Innovative Zinc Battery Technology: Combining Long-Term Energy Storage and Hydrogen Production

The economic viability of hydrogen as an energy source depends on the overall efficiency during generation and high-efficiency storage solutions. In the joint »Zn-H2« project, researchers are working under the direction of the Fraunhofer IZM on a cost-effective zinc battery that can be used both for long-term energy storage and for hydrogen production.

No market-ready battery technology exists today that uses rechargeable zinc electrodes, such as rechargeable alkaline manganese or zinc-air batteries. In collaboration with the start-up Zn2H2, the Technical University of Berlin, and the Fritz Haber Institute of the Max Planck Society, the project team is researching the fundamental working of this innovative zinc battery technology and working on a demonstrator unit.

Dr. Robert Hahn, an expert in battery technologies at Fraunhofer IZM, has met RealIZM to speak about the promising solution for long-term energy storage, hydrogen production and the role of start-ups in the innovation landscape.

The basic idea of the BMBF-funded project »Zn-H2« is, put simply, not to store the hydrogen itself, but to use the reversible zinc deposition process by which hydrogen is released as needed during discharge. For this to work, one needs a storage system that can produce zinc from zinc oxide during charging, but also releases oxygen as a gas. When the power in the battery is used, the reaction runs in the opposite direction: zinc is oxidized with the help of the water present in the electrolytes, and the hydrogen produced is released at the counter electrode.

»In contrast to battery technology, the electrical power and the storable energy are largely decoupled from each other,« explains Dr. Robert Hahn, who is responsible for coordinating the project. »This means that the costs increase significantly less when increasing the storage volume at the same power. It’s about the return on investment!«

With the Zn2H2 system, hydrogen can be produced locally, which virtually eliminates the significant losses associated with storing and transporting hydrogen.

Compared to the use of lithium batteries for home storage, the significantly lower costs of the Zn2H2 system are recouped even with longer energy storage, i.e. with fewer cycles. With the new battery system, solar energy could be stored in the form of zinc and used as needed to produce hydrogen and electricity. The efficiency of the storage solution, that is, when electricity is generated again from the hydrogen produced, is around 50 percent. Although this is lower efficiency than that of lithium batteries, it is still significantly higher than when generating electricity from hydrogen produced with electrolysers and stored in some other way in between.

The Zn2H2 system can be used locally. »The smallest conceivable entity would be a home storage system. Larger units could be used by municipalities, industrial plants, or hospitals,« says Dr. Hahn, describing the range of future deployment scenarios. This type of energy storage would be a game changer. As part of the project, small systems have now been built (10-cell stacks) that can be used to demonstrate the operating parameters as well as the material and energy flows. However, a prerequisite for investments in the production technology for large systems is certainly the production of a larger, functional system (10 to 100 kWh) by the industrial partners in order to test the system under real-life conditions.

In the past year of work on the project, the focus was on developing and making stacks with initially 10 cells. In each stack, the cells are connected in series. Each cell should have the same parameters, and all catalysts should have the same properties. One key to this is the commissioning of a new electroplating facility at Fraunhofer IZM, which enables the reproducible production of catalyst electrodes on larger surfaces. The finished stacks are examined at a special test stand, which can simulate any environmental conditions and load cycles.

The zinc-hydrogen storage system can be produced at one tenth of the cost of lithium batteries and feeds hydrogen and electricity into the energy cycle as needed. | © Zn2H2 Inc.

In regions of the world where power outages are common, solutions with buffer storage are needed to ensure the supply of emergency power for much longer than just a few hours. Another field of application would be the emergency power supply for systems such as cellphone towers. In the event of power outages, these should continue to function. Current energy-autonomous systems located in remote areas without power lines in Africa or Asia have so far only had a reserve of a few hours to days, relying on stationary lead batteries. Here, too, the zinc storage system would bring advantages.

Technological challenges of zinc deposition and bi-functional gas electrodes in the Zn2H2 system

Both processes – reversible zinc deposition and the function of the gas electrode during charging and discharging – bring with them certain technological challenges. The bifunctional gas electrode alternates between producing oxygen (charging) and hydrogen (discharging) over several thousand cycles and operating hours. Catalysts with some similarities are used in alkaline electrolysers, but each electrode always does the same thing in those cases: it functions either as a positive pole (oxygen production) or a negative pole (hydrogen production).

Operating principle of the Zn2H2 system: (a) charging with zinc deposition and oxygen evolution; (b) discharging with zinc dissolution, hydrogen generation and current delivery. | © Zn2H2 Inc.

In the context of the green energy transition, the aim is to connect electrolysers to solar and wind power plants. However, the supply of renewable energy is dependent on the weather. »This means that the electrical input of the electrolysers fluctuates. When connected in a stack, the individual cells can also reverse polarity,” says Dr. Hahn, describing the first of the two challenges. »There is therefore a great deal of research interest in catalysts that remain stable even when the polarity reverses.« The Zn2H2 catalyst was tested at Fraunhofer IZM and found to be stable for more than 5000 oxidation-reduction cycles when switching between hydrogen and oxygen evolution.

The second challenge is to continuously deposit a smooth and stable zinc layer with a high density. One approach would be to use other electrolytes and electrolyte additives. However, these are more expensive than the aqueous KOH electrolytes currently in use. Another possibility would be to use a stable grid or a porous zinc electrode on which the zinc is deposited and dissolves evenly. However, this means that only a fraction of the electrode can be charged or discharged and the energy density is poor.

In order to maximize the energy obtained, the approach taken in the Zn2H2 system is to deposit and discharge the zinc completely smoothly. Charging is done using an adaptive pulse charging algorithm and zinc electrodes with 200 mAh/cm². Despite their coarse-grained surface, they have a high density and are mechanically stable.

»Our system approach has a special feature that conventional zinc batteries don’t. Conventional zinc batteries use metallic zinc, which means that the battery is assembled in a charged state,« explains Dr. Hahn. »In our case, the battery starts with a steel plate without zinc as the negative terminal. Essentially, we start in a discharged state.« The zinc only comes into play through the zinc oxide dissolved in the electrolytes. ZnO paste is located in between the electrodes. The density of the ZnO paste affects the maximum capacity: a paste density of 1.2 g/cm3 corresponds to an energy density for electricity storage of 780 Wh/l.

»If problems arise during zinc deposition, we can discharge the cell at any time and completely remove the zinc.« By restarting the cell, any number of cycles can be repeated. The prerequisite is to carry out a reset or cleaning cycle.

A follow-up project is in the works to determine the ideal charging conditions for smooth zinc deposition. In this project, the electrical parameters during the charging process and variables such as temperature, electrolyte concentration and thickness of the charge will be recorded and evaluated using artificial intelligence (AI) and machine learning (ML).

Science meets industry: Fraunhofer IZM and Zn2H2 Inc. join forces

Ten years ago, research scientists at Fraunhofer IZM were already looking into how primary zinc storage units could be used to generate hydrogen. »Until now, however, we were unable to restore the function of discharged zinc batteries without refilling everything with new zinc powder,« says Dr. Hahn, describing the state of knowledge at the time. Based on publications, Chaim Markheim, CEO of Zn2H2 Inc., and Oren Rosenfeld, development engineer at Zn2H2 Inc., became aware of the previous research work of the Fraunhofer IZM. Zn2H2 holds patents for their unique reversible zinc-hydrogen energy storage concept and zinc electrode advancements.

»With Zn2H2 Inc., we now have an industrial partner at our side that has the know-how about the Zn2H2 system.« The start-up has been an associated institute at Start-a-Factory (SaF) since 2022 and works very closely with Dr. Robert Hahn and his team.

To further improve the fundamentals of the novel zinc battery technology, a comprehensive technical infrastructure is needed, along with access to analytical tools and expertise in rechargeable batteries. To test the functionality of the rechargeable Zn2H2 system, it must be charged and discharged repeatedly and the hydrogen produced in the fuel cell must be converted into electricity. »Thanks to our many years of research in battery technologies, we have a special test laboratory in which we combine our expertise in fuel cells and rechargeable batteries,« explains Dr. Hahn.

Testing of the complete system with zinc hydrogen storage, hydrogen separation, fuel cell and the control electronics developed by Zn2H2 | © Fraunhofer IZM

In addition, there are various testing options, such as the cyclization stations with several hundred test channels for the fully automatic charging and discharging of the cells, and material analysis to routinely examine the composition of the catalysts, for example using energy-dispersive X-ray spectroscopy (EDX). »For everyone involved, being able to quickly examine the laboratory samples is extremely practical in their everyday work.«

Zn2H2 development stages from left to right: functional demonstrator, stack demonstrators with internal electrolyte circulation, stacks with external electrolyte circulation | © Fraunhofer IZM

From left to right: Zn2H2 stacks with external electrolyte circulation; Catalytic electrode for both hydrogen and oxygen production | © Fraunhofer IZM

The stacks are created in close collaboration between the start-up and Fraunhofer IZM’s Start-a-Factory.

The 3D-printed plastic parts needed to build the stack for the demonstrator, for example, are manufactured and continuously developed in the Start-a-Factory, explains Chaim Markheim, CEO of Zn2H2 Inc. The exact design of the integrated channels for the flow of electrolyte plays a very important role. To do this, we had to test various designs.

Fraunhofer IZM also has a small prototype production line to manufacture all the parts needed for the demonstrator itself and to realize the entire assembly of the demonstrator. For example, dispensing robots are used to apply the sealing material to ensure that all seals between the individual cells of the stack are reproducibly tight.

Testing of the complete system with Zn2H2 storage and fuel cell at the Fraunhofer IZM test bench | © Fraunhofer IZM

The role of start-ups in the innovation landscape of Fraunhofer IZM

Robert Hahn is fascinated by the iterative approach of the two founders and their sequential work and continuous rapid improvement of the experimental design: »Personally, I can only recommend to my colleagues at Fraunhofer IZM that they work with start-ups and Start-a-Factory. Nowhere else can you make better use of the full potential of Fraunhofer than here.« Companies that cooperate with Start-a-Factory benefit from the technical infrastructure and expertise of the Fraunhofer IZM. For example, the two founders of Zn2H2 Inc. were able to quickly test and refine their idea in one of the research institute’s laboratories. This avoided the lengthy process of applying for certification for their own chemical laboratory in which to work with KOH and other critical chemicals.

In Dr. Hahn’s experience, industrial projects are more satisfying because the partners believe in their ideas and are highly motivated to bring them to life. In addition, they work with people who understand exactly what is at stake and appreciate the contribution that research institutes make. However, it is essential that the start-ups know exactly what their product is and in which direction they want to develop it.

As the head of the joint research project, Fraunhofer IZM acts as an intermediary between the participating industrial partners and the basic research experts. The former want to get to the very bottom of the fundamentals, while the latter want to present practical results as quickly as possible. The working group »Technical Chemistry, Electrocatalysis – Materials«, headed by Prof. Dr. Peter Strasser at the Technical University of Berlin, and the Fritz Haber Institute of the Max Planck Society, headed by Prof. Dr. Beatriz Roldán Cuenya, are further important project partners who are focusing on the best possible further development of the catalyst. »As an institute for applied research, we understand both sides. I think that’s why we’re getting very good application-oriented results,« summarizes Dr. Hahn.

Zn-H2: Hydrogen storage using reversible electrochemical zinc cells

Partners: Fraunhofer IZM (coordination), Fraunhofer IFAM, Technical University of Berlin, Fritz Haber Institute of the Max Planck Society, SteelPro Maschinenbau Inc., Zn2H2 Inc. (associated)

Duration: 10/2022 – 09/2025

Funding organization: BMBF (Federal Ministry of Education and Research)

Grant number: 03SF0630A

Total funding: approx. 3.8 million euros

Websites: https://www.wasserstoff-leitprojekte.de/zn-h2 and https://zn2h2.com/