Reversible H2/H2O electrochemical conversion mechanisms on the patterned nickel electrodes

The patterned nickel (Ni) electrode enables to quantify the triple-phase boundary (TPB) length and Ni surface area as well as exclude the interference of bulk gas diffusion. In this study, the patterned Ni electrodes are investigated in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H2O/H-2. The experimental test shows the patterned Ni electrode keeps stable and intact only at the specific operating condition due to instability of Ni at the H2O-containing atmosphere. The effects of the temperature, partial pressure of H2O and H-2 on the electrochemical performance are measured. The electrochemical performance has a positive correlation with the temperature, partial pressure of H-2 and H2O. Further, the experimental results are compared with the mechanism containing two-step charge-transfer reaction used in the existing literature. An analytical calculation is performed to indicate the rate-limiting steps may be different for SOFC and SOEC modes. In SOFC mode, H-2 electrochemical oxidation could be dominated by both charge transfer reaction at low polarization voltage and by the charge transfer reaction H(Ni) + O2- (YSZ) -> OH-(YSZ) + (Ni) + e(-) at high polarization voltage, however in SOEC mode, H2O electrochemical reduction is considered to be dominated by H2O(YSZ) + (Ni) + e(-) -> OH-(YSZ) + H(Ni). (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.