Graphite Felt Electrode Modified by Quaternary Ammonium for Vanadium Redox Flow Battery with an Ultra-Long Cycle Life

1. Introduction

With the depletion of fossil energy sources, the application of renewable energy sources for power generation is becoming increasingly widespread. However, owing to the influence of time and climate, the issues of unstable and discontinuous power generation require large-scale energy storage equipment for regulation. Redox flow batteries [1,2] possess high superiority in terms of large-scale energy storage. Among them, VRFBs have already been commercially applied because their electrolytes are all composed of vanadium ions, which have the advantage of extremely strong reversibility of redox reactions, high safety, and a flexible design for independent energy and power [3]. Nevertheless, several issues require continued research in order to improve the electrochemical performance of VRFBs.

Porous biochar is typically used alone or in combination with carbon fiber materials, such as electrodes [4,5,6,7], because of their low cost and advantages in being ecologically friendly and recyclable. Specifically, the mainstream electrodes for VRFBs are composed of three-dimensional carbon materials, such as carbon felt, graphite felt, and electrostatically spun carbon fiber. However, the low vanadium ion redox electrocatalytic activity limits the VRFBs’ ability to operate at high current densities. To solve these problems, optimizing the performance of the electrodes is the key to developing efficient VRFBs. For example, modified carbon fiber electrodes prepared by electrostatic spinning technology enable high-rate VRFBs with current densities of up to or even higher than 500 mA cm⁻² [8,9,10]. Unfortunately, the electrostatic spinning process is complex and costly and is currently not suitable for large-scale applications. To obtain low-cost electrodes that meet the needs of VRFBs with excellent performance, electrode modifications, such as elemental doping, metal modification, and etched porosity, have been performed on existing materials, such as carbon and graphite felts. Additionally, electrode porosity plays a crucial role in the performance of VRB. The etching effect makes it possible to obtain different sizes of pores on carbon electrode materials, which play an important role in the electrolyte transport rate and reaction sites [11,12,13,14,15]. Larger pore size and higher porosity can effectively reduce the concentration polarization of VRFBs while increasing the migration rate of vanadium ions, building carbon-based electrodes with a high electrochemical surface area through the anchoring effect of hydrophilic materials and improving the wettability and electrochemical activity of the carbon...