Biomass-Derived Porous Fe<sub>3</sub>C/Tungsten Carbide/Graphitic Carbon Nanocomposite for Efficient Electrocatalysis of Oxygen Reduction

The oxygen-reduction reaction (ORR) draws an extensive attention in many applications, and there is a growing interest to develop effective ORR electrocatalysts. Iron carbide (Fe<sub>3</sub>C) is a promising alternative to noble metals (e.g., platinum), but its performances need further improvement, and the real role of the Fe<sub>3</sub>C phase remains unclear. In this study, we synthesize Fe<sub>3</sub>C/tungsten carbide/graphitic carbon (Fe<sub>3</sub>C/WC/GC) nanocomposites, with waste biomass (i.e., pomelo peel) serving as carbon source, using a facile, one-step carbon thermal-reduction method. The nanocomposite is characterized by a porous structure consisting of uniform Fe<sub>3</sub>C nanoparticles encased by graphitic carbon (GC) layers with highly dispersed nanosized WC. The Fe<sub>3</sub>C provides the active sites for ORR, while the graphitic layers and WC nanoparticles can stibilize the Fe<sub>3</sub>C surface, preventing it from dissociation in the electrolyte. The Fe<sub>3</sub>C/WC/GC nanocomposite is highly active, selective, and stable toward four-electron ORR in pH-neutral electrolyte, which results in a 67.82% higher power density than that of commercial Pt/C and negligible voltage decay during a long-term phase of a 33 cycle (2200 h) operation of a microbial fuel cell (MFC). The density functional theory (DFT) calculations suggest high activity for splitting the O–O bond of molecular oxygen on the surface of Fe<sub>3</sub>C.