Photo-assisted electrolysis of water using p-type III-V semiconductors

Hancock, Gary Robert (1984) Photo-assisted electrolysis of water using p-type III-V semiconductors. Masters thesis, Memorial University of Newfoundland.

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Abstract

The recent interest in photoelectrochemical methods of solar energy conversion has led to the investigation of many semiconductors for their suitability as electrodes for the process. Attempts have also been made to improve the photoelectrochemical performance of many of these semiconductor electrodes by various surface treatments. -- In the first part of this work, the current density-potential characteristics for the photo-assisted electrolysis of water were compared for p-GaP, P-GaAs and p-InP photocathodes, unplated and with thin electrodeposits (2-200-monolayers) of platinum or palladium. The photocathodes were used in a cell with a platinum anode and 0.5 M H₂SO₄ as electrolyte. For all three semiconductors the noble metal platings resulted in favourable positive potential shifts of the cathodic current-potential curves. Positive energy conversion efficiencies were realized (maximum value of 0.03% in 200 mW.cm⁻² simulated sunlight) for GaP only. It is suggested that the noble metal atoms plated on the surface improve performance by catalytic acceleration of the hydrogen evolution reactions (h.e.r.). -- In the second part of this work photoelectrochemical kinetic parameters were determined for the h.e.r. on the same three semiconductors in 0.5 M H₂SO₄. Room temperature measurements showed the results to conform reasonably well to equations derived by W.L. Ahlgren, based largely on theory developed by J.F. Dewald and H. Gerischer, and modified here for the particular situation of interest. The method involved recording current-potential characteristics at several light intensities and extracting data from them to construct a so-called Ahlgren plot to yield the exchange current density. A jacketed cell was employed later to enable exchange current densities at several temperatures to be determined and used to construct Arrhenius plots. The average values of the exchange current densities (in μA.cm⁻² at 298 K) were 5 x 10⁻², 3 x 10⁻² and 2 x 10⁻⁴ for GaP, GaAs and InP, respectively. The weighted mean activation enthalpies, in kJ.mol⁻¹, were 16.0 for GaP, 25.7 for GaAs, and 61.6 for InP. These results suggest that for the h.e.r. on the bare (unplated) electrodes, GaP is a more suitable electrode than GaAs, and both are much better than InP. -- The agreement with theory and problems of achieving reproducibility are discussed. The significance of the exchange current densities and activation enthalpies determined by this method is explained and a comparison of the exchange current densities with those for the h.e.r. on some metals is presented. Some recent promising work by other researchers is documented and suggestions for future experiments made.

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