Nocera, Daniel GHartnett, Alaina C.2025-09-1820252025-06-052025Hartnett, Alaina C. 2025. Exploring Interfacial Phenomena During the Electrochemical Oxygen Evolution Reaction. Doctoral Dissertation, Harvard University Graduate School of Arts and Sciences.32039409https://dash.harvard.edu/handle/1/42719743Hydrogen plays a critical role in industrial processes such as chemical manufacturing, petroleum refining, and fertilizer synthesis—industries that together contribute approximately 4% of global carbon emissions. These industries currently utilize hydrogen produced via the emissions-intensive steam methane reforming (SMR) process, but production of green hydrogen via electrochemical water-splitting is a promising avenue for wide-scale decarbonization. The production of abundant green hydrogen also has the potential to unlock new use-cases including H2 as a direct iron reductant, H2 as an energy carrier, or H2 as a fuel precursor to decarbonize steel production, shipping, and aviation. However, the emergence of green hydrogen relies upon the development of improved water-splitting catalysts and electrolyzers that exhibit better energy efficiencies (related to the electrochemical overpotential) and lower costs (related to the catalyst materials). Water-splitting involves both the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Of these, improvement of the OER is particularly challenging as it is a kinetically hindered process requiring four electron transfers and often involves operating conditions which corrode and destabilize earth-abundant catalyst materials. Improvement of OER catalysts (OEC’s) and overall electrolyzer systems requires a detailed understanding of processes occurring at the electrode-electrolyte interface and further development of catalyst materials, electrolyzer systems, and techniques for studying the electrochemical interface. This thesis explores multiple phenomena occurring at the electrode-electrolyte interface including (1) the formation of local pH gradients, (2) catalyst surface deconstruction, and (3) fundamental catalyst-water interactions, as well as highlights the development of novel techniques for examining catalysts in operando. First, an acid-stable catalyst was used to investigate the formation of locally acidic environments during OER under varied operating conditions. These experiments culminated in the development of a model for quantifying local pH gradients during OER. Secondly, mixed-metal OER catalyst materials with improved OER activity were developed and the origins of activity enhancement were determined. These materials, comprised of rare-earth cations incorporated into transition-metal oxide host materials, revealed increased surface oxide deconstruction, leading to increased active site density. Surface oxide deconstruction, determined here to be the cause of increased activity, is likely a widespread phenomenon, relevant in many OEC systems. Thirdly, a surface-sensitive X-ray technique, ambient-pressure X-ray photoelectron spectroscopy (APXPS), was utilized to explicitly study the solid-water interface of mixed-metal oxide catalyst films. Interfacial catalyst-water studies reveal that lanthanide incorporation results in an increase in partially negative surface oxygen species. The mixed-metal catalysts have multiple protonation states, resulting in a buffering effect which temporarily prolongs mixed-metal catalyst operation in acidic conditions. Finally, this thesis culminates in the development of a proton-exchange-membrane (PEM) electrolyzer assembly incorporating the mixed-metal catalyst materials developed herein. The advanced technique of operando time-resolved APXPS was utilized to examine mixed-metal OEC operation in an industrial-type electrolyzer system for the first time. This dissertation includes studies ranging from the elucidation of fundamental surface interactions to the engineering of electrolyzer assemblies for advanced characterization. These projects have all furthered our understanding of the OER, enabling future advancements in electrolyzer technologies.application/pdfenElectrochemistryOperando SpectroscopyOxygen EvolutionChemistryMaterials ScienceEnergyThesis or Dissertation2025-09-180000-0001-7920-4708