Sandwich-like multi-scale hierarchical porous carbon with a highly hydroxylated surface for flow batteries

Abstract

High reaction activity and cycling stability are essential to commercialize electrodes for vanadium flow batteries (VFBs) and zinc–bromine flow batteries (ZBFBs). In this work, a sandwich-like multi-scale pore-rich hydroxylated carbon (SPHC), which is made of a highly activated porous middle layer assembled between a pair of protuberance-rich compact outer layers, was proposed and developed to meet both the redox reactions and the mass-charge transport in flow batteries. In this SPHC, (i) the middle layer with the pore-opened integrated structure provides abundant active sites for redox reactions, (ii) the outer layer with a compact surface protects the inside pores against the electrolyte flow, and (iii) the even-distributed protuberances in the outer layer further increase the number of reaction sites. Due to the sandwich-like porous structure that possesses abundant hydroxyl functional groups (25.3%) and a high specific surface area (313.2 m² g⁻¹), the SPHC demonstrates both high electrochemical activity and stability towards vanadium and bromine redox reactions. Additionally, the SPHC-based electrode yields energy efficiencies as high as 77.8% at 200 mA cm⁻² in VFBs and 76.1% at 80 mA cm⁻² in ZBFBs, 28.3% and 8.2% higher than that of a pristine electrode, respectively. More impressively, the SPHC electrode in VFBs has an ultra-low energy efficiency decay of 0.0025% per cycle for over 500 charge–discharge cycles. This work provides a promising approach to develop a high-activity and high-stability electrode that enables great potential applications in flow batteries and other electrochemical energy storage systems.