Date of Award

5-2013

Embargo Period

11-7-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor(s)

Paul Sides

Second Advisor

Dennis Prieve

Abstract

Rapid testing of electrocatalysts and corrosion resistant alloys accelerates discovery 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 a thin-film alloy "library" electrode. Previous work has shown that nanometer scale variations in particle-electrode distance for single particles in electric fields can be measured optically 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 work presented in this dissertation shows that the integrated light intensity in each patch is the sum of the light scattering from all of the particles present in that patch, and that this total measured intensity can be used to infer the current density in the patch during electrochemical experiments.

In addition to scaling the imaging ammeter up to ensembles of particles, the theory for translating measured particle motion to current density has been substantially improved. These improvements involve proper modeling of the current distribution on the electrode below the particles, which has a profound impact on the forces acting on each particle. This work demonstrates that the use of realistic kinetic models for the imaging ammeter is both vital and a discovered opportunity to increase its sensitivity. Finite element analysis was used to explore the variable space of the parameters involved, to better understand the impact of factors such as the current density and solution conductivity on the motion of the particles. Going forward, this information will be leveraged to improve the accuracy of the macroscopic imaging ammeter.

To complete the groundwork for the imaging ammeter laid out in this thesis, proof of concept experiments using a nickel/iron composition spread alloy film were performed. In a 1×5 mm2 area containing alloy compositions from 20% iron to 100% iron, expected trends in electrochemical activity were observed during experiments, i.e. the current density as a function of voltage increased with increasing nickel content on the electrode surface. Future work will probe Fe/Ni alloy compositions with less iron, subsequently moving on to other binary and eventually ternary alloy systems.

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