Electrocatalysis for K3[Fe(CN)6]/K4[Fe(CN)6] reaction of graphite felt with functional groups in flow batteries

Abstract

To address the issue of slow kinetics in the K₃[Fe(CN)₆]/K₄[Fe(CN)₆] positive electrode for alkaline organic redox flow batteries (RFBs), the content of the hydroxyl (–OH), carbonyl (CO), and carboxyl (–COOH) groups on the graphite felt (GF) surface through thermal treatment in air is regulated. The correlation between these functional groups and the electrochemical catalytic behavior was elucidated. GF treated at 400 °C (GF-400 °C) exhibited excellent kinetics and optimal electrochemical performance. In the QXL‖K₄[Fe(CN)₆] flow battery with GF-400 °C, the 1st and 80th discharge capacities under galvanostatic charge–discharge were 0.905 and 0.902 mAh, respectively, with a charge transfer resistance of 0.033 ohm and an apparent diffusion coefficient of 1.116 × 10⁻¹¹ cm² s⁻¹. The molar ratios of C–C, C–OH, CO, and –COOH for GF treated at 25 °C (GF-25 °C) and GF-400 °C were 10 : 1.61 : 0.47 : 0.46 and 10 : 2.49 : 0.64 : 0.58, respectively, indicating increased contents of hydroxyl, carbonyl, and carboxyl groups after 400 °C treatment. The hydroxyl groups serve as active sites for the K₃[Fe(CN)₆]/K₄[Fe(CN)₆] redox reaction. The increased density of hydroxyl active sites on the heat-treated GF surface enhances its catalytic performance. This surface modification approach for enhancing electrochemical performance offers a novel strategy for the directional functionalization of high-activity electrodes in RFBs.