Proton Transport in [BMIM⁺][BF₄^–]/Water Mixtures Near the Percolation Threshold

Abstract

The incorporation of ionic liquids into existing proton exchange membrane (PEM) materials has been shown to enhance thermal stability and improve conductivity at reduced water content. Because proton transport is dictated by an interplay between vehicular diffusion and the Grotthuss mechanism, it is expected that the nanoscale structure of the resulting ionic liquid/water networks will sensitively influence transport properties. In this work, we study proton transport in [BMIM+]­[BF4 –]/water mixtures of systematically varying water volume fraction, focusing on concentrations near the percolation threshold in which water networks are connected over macroscopic length scales. We utilize reactive molecular dynamics within the multistate empirical valence bond (MS-EVB) framework to explicitly model Grotthuss hopping processes. Excellent agreement with experimental conductivity data is obtained within the Nernst–Einstein approximation, indicating that proton transport proceeds in a largely uncorrelated manner even at pH <0. We additionally study the changing topology of the hydrogen-bonded water network in these mixtures using percolation and graph theory analysis. We find that the proton diffusion coefficient and forward hop rate increase linearly with water content at concentrations ranging from dilute through the percolation threshold; surprisingly, we find no deviation in this trend at the percolation transition. The high concentration of BF4 – anions inherently alters the fraction of Eigen and Zundel proton states, producing a net detrimental effect on proton transport rates relative to bulk water. This mechanistic insight is useful for selecting ideal ionic liquid candidates and determining the optimal ionic liquid concentration to incorporate into PEM materials.

Ionic Liquid–Poly(lactic acid) Blends as Green Polymer Electrolyte Membranes

“…As mentioned in Section 3. 44,45 and the resulting smaller water adsorption capacity (Figure 6). Indeed, one can see in Figure 6, more clearly at high RH, that the conductivity of PLA−[C 4 C 1 im][BF 4 ] is 2 to 3 orders of magnitude higher than that for PLA−[C 4 C 1 im]-[PF 6 ] blends of similar nominal IL fractions.…”

Proton Transfer Reaction Rates in Phenol–Ammonia Cluster Cation

“…The present calculations indicate that the proton is transferred along the hydrogen-bond networks in PhOH–(NH 3 ) n + . This type of PT is known as the Grotthuss mechanism. , Baer and co-workers suggested that the PT in ionized water clusters proceeds according to the Grotthuss mechanism . The mechanism of PT is similar in both ionized systems.…”

Engineering the Electrochemical Interface of Oxygen Reduction Electrocatalysts with Ionic Liquids: A Review

“…The main mechanism for proton transport in ILs is the Grotthuss mechanism, which consists of proton experiencing structural diffusion by hopping along hydrogen‐bonded water networks. [ 109 ] Stoppelman et al have studied proton transport in the IL [BMIm][BF4] as a function of water content and have shown that proton conductivity in [BMIm][BF4] increases with water content, as a result of decreased viscosity. [ 109 ] This suggests that when selecting ILs for ORR, on top of proton conductivity in the pure IL, water solubility will determine proton resistance of the overall system.…”

“…[ 109 ] Stoppelman et al have studied proton transport in the IL [BMIm][BF4] as a function of water content and have shown that proton conductivity in [BMIm][BF4] increases with water content, as a result of decreased viscosity. [ 109 ] This suggests that when selecting ILs for ORR, on top of proton conductivity in the pure IL, water solubility will determine proton resistance of the overall system. Our group has observed that the protic IL [EIm][NTF2] offers a higher performance compared with the similar aprotic IL [BMIm][NTF2].…”