Relationship Between the Redox Reactions on a Bipolar Plate and Reverse Current After Alkaline Water Electrolysis

In order to efficiently operate the alkaline water electrolyzers with renewable energy, behaviors of the electrolyzer during start-up or shut-down must be unveiled, because they might be suffered by reverse current that naturally flows. The mechanism of the reverse current in alkaline water electrolyzer having relation between the electrolyzer operating conditions and cell voltage has been investigated using a bipolar-type electrolyzer which consists of two cells. The electrodes were nickel mesh, which are conventional electrodes for alkaline water electrolyzer. The amount of natural reverse current measured during off-load was proportional to the current loaded until just before stopping the operation. The increase in the charge would result from the increasing oxide on the anode of the bipolar plate. Cell voltages were above 1.4 V at all cases just when the electrolyzer is forcibly opened the circuit to stop. The major redox couple of the reverse current would be [NiO2/NiOOH] and [H2/H2O] due to the cell voltage and the redox couples. The open circuit cell voltage of the cathode terminal side cell gradually decreased to 0.3 V, while that of the anode terminal side cell was maintained above 1.1 V. Therefore, nickel oxides on the anode of the bipolar plate would be reduced, and the cathodic active material of hydrogen and nickel for the cathode side of the bipolar plate would be oxidized during the reverse current flows. Ultimately, the reverse current would stop when the redox state of both sides of the bipolar plate had the same oxidation state.