Spent coffee ground-derived hydrochar: An ecologically compatible material for solar-driven H₂O₂ production and...

Solar-driven H₂O₂ photosynthesis via dioxygen activation presents a sustainable pathway to produce energy carriers and purify wastewater, which hinges on the scale-acquired ecologically compatible materials. Herein, a spent coffee ground-based hydrochar (SHTC) photocatalyst was hydrothermally synthesized in dilute H₂SO₄ solution. Mechanism analysis revealed that superoxide radical (•O₂^-) generated via an indirect two-step one-electron involved O₂ photoreduction is responsible for H₂O₂ production, which were systematically evidenced by reactive species trapping experiments, electron paramagnetic resonance, in-situ Fourier transform infrared spectroscopy, rotating disk electrode measurements and theoretical calculation. The sufficient protons endowed by the abundant carboxyl and hydroxy groups of SHTC promoted the successive •O₂^- hydrogenation towards H₂O₂, thus breaking the rate-determining step and realizing superior H₂O₂ photosynthesis. Under solar light, this subtly synthesized SHTC photocatalyst displayed an impressive H₂O₂ production yield (610 µmol L^-1) in pure water. Impressively, the accumulated H₂O₂ yield within 32 h under natural sunlight irradiation reached at 760 µmol L^-1. The produced H₂O₂ could be directly used as the oxidant of traditional Fenton process to degrade typical organic contaminants, including sulfadimidine, p-chlorophenol, and rhodamine B. This work presents the feasibility of environmentally compatible photocatalyst from waste biomass for sustainable H₂O₂ photosynthesis and on site wastewater purification.