Synchrontron Radiation Study On The Structure And Performance Of New Energy Materials

With the rapid development of the global economy and technology, energy and environment have become the two most serious problems facing the international community. Optimizing the energy supply structure and developing new highly efficient, low-carbon, clean energy resources represent an important direction for sustainable development in the world today. As the key to the exploration and power utilization of new energy resources, new energy materials are still in the accumulation stage. Specifically, many problems such as low energy-conversion efficiency, low energy-density and high cost still exist. Further expanding the types of new energy materials and investigating the underlying relationship between structure, composition and performance relationship based on an appropriate material model are of great significance to improve the utilization level of new energy resources and further achieve their widespread use.In this thesis, single-atom catalysts and atomically thick two-dimensional material were designed and synthesized, serving as an entry point to the study on highly efficient new energy materials. Well-defined structure-performance relationship was precisely investigated, aiming to providing new thought for further design and optimization of new energy materials. First, single-site active Co/C3N4 was prepared and realized highly efficient spontaneous overall water splitting. Meanwhile, the inherent mechanism was determined to be effecitve photocarriers separation. Metallic tin nanosheets with atomic-thickness were synthesized for carbon dioxide electroreduction.The coordination-unsaturated surface structure was demonstrated to efficiently stablize the carbon dioxide radical anion in combination of XAFS and electrochemical characterization. In addition, Operando XAFS technique was used to reveal the single-atom structure of Fe in the lcFe-Pt/SiO₂ catalyst and the detailed reaction mechanism for high efficient Preferential Oxidation of CO in H2 ?PROX? reaction condition was further revealed. The main content in this dissertation is as follows:1. Study of single-site active Co/C3N4 for photocatalytic overall water-splittingThe water-splitting process involves complex multi-electron and multi-step reactions, which are very demanding on the photocatalyst material. Few photocatalysts so far can efficiently accomplish overall water splitting without using sacrificial electron donors. In the work of Chapter ?, single-site Co/C3N4 was precisely designed and constructured to effectively promote photocharge separation for highly efficient overall water splitting. Owing to the strong confinement effect of g-C3N4, the hybrid structure can be well trapped to achieve atomically dispersion. Synchrontron Radiation XAFS and high-angle-annular-dark-field scanning transmission electron microscopy?HAADF-STEM? unambiguously revealed the single-site Co1-P4 confined on nitrogenated holey g-C3N4 nanosheets. The hybrid structure induced midgap state in the electronic band structure, as indicated by UV-vis diffuse reflectance spectroscopy ?UV-vis DRS? and synchrotron radiation photoemission spectroscopy ?SRPES?, which not only significantly promoted visible-light harvesting, but also effectively surpass photocarrier recombination and prolong carrier lifetime by -20 times relative to pristine g-C3N4. The developed single-site photocatalyst exhibited steady and efficient water splitting activity with H2 evolution rate of 410.3 ?molh^-1 g^-1 and quantum efficiency as high as 2.2% at 500 nm in the absence of sacrificial reagents and noble metal under simulated sunlight irradiation.2. Study of atomically-thick metallic tin nanosheets for carbon dioxide electroreductionUltrathin nanosheets often exhibit a series of special physical and chemical properties due to the two-dimensional electron confinement effect. In the work of Chapter ?, ultrathin metallic Sn nanosheets were synthesized by means of graphene confinement, resulting in a sandwich structure with an average thickness of 1.4 nm. The corresponding current density for carbon dioxide electroreduction reached 21.1 mA cm-2@^-1.8 V vs. SCE, 13, 2.5 and 2 times higher compared to that of Sn bulk, 15 nm Sn particles and 15 nm Sn particles/graphene physical mixtures, respectively. As shown by Sn K-edxge XAFS analysis and fitting results, Sn in the Sn nanosheets was obvious coordination-unsaturated and the coordination number of Sn-Sn significantly reduced from 2 and 4 to 1.4 and 2.7. Further electrochemical characterization demonstrated this coordination-unsaturated structure can efficiently stabilize the carbon dioxide radical anion, providing an important experimental basis for highly efficient carbon dioxide electroreduction catalyst design.3. Study on single-site iron Fei-Pt/SiO₂ catalysts for preferential oxidation of CO in H2 ?PROX?In situ/Operando XAFS is an important method to gain insight into the dynamic structure evolution of a working catalyst. However, it often affords the ensemble-averaged structure information of all absorption atoms in the catalytic system, selective separation of the catalysts' surface active-site information still remains a great challenge.In spite of high catalytic activity, the single-atom catalysts also possess uniform active sites, providing a simple and ideal model for XAFS investigation on detailed structure-performance relationship. In the work of Chapter V, 1 cFe-Pt/SiO₂ catalyst was precisely prepared for the PROX reaction by ALD, exhibiting a 100% CO selectivity and conversion over a broad temperature range of 198-380 K. Operando synchrontron radiation XAFS measurements were performed and it was first observed that Fe3+ of lcFe-Pt/SiO₂ could even be reduced to Fe2+ under mild conditions of H2 reduction at room temperature. The majority of Fe was in the single-site form regardless of their different valence states. Meanwhile,atomically dispersed Pt-Fei?OH?3 species was determined to function as active sites for PROX reaction condition and the density functional theory further confirmed the high catalytic activity of this structure.