Unravelling the complexities of CO2 tolerance at PtRu/C and PtMo/C

doi:10.1016/j.jpowsour.2007.02.050

Journal of Power Sources

Volume 171, Issue 1, 19 September 2007, Pages 72-78

https://doi.org/10.1016/j.jpowsour.2007.02.050 Get rights and content

Abstract

The CO₂ tolerance exhibited by PtRu/C is known to be greater than that of PtMo/C catalysts in contrast to the trend in CO tolerance. In this manuscript the origins of these differences are investigated in a cyclic voltammetric investigation of the potential dependence of the poisoning of Pt/C, PtRu/C and PtMo/C anode catalyst electrodes in a miniature PEM fuel cell when exposed to pure CO, pure CO₂, or 25% CO₂ in H₂. The results show that the difference in the mechanisms of improved CO tolerance, compared to a Pt/C reference catalyst, of PtRu and PtMo explain the decreased CO₂ tolerance of PtMo compared to PtRu; at PtRu the mechanism is intrinsic (water activation at Ru sites), whilst at PtMo the mechanism relies on the turn-over the Mo(IV/VI) redox couple.

Introduction

It has long been recognised that the anode catalysts used in proton exchange membrane fuel cells (PEMFCs) must have a high tolerance to CO when operating with reformate (impure hydrogen derived from the reformation of hydrocarbons), as even low levels of CO severely poison Pt [1]. Reformate typically contains 20–25% CO₂ and 1% CO, and although the latter can be reduced to 10 ppm to 100 ppm by subsequent clean-up steps, it cannot be eliminated.

Considerable effort has been devoted to the development of CO tolerant catalysts, such as PtRu and PtMo bimetallic alloys, with carbon supported PtMo catalysts being identified as particularly promising, with a greater CO tolerance than PtRu [2], [3], [4], [5]. It has been shown that the Pt-containing catalysts developed for their CO tolerance can also be poisoned by CO₂ to levels beyond those accounted for by simple dilution [6], [7], [8]. However, the trends in CO₂ tolerance of bimetallic alloys do not necessarily follow those for CO tolerance [9], [10], [11]. We have previously reported a difference in the CO₂ tolerance exhibited by PtRu/C and PtMo/C catalysts (operating at 80 °C in 40 ppm CO and 25% CO₂ in H₂) [9]; we found that that the PtRu/C catalyst had better CO₂ tolerance than the PtMo/C catalyst.

Voltammetric studies in acidic solution have indicated that deactivation of the Pt electrode surfaces in the presence of CO₂ is caused by the build up of a CO-like poison formed by the reduction of CO₂ in the presence of adsorbed hydrogen [12]. The reduction of CO₂ at Pt under PEMFC anode conditions has been ascribed to a reaction analogous to the reverse water gas shift reaction (RWGS);CO₂ + 2M-H_ads → M-CO_ads + H₂O + Mwhere M is a surface site on the catalyst. The origins of the adsorbed hydrogen, M-H_ads, can either be the electrochemical reaction,H⁺ + e⁻ + M → M-H_adsor the non-Faradaic chemical (Tafel) reaction,H₂ + 2M → 2M-H_adsJanssen [11] proposed that the chemical mechanism would probably prevail under PEMFC conditions. However, in a recent investigation of CO₂ reduction on carbon supported Pt catalysts in acidic solution Smolinka et al. found a potential dependence of the poisoning of the catalyst surface that indicated that the electrochemical mechanism dominated [13]. They also showed that the result was reproduced in the PEMFC environment.

In the following, we will present and discuss a study of the potential dependence of the poisoning of carbon supported Pt, PtRu, and PtMo catalysts by CO and CO₂ in a model PEMFC environment. Results obtained with pure CO₂ and 25% CO₂ in H₂ will be also compared. The results will be interpreted in terms of the role of the electrochemical versus chemical origins of adsorbed hydrogen in the RWGS reaction and the mechanisms of the tolerances of the catalysts to CO and CO₂.

Section snippets

Catalysts

Three anode catalysts: 39 wt% Pt, 37 wt%/19 wt% Pt/Ru, and 18 wt%/4 wt% Pt/Mo supported on Cabot XC72R furnace carbons were prepared using proprietary methods. The catalysts were characterised for metal content using inductively coupled plasma-emission spectrometry and Pt crystallite size and degree of intermixing of the binary components by powder X-ray diffraction as summarised in Table 1.

Anode catalyst electrodes were prepared by painting aqueous Nafion containing inks of the anode catalysts onto

Results and discussion

The effect of CO in H₂ on the single cell performance is illustrated in Fig. 1, in which the cell voltage as a function of time following switching from H₂ to 100 ppm CO in H₂ is plotted for the three catalysts at a constant current density of 500 mA cm⁻². As previously reported for a similar set of catalysts [9], Pt has the greatest loss in performance, followed by PtRu and PtMo catalysts, both of which suffered much less significant decreases in performance. Similar measurements were conducted

Conclusions

The cyclic voltammetric study presented here has enabled the identification of the origins of the differences in the CO and CO₂ tolerances of carbon supported PtRu and PtMo catalysts. The differences in CO tolerance in the PEMFC environment (PtMo more tolerant than PtRu) are attributed to the difference in mechanism by which the CO tolerance is improved compared to Pt; Ru enabling water activation at lower potentials, the intrinsic mechanism, and Mo serving as a promoter via the turn-over of

Acknowledgements

Nigel Jones and Ed Wright (JMTC) are thanked for their assistance in setting up the miniature fuel cell test stand, Brian Theobald (JMTC) for preparing the catalysts, and the Royal Society for the award of an Industry Fellowship to AER.

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Unravelling the complexities of CO2 tolerance at PtRu/C and PtMo/C

Abstract

Introduction

Section snippets

Catalysts

Results and discussion

Conclusions

Acknowledgements

J. Power Sources

J. Electroanal. Chem.

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Electrochim. Acta

Electrochim. Acta

Electrochim. Acta

Int. J. Appl. Rad. Isot.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

Electrochim. Acta

J. Electroanal. Chem.

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Electrochem. Commun.

Unravelling the complexities of CO₂ tolerance at PtRu/C and PtMo/C