Preparation And Electrochemical Properties Of Lignosulfonate-based Porous Carbon Materials

In this paper,porous carbon nanosheets and N/O/S co-doped porous carbons were prepared via simple strategies using cheap and easily available sodium lignosulfonate and calcium lignosulfonate as carbon precursors.Furthermore,B/N co-doping and microstructure modification were conducted for porous carbon nanosheets and N/O/S co-doped porous carbon,respectively.The physical and chemical properties of as-obtained samples were studied by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy and X-ray diffraction（XRD）measurements,and the electrochemical performance of the corresponding electrode materials for supercapacitors in different electrolytes were also investigated.The porous carbon nanosheets（MNC-x）with developed pore structure and suitable heteroatom-doping were obtained via direct carbonizing sodium lignosulfonatein in N2atmosphere.Compared to MNC-800,the specific surface area,atomic ratio of C/O,graphitization degree and conductivity of MNC-900 increased sharply,which are mainly due to the quinonyl decomposition and the formation of graphitized carbon structures.The high oxygen content（20.1 at.%）and mesopore ratio（71.0%）of MNC-800,large specific surface area(1400 m² g^-1)and superior conductivity(2208 S m^-1)of MNC-900 make them suitable for supercapacitor electrodes in aqueous and organic electrolytes,respectively.Specifically,MNC-800 electrode shows large specific capacitance(196 F g^-1 at 0.25 A g^-1),superior rate capability(170 F g^-1 at 20 A g^-1)and outstanding cycling stability（96%capacitance retention after 10000 cycles）in symmetrical supercapacitor using aqueous electrolyte.The symmetrical supercapacitor based on MNC-900 electrodes shows an impressive energy density of 15.2 Wh kg^-1 at a power density of 9.9 kW kg^-1 in organic electrolyte.N-O-S co-doped hierarchical porous carbons（MGC-x）with large surface area(～1500 m² g^-1),high mesopore ratio（>40%）and heteroatom content were prepared by direct carbonization of calcium lignosulfonate.In three-electrode system,the MGC-700and MGC-800 exhibit high specific capacitances of 417 F g^-1 and 312 F g^-1 at 0.5 A g^-1,and 184 F g^-1 and 170 F g^-1 even at 100 A g^-1 using aqueous electrolyte,respectively.Notably,there exists a critical current density when evaluating the rate capability of MGC-x electrodes,which is based on their ion diffusion and electron transport properties.In two-electrode configuration with aqueous electrolyte,the MGC-700 with ultrahigh mass loading(～13 mg cm^-2,～210μm)can deliver large specific capacitance of 249 F g^-1 and excellent rate capability with capacitance retention of 81.9%from 0.05to 20 A g^-1.The energy and power densities of the supercapacitors based on MGC-700and MGC-800 electrodes with commercial-level mass loading in aqueous electrolyte and organic electrolyte are 5.1 Wh kg^-1/4585.4 W kg^-1 and 9.8 Wh kg^-1/7000 W kg^-1,respectivelyWith aid of the B₂O₃ and（NH₄）₂SO₄,MNC-700 was transformed into B/N co-doped porous carbon nanosheets（BNMNC）by one-step carbonization.The contents of B and N in BNMNC are 6.0 and 7.3 at.%,respectively,with a electrical conductivity of1225 S m^-1.In the three-electrode system,the specific capacitance of BNMNC is as high as 303 F g^-1(0.5 A g^-1)using aqueous electrolyte,and remains 194 F g^-1 at 100A g^-1.In the two-electrode system,the specific capacitance of BNMNC electrode with high mass loading of～7 mg cm^-2 reaches 198 F g^-1 at 0.25 A g^-1,with the capacitance retention of 80.8%at 20 A g^-1.At a discharging time of 360 s,the energy/power densities of the supercapacitor based on BNMNC electrodes are 6.8 Wh kg^-1/53.4 W kg^-1.A novel grafting-template method was proposed to prepare three-dimensional hierarchical porous carbons（MGSEx Ky）,simultaneously possessing high surface area and electrical conductivity.The ethylenediamine was uniformly grafted into calcium lignosulfonate in the form of nitrogen-containing groups under hydrothermal condition,subsequently acting as pore-forming template,which willendow the resultant MGSEx Ky with high atom ratio of C/O and graphitization degree during carbonization procedure.The mesopore/micropore volume ratio of MGSEx Ky can be effectively regulated by changing the dosages of ethylenediamine and KOH.MGSE1K0.5 yields a large specific capacitance of 250 F g^-1 at 0.25 A g^-1 and excellent rate capability(198 F g^-1 at 50 A g^-1)in a two-electrode system using aqueous electrolyte.The energy and power densities of the supercapacitors based on MGSE1K0.5 electrodes in organic electrolyte and ionic liquid electrolyte are 28 Wh kg^-1/10310 W kg^-1 and 38 Wh kg^-1/14.9 kW kg^-1,respectively.