Dissolved Gases–Exciton Interactions Govern Photocatalytic Hydrogen Evolution via Excited‐State Pathway Modulation

Abstract Photocatalytic hydrogen generation offers a promising route for sustainable energy by emulating natural photosynthesis. However, translating promising lab‐scale efficiencies into practical, scalable systems remains a formidable challenge. The efficiency of these scaled systems typically fall below 1%, significantly below their small‐scale benchmarks. Despite extensive exploration of technical and mechanistic factors, one parameter remains overlooked: the dissolved inert gas, primarily argon. Argon is routinely used in lab setups but omitted in practical‐scale system, creating exceptionally high gas‐to‐catalyst molar ratios given low catalyst weight loadings. Here, it is demonstrated that high gas‐to‐catalyst molar ratios actively modulates exciton dynamics by activating triplet‐mediated pathways such as reverse intersystem crossing (RISC) and thermally activated delay fluorescence. For example, in the molecular photocatalyst acridine, a high molar ratio stabilizes triplet excitons and enables RISC to the protonated state within 100 ps. As a result, the hydrogen evolution rate is enhanced by two orders of magnitude compared to conditions with a smaller ratio. Analogous gas‐dependent modulation of exciton dynamics is also observed in carbon nitride, highlighting the general relevance of gas–exciton interactions. The findings identify the gas‐to‐catalyst molar ratio as an overlooked yet crucial barrier to achieving high‐efficiency, scalable photocatalytic systems.