Design And Synchrotron Radiation Technique Study For Highly Efficient Carbon-based Electrocatalyst

Electrocatalysis, as a means of chemical energy to electrical energy conversion,not only provides great benefits for the new energy supply and storage, but also can tackle environmental problems effectively. It has recently attracted tremendous attention. In particular, the rational densign of advanced catalysts is a key point in this field. There are two effective strategies for improving the performance of electrocatlysts: (1) increasing the amount of active sites; (2) tuning the intrinsic activity of catalytic sites. During the synthesis of traditional catalysts, it is used to control the size, morphology, composition and structure to pursue more efficient electrocatalyst. Single atom catalyst (SAC) was proposed in the recent year, and has aroused increasing interest in the field of heterogenous catalysis as the delevop of synthetic strategy. In contrast to conventional nanoparticle-based catalysts, SAC has a maximum efficiency of metal atoms, more sensitive to the size and structure,strong interaction between single metal atoms and supports. It is further determined that the SAC has showed high activity or selectivity in variety of reactions.Meanwhile, the synchrotron radiation (SR) light source has advantages of high resolution, good monochromaticity and strong penetrability. Combined with a varity of SR-based techniques, we can get more information on the species of nearest coordination atoms, bond length, and coordination number. These provide a unique platform for studing new catalysts and their hybrids.In this dissertation, we will primarily focus on the electrochemical applications of SACs. Significantly, it will be fully discussed the relationship between eletrocatalytic performance and catalyst properity, from the view of synthetic strategy, coordination structure, electronic structure, metal-support interaction and dimension of supports. We can precisely tune the amount and type of anchoring sites on the surface of supports, also metal species in order to increase the amount of active sites and improve the intrinsic acitivty at the same time. Depending on SR-based techniques, we give an in-depth understanding of the relationships among catalyst microstructure, electronic structure and its properties. In additional, we have also discussed the relationship between unsaturated sites and catalytic performance.Detailed results were summarized as follows:1. To solve the problem that the loading content of metal atoms is extremely low in SACs, we have proposed an in-situ pyrolysis method to prepare high-density single metal atoms supported on nitrogen doped graphene (SMAs@N-doped graphene), which used dicyandiamide (DICY), glucose and inorganic metal salt as co-precursors. The metal atoms were stabilized on the N-doped graphene by generating a metal-Nx coordination structure. DICY was a critical step to obtain the resultant atomically dispersed catalyst since it confined the polymerization of aromatic carbon intermediates in a cooperative process and slowly released metal atoms at an elevated temperature. Based on the characterization of XAFS and electrochemical measurements, the Fe@N-doped graphene cataltyst contained Fe-Nx structure showed a remarkable electrocatalytic performance for the oxygen reduction reaction in alkaline medium, and a great potential as a cathode catalyst on zinc-air batteries. This work not only offers a feasible approach to design and fabricate highly doped SACs, but also provides new opportunities for a comprehensive study on the high activity of SACs with Metal-Nx structure.2. To confirm the possible synergistic effect for dual-metal supported SACs, we present a facile approach to fabricate porous nickel/iron doped g-C3N4 hybrids coated on carbon nanotube bundles (NiFe@g-C3N4/CNT) via the high-temperature polymerization. The OER performance of NiFe@g-C3N4/CNT is much better than one type of metal (Ni/Fe) doped catalyst. Fro me the Ni/Fe L edge XANES spectra, it suggest that the electronic structure of NiFe@g-C3N4/CNT is very diffierent from that of single metal species catalyst. The oxidation state of Ni atoms is increased but that of Fe is decreased, which is possible the main reason for highly effiecient OER performance. Meanwhile, it is also confirmed that NH4Cl as a precursor play a vital role to form the core-shell structure of g-C3N4/CNT and increase the specific surface area, resulting in high OER performance. This work gives insight on the synergy of bimetallic doped material using a specific catalytic-system and characterized methods.3. To further verify the number of active sites/support and their positive effect on the performance of catalyst, we developed a facile solvothermal method to in-situ introduce more active sulfur ligands into MoS2 layers, increasing the amounts of active sites and triggering in-plane catalytic activity. It was confirmed by X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS).Furthermore, MoS2 nanosheets with a unique "branch-like" structure formed on the surface of carbon nanotube bundles, resulting in a strong electronic coupling between the two compositions by forming the interfacial Mo-S-C bonds. The synergy resulting from these two key factors should accelerate the catalytic kinetics,presenting more efficient HER performance. This work not only offers a more effective route to precisely tune the amounts of catalytic active-sites and enhance the interaction between catalyst and support, but also provides useful insight into the relationships among catalyst microstructure, electronic structure and its properties.4. The catalytic performance of SACs may change significantly due to the low-coordination environment and improved metal-support interactions. Apart from these, we have put forward that the dimension and space utilization of catalyst supports may also have a certain influence on catalytic performance for the first time.Here, we employed a OD onion-like carbon (OLC) to anchor single-atom of Pt(Pt1/OLC) as catalysts for electrochemical hydrogen evolution reaction (HER). For HER evalution, the onsetpotential and overpotential at same current are comparable to that of commercial Pt/C catalyst with 20 wt% Pt loading and better than that of 2D-based Pt1/Graphene catalyst. Based on XAFS and the atomic-resolution high angle annular dark field scanning transmission electron microscopy(HAADF-STEM), it indicates that the OD-OLC support has a higher efficiency in the use of 3D space to increase the chance of collision between active sites and reactants.Furthermore, our first-principles calculations reveal that the OLC act as tiny nano-supercapacitors from the space-charge created during electrochemical charging in the reaction, and electrons get localized around single atom Pt tips causing a strong electric field effect that promotes catalysis activity and reaction kinetics. This work suggests the possible effect on the dimension of loading support, and a new pathway in maximizing atom and space utilization rates of catalytic system via single atom engineering on select substrates.