Aligned hierarchical electrodes for high-performance aqueous redox flow battery

Enhancing the transport properties and enlarging the surface area of electrodes are critical for achieving a high-power-density aqueous redox flow battery. In this work, we design and fabricate a hierarchical and ordered carbon fibrous (CNF-AECF) electrode by in-situ growing a layer of carbon nanofibers on the surface of aligned electrospun carbon fibers. The aligned macroscopic structure provides electrolytes transport pathways, while the highly porous carbon nanofiber layer offers a large specific surface area of up to 108 m2 g−1, enabling abundant active sites for redox reactions. Cyclic voltammetry tests show that the positive side peak potential separation is reduced from 92.77 mV to as low as 55.01 mV, while the negative side peak potential separation is reduced from 92.77 mV to 88.65 mV at the scan rate of 10 mV s−1. The application of the as-prepared material to a vanadium redox flow battery as the positive electrode demonstrates an energy efficiency of 80.1% at the current density of 300 mA cm−2, and 75.0% at the current density of 400 mA cm−2, which is 5.0% and 6.6% higher than that with the pristine electrospun carbon fiber electrodes. All these results show that the custom-made carbon electrode with rationally designed geometric structures and surface properties offer the promise to achieve high power density for aqueous redox flow batteries.