Molybdenum carbide-based electrocatalysts for CO tolerance in proton exchange membrane fuel cell anodes

Highlights

• Stability and CO tolerance of Pt catalysts supported on molibdenum carbide-carbon composites were investigated.

• The prepared molybdenum carbide/carbon phase presented the Mo₂C/C formula.

• The Pt catalyst supported on Mo₂C/C presented higher CO tolerance than a typical PtMo/C material.

• The Pt catalyst supported on Mo₂C/C showed higher stability up to 5000 potential cycles of cyclic voltammetry than a typical PtMo/C material.

Abstract

The activity, stability and CO tolerance of molybdenum carbide-based electrocatalyts were studied in anodes of proton exchange membrane fuel cells (PEMFCs). To this purpose, carbon-supported molybdenum carbide (Mo₂C/C) was prepared by an ultrasonic method, and was used as catalyst support in the anode of a PEMFC. Pt and PtMo nanoparticles were deposited on this Mo₂C/C by the formic acid reduction method. The physical properties of the resulting electrocatalysts were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray absorption near edge structure (XANES), scanning electron microscopy (SEM) and wavelength dispersive 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 were performed to evaluate the stability and CO tolerance of the electrocatalysts. The results obtained for the carbide-based electrocatalysts were compared with those obtained for carbon-supported PtMo and Pt electrocatalysts. It was observed that Pt and PtMo supported on Mo₂C/C present a better stability than PtMo supported on carbon. CV and WDS results evidenced a partial dissolution of Mo from the anode and its migration toward cathode during the cell operation. On the basis of polarization measurements and cyclic voltammograms, it was concluded that the stability of anode electrocatalysts can be improved by the use of molybdenum carbide as catalyst support.

Introduction

Fuel cells are considered as alternative electrical power sources for the near future. Among the various types of fuel cells used today, the most promising is the proton exchange membrane fuel cell (PEMFC), which is particularly useful for portable applications, because of its relatively high power density despite the low temperature of operation [1]. The electrocatalysts most commonly used for hydrogen oxidation in the PEMFC anode mainly contain Pt, which ensures a good electrocatalytic activity. However, the hydrogen obtained from reformate fuel stream contains carbon monoxide (CO) impurities that are easily adsorbed on the Pt surface, thus reducing the Pt surface available for hydrogen adsorption and eventually reducing the cell performance [2]. Therefore, a transition metal such as Ru or Mo is typically added to the Pt in order to increase the CO tolerance of the anode electrocatalysts by desorbing CO molecules from Pt surface [3], [4]. Unfortunately, the high price of these metals, particularly Pt and Ru, and wear of the catalysts, still prevent the large-scale commercialization of PEMFCs. Transition metal carbides have been studied because of their catalytic activities for many chemical reactions including hydrodenitrogenation, hydrodesulfurization, methanol steam reforming and water-gas shift (WGS) reactions [5], [6]. These carbides can be produced having large surface area and catalytic properties resembling those of the Pt-group metals (PGMs) [7], [8].

In particular, molybdenum carbide (Mo₂C) has been investigated as anode electrocatalyst for PEMFCs [9], [10], although its catalytic activity for H₂ electro-oxidation is not high enough to meet the requirements for fuel cell applications [11]. However, the catalytic activity of this carbide can be noticeably improved by the addition of Pt. In a recent study conducted to evaluate the use of molybdenum carbide as electrocatalyst in fuel cell anodes, it has been reported that Pt-Mo_carb supported on carbon black has shown a superior activity for methanol electro-oxidation in comparison with Pt supported on carbon [12]. In another study it was observed that molybdenum carbide has a high activity for the dissociation of methanol and water, as well as a relatively low CO desorption temperature of around 330 K [13]. Moreover, Weigert et al. [9] observed that Pt-modified Mo-C has enhanced activity and stability compared to Pt supported on carbon.

Summarizing, some research data can be found in literature regarding the activity of Mo₂C/C either for methanol oxidation or oxygen reduction, however, the CO tolerance and stability of this material has not been studied in detail. Thus, in this work, molybdenum carbide was prepared and used as support of Pt and PtMo electrocatalysts for hydrogen electro-oxidation in the presence of 100 ppm CO in PEMFC anodes. In addition to physical and electrochemical properties, the stability of these electrocatalysts was studied, in comparison with carbon-supported PtMo and Pt electrocatalysts.