Patent Application
20190115609
Not applicable
The intermittent nature of energy supplied by photovoltaic and wind electricity generation has increased the need for energy storage that allows energy to be stored and retrieved on demand. Batteries provide a convenient mode of energy storage that can be located close to the site of generation. However, batteries can suffer disadvantages associated with their high cost, temperature sensitivity, and insufficient service or cycle life. In addition, as batteries are scaled up to meet the energy storage demands of medium scale applications of tens to hundreds of kilowatt hours there are concerns about the safety of these systems, particularly when the negative electrode incorporates lithium ion technology. Aqueous flow batteries have been developed as a possible solution to the large scale storage of energy. By using active material stored external to the electrode stack they have been proposed as a low cost and readily scalable solution to large scale energy storage. The physical separation of active materials in so called flow type batteries is also a significant factor in improving the safety of the systems for use in a home environment. However, some of these batteries have used toxic halogen materials while other systems have used expensive transition metals such as vanadium or chromium. In general, the costs and the system lifetimes have not met expectations. This is particularly true in kilo Watt hour scale systems that must maintain high performance, even as the increased size of the system dictates more complex electrochemical engineering solutions. In this specific application there is also a need for higher volumetric energy density than is normally associated with medium sized redox flow batteries. In efforts to control costs there has been a recent emphasis on organic redox couples in aqueous media that have the potential for high cycle life and low cost. However, the solubility limitations and relatively low voltages are significant drawbacks to the viability of the systems.
The combination of a high energy density, high cycle life solid battery electrode with a redox flow system separated by an ion exchange membrane can address many of the shortcomings of currently proposed large scale energy storage systems. In particular, the combination of an alkaline nickel hydroxide positive electrode with a quinone based negative redox flow electrode can deliver voltages in excess of 1.2V and can reduce the footprint required within a home by incorporating only one redox flow component per battery or module. For medium sized systems, this can reduce the losses due to shunt currents and electrolyte pumping. In addition this hybrid redox flow battery has high power capability over a wide temperature range since the electrolyte is a highly conductive aqueous hydroxide solution with a wide operational temperature range.
The system may be configured to be limited by the positive nickel hydroxide electrode when maximum energy density and lowest cost is required. For maximum round trip energy efficiency the nickel electrode can operate between 80% and 0% state of charge. Under these conditions the coulombic inefficiency associated with oxygen evolution and fully charging the nickel electrode is avoided. A ready calibration of the end of charge point does not require a sophisticated battery management system since the tank containing the redox flow materials can be monitored using a simple electrochemical reference electrode system that can output voltage values to deliver an accurate and continual state of charge of the system. Alternatively, spectroscopic means could also be used to provide a digital output for charge and discharge control.
A single cell can be set up as in
According to Antelman the redox potential of the nickel hydroxide active material vs. the standard hydrogen electrode is +0.49V. The redox potential of 2, 6 dihydroanthraquinone is determined by Lin et al. as approximately −0.68V. The open circuit voltage of this particular combined system is therefore 1.17V. Using anthraquinones with additional electron donating groups can significantly increase the open circuit voltage by depressing the redox potential of the negative electrode.
A significant advantage of using nickel oxide as the positive electrode is the lack of sensitivity of the open circuit voltage to the state of charge of the electrode.
The sizing of the reservoir of alkaline anthraquinone is determined by the Ampere hour capacity of the nickel hydroxide electrode such that there is at least a 20% coulombic excess Ampere hour capacity associated with the organic electroactive material. This assumes two electrons transferred in the electrochemical reaction for each organic molecule.
During the first formation charge, the 2, 6 dihydroanthraquinone is converted to the reduced form and the nickel hydroxide is converted to the oxidized NiOOH material. Charge Is passed until the nickel hydroxide is fully charged. This will require approximately 20% overcharge with the associated evolution of oxygen which will escape through a low pressure vent inserted into the electrolyte fill hole at the top of the positive electrode compartment. The first discharge will completely discharge the positive electrode and future charging will be carried out to 80% of the positive electrode available capacity. The end point will be determined by the redox potential of the oxidized and reduced material in the negative electrode reservoir. Cycling will then be carried out between 0-80% state of charge on the positive electrode. The 80% state of charge value has been chosen to maintain high coulombic charge efficiency by avoiding the oxygen evolution reaction that occurs at higher states of charge on the nickel oxide electrode. Similarly, it is important to avoid the evolution of hydrogen at the negative electrode by the suitable choice of organic reactants and the solid electrode substrate material. The latter must be free of material that can evolve hydrogen with low overvoltage such as iron, cobalt or nickel. The organic reactant should have reversible kinetics on the substrate and not exhibit a redox potential so negative as to evolve hydrogen during charge or during open circuit stand.
The nickel electrode used in example 1 is formed by impregnating a nickel foam substrate with nickel hydroxide active material in the form of an aqueous paste. The electrode is dried and compressed prior to use in the electrochemical cell. During use, the electrode undergoes some expansion particularly during overcharge. This can result in a loss or degradation of capacity. Physical constraints can be applied to reduce this expansion but this can result in a loss of volumetric energy density. The avoidance of overcharge described in example 1 can significantly moderate the problem. However, the use of more expensive but more dimensionally stable sintered plate nickel hydroxide electrodes can significantly improve the capacity stability. These electrodes are commonly used in aircraft starter nickel-cadmium batteries and are produced by electrochemically or chemically impregnating a powdered porous nickel plaque with nickel hydroxide.
In order to supply higher voltages normally required by devices or loads, the single cell must be arranged electrically in series to provide multiples of the single cell voltage. This can be achieved using single cells connected by external bussing. However, it is more efficient to connect the cells in a bipolar arrangement where the electrical connection between adjacent positive and negative electrodes is achieved through a common electronically conducting “cell” wall as shown in
Provisional Application No. 415,402 titled “An Alkaline Hybrid Redox Flow Battery with High Energy Density” filed on Oct. 31, 2016 U.S. Pat. No. 9,614,245 B2 Narayan et al.U.S. Pat. No. 9,293,777 B2 Wang et al.US 2017/0237091 A1 Zhang et al.
