On The Interfacial Structure And Kinetics Of Oxygen Reduction Reaction At Model Interfaces

The study of oxygen reduction reaction(ORR)kinetics,along with the development of catalysts with higher activity and lower cost,is of great significance for the large-scale commercialization of polymer electrolyte fuel membrane cells(PEMFC)as well as the realization of carbon peak and neutrality targets.At present,the development of ORR catalysts in the laboratory is rapid,but the real commercial application is very limited.The main reasons for this dilemma are as follows:1)The ORR process involves 4-proton and 4-electron transfer;there is still no unified and selfconsistent explanation for experimental results,and it is still in need of a quantitative understanding of the ORR kinetics.2)The interface structure of catalyst layer in PEMFC is significantly different from that in rotation disk electrode(RDE)experiments,but the research on interfacial molecular structure of the catalyst layer is lagging behind.Therefore,it is particularly urgent to reveal the kinetics of ORR and quantitatively describe the effects of catalyst composition and interfacial structure.During my doctoral study,in facing these challenges,single,crystalline electrode/aqueous solution,graphene/polymer electrolyte,and other model interfaces are taken as research objects.Based on the combination of experiment and theory,I adopted conventional electrochemical experiments,sum-frequency generation spectroscopy(SFG),and microkinetic modeling.The relevant work and main conclusions are as follows.(1)Oxygen Reduction Reaction on Rh(111)Single Crystalline Electrode in Acid:The relationship between interfacial structure and ORR activity has long been a hot research topic in the electrochemical field.Adsorption of oxygen-containing species and anions has an important effect on the interfacial structure and ORR kinetics.Based on classical cyclic voltammetry(CV)and RDE techniques,we compared the interfacial structure and ORR kinetics of Rh(111)in 0.1 M HClO4 and 0.5 M H2SO4 using Rh(111)as the model interface.The results show that HSO4-/SO42-,which is chemically adsorbed on Rh(111)surface,occupies the active site and decreased the ORR activity in 0.5 M H2SO4.However,in 0.1 M HClO4,without obvious specific anion adsorption,the accumulation of oxygen-containing species on Rh(111)surface could lead to the decline of ORR activity,but the ORR activity increased after increasing the upper limit potential or holding the Rh(111)at E<0.60 V for 50 s.These results indicate that the adsorption of oxygen-containing species,which are the intermediates of ORR,on the electrode surface has more complicated effects beseids blocking the activte sites;it may also improve the ORR kinetics via changing the double layer structure.(2)A Microkinetic Modeling Study on Oxygen Reduction Reaction on M(111)[M=Pt,Ir,Rh]Single Crystalline Electrode:The volcano plot between binding energies of reaction intermediates and catalytic activities underpins the binding energy approach,a prevailing guideline,in electrocatalysis.Pt,Ir,and Rh belong to the overadsorbing leg of the volcano plot,and adsorb oxygen-containing intermediates in an intensifying order.Therefore,the binding energy approach predicts an activity order of Pt(111)>Ir(111)>Rh(111)for ORR.Herein,single crystal experiments corroborate the binding strength order of Pt(111)<Ir(111)<Rh(111),but give out an ORR activity order of Pt(111)>Rh(111)>Ir(111),contradicting the binding energy approach.To understand this discrepancy,we develop a microkinetic ORR model that complements the standard binding energy approach by including multistep kinetics corrected with double layer effects.Armed with this model,we extract binding energies from CV,quantitatively fit the polarization curves of all three metals,and attribute the higher ORR activity of Rh than Ir to a lower activation barrier of OHad desorption,because Ir possesses a higher magnitude of surface charge inducing a more rigid interfacial environment.Following this line of reasoning,the superior ORR activity of Au@Pt core-shell catalysts are also rationalized.A key message delivered here is that kinetic and double layer effects are nontrivial for ORR on the single crystalline electrodes studied here.(3)Molecular Structures at Nafion Interfaces Investigated by Sum-Frequency Generation Spectroscopy:The interfacial structures of a Nafion-graphene and NafionPt composite,which are used to mimic the interfaces of the catalyst layer in PEMFC,under different humid environments,are investigated by SFG.The Nafion thin-film seems to form a lamellar structure on the graphene surface where the sulfonate moieties(SO3-)of the Nafion at the buried graphene surface probably orientate to the Nafion bulk,the same as that of the free Nafion/air interface.The SFG observation shows that the structure of the buried Nafion/graphene interface is distinct from that on a buried hydrophilic substrate surface such as fused quartz(SiO2)and calcium fluoride(CaF2).The structural differences of the buried Nafion-graphene interfaces significantly affect water adsorption behaviors there.As for the Nafion-Pt interface,in situ electrochemical SFG reveals that SO3-adsorbs on Pt surface through covalent interaction.As the Pt surface becomes more hydrophilic due to the the formation of Pt-oxides,a more ordered distribution of SO3-can be observed at the interface.This new information promises to alleviate some of the significant problems of modeling the PEMFC system.