Electrolysers have many components. A particularly important one is the ionomer, as it helps with the device’s conductivity and, therefore, boost its performance. We’ve been studying it carefully, and we can finally (and proudly) share our latest achievement on this key component, now published in Energy & Fuels.
In the article, we explored two different ways of incorporating the ionomer into an electrolyser. We could demonstrate not only that it significantly improved the performance in both cases, but also that focusing on the interaction between layers is crucial in electrolyser design. This work was led by our partners at TU Berlin, in Germany, with contributions from De Nora, in Italy.
What does an ionomer do?
To put it simply, an ionomer is a type of polymer, a large molecule made up of repeating units. However, an ionomer is a bit special: while some of these repeated units are electrically neutral, others are charged. These charged units help with the transport of ions with the opposite charge.
“For example, an anion-exchange ionomer has positively charged units that enable the transport of negatively charged ions, also called anions, such as hydroxide ions (OH⁻),” explains Arthur Thévenot, from TU Berlin and co-first author of the article.
At ANEMEL, we focus precisely on developing anion exchange membrane (AEM) electrolysers, in which OH⁻ ions pass through the membrane from one electrode to the other. For efficient electrolysis, it’s essential to ensure efficient anion transport. This typically requires the aid of an alkaline electrolyte, that increases the concentration of OH⁻ ions and thus their availability. However, such electrolytes are highly corrosive to electrolyser components, particularly the catalysts, resulting in leakage risks and higher maintenance costs.
The solution lies in designing an electrolyser that achieves good conductivity on its own. And this is where the ionomer comes into play: it provides a steady conductivity, instead of the electrolyte. We must add this is ionic conductivity, as the ionomer enhances the conductivity of ions, OH⁻ in this case, but not the conductivity of electrons themselves. So it acts as an “ion-highway” to boost AEM water electrolysers performance.
But an ionomer does more than this. It also acts as a binder, keeping the catalyst layer stable and preventing it from leaching, especially when using powder catalysts applied directly to the electrode. “You can run an AEM electrolyser without ionomers, but it is a great addition to your system,” says Thévenot.
A comparison of ionomer architectures
Taking this into account, our researchers compared two different ionomer architectures. The first, a well-established method, involved incorporating the ionomer mixed into the catalyst layer. The second approach consisted of applying the ionomer as a separate layer on top of the catalyst layer, acting as an interphase between the latter and the membrane. This one is an innovative approach in AEM water electrolysis.
What they saw is that, for both cases, the ionomer was of great help, even in small quantities. In Thévenot’s words: “Both are very valid approaches, allowing to have a significantly improved performance.”
The secret lies between the layers
We’ve said that this ionomer helps under low OH– concentrations. But what if the hydroxide ions were abundant in your electrolyte? The first results were achieved in low alkaline medium, specifically in 0.01 molar potassium hydroxide (M KOH). This is 100 times less concentrated than the 1 M KOH solution typically used in AEM electrolysers. This higher concentration was set for the second part of the work. The objective: to gain further insight into the role of the ionomer in these different layers.
Surprisingly, the performance improvement persisted. This better performance was even greater when adding the ionomer as a separate top layer than inside the catalyst layer. What does this suggest? That this top layer improved the transport between the catalyst and the membrane.
“One of the most critical steps when designing your electrolyser for operation in low-alkaline medium is to incorporate an ionomer in your membrane-electrode assembly. Moreover, it is also essential to carefully focus on the interactions between the layers,” says Thévenot. “Make sure they’re compatible or optimised enough to allow good ion transport”. Therefore, the key to a better performance lies not only in the layers themselves, but also and mostly between them.
Our team used commercially available compounds, especially for both the ionomer and the membrane, aiming to ensure accessibility and reproducibility for other research groups. More insights, observations and results to compare can lead to better understanding of the mechanistic influence of the ionomer in AEM electrolysers.