Cobalt phosphide embedded N-doped carbon nanopolyhedral as an efficient cathode electrocatalyst in microbial fuel cells

Abstract

Design and synthesis of effective and low-cost electrocatalysts for oxygen reduction reaction (ORR) is of great significance for the performance of microbial fuel cells (MFCs). Herein, we fabricated a cobalt phosphide embedded N-doped carbon nanopolyhedral (CoP@N-C) by simultaneous phosphidation and carbonization of ZIF-67 under N2 atmosphere. The physical and chemical properties of the electrocatalysts were analyzed in detail by scanning electron microscope, transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, Fourier transform infrared spectroscopy and N2 adsorption–desorption isotherms. Results showed that the CoP@N-C had a dodecahedral shape with cobalt, nitrogen and phosphorus incorporating in porous graphical carbon. CoP compound which had a strong electron capture capability and ORR activity was embedded within the electrocatalysts. Introducing of P increased the contents of pyridinic-N, Co2+/Co3+ couples and oxygen vacancies, which provided sufficient catalytic active sites for ORR. The increased metallic Co content and mesoporous/macroporous surface area ensured efficient electron and mass transfer during ORR. Rotating disk electrode measurement revealed that the CoP@N-C exhibited a high ORR catalytic activity comparable to Pt/C, and an efficient four-electron pathway during ORR. When fabricated into activated carbon air cathode, the optimized CoP@N-C produced a maximum power density of 2236.8 mW m−2 in MFCs, which was about 2 times that of the bare activated carbon control. In addition to the superb ORR performance, the cost of the CoP@N-C was only ∼1/3 of commercial Pt/C. This study suggests that CoP@N-C could be an attractive non-precious metal ORR electrocatalyst for the application in high-performance MFCs.