Biomass-derived N, P co-doped biochar as a bifunctional floating cathode for energy-efficient HO electrosynthesis and electro-Fenton catalysis through self-sustained aeration.

HO electrosynthesis via oxygen reduction reaction (ORR) is a green and sustainable strategy for on-site electro-Fenton processes, yet it suffers from high aeration energy consumption and poor O utilization efficiency. Here, N, P self-doped porous carbon (NP/PC) was synthesized from agricultural waste without exogenous dopants or templates to fabricate self-floating gas diffusion electrode (GDE) for HO electrosynthesis. A higher pyrolysis temperature promoted porous structures formation in NP/PC, whereas the N/P functional groups content decreased as pyrolysis temperature increased from 450 °C to 600 °C. The optimized NP/PC-500 exhibited the highest HO selectivity of 81.1 % for its balanced active sites and N/P doping levels. Compared with air-aerated cathode, the fabricated self-floating NP/PC-500-GDE realized self-sustained aeration by utilizing atmospheric O. The rapid O diffusion at the triple-phase interfaces of self-floating NP/PC-500-GDE reduced aeration energy consumption and enabled a higher HO yield (586.7 mg/L), which was 1.6 times that of the air-aerated NP/PC-500-GDE. Besides, the self-floating NP/PC-500-GDE exhibited good scalability with its geometric area scaled up from 6 to 150 cm. Density functional theory calculations and regression analysis verified the pyrrolic N and P-C group as dominant sites for HO electrosynthesis. Notably, the asymmetrical local electric fields induced by N, P self-doping accelerated charge transfer and lowered O adsorption energy. This electro-Fenton system with self-floating NP/PC-500-GDE as cathode achieved efficient degradation of diverse contaminants. This work not only offers a green and low-cost strategy for upcycling biomass waste into functional materials but also proposes a bifunctional self-floating cathode for aeration-free HO electrosynthesis and electro-Fenton catalysis.