Capturing critical gem-diol intermediates and hydride transfer for anodic hydrogen production from 5-hydroxymethylfurfural.

The non-classical anodic H 2 production from 5-hydroxymethylfurfural (HMF) is very appealing for energy-saving H 2 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 (DHMFM 2- ). The DHMFM 2- is easily oxidized to produce H 2 via H - transfer, whereas the DHMFM - is readily oxidized to produce H 2 O via H + transfer. The increases in potential considerably facilitate the DHMFM - oxidation rate, shifting the DHMFM -  ↔ DHMFM 2- equilibrium towards DHMFM - and therefore diminishing anodic H 2 production until it terminates. This work captures the critical intermediate DHMFM 2- leading to hydrogen production from aldehyde, unraveling a key point for designing higher performing systems.