Nitrogen-doped Porous Carbon Via A Template Carbonization Method For High Performance Supercapacitors

Supercapacitors based on a new energy storage device, which can bridge the gapbetween batteries and conventional dielectric capacitors, and are ideal for the rapidstorage and release of energy. Studies have shown that the electrode material plays adecisive impact on its performance. Highly porous carbon has attracted great attentionbecause of its low density, high specific surface area, tunable porous structure, highthermal conductivity, good electrical conductivity, mechanical stability and massproductivity. Herein, we demonstrate a novel template carbonization method to preparenitrogen-doped porous carbon materials which possess high specific surface area,high pore volume and suitable pore size distribution to make their electrochemicalproperties excellent such as high energy density and power density, good ratecapability, long-term cycle stability. The major contents can be summarized asfollows:1. In this work, we have demonstrated a direct carbonization method to prepareporous carbons as electrode materials without activation process, using sodiumcarboxymethyl cellulose (NaCMC) as the carbon source, which are further doped withnitrogen with various mass ratios. Based on X-ray photoelectron spectra, the nitrogenspecies doped mainly include pyridinic N, graphitic N, and pyrrolic N. Besides, themass ratios of NaCMC and CO(NH2)2can exert a decisive impact on the nitrogenspecies, dopant dosages as well as specific surface areas and pore structures. The cyclicvoltammetry and galvanostatic charge-discharge measurements in6mol·L-1KOHaqueous solution reveal that the specific surface areas and capacitive performances havebeen distinctly improved after doping with nitrogen. Taking carbon-N-1:20as example,its SBETcan reach up to858m2·g-1, which is much higher than that of carbon-blank as463m2·g-1, and the corresponding specific capacitance has greatly improved from94.0F·g-1to156.7F·g-1. The present carbons are excellent electrodes candidates for high-rateelectrochemical capacitors.Solid-state method as well as hydrothermal treatment and postannealing method wereapplied to prepare porous carbons, using sodium carboxymethyl starch (abbr. CMS-Na)as carbon source. The as-prepared carbons were further heated with urea to producenitrogen-doped carbons. The nitrogen content within carbons has crucial impact on thesurface areas, pore structures and capacitive properties. The experimental results revealthat the capacitive performances of nitrogen-doped carbons by solid-state method are much better than those by hydrothermal treatment and postannealing method. Specificcapacitance at the current density of1A g1of the carbon-S-N-1:20sample hasimproved up to176.0F g1from147.2F g1(carbon-S-blank); the carbon-H-N-1:20sample has dramatically improved up to170.3F g1from68.3F g1(carbon-H-blank).Furthermore, the carbon-S-N-1:20sample exhibits better rate capability, cyclingstability, and power density.High-performance porous carbons have been prepared as supercapacitor electrodematerials by co-doped with nitrogen and MnOxvia a direct carbonization method, usingsodium butyl naphthalene sulfonate (abbr. BNS-Na) as carbon source. The sampleswere well characterized by means of XRD, FESEM, XPS and BET techniques. It isbelieved that the in situ formed Na6(SO4)2(CO3) in the product would probably serve astemporary template for producing porous structures. The impacts of nitrogen/MnOxcontent as well as the structures upon the capacitive performances were emphaticallydiscussed. It indicates that introducing nitrogen and/or MnOxinto the carbon matrix canremarkably improve their capacitive performance based on the cyclic voltammetry andgalvanostatic charge-discharge measurements in6M KOH aqueous solution. Thespecific capacitances of doped carbons can reach up to ca.167.0~241.8F g-1comparedwith that of the undoped carbon as ca.105.6F g-1.Of these samples, thecarbon-Mn-1:30-N-1:15sample co-doped with nitrogen and MnOxexhibits the highestspecific capacitance and energy density up to241.8F g-1,33.6Wh kg-1, respectively. Inparticular, these carbons also exhibit high intrinsic capacitances (i.e., capacitance persurface area) up to ca.0.66~1.92F m-2.2. Nitrogen-doped porous carbons have been prepared by a direct carbonization ofthe mixture of urea formaldehyde resins (abbr. UF resins) and calcium acetatemonohydrate. The experimental results reveal that the mass ratios of UFresins-to-Ca(OAc)2·H2O and carbonization temperatures can play crucial roles in theformation of porous carbons with various surface areas and structures as well as thecapacitive performances. The UF-Ca-700-3:1sample exhibits much large specificcapacitances ca.334.8F g-1and224.0F g–1at the current density of0.5and1.0A g–1in6.0mol L–1aqueous KOH electrolyte, respectively. Furthermore, the UF-Ca-900-3:1dispalys more superior rate capability, which can possess high specific capacitanceretention as ca.67.1%and51.4%at the high current densities of20and40A g–1,respectively. In addition, a capacity fading lower than1%after5000cycles of chargingand discharging is obtained, indicating its long-term electrochemical stability. Through a template carbonization method, nitrogen-doped porous carbon weresuccessfully achieved by heating urea formaldehyde resins (abbr. UF resins) andmagnesium citrate at800oC, where magnesium citrate involved serves as template. Themass ratio of UF resins and magnesium citrate plays a crucial impact on the surfaceareas, pore structures, and the correlative capacitive behaviors of the final porouscarbons, named as UF-Mg-1:1/1:3/1:5. All present porous carbons take on amorphousfeatures with low graphitization degrees. The specific surface areas of theUF-Mg-1:1/1:3/1:5samples are1062,1059and1117m2g–1, and total pore volumesare1.25,1.67and1.56cm3g–1, respectively. The UF-Mg-1:3sample displays the bestcapacitive performance with large specific capacitance of239.7F g–1at a currentdensity of0.5A g–1and high energy density of33.3Wh kg1in case of the powerdensity as0.25kW kg1. More importantly, it retains high capacitance retention as94.4%after charging-discharging for5000times, clearly indicating good cyclingstability.Spherical nitrogen-doped porous carbons have been prepared through a templatecarbonization method, in which polyacrylamide serves as carbon and nitrogen sources,and calcium acetate as hard template. It reveals that the mass ratio of polyacrylamideand calcium acetate and the carbonization temperature have crucial impacts upon thepore structures and the correlative capacitive performances. Given cyclic voltammetryand galvanostatic charge-discharge measurements, the PAM-Ca-650-1:3sampledisplays the best capacitance performance. It is amorphous with low-graphitizationdegree, possessing total BET surface area of648m2g–1and total pore volume of0.59cm3g–1. At a current density of0.5A g–1, the resultant specific capacitance is194.7Fg1. It exhibits high capacitance retention of97.8%after charging-discharging for5000times. Also, its energy density can reach up to27.0Wh kg1. The polyacrylamide usedis cheap and commercially available, making it promising for large-scale production ofporous carbons containing nitrogen as excellent supercapacitor electrode material.3. As a generalized synthetic protocol, porous carbons have been for the first timeprepared by a direct carbonization of polyacrylate-metal complex. The case ofmagnesium polyacrylate was emphatically studied. It reveals that the carbonizationtemperature can play crucial role in the determination of surface areas, pore structures,surface functionalities of porous carbons as well as the correlative capacitiveperformances. The carbon-Mg-900sample exhibits a high surface area of942m2g–1and large total pore volume of1.90cm3g–1, with high specific capacitance of262.4F g–1at0.5A g–1in6.0mol L–1aqueous KOH electrolyte. Moreover, it displays highcapacitance retention even of33.5%at100A g–1, and long-term cycling ability (~91.3%retention after5000cycles). More importantly, the present synthetic strategy canbe extended to prepare other polyacrylate-metal complexes, such as calciumpolyacrylate and aluminum polyacrylate. The carbon-Al-900sample can exhibit a highsurface area of1556m2g–1and large total pore volume of0.97cm3g–1. To sum up, thecarbon samples derived from magnesium polyacrylate possess the highest capacitiveperformances as supercapacitor electrode materials.In this work, we demonstrate a novel and generalized synthetic approach forproducing nanoporous carbon materials, using adipic acid and zinc powder as rawmaterials. The mass ratio and carbonization temperature have crucial effects on thestructure and electrochemical behavior of the carbon samples. An optimum sample iscarbon-1:2-700, it is amorphous in nature and has a high BET surface area of1426m2g–1and a very large pore volume of5.92cm3g–1. What’s more, the sampletakes on sheet-like structures entirely composed of nanopores. Electrochemicalperformance is measured in a three-electrode system using6mol L1KOH aselectrolyte, and a two-electrode system using [EMIm]BF4/AN as electrolyte,respectively. In the three-electrode system, it delivers a high specific capacitance of373.3F g1at a current density of2A g1. Furthermore, it displays a good cyclingdurability as93.9%after10000cycles. In the two-electrode system, the voltage windowhas been largely broadened and a series of temperature-dependent measurements areadopted. More importantly, the present synthetic method can be extended to otherchemical substances as carbon precursors to produce porous carbon, which can greatlyenrich the field of porous carbon synthesis as well as the application of supercapacitors.4. A hard-soft dual templates method has been developed for the first time to prepareporous carbons by direct carbonization of phenol formaldehyde resins (abbr. PF),Zn(NO3)2·6H2O and polyvinyl butyral (abbr. PVB) at1000oC under Ar gas, in whichPF serves as carbon source. More importantly, Zn(NO3)2·6H2O and PVB acting as hardtemplate and soft template can be readily removed through the evaporation process,resulting in pure carbons without any post-treatment commonly employed. ThePF-Zn-PVB-1:5:1sample has total BET surface area of864m2g–1, and total porevolume of0.76cm3g–1. The electrodes based on the porous carbon exhibit the highspecific capacitances of174.7F g–1and152.8F g–1at the current density of0.5and1.0A g–1, respectively. It also exhibits superior rate capability, with high specific capacitance retention as ca.63.1%at high current densities of20A g–1. Significantly,about96.2%is retained after charging and discharging for10000cycles, evincing itslong-term electrochemical stability. The hard-soft dual templates method proposed inpresent work is straightforward and effective, which can be utilized to synthesize porouscarbons in large scale for the application of high performance supercapacitors.High performance nitrogen-doped porous carbons for supercapacitors, named asgelatin-Mg-Zn-1:5:3, have been successfully prepared via a dual-templatecarbonization method, without any physical/chemical activation process, in whichgelatin serves as both carbon/nitrogen source, and low cost Mg(NO3)2·6H2O andZn(NO3)2·6H2O as dual templates. It is revealed that the carbonization temperature, andthe mass ratio of gelatin-Mg(NO3)2·6H2O-Zn(NO3)2·6H2O plays a crucial role in thedetermination of surface area, pore structures and the correlative capacitive behavior ofthe gelatin-Mg-Zn-1:5:3sample. It displays a high BET surface area of1518m2g–1,large total pore volume4.27cm3g–1, and large average pore width11.3nm. In a threeelectrode system, using6mol L1KOH solution as electrolyte, we can achieve a highspecific capacitance of ca.284.1F g–1at a current density of1A g–1and highcapacitance retention of ca.31.2%is obtained at150A g–1, indicating high ratecapability. It also possesses high capacitance retention of ca.96.1%even aftercharging/discharging for10000cycles. More importantly, a two electrode system, using[EMIm]BF4/AN (weight ratio of1:1) as electrode, has been adopted toward thegelatin-Mg-Zn-1:5:3sample with different operation temperatures of25/50/80°C. As aresult, wide potential windows, broad operation temperatures, high cycling stabilityachieved in two electrode system makes it possible for practical application underextreme circumstances.