The behavior of the leak and reverse currents in a bipolar-type alkaline water electrolyzer has been investigated using a bipolar-type electrolyzer which consists of two cells. The electrodes were nickel mesh, which are the conventional electrodes for alkaline water electrolyzers. The leak circuit could be expressed by a simple equation and a simple equivalent circuit for the cell performance and ionic resistance of the manifolds. The electrolyte was replaced by a gas-free electrolyte after electrolysis to classify the influence of the reverse current into the gas reaction and electrode active material. As a result, the dominant driving force of the reverse current was the active nickel-based materials on the Ni electrode. The redox couples on the electrode surface during the reverse current were estimated based on the measured cell voltages and redox potentials on a nickel electrode. The final potentials of both sides on the bipolar plate for the replacement conditions were higher than those for the non-replacement condition, because the hydrogen of the reductant was removed from the cathode electrolyte, and the balance of the reductant and oxidant would change to the oxidation side.
The performance of separators of ion-exchange membrane (IEM) and diaphragm types in the zero-gap system of alkaline water electrolysis (AWE) was examined using different sizes of cells (i.e., 20, 100 and 1600 cm2). The optimum KOH concentrations for AWE were found to be ca.15 and 25–30 wt% for IEM and diaphragm, respectively, which are different from the KOH concentration (ca. 35 wt% at 80°C) at which the maximum conductivity is obtained. It was confirmed that only anode side feeding operation is available without any serious mixing of both anolyte and catholyte using 1600 cm2 pilot-cell under ca 2.94 kPa differential pressure, typically, the cell voltage of 1600 cm2 pilot-cell was 1.81 V at 0.4 A/cm2 and 82°C with hydrogen purity of 99.96%. The flow rate of electrolyte does not dominate the purity of hydrogen gas produced. The fluctuation of cell voltage in the diaphragm-based AWE process is very large, suggesting that the quality and/or pretreatment of diaphragm dominate the performance of AWE. It was also confirmed that the diffusion amounts through the diaphragm as gas and dissolved species are on the same level.
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