Synthesis And Electrochemical Properties Of Polyaniline-derived Heteroatom-doped Carbon Materials

Fuel cells,supercapacitors,and sodium-ion batteries batteries（SIBs）are important for green sustainable developments.One of the key issues of these energy devices is to develop of new catalysts and electrode materials with high catalytic performance,high capacity,and high stability.The heteroatoms（such as N,B,S,and P）doped carbon materials are widely used in energy fields due to their easy preparations,low costs,and stable chemical properties.In order to greatly simplify the process of preparing heteroatom-doped carbon materials,appropriate heteroatom-containing precursors,such as conductive polymers（CPs）,can be selected.In addition,by further introduction of environmental friend and low conversion potential Mn O,the acquisition of efficient electrode materials can be achieved.Polyaniline（PANI）,as a carbon and nitrogen source with uniform distribution of N-doping sites and the increase in the number of reactive sites,is considered to be the most promising N-containing precursor to preparing N-doped carbon materials.In addition,one-dimensional tubular structures not only have high specific surface areas and thermal stability,but also can provide more active sites by introducing heteroatoms on their walls,with can provide effective transport paths for electrons and Na⁺ions.Sulfur-containing PANI nanotubes were synthesized by self-degradation template method.After treatment with different carbonation technique,the tubular morphology of S/N co-doped carbon nanotubes（S/N-CT）can be well maintained.The interlayer space,specific surface area,and graphitization degree of S/N-CT4-9 obtained by two-step carbonization processes（400 ^℃ at first step and 900^℃ at second step）are higher than that of the S/N-CT9 obtained by one-step carbonization at 900 ^℃.There are the favorable conditions for further improving the electrochemical performance.The onset potential(E_onset=1.01 V vs RHE)of S/N-CT4-9 as electrocatalysts for oxygen reduction reaction（ORR）is highest.Compared with the commercial Pt/C catalyst,the relative current of S/N-CT4-9 is kept as 93%after 32,000 s cycles in the alkaline medium,while only to 26%for the commercial Pt/C.After 4000 cycles,the specific capacitance retention of S/N-CT4-9 as electrode materials for supercapacitor is 93%of its initial specific capacitance,demonstrating its outstanding stability.In addition,S/N-CT4-9 is also used as an anode material for SIBs with excellent Na-storage performance.The reversible discharge capacity of S/N-CT4-9 can reach 340 m Ah/g and 146 m Ah/g under the current density of 0.1 A/g and 5 A/g,respectively.Nitrogen-doped carbon nanotube（NDCT）derived from PANI nanotubes not only showed an excellent electrochemical performance,but also is a carbon matrix with good conductivity.The composites of NDCT and Mn O were obtained by hydrothermal synthesis combined with carbonization method using NDCT and KMn O₄ as precursors,which can not only solve the problem of poor conductivity and serious volume expansion of Mn O during charge/discharge process,but also can improve the electrochemical performance of the composites by the synergistic effect between NDCT and MnO.The experimental results showed that NDCT@Mn O-7 with highest specific surface area（243 m²/g）obtained by calcination at 700 ^℃ provides more active sites for the electrochemical reaction process.When used as ORR electrocatalysts,NDCT@Mn O-7 exhibited a better stability and anti-methanol properties than the commercial Pt/C catalysts in alkaline media.The specific capacitance of NDCT@Mn O-7 as electrode material for supercapacitor can reach 476F/g at scan rate of 2 m V/s,and its capacitance can be kept at 82%of the initial after4000 cycles.The reversible discharge capacity of NDCT@Mn O-7 as anode materials for SIBs can reach 709 and 292 m Ah/g at 0.1 and 5 A/g,respectively.The results of in situ XRD and HRTEM showed that Mn O and Na⁺were changed to metal Mn and Na₂O during the discharge process,and the reverse reaction was carried out during the charging process.It has also been found that PANI derivatives,such as poly（o-toluidine）（POT）and poly（o-methoxyaniline）（POMA）,can be acted as good N-containing precursors to fabricate N-doped carbon materials.In this view,N-doped carbon microspheres（NCMSs）and N-doped carbon spheres（NCS）were obtained from carbonizing POT and POMA by hydrothermal method.It was found that the calcination temperature had few effect on the morphology of NCMSs and NCS,but showed a great influence on their specific surface areas and N species.The highest specific surface area（727.1m²/g）and graphitic-N content were observed in NCMSs-900（carbonization at 900^℃）.For NCS,the NCS-800（carbonization at 800 ^℃）showed the highest surface area（352 m²/g）.NCMSs-900 and NCS-800 exhibited a better stability and anti-methanol performance compared to the commercial Pt/C catalysts during electrocatalytic ORR.The specific capacitance of NCMSs-900 and NCS-800 are 414 and 513 F/g,respectively.In summary,S/N co-doped carbon nanotubes with excellent electrochemical properties can be prepared by direct carbonization of PANI nanotubes and changing carbonization conditions.In addition,the PANI derivatives（POT and POMA）can also be used as N-containing precursors to fabricate nitrogen-doped carbon materials.The NDCT derived from PANI nanotube can also be used as carbon substrate to enhance the conductivity and avoid the serious volume expansion of Mn O during charge/discharge processes,which leads to the acquisition of NDCT Mn O composites with good electrochemical properties.