PEMFCs are current generators the operating principle of which is based on the conversion of chemical energy into electrical power via a catalytic reaction of hydrogen and oxygen. Membrane electrode assemblies (MEAs), commonly called cell cores, are the basic elements of PEMFCs. They are composed of a polymer membrane and of the catalytic layers present on either side of the membrane. The membrane therefore separates the anode compartment and the cathode compartment. The catalytic layers generally consist of platinum nanoparticles supported by carbon aggregates (generally carbon black). Gas diffusion layers (carbon cloth, felt, etc.) are placed on either side of the MEA and serve as electrical conductors and ensure the uniform delivery of reactive gases and the removal of the water produced. At the anode, decomposition of the hydrogen adsorbed on the catalyst produces protons H+ and electrons e−. The protons then pass through the polymer membrane before reacting with oxygen at the cathode. Reaction of the protons and the oxygen at the cathode leads to the formation of water and to the production of heat, as shown in FIGS. 1a and 1b. 
Depending on the method used to produce the hydrogen, the gas may contain impurities. It has been shown that carbon monoxide and sulfur-containing compounds have a particularly adverse effect on the operation of the cell. In this context, maximum concentration thresholds have been set in order to standardize the quality of the hydrogen used in fuel cells: 0.2 μmol/mol for CO and 0.004 μmol/mol for sulfur-containing compounds in the case of automotive applications, for example. These values, which were set by a technical standards committee, are subject to change.
In the case where the hydrogen is produced by reforming, CO is the impurity that is mainly present.
Regarding the impact of CO on the performance of a PEMFC electrode, it is widely recognized that CO poisons platinum-based catalysts. Specifically, this molecule is very strongly adsorbed on catalytic sites, thus rendering them inactive. PEM fuel cells commonly use platinum-based catalysts. The reformed hydrogen feed of a PEM fuel cell may contain traces of CO resulting in a loss in the performance of the stack. This performance loss is due to poisoning of platinized anode catalytic sites by CO which is adsorbed (1) and therefore prevents steps in the hydrogen oxidation reaction (2).
                              Pt          +          CO                →                  Pt          -          CO                                    (        1        )                                                                                                      Pt                  +                                      H                    2                                                  →                                                      2                    ⁢                                                                                  ⁢                    Pt                                    -                  H                                                                                                                          Pt                  +                                      H                    2                                                  →                                  Pt                  -                  H                  +                                      H                    +                                    +                                      ⅇ                    -                                                                                                                                            Pt                  -                  H                                →                                  Pt                  +                                      H                    +                                    +                                      ⅇ                    -                                                                                      }                            (        2        )            
The overall hydrogen oxidation reaction is therefore the following:H2→2H++2e−  (3)
A number of solutions have already been proposed for reducing or eliminating the impact of CO on the performance of the electrode by promoting oxidation of CO to CO2 according to reaction (4).Pt—CO+H2O→Pt+CO2+2H++2e−  (4)
Solution Involving High-Temperature Operation
It has been reported that operating a fuel cell stack at a temperature of 90° C. to 200° C. allows the tolerance to CO to be improved. Li et al. reported a tolerance to 3% CO in hydrogen at 200° C., as described in the article by Q. Li, R. He, J.-A. Gao, J. O. Jensen, N. J. Bjerrum: J. Electrochem. Soc., 150 (2003) A1599-A1605.
Operation at 90° C. to 120° C. nevertheless reduces the durability of the MEAs. Operation at temperatures from 125° C. to 200° C. requires the use of membranes other than the perfluorosulfonic acid (PFSA) membranes commonly used, such as for example polybenzimidazole (PBI) membranes. However, these membranes have the disadvantage of having a lower proton conductivity than PFSA membranes and the performance obtained is therefore not as good.
Solution Involving the Use of a CO-Tolerant Catalyst:
This solution consists in using a CO-tolerant catalyst such as for example a platinum/ruthenium alloy such as described in the by E. Auer, W. Behl, T. Lehmann, U. Stenke: Anode catalyst for fuel cells with polymer electrolyte membranes, US006066410A, 2000.
CO oxidizes at a lower potential on the Pt/Ru alloy than on pure platinum, thus allowing regeneration of the catalytic sites.
Although the Pt/Ru alloy provides a better performance in the presence of CO, this type of catalyst has problems with durability notably due to dissolution of the ruthenium which irreversibly degrades performance.
Solution Involving the Adding of Trace Amounts of Oxygen to the Hydrogen in Order to Oxidize the CO, i.e. “Air Bleeding”
Thus, it has been proposed to inject a low concentration of oxygen into the fuel gas in order to oxidize the CO and thus prevent poisoning of the catalytic sites, as described in the article by S. Gottesfeld, J. Pafford: J. Electrochem. Soc., (1988) 2651-2652.
For example, 4.5% oxygen in hydrogen containing 100 ppm CO allows all the CO to be oxidized and thus the same performance to be obtained as in pure hydrogen.
However, this method has the disadvantage of increasing the risk of degrading performance because of the presence of oxygen at the anode. Specifically, reduction of oxygen at the anode is the cause of the main effects responsible for degradation of PEM cell stacks (corrosion of the carbon support at the cathode, chemical degradation of the membrane, and loss of gas diffusion layer (GDL) hydrophobicity).
USPTO Backgrounds
USPTO Backgrounds