Synthesis Of Low Platinum And Platinum-Free Catalysts And Investigation Of Their Oxygen Reduction Reaction Performance

Oxygen reduction reaction?ORR?is the most important cathode reaction in proton exchange membrane fuel cell?PEMFC?,and its slow reaction kinetics severely limits the energy output of PEMFC.The noble metal platinum?Pt?is currently recognized as the most active ORR catalyst and has been widely studied.However,metal Pt is very expensive and its catalytic activity is easily affected,resulting in a short catalyst life.Therefore,PEMFCs using Pt as an electrode catalytic material have not been widely commercialized so far.Based on the PEMFC cathode catalysts,the main purpose of this thesis is to reduce cost and improve activity and durability of catalyst,two ORR catalysts with low Pt and Pt-free components were designed.For low-Pt catalysts,the amounts of Pt were reduced by controlling the morphology,structure and composition of the Pt-based catalysts to save cost;For Pt-free catalysts,the main strategy was to combine metal organic frameworks?MOFs?with Fe-N-C catalysts.The unique properties of MOFs made the Fe-N-C catalyst have good ORR electrocatalytic performance.By designing the structure of MOFs,the Fe-N-C catalyst with high ORR catalytic activity was prepared,and then the ORR properties of the two types of catalysts were studied.The detailed contents and main results are described as follows:1.Nickel Ion Oriented Fabrication of Spiny PtCu Alloy Octahedral Nanoframes with Enhanced Electrocatalytic ORR PerformancePtCu alloy octahedral nanoframes?PtCu AONFs?catalysts with abundant nanothorns were successfully prepared by a simple one-step hydrothermal method using Ni²⁺as a structure-directing agent.Due to the introduction of the transition metal Cu,the formed alloy structure changed the electronic structure of the Pt surface,and the hollow framework structures improved the specific surface area and Pt atom utilization.In addition,the apex nanothorn structures provided high density of ridges,corners and step active atoms,all of which contributed to the improvement of Pt catalyst activity and stability.Secondly,the hollow frame and alloy structures are beneficial to reduce the amount of precious metal Pt,which achieve cost savings.In order to understand the structure guiding mechanism of Ni²⁺more clearly,a series of mechanism exploration experiments were carried out.It is found that Ni²⁺did not participate in the final product composition,but it played a key role in the formation of hollow structure.A possible hypothesis was proposed as follows:since the adsorption of Ni²⁺changed the surface free energy of the Cu template crystal,many more active sites were formed on the surface,which accelerated reduction rate of the Pt precursor and the atom diffusion of the Cu template intermediate during the replacement reaction.Subsequently,the electrocatalytic ORR performance of PtCu AONFs catalyst was studied under acidic conditions.Compared with commercial Pt/C,PtCu AONFs catalyst had better catalytic activity and stability.At 0.9 V vs.RHE,the mass activity and specific activity of PtCu AONFs were2.8 and 2.6 times higher than that of commercial Pt/C,respectively,and the half-wave potential was 37 mV higher than Pt/C?877 mV?.At the same time,in terms of stability,PtCu AONFs also showed obvious advantages.After 8000 cycles of accelerated durability tests,the half-wave potential of Pt/C decreased by 13 mV,while the half-wave potential of PtCu AONFs remained high,not only not falling,but even better than the initial value.2.MOFs Derivatization Preparation of Pt-free Non-Noble Metal Fe-N-C Catalysts with Improved Electrocatalytic ORR PerformanceIn this study,a class of non-noble metal Fe-N-C catalyst was derived from MOFs.The Fe-TCPP MOFs nanofibers were prepared by a simple one-step reaction using Fe³⁺as the central metal ion and Tetrakis?4-carboxyphenyl?porphyrin?TCPP?as the organic ligand.In this reaction,the thickness of nanofiber is controlled by a surfactant,and the nanofiber is made to grow as much as possible by using pyrazine.Then,the Fe-TCPP MOFs nanofibers were used as precursors to prepare non-precious metal Fe-N-C catalysts by heat treatment at different temperatures.The MOFs skeleton can not only provide an ordered three-dimensional porous structure,but also has a dense and uniformly filled active site due to its cyclical structural properties,which is beneficial to increase the activity of the catalyst.In order to evaluate the Fe-N-C catalyst activity,the electrocatalytic ORR performance of the Fe-N-C catalyst was investigated under alkaline electrolyte and compared with Pt/C.The Fe-N-C catalyst had good ORR performance,and its catalytic performance was close to commercial Pt/C.The initial potential and diffusion current density of Fe-N-C catalyst were comparable to Pt/C?¹.0 V vs RHE,⁵.7 mA/cm²?.Althought,the half-wave potential was about 25 mV lower than Pt/C?885mV?,Fe-N-C catalyst had been greatly improved compared to many other non-precious metal Fe-based catalysts.The number of transferred electrons in the ORR process was calculated to be about 4.06,which indicated that the Fe-N-C catalyzed ORR process was a four-electron reaction pathway.At the same time,the Fe-N-C catalyst was evaluated for stability.After 10,000 cycles of accelerated durability tests,it was found that both the half-wave potential and the limiting current density were reduced,and the decrease was close to Pt/C.Nevertheless,it was found that the ORR process was still close to the four-electron approach after ADT by calculating the number of electron transfer.