A team of researchers at Technical University of Denmark (DTU) may have cracked one of the toughest nuts in sustainable energy: how to make fuel cells light and powerful enough for aerospace applications.
In an interdisciplinary collaboration between DTU Energy and DTU Construct, the researchers have developed a radical redesign of the so-called solid oxide cells (or SOCs) using 3D printing and gyroid geometry. Gyroids are intricate structures mathematically optimised to improve surface area in a given volume and are employed both by engineers for heat exchangers and by nature in structures such as butterfly wings. Gyroidal architecture is structurally robust, has a large surface area and is lightweight. For the first time, DTU scientists have shown how to use the gyroid to make electrochemical conversion devices such as SOCs.
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To power a commercial aeroplane today you need jet fuel. If you retrofit a regular jet, replacing its 70 tonnes of fuel with Li-ion batteries of similar capacity, its weight would be 3,500 tonnes. And so, it wouldn’t take off.
The same has been true for fuel cells, which are mostly confined to flat, heavy stacks that rely on metal parts for sealing and connectivity. So, those are heavy, too. Metal components make up more than three quarters of a fuel cell system’s weight, severely limiting their mobility and consequently, their usefulness in, for example, aerospace applications.
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The DTU scientists may have flipped the script. Professor Vincenzo Esposito from DTU Energy, senior researcher Venkata Karthik Nadimpalli from DTU Construct, and several colleagues from both departments have designed a new fuel cell that is fully ceramic and is built by 3D printing. The printed structure is known as a triply periodic minimal surface and is mathematically optimised for maximum surface and minimum weight.
Their fuel cell – which they dubbed the Monolithic Gyroidal Solid Oxide Cell or the Monolith for short – delivers more than one watt per gram. Not only is this a first, but it also broadens the field of possible fuel cell applications significantly, explained Nadimpalli. ‘Currently, using electricity-based energy conversion, such as batteries and fuel cells, doesn’t make sense for aerospace applications,’ he said. ‘But our new fuel cell design changes that. It’s the first to demonstrate the watts to gram ratio – or specific power – needed for aerospace, while using a sustainable, green technology.’
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Fuel cells are nothing new and their impact is evident in several sectors. While perhaps most visibly in hydrogen cars, they are, for example, also used as power supplies for hospitals and data centres, in ships and as storage to stabilise renewable energy systems. Their ability to switch between power-generating and power-storing modes (electrolysis) makes them highly versatile in several applications.
There are many other reasons why the new fuel cells from the team of DTU scientists may be a game-changer. Apart from the weight being brought down significantly, the system allows gases to flow efficiently through the cell, improves heat distribution and enhances mechanical stability. Switching to electrolysis mode, they produced hydrogen at nearly ten times the rate of conventional designs.
‘We also tested the system in extreme conditions, including temperature swings of 100°C, and repeatedly switched between fuel cell and electrolysis modes. The fuel cells held up impressively, showing no signs of structural failure or layers separating,’ said Esposito.
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The researchers explained that this kind of resilience is vital for space missions such as NASA’s Mars Oxygen ISRU Experiment, which aims to produce oxygen from Mars’ carbon-dioxide-rich atmosphere. This mission currently relies on bulky stacks weighing more than six tonnes. The new design could deliver a similar performance at 800 kilograms, which would significantly lower the costs of launching the equipment.
What makes this design especially compelling is not only its performance but also how it’s made, explained Nadimpalli. ‘While conventional SOC stacks require dozens of manufacturing steps and rely on multiple materials that degrade over time, our monolithic ceramic design is produced in just five steps, where we eliminate the metal and avoid fragile seals,’ he said. ‘Still, I believe that we can improve the system further using thinner electrolytes, cheaper current collectors, such as silver or nickel instead of platinum, and even more compact designs.’
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The research has been published in Nature Energy.