Redox flow is the way to go - Focus on Edwin Otten's research

By Renee Moezelaar

The amount of batteries we use in everyday life keeps on increasing. But we don’t only need them to make our phones or laptops go longer without charging. There is also a growing demand for larger batteries that can store the excess of sustainable energy we don’t immediately need. These are the kind of batteries that Edwin Otten, associate professor of Molecular Inorganic Chemistry at the Stratingh Institute for Chemistry, wants to develop. But he doesn’t think the current lithium-ion systems will do the trick, we need some redox-flow.

Lithium-ion batteries are a great invention that made our everyday life easier. They can store a lot of energy in a small space, and have a relatively long lifetime. But they are not perfect. Just think about the stories of exploding mobile phones or laptops. “The main problem with these lithium-ion batteries is that all the active material is present in a single package”, says Otten. “This makes for a very efficient system, but it also creates a safety concern. If the separator membrane fails then all reactive components are in contact with each other. This leads to internal short-circuit and heat generation.”

Separate tanks

Otten thinks the solution can be found in a different system: the redox-flow battery. This battery consist of two separate tanks filled with molecules in solution, and a small interface in the middle. “This interface is basically a small electrochemical cell”, Otten explains. “When you want to generate or store electricity, you pump the solutions around and they meet at the membrane of this cell. The molecules are oxidized or reduced at the electrode of this small cell, and electrons flow through the membrane.” The size of the interface determines the power output of the battery: “The larger the cell, the more contact there is between the two materials and the more reactions take place.”

The main advantage of these two tanks is that the solutions are not in contact when the pump is turned off. Otten: “The negative and positive sides are separated, and that makes the system relatively safe.” Unfortunately, the separate tanks don’t make the battery last forever: “When you pump the solutions around, sometimes the molecules cross over to the other side of the membrane and react. This really irreversibly kills your battery.”

Organic molecules

There are a few commercially available redox-flow batteries, but they are based on vanadium. This is a relatively expensive metal with a very volatile price, and it is not that common. “These systems are not very widely implemented, mostly because of the cost”, Otten says. “So we are looking for new, cheaper systems, and we think we can make them with organic molecules.” These organic redox-flow batteries have been around for about a decade now, and Otten thinks they can really make a difference: “The elements we use are much more widely available, and you can tune your redox potential or solubility quite easily by adding or changing substituents.” The only problem is finding a suitable molecule: “Not many organic molecules have multiple stable redox states, so that makes it hard. But we are working with a class of stable organic radicals, that hopefully stay together for at least a few hundred cycles of charging and recharging.”

To make the challenge even greater, Otten wants to use the same molecule on both sides of the battery. “Typical organic redox-flow batteries use different molecules in each tank. But we want to use one molecule that you can oxidize and reduce, so when it crosses over the membrane you don’t lose any capacity, just a bit of charge.” If the system works, Otten thinks it will meet a large demand. But he does not have the illusion that he can replace lithium-ion batteries. “Because you have the molecules in solution, it takes up more space than lithium-ion. So you won’t see this kind of battery in cars or phones.” Therefore he focusses more on stationary applications, for instance for large solar or wind farms, or even in the basement of houses. “For those applications it does not matter if the system is slightly bigger, especially if it is safer and has a longer lifetime.”

Mix of systems

If you ask Otten, it is hard to predict where the field of battery-research will be in about ten years. “We are nowhere near the limits, we can still make a lot of improvements in both lithium-ion and redox-flow. We think that organic molecules are the way to go, but you never know.” One thing is for sure, batteries will become even more important than they are right now: “With the transition to more sustainable electricity we need more ways to store this energy. And it does not even really matter what kind of system this will be. Redox-flow, lithium-ion, storing energy in molecules, it is all possible. In the end we need a mix of all available technologies to face the challenge.”