Characterization of Pt-Cu binary catalysts for oxygen reduction for fuel cell applications
Introduction
Fuel cells are attractive power sources for both stationary and electric vehicle applications due to their high conversion efficiencies and low pollution. Among the various types of fuel cells, the proton exchange membrane fuel cells (PEMFC) are the most suitable candidates for electric vehicles as they can be operated at a low temperature of <100 °C. Platinum supported on carbon black is widely used as the electrocatalyst in PEMFC. However, platinum is expensive and the world's supply of Pt is limited. Therefore, how to improve the electrocatalytic activity is a very important issue. It is well known that the cell voltages of both the PEMFC and the direct methanol fuel cell (DMFC) are limited by slow reaction kinetics at the oxygen reduction electrode. The search for oxygen reduction reaction (ORR) catalysts that are more active, less expensive and with greater stability than Pt has resulted in the development of Pt alloys. Some platinum-based binary alloys such as Pt-Fe, Pt-Co, Pt-Ni and Pt-Cr exhibit a higher catalytic activity for the ORR in pure acid electrolytes than pure platinum [1], [2], [3], [4], [5]. Such alloy catalysts could improve the activity toward oxygen reduction by a direct four-electron reaction without involving the intermediate hydrogen peroxide step. The mechanisms for the enhanced activity of platinum alloy catalysts toward oxygen reduction have received much attention in recent years [6], [7], [8], [9]. Toda et al. [10] explained the improvement in the Pt-Fe catalytic activity based on an increase in the d-orbital vacancy, promoting a stronger metal–oxygen interaction and the formation of stronger PtO2 bond with the adsorbed O2 species. The stronger PtO2 bond can cause a weakening and lengthening of the OO bond and an easier scission of the OO bond, resulting in an increase in the reaction rate. Shukla et al. [8] attributed the increased electrocatalytic activity of Pt, Pt-Cr, Pt-Co, Pt-Ni, Pt-Co-Cr and Pt-Co-Ni to a decrease in surface oxides and an enrichment of active Pt sites. Mukerjee et al. [11] explained the enhanced Pt-Cr, Pt-Co, Pt-Ni activity based on the decrease in the Pt–Pt distance and the Pt–Pt coordination numbers. The enhanced electrocatalytic activity can be explained by an electronic factor, i.e. the change of the d-band vacancy in Pt upon alloying and/or by geometric effects (Pt coordination number and Pt–Pt distance).
In the present study, we use transition metal copper, which has received little attention, to synthesize the Pt alloying catalyst because copper has similar atomic radius to those of Ni, Co and Fe. Cu had been studied for oxygen cathode in phosphoric acid fuel cells (PAFC), but the critical issues of improving the ORR activity was not analyzed completely. Therefore, we prepare Pt-Cu/XC-72R and characterize it by various techniques. X-ray diffraction (XRD) characterization is carried out to determine the mean crystalline size and the Pt-Cu alloy effect of these Pt-Cu/C catalysts. Transmission electron microscopy (TEM) is used to investigate the mean particle size of the catalysts. Electrochemical experiments including cyclic voltammetry (CV) are also conducted to characterize these Pt-Cu/C catalysts, to determine the electrochemical area of the catalysts, and to test the ORR activity of the catalysts.
Section snippets
Preparation of the electrocatalysts
The carbon supported Pt-Cu catalysts are prepared by two different procedures. We use two different precursors (CuSO4 and CuCl2·2H2O) for studying the effect of Cu precursor. In procedure 1, the catalysts are prepared by a polyol method [5]. H2PtCl6 and transition metal precursor solution is mixed well and added to carbon ethylene glycol (EG) solution (Pt metal loading: 20 wt.%, Pt:Cu = 3:1 in atomic ratio) under mechanically stirred conditions. 1.0 M NaOH (in EG solution) is added to adjust the pH
Effects of precursor and preparation method on the particle size and dispersion of the catalyst
The metal compositions of the prepared Pt-Cu/C catalysts determined by EDS are shown in Table 1. All of the composition data are averaged values from five locations on the same TEM samples.
From this table, the catalyst samples prepared by procedure 1 are found to have higher Cu loading than those prepared by procedure 2. The Cu loadings of samples 3 and 4 are lower than the setting value. The TEM images of the four samples are shown in Fig. 1. The catalyst particles are found to aggregate
Conclusions
In this study, we use transition metal copper to synthesize Pt alloying catalyst. X-ray diffraction characterization is carried out to determine the average crystalline size and the Pt-Cu alloy effect of these Pt-Cu/C catalysts. Transmission electron microscopy is used to investigate the average particle size of the catalysts. Electrochemical experiments including cyclic voltammetry are also conducted to characterize these Pt-Cu/C catalysts, to determine the electrochemical area of the
Acknowledgements
The financial supports from the National Science Council of Taiwan through Grant NSC93-2212-E-008-004 and from NCU-ITRI Joint Research Center through Grant NCU-ITRI 940401 are greatly acknowledged.
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