AbstractThe non-classical anodic H₂ production from 5-hydroxymethylfurfural (HMF) is very appealing for energy-saving H₂ production with value-added chemical conversion due to the low working potential (~0.1 V vs RHE). However, the reaction mechanism is still not clear due to the lack of direct evidence for the critical intermediates. Herein, the detailed mechanisms are explored in-depth using in situ Raman and Infrared spectroscopy, isotope tracking, and density functional theory calculations. The HMF is observed to form two unique inter-convertible gem-diol intermediates in an alkaline medium: 5-(Dihydroxymethyl)furan-2-methanol anion (DHMFM⁻) and dianion (DHMFM²⁻). The DHMFM²⁻ is easily oxidized to produce H₂ via H⁻ transfer, whereas the DHMFM⁻ is readily oxidized to produce H₂O via H⁺ transfer. The increases in potential considerably facilitate the DHMFM⁻ oxidation rate, shifting the DHMFM⁻ ↔ DHMFM²⁻ equilibrium towards DHMFM⁻ and therefore diminishing anodic H₂ production until it terminates. This work captures the critical intermediate DHMFM²⁻ leading to hydrogen production from aldehyde, unraveling a key point for designing higher performing systems.