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

Abstract

Steady state rotating disk voltammetry provides excellent measurement of permeability for films and layers on electrodes because the hydrodynamic control of rotating disks establishes well defined boundary layers normal to the electrode. For a redox probe present in solution and pre-equilibrated in the layer, voltammetry measures electrolysis current for the probe as the probe transports from solution, through the film, and to the electrode where the probe is electrolyzed. Diagnostic equations relate the steady state, mass transport limited current iss to the rotation rate ω. The incompressible layer is electroinactive about the probe formal potential. Diagnostics are available for a single layer, uniform film (Gough and Leypoldt). Uniform films are homogeneous with no structural features. Here diagnostics are provided for bilayer and multilayer uniform films, where multilayers include discretely and continuously graded films. Consideration of serial mass transport resistances normal to the electrode allows diagnostics for uniform films. Heterogeneous, micro- and nano-structured layers are structured in the plane of the electrode. Consideration of parallel mass transport resistances as part of the layer mass transport resistance yields diagnostics for heterogeneous films. Diagnostics are vetted with literature data. Theoretical and practical advantages and limitations are noted.

Membrane-Coated Electrocatalysts—An Alternative Approach To Achieving Stable and Tunable Electrocatalysis

“…Operation in a supporting electrolyte can greatly reduce the contribution of migration to overall species flux, but this may not be true for MCECs if the supporting electrolyte ions have low D e,j or S j within the MCEC overlayer. Other possible complications include overlayers with depth-dependent D e,j or S j , interactions between ions and/or the overlayer that require the use of concentrated solution theory, overlayers containing pinholes or cracks, , and mass transfer losses in the bulk electrolyte . It is expected that a combination of molecular modeling tools, electroanalytical measurements, and detailed physical characterization of overlayers will be essential for developing more accurate transport models for MCECs.…”

“…More detailed characterization of MCEC structure and properties is also desirable to provide input parameters to first-principles and continuum models. Similarly, measurements of well-defined model MCEC electrodes such as planar electrodes − are highly attractive for validating these models. It can be anticipated that multiphysics, multiscale modeling will be extremely important for elucidating reaction mechanisms and describing the structure−property-performance relationships that underlie the reaction energetics and transport properties of MCECs.…”

“…Other possible complications include overlayers with depth-dependent D e,j or S j , 31 interactions between ions and/or the overlayer that require the use of concentrated solution theory, 40 overlayers containing pinholes or cracks, 33,34 and mass transfer losses in the bulk electrolyte. 31 It is expected that a combination of molecular modeling tools, electroanalytical measurements, and detailed physical characterization of overlayers will be essential for developing more accurate transport models for MCECs. With inspiration from studies of semipermeable polymeric 34 and silicon oxide layers, 35 scanning electrochemical microscopy (SECM) is one electroanalytical tool that is expected to be especially valuable in its ability to quantify permeabilities through ultrathin overlayers and image spatial heterogeneities in overlayer properties due to cracks or pinholes.…”

“…For example, overlayer coatings such as those seen in Figure tend to be nonuniform, making it very difficult to deconvolve electrochemical current associated with species transport through the overlayer from that associated with reaction(s) taking place through pinholes and/or on uncoated portions of the nanoparticles. For this reason, fundamental studies on model planar MCECs, such as that shown in Figure d, can be of great value to quantifying key kinetic and transport parameters that underlie MCEC operation. − …”

Effects of the variation of diffusion coefficients in the potentiometric stripping analysis

“…During the accumulation of amalgam there is an additional equation: The concept of the model was taken from Knoche et al. [ 31 ] It was developed for the permeable film covered electrodes, [ 32–35 ] such as nafion‐coated thin mercury film rotating disk electrode. [ 36,37 ] Its basic assumption is that the flux in the film must be equal to the flux in the diffusion layer (Equations , and ).…”