**1. Introduction**

Concerns about depleting natural resources have been circulating for decades with alarming predictions that have turned out to be less than accurate. What has become clear, however, is the need for a decrease in the utility of a fossil based economy and a focus on a more sus‐ tainable one. This chapter reviews some of the recent progress made in the use of intercon‐ nected carbon nanotubes (CNTs) in the hydrogen, photovoltaics and thermoelectric alternative energy based economies.

The move towards a hydrogen economy is a concept that has gained traction over the last 5 years with advances in hydrogen fuel cells that are economically viable. It is envisaged that the automotive industry will begin to implement measures for the development of vehicles with hydrogen fuel cells as the economy begins to recover. However, such a move will also require a substantial investment in the infrastructure to support these vehicles. Key to the development of such technologies is the need to continuously improve the efficiency, while monitoring the safety. CNTs have been used as frameworks for a number of key areas in the hydrogen economy [1]. The most notable area is that of fuel cell integration, where the tubes are mixed with platinum or palladium to aid in the process of catalysis.

CNTs with palladium attached to their surface have also been used for the construction of hydrogen sensors, expanding the research field from the consumption to the detection of hydrogen. The recent advances in cross-linked CNT papers are stimulating the development of new materials, such as flexible palladium embedded CNT sensors [2] (Fig. 1.). This sec‐ tion of the chapter will explore some of the latest results from the use of interconnected CNTs in hydrogen fuel cells and sensor development.

© 2013 Acquah et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Acquah et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**2. The Hydrogen Economy**

**2.1. Fuel Cells & Hydrogen Storage**

lysts and hydrogen adsorption [9].

form water which is eventually drained from the system.

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

Interconnecting Carbon Nanotubes for a Sustainable Economy

http://dx.doi.org/10.5772/51781

415

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

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‐

problems in the structure, function and safety of this emerging industry?

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

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.

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