Noble-Metal-Free Metal Oxides for Catalyzing Acidic Oxygen and Hydrogen Evolution Reactions: Recent Developments and Future Perspectives

Proton exchange membrane (PEM) water electrolysis has emerged as a highly promising technology for sustainable hydrogen (H2) production, but its widespread application relies heavily on the development of high-performance and cost-effective hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts. Currently, noble-metal-based materials (such as Pt and Ir/Ru oxides) serve as benchmark electrocatalysts for HER and OER in acidic environments, but their high cost and limited availability pose major challenges toward industrialization. Low-cost noble-metal-free metal oxides are an important class of functional materials with rich compositions and structures, offering flexible electronic and crystal structures with tunable physical and chemical properties. In this Review, we present a comprehensive and timely summary of the remarkable progress achieved in the field of noble-metal-free metal oxides for catalyzing acidic HER and OER. First, we provide a brief description of the fundamental concepts of HER and OER in acidic media, including mechanisms, computational activity descriptors, and experimental parameters that are utilized for the evaluation of catalytic performance. Second, an overview is provided covering various types of noble-metal-free metal oxides for HER and OER in acidic media reported so far (e.g., simple transition metal oxides, spinel oxides, rutile antimonate oxides, perovskite oxides, specially structured metal oxides, and mixed-metal-oxide composites), with a focus on the designed strategies for enhancing performance and establishing correlations between properties and activity. Lastly, the challenges and future research directions regarding acidic HER and OER are summarized and discussed.