| dc.description.abstract | While climate change remains a persistent threat, long lasting alternatives must be developed to avoid greenhouse gas emissions. One solution could be the shift to a hydrogen based energy economy, where hydrogen could be won from renewable energy resources and used in fuel cells, where hydrogen and air are converted into electricity, heat and water. For this, a catalyst is needed both at the anode and cathode side of the fuel cell, which is typically platinum supported on carbon. As platinum is a scarce and noble metal with limited resources, it not only drastically increases a fuel cells cost, but also makes them unsustainable. Furthermore, shortcomings such as sluggish oxygen reduction reaction kinetics and low durability, create a high need for alternative materials. The aim of this research was the development of platinum-free electrocatalysts for the oxygen reduction reaction based on carbon materials doped with non-noble metals and/or metal-free heteroatom dopants. For this, an environmentally friendly and low cost approach for producing heteroatom doped materials was used, called hydrothermal carbonisation, where biomass or biomass precursors can be converted into different multifunctional nanoporous carbon materials with a high specific surface area and large pore volume. Materials were synthesised from glucose and ovalbumin, a protein that can be won from chicken egg white, yielding nitrogen doped cryogels. The influence of oxygen activation during the high temperature pyrolysis was investigated on the resulting carbogels and was found to aid higher surface area at the cost of yield loss and less incorporated nitrogen. Additional doping with metal-free heteroatoms sulphur and boron decreased the surface area, while sulphur did not interfere with the structure formation of the cryogel, the presence of a boron precursor caused an altered morphology and higher nitrogen content due to boric acid acting as structure directing agent and ovalbumin being fully utilised as nitrogen source. The sample containing nitrogen and additional boron was found to exhibit the most favourable electrocatalytic activity towards the oxygen reduction reaction due to a higher active sites density. The influence of iron on the glucose-ovalbumin system was further investigated, where the addition of low amounts of ferric iron salt was found to be beneficial for the structure formation during the hydrothermal carbonisation as well as during the high temperature pyrolysis resulting in a higher surface area and better graphitisation, thus drastically improving the oxygen reduction reaction activity. The addition of other heteroatoms, sulphur and boron, to the iron/nitrogen material was further studied and caused a reduction in surface area. Similar to the metal-free versions, sulphur did not interfere with the structure formation, while the boron precursor competed with the iron and ovalbumin to direct the structure resulting in an inhomogeneous morphology. The pure nitrogen/iron doped version exhibited the best catalytic activity in alkaline and acidic media, closely followed by the sample containing additional boron. Due to controllability issues with surface area and dopant amount during the cryo- and carbogel synthesis, another set of materials was synthesised using a hard templating approach. This was carried out to get a better understanding of how different parameters like surface area, dopants and amount of dopant can influence the catalytic activity towards the oxygen reduction reaction. Different silica templates were used to yield varying surface area and pore volume, where a higher surface area only resulted in a better oxygen reduction reaction performance for the undoped control version, while doped carbons did not show any improvement. Different heteroatom dopant precursors were tested for their ability to dope the respective heteroatom into the carbon material. Urea was found to yield the highest nitrogen doping, boric acid was used as boron, allyl disulphide as sulphur and phosphoric acid as phosphorus source. Sulphur doping posed a challenge and resulted in very low dopant amounts, the large atomic size of sulphur was assumed to cause an issue during incorporation. While doping with different concentrations as well as co doping different heteroatoms, it was observed that only nitrogen species, namely pyridinic and graphitic nitrogen, seemed to act as active sites, e.g. a higher dopant amount resulted in a better catalytic activity. Whereas the other heteroatom dopants, boron, sulphur and phosphorus were assumed not to act as actual active sites but rather create defects within the carbon framework which enhanced the oxygen reduction reaction performance. Additional iron doping caused a slight improvement for all heteroatom dopants, though only iron and nitrogen were found to drastically boost the catalytic activity by forming the presumed FeNx active sites. | en_US |
