Feeding Plants with Electricity

Without the chemical element nitrogen (N), life as we know it would be impossible nitrogen is an important component of amino acids and is thus essential for practically all living beings on our planet. In animals, and of course also in humans, amino acids are the basic building blocks of muscles, skin and other tissues. In plants, moreover, nitrogen is an important component of chlorophyll, which enables them to harvest energy through photosynthesis. The biosynthesis of all biologically important nitrogen-containing compounds depends on the supply of nitrogen species such as ammonia (NH 3 ), since neither plants nor animals can absorb nitrogen directly from air (N2 ). Through symbiosis with certain bacteria, some plants (e.g. clover) have found a way, to fixate N2 from air in special root nodules as NH 3 , thus making it bioavailable. In order to ensure a targeted supply of nitrogen in modern agriculture, the Haber-Bosch process was developed about 100 years ago, which allows the conversion of N2 from air into NH3 . This process was a crucial factor in making our modern world possible, because without nitrogen fertiliser, only a fraction of the world`s population could be fed today. The major disadvantage of the Haber-Bosch process is that the hydrogen used is obtained from fossil resources and that it requires an enormous amount of energy due to its reaction conditions. In order to make industrial ammonia production sustainable and CO2 -neutral, research towards alternative approaches has been going on for years. Very often, these approaches have been based on the biological model, but have usually not been able to demonstrate the necessary efficiency. In this project, a completely different approach is to be taken. The aim is to combine a special form of solid oxide electrolysis cells with novel electro-catalysts in order to fixate nitrogen from the air with renewable electricity as ammonia. The special feature of this type of electrolysis cells is that they can provide extremely high hydrogen pressure locally at one of the electrodes, by applying an electrical voltage. This can be used to directly hydrogenate N2 thus obtaining NH3 . The prerequisite for this is the combination of the cells hydrogen electrode with certain catalysts that make it possible to bind N2 from the air and to split its very stable chemical bond. The particular challenge here arises from the complex interactions of the individual materials, which need to be combined and matched in such a way that the desired functionality is obtained with sufficient efficiency. If the endeavour succeeds, it will be possible in the future to convert green electricity with atmospheric nitrogen into plant fertiliser, which can thus be used decentrally all over the world for sustainable food production.

Nitrogen is essential for life on earth, as it is a central component of amino acids, proteins and chlorophyll, but neither plants nor animals can absorb it directly from the air. Only some bacteria are able to convert atmospheric nitrogen (N2) into a bioavailable form. The so-called Haber-Bosch process enables the large-scale industrial conversion of N2 into ammonia (NH3), which is an essential component of nitrogen fertilizer. This process ensures that the world's population is fed, but has considerable ecological disadvantages. In the completed project, a sustainable alternative to ammonia production was researched. The aim was to develop a method that electrochemically converts nitrogen into ammonia using renewable electricity and water. This was to be achieved by using proton-conducting solid oxide electrolysis cells (a special type of electrolysis cell) with innovative electrocatalysts. By applying an electrical voltage, these cells generate extremely high hydrogen pressure locally, which can be used for the direct hydrogenation of N2 to NH3. The project comprised three main aspects: 1. Synthesis of suitable electrode materials: These had to be stable under reducing conditions and conduct protons and electrons. A suitable material was developed, whereby the quantification of the hydrogen content in the material posed an unexpectedly great challenge. To overcome this, Laser-Induced Breakdown Spectroscopy (LIBS) was used as a measurement method and optimized for the requirements of the project. A special measuring chamber was developed for this purpose, which allows hydrogen analysis in oxidic materials using LIBS under different conditions. This establishment of LIBS analysis for precise hydrogen quantification in oxides therefore represents a breakthrough in its own, which will also enable valuable applications in materials research beyond the project. 2. Integration of the materials in electrolysis cells: The novel material was successfully used as a porous electrode in solid oxide electrolysis cells, as demonstrated by the electrochemical pumping of H2 from the anode side of a button cell to its cathode side. The cathode was the novel mixed proton- and electron-conducting material developed by us. 3. Applying a nitrogen hydrogenation catalyst: A suitable catalyst for N2 hydrogenation was successfully integrated into a porous electrode. Nevertheless, NH3 production could not be proven beyond doubt, as the currents under hydrogenation conditions were too low, which is due to the high resistance of the commercial electrolytes used. Overall, the project provided a number of valuable findings, which were incorporated into the submission of an FWF-funded Cluster of Excellence (CoE) "MECS" (Materials for Energy Conversion and Storage). Open questions are now being investigated further as part of this cluster, whereby the project has created an important basis for future research.