果冻传媒

In the search for viable solutions in the energy transition, the focus is not just on finding sustainable energy sources. In fact, solutions that enable us to efficiently store energy are crucial to managing the peaks and troughs in energy supply and demand.

One of the solutions being explored is hydrogen. This can be produced relatively easily from water through electrolysis and can then be stored or used as a fuel or raw material for industry.

Unsurprisingly, a lot of research is being done on electrolysis, especially on how to make this technique affordable, sustainable, and suitable for dealing with a highly variable supply of electricity. At 果冻传媒, we are also doing a lot of research on this, as described in a clear overview from 2021.

Tricky, flexible offerings

As that article explains, sustainable and affordable alkaline electrolyzers find it tricky to cope with a flexible supply of electricity. No wonder that Thijs de Groot 鈥� who recently started working full-time at 果冻传媒 as an associate professor of Sustainable Process Engineering 鈥� and his team have focused their attention on this very issue.

De Groot: 鈥淵ou can make alkaline electrolyzers with relatively cheap raw materials, which makes them very suitable as a sustainable choice. That鈥檚 exactly why I wanted to do research on them 鈥� to see if we can make them suitable for flexible energy storage.鈥�

The challenge: the hydrogen leak

So, what鈥檚 the most difficult thing about these alkaline electrolyzers? To answer this, we have to go back to the basics of what happens in an electrolyzer, where a cathode and an anode are placed in a salt solution with a membrane between them. If you apply enough voltage, a current flows; hydrogen (H₂) forms at the cathode and oxygen (O₂) at the anode.

The membrane is supposed to keep the hydrogen and oxygen from coming together. However, no membrane is able to keep oxygen and hydrogen perfectly apart, which can potentially create dangerous situations.

Explosive mixture

The danger lies in the hydrogen leaking through the membrane and ending up on the oxygen side (known as hydrogen crossover). If the amount of hydrogen in the oxygen exceeds four percent, an explosive mixture is created.

Hydrogen leakage always takes place to some extent but is especially dangerous when the electrolyzer is not running at full power. In that case, less oxygen is made, so the hydrogen leaking through the membrane is not diluted as strongly. The concentration then approaches the explosive limit more quickly.

To mitigate this danger, the preference is now for alkaline electrolyzers to run at full power. They can be turned up or down a bit but cannot handle sudden large changes. This makes it more difficult to connect them directly to solar or wind farms.

It is also tough to operate the electrolyzers with hydrogen at very high pressure because even more hydrogen will then leak through the membrane. But perhaps a change is coming.

The solution: managing the gap

To enhance the flexibility of alkaline electrolyzers and ensure their safe operation when connected to variable power supply, we aim to control the hydrogen crossover. The distance between the membrane and the cathode 鈥� the gap 鈥� is very important to this.

In the past, research has been done on these gaps at electrodes. Those studies focused mainly on the efficiency of electrolysis. And, this efficiency is greatest with a zero gap at the cathode.

More flexibility, no explosion hazard

However, if there is no gap, a lot of gas will move through the membrane. This is due to a high supersaturation of hydrogen near the membrane. Supersaturation is the same phenomenon that occurs when you open a bottle of beer or a carbonated soft drink. So, this supersaturation must be brought down if you want to make your electrolyzer more flexible without the danger of an explosion.

De Groot: 鈥淔or this reason, Rodrigo Lira Garcia Barros 鈥� PhD student and first author of the article 鈥� systematically investigated the effect of gap size during his doctoral research. This concerned, in particular, the effect on the amount of hydrogen moving through the membrane and the performance of the electrolyzer.鈥�

Lira Garcia Barros and de Groot supervised the master鈥檚 student Joost Kraakman, who developed a model for hydrogen crossover and also designed the research set-up. Most of the measurements were then performed by bachelor鈥檚 student Carlijn Sebrechts.

鈥淚t then turned out that with a small but measurable gap at the cathode, we can greatly reduce the leakage of hydrogen through the membrane 鈥� and with an acceptable loss in performance! In the end, you鈥檙e left with a more functional and, above all, more flexible electrolyzer,鈥� says De Groot.

It should be possible to design alkaline electrolyzers that have a higher load flexibility using existing commercial electrodes and diaphragms.

Rodrigo Lira Garcia Barros, PhD student at 果冻传媒

Proof

De Groot: 鈥淭hat鈥檚 the conclusion of our scientific analysis, so it鈥檚 now time for the next step: creating a proof of concept. That鈥檚 what we鈥檙e going to do in our lab, through which we expect to be able to prove that a flexible electrolyzer is scalable to industry.鈥�

Scalable electrolyzers can also be made in different sizes. For example, very large ones could be used for central energy storage, just as power plants now operate centrally in our grid, or smaller ones could accommodate peaks in power generation at the neighborhood or wind farm level.

鈥淚t should be possible to design alkaline electrolyzers that have a higher load flexibility using existing commercial electrodes and diaphragms,鈥� concludes Lira Garcia Barros.

A few years back, I thought that we wouldn鈥檛 be able to make a super-flexible alkaline electrolyzer, but we did it. So, we can meet this challenge as well.

Thijs de Groot, associate professor of Sustainable Process Engineering

The next step: high-pressure electrolysis

De Groot enthusiastically goes on to talk about a new European project that is also starting at 果冻传媒: high-pressure electrolysis. 鈥淭he hydrogen released in electrolysis is a gas. But gas can only be used, stored, and transported efficiently if it is stored at a sufficiently high pressure.鈥�

鈥淲e currently need a compressor for that, in addition to the electrolyzer itself. Those devices are expensive, noisy, and unreliable. So, what if we could make an electrolyzer that immediately delivers the hydrogen at a high pressure?鈥�

鈥淎 few years back, I thought that this wouldn鈥檛 be possible, just as I thought that we wouldn鈥檛 be able to make a super-flexible alkaline electrolyzer. We managed to do the latter, so I鈥檓 very much looking forward to again working with students on this new challenge.鈥�

鈥� by Thijs de Groot et al was published in the International Journal of Hydrogen Energy, (2023), on 25th October 2023. (https://doi.org/10.1016/j.ijhydene.2023.09.280)