The range of applications employing carbon nanotubes for energy storage and conversion include fuel cells, batteries, supercapacitors, solar cells, and thermionic power devices. In fuel cells, carbon nanotubes are likely to be utilized for hydrogen storage and in developing new composite materials for proton exchange membranes. They also represent a promising material for lithium storage in lithium-ion batteries and may even find a use in novel carbon-carbon battery types. Another potential application are supercapacitors, where nanotubes could be used as electrodes in electrochemical double layer capacitors. Nanotube-based composite materials are expected to have a high impact in solar cells industry, while an exotic class of devices is about to exploit thermionic emission of carbon nanotubes for producing electric energy from residual heat.
Therefore, the use of nanotubes in energy storage and conversion applications has the potential to affect several major industries. In the automotive industry, nanotube research could permit hydrogen energy to substitute conventional solutions sooner than expected, replacing internal combustion engines with fuel cells. Since hydrogen can be converted into electricity with only water as emission and with very high operating efficiency, another probable use could be in stationary low scale power plants or energy storage facilities, probably as a complement to solar or wind-based power plants. Solar cells could also benefit from the development of nanotube composites, which allow building of increasingly cheaper, lighter and more efficient photovoltaic cells. In the consumer electronics market, methanol-based micro fuel cells, which are currently developed as energy sources for portable devices, could complement lithium-ion, lithium sulfur, and carbon-carbon batteries employing nanotubes. Carbon nanotubes could also penetrate the aerospace industry, thanks to thermionic power devices, which are able to extract the energy from the residual heat in jet exhaust flow.
Several properties recommend using nanotubes in fuel cells. The abundant pore structure of both individual nanotubes and nanotube bundles is particularly interesting for hydrogen storage. Their excellent conductivity, and the fact that they can impart this property to various composites, positions them as promising additives for proton exchange membranes. Here, the high surface area and thermal conductivity exhibited by carbon nanotubes make them useful also as electrode catalyst supports. Good chemical stability, high mechanical strength and elastic modulus, large surface area, and toughness to weight characteristics allow devising composite components in fuel cells for transport applications, where sturdiness is an important issue.
To put the potential of nanotubes in a proper perspective one has to take into account the relevant figures from a number of other alternative technologies. Presently, La-based or Ti-based alloys can adsorb and desorb hydrogen at temperature ranges below 423 K, but both classes possess a very low gravimetric capacity (<2 wt%). Mg-based alloys, with a calculated maximum capacity of 7.6 wt%, present poor adsorption/desorption kinetics and thermodynamics. Although complex compounds, such as NaAlH4, LiBH4 and LiNH2, show very high theoretical hydrogen storage capacity (for example, 18 wt% in the case of LiBH4), they require relatively high operation temperature and present low kinetics. In addition, they are highly caustic and very sensitive to the atmosphere. Finally, the hydrogen storage capacity of activated carbon and activated carbon fibers reaches 3–6 wt% at cryo-temperatures, but at room temperature the capacity remains well below 1 wt%. As a result, all eyes have turned toward carbon nanotubes.
So far, experimental outcomes of nanotube-based hydrogen storage in gas phase look promising, but inconsistent. Recent experiments offer a broad assortment of results for hydrogen storage capacity, ranging from 0.02 wt% to 8.6 wt%. The lowest value was obtained at room temperature and for atmospheric pressure, on single-walled carbon nanotubes synthesized by arc-discharge and activated with KOH. The highest value represents the hydrogen storage capacity desorbed up to 2000 ˚C by unpurified multi-walled nanotubes produced via CVD. Other promising results also refer to multi-walled nanotubes produced through CVD on anodized Al2O3 template and annealed in argon atmosphere at 900 ˚C. Their large diameter (40 nm) and the opened tips allowed for a storage capacity of 6.46 wt% at 77 K and 10 MPa, and 1.12 wt% at room temperature, but under the same storage pressure of 10 MPa. A high hydrogen uptake of 20 wt% and 14 wt% was achieved in milligram quantities of Li-doped and K-doped multi-walled carbon nanotubes under ambient pressure, but these results were purportedly due to the presence of water. Acid treated aligned multi-walled carbon nanotubes at 300 K and 145 bar showed a capacity of 3.7 wt%. Aligned multi-walled carbon nanotubes with diameters of 50-100 nm exhibited a hydrogen storage capacity of 5-7 wt% at room temperature and under 10 atm pressure. This storage capacity grew up to 13 wt% when nanotubes were pretreated by heating to 300 °C and the catalyst tips were removed. In this case, the release of the adsorbed hydrogen was achieved by heating the samples up to 300 °C.
Nevertheless, the experimental results suggested that hydrogen storage capacity of carbon nanotubes can be further increased by optimizing their geometrical parameters: tube diameter, intertube distance (for single-walled nanotubes) or interlayer distance (for double- or multi-walled species). A better alignment and controlling the purity may also contribute to improved performance in order to achieve the target set by the Department of Energy for gravimetric hydrogen storage capacity by 2010: 6.5 wt%.
The abundant pore structure of both individual nanotubes and nanotube bundles is also highly interesting for the storage of large amounts of lithium ions, whose diameters are only 0.07 nm. Again, good chemical stability, large surface area, high mechanical strength and elastic modulus represent other important features, prolonging the life cycle of nanotube-based batteries. Their high electrical conductivity is likely to improve the power density of electrodes incorporating them. Furthermore, a recent trend promotes using new battery chemistry and architecture, for instance polyphosphonate electrolytes, in order to devise all-carbon batteries employing nanotube-based electrodes. This system also benefits from the lower density of carbon, compared with metal oxide cathodes based on cobalt.
Thus far, the Li-ion battery presented the advantage of high working voltage, high energy density, long life cycle, good environment compatibility and good safety reliability over the old lead–acid battery and the Ni-based battery. In 2000, the market share of Li-ion batteries surpassed the one of Ni–metal hydride and Ni–Cd batteries all together, reaching a global sale of 4 billion dollars in 2003. Li-ion batteries constantly offer higher capacities, larger power densities and longer cycle lives. Currently, the anode of Li-ion batteries is primarily made from various other carbonaceous materials, but carbon nanotubes promise to boost this rate of growth, either by themselves or incorporated into appropriate composite material.
Carbon nanotubes by themselves are able to adsorb a considerable amount of lithium. Nevertheless, the electrochemical performance of carbon nanotubes strongly depends on their structure and morphology, as well as on the level of disorder between nanotube bundles. The reversible capacity of etched multi-walled carbon nanotubes reached 681 mAh/g, exceeding the value obtained for purified multi-walled carbon nanotubes - 351 mAh/g. In the case of opened multi-walled carbon nanotubes, lithium storage capacity may get to 1281 mAh/g. Multi-walled carbon nanotubes with outer diameters of 20–50 nm exhibited a lithium storage capacity of 340 mAh/g. The capacities of single-walled carbon nanotubes vary between 450 mAh/g and 600 mAh/g. Ball milling can further increase these values and reduce the irreversible capacity of lithium storage. Results as high as 1000 mAh/g were reported after a 10 minutes ball milling treatment.
In contrast with other active carbon fibers, which exhibit mostly micropores inaccessible to electrolyte ions, carbon nanotubes contribute with high mesopore (2–50 nm) volume and large specific surface area. In addition, of course, as you have already read, carbon nanotubes and their composites have shown high electrical conductivity and chemical stability.
Compared to batteries, supercapacitors exhibit higher power density. The power density of a supercapacitor is about ten times larger than that of secondary battery. Moreover, their energy density is 1-2 orders of magnitude higher than conventional capacitors. Other advantages are long life cycles and short charge time. So far, high specific surface area activated carbon is normally used for the electrodes of double layer capacitors.
Supercapacitors employing multi-walled carbon nanotubes electrodes achieved a capacitance of 80 F/g. When the nanotubes were subjected to concentrated nitric acid pre-treatment, their specific capacitance was increased to 137 F/g. Other electrodes based on plasma-treated multi-walled carbon nanotubes presented an increased capacitance of almost 207.3 F/g in KOH solution. Pre-treated single-walled carbon nanotubes in a KOH solution have a maximum specific capacitance of 18 F/g, while electrodes made of 350°C-oxidized single-walled carbon nanotubes may exhibit specific capacitances from 30 to 140 F/g. Single-walled carbon nanotubes coated by polypyrrole showed specific capacitances 5–10 times higher than those measured for pure single-walled carbon nanotubes or polypyrrole. A maximum capacitance of 163 F/g in H2SO4 electrolyte was obtained for the polypyrrole-modified multi-walled carbon nanotubes. For carbon nanotube-polypyrrole–poly(3-methyl-thiophene) composite, the measured capacitance was 87 F/g. Double wall carbon nanotube/polyaniline had a specific capacitance of 250 F/g, much higher than that of pure double wall carbon nanotubes (about 36 F/g) and pure polyaniline (about 90 F/g). Nanotube-composites containing nickel oxide exhibited a specific capacitance of 160 F/g. The specific capacitance of impregnated ruthenium oxide on carbon nanotubes was larger than 960 F/g.
Carbon nanotubes have aroused a legitimate interest in the solar cell industry due to their excellent electrical conductivity. Also, carbon nanotubes have the advantage of being able to impart this property to various composite hosts, without impairing their optical transparency. Other attracting features are the semiconducting nature of some nanotube species and photoconduction capabilities.
Thin-film polymeric solar cells reinforced with carbon nanotubes feature greatly improved carrier mobility. Even a small weight percent of nanotubes added to a polymeric thin-film host can dramatically improve the film's electrical conductivity. For instance, the electrical percolation threshold of nanotubes in epoxy-based composites was reported at just 0.0025 wt% of multi-walled carbon nanotubes. Single-walled nanotubes incorporated into poly(3-octylthiophene) yielded larger photocurrent, surpassing by two orders of magnitude that of the pristine diodes. A doubling of the open circuit voltage was also observed. Since single-walled nanotubes can themselves be semiconducting and are very good absorbers in the visible range, they may allow a significant efficiency enhancement in intermediate band solar cells. Photons of energy less than the band gap of the cell may be converted and forced to excite carriers across the energy gap by an intermediate band controlled through the nanotube filling. A recent prototype of Aluminum doped single-walled carbon nanotube-polymer/indium tin oxide thin-film device exhibited promising photoconductive and photovoltaic responses in a broad spectral range, typically from 300 to 1600 nm. A better control upon film preparation and polymer doping will likely permit further improvements in device performance.
In addition, as conductive additives, nanotubes will hopefully deliver the price-drops that will allow solar cells to be manufactured and sold on commercial basis. Therefore, several groups have already shown an interest in the use of carbon nanotubes as electron acceptors in polymeric materials.
Thermionic power devices
Thermionic power generators represent an emerging field of applications for carbon nanotubes. This class of devices converts the residual heat of various processes into electric energy. So far, their use was restricted to situations where the waste heat occurred at high operating temperatures relative to the background temperature, such as exhaust gases in jet engines. Carbon nanotubes are expected to allow thermal to electric conversion even at low temperatures, commonly encountered in the majority of applications resulting in waste heat.
A thermionic power converter consists of an electron emitter (cathode) and an electron collector (anode) placed in a vacuum tube. The cathode is in thermal contact with the heat source, while the anode is coupled with the heat sink. When a voltage is applied across the gap between anode and cathode, it extracts the charged thermions and produces an electrical current. Single-walled carbon nanotubes appear as very promising candidates for reinforcing both electrodes of the device, due to their low percolation threshold, combined with a high electrical conductivity and a remarkable anisotropic thermal conductivity. Early prototypes incorporating titania films with homogeneously embedded carbon nanotubes were already demonstrated, but more efforts are still expected.
Nanotubes are most likely to have an impact in the energy storage and conversion field of applications. Whether their potential for hydrogen storage will be confirmed or not, nanotube-reinforced composites and nanotube-based catalyst supports will likely improve electrodes and proton exchange membranes in fuel cells. Electrodes for lithium batteries are also a sure target and a huge market in the same time. The third major field of research is represented by the solar cell industry, where nanotubes are expected to boost the performances and to significantly lower the costs. Unlike more exotic applications (such as carbon-carbon batteries or thermionic power devices), nanotube-based fuel cells, photovoltaic cells and batteries are expected to hit the market within a decade, as nanotubes’ prices will surely drop.