Nitrogen-doped biochar derived from watermelon rind as oxygen reduction catalyst in air cathode microbial fuel cells

•Watermelon rind acts as both rich-nitrogen source and carbon framework in synthesis.•The high electrochemical performance is ascribed to active CN bonds and high contents of pyridinic-N and graphitic-N.•WRC-700 possesses an outstanding electrochemical active area of 658.90 m2 g−1.•The microbial fuel cell using the WRC-700 cathode achieves the low charge transfer resistance of 20.63 Ω.•WRC-700 is a highly cost-effective and potential replacement to commercial Pt/C for microbial fuel cells.