Ionic separation and recombination processes in water electrolysis and fuel cell devices are of equal importance as the electron transfer processes that occur at the electrode surfaces. We illustrate the basic thermodynamic concepts governing the flow of ions in electrolyzer and fuel cell systems and the effects of pH gradients on the electrochemical phenomena. Particularly, we focus on the use of bipolar membranes, which are composed of anion- and cation-selective membranes enabling operation with different pH environments at the anode versus the cathode. The use of bipolar membranes thus broadens the materials’ availability and could enable low-cost electrolysis systems that operate at very high efficiency. We end by discussing different materials and highlighting key gaps needed to realize such a system.

中文翻译：

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水电解中的离子过程：离子选择性膜的作用

水电解和燃料电池装置中的离子分离和复合过程与发生在电极表面的电子转移过程同等重要。我们阐述了控制电解器和燃料电池系统中离子流以及pH梯度对电化学现象的影响的基本热力学概念。特别是，我们专注于双极膜的使用，该双极膜由阴离子和阳离子选择性膜组成，能够在阳极和阴极的不同pH环境下运行。因此，双极膜的使用拓宽了材料的适用范围，并可以使低成本电解系统以很高的效率运行。最后，我们讨论不同的材料并强调实现该系统所需的关键差距。