Synthesis and Characterization of Low Platinum-Cobalt Alloy Cathode Catalysts by Rapid Annealing and Quenching Method for Fuel Cell Applications

Abstract

Polymer electrolyte membrane fuel cell (PEFC) has tremendous attention due to its attractive properties such as low-temperature operation, high power density and application of solid electrolyte. Thus, it is considered as a reliable power sources for transport, residential and portable applications. However, current PEFC designs encounter problems such as materials compatibility, fabrication cost, performance degradation, heat transfer and so forth. Much research has been carried out to enhance the PEFC performance and durability by improving the electrode in which Pt is commonly used. To further improve the performance of electrode, it is important to enhance the Pt catalyst utilization in the electrode. On the other hand, Pt is extremely rare and expensive, which limits the practical application and wide-scale commercialization of PEFC. Low Pt content catalyst and its alloy have great potential to overcome these limitations by lowering Pt consumption, while its high catalytic activity and durability can be maintained in the meantime.In this study, we prepared the PtCo/C alloys as cathode catalysts by rapid heating and quenching (RHQ) method, and subsequently the relationship between annealing temperature and catalyst activity was investigated. The RHQ method is highly effective at producing catalyst having excellent dispersibility and a smaller average particle diameter. Note that the catalytic activity can be enhanced with increasing the active site via controlling the particles diameter. In order to prepare the PtCo/C catalysts, Pt/C (Pt: 35 wt.%) was synthesized by ethanol reduction method, and subsequently Co was added to Pt/C catalyst (molar ratio of Pt:Co = 1:1). During fabrication, the annealing temperature is increased from 600 ºC to 900 ºC in steps of 100 ºC. The RHQ method was applied in all steps of heat treatment due to obtain the finer PtCo alloy particles. Then, PtCo/C catalysts prepared using temperature ranges of 600 ºC, 700 ºC, 800 ºC and 900 ºC were leached in 0.5 M H2SO4 at 80 ºC for 4 h, followed by annealing at 500 ºC. Powder XRD, TEM, SEM and EDS measurements were performed for physical characterization. Electrochemical properties of PtCo/C alloy catalysts were evaluated by cyclic voltammetry (CV) and accelerated durability test (ADT) by raising and lowering the potential between 0.6 V and 1 V. The membrane electrode assembly (MEA) was prepared using the PtCo/C alloy as the cathode catalyst (0.1 mg-Pt cm−2-MEA), and their performances were investigated to determine the practical applicability of the catalyst at single cell level. On the other hand, a commercial PtCo/C (Pt: 46.4 wt.%, Co: 5.6 wt.%) catalyst was used as a benchmark.Fig.1 shows the XRD patterns obtained for PtCo/C alloy catalysts prepared with varying annealing temperature. As shown in Fig. 1, the peaks are shifted to higher angles compared to those of pure Pt (39.39 º). Broad peaks indicate that alloy particles with small grain sizes are produced. Peak shifting is increased with increasing temperature, indicating a higher degree of alloying. Finally, acid treatment was performed due to remove Co from the surface of PtCo/C alloy catalysts. Indeed, acid-treated catalysts are ascribed to the formation of Pt-rich in the PtCo/C alloy catalysts, resulting in an increased electrochemical active surface area (ECSA) and higher oxygen reduction reaction (ORR) activity. TEM image analysis shows the average diameter of the particles of prepared PtCo/C alloy catalysts and the commercial PtCo/C catalyst is about 3.0 nm and 5.4 nm, respectively. The ECSA of the prepared PtCo/C alloy catalyst estimated from CV curves is 80 m2 g-Pt -1, which is 1.6 times higher than the commercial PtCo/C catalyst. Fig. 2 shows the I-V characteristics of PtCo/C alloy catalyst prepared at 900 ºC in comparison with the commercial PtCo/C catalyst before and after ADT test using single cell. To evaluate performance of PEFC, the cell voltages at low current density region and high current density region are examined. When the current density is low, the performance of PtCo/C alloy and the commercial PtCo/C catalyst are approximately similar. When the current density is increased to the mass transfer region, the MEA prepared with the PtCo/C alloy and the commercial PtCo/C catalysts have degradation in the cell voltage. However, the PtCo/C alloy catalyst prepared at 900 ºC shows comparably best performance at high current density region, even after ADT test. The estimated maximum power density of PtCo/C alloy catalyst is approximately 0.8 W cm−2, comparably higher than that of the commercial PtCo/C catalysts. Therefore, it is suggested that the high catalytic activity and durability can be achieved from the finer particle and high degree of alloying with high annealing temperature. Figure 1