Proton transport behaviour and molecular dynamics in the guanidinium triflate solid and its mixtures with triflic acid

Abstract

Knowledge of the proton transport behaviour in electrolyte materials is crucial for designing and developing novel solid electrolytes for electrochemical device applications such as fuel cells or batteries. In the present work, high proton conductivity (approximately 10⁻³ S cm⁻¹) was observed in the triflic acid (HTf) containing guanidinium triflate (GTf) composites. The proton transport mechanism in the composite was elucidated by comparing the diffusion coefficients obtained from NMR and conductivity measurements. Several orders of magnitude enhancement of conductivity is observed upon addition of HTf to the organic solid, and this appears to follow percolation behaviour with a percolation threshold of approximately 2% HTf. The data support a structural diffusion (or Grotthuss) mechanism of proton transport with a calculated Haven ratio significantly less than unity. ¹³C SUPER and ¹⁴N overtone NMR experiments were used to study the mobility and symmetry of the triflate anion and guanidinium cation respectively at a molecular level. The former experiment shows that the CF₃ group in the anion displays fast and isotropic motion at room temperature. In contrast to the high mobility of the anion group, the ¹⁴N overtone experiments indicate that the guanidinium cation is static in both the pure and the acid-containing GTf samples at room temperature. It is anticipated that these solid-state NMR techniques may be also applied to other organic solid state electrolyte materials to achieve a better understanding of their transport mechanisms and molecular dynamics.