Integrated platinum carbon nanotube based electrocatalyst for high efficiency proton exchange membrane fuel cells 1

Chapter Introduction 1.1 Background Fuel cells are deemed as one of the most important alternative power sources utilizing sustainable energy to replace conventional combustion generators. A fuel cell is an energy conversion device that generates electricity directly from an electrochemical reaction, in which oxygen (or air) and fuels (e.g. hydrogen) combine to form water [1, 2]. Fuel cells differ from ordinary batteries in that they are able to continuously produce electricity as long as fuels are supplied. Moreover, fuel cells convert fuels directly into electricity via an electrochemical process that does not require fuel combustion. Therefore fuel cells are intrinsically more efficient and environment friendly than combustion engines [1, 2]. 1.2 Main Types of Fuel Cells In the last two decades of the 20th Century, fuel cell technologies have achieved significant breakthrough in their applications when the world is facing the shortage of fossil fuels, coal and oil [2]. Owing to the tremendous research effort on fuel cell technologies, several types of fuel cells have been developed for a variety of applications to meet future energy demand, including transportation, stationary, and portable electronic devices. In general, fuel cells can be categorized into five main types according to the specific electrolyte used. The five main types of fuel cells are: proton exchange membrane fuel cells (PEMFC), alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC) [1, 2].   The different characteristics of their respective electrolytes used in the cell configuration determine the particular materials and fuels required for them, as well as their unique properties and applications. Table 1.1 demonstrates the main types of fuel cells and their characteristics and applications [1]. Among the five types of fuel cells, PEMFCs have the most promising potential for domestic uses and other applications in small-scale power generations due to their relatively low operating temperature and compact structure [1, 2]. Fuel Cell Type Proton Exchange Membrane FC (PEMFC) Alkaline FC (AFC) Electrolyte Solid polymer KOH Phosphoric Phosphoric Acid FC acid (PAFC) Molten Carbonate FC (MCFC) Solid Oxide FC (SOFC) Lithium & potassium carbonate Solid oxide Operating Temperatu re Fuel 50−100 °C Pure H2 60−120 °C Pure H2 ~220 °C Pure H2 Electric Applications Efficiency 35−45% Vehicles and portable applications, lower power station. 35−55% Used in military & space vehicles. ~40% Suitable for 200 kW power stations. ~ 650 °C H2, CO, CH4, other hydrocarb on >50% ~1000 °C H2, CO, CH4, other hydrocarb on >50% Suitable for medium to large-scale power stations, up to MW capacity. Suitable for all sizes of power stations, kW to multi-MW. Table 1.1 Main types of fuel cells and their characteristics and applications   1.3 Proton Exchange Membrane Fuel Cells (PEMFCs) Proton Exchange Membrane Fuel Cells are also known as polymer electrolyte membrane fuel cells or polymer electrolyte fuel cells (PEFCs). Unlike other types of fuel cells, a typical PEMFC uses a thin solid polymer membrane as its electrolyte. This membrane is not only an electronic insulator but also an excellent conductor for hydrogen ions, i.e. protons. Owing to its thin and solid properties, the polymer membrane is able to greatly diminish the electrochemical corrosion of electrodes and improve the power densities compared to liquid electrolytes. Furthermore, as it needs water for proton conduction, a PEMFC system usually operates at a relatively low temperature of about 80oC, which allows rapid start-ups of the system from ambient temperature. These distinct advantages of PEMFCs make them particularly suitable for automotive and portable applications. 1.3.1 History of PEMFCs The history of fuel cells can date back to one and a half centuries ago when William R. Grove invented the first fuel cell in 1839 [1]. His setup, so called “gaseous voltaic battery”, included two platinum electrodes covered with inverted tubes that were halfway submerged in a beaker of aqueous sulfuric acid. One of the tubes was filled with hydrogen gas and the other was filled with oxygen. When these electrodes were immersed in a dilute sulfuric acid, a current began to flow between the two electrodes and water was formed in the inverted tubes. In order to increase the voltage produced, Grove linked several of these devices in series and produced a “gas battery”. The name of “fuel cell” had not been introduced until 1889 the chemists Ludwig Mond and Charles Langer used platinum black supported on platinum or gold electrodes as catalyst and introduced a diaphragm to contain the electrolyte between   the electrodes [1]. Historically, the first major application of PEMFCs was to serve as an auxiliary power source in NASA’s Gemini space flights in the 1960s [1]. However, the early version of PEMFCs with a polystyrene sulfonate ion-exchange membrane as electrolyte was unable to give adequate power densities and required operational lifetime. Thereafter the development of this technology remain stagnant for more than ten years until the polystyrene sulfonic acid membrane was replaced by Du Pont’s perflourosulfonic acid membrane (Nafion®) in the 1970s [1]. The utilization of Nafion membrane in PEMFCs boosted the power densities by ten times and the lifetime from two thousand hours to one hundred thousand hours. In the late 1980s and early 1990s, another technical breakthrough was achieved via the 10-fold reduction of platinum loading in PEMFC electrodes. This achievement was first realized by using high surface area carbon particles as platinum support instead of using pure Pt black as electrocatalyst, and also by impregnating a small amount of Nafion ionomers into the catalyst layer of the porous gas diffusion electrode in the Gemini fuel cells [1]. 1.3.2 Structure of PEMFCs As shown in Fig. 1.1 [3], a typical single PEMFC usually consists of four main components: tow current collectors, two gas diffusion layers, two catalyst layers and a solid polymer electrolyte membrane.   (a)   (b) Fig. 1.1 Schematic diagrams of (a) a single PEMFC and (b) a 3-cell PEMFC stack [3]. Current Collectors Current collectors, which are placed at two cell ends to collect current, are used to separate reactant gases, and provide mechanical support and gas flow channels (see Fig. 1.1) [4]. They are also called bipolar plates in multi-cell stacks when each plate is electrically connecting the anode of one cell to the cathode of the adjacent cell. A desirable material for current collector must be electrically and thermally conductive, as well as impermeable to gases. It should also have high resistance to the reactant   gases to avoid corrosion. Presently graphite and stainless steel are the most commonly used materials for current collectors, which provide both high electronic conductivity and corrosion resistance. Current research on current collectors has been focusing on developing novel materials with superior corrosion resistance and lower cost.  Gas Diffusion Layers (GDLs) Gas diffusion layers are a part of a PEMFC electrode and they are in contact with current collectors. They usually consist of a macroporous backing layer and a microporous gas diffusion layer [4]. The macroporous backing layer is either carbon cloth or carbon paper, with thickness ranging from 100 to 300 μm. And the microporous gas diffusion layer is usually composed of nanosized carbon blacks spread on the backing layer. These high porosity media can provide mechanical support as well as pathways for electrons, gas and water in PEMFC electrodes. When reactant gases flow out from the channels in current collectors and reach the gas diffusion layer, they are evenly distributed by the porous structure before reacting in the catalyst layer. In addition, the carbon backing layer is the electronic connection between current collector and electrode. Furthermore, GDLs also provide hydrophobicity for electrodes to facilitate liquid water removal by impregnating Polytetrafluoroethylene (PTFE) into them. Hence the gas diffusion layer is a critical factor in electrode structure design to ensure efficient mass transport in PEMFC electrodes. Catalyst Layers (CLs) Catalyst layers are attached to the microporous gas diffusion layers. They are the most essential part of a PEMFC where the electrochemical reactions take place [4].   The electrochemical reactions in a PEMFC consist of two separate reactions: hydrogen oxidation reaction (HOR) occurring at anode and oxygen reduction reaction (ORR) at cathode. At anode catalyst layer, gaseous hydrogen splits into two protons and the protons then pass through the electrolyte membrane to reach the cathode. While at cathode catalyst layer, oxygen combines with these protons from anode and electrons from external circuit to form water and excess heat. Typically, these two half-reactions would take place very slowly at PEMFC operating temperature. Therefore electrocatalysts are necessary on both anode and cathode to increase the reaction rate of each half-reaction. The best electrocatalyst for each reaction to date is noble metal platinum, a very expensive material. In the 1970s and 1980s the catalyst layer of PEMFCs was made up of pure platinum black and PTFE suspension with a Pt loading up to mg cm-2 [1]. However, it was revealed in later research that it is the catalyst surface area that determines the reaction rate rather than catalyst weight. Thereafter a significant Pt loading reduction to less than 0.4 mg cm-2 was achieved in the late 1990s by synthesizing carbon supported Pt catalyst for PEMFC applications [5]. Porous carbon blacks are widely used as Pt support for their high surface ratio and excellent electrical conductivity. In the most prevalent catalyst layer, composite catalysts consisting of Pt nanoparticles (4nm or smaller) supported on carbon black Vulcan XC72R (ca. 40nm) are usually used with Nafion® ionomers impregnated. The catalyst layers are usually very thin with a thickness of around 10−50 μm, containing electrochemically active regions where three phases − catalysts, ionomers, and reactant gases coexist. In order to improve the utilization of Pt catalyst, optimum catalyst layer structure should be well-maintained to obtain maximum three-phase zones [4].   Polymer Electrolyte Membrane (PEM) A polymer electrolyte membrane is a solid polymer film that separates the anode and cathode catalyst layers. It is a pivotal component of a PEMFC as it not only permits protons to transport from anode to cathode but also insulates electrons to travel through that the free electrons can only reach cathode through external circuit, generating useful electricity. It also separates fuel and oxidant gases from each other thus direct combustion of fuels can be avoided. In addition, a good PEM material should remain chemical and mechanical stability in the hostile fuel cell environment to ensure long-term operation durability. The most commonly used PEM at present is Nafion® series invented by Dupont in 1960’s, due to its high proton conductivity and chemical inertness [2]. The chemical structure of a typical Nafion PEM is composed of a polytetrafluoroethylene (PTFE) chain and a side chain ending with sulphonic acid HSO3. A micro-view of the PEM structure is shown in Fig. 1.2. The fluorocarbon chain usually has a repeating structural unit, i.e. —[CF2–CF2]n—, where n is very large. This chain can provide the PEM with good mechanical strength and chemical stability. On the other hand, the sulphonic acid group HSO3 is highly hydrophilic and is ionically bonded with a SO3- ion and a H+ ion. This is why such structure is called ionomer. When the PEM absorbs water and becomes hydrated, the ionically bonded - H+ ions are relatively weakly attracted to the SO3 group. As a result, the H+ ions are able to move through concentration gradient within the well-hydrated regions of PEM, making this material a very good proton conductor. The proton conductivity of this PEM is strongly correlated with its water content, thus making humidification of the fuel gases a requirement during cell operation. Another requirement of using this PEM is that the operating temperature is limited to the boiling temperature of water to maintain its liquid water content. However, this requirement can lead to a severe mass   transport limitation when excess water accumulates in electrode and blocks gas diffusion pores. Therefore water management is a very important topic in current PEMFC research, especially in developing high-temperature PEM materials as well as optimizing electrode structure [2].   Fig. 1.2 Chemical structure of Nafion membrane [2]. 1.3.3 Basic Thermodynamics and Electrochemistry of PEMFCs As shown previously, a typical PEMFC consists of three core parts: two electrodes (anode and cathode) sandwiched with a solid polymer electrolyte membrane between them. The combination of anode, cathode and membrane corresponds to the heart of a PEMFC, known as membrane electrode assembly (MEA). In a working PEMFC, hydrogen fuel flows into the anode catalyst layer through gas diffusion layer, and it is then split by platinum catalysts into two electrons and two protons (hydrogen ions). The protons can pass through the polymer electrolyte membrane to cathode catalyst layer, whereas the electrons have to travel from the anode to cathode through an external circuit consuming the power generated by the cell. Again with the help of platinum catalysts, the protons and electrons combine with oxygen within the cathode catalyst layer, producing pure water. The   overall electrochemical process of a PEMFC is called "reverse hydrolysis", corresponding to the opposite reaction of hydrolyzing water to form hydrogen and oxygen. The illustration of this process is shown below in Fig. 1.3.   Fig. 1.3 Illustration of electrochemical processes in PEMFCs [2]. As shown in Fig. 1.3, the electrochemical reactions occurring at anode and cathode in a PEMFC can be illustrated as Eq. 1-1 to 1-3 [2]: Anode: H2 2H+ + 2e- Cathode: 2H+ + 2e- + 1/2O2 Overall: H2 + 1/2O2 (1-1) H2O H2O + electricity + heat (1-2) (1-3) According to Eq. 1-3, the electrochemical energy in hydrogen fuel is directly converted into electricity through the PEMFC reaction, producing pure liquid water and heat as the only by-products. As an electrochemical energy converter, a PEMFC must obey the laws and principles of thermodynamics. 10   commercial electrode. However, a notable performance drop was observed at large current densities for their sputtered electrodes. Moreover, they also observed that when the sputtered Pt loading was reduced to 0.04 mg cm-2 the cell performance was visibly lower, presenting a power density of 160 mW cm-2 at the current output of 200 mA cm-2. In 2002, O’Hayre et al. [22] reported their high performance sputterdeposited electrodes with an ultra-low Pt loading about 0.04 mg cm-2. In their study, thin Pt catalyst layers were directly sputter-deposited onto both sides of Nafion membranes. Their MEA obtained by sputter-deposition could give rise to a maximum power output up to three-fifths that of a commercial E-TEK MEA, of which the Pt loading was 0.4 mg cm-2. Later Brault et al. [31] found that the maximum size of the Pt nano-clusters formed on electrodes by sputtering was about nm and the optimum Pt loading was about 0.1 mg cm-2 to give comparable performance as a commercial MEA with a Pt loading of 0.4 mg cm-2. Their results also showed that the influence of the sputter-deposited electrode was greater at cathode than at anode, confirming the results of Weber et al. [28]. They suggested that the sputter-deposition technique can greatly enhance Pt utilization due to the optimum location and distribution of the Pt nanoparticles at the electrolyte-electrode interface. Despite of the high Pt utilization obtained by sputter-deposition technique, however, this method is limited by its nature of surface deposition. It is believed that the electrochemical activity of Pt catalysts significantly depends on their surface area. As such, porous CB-based GDLs are usually fabricated for Pt sputtering process to increase the surface area of the sputter-deposited Pt catalysts. When Nafion membrane was used as substrate, the optimum Pt loading correspond to a Pt film 5−10 nm think, which is about only 0.01−0.02 mg cm-2 [22]. Pt loadings higher than 23   this amount would give rise to a dense Pt film on the membrane, which tremendously led to a reduced Pt surface area and thus a considerably impaired cell performance. Therefore, it was suggested that using suitable substrates with high surface area and porosity for sputtered Pt catalysts is vitally important to optimize the effectiveness of sputter-deposition technique for PEMFC applications. 1.4.2 Carbon Nanotubes as Support for Pt-based Electrocatalysts It is well-known that the electrochemical activity of Pt catalysts greatly depends on the Pt particle size as well as their dispersion on the support material [32]. The intrinsic properties of support materials play an important role in the catalytic performance of Pt catalysts by affecting their structure and morphology. An ideal catalyst support should provide several desirable properties such as: (i) high electrical and thermal conductivity, (ii) high surface area and porosity to ensure reactant gas access to electrocatalysts, and (iii) high electrochemical stability under fuel cell operating conditions [4]. Currently the most prevalent electrocatalyst for PEMFCs is Pt nanoparticles dispersed on carbon black support. In spite of the high surface area of carbon black particles, there remain two main problems for this support material [33]: (i) carbon black-based catalyst may suffer significant mass transport limitations due to its dense structure, which leads to low Pt utilization; (ii) carbon black is susceptible to the hostile PEMFC environment and it would undergo electrochemical oxidation to surface oxides and eventually, to CO2 at cathode, where it is subjected to low pH, high potential, high humidity and high temperatures (80 °C). As carbon black corrodes, the Pt nanoparticles on carbon black will fall off from the support, and consequently dissolve into water or aggregate to larger particles and attach onto adjacent support. This is one of the major mechanisms that cause cell performance 24   degradation due to support oxidation and Pt catalyst loss [34]. Therefore, many research efforts have been devoted to exploring for new catalyst support materials for PEMFC applications. Recently, a number of studies have shown that carbon nanotubes (CNTs) are promising alternative support materials for PEMFC electrocatalysts due to their distinct graphitic characteristics compared to those of carbon blacks [35-44]. CNTs with higher graphite component exhibit superior electronic conductivity, high electrochemical stability and excellent hydrophobility; therefore, they have been proposed as alternative catalyst supports to replace carbon blacks in recent PEMFC research. Many researchers have reported that the CNT supports were able to provide higher corrosion resistance and mass transport capability for PEMFC electrodes [4548]. In addition, Tauster et al. [49] found an effective atomic interaction between the Pt particles and the CNT supports, which could give rise to improved catalytic performance by facilitating the electron transfer process for the PEMFC reactions. Enhanced Pt utilization was thus obtained by reducing the Pt catalyst loading without performance losses. Previous studies on the electrochemical performance of Pt/CNT composite catalysts were performed predominantly via cyclic voltammetry (CV) tests in a simulated PEMFC environment [40-44]. The electrochemical performance of Pt/CNT catalysts is usually compared with that of commercial Pt/VXC72R catalysts. When this is conducted by cyclic voltammetry studies, the performance of Pt/CNT catalysts always show notable activity enhancement for the oxygen reduction reaction (ORR) compared to that of Pt/VXC72R catalysts [40-44]. For example, Kim and coworkers 25   [44] developed a surface thiolation method for achieving high dispersion of Pt nanoparticles on multiwall carbon nanotubes (MWNTs) for electrocatalysts. Highly dispersed Pt nanoparticles with small average particle size (1.5 nm) and narrow size distribution were obtained by the surface thiolation of MWNT supports, in spite of a considerably large Pt loading (40 wt% Pt to MWNT). These highly dispersed Pt nanoparticles showed enhanced and stable electrocatalytic activity in the methanol oxidation and oxygen reduction reactions via CV tests. The authors suggested that the surface thiolation of supports is an effective way to obtain highly dispersed precious metal nanoparticles to enhance electrocatalytic activity. Kongkanand et al. [42] found a significant improvement in the electrocatalytic activity of Pt nanoparticles on ORR achieved by depositing them on single wall carbon nanotubes (SWNTs). The Pt/SWNT films cast on a rotating disk electrode exhibited a lower onset potential and a higher charge transfer rate constant for ORR compared with a commercial Pt/VXC72R catalyst. SWNTs were therefore proposed by the authors as a promising support material for PEMFC applications. When the catalysts were tested in real fuel cell fixtures, however, inconsistent results were reported by several research groups. Some studies reported better performance for Pt/CNT catalysts than for conventional Pt/VXC72R catalysts [50-52], whereas others found Pt/CNT uncompetitive with Pt/VXC72R [21, 35, 39, 53]. For example, Rajalakshmi et al. [35] prepared Pt/CNT catalysts by means of colloidal method. It was found that the as-synthesized catalysts contained both Pt(0) and Pt(IV) species. A high Pt loading of 32.5% on CNTs was obtained when the catalysts were prepared with ethylene glycol and Pt salt. The electrocatalysts were investigated for the ORR in PEMFC tests by analyzing their performance with respect to different 26   CNT pre-treatment processes. The results suggested that surface functionalization of the CNT support is necessary to give a better Pt dispersion with a narrow particle size range. The Pt particles deposited on untreated CNTs were found to easily agglomerate into large particles, resulting in a lower cell performance compared to the commercial Pt/VXC72R catalyst [35]. On the other hand, another benefit from using CNT support for PEMFC electrocatalyst lies in that CNTs are more electrochemically stable than carbon black [54]. It is well-known that the long-term stability of PEMFC operation is of crucial importance for the commercialization of this technology. It order to meet the requirements for its commercial viability, PEMFC stacks for automobile applications is required to ensure a stable operation life over 5,000 hours and at least 40,000 hours for stationary applications [55]. However, few studies on long-term stability were reported in early PEMFC research. It is only since 2004 that PEMFC research emphasis has been shifted from short-term performance to long-term durability [56]. One of the major degradation mechanisms for PEMFC long-term durability was found to be the oxidation of carbon support under PEMFC operating conditions [34]. This oxidation process is further expedited in PEMFCs due to the presence of the Pt catalyst [57]. Kangasniemi et al. [57] observed gradual oxidations of carbon black in H2SO4, which indicated that carbon black support is susceptible to oxidation under acidic environment. Roen et al. [58] detected CO2 in the cathode exhaust gas during potential cycling of a PEMFC while feeding humidified helium to the electrodes. To alleviate carbon support oxidation, extensive electrochemical experimentation has been performed on CNT supports including MWNTs and SWNTs. While a performance improvement is reported in most of the Pt/CNT studies [35-44], 27   electrochemical stability study on CNT support is rarely mentioned. Nevertheless, Wang et al. [59] found that MWNTs show less oxidation at 0.9 V and less Pt surface area loss than VXC72R in durability test under H2SO4. Shao et al. [54] also reported that MWNTs are more oxidation resistant than carbon black under a high potential of 1.2 V in H2SO4. However, it is noteworthy that previous durability studies on CNT support were always performed under simulated PEMFC environment [54, 57-59]. While little evidence is shown in literature that CNT support is more oxidation resistant than carbon black in real PEMFCs, more in situ investigation on the electrochemical stability of CNT support should be carried out to provide sufficient justifications of using this material, considering the higher cost of CNTs than that of carbon blacks. 1.4.3 Synthesis Methods of Pt/CNT-based Electrocatalysts As the Pt/CNT catalysts synthesized via different processes showed distinct catalytic performances in previous studies, further optimization for the synthesis of Pt/CNT composite catalysts is still required to validate the effectiveness of CNTs as the catalyst supports for PEMFC applications. In recent years, a variety of synthesis methods for Pt/CNT composite catalysts have been treated extensively in literature [60-67]. It has been revealed that the microstructure and catalytic activity of the Pt particles supported on CNTs have a strong correlation with their synthesis methods [32]. The most prevalent synthesis methods for Pt/CNT catalysts reported in recent researches were reviewed by Lee et al. [67]. It has indicated that to synthesize Pt catalysts with smaller particle size and more uniform distribution on CNT supports is vitally important in order to enhance the Pt utilization and its catalytic activity on the ORR. To date, well-dispersed, nanosized Pt particles (usually less than nm) have 28   been successfully deposited onto CNT surface through a large variety of wet-chemical processes [61-65]. Most of these methods are basically chemical reduction processes similar to the methods for the synthesis of Pt/CB catalysts as described in Section 1.3.1, where the carbon black is replaced by carbon nanotubes during Pt deposition. Table 1.1 lists some examples of chemical synthesis methods and sputter-deposition method used to prepare Pt/CNT-based electrocatalysts for PEMFC applications. Synthesis Method Support Pt Precursor Pt Particle Size Ref. Chemical precipitation Carbon nanofiber Pt(NH3)4(NO3)2 1−2 nm 61 Colloid MWNT H2PtCl6 2.2 nm 62 Impregnation, ethylene glycol reduction MWNT H2PtCl6, K2PtCl4 2−4 nm 63 Impregnation, acetic acid reduction MWNT Pt (II) acetylacetonate 2−3 nm 64 Electrochemical deposition SWNT K2PtCl4 4−6 nm 65 Sputtering MWNT N-CNT − nm nm 66 26 Table 1.2 Summary of synthesis methods for Pt/CNT-based electrocatalysts for PEMFC electrodes. Chemical methods, such as the colloid method, impregnation and electrochemical deposition, have the advantages of being able to control catalyst particle size, shape and composition [61-65]. However, these procedures all consist of a series of wet chemical processes by which their efficiency is greatly undermined. Physical methods, such as sputter-deposition, have been explored more recently for 29   direct deposition of the Pt catalyst onto CNT supports for PEMFC catalysis [66, 26]. Chen et al. [66] attempted to sputter-deposit Pt particles onto highly graphitized CNTs. The CNT supports were grown on carbon cloth by a bias-assisted plasma enhanced chemical vapor deposition (PECVD) method. They found that the sputtering technique was able to yield a smaller Pt particle size of nm compared to that of 2−5 nm formed by the impregnation deposition method on the same CNT support. However, an analysis of the catalytic performance of the Pt/CNT catalyst was not reported in their work. Sun et al. [26] also used sputtering technique to deposit Pt catalysts on arrayed nitrogen containing CNTs (CNxNT) for micro direct methanol fuel cell applications. The as-deposited Pt particles showed an average diameter of nm and uniform distribution on the CNxNT surface. In addition, the Pt/CNxNT nanocomposites were demonstrated to be electrochemically active for methanol oxidation based on CV curves in H2SO4 solution; however, useful reference catalyst and cell performance evaluation were absent from their study. Although supperdeposition has been revealed as an effective way to obtain nanosized Pt catalysts, little literature has presented relevant results to further confirm the electrochemical activity and stability of the Pt/CNT catalysts in real PEMFC environment. In addition to Pt deposition process, a large number of studies have shown that surface functionalization of CNT support is necessary prior to Pt deposition, due to the inert graphite surface of CNTs [39, 68-75]. In general, Pt particles are unable to wet CNT surface hence anchoring sites are usually generated by oxidizing CNT surface with strong oxidant solutions like HNO3, H2SO4, K2Cr2O7, KMnO4 or H2O2 [69-72]. After surface oxidation, various functional groups such as hydroxyls (−OH), carboxyls (−COOH) and carbonyls (−C=O) are formed on CNT surface to provide 30   abundant anchoring sites for effective Pt deposition [35, 73]. This surface oxidation could also be performed electrochemically [74, 75]. As these surface functional groups have strong attraction forces toward metal ions, it has been found that CNT surface oxidation plays a crucial role in synthesizing Pt/CNT electrocatalysts with small particle size and uniform dispersion. Rajalakshmi et al. [35] reported that more uniform distribution of Pt nanoparticles of size about 3−5 nm was obtained via pretreatment of the CNT support with 70% nitric acid, in comparison to the untreated CNTs. The Pt particle size showed a correlation with the oxidation severity of CNTs, indicating that the effective deposition Pt nanoparticles may be attributed to the strong interaction formed between the Pt precursor and the functional groups on CNT surface. Xu and co-workers [69] also investigated the influence of surface oxidation pre-treatment on Pt/CNT catalyst preparation. They developed a hybrid process, consisting of refluxing with a mixture of HNO3 and H2SO4 solution and immersing in a H2O2 solution for functionalizing CNT surface. They found that the Pt nanoparticles deposited on the CNTs treated by this hybrid process showed a homogeneous size distribution of nm, which gave rise to the best catalytic properties in PEMFC polarization test. Despite the successful synthesis of Pt nanoparticles on CNT supports, the surface oxidation process yet introduces additional complexity to the preparation process that makes the use of CNT support less attractive. Synthesis methods with higher efficiency should be developed for preparing Pt/CNT-based electrocatalysts without sacrificing effective Pt deposition on CNT supports. In this study, the sputter-deposition and thermal chemical vapor deposition (CVD) techniques were combined to efficiently fabricate a Pt catalyst supported on CNTs directly grown on carbon paper as a PEMFC electrocatalyst. The idea of 31   growing CNTs on carbon paper was previously proposed in 2004 by Wang et al. [21], where multi-walled carbon nanotubes (MWNT) were formed on carbon paper via a CVD process using electrodeposited Co catalysts. The electrodeposition method was also used for subsequent Pt deposition onto the MWNTs. However, the polarization performance of the Pt/MWNT-based electrode was lower than that of the conventional ink-process prepared electrode. This is probably due to the large Pt particles formed on the MWNT surface by electrodeposition, which is about 25 nm compared to 2–4 nm from commercial Pt/C catalyst. In their subsequent work [39], enhanced Pt utilization has been obtained when the Pt particle size was reduced to nm via in situ chemical reduction of Pt precursor solution on MWNTs. However, the deposited Pt nanoparticles on MWNTs showed poor dispersion without acid pretreatment of the MWNT surface. As a result, the Pt nanoparticles had a large size distribution range from 2–10 nm. The polarization performance of their Pt/MWNTbased electrode remained very low when the Pt/MWNT-based electrode was not brushed an additional gas diffusion layer on the backside of the carbon paper.  A few research groups later reported some different processes to prepare Pt/CNT-based electrodes with CNTs grown directly on carbon paper [64, 76]. Villers et al. [76] used carbon paper seeded with Co−Ni catalytic particles to grown CNTs via a CVD process. The CNTs were grown for at 800 °C by the decomposition of ethylene on the Co−Ni catalytic particles. Subsequent Pt deposition was carried out by means of immersing the CNTs in a mixture solution containing PtCl4 and PtBr4 diluted in silane, ethanol and H2O composite solvent for h, followed by a chemical reduction process under H2. Later in Saha’s study [64], the CNTs were grown on carbon paper in a CVD reactor also using Co−Ni particles as the growth catalyst. Afterward, they pre-treated the as-grown CNT/carbon paper with nitric acid, and then used glacial 32   acetic acid to reduce a Pt precursor (Pt acetylacetonate) at 110 °C for h. Although the Pt/CNT-based electrodes prepared by the above two methods showed enhanced polarization performance for the ORR in H2/O2 fuel cell tests, it is worth noting that an additional VXC72R-based gas diffusion layer was always needed on the backside of the carbon paper to provide gas permeability and electrode hydrophobicity. Therefore the in situ grown CNT layer with wet-chemically deposited Pt particles simply served as a catalyst layer whereas its synthesis methods usually require multiple steps and repetitive processes. The overall effectiveness of the in situ grown CNT supports still remains a great challenge for PEMFC applications. 1.5 Objectives of this Research Based on the review of previous electrocatalytic studies on Pt/CNT catalysts for PEMFCs in Section 1.3, it is noticeable that a few problematic issues remain unresolved to qualify the CNT supports as an effective replacement of carbon black, notwithstanding the high fabrication cost of CNTs. The main research gaps of the current studies on Pt/CNT catalysts are summarized below:  The electrocatalytic performance of Pt/CNT catalysts is not consistent compared to that of Pt/carbon black catalysts. It has been demonstrated that the electrocatalytic performance of Pt/CNT catalysts significantly depends on their synthesis methods, including CNT surface oxidation and Pt deposition. To develop synthesis methods with higher fabrication-efficiency and catalystperformance remains one of the most pressing research topics.  Growing CNTs directly onto carbon paper as catalyst supports has been reported 33   previously to be an effective way to integrate CNTs into PEMFC electrodes. However, an additional carbon black-based gas diffusion layer is always needed to enhance mass transport whereas it adds further complexity to electrode fabrication.  Currently, the prevailing synthesis methods of Pt/CNT catalysts usually consist of a series of wet-chemical reduction and decontamination processes. Although direct deposition of Pt catalysts on CNTs has been attained recently by sputterdeposition, their electrocatalytic performance in actual fuel cell systems is still unknown.  Although enhanced electrochemical durability of Pt/CNT catalysts has been verified in simulated PEMFC environments in a large number of studies, it is still unclear with regard to their long-term durability under real fuel cell conditions. The aim of this study was to develop a combined fabrication method for Pt/CNT catalysts by thermal CVD process and sputter-deposition technique. The combined fabrication method consists of two main processes to prepare an integrated Pt/CNT layer for PEMFC electrodes: in situ growth of a CNT layer onto carbon paper using thermal CVD technique, and direct deposition of Pt nanoparticles onto the CNT layer by sputtering technique. By using the combined fabrication method, we aimed to address the remaining problematic issues listed above. First of all, the integrated Pt/CNT layer is fabricated to serve as both the gas diffusion layer and the catalyst layer simultaneously, thus eliminating additional CB-based GDLs on electrode. Secondly, the Pt nanoparticles are directly sputter-deposited onto the CNT layer that 34   no surface oxidation of the CNTs is required. Furthermore, its electrocatalytic performance is investigated via a series of activity and durability tests in a single cell system. It may give a better understanding of the electrochemical activity and stability of Pt/CNT catalysts under real fuel cell conditions. Last but not least, the combined fabrication method may provide an efficient fabrication way for integrated Pt/CNT catalysts, which may further facilitate the mass production of PEMFC electrodes. This thesis concentrates on the optimization of the combined fabrication method for Pt/CNT catalysts as well as the morphological and electrochemical characterization of the catalysts. As the Pt/CNT catalysts are integrated into the electrode by direct CNT-growth and Pt-deposition, it is more accurate to perform in situ measurement on the Pt/CNT catalysts. A direct comparison with the results of previous studies tested in simulated PEMFC environments is thus not provided in this study. References [1] G. Hoogers, Fuel Cell Technology Handbook, CRC Press, LLC (2003). [2] J. Larminie, and A. Dicks, Fuel Cell System Explained, John Wiley and Sons Ltd., Chichester (2000). [3] S. Thomas, and M. Zalbowitz, Fuel Cells−Green Power, Los Alamos National Laboratory, USA (2001). [4] F. 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(3), 4 81 (19 64) [8] G Longoni, and P.Chini, J Am Chem Soc., 98, 7225 (19 76) [9] R D Adams, W Wu, R D Adams, and W Wu, Organometallics, 12 , 12 48 (19 93) [10 ] T Teranishi, M Hosoe, T Tanaka, and M Miyake, J Phys Chem B, 10 3, 3 818 (19 99) [11 ] H G Petrow, and R J Allen, United States patent US404 419 3, 19 77 Aug 23 35   [12 ] M Watanabe, M Uchida, and S Motoo, J Electroanal Chem., 229, 395 (19 87) [13 ] O V... Electrochem Soc., 13 4, 14 16 (19 87) [29] S Litster, and G McLean, J Power Sources, 13 0, 61 (2004) [30] S Hirano, J Kim, and S Srinivasan, Electrochim Acta, 42, 15 87 (19 97) [ 31] P Brault1, A Caillard1, A L Thomann1, J Mathias1, C Charles, R W Boswell, S Escribano, J Durand, and T Sauvage, J Physics D: Applied Physics, 37, 3 419 (2004) [32] K Kinoshita, J Electrochem Soc., 13 7, 845 (19 90) [33] H A Gasteiger,... mainly through 17   two strategies: Pt -based alloy catalysts and Pt catalysts on carbon support In the following sections, an overview of supported Pt catalysts will be given including Pt catalysts supported on carbon black (CB) and carbon nanotube (CNT) as well as their diverse synthesis methods 1. 4 .1 Carbon Supported Pt Electrocatalysts for ORR Platinum has been used as the electrocatalyst for PEMFC reactions... Savinova, J Electroanal Chem., 554, 2 41 (2003) [14 ] H Liu, C Song, L Zhang, J Zhang, H Wang, D Wilkinson, et al., J Power Sources, 15 5, 95 (2006) [15 ] Y Zhang, and C Erkey, J Supercrit Fluids, 38, 252 (2006) [16 ] K Shimazu, D Weisshaar, and T Kuwana, J Electroanal Chem., 223, 223 (19 87) [17 ] J V Zoval, J Lee, S Gorer, and R M Penner, J Phys Chem B, 10 2, 11 66 (19 98) [18 ] O Antoine, and R Durand, Electrochem... methods for the synthesis of Pt/CB catalysts as described in Section 1. 3 .1, where the carbon black is replaced by carbon nanotubes during Pt deposition Table 1. 1 lists some examples of chemical synthesis methods and sputter-deposition method used to prepare Pt/CNT -based electrocatalysts for PEMFC applications Synthesis Method Support Pt Precursor Pt Particle Size Ref Chemical precipitation Carbon nanofiber... expensive platinum must be used as catalyst for hydrogen oxidation reaction at low temperature Moreover, the platinum catalyst is extremely sensitive to CO contained in the hydrogen fuel Therefore intensive research on advanced PEMFC electrocatalysts with higher electrocatalytic performance and lower cost are pressingly needed to provide competitive price and reliable performance for PEMFCs [4] 1. 4 Electrocatalytic... is commonly 15   performed as the first characterization step to evaluate fuel cell performance for its convenience and ease of data acquisition and interpretation [1] 1. 4 theoretical equilibrium potential 1. 23 V 1. 2 actual open circuit potential 1. 0 activation polarization Cell potential / V ohmic overpotential 0.8 mass transport overpotential 0.6 0.4 polarization curve 0.2 0.0 0 400 800 12 00 Current... especially for the ORR A suitable electrocatalyst is thus necessary to speed up the reaction kinetics to meet the requirement for practical uses of PEMFCs In the stateof-the-art PEMFCs, platinum (Pt) -based catalysts are the most effective electrocatalysts for both HOR and ORR, owing to the high electrochemical activity and stability of platinum under PEMFC operating conditions However, these Ptbased catalysts... CNT /carbon paper with nitric acid, and then used glacial 32   acetic acid to reduce a Pt precursor (Pt acetylacetonate) at 11 0 °C for 5 h Although the Pt/CNT -based electrodes prepared by the above two methods showed enhanced polarization performance for the ORR in H2/O2 fuel cell tests, it is worth noting that an additional VXC72R -based gas diffusion layer was always needed on the backside of the carbon. .. automobile powered by a PEMFC in 19 93 16   The latest PEMFC vehicle developed by DaimlerChrysler and Ballard has grown to the six generation It has the capability of traveling 15 0 km with one fill of fuel and its highest speed can reach up to 14 0 km/h [1] However, barriers to the commercialization of PEMFCs to date remain formidable due to their exiguous fuel sources and high economic costs One main problem . Cells (PEMFCs) Proton Exchange Membrane Fuel Cells are also known as polymer electrolyte membrane fuel cells or polymer electrolyte fuel cells (PEFCs). Unlike other types of fuel cells, a typical. Suitable for all sizes of power stations, 2 kW to multi-MW. Table 1. 1 Main types of fuel cells and their characteristics and applications 3  1. 3 Proton Exchange Membrane Fuel Cells (PEMFCs). fuel cells can be categorized into five main types according to the specific electrolyte used. The five main types of fuel cells are: proton exchange membrane fuel cells (PEMFC), alkaline fuel