Fabrication and Electrochemical Characterization of Nanoporous Ni-Pd/Al2O3 Composite Films on Al As Anode Materials for Direct Ethanol Fuel Cells

  Introduction. Direct ethanol fuel cells (DEFCs) have received a growing interest in recent years as promising candidates for portable power sources, electric vehicles and transportation applications owing to a high energy density of ethanol (8 kWh kg^-1), less toxicity, and ease in handling and transportation. On the anode electrode, ethanol molecules are completely oxidized into CO₂ involving the release of electrons and the cleavage of the C-C bond. The ORR reaction is kinetically more facile in alkaline medium than in acid medium. It has been proved that Pd possesses a higher activity than Pt for ethanol oxidation in alkaline medium and that the catalytic activity of Pd can been further increased by the addition of a second metal or metal oxide promoter. Among various Pd-based alloys or composites catalysts, the binary PdNi alloys are regarded as the most attractive electrocatalyst for ethanol oxidation in alkaline medium. This is because nickel is non-noble and low-cost metal electrocatalyst with high corrosion resistance in alkaline medium. The present study is aimed at fabricating a novel nanoporous Ni-Pd/Al₂O₃composite film on Al sheets by successive anodizing and electroless-plating processes, to achieve a large surface area and an enhanced activity for ethanol oxidation as anode materials for DEFCs.

    Experimental. Al sheets (99.56%, 20 × 50 × 0.5 mm) were anodized in phosphoric o and citric acidic solutions at 160 and 320 V for 30 min – 2 h, to obtained nanoporous alumina films with different pores sizes and pore densities. Then, then nanoporous alumina films after pore-widening were used as templates in an electroless-plating to achieve nanoporous Ni-Pd alloy films. The electroless deposition was performed in a plating bath mainly containing NiSO₄ and DMAB at 333 K for 15 – 120 sec, after active treatment by successive immersing in SnCl₂ and PdCl₂ solutions. Moreover, in some experiments, the nanoporous Ni-Pd/Al₂O₃composite films were heated at 673 K for 4 h. The morphology, chemical composition, chemical state, and crystalline structure of the specimens obtained at each step were investigated by FE-SEM, EDX, TEM, XRD, and XPS.

    Various nanoporous Ni-Pd/Al₂O₃composite films with and without annealing were used as working electrodes ( 10×10 mm) in cyclic votamentremic tests. A common three-electrode electrochemical cell system was used for the measurements. The counter and reference electrodes were a platinum wire and an Ag/AgCl (saturated KCl, 0.199 V vs. SHE) electrode, respectively. The electrochemical measurements were carried out at ambient temperature using an electrochemical workstation (Ivium Compact stat, Hokudo Denko Co. Ltd).

    Results and Discussion. Fig. 1a-b shows the surface FE-SEM images of (a) a representative nanoporous Ni-Pd/Al₂O₃ composite film on Al, which was fabricated by Ni electroless- plating on a nanoporous anodic alumina film formed in a phosphoric acidic solution, and (b) a flat Ni-Pd film on Al. The Ni-Pd/Al₂O₃composite film reserved the nanoporous structure of anodic alumina film with diameter of 150 – 200 nm and pore interval of around 350 nm. The Ni-Pd alloy film composed of nanoparticles of 50 – 150 nm, thus leading to a large surface area that is much higher than that of a flat film (Fig.1b). The nanoporous morphologies, or the pore size and pore density of porous alumina films can be readily controlled by the anodizing voltage and electrolytes, and the film thickness by anodizing time. The composition of Ni-Pd alloy films varied within 1–60 at%Pd upon the periods of active treatment and electroless deposition. According to XPS analysis, the as-deposited Ni-Pd films were covered with a thin NiO film less than 1 nm thick, due to the highly active nature of the Ni nanoparticles in air.

    Figure 1c shows the cyclic voltammograms of (i, iii) a nanoporous Ni-Pd/Al₂O₃ composite film and (b) a flat Ni-Pd alloy film in 0.1 M NaOH solutions with (i, ii) 0.1 M ethanol and (iii) without ethanol, respectively. In case of 0.1 M NaOH solution, two small peaks appeared at 0.3 – 0.5 V, which can be ascribed to the redox reaction of NiOOH or NiO on the nanoporous Ni-Pd film. Whereas in case of 0.1 M NaOH + 0.1 M EtOH solution, two well-defined oxidation peaks, at -0.14 V in the anodic sweep curve and -0.39 V in the cathodic sweep curve, were observed, which are typically characterized as the ethanol oxidation. The net current densities of the nanoporous Ni-Pd/Al₂O₃ composite film and flat Ni-Pd films at -0.14 V are around 17 and 2 mA cm^-2, respectively, indicating an enhanced activity on ethanol oxidation for the nanoporous films with larger surface area.