The Effect Of Ph Of H₂O₂ On Au,Pt Single Crystal Electrodes

Fuel cells have been considered to be one of the most potential energy sources due to their high energy efficiency,mild reaction conditions,convenient operation,easy to realize automation control,product for water,and environmental friendly.Oxygen Reduction reaction(ORR)is the primary cathodic reaction in fuel cells,however,its high overpotential remains the most critical issue which restricts the commercialization of fuel cells.For reactions which involve the transfer of protons and electrons such as ORR,the variation of pH in the solution may bring multifaceted effects to these reactions:i)it may change the thermodynamic equilibrium potential,ii)it may change the the reactive species and its concentration,iii)it may change the reaction kinetics of electrocatalytic reactions,and iv)it may change may changes the surface properties,such as the type and the coverage of the adsorbate and,excess surface charge.Hence,studying the pH effect will provide very useful information which may help to unravel their reaction mechanisms and factors which may control its kinetics.Hydrogen peroxide(HPO)is one of the possible intermediates of ORR.On Pt group or Pt based electrocatlaysts,which is the best activity for ORR,it is found that there exists a potential range where the ORR is under kinetic and mass transport mixed control.In this mixed potential region,both the oxidation and the reduction of HPO(HPOOR and HPORR)can occur.Understanding the mechanism and kinetics of HPOOR and HPORR will be of great importance in understanding the mechanism and identifying key factors of ORR.However,there remains some difficulties to get a clear understanding of the structure-activity relationship of each reaction on such electrocatalysts.However,on Au electrodes,a very large overpotential is required for HPORR,so there exists two different potential scales for HPOOR and HPORR developed on Au electrocatalysts.,which is different from Pt electrocatalysts.Hence Au electrode is a good model system to study the kinetics and mechanisms of HPORR and HPOOR.In this research work,we investigated the electrochemical behavior of H2O2 on Au single crystal electrodes in the acid and alkaline solutions with different pH by using hanging meniscus rotating disk electrode(HMRDE)system.Then,the reaction mechanism and reaction kinetics of HPOOR and HPORR on Pt(111)have been compared.In the application of HMRDE technology to the study of single crystal electrochemistry,we found out that all different types of factors may affect the reproducibility of data.Consequently,these changes will profoundly affect the electrochemical behaviors of the system,and cause great confusion during the analysis of data.In order to overcome the aforementioned barriers,we have systematically studied the influence of the height of meniscus and the geometric configuration upon the results of the reaction system using HMRDE.The main contents and achievements of this thesis are highlighted here after:1.Study of some factors affecting the electrochemical behaviors on HMRDE configuration:In this chapter,we systematically investigated the effect of the height of the meniscus on the electrochemical behavior in the reaction system.On the premise that the hanging electrode is strictly parallel to the horizontal plane and the electrode edge has no infiltration(that is,the basic CV curves are completely same),we found that the slight changes for the height of meniscus have several possible effects on the electrochemistry:I)For the system affected obviously by mass transfer,for example,a system with gas as reactant(the solubility and low concentration of the reactant),such as ORR,HOR,HPOOR or HPORR on the Pt electrode,is that the lower the height of meniscus,the more conducive to the transfer of the reactant in the liquid phase to the surface of the electrode,and the higher the reaction current of the mixed control area to the limit current zone(the change can be as high as 20 of the total limit current).For this kind of system,the reaction can be conveniently corrected by the normalization of the limiting diffusion current of the reaction;ii)For the reaction which is not in the control of mass transfer,the lower height of meniscus,the greater current,such as HER and HCOOH oxidation reaction,which may be a small amount of high activity on the edge of the electrode makes a great contribution to the current,although the basic CV curves in the supporting electrolyte have no difference.If the height of meniscus is too low,the site of other configurations on the edge of the electrode can take part in the reaction.Meanwhile the electrochemical behavior will completely change from the behavior of the background single crystal.Finally,it is certain that how to minimize the experimental errors caused by the height of the meniscus in the actual experiments.2.The effect of pH on HPOOR and HPORR on Au(111),Au(100)and Pt(111):Through the investigation of the electrochemical behavior of HPOOR and HPORR on Pt(111),Au(100)and Au(111)with pH varying from 1.0 to 13.0 in examined solutions,it is revealed that the change of pH in solution will deeply change the kinetic behavior of HPOOR and HPORR.There are thermodynamics and kinetics reasons behind these changes.On the one hand,for the reaction involving proton and electron transfer,the change of pH will change the thermodynamic equilibrium potential of the reaction.On the other hand,for the reaction of proton and electron separation(decoupling)transfer,the change of the equilibrium potential with the pH may not be the same as the change of reference electrode potential(vs.NHE)with pH.As a fact the driving force of rate determine step of the reaction is different while overpotential of the total reaction is the same.As the rate of reactions changes with overpotential exponentially,the foregoing effect is an important factor in the pH effect in this work.In addition,acid and alkali ionization,and the concentration of the reactants also play important role in the kinetic of proton electron transfer reactions,what's more these factors are influenced by solution pH.For the catalytic reaction,the concentration of OH-in the solution will extremely affect the type,coverage and adsorption configuration of the adsorbent on the electrode surface.And the latter two factors will change the kinetics of reaction to some level.In summary,the results of the pH effect of HPOOR and HPORR,and the results of pH effect on formic acid oxidation,show that the study of the pH effect on the electrocatalytic reaction involving the transfer of proton can help us to optimize the reaction parameters,and can also fast our needed reaction.Conversely,it can also inhibit or reduce the rate of reaction that we do not want to happen.