Mechanism and Kinetics of Ni-Y₂O₃-ZrO₂Hydrogen Electrode for Water Electrolysis Reactions in Solid Oxide Electrolysis Cells

Abstract

Ni-Y 2 O 3 stabilized ZrO 2 (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, R E , for the WRR increases with the dc bias, while in the case of HOR, R E 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 H 2 O 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, R H and in much less extent low frequency electrode polarization resistance, R L with the dc bias indicates that the water electrolysis reaction is kinetically controlled by the reactant supply (e.g., the adsorbed H 2 O species) limited charge transfer process. Solid oxide electrolysis cell (SOEC) as an electrochemical device to convert electricity of renewable energy sources such as solar energy, wind power, hydropower and geothermal power into chemical energy of fuels such as hydrogen and syngas has attracted increasing interests due to the depleting fossil fuel sources, high oil prices and environmental considerations. [1][2][3][4][5][6][7] In the case of water electrolysis to produce hydrogen, steam is introduced to the hydrogen electrode side where it is reduced to hydrogen, while the oxygen ions are migrated through the electrolyte to the air electrode side where they combine to form pure oxygen. Co-electrolysis of steam and CO 2 in an SOEC yields synthesis gas (CO+H 2 ) which in turn can be catalysed to various types of synthetic fuels (such as methane and methanol). [8][9][10][11] SOECs are reversible operation of solid oxide fuel cells (SOFCs), and therefore in principle the electrode and electrolyte materials developed for SOFCs can be utilized for SOECs.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.1,12-14 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 thus substantially enhances the electrocatalytic activity and/or stability fo...

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