Exploration On Electrocatalytic Oxygen Reduction Reaction Pathway Based On H₂O₂ Probe Molecule

Proton Exchange Membrane Fuel Cell?PEMFC?is a kind of electrochemical power generating device featuring high efficiency and using new clean energy,which has broad application prospects in the field of hydrogen fuel cell vehicles,fixed base station,rail transit,etc.However,its cathodic oxygen reduction reaction?ORR?requires a large number of Pt/C catalysts due to the large overpotential and low reaction rate of ORR.Compared with the traditional commercial Pt/C catalyst,non-noble metal ORR catalysts have the advantages of abundant reserve and low cost.And ZIF?zeolitic imidazolate frameworks?based Fe-N-C catalysts have hierarchical pore structure after heat treatment,resulting in large specific surface area and excellent electrochemical performances for ORR.They are regarded as the most promising alternative for noble metal Pt in the field of oxygen reduction catalysis.At present,most reports of ORR catalysts focus on how to improve the activity of catalysts.Because ORR involves four-electrons,relatively few studies have been conducted on the adsorption behavior of O₂or H₂O₂ molecules,in addition to ORR pathways at the surface active sites of different catalysts,especially on the ORR catalyzed by ZIF-derived non-noble metal catalysts.By introducing H₂O₂ probe molecule,this thesis focuses on comparing the differences of catalyst surfaces as well as the differences and similarities of ORR on catalyst surface before and after the introduction of H₂O₂ and the concept of apparent H₂O₂ coverage ratio has been proposed.Meanwhile,combining with the research on hydrogen peroxide reduction reaction?HPRR?on catalyst surface,detailed studies on the ORR pathway on noble metal catalysts Pt,Pd,Au and ZIF-8 derived Fe-N-C non-noble metal catalyst have been conducted.The thesis mainly includes the following research contents:?1?Using the volatilization and elimination of Zn²⁺during high temperature heat treatment to disperse active sites,through pyrolyzing the Fe-doped ZIF-8 precursors,a series of ZIF-8 derived Fe-N-C catalysts?Fe-Z8-C?of different Fe/Zn ratios were prepared and the 7.5Fe-Z8-C of the best ORR catalytic activity was investigated in much details in comparison with benchmark Pt/C catalyst.Through intentionally introducing varied concentrations of H₂O₂ as the probe molecule,as compared to Pt,relatively lower ORR catalytic activity of Fe-Z8-C is not associated with the over-production of intermediate H₂O₂,and in fact,Fe-Z8-C produced significantly lower H₂O₂ amount than that by Pt during ORR.Moreover,we further studied the protonation of ORR on catalyst surface.It is concluded that,in comparison with Pt/C,the first 2e step in ORR?such as oxygen molecule adsorption,activation and reduction into H₂O₂?,rather than the superoxide anion protonation in the first 2e step,nor the HPRR in the second 2e step,is kinetically sluggish on Fe-Z8-C.?2?In order to validate the applicability of the research method used in the former part of the thesis,and to have a deeper understanding of ORR on the catalyst surface,two typical noble metal catalysts,Au and Pd,were adopted for comparative studies.It was found that the H₂O₂ adsorption on the surface of Pd/C catalyst is quite strong,while the H₂O₂ adsorption on the surface of Au/C is extremely weak.Therefore,Au/C is inactive in catalyzing the further reduction of H₂O₂ in ORR,resulting in the corresponding higher ring current than on Pd/C.Meanwhile,by comparing the H₂O₂reduction catalytic activities of the two catalysts and the changes in the H₂O₂ apparent coverage ratios on the catalyst surface,it was confirmed that the ORR on Au is approximately a 2-electron process,while the second step of ORR,i.e.,the reduction of H₂O₂ on Pd surface is the more sluggish step.The difference of adsorption strength with reactant molecules on the two noble metal model catalyst surface has a great influence on the difference of their ORR catalytic activity.Therefore,when synthesizing oxygen reduction catalyst with excellent performance,it is of great importance to properly design and tune the adsorption behavior of H₂O₂ and O₂molecules on the catalyst surface.