Defect-induced electronic structures in carbon electrodes direct electrochemical reactivity

Abstract

Despite intense interest in next-generation carbon-based materials for cost-efficient energy storage, the mechanistic origins of redox-couple-specific activity at defective sites remain poorly understood. The relationship between sp² carbon defects and their induced electrochemical performances remains controversial, therefore, establishing clear structure–activity relationships for carbon electrocatalysts remains challenging. Here, we investigate this relationship using the V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox couples, which allow direct comparison of inner- and outer-sphere electron-transfer kinetics within the same chemical system. Instead of correlating electrochemical activity with unspecified, general parameters (defect density, dopant concentration, or surface treatment), carbon materials with specific defect motifs (edge sites, dislocations, vacancies) were used to correlate individual motifs and their connected electronic structure with performance. Our results demonstrate that defects do not universally enhance electrochemical activity in carbon electrodes. Instead, the effect of defects depends on the electron-transfer mechanism: the V²⁺/V³⁺ couple, with prominent inner sphere character, benefits from chemically active defect sites that introduce electronic states near the Fermi level and strengthen ion–surface interactions, whereas the VO²⁺/VO₂⁺ couple, representing an outer sphere reaction, requires an intact delocalized π-network that enables efficient electronic coupling. Our findings demonstrate that defects are a suitable design handle for influencing the electrochemical performance of vanadium based redox couples. Finally, the effects of the various investigated defect types, depends as well on the character and mechanism of the probed electrochemical reaction.