Cornstalk-Derived Nitrogen-Doped Partly Graphitized Carbon as Efficient Metal-Free Catalyst for Oxygen Reduction Reaction in Microbial Fuel Cells

Abstract

The low electrocatalytic activity for oxygen reduction reaction (ORR) and the high cost of cathode catalyst in microbial fuel cells (MFCs) are the important factors that limit the practical applications. The metal-free nitrogen (N)-doped partly graphitized carbon (NPGC) as cathode catalyst is prepared at different temperatures (700-1050 °C) by using waste cornstalks as the carbon source and melamine as the N source. Scanning electron microscopy, X-ray diffraction, specific surface areas, and transmission electron microscopy have been used, in parallel with electrochemical activity tests including rotating disk electrode (RDE) and power output, to clarify how the active constituents and structure of NPGC influence the MFCs performance. Carbonization temperature has a significant effect on the porous structure and N-doped defects (pyridinic, pyrrolic, and graphitic N), which correspondingly influence the amount of active sites, ORR activity and long-time running durability in MFCs. The abundant functional oxygen-containing groups in the porous structure (1177.76 m2 g-1) of NPGC (1000 °C) contribute to the fast adsorption of molecular O2 onto the carbon skeleton. The N-induced charge delocalization facilitates the chemisorption of O2 and cleavage of O-O bonds to effectively enhance the four-electron O2 reduction on NPGC electrode. The maximum power density of NPGC-1000 is 1122 mW m-2 in MFCs, which is higher than that of Pt/C (988 mW m-2), and only has a decline of 10.2% after 80 days. This work provides a metal-free, high-efficiency, and cost-effective ORR electrocatalyst for MFCs.