(420i) Ensemble Average Electrochemical TIRM: The Impact of Potential Distribution On Electrokinetic Forces

Rapid testing of electrocatalysts and corrosion resistant alloys accelerates investigation of promising new materials. Imaging amperometry, based on the deployment of colloidal particles as probes of the local current density, allows simultaneous electrochemical characterization of the entire composition space represented in an alloy film electrode.  Previous work has shown that variations in particle-electrode distance for single particles in electric fields can be measured using total internal reflection microscopy, and translated into local current density, independent of electrical measurements.  Implementation of this method to enable simultaneous measurements across non-uniform samples involves using a sparse, uniform layer of particles, which requires modification of previously existing theory and methods.  Imaging individual particles for this application is infeasible at the low magnification levels needed to image an entire macroscopic (~1 square cm) sample.  Mapping of electrochemical activity across the surface can be achieved nevertheless by imaging the entire electrode surface and gridding the resulting images into a mosaic of square “patch” areas 100 μm to a side, each containing 15-30 particles.  The measured intensity in each patch is the sum of the scattering from all of the particles present in that patch.  The intensity measured for these ensembles during electrochemical experiments can then be used to infer the current density across that patch.

Further improvements to the theory for translating scattering intensity to current density are proposed here, involving the assumed potential distribution on the electrode below the particles.  This current distribution is governed by three dimensionless groups, which represent the exchange current density on the electrode, the balance between the ohmic and electrode potential drop in the cell, and the particle/wall separation distance.  In the past, the expressions used for electrophoretic and electroosmotic force on the particle involved assuming either a uniform potential distribution or a uniform current distribution under the particle; these represent two limiting cases.  Current work demonstrates that use of an intermediate case may be much better suited for the purposes of the imaging ammeter.  Finite element modeling has been used here to probe a range of intermediate cases and explore the variable space of the dimensionless groups involved, to better understand the impact of parameters such as the current density and solution conductivity on the motion of the particles.