Hydrogen is considered as a clean energy carrier in the global energy transition and development due to its high energy density (142 kJ·g^-1), cleanliness, and sustainability. At present, hydrogen is mainly produced through steam reforming of fossil fuels under harsh conditions, which are restrained by the complex operation and emission of carbon dioxide. Hydrogen production through water electrolysis is an effective and clean method for generating high-purity hydrogen using renewable energy sources without pollution. The electricity utilized in this process is obtained from renewable sources such as solar, hydropower, wind, and nuclear energy, enabling carbon-free and green hydrogen production. With the advancements in renewable energy generation technologies and electrolysis cell techniques over the past a few decades, the production of green hydrogen through water electrolysis is flourishing. Based on the type of electrolyte and operating conditions, water electrolysis technologies are primarily classified into four categories, i.e., alkaline water electrolysis (AWE), proton exchange membrane (PEM) water electrolysis, anion exchange membrane (AEM) water electrolysis, and solid oxide electrolysis cell (SOEC) water electrolysis. PEM water electrolysis technology, distinguished by its prompt start-up and power modulation capabilities, is currently a sole technology capable of producing high-purity hydrogen coupled with renewable energy generation.

This review firstly introduced several major hydrogen production technologies, emphasizing the advantages of PEM water electrolysis technology. Subsequently, we represented the latest developments in iridium (Ir) and ruthenium-based (Ru) oxygen evolution reaction (OER) electrocatalysts in acidic electrolyte, and summarized the strategies for developing acidic OER electrocatalysts with a high performance. Finally, this review highlighted the challenges associated with acidic OER electrocatalysts and outlined prospective avenues for future research.

The electrolysis efficiency of PEM water electrolysis primarily depends on the overpotentials at the anode and cathode. Compared to the hydrogen evolution reaction involving a 2-electron transfer step at the cathode, the OER at the anode involving a 4-electron transfer step is considered as a rate-limiting step. The anodic overpotential is significantly higher than the cathodic overpotential, typically requiring an overpotential of 200-500 mV to drive a current density of 10 mA·cm^-2. Unfortunately, the choice of anodic catalytic materials is greatly limited by the strong acid characteristics and the high anodic electrolysis voltage, which demands that the anodic catalysts can operate stably in a strong acid and oxidizing environment.

According to the OER activity and stability of different metals, the precious metal iridium dioxide (IrO₂) is commercially employed as a benchmark anodic catalyst of PEM electrolysis cells. However, Ir is one of the most expensive metals in the world. For Ir-based catalysts, researchers focus on reducing the Ir content in the catalyst, but the Ir content still generally exceeds 40% (in mass fraction). Therefore, developing efficient, stable, and low-cost non-Ir-based OER catalysts is also important in the development of PEM water electrolysis technology. Compared to Ir, metal Ru possesses a higher OER activity and is only one-tenth the price of Ir, that has a significant potential for application in PEM water electrolysis. In recent years, various types of Ru-based catalysts are developed in the acidic OER field, showing a promising potential as substitutes for Ir-based catalysts. Therefore, this review could provide a detailed overview of the research progress on Ir- and Ru-based catalysts in PEM water electrolysis for hydrogen production.