Precious-Metal-Free Solvothermally Synthesized CeO₂ Nanosphere-Graphitic Carbon Nitride Sheet Composites for Oxygen Reduction Reaction

Microbial fuel cell (MFC) is an emerging renewable energy conversion device that generates electricity from wastewater using microbes as biocatalysts. The oxygen reduction reaction (ORR) occurring at the cathode of MFC requires a low-cost, active, and stable electrocatalyst to overcome the hurdle of upscaling of the MFCs. Herein, we demonstrate the synthesis of a series of earth-abundant and low-cost CeO 2 -gC 3 N 4 composite catalysts with varying molar ratios via a simple solvothermal method without using any harmful chemical reagents. The CeO 2 -gC 3 N 4 composite with an optimal 15% weight of CeO 2 exhibits remarkable ORR activity with low onset and ORR peak potential. The excellent ORR activity of the 15% CeO 2 -gC 3 N 4 composite is ascribed to its high Brunauer–Emmett–Teller (BET) surface area, high content of Ce 3+, oxygen vacancy defects, pyridinic N, low charge transfer resistance, and high electrochemically accessible surface area. The synthesized catalysts are further demonstrated as cathode catalysts in MFC, which show similar ORR activity performance. In particular, the MFC-15 containing the 15% CeO 2 -gC 3 N 4 nanosphere catalyst exhibited the best power performance with operating voltage and open circuit voltage of 321 ± 12 and 743 ± 7 mV, respectively, with a 12.53 W m –3 power density.