Heteroatom Doping Modification And Electrochemical Properties Of Carbon Electrode Materials

Supercapacitors are considered to be one of the most promising energy storage devices due to their high power density,long cycle life,and low maintenance cost.For carbon electrode materials for supercapacitors,there is the problem that their specific capacitance performances,especially the volumetric capacitance performances,are not satisfactory.As for this issue,in this paper heteroatom-enriched carbon materials with moderate specific surface area,hierarchical pore structure and high density were designed and developed in order to explore carbon electrode materials with high gravimetric capacitance and volumetric capacitance properties.The main contents are as follows.Heteroatom-enriched carbon(NOC)was obtained by the carbonization process at high temperature of the aqueous mixture of polyvinylpyrrolidone with high nitrogen and oxygen concentration and melamine formaldedyde resin with high nitrogen concentration.In order to improve its electrochemical performances,the one-step carbonization/activation and washing process was adopted to prepare B/N/O-doped carbon and P/N/O-doped carbon by adding boric acid/urea and phosphoric acid,respectively.The influence of heterotom kinds,species and contents on the morphology,pore structure,and electrochemical properties of materials were systematically investigated.The results of Fourier transform infrared spectra(FTIR),X-ray photoelectron spectroscopy(XPS),Nitrogen adsorption/desorption curves and electrochemical impedance spectroscopy(EIS)show that the boric acid/urea treatment can effectively inhibit the formation of inert B-O and B-N species,increase specific surface area,enrich pore structure,and increase conductivity and diffusion rate of electrolyte ions.By the further washing process,the optimal BNOC-BU-W exhibits high specific surface area and rich pore structure,and remains much active species of N-5,N-6,B-C and O-I,and not lowering density.The results of cyclic voltammetry(CV)and galvanostatic charge/discharge(GC)show that the synthesized BNOC-BU-W exhibits high volumetric capacitance(309 F cm^-3 at 0.5 A g^-1)and high gravimetric capacitance(238F g^-1 at 0.5 A g^-1)and good rate capability in 6 M KOH electrolyte in 6 M KOH electrolyte.BNOC-BU-W//BNOC-BU-W symmetric supercapacitor offers relatively high volumetric energy density(11.5 Wh L^-1at 622 W L^-1)and exhibits a good capacitance retention of 92%up to 3000 charge-discharge cycles.Compared with NOC,the optimal NPOC-30%after phosphoric acid treatment shows the increasing specific surface area and mesopore/macropore volume,which is the crucial for diffusion of electrolyte ions,formation of double-layer capacitance and the improvement of rate capability.And it exhibits the formation of active species of N-6/P=N,N-5,(N-Q)/P-N,O-?,P2 and P3,and not lowering density,which obviously improves the pseudocapacitance.NPOC-30%exhibits high volumetric capacitance(456F cm^-3 at 1 A g^-1),high gravimetric capacitance(304 F g^-1 at 1 A g^-1)and good rate capability in 6 M KOH electrolyte.NPOC-30%//NPOC-30%symmetric supercapacitor also offers relatively high volumetric energy density(11.5 Wh L^-1at 622 W L^-1)and retains as high as 7.8 W h L^-1 at 6294 W L^-1.It exhibits a good capacitance retention of97%up to 6000 charge-discharge cycles.