Role of borate functionalized carbon nanotube catalyst for the performance improvement of vanadium redox flow battery

A borate-group functionalized carbon nanotube (Borate-CNT) catalyst is developed to enhance the performance and long term durability of the vanadium redox flow battery (VRFB). To prepare for the –Borate-CNT catalyst, carboxylic acid groups (COOHs) adopted onto acidified CNT (CA-CNT) are simply co-treated with sodium hydroxide (NaOH) and boric acid (H₃BO₃). As a result, the COOHs are transformed into borate groups. The transformation is verified by XPS analysis, showing the increase in oxygen content and the creation of boron-oxygen bonds. The active sites and catalytic activity of Borate-CNT are increased more than those of the catalysts formed by the single treatment of NaOH and CA-NCT or H₃BO₃ and CA-CNT. This is due to the increase of active sites by the formation of oxygen abundant borate groups and the different electronegativity between the boron and oxygen elements promotes the attraction and subsequent reaction o f vanadium ions. The voltage and energy efficiencies (VE and EE) of the VRFB using Borate-CNT catalyst are better than those of VRFBs using no catalyst or CA-CNT catalyst – even at 200 mA cm⁻² – and the efficiencies of Borate-CNT VRFB are well maintained until 300 cycles, whereas the efficiencies of the no catalyst VRFB are considerably decreased (17% and 16% decreases of the initial values in VE and EE). In addition, Borate-CNT shows the effect of protection by suppressing the chemical aging of carbon felt from toxic acidic electrolyte.