Scanning Electrochemical Microscopy for Kinetic Investigations in Viscous Deep Eutectic Solvents: Identifying Practical Approach Curves and Deviations from Electron Transfer Models

Determining
heterogeneous electrochemical electron transfer (ET)
kinetics in electrolytes with a wide range of physical properties
is of great interest for achieving high-performance redox flow batteries.
Among such electrolytes, concentrated hydrogen-bonded electrolytes
(CoHBEs), including deep eutectic solvents (DESs), have recently garnered
significant attention. Unfortunately, traditional Tafel analysis using
macroelectrodes often encounters issues with mass transfer limitations
in CoHBEs with high viscosities, thereby restricting kinetic analysis
to a narrow potential window. Here, we introduce a methodology for
evaluating ET kinetics in viscous DES using the scanning electrochemical
microscopy (SECM). We first determined practical solutions to SECM
tip positioning in ethaline DES, which yield pseudopositive feedback
responses. Lattice Boltzmann method (LBM) simulations helped us rationalize
the impact of the fluid and concentration fields, as well as tip geometry,
tip approach velocity v, and the solvent viscosity
ηs, on the shape of the approach curves. In addition
to successfully recreating approach curves over a variety of conditions,
we found that approaching a conductor ensured a practical point where
the normalized tip response (NiT = 2) converged at L = 0.7 within ∼10% error regardless of tip velocity.
With positioning capabilities at hand, we investigated the kinetics
of Fe3+/Fe2+ redox couple in aqueous and the
ethaline media. The experimental kinetic results were interpreted
using the Butler–Volmer (BV) and Marcus–Hush–Chidsey
(MHC) models. For ethaline, a nonideal kinetic behavior was observed,
potentially attributed to solvent dynamics within DESs or to the interplay
of chloride anions in the charge transfer process.