Conductive Network Structure Design And Electrochemical Performance Of ZIFs-Derived Porous Carbon Materials

Zeolitic imidazolate frameworks（ZIFs）composed of coordinated transition metal linked by nitrogen-containing organic imidazole ligands possess similar structures to zeolite,which show exceptional mechanical stability,high chemical stability from metal nitrogen bonds,and adjustable pore size and surface area.Due to these excellent intrinsic properties,ZIFs-derived porous carbons obtain high nitrogen doping content,high specific surface area,and abundant porous structure.Therefore,ZIFs-derived porous carbons are considered to be one of the best candidates for supercapacitors and oxygen reduction reaction（ORR）applications.However,most of the synthesized ZIFs present dispersed microcrystalline powders.Even though stable carbon particles can be obtained after pyrolysis,these mono-dispersed ZIFs-derived carbon particles are difficult to form continuous conductive network for fast electron transfer,resulting in low rate performance at high mass loadings.Additionally,ZIFs-derived carbons contain a mass of carbon defects that are not beneficial for the electron transfer.Moreover,these carbon defects can be easily oxidized into CO₂ by the destructive radicals derived from H₂O₂,a by-product from incomplete reduction of O₂,which severely decrease the ORR turnover frequency and durability of catalysts.To solve the above problems,the conductive network structure of the ZIFs-derived porous carbons was constructed through two aspects of structural design:assembly strategy and graphene quantum dots（GQDs）-assisted coordination engineering strategy.The microstructure and atomic coordination structure of the prepared porous carbons were studied by various test methods.The corresponding synthesis mechanism,structure transformation mechanism,and energy storage mechanism were also discussed in depth.The main research contents are as follows:（1）The direct-dried ZIF-8 was used as precursor to prepare 3D interconnected porous carbon through spontaneous merging of nano-sized ZIF-8 polyhedrons during pyrolysis process.The effects of direct drying and freeze drying on the microstructure,surface property,and electron transfer were investigated in detail.The obtained porous carbon from direct dried ZIF-8 precursor shows a continuous conductive network,interconnected micro-/meso-porous structure,abundant N,O heteroatoms,and good hydrophilicity,enabling fast electron and ion transport kinetics for thick electrodes.As a result,even at a high mass loading of 15 mg cm^-2,the as-obtained carbon shows an outstanding rate performance of 137.7 F g^-1 at 100 A g^-1.Furthermore,the assembled symmetric supercapacitor using 1 mol L^-1 Na₂SO₄ electrolyte shows a high energy density of 20.6 Wh kg^-1 at 0.5 k W kg^-1 and 7.6 Wh kg^-1 at 34.3 k W kg^-1,as well as good cycle stability.（2）The mechanical compressed ZIF-8 was used as precursor to prepare gradient porous carbon monolith through thermal annealing.Such a unique structure shows visible distinction from the direct-dried ZIF-8 derived carbon monolith and other reported carbon monoliths,mainly reflecting on the gradient change of carbon skeleton structure and pore size.Specifically,the gradient porous carbon monolith shows large-sized pores and high carbon skeleton strength in the middle region but small-sized pores and low carbon skeleton strength at the edges.The gradient structure can also be optimized through adjusting the packing density of precursors.Based on the morphology,structure,and component analyses,we speculate that the structure transformation mechanism includes（ⅰ）the thermal decomposition of ZIF-8 leads to the release of gaseous decomposition products as well as agglomeration and chemical welding of particles,preliminary forming pore structure;（ⅱ）meanwhile the dense packed ZIF-8 particles leads to the aggregation gradient of carbonaceous gas,forming gradient porous structure;（ⅲ）carbonaceous gas are trapped/adsorbed by the carbon skeleton and redeposited to form gradient carbon skeleton.Benefiting from the large surface area,gradient porous structure,high N and O doping,and high packing density,the resulting carbon achieves much better volumetric performance than the pristine carbon without mechanical compression.（3）The GQDs modified Co-doped ZIF-8 built through coordination engineering strategy were used as precursor to prepare oxygen-rich GQDs-functionalized Co-N-C（G-Co NOC）electrocatalysts.We also prepared the Co-N-C catalyst without GQDs（Co NC）to investigate the effect of GQDs on the microstructure and ORR performance.By contrast,the introduced oxygen-rich GQDs with graphene structures can help to construct a local conductive network and decrease the carbon defects of catalyst,improving the electron transfer efficiency and resistance to the destructive free radicals from H₂O₂.Moreover,the oxygen-rich GQDs can provide a desirable hydrophilic surface for high utilization of active sites,leading to the overall improvement of ORR kinetics.As a result,the G-Co NOC electrocatalyst exhibits high electrocatalytic ORR activity with an onset potential of 0.95 V,a half-wave potential of 0.88 V,and retains a stability of 90.0%after 200 hours,better than the pristine Co-N-C electrocatalyst and commercial Pt/C.