Accelerated discovery of new catalysts for oxygen evolution reaction

• Main Author: Mahmoud G. Ahmed
• Other Authors: Lydia Helena Wong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/164790
• Institution: Nanyang Technological University

Electrochemical water splitting is a promising method to generate hydrogen gas (H2) as a green chemical fuel. However, the efficiency of overall process is limited by the sluggish kinetics of water oxidation reaction or oxygen evolution reaction (OER). As a result, large overpotential values are required to drive the reaction which in turn cause an inevitable rise in the energy consumption and lead to lower energy conversion efficiencies. Thus, exploring efficient OER catalysts is of great importance for energy conversion and storage systems. Synthesis of multi-metal spinel-based oxides is an effective strategy to increase the conductivity and tune the electronic structure and thus improve the OER catalytic performance. However, the rational design of an optimal multi-metal oxide is challenging. Although some studies have reported the synthesis of multi-metal oxides, the coverage of compositional diversity space is limited. The possible number of combinations upon mixing three elements far exceed the number heretofore reported or even that are synthesized and tested by conventional methods. In this thesis, high-throughput synthesis and catalytic measurements are demonstrated as a promising approach to discover new and efficient multi-metal oxide OER catalyst in the unexplored ternary composition spaces. Such high-throughput methods accelerate the synthesis and screening of catalytic activity 10 times more than that performed by traditional methods. The thesis also aims to investigate how electronic structure and synergistic electronic interaction in ternary composition space (Fe-Co-M, M= Cu, Mn and Cr) affect the OER catalytic activity. In this research, Co element is a priori selected because of its unique behavior in catalytic reactions which originates from various valence and spin states. Fe has been previously reported to improve the OER catalytic performance, even though its role is still debatable. The third metal ion is carefully selected based on its electronic configuration to see whether the electronic configuration of ternary oxide can be regulated for enhanced OER performance. For ternary composition space of Fe-Co-Cu oxides, the composition-catalytic activity relationship is well established and the optimum composition with high OER activity is identified. Crystal and electronic structure combined with electrochemical studies on the most active OER catalyst reveal that the cation substitution into spinel oxide synergistically manipulates the electronic states and provides more accessibility to the redox active species, resulting in enhanced OER catalytic performance. In the second composition space (Fe-Co-Mn) oxides, ternary Mn-based spinel oxide (Fe0.3Co1.2Mn1.5O4) is identified for the first time demonstrating high OER catalytic activity. Using a combination of soft-X-ray absorption spectroscopy (sXAS) and electrochemical measurements, Co2+ and Mn3+ in different geometric sites, tetrahedral (Td) and octahedral (Oh) sites, are found to be the main active sites. Fe3+ ion is found to significantly affect the catalytic activity by the confinement of more Co2+ in tetrahedral sites. The Fe3+ ions also promote the oxidation of Co species to form active oxyhydroxide phase. The high spin of Mn3+ ions (t2g3 eg1) can easily bond with OH– ions leading to high OER activity. The Mn-based spinel oxide demonstrates high OER performance and is one of the best Mn-based catalyst reported so far. Unlike Fe-Co-Cu- and Fe-Co-Mn-oxides, the ternary (Fe-Co-Cr) oxide displays unpredicted catalytic activity towards OER. The ternary oxide FexCoyCr(3-(x+y)O4, is identified as the optimum composition with high OER performance. It is found that the formation of non-bonding oxygen states close to the Fermi level triggers the lattice oxygen oxidation. Moreover, the spinel oxide undergoes irreversible surface reconstruction forming Co/Fe(O)OH active layer. The catalyst demonstrates an outstanding stability for over 72 h at high current density of 100 mA cm–2. The results can further help chemists to design an efficient catalyst with active and stable lattice oxygen for oxygen-related applications. In summary, this thesis succeeded in exploring efficient OER catalysts via high-throughput experimentation on unexplored composition spaces. This research demonstrates how the inactive cations affect the OER performance. Although the Co and Mn can occupy both tetrahedral and octahedral sites interchangeably, the highest catalytic performance as a result of combining the two cations is unraveled for the first time by high-throughput methods. Moreover, in the Fe-Co-Cr ternary oxide, we provided, for the first time, experimental spectroscopic evidence for the activation of lattice oxygen. These newly identified ternary-based spinel oxides display a unique behavior and fundamentally contribute to the understanding of such complex OER reaction.