The PH Effects And Its Coupled Effects On Electrocatalysis

Many electrocatalytic reactions with electrochemical substances&energy conversion,such as hydrogen electrode reaction,oxygen electrode reaction,CO2 reduction reaction or small organic molecule oxidation reaction,involve proton and electron transfer.The influence of pH effect on the reaction is mainly reflected in the following two points:1.Changing pH will affect the thermodynamics of the reaction in the whole system or part;2.2.The change of pH can change the structure,charged state,and distribution&configuration of solvated protons at the interface,which profoundly affecting the charge transfer mechanism and dynamics.Although a large number of studies have been carried out on the pH effect of electrocatalytic reactions,the understanding of the pH effect of hydrogen electrode reactions are still controversial.In order to deepen the understanding of the pH effect of hydrogen electrode reaction,this paper took the hydrogen evolution/hydrogen oxidation reaction(HER/HOR)on Pt(111)electrode as the model reaction,combined with the calculation simulation and experiment,and discussed the influence of pH of solution and related coupling effect on the mechanism and dynamics of the HER/HOR reaction.The following results have been achieved:1.The relationship of different form of hydronium ion in the solution and HER dynamics:the results showed that the hydronium ion morphology in the aqueous solution of distribution affected by temperature,water activity,the lower the temperature,the easier the proton complex with hydrogen bond network structure formation,and lower water activity,hydrated proton tend not to form a complex structure.The contribution of various forms to the total reaction is different.In the HER process,the contribution of current comes from the proton(H+· nH2O,n=1,2,3,4)discharge at the low hydration degree.In hydrogen underpotential deposition(H-UPD),protons with complex hydrogen bonds participate in the reaction,while small molecular hydronium ion do not participate in H-UPD.2.PH effect of HER reaction on Pt(111)electrode in acidic medium:In the pH=0 to 4 sulphuric acid or perchloric acid in the solution,we study the influence of pH on HER mechanism and kinetics,and combine the calculation simulation,the reaction interfacial concentration of protons and hydrogen molecule with the variation of the reaction current,through the deduction of instantaneous interfacial concentration of each species on reaction equilibrium potential and current,the influence of the dynamics curves of relationship between reaction current and activation overpotential,found that the reaction mechanism of HER under acid condition was Volmer-Tafel mechanism and in the region of the equilibrium potential,the exchange current density,internal dynamics has nothing to do with pH.The changes of interface electric field and charged state caused by pH change have little influence on HER reaction dynamics.3.Influence of other effects coupled with pH effect on HER dynamics:qualitative explanation and quantitative calculation of similarities and differences of pH effect in different systems and reaction environments were made through the established theoretical model.The results show that:a.The effect of cation addition on water in the interfacial environment leads to the opposite movement trend of the interfacial electric potential when HER and oxide formation reaction;b.Under different pH values,temperature has different influence on HER dynamics,because temperature has direct influence on the hydration proton morphology distribution and internal dynamics;c.model calculation found pH=1,the H/D isotope effect coefficient of HER is 1,in the pH=13,H/D isotope effect coefficient of HER is 1.33,the reason is that when pH increasing,the proportion of water as a proton donor increases,the corresponding energy barrier difference is increased,at the same time isotopes to have influence on the activity of water and other properties,which lead to apparent isotope effect strengthened.