Proton exchange membrane electrolysers (PEMEs) constitute the state-of-the-art in electrolytic water splitting, capable of producing H2 at high current density and efficiency. However, when operating at low current or under increased pressure, H2 and O2 can cross through the membrane which separates the electrodes, creating a potentially explosive gas mixture and reducing the efficiency. Electron-coupled proton buffers (ECPBs) provide a solution to the issues of gas crossover in electrolytic water splitting, by breaking up the oxygen and hydrogen evolution reactions (OER and HER) into two separate steps. This provides benefits to gas purity and improves the operational safety of the electrolyser. Initially, ECPBs were limited to polyoxometalate (POM) compounds, which contain expensive transition metals and have high molecular weights. The use of a cheap organic species as an ECPB was later introduced using hydroquinone sulfonate (HQS); however, this compound was found to be unstable under extended redox cycling, making it unsuitable for practical applications. This thesis details the examination of a host of organic molecules for use as ECPBs, and their development into practical PEME systems. In the first section of work, several classes of organic compounds were investigated to determine their suitability for different modes of ECPB operation. This included anthraquinone-2,7-disulfonic acid (AQDS), which was found to be exceptionally stable under extended redox cycling, providing a lifetime 17 times greater than the previously published HQS. Biphenyl tetrasulfonic acid (BPTS) was found to have a redox potential of 1.037 V vs. the standard hydrogen electrode (SHE), and was subsequently used in a photoelectrochemical cell (PEC). This PEC was able to operate at currents of 0.9 mA∙cm−2 under 1 Sun illumination with zero applied external bias, and the removal of H2 production from the cell eliminates the possibility of explosive gas combinations forming. A sulfonated viologen molecule ((SPr)2V), was found to have a redox potential of −0.392 V vs. SHE, making it capable, in theory, of evolving H2 spontaneously from the reduced solution. Unfortunately, this was not possible due to the chemical stability of the compound. Organic molecules with high pKa functional groups were then investigated at high pH, in the hope of identifying the first ECPB for alkaline water splitting. Although no suitable molecule was identified in the course of this research, the work detailed here provides a solid foundation for future studies. In the second section of work, an ECPB-mediated PEME cell was developed for the first time. This system utilised the AQDS molecule identified in the first section, implementing it into a dual cell PEME which produced H2 and O2 in separate locations. This electrolyser was shown to operate at a similar level to a conventional PEME (in excess of 1.5 A∙cm−2), while producing H2 at higher purity and without cross contamination of the product gases. Through operating the cells independently of one another, H2 was able to be produced at current densities of up to 3.71 A∙cm−2 at 2.0 V. In the third section, similar systems were constructed using the polyoxometalate ECPBs, phosphomolybdic acid (PMA) and silicotungstic acid (STA). These systems were developed to a similar level as the AQDS electrolyser, before being directly compared in terms of performance and cost. Although the conventional PEME was found to have the highest voltage efficiency of the four systems (78.65% at 1 A∙cm−2), it was also found to have the lowest Faradaic efficiency (92.82%), and was the only system examined where crossover of the product gases was observed. Of the three ECPB-based systems, the AQDS PEME was found to have the highest voltage efficiency (54.70% at 1 A∙cm−2) and Faradaic efficiency (>99%), as well as being able to operate at the highest current densities. A mole-for-mole cost-comparison of the different ECPBs revealed AQDS to be just 2.11% and 1.02% of the cost of PMA and STA, respectively. In the final results section, the AQDS PEME was adapted so that the AQDS oxidation cell provided an energy output instead of producing H2, thereby moving away from water splitting and towards electrochemical energy storage. The system developed here is a hybrid between redox flow battery and fuel cell technology, utilising a rechargeable liquid electrolyte alongside the H2O/O2 redox couple. Charging the device proceeded in the same manner as O2 evolution in the AQDS PEME, but the subsequent oxidation of AQDS was then coupled to O2 reduction (forming H2O) instead of proton reduction (forming H2). The system was developed to produce a maximum power density of 124 mW∙cm−2, with a great deal of scope for further improvements.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:754422 |
| Date | January 2018 |
| Creators | Kirkaldy, Niall |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/30842/ |
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