Preparation of Pt-Cu Catalyst Particles Using Electrodeposition Method and Evaluation of Durability and ORR Activity

Introduction Although Proton exchange membrane fuel cells (PEMFCs) are clean and efficient energy converters, the amount of platinum used as the catalyst for the oxygen reduction reaction (ORR) at the cathodic electrode is still large and has to be reduced to drive down cell manufacturing costs. Core-shell structured Pt alloys with relatively abundant transition metals, such as Cu, Co, and Ni, have demonstrated significant improvements for the ORR activities, suggesting one solution to tackle this problem1). However, it has been reported that Pt-M alloys catalysts degrade more easily than pure Pt because of selective dissolution of M, leading to severely reduced performance in long run operation2). Taylor et al. have shown that Pt/C-catalyzed GDL fabrication using the electrodeposition method enables the high Pt utilization ratio in the catalyst layer because it ensures that Pt is deposited only on the electrochemically active area3). Woo et al. have reported considerable single cell performance of Pt-Co catalysts prepared on unloaded GDL using the electrodeposition method4). However, more investigations are needed to clarify their corrosion and degradation behaviors. In this work, we present the preparation of Pt-Cu catalyst particles using the electrodeposition method, and the evaluation of durability and ORR activity using electrochemical measurements such as immersion test and ORR measurement. Experimental Glassy Carbon was used as a substrate on which Pt-Cu nanoparticles were deposited. Direct current electrodeposition of Pt-Cu nanoparticles was conducted in a conventional three-electrode cell. A double junction KCl-saturated Ag / Ag-Cl electrode was used as a reference electrode, and a carbon rod was used as the counter electrode. Electrodeposition was carried out in a plating bath containing a solution of 6 mM K2PtCl4 and various concentrations(1-40 mM) of CuSO4・5H2O dissolved in 0.5M Na2SO4. The parameters for direct current electrodeposition were an electrodeposition potential of -0.05 V vs. SHE and a total charge density of 0.11 C cm−2. Chemical compositions and particle diameters were determined by SEM-EDX. The duration of the immersion test was 3 h in 0.5 M H2SO4, followed by the investigation of the dissolved amount of Cu using ICP-MS. ORR activities were determined by measuring the potentiostatic polarization curve in 0.5 M H2SO4 from OCP to 0.6 V vs. SHE at a scan rate of 0.2 mV s-1. All electrodeposition processes and electrochemical measurements were conducted open to the atmosphere and at 25 oC. Results and Discussion SEM-EDX observations confirmed that various compositions of Pt-Cu were prepared and homogeneous nanoparticles were obtained in each sample. Chemical compositions were controlled from 13 to 85 at. % Cu by changing the concentration of precursor in a plating bath. The amounts of dissolved Cu ions during the immersion test drastically increased with atomic ratio of Cu as shown in Fig. 1(a). The corrosion potential monitored during the immersion test, all Pt-Cu catalysts formed the Pt enriched layer on the surface by selective dissolution of Cu. However, Cu-rich alloys did not suppress the dissolution of Cu. In addition, SEM observations after the immersion test displayed that the surface morphology of Cu-rich Pt-Cu nanoparticles completely changed. Fig. 1(b) shows specific activities at 0.9 V initial against Cu composition. Pt58Cu42 indicated the highest ORR activity, 2.6 times higher than pure Pt. Fe group metal content enhances Oxygen adsorption and weakens O-O bond5) and thus ORR activity is enhanced by the effect of alloying when Cu content increases. From our results, however, the Pt enriched layer which weakens the effect of alloying becomes thicker as Cu content increases because the amount of dissolved Cu ions also increases. As a result, optimum composition is determined by the balance between the effect of alloying and the thickness of the Pt enriched layer. Pulse current electrodeposition of Pt-Cu nanoparticles on GDL will be further discussed in the session. References 1) Mehtap Oezaslana, Peter Strasser, Journal of Power Sources 196 (2011) 5240–5249. 2) S. Mukerjee, S. Srinivasan, Handbook of Fuel Cells, vol2 (2003) 502. 3) E.J. Talyor, E.B. Anderson, N.R.K. Vilambi, J. Electrochem. Soc. 139 (1992) L45. 4) Seunghee Woo, In Kim, Jae Kwang Lee, Sungyool Bong, Jaeyoung Lee, Hasuck Kim, Electrochim. Acta 56 (2011) 3036–3041. 5) T. Toda, H. Igarashi, H. Uchida, and M. Watanabe, J. Electrochem. Soc. 146 (1999) 3750. Figure 1