Preparation Of Ultrafine Porous Carbon Fibers And Its Application In Proton Exchange Membrane Fuel Cells

Porous carbon materials have been received widely attention and application due to its developed pore structure and high conductivity,temperature resistance,acid and alkali resistance and other excellent physical and chemical properties.Ultrafine porous carbon fibers(UFPCFs),a novel kind of porous carbon materials with the characteristics of both porous structure and nano-scaled size,can be expected much better performance in the applications.In this paper,a novel facile and simple approach,combining polymer blend technique and electrospinning process,was explored for preparation of UFPCFs.These carbon fibers were employed as an extra carbon component compounding with commercial Pt/C catalyst to obtain the compound-carbon-supported platinum catalyst for PEMFCs.Furthermore,the electrocatalytic activity and utilization of this home-made catalyst were discussed.The main content and results of this paper are as follows.1.Polyacrylonitrile(PAN)was used as carbon precursor polymer(CPP),while polymethyl methacrylate(PMMA)as thermally decomposed polymer(TDP).They were both dissolved in N,N-dimethylacetamide(DMAc)to prepare the blending solution for the following electrospinning.The electrospinning parameters,i.e.concentration,voltage of electrostatic field,feed rate,receiving distance,were studied and analysed by observing the morphology of nanofiber felts,measuring and counting the fibers diameter and deviation.PAN/PMMA felts with a average diameter of about 199nm and deviation of 2.53%could be prepared under the optimized electrospinning parameters of 10wt.%concentration,18kV voltage,15?L/min feed rate and 18cm distance.2.A series of PAN/PMMA(70/30,80/20,90/10,100/0)blend felts and PAN/POSS felts(98/2,96/4,94/6)were prepared under the optimal electrospinning condition.It was found that the diameter of blend fibers became thiner and the surface roughness enhanced with the increase of PMMA content,while the diameter of PAN/POSS blend fibers hold almostly steady and surface roughness boosted to a great extent.Dynamic mechanical analysis of the PAN/PMMA blend felts indicated the loss peaks of the samples became wider and move towards higher temperature with the addition of PMMA.3.The morphology of blend felts with various content ratio of PAN/PMMA impregnated in acetone showed that pores appeared and existed on the surface of fibers,while the PMMA phase vanished.The quantity and size of these cylinder-shaped pores became larger with the increase of PMMA content.In addition,the diameters of PAN/PMMA blend fibers decreased after acetone-immersion treatment with the respect to PMMA content.4.The dimeters of UFPCFs resulted from the PAN/PMMA blend felts reduced significantly compared with their original size.The sureface roughness and quantities of holes for UFPCFs increased evidently with the increase of PMMA content.Another UFPCFs with obvious pore structure were also prepared by thermal treating the PAN/POSS blend felts.Moreover,the quantities of pores gradually increased and the pore size distribution became more homogeneous with the increase of POSS content.5.The Raman spectra and XRD analysis of the obtained UFPCFs from various carbonization temperature indicated that with increasing temperature of carbonization,the degree of the graphitization became higer and the graphit structure gradually transformed into three-dimensionally ordered lamellar structure.The resistivity of the UFPCFs films decreased with increasing temperature of carbonization.When the temperature was chosen as 1400?,the resistivity of resultant UFPCFs films dropped blow 10 m? cm(reaching 7.92 m? cm),revealing the excellent conductivity of UFPCFs.The resistivity of UFPCFs obtained from thermal treating the PAN/POSS blend felts ranged in 19.98?26.18 m? cm.6.The resultant UFPCFs were employed as an extra carbon component compounding with commercial Pt/C catalyst(HiSPEC 4000)to obtain the platinum based catalyst with compound carbon support(CCS)for PEMFCs,namely CCS-20.For determination of electrochemical surface area(ECSA)of Pt in JM-20,JM-40 and CCS-20 electrocatalysts,cyclic voltammetric measurements were conducted at room temperature and the results indicated that the ECSA of CCS-20 was approximate 300 cm2/mg-Pt larger than that of JM-20,as well as the JM-40,reaching 1255.8 cm2/mg-Pt.The linear sweep voltammetry of JM-20,JM-40 and CCS-20 electrocatalysts was also conducted to evaluate their electrochemical oxygen reduction reaction(ORR)activity at room temperature.The results presented that the onset potential,half-wave potential and the potential at kinetic current density of 0.1mA/cm2 of CCS-20 electrocatalyst were 0.964V,763mV and 0.9V,respectively,much higher than that of JM-20,JM-40 electrocatalysts.It was demonstrated sufficiently that the introduction of UFPCFs into the catalyst(electrode)efficiently enhanced the electrocatalytic activity of the electrocatalyst and kinetics of the ORR.7.Membrane electrode assembly(MEA)was employed to construct a single cell for evaluation of the fuel-cell polarization performce.The MEA febricated from CCS-20 electrocatalyst showed the better performce in the test.The maximum power density of MEA fabricated from CCS-20 electrocatalyst was 24.8%and 52.3%higher than that of MEA fabricated from JM-20 and JM-40 respectively,peaking at 165.4mW/cm2.The improved electrocatalytic activity,utilization efficiency and polarization performce could be explained in two ways.First,chopped-fiber-form UFPCFs constructed an unconsolidated three-dimensional reticular structure in the electrode structure so that the catalyst particles could be homogeneous and secondary dispersion in the network,especially,the vertical dispersion.Thus,the network could efficiently enhance the utilization efficiency,boost the diffusion of chemical specials to allow good reactant flux and prevent the electrode from flooding under humidified conditions.Moreover,the introduction of UFPCFs provided more and better paths for the flow of electrons.