Influence of Hydrogen and Operation Conditions on CO2 Adsorption on Pt and PtRu Catalyst in a PEMFC

Abstract

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 Fuel in PEM Fuel Cells: the Effect of Alkene Impurities on Anode Activity

“…Instead of needing a hydrogen infrastructure, fossil or biofuel is reformed onboard or on-site, resulting in a mixture of hydrogen, CO 2 , nitrogen, water vapor, and traces of CO, sulfur species and hydrocarbons. While CO and sulfur species are well known poisons (1) for the anode of a PEM fuel cell, and CO 2 , although less poisonous, is known as a potential source of CO (2)(3)(4), the effect of hydrocarbon traces has not been as much studied.…”

Reformate Hydrogen Fuel in PEM Fuel Cells: the Effect of Alkene Impurities on Anode Activity

“…Instead of needing a hydrogen infrastructure, fossil or biofuel is reformed on-board or on-site, resulting in a mixture of hydrogen, CO 2 , nitrogen, water vapor and traces of CO, sulfur species and hydrocarbons. While CO and sulfur species are well known poisons [1]for the anode of a PEM fuel cell and CO 2 , although less poisonous, is known as a potential source of CO [2], the effect of hydrocarbon traces has not been studied as much.…”