Membranes and Electrodes for Vanadium Redox Flow Batteries
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Membranes and Electrodes for Vanadium Redox Flow Batteries by Liuyue Cao M.E. in Material Science and Engineering A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy School of Chemical Engineering The University of New South Wales Sydney, Australia December 2018 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Cao First name: Liuyue Other name/s: Abbreviation for degree as given in the University calendar: PhD School: School of Chemical Engineering Faculty: Engineering Title: Membrane and Electrode Materials for Vanadium Redox Flow Batteries Abstract 350 words maximum: (PLEASE TYPE) An Important Issue that has limited the more widespread deployment of the vanadium redox flowbattery (VRFB) technology to date Is the relatively high capital cost. Economic analysis reveals that a high current design with decreased stack size and low--0ost alternative membranes can dramatically reduce the cell cost. To address this, the thesis has (1) evaluated three alternative commercial ion exchange membranes by experimental measurement and mathematic simulation, (2) studied the vanadium redox reaction kinetics on various electrode surfaces to elucidate the effect of surface area and surface functional groups, and (3) developed two approaches to reduce the activation overpotential of the carbon paper electrodes in "zero-gap" cell architecture for high power densities operation. The main results Include: (1) The chemical and mechanical stability, vanadium penneation rates and cell perfonnance of Fumasep� FAP-450 (anion exchange membrane), Fumasep� F930-rfd (cation exchange membrane) and VB2 (cation exchange membrane) were evaluated in the VRFB. Simulations were conducted with experimental data to predict the capacity loss and thennal behaviour of the cells during long-tenn operation. Of the membranes tested, the cation exchange membrane VB2 was found to be the best in tenns of stability, vanadium penneation rates and cell efficiencies. In addition, the simulation results suggest that low and balanced penneations rates between V'·Not and �N02+ ions are beneficial for reduced capacity loss and heat generation. (2) The reaction of the vo2•1 VO2• redox couple with a focus on the effects of electrode surfaceroughness and function groups produced from the pretreatment on the electrochemical behaviour of glassy carbon in vo2•1 VOt solutions was Investigated. Electrochemically scanning to high potentials leads to both surface roughness Increase and oxygen functional group fonnation, but the depression effect of oxygen functional groups on the electrode activity is more dominant. However, electrochemical reduction to negative potentials can reduce the oxygen functional groups and retain the surface area so that the electro-activity of the electrodes can be recovered or even enhanced. Mechanical polishing Introduces more defects which can act as active sites for the vo2•1 VOt redox reaction. (3) Two simple methods to reduce the activation overpotential of thin carbon paper electrodes In the "zero-gap" VRFB architecture were developed. The MoO3 nanosheets, fanned by either directly decorating onto the carbon paper electrode or Indirectly after adding MoO.2· Into the circulating electrolyte, act as active sites for fast electrochemical reactions especially the V2•N3• redox reaction. Both methods Improved the cell perfonnance greatly and achieve similar voltage efficiencies of 85.4%, 81.2% and 78% at current densities of 100, 125 and 150 mAcm·2. Within the tested current density range, the highest discharging power density was around 200mWcm-2 within the voltage range of 0.6-1.7V. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertationin whole or in partin the University libraries in all forms of media. now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and re uire the a roval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: Dedicated To My Family ORIGINALITY STATEMENT ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ COPYRIGHT STATEMENT ‘I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’ Abstract An important issue that has limited the more widespread deployment of the vanadium redox flow battery(VRFB) technology to date is the relatively high capital cost. Economic analysis reveals that a high power density design with decreased stack size and low-cost alternative membranes can dramatically reduce the cell cost. To address this issue, this thesis has (1) evaluated three alternative commercial ion exchange membranes by experimental measurement and mathematic simulation, (2) studied the vanadium redox reaction kinetics on various electrode surfaces to elucidate the effect of surface area and surface functional groups and (3) developed two approaches to reduce the activation overpotential of the carbon paper electrodes in zero-gap cell architecture for high power density operation. The main results include: (1) The chemical and mechanical stability, vanadium permeation rates and cell performance of Fumasepr FAP-450 (anion exchange membrane, 50 µm), Fumasepr F930-rfd (perfluorosulphonic acid cation exchange membrane, 30 µm) and VB2 (perfluorosulphonic cation exchange membrane, 100 µm) were evaluated in the VRFB. The stability tests were carried out by immersing + the membranes in strong acidic and oxidative VO2 solutions for more than 800 days and measuring the dimensional size and weight at intervals. The three membranes exhibited different degrees of swelling and adsorption of the electrolyte. Membrane permeation measurements indicated the following i Abstract ii 2+ + 2+ 3+ trends for the four vanadium ions: VO >VO2 >V >V for the anion 2+ 3+ + 2+ exchange membrane and VO >V >VO2 >V for the cation exchange membranes. Simulations were conducted with the experimental data to predict the capacity loss and thermal behaviour in the long term. The simulation results suggest that more balanced diffusion rates between the 2+ + 3+ 2+ V /VO2 and V /VO couples can diminish the imbalance during the charge and discharge respectively. Moreover, the trends in permeation rates 3+ + 2+ 2+ + 3+ 2+ 2+ such as V <VO2 <VO <V , VO2 <V <V <VO , or completely op- posite ones are more advantageous in retaining capacity since the transfer of vanadium ions from one-half cell can be reversed at different states of charge. To minimize the heat generation during the standby period with pumps off, low and balanced diffusion coefficients for all four vanadium ions or at least 2+ + V and VO2 ions are required. Of all the membranes tested, the cation exchange membrane VB2 (V-Fuel Pty Ltd, Sydney Australia) was found to be the best in terms of chemical and dimensional stability, vanadium permeation rates and cell energy efficiencies. In addition, the simulation results show that the VB2 membrane which possesses low and balanced permeation rates 2+ + 3+ 2+ between V /VO2 and V /VO ions has the lowest capacity loss and heat generation from self-discharge reactions. 2+ + (2) The kinetics of the VO /VO2 redox reactions were investigated on glassy 2+ + cabon in VO /VO2 solutions with a focus on the effects of electrode surface roughness and surface function groups produced by electrode pre-treatment 2+ + on the electrochemical behaviour of glassy carbon in VO /VO2 solutions was investigated. Electrochemically scanning the electrode to high potentials leads to the increasing surface roughness and formation of oxygen functional groups.