A perspective on the role of the three-phase boundary in solid oxide fuel cell electrodes

Within composite electrodes for solid oxide fuel cells (SOFCs), electrochemical reactions take place in the proximity of the so-called three-phase boundary (TPB), the contact perimeter where the electron-conducting, the ion-conducting and the porous phases meet. Strictly speaking, the TPB is a line and efforts have been made to increase its length per unit of electrode volume in order to reduce the activation losses. In this communication, by integrating physically-based modelling, 3D tomography and electrochemical impedance spectroscopy (EIS), a renovated perspective on electrocatalysis in SOFCs is offered, showing that the electrochemical reactions take place within an extended region around the geometrical TPB line. Such an extended region is in the order of 4 nm in Ni/Sc0.2Zr0.9O2.1 (Ni/ScSZ) anodes while approaches hundreds of nanometres in La0.8Sr0.2MnO3-x/Y0.16Zr0.92O2.08 (LSM/YSZ) cathodes. These findings have significant implications for preventing the degradation of nanostructured anodes, which is due to the coarsening of the fractal roughness of Ni nanoparticles, 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). 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 SOFC electrodes.