Structure-activity correlation of thermally activated graphite electrodes for vanadium flow batteries

Abstract

Thermal activation of graphite felts has proven to be a valuable technique for electrodes in vanadium flow batteries to improve their sluggish reaction kinetics. In the underlying work, a novel approach is presented to describe the morphological, microstructural, and chemical changes that occur as a result of the activation process. All surface properties were monitored at different stages of thermal activation and correlated with the electrocatalytic activity. The subsequently developed model consists of a combined ablation and damaging process observed by Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. Initially, the outermost layer of adventitious carbon is removed and sp² layers of graphite are damaged in the oxidative atmosphere, which enhances the electrocatalytic activity by introducing small pores with sharp edges. In later stages, the concentration of reaction sites does not increase further, but the defect geometry changes significantly, leading to lower activity. This new perspective on thermal activation allows several correlations between structural and functional properties of graphite for the vanadium redox couple, describing the importance of structural defects over surface chemistry.

Fig. 1. High-resolution SEM images of GF. A single fiber is displayed for (a) pristine and (b–f) thermally activated GF after (b) 8 h, (c) 16 h, (d) 24 h, (e) 32 h, and (f) 40 h at 400 °C.

Fig. 2. Physicochemical characterization of pristine and thermally activated GF. (a) Example of a deconvoluted Raman spectrum of GF; (b) intensity ratios of the D, G, and D′ bands. (c and d) XPS data of a selected GF, displaying all evaluated surface species in the O 1s and C 1s region; (e) quantification of the carbon–oxygen moieties.

Fig. 3. Electrocatalytic activity of thermally activated GF. (a and b) CV curves recorded in the (a) negative and (b) positive half-cell. (c) Peak separation potential in the negative and positive half-cell. (d) EIS at an applied potential in the negative and positive half-cell.

Fig. 4. Schematic depiction of our three-stage model for the thermal activation of GF.

Fig. 5. Structure–activity correlation of thermally activated GF. Correlation between (a) R_CT and defect type, (b) R_CT and C_V, (c) R_CT and oxygen concentration, (d) C_V and defect type.