Elsevier

Journal of Power Sources

Volume 247, 1 February 2014, Pages 712-720
Journal of Power Sources

Effect of heat treatment on the activity and stability of carbon supported PtMo alloy electrocatalysts for hydrogen oxidation in proton exchange membrane fuel cells

https://doi.org/10.1016/j.jpowsour.2013.08.138Get rights and content

Highlights

  • The CO tolerance and stability of heat-treated carbon supported PtMo (60:40 at.%) catalysts are investigated.

  • The PtMo/C catalyst treated at 600 °C shows higher CO tolerance compared to that of the untreated material.

  • PtMo/C electrocatalysts suffer a partial dissolution of Mo during a 5000 times cycling aging.

  • The stability of the PtMo/C electrocatalyst is improved after the 600 °C heating treatment.

Abstract

The effect of heat treatment on the activity, stability and CO tolerance of PtMo/C catalysts was studied, due to their applicability in the anode of proton exchange membrane fuel cells (PEMFCs). To this purpose, a carbon supported PtMo (60:40) alloy electrocatalyst was synthesized by the formic acid reduction method, and samples of this catalyst were heat-treated at various temperatures ranging between 400 and 700 °C. The 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), cyclic voltammetry (CV), scanning electron microscopy (SEM) and wavelength dispersive X-ray spectroscopy (WDS). Cyclic voltammetry was used to study the stability, and polarization curves were used to investigate the performance of all materials as CO tolerant anode on a PEM single cell text fixture. The catalyst treated at 600 °C, for which the average crystallite size was 16.7 nm, showed the highest hydrogen oxidation activity in the presence of CO, giving an overpotential induced by CO contamination of 100 mV at 1 Acm−2. This catalyst also showed a better stability up to 5000 potential cycles of cyclic voltammetry, as compared to the untreated catalyst. CV, SEM and WDS results indicated that a partial dissolution of Mo and its migration/diffusion from the anode to the cathode occurs during the single cell cycling. Polarization results showed that the catalytic activity and the stability can be improved by a heat treatment, in spite of a growth of the catalyst particles.

Introduction

Proton exchange membrane fuel cells (PEMFCs) have become promising power sources because of their low operating temperature, high power density and environment friendly characteristics [1]. Platinum, the most active electrocatalyst for hydrogen oxidation reaction, is unfortunately susceptible to CO poisoning even at a concentration as low as 10 ppm. Therefore, the effective implementation of PEMFCs depends on the development of anode electrocatalysts capable of tolerating impurities, especially CO, present in reformed hydrocarbon fuel streams.

The poisoning effect on Pt can be decreased by modifying the Pt particles by the addition of a second metal, either by the formation of an alloy or by promotion with a second metal oxide. Nowadays, much research is addressed to the PtRu alloy as anode electrocatalyst, because the PtRu alloy has a large CO tolerance and chemical stability among the catalysts developed so far [2], [3], [4]. However, due to the scarcity of Ru, there is a need to reduce the Ru usage and develop catalysts without Ru species. Research done on alloys of Pt with a non-precious metal, such as Sn, Fe, Co, W or Mo, have found excellent CO tolerance of these materials when reformate gas is used [5], [6], [7], [8]. Among all these bimetallic electrocatalysts, the PtMo alloys showed a most promising CO tolerance as compared to the state-of-art PtRu/C catalyst. Mukerjee et al. [9] reported that PtMo (4:1) catalyst exhibits a two-to-three fold enhancement in CO tolerance compared to PtRu (1:1). A wide range of bimetallic PtMo materials has been prepared and studied: single crystals of the true PtMo alloys [10], carbon supported PtMo catalyst and various Pt electrodes whose surface has been modified either with Mo oxides [10], [11] or some other Mo species. PtMo/C has not been proved only to be an active electrocatalyst when reformate fuel stream is used, but it is also a strong candidate when the ethanol or methanol is used as fuel in PEMFCs. For ethanol oxidation, Anjos et al. [12] observed a shift toward less positive potential in the anodic sweep curve for PtMo when it was compared with pure Pt catalyst. Ordóñez et al. [13] reported an enhanced activity for methanol oxidation on Pt4Mo1/C catalyst, showing low onset potential (0.38 V versus RHE against 0.5 V versus RHE for Pt/C) and high current densities for the methanol oxidation reaction. The mechanism of CO tolerance of the PtMo catalysts has been extensively studied in the literature. Two mechanisms have been reported for the enhancement of CO tolerance of these materials: a conventional bifunctional mechanism, in which the adsorption of CO occurs on Pt sites and oxygen-containing species are generated on Mo sites [14], [15], [16], and an electronic effect that involves the weakening of the Pt–CO bond [16].

While a good CO tolerance of the anode electrocatalysts is a key challenge in fuel cell technology, a high level of catalyst stability is also required to tolerate the dynamic operating conditions involved in practical automotive applications. In particular variations in the voltage over load cycle may trigger a number of degradation processes, including carbon corrosion, Pt sintering and dissolution [17]. In the case of Pt-M alloys (where M = Mo or another metal), the acidic environment of the fuel cell may cause a partial loss of M from the alloy itself, possibly reducing catalytic activity. Therefore it is important to assess the degree of catalyst stability under conditions that simulate the real-life PEMFC anode operating conditions. Although carbon supported PtMo electrocatalysts have shown a CO tolerance up to threefold the tolerance of the state-of-the-art PtRu/C catalysts, their long term stability regarding the CO poisoning is under discussion. It has been shown that bimetallic PtMo/C electrocatalysts are inherently unstable and suffer from the gradual loss of Mo due to its dissolution [7], [8].

One of the major approaches to improve electrocatalyst activity and stability is the thermal treatment [18]. Several heat treatment techniques such as traditional oven/furnace heating, microwave heat treatment, plasma thermal treatment and ultrasonic spray pyrolysis have been applied to prepare and treat PEM fuel cell electrocatalysts [19]. Among these, the traditional oven/furnace heating technique is the most widely used. In general it involves heating the catalyst under an inert (N2, Ar, or He) or reducing (H2) atmosphere, in the temperature range of 80–900 °C for 1–4 h.

In this work, carbon supported PtMo (60:40) catalysts, prepared by the formic acid reduction method, were heat-treated in the temperature range of 400–700 °C, by the traditional oven/furnace heating technique in the presence of reducing (H2) atmosphere for 1 h. The purpose of the present study was to improve the activity and stability of PtMo/C catalysts, especially against load changes. A potential cycling protocol of cyclic voltammetry at a scan rate of 50 mV s−1 ranging from 0.1 to 0.7 V was applied to the anode to stimulate load changes during fuel cell operation. To evaluate the stability of the catalysts, the performances of membrane electrode assemblies (MEAs) were measured before and after potential cycles at the intervals of 1000 potential cycles and the polarization curves were compared.

Section snippets

Experimental

A PtMo/C (60:40 atomic proportion, 20 wt.% metal/C) catalyst was prepared by the formic acid reduction method [3], [4], [5], [6], [7], [20] which consisted of simultaneous reduction of dihydrogen hexachloroplatinate hexahydrate (H2PtCl6.6H2O, Aldrich) and tetrahydrate ammonium molybdate [(NH4)6Mo7O24·4H2O, Mallinckrodt], using formic acid as reducing agent in the presence of carbon (Vulcan XC-72, Cabot). Pt supported on Vulcan XC-72 carbon with 20 wt.% metal/C was supplied by E-TEK. Temperature

Characterization of PtMo/C electrocatalysts

Temperature programmed reduction technique was used to study the reducibility and reduction temperature of oxide species, such as platinum and molybdenum oxides, dispersed on the carbon support. Fig. 1 shows the TPR profile of the as-prepared PtMo/C catalyst, in which two hydrogen consumption peaks can be observed (at around 300 and 700 °C). The lower temperature reduction peak has normally been assigned to the reduction of MoO3 [13], [25],2MoO3 + 6H2 → 2Mo + 6H2Owhereas the higher temperature

Conclusion

A carbon supported PtMo alloy electrocatalyst was synthesized by the formic acid reduction method. The effect of heat-treatment temperature on the catalyst electrocatalytic activity and stability was studied. The heat-treated catalyst showed an enhanced CO tolerance and a better stability as compared to the as-prepared catalyst, particularly due to the stabilization of the catalyst particle sizes at high temperature. Cyclic voltammetry measurements and WDS x-ray analysis showed that the PtMo/C

Acknowledgment

The authors would like to thank the Third World Academy of Science (TWAS), Italy, the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Brazilian Synchrotron Light Laboratory (LNLS), grant #2009/07629-6 from São Paulo Research Foundation (FAPESP), Brazil, and the Secretaría de Ciencia y Técnica (SeCyT) of the Universidad Nacional de Córdoba (UNC), Argentina, for financial supports.

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