Membranes Matter: Preventing Ammonia Crossover during Electrochemical Ammonia Synthesis

The electrochemical nitrogen and nitrate reduction reactions (E-NRR and E-NO₃RR) promise to provide decentralized and fossil-fuel-free ammonia synthesis, and as a result, E-NRR and E-NO₃RR research has surged in recent years. Membrane NH₃/NH₄⁺ crossover during E-NRR and E-NO₃RR decreases Faradaic efficiency and thus the overall yield. During catalyst evaluation, such unaccounted-for crossover results in measurement error. Herein, several commercially available membranes were screened and evaluated for use in ammonia-generating electrolyzers. NH₃/NH₄⁺ crossover of the commonly used cation-exchange membrane (CEM) Nafion 212 was measured in an H-cell architecture and found to be significant. Interestingly, some anion exchange membranes (AEMs) show negligible NH₄⁺ crossover, addressing the problem of measurement error due to NH₄⁺ crossover. Further investigation of select membranes in a zero-gap gas diffusion electrode (GDE)-cell determines that most membranes show significant NH₃ crossover when the cell is in an open circuit. However, uptake and crossover of NH₃ are mitigated when -1.6 V is applied across the GDE-cell. The results of this study present AEMs as a useful alternative to CEMs for H-cell E-NRR and E-NO₃RR electrolyzer studies and present critical insight into membrane crossover in zero-gap GDE-cell E-NRR and E-NO₃RR electrolyzers.