Reversible solid oxide cells (SOCs) are potentially useful for electrical energy storage due to their good storage scalability, but have not been seriously considered due to concerns over round-trip efficiency. Here we propose an SOC storage chemistry where the fuel cycles between H2O–CO2-rich and CH4–H2-rich gases. The unique feature is the formation of CH4 during electrolysis, a less endothermic process than the usual H2- or CO-forming reactions, enabling improved efficiency. Thermodynamic calculations and preliminary experiments show that the CH4-rich storage chemistry is produced during SOC operation at reduced temperature (∼600 °C) and/or increased pressure (∼10 atm). Balance of plant storage system requirements are discussed briefly.
