**2. The Hydrogen Economy**

Many nations are looking into alternative sources of energy to address issues of environ‐ mental responsibility and energy independence. Some of these energy sources include solar power, wind energy, natural gas, and hydrogen. As society explores hydrogen as an alterna‐ tive energy source, the question is how effective can CNTs be in helping to solve some of the problems in the structure, function and safety of this emerging industry?

#### **2.1. Fuel Cells & Hydrogen Storage**

**Figure 1.** A cross-linked CNT paper with embedded Pd nanoparticles that can be used to construct a hydrogen sensor.

414 Syntheses and Applications of Carbon Nanotubes and Their Composites

The field of photovoltaics is regarded as a major contributor to a sustainable economy. How‐ ever; purveyors of large scale solar panels have been experiencing a degree of volatility in the market due in part to the decreasing price of the technology, increased competition and a dependence on government subsidies. At the opposite end of the scale, there is a surge in small solar powered gadgets such as pocket LED torches and mobile device chargers, which adorn many airport convenience outlets. The demand for pocket sized solar powered devices is helping to stimulate research into making the energy conversion process more efficient. There were three major advances in photovoltaics, the development of photovoltaic devices from crystalline silicon, which dominate the commercial market, cadmium telluride (CdTe) and dye sensitized solar cells (DSSCs). CNTs are currently being investigated as a way to enhance electron transfer and replace the standard platinum based counter electrodes, especially with DSSCs. CNT thin films and mats are currently being tested as components of these photovol‐ taic devices. This section of the chapter will explore how the CNTs have been used to en‐ hance dye-sensitized [3], CdTe [4] and silicon [5] based solar cells, and address some the concerns about the race to produce novel photovoltaic devices and the toxic warnings from

the past that may ultimately define the balance between safety and efficiency.

structure, their electrical conductivity, and their axial thermal conductivity [7, 8].

The last section of this chapter will focus on the development of CNT based thermoelectric devices which may bridge the gap between conventional and sustainable economies. Energy loss in the form of heat is clearly an important concept to address, and capturing the heat from combustion engines is one avenue being pursued by research. Around 75% of the ener‐ gy produced from fuel with internal combustion engines is lost to the environment, so it may be possible to recapture some of this energy using a thermoelectric device between the engine coolant system to the exhaust manifold [6]. However, problems have been encoun‐ tered with low efficiency so CNTs have been investigated as a suitable component of ther‐ moelectric devices due to a number of characteristics, such as their low dimensional In the simplest case, a hydrogen fuel cell is comprised of a permeable membrane placed be‐ tween an anode and a cathode. There are various types of fuel cells: polymer electrolyte membrane, direct methanol, alkaline, phosphoric acid, molten carbonate, and solid oxide. Hydrogen fuel cells fall under the polymer electrolyte membrane fuel cell (PEMFC) category and are sometimes also referred to as a proton exchange membrane fuel cell. In a typical PEMFC, the permeable membrane consists of a proton-conductive polymer such as per‐ fluorosulphonic acid, also known commercially as Nafion. The fuel cell works by using a catalyst to oxidize hydrogen at the anode, converting it into a positively charged proton and a negatively charged electron. The electrons travel through a wire creating an electrical cur‐ rent to power a device while the protons travel through the permeable membrane to the cathode. At the cathode, the protons recombine with the electrons and react with oxygen to form water which is eventually drained from the system.

Despite recent advances in research, there are still a few obstacles that need to be overcome in order for fuel cells to become mainstream technology. In order to integrate with existing technologies, fuel cells need to become considerably cheaper. Currently, they are expensive to construct, mainly due to the use of platinum catalysts. According to the United States De‐ partment of Energy, the cost per kilowatt would need to decrease in order for fuel cells to be competitive and economically viable. In order to compete commercially with the combus‐ tion engine, it is estimated that the fuel cell cost would need to be cut to approximately \$25– \$35/kW. Another aspect of fuel cells that needs improvement is the operational lifetime. The permeable membrane is made of a synthetic polymer which is susceptible to chemical deg‐ radation. Reliability in automotive applications, can be defined by the lifetime of a car en‐ gine, approximately 150,000 miles, so research has focused both on improving the efficiency of the catalytic process and the durability of the components. CNTs have been proposed as a substitute to the carbon powder currently used in PEMFCs. (Fig. 2.) CNTs have excellent conductive properties, a low mass density, and robust physical properties making them an ideal and durable material for fuel cell electrodes. Furthermore, nanotubes assembled into such macrostructures have a high surface area making them a suitable substrate for Pt cata‐ lysts and hydrogen adsorption [9].

[14]. We then explored an alternative cross-linking system to avoid the use of thiols and em‐

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Research into hydrogen storage with interconnected CNT networks started by looking in‐ to SWCNTs using a procedure called temperature programmed desorption. Experiments on MWCNTs followed with work focusing on metal doped tubes. However, problems began to arise when increasing values of CNT storage capacities, up to 21 wt%, were reported. A detailed review of the findings can be found by Yunjin Yao and serves as an interesting footnote towards the role of CNTs and the need for a better understanding of their chemis‐ try in materials [15]. In summary, the main concerns were that elevated hydrogen storage percentages may have be due to a number of factors including the insufficient characteriza‐ tion of CNT composites due to the presence of SWCNTs, DWCNTs and MWCNTs with a variety of open and closed ended tubes. Contamination of the CNTs during the process of ultrasonic probe treatments was a concern, because in one example the value for SWCNTs were reported to have a hydrogen storage capacity of around 4.5% at 30 kPa and 70 K, but the ultrasonic probe was made from a titanium alloy that was known to act as a hydrogen

The research field based on water splitting has, not surprisingly, found a niche in the devel‐ opment of the hydrogen economy due to the clean production of hydrogen and oxygen. However this integration has a far more significant impact when combined with hydrogen fuel cells. The waste product of hydrogen fuel cells is water, and it is formed during the re‐ action with oxygen, so the water could fuel the process of splitting and this in turn can fuel

CNTs have been used to enhance the water splitting performance of titania photocatalysts [16] but an alternative use for CNTs has been found in membranes. Nafion, a sulfonated tet‐ rafluoroethylene based fluoropolymer-copolymer, is a membrane that has had commercial success in the fuel cell industry. Research groups are looking into enhancing the properties of the film with the addition of CNTs. Nafion/CNT composites with low concentrations of CNTs have been shown to have an effect on solvent permeation and mechanical stability. At high concentrations of CNTs the membranes have the ability to separate proton and electron conduction pathways in the membrane. Using this concept, many applications can be envis‐ aged for these membranes with one example being that of using sunlight to produce hydro‐ gen from water splitting. Current research has focused on the measurements of the electron and proton transport characteristics of Nafion and MWCNT composite films.[17] These films can be assembled by the addition of Nafion solution to MWCNTs, followed by the dis‐ persion of the MWCNTs in an ultrasonic bath. Various concentrations of MWCNTs were in‐ vestigated to a maximum of 5% MWCNTs by dry weight of Nafion. After the addition of isopropyl alcohol, to further aid the dispersion of the MWCNTs, the slurry was poured into petri-dishes and left to undergo solvent evaporation for 3 hours. The dishes with various

bedded palladium nanocrystals into the cross-linked network [2].

storage material.

**2.2. Water Splitting**

hydrogen cells.

**Figure 2.** Schematic of a CNT composite hydrogen fuel cell.

In 2003, researchers from the University of California, Riverside explored the use of MWCNTs as a carbon support for platinum catalysts in an attempt to maximize Pt interfac‐ ing between all the components in a fuel cell. The problem in conventional fuel cells is that the addition of the polymer tends to isolate the carbon particles reducing electron transport, resulting in the requirement of additional Pt particles to increase the power output. To re‐ solve this issue and improve conductivity, Wang et al. grew nanotubes directly on carbon paper and electrodeposited Pt particles onto the CNTs [10]. Although their experiments pro‐ duced promising results, their CNT based fuel cell still had a lower performance compared to conventional PEMFCs. Despite this low performance, this proof of concept was important to other researchers using CNTs in fuel cells. The following year in 2004, Girishkumar et al. investigated ways to improve the electrodes in direct methanol fuel cells (DMFCs) [11]. Their team developed a way to synthesize SWCNT thin films onto optically transparent electrodes using electrophoretic deposition techniques. It was determined that there was an improvement in catalytic activity mainly due to a larger surface area provided by the CNTs. This high surface area and porosity maximizes interactions between the fuel, electrode, and catalyst interface thereby enhancing Pt utilization and potentially reducing fuel cell manu‐ facturing costs. Li et al. (2006) also explored the use of CNTs in PEMFCs. They developed a facile and cost-effective method for the synthesis of an aligned Pt/CNT film [12]. They were interested in producing oriented CNT films due to enhanced conductivity. It was also sug‐ gested that there would be higher gas permeability and better water removal with aligned nanotubes. The aligned CNTs did show an improvement in Pt utilization as 60% of the met‐ al particles were being used during catalysis [11].

Using covalently cross-linked CNTs is another promising avenue for fuel cell electrodes [13]. Our work at Florida State University focused on the covalent cross-linking of multi-walled carbon nanotubes via a Michael addition reaction mechanism to form thin, flexible mats [14]. We then explored an alternative cross-linking system to avoid the use of thiols and em‐ bedded palladium nanocrystals into the cross-linked network [2].

Research into hydrogen storage with interconnected CNT networks started by looking in‐ to SWCNTs using a procedure called temperature programmed desorption. Experiments on MWCNTs followed with work focusing on metal doped tubes. However, problems began to arise when increasing values of CNT storage capacities, up to 21 wt%, were reported. A detailed review of the findings can be found by Yunjin Yao and serves as an interesting footnote towards the role of CNTs and the need for a better understanding of their chemis‐ try in materials [15]. In summary, the main concerns were that elevated hydrogen storage percentages may have be due to a number of factors including the insufficient characteriza‐ tion of CNT composites due to the presence of SWCNTs, DWCNTs and MWCNTs with a variety of open and closed ended tubes. Contamination of the CNTs during the process of ultrasonic probe treatments was a concern, because in one example the value for SWCNTs were reported to have a hydrogen storage capacity of around 4.5% at 30 kPa and 70 K, but the ultrasonic probe was made from a titanium alloy that was known to act as a hydrogen storage material.

#### **2.2. Water Splitting**

**Figure 2.** Schematic of a CNT composite hydrogen fuel cell.

416 Syntheses and Applications of Carbon Nanotubes and Their Composites

al particles were being used during catalysis [11].

In 2003, researchers from the University of California, Riverside explored the use of MWCNTs as a carbon support for platinum catalysts in an attempt to maximize Pt interfac‐ ing between all the components in a fuel cell. The problem in conventional fuel cells is that the addition of the polymer tends to isolate the carbon particles reducing electron transport, resulting in the requirement of additional Pt particles to increase the power output. To re‐ solve this issue and improve conductivity, Wang et al. grew nanotubes directly on carbon paper and electrodeposited Pt particles onto the CNTs [10]. Although their experiments pro‐ duced promising results, their CNT based fuel cell still had a lower performance compared to conventional PEMFCs. Despite this low performance, this proof of concept was important to other researchers using CNTs in fuel cells. The following year in 2004, Girishkumar et al. investigated ways to improve the electrodes in direct methanol fuel cells (DMFCs) [11]. Their team developed a way to synthesize SWCNT thin films onto optically transparent electrodes using electrophoretic deposition techniques. It was determined that there was an improvement in catalytic activity mainly due to a larger surface area provided by the CNTs. This high surface area and porosity maximizes interactions between the fuel, electrode, and catalyst interface thereby enhancing Pt utilization and potentially reducing fuel cell manu‐ facturing costs. Li et al. (2006) also explored the use of CNTs in PEMFCs. They developed a facile and cost-effective method for the synthesis of an aligned Pt/CNT film [12]. They were interested in producing oriented CNT films due to enhanced conductivity. It was also sug‐ gested that there would be higher gas permeability and better water removal with aligned nanotubes. The aligned CNTs did show an improvement in Pt utilization as 60% of the met‐

Using covalently cross-linked CNTs is another promising avenue for fuel cell electrodes [13]. Our work at Florida State University focused on the covalent cross-linking of multi-walled carbon nanotubes via a Michael addition reaction mechanism to form thin, flexible mats

The research field based on water splitting has, not surprisingly, found a niche in the devel‐ opment of the hydrogen economy due to the clean production of hydrogen and oxygen. However this integration has a far more significant impact when combined with hydrogen fuel cells. The waste product of hydrogen fuel cells is water, and it is formed during the re‐ action with oxygen, so the water could fuel the process of splitting and this in turn can fuel hydrogen cells.

CNTs have been used to enhance the water splitting performance of titania photocatalysts [16] but an alternative use for CNTs has been found in membranes. Nafion, a sulfonated tet‐ rafluoroethylene based fluoropolymer-copolymer, is a membrane that has had commercial success in the fuel cell industry. Research groups are looking into enhancing the properties of the film with the addition of CNTs. Nafion/CNT composites with low concentrations of CNTs have been shown to have an effect on solvent permeation and mechanical stability. At high concentrations of CNTs the membranes have the ability to separate proton and electron conduction pathways in the membrane. Using this concept, many applications can be envis‐ aged for these membranes with one example being that of using sunlight to produce hydro‐ gen from water splitting. Current research has focused on the measurements of the electron and proton transport characteristics of Nafion and MWCNT composite films.[17] These films can be assembled by the addition of Nafion solution to MWCNTs, followed by the dis‐ persion of the MWCNTs in an ultrasonic bath. Various concentrations of MWCNTs were in‐ vestigated to a maximum of 5% MWCNTs by dry weight of Nafion. After the addition of isopropyl alcohol, to further aid the dispersion of the MWCNTs, the slurry was poured into petri-dishes and left to undergo solvent evaporation for 3 hours. The dishes with various CNT concentrations were placed in an oven set to 40 ◦C for a few hours before being washed with deionized water and removed from the petri-dishes.

process of electron transfer because of the amount of water molecules. However mem‐ branes with no MWCNTs demonstrated the best proton conductivity, while the others have slightly lower conductivities. The answer could be as simple as a decrease in the amount of Nafion. Either way, this study has shown great potential for the integration of CNTs for

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Another application of great interest in the field of CNTs is hydrogen sensing. Advance‐ ments in the development of fuel cell design and technology means that a variety of sensors would be required to maintain a safe operational environment. CNTs are an ideal material

One of the first breakthroughs in CNT sensor technology occurred in 2001 when Kong et al. constructed hydrogen sensors by decorating SWCNTs with Pd nanoparticles [18]. Their H2 sensor exhibited significant changes in conductivity when exposed to small amounts of H2 and was able to operate at room temperature. Kong et al. were able to achieve this by depos‐ iting Pd particles on CVD grown SWCNTs via electron beam evaporation methods. When they placed this in a hydrogen atmosphere, a decrease in the CNT conductivity was ob‐ served. It has been proposed that this lower work function promotes electron transport from the Pd NPs into the CNTs resulting in a decreased amount of hole-carriers and conductivity. The reaction is also reversible. Under a hydrogen atmosphere, Pd reacts with H2 to become palladium hydride. The dissolved hydrogen in Pd metal combines with oxygen in air and results in H2O, recovering the electrical characteristics of the sensor. Kong et al. reported that their detector had a limit at 400 ppm, a response time of 5-10 s, and a recovery time of

One design principle of CNT composites that has defined the nature of efficiency is that of aligned CNTs. From aligned thin films of Buckypaper to forests of vertically grown CNTs on substrates, control over the direction of individual tubes and connected bundles is essen‐ tial for unlocking the full potential of the tubes. An investigation was made into the devel‐ opment of aligned CNT sensors using a method involving nanoplating and firing to produce cracks in a CNT composite film, exposing horizontally aligned carbon nanotubes (HACNTs) [19]. This research used arc produced MWCNTs as the basis for the composite film, which was rather enlightening in a field geared towards chemical vapor deposition (CVD) produced tubes. Research with arc produced MWCNTs has almost become a relic of the early years of CNT research. They were made by using a 150 mm long graphite rod for the anode and a graphite disc on a copper block for the cathode. After purification steps the CNTs were acid-oxidized using the standard 3:1 ratio of nitric acid (HNO3) to sulfuric acid (H2SO4) and washed several times in DI water before being dried in air at 120 ◦C. The acid treatment was required to increase the interfacial adhesion between the CNTs and metals. To produce the sensor, a sample of the purified CNTs was dispersed in DI water with a pol‐ yvinylpyrrolidone surfactant (PVP K30), which produced a CNT suspension. The CNT/Ni

for components of sensors due to their durability, and electronic properties.

membrane applications.

**2.3. CNT Hydrogen Sensors**

approximately 400 s [18].

To test the membrane, an artificial leaf system was constructed. (Fig. 3.). The membrane sep‐ arated the anode, which was exposed to sunlight where water droplets were present, and the cathode.

**Figure 3.** Schematic of the water-splitting device. The anode contained a chromophore and an oxygen evolving com‐ plex. The cathode contained a proton reducing catalyst. Image adapted from V. Ijeri *et al.* (2010).

The results highlighted a few points. Firstly pure Nafion exhibited insulating behavior and with increasing MWCNT percentage, a non linear behavior is observed with I–V curves, which is an indication of non-ohmic conductivity. The membranes were tested in both wet (1% H2SO4) and dry conditions to evaluate the electron conductivity. Before and after wet‐ ting the conductivity values increase with increasing filler content, but again without a line‐ ar relationship, which meant a critical concentration at which the membrane changed from insulating to conducting/semiconducting had to be established. This was done by looking at the values higher than 10−1 mS/cm which were obtained when MWCNTs > 3%. The next task was to investigate proton conductivity, and with standard conditions, this was generally low. However, with an increasing MWCNT percentage there was a subsequent increase. Al‐ though the effects of MWCNTs on proton conductivity is still not fully understood, most re‐ searchers will fall back on the semi-empirical quantum mechanical calculations too at least provide an insight into the possible conduction pathways.

When the membranes were subjected to 1% H2SO4 they did show an increase in proton conductivity, which was due to the various proton transfer mechanisms. The hydrogen bonding of the –SO3 groups with an H3O+ ion and water molecules results in a change in the side chains of Nafion. It was difficult to determine the contribution of MWCNTs in the process of electron transfer because of the amount of water molecules. However mem‐ branes with no MWCNTs demonstrated the best proton conductivity, while the others have slightly lower conductivities. The answer could be as simple as a decrease in the amount of Nafion. Either way, this study has shown great potential for the integration of CNTs for membrane applications.

#### **2.3. CNT Hydrogen Sensors**

CNT concentrations were placed in an oven set to 40 ◦C for a few hours before being washed

To test the membrane, an artificial leaf system was constructed. (Fig. 3.). The membrane sep‐ arated the anode, which was exposed to sunlight where water droplets were present, and

**Figure 3.** Schematic of the water-splitting device. The anode contained a chromophore and an oxygen evolving com‐

The results highlighted a few points. Firstly pure Nafion exhibited insulating behavior and with increasing MWCNT percentage, a non linear behavior is observed with I–V curves, which is an indication of non-ohmic conductivity. The membranes were tested in both wet (1% H2SO4) and dry conditions to evaluate the electron conductivity. Before and after wet‐ ting the conductivity values increase with increasing filler content, but again without a line‐ ar relationship, which meant a critical concentration at which the membrane changed from insulating to conducting/semiconducting had to be established. This was done by looking at the values higher than 10−1 mS/cm which were obtained when MWCNTs > 3%. The next task was to investigate proton conductivity, and with standard conditions, this was generally low. However, with an increasing MWCNT percentage there was a subsequent increase. Al‐ though the effects of MWCNTs on proton conductivity is still not fully understood, most re‐ searchers will fall back on the semi-empirical quantum mechanical calculations too at least

When the membranes were subjected to 1% H2SO4 they did show an increase in proton conductivity, which was due to the various proton transfer mechanisms. The hydrogen bonding of the –SO3 groups with an H3O+ ion and water molecules results in a change in the side chains of Nafion. It was difficult to determine the contribution of MWCNTs in the

plex. The cathode contained a proton reducing catalyst. Image adapted from V. Ijeri *et al.* (2010).

provide an insight into the possible conduction pathways.

with deionized water and removed from the petri-dishes.

418 Syntheses and Applications of Carbon Nanotubes and Their Composites

the cathode.

Another application of great interest in the field of CNTs is hydrogen sensing. Advance‐ ments in the development of fuel cell design and technology means that a variety of sensors would be required to maintain a safe operational environment. CNTs are an ideal material for components of sensors due to their durability, and electronic properties.

One of the first breakthroughs in CNT sensor technology occurred in 2001 when Kong et al. constructed hydrogen sensors by decorating SWCNTs with Pd nanoparticles [18]. Their H2 sensor exhibited significant changes in conductivity when exposed to small amounts of H2 and was able to operate at room temperature. Kong et al. were able to achieve this by depos‐ iting Pd particles on CVD grown SWCNTs via electron beam evaporation methods. When they placed this in a hydrogen atmosphere, a decrease in the CNT conductivity was ob‐ served. It has been proposed that this lower work function promotes electron transport from the Pd NPs into the CNTs resulting in a decreased amount of hole-carriers and conductivity. The reaction is also reversible. Under a hydrogen atmosphere, Pd reacts with H2 to become palladium hydride. The dissolved hydrogen in Pd metal combines with oxygen in air and results in H2O, recovering the electrical characteristics of the sensor. Kong et al. reported that their detector had a limit at 400 ppm, a response time of 5-10 s, and a recovery time of approximately 400 s [18].

One design principle of CNT composites that has defined the nature of efficiency is that of aligned CNTs. From aligned thin films of Buckypaper to forests of vertically grown CNTs on substrates, control over the direction of individual tubes and connected bundles is essen‐ tial for unlocking the full potential of the tubes. An investigation was made into the devel‐ opment of aligned CNT sensors using a method involving nanoplating and firing to produce cracks in a CNT composite film, exposing horizontally aligned carbon nanotubes (HACNTs) [19]. This research used arc produced MWCNTs as the basis for the composite film, which was rather enlightening in a field geared towards chemical vapor deposition (CVD) produced tubes. Research with arc produced MWCNTs has almost become a relic of the early years of CNT research. They were made by using a 150 mm long graphite rod for the anode and a graphite disc on a copper block for the cathode. After purification steps the CNTs were acid-oxidized using the standard 3:1 ratio of nitric acid (HNO3) to sulfuric acid (H2SO4) and washed several times in DI water before being dried in air at 120 ◦C. The acid treatment was required to increase the interfacial adhesion between the CNTs and metals. To produce the sensor, a sample of the purified CNTs was dispersed in DI water with a pol‐ yvinylpyrrolidone surfactant (PVP K30), which produced a CNT suspension. The CNT/Ni composite was produced by the addition of nickel sulfate solution containing sodium phos‐ phinate, maleic acid disodium salt hydrate, citric acid monohydrate, lead(II) acetate trihy‐ drate and sodium acetate trihydrate. The composite film was produced on a glass substrate by the immersion of the glass, with palladium particles on the surface, into the CNT/Ni sol‐ ution for 60 seconds before drying the substrate at 100 ◦C to induce cracks in the film (Fig. 4.) exposing horizontally aligned CNTs. 18 Finger platinum electrodes were then deposited by DC sputtering to complete the sensor.

**Figure 4.** Schematic of the cracked composite film exposing horizontally aligned CNTs.

These results are described in two papers and although the idea of horizontal alignment is important, it is difficult to accurately quantify the results of the papers since in both cases there is an abundance of nanoparticle palladium in both the CNT/Ni system (Pd deposited on the glass) [19] and the Pd/CNT/Ni (Pd deposited on the CNT/Ni film) system [20]. Fig. 5. shows the process of assembly for the sensors, which use a similar procedure in both of the research papers.

**Figure 5.** Schematic of the steps involved in the construction of hydrogen sensors on glass substrates with the use of the Pd nanoparticle functionalized CNT/Ni composite film. Image adapted from Lin *et al.* (2012). Schematic illustration

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In another example, a hydrogen sensor was constructed using SWCNTs and chitosan (CHIT).[21] The CHIT which covered the SWCNTs was able to filter out polar molecules and allow hydrogen to flow to the SWCNTs. The CHIT conjugate which is porous is insulat‐ ing by nature, but can be made water soluble in an acidic environment which is then useful for making a film. Additional benefits can be found in the many functional hydroxyl (–OH) and amino (–NH2) groups that react with analytes, so the effect Of a CHIT conjugate with SWCNTs for the development of a hydrogen sensor was investigated. The CHIT film was prepared by making a 2 wt% solution dissolving CHIT in a 5% acetic acid solution. This was used to coat a glass substrate or SWCNTs depending on the sensor preparation and fol‐ lowed by the removal of solvent to form the films. To evaluate the sensor performance three

of a HACNT-based gas sensor on glass substrate. Image adapted from B-R Huang *et al.* (2012).

The HACNT-based sensors were also shown to have a sensitivity response to carbon diox‐ ide, methane and ethene with a gas concentration of 200 ppm, with the highest sensitivity for H2. One of the points raised in this research, that was fundamental to the mechanism of sensing, was the role of atomized hydrogen. These atoms, produced by the metal particles, migrated to the sidewalls and the defects of CNTs, diffusing into the lattice of nanoparticles. It was stated that a dipole layer formed at that interface and affected the charge-carrier con‐ centration, and the hydrogen atoms donated their electrons to the CNTs, which resulted in a decrease in conductivity.

Interconnecting Carbon Nanotubes for a Sustainable Economy http://dx.doi.org/10.5772/51781 421

composite was produced by the addition of nickel sulfate solution containing sodium phos‐ phinate, maleic acid disodium salt hydrate, citric acid monohydrate, lead(II) acetate trihy‐ drate and sodium acetate trihydrate. The composite film was produced on a glass substrate by the immersion of the glass, with palladium particles on the surface, into the CNT/Ni sol‐ ution for 60 seconds before drying the substrate at 100 ◦C to induce cracks in the film (Fig. 4.) exposing horizontally aligned CNTs. 18 Finger platinum electrodes were then deposited

by DC sputtering to complete the sensor.

420 Syntheses and Applications of Carbon Nanotubes and Their Composites

research papers.

decrease in conductivity.

**Figure 4.** Schematic of the cracked composite film exposing horizontally aligned CNTs.

These results are described in two papers and although the idea of horizontal alignment is important, it is difficult to accurately quantify the results of the papers since in both cases there is an abundance of nanoparticle palladium in both the CNT/Ni system (Pd deposited on the glass) [19] and the Pd/CNT/Ni (Pd deposited on the CNT/Ni film) system [20]. Fig. 5. shows the process of assembly for the sensors, which use a similar procedure in both of the

The HACNT-based sensors were also shown to have a sensitivity response to carbon diox‐ ide, methane and ethene with a gas concentration of 200 ppm, with the highest sensitivity for H2. One of the points raised in this research, that was fundamental to the mechanism of sensing, was the role of atomized hydrogen. These atoms, produced by the metal particles, migrated to the sidewalls and the defects of CNTs, diffusing into the lattice of nanoparticles. It was stated that a dipole layer formed at that interface and affected the charge-carrier con‐ centration, and the hydrogen atoms donated their electrons to the CNTs, which resulted in a

**Figure 5.** Schematic of the steps involved in the construction of hydrogen sensors on glass substrates with the use of the Pd nanoparticle functionalized CNT/Ni composite film. Image adapted from Lin *et al.* (2012). Schematic illustration of a HACNT-based gas sensor on glass substrate. Image adapted from B-R Huang *et al.* (2012).

In another example, a hydrogen sensor was constructed using SWCNTs and chitosan (CHIT).[21] The CHIT which covered the SWCNTs was able to filter out polar molecules and allow hydrogen to flow to the SWCNTs. The CHIT conjugate which is porous is insulat‐ ing by nature, but can be made water soluble in an acidic environment which is then useful for making a film. Additional benefits can be found in the many functional hydroxyl (–OH) and amino (–NH2) groups that react with analytes, so the effect Of a CHIT conjugate with SWCNTs for the development of a hydrogen sensor was investigated. The CHIT film was prepared by making a 2 wt% solution dissolving CHIT in a 5% acetic acid solution. This was used to coat a glass substrate or SWCNTs depending on the sensor preparation and fol‐ lowed by the removal of solvent to form the films. To evaluate the sensor performance three different types were made (Fig. 6.). The Type I sensor was assembled simply by depositing SWCNTs onto the glass substrate with Pt electrodes placed by sputter deposition. The Type II sensor was assembled by casting the glass slide with a film of CHIT before being placed into an arc-discharge chamber to deposit SWCNTs. The Pt electrodes were added in a simi‐ lar method. The Type III sensor was assembled using the initial preparation for a Type I sen‐ sor followed by CHIT film coating and Pt electrode deposition. There were slight differences in the interaction of the CHIT film with the SWCNTs. In the Type II sensor, there was some mixing of the CNTs with CHIT but only at the interface. With the Type III sensor, the CNTs were immersed in the CHIT matrix.

terials been lost? It is well known that platinum and palladium are extremely important to the fuel cell and sensor industries, with CNTs enhancing their properties, but an increase in alternative energy devices based on these metals, whatever the concentration, may cause is‐

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The research field of photovoltaics has certainly become a hot topic over the last few years with a lot of attention based on increasing the efficiency of dye sensitized solar cells (DSSCs) in the hope that they will one day be as prevalent as the silicon based alternative. CNTs are an important addition to the field of photovoltaics with the focus on the nanotubes acting as

If there were an enclave for truly beautiful chemistry, then the research behind dye sensi‐ tized solar cells (DSSCs) would clearly be the centerpiece. The chemistry behind the opera‐ tion of these devices is inspiring a generation of researchers to address the concerns of renewable energy with a different approach to the well established silicon based solar cells. Generally, the DSSCs are comprised of an anode, electrolyte and cathode. The anode is usu‐ ally assembled from nano-crystalline titania particles (TiO2) and a dye attached to the parti‐ cles. The cathode, also known as the counter electrode (CE), is where the catalysis must occur and typically contains platinum. The iodide electrolyte facilitates the iodide/triiodide redox couple where after the excitation of the dye and loss of an electron, it regains one from iodide, oxidizing it to triiodide. The best reported efficiency for DSSCs is 11.4% as docu‐

CNTs have been used as a potential replacement for the platinum based CE. In a study by Jo et al. (2012), interconnected ordered mesoporous carbon–carbon nanotube nanocomposites were used to demonstrate Pt-like CE behavior in a dye-sensitized solar cell [22]. CNT fibers have been used as a conductive material to support the dye-impregnated TiO2 particles. The CNTs were first spun from an array synthesized by chemical vapor deposition and resulted in highly aligned macroscopic fibers [23]. The research was novel in the application of these

The CNT/TiO2 composite fiber was produced by submersing the pure CNT fiber in a TiO2 colloid solution which was followed by sintering at 500 °C for 60 min. The thickness of TiO2 layer was determined to be between 4 and 30 µm, depending on the submersion time. The dye used for the cell was cis-diisothiocyanato-bis (2,2′-bipyridyl-4,4′-dicarboxylato) rutheni‐ um(II) bis (tetrabutylammonium) which is better known as N719. For DSSCs with a metal

couple does eventually cause corrosion, but the CNT fibers exhibit a high stabil‐

p-type materials or enhancing/replacing the counter electrodes.

mented by the National Institute for Material Science (NIMS).

fibers as both the working electrode and the counter electrode.

sues of sustainability in the future.

**3.1. Dye Sensitized Solar Cells**

CE the I<sup>−</sup>

/I3 <sup>−</sup>

**3. Photovoltaics**

Resistance measurements of the films were made between the electrodes, and the values were around 100 Ω for Type I and II films and around 106 Ω for the Type III film. The high resistance could be accounted for by the contact of the electrode with chitosan, although it was noted by the authors that ohmic contacts were present.

**Figure 6.** Diagram of the 3 types of sensors. Image adapted from Li *et al.* (2010).

The response of the sensors was measured at room temperature and the results showed 15, 33, and 520% for Type I, Type II, and Type III sensors, respectively. One interesting point made by the authors was that although the Pd decoration of SWCNTs is typically used to enhance hydrogen sensing, the response can be less than the effect of chitosan at 4% H2 gas. This research provided an important step towards the use of CNTs in sensors without the requirement of Pd.

In summary, the use of CNTs in the hydrogen economy has highlighted some interesting points. Is the race to develop more efficient hydrogen powered devices really producing a sustainable economy? And has the focus on reducing the utility of some of the rare raw ma‐ terials been lost? It is well known that platinum and palladium are extremely important to the fuel cell and sensor industries, with CNTs enhancing their properties, but an increase in alternative energy devices based on these metals, whatever the concentration, may cause is‐ sues of sustainability in the future.
