Spinel-Type Transition Metal Oxides as Heterogeneous Water Oxidation Catalysts

Abstract

Water oxidation, the bottleneck of the water splitting reaction, requires the development of stable, economic but also efficient water oxidation catalysts. Spinel-type Co3O4 is amongst the most promising materials in this context, due to its low cost and robustness. However, there is not only one clearly defined, unique compound referred to as Co3O4: the material in fact can exhibit very different characteristics, with respect to crystallinity, defects, doping, particle size, oxidation state, or surface area, just to name the most important ones. These parameters were found to largely influence the catalytic behavior of the corresponding spinels and they can be tuned by varying and adapting the synthetic conditions. Therefore, three different approaches were applied throughout this thesis to investigate the influence of synthesis parameters on the emerging spinels and the resulting relationships between materials properties and catalytic activity.
First, spinel-type Co3O4 materials prepared by nine different, well-established synthetic protocols are compared. The resulting cobalt oxides were characterized with a wide range of analytical methods, including XAS, XPS, PXRD, Raman, BET, and TEM to investigate the connections between materials properties and efficient catalysis. Going beyond most literature reports, the catalytic activity was tested with all of the three typical water oxidation methods: electrocatalytically, chemically and photocatalytically. Different activity trends were found for the three test methods. Whereas no clear influence of the synthetic protocol on the electrocatalytic activity was evident, the chemical and photocatalytic approaches showed significant differences between the samples. Especially for chemical water oxidation, the importance of the surface area, the increase in disorder and the decrease in oxidation states of the materials exhibited by samples synthesized at lower temperatures, was unequivocal.
The next study was in fact a close-up investigation, following up on the previous one: microwave hydrothermal synthesis, which is a very attractive and time-saving synthesis method that can accelerate reaction times and reduce reaction temperatures significantly, was applied. Hence, the parameters were varied only within a single preparative approach. The resulting spinels were examined with respect to materials characteristics like crystallinity, oxidation state and surface area with various analytical methods. Furthermore, their water oxidation activity in electrocatalytic and chemical oxidation setups was investigated. For both oxidation methods the same trends regarding the synthesis parameters were found, showing higher activity for lower synthesis temperatures, lower precursor concentrations, addition of hydrogen peroxide and shorter ramping and reaction time.
In the last part of this thesis, synergistic effects between cobalt and other transition metals as well as the influence of the addition of redox inert gallium ions into the spinel matrix were studied. Especially the incorporation of around one third of iron was slightly beneficial for the water oxidation activity. However, most metal combinations were not able to compete with the plain Co3O4 matrix, also not with the use of Ga3+ as a redox-inert mimic of Ca2+ in photosystem II. Interestingly, metallic cobalt, which was formed as a side product in one synthesis, increased the activity significantly and was shown to exhibit considerable water oxidation abilities. These results merit deeper investigations with respect to the interplay and relative activities of cobalt materials with oxidic and metallic properties.
Overall, the influence of spinel properties, which were varied through synthesis conditions or starting materials, on different approaches of catalytic water oxidation is reported in the main part of this thesis. It was shown that less-crystalline materials with more defects and higher surface area exhibit the highest water oxidation activities. All in all, the investigations demonstrate that both preparative approach and selected water oxidation test assays can exert substantial influence on the catalytic activity and assessment of cobalt oxide catalysts. These results are significant for future comparative studies on the optimization of oxide-based water oxidation catalysts with respect to technical applications.