EXPLAINER
If by any chance you have seen any of the ASTERISK promotional materials (including our very cool animation and video), you’re probably already familiar with anion exchange membranes. In fact, it’s also hidden in the full title of our project: ASTERISK stands for “Integrated process for seawater electrolysis using a PGM-free anion exchange membrane stack”. Apparently, there wasn’t a shorter title available!
However prominent, perhaps we haven’t spent much time explaining what an anion exchange membrane actually is, how it works, and, above all, what sets ASTERISK apart from conventional and commercial designs.
Until now.
Let’s start by asking ourselves the most basic question: What exactly is a membrane?
There is always a beginning and an end
The simplest way to understand the nature of a membrane is to see it as a physical barrier that separates two spaces. It is, therefore, an intersection.
There are many types of membranes, but their function is always the same: to separate stuff. However, most membranes also have the ability to filter, and to filter selectively: in our own cells, the membrane allows certain substances, such as nutrients, to enter, and ejects others outside as waste.
Therefore, a membrane is a permeable and selective barrier.
Seems simple, right? But…how’s a cell membrane connected to catalysts and electrolysers?
Our very selective, special heart
While in biology the membrane surrounds the cell, in an electrolyser the membrane is in the middle, at the very heart of the device. There, it plays a key role in producing green hydrogen.
In water electrolysis, electrons flow to the cathode, and a catalyst combines with protons to produce hydrogen. In the anode, another catalyst creates oxygen and electrons, which close the circuit. The membrane maintains hydrogen and oxygen separated, a significant responsibility since it’s a pretty explosive mixture. Moreover, mixing hydrogen and oxygen would only produce water again – a bit counterproductive.
Whilst keeping the gases separate, the membrane stays selective and still filters ions (electrically charged particles), which move between the electrodes, pushed by the electric current applied to the electrolyser. Just like the membrane of our cells, the heart of our electrolyser is a barrier blocking stuff, but permitting the passage of important particles, all at the same time.
Let’s take a look at our membrane!
Right, now that we understand a bit better what membranes do inside the electrolyser, let’s talk about anion exchange membranes, the ones our researchers currently develop in ASTERISK.
There are four major membrane types: alkaline membranes (AEL), solid-oxide membranes (SOEC), proton exchange membranes (PEM), and anion exchange membranes (AEM). We’ll focus on the latter.
We explained earlier that ions are particles with an electric charge. If the electric charge is negative, we have anions, on the other hand cations carry positive charge.
Anion exchange membranes (AEMs) focus on transporting anions, negative ions. In the cathode, when water interacts with the electrons, it generates molecules of hydrogen gas (H₂) and hydroxide anions (OH⁻). Our membrane is packed with positively charged cations, which immediately attract the anions and allow the advance towards the anode. Once there, they react in the electrode to form oxygen (O₂) and water.
AEMs have become a very attractive option in recent years for several reasons, all of them closely related:
PEM electrolysers operate in highly acidic environments, requiring scarce and expensive precious metals to prevent corrosion. In contrast, our AEM technology operates in alkaline conditions. This enables the use of catalysts made from earth-abundant materials, such as iron or nickel, instead of platinum. This is not only more sustainable, but also enables efficient electrolysis with cheaper technology, ready to work in very different contexts, including offshore. Finally, anion exchange membranes are compatible with intermittent energy sources such as solar or wind, ensuring that we produce hydrogen using only renewable energy as a feedstock.
And… if AEM have so many advantages, why aren’t these membranes widely used today?
I’m glad you asked that question, imaginary reader whom I’m using as a narrative resource! That’s fair: everything has its downside, and AEMs are certainly no exception.
The application of these AEM for seawater electrolysis is still in fairly early stages, especially when compared to other technologies. The reasons vary, but we can summarise them:
Up until now, AEM have faced two lethal enemies: ionic conductivity and mechanical stability. Because anions need to cross the membrane very quickly, to ensure electrolysis is steady and smooth, our researchers design extremely thin membranes and load them to the brim with positively charged functional groups to increase the attraction of negative ions.
The trouble is, if we do this, our membrane becomes far too hydrophilic – it attracts much more water and swells up, losing mechanical strength and becoming brittle. On the flip side, if you design a more rigid membrane or one with lower mechanical strength, ionic conductivity drops drastically. It’s complicated to compromise!
Luckily, at ASTERISK we love finding solutions to difficult problems. After all, we are scientists! Our innovative idea involves applying strategies that have been successfully developed in other similar technologies, such as proton exchange membranes, which historically have much more literature and research behind them. The goal is to find a perfect recipe to greater mechanical strength without compromising the conductivity of ions through our membrane. In this regard, we are working with brilliant partners such as Newcastle University, Technion, and Novamea, all world experts in the development of AEM technology.
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