Bmpy or Bmim: which is better for H2 sensing?

YN He and T Glossmann and XQ Zeng and W Lai, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 27, 10962-10978 (2025).

DOI: 10.1039/d5cp00642b

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, BmpyNTf2 and BmimNTf2, experimental results reveal that BmimNTf2 exhibits a higher ionic diffusivity and conductivity than BmpyNTf2. However, recent hydrogen sensing tests demonstrate that BmpyNTf2 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 BmpyNTf2 than in BmimNTf2. Collectively, these results provide insights into why BmpyNTf2 is a more sensitive electrolyte material than BmimNTf2.

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