Hydrogen evolution reaction (HER) in electrocatalytic water splitting for producing green hydrogen has recently emerged as a promising approach to address the current energy and environmental challenges. In this study, we prepared and characterized the mononuclear Pt-polyoxometalate clusters [Pt₁Mo₆(OH)₆O₁₈](NH₄)₂ (PtMo₆) and [Pt₁Mo₆(OH)₃O₁₈)O₃C₅H₉]TBA₂ (PtMo₆-L, TBA: tetrabutylammonium) as HER electrocatalysts with enhanced activity. The HER performance of PtMo₆-L coated onto glassy carbon is superior to that of naked PtMo₆ and comparable to that of commercial 20 wt.% Pt/C, with an overpotential of 37 mV@10 mA·cm⁻² and a Tafel slope of 42 mV·dec⁻¹. The HER follows the Heyrovsky–Volmer mechanism over PtMo₆-L, similar to Pt/C, but the Tafel–Volmer mechanism over PtMo₆. Experimental and density functional theory calculation results indicate that the Pt–O–H group in the polyoxometalate clusters is the active site for the HER. Moreover, mechanistic studies reveal that the negatively charged C₅H₉-bonded O1 in PtMo₆-L is prone to proton adsorption and activation, thereby enhancing electrocatalytic HER activity. Overall, this study provides new insights into the polyoxometalate structure–electrocatalytic property relationship at the precise atomic and molecular levels, advancing HER research.

Covalent organic frameworks (COFs) are increasingly recognized as promising crystalline platforms for solar-driven H₂ evolution because of their unique architectures, extended conjugation characteristics, adjustable pore environments, and structural regularity. Nevertheless, unmodified frameworks generally exhibit inadequate photocatalytic functionality and rapid recombination of photoinduced charge carriers, limiting their performance during H₂ evolution processes. To address these limitations, extensive investigations have explored the incorporation of metallic entities into COF networks to improve light absorption, carrier mobility, interfacial redox behavior, and surface catalytic dynamics. Despite rapid developments in this area, an integrated understanding linking metallic incorporation approaches with H₂ evolution activity remains insufficient. This review therefore provides a comprehensive overview connecting the structural characteristics and catalytic functions of metal-containing COF systems for photocatalytic H₂ evolution. The fundamental chemistry and framework features of COFs are first introduced, followed by representative methodologies for incorporating metallic species and their corresponding functional effects in regulating light absorption, charge separation, proton reduction, and H₂ evolution kinetics. Finally, future research directions toward constructing highly efficient and durable COF-based photocatalysts for sustainable H₂ evolution are critically highlighted.

Iridium-based materials are less studied in proton exchange membrane water electrolyzers (PEMWEs), which are crucial for the generation of green hydrogen, due to their rarity and high cost. Herein, doping Cr with cobalt oxide resulted in rich oxygen vacancies with the Co²⁺/Cr³⁺ redox couples and surface hydroxyl groups, which considerably enhance the capacity of the material to adsorb and activate oxygen intermediates. Due to its structural and electrical characteristics, CrCo₈O_x has proven to be a stable and effective catalyst for acidic oxygen evolution reaction (OER) and high-performance PEMWEs applications. A high current density of 100 mA·cm⁻² was attained by the CrCo₈O_x anode catalyst and maintained for 130 h, whereas a current density of 300 mA·cm⁻² was maintained for 20 h. In situ Fourier transform infrared (FT-IR) spectroscopy combined with density functional theory (DFT) calculations corroborate the oxide path mechanism (OPM) over pristine Co₃O₄ rather than the associative electron mechanism (AEM) over Cr-doped CrCo₈O_x composites. Such outstanding results indicate a bright future for industrial deployment and provide broad recommendations for developing PEMWEs with high efficiency.

Atomically precise metal nanoclusters, consisting of tens to hundreds of metal atoms, represent a unique class of catalytic materials with well-defined electronic structures and tunable surface chemistry. These features enable nanoclusters to act as versatile components in photocatalytic systems, where they regulate light absorption, charge separation, and interfacial reaction dynamics. Photocatalysis provides a sustainable pathway for converting solar energy into fuels and value-added chemicals; however, its practical application is limited by intrinsic thermodynamic and kinetic barriers, as well as catalyst stability and selectivity challenges. Recent advances have spotlighted the integration of nanoclusters with extended porous frameworks, including metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), as a powerful strategy to overcome these limitations. These hybrid architectures allow precise control over active-site geometry, electronic environments, and substrate accessibility, while promoting synergistic effects such as enhanced charge transport and stabilization of reactive intermediates. This review highlights emerging synthetic methodologies, modification strategies, and recent photocatalytic applications (CO₂ reduction, H₂ evolution, H₂O₂ production, organic synthesis, pollutant removal, CH₄ conversion, etc) reported over the past three years. We discuss mechanistic insights, structure–function relationships, and critical challenges, including conductivity, robustness, and scalability. Finally, we propose integrated design principles for constructing hybrid nanocluster–framework photocatalysts with optimized efficiency, selectivity, and durability, offering a roadmap for the rational development of next-generation energy-conversion and chemical-synthesis platforms.