Large Bandgap Semiconductors for Solar Water Splitting

Publication: ResearchPh.D. thesis – Annual report year: 2015

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Photoelectrochemical water splitting represents an eco-friendly technology that could enable the production of hydrogen using water as reactant and solar energy as primary energy source. The exploitation of solar energy for the production of hydrogen would help modern society to reduce the reliance on fossil fuels as primary feedstock for hydrogen production and diminish the emission of greenhouse gases in the atmosphere, weakening the global warming phenomenon.The dissertation reports the development of GaP (gallium phosphide) photocathodes as a large bandgap semiconductor for photoelectrochemical water splitting devices having tandem design. The increase of the photovoltage produced by GaP under illumination was the main goal of this work. GaP has a bandgap of 2.25 eV and could in theory produce a photovoltage of approximately 1.7 V. Instead, the photovoltage produced by the semiconductor is limited to ≈ 0.35 V when utilized to produce hydrogen in acidic electrolyte with Pt catalyst. The formation of an effective p–n heterojunction between p−GaP and sputter deposited n−TiO2 or n−Nb2O5 enables the photocathode to produce a photovoltage of 0.70 V. The large built-in potential formed at the junction and the high donor density of the metal oxides determines the effectiveness of the p–n heterojunction approach. Moreover, TiO2 protects the GaP from corrosion in acidic electrolyte and enables the photocathode to perform continuous hydrogen generation for a time period of 24 hours.The photocurrent density generated by GaP was increased by more than 60% by electrochemical etching of the surface. The etching process produces a rough microstructured surface that increases the optical path length of the incident photons and the collection of photogenerated electrons.Furthermore, the synthesis of BiVO4 (bismuth vanadate) was investigated in view of combining this 2.4 eV large bandgap semiconductor with a Si back-illuminated photocathode. A device obtained by mechanical stacking of BiVO4 photoanode and standard Si photocathode performs non-assisted water splitting under illumination with Solar-to-Hydrogen efficiency lower than 0.5%. In addition, BiVO4 was synthesized on the back-side of a Si back-illuminated photocathode to produce a preliminary monolithic solar water splitting device.The Faradaic efficiency of different types of catalysts for the electrochemical production of hydrogen or oxygen was evaluated with gas chromatography analysis. The gas chromatograph interfaced with the electrochemical cell provided qualitative and quantitative measurements of the reaction products produced during the electrochemical experiments. The results show high efficiencies for Pt as hydrogen evolution catalyst and for IrOx and RuO2 as oxygen evolution catalysts in acidic electrolytes
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark (DTU)
Number of pages143
StatePublished - 2014
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