**4. Carbon nanotubes for a decrease in the amount of noble metal catalysts in fuel cells**

Among the different types of fuel cells, PEM fuel cell operates at low temperatures around 100°C. For small portable application requiring less than 10 kW, they are more suitable than higher powering solid oxide fuel cells (functioning at 700°C) due to the possible use of usual materials for electronic connectors (mainly based on carbon) and membrane. However the necessary use of platinum-based catalysts on electronic connectors to accelerate the rate of dihydrogen oxidation and oxygen reduction is a real brake towards the fuel cell develop‐ ment. Platinum is scarce enough on earth to be a limiting factor in case of large scale devel‐ opment of fuel cells. Consequently platinum currently accounts for 25% in the total cost of a fuel cell. Over the past five years, the price of platinum has ranged from just below \$800 to more than \$2,200 an ounce. Carbon black particles offer a high surface area support, able to decrease the amount of platinum particles. But they suffer from mass transfer limitations and strong carbon corrosion.

Among the low-cost alternatives to platinum, carbon appears to be the most promising. Due to their nano-structure and unique chemical and physical properties, CNTs have appeared 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:


mal energy transfer through the nanotubes may also help reduce hydriding and dehydrid‐

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

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

Among the different types of fuel cells, PEM fuel cell operates at low temperatures around 100°C. For small portable application requiring less than 10 kW, they are more suitable than higher powering solid oxide fuel cells (functioning at 700°C) due to the possible use of usual materials for electronic connectors (mainly based on carbon) and membrane. However the necessary use of platinum-based catalysts on electronic connectors to accelerate the rate of dihydrogen oxidation and oxygen reduction is a real brake towards the fuel cell develop‐ ment. Platinum is scarce enough on earth to be a limiting factor in case of large scale devel‐ opment of fuel cells. Consequently platinum currently accounts for 25% in the total cost of a fuel cell. Over the past five years, the price of platinum has ranged from just below \$800 to more than \$2,200 an ounce. Carbon black particles offer a high surface area support, able to decrease the amount of platinum particles. But they suffer from mass transfer limitations

Among the low-cost alternatives to platinum, carbon appears to be the most promising. Due to their nano-structure and unique chemical and physical properties, CNTs have appeared

H bonds on SWCNT

the CNT wall or the tube must be opened. Hydrogen forms stable C\_

**4. Carbon nanotubes for a decrease in the amount of noble metal**

Efficiency and Renewable Energy [26].

438 Syntheses and Applications of Carbon Nanotubes and Their Composites

**catalysts in fuel cells**

and strong carbon corrosion.

ing times.

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

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 a hydrogen economy.
