Enhanced Gas Diffusion in Reversible Solid Oxide Cell Fabricated by Phase-Inversion Tape Casting

Abstract

Electrochemical characteristics of a hydrogen-electrode-supported solid oxide cell (SOC) fabricated by phase-inversion tape casting are experimentally evaluated in both fuel cell (FC) and electrolysis cell (EC) operations. Particular focus is on the effect of the microchannels formed in the hydrogen electrode on the gas diffusion performance in the electrode. Current-voltage and impedance characteristics are measured and compared with those obtained from an SOC fabricated by conventional tape casting technique. The activation resistance is found to be almost identical between the tested cells because both cells are fabricated with a spin-coated functional layer in the hydrogen electrode. On the other hand, the concentration resistance is significantly decreased in the cell fabricated by the phase-inversion tape casting because the microchannels in the hydrogen electrode enhances the gas diffusion performance. This is more remarkable when the cells are operated in diluted fuel composition and with higher current densities. The asymmetric behavior is observed in the current-voltage characteristics of the cells, meaning that the overpotentials in the FC and EC modes are not the same at the same FC and EC currents. The overpotential is found to be always larger in EC mode, however, the extent of asymmetry is reduced in the cell fabricated by the phase-inversion tape casting. This is mainly because the microchannels in the hydrogen electrode reduces the gas diffusion resistance associated with the steam transport in EC mode. Microstructure of the fabricated cells is also analyzed in micro- and nano-scale to quantify the structure of the microchannels formed in the hydrogen-electrode. The microchannels are initiated below the immersed surface during the phase-inversion process, and then grow towards the bottom of the layer. The average diameter of the microchannels increases, whereas its number density decreases. Also, the total porosity in the phase-inversion electrode is found to be almost the same as that in the conventional cell.