Fabrication and Evaluation of Perovskite LaCrO3 Nanoparticles as Catalyst for Electrochemical Ammonia Synthesis

Abstract: As our way of living exploits the resources of the earth and our energy use keeps on increasing, we are now facing a climate crisis never seen before in modern time. To face these consequences and try to ease the harm we need to transform our extraction and use of energy. The use of chemical energy carriers and fuel cells offers the possibility of a completely green solution for energy distribution and conversion. A fuel cell converts chemical energy into electrical energy through a series of chemical and electrochemical reactions. Hydrogen has been the dominating fuel for fuel cells but a large obstacle for commercialization is the distribution infrastructure that needs to be planted. This problem has opened the way for another chemical energy carrier with a widely developed infrastructure. Ammonia (NH3) is one of the most produced chemical substances globally, mainly used for fertilizers. It’s easier to store and transport than hydrogen but the big challenge is the industrial ammonia synthesis, a heavily energy consuming process. This Master’s thesis focus on the production of an active and selective catalyst for the electrochemical ammonia synthesis. A primary study was performed to investigate previous reported results of catalyst designs for the nitrogen reduction reaction (NRR). Noble metals have been proved to be stable and active catalysts but to decrease the cost of the material, transition metals offers a great alternative. To reach the performance as for the traditional Haber-Bosch process the catalyst need to be optimized. One approach to this is a nanoscale design of the catalyst to improve the availability of the active sites and the mass transfer. The perovskite LaCrO3 is a ceramic material with great properties for heterogenous catalysis. The perovskite material is thermally stable with a great flexibility in tailoring the structure down to atom level. The highest ammonia yield was obtained at -0.8 V vs. RHE with an ammonia formation rate of 24.8 μg h−1 mg−1cat, and a Faradic efficiency of 15%. The results offer a great alternative with the easily produced and low-cost perovskite structured electrocatalysts for NRR. To find the perfect NRR electrocatalyst experimental and theoretical work needs to be furthered improved. One important aspect of this work is to get detailed material characterizations to investigate structure, particle size, surface properties, and electrical conductivity of the catalysts. From this, a relation between the catalytic activity and solid-state properties can be drawn and further material design strategies can be conducted. A detailed characterization of the as-produced LaCrO3 crystals are therefore also conducted in the experimental part of this thesis, together with the electrochemical ammonia synthesis testing.