Preparation of H3PO4–intercalation compounds of

carbon fibers through electrolysis and their exfoliation

Masahiro TOYODA1, Asuka NAKAMURA2 Yumi NANBU2

1 Department of Applied Chemistry, Faculty of Engineering, Oita University,Dannoharu, Oita 870-1192 Japan

2 Fukui National College of Technology, Geshi, Sabae 916-8507 Japan

Corresponding author e-mail address: toyoda22@cc.oita-u.ac.jp

IntroductionExfoliation of graphite flakes has been carried out by using different intercalates

such as sulfuric acid [1], iron chloride [2,3], formic acid [4], and so on, and itsmorphology [5,6] and pore size distribution [7] have been also studied and reported.On the other hands, carbon fibers and their intercalation compounds are expected tomake full use of such features, composite materials, adsorption materials andelectrode materials by using its excellent characteristics. Though the reports on theexfoliation of carbon fibers through their intercalation compounds were very limited[8,9]. And their reports refer only to the intercalation of the VGCF with short fibersrather than that with continuous ones. Exfoliation of pitch-based carbon fibers heat-treated at a high temperature was firstly reported by Anderson and Chung [8]. Fromobservation of their SEM micrographs marked exfoliation as observed in naturalgraphite flakes and vapor-grown carbon fibers was not recognized. Recently wesucceeded in the formation of intercalation compounds of various carbon fibers byelectrochemical methods in acid electrolyte, and also in marked exfoliation of them byheat-treatment [10-13]. Its unique exfoliated morphology of carbon fibers wasapplied and reported electric double layer capacitor, the capacitance reached 555 F/gin concentrated H2SO4 electrolyte [14-17].

Preparation of the intercalation of carbon fibers by electrochemical processusing H3PO4 electrolyte have never been carried out and reported. In the presentwork, H3PO4 electrolysis could successfully intercalated into mesophase-pitch-basedcarbon fibers and they could be exfoliated by heating from 400 to 1000 oC.Preparation of the intercalation compounds of H3PO4 – carbon fibers and theirexfoliation morphology are reported and discussed.

Experimental Mesophase-pitch-based carbon fibers, having corrugate radial texture in their

cross-sections and heat-treated at 2800 oC were used in the present study. Sizingagent on the surface of these carbon fibers was removed by heating at 700 oC for 3 hin a flow of N2. Intercalation reaction was carried out by electrolysis in H3PO4

electrolyte with various concentrations at a constant current of 0.5 A using thepotentiostat/galvanostat. A bundle of carbon fibers with a length of 30 mm was usedand one end was put between two platinum plates by using PTFE sealing tape toensure the electrical contact, the other end was kept free. The carbon fiber wasanodically polarized in the H3PO4 electrolyte solution at a constant current by using

a potentiostat/galvanostat. The counter and reference electrodes were platinum andAg/AgCl electrode, respectively. Carbon fibers after electrolysis were rinsed withwater and ethanol, and then dried at room temperature completely. Carbon fibersthus electrolyzed were rapidly inserted into a tubular furnace kept at 400 to 1000 oCand held for 60 to 5 sec to be exfoliated. Carbon fibers after the electrolysis werestudied with X-ray diffraction (XRD: Rigaku, RAD) to confirm the occurrence ofintercalation into carbon fibers. Weight changes with intercalation reaction werealso carried out after rinsed with water and then dried. Morphological changes incarbon fibers after electrolysis and after exfoliation were observed under scanningelectron microscope (SEM: Hitachi, S-4100).

Results and Discussion Preparation of intercalation compounds using H3PO4 electrolyte solution in

mesophase-pitch-based carbon fibers was successfully carried out by electrolysis inless than 10 mol/dm3 its electrolyte solution. Changes in potential were found todepend on preparation of intercalation compounds of carbon fibers with H3PO4

electrolyte solution; at the higher concentration of it, potential saturated at the lesselectrical charge supplied. On XRD patterns of the carbon fibers, after electrolysisup to the plateau potentials, electric charges of 1200 C, in the H3PO4 electrolytesolution are shown in Fig. 2. Its carbon fibers, an additional peak around in 2θ = 9 o

is observed after electrolysis, which corresponds to an interlayer spacing of about 0.9nm. This new peak was reasonably supposed to be due to the intercalation intointerspacing of carbon layers.

The exfoliation was confirmed to occur immediately after insertion at thetemperature above 400 oC, original single fiber splitting into a number of thin filaments.Although keeping for around 10 sec at a temperature above 600 oC looked enough toexfoliate the carbon fibers, holding time of 60 sec was necessary for the apparentexfoliation at 400 oC, and even so still the degree of exfoliation seemed to be low, asshown on the fiber exfoliated at 400 oC for 60 sec in SEM micrograph. Morphologychanges with exfoliation at 1000 oC is shown in Fig. 2(a), (b) and (c). Theremarkable changes to the fibril morphology could not be recognized different fromthe case of intercalation compounds electrolyzed through the nitric acid electrolytesolution. However, there were a lot of cracks with fiber axis in the surface ofexfoliated carbon fibers after heat-treatment on Fig. 2(c). Since the thermaldecomposition temperature of phosphoric acid intercalated into graphite layer ofcarbon fiber is high, it was suggested that there was a difference as an exfoliation.Suitable synthesis condition of the intercalation compounds was determined to be theconcentration of electrolyte of 5 mol/dm3 at the electrolysis.

Fig. 1 XRD patterns of after electrolysis of carbon fibers

Fig. 2 (a) Surface and cross sectional morphology of pristine carbon fiber

Fig. 2 (b) Surface morphology of after Fig. 2 (c) Surface morphology of afterexfoliation of carbon fiber exfoliation of carbon fiber with high

magnification

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