Platinum Catalyst Poisoning by Traces of Co in the Hydrogen

Several methods of selective oxidation of CO impurities in hydrogen have been suggested. One of them (Carrette et al., 2002) involved the application of periodic current pulses of alternating sign to the hydrogen electrode. The potential of the anode then shifts periodically in the positive direction, which causes oxidation of the adsorbed CO species and their desorption from the catalyst surface. This method is actually effective over short times, but CO adsorption repeats after cessation of the pulse. 

For fuel-cell operation, most often technical hydrogen obtained by the conversion of primary fuels such as methanol or petroleum products is used, rather than pure hydrogen obtained by electrolysis. Technical hydrogen always contains carbon monoxide and a number of other impurities, even after an initial purification. In the first experiments conducted in the mid-1980s it was shown that traces of CO in hydrogen used for the operation of fuel cells with phosphoric acid electrolyte lead to a marked increase in the hydrogen electrodepolarization. 

An increase in polarization is noticeable already in the presence of traces of carbon monoxide (less than 10 ppm). With 25 ppm of carbon monoxide and a current density of 600 mA/cm, the polarization of the electrode increases by 0.2-0.3 V, implying a loss of about 30-40% of electfical power. A more thorough elimination of carbon monoxide from hydrogen is difficult to attain. This electrode polarization is because of the fact that the platinum catalyst is a good adsorbent for carbon monoxide. On carbon monoxide adsorption, the fraction of the surface that is available for 

PROTON-EXCHANGE MEMBRANE FUEL CELLS (PEMFC) 

Still another way of fighting hydrogen catalyst poisoning by carbon monoxide impurities is that of raising the operating temperature. 

Raising the temperature, the adsorption equilibrium between hydrogen and carbon monoxide, jointly adsorbing on platinum, shifts in favor of hydrogen adsorption. This raises the highest admissible threshold concentration of carbon monoxide. The effect could be seen in fuel cells with phosphoric acid electrolyte, which work at temperatures of about 180-200°C and admit carbon monoxide concentrations in hydrogen as high as 100 ppm, despite the fact that platinum catalysts are used. 

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