| Proton Exchange Membrane Fuel Cell(PEMFC)has potentially broad application in the fields of military,aerospace,communication and transportation due to its various advantages,such as high-energy density,low operating temperature,fast start-up,safety and durability,and continuous high-power output.Compared with the anode hydrogen oxidation,the kinetics of the cathode oxygen reduction reaction(ORR)is very sluggish so that a large amount of Pt has to be used as a catalyst to ensure that it proceeds rapidly.Yet the high price and limited reserves of Pt have severely hampered the widespread commercialization of PEMFC.In addition,the lifetime of the PEMFC is also seriously affected by the poor stability of the Pt/C catalyst under ORR conditions.Aiming at these problems of electrocatalytic oxygen reduction and ORR catalysts,Pt monolayer(Pt ML)core-shell catalysts were synthesized,and their oxygen reduction properties and the structural effects between the core and Pt ML shell were investigated in this dissertation.The effects of different electrochemical polarization methods and related parameters on the ORR activity of the catalyst,i.e.,Pt/C,which was evaluated by the thin-film rotating disk electrode(TF-RDE)methodology,were studied systematically,and the error sources and correction methods of the TF-RDE methodology were discussed in detail.It was found that the potentiostatic step method is not suitable for the TF-RDE methodology to evaluate the ORR activity of the catalyst;However,the stable and reproducible ORR polarization curves could be obtained by the potentiodynamically-polarized cyclic voltammetry,but in order to obtain reliable experimental results,the ORR polarization curve must be processed by the background current subtraction and IR compensation.Based on these studies,the following potentiodynamic parameters were provided for an accurate assessment of the ORR activity of catalyst:scan rate 10 m V s-1;scan region:0?0.05 V to OCP(1.00?1.05 V).In addition,it was proposed that the quality of the as-prepared catalyst thin-film can be determined by comparing the ORR polarization curves of the polycrystalline Pt electrode and the catalyst thin-film electrode,and pointed out that the cleanliness of the electrolytic cell system can be confirmed by measuring the electrochemical surface area(ECSA)-specific activity of the polycrystalline Pt electrode,so as to ensure the reliability of the assessment results.The Au-Ni bimetallic nanoparticles with alloy and core-shell structure were firstly synthesized by a fast co-reduction method and subsequent thermal annealing,and then a Pt ML was constructed on these two nanoparticles to form new catalysts,viz.Pt ML/Au Ni-a/C and Pt ML/Au Ni-cs/C,by the galvanic displacement with Pt of an underpotentially deposited(UPD)Cu monolayer.In the same way,the Pt ML supported on the pure Au nanoparticles,viz.Pt ML/Au/C,was also fabricated.Finally,the effects of these three substrates on the ORR activity and stability of the Pt ML were investigated.For the Pt ML/Au/C catalyst,although its Pt mass activity was improved,its total Pt and Au mass activity was still low relative to the commercial E-TEK 20wt%Pt/C catalyst.However,its ORR activity could be significantly enhanced by introducing Ni atoms,relatively smaller in diameter,into the Au nanoparticles.This is because the Au lattice in Au-Ni bimetallic nanoparticles was contracted,which induces the lattice shrinkage of the surface Pt monolayer and reduces the binding energy of oxygen species.It was further found by the synchrotron X-ray diffraction and X-ray absorption studies that in the Au Ni alloyed nanoparticles,the lattice contraction of Au was more obvious and the electronic interaction between Au and Ni was also relatively stronger.Accordingly,the Pt ML/Au Ni-a/C catalyst exhibited the highest ORR activity.Its total Pt and Au mass activity,and specific activity were1.5 and 4.7 times as much as the commercial E-TEK 20 wt%Pt/C,respectively.The Pt ML/Pd/C catalyst was synthesized by the underpotential deposition combined galvanic displacement techniques.Then the changes of its ECSA and ORR activity,and the compositional-and structural-evolution of the Pt ML/Pd core-shell nanoparticles during the stability test were studied systematically.The results showed that the mass/dollar activity of the Pt ML/Pd/C catalyst displayed a volcano-like profile in the stability test up to 100,000 cycles;more importantly,the overall loss of the mass/dollar activity was as low as 17%of the initial value,significantly superior to the commercial Pt/C catalyst.Further,it was revealed by combining physical characterizations with model analyses that the freshly-prepared Pt ML did not entirely encompass the whole Pd core,and it was actually a disconnected Pt submonolayer configuration;in the initial stage of the stability test,the exposed Pd atoms gradually dissolved and caused the re-arrangement of the surface Pt atoms to form a complete covering layer on the Pd core?that is,the self-healing of Pt monolayer.Due to the formation of a highly-coordinated,contiguous Pt monolayer,the ORR activity of Pt ML/Pd/C catalyst was enhanced as a consequence.During the subsequent stability test,the electrochemical Ostwald ripening of the Pt ML/Pd nanoparticles was significantly retarded by its structural self-retaining property specific to Pt monolayer catalysts,thereby reducing the ORR activity decay. |
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