The Dutch Electrolyzer

With its ‘Dutch Electrolyzer’, EIRES wants to demonstrate what is needed to accelerate the energy transition. Bert Vreman, research professional at Nobian and parttime professor at the Power & Flow group of TU/e’s Department of Mechanical Engineering, explains the mechanical engineering challenges associated with realizing cost-effective, industrial-scale electrolysis for green hydrogen production.

With the Dutch Electrolyzer as an iconic project, EIRES wants to connect basic research in the fields of electrochemical reactor engineering, electrocatalysis, electricity conversion and process systems engineering at TU/e to the Brainport high tech industry. The aim is to learn step-by-step how to upscale a commercially viable, renewable electricity-fed, hydrogen-producing electrolyzer to a megawatt scale. Hydrogen is not only key as a basic fuel to re-generate electricity through fuel cells, but also is a major building block for the chemical industry.

Bert Vreman starts by debunking a common misconception: ‘Industrial-scale electrolyzers producing hydrogen already exist, that is not the new part here.’ Large-scale electrolyzers are commonly used to produce caustic soda, chlorine and hydrogen from sodium chloride solutions. Still, implementing electrolyzers that are able to produce green hydrogen from water at large scale is far from simple. The key words here are efficiency and cost-effectiveness, since at the moment, producing hydrogen from natural gas is still cheaper than through water electrolysis.

Simple in theory, hard in practice
The idea of producing hydrogen via water electrolysis is simple: Dip two conductive metal strips in an electrolyte solution and apply a voltage to them. One of the electrodes attracts positive ions; there gaseous hydrogen is formed. The other does the same with the negative ions, leading to oxygen formation. Collect the hydrogen containing gas bubbles and you’re done. But at the moment, this process cannot compete with natural gas-based hydrogen production.

One of the problems is that the materials currently used for the electrodes and catalysts are too expensive, and too rare. Research within the EIRES focus area Chemistry for Sustainable Energy Systems thus focuses on designing and engineering cost-effective materials to replace expensive metals and compounds such as platinum and iridium oxide.

Another problem is that the electrolysis process is not as effective as it could be in theory. That is where the EIRES focus area Engineering for Sustainable Energy Systems comes in. ‘We are concentrating on improving the efficiency of the process,’ Vreman tells. The gaseous bubbles that are formed play an important role here, since if the liquid electrolyte contains bubbles, the electric current cannot pass through it as easily. But these bubbles are largely terra incognita; many fundamental questions are still open, Vreman explains. ‘How are they formed? How do they move along the electrode? What determines how long they will stick to the electrode, blocking its surface and effectively decreasing its activity? How large do they get, and how do they influence the liquid flow behavior?’

Impossible to see
What complicates the research tremendously, is that it is impossible to put a camera on the electrolyzers and observe what happens, Vreman says. ‘The bubbles are too small, and there are too many of them. With optical methods, you don’t see a thing in electrolyzers with macro electrodes at high current density.’ That is why the scientists use experiments with small-scale simplified systems and simulations based on physical and chemical models to get an idea of what happens.

‘One of the properties we need to study is the coalescence behavior of the bubbles,’ Vreman tells, ‘so how, when and where several individual bubbles melt together to form a bigger bubble. To be able to model this behavior, we for example need to know the chemical properties of the electrolyte in the vicinity of the bubbles, and the electrical charges on the bubble surface. But these also cannot be measured directly.’ Besides the problem of the lack of experimental data, building realistic bubble models is also complicated because of the interconnectedness of the multitude of parameters that influence the formation and behavior of the bubble. ‘Think of the contact angle between the bubble and the electrode, the material and geometric properties of the electrode, the chemical composition of the electrolyte and the bubbles, the coalescence, the high gas fraction, the effect of temperature, the effect of high pressure …’

So where to start when you want to untangle such a tight and complex knot? ‘We took an experiment from literature where the gas fraction could be measured, and we tried to reproduce that with our simulations. And to gain some experimental data, we experiment with small scale, single bubble set ups. In addition, together with TU Dresden, we are currently looking into the possibility of micro-CT imaging technology to get a visual on the bubbles after all.’

Close collaborations between disciplines
Understanding bubble behavior is a multidisciplinary endeavor par excellence, says Vreman. ‘Within TU/e, there is a close collaboration between the departments of Mechanical Engineering (with Niels Deen, Yali Tang, Aled Meulenbroek, Faeze Khalighi and Peter Dung) and of Chemical Engineering & Chemistry (with Thijs de Groot, John van der Schaaf, Rodrigo Lira Garcia Barros and others). Take the Alkaliboost project, which is a collaboration between TU/e Mechanical Engineering, Chemical Engineering and HyCC (Hydrogen Chemistry Company). Another collaboration is the Bubblelectric project, which is carried out by a consortium consisting of TU/e, TU Delft, University of Twente, Shell, Nouryon and Nobian. There are also two PhD students from TU/e working on that project: one at Mechanical Engineering and one at Chemical Engineering & Chemistry.’

Studying bubbles is a particularly though area of research, Vreman says. ‘We only take little steps at a time. And to be frank: After the efficiency gain due to the application of zero gap technology (in which the electrodes are pressed onto the membrane in between), we are not sure how much there is still to gain in terms of efficiency by understanding and controlling the bubble dynamics. And suppose we were able to significantly improve the efficiency by removing bubbles faster by increasing the liquid circulation through the electrolyzer, if as a result of our efforts we would need a large and expensive separator to extract the hydrogen, we might not have won anything in the end.’

Still, Vreman thinks there are good reasons to keep investing in this type of research. ‘Bubble dynamics is a very interesting phenomenon for many more applications than water electrolysis alone. It also plays a role in brine electrolysis, in lithium chloride electrolysis, in boiling and evaporation processes, or in bubble column reactors. By the way, what many people do not realize is that green hydrogen production via electrolysis is not just the future, it is also today: in the industrial process to produce caustic soda and chlorine, we also make hydrogen. So when we use electricity from renewable sources to power that process, we are producing green hydrogen already.’