Chemical modification of graphite electrode materials for vanadium redox flow battery application—part II. Acid treatments

Abstract

The surface modification of graphite felt with concentrated sulphuric acid has led to dramatic improvement in the electroactivity of this material in the vanadium redox cell. A cell resistance of 2.50 Ω cm 2 and energy efficiency of 91% were obtained in the vanadium cell, after the graphite felt electrode material was treated with boiling concentrated sulphuric acid for 5h. The electrochemical activity of the graphite felt was found to increase with increasing sulphuric acid concentration. Treatment of graphite felt with nitric acid and mixtures of sulphuric and nitric acids, however, led to higher cell resistance values compared with the sulphuric acid treatment. Surface analysis of treated and untreated felts using the XPS method has shown that the functional groups CO and CO increased dramatically compared with untreated samples. A sequence reaction mechanism was proposed for the VO 2+ ⇌ VO + 2 and V 2+ ⇌ V 3+ charge-discharge reactions occurring in the positive and negative 1 2 -cells, respectively of the vanadium redox battery.