Activity and Stability of Molybdenum-Containing Dispersed Pt Catalysts for CO Tolerance in Proton Exchange Membrane Fuel Cell Anodes

Previous work conducted on carbon supported bimetallic (Pt-M/C, where M = Sn, Fe, Co, W or Mo, etc) catalysts have demonstrated the considerable CO tolerance of these materials when reformate hydrogen gas is used to feed the proton exchange membrane fuel cell (PEMFC) anodes. Among these bimetallic electrocatalysts, Pt-Mo composites have shown the most promising CO tolerance. This includes carbon supported Pt-Mo nanoparticles and various Pt electrodes whose surface was modified either with Mo oxides or some other Mo species. While a good CO tolerance of the anode electrocatalysts is a key challenge in the low temperature fuel cell technology, a high level of catalyst stability is also required to tolerate dynamic operating conditions involved in practical applications. Studies have shown that carbon supported Pt-Mo electrocatalysts present a CO tolerance up to threefold the tolerance of the state-of-the-art Pt-Ru/C catalysts, but their long term stability regarding the CO poisoning is under open discussion. This work reviews attempts to improve the activity and stability of Mo-containing dispersed Pt catalysts for CO tolerance in PEMFC anodes, made by using the following approaches: (1) application of a heat treatment at various temperatures ranging from 400 to 700 °C on carbon supported Pt-Mo (60:40, Pt:Mo) electrocatalyst; (2) deposition of Pt and Pt-Mo nanoparticles on carbon-supported molybdenum carbides and oxides (Mo₂C/C, MoO_x/C) prepared by different methods, and; (3) employing ternary and quaternary materials formed by Pt-Mo-Fe/C, Pt-Mo-Ru/C and Pt-Mo-Ru-Fe/C.

Depositions of the metallic particles on the different substrates were made by a formic acid reduction process. Catalyst samples were characterized by temperature programmed reduction (TPR), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM) and wavelength dispersive X-ray spectroscopy (WDS). Electrochemical characterizations were carried out by single cell polarization measurements, CO stripping, cyclic voltammetry (CV), and online mass spectrometry (OLMS). CV and OLMS experiments have been performed to evaluate the stability and CO tolerance of the electrocatalysts.

The Pt-Mo/C catalyst treated at 600 °C, for which the average crystallite size was 16.7 nm, showed a better stability up to 5000 potential cycles of cyclic voltammetry and a higher hydrogen oxidation activity in the presence of CO, compared to untreated Pt-Mo/C, giving an overpotential induced by CO contamination of 100 mV at 1 Acm^-2. Similar observations were made for the PtMoRuFe/C, PtMoFe/C and PtMoRu/C electrocatalysts. In the cases of Pt and Pt-Mo supported on Mo₂C/C, and also for PtMoRuFe/C, PtMoFe/C and PtMoRu/C the results also evidenced a better stability than Pt-Mo supported on carbon. In all cases, a partial dissolution of Mo from the anode and its migration toward cathode during the cell operation was observed. This was also seen for Ru and Fe, in the cases of ternary and quaternary materials.

On the basis of polarization measurements and cyclic voltammograms, it is concluded that the stability of anode electrocatalysts can be improved either by heat treatment of Pt-Mo/C, or by using molybdenum carbide as catalyst support or even by using Mo-containing ternary and quaternary electrocatalysts.

Acknowledgments: This work has been supported by Fundação de Amparo a Pesquisa do Estado de São Paulo and Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil.