| Under the impetus of global energy security and sustainable development technology,the development and design of novel renewable energy storage and transformation system is of great significance in dealing with the energy and environment crisis.Surface /interface catalysis is the core of various reactions,such as the hydrogen evolution reaction(HER),the oxygen evolution reaction(OER)in the electrolytic water and the important oxygen reduction reaction(ORR)in the fuel cell.Therefore,establishing a clear platform relationship between the structural characteristics of the surface and the electrocatalytic activity is the only way to apply high activity,long-life and low-cost catalyst materials to practical fields.Since 2002,Daihatsu first found that Pd could be in and out of La Fe0.57Co0.38Pd0.05O3 crystal lattice reversibly in a high temperature redox atmosphere,and maintain a stable and efficient catalytic activity,so the “exsolution behavior” has attracted wide attention in the scientific community.However,the above high temperature reduction method is limited by the doping element composition,preparation conditions and cost.This research combined pulsed electric current technology and perovskite oxides coordinated control new interface structure,mainly around the material intrinsic properties such as nonstoichiometric,degree of crystallinity and the external environment such as the electric fields,to realize the controlled exsolution of transition metal elements at perovskite B-site,the transfer of matrix structure and electrochemical properties of perovskite under the pulsed current treatment.All samples in this study were prepared by sol-gel method.Firstly,based on the nominal oxygen metering ratio and non-oxygen metering ratio,different lattice defect states,and based on a large number of pulsed experimental datas,the influence of pulsed electric current on the composition,structure,shape,size and performance of the exsolved phase under different oxygen metering ratio was explored.According to the results,for the A-site deficiency,the effective pulsed electric current intervention will promote the exsolution of transition metal elemental Ni at B-site and inhibit the appearance of AO phase,and the matrix perovskite structure remains stable,consistent with the conclusion of high-temperature reduction method.In the theoretical standard oxygen metering ratio,after the same pulsed current treatment,the precipitation of Ni-Ti metal nanoparticles will be accompanied by the precipitation of rare earth elements such as La2O3 and La,and the matrix structure remains stable.After precipitation,the matrix morphology was relatively flat,and the exsolved particles were observed,which were closely connected with the material matrix,and the size distribution was basically uniform.The surface supported nanoparticles of perovskite are the key factors to improve the cycling stability of LSTN/LSTN-powders and the electrocatalytic activity of oxygen evolution.The influence of calcination temperature on pulsed electric current to promote the exsolution of B-site perovskite nanoparticles was studied.It was found that the calcination temperature changed the crystalline degree and structural defects of the matrix,and also changed the exsolution selectivity of metal nanoparticles.At 800?,the effect is most significant under the pulse electric current parameters of 500V-3Hz-90 s.XRD and SEM analysis have proved the existence of Ni nanoparticles,which is the minimum size reported currently about 4nm.But any exsolved particles were not found under calcination at 1000? and 1200?.Electrochemical tests showed that the exsolved active materials had better reversibility and larger catalytic activity area,the initial oxygen evolution potential is the most negative(?0.45 V vs.SCE),and the peak current is as high as 701.5m A.The exsolution of Ni nanoparticles significantly improved the OER catalytic activity and electrode reaction kinetics of perovskite oxides.For the introduction of A-site deficiency,La0.52Sr0.28Ti0.94Ni0.06O3,500V-3Hz-90 s is the best pulsed parameters for Ni nanoparticles to be exsolved and embedded into perovskite matrix.When the pulsed voltage was changed to 300V/500V/ 700V-3Hz-90 s,when U=500V,obviously,Ni(111)crystal diffraction peak was observed.However,when the voltage is increased to 700 V,the pulsed sintering mechanism dominates and the Ni characteristic diffraction peak disappears;when the pulsed time was changed to 500V-3Hz-30s/60s/90 s,the diffraction peak of Ni appeared in all samples.With the extension of time,the reduction degree of Ni increased and the size of exsolved particles decreased sharply(250nm?4nm),indicating that there were different action mechanisms for the reduction of Ni nanoparticles by pulsed current treatment,and the shape reconstruction caused by high density pulse affects the size of the exsolved particles;when pulsed frequency is changed to 700V-2Hz/3Hz/4Hz-90 s,no Ni particles precipitated,but it leads to sinter and collapse of the matrix morphology of perovskite,which provides a new idea for inhibiting superfluous precipitation of nanometals at A and B sites.The electrochemical test results showed that the electrocatalytic activity of the active materials was improved after applying the pulsed electric current,whether it was exsolved or not.The active material embedded with nanoparticles is the key to improve the electrocatalytic activity.In conclusion,the new method of regulating and modifying the surface state of perovskite by pulsed electric current technology will provide a feasible scheme for the design and development of more complex oxides surface problems,provide sufficient data support and practical methods to realize a wide range of applications of advanced energy materials. |
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