Local Structure Analysis and in situ Characterization Studies of Transition Metal Oxides for Oxygen Evolution Reaction

In the context of building a sustainable economy, hydrogen has emerged as a potential fuel for the future. Among the various methods for producing hydrogen, water splitting stands out particularly as a clean approach. Therefore, the development of active electrocatalysts and their fundamental studies for the oxygen evolution reaction (OER) have received tremendous attention. This work focuses on the investigation of two interesting categories of transition metal oxide-based electrocatalysts: (i) perovskites and (ii) Co/Ni-B oxides. The degree of crystallinity and phase purity play an important role in tuning the electrocatalytic activity of the catalysts. Therefore, it is important to tune the synthesis to obtain good control over the size, composition, and crystallinity. Chapter 3 reports a detailed analysis of the structural evolution of perovskites via solution combustion synthesis using in situ diffraction and total scattering experiments. It also discusses the detailed characterization of the obtained perovskites at the atomic, local, and bulk scales. LaNiO3 is used as the model system in this study. This chapter provides new insights into the design principles of perovskites. Chapter 4 focuses on a detailed study of surface reconstruction and dynamic active site evolution in LaNiO3 perovskite during OER. A combination of ex situ and in situ characterization techniques is used to gain a comprehensive understanding of the perovskites. Identical location scanning transmission electron microscopy shows how the catalyst surface has changed before and after OER. Electron energy loss spectroscopy provides information on how the oxidation state and local environment of the elements have changed during OER. In situ electrochemical Raman spectroscopy helps to monitor the local changes in the perovskite with the application of voltage, monitoring the real-time surface evolution, active species formation, and surface reconstruction in the system. Due to the affinity of the active Ni centers for the electrolyte impurities, especially Fe, this chapter also discusses the iron incorporation dynamics of LaNiO3. This chapter establishes functional links between the perovskite properties, the influence of electrolyte impurities, the surface evolution, and the reaction mechanism. Heteroatom doping is known to increase the OER activity. Studies show that the introduction of Fe into rare earth perovskites such as LaNiO3 enhances the intrinsic OER activity. In Chapter 5, a series of LaNixFe1-xO3 perovskites are studied in detail to decipher the surface evolution and active species formation during OER. To address the structural complexity and sensitivity of the short-lived active species formed during OER, in situ electrochemical surface-enhanced Raman spectroscopy and electron energy loss spectroscopy are employed. The affinity of the active Fe centers for the presence of Ni impurities in the electrolyte is also discussed in detail. This chapter provides a deeper insight into the Ni-Fe synergy during OER. Recent studies have shown that the incorporation of more electronegative elements such as boron into transition metal oxides facilitates charge transfer and improves OER performance. Also, the amorphous materials have shown better OER performance compared to their crystalline counterparts. Chapter 6 discusses the structural evolution of partially amorphous Co8Ni-B oxides using X-ray total scattering experiments (and subsequent PDF analysis) in combination with X-ray absorption spectroscopy. The morphology and crystallinity of the samples obtained at different temperatures are studied by electron microscopy. The structural information obtained at different temperatures is further correlated with the OER activity. This chapter highlights the structure-property relationships of these electrocatalysts.