Pt-decorated nanoporous gold for glucose electrooxidation in neutral and alkaline solutions
© Yan et al; licensee Springer. 2011
Received: 1 February 2011
Accepted: 7 April 2011
Published: 7 April 2011
Exploiting electrocatalysts with high activity for glucose oxidation is of central importance for practical applications such as glucose fuel cell. Pt-decorated nanoporous gold (NPG-Pt), created by depositing a thin layer of Pt on NPG surface, was proposed as an active electrode for glucose electrooxidation in neutral and alkaline solutions. The structure and surface properties of NPG-Pt were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and cyclic voltammetry (CV). The electrocatalytic activity toward glucose oxidation in neutral and alkaline solutions was evaluated, which was found to depend strongly on the surface structure of NPG-Pt. A direct glucose fuel cell (DGFC) was performed based on the novel membrane electrode materials. With a low precious metal load of less than 0.3 mg cm-2 Au and 60 μg cm-2 Pt in anode and commercial Pt/C in cathode, the performance of DGFC in alkaline is much better than that in neutral condition.
Glucose is widely used in modern life and industry as a nontoxic, inexpensive, and renewable resource. Since Rao and Drake  first reported the glucose oxidation on platinized-Pt electrodes in phosphate buffer solution in the 1960s, electrocatalytic oxidation of glucose has been extensively investigated as a key reaction in the fields of sensors [2, 3] and fuel cells [4, 5]. Great efforts have been made to develop catalytically active electrode materials for this reaction in the past two decades. As one of the most studied electrocatalyst, Pt was found to exhibit considerable activity for glucose oxidation at a negative potential in neutral and alkaline solutions . However, systematical study showed that this electrocatalytic process was subject to serious poisoning due to adsorbed intermediates from the oxidation of glucose . To mitigate the poisoning effect, Pt-based bimetallic catalysts such as Pt-Pb [8, 9], Pt-Ru [10, 11], and Pt-Au [4, 12], have been developed to improve the electrocatalytic activity and selectivity. On the other hand, it is increasingly realized that glucose electrooxidation is sensitive to surface structure of the electrocatalyst. For example, Adzic et al. found that this reaction strongly depended on the crystallographic orientation of the Pt electrode surface . Thus, significant attention has been focused on exploiting the potential applications of the nanostructured materials with special surface properties for glucose oxidation. Besides the widely used nanoparticles [14, 15], many other nanostructures were also studied, such as carbon nanotubes , ordered Pt nanotube arrays , mesoporous Pt electrodes , and nanoporous Pt-Pb and Pt-Ir networks [8, 19]. While these unique nanostructures exhibited considerable advantages as compared to traditional electrodes, they were mainly employed for glucose electrochemical detection. Exploiting nanostructures for potential applications in glucose fuel cell is still highly desirable.
Recently, Erlebacher and co-workers reported an interesting type of membrane electrode materials called nanoporous gold (NPG) leaves which could be made by chemically etching the white gold (AgAu alloy) leaves in corrosive medium . Coupled with surface functionalization with other catalytically active material, such as Pt, the 100-nm-thick high surface area electrode materials demonstrated superior activities toward a series of important electrochemical reaction including methanol oxidation [21, 22] and formic acid oxidation . Preliminary studies also proved they could work as promising electrocatalysts in proton exchange membrane fuel cells at ultra-low Pt loading [24, 25]. Here, we focus on their electrocatalytic properties toward glucose oxidation and its application in alkaline glucose fuel cells.
Reagents and apparatus
All chemicals were of analytical grade and used as purchased without further purification. D-Glucose, NaOH, HNO3 (65%), Na2HPO4·12H2O, NaH2PO4·2H2O, and H2PtCl6·6H2O were obtained from Sinopharm Chemical Reagent Co., Ltd. Au/Ag alloy (50:50, wt%) leaves with thickness of 100 nm (Sepp Leaf Products, New York) were used for NPG fabrication. Ultrapure water (18.2 MΩ) was used throughout the experiments and 0.1 M PBS was prepared with pH 7.4. The composition of NPG-Pt sample was determined by an IRIS Advantage inductively coupled plasma-atomic emission spectrometry (ICP-AES). The surface structure of NPG-Pt was observed JSM-6700F SEM and JEM-2100 TEM. The crystallographic information was obtained with XRD (Bruker D8 Advance X-ray diffractometer, Cu Kα radiation λ = 1.5418 Å) at a 0.02°/s scan rate. All electrochemical measurements were performed at room temperate in a traditional three-electrode electrochemical cell with a CHI 760C electrochemical workstation (Shanghai). Mercury sulfate electrode (MSE) was selected as reference electrode in all the electrochemical measurements, and a pure Pt foil as the counter electrode. Both PBS and the mixed solutions were purged with high pure nitrogen (99.999%) for 30 min prior to measuring.
Membrane electrode assembly (MEA) was prepared by attaching NPG-Pt to carbon paper (TGP-H-060, Toray, Japan) first, and then hot-pressed onto one side of a Nafion 115 membrane and commercial Pt/C (60 wt%, Johnson Matthey, UK) onto another side at 110°C and 1.5 MPa for 195 s. As-prepared MEAs were then assembled between high purity graphite plates as flow and current collecting plates, which have single channel serpentine flow pattern. The anolyte was pumped to anode by peristaltic pump, while pure oxygen was fed to the cathode without humidification by a massflow controller. The cell temperature was controlled through a temperature controller and monitored by thermocouples buried in the graphite blocks. The steady state polarization curves were recorded by automatic Electric Load (PLZ 70UA, Japan).
Preparation of NPG and NPG-Pt electrodes
NPG was made by dealloying commercial 12-carat white gold membrane in concentrated nitric acid for 20 min at 30°C . Subsequently, NPG were immediately transferred to ultrapure water and repeatedly washed to remove Ag+ and NO3 -. NPG-Pt samples were prepared by floating the as-prepared NPG membranes at the interface between the H2PtCl6 (1 g/L, pH = 10) solution and the vapor of hydrazine hydrate (85%) in a closed system . Deposition reaction occurred uniformly on the surface of NPG. The amount of Pt deposited onto the NPG substrate gradually accumulates with increasing plating time. The as-prepared NPG-Pt (loading of 0.1 mg cm-2 Au and 20 μg cm-2 Pt) samples were transferred into ultrapure water as soon as the plating reaction finished. Then NPG-Pt membranes were affixed onto the clean GC electrode (4 mm in diameter) and fixed with 2 μL dilute nafion solution (0.5 wt%). The as-prepared NPG-Pt electrode was dried at room temperature for 24 h before measurements.
Results and discussion
Surface and crystal structure of the NPG-Pt
Electrochemical characteristics of NPG-Pt in PBS
Electrocatalytic properties of NPG-Pt for glucose oxidation in neutral and alkaline solutions
DGFCs in neutral and alkaline solution
It also can be seen that the maximum power densities in alkaline (Figure 7b) was 4.4 mW cm-2 which is about 24 times than that in neutral solution (0.18 mW cm-2, Figure 7a). This should be mainly attributed to quicker reaction rate on the NPG-Pt in alkaline than that in neutral solution for glucose oxidation which was in line with the results of 3.3 above.
NPG-Pt membranes, a type of porous Au-Pt bimetallic nanostructures, were fabricated by chemically plating thin layer of Pt on NPG and were studied for glucose electrooxidation and the application in fuel cell. Taking advantage of the unique structure and high surface area, NPG-Pt exhibits considerable activity toward this reaction in neutral and alkaline solutions. In addition, glucose oxidation on NPG-Pt was found to be a surface sensitive process and Au-Pt surface alloy is highly active for oxidizing the adsorbed intermediate species resulted from the glucose electroadsorption. This means we could further improve the catalytic performance of NPG-Pt by tailoring the surface composite and structure. The results of DGFC test indicated that NPG-Pt is expected as a promising low precious metal loading electrocatalyst for application in glucose fuel cells.
direct glucose fuel cell
inductively coupled plasma-atomic emission spectrometry
membrane electrode assembly
mercury sulfate electrode
Pt-decorated nanoporous gold
scanning electron microscopy
transmission electron microscopy
X-ray powder diffraction.
This work was supported by the Ph.D. Programs Foundation of the MOE (20090131110019). We thank Prof. Y. Ding and HouYi Ma for valuable discussions and for sharing their nanomaterials and facilities.
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