N,P co-doped carbon microsphere as superior electrocatalyst for VO2+/VO2+ redox reaction

In this paper, N,P co-doped carbon microsphere (CMS-NP) was investigated as electrocatalyst for VO2+/VO2+ redox reaction for vanadium redox flow batteries. CMS-NP was prepared by hydrothermal treatment followed by calcination with (NH4)2HPO4 as N and P sources. X-ray photoelectron spectroscopy confirms the incorporation of N and P atoms into carbon frameworks of carbon microsphere (CMS), and co-doping has no obvious influence on its morphology. The VO2+/VO2+ redox reaction on CMS-NP exhibits the best electrochemical kinetics compared with pristine CMS and N mono-doped CMS. The cell using CMS samples as electrocatalyst was assembled and its electrochemical performance was evaluated. The comprehensive energy storage performances of the cell are improved by using CMS samples, leading to higher discharge capacity and energy efficiency. The average energy efficiency of the cell increases by 5.1% at 50 mA cm−2 by using CMS-NP. The superior electrocatalytic properties of CMS-NP is mainly ascribed to the increased total number of heteroatoms as active sites and synergistic effect of N,P co-doping resulting from the unique structural features. Our study reveals that N,P co-doping is beneficial for the electrochemical kinetics of VO2+/VO2+ redox reaction on carbon frameworks, and CMS-NP can be employed as superior electrocatalyst in positive electrode.