Closed-circuit electrochemical hydrogen pump for the investigation of pressure and isotope effects in the hydrogen-palladium system: an experimental and computational study

Isotope effects between hydrogen and deuterium are key to various applications, including isotope separation for the nuclear industry and cold fusion. While molecular deuterium is rarely studied due to its high cost and is typically replaced by electrolytes like LiOD or NaOD, advances in hydrogen technologies enable direct investigation of isotope effects. Here, we introduce a novel closed-circuit electrochemical hydrogen pump for low-cost, long-duration studies of the hydrogen-palladium system under controlled H2/D2 partial pressures (0.5 – 2.5 bar) at high current densities (100 mA.cm-2). Pressure-dependent changes in capacitance and ohmic resistance suggest compression of the catalyst layers and narrowing of water channels within the membrane. Comparative measurements reveal clear isotope effects: ~25% and ~10% lower anodic charge transfer and ohmic resistances with H₂, yielding ~1.6× higher exchange current densities across all pressures. Additional effects on mass transport resistance further highlight the kinetic advantage of protium. Furthermore, density functional theory calculations were performed on PdHx and PdDx model systems, wherein the thermodynamic phase and surface Pourbaix diagrams confirmed stable hydride/deuteride formation at the experimental conditions, consistent with changes in surface coverage inferred from capacitance trends. Reaction pathway analysis identified the Volmer–Heyrovsky mechanism, with the Heyrovsky step as rate-determining, in agreement with measured Tafel slopes. Electronic structure analysis shows that subsurface hydrogen weakens surface binding via d-band shifts, enhancing H2/D2 desorption. Altogether, this study integrates the design and validation of a novel hydrogen pump system and atomic-scale modelling to provide a multiscale understanding of isotope effects on metal-hydrogen systems under electrochemical conditions.

氢与氘之间的同位素效应是诸多核心应用的关键基础，涵盖核工业同位素分离与冷聚变领域。由于分子氘成本高昂，相关研究鲜有开展，通常以LiOD（氘氧化锂）、NaOD（氘氧化钠）等电解质替代开展实验。随着氢能技术的进步，如今已可直接开展同位素效应的相关研究。本文介绍一款新型闭环电化学氢泵（closed-circuit electrochemical hydrogen pump），可在高电流密度（100 mA·cm⁻²）、可控H₂/D₂分压（0.5–2.5 bar）的条件下，以低成本开展长期的氢-钯体系研究。

电容与欧姆电阻随压力的变化表明，催化层发生了压缩，且膜内水通道出现收窄。对比测试结果清晰展现了同位素效应：使用H₂时，阳极电荷转移阻力与欧姆电阻分别降低约25%与10%，对应所有压力下的交换电流密度提升约1.6倍。传质阻力方面的额外差异进一步凸显了氕的动力学优势。

此外，本研究针对PdHₓ与PdDₓ模型体系开展了密度泛函理论（density functional theory）计算，通过热力学相图与表面布拜图（Pourbaix diagrams）证实，在实验条件下可稳定生成氢化物/氘化物，该结果与通过电容趋势推导得到的表面覆盖率变化一致。反应路径分析表明体系遵循沃尔默-海罗夫斯基机理（Volmer–Heyrovsky mechanism），其中海罗夫斯基步骤为速率决定步骤，与实测的塔菲尔斜率（Tafel slopes）结果相符。电子结构分析显示，亚表面氢通过d带位移削弱了表面结合能，从而促进H₂/D₂的脱附。

综上，本研究完成了新型氢泵系统的设计与验证，并结合原子尺度建模，实现了电化学条件下金属-氢体系同位素效应的多尺度认知。