Synthesis And Capacitance Performances Of Humate-based MOFs Derived Electrode Materials

Metal-organic framework materials(MOFs)are a family of nanoporous materials composed of various metal ions and organic ligands by means of self-assembly.Due to its rich and adjustable architectures,diverse pores and high specific surface area,it can be widely used in drug carriers,gas absorption,catalysis,supercapacitors and other fields.When used as electrode materials for supercapacitors,MOFs often exhibit high specific capacitances.However,their capacitance performances are always unstable due to the poor structural stability and conductivity.MOFs-derived layer double hydroxides(LDHs)and carbon materials always have stable structures and capacitive performances.Humic acid is a natural organic mineral with wide sources,owning similar structure and activity to GO.And it is easily combined with metal ions,which can be pyrolyzed to prepare graphene.Here,HA-MOFs were first synthesized by natural precipitation,direct agitation or solvothermal method,using the purified sodium humate(p-HA-Na)or anthracite-based humic acid(a-HA)as the substrate,2-MIM,BTC and BDC as organic ligands and Ni2+,Co2+ or Fe2+ as metal ions.Then HA-NiCo-LDHs were then derived by a protic solvothermal approach and nitrogen-doped carbon skeleton materials(NCS)were evolved by a gradient calcination method.And we investigated the universality of the protic solvothermal approach or gradient calcination method for the derivation of LDHs or NCS from HA-MOFs templates formed by different humic acids,ligands and metal ions.The morphologies and structures of the derived materials were characterized by SEM,TEM,XRD,Raman,FTIR,XPS,BET and elemental analysis.The capacitive performances of the derived materials were studied by cyclic voltammetry(CV),galvanostatic charge-discharge(GCD),electrochemical impedance spectroscopy(EIS)and long cycle charge-discharge test.The detailed research results are as follows:1)HA-ZIF-67(a-HA-ZIF-67),HA-Ni-BTC and HA-Ni-BDC templates were first synthesized by natural precipitation,direct agitation or solvothermal method,respectively.Then HA-NiCo-LDH and a-HA-NiCo-LDH composites were successfully derived by a protic solvothermal method.All the synthesized templates showed the guiding role of the HA.Therein,HA-ZIF-67 exhibited a polyhedral structure on the HA surface while HA-Ni-BTC and HA-Ni-BDC were assembled into a laminated bundle structure on the HA surface.In addition,HA-NiCo-LDH-1 inherited the morphology of the HA-ZIF-67 template and evolved into a dodecahedral cage structure in situ after the solvothermal reaction.It showed excellent capacitance performances and good cycling stability when used as electrode materials.It originates that p-HA-Na is a carboxylate salt with strong polarity and high activity.It can be more easily dissolved and better dispersed in water,which is conducive to forming lamellar micelles,further constructing the HA-ZIF-67 template with a more ordered assembly structure,and purposefully deriving the HA-NiCo-LDH-1 with lower internal resistance and higher capacitance from the HA-ZIF-67 template.HA-NiCo-LDH-1 exhibited a high specific capacitance of 1083 C g-1 at 0.5 A g-1,high rate capability(20 A g-1,559 C g-1)and good cycle stability(5 A g-1,10000 cycles,77%).The assembled HA-NiCo-LDH-1//AC asymmetric supercapacitor also showed a high specific capacitance of 231 C g-1 at 0.5 A g-1,high rate capability(10 A g-1,120 C g-1),high cycle stability(5 A g-1,5000 cycles,98%)and high specific energy density of 44.9 Wh kg-1 at a power density of 348 W kg-1.2)Using HA-Ni-BTC and HA-Fe-BTC templates as carbon sources and melamine as nitrogen source,a series of nitrogen-doped carbon skeleton materials were synthesized via a gradient calcination method by adjusting the calcination temperature and the amount of melamine.When the mass ratio of the HA-Ni-BTC/melamine was controlled to 1:3,the three-dimensional NCS-Ⅰ-700 composed of carbon nanotubes,graphene and porous carbon was successfully prepared via a pyrolysis process at 700℃.HA-Ni-BTC can not only serve as a sacrificial template,but also provide metal catalysts to promote the formation of carbon nanotubes and graphene.The NCS-Ⅰ-700 has abundant mesopores(average pore size of 6.1 nm),a large specific surface(296.5 m2/g)and a high nitrogen dosage(11.8 wt%).When used as electrode material,NCS-Ⅰ-700 had a high specific capacitance of 357 C g-1 at 0.5 A g-1,high rate capability and cycle stability(5 A g-1,10000 cycles,94%).The assembled NCS-Ⅰ-700//AC asymmetric supercapacitor also had a high specific capacitance of 148 C g-1 at 1 A g-1,high rate capability(25 A g-1,88 C g-1),good cycle stability(5 A g-1,10000 cycles,94.8%)and high energy density(300 W kg-1,12.3 Wh kg-1).When the mass ratio of the HA-Fe-BTC/melamine was adjusted to 1:3,the prepared NCS-Ⅱ-750 with a nitrogen dosage of 6.0 wt%was achieved by the pyrolysis at 750℃.The NCS-Ⅱ-750 also exhibited a rich mesoporous structure,large specific surface(278.7 m2/g)and high specific capacitance(0.5 A g-1,347 C g-1).The assembled NCS-Ⅱ-750//AC asymmetric supercapacitor demonstrated a high specific capacitance,rate capability(1 A g-1,130 C g-1;50 A g-1,81 C g-1),cycle stability and energy density.