Abstract
Ni-Y2O3 stabilized ZrO2 (Ni-YSZ) cermet is the most commonly used hydrogen electrode for hydrogen oxidation reaction (HOR) under solid oxide fuel cell (SOFC) mode and water reduction reaction (WRR) under solid oxide electrolysis cell (SOEC) mode. Here we studied the electrocatalytic activity of Ni-YSZ electrodes as a function of Ni content, water concentration and dc bias for WRR and HOR under SOEC and SOFC modes, respectively. The activity of Ni-YSZ cermet increases significantly with the increase of YSZ content due to the enhanced three phase boundaries (TPB). The electrode activity for the WRR and in less degree for the HOR increases with the increase of steam concentration. The electrode polarization resistance, RE, for the WRR increases with the dc bias, while in the case of HOR, RE decreases with the dc bias, demonstrating that kinetically the WRR and HOR is not reversible on the Ni-YSZ cermet electrodes under SOFC and SOEC operation modes. The WRR can be described by two electrode processes associated with the H2O adsorption and diffusion on the oxygen-covered Ni or YSZ surface in the vicinities of TPB, followed by the charge transfer. The significant increase of high frequency electrode polarization resistance, RH and in much less extent low frequency electrode polarization resistance, RL with the dc bias indicates that the water electrolysis reaction is kinetically controlled by the reactant supply (e.g., the adsorbed H2O species) limited charge transfer process.
Highlights
We report the results on the dependence of electrocatalytic activity of Ni-yttria-stabilized zirconia (Ni-YSZ) hydrogen electrodes as a function of operating temperature, water concentration and dc bias, and new reaction mechanism of water electrolysis reaction on Ni-YSZ cermet electrodes has been proposed
Electrocatalytic activity of Ni-YSZ hydrogen electrodes is dependent on Ni content, operating temperature, water concentration and dc bias for the water reduction reaction (WRR) under Solid oxide electrolysis cell (SOEC) mode and hydrogen oxidation reaction (HOR) under solid oxide fuel cells (SOFCs) mode
The impedance behavior for HOR and WRR on Ni-YSZ electrodes is characterized by two separable impedance arcs at high and low frequencies, indicating HOR and WRR on Ni-YSZ electrode are controlled by two rate limiting processes
Summary
Similar to SOFCs, Ni-yttria-stabilized zirconia (Ni-YSZ) cermets are the most common hydrogen electrodes for SOECs, due to its high electrical conductivity, high electrocatalytic activity, high thermal and structural stability and low price.[1,12,13,14] The incorporation of electrocatalytically active nanoparticles such as doped ceria oxides and Rh metal in the Ni-YSZ hydrogen electrodes enhances the ionic conductivity and three phase boundaries (TPBs), and substantially enhances the electrocatalytic activity and/or stability for the water electrolysis or water reduction reaction (WRR).[15,16,17] Nanostructured Ni infiltrated samaria doped ceria (SDC) can increase the electrocatalytic activity by increasing the reaction area.[18,19]. The high activation energy value of RH observed in this study again indicates that the electrode process associated with high frequencies is a charge transfer reaction at the TPB, close to that of the HOR at the Ni-YSZ cermet electrode.[27]
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