Activating T2g orbitals by tetrahedral vacancy in spinel oxides to promote water oxidation

• Main Author: Shen Jingjing
• Other Authors: XU Zhichuan, Jason
• Format: Thesis-Master by Research
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Energy materials; Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/154821
• Institution: Nanyang Technological University

Spinel oxides have been substantially explored as OER catalysts due to their comparable activity compared to noble electrocatalyst. The octahedral site of the spinel oxide is commonly reported to be more catalytically critical than the tetrahedral site, thus many research groups have focused on modifying the octahedral site to enhance OER performance. Besides octahedral modification, altering the tetrahedral sites also can indirectly influence the octahedron and affect the OER activity. By replacing Zn with a low electronegative metal such as lithium, the electron cloud of oxygen will be more polarized towards the octahedron, thereby introducing higher covalency of Co-O and causing OER performance to be enhanced. Besides, lithium cation does not participate in metal and oxygen hybridization at the energy region near-Fermi-level because the energy of the Li 2s orbitals is very low and far away from the Fermi level. Moreover, lithium metal vacancy can be easily induced by high temperature since it is very light. These results encouraged the investigation of whether an even lower electronegativity can be achieved in the tetrahedral site. Therefore, theoretical calculation was first employed to investigate the relationship between the loss of metal tetrahedral sites and the Co 3d and O 2p band centers relative to the Fermi level, which concluded that the tetrahedral vacancy in the electrocatalyst could induce t2g orbital activation. To design an electrocatalyst with tetrahedral vacancies, LiCo2O4 has been synthesized at different annealing temperatures. Various characterization techniques such as ICP, XPS, and XAS techniques were employed to identify the tetrahedral site vacancies. ssNMR, and magnetic analysis have shown the presence of paramagnetic Co3+ (intermediate or high spin) in LiCo2O4 400 °C, which means that t2g orbital is activated. Moreover, EELS analysis has shown that empty states are induced in the t2g orbital. Electrochemical measurements reveal that the OER activity of the LiCo2O4 400 °C is superior to that of CoOOH and IrO2. The recommendations have been proposed to conduct theoretical calculations to calculate the binding strength of adsorbates to understand the rate-determining step in OER. Furthermore, OER performance can be systematically investigated by doping other transition metals in the octahedral site. Further follow-up experiments should also be conducted on the 500 °C sample to study surface reconstruction.