The Construction And Performance Of Pt-low-loading Or Pt-free Electrocatalysts For Oxygen Reduction Reaction

Low temperature hydrogen oxygen fuel cells are ideal candidate for mobile and stationary power generation due to their high-energy efficiency and environmentally benign process.Pt-containing materials are currently the best electrocatalysts for fuel cells.However,the commercialization of fuel cells is significantly hampered by scarcity,expensiveness,and poor electrochemical stability of Pt.Therefore,design and fabrication of low-cost but highly efficient catalysts for ORR turns out to be an important research subject in fuel cell technologies.Current studies on ORR catalysts have been devided into two main-lines aiming at weakening or even fully eliminating the dependency on noble Pt:developing catalysts that contain only tiny amounts of platinum?low-Pt?or do not contain platinum?Pt-free?.Based on above two principal lines,and taking the primary issues exsisted in low-Pt and Pt-free research field into account,three works were carried out in this thesis as followed:Firstly,in order to tackle both durability and activity issues of traditional Pt/C catalyst,we design a new catalyst by anchoring Pt NPs between the layers of etched montmorillonite?e-MMT?via Pt ion exchange plus in situ chemical reduction in the e-MMT.The e-MMT,produced by partial etching off MMT via hydrofluoric acid,contains defects newly created plus native defects and is very active for binding foreign elements.This efficiently binding effect and the space-confinement between e-MMT layers provide a strong intercalation between Pt NPs and e-MMT to restrain migration,dissolution and agglomeration of Pt NPs.The transmission electron microscopy?TEM?images and X-ray diffraction?XRD?patterns show that nearly 2.5 nm Pt NPs are dispersed in the layers of e-MMT.The Pt/e-MMT catalyst exhibits the ORR activity approximately 1.8 times greater than that of commercial Pt/C.After 1600 cycles,the Pt/e-MMT-40%catalyst showed 31.0%diminution in the Pt ECSA,whereas the loss of Pt ECSA was as high as 49.1%for Pt/C catalyst.The X-ray photoelectron spectroscopy results reveal the reason for improvement of electrocatalysis performance is that change the electronic structures of Pt,and then tune the adsorption behavior of the oxygen-containing intermediate species on the Pt surface,and thus the oxidation potential of Pt is promoted.Therefore,Pt/e-MMT catalyst possesses good ORR performance and durability.Secondly,the nitrogen-doped carbon catalysts are often suffered from low nitrogen utilization,we report a phase-transition-assisted strategy that uses gas-liquid separation coupled with a gas-liquid interfacial reaction during the phase transition of precursors for constructing hierarchically porous carbon materials?HPCMs?with tunable interconnected macropore channels,and simultaneously conducting the formation of active sites along these channel walls for the oxygen reduction reaction?ORR?.A HPCM with an ultrahigh specific surface area of 1141.41 m² g^-1 and a highly interconnected macroporous network was produced,in which the active sites are exactly located on the masstransport channels.By adjusting the precursor's solidification rates,the porosity of HPCMs can be exactly tuned from the micrometer to the nanometer level.Such an excellent structure,by benefiting the fast mass transport and maximizing the utilization of active sites,leads to an excellent ORR performance with a half-wave potential of 0.901 V which exceeds that of the state-of-the-art Pt/C catalyst by 40 mV.Finally,in order to further boost ORR activity of the most promising candidate,i.e.,Fe/N/C catalyst,and provide deep insights into its nature of active sites in acid medium,we design a facile strategy for the formation of Fe-N-CNTs with abundant active sites derive from a new metal organic framework.The ORR performance of Fe-N-CNTs enhances by the increasing of intrinsic activity and the promotion of the mass transfer efficiency.Meanwhile,we study the formation mechanism of CNTs and reveal the structure-activity relationship between CNTs and ORR.Integrated with the results acquired by ORR tests,XRD and XPS,we deeply analyzed the effect and contribution of the surface area,morphology and ORR performance by different iron salts species.The three-dimensional network structure is conducive for mass transport and high utilization of the active sites.The PEMFC with the invented cathode catalyst outputs a peak power of 724.8 mW·cm^-2,which is among the best non-precious metal catalysts for the ORR having been reported so far.