Highly active, bi-functional and metal-free B₄C-nanoparticle-modified graphite felt electrodes for vanadium redox flow batteries

The potential of B₄C as a metal-free catalyst for vanadium redox reactions is investigated by first-principles calculations. Results show that the central carbon atom of B₄C can act as a highly active reaction site for redox reactions, due primarily to the abundant unpaired electrons around it. The catalytic effect is then verified experimentally by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests, both of which demonstrate that B₄C nanoparticles can enhance the kinetics for both V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox reactions, indicating a bi-functional effect. The B₄C-nanoparticle-modified graphite felt electrodes are finally prepared and tested in vanadium redox flow batteries (VRFBs). It is shown that the batteries with the prepared electrodes exhibit energy efficiencies of 88.9% and 80.0% at the current densities of 80 and 160 mA cm⁻², which are 16.6% and 18.8% higher than those with the original graphite felt electrodes. With a further increase in current densities to 240 and 320 mA cm⁻², the batteries can still maintain energy efficiencies of 72.0% and 63.8%, respectively. All these results show that the B₄C-nanoparticle-modified graphite felt electrode outperforms existing metal-free catalyst modified electrodes, and thus can be promising electrodes for VRFBs.