Investigations into Nanostructured Materials for Water Splitting and Direct Solar Energy Harvesting

This thesis deals with different aspects of renewable energy transformation and storage concepts and connects them. Although renewable energy installations are growing worldwide, their use is limited by storage technologies. As such, water splitting for hydrogen production and battery technologies are research areas with high potential impact. Especially earth abundant material solutions and integrated technologies that combine solar energy absorption and transformation operando such as photoelectrochemical cells or solar batteries are highly promising concepts. The technical feasibility of large scale electrochemical renewable fuel generation is most limited by overpotential losses at catalysts, their often low abundance and limited stability. For this purpose, we studied delafossite oxides, making use of an intrinsically expanded noble metal sublattice. We observed an operando increasing activity for the materials with the lowest stability in acidic media, outperforming even bulk Pt in the interesting low overpotential region. Investigations into the growth of a corrosion induced cap layers revealed substrate induced strain effects, modifying the ability of Pd to catalyze the hydrogen evolution reaction. The kinetically facilitated and stabilized phase transition to β-PdHx appears to be induced by these strain effects, opening up an alternative materials perspective for HER and an interesting pathway to modify material properties and phase transition barriers. Second, we studied the photocatalytic activity origins of covalent organic frameworks (COFs). This family of crystalline porous polymers enables a bottom-up design of periodic organic structures for engineering opto-electronic properties on the molecular level. An application is photocatalysis. Here, the activity description often lacks knowledge about intrinsic optoelectronic properties, which consequently rely on calculations. This work is one of the first in the field addressing these issues and actually measuring photocurrents, which are related to photocatalytic activities. Besides, these currents were used to adapt band position determination theories, which were used for classical semiconductors, to COFs for the first time. Third, we investigated and explained the direct solar energy storage properties of new, 2D carbon nitrides (CNx), namely cyanamide (NCN-)-functionalized poly(heptazine imide) polymers (NCN-PHI). CNx are organic based semiconductors showing promise in photocatalytic applications. After having shown that photocatalytic hydrogen evolution could be triggered with significant time delays, we revealed the storage process of this material and have shown that it is suitable for new types of direct solar batteries, where the absorber and the storage medium are combined in the very same material. Besides this conceptual novelty, we reveal many interesting material features, such as light induced conductivity increase, the compatibility with different earth abundant ions and the interesting use as a (solar) battery electrode in aqueous media, where the material allows for larger potential windows of water based batteries, affecting energy densities positively.