Zero-gap bipolar membrane water electrolyzers: Principles, challenges and practical insights

Producing hydrogen through the electrolysis of water is an attractive method for storing intermittent, renewably-generated power in the form of a clean-burning fuel. Amongst the various types of electrolyzer that have been proposed, those employing bipolar membranes have the unique advantage of allowing the water splitting half-reactions to proceed under their ໿kinetically most favourable conditions, thanks to the ability of these membranes to maintain a pH gradient. Maintaining such a pH gradient enables the oxygen evolution reaction to occur in basic conditions whilst the hydrogen evolution reaction occurs in acidic conditions. However, bipolar membrane electrolysis technology remains at an early stage of development and faces several challenges, including high potential losses and low energy efficiency caused by insufficient water transport across (and ion crossover within) the bipolar membrane. In this review, the strategies and methods employed to address these challenges and to enhance the performance and efficiency of bipolar membrane water electrolysis are discussed. The scope of this review is primarily on zero-gap bipolar membrane/interface water electrolyzers, as this configuration is arguably the most relevant bipolar membrane/interface electrolyzer architecture for scaled-up devices and commercial hydrogen production.