The wide-spread application of PEM fuel cells for transportation and residential use implies the usage of ready available fuels as gasoline, natural gas and methanol instead of pure hydrogen [1]. Hydrogen is generated in the fuel cell system by the consecutive reforming and cleaning of carbon based fuels leading to fuel cell fuels containing hydrogen with concentration of only 3075%, depending on the primary fuel and the reforming process used [2-5]. Other main gas components are carbon dioxide, nitrogen, water, incompletely converted fuel and carbon monoxide [5,6], while trace amounts of ammonia [1], hydrogen cyanide [1] and hydrogen sulfide [1] are reported as well. Both sulfur containing compounds [7,8] and carbon monoxide [8-10], have already at concentration levels of 1-10 ppm a measurable negative influence on the catalyst performance, while ammonia was demonstrated to have an irreversible effect on the proton conductivity of the electrode [11]. Apart from its dilution effect, methane is regarded to be inert [12]. The influence of carbon dioxide on PEM fuel cell performance, which can be present in concentrations up to 25%, is only described in a limited number of papers [9,13,14], primarily because its effect is thought to be negligible compared to that of CO. While carbon dioxide itself is regarded as inert, the formation of carbon monoxide by a reverse water-gas shift reaction leads to a negative impact of carbon dioxide on the performance of fuel cell anodes [9,13,14]. As the concentration of carbon dioxide is in the order of 10-20%, already a very small reverse water-gas shift can lead to carbon monoxide concentrations which are comparable to the concentration directly emitted by the fuel cell processor, being in the range of 10-100 ppm. The present paper focuses on the effect of carbon dioxide on fuel cell performance at conditions relevant for stationary and mobile applications, as studied by the combination of fuel cell experiments, studies of carbon dioxide interaction with platinum and platinum-ruthenium surfaces by cyclic voltammetry and thermodynamic calculation of the carbon monoxide equilibrium concentration as a function of cell temperature and the anode gas composition.

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