The massive growth of population around the world accompanied by a tremendous industrial development result in an enormous demand for global energy consumption. In addition, the evolution of the world towards smart and connected technologies and the daily need for electronic devices such as laptops, smartphones and tablets require highly efficient energy storage systems.¹ Electrochemical double layer capacitors (EDLCs), also called supercapacitors, are energy storage devices that store energy via adsorption-desorption of ions at the electrode/electrolyte interface. Nanoporous carbons such as activated carbons (ACs) with high specific surface area (more than 1500 m² g^-1) and good electrical and electrochemical properties have been extensively used as electrode material for EDLCs. Depending on the pore size distribution and the surface functional groups, ACs can show high capacitances in aqueous electrolytes, ranging from 100 to 300 F g^-1.² Nevertheless, their relatively low energy density compared to batteries is considered a major challenge for supercapacitors. ^2,3 Therefore, understanding the charge storage mechanism in nanoporous carbon-based electrodes and exploring the attractive properties on the nanometric scale is a very effective way to boost the performance and to develop the next generation of EDLCs.

In this work, we investigate the influence of surface chemistry on heterogeneous electron transfer kinetics for activated carbons using scanning electrochemical microscopy (SECM) equipped with Pt-ultra microelectrode (Pt-UME). Using a thermal treatment in air and under argon, we oxidized and defunctionalized the surface of a commercial activated carbon. Both treatments showed a slight decrease of the specific surface area of the material, without altering its porosity and morphology. The nanogap voltammetry and approach curves obtained by SECM were used to characterize the kinetics constants at the ferrocenemethanol / AC interface (Figure 1) in aqueous electrolytes. The rate constant for the untreated AC was 3.4 10^-2 cm s^-1 and was increased by about 2.5 and 1.5 times, respectively, after oxidizing and defunctionalizing the surface. SECM results showed that the heat treatment increases the conductivity of AC, and the oxidation of the surface increases the wettability and accessibility of the pores, and promote the electron transfer. The rate constant values obtained by SECM are higher, by an order of magnitude, than those obtained by conventional electrochemical measurements. The local information obtained by SECM is valuable for understanding the nanoscale interfacial mechanisms in porous carbon electrodes and promoting their electrochemical performance.

Funding: This project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no.714581).

References

1. Simon, P. & Gogotsi, Y. Perspectives for electrochemical capacitors and related devices. Nat. Mater. 19, 1151–1163 (2020).
2. Shao, H., Wu, Y.-C., Lin, Z., Taberna, P.-L. & Simon, P. Nanoporous carbon for electrochemical capacitive energy storage. Chem. Soc. Rev. 49, 3005–3039 (2020).
3. Xu, K. et al. Computational Insights into Charge Storage Mechanisms of Supercapacitors. ENERGY & ENVIRONMENTAL MATERIALS 3, 235–246 (2020).