Hydrogen permeation characteristics in proton exchange membrane water electrolyzers under time-varying current density: experimental and numerical study

Proton exchange membrane water electrolysis is well suited for renewable hydrogen production under fluctuating power inputs because of its rapid dynamic response. However, hydrogen permeation characteristics and associated outlet gas-purity behavior under time-varying current densities remain insufficiently quantified. Experiments quantitatively characterize the hydrogen concentration in the outlet oxygen stream in response to current-density step changes and under pressure-asymmetric operation, with membrane-integrity loss examined as an abnormal case. A three-dimensional non-isothermal two-phase model analyzes internal fields at representative conditions. The measurements reveal markedly different response times following current-density increases and decreases: 65–120 s and 156–483 s, respectively. Following current-density decreases, the response times exceed estimated downstream-system upper bounds by 59.9–341.2 s, showing that downstream gas replacement and sensor response alone cannot account for the delay. Further analysis indicates that the prolonged response times result from the combined effects of reduced oxygen dilution, slower downstream gas replacement, and the gradual release of hydrogen stored in the membrane. Numerical results show that local hydrogen concentration increases with current density and exhibits spatial non-uniformity. Pressure-asymmetry testing and the membrane-failure case further indicate that the measured outlet concentration is sensitive to pressure differences and membrane integrity. These findings provide a basis for gas-purity monitoring and operating-condition management under time-varying current densities.