Elucidating the increased ohmic resistances in zero-gap alkaline water electrolysis

This is the metadata for the research paper "Elucidating the increased ohmic resistances in zero-gap alkaline water electrolysis". In this study we elucidate the possible reasons for the increased ohmic resistances in zero-gap alkaline water electrolysis. We critically assess the methodology for estimating the ohmic resistance of zero-gap electrolyzers as reported in literature. In order to monitor variations in ohmic resistance during zero-gap alkaline electrolysis, we employ electrochemical impedance spectroscopy (EIS) at both low and high current densities. Both 2- and 4-terminal configurations are used to address additional contact or cable resistance. Furthermore, we evaluate the diaphragm wettability and the bubble behavior both outside and inside the diaphragm using a photographic method to better understand the possible role of bubbles. Our intention is that these findings will help to open the way for electrolyzers with reduced ohmic resistance, enabling efficient operation at higher current densities. ABSTRACT This study investigates the increased ohmic resistances observed in zero-gap alkaline water electrolyzers, aiming to provide insights that can help enhance electrolyzer efficiency and enable operation at higher current densities. Electrochemical impedance spectroscopy (EIS) has been employed in combination with chronopotentiometry, utilizing a custom-designed flow cell with nickel perforated electrodes and a Zirfon UTP 500 diaphragm. Observed differences in area-ohmic resistance values obtained through I-V fitting and EIS, are ascribed to a non-linear Tafel slope at higher current densities. Ohmic resistance values measured with EIS are up to 27% higher than the ex-situ determined value, a significantly smaller percentage than expected based on previous studies. The presence of bubbles outside and inside the diaphragm is identified as the key factor contributing to this increased resistance. We recommend the use of an improved fitting approach, accounting for non-linear Tafel behavior, and the use of a 4-terminal configuration when performing EIS measurements to minimize cable and contact resistance.