| Developing renewable and zero-emission clean energy is becoming increasingly important in mitigating global warming and environmental deterioration.Hydrogen,as a clean energy,with environmentally friendly,high energy density and product purity,has the vast potential to solve the energy crisis.Electrocatalytic hydrogen evolution reaction（HER）is an effective strategy for producing hydrogen,and Platinum-group noble metals are considered the best HER catalysts.However,their large-scale application is limited due to their scarcity in the earth’s crust as well as their inadequate durability.Recently,single-atom catalysts（SACs）have attracted much attention due to their homogeneous active sites,high catalytic selectivity,high atom-utilization efficiency and the use of few noble metals,and have proven to be an effective strategy for reducing the use of noble metals and lowering catalyst costs.However,there are still several challenges and issues in the study of SACs.These can be specifically divided into the following categories:i)the issue of the type of support being relatively homogeneous;ii)the issue of the aggregation of single atoms in SACs;iii)the issue of low catalytic efficiency due to low metal mass loading;iv)the issue of active sites being isolated from each other.Therefore,it is of utmost urgency and significance to study the support materials of SACs,expand the SACs support library,develop novel SACs structures that can withstand harsh reaction conditions for extended periods,establish efficient,well-controlled and reproducible methods for synthesizing SACs with significantly improved loading efficiency,design binary or multi-component SACs with synergistic catalytic sites,and investigate their corresponding catalytic reaction mechanisms.To address the aforementioned issues,this thesis starts from the following four key areas:i)focusing on transition metal nitrides,taking vanadium nitride（VN）as a specific example,investigating the potential of transition metal nitrides as support materials for noble metal SACs;ii)preparing in-lattice-substituted SACs by selecting guest metal atoms with similar atomic radii to the support atoms to enhance the durability of these catalysts under highly reducible conditions;iii)developing a novel ice-photochemical-freezing（IPF）method for synthesizing SACs to improve loading efficiency,in combination with high specific surface area supports,to achieve high-loading SACs fabrication;iv)rationally designing binary synergistic catalytic sites to enhance the catalytic reaction kinetics of alkaline HER.The main research objectives of this thesis are outlined as follows:（1）To expand the SACs support library,this thesis takes transition metal nitrides（using VN as an example）as the support and predicts through first-principles density functional theory（DFT）calculations that the binding energy between Platinum-group noble metal atoms and the VN support surface is lower than that on N-doped graphene supports,revealing the potential of the VN support for loading Platinum-group noble metal atoms.Systematic characterization studies show that the VN support has abundant N unsaturated sites（with a site density much higher than that of N-doped graphene supports）,porous morphology characteristics,as well as metallic electronic transport behavior.Through atomic-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）and synchrotron X-ray absorption spectroscopy（XAS）characterization,the successful fabrication of Pt-based SACs was verified from both micro and macro perspectives,and the universality of VN supports for Pt-group noble metal loading was verified by HAADF-STEM characterization.Finally,taking Pt-based SACs as an example,their activity and stability in electrocatalytic acidic HER applications were compared with commercial Pt/C catalysts.（2）To address the aggregation issue of SACs under highly reducible conditions,we prepared in-lattice-substituted Co SACs using VN as support by introducing Co metal atoms with a similar radius to V atoms.The lattice substitution characteristics of Co atoms in the VN support were confirmed by HAADF-STEM and synchrotron XAS characterization.Compared with the supported Co SACs,the in-lattice-substituted Co SACs showed excellent durability in the alkaline HER even at high current densities up to-1000 m A cm-2.The mechanism for superior durability was investigated by ex-situ XAS and identical-location TEM（IL-TEM）techniques.Finally,we assembled an alkaline anion exchange membrane electrolyzer to evaluate the potential of in-lattice-substituted Co SACs in practical applications and confirmed its high activity and long-term stability under actual working conditions.（3）To address the low-loading issue of SACs,we restricted the metal atoms in a solid-state ice precursor to prevent metal aggregation and directly converted the solid-state ice into a gas state to preserve the single metal atoms on the support surface,significantly improving the SACs loading efficiency.The effectiveness of the IPF method for enhancing SACs loading efficiency was verified by comparing the theoretical and experimental loading differences.The single-dispersion characteristic of Pt single atoms on the Ti N support surface was confirmed by aberration-corrected HAADF-STEM analysis.Additionally,we investigated the universality of the IPF synthesis strategy by introducing it on both defect-free single-layer graphene and defect-rich amorphous carbon film supports,thus confirming the versatility of the technique.In the application of acidic HER,high-loading Pt SACs showed significantly higher activity and stability than the low-loading counterpart.（4）To address the lack of synergistic active sites in SACs,we prepared corresponding binary and ternary SACs by introducing the precursor of binary Pt Pd and Pt Ru,as well as ternary Pt Pd Ir species,into the VN support.Binary Pt Pd,Pt Ru,and ternary Pt Pd Ir atoms were confirmed by aberration-corrected HAADF-STEM analysis,verifying the successful preparation of binary and ternary SACs.Taking the binary Pt Ru SACs supported on VN as an example,we explored the synergistic enhancement effect of binary SACs on acidic HER activity and stability.To address the slow kinetics issue of alkaline HER,we explored the dispersion of binary Pt Co on the Ti N support surface by introducing Co metal atoms with appropriate oxophilicity and Pt metal atoms with low hydrogen binding energy and increasing the density of binary Pt Co atoms using the IPF synthesis strategy.Finally,we investigated the alkaline HER activity,stability,and kinetic mechanism of binary Pt Co SACs in alkaline HER applications.Based on the above research results,this thesis has profound reference significance in the aspects of how to design an ideal SACs support,how to effectively improve the stability of SACs under harsh conditions,how to increase metal loading of SACs to meet industrial demand,and how to design multi-element SACs for complex catalytic reactions rationally.On the other hand,this thesis expects to expand the existing SACs synthesis system and improve the current understanding of the structure-activity relationship（i.e.,"Support—Metal atom—Catalytic activity"）of SACs. |
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