Cyclic Voltammetric Diagnostics for Inert, Uniform Density Films

Abstract

Quantitative characterization of uniform density, electrochemically inert films on electrodes is achieved by cyclic voltammetry (CV) of a redox probe that partitions from electrolyte into the film. Electrochemically inert films generate no faradaic current in the voltammetric window of the probe. In simulation models, probes pre-equilibrate into films, electrolyze at electrodes, diffuse in film and solution, and extract across film solution interfaces. Film thickness is ℓ. Diffusion length δ approximates distance from the electrode where voltammetry perturbs probe concentration; δ∝ν− 1/2 for scan rate ν. At high ν, δ < ℓ and voltammetric morphologies are typical of semi-infinite linear diffusion. As ν slows, δ ≳ ℓ and CV morphologies can change with relative probe flux in the film and solution. For higher solution flux, voltammograms assume sigmoidal (S-shaped) characteristics; higher film flux generates gaussian (thin layer CV) characteristics. For film and solution diffusion coefficients Df and Ds and κ the equilibrium ratio of probe concentration in film to solution, diagnostics yield κ√(Df/Ds) and ℓ2/Df. Because diagnostics apply for all ν, films are fully parameterized by CV alone. Without these diagnostics, full characterization requires a second, steady state voltammetric measurement. Diagnostics are vetted with [Ru(bpy)3]2 + (probe) in inert polymer films of Nafion and of poly(styrenesulfonate).

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions

“…The observation of a steady-state voltammetric response for Ru­(NH 3 ) 6 3+ reduction, following the partial removal of the La­(OH) 3 film by scanning to positive potentials, was surprising as La­(OH) 3 is reported to be an electronically insulating material . There are several possible mechanisms that may be responsible for this phenomenon, namely, (i) there are pinholes in the films through which Ru­(NH 3 ) 6 3+ is diffusing; − (ii) La­(OH) 3 is ionically conducting and Ru­(NH 3 ) 6 3+ is transported through the film; − or (iii) the film is sufficiently thin to allow electron tunneling between the underlying Pt electrode surface and redox molecule. − To explore these mechanisms, we performed steady-state voltammetric experiments using three additional outer-sphere redox species: the neutral ferrocene methanol (FcMeOH) and negatively charged ferrocyanide and ferricyanide.…”

“…If the Ln­(OH) 3 film contained pinholes greater than molecular dimensions or if large areas of the Pt surface were uncovered, then it would be expected that the reduction in currents for the two redox species would be approximately equal. The finding that the degree of attenuation of the current is dependent upon the redox couple suggests that the current is limited by either diffusion of the redox species through the film − or by electron tunneling across the film. − …”

Electron Hopping of Tris (2,2′-bipyridyl) Transition Metal Complexes M(bpy)2/33in Nafion

“…Binding by z sulfonates is sufficiently strong that physical motion of M(bpy) z+ 3 is hindered and yet for some metals M z+ , cyclic voltammetric currents (flux) for M(bpy) z+ 3 in Nafion can equal or exceed flux of M(bpy) z+ 3 1 mM in solution. 12 High M(bpy) z+ 3 concentration in hydrated domains narrows the gap between adjacent M(bpy) z+ 3 moieties sufficiently to allow efficient electron hopping (self exchange). Self exchange augments measured diffusion coefficient.…”

Film Permeation by Rotating Disk Voltammetry at Electrodes Modified with Electrochemically Inert Layers and Heterogeneous Composites

“…The equation for the chronoamperometric response for permeation of a probe through an inert, single layer, uniform film is reported by Peerce and Bard. 22 A method to extract κ D f and 2 /D f from cyclic voltammetry is reported by Knoche et al 23 Given P = κD f / , κ D f and perhaps 2 /D f , the three parameters κ, D f , and cannot be determined individually without additional information. Typically, this is .…”