Influence of gas holdup on ohmic resistance in a mid-scale pressurized alkaline water electrolyzer

This study investigates the impact of gas holdup on ohmic resistance of a zero-gap alkaline water electrolyzer (AWE) operating under pressurized conditions. A custom-built 150-cm 2 cell equipped with nickel perforated electrodes and a Zirfon UTP 500 diaphragm was tested at pressures ranging from 1 to 15 bar and temperatures of 25 °C and 60 °C. The electrodes have a height of 0.5 m, mimicking the vertical scale of industrial electrolyzers to better capture possible bubble effects. An empirical correlation for hydrogen holdup in the bulk electrolyte was obtained, indicating a nonlinear dependence on current density and pressure. The results deviated from the Bankoff model, which overestimated the gas holdup compared to experimental data. Electrochemical performance was evaluated using chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) up to 0.5 A/cm 2 . Results show that increasing pressure reduces ohmic resistance. This reduction cannot be directly attributed to the reduced amount of gas in the bulk electrolyte. Instead, gas accumulation in the electrode-diaphragm gap appears to be a dominant factor. Ohmic resistance exceeded the intrinsic diaphragm resistance, probably due to bubbles in the imperfect zero-gap and/or contact resistance. Analysis of the ohmic resistance and gas fraction in gaps of different sizes suggests that local gas holdup in the narrow gap could be in the range of 70-80%. Results also suggest that increasing pressure effectively reduces gas holdup in the bulk electrolyte, but it is likely to have only a limited effect on gas amount reduction in the imperfect zero-gap. These findings highlight the importance of optimizing electrode-diaphragm gap and bubble dynamics to minimize ohmic losses and, hence, increasing efficiency of industrial-scale electrolyzers. • Increased pressure reduces ohmic overpotential at high current densities. • Experimental data show lower gas holdup than predicted by Bankoff model. • High ohmic resistance likely caused by high gas holdup in electrode-diaphragm gap. • Long electrodes experience greater bubble-induced ohmic losses than short electrodes.