Pt modified tungsten carbide as anode electrocatalyst for hydrogen oxidation in proton exchange membrane fuel cell: CO tolerance and stability
Introduction
In recent years, proton exchange membrane fuel cells (PEMFCs) have been recognized as the most feasible power source for low/zero-emission electric vehicles and stationary applications [1]. In such type of fuel cells, H2 is used as fuel and O2/air as oxidant [2]. Utilization of pure hydrogen as fuel is the simplest and most efficient way for PEMFCs, but the infrastructure required is limited due to the production cost and storage difficulties. Alternatively, hydrogen may be obtained from reformed fuels, such as steam-reformed methanol, ethanol or natural gas. A potential problem which arises from this system is the production of small amount of impurities particularly carbon monoxide, which strongly adsorb on the Pt catalyst, usually employed in the anode [3], blocking the sites for the hydrogen adsorption and oxidation. Therefore, Pt is modified with other elements such as Ru or Mo for the PEMFCs so as to search for enhanced CO tolerant materials [4], [5]. Unfortunately, the relative high cost and insufficient durability of these elements still hinder the large scale commercialization of PEMFCs. For example, Antolini [6] observed the dissolution of Ru from the Pt–Ru anode catalyst and its presence in the cathode side. Other studies have shown the dissolution of Mo from the Pt–Mo anode and its transfer through the electrolyte membrane to the cathode side [7], [8]. Therefore, most robust materials are required to provide not only stability to the anode catalysts, but also ability to tolerate trace amounts of CO. Because the catalytic properties of transition metal carbides have been found to be similar to those of precious metals like Pt [9], extensive studies have been carried out using transition metal carbides, particularly WC, as catalyst supports to improve the catalytic performance and minimize usage of precious metals. As electrocatalysts, they are also known to be highly resistant to CO poisoning and stable in acidic and basic solutions [10]. This is most probably due to the active surface of WC toward the dissociation of H2O to produce surface hydroxyl group [11], which are critical for the subsequent oxidation of CO. If used as support, WC helps to increase the dispersion of precious metals [12].
There have been recent studies to evaluate the use of tungsten carbide as catalyst for fuel cells and it has been shown that Pt supported on tungsten carbide presents superior activity for both, the methanol electro-oxidation and oxygen reduction reactions [13], [14]. In the work performed by Chhina et al. [15] a higher activity and stability for oxygen reduction reaction before and after 100 oxidation cycles was shown by the Pt/WC as compared to Pt/C. In another study, it has been observed that Pt supported on tungsten carbide shows higher activity for electrochemical oxidation of methanol than a commercial carbon supported PtRu electrocatalyst [16]. Lee et al. [17] reported that Pd/WC shows improved activity for the electro-oxidation of methanol as compared to a Pd/C catalyst in an alkaline media. Similarly, in the work conducted by Moon and co-workers [18], it has been demonstrated that Pd supported on mesoporous tungsten carbide showed a more negative peak potential compared to Pd/C in the CVs for methanol electrooxidation. They also evaluated the stability of these electrocatalysts in alkaline solution containing methanol and reported only 2.2% decrease in current density for Pd/meso-WC after stability test compared to a decrease of 26% for Pd/C. In a subsequent study, it has been shown that tungsten carbide alone has low activity for the methanol and hydrogen oxidations [19]. The hydrogen oxidation activity of WC-based anode in absence of noble metals was also evaluated by Yanga and Wang [20]. They observed that WC has very low activity toward hydrogen oxidation. However, the activity could be improved significantly by adding a small amount of Pt to tungsten carbide [21], [22]. Hence, in the work performed by Ham and co-workers, the Pt/WC catalyst showed two times higher activity per mass of Pt for hydrogen oxidation compared to a commercial Pt/C [23].
Similarly, Kelly et al. [24] investigated the hydrogen evolution activity of Pd supported on tungsten and molybdenum carbide. In their work a superior activity was noted for Pd/C in contrast to bare carbides. Nevertheless, only the addition of monolayer of Pd to these carbides doubled the values of corresponding current density for the resultant carbide based electrocatalysts. Furthermore, the activity and stability of Pt supported on tungsten carbide has been tested for hydrogen evolution and oxidation reaction by Liu and Mustain [25]. Although they found a very little difference in the activity of Pt/WC and Pt/C catalysts for hydrogen evolution reaction, the stability of Pt/WC was far better than that of Pt/C. A loss of only 4% in activity was observed for Pt/WC which was very small in comparison to loss in activity of more than 20% for Pt/C.
Summarizing, there has been some research data in the literature on the use of tungsten carbide as anode materials, either for hydrogen oxidation/evolution or for methanol oxidation. However, the CO tolerance and stability of this material as anode electrocatalyst in a real fuel cell environment has not been studied in detail. Thus in this work, carbon supported tungsten carbide prepared by a simple impregnation method was used as Pt catalyst supports in the anode of a PEMFC. The Pt was supported on this carbide by a formic acid reduction method. The resulting catalyst was first tested for hydrogen oxidation in the presence of pure hydrogen and hydrogen containing CO and then its stability was evaluated by an accelerated stress test [26], applying an electrode potential cycling, from a low to a high potential. The results obtained were compared with carbon supported PtW catalyst, which was also prepared by formic acid reduction method.
Section snippets
Electrocatalysts preparation
Tungsten carbide supports with different W/C wt. percentages (10, 20 and 30) were prepared by a simple impregnation method [27]. Briefly, the tungsten precursor (WCl6, Aldrich) and carbon black (Vulcan XC-72) were added in to ethanol and then the composite was impregnated for 3 h at room temperature. The mixture was then heated at 70 °C, until the ethanol was evaporated completely. The resultant solid was transferred in to a quartz reactor, placed in a tubular furnace and heat treated under CH4/H2
Physical characterization of the electrocatalysts
The wt.% of the different components of the catalysts as obtained by EDX are given in Table 1. The tungsten contents in the tungsten carbide supports are less than the target values of 10, 20 and 30 wt.% and this was also observed for the Pt loading of the Pt/WC/C, whereas the wt.% of PtW/C is very close to the desired value, with a Pt:W atomic ratio quite closed to the expected value (3:2). In all cases, the ranges of the catalyst contents were considered satisfactory for the purpose of the
Conclusions
WC/C has been synthesized by a simple impregnation method and characterized physically by XRD, EDS and TEM and electrochemically by polarization curves, cyclic voltammetry, online mass spectrometry and CO stripping experiments. Pt supported on this material has shown a higher activity for electrochemical oxidation of hydrogen with and without CO than a PtW/C catalyst. As catalyst support, WC/C showed to be somewhat more stable against oxidation than Vulcan XC-72 carbon, as evidenced by
Acknowledgments
The authors would like to thank the Third World Academy of Science (TWAS), Italy, the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP - Proc. 2013/16930-7), Brazil, for financial supports.
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