VRFB Development highlights
Initial work on the Vanadium Redox Flow Battery (VRFB) at UNSW began. While other researchers (NASA) had previously proposed the use of vanadium redox couples for redox cell applications, this was previously believed to be impractical due to the very low solubility of V(V) compounds which would have restricted the concentration of the vanadium electrolyte to a level much too low for practical use. The UNSW breakthrough came when it was discovered that highly concentrated V(V) solutions could be prepared in sulphuric acid. They found it was possible to prepare a highly concentrated solution which, unexpectedly, did not precipitate over a reasonable temperature range. This meant that reasonable vanadium solution concentrations could be achieved for practical systems.
The technology was taken from the initial concept stage through the development and demonstration of several 1-4 kW prototypes in stationary and electric vehicle applications over a 15 year period at UNSW. A further milestone in the UNSW R&D program, was the development of a low cost process for producing vanadium electrolyte from the vanadium oxide raw material.
Generation I Invention of the first all-vanadium redox flow cell by Prof Skyllas-Kazacos and co-workers back in 1985 at the University of New South Whales in Sydney, Australia. The UNSW Vanadium Redox Flow Battery technology is a proven, economically attractive and low-maintenance solution, with significant benefits over the obsolete lead-acid battery technology. With a global focus on climate and adoption of cyclic renewable energy generation sources such as wind and solar continues to increase, the demand for large scale energy storage technologies is growing dramatically.
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The redox-flow battery differs from the usual storage battery in that the energy-bearing chemicals are not stored within the battery container, but are in a separate liquid reservoir(s).
The system is very simple (Fig. 1); it consists of two tanks, each containing an active species in different oxidation states. Each fluid passes in a half-cell (a membrane divides the entire cell) and is returned to the reser voir. In the half-cell there is electrochemical exchange with the electrodes which permits the output or input of current, for example, Fe3+ + e- = Fe2+
and Cr2+ - e- = Cr3+. Usually the four species (ions) are in solution and some times gaseous elements are involved.
In other cases metallic species are deposited in one half-cell. Such sys tems are referred to as being "hybrid" or "redox-hybrid".
In 1971, Ashimura and Miyake published in Denki Kagaku a paper dealing with the polarization characteristics of redox type fuel cell cathodes at a flow-through porous carbon electrode . Later they also studied the regeneration of Fe3+/Fe2+ catholyte in the sulphuric acid of a redox-type fuel cell, and the polarization characteristics of a redox-type fuel cell cathode at a flow-through porous carbon electrode [4, 5].
Fig. 1. Electrically rechargeable redox-flow cell
NASA (National Aeronautics and Space Administration, U.S.A.) founded the Lewis Research Center at Cleveland, Ohio, U.S.A. with the object of researching electrically rechargeable redox flow cells.
NASA also placed development contracts with Exxon Company (Linden, NJ, U.S.A.) , Giner Ind. (Waltham, MA, U.S.A.) , and with Gel Inc. (Durham, NC, U.S.A.) for the hybrid version, and with Ionics Inc. for membrane development.
The attractive features of an electrically rechargeable redox flow system are:
(a) simple electrode reactions,
(b) favorable exchange currents (for some redox couples);
(c) no high temperatures required;
(d) no cycle life limitations (for the redox couples);
(e) electrochemically reversible reactions (some redox couples); (f ) very high overall energy efficiency.
Their main disadvantage is a low energy density in comparison with the more usual secondary battery systems.
From 1973-1979 the Lewis Research Center produced several reports in this field, covering the topics:
(i) screening of redox couples
(ii) electrochemical diagnostics
(iii) kinetic problems
(iv) membrane development
(v) electrodes (graphite, graphite felt, reticulated vitreous carbon, carbon foam, carbon cloth)
(vi) component screening and life testing
(vii) system studies
The reports included an electrically rechargeable redox flow cell ; electrochemical behavior of 0.2 - 0.3 M ferrous chloride-ferric mixtures on edge-on pyrolitic graphite rotated disk electrodes ; redox flow cell development and demonstration project -calendar year 1976 report - (10) the redox flow system for solar photovoltaic energy storage ; factors affecting the open-circuit voltage and electrode kinetics of some iron/ titanium redox flow cells , and redox flow batteries , by Thaller, Gahn, Miller and others. Many elements were tested, titanium (Ti3+/Ti02+), iron (Fe2+/Fe3+), chromium (Cr2+/Cr3+). Of these, the iron-chromium system
(Fe3+ + Cr2+ / Fe2+ + Cr3+) seemed to be the most promising for the global reaction and much effort was expended in this direction.
During this period we also find contributions from Beccu (Battelle Institute, Geneva, Switzerland) , who examined the redox systems with regard to secondary batteries, Warshay, who estimated the costs of electrochemical energy storage , Weaver - The G.E.L. iron redox cell: a report of initial cycling tests - , and from Zito (The iron-redox battery in a large solar-photovoltaic application) 
Roy and Kaplan (Oak Ridge National Laboratory, Tenn., U.S.A.) analysed the performance capabilities of redox-flow storage in 1979 . In this period there were also some NASA patents on electrically recharge able redox flow cells , on gels as battery separators , and on an electrochemical cell for rebalancing a redox flow system 
The characteristics of a "soluble" iron/titanium battery system were studied by Savinell et al. (Department of Chemical Engineering, Pittsburgh, Penn., U.S.A.) , while improvements in redox flow cell storage systems are proposed by Thaller for NASA [23, 24].
Roberts (Mitre Corporation, McLean, VA, U.S.A.) reviewed the status of the Department of Energy program on Electrochemical Storage Systems in a report issued in August 1979 
In early 1980 a patent was granted to the Italian 0. De Nora Electro chemical Plants (Milan, Italy) on an electric storage battery  and another to Giner Inc. on catalyst surfaces for the chromous/chromic redox couple  in the iron-chromium flow cell.
Some evaluations of redox systems (chromium and titanium) were made by Nozaki and coworkers  of the National Electrotechnical Laboratory (Tokio, Japan). The G.E.L. Inc. was granted a patent on the iron hybrid flow cell 
A new report from NASA  described a redox system based on Fe and Cr chlorides redox couples. A little later NASA was granted a patent on gels as battery separators for soluble electrodes  and another con cerning improvement and scale-up of the NASA redox storage system (up to 1kW) 
NASA presented a pre-prototype redox storage system, based on iron-chromium, for a photovoltaic stand-alone application 
Some contribution was made by the University of Akron (OH, U.S.A.) when Savinell investigated factors affecting the performance of the iron redox battery , the enhancing performance of the titanium(III)/ titanium(IV) couple for redox battery applications , and also evaluated a hybrid redox-halogen (Cr-chlorine) for use in an energy storage device 
Chen (Department of Chemistry, Austin, TX, U.S.A.) published work on redox couple iron(IIl)-iron(II) complexes with o-phenanthroline 
In February 1981, an extensive report was prepared by Nanis (Electro chemistry Group, SRI International, U.S.A.) for the Electric Power Re search Institute (Palo Alto, CA, U.S.A.). This concerned work which was presented at a workshop on electrodes for flowing solution batteries at Tampa, FL, U.S.A., in November 1979 
A theoretical study, comparing flow-through and flow-by porous electrodes for redox energy storage, was published by Trainham and New man (Department of Chemical Engineering, Berkeley, CA, U.S.A.) . Also in 1981 Dol signed a French patent on redox batteries with couples of iron and manganese ions 
Advances in membrane technology for the NASA redox energy storage systems were studied by Ling of Lewis Center 
Stalnaker (NASA) presented a report in which he described stacks, each consisting of forty active cells, for a 1kW pre-prototype system (Fe Cr) 
A report from Italy by Buzzanca (Cise, Milan, Italy ) analyzed redox flow batteries . Hagedorn analyzed redox storage systems, in particular iron and chromium chloride redox couples, specifically for solar applications and announced the construction and testing of a 1 kW system integrated with a solar photovoltaic array 
In January 1982, two Japanese patents by the Agency of Industrial Sciences and Tchnology were issued. The first was for a manganese (cathode) - chromium (anode) system . The second was for chromium or titan nium coupled with bromine, for which a battery output of 0.9 V at 10 mA cm-2 was claimed 
Nozaki and coworkers (Energy Division, Electrotechnical Laboratory, Ibaraki, Japan ) presented test results and scale-up of systems containing Fe2+/Fe3+- Cr2+/Cr3+ and a screening of over forty types of carbon fibers as a potential cathode material in a report by Roberts (279 pp.) on the status of the DOE battery and electrochemical technology program, redox and Zn-Br batteries were reviewed 
Giner Inc. presented data at the Dechema meeting in 1982, in partic ular the development of Cr3+/Cr2+ redox reactions in the iron-chromium battery . Savinell et al. described the operating performance of an iron-titanium stationary redox battery in the presence of lead 
Catalytic electrodes (ZrC electrodes) for redox flow cell energy storage devices were studied by Yang (51] of the Department of Energy, Brook haven National Laboratory (New York, U.S.A)
Oei, a researcher of the Ford Motor Company (Dearborn, MI, U.S.A.), published exploratory experiments with redox cells utilizing VO/V02+- Sn2+/Sn4+; VO/VO2+- Fe2+/Fe3+ redox couples 
A singular way of recharging redox batteries was proposed by Denno (Institute of Technology, NJ, U.S.A.) utilizing ocean thermal energy 
An extensive report (314 pp.) on solution redox couples for electro chemical energy storage was written by Chen (University of Austin, TX, U.S.A.) 
A flowing electrolyte battery was presented by Butler (Sandia Labora tory, NM, U.S.A.) at the 17th IECEC (1982) (55]
At the beginning of 1983 a patent was granted to Savinell on a chrome halogen energy storage device . Nozaki continued to work on a flow type secondary battery using Fe2+/Fe3+ and Cr2+/Cr3+ 
A patent from NASA on zirconium carbide as an electrocatalyst for the chromous/chromic redox couple appeared in 1983 .
A contribution to European research was made by Cnobloch (Siemens A-G, Erlangen, F.R.G.) who studied a redox battery based on Fe3+/Fe2+ and Cr2+/Cr3+ 
The development of a circulating zinc-bromine battery was described by Bellows (Exxon Research, Linden, NJ, U.S.A.) .
At the 18th I.E.C.E. Conf . Nozaki described a 1kW redox-flow battery of 96 bipolar cells for which seventy varieties of carbon fiber electrode ma terials had been screened  At the same meeting Gahn discussed a small cell using mixed reactant solutions at 65 °C, with a chromium electrode catalyzed with Bi or Bi-Pb . Butler and coworkers (Sandia Laboratory, Albuquerque, NM, U.S.A.) presented an evaluation of zinc-bromine pro totype batteries 
Studies of the iron-chromium redox cell, and the development of an efficient electrode for Cr3+/Cr2+ redox reactions were presented by Giner (Giner Inc., Waltham, MA, U.S.A.)  A mathematical model of NASA's redox flow cell was described by Watts and Fedkiw (Department of Chemical Engineering, Raleigh, NC, U.S.A.) 
Iron-titanium redox and hybrid iron secondary batteries was the title of a report by Bartolozzi and coworkers (Department of Chemical Engi neering, Pisa, Italy ) 
An improved mathematical. model of the iron-chromium redox battery was made by Fedkiw. This was based on the porous electrode theory and incorporates redox kinetics, mass transfer, and ohmic effects, as well as the parasitic hydrogen reaction which occurs in the chromium electrode 
Nozaki again described the constructed and tested 1kW system  Yeo (Pinnacle Institute, Cupertino, CA, U.S.A.) analysed the economics of redox cells including Zn-Cl, Zn-Br, H-Cl, H-Br, Fe/Cr redox, Fe/Fe redox, Zn/ferricyanide redox, acid Fe/ Zn redox and Cr/Cl 
Fouling mechanisms of separator membranes for the iron-chromium redox battery have been discussed by Assink (Sandia Lab., Albuquerque, NM, U.S.A.) . Bartolozzi and Marconi presented an experimental study using a model of an iron hybrid cell 
A review, with sixteen references on redox batteries for solar energy storage, was prepared by Ritchie (Department of Chemistry, Nedlands, Australia)  The discharge performance of the titanium/iron redox flow system was studied by Wang (Department of Chemical Engineering, Hsinchu, Taiwan)  Nozaki presented a description of the complete system (tanks, pumps, piping and flow-control units) of the 1 kW cell using iron-chromium elec trolytes 
Many prototypes, zinc/ bromine; redox; zinc/ferricyanide flowing electrolyte batteries and cells, were discussed by Butler at the I.E.C.E. Conf . At the same meeting Nozaki also gave more information on his unit consisting of thirty bipolar cells with an electrode area of 3000 cm 2 . A patent relating to redox batteries, including bromine and chromium, was granted to Giner (Electric Power Research Institute Inc., U.S.A.) 
Voltage drop and electrical resistivity measurements of ion-exchange membranes used in redox-flow batteries were studied by Ohya (Faculty of Engineering, Yokohama, Japan )  Remick (Institut of Gas Technology, U.S.A.) patented a cell using sulfide-polysulfide as anolyte and chloride-chlorine as catholyte . Reactivation of complex compounds by amines participating in the redox reaction was the subject of a German patent by Cheng and Reiner 
The problem of reactivation in iron-chromium redox cells was inves tigated by Cheng (Institute of Chemistry, Peking, Peoples' Republic of China) and coworkers  A general energy balance for battery systems was described by Bernardi and Newman (Department of Chemical Engineering, Berkeley, CA, U.S.A.)  Sumitomo Electric Industries (Japan) patented a general system of redox flow batteries , with a separator consisting of a cation-exchange membrane, in January 1985 Cheng and Hollax patented an iron-chromium type redox battery using thallium and gold for electrocatalytic acceleration of the processes . Siemens A-G (F.R.G.) patented a redox cell (iron-chromium) with graphite electrodes catalyzed with gold and lead . The chromium complex in the Fe-Cr redox energy storage system was identified by John son (Department of Chemistry, Spring Arbor, MI, U.S.A.) . The Mitsui Engineering and Shipbuilding Co. Ltd. (Japan) patented a system for the determination of the state-of-charge of a redox battery  and general techniques for the construction of a redox cell . The status of storage battery development and potential applications was studied by Mathur (Institute of Electrochemistry, Karaikudi, India) 
Ohya (Faculty of Engineering, Yokohama, Japan) published studies involving factors influencing the electrical resistivity of anion exchange membranes used in a redox-flow· battery . Mitsui patented some redox batteries containing Fe, Cu, Sn, Ni or halogens . A study of the vanadium(Il)/vanadium(III) redox couple for flow cell application was made by Sum (Department of Chemical Engineering, Kensington, Australia) . At the 20th l.E.C.E.Conf , Miami, CA, U.S.A., August, 1985, Kanazashi (Meidensha Electrical Mfg. Co. Ltd., Tokyo, Japan) discussed the perfor mance of a 1kW Zn-Br battery 
The cycling performance at 65 °C of the iron-chromium redox energy storage system was investigated by Gahn (Lewis Center, NASA, Cleveland, OH, U.S.A.) 
Results of research and development of a 10 kW-class redox flow battery were presented by Mitsui 
The iron-chromium (graphite electrodes) redox battery was studied by Aldaz (Department of Chemistry, Alicante, Spain) 
Sources: University of New South Whales, Journal of Power Sources