Considering future hydrogen society without CO₂ emission, water electrolysis will take an important position in total hydrogen technology chain. Alkaline water electrolysis (AWE) is one of the candidate technologies to produce hydrogen on a large scale from renewable energy. The performance of AWE process is mainly achieved by four factors, i.e., anode, cathode, separator and cell structure. Although the each factor affects mainly the cell voltage, separator has an effect for both cell voltage and generated gas purity. Therefore, in order to achieve high purity of the gas from the AWE electrolyzer, the characteristic or performance of separator is important in the system.

Our target is to achieve 1.80 V of cell voltage at 0.4 A/cm², 80℃ and over 99.9% hydrogen purity. Many separators were screened. A separator of diaphragm type showed good performance for AWE. The evaluation of separator was carried out by using different sizes of cells such as 0.2, 1.0 and 16 dm²depending on the aim of evaluation. The structure of each test cell is based on zero-gap concept to reduce gas effect in the electrolysis.

The cell voltage of 16 dm² bench cell was 1.81 V at current density 0.4 A/cm², temperature 80℃ and hydrogen purity 99.95% and 1.94 V at current density 0.6 A/cm², temperature 80℃ and hydrogen purity 99.96%. It was confirmed that only single side feeding to cathode is available for the cell operation without any serious problems under Δ300 mmH₂O differential pressure (between anode chamber <low pressure> and cathode chamber<high pressure>). The result indicates that cathode electrolyte can easily pass through the diaphragm from cathode side to anode side.

The cell equipped with diaphragm without hydrophilic treatment showed a large fluctuation in cell voltage. It suggests that hydrophilic treatment of membrane is very important factor to get steady operation in the actual large-scale plant. The flow rate of electrolyte (0.3 to 0.45 mL/min-cm²) does not dominate the purity of hydrogen gas produced. It was also confirmed that the diffusion amounts of gas through the diaphragm as gas (bubble) and solved species (gas) in the electrolyte are on the same level (H₂ gas: dissolved H₂ gas = 1:1.25).