A multiphysics model of a proton exchange membrane acid-alkaline electrolyzer

The high energy requirement of hydrogen generation via water splitting has motivated the development of acid-alkaline electrolyzers, which have a lower thermodynamic voltage requirement than conventional electrolyzers. Proton exchange membrane acid-alkaline electrolyzers have been reported in literature, but its reactions and ion transport mechanisms are still unknown. In this work, we developed a multiphysics model of a proton exchange membrane acid-alkaline electrolyzer to elucidate the mechanism of operation. The model showed that Na+ crossover from the anolyte to the catholyte is the primary mechanism for retaining electroneutrality, in contrast with the prevailing hypothesis that H+ is the primary charge carrier. Moreover, we found that H+ is transported from the catholyte to the anolyte, which is counterproductive towards maintaining electroneutrality and results in the undesired acid-base neutralization reaction. Increasing the applied current reduces H+ crossover, thereby demonstrating a tradeoff between power consumption and side reaction minimization. As the cell is operated, the catholyte composition changes from H2SO4 to a mixture of NaHSO4 and Na2SO4, which in turn reduces the overall efficiency. Therefore, in addition to water, proton exchange membrane acid-alkaline electrolyzers will require constant feeding of fresh electrolyte to maintain its performance, and this poses a barrier towards its practical use.

Automated summary

This study developed a multiphysics model to elucidate the operating mechanism of a proton exchange membrane acid-alkaline electrolyzer, revealing that Na+ crossover is the primary mechanism for maintaining electroneutrality. Increasing the applied current reduces H+ crossover, but also decreases the overall efficiency.