Quantitative Ananlysis of Complex Electrode Kinetics

Author:Chen Wei

Supervisor:chen yan xia


Degree Year:2019





Electrocatalysis plays an important role in energy conversion,environment protection,analysis,etc.The electrochemical energy conversion reactions,such as oxygen reduction reaction and small organic molecule oxidation reaction,are not only the primary electrode reactions for fuel cell devices in application,but also model reactions in fundamental research.In order to overcome the complexity of electrocatalytic reactions,the in-situ electrochemical spectrometric techniques,which can help obtain information about reactant/intermediate/product,are invented and developed in the past decades.However,even great progress has been made,most of the studies and analysis of data usually ends up at qualitative or semi-quantitative due to that fact that the methodology of quantitative analysis is still immature.In this thesis,we take oxygen reaction reaction and small organic molecule oxidation reaction as model reactions,to investigate the methodology of quantitative analysis the complex electrocatalytic reactions by rotating disk electrode and differential electrochemical mass spectrometry.Beside the development of methodology,the new insights of quantitative results on electrocatalysis are also provide and discussed.The main content includes:A.The methodology of quantification of the reaction kinetics by DEMS:The effect of mass transfer and solution composition on the detected mass signal,background mass signal correction and the calibration constant as well as the accuracy for the quantitative analysis of reaction kinetics are systematically studied and analyzed.With flow rate increased,mass signal calibration constant rapidly reduces,which suggest that the mass transfer condition should be the same between calibration experiments and activity measurements.Background signal changes during the swich of solution.Volatite species of high concentration will reduce the ionization probablility of other species and then reduce their corresponding mass signal.B.Kinetic analysis of reaction contolled by mass transfer of multiple species:A method to calculate the kinetic current density and rate constant for reactions whose kinetics are affected by the mass transfer of two or more reactants is proposed.The intrinsic pH dependent ORR kinetics at Pt(111)in un-buffered solutions under conditions where its kinetics are affected by mass transfer of both H+ and O2 are deduced.After normalization of the concentration,the rate constant of ORR was found to increase one magnitude per pH in the pH region from 1 to 3.Possible origin for this is that the rate determining step of ORR is proton-eletron decoupled transfer.C.Kinetic analysis of competition reaction system:The competition between HER and ORR at Pt electrode in weakly acidic solution are studied using DEMS and RRDE technique.The contribution of ORR producing H2O2,ORR producing H2O and HER to overall cathodic current measured at low potentials when both ORR and HER are thermodynamically allowed are quantified.Oxygen reduction is superivor than HER to consume proton and only the rest of proton can be used for HER.With potential negative shift across the onset of HER,the current of ORR producing H2O2 first increases than decreases.The cleavage of O-O bond is found to be hindered by surface adsorbed hydrogen,which makes ORR to produce H2O2.