Date of Award

Spring 5-1-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

School

Mathematics and Natural Sciences

Committee Chair

Wujian Miao

Committee Chair Department

Chemistry and Biochemistry

Committee Member 2

Song Guo

Committee Member 2 Department

Chemistry and Biochemistry

Committee Member 3

Douglas S. Masterson

Committee Member 3 Department

Chemistry and Biochemistry

Committee Member 4

Sarah E. Morgan

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

Karl J. Wallace

Committee Member 5 Department

Chemistry and Biochemistry

Abstract

Photoelectrochemical (PEC) water splitting makes direct use of solar energy incident on semiconductor photoelectrodes, and it is a convenient, economic option to produce high purity hydrogen at low temperatures. The use of multiple light absorbers can increase overall solar energy utilization and provide a solution to the trade-off between overall band gap and band edge positioning of photoelectrodes specific to solar water oxidation and water reduction. The study of non-noble metal based catalysts for hydrogen evolution reaction (HER) and oxygen evolution reactions (OER) are essential for economic practical commercialization of photo-electrolyzers. This dissertation focuses on the use of a variety of non-noble transition metal chalcogenide based heterogeneous co-catalysts, and solution based water redox catalysts immobilized in polyoxometalate (POM) ensembles for water splitting in single and multiple absorber based systems. With a brief discussion on background and literature review in Chapter I, Chapter II reports PEC studies on TiO2 photoanode in contact with solution-phase nickel-POM co-catalyst viz. K10H2[Ni5(OH)6(OH2)3(Si2W18O66)]•34H2O (Ni5-POM) for water oxidation, whereas Chapter III presents PEC studies on CuxSe photocathode using cobalt-POM co-catalyst viz. [Co9(OH)3(H2O)6(HPO4)2(PW9O34)3]16-(Co9-POM) for water reduction. Chapters IV focuses on the use of electrodeposited transition metal selenide based co-catalysts for solar water reduction with Cu2O photocathodes, and Chapter V demonstrates the results of solar water oxidation using transition metal phosphide and selenide based co-catalysts electrodeposited on Fe2O3 and BiVO4 photoanodes. UV-Vis spectroscopy, scanning electron microscopy, and energy dispersive energy diffraction (EDX) were used for characterization of photoelectrodes and heterogeneous co-catalysts. Synthesized metal-POMs were characterized using FT-IR and mass spectrometry. PEC measurements conducted under simulated solar irradiation provided evidence of significantly increased photocurrents, lowered onset potential and increased electron-hole separation in the presence of co-catalysts. Production of oxygen during solar water oxidation was verified with a dissolved oxygen sensor, and the stability of photoelectrodes were examined with multi-potential step experiments. An approximate 10 times increase in photocurrent (i.e., from ~0.008 to 0.08 mA/cm2) was observed under zero biased water splitting at TiO­2 photoanodes upon the addition of Ni5-POM co-catalyst to the electrolyte solution. A maximum water reduction photocurrent of ~ 2 mA/cm2 at -0.45 V vs Ag/AgCl was observed from Cu2O photocathodes with electrodeposited MnSe as water reduction co-catalyst. In Chapter VI, multiple-absorber configurations based on the above studied metal oxide photoanode/photocathode and a commercially available amorphous Si triple junction (3 jn Si) photoanode with Ni- or Co-POM solution-phase co-catalysts or heterogeneous transition metal chalcogenide co-catalysts are presented. The maximum zero bias photocurrents of -3.50 and -0.60 mA/cm2 were observed from the tandem cells of 3 jn Si/Ni5-POM [photoanode] : FTO/Cu2O-MnSe [photocathode] and FTO/TiO2/Ni5-POM [photoanode] : FTO/Cu2O-MnSe [photocathode] water splitting systems, respectively. As of date, there have been limited studies on the use of non-noble metal POMs and chalcogenide based catalysts for PEC water splitting, and the large unbiased photocurrents obtained from this dissertation with respective to those shown in recent literature are clearly promising. Hence, the present studies provide a useful insight into the investigation of more efficient and commercially viable tandem water splitting systems in the future as discussed in Chapter VII.

ORCID ID

0000-0002-1288-577X

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