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

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 sp(2) 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.