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Electrochemical properties of earth abundant catalysts for efficient water electrolysis

Delgado, Dario (2016) Electrochemical properties of earth abundant catalysts for efficient water electrolysis. PhD thesis, Murdoch University.

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Abstract

The hydrogen economy is attracting a great deal of attention from governments and major oil companies. Hydrogen is seen as a solution to the problems arising from the current unsustainable fossil fuel economy. Hydrogen generation can be done thermochemically, electrochemically and biochemically. Of all these options, hydrogen generation using renewable energy inputs to split water electrochemically into hydrogen and oxygen is potentially attractive on a commercial scale. Water electrolysis has two reactions happening simultaneously, the hydrogen and oxygen evolution reactions (i.e. HER and OER) from the cathode and anode respectively. Most of the relevant published work supports the use of platinum group metals for the HER and platinum group oxides for the OER. Platinum group materials are expensive thus a cheap substitute is needed. In this respect, lower cost substitutes (e.g., manganese dioxide, Raney cobalt and Raney nickel) were investigated possessing the following characteristics: (a) stable in the reaction environment; (b) environmentally friendly, (c) good catalytic activity and (d) earth abundant.

The catalytic activity of materials in general can be enhanced by modifying their geometric and electronic factors. In the case of manganese dioxide (MnO2), the electronic factor has been modified by changing its crystalline structure and chemical composition through a range of additives. In the case of nickel and cobalt as raw materials, their geometric factor has been modified by increasing the surface areas with the use of Raney powders. The electrochemical characteristics of the above materials were investigated by subjecting them to linear voltammetry and electrochemical impedance spectroscopy. The materials were also characterized by physical techniques using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS) analyses.

The obtained electrochemical results revealed that the best overpotentials found for Raney Ni and Raney Co in alkaline HER were -190 mV and -270 mV (i.e. n100 at 100 mA cm-2) respectively. The presence of Mo, Co and V as additives in the parent y type MnO2 substantially improved the catalytic activity towards acid and alkaline OER. In particular, for acid OER the Co/Mo bimetallic addition to y-MnO2 resulted in 305 mV (at n100) which is competitive to that of commercially available DSA® which is 341 mV (at n100). The physico-chemical characterization of the material before and after electrochemical experiments confirmed the stability in aqueous media. The effectiveness of the modified Watts bath for the deposition of Raney cobalt and Raney nickel, has been established through overpotential curves. The presence of oxide species lowers the efficiency of the electrode which increases the overpotential for the HER on Raney based electrodes. A porous Raney surface type enhances the available area for the hydrogen evolution reaction to occur which increases its energy efficiency (i.e. n100). Recommendations for further developmental work for such catalysts are made.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor: Minakshi, Manickam and Senanayake, Gamini
URI: http://researchrepository.murdoch.edu.au/id/eprint/30718
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