Redox Flow Batteries as a Key to the Energy Transition?

Interview on Battery Day, 18 February 2026

18 February 2026 – Redox flow batteries are considered a promising technology for storing renewable energy. But what exactly is behind them – and why are they gaining renewed attention right now? On Battery Day on 18 February 2026, we spoke to Dr Mariano Grünebaum from Helmholtz Institute in Münster (HI MS) of Forschungszentrum Jülich.

What is a redox flow battery, and how does it differ from lithium-ion technology?

A redox flow battery stores energy in liquid electrolytes that are kept in external tanks. During operation, these liquids are pumped through a so-called reactor or “stack,” where the electrochemical reaction takes place.

The key difference compared to lithium-ion batteries is that in redox flow systems, energy and power can be scaled independently. The amount of stored energy depends mainly on the size of the tanks, while the power output is determined by the size of the stack. In lithium-ion batteries, by contrast, energy and power are fixed within the cell – increasing capacity usually means adding more cells or modules.

Which applications are redox flow batteries particularly suited for?

Redox flow batteries are especially well suited for large-scale stationary storage, meaning systems designed to provide energy over several hours. They are robust, long-lasting, and considered particularly safe. This makes them highly attractive for storing wind and solar power in a grid-supportive way – absorbing surplus energy and feeding it back into the grid when needed.

What research questions are at the center of your work?

At Helmholtz Institute Münster, we are working on all-organic redox flow batteries as well as zinc-based systems, such as so-called Zn-flow approaches. Key questions include: How can electrolytes and functional additives be improved? How can unwanted “crossover” – the mixing of the electrolytes – be reduced? And how can we develop reproducible testing methods all the way to implementation in real battery cells?

Why are redox flow batteries coming back into focus right now?

With the growing share of renewable energy sources, the need for storage technologies that can provide energy not just for minutes, but for several hours, is increasing. At the same time, topics such as safety, raw material availability, and long service life are becoming ever more important. This is exactly where redox flow batteries can play to their strengths.

Have there been particularly exciting advances in recent years?

Yes, especially in new electrolyte materials. Organic redox molecules are becoming more stable and more soluble. Zinc-based systems are showing progress in controlling side reactions. And there is also major development in membranes, as they are often the key factor for both lifetime and cost.

What are currently the biggest challenges?

One central challenge lies in the membranes that separate the two electrolytes. They must function like a filter: ions need to pass through so that the battery can operate, but the active materials must not mix. Very dense membranes can prevent crossover, but they often increase resistance and reduce efficiency. More permeable membranes improve performance, but may lead more quickly to capacity loss.

In addition, long-term stability remains a key issue. For zinc-based systems, hydrogen evolution and uniform metal deposition are further challenges. From an economic perspective, overall system costs and real-world field experience will be crucial for widespread deployment.

If you could wish for one solution: what would ideally be solved tomorrow?

Clearly: a membrane that is simultaneously inexpensive, highly selective, low-resistance, and extremely stable. That would ease many problems at once – from capacity loss and efficiency to overall costs.

How do redox flow batteries compare with lithium-ion batteries?

Lithium-ion batteries are particularly strong when compactness is required and are often used for storage durations of one to four hours. Redox flow batteries show their advantages when safety, lifetime, and long storage durations of many hours are the priority – and when capacity needs to be scaled flexibly via larger tanks.

What role do raw material availability and recycling play?

A very important one, because stationary storage will be needed in enormous quantities. Flow systems have the advantage that their active materials are liquid. They can be processed, recycled, or brought back into balance through so-called “rebalancing.” Organic and zinc-based concepts also aim for improved resilience in terms of raw material supply.

What is the most common misconception you encounter?

Many people think: “Redox flow automatically means vanadium.” In reality, there are many alternative chemistries, including organic and zinc-based systems. Another misconception is: “Flow batteries are slow or inefficient.” In fact, performance depends strongly on stack design, membrane properties, and operating conditions.

What needs to happen for redox flow batteries to be more widely used in five to ten years?

Key factors include cost reduction through scaling and standardization, more long-term data from real-world operation, improved membranes, and more stable electrolytes. Clear testing standards will also help ensure that results are comparable between laboratories and industry.

Thank you very much!