Efficient and stable interfaces for photoelectrochemical energy conversion

• Applicant: Dr. Johanna Eichhorn
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 428591260

A promising strategy for directly converting sunlight into storable fuels is photoelectrochemical conversion using semiconductor photoelectrodes. However, major unsolved problems of these photosystems are poor efficiency and material instability under operation. Current strategies to overcome this limitation focus on protecting/passivating the photoelectrode surface with conformal, ultra-thin functional coatings. Designing such heteroarchitectures is in general a non-trivial task, since it cannot be assumed a priori that the photoactivity of a semiconductor/electrolyte junction will be maintained with the application of a conformal surface coating. Furthermore, it is also crucial to realize that no corrosion protection coating or engineered interface is perfect, and microscopic imperfections can lead to a catastrophic system failure. Hence, for achieving stable and efficient operation, the proposed project will explore the synergistic combination of intrinsic self-passivation of semiconductor surfaces and functional coatings deposited by plasma enhanced atomic layer deposition (PE-ALD). The self-passivation mechanism of the photoelectrode forms chemically stable regions in areas of vulnerability and thus stops material degradation which will increase the overall stability of the multilayer photoelectrode. In these multilayer systems, the interface properties will be tailored to enable efficient charge transfer and to maintain the photoactivity of the semiconductor, while the surface coatings will be tailored to provide durable surfaces on the photoelectrodes with improved oxygen evolution reaction kinetics. In the first funding period, the project initially focused on Ta3N5 as semiconductor photoelectrode and CoOx as catalytically active coating. In the second funding period, we will also explore the combination of Ta3N5 with novel nitride catalyst coatings (Co4N) to mitigate the current limitations associated with combined oxide-nitride multilayer photoelectrodes. For both systems, we will systematically characterize structure, chemical composition, and defect states at the interface as well as their impact on charge transfer and transport mechanism to understand efficiency limitations in these multilayer photoelectrodes. For the nitride catalyst, the behavior under realistic operation conditions will also be analyzed to understand the basic working mechanism of these novel catalytic coatings. Lastly, we will quantify the effectiveness of the self-passivation strategy to provide insights into possible failure mechanisms. To this end, we will elucidate the roles of different deterministically introduced defects on the stability of the PE-ALD layers. Overall, this understanding will advance the development of novel photoelectrochemical systems with improved efficiency and stability. The proposed methodology provides a general strategy independent of the specific material system.

• DFG Programme: Research Grants