Investigation of the performance decay of anodic PtRu catalyst with working time of direct methanol fuel cells

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Abstract

Life tests of direct methanol fuel cells (DMFC) were carried out with three individual single cells at a current density of 100 mA cm−2 for three different times under ambient pressure and at a cell temperature of 60 °C. X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) were used to characterize the anodic PtRu catalysts before and after the life tests. XRD results showed that the particle sizes of anodic catalysts increased from an original value of 2.8 to 3.0, 3.2, and 3.3 nm, whereas their lattice parameters first increased and then decreased from an original value of 3.8761 to 3.8879, 3.8777, and 3.8739 Å before and after 117, 210, and 312 working hours, respectively. XPS results indicated that during cells’ working the contents of Pt and Ru oxides in anodic catalysts increased, but the metal content gradually decreased with test times. Polarization curves, power density curves, and in situ CO-stripping cyclic voltammetric (CV) curves were also plotted to evaluate the performances of fuel cells and electrochemically active surface areas (SEAS) of anodic catalysts before and after life tests. After different time tests, the performances of DMFC lowered to different extents and could not recover their initial performances. The SEAS of anodic catalyst decreased slightly by 4.78 and 9.03 m2 g−1 after 117 and 312 working hours, respectively. The utilization of anodic catalysts lowered slightly. This indicates that the change of (SEAS) and utilization of anodic catalysts are not the main factors affecting the performance degradation of DMFC. The dissolution of Ru metal from anodic catalysts surface could be one of the main factors for the performance degradation of the PtRu black catalyst.

Introduction

Direct methanol fuel cells (DMFC) are attractive for several applications in view of their lower weight and volume compared with indirect fuel cells [1]. Investigations of DMFC are mainly focused on the catalysts [2], [3], methanol crossover [4], [5], [6], carbon support [7], [8], Nafion modification [9], the optimization of gas electrode layer [10], and so on in the past decades. Significant advances have been made for DMFC developments [11], [12], [13]. But, at present, their performance cannot meet the demand of DMFC commercialization. In addition, the cost of DMFC is still high because of the expensive noble metals of Pt and Ru used as catalysts in cathodes and anodes. The resource of platinum is limited, and that of ruthenium is very rare, limiting the commercialization of DMFC as well. Hence, it is very worth enhancing the activity and prolonging the life of catalysts for DMFC and lowering the cost of DMFC by using new techniques. The performance degradation rates of DMFC, generally higher than that of hydrogen PEMFC, are typically in the range of 10–25 μV h−1 [14]. The commercialization of DMFC demands a stable operation for at least thousands of hours, e.g. 5000–40,000 h usually required for fuel cell vehicles and residential power generators, which is not easy to achieve [15]. Most of the fundamental mechanisms determining the life of DMFC are poorly understood, such as membrane electrode assembly (MEA) failure mechanisms including the growth and corrosion of catalytic particles resulting in compositional changes, poisoning of electrocatalysts by accumulated intermediates from methanol electro-oxidation or impurities, the aging of polymer electrolyte membrane, and changes in the hydrophobic/hydrophilic properties in the catalyst layers and diffusion layers [16], [17]. Preliminary research results indicated that the electrocatalysts’ stability plays an important role in the long-term operation of fuel cells [18]. The degradation of catalysts in DMFC proceeds gradually, and its degree is time-dependent. DMFC cathodes operate in a more corrosive environment of high water content, low pH value (<1), high temperature (50–90 °C), and high potentials (0.6–1.2 V) coupled with operating oxygen partial pressures [19]. The carbon support can also be oxidized in the case of fuel starvation [14]. So, the agglomeration and dissolution of the catalyst in cathodes are more serious than that in anodes. Some research results showed, however, that the agglomeration of catalyst in anodes is more serious than that in cathodes, because methanol might be more aggressive towards the catalyst than water produced in DMFC [20]. With respect to the agglomeration of catalysts, there are two mechanisms for particle size growth, i.e. Ostwald ripening and coalescence [21], [22], [23], [24], [25]. Although coalescence has been reported to be insignificant for carbon supported catalysts at temperatures below 500 °C, or in a gas phase [22], this may not be the case with DMFC where metal black catalysts are typically used. The primary growth mechanism, i.e. Ostwald ripening, would be expected to be more serious at the cathode [24]. However, the aging intrinsical mechanism of anodic catalysts is not clear, the changing characteristics of PtRu with time during DMFC operation are not discussed in detail yet. It is necessary to explore the aging regularity and its mechanism for anodic catalysts at different periods of time. In the present paper, we carry out the short-term life tests of DMFC of 117, 210, and 312 h, respectively, with three single DMFC operated at 60 °C using Johnson Matthey unsupported Pt and PtRu catalysts. Electrochemical characterizations of anodic catalysts were performed, the results of X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) characterizations obtained prior to and after tests were discussed.

Section snippets

MEA preparation

Johnson Matthey Pt black and PtRu black (Johnson Matthey Co.) were used as catalysts for the cathode and anode, respectively. PtRu black and 5 wt.% Nafion® ionomer solution (DuPont Co., EW = 1100) were mixed in isopropanol alcohol solution to form a homogeneous catalyst black suspension for the anode. The cathodic catalyst ink was prepared similarly with Pt black, Nafion® ionomer, and PTFE latex. The Nafion® contents in both anodic and cathodic catalyst layers were 20 wt.%. The catalyst inks were

Results and discussion

The life tests of three single cells were carried out at a cell temperature of 60 °C at 100 mA cm−2. As shown in Fig. 2, during each continuous discharge process, the majority of voltage losses occur in the first few hours and then its decline becomes less significant. The initial rapid performance loss was attributed to the non-equilibrium state among ruthenium oxides [33], [34]. During the life tests, continuous discharges were interrupted by refilling the methanol solution, resulting in the

Conclusions

Life tests of three single cells of direct methanol fuel cells were carried out at a current density of 100 mA cm−2 under ambient pressure and a cell temperature of 60 °C. After the long period of operations and tests, the performances of the DMFC lowered to different extents and could not recover the initial performance. The losses of their initial maximum power densities increased with their operation times. After running for more than several hundreds of hours, the particle sizes of catalysts

Acknowledgements

This work is supported financially by the National Natural Science Foundation of China (Grant No. 20606007), Postdoctoral Science-Research Developmental Foundation of Heilongjiang Province of China (LBH-Q07044), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (2008) and Harbin Innovation Science Foundation for Youths (2007RFQXG042).

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