Uy

Cathode

Fig. 10.15. Schematic diagram of MEA and single cell testing apparatus (Iyuke et al. 2003)

10.6.6 Fuel Cells

In the year 2004, especially at the last quarter of the year, motorists and other forms of energy generators from fossil fuel around the world whimpered as prices of petroleum products especially petrol soared to unprecedented levels. Consumers at homes stared in disbelief as gas and electricity prices increased with no glimpse of immediate end. In spite of these problems coupled with environmental pollution problems such as global warming caused by the exhaust releases from the internal combustion engines, the world has no option other than to look into a safe and environmental friendly hydrogen fuel cell technology for a lasting solution. Thus the overwhelming advantages of proton exchange membrane (PEM) fuel cell amongst its four competing potential commercialisation counterparts (alkaline, phosphoric acid, molten carbonate and solid oxide fuel cells) have aroused concerns in research and development in energy and environment. PEM fuel cell (Fig. 10.15) is one of the currently known five types of fuel cell that convert chemical energy of fuels (e.g. H2 and O2) directly into electricity, heat and water. PEM fuel cell is commonly claimed as the most promising fuel cell due to its portable power and residential applications. This section therefore discusses the roles of carbon nanotubes in acquiring better performed and cost saving in size reduction of the membrane electrode assembly (MEA) and hydrogen storage, which have remained teething problems in fuel cells commercialisation and production efforts.

10.6.7 Membrane Electrode Assembly

One major component in the PEM fuel cell is the MEA often referred to as the heart of the PEM fuel cell. The MEA consists of a sheet of proton conducting polymer electrolyte membrane with two Pt/C electrodes, which are the anode and cathode bonded to the opposite sides of the membrane sheet. The arrangement is then compressed on both sides by grooved bipolar plates or grooved end plates in the case of single cell, to transport the H2 and O2/air respectively to the electrodes, as shown in Fig. 10.15 (Iyuke et al. 2003). Perfluorinated sulfonated cation exchanger membrane (NafionTM) has been commonly used in MEA for PEM fuel cell to provide mechanical support, insulation and as solid electrolyte for H+ transport. These qualities have been improved further in the study of Marken and co-workers where the electrochemical behaviour of MWNTs/NafionTM modified electrodes in 5 mM Fe(CN)l" solution and 0.1 MKCl. The cyclic voltammograms obtained for NafionTM alone and MWNTs/NafionTM blends are presented in Fig. 10.16(a)&(b), respectively showing the oxidation and reduction peaks observed are at +0.25 and +0.19 V having 63mV inter peaks separation at scanning rate of 0.1 V/s with the modified MWNT/NafionTM electrode. In comparing Fig. 10.16(a & b), higher peak current responses for oxidation and reduction and inter peaks separation of 290 mV for the NafionTM coated glassy carbon electrode due to the incorporated MWNTs were observed (Tsai et al. 2004). It was thus explained that the higher activity of the MWNTs/NafionTM modified electrode was due to the higher surface electroactivity provided by the MWNTs as compared to the electrode coated with only NafionTM. These observed improved qualities therefore justify better workability, life span, chemical, pressure and heat stability of the MWNTs/NafionTM coated electrode as membrane for PEM fuel cell application. On the other hand, Pt/graphite electrode has been the usual electrocatalytic electrode in PEM fuel cell (Iyuke et al 2003). However, in the electrochemical and electrocatalytic studies in a PEM fuel cell (Tang et al. 2004) of a finely and highly dispersed Pt on carbon nanotubes (Pt/CNT) electrode as compared to Pt/graphite electrode showed a remarkable superiority over the latter. The insert of Fig. 10.16(c) presents the TEM image of Pt dispersed on the CNTs. Line I represents typical cyclic voltammogram of CNT electrode, which is the background current with characterised capacity current, and in this case larger than the graphite electrode, Line II. Thus these results are considered breakthrough in obtaining elegant MEA, which would result into miniaturised PEM fuel cell stack.

20 pA

20 pA

.Sim

.Sim

Potential (V)

Fig. 10.16. Membrane electrode assembly (MEA) components and performance comparison (a) cyclic voltammetric response for modified Nafion™, (b) cyclic voltammetric response for MWNTs/ NafionTM modified glassy carbon electrode, and (Tsai et al. 2004), (c) linear voltammograms of Pt/CNT and Pt/graphite electrodes in 50 cm3/min air flow bubbled H2SO4 aqueous solution: Line I, CNT electrode; Line II, graphite electrode; Insert is the TEM image showing Pt catalyst dispersion on well aligned CNTs (Tang et al. 2004)

10.6.8 Mechanical and Electrical Reinforcement of Bipolar Plates with CNTs

As mentioned earlier, cost is one of the key issues hampering or delaying full bloom of PEM fuel cell application in the automobile. The high cost is not only from the MEA but also the bipolar plates. Hitherto, these plates are machined from PocoTM graphite or carbon/polymer composites, which are brittle during machining of the flow channels (Fig. 10.15). However, efforts to research into other potential materials such as metal-based bipolar plates, carbon-filled polymers or carbon-carbon composites have not yielded adequate success because cost remains high and performance is still unsatisfactory. Obviously the metalbased plates would succumb to corrosion in fuel cell stack environment, and the cations that will be released will not only enhance membrane resistance but also lead to the electrode catalyst poisoning. As for polymer filled with carbon, the carbon load would normally be greater than 50% by volume to attain good electrical conductivity for the bipolar plate (Barbir et al. 1999). Unfortunately, injection moulding proposed for mass and economic manufacturing of bipolar plates will be constrained by the high carbon concentrations due to difficulty in processing. Meanwhile, improvising with compression moulding will trade off the benefits expected because it is slow, as it requires cooling and products discharging (Barbir et al. 1999). Furthermore, high carbon concentration in the polymer would substantially decrease the strength and ductility and also impose adverse effects on the tensile strengths of the composites. However, Wu and Shaw (2004a) indicated some rays of hope from the problems when they demonstrated triple continuous carbon-filled polymer blends in injection moulding of polyethylene terephthalate (PET)/polyvinylidine fluoride (PVDF) blended with CNTs. They observed that the blend exhibited 2500%, 36% and 320% improvement in electrical conductivity, tensile strength and elongation, respectively over PET blended with CNTs at the same carbon concentration. In an increased effort to affirm the earlier claims with similar blends, Wu and Shaw (2004b)obtained improved mechanical and electrical properties for CNT-filled polymers over ordinary polymer blends as presented in Table 10.4.

Table 10.4. Tensile strength, electrical conductivity and resistivity of polymer and CNT-polymer blends (Wu and Shaw 2004b) for PEM fuel cell bipolar plates

Parameter

PET

PVDF

CNT-filled

CNT-filled

% improvement

PET/PVDF

PET

in CNT-filled

(6 vol.%

(6 vol.%

PET/PVDF over

CNT)

CNT)

CNT-filled PET

Elongation at the

2.2

1400

5.1

1.2

325

rate of break 9%)

Tensile stress at the

34

32

-

25

-

rate of yield (MPa)

Tensile stress at the

34

54

34

25

36

rate of break (MPa)

Conductivity (S/cm)

0.059

0.0023

Resistivity, p (cm)

16.95

430

10.6.9 Hydrogen Storage in CNTs

The world today is in search for a novel material to store hydrogen as fuel for a clean, renewable and environmental friendly technology, such as fuel cell, automotive, stationary power generation etc. Quite a number of publications have proposed materials ranging from metals and alloys in their hydride forms, carbons to cryogenic methods for hydrogen adsorption. Amongst these prospective candidates, carbon materials such as carbon nanotube and graphite nanofibre (GNF) are receiving the most attention due to their large capacity to adsorb hydrogen, though still controversial. The United States department of Energy has set targets of 6.5 wt% and 62 kg H2/m3 for on-board hydrogen storage at ambient temperature for fuel cell powered vehicles. It has been envisaged that a compact passenger vehicle powered by fuel cell would require 4 kg H2 for a 400 km driving range (Yang 2003). Since the first report on hydrogen storage in SWNTs by Dillon et al (1997) quite a number of works have been reported in the literature. Thus in conjunction with the review by Poole Jr and Owens (2003) subsequent experimental results on H2 storage in SWNTS, MWNTs, GNFs are presented in chronological order in Table 10.5. In general the vast evidences available affirm the progressive developments of hydrogen storage for PEM fuel cell that will alleviate the petroleum reserves and price problems as well as the threatening global environmental issues.

Table 10.5. Chronology of hydrogen storage reports in carbon nanotubes (SWNT and MWNT) and graphite nanofibres (GNF)

Material

H2 wt %

Conditions

Author &

storage

T(K), P(MPa)

Reference

SWNT

~5-10

133, 0.04

Dillon et al. 1997

MWNT

11.26

300, 9

Chambers et al. 1998

GNF

67.55

300, 12

Chambers et al. 1998

SWNT

~8

80, 11.2

Ye et al. 1999

SWNT

4.2

300, 10.1

Liu et al. 1999

GNF

35

300, 11

Park et al. 1999

Li/MWNT

20

473-673. 0.1

Chen et al. 1999

K/MWNT

14

300, 0.1

Chen et al. 1999

Li/MWNT

2.5

473-673, 0.1

Yang et al 2000

K/MWNT

1.8

300, 0.1

Yang et al 2000

K/MWNT

1.3

300, 0.1

Pinkerton 2000

MWNT

3.98

300, 10

Li et al. 2001

GNF

>0.7

296, 11

Tibbetts et al. 2001

SWNT

0.05

296, 3.59

Tibbetts et al 2001

GNF

0.7

295, 10.5

Poirier et al. 2001

SWNT

1.5

300, 0.08

Hirscher et al. 2001

GNF

~15

300, 12

Gupta & Srivastava 2001

GNF

6.5

300, 12

Browning et al 2002

MWNT

0.65

300-373, 0.1

Lueking & Yang 2002

MWNT

3.6

298, 7

Lueking & Yang 2002

SWNT

1.0

253, 6

Luxembourg et al 2004

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