Studies on the electrochemical synthesis and modification of semiconductor photoanodes for improved photoelectrochemical water splitting

Kenneth James McDonald, Purdue University

Abstract

Solar energy conversion offers the potential to power the planet using the 3.8 x 1026 J/h of power the sun offers everyday. Although only a small portion of the sun's energy hits the surface of the earth (3x10 -8 percent), one hour of that energy is capable of meeting the current demand for energy in a year. Utilizing the suns energy requires effective means of harvesting solar photons and storing the collected energy. One device capable of accomplishing both tasks is a photoelectrochemical cell (PEC) designed to split water, which can absorb solar photons and store the energy in the chemical bonds of H2 and O2. However, no one material has yet been able to effectively accomplish both the reduction and oxidation of water for a PEC. One possible solution is to design a photoelectrochemical diode which cuts the overall water splitting reaction into two half reactions using a p-type material as the photocathode to reduce water and an n-type material as the photoanode to oxidize water. Though the production of H2 is the more desirable product the oxygen evolution reaction (OER) is the more kinetically limiting reaction and can hinder production of H2. Therefore effective photoanode materials need to be studied. This work focuses on the development of electrochemical synthesis routes and surface modifications of semiconductor photoanode materials to improve their photocatalytic properties towards water oxidation. Fe2O3 photoanodes were modified by photodepositing a Co-phosphate oxygen evolution catalyst (Co-Pi OEC) onto the surface using two different circuit conditions. The addition of Co-Pi OEC improved surface kinetics towards OER resulting in improved photocurrent. The surface of Fe 2O3 photoanodes was also modified using a simple solution thermal treatment to convert the surface of the Fe2O3 electrode to ZnFe2O4, another n-type material. The formation of the Fe2O3/ZnFe2O4 composite electrode demonstrated enhanced photocurrent compared to Fe2O 3 alone. BiVO4 photoanodes were prepared via a simple solution thermal treatment of BiOI that were electrodeposited for the first time by a cathodic method. When combined with FeOOH as an oxygen evolution catalyst (OEC) the BiVO4/FeOOH demonstrated remarkable photocurrent properties with little applied bias. FeOOH was also cathodically electrodeposited for the first time from a near neutral medium. The resulting electrodes were tested as an OEC and as a template for the formation of both photoanode and photocathode materials. Lastly, a series of copper hydroxyl double salt (CHDS) electrodes were cathodically electrodeposited, many of them for the first time. Although not known as photoanode materials, CHDSs have a unique layered structure that can be used as an ion exchange material which was tested and will be discussed.

Degree

Ph.D.

Advisors

Choi, Purdue University.

Subject Area

Inorganic chemistry

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