Do electrochemical reactions really take place at the three-phase boundary in solid oxide fuel cells?

Within composite electrodes for solid oxide fuel cells (SOFCs), electrochemical reactions between gas species and charge carriers take place in the proximity of the three-phase boundary (TPB), which is the contact perimeter among the electron-conducting phase, the ion-conducting phase and the porous phase. The TPB reaction zone is conventionally regarded as a mono-dimensional line and efforts have been made to increase its length to reduce the activation losses. In this study, by using physically-based modelling, 3D tomography and impedance spectroscopy, we show that the electrochemical reactions take place within an extended region around the geometrical TPB line, as in Fig. 1. Such an extended region is in the order of 4 nm in Ni-YSZ anodes [1] while approaches ca. 200 nm in LSM-YSZ cathodes [2]. These findings have significant implications for preventing the degradation of nano-structured anodes, which is due to the coarsening of the fractal roughness of Ni nanoparticles [1], as well as for the optimisation of composite cathodes, indicating that the adsorption and surface diffusion of oxygen limit the rate of the oxygen reduction reaction (ORR) [2]. In both anodes and cathodes, the results point out that the surface properties of the materials are key in determining the performance and lifetime of SOFCs, demonstrating that the three-phase boundary paradigm must be abandoned. [1] A. Bertei, E. Ruiz-Trejo, K. Kareh, V. Yufit, X. Wang, F. Tariq, and N.P. Brandon, Nano Energy 38 (2017) 526-536. [2] A. Bertei, M.P. Carpanese, D. Clematis, A. Barbucci, M.Z. Bazant, and C. Nicolella, Solid State Ionics 303 (2017) 181-190.