Electrochemical Synthesis of Mesoporous CoPt Nanowires for Methanol Oxidation

Abstract

A new electrochemical method to synthesize mesoporous nanowires of alloys has been developed. Electrochemical deposition in ionic liquid-in-water (IL/W) microemulsion has been successful to grow mesoporous CoPt nanowires in the interior of polycarbonate membranes. The viscosity of the medium was high, but it did not avoid the entrance of the microemulsion in the interior of the membrane's channels. The structure of the IL/W microemulsions, with droplets of ionic liquid (4 nm average diameter) dispersed in CoPt aqueous solution, defined the structure of the nanowires, with pores of a few nanometers, because CoPt alloy deposited only from the aqueous component of the microemulsion. The electrodeposition in IL/W microemulsion allows obtaining mesoporous structures in which the small pores must correspond to the size of the droplets of the electrolytic aqueous component of the microemulsion. The IL main phase is like a template for the confined electrodeposition. The comparison of the electrocatalytic behaviours towards methanol oxidation of mesoporous and compact CoPt nanowires of the same composition, demonstrated the porosity of the material. For the same material mass, the CoPt mesoporous nanowires present a surface area 16 times greater than compact ones, and comparable to that observed for commercial carbon-supported platinum nanoparticles.

Keywords: CoPt alloy; electrodeposition; ionic liquid DMFC; mesoporous nanowires; microemulsion.

Figure 1

Schematic representation of electrochemical synthesis of mesoporous and non-mesoporous CoPt nanowires on polycarbonate membranes coated with gold layer.

Figure 2

Cyclic voltammetry at 25 °C and stationary conditions on Si/Ti (15 nm)/Au (100 nm) of (A) CoPt solution (W) and (B) CoPt solution (W), aqueous solution–surfactant system (79W:21S) and IL/W microemulsion.

Figure 3

(A) Cyclic voltammetrie sand (B) chronoamperometric curves at 25 °C on Au sputtered 20 µm-thick polycarbonate membranes with 200 nm pore diameters size. Geometrical area has been used to calculate current density.

Figure 4

TEM micrographs of CoPt nanowires prepared in (A) aqueous solution (W), (B) aqueous solution–surfactant system (79W:21S) and (C) IL/W microemulsion systems at 25 °C on Au sputtered 20 µm-thick polycarbonate membranes with 200 nm pore diameters size after circulating the same charge. The first micrographs in each series correspond to a general overview of CoPt nanowires; the second one corresponds to a magnification of a central part of nanowire. In addition, the latter corresponds to a magnification of the edge of a nanowire.

Figure 5

Cyclic voltammograms for methanol oxidation on CoPt nanowires obtained from W and IL/W systems. Scans were recorded in Ar saturated 1.0 M CH₃OH/0.5 M H₂SO₄ at 100 mVs⁻¹. Current density calculated using (A) catalyst’s mass, and (B) electrochemically active area. Inset shows the region used to calculate electrochemically active area (in red) from a cyclic voltammogram in 0.5 M H₂SO₄ at 100 mV s⁻¹.