Study On The Electrocatalytic Hydrogen Evolution Mechanism Of Step TiO₂ Anchored Nanoclusters/Single Atoms

The depletion of resources and environmental pollution caused by the excessive use of fossil fuels have aroused wide attention to the sustainable development of environmentally friendly energy.Hydrogen（H₂）is considered as a green and clean renewable energy because of its high calorific value and no pollution from combustion products.Electrolytic water hydrogen production technology has the advantages of high hydrogen production efficiency,clean environmental protection and so on.It is one of the effective ways to realize large-scale industrial hydrogen production.In the two half reactions that constitute electrolytic water,hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）,there are high power consumption phenomena caused by kinetic retardation.A lot of research work is devoted to the design and preparation of efficient electrocatalysts to solve this problem.From the production cost and catalytic performance,it is an effective means to design high efficiency catalysts based on non-precious metals or to improve the atomic utilization rate of precious metals catalysts by reducing the size.In this paper,we focus on the development of nano clusters and monatomic catalysts with high efficiency.On the one hand,the atomic utilization rate of catalytic active center can be improved by using the small size effect of catalyst.On the other hand,the chemical environment of the catalytic active center is affected by the surface interface engineering regulation of the substrate and the coordination environment of the metal center,so as to optimize the reaction kinetics and improve the catalytic activity.The main contents are as follows:（1）TiO₂ nanowires（S-TiO₂）with step structure were prepared by simple acid etching method,and the surface structures of the samples were characterized by scanning electron microscopy（SEM）and high resolution transmission electron microscopy（HRTEM）.Then,CoP₂ nanoclusters（CoP₂/S-TiO₂）were supported at S-TiO₂ step sites by impregnation and phosphating.The results of XPS test show that Co atoms in CoP₂/S-TiO₂ have lower valence,indicating that the step structure affects the electron density of Co atoms in the nanoclusters.The results showed that the overpotential required by CoP₂/S-TiO₂（η100=208 mV）decreased by 87 mV when the current density was 100 mA cm^-2,compared with the sample with smooth surface（CoP₂/F-TiO₂,η100=295 mV）.Moreover.CoP₂/S-TiO₂ also has smaller impedance and Tafel slope,indicating that the step structure of the substrate enables CoP₂ nanoclusters to show more favorable HER reaction kinetics,thus improving the catalytic performance.（2）Reducing catalyst size,preparing single atom catalysts（SACs）.and keeping the P coordination environment of Co center unchanged,while improving the utilization rate of atoms,can further explore the regulation effect of step configuration on the electronic structure of supported metal.The monatomic morphology of Co and the coordination number of P were determined by means of XAS.XPS and spherical electron microscopy,which showed that the Co-P₃ monatomic catalyst（Co-P₃/S-TiO₂）was successfully anchored on the surface of S-TiO₂.Density functional theory（DFT）calculation revealed that the P element at the edge site of the step has higher activity,which makes the Co atom at the edge site have better water adsorption and dissociation energy.The electrochemical test results show that Co-P₃/S-TiO₂ has excellent catalytic activity for HER.At 100 mA cm^-2,the overpotential is 1 86 mV.Compared with the nano-cluster sample（CoP₂/S-TiO₂）.the performance is improved.and it is significantly better than the smooth surface sample（Co-P₃/F-TiO₂）.η100=267 mV).It is confirmed that the step structure and P coordination in the substrate can significantly improve the performance of catalyst HER.and it also confirms the huge potential of SACs.which can improve the catalytic performance while improving the atomic utilization rate.（3）To further confirm the superiority of TiO₂（S-TiO₂）with surface rich step structure as a single atom catalyst base,Pt SACs/S-TiO₂ was anchored on S-TiO₂ nanowire array for electrocatalytic hydrogen evolution（HER）reaction.The monoatomic morphology of Pt was demonstrated by XRD,HRTEM,spheroidal correction of high resolution HAADF-STEM.EPR and Raman spectroscopy.The results of XPS test show that the unique step structure of S-TiO₂ surface can promote the electron transfer from Ti and O atoms to Pt atomic sites,so that the catalyst has stronger metal-support interaction.Electrochemical tests showed that Pt SACs/S-TiO₂(69 A mg^-1)had significantly better activity per unit mass than commercial Pt/C(17 A mg^-1),and HER catalytic performance was much higher than that of smooth surface samples（Pt SACs/F-TiO₂）.