Status of the Vanadium Redox Battery Development Programme

The vanadium battery is a redox flow battery system which was pioneered by Skyllas-Kazacos et. al. (1) and is currently under development at the Vanadium Battery Development Laboratory, in the School of Chemical Engineering and Industrial Chemistry at the University of New South Wales.

Redox flow battery systems have been investigated by a number of world government and independent research organisations over the last two decades. An increase in interest in recent years has seen certain systems reaching the demonstration stage. The primary components of the redox flow system are illustrated in Figure 1.

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Figure 1: The redox flow battery system.

A redox flow cell consists of two half-cells, one positive and one negative that are separated by a membrane. To enable electric charge transfer in and out of the system each half-cell contains an iert electrode. The energy is stored in the positive and negative half-cell electrolytes which are pumped around the system. The flow through the redox cell stack is parallel ant the half-cell electrolytes are stored in separate storage tanks. An overview on redox flow batteries has benn presented by Ritchie and Sira (2) and a historical bibliography an their development by Bartolozzi (3).

Energy in batteries is produced when electrons flow between the positive and negative species. In flow batteries this corresponds to the two redox species which have different electrochemical potentials.

In conventional energy storage systems solid state electrode reactions are employed as in the lead-acid battery. In redox flow cells the redox couples are all soluble solution species.

For practical application high currents and voltages are generally required. Redox cell can be stacked in series to increase the voltage and the cells can be electrically connencted in parallel for high currents. In connecting individual cells in series to form a battery stack, bipolar electrodes are employed and flow to the cells is hydraulically in parallel performed by the use of manifolds.

In most redox flow batteries different metal species are employed in the positive and negative half-cell electrolytes as in the case of the iron-chromium (Fe-Cr) redox cell. Results on the research and development of a 10kW class Iron-Chromium redox flow battery were presented byHamamoto et. al. (4). Cross-contamination of the electrolyte can occur by the ions crossing through the membrane separator resulting in a decrease in battery capacity.

The vanadium redox battery employs vanadium ions in both half-cell electrolytes. The V(II)/V(II) redox couple which was investigated by Sum and Skyllas-Kazacos (5) is employed in the negative electrolyte. The positive electrolyte employs the V(IV)/V(V) redox couple which was investigated by Sum et. al. (6). If solution cross-over occurs the vanadium half-cell solutions can be remixed and the system brought back to its original state.

The following half-cell reactions are involved in the vanadium redox cell:

At the positive electrode:

V(V) - e ---¯ V(IV) (discharge) V(V) - e ®--- V(IV) (charge)

E(null) = 1.00 V (1)

At the negative electrode:

V(III) - e ---¯ V(II) (charge) V(III) - e ®--- V(II) (discharge)

E(null) = -0.26 V (2)

At the concentration of 1 mole per litre for each vanadium species and 25degC the standard cell potential is 1.26 Volts. Under operating conditions the actual open-circuit cell voltage obtained at 50% state of charge is 1.4 Volts.

The relatively fast kinetics of the vanadium redox couples allow high coulombic and voltage efficiencies to be obtained but the value of these efficiencies also depends on the internal resistance of the cell.

One of the most important features of redox flow batteries is that by using solutions to store the energy the system power and the energy storage capacity are independent. The vanadium redox battery can therefore be tailored to specific storage applications.

- The solutions have an indefinite life so only the mechanical components need replacement at the end of their life.

- Instant recharge is possible by replacing the spent electrolytes which makes the system ideal for electric vehicle applications.

- The system capacity can be increased by increasing the volume of solution.

- The vanadium battery can be fully discharged without any detrimental effects.

- The cost per kWh decreases as the energy capacity increases, making large scale applications cost effective.

- The system can operate between restricted voltage ranges by the use of trim cells.

- The capacity of the whole system can be monitored in line by monitoring the state of charge of the electrolytes.

- The vanadium battery system is environmentally friendly since no waste products are produced.

A vanadium battery bipolar stack illustrated in Figure 2 showing the individual call stack components.

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Figure 2: Vanadium battery bipolar stack

The individual cell components have all undergone development at UNSW in their own right. Optimisation and manufacturing techniques for large scale commercialisation applications have also been considered for the electrodes and flowframes.

3.1. Electrodes

There are two types of electrodes used in the vanadium battery stack. The end electrodes are used as the first and last electrodes in the stack while the remainder are bipolar electrodes. Studies on fabrication and activation of conducting plastic electrodes were undertaken and presented by Zhong et. al. (7).

The bipolar electrodes consist of an electrically conductive graphite impregnated polymer sheet approximately 0.7 mm in thinckness. To each side of the polymer sheet graphite felt is heat bonded to allow the electrode formed to be a flow-through electrode with a very large contact area.

The end electrodes are similar to the bipolar electrodes except that felt in only bonded to one side and the other side is copper plated to allow unrestricted electrical transport to the copper current collectors.

The main problem with conducting plastic electrodes is to find a material that is nor only an excellent electrical conductore but is also resistant and impermeable to the electrolyte. the electrode must also be of sufficient mechanical strength to be able to tolerate any pressure changes in the stack.

The development of these electrodes has reached the stage where polyethylene base bipolar electrodes and end electrodes with area resistivity as low as 0.6 and 0.8Ohm/square cm respectively have been obtained.

A polypropylene base material has also been sheet extruded and resistivities in the range of 0.5-0.6 Ohm/square cm have been obtained for bipolar electrodes by Haddadi-Asl et. al. (8). Sheet extrusion for electrode preparation provides a uniform thickness of conductive polymer sheet and large quantities can be produced in single production runs at costs as low as $1 per square metre.

3.2. Flowframes

The electrolyte in the vanadium redox battery is introduced into and out of each half-cell by the flowframe. The flowframe determines the cell thickness and also supports the cell structure.

Up until recently the latest vanadium battery stacks were of an external fed design. This simply relates to each cell being fed individually from a common external manifold. The flow frames were made of 5mm polypropylene and 0.8mm neoprene rubber was used for the gaskets.

The polypropylene flow frames were machined out of full sheets of polypropylene and a primary inlet and outlet manifold were heat welded to each flowframe.

In the latest design of the vanadium battery stack an internal flow distribution system has been developed which leads to a more compact and robust stack assembly. The flowframes are injection moulded resulting in a high level of quality control and a much lower cost per flowframe. the initial capital outlay of the flowframe mould will be quickly recovered when commercial production commences.

The flowframe construction material is santoprene and the hardness of this material has been optimised resulting in the elimination of gaskets in the battery stack.

A battery stack assembled using santoprene flowframes has been extensively leak tested. Latest results show that this material provides a means of obtaining a leakproof stack which had previously been a concern.

4. Current Applications and Designs

4.1. Energy Storage - Solar Demonstration House

A solar demonstration house has been built on the grounds of tthe Tha Gypsum factory in Thailand. the house has been designed with total energy self sufficiency in mind. This house employs solar cell arrays on the roof for the collection and conversion of energy and a vanadium battery system for storing the electrical energy collected.

The solar demonstration house is totally energy self sufficient and the vanadium battery is housed in the battery room. The vanadium battery has been used to power the lighting and airconditioning as well as other general appliances.

The specifications of the vanadium battery in this application is set out in table 1.

Table 1: Specifications of the Solar Demonstration House Vanadium Battery:

Number of battery stacks 1

Number of cells 36

Volume of electrolyte per half-cell 200 l total: 400 l

Peak Power* 4.9 kW

Capacity* 13.0kWh

Cell flow distribution design: external

*These values are theoretical values based on a cell resistance of 2 Ohm per square centimetre and electrolytes with a 2M vanadium concentration. The peak power was calculated using a discharge current density of 67 mA per square centimetre corresponding to 100A.

This system has shown promising results for the application of the vanadium redox battery in energy self sufficient housing. If the energy needs of the house increases at a later date the capacity of the system can simpy be increased by adding extra electrolyte. The benefits of such a system in South East Asia are that it will provide reliable power for individual dwellings and commercial buildings.

4.2. Mobile Application - Electric Golf Cart

A commercially available golf cart powered by lead-acid batteries was obtained for the development of a vanadium battery powered golf cart from Deep Down Distributors P/L. The golf cart was originally powered by six 6 Volt lead-acid batteries that were stored under the seat.

The specifications of the vanadium battery designed for the golf cart are shown in Table 2. The actual electrode area of the battery was determined by the mould already develped for santoprene flowframes that are prepared by injection moulding as described above in Section 3.2.

Due to severe budget restrictions for this small demonstration project the electrodes and flowframes had to be fabricated using the same moulds as manufactured for the Solar House battery project.

The size of the flowframes used in the golf cart vanadium battery are for electrodes with an area of 1500 square centimetres. The resulting battery is thus oversizes for the golf cart. The optimum size for a vanadium battery specifically designed for the golf cart would thus be approximately one quarter that used for this initial trial.

Table 2 Specifications of the Electric Golf Cart Vanadium Battery

Number of battery stacks 1

Number of cells 30

Volume of electrolyte per half cell 60 l total: 120 l

Peak Power 4.1 kW*

Capacity 3.9 kWh*

Cell flow distribution design: internal

*These values are theoretical values based on a cell resistance of 2 Ohm per square centimetre and electrolytes with a 2M vanadium concentration. The peak power was calculated using a discharge current density of 67 mA per square centimetre corresponding to 100A.

The 2 pumps to pump the electrolyte around the vanadium battery system were 240V AC and a battery monitor-inverter was developed and used to power the pumps of the battery system. The pumps through the inverter were found to sonsume a current of 7.5A total for both pumps at an operating pressure of 45kPa each.

Preliminary road trials of the vanadium battery powered golf cart have already been undertaken. The golf cart was found to perform exeptionally well carrying two passengers with ease and a total vehicle weight including passengers in excess of 400 kgs.

In the first preliminary road trials the battery voltage for the stationary vehicle with the pumps off was 41.4V. the battery voltage for the stationary vehicle with the pumps on was 38.9V and the battery voltage for the moving vehicle on a flat road was 37.6V.

The vehicle can also be run with the pumps off, running simply of the charge available in the battery stack. This will obviously limit the distance that can be travelled however, in this case the battery voltage only decreased from 41.4V for the stationary vehicle to 40. 7V when the vehicle was moving.

The pumps can therefore be run intermittently to conserve power with the preferred option being the employment of DC pumps. The AC pumps were employed in the current trials due to suitable DC pumps so far proving difficult to acquire.

The vanadium battery powered golf cart will soon undergo endurance testing as well as acceleration and maximum speed trials.

4.3. Back-up Battery - Submarine

Submarine back-up batteries in the present design consist of NiCad cells. There are 2 identical niCad banks each consisting of 20 cells providing 24V.

A vanadium battery system is currently being developed for this application. There are certain major requirements stipulated by the Department of Defence for the vanadium battery system. These requirements are that the battery has the ability to be charged and discharged between 5% and 95% of the rated capacity for a current range of 0-160A while remaining in the voltage range of 22-28V.

The preliminary design of the vanadium battery system comprised of 2 identical banks each formed from two, 20 cell stacks. The two, 20 cell stacks would be electrically connected in parallel. This connection is neccessary to permit the voltage to be in the desired range. Each stack only needs to support a maximum current of 80A when considering that the total maximum current the two stacks need to provide is 160A.

A computer simulation programme was developed to simulate the performance of the vanadium battery in the submarine back-up battery application. The main purpose of this simulation was to detect whether the stipulated voltage range of 22-28V could be met over the current range of 0-160A. The back-up battery is to be connected to the submarine instrumentation continuously even in the charge cycle.

The simulation carried out on one 20 cell stack revealed that the open circuit voltage (OCV) for the 20 cell stack was over 28V and that this voltage and that this voltage was also exceeded when charging at the maximum of 80A on one stack.

Further simulations suggested that one way to overcome the exceeding voltages was to use 19 instead of 20 cells in the stack to bring the OCV within the prescribed voltage range. During charging a tapping cell in the stack could be used that would bring the voltages obtained during charging within the voltage range required. A tapping cell is to be employed at cell 17 in the 19 cell stack and Table 3 gives the specifications for one bank. Table 4 illustrates the stack voltages are in line with the voltage range required over the conditions to be expected during operation of the vanadium back-up battery system.

The use of a tapping cell thus enables the difference between the charging and discharge voltage to be minimised and any variance at the outer extremities of the voltage range may be overcome by further refinement.

Table 3 Specifications of the submarine back-up battery:

Number of battery stacks 2 Stack connection parallel Number of cells (total) 38 Tapping cell (in each stack) Cell No. 17 Volume of electrolyte per half cell 70 l total: 140 l Peak Power 4.2 kW* Cell flow distribution design: internal

*This value is a theoretical value based on a cell resistance of 2 Ohm per square centimetre and electrolytes with a 2M vanadium concentration. The peak power was calculated using a discharge current density of 53 mA per square centimetre corresponding to 80A.

Table 4 Vanadium back-up battery stack voltages under open circuit, discharge with 19 cells and charge with 17 cells. SOC (%) Stack OCV (V, 19 cells) 95% 28.53 50% 25.65 5% 22.77

SOC (%) Discharge voltages at different currents (V) for a 19 cell stack. 80A 40A 5A 95% 26.50 27.51 28.40 50% 23.62 24.64 25.52 5% 20.75 21.76 22.65

SOC (%) Charge voltages at different currents (V) using 17 cells 80A 40A 95% 27.56 26.88 50% 24.99 24.31 5% 22.42 21.74

The vanadium redox flow battery system has undergone optimisation and the manufacture of various components have been streamlined and designed to meet the needs for full scale commercial production.

This system has already been used in a domestic load levelling application and has shown that energy self sufficient housing is not a future possibility but indeed a reality. An electric vanadium golf cart has been completed and initial road trials indicate that this battery system shows great promise for specialised traction applications although further research to increase energy density is required before it can be used in commuter vehicles. Back-up power systems are incorporated in virtually all industries and a vanadium bach-up battery system is currently under development for use in submarines.

The vanadium redox flow battery system has demonstrated an ability to be applied in various energy storage applications. As the development continues more applications will reveal its full versatility and potential.

1. Skyllas-Kazacos M., Rychick M. and Robins R., "All-Vanadium Redox Battery", US Patent No. 4 786 567, November (1988) 1-22

2. Ritchie I.M. and Siira O.T., "Redox Batteries - An Overview", Proceedings of the 8th Biennial Congress of the International Solar Energy Society, Solar World Congress, Vol 3 (1983) 1732-1737.

3. Bartolozzi M., "Development of Redox Flow Batteries. A Historical Bibliography", Journal of Power Sources, Vol. 27 (1989) 219-234

4. Hamamoto O., Takabatake M., Yoshitake M. and Misaki H., "Research and Development of 10 kW Class redox Flow Battery", Proceedings of the 20th Intersociety Energy Conversion Engineering Conference, Vol 2, (1985) 98-104.

5. Sum E. and Skyllas-Kazacos M., "A study of the V(II)/V(III) Redox Couple for Redox Flow Cell Applications", Journal of Power Sources, Vol. 15 (1985) 179-190.

6. Sum E., Rychcik M. and Skyllas-Kazacos M. "Investigation of the V9V)/V(IV) System for Use in the Positive Half-Cell of a Redox Battery", Journal of Power Sources, Vol. 16 (1985) 85-95.

7. Zhong S., Kazacos M., Burford R.P. and Skyllas-Kazacos M., "Fabrication and Activatioin studies of Conducting Plastic Composite Electrodes for Redox Cells" Journal of Power Sources, Vol. 36(1991) 29-43.

8. Haddadi-Asl V., Kazacos M. and Skyllas-Kazacos M. "Conductive Carbon-Polypropylene Composite Electrodes for Vanadium Redox Battery", Journal of Applied Electrochemistry, In Press.

7. Acknowledgements

The development of the vanadium redox battery has been funded by NERDDC, ERDC, NSW Office of Energy and Mount Resources.

Support for the golf cart project was also provided by Pacific Power and Deep Down Distributors P/L. The authors are grateful to Formica Australia P/L for donating the end plates in the battery stacks.