Nanostructured iridium oxide electrocatalysts for PEM electrolysis

Kost, Melisande (2025): Nanostructured iridium oxide electrocatalysts for PEM electrolysis. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

In light of the growing demand for green and renewable energy, hydrogen has emerged as a promising energy carrier for the near future. Given its extensive range of applications, from basic chemical synthesis feedstock source to re-electrification with a fuel cell, the demand for green hydrogen is anticipated to grow significantly in the future. This thesis presents a number of projects that facilitate the efficient conversion of electrical energy to chemical energy with the acidic proton exchange membrane electrolysis of water for the purpose of efficiently producing hydrogen. The aforementioned approaches place an emphasis on sustainability with respect to the materials utilized, or alternatively, demonstrate a significant reduction in the consumption of rare and non-abundant iridium metal, which serves as the active component of the employed catalysts. The synthesized materials were subjected to extensive examination with regard to their performance and stability through the utilization of a range of physical analytical techniques, including electrochemistry ranging from static wet-cell experiments to single cell electrolyzers, X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy techniques. The overarching theme that emerges from this work is the impact of nanostructuring materials on their catalytic performance. In the initial project discussed in this thesis, a SnO₂ filament morphology was electrospun and wet-chemically coated with IrO(OH)x to create a composite material that acts as a catalyst for the oxygen evolution reaction. The filaments were oxidized at varying temperatures in air to ascertain the optimal temperature range for achieving high interconnectivity and crystallization of the IrO₂ layer, which resulted in enhanced conductivities and associated activities compared to the non-oxidized compound by establishing percolation pathways for electrons. Transmission electron microscopy was employed to elucidate the macroscopic processes undergone by iridium oxide at varying temperatures. An optimal temperature of 375 °C was identified, at which the interconnectivity and crystallinity were in an optimal state. At lower temperatures, the degree of crystallinity was insufficient for optimal conductivity and activity in the context of catalysis. Conversely, at higher temperatures, the increased degree of crystallinity and formation of larger crystallites resulted in the formation of isolated IrO₂ crystallites on the filament. This resulted in a reduction of conductivity and, consequently, in a decline of catalytic activity. In the subsequent project, the insights gained from the initial project were integrated, and the impact of nanostructuring the iridium oxide phase on catalytic behavior was further elucidated. Specifically, the stability of an epitaxial catalyst-support system in comparison to a non-epitaxial catalyst was investigated with regard to the oxygen evolution reaction in proton exchange membrane electrolysis. IrO₂ and SnO₂ were utilized, both of which possess highly similar lattice parameters and exhibit a proclivity for epitaxy. Conversely, IrO₂ on TiO₂, which predominantly comprised the anatase structure, renders epitaxy nearly impossible. The impact of firm anchoring and good contact, which arises from the epitaxial growth of IrO₂ on SnO₂, on the stability and activity of the catalyst was examined, particularly at lower iridium contents. Electrochemical experiments conducted with a corresponding membrane electrode assembly demonstrated enhanced activity in comparison to non-epitaxially grown IrO₂ layers on nanoparticulate TiO2 with a comparable iridium content. Further in-depth and long-term measurements are required to validate enhanced stability. In the final and concluding project of this thesis, an investigation was conducted into the potential for coating novel materials with IrO₂ through nanostructuring. An iridium oxide layer with a thickness of only a few atom layers was deposited on a highly crystalline silicified DNA origami architecture using atomic layer deposition (ALD). Furthermore, ZnO and TiO₂ were also deposited on pure (non-silicified) DNA origami. Scanning electron microscopy demonstrated the successful complete and homogeneous coating of the various compounds, penetrating fully into the DNA origami crystal structure and indicating consistent coverage. Proof-of-principle experiments for an application in acidic water electrolysis indicate that this DNA origami-based system exhibits promising stability, as evidenced by post-catalytic scanning electron microscopy imaging. Promising prospective applications of this system may be achieved through the skillful engineering of DNA origami crystals with larger pores or a consistent crystal film growth on suitable substrates. Furthermore, the nanostructuring of the material was achieved by coating the DNA origami with a thin IrO2 layer to split water. This development has opened up new paths for the functionalization of the material, paving the way for potential applications in areas such as photonic crystals. In conclusion, this work has demonstrated that nanostructuring of diverse support materials represents a promising strategy for the development of low-iridium containing catalysts for the oxygen evolution reaction in proton exchange membrane water electrolysis. These catalysts are distinguished by high activity and stability. An additional emphasis is placed on enabling synthesis methods such as a wet chemical process which yielded reproducible coating of semiconductor oxide supports on a range of solvent volumes producing catalysts from a few mg up to 5 g to establish electrolysis for producing green hydrogen to ultimately meet global energy demand. The comprehensive electrochemical and materials science characterization results enhance our understanding regarding the factors contributing to activity and stability of low-iridium water electrolysis catalysts. This enables the overcoming of morphological constraints, and subsequent studies based on this work can advance the understanding of energy conversion processes with the ultimate goal of further reducing the iridium content of oxygen evolution catalysts.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 12. Juni 2025
• 1. Bericht­erstatter:in: Bein, Thomas