Investigation of Charge–Discharging Behavior of Metal Oxide–Based Anode Electrocatalysts for Alkaline Water Electrolysis to Suppress Degradation due to Reverse Current

In the bipolar-type alkaline water electrolysis powered by renewable energy, electrocatalysts are degraded by repeated potential change associated with the generation of reverse current. If an electrode has large discharge capacity, the opposite electrode on the same bipolar plate is degraded by the reverse current. In this study, discharge capacity of various transition metal-based electrocatalysts was investigated to clarify the determining factors of electrocatalysts on the reverse current and durability. The discharge capacities from 1.5 to 0.5 V vs. RHE (Q_dc,0.5) of electrocatalysts are proportional to the surface area in most cases. The proportionality coefficient, corresponding to the specific capacity, is 1.0 C·m^–2 for Co₃O₄ and 2.3 C·m^–2 for manganese-based electrocatalysts. The substitution of Co³⁺ in Co₃O₄ with Ni³⁺ increased Q_dc,0.5. The upper limit of theoretical specific capacity for Co₃O₄ is estimated to be 1.699 C·m^–2, meaning the former and latter cases correspond to 2- and 1-electron reactions, respectively, per a cation at the surface. The discharge capacities of the elctrocatalysts increased because of the dissolution and recrystallization of nickel and/or cobalt into metal hydroxides. The increase in the capacities of Co₃O₄ and NiCo₂O₄ during ten charge–discharge cycles was below 2–9% and 0.5–38%, respectively. Therefore, if a cathode electrocatalyst with relatively low redox durability is used on the one side of a bipolar plate, it is necessary to control optimum discharge capacity of the anode by changing surface area and constituent metal cations to minimize the generation of reverse current.

Graphical Abstract