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Disputation Si Liu

Apr 26, 2024 | 11:30 AM
Thema der Dissertation:
Activity determinants and limiting factors in neutral-pH water oxidation investigated for an electrocatalytic cobalt-phosphate system
Abstract: The electrocatalytic oxygen evolution reaction (OER) is crucial in the sustainable production of hydrogen and other chemicals. To reduce operational risks and enable direct coupling to electrochemical CO2 reduction, neutral-pH OER is essential, but it is typically hampered by low current densities. Therefore, in this thesis, an amorphous CoCat material combined with a potassium dihydrogen phosphate (KPi) electrolyte is chosen as a model system to explore the activity determinants and limiting factors in OER at neutral pH.
Comprehensive analysis of current-potential curves combined with operando Raman spectroscopy as well as numerical simulation revealed that the macroscopic proton transport limitations in the bulk electrolyte determine catalyst-internal OER rates. Two pathways, buffer pathway and water pathway, facilitate electrolyte proton transfer. A proof-of-principle experiment shows that high current densities exceeding 500 mA cm−2 are achievable. Non-diffusive, convective flows dominate electrolyte proton transport in a stagnant electrolyser investigated by spatially-resolved operando Raman spectroscopy. Importantly, microscopic determinants of catalyst-internal redox and reaction chemistry show a non-conventional electrocatalysis rate mechanism, where it is the chemical state of the equilibrated material that governs the rate of catalysis rather than electrode potential. The rate of catalysis depends strictly exponentially on the CoIV concentration through electrochemical experimentation combined with X-ray absorption (XAS) and visible-light spectroscopy in operando experiments. Meanwhile, novel potassium K-edge XAS experiment and time-solved operando XAS at Co K-edge exclude the involvement of catalyst-internal redox-inert ions (potassium) in charge compensation events of cobalt oxidation. The results reported in this thesis are of general importance for an extended class of catalyst materials. The understanding of limiting factors for neutral water oxidation facilitates the knowledge-guided design of efficient, technologically relevant catalysts and water electrolyzers.

Time & Location

Apr 26, 2024 | 11:30 AM

FB-Raum (1.1.16)
Fachbereich Physik, Arnimallee 14, 14195 Berlin