Preparation And Electrochemical Properties Of Protein-derived N-doped Porous Carbon And Its Composites

Nitrogen-doped porous carbon（NPC）materials have a wide range of applications in the field of electrochemistry such as supercapacitor electrode materials and oxygen reduction catalysts due to their improved characteristics after doping of heterogeneous elements,high specific surface area,abundant pore structure,good electrical conductivity and high chemical stability.Exploring low-cost raw materials and simple preparation methods to control the specific surface area,pore structure,and surface properties and achieve excellent electrochemical performance has become the goal of those research fields.In order to simplify the preparation process of nitrogen-doped mesoporous carbon,N-doped mesoporous carbon（NMPC）was prepared by a one-step method of pyrolyzing the mixture of milk powder and potassium hydroxide without templates.The NMPC with microporous and mesoporous had a specific surface area of 2145.5 m²?g^-1 and a pore volume of 1.25 m²?g^-1.X-ray photoelectron spectroscopy analysis showed that the nitrogen content of NMPC was 2.4 at%.The NMPC exhibited a high specific capacitance(396.5F?g^-1at 0.2 A?g^-1)and excellent stability(capacitance retention of 95.9%after 2000 cycles at 50 mV?s^-1)as a supercapacitor electrode material.Moreover,used as an ORR catalyst,NMPC exhibited a high initial potential of 0 V（vs.Hg/HgO）,a half-wave potential of-0.095 V,and a limiting current density of 4.7 mA?cm^-2,and the number of reactive electrons is greater than 3.7,very Catalytic performance close to 20 wt%Pt/C.The results demonstrate that the NMPC is a promising electrode material for supercapacitors and fuel cells.To optimize the pore structure and improve the rate performance of the NPC materials,simultaneously simplify the preparation process of the N-doped porous carbon,N-doped hierarchical porous carbon（NPC-800）was synthesized via pyrolysis of cobalt-containing casein obtained by extraction from pure milk with the auxiliary of cobalt ions.The cobalt element in casein promoted the specific surface area and N element doping amount of casein-derived porous carbon.The N-doped porous carbon obtained by carbonization at 800oC（NPC-800）possessed a specific surface area of 886.7 m²?g^-1 and the mesoporous size was centered at 2.7 and 7 nm.X-ray photoelectron spectroscopy analysis showed that the nitrogen content of NPC-800 was 3.29 at%.The NPC-800 was explored as a symmetric supercapacitor,which exhibited a specific capacity of 380 F?g^-1 at0.5 A?g^-1,218 F?g^-1 at a current density of 20 A?g^-1,and the high capacitance retention of91.3%after charging/discharging 5000 cycles.The composite material with electrochemical double layer capacitance and pseudocapacitance was prepared by using transition metal ions adsorbed in casein,realizing the preparation of high specific capacitance electrode material and maximizing the utilization of raw materials.The Fe³⁺adsorbed casein obtained by extracting from the milk and the melamine were fully dissolved in the KOH solution,and the precursor obtained after lyophilization was carbonized at 600oC to obtain Prussian blue nanocubes/N-doped porous（PB/NPC-600）composite material.The preparation of PB/NPC-600 was more simplified than the two-step compounding of traditional solution chemical synthesis.In the composite material,PB nanocubes were embedded on the surface of NPC particles with a size of about 50 nm,and its content was about 15 wt%of the composite.In the application of supercapacitor electrode materials,PB/NPC-600showed very good capacitance reversibility and the specific capacitance of 398.9 F?g^-1 at0.5 A?g^-1.At a power density of 15.2 kW?kg^-1,the energy density of the PB/NPC-600symmetric supercapacitor could be stored up to 29.5 Wh?kg^-1,and its performances were far superior to the PB+NPC prepared by traditional solution chemical synthesis.PB/NPC-600 composite exhibited excellent catalytic ORR performance close to Pt-based catalysts in catalytic ORR applications due to its special composite structure and functional components,such as the initial potential of 0.93 V（vs.RHE）,the limiting current density of 5.79 mA?cm^-2,the number of reactive electrons of close to 4,and the good stability.In order to eliminate the contact resistance generated between NPC particles,the three-dimensional（3-D）conductive graphene network was used to organically combine NPC to reduce the internal resistance of the electrode and improve the charge and discharge efficiency of the supercapacitor.Casein and graphene oxide（GO）were dissolved in KOH solution,and the hydrolyzate is obtained by utilizing the reaction of GO and the polypeptide and amino acid produced by hydrolysis of casein.After lyophilization,the precursors was carbonized at 700oC and then washed with deionized water several times to obtain N-doped porous carbon/holey graphene（NPC/HG）composite materials.The NPC/HG-2 showed a specific surface area of 1135.3 m²?g^-1 and possessed a microporous,mesoporous and macroporous multiple pore structure.The images of SEM and TEM showed that NPC/HG-2 had a complete 3-D crosslinked structure and many holes appeared on the 2-D plane of graphene.XPS elemental analysis showed that the N doping amount was up to 6 at%.Used as the symmetrical supercapacitor electrode material,the NPC/HG-2 showed the specific capacitance of 316.6 F?g^-1 at 1 A?g^-1.The current density increased from 1 to 50 A?g^-1,and the capacitance retention rate was up to82%.Moreover,the energy density was 29.33 kWh?kg^-1 at a power density of 42.75kW?kg^-1.The NPC/HG-2 was assembled as a PVA/KOH solid supercapacitor with a single cell voltage of up to 1.4 V.