Working together with external partners, chemists, materials scientists and chemical engineers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have developed an innovative microcell for water electrolysis. They hope to replace standard plate electrodes used to split water into oxygen and hydrogen with compact, nanostructured tubes. They aim to simplify production, increase flexibility of use and avoid the need for expensive precious metals. A total of 2.5 million euros in funding has been earmarked for the 'Tubulyze' project which is due to start in early 2019 and is planned to run for four years.
Hydrogen is considered a promising means of saving and providing energy in an environmentally friendly and sustainable way. The element is available in virtually unlimited quantities in water molecules, but it is not easy to extract. Expensive and complex electrolysis procedures are needed to split water into oxygen and hydrogen. Mostly, large plate electrodes coated with catalysts are plunged into huge basins of water. In order to allow chemical electrolysis to take place under such highly corrosive conditions, catalysts made of expensive precious metals such as iridium and platinum are used. The membrane needed for ions to be exchanged between the anode and cathode is another costly factor.
Tube cell with ultra-thin catalyst layer
Engineers and chemists at FAU are now researching an electrolytic cell which avoids considerable disadvantages entailed by standard technology. Their idea: the cell takes the form of a tube, not a plate. At its core is an electrode made of porous titan, produced using a 3D printing process at the Chair of Materials Science and Technology of Metals (Prof. Dr. Carolin Körner). The surface of the electrode is then nanostructured and coated with an ultrathin catalyst layer - in this specific instance iridium - using atomic layer deposition. 'We can determine exactly how thick the coating layer should be, all the way down to the scale of an atom,' explains Prof. Dr. Julien Bachmann from the Chair of Chemistry of Thin Film Materials, who is responsible for coordinating the project. 'This allows us to work as cost-effectively as possible, as there is not a linear correlation between a thicker catalyst layer and increased current or greater output.'
Compact: Layers lie directly on top of each other
One decisive advantage of the tubular structure is that the ion exchange membrane can be extruded directly onto the titan electrode. This makes the connection to the carrier electrode considerably more robust and less expensive than planar models. A carbon fleece coated with platinum acts as a cathode for splitting water. The electrolytic tube is wrapped in conductive composite material which acts as an electron conductor. 'The individual layers can be combined in just a few steps, making the electrolytic tube both compact and inexpensive,' says Julien Bachmann. 'In addition, the number of tubes used can be adapted to the amount of hydrogen required much more flexibly than is possible with large plate electrodes.'
Objective: industrial application in four to eight years
The Federal Ministry of Education and Research (BMBF) has provided a total of 2.5 million euros for the interdisciplinary research project 'Tubulyze', which is planned to run for four years. A few improvements are still needed before it is ready for industrial application, however, for example concerning the microstructure of the inner titan anodes. Here the researchers have to weigh up the different properties of the material. If it is highly porous, the water can flow through it better, but the electrons do not flow so well. If it is not so porous, the opposite is true. Researchers from the working group led by Prof. Dr. Jens Harting at Helmholtz Institute for Renewable Energies Erlangen-Nürnberg (HI ERN) are therefore investigating the optimal geometry which will offer the best balance between flow of water and electricity whilst ensuring the maximised area for electron exchange. The Hamburg University of Applied Sciences builds the electrolytic cell from the various components and runs tests, the Dechema-Forschungsinstitut in Frankfurt (Main) carries out stability tests. Julien Bachmann: 'Our aim is to establish a simple process for manufacturing tubular electrolytic cells which saves materials and makes storing energy using water electrolysis a more attractive and inexpensive option.'