ZIF-modified tailored multiscale-pore N-doped graphite felt for high performance vanadium redox flow batteries

Achieving both high electrochemical activity and local mass transport ability in porous electrode is critical to improve the performance of vanadium redox flow battery. In this work, a nitrogen-doped hierarchical multi-porous graphite felt electrode with optimal tunable pore size distribution is prepared by attachment of zeolitic imidazolate framework-8 and subsequent annealing on graphite felt. The constructed macro/meso-scale pores in this tailored multi-porous structure facilitates the diffusion of active species from bulk solution to the electrode-electrolyte interface, increasing the accessibility of active sites. Meanwhile, numerous micropores containing functional groups and defects on nanoscale surface can boost the number of effective active sites and catalytic activity, thus enhancing the reaction kinetics. The tests exhibit that the rationally designed electrode realizes outstanding battery performance at high current densities. It achieves an energy efficiency of 83.7 % at 300 mA cm⁻² and 79.4 % at 400 mA cm⁻², and a peak power density of 1215.8 mW cm⁻². Nevertheless, it even enables the battery operation at 500 mA cm⁻² with a considerable energy efficiency of 74.8 %. Remarkably, its long-term stability was also proved by stably cycling over 1000 cycles at 300 mA cm⁻² with a low energy efficiency decay rate of 0.00667 % per cycle. This work provides a feasible method for electrode design to meet future vanadium redox flow battery requirements for high power and efficiency.