Nanotube Buckypaper Electrodes for PEM Fuel Cell Applications Chung-Lin Ku

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2007 Nanotube Buckypaper Electrodes for PEM Fuel Cell Applications Chung-Lin Ku

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THE FLORIDA STATE UNIVERSITY

COLLEGE OF ENGINEERING

NANOTUBE BUCKYPAPER ELECTRODES

FOR PEM FUEL CELL APPLICATIONS

By CHUNG-LIN KU

A Thesis submitted to the Department of Industrial and Manufacturing Engineering in partial fulfillment of the requirements for the degree of Master of Science

Degree Awarded: Fall Semester, 2007

The members of the committee approve the thesis of Chung-Lin Ku defended on November 06, 2007.

______Zhiyong Liang Professor Directing Thesis

______Jim P. Zheng Outside Committee Member

______Ben Wang Committee Member

______Chuck Zhang Committee Member

Approved:

______Chuck Zhang, Chair, Department of Industrial and Manufacturing Engineering

______Ching-Jen Chen, Dean, FAMU-FSU College of Engineering

The Office of Graduate Studies has verified and approved the above named committee members.

BIOGRAPHICAL SKETCH

Chung-Lin (Daphne) Ku is currently a master’s student in Industrial Engineering at FAMU-FSU College of Engineering and expecting graduation in December, 2007. She was born in October of 1982. She received her Bachelor’s of Engineering in Industrial Engineering in June 2005 from Tung-Hai University at Tai-Chuang, Taiwan. Her research interests include nanotube technologies, design of experiments, supply chain management, quality control, reliability, and applied optimization. Her favorite hobbies include badminton, music and movies. She is looking forward to pursue a career in the field of Industrial Engineering. For more information, please feel free to contact her at [email protected].

TABLE OF CONTENTS

List of Tables...... vi List of Figures...... vii Abstract...... x

CHAPTER 1 INTRODUCTION...... 1

1.1 Motivation...... 2 1.2 Problem Statement...... 3 1.3 Research Objectives...... …...... 4

CHAPTER 2 LITERATURE REVIEWS...... 6

2.1 Introduction of Fuel Cells…………………………………………………….……….6 2.1.1 Principle...... 6 2.1.2 Types of Fuel Cells...... 8 2.1.3 Proton Exchange Membrane Fuel Cells (PEMFCs)...... 9 2.1.4 Electrodes...... 10 2.2 Carbon Nanotube (CNT) and Nanofiber (CNF)...... 13 2.2.1 Good Candidates for Catalyst Supports……………………………………..13 2.2.2 CNT Buckypaper……………….…………………………………….………16 2.3 CNT Application Studies in PEMFCs...... 19 2.3.1 Synthesis Methods for Pt/ CNT Electrocatalysts...... 20 2.3.2 CNT Supports for PEMFC Catalysts...... 24 2.4 Conclusion...... 29

CHAPTER 3 EXPERIMENTAL………...…...... 31

3.1 Preparation of Mixed CNT Buckypapers...... 31 3.1.1 Fabrication Methodology...... 31 3.1.2 Characterization Methods...... 33 3.2 Preparation of Pt/BP Electrodes...... 33 3.2.1 Electrodepositions of Pt Nanoparticles...... 33 3.2.2 Electrochemical and Structural Analyses of Pt/BP Samples………...... 36

CHAPTER 4 RESULTS AND DISCUSSION...... 38

4.1 Surface Morphology of Pt/Mixed BP Electrodes...... 38 4.1.1 SEM Analysis of the Specimens…………...... 38

iv 4.1.2 XRD Analysis of the Specimens...... 45 4.2 Electrocatalytic Properties of Pt/Mixed BP Electrodes...... 47 4.2.1 Pt Specific Surface Area...... 47 4.2.2 Pt Utilization Analysis...... 52 4.3 Effects of Mixed BPs Nanostructure on Pt Deposition……...... 54 4.4 Cost Analysis...... 54 4.5 Ideal Electrode Microstructure for PEMFC...... 58 CHAPTER 5 CONCLUSIONS...... 61

CHAPTER 6 FUTURE WORKS...... 62

REFERENCES...... 63 BIOGRAPHICAL SKETCH...... 72

LIST OF TABLES

Table 2.1 Pore channel types and composition in the negative of H2/O2 fuel cell...... 10

Table 2.2 Current Pt loading of anode and cathode...... ….12

Table 2.3 Adsorption properties of CNT, CNF, and carbon black……………………....15

Table 2.4: BET surface area analyzer………………………………………………...... 18

Table 2.5: Electrical conductivity of mixed buckypapers……………………………….18

Table 2.6 Summarization of synthesis methods for CNT-supported Pt electrocatalysts...24

Table 2.7 Summarization of Pt particle size on CNT supports by electrodeposition……29

Table 3.1 The weight ratio information of all buckypaper samples……………………..33

Table 4.1 Pt particle size calculated by Debye-Scherrer formula……….………...……..46

Table 4.2 Material cost analysis………………………………………………………….55

Table 4.3 Theoretical Pt loadings of Pt/S89M72 BP…………………………………….55

Table 4.4 Prediction of material cost in 2010 and 2020…………………………………57

LIST OF FIGURES

Figure 1.1 Scheme of platinum on carbon support...... 4

Figure 2.1 Scheme of a fuel cell and basic operating principles...... 7

Figure 2.2 The components of a single PEM fuel cell...... 10

Figure 2.3 Electrode structure of a conventional design and SEM micrograph of carbon cloth (GDL)……………………………………………………………………...... 11

Figure 2.4 Schematic configuration of the MEA………………………………………...12

Figure 2.5 Schematic representations of nanotube and nanofiber structures………..…...14

Figure 2.6 SWNT buckypaper, TEM image of SWNT powder, and SEM images of SWNT buckypaper surface……………..………………………………………………..17

Figure 2.7Scheme of the structures of a MWNT and a carbon black particle, and their structures after electrochemical oxidation; comparison of Pt active surface area loss for the case of Vulcan XC-72 and MWNT during oxidation treatment……………….…….19

Figure 2.8 Typical synthesis process and explanation of Pt/ CNT catalyst by the chemical reduction method………………………...………………………………………………20

Figure 2.9 SEM micrograph of carbon cloth after impregnation, and SEM image of the surface of a Pt-sputtered GDE with an equivalent film of thickness of 3 nm…………...21

Figure 2.10 The synthesis procedure for Pt/CNT catalysts by microwave heated polyol process……………………………………………………………………………………22

Figure 2.11 Schematic diagram of three-step electrochemical synthesis of Pt particles on CNTs…………...……………………………………………………………...…………23

Figure 2.12 SEM image of the well-aligned carbon nanotube arrays on Ti substrate…...25

2.13 Chronoamperometric curves for 30wt% Pt/Vulcan XC-72, 30wt% Pt/MWNT, Vulcan XC-72, and MWNT……………………….…………………………...………...26

Figure 2.14 TEM micrographs and histogram of Pt particle size distribution on Pt/Vulcan XC-72 and Pt/MWNTs before and after durability test….…………...………………….27

vii Figure 2.15 Electrical conductivity as a function of thickness for SWNT papers……….27

Figure 2.16 SEM images of an in-house prepared high-metal-content Pt-Ru/CNT, and a porous catalyst layer with a buckypaper texture by vacuum filtration.………………….28

Figure 3.1 Manufacturing process of CNT buckypaper…………………………………32

Figure 3.2 Cyclic voltammograms on the BPs with/ without electro-oxidation….……...34

Figure 3.3 Preparation process of Pt/BP electrodes and cyclic voltammetry test…….....35

Figure 3.4 Three electrode cell…………………………………………………………..35

Figure 3.5 Mechanism of Pt deposited on CNT…….…………………………………...36

Figure 3.6 Hydrogen adsorption/ desorption peaks of CV………………………………37

Figure 3.7 Three-electrode arrangement of CV test……………………………………..37

Figure 4.1 SEM micrographs and estimated distributions of particle size of Pt/SWNT86, Pt/SWNT89………………………………………………………………………………39

Figure 4.2 FT-IR spectrum, Raman spectrum of SWNT 86 and SWNT 89 buckypaper before and after burned surfactant……………………...……………………………..…39

Figure 4.3 SEM micrographs and distributions of particle size of Pt/SM (3:1), Pt/SM (1:1), and Pt/SM (1:3) (S: SWNT86, and M: MWNT72)…….………….………………40

Figure 4.4 SEM micrographs and distributions of particle size of Pt/SF (3:1), Pt/SF (1:1), and Pt/SF (1:3) (S: SWNT86, and F: CNF19)………….………………………………..41

Figure 4.5 Thermogravimetric Analysis (TGA) of SWNT86, SWNT89, MWNT90, and CNF19……………………………………………………………………………………42

Figure 4.6 SEM micrographs of (a) Pt/S335M72 (1:3), and (b) Pt/S89M72 (1:3); the distributions of particle size of (c) Pt/ S335M72 (1:3), and (d) Pt/S89M72 (1:3)………..43

Figure 4.7 SEM micrographs and the distributions of particle size of Pt/MWNT90, Pt/S89M72 (1:5), and Pt/S89M72 (1:7)…….………...……………..…………...……….44

Figure 4.8 SEM micrographs of Pt/SCB (7:1), Pt/SCB (3:1), and Pt/SCB (1:1) (S: SWNT86, and CB: Vulcan XC-72)……………………………………………….……...45

Figure 4.9 Powder X-ray diffraction (XRD) patterns of Pt/mixed BPs electrodes..…...... 46

Figure 4.10 Cyclic voltammograms on Pt/mixed BP…………………………..……...... 48

viii

Figure 4.11 Summary of the electrochemical surface area (ECSA) of the Pt/mixed BP electrodes………………………………………………………………………………...48 Figure 4.12 SEM micrographs of SWNT89+MWNT72 buckypaper (1:3) on top side, bottom side…………………………………………….………………………………....49

Figure 4.13 Scheme of the SWNT89+MWNT72 (1:3) buckypaper…………...…...…....50

Figure 4.14 SEM micrographs of the different contents: a drop, 50 ml, 300 ml, 1000 ml of suspension (SWNT89+MWNT72, weight ratio 1:5) filtrating through the membrane………………………………………………………………………………...51

Figure 4.15 SEM micrographs of the membranes of SWNT89+MWNT72 (1:7) and MWNT90 after buckypaper was peeled…………………………………………………52

Figure 4.16 Utilization of Pt/CB and Pt/ Mixed BPs…….……………………………...53

Figure 4.17 Electrical conductivity of buckypaper vs. Pt loading………….…………...54

Figure 4.18 Estimated Pt cost and total material cost of Pt/CB and three different weight ratios of Pt/SWNT89+MWNT72..……………………………………………………….56

Figure 4.19 Pt price from 2002 to 2006………………………………………………….56

Figure 4.20 Estimated total material cost of Pt/CB and Pt/SWNT89+MWNT72 in 2010 and 2020…………………………………….……………………………………………57

Figure 4.21 Optimum electrode microstructure for PEMFC………………………….…58

Figure 4.22 Micro-surface phenomenon of SWNT+MWNT mixed BP (1:3)…...... …….59

Figure 4.23 SEM micrographs of Pt/SWNT89MWNT72 (1:3) 1200 ml with MWNT72 800 ml on bottom side and top side…………...…………………………………………60

Figure 4.24 ECSA comparisons of Pt/S89+M72 BP with double layer BP……………..60

Figure 4.25 Cost comparisons of Pt/S89+M72 BP with double layer BP……………….60

ABSTRACT

Many researchers proposed the use of carbon nanotubes as an advanced metal catalyst support for electrocatalysis applications. In this research, buckypaper (thin film of preformed nanotube network) electrodes with different weight ratios of carbon nanomaterials, including SWNT, MWNT, CNF, and Vulcan XC-72 (CB), were fabricated and compared by their electrochemical properties using cyclic voltammetry (CV) test. Platinum (Pt) nanoparticles were successfully electrodeposited on the mixed

buckypapers in mixed ethylene glycol, H2PtCl6, and H2SO4 aqueous solutions by applying a potential pulse at 0.2 and -0.25 V, forming Pt/mixed buckypaper electrodes. The dispersion and particle size of Pt nanoparticles on the buckypapers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The average diameter of Pt nanoparticles on the buckypapers was 10 nm. Surface areas of the Pt nanoparticles on the mixed buckypapers were determined by cyclic voltammogram measurements in 0.5 M H2SO4 solution, and electrocatalytic performances of the resultant buckypaper electrodes were observed. Compared to the Pt/CB electrodes, the Pt/SWNT+MWNT buckypaper electrode exhibits higher electrocatalytic performance. The highest electrochemical surface area (ECSA) of Pt/SWNT+MWNT (1:3) electrodes reaches 43.7m2/g and is about 1.6 times higher than that of the Pt/CB electrode. This may be attributed to the small particle size and good dispersion of platinum, high conducting property of carbon nanotubes, special deposition phenomenon, and unique three–dimension electrode structure. The research results suggest that mixed buckypapers are good candidates for catalyst supports in fuel cell applications because of their high electrocatalytic

x performance. The reduction of the amount of precious metal catalyst (Pt) needed is important for real-world applications. Further research into the optimization of Pt deposition and nanostructure of mixed buckypapers could lead to highly efficient and potentially affordable electrodes for fuel cell applications.

CHAPTER 1

INTRODUCTION

As a potential technology for environmental friendly and highly efficient electric power generation, proton exchange membrane fuel cells (PEMFC) have been attracting enormous interest in various applications, such as low/zero-emission vehicles, distributed home power generators, and power sources for small portable electronics [1-4]. However, one of the challenges in the commercialization of PEMFC is the high cost of noble metals used as catalyst. Platinum (Pt) is the most popular catalyst used in PEMFC. Decreasing the amount of Pt used in a PEMFC via the increase of utilization efficiency has been one of the major concerns during the past decades [5]. To effectively utilize Pt catalyst, Pt must have simultaneous access to three-phase interface which are gas, electron-conducting medium, and proton-conducting medium in a PEMFC system. Therefore, electrodes play an important role of a fuel cell. An effective electrode is one that properly balances the transport process required for an operational fuel cell [6]. The electrode must be porous, electronically and ionically conducting, electrochemically active, and have a high surface area [7]. The catalytic activity, physical properties and electrochemical performance must be stable for the duration of the desired cell life. Carbon nanotubes were discovered in 1991 by the Japanese electron microscopist, Sumio Iijima, who was studying the material deposited on the cathode during the arc- evaporation synthesis of fullerenes [8]. Since the discovery of carbon nanotubes, considerable researches have been done in the fields of electrical and electronic engineering, physical, chemistry, materials science and engineering due to the facts that

1 carbon nanotubes possess unprecedented electronic, physical, biological, and mechanical properties in a single material [9]. It has been mentioned that the efficient Pt loading on electrodes is an important technique for the development of PEMFC. Carbon nanotubes (CNTs) are promising for developing efficient electrodes for the fuel cell applications due to their unique and exceptional properties. High-Performance Materials Institute (HPMI) has established a comprehensive set of analysis, processing and testing systems for nanocomposite materials. It also has a lot of experience on nanotube dispersion and functionalization, such as fluorination, E-beam and hydrogen peroxide oxidization. Additionally, a unique technology has been developed to produce nanotube buckypapers, which are thin sheets with 10-40 m in thickness and as large as 60 in² in area. Buckypaper is a very useful platform to harvest the exceptional properties of nanotubes. HPMI will take the advantages of existing buckypaper expertise to investigate and develop new and high performance electrodes for fuel cell applications.

1.1 Motivation

Fuel cell is one of the exciting researches of major efforts to develop clean and renewed energy. There are many different types of fuel cells, such as alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC), proton exchange membrane fuel cells (PEMFC), molten carbonate fuel cells (MCFC), and solid oxide fuel cells. Basically, they are classified by the temperature they operate and electrolyte they use. Proton exchange membrane fuel cell (PEMFC) probably has the widest present and research of all existing fuel cells because of its many advantages and various applications. However, there still exist some problems that need to be solved in the present fuel cell technologies, such as platinum dispersion and utilization, manufacturing process cost, CO poison, and degradation etc. It has been demonstrated that some performances improved when using CNTs as catalyst supports in the fuel cells compared to the conventional electrode using Vulcan XC-72 [10], such as platinum dispersion and durability. High quality nanotube buckypapers are characterized by uniform tube network

2 formation, small rope size, evenly distributed pore structure, and smooth surface quality. Using buckypaper as electrode in proton exchange membrane (PEM) fuel cells is considered very promising because of high Pt utilization rate and large specific three- phase boundary (TPB) length.

1.2 Problem Statement

For commercialization and the use of PEM fuel cells, there are many barriers, including high cost of noble metals used as catalysts (e.g., platinum) [11], the production costs of the proton exchange membrane (PEM) [12], water management, flow control, temperature management, durability, service life and limited carbon monoxide tolerance of the anode [13]. Current carbon black (i.e., Vulcan XC-72) based electrodes and catalyst techniques cannot provide adequate properties to meet some requirements, such as Pt utilization and durability. Therefore, high performance electrode concepts and techniques must be developed. It has been calculated that Pt contained electrode cost 71% of all stack components in the PEMFC system [14]. Besides, the platinum cost has been increasing from $29/per gram in 2005 to $35/per gram in 2007 [14]. People have tried to reduce platinum particle size to nano-scale by using small, conductive, and high corrosive resistant catalyst support (Figure 1.1), such as Vulcan XC-72 which is the most common catalyst support in use at present. However, carbon black still has two major problems. Firstly, some platinum particles could dispose into the micro pores of carbon blacks which are hard to be accessed by water and gas, and then the usage of platinum will decrease. Secondly, carbon black could be corroded under severe condition of cathode which results in low cell stability and short service life.

Figure 1.1 Scheme of platinum on carbon support

1.3 Research Objectives

In this project, we will demonstrate a technique to improve the catalyst efficiency, reduce the catalyst loading, and improve the durability of PEM fuel cells by using fine Pt particle deposition on buckypaper made with carbon nanotubes (CNTs) or nanofibers (CNFs). The specific objects of the research are as follows: 1) Develop CNT buckypaper for PEM fuel cell electrode applications The key component of this research is to develop CNT buckypaper for electrode application in PEM fuel cell. The buckypaper will play both roles as the supporting substrate for catalyst materials and gas diffusion layer. The buckypaper is highly electrically conductive and stable at high temperature (~400oC). The porosity and thickness of the buckypaper are also controllable. The Pt catalyst particles will be deposited on buckypaper by electrodeposition method. To investigate and understand the relationships between buckypaper properties and electrode performance, we will fabricate and test different buckypapers for PEM fuel cell electrode applications. Due to the issues of high SWNT cost and pore size control, mixed (SWNT+MWNT) buckypapers could solve those problems and are primarily used in this research.

4 2) Demonstrate a new catalyst structure comprised of CNT buckypaper and Pt catalyst Through this research, the loading of catalyst materials will be reduced by depositing on buckypaper surface. The effectiveness of the catalyst will be significantly increased through maximization of the effective area of three-phase interface, and the durability of the fuel cell will be improved due to the highly electrochemically stability of CNTs. The characteristics of such catalyst electrode include: localization of the electrocatalyst on the uppermost surface, thin active layer, and ultra-low level of Pt loading (cathode <0.1 mg/cm2). The research will investigate these properties. 3) Demonstrate efficiency and durability improvement of CNT buckypaper based electrodes Characterizations of CNT buckypaper electrodes, such as efficiency and durability, will be conducted and compared to the conventional electrode using Vulcan XC-72. The mechanisms of the performance improvements will be discussed.

CHAPTER 2 LITERATURE REVIEWS

2.1 Brief Introduction of Fuel Cells

In 1839, William Robert Grove [15] discovered the fuel cell operating principle: the gaseous fuels that generated electricity; actually, a Swiss scientist Christian F. Shoenbein [16] independently discovered the very same effect a year before. In 1937, Francis T. Bacon started working on practical fuel cells and developed a 6 kW fuel cell by the end of the 1950s. The first practical fuel cell application happened in the U.S. space program. In the early 1960s, General Electric developed the first polymer membrane fuel cells used in the Gemini Program before the Apollo Space Program used the fuel cells. In the mid- 1960s, General Motors experimented with a fuel cell-powered van. In 1989, Perry Energy Company demonstrated a polymer electrolyte membrane (PEM) fuel cell-powered submarine. In 1993, Ballard Power Systems demonstrated fuel cell-powered bus. Energy Partners, a successor of Perry Energy Systems, demonstrated the first passenger car running on the PEM fuel cells in the same year [17]. The number of fuel cell-related patents primarily in U.S. and Japan increasing dramatically shows continuous interest and development of fuel cell technologies and applications [18, 19].

2.1.1 Principle A fuel cell is an electrochemical energy converter which converts the chemical energy of fuel, such as hydrogen, directly into electricity and heat. The anode and the cathode, on both sides of the membrane, have the electrochemical reactions in fuel cells simultaneously. The basic reactions of fuel cells are described in the following, but these

6 reactions may have several intermediate steps and side reactions.

+ - At the anode: H2 Æ 2 H + 2 e (1) 1 + - At the cathode: /2 O2 + 2 H + 2 e Æ H2O (2) 1 Overall: H2 + /2 O2 Æ H2O (3)

Figure 2.1 shows the scheme of a fuel cell and the basic operating principles [20]. Electrochemical reactions happen on the surface of the catalyst at the surface between the electrolyte and the porous electrodes. Hydrogen is fed into the anode (negative); oxygen is fed into the cathode (positive). Protons pass through the electrolyte to reach cathode, whereas the electrons travel through the outside circuit to go to another side of the electrolyte. The byproduct, water, is created after protons, electrons, and oxygen meet on the catalyst between cathode and electrolyte, and then pushed out of the fuel cell with excess flow of oxygen.

Figure 2.1 Scheme of a fuel cell and basic operating principles [20]

7 In some aspects, a fuel cell is similar to a battery which has an electrolyte, and negative and positive electrodes, and it generates DC electricity through electrochemical reactions. However, the reactants consumed in fuel cells must be replenished, while batteries store electrical energy chemically in a closed system. Therefore, a battery may be discharged which happens when the reactants stored in the cells are depleted, or be rechargeable which means that the electrochemical reactions may proceed in reverse when external electricity is applied. A fuel cell does not need to be discharged as long as the reactants, fuel and oxidant are supplied. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols; other oxidants include air, chlorine and chlorine dioxide. A fuel cell is a high efficient energy converter because the basic operating principle is from chemical energy to electrical energy directly. The byproduct in the hydrogen fuel cell is just water and it causes no pollution. There are other features of fuel cell, such as lower noise, abundant reactants and plenty application possibilities.

2.1.2 Types of Fuel Cells

Different types of fuel cells are classified by types of electrolyte they use, and levels of temperature they operate, namely:

1) Alkaline fuel cells (AFC) use concentration of 85 wt% KOH as the electrolyte for high temperature operation (250°C) and less concentration of 35-50 wt% for lower temperature operation (<120°C). 2) Phosphoric acid fuel cells (PAFC) use concentration of near 100% phosphoric acid as the electrolyte, and operating temperature is typically between 150 °C and 220°C. 3) Polymer electrolyte membrane or proton exchange membrane fuel cells (PEMFC) use a thin proton conductive polymer membrane as the electrolyte, and operating temperature is typically between 60 °C and 80°C.

4) Molten carbonate fuel cells (MCFC) use the carbonate (LiKCO3, LiNaCO3 etc)

electrolyte composed of a ceramic matrix of LiAlO2, and operating temperature is typically between 600 and 700°C.

5) Solid oxide fuel cells (SOFC) use a solid, nonporous metal oxide, usually Y2O3-

stabilized ZrO2 (YSZ) as their electrolyte, and the ionic conduction by oxygen ions

8 take place under the operating temperature between 800 and 1000°C. Depending on the operation temperature, AFC and PEMFC belong to low temperature fuel cells; PAFC belongs to intermediate temperature fuel cell; MCFC and SOFC belong to high temperature fuel cells. Different types of fuel cells have different requirements and conditions. Basically, the operation temperatures of different fuel cells affect the types of catalyst in chemical reactions, and also affect the types of fuels. In addition to the five basic classifications, there is a new developing type of fuel cell, direct methanol fuel cell (DMFC), which can be applied in portable equipments, such as cell phones and laptop computers.

2.1.3 Proton Exchange Membrane Fuel Cells (PEMFCs)

PEM fuel cells probably have the largest range of the present and great potential applications from all existing fuel cell systems [21]. The electrolyte of PEM fuel cells is the solid polymer, and that is the reason why it is also called polymer electrolyte fuel cell (PEFC) or solid polymer fuel cell (SPFC). In addition to its clean and pollution-free, energy conversion efficiency as the general characteristics of the other fuel cells, it still possesses advantages of atmospheric operation temperature and fast start, and there is no electrolytic liquid to corrode and overflow. Therefore, PEM fuel cells are a potential candidate for automotive applications, distributed home power generators, and portable power electronic applications as well [1-4]. The primary elements which form the fuel cell include electrode, electrolyte, and current collector shown in Figure 2.2. The electrode is the place where electrochemical reaction takes place, which can be divided into two parts of a negative pole (anode) and positive pole (cathode). The electrode of the fuel cell is the porous structure, and the thickness is generally between 200 and 500 m. Property of electrodes depends on the performances of catalyst, the material of the electrode, and the making of the electrode, etc.

Figure 2.2 The components of a single PEM fuel cell

The function of electrolyte is to separate the oxidant and reducing agent, and to conduct the ion at the same time. With present technology, the thickness of electrolyte is generally in several dozen micrometers to several hundred micrometers. PEM fuel cells use a thin proton conductive polymer membrane, perfluorosulfonated acid polymer, as the electrolyte.

2.1.4 Electrodes Electrode is the place where electrochemical reaction takes place, which means that the fuel is oxidized in the anode, and the oxidant is reduced in the cathode. From the cathode oxidant reduction in Equation (2), we could see the conduction of reactants and product through the electrodes. Table 2.1 shows the types of pore channel and composition in the negative of H2/O2 fuel cell.

Table 2.1 Pore channel types and composition in the negative of H2/O2 fuel cell [17] Types of channel Composition of the channel Description Electron transport Catalyst and support Electric catalyst and support channel (e-) transmit electrons Gas diffusion Vacant pore Gas pore made by hydrophobic channel (O2) material Proton transport Electrolyte Pore infiltered by electrolyte or channel (H+) ion exchange resin Water transport Hydrophobic pore Water left from electrode by channel (H2O) hydrophobic pore

10 The electrode used in PEMFC is the porous medium of a kind of multi-layer structure shown in Figure 2.3 (a). The early manufacturing method is to press platinum black on gas diffusion layer (GDL, Figure 2.3 (b)) directly, and then progressively press platinum/carbon black on GDL, like PAFC.

Figure 2.3 (a) Electrode structure of a conventional design [19] and (b) SEM micrograph of carbon cloth (GDL) [22]

GDL used in PEMFC is the porous material, like carbon paper or carbon cloth which supports catalyst, and its thickness is normally between 100-300 m. It has two main functions: First function is that due to porous structure of GDL, reactive gases spread and enter electrodes smoothly and are distributed evenly on catalyst layer, in order to offer the greatest electric area of chemical reaction. The second function is to conduct the electron produced after reacting from anode, in order to enter the external circuit and channel the electron that the external circuit comes into cathode at the same time. Therefore, GDL must be porous material with good electrical conductivity. In addition, it will produce water in the cathode when PEMFC operates, and GDL will have function of auxiliary water management at the same time. There are several steps for the common manufacturing process of electrode in PEMFC (Figure 2.3 (a)). Briefly, carbon paper firstly is teflonized by PTFE solution to become the hydrophobic network. The mixture of carbon black (CB) and PTFE solution is then applied to the carbon paper to form GDL. Finally, the mixture of Pt/CB catalyst and Nafion solution is applied to GDL surface to form catalyst layer.

11 The supports used in catalyst, carbon materials, attract a growing interest due to their specific characteristics; (i) resistance to acid/basic media, (ii) possibility to control, up to certain limits, the porosity and surface chemistry, and (iii) easy recovery of noble metals to lower environmental impact [23-25]. Until now, most manufacturers have adopted the platinum/carbon black technology with binder to apply the coating on the GDL at both sides of the pretreated Nafion membrane, followed by hot-pressing, which is called membrane-electrode assembly (MEA, Figure 2.4) [26]. Like this, they can condense platinum on the intersection of the electrode and membrane, and form three- dimensional network structure. Table 2.2 shows the current Pt loading of anode and cathode. In the present technology, the Pt loading could decrease to 0.05 mg/cm2 in the anode, but it still higher than 0.4 mg/cm2 in the cathode because of poor activity for oxidation reduction reaction (ORR).

Figure 2.4 Schematic configuration of the MEA [26]

Table 2.2 Current Pt loading of anode and cathode [11] Anode Cathode From 0.2-0.4 mg/cm2 to 0.05 mg/cm2 >0.4 mg/cm2 poor activity for ORR

However, there exist some problems and disadvantages when using carbon black based electrodes in the fuel cells. First of all, it has a large portion of micro pores, and the

12 platinum deposited in these pores would be hard to be accessed by gas and water. Therefore, the usage of platinum will be reduced. Secondly, carbon black could be corroded under the severe condition of cathode resulting in low cell stability.

2.2 Carbon Nanotube (CNT) and Nanofiber (CNF)

The potentiality of using CNT or CNF as catalyst supports in fuel cells has already been investigated [27-29]. The production of carbon nanofibers is even older and the first report date is more than a century [30, 31]. Since carbon nanotubes (CNTs) were first discovered by Iijima in 1991 [32], there are a great number of publications each year and of interest for many applications, such as hydrogen storage, chemical sensors, batteries, etc. CNTs play important roles in nanoscience and nanotechnology, because of their special properties, making them suitable for many applications. Researchers also studied nanotube applications in the electrodes of fuel cell [33] and the supported catalyst where the fluid- phase reaction happened [34].

2.2.1 Good Candidates for Catalyst Supports In order to discuss the use of carbon nanotubes and nanofibers as efficient catalyst supports, it is important to analyze their properties with respect to catalytic requirements in the following: 1) Structural features In general, there are two types of carbon nanotubes. Depending on the conditions that they are synthesized, they assemble either as several coaxially arranged graphene sheets rolled into a cylinder, called multi-walled nanotubes (MWNTs, diameter 5-50 nm) or a single graphene sheet rolled into a cylinder, called single-walled nanotubes (SWNTs, diameter 1-2 nm ). A schematic diagram of both single- and multi- walled nanotubes is shown in Figure 2.5 (a) and (b) [37]. Because of van der Waals forces, both SWNTs and MWNTs can self-assemble to form bundles (diameter 10-100 nm) or thin film called buckypaper.

Figure 2.5 Schematic representations of (a) single-walled carbon nanotube (SWNT), and (b) multi-walled nanotube (MWNT) [37]; schematic representations of the three forms of graphitic nanofibres: (c) platelet, (d) ribbon and (e) herringbone structures [38]; (f) TEM showing the structure of a VGCNF with the cylindrical hollow core at the center; (g) TEM of the VGCNF wall showing the canted graphene planes comprising the “stacked cup” structure [36]

Depending on differing for the disposition of the graphene layers, there are mainly three types of CNFs: the platelet CNF (CNF-P) exhibit graphene layers perpendicular to the growth axis, the ribbon-like CNF (CNF-R) display parallel graphene layers to the growth axis, and herringbone nanofibers have graphenelayers stacked obliquely in respect to the growth axis (see Figure 2.5 (c)~(e)) [38]. Previously, three different geometries of carbon nanofibers (e.g., platelet, herringbone, and tubular) were extensively investigated from the early 1970s to the present [35]. In 1991, Applied Sciences, Inc. (apsci.com) began marketing a VGCNF developed in collaboration with General Motors Research. The new type of carbon nanofiber, stacked-cup carbon nanofiber, exhibited a unique morphology of stacked, truncated conical grapheme layers along the fiber length shown in Figure 2.5 (f) [36]. A hollow core is surrounded by a

14 cylindrical fiber grown from a catalyst particle and comprised of graphite basal planes stacked at about 25o from the longitudinal axis of the fiber [36]. Figure 2.5 (g) shows the interior wall of a nanofiber; the nested conical graphene planes are clearly canted with respect to the longitudinal fiber axis [36]. Generally, the diameters of CNF are larger than the ones presented by nanotubes and can easily reach 500 nm [37]. 2) Adsorption properties Adsorption properties for SWNT usually found in bundles or ropes; for MWNT could occur either on or in the tube or between the aggregated MWNT. Some papers mentioning the adsorption of nitrogen on MWNT [39, 40] and SWNT [41] have emphasized the porous nature of these materials. Several studies have shown that, at low coverage, the binding energy of the adsorbate on SWNT is between 25 and 75 % higher than the monolayer binding energy on graphite. The observed change of the binding energy can be attributed to an increase of the effective coordination in binding sites, as the groove sites, in SWNT bundles [42, 43]. Typically, the specific surface area of SWNT (400-900 m2g-1) is often larger than that of MWNT (200-400m2g-1) shown in Table 2.3. For CNF, the surface area can range from 10 to 200 m2g-1. Carbon nanotubes and nanofibers show no micropore surface area [37]. Besides, hydrophobicity of carbon nanotube is much better than that of carbon black for fuel cell applications.

Table 2.3 Adsorption properties of CNT, CNF, and carbon black [39, 44, 45] SWNT MWNT CNF Carbon black Surface area (m2/g) 400-900 200-400 10-200 230 Micropore surface area (m2/g) None None 87 Hydrophobicity Excellent - Poor

3) Electronic properties Some studies have discussed about electronic properties of SWNTs [46] which behave like pure quantum wires where the electrons are confined along the tube axis, and

15 MWNTs have 2D-quantum transport features at low temperature [47]. However, CNF are often studied as conductive substrate which can exert electronic perturbation similar to those of graphite [48]. Further, the conductive supports exhibit clear differences between active carbon and nanotubes for catalyst application. The interaction of transition metal atoms with CNT and graphite presented major differences [49]. It has been shown that the binding sites are depending on the structure of the support: the studies conducted over nickel exhibit that the most stable anchoring sites vary sensibly between graphite and SWNT due to the different curvature of the surfaces where the active species can be deposited. In addition, the curvature affects significantly the value of magnetic moments on the nickel atoms on the nanotube’s wall and the charge transfer direction between nickel and carbon can be inverted. Hence, the possibility of unique metal-support interaction has to be taken into account. The unique structure features, adsorption, and electronic properties suggest that CNTs and CNFs are suitable and competitive materials for electrodes and catalyst supports by comparison with activated carbons [37]. Especially, as long as the properties of activated carbons are still difficult to control and their microporosity has often slowed down catalysts development, CNTs and CNFs could replace activated carbons in liquid- phase reactions. Table 2.3 summarizes the main properties of CNT and CNF and compares to the ones of carbon black [39, 44, 45].

2.2.2 CNT Buckypaper Buckypapers, freestanding thin membranes formed with controlled and dispersed porous network of carbon nanotubes (CNTs), was first proposed in 1998 [50]. Buckypapers are typically produced by a conventional filtration process where CNT dispersed in aqueous medium were filtered through a membrane filter with the aid of vacuum, fabricating a random CNT bundles or ropes network on the membrane shown in Figure 2.6 (a). The SWNT ropes are very long and present a highly entangled network structure (Figure 2.6 (b)) [51]. After being made as SWNT buckypapers, the papers exhibit a very dense structure with a coarse surface in Figure 2.6 (c).

Figure 2.6 (a) SWNT buckypaper, (b) TEM image of SWNT powder [51], (c) SEM images of SWNT buckypaper surface

Basically, the surface quality of buckypaper is very important for various applications. The average rope size used to evaluate the dispersion effect in the suspension preparation process is the most important performance of buckypaper quality. The pore size here is defined as the space between ropes and is used to evaluate the uniformity of tube dispersion in the buckypaper. For example, uniform rope size and porous structure of buckypaper present well tube dispersion of the suspension [52]. During buckypaper filtration, the tube network in the buckypaepr is composed of continuous CNT ropes, because of tube self-assembly induced by van der Waals force [53]. Consequently, the nanotube rope diameter of buckypapers varies from tens to several hundreds of nanometers, depending on the state of tube bundle exfoliation and dispersion in the buckypaper suspension process. The thickness of the buckypapers produced in HPMI, from 10 m to 50 m, depends on the volume of suspension filtrated through the membrane. The more suspension is filtrated, the larger the thickness of buckypapers. The BET surface areas of the pure SWNT buckypaper and of the different weight ratios of mixed buckypapers (SWNT+CNF and SWNT+MWNT) have been measured shown in Table 2.4 [44]. The pure SWNT buckypaper has higher surface area than other mixed buckypapers.

17 Table 2.4: BET surface area analyzer [44] (Tristar 3000 from Micrometritics Analytical Services) SWNT SWNT:CNF SWNT:MWNT SWNT XP-PF-036 1:3 1:3 XP-PF-102 BET Surface 271.63 89.59 182.72 433.71 Area (m2/g) 388.27 101.32 214.22 522.24

To measure the electrical conductivity of buckypapers, the four-probe electrical resistance (ER) measurement is the most general method. It has been measured that the aligned buckypaper and random SWNT buckypaper have lower resistivity than those of the mixed ones [44]. Furthermore, the buckypaper washed by isopropanol presents lower resistivity as well (Table 2.5).

Table 2.5: Electrical conductivity of mixed buckypapers [44]

Gas and liquid permeability of buckypapers is one of the least investigated areas associated with buckypapers. The gas transport property measurement showed that the gas permeant behavior in buckypapers has a strong dependence on pressure, and apparent buckypaper permeability rises quickly as pressure is increased [54]. Researchers have demonstrated using CNT films in a wide variety of energy-related applications, such as electrodes [51, 55-57], fuel cells [58, 59], solar cells [60-62],

18 electromechanical actuator [63], and transistors [64] etc. Besides, it can get higher platinum utilization rate and larger three-phase boundary (TPB) length by using CNT films as the catalyst support when compared to the conventional slurry or ink process.

2.3 CNT Application Studies in PEMFCs

Li et al. have observed the higher resistance of corrosion when using CNT based electrode. The observed durability demonstrated that CNTs would be a better catalyst support in PEM fuel cells in which the commonly used carbon black often undergoes severe electrochemical corrosion shown in the Figure 2.7 (a) [65]. Wang et al. have also observed that MWNT shows lower loss of Pt surface area and oxygen reduction reaction activity when used as fuel cell catalyst support in Figure 2.7 (b) [10].

Figure 2.7 (a) Scheme of the structures of a MWNT and a carbon black particle, and their structures after electrochemical oxidation [65]; (b) Comparison of Pt active surface area loss for the case of Vulcan XC-72 and MWNT at different time intervals during oxidation treatment [10]

19 2.3.1 Synthesis Methods for Pt/ CNT Electrocatalysts For Pt/ CNT catalyst synthesis, the CNT surface treatment to create the functional groups for Pt particle deposition is an important step towards an active fuel cell electrode catalyst. The following descriptions will give the brief concepts and explanations of the different synthesis methods for Pt/CNT electrocatalysts in the literature reports. 1) Impregnation method (also called electroless deposition or chemical reduction method) Impregnation is the most common method to prepare highly dispersed Pt nanoparticles on the carbon supports for PEMFC catalysts [66-68]. Figure 2.8 shows the typical preparation procedure and explanation of the chemical reduction method for Pt/ CNT catalyst.

Synthesis process Explanation To create surface active sites and improve the Surface-oxidation of CNTs by dispersion of catalyst metals on CNT surfaces. HNO3 or H2SO4 or a mixture of two Chemical interaction between anchoring catalyst metal ions with a CNT functional Stirring with H2PtCl6 surface. or K2PtCl4 Upon the addition of a reductive agent such as Pt reduction by HCHO or ethylene glycol (EG) into the HCHO or EG reaction system, the surface Pt ions could be reduced to well-dispersed Pt metal nanoparticles, resulting in a Pt/ CNT catalyst. Filter with deionized Washing and drying. water & dry

Figure 2.8 A typical synthesis process and explanation of Pt/ CNT catalyst by the chemical reduction method [66-68]

Han et al. investigated the effect of HNO3 concentration on the introduction of functionalities [69]. They found that the surface of CNTs had more anchoring sites for

20 metal deposition that result the catalyst particles with smaller size and more homogeneous dispersion when increasing the nitric acid concentration. Li et al. [70]

treated CNTs by a HNO3 - H2SO4 mixture which is more beneficial to Pt dispersion than

the ones treated by HNO3 alone because of its higher oxygen surface concentration determined by X-ray photoelectron spectroscopy (XPS) analysis as well as cyclic voltammetry. Figure 2.9 (a) shows that the catalytic suspension was spread on the support resulting in a homogeneous layer that covers completely the fibers of the carbon cloth. It causes a compact film that decreases the electrode porosity. The preparation of the catalyst suspension (‘ink’) was made by mixing the catalyst (20 wt.% Pt/C supplied by ETEK), the ionomer (Nafion, supplied by ALDRICH, 5 wt.% solution in a mixture of lower aliphatic alcohols and deionised water) and different solvents (n-butylacetalte, ethanol and glycerol) [22].

Figure 2.9 (a) SEM image of carbon cloth after impregnation [22], (b) SEM image of the surface of a Pt-sputtered GDE with an equivalent film of thickness of 3 nm [76]

2) Sonochemical technique In order to obtain more surface functional groups, the sonochemical technique can improve the oxidation of the CNT surface by strong nitric and sulphuric acids [71]. This process can break the big CNT bundles by sonicating in an ultrasonic bath and disperse

21 much better than conventional process. This Pt deposition was almost the same as the conventional impregnation method as described in Figure 2.8. 3) Microwave-assisted heating polyol process Several research groups have investigated the synthesis of Pt/ CNT catalyst using a microwave heated polyol process shown in Figure 2.10 [72-75]. Heating by microwaves can accelerate the reduction of the metal precursor ions and the nucleation of the metal particles. Besides, the homogenous microwave heating can also reduce the temperature and concentration gradients in the reacting sample solution, resulting in a more uniform environment for the nucleation and growth of metal particles [72, 74].

Process Condition

Mixture solution of H2PtCl6 +EG ← KOH to adjust PH ← Add purified CNT Sonicate

Heating In a microwave oven

Filter/wash with acetone and deionized water

Dry at 373 K in vacuum oven Figure 2.10 The synthesis procedure for Pt/ CNT catalysts by microwave heated polyol process [72]

Cyclic voltammetry showed that microwave synthesized Pt/CNTs catalyst exhibited higher catalytic activity for methanol electrooxidation at room temperature than a commercial E-TEK Pt/C catalyst [74]. A DMFC using as-prepared Pt-Ru/CNTs catalyst as anode exhibited a high power density of 117 mW/cm2 at 70 ºC [73]. 4) Electrodeposition Several groups have reported the synthesis of Pt/CNT catalyst by using the electrodeposition method [77-79]. Guo et al. have used a three-step process including the

22 electrodeposition method to prepare SWNT-supported Pt catalysts for methanol oxidation [77] explained and shown in the Figure 2.11. Guo et al. have demonstrated that the as- prepared Pt/SWNT catalyst has Pt nanoparticles of 4-6 nm, and showed excellent electrocatalytic activity for MOR and good stability [77].

First step Second step Final step To create the surface To form surface octahedral The octahedral complex of Pt functional groups Pt (IV) complex which are (IV) on the SWNT surface are used as the precursors electrochemically reduced to Pt metal Figure 2.11 Schematic diagram of the three-step electrochemical synthesis of Pt nanoparticles on CNTs [77]

5) Sputter deposition technique Sputter deposition technique (SEM image in Figure 2.9 (b)) has also been employed to prepare Pt-loaded CNT catalyst by several groups. In Chen’s study [80], the MWNTs were fabricated directly on the carbon cloth utilizing bias-assisted microwave plasma enhanced CVD (MWCVD); Pt nanoparticles were then sputtered on MWNT surface. The advantages of sputter deposition technique are depositing small and uniform Pt nanoparticles, generating a thinner catalyst layer, and reducing the Pt loading. However, this method still has some challenges for electrode mass production. 6) Other methods Beside the methods mentioned above, there are other methods which have been applied for synthesis of CNT supported metal nanoparticles composites. Oh et al. prepared CNT-supported Ag, Pd and Pt-Ru catalysts using the -irradiation technique

23 [81]. Lee et al. reported a novel preparation method, the self-regulated reduction of surfactant method, for Pt/ CNT catalyst synthesis [82]. Yoshitake et al. have reported the colloid method for the synthesis of Pt/CNT catalysts in their PEMFC cathode preparation by single-wall carbon nanohorns (SWNH) as the support [83]. For the synthesis of CNT-supported Pt catalysts, several catalyst synthesis methods such as impregnation method, sonochemical technique, microwave heated polyol process, electrodeposition, sputter-deposition technique, and others have been explored and summarized above. The created functional groups by the acid treatment, which have a strong effect on the formation of the favorable morphology of catalysts, can serve as the anchoring sites for Pt ions [68]. The sonochemical technique can also provide an effective way to increase CNT surface functional sites during the oxidation treatment in acidic solution [71]. The sputter-deposition technique can provide low Pt loading and thinner catalyst layer [80]. Table 2.6 shows the summarization of synthesis methods for CNT-supported Pt electrocatalysts.

Table 2.6 Summarization of synthesis methods for CNT-supported Pt electrocatalysts Synthesis method Catalyst Mean metal particle Particle size Ref size (nm) range (nm) Impregnation Pt/MWNT - 3-5 [68] 2.6 2-5 [68] - 1-5 [66] 4.5 1-9 [84] Sonochemical Pt/MWNT 3.6 - [71] Technique 2.7 - [71] Microwave Pt/CNT 3.8 2.6 [72] Electrodeposition Pt/SWNT 5.3 4-6 [77] Sputter-deposition Pt/MWNT ~2 - [80] γ-irradiation Pt-Ru/SWNT 15 - [81] Self-regulated Pt/SWNT 1.8 1-3 [82] Colloid Pt/SWNH 2 - [83]

2.3.2 CNT Supports for PEMFC Catalysts Carbon nanotubes (CNTs) are being widely studied for electrochemical supercapacitor electrodes due to their unique properties and structure, which include high

24 surface area [85], high conductivity [86, 87], and outstanding chemical stability. The common catalysts in the present PEMFC technology are Pt and Pt–Ru, but they are expensive and insufficient to commercialize as resources. The efficient Pt loading is thus an important technique for the development of PEMFC. One of the possibilities is the application of carbon nanotubes as a support of Pt catalysts to reduce Pt loading [88]. The “CO poison” of Pt is another reason inhibiting the commercialization of PEMFC due to either strong adsorption of impurity CO in hydrogen fuel for PEMFC or production of linear intermediate CO during methanol dissociation for DMFC. To solve this problem, advanced electrocatalyst design relies on the “bifunctional approach”, in

which a second compound such as M or MxOy (M = Ru, Sn, Au, Ir. . .) assists the oxidation of CO or CHO species by adsorption of oxygen-containing species close to the poisoned Pt sites. In this case, CNTs serving as catalyst support is expected to make an enhancement of catalyst activity due to their high conductivity and large surface area. Wang et al. have firstly grown MWNTs on carbon paper through CVD with Co as a catalyst support and then subsequently electrodeposited Pt on MWNTs surface to act as PEMFC electrodes [78]. Tang et al. have used the well-aligned carbon nanotubes (Figure 2.12) growing on Ti substrate as catalyst support for DMFC electrode [89]. The cyclic voltammograms of methanol oxidation results imply that the Pt/aligned-CNT electrode has higher electrocatalytic activity for methanol oxidation than Pt/tangled-CNT and Pt/graphite electrodes at both low and high Pt loading. This may be attributed to that well-aligned CNT electrode can provide a high accessible surface area of Pt and good conductivity.

Figure 2.12 SEM image of the well-aligned carbon nanotube arrays on Ti substrate [89]

25 Xin et al.[10] have found that the MWNT indeed shows 30% less corrosion (oxidation) current under same condition by comparing the chronoamperometric curves of MWNT and Vulcan XC-72 (curve d and c in Figure 2.13).

Figure 2.13 Chronoamperometric curves for (a) 30wt% Pt/Vulcan XC-72, (b) 30wt% Pt/MWNT, (c) Vulcan XC-72, and (d) MWNT, measured at 0.9V and 60°C in N2 purged 0.5 M H2SO4 [10]

The morphology of the Pt before and after the oxidation treatment was examined by TEM (Figure 2.14). Specifically, for the Pt/Vulcan XC-72 before oxidation treatment, the Pt nanoparticles show a narrow size distribution with an average size of 2.5 nm (Figure 2.14 (a)). After 168 hr oxidation treatment, the particle size distribution becomes broader and the average size is around 5–6 nm (Figure 2.14 (b)). In contrast, for MWNT, Pt particle size barely shows any increase in most regions (Figures 2.14 (c) and (d)). However, some aggregation of Pt nanoparticles is indeed observed in some isolated areas on the MWNT surface (Figure 2.14 (d)) as opposed to the uniform aggregation in the case of Vulcan XC-72. The difference in aggregation phenomena in the case of Vulcan XC-72 and MWNT is attributed to the higher corrosion resistance of the MWNT [10].

Figure 2.14 TEM micrographs and histogram of Pt particle size distribution on (a) Pt/Vulcan XC-72 before durability test, (b) 30 wt% Pt/Vulcan XC-72 after durability test for 168 h, (c) Pt/MWNTs before durability test, and (d) 30 wt% Pt/MWNTs after durability test for 168 h. Inset of (d) shows a typical region [10]

The conductivity of the substrate is one of the most important parameters in selecting electrode substrates. Ng at el. has used SWNT paper as anode for lithium-ion battery [52]. Interestingly, they found that the conductivity was improved when 10 wt.% carbon black was added as electrode conductor (Figure 2.15). Furthermore, it can be seen that the conductivity decreased as the thickness of the carbon nanopaper increased for samples both with and without carbon black [52].

Figure 2.15 Electrical conductivity as a function of thickness for SWNT papers [52]

27 Jeng et al. has used CNT-supported high-metal-content electrocayalyst incorporated with the DMFC anode [90]. The in-house prepared CNT-supported Pt-Ru catalyst is illustrated in Figure 2.16 (a). After conducting the vacuum filtration, it can be seen that the catalyst layer on the anode formed a carbon nanotube paper or “buckypaper” type structure [91, 92] with the carbon nanotubes tangled together in bundles and oriented randomly forming a rather porous structure as shown in Figure 2.16 (b). Overall, the DMFC incorporated with the high-metal-content catalyst exhibited a much better performance. A power density of >100mWcm−2 can be obtained at 80oC. The electrochemical impedance spectroscopy also indicated that the anode incorporated with a high-metal-content catalyst exhibited a much lower resistance and the catalyst could be fully penetrated and utilized in the DMFC operations [90].

Figure 2.16 SEM images of (a) an in-house prepared high-metal-content Pt-Ru/CNT, and (b) a porous catalyst layer with a buckypaper texture on the anode using a vacuum filtration approach [90]

Table 2.7 shows the summarization of platinum particle size on CNT supports by electrodeposition method. Electrodeposited platinum catalysts on CNTs were tested by cyclic voltammetry and the results showed that platinum had smaller particle size and better electrocatalytic activity. This may be attributed to the unique structure and high surface area of carbon nanotubes and also suggests that CNTs have application potential as catalyst supports in fuel cell.

28 Table 2.7 Summarization of Pt particle size on CNT supports by electrodeposition Catalyst Pt particle Comment Reference: size (nm) Pt/CNT/graphite 60-80 nm CNTs grown directly on graphite He et al. [93] Pt/graphite 150 nm disk by CVD Pt/aligned-CNT 30-70 nm Well-aligned CNTs arrays (D:60 Tang et al. [89] Pt/tangled CNT/graphite 30-100 nm nm, L:3-4m) by CVD Pt/graphite 100-250 nm Pt-Ru/CNT/graphite 60-80 nm CNTs grown directly on graphite He et al. [79] substrate by CVD PtCo/MWNT/carbon 25 nm MWNTs grown directly on carbon Wang et al. [78] paper paper by CVD with electrodeposited cobalt Pt/CNT/carbon cloth 4.5-9.5nm CNTs grown directly on carbon Tsai et al. [94] Pt-Ru/CNT/carbon cloth 4.8-5.2nm cloth by CVD Pt/SWNT 5-10 nm SWNT paste electrode Wu et al. [95] Pt/MWNT 5-10 nm MWNT paste electrode

The exciting new materials, carbon nanotubes, which have unique structure and extraordinary electrical, thermal, and mechanical properties, have been demonstrated to be feasible as catalyst supports for PEMFC electrode. It is believed that CNTs is the very promising material to improve performance and reduce cost for the commercialization of PEM fuel cell.

2.4 Conclusion

PEM fuel cells are now attracting enormous interest for various applications. The high cost of platinum as catalyst is one of the challenges for commercialization of PEMFCs. People have tried to decrease the platinum loading via the increase of the utilization efficiency of platinum for years. The electrocatalytic activity of platinum particles is dependent on many factors, such as particle size and dispersion [96-98], preparation methods [99], and supporting materials [100], etc. Generally, the small particle size and high dispersion of Pt will result in high electrocatalytic activity. Simultaneously, reduction of the noble metal loading is

29 also necessary for the decrease of cost and generation of high power density [101]. Furthermore, the effect of the microstructure of the supporting materials on activity of the catalyst has been addressed [102]. Several papers have focused on the application of carbon nanotubes in fuel cell as catalyst supports and electrode materials [103-105]. Carbon nanotubes possess many exceptional properties, such as high electrical conductivity, high chemical stability as well as extremely high mechanical strength and modulus [106-108]. Both SWNTs and MWNTs have the ability to promote electron- transfer reactions when used as an electrode material in electrochemical reactions [106- 108]. Some results indicated that the CNTs support induced higher catalytic activity than the conventional Vulcan XC-72 carbon support. However, the reason why the CNTs support is more suitable for Pt particles toward high durability is still unclear. In the catalyst layer of a Pt-based conventional fuel cell prepared by the ink-process, there is still a significant portion of Pt being isolated from the external circuit and/or the PEM, resulting in a low Pt utilization [78]. Besides, a common problem has been that the necessary addition of Nafion for proton transport tends to isolate carbon particles in the catalyst layer, leading to poor electron transport. Due to their unique structural, mechanical, and electrical properties, buckypapers could replace traditional carbon powders as electrode in PEMFCs, and then subsequently electrodepositing Pt on the buckypaper showed the potential to improve the Pt utilization by securing the electronic route from Pt to the supporting electrode in a PEMFC.

CHAPTER 3 EXPERIMENTALS

3.1 Preparation of Mixed CNT Buckypapers

The MWNTs (I.D., 5-10 nm; O.D., 60-100 nm, length, 0.5-500 μm) used in experiments were produced by chemical vapor deposition (CVD) method from Sigma- Aldrich. The SWNTs used in the research were produced by Cabon Nanotechnologies Inc. (CNI). Based on CNI technical data, the individual tubes are around 0.8-1.2 nm in diameter and 100-1000 nm in length. They are purified SWNT products with a residual metal content of 3-12wt%. Carbon nanofiber (CNF) with diameter s of 100-200 nm and length in between 30-100m were produced by CVD from Applied Sciences Inc. (ASI). All materials were used without further purification. For comparison purpose, carbon black powders (Vulcan XC-72, FuelCellStore) were also used without any further treatment.

3.1.1 Fabrication Methodology The SWNT+MWNT, SWNT+CNF, and SWNT+CB mixed buckypaper were produced by multiple steps of dispersion and suspension filtration using a membrane of 90 mm in diameter (Figure 3.1). The CNT suspension manufacturing process is described as follows: starting materials (CNT, CNF, or CB, 80 mg in total weight) were ground with few drops of Triton-X 100 in mortars, and added some water to sonicate for 3 minutes. Triton-X 100 was added as surfactant to prevent nanoparticle re-aggregation. The 800 mg surfactant sonicated with 100 ml distilled water for 3 minutes; then it mixed with the sonicated material paste, and diluted to 200 ml with distilled water and sonicated for 10 minutes.

31 Then, distilled water was added to 500 ml, and then conducted 15 minutes sonication; added 1000 ml water and then another 15 minutes sonication. At last, it was evenly separated into two 1000 ml beakers, and 500 ml distilled water was added to each one and conducted 15 minutes sonication. The process was followed by vacuum filtering, with the suspension passing through a Nylon membrane (Millipore, 0.45 m in pore size) under full vacuum to form a buckypaper on the surface of the membrane. The formed buckypaper was washed with 100 ml isopropanal which was an effective method removed the residual surfactant. A thick CNT layer formed after dried and was ready to be peeled off from the membrane, producing a freestanding mate, or CNT buckypaper (BP).

Figure 3.1 Manufacturing process of CNT buckypaper

Table 3.1 shows the weight ratio information of all samples produced. At the start, the different weight ratios of mixed buckypapers were fabricated with the same SWNT batch, called SWNT 86. Other mixed buckypapers using another batch SWNT, called SWNT 89 for higher weight ratio of MWNT were fabricated as well as the pure MWNT 90 buckypaper. Besides, for comparisons, mixed buckypaper of SWNT batch # 335 (high purified SWNTs) and MWNT 72, and double-layer mixed buckypapers (1200 ml SWNT 89+MWNT 72 suspension followed with 800 ml MWNT 72 suspension) were also fabricated.

32 Table 3.1 The weight ratio information of all buckypaper samples Pure 3:2 7:1 3:1 1:1 1:3 1:5 1:7 SWNT86 ● SWNT86+MWNT72 ● ● ● SWNT86+CNF19 ● ● ● SWNT86+Vulcan XC72 ● ● ● SWNT89 ● SWNT89+MWNT72 ● ● ● SWNT89+CNF19 ● MWNT90 ● SWNT335+MWNT72 ● SWNT89+MWNT72:MWNT72 ●

3.1.2 Characterization Methods The purity of starting carbon material was determined by the thermogravimetric analysis (TGA). For the electrical conductivity of mixed buckypapers, each sample was tested by four-probe electrical resistance (ER) measurement. BET surface areas of the samples have been tested by Tristar 3000 from Micrometritics Analytical Services. The nanostructures of mixed buckypapers were characterized using a scanning electron microscope (SEM) (JEOL JSM 7401F). The functionalization degrees on buckypaper were tested by Fourier transform spectroscopy (FT-IR) and Raman spectroscopy.

3.2 Preparation of Pt/BP Electrodes

3.2.1 Electrodepositions of Pt Nanoparticles The mixed buckypapers were washed with iso-propanol to remove the most of surfactant before preparing Pt/BP electrodes. Surface activation of buckypaper by electro-oxidation or acid treatment was omitted, because it did not display any improvement of ECSA after electro-oxidation (EO) by 10 and 50 cycles in our preliminary studies, as shown in Figure 3.2.

Figure 3.2 Cyclic voltammograms on the BPs with/ without electro-oxidation in the potential range -0.25 ~ 1.0 V vs. SCE (EO: electro-oxidation)

For the synthesis of CNT-supported Pt catalysts, several catalyst synthesis methods such as impregnation method, sonochemical technique, microwave heated polyol process, electrodeposition, sputter-deposition technique, and others have been mentioned in the literature review. However, the first three techniques are easy to destroy buckypaper badly under stirring in solutions. In this research, we adopt electrodeposition method to do Pt deposition on buckypaper surface. Figure 3.3 shows the preparation process of Pt/BP electrodes. Using the tube furnace, the surfactant of buckypapers was further burned out in nitrogen at 400oC for two hours for all samples. For electrodepostion method, a three-electrode cell was setup with saturated calomel electrode (SCE) as reference electrode and platinum foil as counter electrode as shown in Figure 3.4. The working electrode was a piece of buckypaper mated with a Pt support where Parafilm was used to hold them together to ensure the electrical contact. The platinum was electrochemically deposited on buckypaper by a pulse potential (0.2 V for 3s and -0.25 V for 1s) in a solution of 10 mM H2PtCl6, 0.1 M H2SO4, and 0.5M ethylene glycol (EG). The concept of Pt deposited on CNT is shown in Figure 3.5. The amount of Pt loading was determined by weighing the mass increase after deposition.

Figure 3.3 Preparation process of Pt/BP electrodes and cyclic voltammetry (CV) test

Figure 3.4 Three electrode cell

Figure 3.5 Mechanism of Pt deposition on CNT

3.2.2 Electrochemical and Structural Characterization of the Pt/ BP Samples Pt electrochemical surface area was determined by measuring the area under the

hydrogen adsorption/desorption peaks of CV curve in 0.5 M H2SO4 (eliminating the charge for the double layer, Figure 3.6). The Pt specific surface area, which represents the Pt surface area per unit amount of Pt, was calculated from the Pt surface area and the total deposition amount of Pt, which was calculated from the Pt deposition charge. Voltammetric and amperometric measurements were performed using a Solartron 1287 electrochemical interface in a three-electrode arrangement in Figure 3.7. Silver chloride electrode was used as reference electrode and platinum gauze as counter electrode. To prepare working electrode, a piece of Pt/BP electrode sample was stick on the top of glassy carbon electrode (0.07 cm2) by a drop of 0.5% Nafion solution. The electrolyte solution is 0.5 M H2SO4, which was thoroughly deaerated by bubbling N2 during 30 minutes and a N2 atmosphere was kept over the solution during the test. The potential ranged from -0.3V to +1.0 V at a scan rate of 50 mV/min.

Figure 3.6 Hydrogen adsorption/ desorption peaks of CV (Q: the charge for hydrogen adsorption (mC/cm2))

Figure 3.7 Three-electrode arrangement of CV test

For comparison, a paste electrode of Pt/CB was prepared to be applied to a glassy carbon rotating disc electrode and air-dried for 1 minute at 80oC and test by CV at the same experimental condition. All experiments were carried out at ambient temperature. All the potentials of electrodeposition were referred to saturated calomel electrode (SCE) and all the potentials of CV test were referred to Ag/AgCl. The morphologies and structures of the specimens were investigated by scanning electron microscopy (SEM) and X-ray diffractometer (XRD) using a CuKα source (= 1.542 Å).

CHAPTER 4 RESULTS AND DISCUSSION

4.1 Surface Morphology of Pt/Mixed BP Electrodes

4.1.1 SEM Analysis of the Specimens The microstructures of the specimens have been investigated by SEM. The results are presented in Figure 4.1, 4.3, 4.4, and 4.6, respectively. Well-dispersed Pt nanoparticles were observed on the buckypapers with a diameter of about 10 nm. Figure 4.1 shows the SEM micrographs and the measured particle size distributions of Pt catalyst deposited on the two batches of SWNT 86 and SWNT 89 buckypapers. It can be seen that the Pt particle size of the SWNT 86 is kind of larger than that of the SWNT 89. Furthermore, from the FT-IR spectrum in Figure 4.2 (a), we can see the slight carboxyl group shown on the curve of SWNT 86 buckypaper. FT-IR tests before and after surfactant burned did not exhibit the obvious functional groups on the surface of buckypaper. From the Raman spectrum in Figure 4.2 (b), the ratio of ID/IG in SWNT 86 buckypaper before and after surfactant burned is obviously larger than that in SWNT 89 buckypaper respectively. It means that there are more defects on the surface of SWNT 86 buckypaper. In addition, it was also observed that the ratio of ID/IG is slightly down after surfactant burned of either SWNT 86 or SWNT 89. Figure 4.3 and 4.4 are the SEM micrographs of three different weight ratios (3:1, 1:1, and 1:3) of SWNT86+MWNT72 and SWNT86+CNF19 buckypapers, and their Pt particle size distribution, respectively. It can be seen that the Pt particle size is slightly getting smaller when the weight ratio of SWNT decreases. Besides, it can be also seen that more Pt nanoparticles were deposited on SWNTs instead of MWNTs or CNFs in Figure 4.3 (c) and 4.4 (c).

Figure 4.1 SEM micrographs of (a) Pt/SWNT86 (S86), (b) Pt/SWNT89 (S89), the measured distributions of particle size of (c) Pt/SWNT86, and (d) Pt/SWNT89

Figure 4.2 (a) FT-IR spectrum, (b) Raman spectrum of SWNT 86 and SWNT 89 buckypaper before and after burned surfactant (b: surfactant burned)

Figure 4.3 SEM micrographs of (a) Pt/SM (3:1), (b) Pt/SM (1:1), and (c) Pt/SM (1:3); the distributions of particle size of (d) Pt/SM (3:1), (e) Pt/SM (1:1), and (f) Pt/SM (1:3) shown in the micrographs (S: SWNT86, and M: MWNT72)

Figure 4.4 SEM micrographs of (a) Pt/SF (3:1), (b) Pt/SF (1:1), and (c) Pt/SF (1:3); the distributions of particle size of (d) Pt/SF (3:1), (e) Pt/SF (1:1), and (f) Pt/SF (1:3) shown in SEM micrographs (S: SWNT86, and F: CNF19)

The possible reason that resulted in more Pt particles deposited on SWNTs may be the metal exchanged. In other words, some irons left in SWNTs were replaced by some platinic ions in the electrolyte.

41 From the thermogravimetric analysis (TGA) of SWNT86, SWNT89, MWNT90, and CNF19 shown in Figure 4.5 respectively, it exhibits that the content of residual metal in SWNT is about 10-15 % which is much higher than that in MWNT or CNF. In other words, Pt may have more anchoring sites of the residual metal on SWNT. This may explain the reason why more Pt particles were deposited on the SWNTs.

Figure 4.5 Thermogravimetric Analysis (TGA) of SWNT86, SWNT89, MWNT90, and CNF19

In order to observe the Pt particles deposition phenomenon, Pt deposited on the high purity of SWNT 335 mixed with MWNT 72 mixed buckypaper has been investigated in the Figure 4.6 (a). It can be seen that slightly larger Pt particle size was formed on Pt/S335M72 than that on Pt/S89M72. Besides, it can be observed that little lower deposition density on the Pt/S335M72 as well.

Figure 4.6 SEM micrographs of (a) Pt/S335M72 (1:3), and (b) Pt/S89M72 (1:3); the distributions of particle size of (c) Pt/ S335M72 (1:3), and (d) Pt/S89M72 (1:3)

Moreover, the Pt particle size on MWNT 90 buckypaper (~20nm) is much larger than that of other specimens shown in Figure 4.7 (a) and (d). It can be explained that the residual metal in MWNT (~1%) is lower in Figure 4.5, resulting in less anchoring sites for Pt deposition without electro-oxidation. The Pt particle sizes on Pt/SWNT89+MWNT72 (1:5) and Pt/SWNT89+MWNT72 (1:7) mixed buckypapers in Figure 4.7 (b) and (c) are slightly larger than that of Pt/SWNT89+MWNT72 (1:3) mixed buckypaper in Figure 4.3 (c), the possible reason is that the weight ratio of SWNT is too less to offer enough anchoring sites for Pt deposition without electro-oxidation.

Figure 4.7 SEM micrographs of (a) Pt/MWNT90, (b) Pt/S89M72 (1:5), and (c) Pt/S89M72 (1:7); the distributions of particle size of (d) Pt/MWNT90, (e) Pt/S89M72 (1:5), and (f) Pt/S89M72 (1:7) shown in SEM micrographs

The motivation of mixing Vulcan XC-72 with SWNT in buckypaper is to increase contact in the network. However, the particle size of carbon black seems too big (~30 nm) in the buckypaper. In addition, the Vulcan XC-72 particles were agglomerated from Figure 4.8, instead of well dispersion in SWNT as we expected.

Figure 4.8 SEM micrographs of (a) Pt/SCB (7:1), (b) Pt/SCB (3:1), and (c) Pt/SCB (1:1) (S: SWNT86, and CB: Vulcan XC-72)

4.1.2 XRD Analysis of the Specimens Figure 4.9 shows the XRD patterns of Pt deposited on the mixed buckypapers. The

Debye-Scherrer formula is used to represent the size of the Pt crystallites (Dp). is the wavelength of the radiation Cu Kα = 1.542 Å, 1/2(degree) means FWHM (full width at half maximum), and θ (degree) is the position of the maximum of diffraction. The

calculated and estimated sizes of the Pt crystallites (Dp) were shown in Table 4.1. The sizes of Pt crystallites of SWNT86+MWNT72 (1:3) and SWNT86+CNF19 (1:3) buckypaper were smaller than those of other weight ratios of mixed buckypapers. The same results also can be observed in the SEM images (Figure 4.3 and 4.4).

45 .0 94λ Debye − Scherrer formula : Dp (nm) = (4) β 1 cosθ 2

: Wavelength of the radiation Cu Kα = 1.542 Å 1/2(degree): FWHM (full width at half maximum) θ (degree): The position of the maximum of diffraction

Figure 4.9 Powder X-ray diffraction (XRD) patterns of Pt/mixed BPs electrodes

Table 4.1 Pt Particle size calculated by Debye-Scherrer formula Types of Mixed BPs Dp (nm) SWNT86 9.3 SWNT86+MWNT72 (1:3) 5.9 SWNT86+CNF19 (1:3) 6.5 SWNT86+CB (1:1) 7.4 SWNT86+CB (3:1) 10.2

46 4.2 Electrocatalytic Properties of Pt/Mixed BP Electrodes

4.2.1 Pt Specific Surface Area Figure 4.10 shows the cyclic voltammograms (CV) of Pt deposited on mixed buckypaper in the potential range of -0.25 ~ 1.0 V vs. Ag/AgCl. Obviously, the area of hydrogen adsorption peak of the CV measurements is getting larger when the weight ratio of SWNT decreases. Besides, the double layer charge is thicker when the weight ratio of SWNT increases. The Pt electrochemical surface area (ECSA) determined by measuring the area under the hydrogen adsorption/desorption peaks of the CV curves in

0.5 M H2SO4 :

(5)

Where [Pt]: the platinum loading (mg/cm2) in the electrode Q: the charge for hydrogen adsorption (mC/cm2) 0.21: the charge required to oxidize a monolayer of H2 on bright Pt

Figure 4.11 shows the summary of the electrochemical surface area (ECSA) of the Pt/mixed buckypaper electrodes. The mean ECSA of commercial Pt/CB tested by the same experimental condition is 27.3 m2g-1. The ECSA value of Pt/SWNT89 is four times higher than that of Pt/SWNT86, and a little higher than that of Pt/MWNT90. Furthermore, when the weight ratio changes from 3:1 to 1:3, the ECSA value on either Pt/SWNT86+MWNT72 or Pt/SWNT86+CNF19 is gradually moving up. Besides, the ECSA value of Pt/SWNT86+Vulcan XC-72 is also increasing from weight ratio 7:1 to 1:1. Due to the much higher ECSA value of Pt/SWNT89 than that of Pt/SWNT86, three different weight ratios of Pt/SWNT89+MWNT72 buckypapers were investigated.

Figure 4.10 Cyclic voltammograms on Pt/mixed BP in the potential range -0.25 ~ 1.0 V vs. Ag/AgCl

Figure 4.11 Summary of the electrochemical surface area (ECSA) of the Pt/Mixed BP Electrodes

48 At the weight ratio of 1:3, the ECSA values of Pt/SWNT89+MWNT72 or Pt/SWNT89+CNF19 are higher than those of Pt/SWNT86+MWNT72 or Pt/SWNT86+CNF19. However, the ECSA values of Pt/SWNT89+MWNT72 are starting going down from weight ratio of 1:5 and 1:7. Pt/SWNT+MWNT (1:3) buckypapers have been exhibited higher electrocatalytic activity, and the highest ECSA of Pt/SWNT89+MWNT72 (1:3) reaches 43.7 m2/g which is about 1.6 times higher than that of Pt/CB electrode. After observing the SEM micrographs of Pt/SWNT89+MWNT72 (1:3) in Figure 4.12, it has been noticed the different microstructures of the top and bottom sides of the mixed buckypaper. The bottom side of the buckypaper means the side that was peeled from membrane in Figure 4.13; another side is the top side of buckypaper. It showed that more SWNTs were deposited and entangled on the top side of mixed buckypaper (1:3) in Figure 4.12 (a); on the contrary, more MWNTs were on the bottom side seen in Figure 4.12 (b). In other words, more Pt catalysts were deposited on the top side of mixed buckypaper (1:3). It may be explained the reason why Pt/SWNT+MWNT (1:3) buckypapers have been exhibited higher electrocatalytic activity.

Figure 4.12 SEM micrographs of SWNT89+MWNT72 buckypaper (1:3) on (a) top side, (b) bottom side,

Figure 4.13 SEM micrographs of SWNT89+MWNT72 buckypaper (1:3) on (a) top side, (b) bottom side, and (c) scheme of the SWNT89+MWNT72 (1:3) buckypaper

Moreover, in order to observe the microstructure of the top and bottom sides of the mixed buckypaper, the SEM micrographs of the different SWNT ratios (SWNT89+MWNT72, weight ratio of 1:5) samples were investigated. At first, a drop of suspension dripped on the membrane in Figure 4.14 (a). A few SWNTs formed a small part of network on the membrane, and some SWNTs adhered to the membrane firmly. Also, the pore size of the membrane (0.45) compared to SWNT is pretty large, and some SWNTs may flew away through the pores. However, the MWNTs were stuck in and across the pores of the membrane. After filtrating suspension of 50ml in Figure 4.14 (b), more SWNTs entwined and adhered to the structure of the membrane tightly, and start to form the network base which entangles with MWNTs. The pore structure of membrane is covered by the entangled network of SWNT and MWNT after filtrating 300 ml suspension in Figure 4.14 (c). The complete and well mixed network was formed after filtrating suspension of 1000 ml in Figure 4.14 (d). By inspecting the deposition process, we know that the SWNTs and MWNTs were entangled and deposited gradually by vacuum filtration, instead of being deposited as two obvious layers which SWNTs are on the top and MWNTs are on the bottom.

Figure 4.14 SEM micrographs of the different contents: (a) a drop, (b) 50 ml, (c) 300 ml, (d) 1000 ml of suspension (SWNT89+MWNT72, weight ratio 1:5) filtrating through the membrane

In order to look for the possible reason of the different top and bottom structures of SWNT89+MWNT72 buckypaper (1:3), we also observed the membrane after buckypaper was peeled. At the beginning of suspension filtrating through the membrane, it has been observed that the SWNTs entwined and adhered to the structure of the membrane tightly in Figure 4.14 (a) and (b). This may make the first deposited layer of the most SWNTs still remain on the membrane after buckypaper was peeled shown in Figure 4.15 (a). However, it presents that MWNTs was torn and lead to the fluffy structure (Figure 4.15 (b)).

Figure 4.15 SEM micrographs of the membranes of (a) SWNT89+MWNT72 (1:7), (b) MWNT90 after buckypaper was peeled

Therefore, the morphology of the mixed buckypaper bottom side after peeling from membrane and the loss of SWNTs at the filtration beginning may be the reasons that cause the microstructure differences in top and bottom sides of the SWNT89+MWNT72 buckypaper (1:3). However, the ECSA values of Pt/SWNT89+MWNT72 with weight ratio of 1:5 and 1:7 are not improving or even keeping the same value as the Pt/SWNT89+MWNT72 (1:3), but presenting the trend of going down. The first possible reason is that it could not form the apparent top and bottom structure like SWNT89+MWNT72 (1:3) when the content of SWNTs is getting less. Besides, it may be insufficient anchoring sites for Pt deposition which results in the larger Pt particle size if the content of SWNT is too less and the sample without surface activation in Figure 4.7 (b) and (c).

4.2.2 Pt Utilization In the conventional electrodes, Pt utilization in current commercially offered prototype fuel cells remains very low (20-30%) [78]. Based on the theoretical specific surface area of platinum under assuming platinum as a sphere. Therefore, Pt utilization (%) is defined and calculated as ECSA divided by the theoretical specific surface area of platinum.

52 Theoretical specific surface area (m2 / g)

S S 4π R2 3 6 = = = = = (6) 4 3 m ρ ⋅V ρ ⋅ 3 π R ρ ⋅ R ρ ⋅ D ECSA Pt Utilization(%) = (7) S / m Assume: platinum sphere S: surface area of platinum sphere m: mass of platinum particle 3 ρ: density of platinum (ρPt=21.1 g/cm ) R: radius of platinum particle D: diameter of platinum particle

Figure 4.16 shows the summary of Pt utilization of the specimens. The mean Pt utilization of the Pt/CB is about 21%, and the most BP samples display much higher Pt utilization.

Figure 4.16 Utilization of Pt/CB and Pt/ Mixed BPs

53 4.3 Effects of Mixed BPs Nanostructure on Pt Deposition

Figure 4.17 shows the relation of the electrical conductivity and Pt loading. It can be seen that the conductivity is increasing when the weight ratio of SWNT rises for these three different compositions (SWNT+MWNT, SWNT+CNF, and SWNT+Vulcan XC- 72). At the same time, Pt loading would also be going up when the conductivity increases. It could be explained that the deposition current increases if conductivity of buckypaper is larger which results in rising up Pt loading.

Figure 4.17 Electrical Conductivity of buckypaper vs. Pt Loading

4.4 Cost Analysis

Table 4.2 shows the present material costs. The cost of Vulcan XC-72 is much cheaper than that of carbon nanotubes. The cost of CNF is just three times higher than that of Vulcan XC-72. The cost of platinum in this year is $35/g.

54 Table 4.2 Material cost analysis Types of Material Cost ($/g) SWNT 500 MWNT 7 CNF 0.21 Vulcan XC-72 0.07 Pt 35

The theoretical Pt loading were calculated under the same electrode chemical surface area of Pt per unit electrode area in Table 4.3. The estimated Pt cost and total material cost of Pt/CB and Pt/SWNT89+MWNT72 samples with three different weight ratios under assuming total weight of material composition as one gram was shown in Figure 4.18. Even though the Pt cost of Pt/SWNT89+MWNT72 is lower than that of Pt/CB, however, the total material cost of Pt/SWNT89+MWNT72 is quite higher than that of Pt/CB. It could not save cost or gain any profit of using Pt/SWNT89+MWNT72 for the time being.

Table 4.3 Theoretical Pt loadings of Pt/S89M72 BP Pt loading (mg/cm2) ECSA (m2/g) Pt/C 0.20 27.83 Pt/S89M72 BP (1:3) 0.13 43.70 Pt/S89M72 BP (1:5) 0.17 32.20 Pt/S89M72 BP (1:7) 0.19 29.80 (Under same electrode chemical surface area of Pt per unit electrode area)

However, platinum is a precious and extremely rare metal. From the collected data of Pt price in the past five years in Figure 4.19, the price of Pt was exhibited a sharply the increasing trend [109]. In addition, according to the nanotube production survey, it predicted that the annual growth rate and output value will be over 60% from 2004 to 2010, and carbon nanotubes price will decrease by a factor 10-100 in the next 5 years, respectively [110].

Figure 4.18 Estimated Pt cost and total material cost of Pt/C and three different weight ratios of Pt/SWNT89+MWNT72 under assuming total weight of material composition is 1 gram

1200 1200

1050

900 899.51 848.76 750 694.44 600 542.56

Pt price (dollars troy ounce) troy (dollars Pt price 450

300 2001 2002 2003 2004 2005 2006 (Year)

Figure 4.19 Pt price from 2002 to 2006 [109]

56 Based on the collected Pt price in past five years and nanotube production survey, it assumes that the Pt price increases $150/ounce per year on account of the mean growing value as $150/ounce, and carbon nanotubes (CNTs) will be decrease by a factor 10 in 2010 and by a factor 100 in 2020. The prediction of the material costs in 2010 and 2020 is shown in Table 4.4. Therefore, the estimated total material cost of Pt/CB and Pt/SWNT89+MWNT72 in 2010 and 2020 were calculated in Figure 4.20. It is shown that the total material cost of Pt/CB is almost the same as that of Pt/SWNT89+MWNT72 in 2010. In a long term aspect, it will benefit to invest nanotube buckypaper electrodes.

Table 4.4 Prediction of material cost in 2010 and 2020 2007 2010 2020 SWNT ($/g) 500 50 (10 times↓) 5 (100 times↓) MWNT ($/g) 7 0.7 (10 times↓) Vulcan XC72 ($/g) 0.07 Pt ($/g) 35 60 (1.7 times↑) 110 (3.1 times↑)

140

120

100 Pt/C S89M72(1:3) 80 S89M72(1:5) S89M72(1:7) 60

Total material cost ($) 20

0 2005 2010 2015 2020 Year

Figure 4.20 Estimated total material cost of Pt/CB and Pt/SWNT89+MWNT72 in 2010 and 2020

57 4.5 Optimum Electrode Microstructure for PEMFC

Figure 4.21 shows the optimum electrode microstructure for PEMFC. The metallic catalyst (Pt) can be deposited as a thin monolayer of 1-2μ on support, resulting in better electrochemical reactions. Another side of larger pore size from electrode is easier for gas to be passing through shown in Figure 4.21.

Figure 4.21 Optimum electrode microstructure for PEMFC

Due to the high electrocatalytic activity and minimization of the precious metallic catalyst (Pt) usage on the SWNT+MWNT buckypaper (1:3) electrode, it is interesting to understand its microstructure effect. It is showed that the buckypaper electrode top and bottom surfaces are different, as shown in Figure 4.13. It has been discussed that the morphology of the mixed buckypaper bottom side after peeling from membrane and loss of SWNTs at the beginning filtration are the possible reasons for forming this unique electrode structures as shown in Figure 4.22.

Figure 4.22 Micro-surface phenomenon of SWNT+MWNT mixed BP (1:3)

In order to further observe nanostructure effect, a double-layer mixed buckypaper electrode was fabricated and tested. The double-layer mixed buckypaper comprised of SWNT89+MWNT72 (1:3) by filtrating 1200 ml suspension first shown Figure 23 (a). And then, 800 ml MWNT 72 suspension was filtered shown in Figure 23 (b). The ECSA value of this double-layer mixed buckypaper electrode is 36.59 m2g-1. It is slightly higher than that of the Pt/SWNT89MWNT72 (1:5) with lower content of SWNT in Figure 4.24. In other words, it can save more cost but reaches almost the same ECSA value when it compared to Pt/S89M72 (1:3) shown in Figure 4.25. Based on the high electrocatalytic activity of the double-layer microstructure of the SWNT+MWNT mixed buckypaper, an optimal microstructure of nanotube electrode for PEMFC can be proposed. Therefore, the microstructure of SWNT+MWNT may be an efficient electrode material for the electrochemical reactions, comparing to conventional Pt/CB electrodes.

Figure 4.23 SEM micrographs of Pt/SWNT89MWNT72 (1:3) 1200 ml with MWNT72 800 ml on (a) bottom side and (b) top side

Figure 4.24 ECSA comparisons of Pt/S89+M72 BP with double layer BP

Figure 4.25 Cost comparisons of Pt/S89+M72 BP with double layer BP

CHAPTER 5 CONCLUSIONS

The dispersion and electrocatalytic properties of platinum on mixed buckypapers were investigated. Nano-sized (~10 nm) catalysts of Pt particles were successfully electrodeposited on the surface of the mixed buckypapers by electrdeposition in mixed ethylene glycol, H2PtCl6, and H2SO4 aqueous solution. The electrocatalytic activities of the Pt/ mixed buckypapers were measured by cyclic voltammetry. The results show that the Pt/SWNT+MWNT buckypaper electrode exhibits higher electrocatalytic activity than that of the Pt/CB samples. The highest ECSA of the Pt/SWNT+MWNT (1:3) electrode reaches 43.7m2/g, which is about 1.6 times higher than that of the Pt/CB electrode. Pt utilization of the Pt/buckypaper electrodes were increased up to 70% compared to that of the Pt/CB samples. This may be attributed to small size and good dispersion of Pt particles, good conductivity properties of carbon nanotube network, effective deposition process, and unique microstructure of the Pt/mixed BP electrodes. Particularly, the gradient microstructures of the Pt/mixed BP electrodes provided a more optimized structure for reactions in fuel cell. The small pore and high Pt concentration on the reaction surface with more SWNTs lead to high Pt utilization and the opposite surface consisted of more MWNTs or CNFs to form large pores to facilitate gas transport. The results show that the electrodeposition of Pt nanoparticles on mixed SWNT+MWNT buckypaper by applying potential pulse provides an efficient way to prepare good performance electrodes. The Pt/SWNT+MWNT electrode is potentially a novel electrode material for applications in fuel cells. Further research into the optimization of Pt deposition and microstructure of the mixed buckypapers can lead to produce highly efficient and potentially affordable electrodes for fuel cell commercialization.

CHAPTER 6 FUTURE WORKS

Due to the micro-surface phenomenon on top and bottom side of SWNT+MWNT buckypaper (1:3), it reaches high electrocatalytic activity and minimization of the precious metallic catalyst (Pt). However, the possible reason of insufficient content of SWNT in SWNT+MWNT buckypaper (1:5) and (1:7) results in forming the unobvious top and bottom deposition nanostructure. It can be attempted to filtrate membrane on the first layer of SWNT by beginning of SWNT suspension and then the second layer of MWNT by following MWNT suspension to form the two-layer deposition of SWNT+MWNT buckypaper in weight ratio of 1:5 and 1:7 for using as the catalyst support in fuel cells. Even though the MWNT buckypaper did not show high electrocatalytic performance, it still can be seen that MWNT buckypaper is looser and larger pore size than those of SWNT from SEM micrographs. The larger Pt particles formed on MWNT buckypaper has been observed, and it may be attributed to the insufficient anchoring sites for Pt deposition by using high purity of MWNT. The electro-oxidation or acid treatment is suggested for MWNT buckypaper or high weight ratio of MWNT mixed buckypaper before Pt electrodepositon. . Further optimization of the Pt deposition and Pt nanoparticle formation on the mixed buckypapers can be expected to generate highly efficient electrocatalyst for further applications in the field of fuel cells.

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