A transient simulation study of a reversible solid oxide hydrogen planar cell (rSOC): optimising switching strategy between fuel cell and electrolyser modes

• A transient model is developed to study the mode-switching strategies of rSOCs. • Mode-switching are studied under varying voltage along with fuel/steam (F/S) ratio. • Slow oxygen diffusion is the primary factor influencing switching speed. • Linear voltage switching exhibits larger overshoots with shorter switch times. • Exponential function surpasses linear ones, offering faster and smoother transitions. Reversible solid oxide cells (rSOCs) require frequent mode-switching between fuel cell (FC) and electrolysis cell (EC) modes to accommodate the intermittency of renewable energy and the fluctuation of electrical loads. However, mode-switching introduces complex dynamic processes involving electrochemical reactions, heat transfer, and mass transport, which can lead to critical issues, such as current overshoot, performance decline, cell failure or even safety issues. To investigate and optimise the transient behaviour during mode-switching, a 2D transient multiphysics model of an anode-supported planar rSOC cell was developed, incorporating reversible electrochemistry, charge transport, and coupled momentum/mass/heat transport. The model investigates the dual influence of voltage switch and fuel/steam (F/S) ratio on mode switching. The study revealed that higher hydrogen fractions in FC-to-EC transitions and higher steam fractions in EC-to-FC transitions induce larger current overshoots and longer relaxation times. Rapid voltage switching exacerbates overshoot due to insufficient time for species redistribution, while oxygen diffusion emerges as a key limiting factor for switching speed. Coordinated adjustment of gas composition along with voltage switching reduces or eliminates overshoot, with exponential voltage transitions producing smoother and faster responses than linear switching. Furthermore, combining linear F/S control with exponential voltage switching demonstrated the potential to achieve optimal switching strategies. These findings provide valuable insights for improving rSOC performance, efficiency, and longevity under dynamic operating conditions. The proposed control approaches serve as guidance for developing advanced strategies to integrate rSOC systems effectively with intermittent renewable energy sources.