In this study, multi-wall carbon nanotubes (MWNTs) are used as catalyst support. Attractive properties of nanotubes related with their good electrical conductivity and high specific surface area have been demonstrated in the electrochemical energy storage systems. Some researches have already used the application of carbon nanotubes powder as electrocatalyst, Pt and Pt/Ru, support for cathodic and anodic reactions in fuel cell [18–28]. However, there was little research that CNTs were fabricated on carbon cloth directly. The method in the electrode of DMFC is easy to constructed and sticking on the carbon cloth tightly.
And then, the other researches were reported that the chemical modifications of the surface of CNTs powder by using HNO3 or H2SO4-HNO3 could improve the metal particle dispersion, increase the available metal specific surface area, reduce the amounts of the expensive active metal component, and open the cap of CNTs[29–30]. However, there was a lack of the correlation of the CNTs texture and structure with the chemical and electrochemical analysis.
Our purpose is to find which chemical solution, temperature, concentration, and time parameter is the best way to modify CNTs in order to rise the efficiency of cathode in the DMFC.
Chapter 2 Fundamentals of MEA for DMFC and Modification of Carbon nanotube (CNT)
2.1 Assemble of MEA for DMFC
Typically, the DMFC is composed of a membrane electrode assembly (MEA) and two graphite flow-field plates, which are pressed against the MEA. The central component of the DMFC is the MEA composed of membrane, catalyst, and gas diffusion layers. Each of these layers has special functions in the DMFC and follows to explain its functions. For the interaction of the layers in the MEA it is important to define the respective functions of the individual components. Fig. 2.1 shows the description of the components in the DMFC. Fig. 2.2 shows the structure of the MEA.
2.1.1 Proton Exchange Membrane
The proton exchange membrane for DMFC is commercially available from DuPont Incorporation and has a commercial name, Nafion®. This organic proton-conductive membrane is a sulfonic acid-based perfluorinated polymer or polystryene sulfonate polymer. It is used as electrolyte and the protons (cations) are allowed to permeate through it, but anions are rejected.
The Nafion® structure is composed of three different parts, namely, rigid hydrophobic backbone, flexible perfluorocarbon, and hydrated ionic cluster region. The chemical formula and physical structure of the membrane is shown in Fig. 2.3.
2.1.2 Catalyst Layer
The catalyst layer is in direct contact with the membrane and the gas diffusion layer. It is also referred to as the active layer. In both the anode and cathode, the catalyst layer is the location of the half-cell reaction in a DMFC. The catalyst layer is either applied to the membrane or to the gas diffusion layer. In either case, the objective is to place the catalyst particles,
platinum or platinum alloys, within close proximity of the membrane.
2.1.3 Gas diffusion layer
The porous gas diffusion layer in DMFC ensures that reactants effectively diffuse to the catalyst layer. In addition, the gas diffusion layer is the electrical conductor that transports electrons to and from the catalyst layer. Typically, gas diffusion layers are constructed from porous carbon paper, or carbon cloth, with a thickness in the range of 100–300 mm. The gas diffusion layer also assists in water management. In addition, gas diffusion layers are typically wet-proofed and could ensure that the pores of the gas diffusion layer do not become congested with liquid water.
Fig.2.1 The schematic of a unit cell of DMFC
[www.eng.wayne.edu/ page.php?id=1740]
Fig.2.2 MEA structure
[www.hidrotec-fuelcell.com.ar/ mea_esp.htm]
Fig.2.3 The chemical formula and physical structure of the membrane Nafion®
[Dupont Products information about Nafion ]
2.2 Principle of DMFC
Fig.2.4 Illustration of DMFC in Principles [www.echem.titech.ac.jp/ ~dmfc/C01/C01mokuteki.html]
The schematic of a unit cell DMFC and its principle are in the Fig. 2.4. As soon as methanol attains to the surface of anode, it may be oxidized by electrocatalyst particle Pt and provide CO2, H+, and six electrons. The follow is the location of the half-cell reaction in the anode for DMFC.
CH3OH + H2O → CO2 + 6H+ + 6e- E°=0.046V (1)
In the other hand, when electrons reach the cathode by external current loop, H+ ions transfer the proton exchange membrane to the cathode surface and provide water with oxygen from the air. The follow is the location of the half-cell reaction in the cathode for DMFC.
3/2O2 + 6H+ + 6e- → 3H2O E°=1.229V (2) Finally, methanol is oxidized in the overall reaction for DMFC.
CH3OH + 3/2O2 → 2H2O + CO2 E°=1.183V (3)
As the assumption of the absorption, the detail reaction mechanism on the surface of the electrocatalyst Pt is below the equation.
Pt + CH3OH → Pt-(CH3OH)ads (4)
Pt-(CH3OH)ads → Pt-(CH2OH)ads + H+ + e- (5)
Pt-(CH2OH)ads → Pt-(CHOH)ads + H+ + e-
(6)
Pt-(CHOH)ads → Pt-(COH)ads + H+ + e-
(7)
Pt-(CHO)ads → Pt-(CO)ads + H+ + e-
(8)
From all above equations, we could find that it is the complex in this reaction mechanism in the anode. There are many intermedium in the sequence reactions. The intermedium, Pt-(CO)ads, is formed by CO and Pt when all H+ leave the surface of Pt. Furthermore, Pt itself gets out of the ability for oxidizing methanol. It is named Poison Effect. In order to solve the problem, the second electrocatalyst, transitional metal (Sn、Ru、Re、Mo), is added to assist water segregate.
Then, it allows Pt-(CO)ads to be reduced as Pt.
M-(OH)ads + Pt-(CO)ads → Pt + M + CO2 + H+ + e-
(9)
Furthermore, some researches use three kinds of metal as the electrocatalyst on the anode.
In the other hand, the electrocatalyst, Pt, is the single kind of metal on the cathode.
2.3 Growth and Characterization of Carbon Nanotube
In 1991, Iijima of the NEC Laboratory in Japan reported the first observation of multi-walled carbon nanotubes (MWNTs) in carbon-soot made by arc-discharge. About two years later, he made the observation of single-walled carbon nanotubes (SWNTs). The past decade witnessed significant research efforts in efficient and high-yield nanotube growth methods. The success in nanotube growth has led to the wide availability of nanotube materials, and is a main catalyst behind the recent progress in basis physics studies and applications of nanotubes.
Nanotubes can be utilized individually or as an ensemble to build functional device prototypes, as has been demonstrated by many research groups. Ensembles of nanotubes have been used for field emission based flat-panel display, composite materials with improved mechanical properties and electromechanical actuators. Bulk quantities of nanotubes have also been suggested to be useful as high-capacity hydrogen storage media. Individual nanotubes have been used for field emission sources, tips for scanning probe microscopy and nano-tweezers.
Nanotubes also have significant potential as the central elements of nano-electronic devices including field effect transistors, single-electron transistors and rectifying diodes.
2.3.1Growth Methods
Arc-Discharge
In arc-discharge, carbon atoms are evaporated by plasma of helium gas ignited by high currents passed through opposing carbon anode and cathode in Fig. 2.5(a). Arc-discharge has been developed into an excellent method for producing both high quality multi-walled nanotubes and single-walled nanotubes. MWNTs can be obtained by controlling the growth conditions such as the pressure of inert gas in the discharge chamber and the arcing current. In1992, a breakthrough in MWNTs growth by arc-discharge was first made by Ebbesen and Ajayan who
MWNTs have lengths on the order of ten microns and diameters in the range of 5-30nm. The nanotubes aretypically bound together by strong van der Waals interactions and form tight bundles. MWNTs produced by arc-discharge are very straight, indicative of their high crystalline.
For as grown materials, there are few defects such as pentagons or heptagons existing on the sidewalls of the nanotubes. The by-product of the arc-discharge growth process is multi-layered graphitic heating the as grown material in an oxygen environment to oxidize away the graphitic particles. The polyhedron graphitic particles exhibit higher oxidation rate than MWNTs;
nevertheless, the oxidation purification process also removes an appreciable amount of nanotubes.
Laser Ablation
The method utilized intense laser pulses to ablate a carbon target containing 0.5 atomic percent of nickel and cobalt. The target is placed in a tube-furnace heated to 1200oC in Fig.
2.5(b). During laser ablation, a flow of inert gas is passed through the growth chamber to carry the grown nanotubes downstream to be collected on a cold finger.
Chemical Vapor Deposition (CVD)
A schematic experimental setup for CVD growth is depicted in Fig. 2.5(c). The growth process involves heating a catalyst material to high temperatures in a tube furnace and flowing a hydrocarbon gas through the tube reactor for a period of time. Materials grown over the catalyst are collected upon cooling the system to room temperature. The key parameters in nanotube CVD growth are the hydrocarbons, catalysts and growth temperature. The active catalytic species are typically transition-metal nanoparticles formed on a support material such as silicon.
Fig. 2.5 (a)-(c) Schematic experimental setups for nanotube growth methods
2.3.2 The growth mechanism of Carbon nanotubes
The general nanotube growth mechanism in a CVD process involves the dissociation of hydrocarbon molecules catalyzed by the transition metal, and dissolution and saturation of carbon atoms in the metal nanoparticle in Fig. 2.6. The precipitation of carbon from the saturated metal particle leads to the formation of tubular carbon solids in sp2 structure. Tube formation is favored over other forms of carbon such as graphitic sheets with open edges. This is because a tube contains no dangling bonds and therefore is in a low energy form. For MWNTs growth, most of the CVD methods employ ethylene or acetylene as the carbon feedstock and the growth temperature is typically in the range of 550-750oC. Iron, nickel or cobalt nanoparticles are often used as catalyst. The rationale for choosing these metals as catalyst for CVD growth of nanotubes lies in the phase diagrams for the metals and carbon. At high temperatures, carbon has finite solubility in these metals, which leads to the formation of metal-carbon solutions and therefore the aforementioned growth mechanism. Noticeably, iron, cobalt and nickel are also the favored catalytic metals used in laser ablation and arc-discharge. This simple fact may hint that the laser, discharge and CVD growth methods may share a common nanotube growth mechanism, although very different approaches are used to provide carbon feedstock.
Fig. 2.6 Two general growth modes of nanotube in chemical vapor deposition
2.4 Modification of Carbon Nanotube
The components of the produced gas were identified as CO, CO2 and NO by the gas chromatographt (GC) and gas chromatography-mass spectrometry (GCMS) analyses. This implies that the carbon atoms constituting the MWNTs were partly removed by HNO3 oxidation [32].
2HNO3 + 3C (MWNTs) →3CO + 2NO + H2O
(10)
4HNO3 + 3C (MWNTs) → 3CO2 + 4NO + 2H2O (11)
The reaction pathways for the functional group formation most probably involve the following two reactions in Fig. 2.7: (i) hydration of the olefinic C=C moieties released from the conjugation network by the decarbonization due to HNO3 oxidation. The nanotube sidewalls and more active top of the nanotube form the COOH and C–OH bonds) (ii) A hydration–dehydration equilibrium between adjoining C–OH groups and C=O ether groups.
Fig. 2.7 MWNTs are modified by HNO3
Chapter 3 Experimental procedures
New cathode material for direct Methanol Fuel Cells (DMFC) bases on multi-wall carbon nanotubes (MWNTs), aiming to improving the performance of the cell is analyzed. In this study, MWNTs are fabricated directly on carbon cloth by Microwave plasma enhanced chemical vapor deposition (MPECVD) and then functionalized by several chemical solutions. Therefore MWNTs can be functionalized with groups such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (>C=O) that are necessary to anchor metal ions to the tube. Pt catalyst is electroless deposited on MWNTs using a chloroplatinic acid solution, based on H2(PtCl6)*6H2O. The morphology of MWNTs and Pt nanoparticles are analyzed by SEM, TEM, and XRD.
Surface-to-depth analysis of functionalized multi-wall carbon nanotubes is achieved by high resolution x-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR). Finally, half-cell test is determined by CV to compare its efficiency different. Fig. 3.1 shows the main experimental procedures.
Fig.3.1 Flow chart of experimental procedures Carbon cloth
3.1 Fabrication of carbon nanotube on carbon cloth
MWNTs on carbon cloth are fabricated by MPECVD without bias. The 20nm of Fe film is deposited on carbon cloth as catalyst for growth of carbon nanotubes by using ion-beam sputter.
Its sample size is 0.5*0.5 cm2 and six samples are fabricated in the chamber together in order to keep the same condition.
The chamber is evacuated at pressure of ~10-2 Torr with a mechanical pump. The reactive gases are mixture of H2 and CH4, which held a ratio of 90/30 sccm to a pressure set at 10 Torr.
The microwave power of 300W is applied to light the plasma which let the temperature reaches up to 500°C ~ 600°C for 20 min without bias-assisted. Fig. 3.2 shows schematic diagram of the MPECVD system. Table 3.1 reports the parameters of the growth of MWNTs.
Fig. 3.2 Schematic diagram of the MPECVD system
Table 3.1 The parameters of the growth of MWNTs
Metallic catalyst
Reactive gases
Power Pressure Reaction time
Bias
Fe: ~20nm CH4: 30 sccm H2: 90 sccm
300 W 10 Torr 20 min 0 V
3.2 Modification of carbon nanotube on carbon cloth
MWNTs on carbon cloth are functionalized by several chemical solutions in the sample tube with sand bath. The parameters of chemical modification are chemical solutions (HNO3, H2SO4, and KOH), temperature (800C, 900C, and 1000C), concentration (2M and 14M), and time (0 hr, 6 hr, 12 hr, 18 hr, 24 hr, and 48 hr). Then, the sample is collected after immersing with deionized water until the filtrate pH became nearly the pristine pH. Fig. 3.3 shows schematic diagram of the chemical modification
Fig. 3.3 Schematic diagram of the chemical modification
3.3 Dispersion of Pt on prepared carbon cloth
The prepared carbon cloth is immersed in a chemical solution containing H2PtCl6·6H2O, PVP-4000, and ethylene glycol mixture diluted with acetone. Polymer (PVP) is the protection agent to limit metal particle growth spacing. Pt is deposited by synthesis from H2PtCl6·6H2O in ethylene glycol solution under 1600C for 3 hr, followed by filtration with acetone for 8 times and sintered for 1 hr at 2500C. This method, polyol process, is expected a uniformly homogeneous nucleation and growth mechanism of nanoparticle. The reaction dominates by temperature control and solvent plays as reductant as well. Fig 3.4 shows the experimental procedures of polyol process.
Fig. 3.4 The experimental procedures of polyol process
3.4 Analysis Instruments
3.4.1 Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) is used to observe the surface morphology of wide range kinds of objects. There are many advantages including of easy sample preparation, high image resolution, large depth of field, and high magnification. [33]
The SEM image is that signals (secondary electrons and backscattered electrons) emit from the sample surface as the sample is bombarded by the high energy incident electrons. The fabricating CNT morphology and the dispersing Pt on CNT morphology could be observed by JEOL JSM6500F in NCTU with field emission electron source and 15kV accelerate voltage.
Fig.3.5 Diagram of a Scanning Electron Microscopy [http://www.le.imm.cnr.it/sito/laboratories/jsm6500f.html]
3.4.2 Transmission Electron Microscopy (TEM)
In a typical TEM a static beam of electrons at 100-400kV accelerating voltage illuminate a region of an electron transparent specimen which is immersed in the objective lens of the microscope. The transmitted and diffracted electrons are recombined by the objective lens to form a diffraction patter in the back focal plane of that lens and a magnified image of the sample in its image plane. [33]
The raw MWNTs and the modified MWNTs morphology could be compared by TEM using
a JEOL JEM 4000 system in NCTU operating at 200kV. And the particle morphology, size and size distribution of Pt nanoparticles dispersed on the surface of MWNTs are also characterized by TEM.
3.4.3 X-ray Photoelectron Spectroscopy (XPS)
The phenomenon is based on the photoelectric effect. The concept of the photon was used to describe the ejection of electrons from a surface when photons impinge upon it. The XPS technique is highly surface specific (< 5nm) due to the short range of the photoelectrons that are excited from the solid. The energy of the photoelectrons leaving the sample is determined using a Spherical Capacitor Analyzer (SCA) this gives a spectrum with a series of photoelectron peaks.
The binding energy of the peaks is characteristic of each element. The peak areas can be used (with appropriate sensitivity factors) to determine the composition of the materials surface. [33]
XPS could determine the difference of the raw CNT and the modified CNT due to the element C chemical shifts. Furthermore, it may ensure if Pt is reductive by the same way. In this study, XPS analysis is carried out on a ESCA PHI 1600 using an Mg Kα X-ray source in NTHU.
Fig.3.6 Diagram of a X-ray Photoelectron Spectroscopy [http://www.nscric.nthu.edu.tw/Other/augeresca/auesca.html]
3.4.4 Fourier Transform Infrared Spectrometer (FTIR)
Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify organic and inorganic materials. This technique measures the absorption of various infrared light wavelengths by the material of interest. These infrared absorption bands identify specific molecular components and structures. [33]
The functional groups on the surface of MWNTs modified by chemical solution could be determined by FTIR. The FTIR measurements are performed on a PROTEGE 460 series FTIR apparatus by transmission spectroscopy in NCTU.
Fig.3.7 Diagram of a Fourier Transform Infrared Spectrometer [http://www.forumsci.co.il/HPLC/FTIR_page.html]
3.4.5 Cyclic Voltammetry (CV) Potentiostat
A potentiostat is an electronic device that controls the voltage difference between a working electrode and a reference electrode. Both electrodes are contained in an electrochemical cell.
The potentiostat implements this control by injecting current into the cell through an auxiliary, or counter, electrode. In almost all applications, the potentiostat measures the current flow between the working and auxiliary electrodes. The controlled variable in a potentiostat is the cell potential
and the measured variable is the cell current.
The CHI Version 5.01 system in the potentiostat in NCTU is used to measure the electrochemical specific surface area of the dispersive Pt on the surface of MWNTs for the fuel-cell electrodes. From the CV, the charge equivalent to the area under the hydrogen desorption region is evaluated and the electrochemical specific surface area is calculated assuming that the charge is required for the adsorption-desorption of a monolayer of atomic hydrogen on the surface.
Fig.3.8 Schematic of a Cyclic Voltammetry (CV) Potentiostat [http://www.gamry.com/App_Notes/Potentiostat_Primer.htm#Workking]
3.4.6 Energy Dispersive X-ray (EDX)
It is a technique used for identifying the elemental composition of the specimen, or an area of interest thereof. The EDX analysis system works as an integrated feature of SEM (JEOL JSM6500F) in NCTU. An EDX spectrum plot not only identifies the element corresponding to each of its peaks, but the type of X-ray to which it corresponds as well. For example, a peak
corresponding to the amount of energy possessed by X-rays emitted by an electron in the L-shell going down to the K-shell is identified as a K-Alpha peak. The peak corresponding to X-rays emitted by M-shell electrons going to the K-shell is identified as a K-Beta peak. [33]
EDX measurements show the element on the MWNTs and the content of Pt/MWNTs. It appears that the difference content of Pt is on the raw MWNTs and the modified MWNTs.
Fig.3.9 Elements in an EDX spectrum are identified based on the energy content of the X-rays
[http://www.semiconfareast.com/edxwdx.htm]
3.4.7 X-Ray Diffraction (XRD)
X-ray Diffraction (XRD) is one of the primary techniques used by solid state chemists to characterize materials. XRD can provide information about crystalline structure and particle size in a sample even when the crystallite size is too small for single crystal x-ray diffraction. [33]
An X-ray beam hits a sample and is diffracted. We can observe the diffraction peaks when the distances between the planes of the atoms apply to Bragg's Law. Bragg's Law is:
nλ =2dsinθ (1) Where the integer n is the order of the diffracted beam, is the wavelength of the incident
nλ =2dsinθ (1) Where the integer n is the order of the diffracted beam, is the wavelength of the incident