Development of a Separation-Factor-Constrained Genetic-Algorithm Kinetic Analysis for Isotopic Hydrogen Evolution Reactions

Abstract The isotopic evolution of hydrogen/deuterium in mixed H2O/D2O electrolytes provides mechanistic insights that cannot be obtained from conventional current–potential analysis alone. In this study, we present a constrained kinetic analysis framework for the isotopic hydrogen evolution reaction, integrating an isotopically extended Volmer–Heyrovsky–Tafel model with a genetic algorithm-based parameter estimation approach. This methodology allows us to fitting the electrochemical current response while imposing experimentally determined isotopic selectivity, acquired through electrochemical mass spectrometry, as an additional physical constraint. We evaluated this framework using electrochemical data from Au, Pt, and Ni–Mo electrodes in electrolytes with different D2O ratios. The fitted kinetic parameters aligned with the anticipated isotope-effect trend, indicating that pathways involving H were kinetically favored over those involving HDO or D2O. Significantly, the imposed selectivity constraint affected the fitted parameters in a metal-dependent manner, particularly influencing those parameters related to product-forming isotopic branching. This finding suggests that isotopic selectivity predominantly constrains the branching kinetics that determine the distribution among H2, HD, and D2. Additionally, a comparison of parameter-free penalty and violation-constraint-handling methods indicated that violation-constraint handling yields more kinetically plausible parameter sets when isotopic selectivity is treated as a fundamental constraint. These results establish aseparation factor-constrained kinetic fitting as a practical strategy for resolving product-forming branching kinetics in multistep electrocatalytic HER, providing mechanistic information inaccessible from conventional current–potential analyses.