Quaternary ammonium ionic liquids based on -SO3H with different alkyl chain lengths as electrolytes for fuel cells and the mechanism for proton transfer in ionic liquid

Abstract

N, N, N-trimethyl propylsulphonate ammonium hydrosulfate (NC3SO3HHSO4) and N, N, N-trimethyl butyl sulphonate ammonium hydrosulfate (NC4SO3HHSO4) have been synthesized as electrolytes for the fuel cell. The results indicate that the single cell with NC3SO3HHSO4 exhibits a higher maximum power density (MPD) compared to that of the cell with NC4SO3HHSO4. The MPD achieved using NC3SO3HHSO4 and NC4SO3HHSO4 at 50°C were 48.5 mW·cm−2 and 38.2 mW·cm−2, respectively. The diffusion of ions becomes rapid for ionic liquids (ILs) due to their low viscosity, high conductivity, and chemical structure. Furthermore, quantum chemistry calculations provide detailed insights that elucidate the mechanisms underlying the improvement of cell performance. The proton transfer mechanisms in NC3SO3HHSO4 and NC4SO3HHSO4 are further investigated, including the proton transfer between H2SO4 and HSO4−, the proton transfer between cations and their corresponding neutral molecules, as well as the proton transfer between cations and anions. The cell performance improvement mainly depends on proton transfer between cations and their corresponding neutral molecules. The activation energy barriers for the NC3SO3H+-NC3SO3 complex, between the initial structures and the transition state, and between the final structures and the transition state, are 0.67 kcal·mol−1 and 1.55 kcal·mol−1, respectively. However, proton transfer from an NC4SO3H+ to an NC4SO3 does not occur. The difficulty in proton transfer may be attributed to the steric hindrance effect of cations. Therefore, it can be inferred that the steric hindrance effect of cations significantly impacts the mechanism of proton transfer.