Membrane Fuel Cells are also called Proton
Exchange Membrane Fuel Cells (PEFC). As the name reveals, they use a solid polymer
membrane as the electrolyte, where protons are transported from the anode to
the cathode. Most membranes are based on a
sulphonated fluoropolymer, e.g. poly-tetrafluoroethylene (PTFE), which is
chemically inert in reducing and oxidising environments. In most cases, the membrane
itself is the temperature-limiting component. The maximum
operating temperature is usually around 120-130°C for PTFE-based membranes,
but can reach over 200°C for other types (poly-benzimidasol (PBI)
membranes). Due to the low operating temperature, it is necessary to include noble metals such as platinum (Pt)
and ruthenium (Ru) in the electrodes. Pure hydrogen or reformed hydrogen from
fossil fuels (e.g. natural gas) is used as the anode fuel and the cathode can be operated with air
as the oxygen source. The electrochemical reduction reaction of oxygen and
hydrogen are shown below:
oxidation of hydrogen:
H22H+ + 2e-
reduction of oxygen:
+ 2H+ + 2e-
addition to the produced water, water is also transported through the
membrane together with protons, so the cathode air flow channels can be filled
with water (also called flooding). Since proton transport through the
membrane is dependent on the humidity of the membrane, water management in
a key issue. Polymer electrolyte membrane fuel cells can be used as energy
converters in all kinds of
systems, from portable electronics
applications to stationary power plants. Their main advantages are the low
operating temperature, quick start-up, high power density and system
kW portable PEMFC, Ballard.
W portable DMFC, Smart Fuel Cell.
bipolar plates (flow-fields) Schunk.
PEMFC can also be operated on a methanol-water mixture and is then called a Direct Methanol Fuel
Cell (DMFC). Methanol is fed either as a liquid or a gas, mostly depending on operating conditions and application.
One drawback of the DMFC is the more complicated and much slower anode
oxidation of methanol compared to hydrogen. Ruthenium is added to the
anode as a binary catalyst to prevent carbon monoxide poisoning during the
reaction, see below:
oxidation of methanol:
CO2 + 6H+ + 6e-
reduction of oxygen:
+ 6H+ + 6e-
physical properties between methanol and water leads to a leakage of
methanol from the anode through the membrane to the cathode. This effect,
called methanol crossover, results in a mixed potential on the cathode and a
reduction in overall cell voltage. Due to this and the much slower oxidation of
methanol than hydrogen, the performance (power density) of the DMFC
is lower than the PEMFC. However, the easy storage of high energy
density fuel and simple system design makes it very attractive
as a substitute for rechargeable batteries for portable applications.
phone DMFC concept, Motorola.
mobile phone charger unit, ZSW.
5, PEMFC with methanol reformer, Daimler Chrysler.
units are usually equipped with a reformer for conversion of fossil fuels to
hydrogen. After this process, the fuel gas contains certain amounts of
carbon oxides and sulphides,
which all poison the fuel cell. Carbon monoxide (CO), for example, adsorbs on the catalyst particles,
thus blocking further hydrogen access. To minimise the effect of CO, the amount
in the anode fuel should
be less than 20 ppm. The tolerance for CO is in the ppm range for low-temperature PEMFC but can be
many thousands of ppm for emerging high-temperature
designs. Therefore, much research is focussed on developing better reformers and
clean-up systems in addition to CO-tolerant catalysts for fuel cells.