Methanol crossover through PtRu/Nafion composite membrane for a direct methanol fuel cell

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Abstract

This study examined methanol crossover through PtRu/Nafion composite membranes for the direct methanol fuel cell. For this purpose, 0.03, 0.05 and 0.10 wt% PtRu/Nafion composite membranes were fabricated using a solution impregnation method. The composite membrane was characterized by inductively coupled plasma-mass spectroscopy and thermo-gravimetric analysis. The methanol permeability and proton conductivity of the composite membranes were measured by gas chromatography and impedance spectroscopy, respectively. In addition, the composite membrane performance was evaluated using a single cell test. The proton conductivity of the composite membrane decreased with increasing number of PtRu particles embedded in the pure Nafion membrane, while the level of methanol permeation was retarded. From the results of the single cell test, the maximum performance of the composite membrane was approximately 27% and 31% higher than that of the pure Nafion membrane at an operating temperature of 30 and 45 °C, respectively. The optimum loading of PtRu was determined to be 0.05 wt% PtRu/Nafion composite membrane.The PtRu particles embedded in the Nafion membrane act as a barrier against methanol crossover by the chemical oxidation of methanol on embedded PtRu particles and by reducing the proton conduction pathway.

Introduction

Direct methanol fuel cells (DMFCs) are promising candidates for applications in portable power sources, electric vehicles and transport applications because they do not require any fuel processor and can be operated at room temperature [1], [2], [3]. However, the DMFC is hindered by methanol crossover through the electrolyte membrane as a result of diffusion and electro-osmotic drag. This results in a mixed cathode potential, as well as a reduction in power output and fuel utilization.

Therefore, the suppression of methanol crossover has been a major research focus, and various methods for reducing methanol crossover have been examined including the development of a new electrolyte [4], the surface modification of a Nafion membrane [5] and the incorporation of Pt and hygroscopic oxides into the Nafion membrane [6]. In order to reduce methanol crossover, Uchida et al. [7] developed Pt-dispersed polymer electrolyte membrane (PEM) and Prabhuram et al. [8] modified the Nafion membrane by sputtering Pd and a Pd–Cu alloy. Pt, Pd and Pd alloy oxidize methanol either chemically or electrochemically and impede the migration of water and methanol through the membrane by filling the pores in the membrane. In general, PtRu is an active electrocatalyst for the DMFC anode, while Pt is not because of CO poisoning. Instead of incorporating Pt or Pd in the PEM, PtRu might be a good material for decreasing methanol crossover in the view point of the CO poisoning resistance.

In this study, PtRu/Nafion composite membranes were fabricated in order to reduce methanol crossover through the PEM via solution impregnation and reduction methods [9]. For this purpose, the PtRu loading in the membrane was measured by ICP-MS, and the thermal properties, methanol permeability and the proton conductivity of the composite membrane were characterized. In addition, the single cell performance was evaluated as a function of the PtRu loading. The variation in the proton conductivity, methanol permeability and single cell performance with PtRu loading amount is discussed in terms of the effect of the PtRu particles embedded in the Nafion membrane.

Section snippets

Preparation of composite membrane

The Nafion® 115 membrane was pre-treated in a 5 wt% aqueous H2O2 solution for 60 min at 80 °C, and washed repeatedly with deionized water. In order to obtain the membrane in the Na+ form, the membrane was boiled in a 0.5 M solution of NaCl at 80 °C for 60 min and finally rinsed repeatedly with deionized water for 6 h. The washed membrane in the Na+ form was dried in a vacuum oven for 24 h at 80 °C. Pt(NH3)4Cl2 (Aldrich Chemical co.) in DI water and RuCl3 (Alfa co.) in 13 M NH4OH were used as the Pt and

Results and discussion

The loading of the PtRu embedded in the Nafion membrane was analyzed by ICP-MS. Table 1 shows PtRu loading as a function of impregnation solution concentration. The weight and atomic ratios of Pt and Ru were 0.66:0.34 and 1:0.98, respectively, which is almost the same ratio as used in the commercialized PtRu/C catalyst for the reformed gas and DMFC. The PtRu loading increased linearly with increasing Pt and Ru concentration in the impregnation solutions. However, the weight and atomic loading

Conclusions

This study examined the methanol crossover through the PtRu/Nafion composite membrane in a DMFC. The composite membrane was fabricated using a solution impregnation method. The composite membrane performance was evaluated by a single cell test. The concentration of PtRu particles impregnated in the pure Nafion membrane was controlled by the concentration of the impregnation solution. The proton conductivity of the composite membrane decreased with increasing number of PtRu particles embedded in

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