PEM fuel cell Pt anode inhibition by carbon monoxide: Non-uniform behaviour of the cell caused by the finite hydrogen excess

Inhibition of platinum surfaces by carbon monoxide, in particular in polymer membrane electrolyte fuel cells (PEMFC) has been observed for decades by electrochemists. Significant effects have been observed in the hydrogen stream fed to the anode of the fuel cell with concentrations ranging from 1 to 100 ppm depending on the operating conditions e.g. temperature, pressure and excess in reacting gases. As a matter of fact, the gas composition and the surface coverage by CO and H2 vary in the cell, because of the hydrogen consumption at the anode: this is to result to non-uniform distributions of electrode poisoning, current density, and overvoltage, from the inlet to the outlet of the cell. A simple 1D-model has been developed for prediction of the profiles of the above variables in the fuel cells, with the support of experimental data obtained with a 25 cm2 PEMFC: interpretation of polarization curves and impedance spectra yielded the kinetic laws of the two electrode reactions, with both neat hydrogen and CO-containing hydrogen at ppm levels. Simulations show that for low excess in hydrogen – as for practical use of fuel cells – the coverage fractions of the various species can greatly vary in the cell, resulting in non-uniform distributions of current density in the cell and enhanced electrode poisoning near the cell outlet. In contrast working with very high hydrogen excess, as can be done at bench scale, leads to uniform behaviour of the cell, and far less visibility of the anode poisoning by carbon monoxide.