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

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 m²g⁻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⁻²in VFBs and 76.1% at 80 mA cm⁻²in 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.