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