University of Wollongong Research Online University of Wollongong Thesis Collection 2017+ University of Wollongong Thesis Collections 2018 Fabrication and Study of Gortex-based Gas Diffusion Electrodes in Hydrogen-Oxygen Fuel Cells and Electrolysers Prerna Tiwari Follow this and additional works at: https://ro.uow.edu.au/theses1 University of Wollongong Copyright Warning You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorise you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part of this work may be reproduced by any process, nor may any other exclusive right be exercised, without the permission of the author. Copyright owners are entitled to take legal action against persons who infringe their copyright. 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For further information contact the UOW Library: [email protected] Fabrication and Study of Gortex-based Gas Diffusion Electrodes in Hydrogen-Oxygen Fuel Cells and Electrolysers Prerna Tiwari Supervisors: Gerhard F. Swiegers, Klaudia Wagner and Gordon G. Wallace This thesis is presented as part of the requirement for the conferral of the degree: Doctor of Philosophy in Science This research has been conducted with the support of the Australian Government Research Training Program Scholarship The University of Wollongong Intelligent Polymer Research Institute and ARC Centre of Excellence for Electromaterials Science May 2018 Abstract A growing imperative in the modern world is the development of new or improved technologies that: (i) can store energy efficiently, and (ii) deliver electricity on demand, at peak and off-peak times, whilst, still being (iii) compatible with the goal of sustainable development. One promising approach in this respect is to use hydrogen gas as an energy storage medium. Hydrogen can be manufactured from water, by the application of electrical energy, in an electrolyser. Hydrogen can also be converted back into water, with the production of electrical energy, in a fuel cell. However, present commercial electrolysers and fuel cells are too electrically inefficient and too costly to make such a round-trip storage and release of electrical energy, viable. In this work, a new approach to the problem has been studied using a novel class of gas diffusion electrode based on a Gortex substrate. We report the fabrication, characterization, and operation in fuel cells and electrolysers, of gas diffusion electrodes comprising of finely-pored GE PrevailTM expanded PTFE (ePTFE) (‘Gortex’) membranes over- coated with a wide range of precious metal and transition metal / metal oxide catalysts (PTFE=poly(tetrafluoroethylene). The coatings also incorporated carbon black and PTFE binder, with a fine Ni mesh as a current carrier. The fuel cells and electrolysers generally employed aqueous alkaline 6 M KOH as electrolyte, although selected acid systems were also examined. The major findings of this work are summarised below. The key distinctive features of Gortex-based gas diffusion electrodes relative to conventional gas diffusion electrodes, are as follows: • Their gas handling pores are more uniform, more hydrophobic and may be smaller than conventional gas diffusion electrodes. • As a result, they exhibit Water-Entry Pressures (water →gas side) that are ca. 20-times higher than conventional gas diffusion electrodes, which makes them, effectively, “leak-proof” and “flood-proof”. This is a consequence of the water-repellent nature of Gortex. • Their Bubble Point Pressures (gas →water side) are lower; this appears to be a consequence of an affinity by Gortex for gas bubbles (i.e. it is “bubble-philic”). • Because of the above properties, Gortex-based gas diffusion electrodes can, generally, be employed in electrochemical cells without need for an inter-electrode diaphragm / ionomer. This includes in water electrolyzers that normally require such diaphragms / ionomers to separate the hydrogen and oxygen bubbles produced (to avoid the creation of an explosive mixture). • Gortex-based gas diffusion electrodes display high gas permeabilities, that are >2- orders of magnitude greater than would be needed in any electrochemical liquid-to-gas or gas-to-liquid transformation. • Gortex-based gas diffusion electrodes are compatible with acid or base liquid electrolytes. • Gortex-based gas diffusion electrodes may be capable of operating at temperatures of up to ca. 300 oC (which is the limit for expanded PTFE, depending on the backing material used on the Gortex). • Gortex-based gas diffusion electrodes may generally exhibit high surface / active areas with a well-defined three-way, solid-liquid-gas interface. I The major distinctive features of alkaline water electrolysers with Gortex-based gas diffusion anodes and cathodes can be summarised as follows: • The presence of the Gortex substrate generally acts to diminish the bubble overpotential at the electrodes, particularly as the temperature is increased to 80 oC and above. • This may result in direct conversion of water to a product gas at the Gortex-based electrode, without the formation of gas bubbles in the electrolyte. That is, the electrodes may split water in a “bubble-free” manner. • When operated at 80 oC and above, the activation overpotential of an oxygen-generating Gortex-based anode may decline substantially, thereby significant decreasing the cell activation overpotential. Cell activation overpotentials as low as 0.09 V were measured in some electrolysers (2-electrode). • Electrolysers with Gortex-based gas diffusion electrodes may operate below the thermoneutral voltage for water-splitting, in an endothermic manner. Electrolysers (2- electrode) that operated at 10 mA cm-2 at 80 oC for 1 h at voltages as low as 1.23-1.27 V were developed. • At 20 oC, the activity of Gortex-based water electrolysis catalysts generally followed the trends present in their equivalent volcano plots. At 80 oC, the order of catalytic activity may be different. • Electrolysers with earth-abundant catalysts at anode and cathode performed well with 6 M KOH electrolyte. For example, the spinel catalyst, NiCo2O4, displayed a Tafel slope of 70 mV dec-1 at 80 oC for oxygen evolution when incorporated into a Gortex-based gas diffusion electrode. This is low in comparison to Pt in conventional electrolysers. The major distinctive features of alkaline fuel cells with Gortex-based gas diffusion electrodes as anode and cathode can be summarised as follows: • The presence of the Gortex substrate generally imparted fuel cell electrodes with an unusually efficient three-way solid-liquid-gas interface. This was evidenced by: o the ability of Gortex-based anodes to efficiently extract hydrogen gas from highly dilute mixtures (<5%) with the inert gas, methane. 2 o the low Pt loadings (0.16 mg per cm ) that could be used to achieve peak performance. (The power densities and overpotentials at such low loadings were not, however, as high as PEM fuel cells operating at peak performance with notably higher Pt loadings). o the low Tafel slopes exhibited by many of the tested catalysts for oxygen reduction at low current densities (<50 mA cm-2). For example, an Earth- abundant perovskite catalyst displayed a Tafel slope of only 43 mV dec-1, against the standard benchmark of 60 mV dec-1 for Pt in conventional fuel cells. At higher current densities (>50 mA cm-2), Tafel slopes of >100 mV dec-1 were observed that were, nevertheless, lower than the Pt benchmark in conventional fuel cells under the same conditions. 2 o the low charge transfer resistances (RCT) (0.2-0.4 Ω cm ) observed using electrochemical impedance spectroscopy at 20 oC for fuel cells with Gortex- based precious metal catalysts, even under conditions where the fuel was supplied in highly dilute mixtures. The charge transfer resistances were only moderately decreased at 80 oC (0.2-0.3 Ω cm2). The use of perovskite and o spinel catalysts resulted in a greater temperature dependence (RCT at 20 C 0.6- II 2 o 2 0.8 Ω cm ; RCT at 80 C 0.2-0.3 Ω cm ). Under comparable conditions, PEM fuel cells appear to generally display higher charge transfer resistances that are more sensitive to changes in the temperature and concentration of the fuel. o the capacity to mitigate the effect of supplying fuel in high dilutions by merely increasing the overall flow-through rate at which fuel was supplied. • Fuel cells with Gortex-based gas diffusion electrodes as anode and cathode were generally resistant to CO2 poisoning. The major distinctive features of discrete regenerative fuel cell – electrolysers (DRFCs) employing an electrolyser unit and a fuel cell unit, each equipped with Gortex-based gas diffusion electrodes, can be summarised as follows: • DRFCs equipped with Gortex-based gas diffusion electrodes generally displayed unusually high round-trip energy efficiencies. The best result at 80 oC and 10 mA cm-2 in each direction was a round-trip efficiency of 73.5%, assuming complete conservation of heat. DRFCs with round trip efficiencies of 64-72% at 80 oC and 10 mA cm-2 in each direction were obtained with several different catalyst combinations.