Controllable Synthesis And Electrocatalysis Studies Of Non-Precious Metal-Based Oxygen Reduction Reaction Catalysts

Fuel cells have been suggested as one of the most important strategies to solve the potential energy crisis,thanks to their large energy density,negligible emission of greenhouse gases,and significantly improved utilization ratio of fuels.However,the great challenge for the broad applications of fuel cells lies in naturally sluggish oxygen reduction reaction?ORR?at the cathode and the high cost derived from using of precious metal Pt-based ORR catalysts.To this end,the development of non-precious metal-based ORR catalysts as low-cost alternatives to Pt-based materials has been widely considered as the most promising strategy to promote the introduction of fuel cell powered vehicles to the large-scale market.To achieve this goal,this dissertation is mainly focused on the exploration of the relationship between the physicochemical properties of catalysts and their catalytic performance,and subsequently synthesized a series of ORR catalysts with outstanding ORR catalytic performance.The main points are summarized as follows:1.Majorities of currently studied Metal-Nitrogen-Carbon?M-N-C?materials exhibited large size and polydispersity,and thus the active sites are not able to be exposed thoroughly,leading to suboptimal ORR catalytic performance.Extensive efforts have been made to reveal the correlation of active site and activity.However,it remains unclear to what extent the particle size will influence the ORR activity of M-N-C materials.To the best of our knowledge,controllable synthesis of M-N-C catalysts with high-density activity sites has not been reported.Here we develop a straightforward strategy for the large-scale manufacture of a monodisperse and size-controlled Co-N-C catalyst by using a Co-containing ZIF?ZIF-67?as the precursor.The strategy has multiple unique features:?1?the precursor ZIF-67 has the well-controlled size,a much larger surface area?1904 m2/g?,and specifically,a regular dodecahedron structure due to the sufficient coordination between Co2+ ions and N atoms in the imidazolate ligands at the molecular level;?2?after carbonization,the resultant Nano-P-ZIF-67 can preserve its well-defined rhombic dodecahedron morphology and retain the large surface area,porous structure,and more strikingly,high-density and evenly distributed Co-N active sites across the whole framework;?3?the well-controlled size create an opportunity for the evaluation of the size effect on the ORR catalytic activity of the Nano-P-ZIF-67;?4?only by optimizing the particle size,the Nano-P-ZIF-67 can outperform commercial Pt/C in terms of all evaluating indicators for ORR catalysts in alkaline solution including higher catalytic activity,superior durability,and higher catalytic selectivity for cathode reactions;?5?Nano-P-ZIF-67 shows a comparable catalytic activity to commercial Pt/C in acidic media;and?6?neither complex operations nor expensive equipment is involved in this strategy,and the preparation of the Nano-P-ZIF-67 can be extended to large scale owing to its ease of operation,excellent reproducibility,and high yield,enabling industry production at a low cost.2.Nitrogen-doped carbon materials have become one of most promising ORR electrocatalyst,given their favorable properties such as high catalytic activity,facile preparation,among others.However,the ORR activities of N-doped carbon materials are lower than expected.The graphitization degree?electroconductivity?and the surface area?sufficient exposure of the active sites?are widely recognized as two fundamentally crucial factors for the catalytic performance of N-doped carbon nanomaterials.Conventional methods are hard to prepare N-doped carbon materials with a large surface area and a high degree of graphitization,simultaneously.We here introduce an Ostwald ripening mechanism-driven method to produce monodisperse N-graphite hollow nanoparticles with significantly improved graphitization and a dramatically enlarged surface area.Different from previously reported N-doped carbon materials,the present strategy first involves coating of large-size Fe₃O₄ nanoparticles?100 nm in diameter?with an N-rich precursor?polydopamine?.The inner large Fe₃O₄ nanoparticles can provide efficient and degradable templates that favor the formation of interior cavity and simultaneously catalyze and modulate the graphitization behavior of the polydopamine shell.The resultant N-graphite hollow nanoparticles exhibit dramatically enhanced graphitization and a large surface area of 1064 m2/g,leading to remarkably improved electrochemical performance as compared to the commercial Pt/C.More impressively,the assembled cell using the N-graphite hollow nanoparticles shows a power density of 47.2 m W/cm²,which is almost twice the value of Pt/C-based cell.3.The cost of the fuel cells largely dependents on the cost of the ORR catalyst materials.Consequently,the preparation of cost-effective yet high-performance ORR catalysts is of great significance.Tofukasu is a biological waste that contains abundant C,N,and O atoms,and thus it can be used as both the carbon and nitrogen resources of non-precious metal-based ORR catalysts.In this part,the tofukasu was directly explored as the precursors for the synthesis of high-performance non-precious metal-based ORR catalysts.After exhaustive characterization including the size,morphology,pore size,and element contents,BD-700-A3 is selected.This catalyst possesses cross-linking large pores,a large surface area?2873 m2/g?,and a high level of ORR active N.These favorable properties enable BD-700-A3 to show higher ORR activity,stronger methanol tolerance,and better catalytic stability than commercially available Pt-based materials under alkaline conditions.BD-700-A3 has been further assembled into a cell,and the cell shows a power density of 46.3 m W/cm²,which is much higher than that of Pt/C-based cell.