Mass Transport Studies and Hydrogen Evolution at a Platinum Electrode Using Bis(trifluoromethanesulfonyl)imide as the Proton Source in Ionic Liquids and Conventional Solvents

Abstract

The hydrogen evolution reaction at a platinum electrode and proton transport using bis(trifluoromethanesulfonyl)imide (H[NTf2]) as the proton source has been investigated in six aprotic [NTf2]− ionic liquids (ILs), a protic [NTf2]− IL, propylene carbonate, and water. Proton reduction to form hydrogen in ILs is highly sensitive to the surface state of the platinum electrode, requiring oxidative preconditioning at ∼2 V versus Fc/Fc+ (Fc = ferrocene) to “activate” the electrode and achieve reproducible voltammetry. The hydrogen evolution reaction at a preconditioned electrode in ILs has been simulated by combining the classical Volmer, H+ + e– ⇌ H* (E10,ks,1,α1), and Heyrovsky reactions, H+ + H* + e– ⇌ H2 (E20,ks,2,α2), where E0, ks, and α are the standard potential, standard heterogeneous electron-transfer rate constant, and charge transfer coefficient, respectively. The Volmer reaction is the rate-determining step on platinum in IL media and the formal potential of the H+/H2 process is insensitive to the identity of the IL cation, lying at approximately −30 mV with respect to the Fc/Fc+ process. Proton transport in the ILs investigated obeys the Stokes–Einstein equation. Furthermore, the proton diffusion coefficient (measured electrochemically) is essentially identical to the self-diffusion coefficient of [NTf2]− (measured with pulsed field gradient spin–echo NMR), indicating that the protons are transported as undissociated H[NTf2]. The Stokes radius of H[NTf2] is estimated to be 3.24 Å from the slope of a Stokes–Einstein plot, in agreement with the literature. Proton transport in propylene carbonate follows the same Stokesian relationship, implying that H[NTf2] also is not dissociated in this molecular solvent. Finally, proton transport in water follows the well-established Grotthuss mechanism, as expected. Although we have shown that there is no Grotthuss-like proton conduction mechanism operating in the investigated ILs, this work lays the foundation for further studies on proton reduction, activity, and transport in this class of solvent. Ultimately, the goal is to develop an IL formulation which facilitates facile anhydrous proton conduction over a wide temperature range.