Proton Exchange Membrane Fuel Cell
This type of fuel cell also known as the Polymer Electrolyte Membrane Fuel Cell consists of a proton conducting membrane, such as a perfluorosulphonic acid polymer as the electrolyte which has good proton conducting properties, contained between two Pt impregnated porous electrodes. The back of the electrodes are coated with a hydrophobic compound such as TeflonR forming a wet proof coating which provides a gas diffusion path to the catalyst layer. Within the cell, H2 at the anode provides protons and releases electrons which pass through the external circuit to reach the cathode. The protons solvate with water molecules and diffuse through the membrane to the cathode to react with the O2 while picking up electrons and forming water.
Thus, the electrochemical reactions occurring in a PEMFC are:
at the anode:
H2 = 2H+ + 2e-
at the cathode:
1/2O2 + 2H+ + 2e- = H2O
with the overall cell reaction: l/2O2 + H2 = H20
Note that the fuel cell operates on H2 while only a few ppm of CO may be tolerated by the Pt catalysis at its operating temperature of 175 deg F or 80 deg C. If a hydrocarbon fuel such as natural gas is used as a fuel, reforming of the fuel by the reaction:
CH4 + H2O = 3H2 + CO
(in the case of natural gas), followed by shifting of the reformate by the reaction:
CO + H2O = H2 + CO2,
and removal of the unconverted CO to ppm levels are required to generate the required fuel by the cell. Any sulfur compounds present in a hydrocarbon fuel have to be removed prior to use in the reformer to a concentration of <0.1 ppmV. There is a limit on the CO concentration in the fuel gas to the fuel cell which is <10ppmV.
Some of the advantages of the cell are that it may be operated at high current densities resulting in a cell that has a fast start capability, compact and light weight design, and that there is no corrosive fluid spillage hazard because the only liquid present in the cell is water. Thus, a PEMFC is well suited for use in vehicles. A disadvantage associated with this type of fuel cell, however, is that Pt catalysts are required as promoters for the electrochemical reaction.
The ionic conductivity of the electrolyte increases with the water content. It is necessary to maintain a high enough water content in the electrolyte to avoid membrane dehydration and maintain proper ion conductivity without flooding the electrodes. Thus, the balance between production of the water by the oxidation of the H2 and its evaporation has to be controlled.
PEFCs are capable of operation at pressures from 0.10 to 1.0 MPa (10 to 100 psig) and with suitable current collectors and supporting structure, these fuel cells may be capable of operating at pressures as high as 3000 psi.
The conversion efficiency of fuel bound energy to electricity of a PEMFC is 40 to 47% on a fuel (natural gas) LHV basis.