Stationary and transient characterisation and optimisation of reversible solid oxide cells (rSOCs)

<p dir="ltr">The reversible solid oxide cell (rSOC) exhibits a unique capability to function as either a fuel cell (FC) or an electrolyser (EC) within a single device. This dual capability opens up a wide range of potential applications, including grid balancing and energy solutions for buildings and transportation.</p><p dir="ltr"><br>This research evaluates the optimisation of the rSOC's performance in both steady-state and transient mode-switching processes. It examines the diverse internal parameter distributions within the rSOC under its two operational modes and during the transitions between these modes through experimental and simulation methodologies. These methodologies encompass steady-state and transient modelling, empirical testing, and characterisation techniques such as Electrochemical Impedance Spectroscopy (EIS), Equivalent Circuit Modelling (ECM), and Distribution of Relaxation Time (DRT).</p><p dir="ltr"><br>To commence, a porosity study optimises porous electrodes employing a multiphysics 3D steady model of an anode-supported planar rSOC. Various electrode microstructures, including homogeneous and functionally graded porosity distributions, are applied to evaluate the I-V performance in both FC and EC modes. Results show that increasing homogeneous electrode porosity enhances mass transport but does not always improve I-V performance. The optimal porosity range is between 0.5 and 0.7 under the working conditions of this study. Functionally graded porosity, especially under gas channel ribs, significantly improves performance by enhancing mass transport.</p><p dir="ltr"><br>Optimised porosity not only boosts steady-state current but also influences transient mode-switching by affecting gas diffusion rates and active surface areas. Enhancing these factors reduces diffusion resistance and improves dynamic performance. Besides the steady-state, the transient mode-switching process is also crucial for optimising the overall performance of rSOCs. Consequently, an experimental and computational study is conducted to investigate rSOCs mode-switching process.</p><p dir="ltr"><br>Empirical tests investigate mode-switching (FC to EC and vice versa) under varied conditions. Steady-state I-V performance is characterised using EIS, DRT, and ECM to identify major losses and analyse cell behaviours. Both switching directions are considered due to the asymmetrical behaviour of conventional rSOC catalysts. The switching conditions being studied include synchronous and non-synchronous changes in fuel/steam (F/S) compositions with voltage switches. Experimental results reveal current overshoots during mode-switching can be mitigated by adjusting the F/S ratio at fuel inlet. A correlation is observed between current and operating temperature during switching, and ohmic and polarisation losses are analysed through EIS, DRT, and ECM.<br>Finally, a computational study is conducted employing a validated transient two-dimensional (2D) Multiphysics model. Linear and exponential functions are employed to regulate the voltage and the F/S ratio at the gas inlet. A proposed control logic for F/S ratios and cell voltage shows potential for optimising switch strategies, enabling faster, smoother transitions with minimal overshoot.</p><p dir="ltr"><br>This thesis presents novel findings on porosity distribution and transient mode-switching. It updates prior studies focused on FC mode and changing porosity merely on laminated layers. The expanded switching conditions and innovative control logic offer enhanced insights into rSOC behaviour and performance optimisation.</p>