Understanding the oxygen evolution reaction (OER) for co based transition metal oxides / hydroxides in alkaline electrolytes

• Main Author: Duan, Yan
• Other Authors: XU Zhichuan, Jason
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/157923
• Institution: Nanyang Technological University

The development of efficient electrocatalysts to lower the overpotential of oxygen evolution reaction (OER) is of fundamental importance in improving the overall efficiency of fuel production by water electrolysis. Among a plethora of catalysts being studied on, transition metal oxides / hydroxides that exhibit reasonable activity and stability in alkaline electrolyte have been identified as catalysts to potentially overpass the activity of expensive Ir- and Ru- based oxides. Understanding the oxygen evolution reaction (OER) for transition metal oxides / hydroxides in alkaline electrolytes paves the way for better design of low cost and highly efficient electrocatalysts. As discussed by Trasatti, “A true theory of electrocatalysis will not be available until activity can be calculated a prior from some known properties of the materials”. Substantial efforts have been devoted to understanding the factors that determine the OER activity and searching for activity descriptors. Different parameters have been studied throughout the years and descriptors including enthalpy of transition from a lower to higher oxide states, the binding energy of surface oxygen, the number of 3d electron for bulk transition metal cations, eg occupancy number of transition metal cations and covalency bonding transition metal ions to oxygen have been reported as OER activity descriptors. However, for some very active transition metal oxides / hydroxides, their surface can be changed when interacting with the surrounding electrolyte. Hence, interfacial descriptors, examples including surface redox transition from lower to higher oxidation state of the oxide precedent to OER, the potential of zero charge or the change in a work function on the adsorption through the water layer have been developed. This dissertation, with three different work on Co-based oxides / hydroxides, studies and deepens the understanding of the bulk properties, surface properties of materials and interfacial properties on OER. Firstly, with Fe substitution, it addresses tuning the eg configuration of metal cations in LaCoO3 where adjusting the metal 3d oxygen 2p covalency can bring benefits to the OER performance. Secondly, with Ni substitution in ZnCo2O4, it demonstrates a change in relative position of O p-band and MOh d-band centre which induces a change in stability as well as the possibility for lattice oxygen to participate in the OER. Stable ZnCo2-xNixO4 with small amount of Ni substitution follows adsorbates evolving mechanism (AEM) under OER conditions. Lattice oxygen participate (LOM) in the OER of metastable ZnCo2-xNixO4 with larger amount of Ni substitution. Lattice oxygen participating mechanism would give rise to the continuous formation of oxyhydroxide as surface-active species and consequently enhance the OER activity. Finally, with La1-xSrxCoO3 series, CoOOH and Fe-containing CoOOH as examples, the impact of the electrolyte has been explored by the study of reaction kinetics parameters. With a better understanding of how material properties and dynamic environment influence the OER activity and mechanism, we can obtain more efficient OER catalysts for better energy infrastructure.