The Preparation,Performance,and Application Investigation Of Carbon-based Oxygen Electrocatalysts

The fuel cells and metal-air batteries have the advantages of high energy density,environmental friendliness,etc.However,the slow reaction kinetics of the oxygen electrode severely limits their energy conversion efficiency.Precious metals such as platinum,ruthenium and iridium are the most efficient oxygen electrocatalysts,but the large-scale commercialization of precious metal catalysts is seriously limited by the shortcomings,such as lack of resources,high cost and poor stability.Therefore,the development of low cost,highly efficient,and stable alternative non-noble metal based catalysts is of critical importance and urgently needed for their widespread use.In this paper,selecting carbon-based non-precious metal materials as the object of study,by optimizing the experimental program to control the morphology and microstructure of the materials,to adjust the abundance and activity of the catalytic active sites,to fabricate an integrated electrode to improve the electrical conductivity and mechanical stability,we establish the structure-activity relationship achieving the optimal electrochemical performance,and explore their actual application in various devices.The work in this paper is mainly focused on the following aspects:1.Fe-N doped mesoporous carbon microspheres?Fe-NMCSs?electrocatalyst is firstly synthesized using a facile in situ replication and polymerization strategy,wherein the mesoporous ferroferric oxide?Fe₃O₄?microspheres are employed as multifunctional template-mesoporous structure-directing agent,source of Fe³⁺ions as oxidation agent for polymerization of pyrrole,and especially precursor of Fe doping.This strategy highly improves the utilization and digestion of the templates.Unexpectedly,the obtained catalyst exhibits superior oxygen reduction reaction performances,including high activity,superior durability,and good selectivity even in comparison to commercial Pt/C catalysts.Especially,benefiting from the open framework of mesoporous microspheres and effective Fe doping,the catalyst also can be successfully employed as a promising cathode catalyst both in real alkaline and proton fuel cells,giving a power density as high as 506 and 463 mW cm^-2,respectively.2.With the idea of optimizing Fe-N doped carbon?M-Nx/C?type oxygen reduction electrocatalysts based on precursor-controlled synthesis in mind,an efficient liquid nitrogen assistant extremely rapid freeze drying method inspired by a natural principle in sea ice is developed to anchor cheap iron-EDTA complex on graphene to realize microstructural homogeneity of the derived Fe-N-C electrocatalyst.As a result,a high catalytic activity of self-supporting iron-nitrogen doped porous carbon sheet oxygen reduction electrocatalyst was synthesized by high-temperature carbonization of the precursor and subsequent acid treatment to solve the problem of poor uniformity and agglomeration of catalysts.The hierarchical porous structure with nanosheet morphology and high specific surface area ensure the exposure of the catalytic active sites and high mass transfer efficiency,resulting in superior catalytic performance in alkaline electrolyte,superior to commercial Pt/C catalysts,including the positive onset potential and half-wave potential,and the longer stability.3.Novel inexpensive and mass-produced iron chelated urea resin hydrogel?Fe-UFR?is synthesized to construct a uniform composite system of polymer and transition metal,the high compatibility and coordinating capability between carboxyl group,amide group and metal ion is utilized to achieve a uniform distribution of metal particles in the carbon atom-scale precursor during the sol-gel process,which prevents agglomeration of the metal particles and guarantees the microstructural homogeneity during the pyrolysis synthesis.Fe-N doped porous carbon bifunctional catalyst was synthesized by high temperature pyrolysis and carbonization.Combination of different channels with a uniform loading of high activitve sites to achieve the optimal combination of mass transfer and activity in oxygen reduction and evolution test realizes both high activity and stability,rendering it high-performance cheap bifunctional electrocatalysts.Furthermore,much better performances including low charge and discharge over-voltage,higher power density and better cyclic stability are obtained in the rechargeable Zn-air batteries.4.A three-dimensional?3D?integrated free-standing bifunctional oxygen electrode is first successfully developed,through one step carbonization of string of ZIF-67 on polypyrrole nanofibers network rooted on carbon cloths under N₂ atmosphere.N-doped carbon fiber networks derived from polypyrrole nanofibers are intertwined on carbon cloth to form a stable three-dimensional interconnected conductive network.High temperature carbonization not only synthesized N-doped carbon and Co-Nx/C active sites with high catalytic activity for oxygen reduction,but also obtained Co₄N with high catalytic activity for oxygen evolution.In addition,the unique polyhedral porous nitrogen-doped carbon backbone anchored Co₄N particles and Co-Nx/C active sites on the three-dimensionally interconnected nitrogen-doped carbon fiber network,further increasing the electrode structure stability.Benefiting from these advantages,Co₄N/CNW/CC electrodes exhibited excellent bifunctional catalytic activity including lower oxygen evolution potential,postive half-wave potential for oxygen reduction,long cycle stability and high selectivity,which ensure an exceptionally high performance both for primary and rechargeable Zn-air batteries.Furthermore,the Co₄N/CNW/CC electrode based cable-type Zn-air battery displays excellent rechargeable performance at a high current density,the flexible characteristics make it a huge potential in wearable electronic devices.