1 Overview

As noted in an earlier chapter, a large area of potential applications of CNTs are batteries and energy devices , including, especially, Li batteries , supercapacitors, solar cells, and materials for hydrogen storage. Many of the applications in such areas as Li batteries have originated in simple replacement of already established additives such as activated-C and graphite.

2 Batteries, Including Li Batteries

To date, the foremost area of effort for use of CNTs in batteries and energy devices has been in the field of Li batteries , mainly secondary (rechargeable) batteries . In this area, CNTs have found some limited commercial use as well, mainly as additives to cathode materials such as LiCoOx and, less frequently, additives to Li-intercalated graphite anodes.

For example, a 1 w/w% CNT loading in LiCoO2 cathodes and graphite anodes has been shown to increase electrical connectivity and mechanical integrity of these electrodes, thus, ultimately, leading to higher battery rate capability and cycle life [165,166,167]. Many lab studies have shown very exceptional performance for Li secondary batteries incorporating CNTs in one or both electrodes. Examples of such laboratory studies include MWCNT/CuO anodes with storage capacities up to 700 mAh/g [168], MWCNT-V2O5 composite electrodes with specific capacities of more than 400 mAh/g [169], and electrodes containing ball-milled SWCNTs with claimed capacities of up to 1000 mAh/g [170]. Unfortunately, such studies have not been translatable, to date, to enhance performance in practical, commercially viable Li batteries .

More recently, exotic batteries , such as “paper (Li) batteries, ” incorporating CNTs have been demonstrated wherein CNT films function as current collectors for anode as well as cathode [171,172,173].

Chen et al. [174] recently described a Li battery based on a 3D-interconnected, hybrid hydrogel comprised of a TiO2 interspersed in a CNT-conducting polymer (CP) composite, with the CP being poly(ethylene dioxythiophene) doped with poly(styrene sulfonate). The flexible Li-CNT-CP electrode appeared to demonstrate excellent Li-ion intercalation therein, high cycling stability, a capacity of 76 mAh/g in 40 s of charge/discharge, and a high areal capacity (2.2 mAh/cm2 at a 0.1C rate of discharge). Lee et al. [175] used a layer-by-layer approach to assemble a several-μm-thick electrode that consisted of additive-free, densely packed, and functionalized MWCNTs. The electrode displayed a gravimetric capacity of ∼200 mAh/g(electrode), power of 100 kW/kg(electrode), and a lifetime in excess of thousands of cycles. Guo et al. [176] described sulfur-impregnated, disordered CNT cathodes for Li-sulfur batteries with a unique stabilization mechanism for the sulfur in carbon and claimed superior cyclability and coulombic efficiency. However, to date, none of this work has been translated into a commercial, practical Li battery or supercapacitor.

CNTs have also been studied for use in lead-acid batteries . In one study, with incorporation of ca. 0.5–1 w/w% of CNTs into the cathode, the cycling characteristics of the battery were improved nearly twofold as compared to a control battery having no CNTs (1000 vs. 600 cycles to 40% discharge capacity) [177]; the proposed explanation for the improved performance was the ability of CNTs to act as a better physical binder than graphite.

3 Capacitors and Supercapacitors

Early work demonstrated that supercapacitors incorporating MWCNTs could achieve specific capacitances of up to 200 F/g and power densities of more than 8 kW/kg [178,179,180,181]. Some of the simplest methods of fabrication of CNT-based supercapacitors involve electrophoretic deposition from a CNT suspension or directly grown on a graphite or Ni substrate [182]. Attempts to improve performance via such methods as introduction of Ag nanoparticles, MnO2, or the conducting polymer poly(aniline) have produced mixed or irreproducible results [183,184,185,186].

Excellent performance has been demonstrated for supercapacitors incorporating forest-grown, binder, and additive-free SWCNTs, specifically an energy density of 16 Wh/kg and a power density of 10 kW/kg for a 40-F supercapacitor with a maximum voltage of 3.5 V, with a forecast lifetime of about 15 years based on accelerated life testing [187]. Nevertheless, the high cost of the SWCNTs used was perceived as a major impediment to commercialization. Liu et al. [188] fabricated supercapacitors by casting SWCNTs from suspensions in several solvents on the surface of Pt or Au electrodes. They observed an increase in the effective capacitance of electrodes with SWCNT films at 0.5 V (vs. a Ag quasi reference electrode) of 283 F/g, approximately twice that of carbon electrodes in nonaqueous solvents.

Once again, however, as of this writing, none of the above supercapacitor work has been translated into a commercial, practical supercapacitor.

4 Fuel Cells

For standard fuel cells that combine H2 and O2 to produce water, CNTs have been studied for use as a catalyst support for potential reduction of Pt usage by up to 60% as compared with carbon black and, possibly in the future, complete elimination of Pt use [189,190, – 191]. They have also been studied for use in methanol fuel cells [197]; in this case, MWCNTs were grown on C-fibers using a CVD method, followed by deposition of Pt particles of about 1.2 nm particle size.

5 Solar Cells

CNTs have been studied for use in organic solar cells, as a means to reduce undesired carrier recombination and enhance resistance to photooxidation. Also for potential applications in solar cells, CNTs have been studied for incorporation into CNT-Si heterojunctions, due to the observation of efficient multiple-exciton generation at p-n junctions formed within individual CNTs [192, 193].

6 Storage of Hydrogen and Other Gases

One of the predicted properties of CNTs, said to arise from their cylindrical shape and geometry and nanoscale dimensions, is the potential to store gaseous H2 at liquid-H2 densities [96, 194, 195]; one of the postulated mechanisms for this is capillary action on a nanoscale in CNTs. Since the initial studies of H2 storage in CNTs [196, 197], many strides have been made in this field. Nevertheless, experimentally measured H2 storage parameters for “medium-purity” SWCNTs remain at a maximum level of w/w% of about 4.2%, at a temperature of 300 K and pressure of 10.1 MPa, as of this writing; these may be compared with corresponding values for graphite of 4.52% at 298 K and 11.35 MPa [96]. Clearly, significant improvement in the practical performance of CNTs is required before they can be considered seriously for use in H2 storage.

It should also be noted that many irreproducible reports of high H2 storage capacity in CNT-based materials have been questioned on the basis of the fact that they could arise due to metallic impurities (e.g., Ti) in the catalytically produced CNTs [12].

CNTs have also been studied for storage of other gases, e.g., Ar, He, and SF6, with very mixed results [12].

7 Problems and Exercises

  1. 1.

    Briefly enumerate the method of functioning of secondary (rechargeable) Li batteries and identify their major components. Then identify which components and/or processes in such batteries are most amenable to improvement with the addition of CNTs, and identify how.

  2. 2.

    Briefly enumerate the method of functioning of supercapacitors. Then identify which components and/or processes in such supercapacitors are most amenable to improvement with the addition of CNTs and identify how. Enumerate the typical applications of supercapacitors.

  3. 3.

    In which component of fuel cells can the introduction of CNTs lead to improvement in their function and how?

  4. 4.

    Construct a schematic diagram of a solar cell and describe the specific function of CNT layers in its improvement (e.g., via prevention of carrier recombination).

  5. 5.

    What was the status of the use of CNTs in hydrogen storage, as of 2016? And as of your reading today?