Revealing the Performance Enhancement of Oxygenated Carbonaceous Materials for Vanadium Redox Flow Batteries: Functional Groups or Specific Surface Area?

Abstract It is widely recognized that the oxygenated carbonaceous materials improve the performance of vanadium redox flow batteries (VRFBs). However, whether the enhanced vanadium redox reactions are due to the existence of grafted oxygen‐containing functional groups (OCFGs) or the enlarged specific surface areas (SSAs) during the course of surface treatments is still under dispute. To determine which factor is indeed responsible for the enhanced redox reactions, a set amount of OCFGs is intentionally grafted on the model carbon electrode without modifying the surface area using an alkaline‐mediated hydrothermal treatment. Experimental results, using the peak potential separation and the activation energy as electroactivity descriptors in the three‐electrode cell, demonstrate that OCFGs promote the V(II)/V(III) redox reaction, while exhibiting a minute catalytic effect on the V(IV)/V(V) redox reaction. The superior electroactivity is also testified by a flow battery test, in which the energy efficiency as well as the charge/discharge capacity of VRFBs with hydrothermally treated samples is significantly higher than those of VRFBs with the pristine one. Through these experiments, it is verified that OCFGs contribute to the electroactivity promotion for carbon electrodes rather than the enhanced SSA. The findings can guide the effort to design high‐performing carbonaceous electrodes for VRFBs.