Cost-effective and methanol tolerant carbon-supported Pd and Pd-Co cathode catalysts for DMFCs

Carbon (Ketjenblack, KB)-supported Pd-based catalysts such as Pd/C, Pd4Co1/C and Pd10Co1/C were prepared by the sulphite complex route [1] and physico-chemically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS). The electrochemical investigation was carried out in half cell and direct methanol fuel cell (DMFC) to evaluate the performance, the tolerance to permeated methanol and the durability of the Pd-based electrocatalysts, in particular operating with high methanol concentration (10 M). For comparison, a commercial 30% Pt/C catalyst was also electrochemically investigated. The advantage of using a high methanol concentration in DMFCs is related to a high energy density. Unfortunately, methanol crossover causes a mixed potential at Pt cathode catalysts reducing the overall cell efficiency, which is exacerbated with high methanol concentration at the anode side. Pd and Pd-Co alloys based electrocatalysts exhibited high methanol tolerance properties, as evidenced in the half cell characterization, which led also to high performances in single cell configuration (DMFC). In Figure 1a, a comparison of oxygen reduction reaction (ORR) polarization curves for the different catalysts, in the presence of 0.1 M methanol in the oxygen-saturated acidic solution, is shown. In the presence of methanol, the Pd/KB and the Pd10Co1/KB catalysts show the most positive onset and half-wave potentials and reach the largest limiting current density, compared to those obtained with Pd4Co1/KB and with Pt/C. The behavior of Pd4Co1/KB is intermediate between the other Pd-based catalysts and the Pt-based one in the presence of methanol, indicating that a larger amount of Co in the alloy causes a lower tolerance of Pd catalysts to the presence of the alcohol. DMFC polarization and power density curves carried out at 60 °C in the presence of 10 M methanol concentration are shown in Figure 1b. At the highest concentration tested of 10M, Pd/KB and Pd10Co1/KB cathode-based membrane electrodes assemblies (MEAs) show a better performance than Pt/C. Thus, Pd-based electrodes represent a reliable way to minimize the cost in low temperature fuel cells, providing a higher performance than that of Pt-based electrodes at high methanol concentration. Moreover, the durability tests highlighted that including a small amount of cobalt in the crystalline structure of Pd resulted in better durability in terms of resistance to corrosion phenomena. [1] A.S.Aricò, A. Stassi, C. D'Urso, D. Sebastiàn, V. Baglio, Chem. Eur. J. 20 (2014) 10679