Morphology Control Of Metal-Organic-Frameworks Based On Crystallization Process And Electrocatalytic ORR Performance Of Derived Carbon-Based Catalysts

With the rapid progress of hydrogen energy,series of important measures,including developing highly active,stable,and durable non-precious ORR catalysts with earth-abundant elements,have been implemented to reduce fuel cell costs and promote the commercial application of hydrogen fuel cells.Among numerous candidates,transition metal and nitrogen co-doped carbon-based composite materials（M-N-C）are widely regarded as one of the most promising non-precious metal catalysts for the ORR.Nevertheless,the controllable synthesis of M-N-C catalysts with optimal micro-and macro-structures remains a great challenge.Metal-organic frameworks（MOF）,a type of novel porous crystalline material assembled by metal ions and organic ligands,are well known for their high surface area,well-organized porous structure or tailorable structure,and have been considered to be an ideal precursor for the construction of high-efficiency M-N-C catalysts.The result of previous research proves that the morphology of MOF has a pivotal influence on the structure and morphology of their derivatives.Nevertheless,current structural regulations of MOF are more concentrated on the micro-molecular level,and a few studies were conducted to regulate the meso-or macro-morphology of MOF.This thesis starts with the crystal growth process of MOF.That is,the self-assembly process of the MOF crystal can be artificially regulated by controlling the release rate of metal salt,adjusting the reaction time,and changing the content of the modifier.Based on the above,a plurality of MOF with ultra-fine structure,1D monodisperse nanofiber and 3D urchin-like morphological characteristics had been fabricated.Hence,the effect of synthesis conditions and rules on the growth kinetics of MOF crystals with special morphology and structure were studied.Using the above-mentioned MOF as precursors,a series of carbon-based oxygen reduction electrocatalysts with excellent structure and composition were constructed,and reveal the influence of the intrinsic structure/morphology of MOF on the composition,structure of the obtained catalyst,and the promoting effect of excellent structural characteristics on electrocatalytic performance for ORR.Firstly,a sustained-release method for regulating the crystallization rate of ZIF was proposed.Co-doped basic zinc acetate,which shows solubility differences in diverse mixed solvents,was employed as the metal source to adjust the crystallization rate of ZIF.Based on this,a series of Zn/Co-ZIF with different sizes were synthesized,and the smallest average particle size of a single particle could reach 50 nm.By using the Zn/Co-ZIF with best structure as the precursor（UF Zn/Co-ZIF）,a carbon-based nanocatalyst（UF Co-N-C）with ultra-fine grain size（50 nm）,hierarchical porous structure,high surface area(765 m² g^-1)and Co quantum dot center can be obtained after one-step pyrolysis.When used as an ORR catalyst,the UF Co-N-C possesses significantly high ORR activity in alkaline media and displays strong ORR performance similar to that of Pt/C in acidic media,the outstanding ORR performance of UF Co-N-C could be attributed to the simultaneous optimization of both excellent external structures and highly dispersed active sites.Besides,the controllable crystallization process of ZIF not only proves the feasibility of the sustained-release strategy in regulating the crystallization rate of MOF,but the proposed method also provides a worthy reference for the preparation of high-efficiency carbon-based catalysts.Secondly,we proposed an in-situ recrystallization strategy for the fabrication of an urchin-like superstructure MOF composed of 1D MOF nanofibers.A 3D bulk MOF（Zn（H2O）2（C8H4O6）,termed as Zn-DHTA）was selected as the research object.The crystal structure of Zn-DHTA can be modulated significantly upon the addition of methanol or urea,and thus that leads to a remarkable improvement of the morphology of Zn-DHTA after recrystallization reaction.Specifically,a 3D urchin-like superstructure MOF（US-MOFNF）composed of nanofiber which transformed from the original 3D block morphology can be obtained during the recrystallization process.In addition,1D monodisperse nanofibers MOF（MOFNF）were also fabricated by adjusting the types of regulators.Subsequently,a certain amount of Co element was successfully doped into the US-MOFNF to prepare the Zn/Co bimetallic urchin-like superstructure MOF during the synthesis process.The Co and N co-doping carbon-based nanofiber material（Co-N-CNF）can be fabricated via a pyrolysis process by using dicyandiamide as the supplementary nitrogen source.Attributing to multi-characteristics of morphology or structure,the metal active center of the Co-N-CNF catalyst can be fully exposed,thus endowing it with excellent electrocatalytic ORR activity and stability in alkaline medium.Finally,choosing the above-mentioned US-MOFNF with structural/morphological advantages as a template,and then the polypyrrole was coated on the external surface of US-MOFNF through a simple polymerization reaction subsequently.Fe-N-C catalysts with one-dimentional carbon nanotube structure can be obtained via a one-step pyrolysis,which possess a large specific surface area(737 m² g^-1)and hierarchical porosity after carbonization.The Fe element in polypyrrole underwent a series of reactions such as oxidation and reduction during pyrolysis process and transformed into a core-shell Fe3C@Fe3C crystal structure.When applied to the electrocatalytic oxygen reduction reaction,the obtained nanotube catalyst exhibits excellent alkaline performance（half-wave potential of 0.902 V）and stability（current density only attenuates 5%in 80,000 s after long-term testing）.Ascribing to the difference in the pyrolysis rate and the thermal stress between US-MOFNF and polypyrrole,the interior MOF gradually decomposed into the macroporous channel as the increasing pyrolysis temperature,while the exterior polypyrrole coating layer was transformed into carbon nanotubes.In addition,the proposed polymer coating strategy was extended to the fabrication of carbon nanotubes by using 1D Co-MOF-74 as a template,and successfully fabricated a carbon-based catalyst with nanotube structure and Co3Fe7 metal center after high-temperature pyrolysis.This fully illustrates the university of the polypyrrole coating method for the fabrication of 1D nanotube catalysts and the prospect for the construction of carbon-based catalysts with specific metal active centers and hierarchical porous structure.The above results reveal that the synthetic strategy that oriented to adjust the crystal growth process of MOF provides a significant route to improve the morphology and mesostructure of MOF and promote the local microstructure and activity of MOF-derived carbon-based materials to a large extent.The simultaneous construction of remarkable micro-nano structures ultimately endows the derived carbon material with excellent electrocatalytic oxygen reduction performance.Therefore,this thesis not only provides a fire-new path for the construction of non-noble metal oxygen reduction catalysts,but opens up new ideas for the synthesis of other functional carbon-based materials.