Effects of nitrogen doping on the electrochemical performance of graphite felts for vanadium redox flow batteries

Vanadium redox flow batteries (VRFB), originally proposed by Skyllas-Kazacos et al., have been considered as one of the most promising energy storage systems for intermittently renewable energy. However, the poor electrochemical activity and hydrophobicity of graphite felt electrode greatly limit energy storage efficiency of VRFB system. In this paper, two nitrogen-doped (N-doped) graphite felts, obtained by heat-treating in an NH3 atmosphere at 600 degrees C and 900 degrees C, have been investigated as electrodes with high electrochemical performance for vanadium redox flow batteries. In particular, the one obtained at 900 degrees C exhibits an excellent electrochemical activity for both V2+/V3+ and VO2+/VO2+ redox couples. The cells with different graphite felt electrodes were assembled, and the charge-discharge performance was evaluated. The cell with the N-doped graphite felts has larger discharge capacity, discharge capacity retention, and energy efficiency, especially with the sample treated at 900 degrees C. The average energy efficiency of the cell with the 900 degrees C treated N-doped graphite felts is 86.47%, 5.44% higher than that of the cell with the pristine graphite felt electrodes. These enhanced electrochemical properties of the N-doped graphite felt electrodes are attributed to the increased electrical conductivity, more active sites, and better wettability provided by the introduction of the nitrogenous groups on the surface of graphite felts. It indicates that N-doped graphite felts have promising application prospect in VRFB. Copyright (c) 2015 John Wiley & Sons, Ltd.