Electrochemical storage of energy in carbon nanotubes and nanostructured carbons
Introduction
At the beginning of this new Millennium, the atmospheric pollution by combustion engines of automotive systems is among the most important industrial problems to be solved. Urgently, it is necessary to start with a world-wide application of environment friendly electric power sources. On the other hand, there is also a big demand for high energy density batteries or fuel cells for portable electronic devices. Actually, most of the devices available on the market in the next 10 years, together with portable systems (mobile phones, lap-tops, camcorders, etc.) should be mainly equipped with lithium-ion, nickel–metal hydride (Ni–MH) batteries or fuel cells [1].
Even if lithium-ion batteries, which combine high power and energy density, appear as the most promising system, there is still a need to improve the electrode materials for achieving the highest capacity while maintaining good electrochemical characteristics. Presently, for the negative electrode, only graphite-based composites are able to fulfil the requirements, i.e., a relatively high reversible capacity at a potential close to metallic lithium and a moderate irreversible capacity.
In the case of Ni–MH batteries, performance is limited by the hydrogen capacity of the anodic material, which does not exceed ca. 1.5 wt.% at room temperature for the most popular LaNi5-based alloys [2]. While weight and cost preclude the use of most of the hydrogen-absorbing alloys for automotive applications, recent works seem to indicate good hydrogen storage capability of nanostructured carbon materials when they are applied for cathodic electrodecomposition of water [3].
The hydrogen–oxygen (air) fuel cells have been developed for many years in spacecraft application, and efforts are underway to provide them to more spread commercial use. They can be used in a wide area of applications, from generating electricity for a utility power plant to running an all-electric or hybrid car to powering a cell phone [4]. It is estimated that using a fuel cell in an all-electric car would be required to store about 3 kg of hydrogen for 500 km driving range. Since the present technologies using high pressure or cryogenic vessels [5] do not allow safe storage and delivery of hydrogen at a reasonable cost, other solutions are strongly required. Recently, an important storage capacity of hydrogen has been claimed for graphite nanofibers and single-wall carbon nanotubes [6].
The last example of modern power sources where carbon materials find an application is the supercapacitor. This kind of cell, characterised by a low energy density, should be mainly used in hybrid systems, providing the high power that a lithium battery is unable to deliver reversibly. In this case, the electrode materials are based on high specific surface area nanostructured carbons.
It turns out that carbons play a key-role in all the storage devices considered above. However, there is still an important search for new or optimised carbons, especially with well-controlled nanostructure and surface functionality, in order to improve the storage capacity and the cycleability of the power supply. Carbon nanotubes, have been carefully considered for all these applications, where their unique morphology (e.g., helical or fishbone arrangement of graphitic layers, presence of a central canal, entanglement, bundles formation, etc.) is expected to be profitable for getting improved performance. Multi-walled nanotubes or nanofilaments (MWNTs) and single-walled nanotubes (SWNTs) have been used for lithium storage [7], [8], [9], [10], [11], [12], electrochemical capacitors [13], [14], [15], [16], [17], electrochemical hydrogen storage [18], [19] and some redox reactions [20], [21] applicable for fuel cells.
In this paper, electrochemical applications of nanotubes in energy conversion will be critically discussed and compared to other nanostructured carbons, with some attention for future perspectives. Especially our results on the use of multiwalled nanotubes for lithium insertion and supercapacitors will be presented.
Section snippets
Nanotubular materials
Catalytic MWNTs have been synthesized by decomposition of acetylene using cobalt supported on silica at 700 and 900 °C (A/CoSi700, A/CoSi900) or on zeolite at 600 °C (A/CoNaY600) [22], [23], [24], [25]. The sample A/CoSi900 was annealed at 2400 °C for 1 h under argon atmosphere, in order to modify its nanostructure. Chemical vapour deposition of propylene at 800 °C within an alumina membrane supplied bamboo-like nanotubes (P/Al800) [26]. In all cases, after the preparation of the nanotubes, the
Lithium storage in carbon nanotubes
The great interest in lithium insertion into different kinds of carbons is connected with the rapid development of lithium-ion batteries in the world market. Intensive research is still focused on the optimisation of anodic carbon materials for getting better performance of this system.
The electrodes of lithium-ion batteries are based on intercalation materials between which lithium ions are transferred through the electrolytic medium during charge and discharge [27]. The cathodic (positive
Supercapacitors from nanotubular materials
In the electrochemical capacitors, the electrical charge is accumulated in the double layer mainly by electrostatic forces [38]. The stored energy is based on the separation of charged species across the electrode–solution interface. The electrochemical capacitor contains one positive electrode with electron deficiency and the second one with electron excess (negative). The amount of electrical energy W accumulated in such a capacitor is proportional to capacitance C and voltage U (W=1/2CU2).
Electrochemical storage of hydrogen
Among all the trials for application of nanotubes in energy conversion, especially storage of hydrogen supplied a lot of controversy and discrepancy in experimental and theoretical results taking into account the amount of sorbed gas, forces and sites for its accumulation [39]. Already about 20 years ago, Carpetis and Peschka reported relatively important adsorption of ca. 3–6 g of hydrogen for 100 g of high surface area activated carbons at 78 K and up to 40 bar of hydrogen pressure [40].
Conclusion
The mesoporous character of carbon nanotubes plays a dominant role in their electrochemical properties. Compared to conventional carbon materials, carbon nanotubes have higher rate of electron transfer. Their entangled network and central canal are at the origin of pseudocapacitive effects, allowing an easy access of the ions to the electrode/electrolyte interface.
Relatively high values for lithium insertion degree up to x=2.1 (in LixC6) but with a significant hysteresis confirm that lithium
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