Water based carbon nanotube (CNT) dispersion was produced by wet-jet milling method. Commercial CNT was originally agglomerated at the particle size of less than 1 mm. The wet-jet milling process exfoliated CNTs from the agglomerates and dispersed them into water. Sedimentation of the CNTs in the dispersion fluid was not observed for more than a month. The produced CNT dispersion was characterized by the SEM and the viscometer. CNT/PTFE composite film was formed with the CNT dispersion in this study. The electrical conductivity of the composite film increased to 10 times when the CNT dispersion, which was produced by the wet-jet milling method, was used as a constituent of the film. Moreover, the composite film was applied to bipolar plate of fuel cell and increased the output power of the fuel cell to 1.3 times.
Carbon nanotubes (CNTs) have attracted considerable attention due to their exceptional electronic, mechanical, optical, and chemical properties [
CNT dispersion is fabricated by various methods such as sonication [
Wet-jet milling method is recently employed to disperse CNTs in various liquid medium [
In this study, CNTs were dispersed in water by wet-jet milling method. The produced CNT dispersion was investigated by the SEM and the viscometer. Moreover, the CNT/PTFE composite film was formed with the CNT dispersion fluid. The electrical conductivity of the composite film was evaluated. From these evaluations, the dispersion process of the CNTs by the wet-jet milling method and the correlation between the viscosity of CNT dispersion and the electrical conductivity of the CNT/PTFE composite film are discussed. Moreover, the CNT/PTFE composite film was coated on bipolar plates of fuel cell. Influence of the film coating on the output power and the inner impedance of the fuel cell is discussed.
Multiwall carbon nanotube (MWCNT), Nanocyl NC7000 (Nanocyl S.A., Sambreville, Belgium), was used as a constituent of the dispersion fluid. The CNT and a dispersant were added into water and mixed by planetary centrifugal mixer (Thinky Co.) for 30 min. Polyoxyethylene (100) stearyl ether (Brij S100, Sigma-Aldrich) was used as the dispersant. The concentrations of both the CNT and the dispersant were 1 wt.%. The CNT was dispersed into water using high-pressure wet-jet milling equipment (Nano-Jet Pal, JN 5, Jokoh Co., Japan).
Figure
Schematic diagram of a dispersion unit of the wet-jet milling equipment.
The rheological characteristics of the CNT dispersion fluids were measured using a Sine-wave Vibro Viscometer (SV-10, A&D, Japan) at 24°C. Moreover, the size of CNT particles in the dispersion fluid was observed by scanning electron microscopy (SEM) (FE-SEM S-4800, Hitachi High-Technologies Co.). For the SEM observation, the CNT dispersion fluid was dropped on silicon substrate and was dried as sample preparation.
CNT/PTFE composite film was formed from CNT and polytetrafluoroethylene (PTFE) dispersion fluids. The CNT dispersion was produced by the wet-jet milling method. The separate CNT and PTFE dispersions were mixed and stirred by ultrasonic agitation for 20 min. This mixed fluid was applied to glass substrate by doctor blade method. The sample was dried in the atmosphere for 30 min then heated at 350°C for 5 min. The CNT/PTFE composite film with the thickness of 10–20
The CNT/PTFE composite film with the CNT concentration of 25 wt.% was applied to stainless steel (316SS) bipolar plates of fuel cell. The polymer electrolyte fuel cell (PEMFC) was assembled with the bipolar plates, which were coated with the CNT/PTFE composite film. Nafion 117 was used as the polymer electrolyte of the PEMFC. The anode and the cathode electrodes contained platinum catalyst, which was supported on acetylene black powder at the loading of 0.25 mg/cm2. Carbon paper was used as gas diffusion layer. The membrane electrode assembly (MEA) was 4 × 4 cm2 in the size. Humidified hydrogen and oxygen gases were flowed into the anode and the cathode electrodes, respectively, both at 1000 mL/min. Characterization of the output power and the inner impedance was performed to the fuel cell.
Figure
Photograph for (a) the CNT suspension without using wet-jet milling process and (b) the CNT dispersion with using wet-jet milling process.
Figure
Low magnification SEM images of CNT in the suspension (a) before mixing and (b) after mixing using planetary centrifugal mixer. SEM images of CNT in the dispersion after wet-jet milling process at various processing numbers, (c) 1 time, (d) 4 times, (e) 30 times, and (f) 70 times.
Before mixing
After mixing
1 time
4 times
30 times
70 times
Figures
The size and the number of CNT particles decreased with an increase in the number of the jet-milling processes. CNT particles were not observed for the samples above the processing times number of 30. The roughness of the region existing in the individual CNTs also decreased with an increase in the processing number. Above the processing times number of 30, the surface was smooth enough not to be observed by the low magnification SEM investigation, although the exfoliated single CNTs existed on the substrate. The smooth surface indicates that the exfoliated single CNTs were uniformly dispersed in water by using the wet-jet milling method.
Figure
High magnification SEM images of CNT in the dispersion (a) before and (b) after the wet-jet milling process of 70 times.
Before wet-jet milling
After wet-jet milling
Figure
(a) Viscosity of CNT dispersion formed by the wet-jet milling process at various processing numbers and (b) electrical conductivity of the CNT/PTFE composite film. The CNT/PTFE composite film was formed from the CNT dispersion, produced by the wet-jet milling at various processing numbers.
The CNT/PTFE composite film was formed with the CNT dispersion. Figure
The suspension without the wet-jet milling process contains the large agglomerates of CNTs. Few individual CNTs are contained in the suspension, although the planetary centrifugal mixing is carried out. This CNT suspension shows the low viscosity similar to pure water, because the CNT agglomerates with the size of 300
The wet-jet milling process exfoliates individual CNTs from the agglomerates, in addition to a decrease in the number and the size of agglomerates. The dispersion containing the individual CNTs, which is formed by the jet-milling process of 1 time, shows high viscosity of 48 mPa·s. The increase of the viscosity results from an increase in external specific surface of CNT [
The electrical conductivity of the CNT/PTFE composite film increases to 22 S/cm with an increase in the process time of the wet-jet milling, which is used for producing the CNT dispersion. The wet-jet milling process uniformly disperses CNTs in the water. Therefore, it is possible to distribute the CNTs in the CNT/PTFE composite film uniformly, when the CNT dispersion, which applied the jet-milling at higher process time, is used as a constituent of the composite film. The uniform distribution of the CNTs in the composite film increases the electrical conductivity of the film, even if the CNT in the film has the same concentration.
The CNT/PTFE composite film was formed with the CNT dispersion, which was produced by the jet-milling process, and was applied to the stainless steel (SS) bipolar plate of fuel cell. Figure
Dependence of the current on (a) the output voltage and (b) the output power for the fuel cells assembled with the bare stainless steel bipolar plate (without coating) or the stainless steel bipolar plate coated with the CNT/PTFE composite film.
Figure
Figure
Cole-cole plots for the impedance measurement of the fuel cells assembled with the bare stainless steel bipolar plates or those coated with the CNT/PTFE composite film.
The CNT dispersion was successfully formed by the wet-jet milling method. The dispersion had features such as time stability and low viscosity, although the CNT dispersion has high CNT concentration of 1 wt.%. The CNT/PTFE composite film shows the high electrical conductivity of 22 S/cm, when the CNT dispersion, which is produced by the wet-jet milling method, is used as a constituent of the composite film. These results are because the wet-jet milling process exfoliated individual CNTs from the CNT agglomerates and disperses them to the water uniformly.
The highly conductive CNT/PTFE composite film was applied to the bipolar plate of fuel cell. The coating of the composite film increased the output power of fuel cell to 1.3 times, because of a decrease in the contact resistance between bipolar plate and MEA. The improved output power results from forming good CNT dispersion as a constituent of the CNT/PTFE composite film by the wet-jet milling process.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by JSPN KAKENHI Grant no. 26420683. The author would like to thank Mr. Y. Miyamoto of the Technical Service Coordination Office, Tokai University, for the SEM investigation and Mr. K. Sano of Jokoh Co., Ltd., for technical supports of wet-jet milling method.