Spatiotemporal Mapping of Diffusion Layers Using Synchrotron Infrared Radiation

Synchrotron infrared (SIR) radiation has been employed to map the diffusion layer surrounding a platinum working electrode. A thin-cavity transmission cell containing a raised, 12 μm platinum working electrode is employed to generate a two-dimensional diffusion space. The use of a prototypical redox system, i.e. the diffusion controlled reduction of ferricyanide (Ox) and concurrent production of ferrocyanide (Red), allows for a proof of principle evaluation of the viability of SIR for simultaneous mapping (in time and space) of the concentrations of species in the diffusion layer. Diffusion coefficients for the two species in the redox couple are extracted by comparing the experimental results with numerical simulations using finite elements. Absolute values of D_Ox = 4.5 X 10⁻⁶ cm² s⁻¹ and D_Red = 3.6 X 10⁻⁶ cm² s⁻¹ have been obtained which are systematically lower by about 30% than those independently determined from electrochemical measurements in the 0.10 M NaF supporting electrolyte. However, their ratio is in excellent agreement with accepted values. Deviations are attributed to heterogeneity in the SIR beam's intensity profile as well as difficulties in accurately accounting for the working electrode's pronounced edge effects. Implications for future IR spectroelectrochemical studies of chemical reactions in electrochemically generated diffusion layers are discussed