S-Scheme Charge Separation Enables Photocarrier-Regulated Aqueous NH₄ ⁺ Storage in g-C₃N₄@WO₃ Heterojunctions

Aqueous ammonium-ion (NH₄ ⁺) batteries are promising for sustainable energy storage, yet their performance is severely constrained by sluggish reaction kinetics. Photo-assisted strategies offer a compelling route to overcome these limitations; however, single-semiconductor electrodes suffer from rapid photogenerated carrier recombination, resulting in poor light-utilization efficiency. Here, we develop a g-C₃N₄@WO₃ S-scheme heterojunction (CNW) that unlocks efficient light-to-charge conversion for photocarrier-regulated reversible NH₄ ⁺ storage. Characterization and simulation reveal that Fermi level alignment and band bending at the CNW interface establish a built-in electric field, driving S-scheme charge separation with selective recombination of low-energy carriers and retention of high-energy electrons on g-C₃N₄ and holes on WO₃. This preserves strong redox capability, thereby enabling efficient utilization of photoexcited carriers in the electrochemical storage process. Together with pronounced interfacial orbital coupling, the resulting photocarrier redistribution accelerates NH₄ ⁺ adsorption and electron/ion transport. Consequently, the CNW electrode delivers a 187% capacity enhancement under illumination, achieving 452 mAh g^-1 at 2 A g^-1 with excellent cycling stability, surpassing all currently reported W-based and C-based materials for NH₄ ⁺ storage. Correlative electrochemical, spectroscopic, and theoretical analyses decouple photoelectric and thermal effects, revealing a photoelectric-dominated, thermally assisted synergistic mechanism that simultaneously enhances NH₄ ⁺ storage kinetics and capacity.