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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.

Chlorine Engineers Corp., Ltd. (CEC) is special electrochemical company in Japan and was established in 1973, when mercury process of chlor-alkali industry was abandoned by Japanese government. Since then, CEC has accumulated electrolysis technology and has an interest for future hydrogen society. In the background, zero gap system was applied for cell evaluation of alkali water electrolysis based on CEC's technology. A special electric collector for cathode was used to create zero gap without damaging the separator. Cell voltage was 1.76V at 40A/dm 2 , 80 deg.C. Raney Ni alloy coating had advantage for oxygen overvoltage. (100mV -200mV saving against Ni metal) Thermal decomposition coating of cathode showed low hydrogen overvoltage (around 100mV). Ion exchange membranes N117 and F8020 showed high cell voltage (over 2.2V) but high purity of H 2 gas (over 99.95%). Porous polyolefin film showed low cell voltage but had less durability.

There are four main factors in the Alkaline Water Electrolysis (AWE) for contributing to the cell performance. One is cell structure which is related to the uniform electrolyte concentration with proper internal circulation of electrolyte in the cell room or the out gassing from electrode, separator and cell chamber. Second is cathode property such as low cathode overpotential and long life. Third is anode property as cathode. Forth is the characteristic of separator which is the most significant influence for total cell performance (cell voltage, hydrogen gas purity and life). Then, various separators and several anode materials were evaluated to confirm the performance. Although, thin IEM (25μm thickness) of polyethylene base showed the lowest cell voltage 1.7V at 0.4A/cm 2 , the film did not have durability against continuous operation because of deterioration of base film. Raney Ni coating and Spinel type coating for anode achieved 100mV and 50mV, respectively.