Abstract
Artificial photosynthesis is a promising strategy for renewable energy production by converting sunlight into chemical fuels. The water oxidation half-reaction is considered as the bottleneck of this process. Therefore, the development of stable, efficient, and economic catalysts is an essential part of this research. However, this can only be achieved by an in-depth understanding of the catalysts with respect to their formation pathways and operational mechanisms. To this end, in the present doctoral thesis Co-based heterogeneous water oxidation catalysts, especially spinel-type cobalt oxide, were systematically investigated.
In the first project, the hydrothermal growth mechanism of spinel-type Co3O4 was studied by monitoring the reaction with in situ EDXRD and evaluating the kinetic data. A change of growth mechanism for reaction temperatures above 185 °C is revealed, which influences the morphology and the photocatalytic water oxidation performance. Additionally, complementary ex situ quenching experiments were performed to compare the in situ data with conventional hydrothermally synthesized materials. The quenching experiments were crucial to evaluate the influence of amorphous phases on the catalytic performance.
In the second project, a three-step approach was applied to study (1) the impact of the preparative method on the properties of the catalyst and (2) its relationship towards the water oxidation performance (3) as a function of the applied driving force. Spinel-type Co3O4 was synthesized via nine different synthetic routes and characterized by various analytical methods. The resulting catalytic performance was assessed by electrocatalytic, photocatalytic, and chemical oxidation for comparison. The applied test method undoubtedly influenced the performance. Electrocatalytic tests showed very similar activities and photocatalytic tests did not show clear well-defined property-activity correlations. However, the chemical water oxidation activity was increased by a decrease in oxidation states of the cobalt centers as well as through an increase in disorder and surface area.
In the last project, the formation and structure of highly active CoOx NPs were investigated, which have been a well-known but little understood benchmark for decades of Co-based water oxidation catalyst research. To this end, simple Co(NO3)2 was added to the photocatalytic test assay under varying conditions and the formed precipitates were quenched at different time intervals. The quenched Co-based species were characterized by different techniques and tested for their photocatalytic water oxidation activity. Interestingly, the amorphous starting material transforms into Co3O4 within the first minute of illumination and then further transforms into CoOOH. Moreover, the results demonstrate that these transformations have a major influence on the OER recycling performance. Most importantly, these systematic investigations clearly outline the highly interesting trend that simple Co2+-salts exhibit an activity similar to or even higher than the current performance state of the art for Co-based heterogeneous and homogeneous (pre-)catalysts.
Overall, it was demonstrated how the efficient correlation between an in situ technique and ex situ analyses helps to optimize the hydrothermal synthesis of key energy materials. Furthermore, the assessment of the preparative history, as well as the applied catalytic test method, are essential for comprehensive catalyst design. Lastly, simple Co2+-salts go through structural changes to form “self-activated” nanoparticles, which are remarkably active compared to other tailored homogeneous and heterogeneous WOCs.
Reith, Lukas