Study On The Interface Structure And Process Of Pt-based Single Crystal Electrode In Acid Solution

The electrochemical material-energy conversion technology represented by fuel cells,lithium ion batteries,etc.has the advantages of cleanliness and high efficiency.If it can be applied on a large scale with clean fuel production technology using wind energy and solar energy,it will effectively solve the energy problems of oil shortage and environmental problems such as urban smog pollution.The electrocatalyst/electrolyte interface is the core that determines the performance of the electrochemical material-energy conversion device.On one hand,the electronic and geometric structure of the electrocatalyst/electrolyte interface is an important factor that determines the electrochemical mechanism and kinetics occurring at the interface;On the other hand,the surface structure on the side of the electrocatalyst is also prone to a series of changes during the reaction,which is accompanied by possible changes in reaction mechanism and kinetics.Therefore,understanding the catalyst/electrolyte interface,especially the structure and dynamic changes of the catalyst in the reaction state is the prerequisite for correctly revealing structure-activity relationship of the catalyst.Due to the confirmed surface structure,the model single crystal electrode has high controllability and reproducibility in experiments,and is relatively sensitive to small changes in microstructure.It is very suitable for exploring the structure-activity relationship of electrocatalysts and the dynamic change of electrocatalyst structure during the reaction.Electrochemical scanning tunneling microscope(EC-STM)can directly observe the structure of the electrode solution interface at the atomic and molecular levels,among them,the fast scanning tunneling microscope(In situ\fideo-STM)technology can also directly observe and track the electrode solution interface structure and dynamic change behavior as well as the dynamic behavior of related interface processes on the millisecond time scale.In this paper,I mainly used in-situ(fast)scanning tunneling microscope technology and combined with conventional electrochemical methods to deeply study the electrochemical interface structure and dynamic change behavior of Pt(hkl)single crystal electrode,the structure of the adsorption layer such as CO and its dynamic changes in the oxidation process on Pt(hkl)electrodes,as well as the oxygen reduction process on the Ir(hkl)single crystal electrode.The main results obtained are as follows:(1)Electrochemical in-situ STM characterization of the surface structure of the Pt(hkl)electrode itself and its surface CO and I-ion adsorption layer structure in the aqueous electrolyte solution:The structures of Pt(332)and Pt(997),two vicinal surfaces to the(111)plane,are examined by high-resolution STM under potential control in 0.5 M H2SO4 solution containing iodide or carbon monoxide.These electrodes are annealed by a hydrogen flame and quenched in Millipore water,giving rise to rough step edges with poorly defined atomic structures at 0.1 V(vs reversible hydrogen electrode)in 0.5 M H2SO4.However,step edges are sharpened and aligned in the(111)direction after adsorption of CO on both electrodes.In contrast,seesaw-like step lines are produced by iodine adsorption.These indicate that the step structure and mobility of Pt atoms are markedly influenced by the adsorbate.Pt(997)and Pt(332)electrodes with(111)facets that are 8 and 5 Pt atoms wide,respectively,afford(?7 ×?7)R19.1°-? and(?3×?3)R30°-? structures at 0.1 V.In comparison,the(2 × 2)-3CO structure,as seen on Pt(111),is found on the(111)terrace of both Pt electrodes,while STM results yield linearly bonded and threefold bonded CO molecules at the peaks and troughs of steps on Pt(332)and Pt(997),respectively.In-situ STM shows that the CO molecules are more loosely adsorbed on the lower edge of the step than on the upper edge of the step.This facilitates the formation of OH species at these sites and their subsequent reaction with CO molecules at a positive potential.These may be the reason why the stepped Pt surface has higher CO oxidation activity.(2)Dynamic phase transition process of the saturated CO adsorption layer on Pt(111)in sulfuric acid solution with the change of electrode potential:A study of the phase transition of a saturated CO adsorption layer on Pt(111)in sulfuric acid solution revealed that the saturated(2 × 2)-3CO adsorption layer was converted to(?19×?19)R23.4°-13CO before oxidation.During the phase transition,there is a highly dynamic apparent(1×1)-CO interphase structure,which is easily identified as a true high-density adsorption phase in conventional STM observations.However,Video-STM results show that this is just a dynamic effect caused by a small number of point defects caused by the pre-oxidation of the CO adsorption layer on the surface.Combining the results of DFT calculations,we infer that the lateral dynamics of the CO adsorption layer on the electrode surface during the CO pre-oxidation process is very fast.Even when(?19×?19)phase features appears,the surface adsorption layer still needs a self-restructuring process until it finally transforms into a stable(?19 ×?19)adsorption layer structure.This provides a new basis for the mean field theory to elucidate CO oxidation kinetics.In addition,on the highly dynamic COad layer,occasionally we can also observe relatively slow kinetic surface vacancies.After careful observation and analysis,it was found that the vacancies belong to the defects in the atoms of the Pt surface layer,which originated from the interaction between the step edge CO and the highly active sites such as the step.This shows that even at a low potential,the highly dynamic CO adsorbed on the Pt(111)surface can still affect the surface structure and stability of the Pt electrode to a certain extent.(3)Oxygen reduction reaction(ORR)kinetic behavior of Ir(hkl)electrode:The oxygen reduction reaction(ORR)at iridium single crystalline electrodes,Ir(111)and Ir(332),in 0.1 M HClO4 and 0.5 M H2SO4 solution,was studied using cyclic voltammetry and potential step chronoamperometry under a hanging-meniscus rotating disk electrode configuration.The results are compared to the ORR behaviors observed on platinum single crystal electrodes with the same surface orientation.We found that i)The kinetics for ORR on Ir(111)are significantly slower than those on Pt(111),the onset potential and half-wave potential for ORR are ca.100 mV and 370 mV more negative than those on Pt(111),respectively;ii)in 0.1 M HClO4 only H2O2 is formed on Ir(111)at E>0.55 V,and when E<0.4 V the major product is H2O,in 0.5 M H2SO4 a significant amount of H2O2 is produced even the potential is as low as 0.15 V;iii)In a solution containing 2 mM H2O2 in saturated O2,the kinetics of hydrogen peroxide redox reaction(HPOOR,HPORR)and ORR on Ir(111)is higher than that in a solution with only H2O2 or saturated O2;iv)Ir(332)exhibits lower ORR activity than Ir(111),with a half-wave potential that is ca.30 mV more negative,and the diffusion-limited ORR current is not reached from 0.8 V down to 0.05 V,indicating that at Ir(332),incomplete reduction of O2 to H2O2 occurs in a wide potential region;v)significant decay of ORR current with potential scan rate and reaction time in the current transient measurements are observed on both Ir(111)and Ir(332),similar to those observed on Pt(111)and Pt(332)electrodes.These analyses indicate that the strong interaction between oxygen species and Ir(hkl)is responsible for the low ORR activity and poor stability of the Ir(hkl)electrode.