(366f) Ensemble Average TIRM: Theory and Application In Imaging Amperometry

Rapid testing of electrocatalysts and corrosion resistant alloys accelerates discovery of promising new materials. Imaging amperometry, 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 changing electric fields can be measured using total internal reflection microscopy (TIRM), and used to determine the electrochemical current passing around the particle, independent of electrical measurements.  [1,2]

Improving this method to enable simultaneous measurements across inhomogeneous samples involves imaging evanescent wave scattering from a two-dimensional random ensemble of particles levitated above the working electrode in electrochemical TIRM experiments.  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 by dividing the electrode surface into a mosaic of square “patch” areas 100 μm to a side, each containing 15-30 particles.  The measured intensity for each patch is the sum of the scattering from all of the particles present in that patch.  Due to the non-symmetric nature of the potential energy well of the particles in the vicinity of the wall, there may be significant discrepancies between data collected for individual particles and ensemble-average data of those same particles. 

Theory has been developed for analysis of ensemble-average TIRM data using elements of the single-particle theory and considerations necessary for ensembles. Conversion of scattering intensity to current density requires knowing the scattering intensity at the most probable height of the particles at open circuit, but the mean intensity is the value measured in ensemble average experiments. In some cases these two values are indistinguishable but this is not typical for many experimental conditions.  The results of this recent study demonstrate how to correct for this difference. The adjustment to ensembles has been verified with simulation and experiment.  The new theory predicts that the variation between the currents calculated from ensemble and single-particle scattering data is a strong function of the weight of the particles in the ensemble; this conclusion is supported by experimental and simulation results presented. 

Patch-level electrochemical TIRM experiments were conducted with polystyrene beads levitated over a tin doped indium oxide electrode in alkaline solution, at open circuit and during cyclic voltammetry (CV) experiments probing the oxygen evolution reaction.  The measured raw TIRM scattering intensities of particle ensembles appropriately increase  and decrease with corresponding values of applied potential.  Furthermore, the new theory clearly converts experimentally measured TIRM intensities to local current densities approximately equal to the current density measured by traditional methods.  The focus in this work is on determining the correct physics governing the particle movements in order to maximize the fidelity of the derived current densities. These same experiments were modeled using Brownian dynamics simulations to examine the interaction between the scattering from individual particles and that of the ensemble average.  These investigations represent important foundational steps towards the use of the macroscopic imaging ammeter.

1. P J Sides, C L Wirth, D C Prieve "An Imaging Ammeter for Electrochemical Measurements," Electrochemical and Solid State Letters 13 F10-F12 (2010).

2. CL Wirth, PJ Sides, DC Prieve "The Imaging Ammeter," J Coll Interf. Sci.  357 1-12 (2011).