Grotthuss vs. vehicle mechanism: Insights into ion conduction in different pore structures

Abstract

In this study, we investigated the ion conduction of protons and sulfate ions within various pore structures using an aqueous H₂SO₄ electrolyte. Proton conduction in aqueous electrolytes follows the Grotthuss mechanism, whereas sulfate ion conduction proceeds via the vehicle mechanism. These conduction mechanisms play a crucial role in the performance of electric double-layer capacitors, fuel cells, and related energy storage devices. Two redox-active materials, benzoquinone and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) derivatives, undergo reversible redox reactions with protons and sulfate ions acting as counterions, respectively. These materials were hybridized into the pores of three porous carbons with varying pore structures and sizes. The two porous carbons are activated carbons, one containing only micropores and the other containing both micropores and mesopores. Additionally, a microporous carbon with three-dimensionally ordered and interconnected 1.2-nm micropores was used. The hybridized benzoquinone and TEMPO derivatives underwent reversible redox reactions within the pores, functioning as electrode materials in electrochemical capacitors. The reversible redox reactions involve the counterion diffusion of protons and sulfate ions within the pores of the porous carbons. The rate of these reversible redox reactions depended on the ion conduction within the pores. Ion conduction was assessed by examining the charge/discharge performance of the hybrids.