Polarisation mechanism of the misfit Ca-cobaltite electrode for reversible solid oxide cells

•C349 electrode full mechanism is clarified for r-SOCs.•The very fast oxygen exchange overcomes the diffusion limitation of C349.•C349 performance is higher under anodic polarisation. Reversible Solid Oxide Cells (r-SOCs) are promising devices for energy production and management, with the misfit calcium cobaltite electrode ([Ca2CoO3−δ]q[CoO2], commonly designated as Ca3Co4O9, C349), being proposed as a potential oxygen electrode for these devices. To be able to correctly assess this possibility, the processes that limit its polarisation behaviour need to be clarified under relevant conditions of reversible operation. In the current work, we assess the electrochemical characteristics of the C349 electrode by electrochemical impedance spectroscopy using a 3-probe cell configuration. The impedance data is analysed by the use of an equivalent circuit model (EQM) including a Gerischer element to describe the diffusion and surface-exchange properties of the C349 electrodes under applied polarisation, in both cathodic and anodic modes of operation. This analysis is complemented by distribution function of relaxation times (DFRT) and Kramers-Kronig (KK) analyses, revealing that two major processes dominate the total polarisation resistance. Under cathodic polarisation, surface diffusion is rate-limiting upon moderate applied potentials, while, under anodic polarisation, bulk diffusion is rate-limiting. Overall, the performance of C349 was found to not be prevented by the diffusion limiting characteristics of this material, due to compensation of this feature by very fast oxygen exchange. Moreover, the performance of C349 was found to be enhanced under anodic polarisation, due to the easier removal of electrons during oxygen evolution. This work, therefore, provides the first full description of the electrochemical behaviour of the C349 electrode under applied polarisation conditions for r-SOC applications and highlights the high potential of this material for this application. [Display omitted]