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10 Hydrogen Storage and Fuel Cells

10 Hydrogen Storage and Fuel Cells

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10.10



Hydrogen Storage and Fuel Cells



517



Fig. 10.41 Design and electricity-generating mechanism of the fiber-based nanogenerator driven

by a low-frequency, external pulling force. (a) Schematic experimental setup of the fiber-based

nanogenerator. (b) An optical image of a pair of entangled fibers, one of which is coated with Au

(darker contrast). (c) Scanning electron image at the “teeth-to teeth” interface of two fibers covered

by nanowires (NWs), with the top one coated with Au which serve as the conductive “tips” that

deflect/bend the bottom ZnO nanowires. (d) Schematic of the teeth-to-teeth contact between the

two fibers covered by nanowires. (e) The piezoelectric potential created across nanowire I and

II under the pulling of the top fiber by an external force. The side with the positive piezoelectric

potential does not allow the flow of current owing to the existence of a reverse-bias Schottky barrier.

Once the nanowire is pushed to bend far enough to reach the other Au-coated nanowire, electrons in

the external circuit will be driven to flow through the uncoated nanowire due to the forward-biased

Schottky barrier at the interface. (f) When the top fiber is further pulled, the Au-coated nanowires

may scrub across the uncoated nanowires. Once the two types of nanowires are in final contact,

at the last moment, the interface is forward biased Schottky, resulting in further output of electric

current, as indicated by arrows. The output current is the sum of all the contributions from all the

nanowires, while the output voltage is determined by one nanowire. (Reprinted with permission

from [10.105]. © 2008 Nature Publishing Group)



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Nanochemistry



hydrogen fuel-cell vehicles. Experimental results show that grain refining of storage

materials, especially to the nanoscale, significantly improves the kinetics of hybriding and dehybriding of metals and alloys. The desorption energy of MgH2 , which

is 74 kJ/mol H2 for the bulk, decreases to 65.3 kJ/mol H2 for nanowires with a

30– 50 nm diameter which is consistent with theoretical predictions (see [10.106]).

TiF3 -catalyzed MgH2 nanoparticles desorb 4.5 wt% hydrogen in 6 min at 573 K, and

hydrogen uptake can take place even at room temperature. The activation energy

for MgH2 decomposition was found to decrease significantly owing to the addition of the nanosized catalyst. In Mg/MmM5 /Mg multilayer films (see Fig. 10.42),

where MmM5 is a LaNi5 -based rare-earth alloy, the hydrogen sorption temperature

of the Mg layer is dramatically decreased, presumably due to the elastic interaction

between the nanostructured Mg and MmM5 layers (see [10.106]).

Polymer electrolyte membrane (PEM) fuel cells and hydrogen are promising

candidates as power sources for cars with low-level emission of greenhouse gases

as they function in the appropriate operating temperature range (80–120◦ C). To

date, the best performing PEMs in H2 fuel cells have been composed of Nafion, a

perfluoro-sulphonated copolymer developed by DuPont in the 1960s (see [10.107]).



Fig. 10.42 Transmission electron micrograph of cross section of MmM5 /Mg/MmM5 multilayer

film. MmM5 is a LaNi5 -based rare-earth alloy; the inset is a scanning electron micrograph.

(Reprinted with permission from [10.106]. © 2008 Elsevier)



10.11



Lithium Ion Batteries and Supercapacitors



519



For explaining the electrochemical reactions during fuel–cell operation, a model of

a tubular structure of Nafion, based on x-ray scattering data, has been suggested

[10.108]. This structure comprises an array of oriented ionic nanochannels embedded within a locally aligned polymer matrix. The confined ions can thus diffuse

easily in the tubular nanostructure. The specific development of a fabrication process that can create a macroscopic orientation of the proton channels could make

Nafion the ideal PEM material [10.107].



10.11 Lithium Ion Batteries and Supercapacitors

Crucial environmental issues, which require the production and storage of renewable energy, together with the rapid advance and eagerness from the automotive

industry toward the commercialization of hybrid electric vehicles and plug-in electric vehicles, have combined to make the development of improved rechargeable

lithium batteries a worldwide imperative (see [10.109]). Whereas the Li-ion battery

has conquered the portable market with an annual volume of $5 billion [10.110], its

implementation into electric transportation is constantly postponed due to performance, cost, and more importantly, safety issues [10.109]. To address these issues,

positive electrode materials are needed that are naturally abundant and react with Li

at voltages within the thermodynamic stability range of non-aqueous Li-based electrolytes [10.109]. Some recent approaches for nanostructured Li battery electrodes

will be discussed in the following.



10.11.1 Carbon Nanotube Cathodes

A combination of carbon nanotubes and nanoporous cellulose is suggested [10.111]

for the fabrication of paper-thin and flexible batteries (Fig. 10.43). Vertical carbon

nanotubes are grown on a silicon substrate cathode and impregnated with cellulose to form a nanocomposite paper, which is a few tens of micrometers thick,

can be rolled up, twisted and bent to any curvature, and then returned to its original shape. This battery can light up a tiny light-emitting diode over several tens of

charge/discharge cycles [10.111].



10.11.2 Tin-Based Anodes

Sony Comp. showed that amorphous nanostructured tin anodes (NP-FP71 camcorder battery) can be readily recharged (Fig. 10.44) and have a 30% higher

volumetric energy density than carbon anodes [10.114]. Mesoporous carbon–

tin nanocomposites have been synthesized as anode material for Li-ion batteries

(Fig. 10.44b). The mesoporous structure of the carbon can effectively buffer the

volume changes during the Li–Sn alloying and de-alloying cycles. The specific



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Nanochemistry



Fig. 10.43 A lithium-ion battery with a carbon nanotube cathode grown on a silicon substrate

and cellulose infiltrated with the electrolyte. Electricity is produced when lithium on the anode is

oxidized to form lithium ions, which are inserted into the nanotube cathode. Charging occurs when

the lithium ions move in the opposite direction and are deposited as lithium metal on the anode.

(Reprinted with permission from [10.112]. © 2007 Nature Publishing Group)



capacity of Sn in the composite electrode is calculated to be 0.96 Ahg−1 , which

is 96.9% of the theoretical capacity (0.99 Ahg−1 ) [10.115].



10.11.3 LiFePO4 Cathodes

Electrodes made of LiFePO4 nanoparticles (40 nm) [10.116, 10.109] formed

by a low-temperature precipitation process (Fig. 10.45) exhibit sloping voltage

charge/discharge curves, characteristic of a single-phase behavior. The presence



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