Atomic level tailoring of two-dimensional metallenes for high performance electrocatalytic conversion

• Main Author: P Prabhu
• Other Authors: Lee Jong-Min
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Chemistry; Engineering; Electrocatalysis; Hydrogen evolution reaction; Oxygen reduction reaction; Electrochemistry; Metallenes
• Online Access: https://hdl.handle.net/10356/181057
• Institution: Nanyang Technological University

Dependence on non-renewable fossil fuels to meet the escalating global energy consumptions has raised concerns over its eventual depletion. Besides, combustion of these carbonaceous resources releases extensive greenhouse gases, thereby harming our environment. These perspectives question a sustainable future, necessitating solutions that can effectively address the energy and environmental crisis. Electrocatalysis presents a promising avenue as electricity derived from renewable energy can be utilized to drive specific molecular conversions from earth abundant feedstocks into clean electro-fuels or value-added chemicals. Water electrolysis, for instance, presents a sustainable and environmentally benign pathway for generation of hydrogen gas, a clean fuel which produces only water as a byproduct. Else, storage systems such as rechargeable batteries are indispensable as electricity from renewables can be efficiently stored as chemical energy, and later utilized on point of need. Water electrolysers and rechargeable batteries certainly hold tremendous merit in bringing us closer towards a sustainable future. At the heart of these technologies lie electrocatalytic conversion reactions, namely the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), which ultimately govern the overall efficiency of these techniques. As these reactions are thermodynamically unfavorable in nature and kinetically sluggish, high-performance electrocatalysts are critical for accelerating the reaction kinetics and alleviating excessive overpotentials. To date, state-of-the-art catalysts for HER and ORR are based on platinum (Pt) and its alloys, giving rise to excellent intrinsic activities and superior reaction kinetics due to optimum binding energy with hydrogen and oxygen intermediates. Nonetheless, the extensive cost, coalescence behavior and poor electrochemical durability raises ineluctable concerns over the viability of Pt-based electrocatalysts. In this regard, high-performance electrocatalysts that exhibit excellent catalytic activity and durability are urgently required for use as alternatives to Pt. Metallenes, atomically thin two-dimensional metals, demonstrate intriguing physicochemical properties including high specific surface area, dense and readily accessible active sites, maximized atomic utilization and high surface reactivity due to atomic under-coordination, thereby rendering them fascinating materials for electrocatalytic applications. Furthermore, through suitable physicochemical modification strategies, the inherent activity of surface atoms may be effectively refined to further drive enhancements in their electrocatalytic capabilities. The tailoring of catalytic surfaces, surface topologies and chemical compositions at the atomic level is critical for attaining advanced metallene materials endowed with superior catalytic performance as will be demonstrated through two such strategies, first being the functionalization with single atoms and second, the surficial confinement of sub-nanometric clusters. In the first part of this dissertation, three kinds of rhodium (Rh) metallene variants, namely Rh-O-W, Rh- O-Mo and Rh-O-Cr metallenes, each incorporated with secondary metal single atoms (i.e., tungsten (W), molybdenum (Mo) and chromium (Cr) single atoms, respectively), was developed through a facile solvothermal reduction methodology. Through consideration of extensive variables such as reaction temperature, reducing environment, surface capping agent, concentration of secondary metal precursors, amongst others, electrocatalysts with a desirable morphology and functionalized single atoms was achieved. For Rh-O-W metallenes, the presence of single atomic W sites was verified through extensive characterizations. Importantly, the nature and coordination environment of single atoms was probed for through ex-situ XAS characterization. The W single atoms was found to coordinate through oxygen bonding and thus, oxygen-bridged single atomic W stabilized on surficial Rh is evidenced to be the most probable configuration. In the second part of this dissertation, the electrochemical performance for pH-universal HER and origin of electrocatalytic activity for as-synthesized Rh based electrocatalysts was investigated. Rh-O-W metallene was demonstrated to render excellent functionality as high-performance electrocatalyst for pH- universal HER outperforming that of even benchmark Pt/C catalysts and numerous other precious-metal based catalysts reported in literature. Combined operando XAS characterization and DFT calculations evidence that strong electron interactions between -O-W and Rh surface atoms in Rh-O-W metallene drive electron localization at Rh active sites, thus altering the surface reactivity and facilitating favorable energetics for hydrogen and water adsorption. The Rh-O-W metallene delivers high-performance capacity for pH-universal HER through enhanced water adsorption and dissociation as well as hydrogen adsorption and formation behaviors. In the third part, the confinement of high-density subnanometric osmium clusters (Os NCs) on the surficial layer of atomically thin palladium (Pd) metallenes was demonstrated through a facile solvothermal reduction methodology. Through consideration of extensive variables such as reaction temperature, reducing environment, surface capping agent, concentration of metal precursors, amongst others, Os@Pd metallenes with a desirable morphology was achieved. The presence and physicochemical properties of sub-nanometric Os NCs confined on Pd lattice was validated through extensive characterizations. Importantly, quantitative EXAFS curve fitting analysis performed for the structural parameters surrounding the Os atoms revealed that Os clusters with varied structural geometries can be realized, simply via varying the Os precursor content in the initial reaction medium. Finally, the bifunctional performance of Os@Pd metallenes for alkaline HER and ORR was investigated. Os5%@Pd metallenes, constituting two-atomic-layer Os18 clusters stabilized on Pd surface, was found to deliver an optimal alkaline HER performance with a remarkably low overpotential and a superior TOF value, surpassing that of even benchmark Pt/C. In conjunction, Os5%@Pd metallene also renders exceptional functionality for ORR, giving rise to outstanding half-wave potentials, outperforming that of even benchmark Pt/C and most other reported precious-metal catalysts in literature. DFT calculations reveal a strong charge redistribution taking place from Os to Pd atoms, thus forming distinct active centers with appropriate d-band centers. In the case of electron deficient Os clusters, the upshifted Os d-band edge facilitates readily accommodation and activation of electron-rich H2O in the Volmer step, thus guiding alkaline HER with enhanced kinetics. Whereas the downshift in Pd d-band center as induced by Os cluster confinement, weakens interaction of oxygenated species with Pd surface sites, thus making way for optimized free energetics along the ORR reaction pathway.