Nanostructured Architectures for Solar Thermal and Photocatalytic Applications

Abstract

One of the most pressing current societal needs is to transition from non-renewable to renewable energy sources. Photovoltaic- and wind-based electricity generation is rising in prevalence, but electricity alone cannot address many demands in industrial and transportation sectors. Two promising alternative approaches are concentrating solar thermal (CST) systems and solar fuels generation. CST can be used to generate heat for industrial processes. Solar fuels, or sunlight-derived fuels, can be used to produce chemical feedstocks for industrial or transportation demands. Currently, the efficiency and cost of these processes prevent widespread commercialization. One strategy to improve the utilization of photon energy for thermal and chemical transformations is the implementation of nanostructures, which can enable light manipulation across the solar spectrum. The goal of this thesis is to develop fundamental understanding and rational design concepts which help enable the use of nanostructures for these applications, and to design, fabricate, and test potential engineering solutions.

In the first section of this thesis, silica aerogels, a promising material for next-generation CST systems, are modified using atomic layer deposition (ALD) to induce structural stability at high temperatures. The ALD process parameters are tuned to investigate the effect on modification conformality, and a reaction-diffusion model is developed. This model compares favorably with the experimental results, and enables the tunability of throughput and raw material use in the manufacturing process, which are not often accounted for in ALD models. This is the first report that accounts for multiple precursor doses and precursor depletion in the quasi-static-mode ALD method. The ALD-modified aerogels are more resistant to structural changes at high temperatures, and, when coupled with a blackbody absorber, are estimated to have the highest receiver efficiency of any intermediate-concentration-ratio system characterized at ≥700°C.

The second section focuses on tuning geometric parameters of nanostructured bismuth vanadate (BVO)-based photoanodes for photoelectrochemical applications to make solar fuels. BVO photoanodes with varying nanowire (NW) lengths, inter-NW spacings, and shell thicknesses are fabricated and characterized for optical and photoelectrochemical performance. Electromagnetic simulations were used to evaluate spatial heterogeneity of light absorption. The optimal geometry demonstrated a record-high photocurrent for sulfite oxidation for ALD-based photoanodes under 1 sun illumination. Design criteria for photoanodes were developed, focusing on the manipulation of spatial distribution of light absorption and electrochemically active surface area.

The third section involves photocatalytic self-cleaning transparent surfaces. Pt-modified TiO2 photocatalysts that utilize near-ultraviolet light were incorporated into transparent films, to enable tunable visible-light-driven self-cleaning while maintaining broadband transparency. Monochromatic illumination, which is not often used in photocatalysis, enabled an improved understanding of the spectral dependence of photoactivity. The intentional design of these self-cleaning films enables new use cases, as the majority of prior studies on photocatalytic visible-light-driven self-cleaning surfaces report relatively low transparencies.

The fourth section focuses on CO2 reduction for solar fuels synthesis. A versatile reactor enables the probing of temperature and pressures conditions under illumination which have not been reported in literature to date. An enhancement in the CH4 formation rate was observed upon illumination, although the enhancement mechanism is still under investigation, making solar-to-fuel efficiency calculations preliminary. Light inputs enabled significant reductions in pressure compared to dark controls.

Overall, this thesis provides insight into the factors limiting various solar energy technologies, and develops informed nanostructured materials systems as a platform for the development of further understanding and commercial energy solutions.