Proton exchange membrane water electrolyzer (PEMWE) represents a promising technology for the sustainable production of hydrogen, which is capable of efficiently coupling to intermittent electricity from renewable energy sources (e.g., solar and wind). The technology with compact stack structure has many notable advantages, including large current density, high hydrogen purity, and great conversion efficiency. However, the use of expensive electrocatalysts and construction materials leads to high hydrogen production costs and limited application. In this review, recent advances made in key materials of PEMWE are summarized. First, we present a brief overview about the basic principles, thermodynamics, and reaction kinetics of PEMWE. We then describe the cell components of PEMWE and their respective functions, as well as discuss the research status of key materials such as membrane, electrocatalysts, membrane electrode assemblies, gas diffusion layer, and bipolar plate. We also attempt to clarify the degradation mechanisms of PEMWE under a real operating environment, including catalyst degradation, membrane degradation, bipolar plate degradation, and gas diffusion layer degradation. We finally propose several future directions for developing PEMWE through devoting more efforts to the key materials.

Crystal phase is an intrinsic structural parameter to determine the physicochemical properties and functionalities of materials. The unconventional phases of materials with distinct atomic arrangements from their thermodynamically stable phases have attracted enormous attention. Phase engineering has recently made fruitful achievements in electrocatalysis field to optimize the performance of various electrochemical reactions. In this review, theoretical and experimental advances made in phase engineering of electrocatalysts are summarized. First, we introduce basic understanding on crystal phases of catalysts to show the dialectical relationship between bulk phase and surface catalytic layer, and highlight the multiple functions of phase engineering in catalysis studies. We then describe phase-controlled synthesis of materials through various experimental methods such as wet-chemical method, phase transition, and template growth. As a focus, we discuss the wide usage of phase engineering strategy in different kinds of electrocatalytic materials, and particular emphasis is given to establishment of reasonable crystal phase-activity relationship. Finally, we propose several future directions for developing more desirable electrocatalysts by rational crystal phase design.