Polymer Electrolyte Membrane Fuel Cells - PEMFC
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Polymer Electrolyte 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 the oxidation reaction of hydrogen are shown below: Anode oxidation of hydrogen: H2
Cathode reduction of oxygen: ½O2
+ 2H+ + 2e- Total PEMFC reaction:
H2 +
½O2
In 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 PEMFC is a key issue. Polymer electrolyte membrane fuel cells can be used as energy converters in all kinds of systems, from portable electronics and automotive applications to stationary power plants. Their main advantages are the low operating temperature, quick start-up, high power density and system simplicity. |
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1 kW portable PEMFC, Ballard. |
25 W portable DMFC, Smart Fuel Cell. |
Graphite bipolar plates (flow-fields) Schunk. |
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The 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: Anode oxidation of methanol: CH3OH
+ H2O Cathode reduction of oxygen: 1½O2
+ 6H+ + 6e- Total DMFC reaction: CH3OH
+ 1½O2
The similarity in 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. |
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Mobile phone DMFC concept, Motorola. |
DMFC mobile phone charger unit, ZSW. |
Necar 5, PEMFC with methanol reformer, Daimler Chrysler. |
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Larger 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. |
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