Unveiling proton pathways between the anode and cathode in PEM electrolyzer cells via direct reaction visualization†

Proton exchange membrane electrolyzer cells (PEMECs) represent a promising technology for producing green hydrogen to support global carbon neutrality targets. However, the intricate details of proton transport within PEMECs have long remained shrouded in mystery due to the lack of direct observation tools. In this work, by developing a novel visualizable coplanar electrode for the PEMEC (CPE-PEMEC) and well-designed anode electrodes with controllable conductivity levels, we discovered through direct observation of the reaction that proton transport exhibits regional mismatches between the anode and cathode, as evidenced by notable differences in bubble generation on the anode and cathode electrodes. This observation challenges the conventional belief in parallel proton transfer between the cathode and anode via the proton exchange membrane (PEM). Contrarily, we propose a fixed-segment transfer mechanism, wherein protons are transferred between corresponding reaction area segments on the cathode and anode electrodes. This revelation enables the prediction and control of proton pathways within PEMECs for the first time. This work not only deepens our understanding of detailed reaction mechanisms in PEMECs but also potentially charts a course for the optimization of next-generation PEMECs and other energy devices.