| The depletion of fossil fuels and the deterioration of the environment have stimulated demands for renewable and clean energy supplies.Hydrogen is considered to be one of the most promising clean energy sources due to the advantages of comprehensive sources,zero pollution,and high calorific value,which has attracted extensive attention from scientists.Hydrogen production via electrochemically driven water splitting is one of the most promising alternatives to convert intermittent resources.This technology can use renewable energy such as solar,wind,and tidal energy to directly electrolyze water that exists widely in nature into high-purity hydrogen.Meanwhile,the electrolyzing process is mild,controllable,clean,and pollution-free.The critical bottleneck of water splitting lies in the oxygen evolution reaction at the anode since the four-electron process is kinetically more sluggish than the two-electron process of hydrogen evolution at the cathode.In addition,the large-scale deployment of this technology is restrained by the high cost of using noble metal oxides as catalysts.Therefore,it is of great significance to develop highly efficient and low-cost OER catalysts for promoting the hydrogen industry.Single-atom catalysts are an ideal model for the in-depth study of reaction mechanisms and catalytic performance with maximized atom utilization,uniform active sites,and unique electronic structures.In addition to changing the coordination structure of single atoms,modulating the single-atom anchoring site local environments can also regulate the properties of single atoms and improve their performance.This dissertation focused on modulating the single-atoms site local environment of cobalt-based single-atom catalysts to optimize the OER performance and metal loadings of single atoms.The modulating strategies are including fabricating cationic vacancies,constructing disordered regions,and introducing highly electronegative cations.Combining synchrotron radiation characterization techniques and theoretical calculations,in-depth mechanistic understandings of the correlations between single-atom anchoring site local environment and OER performance were obtained.The specific studies were made from the following aspects:1.The hydrogen-bonding interaction between single-atom species and oxygenated intermediates was modulated to break the universal scaling relation in the OER by fabricating cationic vacancies in the single-atom site local environment.First,the lattice of CoOOH was distorted by heterogeneous Cu atom doping.Synchrotron radiation X-ray absorption spectroscopy revealed the presence of cationic vacancies in the local environment of single-atom site.Subsequently,Ir single atoms were anchored onto the oxygen vacancies of CoOOH and Co0.8Cu0.2OOH surfaces by electrochemical anodic deposition,respectively.Electrochemical measurements exhibited a significantly improved OER performance of Ir1/Co0.8Cu0.2OOH.Specifically,Ir1/Co0.8Cu0.2OOH delivered a current density of 10 mA cm-2 merely with an overpotential of 228 mV,which was significantly 68 mV lower than that of Ir1/CoOOH.As revealed by further mechanistic studies,Irl/Co0.8Cu0.2OOH with cationic vacancies adjacent to the single-atom sites regulated the strength of hydrogen-bonding interactions between single-atom species and*OH intermediates,thereby differentiating the similar adsorption behaviors of*OH and*OOH intermediates.The differentiated adsorption behavior reduced the free energy difference between*OH and*OOH to 2.76 eV,thereby breaking the universal scaling relation in OER.2.Based on the in-situ leaching characteristics of F ions during OER,combined with the electrochemical anodic deposition technology,disordered regions were constructed in the single-atom site local environment,which significantly improved the single-atom loading and optimized the electronic structure of the active sites.F ions were introduced into the Co(OH)2 lattice by the wet chemical method,and the in-situ leaching of F ions during the OER would construct disordered regions in the single-atom site local environment.Electrochemical measurements showed that the specific activity of Ir1/CoOOH(F)was 2.7 times that of Ir1/CoOOH at an overpotential of 400 mV.As revealed by synchrotron radiation X-ray absorption spectroscopy and mechanism studies,a large number of oxygen vacancies in the disordered regions provided abundant anchoring sites for single atoms,which increased the loading amounts of single-atom by 39%.In addition,the existence of disordered regions elevated the valence state of Co,thereby optimizing the adsorption of*OH intermediates.This work not only provided a practical idea for improving the catalytic performance but also offered a preliminary exploration for further increasing the single-atom metal loadings and preparing ultra-high-density single-atom catalysts.3.The introduction of Ga ions in the single-atom site local environment optimized the electronic structure of anchoring sites,thus bringing the preparation of high-density single-atom catalysts into practice.The introduction of Ga ions affected the symmetry of the CoOOH crystal structure,thereby constructing a large number of oxygen vacancies,thus providing abundant anchoring sites for Ir single atoms.Elemental contents analysis showed that the single-atom loading amounts of Ir1/CoGaOOH were 12.20 wt%,which was 5.08 times that of Ir1/CoOOH.As revealed by synchrotron radiation X-ray absorption spectroscopy and theoretical calculation,the electron delocalization effect generated by Ga ions induced more delocalized positive charges around the oxygen vacancies,thereby enhancing the binding ability between the oxygen vacancies and the single-atom precursors,thus forming highly stable single-atom structures.Furthermore,electrochemical measurements exhibited that the OER current density of Ir1/CoGaOOH showed no apparent attenuation at a current density of 10 mA cm-2 over 2000 h.Moreover,we successfully anchored Pt single atoms onto the surface of CoGaOOH through the same strategy,which proved that the synthesis strategy of ultra-high-density single-atom catalysts has a certain degree of universality.This work provided a new strategy for preparing ultra-high-density single-atom catalysts for oxygen evolution. |
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