( Invited ) Mass-Transfer Resistance across Bubble Layers on Wire- and Mesh-Type Electrodes during Alkaline Water Electrolysis

Abstract

A major limitation of the practical implementation of alkaline water electrolysis (AWE) is the substantial increase in overvoltage caused by gas bubbles adhering to, and accumulating on the electrode surface under high current-density operation. Such bubble accumulation inhibits ion transport and blocks active electrochemical sites, resulting in lowered energy efficiency. Because the extent of bubble coverage depends on both the applied current density and the geometric properties of the electrode, the bubble-induced contribution to total overvoltage can vary significantly from system to system. Therefore, when evaluating catalyst or electrode performance, it is essential to consider not only intrinsic catalytic activity, but also to carefully identify and quantitatively separate the bubble-related mass-transfer overvoltage from the overall cell polarization. Our group previously developed the mass-transfer models in bubble layer based on electrochemical impedance spectroscopy (EIS) using simple Ni wire electrodes, combined with equivalent-circuit analysis for the anode/OER and cathode/HER, respectively [1, 2]. These studies enabled the isolation of the bubble contribution. For OER, the measured i-V curve deviates from the expected Tafel line at current densities above approximately 0.2 A cm ‒2 , indicating the onset of a mass-transfer limitation associated with bubble accumulation. A comparable deviation is observed for HER at current densities exceeding roughly 0.3 A cm ‒2 . In both cases, the mass‐transfer resistance was found to consist of two key components: diffusion resistance and ohmic migration resistance within the bubble layer. The migration resistance 𝑅 sb was successfully interpreted in terms of the thickness and void fraction of the bubble layer, the conductivity of the electrolyte, and a tortuosity model characterizing the ion-migration pathways through the inter-bubble gaps. Diffusion transport differs between OER and HER: for OER, the diffusion of hydroxide ions was modeled by a finite-length Warburg impedance element, reflecting the restricted diffusive boundary created by the bubble layer. For HER, two distinct inductive elements were identified at high and low frequency, suggesting that gas evolution dynamically enhances the reaction rate in a frequency-dependent manner. The charge-transfer resistance (𝑅 ct ) at the electrode surface was proven to scale inversely with current density, indicating that 𝑅 ct becomes relatively insensitive to bubble coverage and that nearly all accessible electrode surface remains electrochemically active even beyond 𝑖 geo = 0.5 A cm −2 . These two mass-transfer models were subsequently extended to a cell-type AWE configuration [3], operated in 2 M KOH at 30 °C. Both CV and EIS measurements were performed under control of anodic potential vs. RHE, allowing simultaneous acquisition of three i-V curves and three Cole-Cole plots corresponding to anodic potential, cathodic potential, and whole-cell voltage. From the three i-V curves, the cell voltage could be decomposed into seven distinct contributions: the theoretical decomposition voltage, the reaction overpotential and mass-transfer overpotential for both anode and cathode, the diaphragm resistance, and the mass-transfer overpotential arising near the diaphragm. Subsequent analysis of the Cole-Cole plots allowed the mass-transfer overpotential to be further separated into diffusion- and ohmic-type components. Through these procedures, we were able to quantitatively demonstrate that bubble-derived mass-transfer overpotential accounts for approximately 30–50% of the total cell overvoltage under high-current density conditions, depending strongly on electrode geometry and gas-release behavior. This finding highlights that bubble management—whether by electrode structuring, surface modification, hydrodynamic control, or cell design—can have an impact comparable to improving intrinsic catalyst performance. Accordingly, the methodology presented here provides a rational framework for diagnosing limiting mechanisms in practical AWE devices and may serve as a useful benchmark when screening electrode materials for high-current industrial hydrogen production. Acknowledgments A part of this study was supported by the New Energy and Industrial Technology Development Organization (NEDO, JPNP25002). References [1] H. Ikeda, R.Misumi, Y. Nishiki, Y. Kuroda, S. Mitsushima, Electrochimica Acta , 430, 141053 2022 [2] D. Kitajima, R. Misumi, Y. Kuroda, S. Mithushima, Electrochimica Acta , 502, 144772, 2024 [3] Y. Nishio, R. Misumi, S. Mithushima, PRiME 2024 , I01B-2773, 2024