Mechanism And Kinetics Study On The Oxygen Reduction Reaction And Electrochemical Reaction Of Hydrongen Peroxide On Pt Based Electrocatalysts

Fuel cell was widely concerned as it's high efficient and environment-friendly.The oxygen reduction reaction?ORR?is the primary cathodic reaction for polymer electrolyte membrane fuel cells?PEMFCs?.However,the air cathode remains the major contributor to voltage loss,which limits the PEMFCs' performance.Thus it's an important issue to find some better and cheaper ORR catalysts.In order to improve the ORR activity of catalysts,it's necessary to study the mechanism and kenitics of ORR at the best catalysts?i.e.Pt?.For the high specific area,nanomaterials were used as the catalysts in practice system.But in order to get reliable conclusion and analyse the data easier,the model study on single crystallian is better choice in mechanism and kinetics study.ORR is a 4-electrons reaction with several elementary reaction and complicated mechanism.Thus it will be great helpful to study the kinectics and mechanism of some simpler reduction for comparation,such as electrochemical reaction of H₂O₂.And hydrogen peroxide is one of the possible intermediates for ORR.Thus,we studied three topics in this work as fellow:1.ORR study at Pt stepped surface:Oxygen reduction reactions?ORR?at Pt?111?,Pt?332?and Pt?331?in 0.1 M HCI04 are examined by cyclic voltammetry?CV?and potential step chronoamperometry,in order to understand the structure-activity relationship and the long term stability.Our results confirm that the ORR current recorded in the first positive-going potential scan in 0.1 M HCI04 immediately after annealing the working electrodes decreases in the order of Pt?331?>Pt?332?>Pt?111?.The current transients obtained after stepping to 0.85 V reveal that ORR current decreases with reaction time.The deactivation rate for the ORR increases in the order of Pt?111?<Pt?332?<Pt?331?,which leads to the opposite trend of ORR activity at longer reaction time during potential step measurements or under slow-scan CV.Further experiments with a rotating ring disk electrode reveal that the activity decay is accompanied by an increase in the current efficiency of the 2e-ORR pathway by ca.20%.The results are explained by the faster dissociative adsorption of 02 and slower desorption of O-containing intermediates at stepped surfaces,resulting in higher coverage by O-containing intermediates at Pt?331?and Pt?332?than at Pt?111?.This leads to the significant degradation of the ORR activity at the stepped surface.The present study reveals that the activity trends derived from the polarization curves recorded in the first positive-going potential scan under a relatively high scan rate?? 20 mV/s?,which are usually used to evaluate the activities of catalysts,cannot be simply taken as a guideline for the rational design of improved catalysts for ORR.2.Kinetics study of electrochemical reaction of H₂O₂:We first compared the reaction kinectics of H₂O₂ at Au?111?and Au?100?.Then the ORR at Au?100?was discussed,we found that:i)the kinetics for the redox reaction of hydrongen peroxide oxidation reaction?HPOOR?at Au?100?is asymmetric,the increase in the rate with overpotential the forward reaction is very slow,while that for the backward reaction is very fast;ii)O₂ reduction to H₂O₂ has a H-D kinetic isotope effect of more than two times,iii)The high overpotential at the onset for ORR at Au?100?is limited by the thermodynamic equilibrium potential for HPOOR due to the fast kinetics of H₂O₂ oxidation.The slow rate for ORR Au?100?at potentials below the onset potential is limited by the slow kinetics for the hydrogenation of O₂,H02 and the scission of 0-0 bond in H₂O₂.At Pt electrode,we studied the kenitics of the reaction of H₂O₂ at different mass transport condition.And we divided the hydrongen peroxide reduction reaction?HPORR?,HPOOR and ORR currents from the sum reaction current for first time,and discussed their kinetics.At last we discussed the implication to the ORR kinetics:i)02 can decompose at Pt?111?at E>1 V,but the thermodynamic equilibrium of H₂O dicompose inhibits the further reduction;ii)The ORR at Pt don't pass by the H₂O₂ mechanism at E>Emix;iii)HPORR and ORR are inhibited by same factor,i.e.the thermodynamic equilibrium of H₂O dicompose;iv)The broken O-O bond of O₂ at Pt surface is harder than the 0-0 bond of H₂O₂,thus the bottleneck of the ORR pass through the H₂O₂ mechanism is before the O-O bond change into single bond.3.Enhanced Activity of Au@Pt Core-Shell Nanoparticles toward ORR:Nanoparticles with Au core and 1-2 monolayers Pt shell?denoted as Au@1-2 ML Pt?are found to have enhanced activity toward oxygen reduction reaction?ORR?comparing to commercial Pt nanoparticles and pc-Pt electrodes in acidic electrolyte,i.e.,the half wave potential is ca.30 mV positively shifted.Increase of d-electrons for Pt shell is confirmed by the X-ray Photon emission spectroscopy and the blue-shift in Pt-CO as well as the red-shift in C-O stretching frequencies measured by surface enhanced Raman spectroscopy.A positive electronic effect induced by the hybridization between Pt and Au at the alloyed surface,which leads to the acceleration of O-O bond breaking kinetics,as well as the reduction of the binding energy of Pt-OHad/Oad,is suggested to be the origin for the enhanced ORR activity of Au@1-2 ML Pt over that of commercial Pt/C nanocatalysts.