Development of nanostructured metal oxides as photoelectrodes and water oxidation catalysts for solar water splitting applications [electronic resource]
Keywords: Chemical Engineering.

Solar water electrolysis, which can be categorized as either a photovoltaic (PV)-electrolyzer or a photoelectrochemical (PEC) water splitting cell, is a technology that provides both solar energy capture and storage. A solar water electrolysis device utilizes photoabsorbing semiconductors in series with electrocatalysts to convert solar energy and water into hydrogen, an alternative fuel and an important chemical feedstock. Hydrogen offers a sustainable alternative pathway for replacing our fossil fuel based energy and transportation sectors, combating the rise of anthropogenic CO2 emission. The widespread implementation of solar water electrolysis is, however, currently hindered by many scientific and engineering challenges. Two of the main challenges are (1) the realization of efficient and cost-effective PEC photoabsorber materials, and (2) the development of highly active catalysts for the oxygen evolution reaction (OER), which is required in both a PV-electrolyzer and a PEC cell. To address the challenge of efficient and cost-effective photoabsorbers for PEC photoelectrode realization three metal oxide photoanode materials: hematite (α-Fe2O3), tungsten oxide (WO3), and bismuth vanadate (BiVO4) were investigated as candidates for top photoabsorber to be paired with silicon (Si). To overcome the poor charge transport properties, a common performance limiting factor of these metal oxides, we developed conductive high surface area scaffolds to be used as majority carrier current collectors. This general strategy allows for an independent maximization of charge extraction and optical density of the photoactive films. Additionally, we show that interfacial engineering at the semiconductor/substrate interface is critical to this approach as it provides chemical compatibility to the otherwise incompatible substrates as well as reducing the interfacial recombination and shunt loss in the devices. Finally, employing both nanostructuring and interfacial engineering, a model tandem p+n Si Core/W-doped BiVO4 shell photoanode architecture capable of spontaneous solar water splitting without any precious metal catalyst was demonstrated. To investigate the challenge associated with the OER which is critical to both PEC and PV-electrolysis research, NiOOH-based OER catalysts were studied. We synthesized highly active NiOOH-based catalysts and demonstrated that the geometric current densities, especially at higher loadings, were exceptionally enhanced on Au substrates relative to the analogous glassy carbon (GC) substrates. Utilizing a systematic loading study in combination with the use of both in situ and ex situ characterizations, the origins of the high activity of NiOOH-based catalysts on Au substrates was attributed to a greater number of electrochemical active sites due to the more homogenous distribution of the catalyst films. This investigation clarified how substrates can influence the overall activity of the catalysts, which is critical to device integration. In summary, this thesis addresses two of the main challenges facing solar water splitting research: development of the photoabsorber and oxygen evolution catalyst. The first part of the thesis presents design strategies to mitigate performance limitations of metal oxide-based semiconductors, enabling a bias-free tandem PEC water splitting device. The latter part demonstrates a systematic methodology for investigating the mass activity and site specific activity of NiOOH-based OER catalysts, providing an insight into the effect of catalyst-substrate interactions. These strategies can be applied to the development of other photoabsorber or catalyst systems, accelerating the development of efficient and cost-effective solar water electrolysis devices.