By Will Soutter
Topics Covered
What is a fuel cell?
Nanotechnology for
Fuel Cell Catalysts
Carbon Nanotube Catalysts
Self-Cleaning Catalysts
Thin-Film Electrolyte
Membranes
References
What is a fuel cell?
A fuel cell is a device which converts a fuel directly
into electricity in an electrochemical reaction. This is in contrast to
most methods of generating electricity, which use the heat from burning
fuel to generate electricity mechanically.
Fuel cells have the potential to be an incredibly efficient power
source. They can theoretically operate on a wide range of fuels, and
the technology can be scaled from portable fuel cells in laptops, up to
huge stationary installations to power data centres.
There are many limitations preventing fuel cells from reaching
widespread commercial use, however. Expensive materials such as
platinum are needed for the electrode catalysts. Fuels other than
hydrogen can cause fouling of the electrodes, and hydrogen is costly to
produce and difficult to store. The most efficient types of fuel cell
operate at very high temperatures, which reduces their lifespan due to
corrosion of the fuel cell components.
Nanotechnology may be able to ease many of these
problems. Recent nanotechnology research has produced a number of
promising nanomaterials which could make fuel cells cheaper, lighter
and more efficient.
.jpg)
Figure 1. Schematic of a hydrogen fuel cell.
The two electrodes act as catalysts for the chemical reaction, as well
as passing charged particles across the electrolyte. The electrolyte is
usually a conducting solid ceramic, aqueous solution, or molten
phosphoric acid.
Nanotechnology
for Fuel Cell Catalysts
The catalytic electrodes in fuel cells are most often made from
platinum. It has been known for some time that using platinum
nanoparticles instead of a solid platinum surface increases efficiency,
and allows much less metal to be used.
One potential improvement to the current technology is to support
the platinum nanoparticles on a porous sufrace, such as an activated
carbon, or nanostructures like carbon nanotubes or nanowalls. This
further increases the accessibility of the platinum surfaces,
decreasing the amount of the expensive metal which is needed to make an
effective catalytic electrode.
Carbon Nanotube Catalysts
Modified carbon nanotubes may also be able to replace platinum in
fuel cells altogether. Fabrication technology for carbon nanotubes is
advancing rapidly, and the cheap and abundant raw material means that
catalysts based on carbon nanotubes can be produced for a fraction of
the cost of platinum catalysts. As platinum currently accounts for at
least 25% of the cost of commercial fuel cells, adoption of these
catalysts will remove a major barrier to many applications of fuel
cells.
By doping carbon nanotubes with nitrogen, or coating them in an
electron-withdrawing polymer (polydiallyldimethylammonium chloride, or
PDDA),
the electronic properties of the nanotubes can be altered so as to
make them effective as a catalyst.
The electrocatalytic activity of these modified nanotubes can actually
be superior to that of platinum - the power output of a fuel cell using
carbon nanotube electrodes is equal to or greater than that from the
platinum equivalent.
The nanotube electrodes are also more robust. Their catalytic
activity is not damaged by carbon monoxide or the crossover effect when
using methanol as the fuel, unlike platinum, which improves the
lifetime of the cell.
.jpg)
Figure 2. Catalytic electrodes based on
modified carbon nanotubes could make fuel cells significantly cheaper
and able to cope with more diverse fuels. Image credit: NCNR.NIST.gov
Self-Cleaning Catalysts
Current commercial fuel cells can only run on a limited range of fuels.
Most use hydrogen, and some are able to use methanol or natural gas.
Whilst other hydrocarbon fuels would be cheaper, they produce carbon
monoxide and carbon soot deposits, which poison the electrodes of the
fuel cell after a short amount of time. Running at higher temperatures
(>900C) can reduce the amount of these poisons which is produced,
but that increases the stress on the structure of the cell, requiring
costly materials for the interconnects and other important components.
In 2011, researchers at Georgia Tech announced that they could
modify the surface of a standard electrode to remove carbon deposits as
they form using barium oxide nanoparticles. The nanoparticle coating
forms "islands" on the surface, leaving plenty of space for the normal
reaction to take place, whilst initiating an oxidation reaction that
clears the catalyst surface of any deposits.
This development will allow fuel cells to be run on coal gas,
produced by gasification of coal, at a much lower temperature than
previously, allowing savings on the materials used in the rest of the
cell. This is also an incredibly clean way to use coal to generate
energy - the exhaust stream is almost pure carbon dioxide, allowing
easier operation of carbon sequestration technologies, without any
intermediate purification steps.
A similar fuel gas can also be produced by gasification of any
carbon-rich material, including biomass such as wood or waste straw.
Running fuel cells on these fuels could be a great route to large-scale
production of carbon-neutral energy.
.jpg)
Figure 3. Large fuel cell installation. Solid
oxide fuel cells are already beginning to be used to efficiently
produce energy for large buildings, warehouses and data centers. Waste
heat from the fuel cells is captured and used to heat the buildings,
resulting in up to 90% efficiency overall. Image credit: EERE.Energy.gov
Thin-Film Electrolyte
Membranes
Most commercial fuel cell designs use a solid electrolyte, such as a
conducting ceramic like yttria-stabilized zirconia (YSZ), or a polymer
like perfluorosulfonic acid. Fuel cells of this type have to operate at
very high temperatures. This causes a problem, as the performance of
the cell is improved by making the electrolyte layer thinner, reducing
its electrical resistance. Very thin electrolyte films, however, suffer
mechanical damage at the temperatures required for efficient cell
operation.
In 2010, Harvard researchers developed a method of stabilizing a
thin film of YSZ, just 100 nm thick. This was accomplished by
depositing a micrometer-scale metal grid onto the YSZ film. This method
allowed films with lateral dimensions up to the centimetre range. These
ultra-thin electrolyte layers could significantly increase the layer
density of fuel cells, making the overall size and weight of large
installations much more favourable.
References