Redox flow batteries

Antoni Forner-Cuenca is an Assistant Professor in the Department of Chemical Engineering and Chemistry at TU/e. As extensive as his title is, as big are Antoni's plans and ambitions. His favorite part of his job is to interact with his students. And this project is a perfect opportunity for that. He has recruited a team of students that aims to seriously speed up the energy transition. Together they want to make great strides in storing electricity, employ hydrogen to make transport CO2-free and develop very creative routes to reduce CO2 emissions. 

This story is about the project behind the first ambition: storing energy in batteries. This can and does happen now, but there are still too many barriers that prevent it from happening on a large scale. And that has a lot to do with the process by which batteries store and provide electricity. The improved version of Redox Flow battery that Antoni and his team are developing is going to drastically change that playing field. And moreover: level the playing field for those countries that don't have access to rare metals or other ingredients of conventional batteries. 

Anodes and cathodes
The rechargeable batteries we use today are so-called lithium-ion batteries. They are storage systems composed of an anode and a cathode, a separator between the two electrodes; and an electrolyte filling the remaining space in the battery. The anode and cathode are capable of storing lithium ions. Energy is stored and released as lithium ions travel between these electrodes through the electrolyte.

In these type of batteries, lithium ions intercalate in carbonaceous electrodes, leading to expansion and contraction. Over time that process reduces the battery’s lifetime. Antoni explains: "Just look at your phone. It shows the 'health' of your battery. That is the capacity in relation to the 100% on which the battery started. Per charge cycle, the battery is losing capacity. That's wear and tear from frequent charging and discharging, along with a degradation of chemical reactions in the electrolyte. " 

Not fit for large scale applications. 
Along with some unfavourable effects on the environment, these types of batteries have their limitations in size and application. Antoni acknowledges that, without dismissing the concept of lithium-ion batteries: “It’s a wonderful technology that has transformed the way we live. It’s just not right for large-scale storage technologies due to material limitations, scale up problems and durability.” 

On top of that, they use some scarse material such as cobalt. Which makes them expensive. Antoni: "You want to eliminate geopolitical and economic influences. So that you don't remain dependent on materials that can only be found in a few places.". He describes his ambitious solution to that problem of scarcity: "An open system based on ordinary air and ordinary iron. Both can be found everywhere on earth. This way, everyone will soon have an equal opportunity to store energy. We are not there yet, but we are making steps in the right direction with our research. " 

The solution begins at NASA
About 50 years ago, NASA invented the Redox-Flow battery. A concept in which the membrane still plays a leading role, but where the negatively charged liquid and the positive variant are stored in different containers, as opposed to the 'all in one container' storage in conventional batteries. 

Antoni points out the advantages: "That way of storing means that you can decouple power and capacity. With conventional batteries, if you need more capacity you need to increase the number of cells and with that the total power. " 

Customization for larger applications
The technology is especially suited to large-scale storage, Antoni explains: "Large parks with solar panels, for example. These often produce more energy than is needed during certain times of the day and year. The rest can then be stored. This also relieves the electricity network, which is not really equipped to handle strongly fluctuating and growing peaks and drops. We can adjust the capacity for each battery. Because for some applications, such as data centers, you simply need much more power than for a few households, for example. " 

In addition, the system is modular: "With additional or larger vessels of liquid, you scale up fairly quickly. We will be able to create storage forms that will really help us to use energy more sustainably."

He expects that within 5 and 10 years we will see the first large storage forms based on this technology. 

Porous electrodes

Antoni gets visibly excited when we discuss this topic. Porous electrodes open doors to more power density. But it is searching for the right degree of porosity; for the right size and shape of the holes in the surface of the electrodes. "It's architecture at the nano level," he explains. The idea is to increase the surface area of electrodes as efficiently as possible. "You actually make it artificially larger in this way, so you can get more out of it," he says. Says Antoni, "It's important to get a good balance between large and smaller pores," he explains. "The big ones are for the fast transport of electrolyte, the smaller ones for finer distribution and high surface area." 

Such a precarious balance requires a sophisticated design process. "For that, we use predictive design. We feed the theory into a computer and we ask it to design the perfect electrode." Antoni explains. 

On to a greener, fossil-free future

Antoni sees many opportunities for electricity storage in the new version of the Redox Flow Battery. "If we can store energy in multiple places for multiple uses, we will make maximum use of all that renewable energy that is now being generated, relieve the burden on the electricity grid and make real steps towards true sustainability", says a scientist who, together with his team, is bringing closer a reality that is now still mostly a dream. With solutions that will make our planet cleaner, greener and less dependent on fossil fuels.