the cell units. This is useful in achieving a relatively more consistenttemperature along the length of the cell units when the cell units are operated at endothermic voltages. In turn, this enables the temperature of the outlet of the first fluid volume (fuel volume) 608, and the downstream part of the first fluid volume 608, to maintain a relatively higher temperature (for a given inlet temperature) than would normally be the case for an endothermic voltage. Selectivity for the endothermic conversion of hydrogen and carbon dioxide to carbon monoxide (Reaction 1) is improved. Simultaneously, the peak temperature experienced by the electrolyser cell stack is minimised compared to running the stack in a co-flow arrangement with an elevated fuel inlet temperature to achieve the same fuel outlet temperature in the first fluid volume (fuel volume) 608. A co-flow arrangement is where the fluid (fuel and products thereof) in the first fluid volume 608 flows in the same direction as the fluid (oxygen and sweep gas) in the second fluid volume 609).

[0151] The interconnect 602 includes upwards and downwards dimpled protrusions 610, 611. The upwards dimpled protrusions 610 extend up from a mid-plane region of the interconnect 602 to the support structure 601 of its own cell unit 600. The downwards dimpled protrusions 611 extend down from the mid-plane region to the counter-electrode (oxygen electrode) 605 of a neighbouring / adjacent cell unit stacked the cell unit 600. The use of dimpled protrusions 610, 611 provides a reduced restriction to mixing in the fluid volume as compared with known channelled interconnects. They also provide a reduced contact area of the interconnect 602 against the cell layer. This reduced contact area means that there is reduced blocking of access to the support structure 601 from the fluid volume by the dimpled protrusions, thereby improving the rate of hydrogen production of each cell unit 600.

[0152] Fig. 7 is a plot which illustrates the effect of steam utilisation on normalised carbon monoxide production at a fixed temperature, in this case of 630°C fuel volume outlet temperature at conditions approximating a counter-flow arrangement with a endothermic voltage (production being normalised by the maximum). Fig. 7 depicts the result of increasing concentration of steam in the fuel volume (i.e., lower steam utilisation) which suppresses the methanation reaction by shifting its equilibrium toward reactants (carbon monoxide and hydrogen), thereby improving carbon monoxide selectivity within the stack. Fig. 7 also depicts the result of the water-gas shift reaction which drives carbon monoxide toward carbon dioxide at high concentrations of steam, reducing net carbon dioxide conversion. Fig. 7 shows that these competing effects produce a broad 'sweet spot' for carbon monoxide production between approximately 50% and 80% steam utilisation, with peak production between 55 and 65% steam utilisation.

[0153] The present disclosure is not to be limited by the above-described aspects and embodiments, and that many variations are within the scope of the appended claims. The various aspects andembodiments may be combined if necessary and appropriate. The drawings serve as exemplary illustrations of the disclosure only, to aid understanding of the same.

Claims

CLAIMS1. A method for operating a system comprising an electrolyser cell stack of electrolyser cell units, the electrolyser cell stack comprising a fuel volume and an oxygen volume, the method comprising: providing fuel to the fuel volume of the electrolyser cell stack, wherein the fuel comprises steam and carbon dioxide, wherein the fuel is provided to the fuel volume at a first temperature; powering the electrolyser cell stack with electrical energy thereby converting, at least partially, the steam into hydrogen and oxygen, wherein hydrogen is released into the fuel volume and oxygen is released into the oxygen volume; andconverting, at least partially, the carbon dioxide and hydrogen into carbon monoxide and water in the fuel volume;wherein powering the electrolyser cell stack comprises controlling a voltage supplied at an endothermic value.

2. The method according to claim 1, wherein the first temperature is at most 970 K, preferably between 870 and 970 K, preferably between 870 and 950 K, and more preferably between 870 and 920 K.

3. The method according to claim 1 or 2, wherein the first temperature is within 15% of a maximum rated steady state temperature.

4. The method according to any preceding claim, wherein each electrolyser cell unit comprises a fuel electrode in fluidic communication with the fuel volume, a counter-electrode in fluidic communication with the oxygen volume, and an electrolyte separating the fuel electrode and the counter-electrode, the electrolyte comprising ceria, preferably doped ceria.

5. The method according to any preceding claim, wherein a steam utilisation is between 50% and 80%.

6. The method according to any preceding claim, wherein a steam utilisation is at most 75%, wherein the method further comprises transferring heat from the steam to the electrolyser cell stack.

7. The method according to any preceding claim, further comprising providing a sweep gas to the oxygen volume of the electrolyser cell stack, wherein the sweep gas is provided at a third temperature and the third temperature is substantially equal to the first temperature.

8. The method according to claim 7, wherein providing the sweep gas comprises flowing the sweep gas through the oxygen volume in an opposite direction to the direction of flow of fluid through the fuel volume.

9. The method according to claim 8, wherein off-gas is exhausted from the oxygen volume, the off-gas comprising the sweep gas and oxygen as a product of the electrolyser cell units, wherein at least 30% by volume of the off-gas is sweep gas.

10. The method according to any preceding claim, wherein off-gas is exhausted from the fuel volume at a second temperature lower than the first temperature, and the second temperature is between 5 and 60K lower than the first temperature.

11. The method according to any preceding claim, wherein off-gas is exhausted from the fuel volume, and wherein the off-gas comprises hydrogen and carbon monoxide produced in the electrolyser cell stack and unconsumed carbon dioxide and steam, the method further comprising separating the carbon dioxide and steam from the off-gas and recirculating the carbon dioxide and steam to the fuel volume of the electrolyser cell stack.

12. The method according to claim 11, wherein separating the steam from the off-gas comprises condensing the steam to liquid water.

13. The method according to claim 11 or 12, wherein separating the carbon dioxide from the off-gas is performed by an amine absorption unit.

14. The method according to any preceding claim, further comprising providing hydrogen and carbon monoxide from the electrolyser cell stack to a Fischer-Tropsch reactor unit for performing a Fischer-Tropsch reaction, thereby producing a mixture of hydrocarbons as a product of the Fischer-Tropsch reaction.

15. The method according to claim 14, comprising transferring heat produced by the Fisher-Tropsch reactor unit to the fuel, prior to providing the fuel to the fuel volume of the electrolyser cell stack.

16. The method according to claim 14 or 15, further comprising using the mixture of hydrocarbons as a fuel.

17. The method according to any preceding claim, further comprising controlling the voltage supplied to be between 60 and 95% of a thermoneutral voltage.

18. The method according to any preceding claim, further comprising controlling the voltage supplied at a first voltage for a first time period and subsequently controlling the voltage supplied at a second voltage for a second time period, wherein the first voltage is lower than the second voltage and is lower than a or the thermoneutral voltage.

19. The method according to according to claim 18, further comprising one or more of: reducing the temperature of fuel provided to the fuel volume for the second time period relative to the first timeperiod; increasing a or the steam utilisation for the second time period relative to the first time period; and decreasing a flow rate of a or the sweep gas to the oxygen volume for the second time period relative to the first time period.

20. The method according to any preceding claim, wherein the electrolyser cell stack comprises a reverse water-gas shift catalyst in the fuel volume to convert, at least partially, the carbon dioxide and hydrogen into carbon monoxide and water.

21. The method according to any preceding claim, wherein the system comprises a plurality of electrolyser cell stacks.

22. A controller configured to perform the method of any of the preceding claims or an electrolyser system comprising a controller configured to perform the method of any of the preceding claims.

23. A system comprising:an electrolyser cell stack of electrolyser cell units, the electrolyser cell stack comprising a fuel volume and an oxygen volume, a fuel volume inlet and a fuel volume outlet in fluidic communication with the fuel volume, and an oxygen volume inlet and an oxygen volume outlet in fluidic communication with the oxygen volume;a fuel supply line in fluidic communication with the fuel volume inlet for providing fuel to the fuel volume of the electrolyser cell stack at a first temperature, wherein the fuel is configured to comprise steam and carbon dioxide;an off-gas line in fluidic communication with the fuel volume outlet for exhausting off-gas from the fuel volume;a power supply unit configured to provide the electrolyser cell stack with electrical energy to convert, at least partially, steam into hydrogen and oxygen, wherein hydrogen is configured to be released into the fuel volume and oxygen is configured to be released into the oxygen volume;the electrolyser cell stack configured to convert, at least partially, carbon dioxide and hydrogen into carbon monoxide and water in the fuel volume; andthe power supply unit configured to power the electrolyser cell stack by controlling a voltage supplied at an endothermic value.

24. A system comprisingan electrolyser cell stack of electrolyser cell units, the electrolyser cell stack comprising a fuel volume and an oxygen volume, a fuel volume inlet and a fuel volume outlet in fluidic communication with the fuel volume, and an oxygen volume inlet and an oxygen volume outlet in fluidiccommunication with the oxygen volume, the electrolyser cell stack configured to be provided with fuel comprising steam and carbon dioxide;a separation unit;a Fischer-Tropsch reactor unit for performing a Fischer-Tropsch reaction;an off-gas line in fluidic communication with the fuel volume outlet of the electrolyser cell stack and configured to route off-gas from the fuel volume of the electrolyser stack to the separation unit;wherein the separation unit is configured to separate carbon dioxide from the off-gas from the fuel volume, and the system is configured to route carbon dioxide from the separation unit to the fuel volume inlet of the electrolyser cell stack;the system further comprising a purified off-gas line in fluidic communication with the separation unit and the Fischer-Tropsch reactor unit, the purified off-gas line configured to route hydrogen and carbon monoxide from the separation unit to the Fischer-Tropsch reactor unit.

25. The system according to claim 24, further comprising a heat exchanger configured to transfer heat from off-gas from the fuel volume to fuel upstream of the fuel volume inlet.

26. The system according to claim 24 or 25, further comprising a fuel supply line in fluidic communication with the fuel volume inlet for providing fuel to the fuel volume of the electrolyser stack, wherein the fuel comprises steam and carbon dioxide.

27. The system according to claim 26, further comprising a heat integration system, configured to transfer heat from the Fischer-Tropsch reactor unit to the fuel supply line to the electrolyser cell stack.

28. The system according to claim 26 or 27, further comprising a condenser in the off-gas line downstream of the fuel volume outlet, configured to condense water out of the off-gas from the fuel volume and route condensed water to the fuel supply line.

29. The system according to any of claims 26 to 28, comprising a compressor in the off-gas line downstream of the fuel volume outlet, configured to compress off-gas to a pressure of between 2,000 and 3,000 kPa.

30. The system according to claim 29, wherein the system is configured to route heat produced from compression of off-gas to the fuel supply line.

31. The system according to any of claims 24 to 30, wherein the separation unit is an amine absorption unit.

32. The system according to claim 31 when dependent upon claim 27, wherein the amine absorption unit is a component part of the heat integration system.

33. The system according to any of claims 24 to 32, wherein the fuel and oxygen volumes of the electrolyser cell units are arranged in counterflow.

34. The system according to any of claims 24 to 33, comprising a reverse water-gas shift catalyst in the fuel volume of the electrolyser cell stack.

35. The system according to any of claims 24 to 34, comprising a plurality of electrolyser cell stacks, wherein the fuel volume outlets of respective electrolyser cell stacks are fluidically connected to one another to form the off-gas line.

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