Electro chemical reactions always consist of two half reactions taking place in anode and cathode electrodes. In polymer electrolyte membrane fuel cells or PEFCs, hydrogen ions produced in anode electrode travel through the ion conductive membrane and reach to the cathode electrode and the released electrons in anode, will go to cathode through an external circuit. These electrons and hydrogen ions react with oxygen producing heat and water in cathode.
H2 —–> 2H+ + 2e– Anode reaction
½ O2 + 2H+ +2e– ——> H2O cathode reaction
These half reactions proceed very slowly and at low temperature of about 80 degrees centigrade. Platinum is commonly used as catalyst for these reactions. It provides places to hydrogen decomposition easier:
H2 + 2Pt ——> 2 (Pt – H)
2 (Pt – H) ——> 2Pt + 2H+ +2e–
Controlling water and fuel cell performance:
Amount of water in the cell poles is one of the effective factors on cell performance. On one hand, the produced water must come out to avoid obstruction in reaction zones using air flow through the cell. On the other hand, the fuel entering the cell must be moist so that the membrane can stay in hydrated form. Also, lack of water will reduce the membrane ion conductivity and consequently the fuel flow drops through the cell.
Feeding air and oxidants to fuel cells:
Since air flow is used to feed oxidants to the cell, and air includes only 21% oxygen, almost 200% excess air should be fed to the cathode of the cell to react with hydrogen. It is better to use compressed air (30 to 70 psig) instead of high flow rates of air. Compressed air has the following advantages:
- Increasing oxygen partial pressure will increase the output voltage of the cell. Voltage will increase up to 0.045V if temperature rises to 100 degrees centigrade simultaneously with pressure.
- As lower volumetric flow rates of air is needed in comparison with compressed air, less amount of water per mole of air is needed to saturate the air.
- Pressure difference between compressed air and atmospheric air is too high that increases the water exit flow rate from the cell.
It’s important to note that an air compressor is needed to provide the compressed air and since the compressor power is supplied from the cell itself, there will be a voltage drop of about 120 mV in cell output. If several cells are used together as a small 0.5 to 1 kW power plant, and the air pressure drop through the cells is less than 5 psig, it’s possible to use a blower instead of a compressor to feed air to cells.
Operating temperature of polymer electrolyte cells
These cells usually operate at 70 to 80 degrees Celsius, but if the temperature is reduced to 50 degrees, following advantages could be obtained:
- Since vapor pressure changes exponentially with temperature, it will be easier to moist the air. For instance, the amount of water needed to most unit volume of air at 80 degrees Celsius if four times bigger than the amount of water needed at 50 degrees. So, smaller and lighter air humidifier system is needed.
- Cell temperature should rise to a certain level to start the operation. Reducing the operating temperature will shorten the time needed to start the operation.
- Reducing the operating temperature will reduce the thermal and mechanical tensions on the cell and fittings.
- High temperature will increase the water vapor pressure in the ion exchange membrane and affects its performance negatively.
- At lower temperatures, corrosion and oxidation rate of cell components will be slower and the cell will have loner lifetime .