"Living"/Controlled Synthesis Of Polyacrylonitrile Mediated By Organometallic Salt And Investigation On Electrode Materials As Electrochemical Capacitors

Currently, the consumption of the global energy became more serious because of the rapid development of economic and the fast-growing population worldwide. Electrochemical capacitors(EC) have distinguished themselves due to their high power energy, long cycle life, etc. Polyacrylonitrile-based activated carbon fibers(PAN-based ACFs) have been one of the optimal electrode materials owning to good chemical stability, high electrical conductivity and well environmental compatibility, etc. Contrast with conventional radical polymerization(RP) methods, “living”/controlled radical polymerization(LRP) technique can obtain well-defined polymers with the predictable molecular weights(Mn) and relative polydispersity index(PDI). It is usefull for material application.Organic metal-mediated LRP techniques possessed the potential application for easy operation, cheap raw material, etc. Two mechanisms can well account for the reaction processes, i.e., reversible termination(RT) and degenerative chain transfer(DT). In this work, well-defined PAN was first synthesized via the organic metal-mediated LRP technique. PAN nanofibers were obtained via electrospinning using the PAN/N,N-dimethylformaide(DMF) homogeneous solution. PAN-based ACFs were yielded after pre-oxidation and direct activation. Gel permeation chromatograph(GPC) was used to measure polymer’s Mn and PDI. The chain-end fidelity was determined by 1H NMR. The surface morphologies of samples were characterized by high and low vacuum scanning electron microscopy. X-ray diffraction(XRD) was used to analyze the crystal structure of samples. The specific surface areas(SA) were calculated based on BET method. The electrochemical measurements were carried out using CHI 660 C electrochemical workstation.1. Polymerization of acrylonitrile(AN) was mediated by cobalt acetylacetonate(Co(acac)2), with 2,2’-azodiisobutyronitrile(AIBN) as initiator, N,N-dimethylformaide(DMF) as the solvent at 60 oC. The experiments demonstrated that the monomer consumption lived up to the first order rate law. The Mn of PAN increased with conversion(conv.) and the PDI was 1.26-1.52. As presented by SEM images, PAN nanofibers were long enough and homogeneous. The diameter of nanofibers was mostly distributed at 350 nm. Diffraction peaks in XRD spectrogram indicated that PAN-based ACFs were amorphous carbon materials. BET surface area of PAN-based ACFs was as high as 982 m2/g with the mass ratio of PAN pre-oxidized fibers/KOH 1:2 at the same activation temperature(600 oC) and time(1.5 h). The evaluation of electrochemical performance of the PAN-based ACFs was investigated with potassium hydroxide(KOH) as electrolyte. The results showed that the maximum capacitance was 125 F/g when the scan rate was 5 mV/s. The electrode showed well stabilized capacitance after 500 cycles.2. Polymerization of AN in the presence of AIBN as the initiator and bis(acetylacetonato) iron(II)(Fe(acac)2) as the mediator in DMF solvent was investigated at 60 oC. The polymerization exhibited first-order kinetics characteristic. The Mn of PAN increased with the increasing conv. and PDI of PAN distributed at 1.40-1.64. 1H NMR indicated that Fe(acac)2 was successfully end-capped on the PAN chain. The polymerization owned "living" /controlled features. PAN nanofibers were obtained via electrospinning technique. PAN-based ACFs were obtained via sodium hydroxide(NaOH) direction activation method with different mass ratios of PAN/NaOH 1:1, 1:2, 1:4, respectively. SEM exhibited that the nanofibers were long and continuous, and their diameter was mostly distributed at 300 nm. As assessed by XRD spectrum, the crystallization and orientation fading away as the usage of NaOH increased. BET surface area increased as the activation rates increased. The maximum SA was 1165 m2/g. Cyclic voltammetry(CV) curves presented quasi rectangle without any obvious redox peak. The capacitance was as high as 167 F/g when the scan rate reached 30 mV/s. EC showed better electrochemical stability after 500 cycles.3. Polymerization of AN was conducted in the presence of azobisisoheptonitrile(V-65) with Fe(acac)2 as the mediator, DMF as the solvent at 40 oC. Polymerization kinetics fitted first order linear relation. Mn of polymer increased as the conv. increased. The PDI distributed at the range of 1.48-1.60. The polymerization manifested "living" /controlled characteristic. PAN-based ACFs were prepared via NaOH direct activation after electrospinning of PAN and preoxidation process. SEM showed that the nanofibers were continuous without breakage, and their diameter was mostly distributed at 300 nm. XRD spectrogram indicated that electrode material was kind of amorphous carbon materials. The maximum SA was 1305 m2/g with higher activation ratio. Electrochemical tests indicated that the highest capacitance was 200 F/g when the scan rate reached 20 mV/s. EC showed better electrochemical stability after 500 cycles.