- Title
- Solar hydrogen: a fundamental and applied electrochemical investigation of the hybrid sulfur cycle electrolyser
- Creator
- O'Brien, Jessica Alice
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2011
- Description
- Research Doctorate - Doctor of Philosphy (PhD)
- Description
- Fundamental and applied investigations of the electrochemical oxidation of aqueous sulfur dioxide have been carried out. This reaction is of key importance for the further development of the hybrid sulfur cycle for large scale, zero-emission solar hydrogen production. The cycle involves both thermal and electrochemical reactions including the thermal decomposition of sulfuric acid and the electrolysis of aqueous sulfur dioxide (anodic reaction) and water (cathodic reaction). Oxygen is liberated during the thermal decomposition step and hydrogen is generated electrolytically with water as the only input. The cycle is therefore a water splitting cycle which has a high efficiency due to the use of sulfur species intermediates. The acid electrolyser for the cycle has been identified as limiting efficiency and cost. However, catalyst development for the controlling reaction of this electrolysis, sulfur dioxide electro-oxidation, has had limited attention in the literature despite its inefficiency using the commonly employed platinum electrocatalyst. The large overpotential observed on platinum means an excess of electrical input is required to operate the electrolyser at a specified current density. A lack of fundamental understanding of the reaction on platinum and other electrode substrates means the origin of the limited oxidative behaviour is not known and catalyst development in this area has previously been limited to simple screening of a multitude of substrates. Understanding of the oxidative mechanism and catalysis on a platinum electrode has been identified as the key to catalyst development in this work and fundamental investigations of how the oxidation of aqueous sulfur dioxide proceeds have been carried out. Literature in this area demonstrates inconsistencies between studies and does not appear to systematically evaluate the mechanism on various electrodes. Synthesis of the literature here has in part explained these inconsistencies, mainly through determination of the variable formation of sulfur which can act both catalytically and to inhibit the reaction on platinum and gold electrodes studied depending on coverage. Systematic analysis of sulfur formation on a platinum electrode in this work has revealed sensitivity of sulfur formation, and thus electrode activity, to not only the extent of cathodic polarisation but also to the acid concentration employed. The oxidation pathway on a catalytically active (i.e. sulfur modified) platinum electrode was determined to be different to the oxidation pathway preferred on an unmodified and inactive platinum electrode. The catalytic scenario on platinum has been examined in detail and is seen to exhibit an oscillatory phenomenon under certain conditions. Analysis of the oscillation determined its electrochemical nature and a dual path mechanism is proposed for the oxidation involving both direct sulfate formation and the intermediate formation of a dithionate species. The reaction pathway was further investigated by comparing performance on platinum to that on gold, the latter previously seen to display a high activity. A gold electrocatalyst does not need to be sulfur modified to display catalytic activity, as is the case for platinum, meaning the substrate is inherently more active. Analysis of the diffusion behaviour and reaction order on these substrates suggested that different limitations control the oxidation on each; however it was proposed that an essentially similar oxidation pathway is followed for an activated electrode. Mass changes were monitored at the electrode surfaces using the electrochemical quartz crystal microbalance and found to support this proposal. The mass build up at the platinum electrode surface during oxidation was attributed to the slow dissociation of intermediate dithionate formed at the electrode surface, while the mass loss on gold suggested a faster desorption of these species and a diffusion limited reaction. Synthesis of results on platinum and gold led to a proposal for the catalytic dependence of the reaction on specific substrates. Activity is explained in terms of alteration of the specific adsorption strength and geometry of reactant sulfur dioxide on the electrode surface, which is influenced by both sulfur formation and electrode character. The adsorption strength of sulfur dioxide has therefore been identified as a key factor in the development of an active catalyst, with weak adsorption resulting in high electrochemical activity while strong adsorption results in a limited oxidation pathway as observed on unmodified platinum. Bimetallic catalysts, previously shown to alter adsorption properties of substrates through electronic or ensemble effects, were then investigated. Promising catalyst combinations screened included the noble and transition metals platinum, gold, palladium, rhodium, cobalt and chromium. These were prepared on both an inactive fluorinated tin oxide (FTO) substrate for fundamental behaviour of the unsupported metal and also as carbon supported catalysts. Significant differences in the oxidation onset potential were observed between the different unsupported catalysts with the PtAu mixture showing a significantly improved onset. However, this activity did not translate to the carbon supported catalysts, which exhibited similar onset potentials and activation, suggesting a limitation by the carbon support. Despite analysis of the bimetallic catalysts and apparent advantages of the unsupported bimetallic mixtures over pure metals, the most active catalyst on a carbon support was found to be gold. This catalyst was tested under saturated sulfur dioxide and increased acid concentration with high activity observed.
- Subject
- solar hydrogen; fundamental; applied; electrochemical investigation
- Identifier
- http://hdl.handle.net/1959.13/1484594
- Identifier
- uon:51380
- Rights
- Copyright 2011 Jessica Alice O'Brien
- Language
- eng
- Full Text
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