Fuel
Age
Viswanathan
Krishnan gives an insight into a novel direct methanol fuel cell using
a tubular polymer electrolyte
In
the ever-evolving field of information technology, the demands for power
sources in mobile devices are steadily increasing. And fuel cells are
being considered as the promising candidates for supplying power to
advanced portable electronic devices such as cell phones, PDAs, digital
cameras and laptop computers, thanks to their potentially high power
density and capacity. Direct methanol fuel cells (DMFCs) are currently
being considered for use in power supplies for consumer electronics
and portable applications. These fuel cells also use polymer electrolyte
membrane as proton exchange membrane fuel cells do. The only difference
is that DMFCs do not require reformers to reform hydrocarbon or other
fuels to liberate hydrogen. Instead, methanol is fed at the anode and
hydrogen is stripped off from methanol by the electrolyte’s catalyst.
The ionisation of hydrogen takes place at the anode to release hydrogen
ions while the electrons flow through the circuit generating power.
This electrochemical process produces electricity and vents out carbon
dioxide and water as by-products. The by-product water is obtained when
the protons permeate through the electrolyte to react with oxygen and
electrons from the external circuit. DMFCs are especially advantageous
compared to reformer type fuel cells because it is easier to handle
methanol as a fuel than hydrogen, and moreover, because the system can
be simplified. Additionally, larger energy content is produced compared
to secondary batteries such as lithium. Motivated by benefits of technology,
researchers at the National Institute of Advanced Industrial Science
and Technology (AIST), Japan have developed smallscale DMFCs for portable
devices. The research team led by Tatsuhiro Okada, Senior Researcher,
AIST has developed a novel small-scale DMFC that comprises tubular polymer
electrolytes. The tubular polymer electrolyte is made of perfluorosulfonic
acid polymer material. Pt-Ru black as the anode catalyst, and Ptblack
as the cathode catalysts are loaded inside andoutside the tubular surface,
and hot pressing of membrane-electrode structures is optimised after
which an output power of 15-20 mW/cm2 is attained using 1-5 mol/L methanol
with air breathing, at an ambient temperature and pressure. Additionally,
a new method is proposed to prepare the membraneelectrode assembly (MEA)
by chemical plating, to ease the fabrication of tubular DMFCs. DMFCs
using microtubular electrolyte membranes – a technology established
at the AIST, have been successfully demonstrated. The advantages of
tubular type DMFCs are as follows:
1. Larger electrode surface to volume ratio
2. Better sealing of the fuel chamber inside the tube without the need
to use separators
3. Higher flexibility in shape compared to planar type fuel cells.
To improve the power of tubular DMFCs, hot pressing is performed using
a pair of plates with a groove and hot pressed at optimised conditions.
The peak power density of hot-pressed MEA is about six times higher
than that of a non-hot pressed MEA, and reaches 15-20 mW/cm2 at a voltage
of 200 mV. The ohmic resistance of the hot pressed MEA, as measured
by AC impedance, is about half of that as a non-hot pressed one. Thus,
the hot pressing process is very effective in improving the contact
between the electrolyte and the catalyst layer, and increases the power
density in microtubular DMFCs. When the cell was connected to a cylinder
pump to supply the...
....CONTD
