This paper discusses the proper measure of the electrochemically active area (ECA) of carbon supported Pt catalyst in polymer electrolyte fuel cells employing in situ cyclic voltammetry. The charges of the hydrogen underpotential deposition (normalHupd) and CO stripping peaks obtained in situ are compared, and the influence of operation temperature (25–80°C) and relative humidity (40%–90%) is discussed. The results show that the charges of the Hupd decrease with rising temperature, while the corresponding charges of the CO stripping peak are essentially independent of temperature, at least at high relative humidity. The unexpectedly small Hupd charges are explained by the significant overlap with the hydrogen evolution reaction in a fuel cell at elevated temperatures. According to our results, it is proposed that a more reliable value of Pt ECA is estimated from the CO stripping charge. However, with decreasing humidity the charges of both Hupd and CO stripping peaks decrease, which is probably an effect of increasing blockage of Pt active sites by hydrophobic domains in the electrode ionomer. Some implications of varying cell conditions on the estimated Pt ECA and its correlation with fuel cell activity are discussed in an example from a fuel cell degradation test.

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CO 2 is a major component in reformate gas and can, as a source of CO, be a catalyst poison in polymer electrolyte membrane fuel cells. The effect of CO 2 on cell performance is not fully understood in the presence of hydrogen. This paper addresses the influence of hydrogen on CO 2 adsorption on Pt/C and PtRu/C catalysts. The results show that the reduction and adsorption of CO 2 is slow but increases if hydrogen is present, especially on PtRu/C. Further, exposure to a CO 2 and H 2 mixture at 0.15 V on PtRu/C results in current oscillations, which are dependent on operation conditions. Reformate gas inevitably contains 10-30% CO 2 and traces of CO, even after several purification steps. While numerous studies exist on the influence of CO on fuel cell performance, see reviews 1-3 and references therein, the effect of CO 2 is less studied. Although CO 2 is potentially harmless in a fuel cell, studies have shown 4,5 that 20-25% CO 2 in the hydrogen fuel induces performance losses that cannot be explained by dilution only. It is generally argued that CO 2 is transformed to CO in the fuel cell, due to the reverse water-gas shift (RWGS) reaction 4or by an electrochemical reduction mechanism, 6 such asalthough the latter has been questioned due to its low standard potential of −0.2 V. 7 According to thermodynamic calculations the RWGS reaction would give rise to CO concentrations of about 15-100 ppm in equilibrium with a 20-25% CO 2 /H 2 gas mixture 2,8,9 depending on temperature and humidity. However, the moderate effects of CO 2 in operating fuel cells 8,10 indicates that much lower concentrations must be present, presumably due to slow kinetics of both the RWGS reaction 9,11 and the electroreduction of CO 2 8 at PEM fuel cell conditions. Using PtRu as a catalyst instead of Pt has been shown to improve the tolerance of CO 2 in the hydrogen gas, 4,5,9,12 but there is some controversy as to whether it is only due to the improved tolerance to CO,11,12 or if Ru somehow hinders the transformation of CO 2 to CO. 4,9 This study sheds further light on the interplay between hydrogen and CO 2 in a fuel cell both on Pt/C and PtRu/C catalyst and how this is affected by operation conditions, in particular the influence of relative humidity and the presence or absence of H 2 . ExperimentalMembrane electrode assemblies (MEA) were prepared using Tanaka catalysts (TEC10E40E, 36% Pt, and TEC61E54, 29.7% Pt and 23.0% Ru (1:1.5) both on high surface carbon) and N115 membranes, as previously described. 13,14 The experimental setup was identical to that used in a previous study, 14 with a cell housing from ElectroChem, Inc connected to a PAR potentiostat. The MEA was activated according to the previously described procedure, 13,14 except for the adjustment of the higher potential limit in the case of PtRu from 1.2 to 0.9 V, to avoid Ru oxidation and subsequent dissolution and loss of catalyst.As a standard experiment, CO 2 was allowed to enter the cell at a potential hold at 0.15 V for 30 min followed by a 5 min purge with N 2 . These time...

Reformate hydrogen contains many impurities, some are well known while others have been less studied. Hydrocarbons are possible impurities in reformate hydrogen and are among those less studied. This study if aimed at alkenes, with special focus on propene. Adsorption and desorption on the Pt catalyst is studied using stripping cyclic voltammetry combined with mass spectrometry. The results show that although the effect of propene in the presence of hydrogen is expected to be minimal, adsorption and blockage of catalytic sites cannot be ruled out. A small amount of ad-species is formed on Pt at low adsorption potentials, and in the presence of hydrogen, although suppression of the hydrogen desorption peak was minimal if hydrogen was adsorbed on the Pt catalyst prior to exposure.