Water electrolysis is attracting attention as a technology for producing hydrogen and chemical fuels using renewable energy. Because even trace amounts of chloride ions remaining in the electrolyte can generate toxic and corrosive chlorine gas, electrolysis normally requires extremely pure water. However, fresh water is in short supply globally, and there is growing demand for technology that can electrolyze lower-purity water containing trace chloride ions while minimizing chlorine gas generation.
A research group led by Team Director Ryuhei Nakamura at the RIKEN Center for Sustainable Resource Science discovered that changing the type of alkali metal ion added to the electrolyte significantly affects the maximum rate of chlorine gas generation. They found that when lithium ions (Li+) are present in an electrolyte containing trace residual chloride ions, chlorine gas generation can be suppressed by 33 percent compared with when cesium ions (Cs+) are present. To explain this phenomenon, the group proposed a model in which a new "diffusion layer"—one that depends on the type of alkali metal ion added—exists inside the conventional "diffusion layer" that forms a solute concentration gradient around the electrode. They showed that with Li+, this diffusion layer is thicker than with Cs+, making it harder for chloride ions to pass through and reducing the number that reaches the electrode.
This finding offers a new perspective that the diffusion rate of chloride ions is influenced by the structure formed by ions and water molecules in aqueous solution. As a technological foundation for using lower-purity water in electrolysis, this work is expected to contribute to achieving the Sustainable Development Goals (SDGs).
(Article: Masanori Nakajo)
