[Bmpy] or [Bmim]: which is better for H2 sensing?

Abstract

Ionic liquids (ILs) have been found to be a good type of electrolyte material to fabricate highly sensitive H₂ sensors, accredited to their minimal vapor pressure and array of tunable physicochemical properties. Of the two IL molecules commonly used, [Bmpy][NTf₂] and [Bmim][NTf₂], experimental results reveal that [Bmim][NTf₂] exhibits a higher ionic diffusivity and conductivity than [Bmpy][NTf₂]. However, recent hydrogen sensing tests demonstrate that [Bmpy][NTf₂] based sensors are more sensitive instead. Until now, this seemingly contradictory phenomenon has lacked a reasonable explanation because of the spatial and temporal limitations of current experimental techniques. Thus, molecular dynamics (MD) simulations were used in this work to examine the electric double layer (EDL) structure and H₂ diffusion in the EDL for the two IL species. With the use of multiple descriptors like IL number distributions, orientation distributions, etc., the electrolyte|electrode heterostructure can be categorized into three distinct regions: the 1st EDL, the 2nd EDL, and the bulk phase. The self-diffusion coefficients of IL cations and anions for each region are then calculated and compared, which is, as per our knowledge, the first time that the diffusion-related differences in the different regions of the electrolyte|electrode interphase have been addressed. As compared to [Bmim], [Bmpy] cations demonstrate a more scattered orientation distribution within the 1st EDL, which allows more H₂ transport pathways to the electrode and thus leads to a higher possibility of H₂ redox reaction. Furthermore, H₂ molecules show a slightly higher bulk solubility and higher probability density in the 1st EDL of the positive electrode (PE) in [Bmpy][NTf₂] than in [Bmim][NTf₂]. Collectively, these results provide insights into why [Bmpy][NTf₂] is a more sensitive electrolyte material than [Bmim][NTf₂].