• Frisch, Marvin Lionel

Abstract

Large-scale production of hydrogen (H2) by renewable energy powered water electrolysis represents a promising strategy to cut global carbon dioxide emissions and ease fossil fuel depletion. However, state-of-the-art electrolyzers and fuel cells employ high loadings of rare and expensive noble metals (e.g. Ir, Pt) as active electrocatalysts, hampering wide-scale adoption of industrial electrolysis and electricity generation, respectively. The design of nanostructured high-surface-area electrocatalysts with control over composition, morphology, porosity, crystallinity and surface sites can promote the overall efficiency of both processes. In water electrolysis, the oxygen evolution reaction (OER) is considered the main bottleneck due to its sluggish kinetics and high overpotentials. This thesis contributes to an optimized utilization of OER-active species based on a rational design of surface-modified electrode coatings. A sophisticated concept is presented, which proposes transition metal oxides and carbides with template-controlled pore structures as conductive supports for both precious (IrOx) and non-precious (NiO) metal species introduced as ultra-thin surface coatings via conformal atomic layer deposition (ALD). The optimized catalyst-support systems show outstanding mass-specific OER activities and outperform literature-reported catalysts as well as commercial references. Bulk, surface-sensitive and local characterization techniques are combined for the deduction of structure-property-activity correlations that are further evaluated by theoretical calculations in order to improve the electrical properties of the support. In this context, a decisive impact of matching the interfacial properties between semiconducting, oxidic supports and the ALD surface layer is found. Employing soft-templated mesoporous tungsten carbide (mp. WCx) films synthesized via an innovative approach as supports with metallic conductivity can significantly enhance the catalytic performance for both alkaline (NiO-ALD) and acidic (IrOx- ALD) OER. The excellent conformality of ALD guarantees the formation of a dense surface layer which is shown to prevent the underlying support from oxidative dissolution and/or degradation during electrocatalysis. A basic understanding of the ALD parameters is used to adjust the properties of the deposited surface species, the morphology as well as the layer thickness, and hence serves as the bedrock in application-driven catalyst design. The concept of improving the utilization of catalytically active species via conformal ALD on mesoporous metal oxides and carbides with tailored electronic properties is exemplarily proven by the successful development of highly efficient, i.e. active and stable, electrocatalysts for both alkaline and acidic OER.

Topics

• impedance spectroscopy
• pore
• morphology
• surface
• Carbon
• Oxygen
• carbide
• Hydrogen
• porosity
• tungsten
• crystallinity
• atomic layer deposition