Highly Ordered Pd Nanowire Arrays as Effective Electrocatalysts for Ethanol Oxidation in Direct Alcohol Fuel Cells

Abstract

Direct alcohol fuel cells (DAFCs) based on liquid fuels have attracted enormous attention as power sources for portable electronic devices and fuel-cell vehicles owing to the much higher energy density of liquid fuels than gaseous fuels such as hydrogen (e.g., the energy densities of ethanol and methanol are 6.34 kWh L and 4.82 kWh L, respectively, as compared to 0.53 kWh L for gaseous hydrogen at 20 MPa. Among various liquid fuels, ethanol is particularly attractive because it is less toxic than methanol and can be produced in large quantities from agricultural products. Ethanol is also the major renewable biofuel from the fermentation of biomass. Pt and Pt-based catalysts such as PtRu/C have been extensively investigated as electrocatalysts for the electrooxidation of liquid fuels such as methanol and ethanol. However, the high cost and limited supply of Pt constitute a major barrier to the development of DAFCs. We studied recently Ptfree electrocatalysts for the electrooxidation reactions of ethanol and methanol, and the results revealed that Pd is a good electrocatalyst for ethanol oxidation in alkaline media. Metal nanowire arrays (NWAs) have attracted much attention and interest owing to their excellent physical and chemical properties and have been extensively investigated for applications such as high-density magnetic recording devices and sensors. Ordered nanowire or nanotube arrays have also been applied for methanol oxidation and H2O2 electrocatalytic reduction because of their high active surface area. Among various techniques to synthesize NWAs, the anodized aluminum oxide (AAO) template method is probably the simplest and most versatile approach to creating highly ordered metal NWAs with uniform and tunable porous structure and good mechanical and thermal stability. Electrodeposition has been shown to be an efficient method for the growth of uniform and continuous metallic nanowire arrays. Despite the exceptional physical, chemical, and electrical properties of metal NWAs, there is little information on the electrocatalytic properties of Pd NWAs. Here, we report the fabrication of highly ordered Pd NWA electrodes by the AAO templateelectrodeposition method, and the results show that Pd NWAs are highly active for ethanol oxidation in alkaline media, demonstrating the potential of applying Pd NWAs as effective electrocatalysts for DAFCs. Figure 1 shows typical scanning electron microscopy (SEM) images of Pd NWAs after the AAO template has been fully dissolved. The Pd nanowires (NWs) are highly ordered with uniform diameter and length. The average length and diameter of the Pd NWs are ca. 800 and ca. 80 nm, respectively. The NWs are uniform, well isolated, parallel to one another, and standing vertically to the electrode substrate surface. The hexagonal shape of the Pd NWs is due to the AAO porous structure during anodization. The X-ray diffraction (XRD) pattern (inset in Fig. 1a) indicates that the Pd NWAs exhibit a typical face-centered cubic (fcc) lattice structure. The strong diffraction peaks at 40.10°, 46.49°, and 68.08° correspond to the (111), (200), and (220) facets of Pd. This indicates that Pd NWAs have been successfully fabricated. Figure 2a shows cyclic voltammograms (CVs) of ethanol oxidation in a 1.0 M KOH + 1.0 M C2H5OH solution on a Pd film electrode (curve a, Pd loading: 1.10 mg cm), a Pd NWA electrode (curve c, Pd loading: 0.24 mg cm), and an E-TEK PtRu/C electrode (curve b, Pt loading: 0.24 mg cm). The cyclic voltammograms of the Pd film, E-TEK PtRu/C, and Pd NWA electrodes in 1.0 M KOH solution without ethanol are shown in Figure 2b. In the CVs obtained in 1.0 M KOH electrolyte solution, the anodic peaks appearing between –0.73 and –0.53 V versus Hg/HgO on Pd and –0.8 and –0.5 V versus Hg/HgO on Pt originate from the desorption of atomic hydrogen on the electrocatalysts (Fig. 2b). Thus the area of H desorption after the deduction of the double layer region on the CV curves represents the charge passed for the H desorption, QH, and is proportional to the electrochemically active area (EAA) of the electrocatalysts. The value QH = 10.6 mC cm –2 for the Pd NWA electrode is much higher than 3.4 mC cm for the Pd film electrode and 4.6 mC cm for the E-TEK PtRu/C electrode. This shows that the Pd NWA electrode has high EAA, most likely due to the well-defined and uniform porous structure of the nanowires in the arrays (Fig. 1). Such well-defined nanowire structure enhances the active sites for the electrooxidation reaction of ethanol. The high electrocatalytic activity of the Pd NWA electrode is also indicated by its superior performance for the electrooxC O M M U N IC A TI O N