**3. Carbon nanotubes for safe and efficient H2 storage**

ties, thus tuning the platform properties for enzyme immobilization. The end of the tubes is capped by a fullerene-type hemisphere that yields selective functionalization of the CNTs [14].

With the objective of dihydrogen as a future green energy vector, this review focuses on the last developments in the fuel -and more especially biofuel- cell field thanks to the advanta‐ geous use of carbon nanotubes. In a first part, carbon nanotubes for H2 storage enhancement are discussed. Then fuel cells in which carbon nanotubes help to decrease the amount of high cost noble metal catalysts are described. Green H2 economy is then emphasized consid‐ ering the key role of hydrogenase, the enzyme responsible for dihydrogen conversion. This requires the functional immobilization of the biocatalysts onto electrodes. The use of carbon nanotubes in this immobilization step is underlined, including the modes of carbon nano‐ tube functionalization and enzyme or microbes grafting. Then the advantages of developing biofuel cells in which chemical catalysts are replaced by enzymes or microbes are described. A short review of the sugar/O2 biofuel cells, the most widely investigated biofuel cell, is giv‐ en with a particular attention on the devices based on carbon nanotube-modified bioelectro‐ des. The last developments based on carbon nanotube networks for hydrogenase immobilization, or mimicking synthetic complex immobilization, in view of efficient dihy‐ drogen catalytic oxidation are finally described in order to allow the design of a future H2/O2

The discovery of carbon nanotubes (CNTs) has induced breakthroughs in many scientific domains, including H2 economy, biosensors, bioelectrochemistry…This is due to their re‐ markable properties, such as good electronic, mechanical and thermal properties. Their nanometric size compares with that of proteins and enzymes, offering the possibility of elec‐ trical connection. Their large developed surface area allows the development of devices in smaller volumes. SWCNTs are sp2 hybridized carbon in a hexagonal honeycomb structure that is rolled into hollow tube morphology [15]. MWCNTs are multiple concentric tubes en‐ circling each other [5]. Depending on the chirality, CNTs can be metallic or semiconducting. The distinction between metallic and semiconducting is very important for application, but the physical separation of allotropes is one of the most difficult challenges to overcome. In MWCNTs, a single metallic layer results in the entire nanotubes metallic behavior. Most of‐ ten mixtures of these two forms are present in CNTs preparation. More information on the physical and electronic structures can be found in many published reviews [16]. CNTs are produced by various methods such as arc discharge, laser ablation, and chemical vapor dep‐ osition (CVD). Commercially CNTs are generally produced by CVD during the pyrolysis of hydrocarbon gases at high temperature. The control of synthesis parameters (reagent gas, T °, metal catalysts) allows for the control of CNT properties. Metal impurities may remain in the CNTs sample, thus requiring purification steps. CNTs may be treated to functionalize

**2. Carbon nanotubes: an attractive carbon material**

436 Syntheses and Applications of Carbon Nanotubes and Their Composites

biofuel cell.

the surface.

The use of H2 in fuel cells to generate electricity has been proved early in the middle of the nineteenth century. Surprisingly this discovery by C. Schönbein in 1839 of current genera‐ tion by use of H2 and O2 in sulphuric acid was applied by NASA only late in 1960. Despite intensive studies over the last two decades, fuel cells still suffer from high cost and low du‐ rability. The first difficulty responsible for this slow large scale development lies on dihy‐ drogen storage and transportation, both regarded as bottlenecks considering dihydrogen specific volumic density as a gas. For convenience the gas must be intensely pressurized to several hundred atmospheres and stored in a pressure vessel. The ways to store dihydrogen with minimum hazard are under liquid state under cryogenic temperatures (at a tempera‐ ture of -253 °C), or more efficiently in a solid state. Storage of dihydrogen in hydride form uses an alloy that can absorb and hold large amounts of dihydrogen by bonding with hy‐ drogen and forming hydrides. A dihydrogen storage alloy is capable of absorbing and re‐ leasing dihydrogen without compromising its own structure, according to the reaction: M + H2 ↔ MH2, where M represents the metal and H, hydrogen. Qualities that make these al‐ loys useful include their ability to absorb and release large amounts of dihydrogen gas many times without deteriorating, and their selectivity toward dihydrogen only. In addi‐ tion, their absorption and release rates can be controlled by adjusting temperature or pres‐ sure. The dihydrogen storage alloys in common use occurs in four different forms: AB5 (e.g., LaNi5), AB (e.g., FeTi), A2B (e.g., Mg2Ni) and AB2 (e.g., ZrV2). Metal hydrides, such as MgH2, Mg2NiH4 or LiBH4, constitute secure reserves of dihydrogen [17-19]. Dihydrogen is released from MH2 upon increase in temperature and/or decrease in pressure.


**Table 1.** H density as a function of storage method.

Much progress has been made during the last years in that domain, including the high‐ light of the advantages offered by using CNTs. An efficient approach appears to be the formulation of new carbon/transition metal catalyst composites of specific composition and molecular structure, which can greatly stimulate and improve the chemical reactions involv‐ ing dihydrogen relocation in alkali-metal aluminium materials. Absorption kinetics and dihydrogen storage capacity were shown to be enhanced by mixing MH2 with SWCNTs as a result of an increase in interfacial area, decrease in MH2 particle agglomeration and nanoplatform for efficient H2 diffusion [20, 21]. The hydriding and dehydriding kinetics of SWCNT/catalyzed sodium aluminium composite were found to be much better than those of the material ground without carbon additives. Temperature of H2 desorption was low‐ ered [22]. The presence of carbon creates new dihydrogen transition sites and the high dihydrogen diffusivity of the nanotubes facilitates hydrogen atom transition. Faster ther‐ mal energy transfer through the nanotubes may also help reduce hydriding and dehydrid‐ ing times.

as ideal supporting materials to improve both catalytic activity and electrode stability. The enhancement of fuel cell performances by using CNT/Pt or Pt-alloy catalysts may arise from:

Carbon Nanotube-Enzyme Biohybrids in a Green Hydrogen Economy

http://dx.doi.org/10.5772/51782

439

Various CNT-Pt composites were used to reduce the platinum amount while preserving high catalytic activity in PEM fuel cells. Platinum nanodots sputter-deposited on a CNTgrown carbon paper [27], or deposited on functionalized MWCNTs [28] exhibited great im‐ provement in cell performance compared to platinum on carbon black. This was primarily attributed to high porosity and high surface area developed by the CNT layer. Compared to a commercial Pt/carbon black catalyst, Pt/SWCNT films cast on a rotating disk electrode was shown to exhibit a lower onset potential and a higher electron-transfer rate constant for oxy‐ gen reduction. Improved stability of the SWCNT support was also confirmed from the mini‐ mal change in the oxygen reduction current during repeated cycling over a period of 36 h [29]. Platinum particles deposited on MWCNT encapsulated in micellar surfactant were also explored as efficient catalysts for fuel cells [30, 31]. An in situ synthetic method was reported for preparing and decorating metal nanoparticles at sidewalls of sodium dodecyl sulfate mi‐ celle functionalized SWCNTs/MWCNTs. Accelerated durability evaluation was carried out by conducting 1500 potential cycles between 0.1 and 1.2 V at 80°C. These nanocomposites were demonstrated to yield a high fuel cell performance with enhanced durability. The membrane electrode assembly with Pt/MWCNTs showed superior performance stability with a power density degradation of only 30% compared to commercial Pt/C (70%) after po‐ tential cycles. Identically electrocatalytically active platinum nanoparticles on CNTs with en‐ hanced nucleation and stability have been demonstrated through introduction of electronconducting polyaniline (PANI) [32]. A bridge between the Pt nanoparticles and MWCNTs walls was demonstrated with the presence of platinum nitride bonding and π-π bonding. The synthesized PANI was found to wrap around the CNT as a result of π-π bonding, and highly dispersed Pt nanoparticles were loaded onto the CNT with narrowly distributed par‐ ticle sizes ranging from 2.0 to 4.0 nm. The Pt-PANI/CNT catalysts were electroactive and ex‐ hibited excellent electrochemical stability, therefore constitute promising potential applications in proton exchange membrane fuel cells. Strong evidence thus emerges that CNTs/Pt composites are efficient as catalysts for fuel cells. Although platinum content has been dramatically decreased, industrials consider that further optimization is mandatory for a large scale fuel cell production. In addition Nafion® membrane between the cathodic and anodic compartment delays the large scale application of fuel cells, due to cost and problem of mass transfer. Breakthrough research towards these two bottlenecks could surely enforce

**i.** higher dispersion of Pt nanoparticles,

**ii.** increased electron transfer rates,

**iii.** porous structure of CNT layers.

a hydrogen economy.

Dihydrogen can be stored through physisorption on CNTs, based on Van der Waals interac‐ tion. Based on the surface area of a single graphene sheet, the maximum value for the stor‐ age of dihydrogen capacity is around 3 wt%. Dihydrogen can also be stored through chemisorption in CNTs matrix. If the π-bonding between carbon atoms were fully utilized, every carbon atom could be a site for chemisorption of one hydrogen atom. Dillon et al. first reported in 1997 dihydrogen storage in SWCNT networks [23]. Both SWCNTs and MWCNTs store dihydrogen in microscopic pores on the tubes [24, 25]. Similar to metal hy‐ drides in their mechanism for storing and releasing dihydrogen, the carbon nanotubes hold the potential to store a significant volume of dihydrogen. The storage capacity is dependent on many parameters of the CNTs, including their structure, structure defects, pretreatment, purification, geometry (surface area, tube diameter, length), arrangement of tubes in bun‐ dles, storage pressure, temperature,…Dihydrogen uptake varies linearly with tube diame‐ ter, because the uptake is proportional to the surface area, *i.e.* the number of carbon atoms. The adsorption sites exist inside and outside the tube, between tubes in bundles, between the shells in MWCNTs. For dihydrogen storage into the tube dihydrogen must pass through the CNT wall or the tube must be opened. Hydrogen forms stable C\_ H bonds on SWCNT surface at room temperature that can dissociate above 200°C. According to SWCNT diame‐ ter 100% hydrogenation can be obtained, thus more than 7 wt % dihydrogen storage capaci‐ ty, which is above the target fixed by the US Department of Energy's Office of Energy Efficiency and Renewable Energy [26].
