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 cm(2)). 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 degrees 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 cm(2) pilot-cell under ca 2.94 kPa differential pressure, typically, the cell voltage of 1600 cm(2) pilot-cell was 1.81 V at 0.4 A/cm(2) and 82 degrees 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. (C) 2016 The Electrochemical Society. All rights reserved.