Magnetized manganese-doped watermelon rind biochar as a novel low-cost catalyst for improving oxygen reduction reaction in microbial fuel cells

Microbial fuel cells (MFCs) are promising equipment for water treatment and power generation. The catalyst used in the oxygen reduction reaction (ORR) at the cathode is a critical factor for efficacy of MFCs. Therefore, it is important to develop cost-effective cathode catalysts to enhance application of MFCs. In the current study, a novel cathode catalyst was developed, which was annealed with watermelon rind as raw material and transition metals including iron, and manganese were introduced. The 700Mn/Fe@WRC catalyst, which was annealed at 700 °C, exhibited excellent electrochemical performance. The high relative content of pyridine nitrogen caused by the inherent nitrogen element of the watermelon rind and the high content of iron and manganese elements introduced resulted in increase in electrochemical surface area to 657.6 m²/g. The number of electrons transferred ORR was 3.96, indicating that ORR occurs through a four-electron pathway. The maximum power density of MFCs was 399.3 ± 7.4 mW/m² with a fitting total internal resistance of 15.242 Ω, and the removal efficiency of COD was 97.1 ± 1.2%. The cost of the 700Mn/Fe@WRC catalyst was approximately 0.15 $/g, which is significantly lower compared with Pt/C (33.0 $/g). Experimental verification showed that the 700Mn/Fe@WRC prepared using the economical watermelon rind biochar (WRC) is an excellent substitute for non-precious metal catalysts used in MFCs.