Date of Award

1988

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Robert J. Gale

Abstract

A study is presented of the utility of pulse derived waveforms for use in impedance analysis of electrochemical systems. Factors to be considered in optimization of pulse duration and also in the proper implementation of the FFT are discussed. Impedance results are presented for model circuits and for two nonfaradaic electrochemical systems--n-TiO$\sb2$ electrode in aqueous Na$\sb2$SO$\sb4$ and LaF$\sb3$ electrode in aqueous fluoride solution. The techniques developed were applied to model circuits composed of aperiodic elements in order to quantify the decrease in precision with increasing frequency associated with pulse type waveforms. Using 8 bit digital sampling resolution and 512 point double precision Fourier transformation, model circuit results showed a relative standard deviation of approximately 0.5% for the first 20 harmonics and less than 2.5% for all harmonics up to the 50th. Modification of the waveform by analog high-pass filtering extended the useful range to almost 2 decades. Impedance measurements by pulse-Fourier transformation agreed with steady state a.c. results to within 1% at frequencies up to 100 kHz. The technique was used to study the n-TiO$\sb2$/electrolyte interface via Mott-Schottky analysis. Reliable estimates were obtained for donor densities in n-TiO$\sb2$ crystals. Investigation of the shift in flatband potentials with pH showed nernstian behavior although the data showed a high degree of scatter. Impedance studies were made of the fluoride selective LaF$\sb3$ membrane electrode in order to investigate reported concentration dependent conductance. Conductance was measured over frequencies between 500 Hz and 50 kHz. Studies were made using a standard addition method and an electrode immersion method. For the standard addition method, an apparent concentration dependence was seen in the frequency regime corresponding to a hydrated surface film on the membrane. Such concentration dependence was not seen for the immersion method. Relative standard deviations obtained for conductance values were on the order of 0.5% for frequencies up to 25 kHz.

Pages

234

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