CO tolerance of proton exchange membrane fuel cells with Pt/C and PtMo/C anodes operating at high temperatures: A mass spectrometry investigation

Abstract

The performance of proton exchange membrane fuel cells (PEMFC) with Pt/C and PtMo/C anodes has been investigated using single cell polarization and on line mass spectrometry (OLMS) measurements in a wide range of temperature (70–105 °C) for the system supplied with hydrogen containing different amounts of CO. As expected a higher CO tolerance is observed at higher temperatures for both catalysts. The anode exit gas analysis revealed that CO₂ is produced already at the cell open circuit potential, and it increases with the increase of the anode overpotential. The CO tolerance phenomena are assigned to different processes depending on the catalyst nature. For the Pt/C containing anodes, at temperatures above 80 °C, thermal desorption, reduced CO oxidation potential and CO oxidation by O₂ crossover are responsible for enhanced tolerance, whilst PtMo/C shows greater tolerance due the occurrence of a MoO_x-mediated water gas shift reaction (WGS), which is activated at high temperatures. Although the occurrence of WGS leads to the anode poisoning in the presence of CO₂, the polarization results show that only small additive contamination effect occurs by the combined presence of CO + CO₂ in the hydrogen stream.

Introduction

Proton exchange membrane fuel cells (PEMFC) have been gaining large attention as an alternative electric power source, since they are very efficient for the conversion of chemical energy into electrical energy [1]. However, before the effective use of this technology, the problem of large power losses caused by low levels of carbon monoxide have to be solved, particularly when reformate hydrogen is used as anode reactant. In such a case, a strong CO adsorption [2], [3], [4] occurs on the Pt anode catalyst, which severely hinders the adsorption and oxidation of hydrogen [5]. Additionally, the operating environment/conditions are known to have an impact on the fuel cell's durability, and this includes exposure to impurities (on both, the anode and cathode), start-up from subfreezing conditions, and high operation temperatures [6].

In the CO tolerance context, the operating temperature [7] and the oxygen crossover [8] are shown to be important factors for improving the PEMFC performance, but some other approaches have been attempted to reduce the poisoning problem [9], [10], [11]. So, several Pt-based catalysts have been investigated to improve the CO tolerance, such as Pt–Fe [12], Pt–Ru [13], [14], [15], Pt–Mo [7], [14], [15], Pt–W [16] and Pd–Pt [17]. The higher CO tolerance of these materials, as compared to Pt alone, is usually assigned to two distinct mechanisms: the so-called bifunctional and electronic mechanisms. In the first, the presence of a second metal promotes the electro-oxidation of CO to CO₂ after a spillover process of Pt–CO to OH-species formed on the oxophilic sites on the second metal [18]. As for the electronic effect [19], [20], the presence of the second metal modifies the Pt electronic properties and thus changes the CO chemisorption properties, ultimately reducing the CO coverage and leaving freer Pt sites available for the H₂ oxidation. Also, other known ways to reduce the CO poisoning is the promotion of CO oxidation by oxygen either deliberated introduced in the fuel stream or coming from the cathode after crossing the membrane [8], and the occurrence of the water gas shift process (WGS) which corresponds to the reaction of CO with water catalyzed by specific catalysts, particularly on PtMo/C [14]. This catalyst had been also tested for the performance on hydrogen reformate (40 ppm CO, 25% CO₂) [15], where the occurrence of the reverse WGS process is proposed for explaining the poisoning effect observed for CO₂.

This work presents results showing how the increase of temperature (up to 105 °C), and the CO and CO₂ concentrations (ranging from 0.01 to 2% CO and 25% CO₂) affect the CO tolerance of PEMFC anodes formed by Pt/C and PtMo/C. By using on line mass spectrometry (OLMS) it was possible to elucidate the reactions and processes involving CO, occurring in parallel with the hydrogen oxidation reaction. The results provide new insights to the CO tolerance mechanism for PEM single cells as well on the extension of O₂ crossover from the cathode to the anode, and its role on the CO oxidation.