Zinc-air batteries for the grid
Until now, zinc-air batteries have mostly been used as small disposable batteries, for example for hearing aids. The electrochemical reaction of zinc with atmospheric oxygen offers, however, considerable potential for building rechargeable storage systems with a high energy density for large energy volumes. Until now it has not been possible to implement it technically. In the “ZnPLUS” project, scientists want to utilise new concepts and components to develop an economic zinc-air energy storage system in the megawatt range.
|Project status||Project finished|
|Project duration||September 2012 until August 2015|
Zinc-air energy storage systems are especially suited for stationary applications due to their high specific energy content (1100Wh/kg), low raw material costs, sufficient global availability of technical grade zinc, high operational safety and environmental compatibility (complete recycling of the active materials zinc, silver and potassium hydroxide is possible, no toxicity to humans). They compare favorably to lead-acid or nickel-cadmium systems that use toxic active materials as well as redox flow battery systems that use the rare element vanadium. They could be used for energy management of industrial sites, integration of decentralized power facilities and tertialy grid regulation.
Concepts for large rechargeable batteries
The advantages of Zinc-air energy storage systems have not yet been realized in rechargeable systems on a large scale due to an insufficient number of charge/discharge cycles caused in many cases by dendrite formation on the zinc anode and subsequent shortcircuiting of the cell.The goal of ZnPLUS is the systematic scientific evaluation of several promising concepts for stationary zinc-air systems to store large energy amounts. After three years at the completion of the project a choice for the most promising design will be made and the scale-up in a technical demonstration scale may be carried out in a subsequent project.
The critical properties of zinc-air systems that have caused failure in earlier R&D efforts are the stability of the gas diffusion electrode, dendrite growth on the zinc anode, the stability and regeneration of the zinc slurry, the cell geometry and the battery management strategies. The ZnPLUS concept will use a parallel development and optimization of electrocatalysts, cell components and cell geometry based on rheological and electrochemical simulations combined with experimental validation.
Test cells undergoing benchmarking
Five different cell types are currently under construction. Standard components will be installed initially in order to confirm the benchmark performance. The improved components such as gas diffusion electrodes, zinc slurries and structured current conductors that will be developed during the project will then be incorporated in the various cell designs and evaluated. The cell geometry will be further refined based on model and simulation calculations to optimize the cell performance profile. A selection of the best system will be made at the end of the project. This system may then be tested in a larger scale in a subsequent project.
ZnPLUS will provide an extensive evaluation of five cell design concepts with validated assessments on their cycling stability, storage capacity and performance level. Potential efficiency (VE), current efficiency(CE) and energy efficiency (EE=VE*CE) are three parameters that will be evaluated and will provide suggestions for component improvements and a selection between cells with massive or slurry anodes as well as cells with two or three electrode arrangements. The following performance specifications should be met in the laboratory scale:
- 60% energy efficiency,
- 4 kA/m² current density during discharge,
- at least 500 cycles without performance drop
Supporting economic analysis
The economic viability of a stationary energy storage system is together with energy efficiency and reliability a critical success factor. ZnPLUS will provide an ongoing economic evaluation of all project cell concepts including investment and operational costs as well as benchmark data in comparison with other competing storage technologies.
Technology Benchmarking, Market Analysis and Economical Evaluation
Bayer MaterialScience AG (BMS) is the project coordinator and evaluates the project concepts together with the other industrial partners Grillo and ThyssenKrupp Uhde as to their scalability and economic viability. BMS provides its' proprietary oxygen depolarizing cathode (ODC) as a benchmark for the further development of gas diffusion electrodes. BMS supports the catalyst development work for oxygen reduction and oxygen evolution electrodes at the University of Saarland (UdS). The new catalysts will be processed in test electrodes and tested in charge and discharge experiments.
Massive Zinc Electrodes
Grillo Werke AG (GRI) develops and characterizes the zinc anode. Different Zn-alloys as well as zinc powders with varying particle size distributions and shapes will be tested regarding their rechargeability. GRI will develop special alloys to minimize dendrite formation during charging thus reducing the cell tendency to discharge via micro-shortcircuiting. GRI optimizes zinc powders and develops zinc slurry formulations with improved rheological characteristics. GRI will also develop recycling concepts for zinc anodes and zinc slurries.
Zinc Slurry and Current Conductor
The Center for Fuel Cell Technology (ZBT) is responsible for flowstream simulations and measurements on slurry cells as well as the development of a segmented test cell that will allow spatially defined current-voltage measurements. The models and data thus generated will be useful in the evaluation and optimization of the structured current conductors and the rheological properties of the zinc slurries.
The Energy Technology Laboratory of Duisburg-Essen University (UDE)will use its' long experience in fuel cell and battery technology research to develop and optimize zinc slurry cell design and the current conductor used therein. Long term cycling tests will be carried out at UDE to evaluate the performance reliability of the various cell designs.
Cell Design and Testing
The Institute of Chemical Process Technology at Clausthal University of Technology (TUC) is focussing on cell designs with plate anodes and separate electrodes for charging and discharging the battery. The performance profile of such cell systems will be investigated by varying different operational parameters such as temperature, oxygen concentration, and electrolyte concentration. The efficiencies during charge-discharge cycles will be also determined. TUC will use the experimental data to develop an improved electrochemical model for the zinc-air cell together with Niederrhein University of Applied Sciences (HNR). This model can then be used for battery optimization and scaleup.
Saarland University (UdS) will develop catalysts for oxygen reduction (ORE) and oxygen evolution (OEE)electrodes. Perovskites and Spinels will be investigated for the oxygen reduction reaction, whereas new oxide materials will be tested for oxygen evolution. Catalyst synthesis will be performed using sol-gel or electrodeposition techniques. The catalysts will be characterized electrochemically, chemically and structurally. Catalysts with promising activity will then be provided to BMS, TUC and UDE to be tested in gas diffusion electrodes.
Process Modelling and Simulation
Niederrhein University of Applied Sciences (HNR) will support the other partners to define the critical parameters influencing cell performance by using statistical planning of experiments and developing validated numerical models that can be used for simulation of experiments. First, already existing electrochemical and rheological models will be adapted to predict experimental results for cell performance. Second, methods of multivariant optimization and sensitivity analysis will be used to maximize the electrochemical efficiency of the zinc-air cells. Thirdly, further numerical optimization steps will be used to improve system properties and develop battery management strategies.