Preparation Of Iron Group Metal Species (Fe,Co)@Doped Carbon Matrices Electrocatalysts For The Oxygen Reduction Reaction

With the needs of the new generation device to satisfy the energy shortage,fuel cells has attracted much attention.However,the sluggish kinetics severely influence the overall performance of the devices.It's important to search a highly efficient cathode catalyst.Till now,platinum?Pt?-based catalysts were still thought as the most active ORR electrocatalysts.However,low reserves and high cost of platinum inhibited the development and commercialization of energy storage and conversion devices.Hence,intensive efforts were focused on exploring non-precious-metal catalyst?NPMCs?alternatives with the comparable activity,good durability and high tolerance against fuel molecules.Our work used simple pyrolysis method to prepare?the Core-shell Fe-Co@Nitrogen-dopedMultilayerGrapheneNanocubes?and?theIron Sulfides/Nitrogen and Sulfur Dual-doped Mesoporous Graphitic Carbon Spheres?.Besides,both of two kinds of doped carbon matrices and transition metal composite catalyst,have the features of the low cost,the high activity and the excellent stability.The main contents of this work are described as follows:?1?Core-shell Fe-Co@Nitrogen-doped Multilayer Graphene Nanocubes Derived from Polydopamine-encapsulated Metal-organic Frameworks with Efficient Catalytic Activity for the Oxygen Reduction ReactionAs a sub-class of metal-organic frameworks?MOFs?,Prussian blue analogues?PBA?,with the chemical formula M₃^II[M^III?CN?₆]₂(M^III and M^III both are transition metal cations),are believed to be a promising precursor/template to prepare porous M-N-C catalysts with the controllable component and structure.We have successfully developed a simple process to synthesize core-shell structured Fe-Co@NMG NCs catalyst through an one-step pyrolysis route of the Fe₃[Co?CN?₆]₂@PDA NCs.The influence of the temperature during the transformation from Fe₃[Co?CN?₆]₂ NCs to Fe-Co NCs on the ORR catalytic activities were investigated.Comparing the ORR performance between Fe-Co@NMG and Fe-Co NCs,we can easily find the influence of the surface multilayer graphite carbon on the ORR.Introducing NMG at the surface of?Fe,Co?NCs as a protective layer does not influence the ORR mechanism and cooperatively enhances catalytic activity.Because of the synergistic effect of Fe-Co core and NMG shell,the Fe-Co@NMG NCs as an ORR catalyst exhibited excellent catalytic activity with the onset and half-wave potentials of 0.92 and 0.86 V,which is well comparable with that of the commercial Pt/C catalyst.More importantly,it showed much more outstanding methanol tolerance and cycle stability performance.?2?Iron Sulfides@Nitrogen and Sulfur Dual-doped Mesoporous Graphitic Carbon Spheres:A Promising Electrocatalyst for the Oxygen Reduction Reaction in Alkaline and Acidic MediaIn the system of Fe-Co@NMG NCs,lots of active sites were wrapped by carbon layers,and are difficult to be completely involved in catalyzing the ORR.Hence,we deisgn and synthesize a raisin bread-like electrocatalyst composed of iron sulfides(Fe_1-xS)and nitrogen and sulfur dual-doped mesoporous graphitic carbon spheres?N,S-MGCS?via a two-step pyrolysis process.By controlling the Fe²⁺content,we could have investigated the influences of the particle sizes and the contents of iron sulfides on the ORR catalytic activites.The resultant Fe_1-xS/N,S-MGCS has been demonstrated good electrocatalytic activities towards the oxygen reduction reaction?ORR?in alkaline and acidic media,of which the catalytic activities are closely related to the content and distribtuion of iron sulfide species.The most optimized electrocatalytic activity achieved over(Fe_1-xS/N,S-MGCS)_0.2 even outperforms that of commerical Pt/C catalyst in alkaline medium,and is comparable to the highest catalytic activities of the as-reported non-precious metal catalysts in acidic medium.The remarkable catalytic performance is ascribed by that,iron sulfide nanocrystals are introduced into highly conductive carbon supports for significantly enhancing the reactivity,and the raisin bread-like structure is constructed for improving the characteristics of electron and ion transport and inhibiting the self-aggregration and leaching of iron sulfide nanocrystals.