Solar Energy Storage in Polyoxometalate for On-Demand Hydrogen Transportation and Evolution

Widespread application of solar-driven hydrogen production is hindered by critical limitations: the inherent safety risks of high-pressure H₂ storage/transport, and the intermittent nature of solar energy. To address these challenges, we develop a solar energy storage and on-demand hydrogen production system by synergistically integrating commercial polyoxometalate (NH₄)₆H₂W₁₂O₄₀ (W₁₂) with graphitic carbon nitride (g-C₃N₄). This system demonstrates remarkable efficiency in storing solar energy as electrons within W₁₂ during illumination. These stored electrons can be released on demand in the dark through Pt/C activation to produce H₂, achieving a highly efficient hydrogen evolution rate of 3220 µmol g^-1 h^-1 under dark photocatalytic conditions. Notably, the system maintains robust performance under real-world conditions, delivering an outdoor hydrogen evolution rate of 954 µmol g^-1 h^-1 under natural sunlight irradiation. Detailed analysis reveals an optimized electron storage-release pathway, facilitated by the favorable alignment between the reduction potential of W₁₂ and the band structure of g-C₃N₄, as well as the electrostatic self-assembly driven by oppositely charged g-C₃N₄ and W₁₂. In this system, solar energy is facilely stored as stabilized electrons within the POM-semiconductor suspension, which can then be efficiently transported to light-deficient environments and controllably released for on-demand hydrogen evolution.