|
|
| Fig. 1: Various types of carbon nanotubes, with the specific structure dictating the molecule's properties. (Source: Wikimedia Commons) |
The rising global energy consumption coupled with the negative environmental effects from fossil fuel use increases the need for renewable energy sources. The global energy consumption in 2012 was 549 quadrillion BTUs (5.79 × 1020 joules), and is expected to increase to 813 quadrillion BTUs (8.58 × 1020 joules) by 2040, a 51.7% climb in world energy use. [1] This has resulted in a dramatic increase in carbon dioxide and other greenhouse gases in the atmosphere, which has devastating environmental effects. Newer, more efficient renewable energy conversion and storage technologies are required to keep up with this rapid growth in demand, and carbon nanotubes (CNTs) have emerged as a promising contender for these applications. Due to their superior electrical, mechanical, and chemical properties, CNTs boast potentially significant improvements in applications for photovoltaics, supercapacitors, and lithium ion batteries. [2]
CNTs are an allotrope of carbon composed of either single or multiple sheets of graphene rolled into a cylinder. Their unique properties are attributed to the quantum effects caused by their high aspect ratio: diameters range between 0.2 - 5 nm and lengths from 10 nm - 1 cm. In addition, the direction around which CNTs are rolled, which dictates their specific structure, causes CNTs with different structures to exhibit different properties (see Fig. 1). Leveraging their properties, including high mechanical strength, elasticity, surface-to-volume ratio, thermal conductivity, and electron mobility, may improve current and potential energy conversion and storage applications.
The use of CNTs as electrodes and electrolytes has been widely researched due to their unique properties. However, their applications in energy conversion and storage technologies has a more extensive range outside of these uses.
CNTs hold a promising future in upcoming photovoltaic technologies. Solar cells are popular in the renewable energy sphere due to their low cost, ease of installation, and the abundance of solar energy. However, their low power conversion efficiencies are the main target of improvement with new technology. Current silicon solar cells range in efficiency from 21.9 - 26.7%. [3] Other types of cells, such as GaAs thin film cells, have improved efficiencies of nearly 30%, however are much more expensive to fabricate. CNTs have multiple potential applications in improving photovoltaics including use as transparent conducting films (TCFs), and electrode and electrolyte materials in dye-sensitized solar cells (DSSCs). TCFs cover the exposed surface of solar cells and thus require high electrical conductivity and transparence to visible light. [2] The most prevalent TCFs, indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO), have strong optoelectronic properties, however suffer from limited mechanical workability and high manufacturing costs. CNTs offer these same properties with the addition of better mechanical strength, flexibility, chemical stability, and lower fabrication costs. Because of this, CNTs are being widely investigated as promising replacements for traditional TCF materials. The excellent electrical and chemical properties of CNTs can also be utilized in electrode and electrolyte materials in DSSCs, which are a type of low-cost, thin film solar cells. In these applications, the fast charge transfer rate of CNTs can improve the power conversion efficiency of photovoltaics.
The incorporation of CNTs into electrodes of lithium-ion batteries (LIBs) are a pathway to the improvement of energy storage technologies. LIBs have revolutionized the battery industry with their high capacity, power capability, and portability. [4] However, further work is needed to improve their power density and safety. CNTs can enhance battery electrode performance with their strong electrical and thermal conductivities, mechanical flexibility, and high surface areas. These factors make CNTs a promising candidate for anode material in LIBs.
With these advantages, CNTs pose challenges for their widespread incorporation into energy systems. Mainly, CNTs must be able to be produced at large scales reliably and at low cost. A current disadvantage is the inability to precisely control the specific structure of an entire batch of CNTs. Because the properties are highly dependent on the exact structure, this impedes the reproducibility and reliability required by energy systems.
Overall, CNTs present an attractive alternative to current methods of energy conversion and storage. The increasing demand for renewable energy sources requires significant improvement in current technologies, and the use of CNTs are a novel way to do so. They have excellent electrical, thermal, and mechanical properties dictated by their nanoscale structure, and can contribute to environmentally friendly energy solutions.
© Kelsey Pian. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Chu, Y. Cui and N. Liu, "The Path Towards Sustainable Energy," Nat. Mater. 16, 16 (2017).
[2] M. A. Green et al., "Solar Cell Efficiency Tables (Version 52)," Prog. Photovolt. Res. Appl. 26, 427 (2018).
[3] S. Kumar et al., "Carbon Nanotubes: A Potential Material for Energy Conversion and Storage," Prog. Energy Combust. 64, 219 (2018).
[4] P. Sehrawat, C. Julien, and S. S. Islam, "Carbon Nanotubes in Li-Ion Batteries: A Review," Mater. Sci. Eng. B 213, 12 (2016).
