Date on Master's Thesis/Doctoral Dissertation

5-2023

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Sunkara, Mahendra

Committee Co-Chair (if applicable)

Spurgeon, Joshua

Committee Member

Gupta, Gautam

Committee Member

Sathitsuksanoh, Noppadon

Committee Member

Zamborini, Francis

Author's Keywords

solar water-splitting; methylene blue; in situ magnetic alignment; tandem semiconductor microwire slurry; photochemistry; solar energy storage

Abstract

The intermittent nature of the Sun makes it difficult to use it as a primary source of electricity and often needs to be supplemented by electricity from the grid which comes from fossil fuels. This motivates the need for solar energy storage. Photoelectrochemical (PEC) water-splitting has been explored as a means to convert solar energy into hydrogen (and oxygen), which can be stored as fuel. The current method of coupling PV and electrolyzer units has been widely commercialized, however, the cost of H2 generated is far from the target of $1/kg set by the DOE under the Energy EarthShot initiative. One of the approaches to reducing the cost associated with electrical connections and panel installation (as in the case of coupled PV + Electrolyzer) is using a semiconductor-based anode and cathode in direct contact with the electrolyte along with catalysts to aid the water-splitting reaction. The initial part of this dissertation explores a relatively low-cost and abundant Ni-based molecular catalyst (NiATSM) for H2 production reaction on a p-Si photocathode. However, various techno-economic analyses suggest that semiconductor particulate slurry reactors are a potential route for reducing the cost of H2 even further. This dissertation also explores the fabrication and characterization of a novel tandem semiconductor particle slurry that has the potential to produce cost-effective hydrogen by achieving high solar-to-hydrogen (STH) efficiencies. In this work, a tandem-junction microwire array was fabricated by coating p+n-Si radial homojunction microwire arrays sequentially with fluorine-doped tin oxide (FTO) and titanium dioxide (TiO2). These tandem microwires were cleaved from the growth substrate to form a Si/TiO2 tandem particulate slurry. Ni hydrogen evolution catalyst (HEC) was selectively photodeposited at the exposed Si microwire core to serve as the cathode site as well as a handle for magnetic orientation. Since the Ni bulb at one end of the microwire would be highly opaque and lead to poor performance for a tandem microwire in an upside-down arrangement, a magnetic field was used to control the orientation of the particle. The ferromagnetic Ni HEC bulb was magnetized and used as a handle for the alignment of tandem microwires under active dispersion of the particles with uplifting bubbles. The photoactivity of the tandem slurry was also tested via photodegradation of methylene blue indicator dye. Light management strategies, to improve the degradation rate and light absorptance in the slurry, were also explored. Optical properties of the slurry were measured under varying reactor conditions such as particle concentration, uplifting bubble flowrate, etc. The results indicate that particle suspension reactors may provide the lowest-cost option for solar hydrogen generation.

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