Reversible solid oxide cell

A reversible solid oxide cell (rSOC) is a solid-state electrochemical device that is operated alternatively as a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC). Similarly to SOFCs, rSOCs are made of a dense electrolyte sandwiched between two porous electrodes. Their operating temperature ranges from 600 °C to 900 °C, hence they benefit from enhanced kinetics of the reactions and increased efficiency with respect to low-temperature electrochemical technologies. When utilized as a fuel cell, the reversible solid oxide cell is capable of oxidizing one or more gaseous fuels to produce electricity and heat. When used as an electrolysis cell, the same device can consume electricity and heat to convert back the products of the oxidation reaction into valuable fuels. These gaseous fuels can be pressurized and stored for a later use. For this reason, rSOCs are recently receiving increased attention due to their potential as an energy storage solution on the seasonal scale.

A reversible solid oxide cell (rSOC) is a solid-state electrochemical device that is operated alternatively as a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC). Similarly to SOFCs, rSOCs are made of a dense electrolyte sandwiched between two porous electrodes. Their operating temperature ranges from 600 °C to 900 °C, hence they benefit from enhanced kinetics of the reactions and increased efficiency with respect to low-temperature electrochemical technologies. When utilized as a fuel cell, the reversible solid oxide cell is capable of oxidizing one or more gaseous fuels to produce electricity and heat. When used as an electrolysis cell, the same device can consume electricity and heat to convert back the products of the oxidation reaction into valuable fuels. These gaseous fuels can be pressurized and stored for a later use. For this reason, rSOCs are recently receiving increased attention due to their potential as an energy storage solution on the seasonal scale. V_{TN} = \frac{\Delta H^0}{zF} During the electrolysis operation: if V_{SOEC} , the reaction is endothermic; if V_{SOEC} > V_{TN} , the reaction is exothermic. The fuel cell operation, instead, is always exothermic. H2 + 1/2 O2 H2O where the forward reaction occurs during SOFC mode, and the backward reaction during SOEC mode. On the fuel electrode, hydrogen oxidation (forward reaction) takes in SOFC mode and water reduction (backward reaction) takes plain SOEC mode: H2 + O^2- H2O + 2e- On the oxygen electrode, oxygen reduction (forward reaction) occurs in SOFC mode and oxide ions oxidation (backward reaction) occurs in SOEC mode: O2 +2e- O^2- The thermoneutral voltage for steam electrolysis is equal to 1.29 V.

Carbonaceous reactants

Differently than low-temperature electrochemical technologies, rSOCs can process also carbon containing species with reduced risk of catalyst poisoning. Methane can be internally reformed on the Ni particles to produce hydrogen, similarly to what happens in steam reforming reactors. Subsequently, the produced hydrogen can undergo the electro-oxidation. Moreover, when working in SOEC modality, water and carbon dioxide can be co-electrolyzed to generate hydrogen and carbon monoxide to form syngas mixtures with various composition. The reactions taking place on the oxygen electrode are the same considered for the hydrogen/steam case. Even if characterized by much slower kinetics with respect to the one involving hydrogen and steam, the direct electro-oxidation of carbon monoxide (forward reaction) or the direct electro-reduction of carbon dioxide (backward reaction) can be considered as well: CO + 1/2 O2 CO2 The thermoneutral voltage of the CO2 electrolysis is equal to 1.48 V. One useful way to depict the cycling between SOFC and SOEC mode of the rSOC operation with carbonaceous reactants is the C-H-O ternary diagram. Each point in the diagram represents a gas mixture with a different number of carbon, hydrogen or oxygen atoms. When dealing with the operation on reversible solid oxide cells, three distinct regions can be distinguished in the graph. For different operating conditions (i.e., different temperature and pressure), distinct boundary lines between these regions can be drawn. The three regions are: the carbon deposition region: gas mixtures lying in this region are characterized by compositions that are prone to carbon deposition on the fuel electrode; the fully oxidized region: this region is characterized by gas mixtures that are fully oxidized, hence they cannot be used as fuels in the rSOC; the operating region: this region is characterized by gas mixtures that are suitable for the rSOC operation. In the operating region, the fuel mixture and the exhaust mixture can be depicted. These two points are connected by a line which runs through points characterized by a constant H/C ratio. In fact, during the rSOC operation in both modalities, the gases on the fuel electrode exchange with the oxygen electrode only oxygen atoms, while hydrogen and carbon are confined inside the fuel electrode. During the SOFC operation, the composition of the gas in the fuel electrode moves towards the boundary line of the fully oxidized region, increasing its oxygen content. During SOEC operation, on the other hand, the gas mixture evolves away from the fully oxidized region towards the carbon deposition region, while reducing its oxygen content. N2 + 3H2 2NH3 is completely shifted towards the left at their higher than 600 °C working temperature. For this reason, for clean ammonia production, hydrogen production via electrolysis must be coupled with nitrogen production from air with hydrogen oxidation and subsequent water separation. \eta_{RT,cell} = \frac{Q_{FC}V_{FC}}{Q_{EC}V_{EC}} where Q is the charge supplied or consumed during the reactions, and V is the operating voltage. If the assumption of no current or reactants leakage is made, the exchanged charges during the reactions can be assumed to be equal. Then, the roundtrip efficiency can be written as: \eta_{RT,cell} = \frac{V_{FC}}{V_{EC}} To maximize the roundtrip efficiency, the two operating voltages must be as close as possible. This condition can be achieved by operating the rSOC with low current densities in both modalities. In SOFC mode this is easily pursuable, while in SOEC mode a too low voltage may lead to an endothermic operation. If the operating voltage in SOEC mode is lower than the thermoneutral voltage, additional heat sources at high temperature are needed to sustain the reaction. These could come from waste industrial heat or from nuclear reactors. If not easily accessible, though, electrical heating is necessary. This can be supplied by external additions or by operating the cell with an operating voltage higher than the thermoneutral one. Both solutions, though, would inevitably lower the roundtrip efficiency of the rSOC. For this reason, in reversible operation, the thermoneutral voltage poses significant limitations in achieving high roundtrip efficiencies. On the other hand, the thermoneutral voltage is greatly affected by the reaction chemistry. It has been demonstrated that increasing the yield of methane in the electrolysis operation can substantially decreases the thermoneutral voltage and heat demand of the reaction. For conventional electrolyzers (operating at atmospheric pressure and 750 °C), the methane content in the products is very low. It can be increased effectively by lowering the operating temperature to 600 °C and increasing the operating pressure up to 10 bar. For example, the thermoneutral voltage is equal to 1.27 V at 750 °C and 1 bar, while it becomes equal to 1.07 V at 600 °C and 10 bar. In these conditions, the rSOC can even be operated in exothermic mode at reduced voltages, permitting to produce additional heat at high temperature. This result becomes very helpful in the design of high efficiency rSOC systems for energy storage purposes. CO + 3H2 CH4 + H2O \qquad \Delta H^0=-206\, \mathrm{kJ/mol} . The syngas that produced by the co-electrolysis can undergo a further reaction in one or multiple methanation reactors to produce methane and generate low-temperature heat for water evaporation. In addition, the formation of methane in such systems may be beneficial to the size of the tanks used for storing the fuels. In fact, methane is characterized by a higher volumetric energy density than hydrogen in the gaseous form. When computing the roundtrip efficiencies of rSOC systems, the definition must take into account the net electric consumption (or additional electric production) of other components inside the system. The set of these component is regarded as balance of plant (BOP), and may comprehend pumps, compressors, expanders or fans, needed for fluid circulation and processing inside the system. Therefore, the system roundtrip efficiency can be defined as: \eta_{RT,system} = \frac{P_{el,FC}+P_{BOP,FC}}{P_{el,EC}+P_{BOP,EC}} where: P _{el,FC} is the electric power produced in SOFC mode; P _{el,EC} is the electric power consumed in SOEC mode; P _{BOP,FC} is the net electric power consumption (negative) or production (positive) by the BOP in FC mode; P _{BOP,EC} is the net electric power consumption (negative) or production (positive) by the BOP in EC mode. The roundtrip efficiencies achievable with rSOC systems operating with steam and hydrogen can reach values in the order of 60%. On the other hand, systems exploiting the beneficial effects of methane formation, either inside the rSOC or in external reactors, can reach roundtrip efficiencies in the order of 70% and beyond.

See also

High temperature electrolysis Solid oxide fuel cell Solid oxide electrolyzer cell Power-to-gas Energy storage Grid energy storage Thermal energy storage Hydrogen technologies

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