Greater current density for zinc-air batteries
Rechargeable zinc-air storage systems have yet to achieve satisfactory charge and discharge cycles. This provides the starting point for the ZnPLUS project: researchers have increased the current density by using a modified zinc slurry. This makes the batteries easier to maintain and improves their performance reliability – with simultaneously lower costs.
Zinc-air batteries offer many advantages as stationary storage systems. These include a high specific energy content (1,100 Wh/kg) and the low costs of the active materials. Zinc-air batteries are environmentally friendly because the active materials silver (Ag), zinc (Zn) and potassium hydroxide (KOH) can be completely recycled. In addition, these materials are non-toxic to humans. Zinc is also sufficiently available and has advantages in terms of safety. The technology competes with lead-acid and nickel-cadmium batteries – both of which contain toxic materials. Another competitor are redox flow systems, which are still in development. However, the vanadium required for them is scarce.
Until now, it has not been possible to realise the aforementioned advantages with rechargeable zinc-air energy storage systems. The main weakness of the zinc-air batteries is that they do not simultaneously have a high capacity and cycle stability. In the ZnPLUS research project, scientists have therefore developed a promising concept for stationary, rechargeable zinc-air batteries for electrochemically storing large amounts of energy. These are intended to be used, for example, for the efficient and economical energy management of chemical plants, for decentralised power supply units and for tertiary grid control, i.e. for providing minutes reserve. According to the researchers, they have managed to create a competitive cell design that is scalable up to the megawatt range.
Modular cell design
The modular cell system makes it possible to characterise different operating concepts for zinc-air batteries – such as separate discharge cells, separate charge cells and the operation in a three-electrode arrangement with interchangeable components. For the discharging and charging current collectors, materials have been determined that have a high efficiency in discharge mode and low overvoltage and zinc build-up when charging.
At Clausthal University of Technology, extensive investigations of the passivation phenomena were conducted on cells with solid zinc electrodes and optimum operating parameters for the cell discharging were determined. Through the use of metal foams as the carrier structure for the zinc, the cells with solid zinc electrodes achieved power densities equivalent to slurry cells. A stationary physicochemical model developed by Clausthal University of Technology explains the basic mechanisms. It is used to optimise the cell design and select the operating conditions.
Improved zinc slurry
In order to increase the performance of the preferred Zn slurry cell, the project partners improved the slurry formulation. First of all they developed a manufacturing method that enabled all slurry components to be processed homogeneously in order to then produce and evaluate 120 slurry formulations. The result of the test evaluations: They managed to identify the main parameter correlations and develop analogous mathematical models. The slurry formulation was optimised on the basis of these meta models, which has led to highly promising formulations that could improve both the stability and the dischargeability of the Zn slurry.
Parallel to the experimental studies, the scientists also investigated the complex flow behaviour of the zinc slurry in the anode geometries in numerical and analytical terms in order to determine the relationship between the cell performance and flow conditions and thus find optimised parameters for the slurry composition, flow geometries and operating conditions.
Based on a technology map for energy storage systems, economic indicators for the competitiveness of Zn-air energy storage systems were defined. To meet the economic demands, the preferred Zn slurry system requires further optimisation, whereby the cost optimisation of the oxygen depolarised cathodes (ODC) technology also plays an important role. To this end, various new oxygen reduction and development catalysts have already been synthesised at Saarland University and tested at a laboratory scale.
Multi-cell module planned in follow-up project
Future development is focusing on the cost optimisation and long-term monitoring of the ZnPlus Zn slurry cell. As part of a follow-up project, the aim is to build a robust multi-cell module and to validate it under realistic conditions. Involved in the project are Covestro Deutschland (formerly Bayer MaterialScience), Grillo-Werke AG, Clausthal University of Technology, Duisburg-Essen University, the Centre for Fuel Cell Technology, Saarland University and the Hochschule Niederrhein. ThyssenKrupp Industrial Solutions helped to economically assess the system as an associated partner.