Energy technology: Hydrogen quick and clean

Energy technology: Hydrogen quick and clean

Rich Masel


Rich Masel is in the Departments of Chemical and Biomolecular
Engineering, and Electrical and Computer Engineering, University of
Illinois at Urbana-Champaign, 600 South Mathews, Urbana, Illinois
61801, USA.
r-masel@xxxxxxxx


Systems for producing pure hydrogen for fuel cells from methanol run
into problems with energy efficiency and short lifetimes. Unless, that
is, you combine the right catalyst and the right purification membrane.

While the power demands of portable electronic gadgets - laptops,
mobile telephones, iPods and the like - have exploded in the past few
years, the storage capacity of the batteries used to power them has not
kept pace. In a worldwide push to find alternative power sources,
systems based on fuel-cell technologies, with their potentially much
higher energy-storage densities, have been receiving considerable
attention.

So far, however, that attention has not translated into practical
systems. Writing in Advanced Materials, Benjamin Wilhite and
colleagues1 report a substantial advance towards that goal. They have
developed a catalytic system that allows them to produce pure hydrogen,
the basic power source of proton-exchange membrane fuel cells, from
methanol at a much higher rate per unit volume than has been achieved
before.

In the way they work, fuel cells are much like batteries: they use
electrochemical reactions to produce electricity. First, a reaction on
an anode coated with a catalyst such as platinum produces electrons.
These electrons pass through a circuit, delivering power to a device.
They then return to the fuel cell, where they are collected by a second
reaction on a catalytic cathode, generally producing water as a waste
product. For example, in proton-exchange membrane fuel cells, the
reactions are 2H24H+4e- (anode) and 4H++4e-+O22H2O (cathode).

Because of its lightness and the large amounts of heat energy it
releases on combustion, hydrogen can supply much more energy per unit
weight than can the lithium batteries conventionally used in laptops
and similar electronic devices. Thus hydrogen fuel cells are an
attractive proposition to supplement, or even replace, such batteries.

The catch is that it is difficult to find a way to store or produce
enough pure hydrogen in the laptop. One could imagine putting a tank of
very pure hydrogen in the laptop and refilling it at filling stations,
rather like those proposed for cars. But in practice, the explosive
nature of hydrogen rules that possibility out. One could not, for
example, countenance taking a laptop with a hydrogen cylinder on to an
aeroplane. In fact, of possible fuels, so far only formic acid and
methanol have been approved by the dangerous-goods committee of the
International Civil Aviation Organization for use in aeroplanes and
storage in luggage2. Small butane cylinders are allowed in the
passenger compartment, but not in the hold. Hydrogen cylinders, metal
hydrides and borohydrides are currently banned. A hydrogen-powered
laptop, therefore, would need to generate its hydrogen internally from
methanol or formic acid.

There have been many suggestions3,4,5 for how to generate hydrogen from
methanol, which has the chemical formula CH3OH. A common approach is to
perform 'steam reforming' via the reaction CH3OH+H2OCO2+3H2. The catch
here is that a side-reaction yields carbon monoxide: CH3OHCO+2H2.
Carbon monoxide is highly poisonous for the catalysts used in fuel
cells: if as much as one part per million of carbon monoxide is mixed
in with the hydrogen that reaches the anode, this will, over a few
weeks of continuous operation, poison all the active sites on the
catalyst and effectively kill the fuel cell. A trace presence of carbon
dioxide can also be fatal, as this produces carbon monoxide through the
reverse water-gas shift reaction: CO2+H2CO+H2O.

In the past, differentially permeable membranes have been used to
purify hydrogen of such trace gases, with limited success. Membranes
made from the lattice-structured mineral zeolite or mesoporous silica
allow too much carbon monoxide through for practical use. Palladium and
palladium-silver membranes produce pure hydrogen, but so far have been
found to degrade quickly in the presence of carbon monoxide and
methanol. A further problem is that it takes considerable energy to
push hydrogen through a membrane that is of the thickness required for
mechanical stability. Thus, no one has yet been able to produce
hydrogen of sufficient purity at a high enough rate to power a laptop
for an extended period.

Fig. 1



Figure 1 | All in one.


Wilhite et al.1 found that, by building a composite structure that
combined a hydrogen-permeable palladium-silver membrane with a
catalyst for the hydrogen-producing methanol-oxidation reaction
2CH3OH+O22CO2+4H2 (Fig. 1), they could prevent the membrane from
degrading. Protective layers are often used6 in commercial applications
to protect sensitive materials from corrosive gases by forming an
impenetrable barrier in front of them. But such an impermeable system
is of no use in a fuel-cell system, where hydrogen must pass through
both the protective layer and the membrane to reach the anode of the
fuel cell.

But Wilhite and colleagues use a permeable catalytic layer that assists
in the oxidation of most carbon monoxide and methanol, rendering them
harmless before they reach the membrane, while letting hydrogen past.
Any traces of the corrosive gases that reach the membrane are small
enough to be oxidized there before they can do any damage, leaving the
hydrogen to filter through. Thus, hydrogen can be both produced and
purified in one stage. That is important for portable devices, but this
all-in-one approach to corrosion protection could also be useful in
many other chemically reacting systems.

As with most scientific endeavour, progress comes in small steps.
Wilhite et al. have demonstrated1 that their membrane is stable for
days, as opposed to a few hours with an unprotected membrane. But a
laptop's power supply needs to be stable for years. More thermal
management is needed, too: the temperature in the authors' system is
about 400 °C, higher than you would be comfortable with in your lap or
pocket. Nevertheless, this step represents a significant advance
towards practical portable fuel-cell power systems.

References
1. Wilhite, B. A., Weiss, S. E., Ying, J. Y., Schmidt, M. A. &
Jensen, K. F. Adv. Mater. 18, 1701-1704 (2006). Article

2. International Civil Aviation Organization Report AN-WP/8084 (18
January 2006).

3. Holladay, J. D., Wang, Y. & Jones, E. Chem. Rev. 104, 4767-4789
(2004). Article

4. Masel, R. I., Gold, S. & Zheng, N. in Encyclopedia of Chemical
Processing (ed. Lee, S.) 1643-1661 (Taylor & Francis, New York,
2005).

5. Ferreira-Aparicio, P., Benito, M. J. & Sanz, J. L. Catal. Rev. Sci.
Eng. 47, 491-588 (2005).

6. Khaladkar, P. R., Hare, C. H. & Norsworthy, R. in Uhlig's Corrosion
Handbook (ed. Revie, R. W.) 965-1069 (Wiley, New York, 2000).


http://www.nature.com/materials/news/newsandviews/060803/journal/442521a.html

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