Carbon Nanofibers Coated with MOF-Derived Carbon Nanostructures for Vanadium Redox Flow Batteries with Enhanced Electrochemical Activity and Power Density

A unique synthetic approach has been introduced where nanostructurally grown zinc layered double hydroxide on graphitic carbon felt (CF) is converted into a zeolitic imidazolate framework (ZIF-8), and then, subsequent carbonization resulted in a N/O-functionalized porous carbon electrode (N,O/CF). Because of the presence of N/O-containing functional groups and deposition of ZIF-8-derived nanoporous carbon on the CF, the N,O/CF is found to be highly hydrophilic in nature with a large surface area. Cyclic voltammetry measurements with N,O/CF suggest the fast electrochemical kinetics of V(IV) ↔ V(V) reactions. Polarization curves and electrochemical impedance spectroscopy measurements of the vanadium redox flow battery (VRFB) assembly illustrate the significant decrease in charge transfer resistance at electrode surfaces. At 50 mL/min electrolyte flow rate, N,O/CF delivers energy efficiencies of 83.11 and 76.66% at current densities of 40 and 80 mA/cm2, respectively. The values are 82.59 and 76.39%, respectively, at 100 mL/min, showing the negligible effect of the flow rate. The power density of VRFBs at various electrolyte flow rates is also presented, which increases with increasing flow rates and is higher for N,O/CF (∼821 mW/cm2) than for bare CF (606 mW/cm2). The stability test confirms the retention of energy, voltage, and coulombic efficiencies after recycling of the electrode. The above-mentioned findings of improved performance of VRFBs with employing the N,O/CF electrode are a cumulative effect of enhanced nanoporosity, an increased number of catalytic active sites, high wettability, and low charge transfer resistance.