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

Abstract

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 (Qdc,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 Qdc,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

(Invited) Shunt-currents in Alkaline Water-Electrolyzers and Renewable Energy

“…It is obvious that the centre-electrodes must discharge must faster compared to the end-electrodes. Neighbouring anodes and cathodes sharing the same bipolar plate must also process the same current when passing through the bipolar plate (14). Modelling the cell voltages under discharge in the combination with shuntcurrents on larger stacks, i.e.…”

“…The second major implication of shunt-current and large electrolyzer stacks in connection to renewable energy operation and intermittent operation, is the electrode stability during shutdown. The discharge of hydrogen-and oxygen-electrodes and how much one or the other will discharge as discussed in (12,13,14,15) is perhaps better understood by simply inspecting the cyclic voltammogram of a typical hydrogen-and oxygen-electrode (using the term anode and cathode may get confusing when discussing current reversal as the hydrogen-electrode switches from being a cathode under operation to become an anode under current reversal and visa versa for the oxygen-electrode). Figure 5 shows the cyclic voltammogram of a platinum group metal (PGM)-based hydrogen electrode and a bare nickel as the oxygen electrode after being used in a AWE system, measured at 20 mV/s in 1 M KOH at room temperature and measured against a Gaskatel Reversible Hydrogen Electrode (RHE).…”

“…It is also listed a few materials and material compositions that are vulnerable to revers currents, e.g. cobalt molybdate cathode, nickel molybdate cathode, unmodified nickel cathodes, mixed oxide of nickel and cobalt as anodes, lanthanum strontium-cobalt perovskite as anodes and nickel-cobalt spinel materials (14). four cells, the ionic current could be measured during discharge and the cells were equipped with reference electrodes to measure both oxygen-and hydrogen-electrode for each cell.…”

“…K. Oda et.al. (14) synthesised different OER catalysts based on the oxides of Ni, Co and Mn, and studied their discharge characteristics as function of redox properties and surface area and argued that the OER catalyst must be matched correctly with the HER catalyst on the opposite side of the bipolar plate to avoid disintegration of one of the catalysts.…”