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

Area of Science:

• Electrochemistry and Materials Science
• Computational Chemistry and Nanotechnology

Background:

• Ionic liquids (ILs) are promising electrolyte materials for hydrogen (H2) sensors due to their low vapor pressure and tunable properties.
• While [Bmim][NTf2] shows higher ionic conductivity than [Bmpy][NTf2], experimental H2 sensors based on [Bmpy][NTf2] exhibit superior sensitivity, a discrepancy lacking explanation.
• Current experimental techniques have limitations in spatially and temporally resolving the phenomena at the electrolyte-electrode interface.

Purpose of the Study:

• To investigate the electric double layer (EDL) structure and H2 diffusion mechanisms in [Bmpy][NTf2] and [Bmim][NTf2] using molecular dynamics (MD) simulations.
• To elucidate the reasons behind the higher sensitivity of [Bmpy][NTf2]-based H2 sensors compared to those using [Bmim][NTf2].

Main Methods:

• Molecular dynamics (MD) simulations were employed to model the electrolyte|electrode heterostructure, categorizing it into three regions: 1st EDL, 2nd EDL, and bulk phase.
• Analysis included IL number and orientation distributions to characterize the EDL structure.
• Self-diffusion coefficients of IL cations and anions were calculated for each region, alongside H2 solubility and probability density.

Main Results:

• The [Bmpy] cation exhibits a more scattered orientation in the 1st EDL compared to [Bmim], creating more H2 transport pathways to the electrode.
• H2 molecules show higher bulk solubility and increased probability density in the 1st EDL of the positive electrode for [Bmpy][NTf2] than for [Bmim][NTf2].
• This study provides the first regional analysis of diffusion differences at the electrolyte|electrode interphase.

Conclusions:

• The enhanced H2 transport and interaction within the EDL of [Bmpy][NTf2], attributed to the [Bmpy] cation's orientation, explain its superior performance in H2 sensors.
• MD simulations offer valuable insights into the structure-property relationships of ILs for designing advanced electrochemical sensors.