Polymeric membrane materials for green hydrogen production by water electrolysis: Progress and suggestions for future development

Abstract

<p indent="0mm">Green hydrogen has gained much interest due to its low cost, sustainability, and environmental friendliness, especially when combined with water electrolysis technology powered by renewable energy resources. It has been recognized as one of the perfect solutions to achieve the goal of near-zero carbon emissions. According to the type of materials used to separate the anode and cathode and the ionic species it conduct, the electrolyzers can be divided into several categories, i.e., alkaline water electrolyzer (AWE) that involve the use of liquid electrolyte, proton exchange membrane water electrolyzer (PEMWE), solid oxide electrolyzer (SOE), and anion exchange membrane water electrolyzer (AEMWE). As the key component in different water electrolysis technologies, polymeric membrane materials, including proton exchange membrane (PEM), anion exchange membrane (AEM), and ion-solvating membranes (ISM), are of great importance, which serve as the ionic conductor and gas separator. Thus, the efficiency and durability of water electrolyzers are mainly determined by the properties of membranes, such as ionic conductivity, chemical stability, and mechanical properties. However, these unfavorable performance parameters of membranes still limited the worldwide commercialization of water electrolysis for the production of green hydrogen. In a typical AWE, a porous diaphragm made of asbestos or composite ceramic (or asbestos)/polymer materials (Zirfon, a state-of-the-art diaphragm) is used to separate the gas product and transport hydroxide ions. Although the mature AWE technology shows higher durability, low capital cost, and high compatibility with non-noble metal catalysts, they operate at low current densities lying between <sc>0.3−0.4 A cm⁻²,</sc> owning to the high ionic resistance and high gas permeation of the non-ionic separator membranes. The replacement of porous diaphragm with ionic polymeric membranes, such as PEM, AEM, and ISM based on polybenzimidazoles have attracted increasing attention in water electrolysers, due to their effectiveness of ion transport and gas tightness of the dense membrane. The acidic PEM allow the operation of water splitting with higher efficiency and current densities <sc>(500−2000 A cm⁻²).</sc> However, large-scale implementation of PEMWE technology is limited by the expensive PEM and precious platinum group metal (PGM) catalysts. When working under basic environment, the AWE using solid AEM and ISM combines the merits of traditional AWE and PEMWE, i.e., an alkaline working environment allows for the use of PGM-free catalysis and the solid hydroxide ion conducting membrane reduce the ionic resistance of the cells. Thus, the design of AEM and ISM materials plays a crucial role in the overall performance and durability of electrolytic cells. Currently, compared with PEMWE, the AEMs and ISMs with sufficient conductivity and satisfactory stability are still highly needed, due to the well-recognized vulnerable functional cations and polymer backbones in hot and alkali aqueous solutions. Thus, numerous chemical designs on AEMs are carried out. In this review, we summarized the research progress of polymeric membranes in water electrolysis for hydrogen production. We first compared the properties of membranes and electrolyzer device performance using different types of membranes, and analyzed the relationship between polymer structure and device performance; then, after analyzing the development and the technical advantages and disadvantages of PEM, AEM, and ISMs, the technical limitations and future developing trends of these technical routes were discussed. Finally, we also give a brief prospect on how to guide and encourage the future development of various technical pathways through the policy guidance, so as to realize the large-scale market penetration of water electrolysis technology for green hydrogen production.