Facile synthesis of hollow carbon spheres by gas-steamed bifunctional NH4F for efficient cathodes in microbial fuel cells

A facile gas-steamed strategy is reported for preparing heteroatom dual-doped hierarchical porous hollow carbon catalysts via carbonization of a mixture of carbon sphere precursor and ammonium fluoride (NH4F). Notably, NH4F can be decomposed into NH3 and HF by pyrolysis, in which HF gas can etch SiO2 pellets to form hollow structure while the N and F atoms can be introduced at the same time. The FCS-900 exhibits admirable electrocatalytic properties with the highest onset potential and limiting current density in neutral electrolytes (0.944 V vs. RHE and 6.44 mA cm−2). In comparison to MFC-Pt, much higher output voltage and power density (0.617 V and 1093.6 ± 6.26 mW m−2) are obtained by MFC-900. Such results can be attributed to the largest specific surface area of FCS-900 to supply exposed active sites and fast transportation channels. Based on X-ray photoelectron spectroscopy, the FCS-900 catalyst possesses the active substances of pyridinic/graphitic N and C-F bonds. The synergism of N and F can effectively facilitate the adsorption of O2 during ORR, as further supported by density functional theory calculation. The facile and green synthesis strategy can be extended to design metal-free carbon-based electrocatalysts with superior electrocatalytic performance.