The yin and yang of the hydrogen world
Yin and Yang: these are fuel cells and electrolysers for the world of hydrogen. Graphic: AdobeStock
Electrolysers and fuel cells are something like the yin and yang within the hydrogen world. They represent two processes that are contrary to one another but nevertheless complement each other perfectly. With the help of electrical and thermal energy, an electrolyser splits water (H2O) into hydrogen (H2) and oxygen (O2). The fuel cell is the command reverse: hydrogen and oxygen combine to form water. During the reaction, a large part of the electrical energy that was necessary for the electrolysis is recovered and made available for use. That sounds pretty straight forward.
Energy converter
Both principles have been known since the first half of the 19th century. As climate research has been finding more and more evidence since the late 1980s that humans are disrupting the natural balance of the climate by burning fossil fuels such as coal, oil and gas, science has been working harder to develop energy storage and conversion systems that do not further destabilise the climate. Electrolysers and fuel cells are two crucial technologies that are necessary for hydrogen to be able to join batteries as a pillar of the climate-friendly energy system of the future.
PEM systems
The PEM electrolyser and the PEM fuel cell – PEM stands for Proton Exchange Membrane – are still comparatively new. And they are the hope for the hydrogen economy of the future. ‘They have good load flexibility,’ says Dr Holger Janßen, group leader for stacks and systems at the Institute of Energy Technologies (IET 4) at Forschungszentrum Jülich, describing a major advantage of PEM technology. ‘So they cope well with electricity that fluctuates a lot. It can also be operated well at partial load when little green electricity is available.’ This is important for the green energy economy of the future, which is to be supplied with renewable energy. And this is not available in constant quantities because the sun does not always shine and the wind does not always blow.
The most expensive metal of all
However, electrolysers and fuel cells with proton exchange membranes continue to pose a challenge for research. ‘The membrane electrode unit, the heart of every PEM cell, consists of critical expensive materials,’ explains Holger Janßen, primarily referring to the metal iridium, which is a bottleneck for electrolysis. Platinum is used in PEM fuel cells – it is also expensive, but less critical than iridium. ‘Since it has become clear that we want to produce more hydrogen and that PEM electrolysis will play a major role in this, the price of iridium on the metal exchange has risen dramatically.’ Over the past ten years, it has increased by more than 800 per cent. A troy ounce (31.1 grams) of iridium currently costs more than 4,200 euros. By comparison, gold was around 2,150 euros. ‘Even for us, who only need small scientific quantities, this is a problem,’ says Holger Janßen.
Less than ten tonnes per year
The reason for the dramatic increase that occurred at the end of 2020 is the rarity of iridium compared to the sharp increase in demand. Natural deposits are minimal. Iridium makes up only 0.022 billionth of the earth’s crust. The occurrence is probably due, among other things, to the impact of an asteroid that wiped out most dinosaur species 66 million years ago. However, iridium can still be obtained because it is a residue in the production of platinum. Nevertheless, the amount obtained, which is less than ten tonnes per year worldwide, is clearly disproportionate to the demand of over 200 tonnes.
Fuel cells and electrolysers are energy converters that transform electricity into hydrogen – and back again. Graphic: Forschungszentrum Jülich/Reisen
PEM electrolysis technology requires a gas-tight membrane to separate the anode and cathode areas of the cell. It works in an acidic environment. Therefore, it is important that the materials do not rust. No metal rusts as little as iridium. In contrast to alkaline electrolysis, in which a negative ion migrates from the cathode to the anode, the ion transport process is different here: a positive hydrogen ion migrates through the proton-conducting membrane from the anode to the cathode. The result is the same: oxygen collects at the anode and hydrogen at the cathode. Everything exactly the same, only in reverse – this is how the PEM fuel cell works. Hydrogen and oxygen combine to form water again. ‘We already know PEM fuel cells quite well from their application. For example, they are used in mobility in cars, trucks and buses,’ explains Holger Janßen. The technology has now been developed to such an extent that it has made the leap into application and into series production. The strength of PEM technology is also evident in mobile applications: it can be used in small, decentralised units. And it is robust because it can withstand vibrations and shocks.
Electrolysers will play a central role in the hydrogen economy of the future. Photo: Forschungszentrum Jülich/Limbach
The best of both worlds
Holger Janßen sees the fastest possible route to a climate-friendly future, in which hydrogen plays a key role alongside battery storage, as being the interaction of two electrolysis technologies. ‘In principle, this is comparable to fossil fuel energy production: coal-fired power plants are used here for the necessary base load. And gas-fired power plants are used for dynamic peaks. In the future, large alkaline electrolysers could provide the base load. The technology is proven, comparatively inexpensive and can be scaled up at a reasonable cost. PEM electrolysers can then cover the peaks. They are flexible in terms of load and can be ramped up and down quickly.’ This basically applies to both types of electrolyser. This is because they work at a comparatively low temperature in the range of 50 to 80 degrees Celsius. This does not place any further special demands on the material. Furthermore, the electrolysers do not have to be insulated at great expense. This is in stark contrast to high-temperature applications.
Another line of research, not only being pursued by the researchers in Jülich, is to combine alkaline electrolysis and PEM electrolysis. Not by using them side by side, but by actually combining the properties of both technologies into a new one.
This new approach is called an anion exchange membrane (AEM) electrolysis. Unlike PEM electrolysis, protons do not migrate through a membrane to the cathode, although the electrolyser is constructed like a proton exchange membrane system. Instead, an anion migrates to the anode, as in alkaline electrolysis.
The advantage that scientists are hoping for: the iridium is no longer necessary and is replaced by nickel. This is cheaper than iridium and available in larger quantities. ‘There are no large-scale AEM systems in industrial use yet, only smaller demonstrators. So it will take some time before this technology is available on the market. But we are convinced that AEM electrolysers can play an important role in the 2030s,’ says Holger Janßen optimistically.
Dr. Holger Janßen, group leader for stacks and systems, Institute of Energy Technologies (IET-4). Photo: Forschungszentrum Jülich/Kreklau
Research into solid oxide systems, which work at high temperatures, is far from over. There are SOEC and SOFC systems. SOEC stands for Solid Oxide Electrolysis Cell, i.e. a solid oxide electrolysis cell. SOFC stands for Solid Oxide Fuel Cell. These systems are now being used on an industrial scale, for example in the HC-H2 demonstration project Multi-SOFC at the hospital in Erkelenz. The high temperatures at which the systems operate have long been a concern for researchers. They range from 500 to 1,000 degrees Celsius. ‘Solid oxide reactors start at ambient temperature and then heat up sharply. This affects the material because it expands considerably and can be damaged,’ says Holger Janßen, describing the challenge that research had to overcome. Nevertheless, the approach is worthwhile. ’Wherever high temperatures are present anyway, or can be easily decoupled, solid oxide systems make sense.’
High-temperature applications
This makes sense in many industrial environments, for example. High-temperature energy converters achieve the highest levels of efficiency, which means that the greatest amount of electricity can be obtained at the end of the process. They are also the only hydrogen technology that can guarantee a heat supply in addition to an electricity supply. ‘High-temperature systems are useful where they can run as continuously as possible. Frequent start-ups and shutdowns take too long due to the high temperature level required,’ says Holger Janßen, describing the scenarios that come into question. A second requirement: high-temperature cells only make sense for applications in which they are exposed to as little vibration as possible. ’This is where ceramics are used, which are relatively brittle and can be damaged if they are exposed to too much vibration.’ So the technology is not suitable for cars and lorries, for example. However, it makes more sense for larger ships.
The technologies complement each other
‘In the case of high-temperature applications and PEM technology, research will achieve further improvements in the future. This will ensure that the technology becomes cheaper and more efficient. But the technologies for electrolysers and fuel cells that are already available today can already meet the demand that is necessary for hydrogen to become an important part of the climate-friendly energy industry of the future,’ says Holger Janßen. He also draws a second important conclusion: ‘Hydrogen is part of the future of energy storage and it complements battery storage perfectly. And that’s exactly how hydrogen technologies complement each other. Where one is less suitable, the other has its strengths. This is a good basis on which research and development can build.’