Nanostructured Materials for Ammonia Splitting - A Circular Hydrogen Production

One of the biggest challenges of our century is to produce clean and sustainable energy with a Circular Economy approach. Ammonia from sewage sludge is opening new prospects as fuel and in power generation thanks to its carbon-free molecular structure which makes it a good substitute for fossil fuels in decarbonization efforts. This PhD thesis is focused on the design and development of more sustainable electrocatalysts for ammonia splitting. In the Chapter 1, an overview of ammonia-based economy is illustrated: ammonia can be recovered as byproduct from sewage sludge treatment and represent an important energy vector for transport and electricity generation. Among different technologies, electrolysis is identified as the most suitable to produce hydrogen from ammonia in addition to being perfectly aligned with the concept and practice of circular economy. Its splitting into N2 and H2 requires much less energy than water splitting. A state-of-art of materials used so far as anodes for ammonia oxidation is presented highlighting pros and cons. Chapters 2 and 3 deal with the design and development of two distinct classes of nanostructured inorganic materials to use as anode for electrochemical ammonia oxidation. The main idea is to reduce the use of noble metals, favor the implementation of earth abundant elements and at the same time improve the selectivity towards dinitrogen and the resistance to poisoning. Chapter 2 concerns Pt nanostructures grown on nickel foam (NF). The design of these materials aims to reduce the amount of platinum without affect efficiency. Two synthetic methods, galvanic displacement and pulsed electrodeposition, are employed to fabricate Pt nanostructures on NF. Both methods result in a high αPt, with the highest value of 210 cm² achieved using galvanic displacement. Despite its low Pt content, the Pt/NF systems demonstrate excellent selectivity for ammonia oxidation, achieving a Faradaic efficiency of 94% very similar to that of a black platinum but a lower tendency to poisoning. Chapter 3 focuses on noble-metal-free electrocatalysts based on doped Ni(OH)2 developed using Design of Experiments (DoE). DoE is a statistical method used to find the best combination among different factors which affects the experiment to obtain the desired physical and chemical properties. The use of DoE demonstrates the benefits of using a well-organized experimental approach, suggesting the repetition of each experiment for statistical reliability and recommending a random order of syntheses to minimize external factors and errors. The investigation of Cu- and F-doped nickel hydroxide yields unexpected findings, showing that fluoride is not essential to improve Ni(OH)2 electrochemical properties, as reported in literature, but it could even hinder its performance towards ammonia oxidation reaction. On the contrary, minimal amount of Cu significantly improves current performance in linear sweep voltammetry (LSV) and chronoamperometry (CAs), doubling the current compared to bare NF. These results are particularly notable when compared to Pt/NF samples, which exhibit high selectivity for nitrogen at much lower overpotentials (0.8 V less) but suffer from poor long-term stability and low currents. Ni(OH)2-based materials show good stability even at high ammonia concentrations. Finally, Chapter 4 concerns a preliminary investigation on the development of photoanode based on Pt deposited on titania nanorods for ammonia oxidation in a photoelectrochemical cell. The main idea is to improve the absorbance in visible region of solar spectrum by engineering the band gap by doping titania with iron ions and/or annealing.