Grotthuss vs. Vehicle Mechanism: Insights into Ion Conduction in Different Pore Structures

Abstract

In this study, we study ion conduction of protons and sulfate ions within different pore structures in aqueous H₂SO₄ electrolyte. Proton and sulfate ion conduction in aqueous electrolytes proceeds based on the Grotthuss and vehicle mechanisms, respectively, and their ion conduction plays an important role in the performance of electric double-layer capacitors, fuel cells, and others. Two redox-active materials, benzoquinone and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) derivatives, undergo reversible redox reactions involving protons and sulfate ions as counterions, respectively, and are hybridized in the pores of three porous carbons with different pore structures and sizes. The two porous carbons are activated carbons with only micropores and both micropores and mesopores and another is microporous carbon with three-dimensionally ordered and interconnected 1.2 nm micropores. The hybridized benzoquinone and TEMPO derivatives undergo reversible redox reactions within the pores and these hybrids are used as electrode materials in electrochemical capacitors. The reversible redox reactions involve counterion diffusion of protons and sulfate ions in the pores of the porous carbons, and how quickly the reversible redox reactions proceed depends on their ion conduction within the pores. The ion conduction is discussed by evaluating the charge/discharge performances of the hybrids.