Standard oxidation-reduction reactions such as those of ferrocyanide and ferrocene have long been employed in evaluating and comparing new electrode structures with more traditional configurations. A variety of nanostructured carbon electrodes developed in recent years have been reported to exhibit faster electron transfer kinetics than more traditional carbon structures when studied with these redox reactions. This type of comparison has not been widely explored for nanostructured platinum electrodes that have become increasingly common. In this work, a platinum nanotubule array electrode was fabricated via a simple template-based process and evaluated using the standard ferrocyanide redox reaction. The nanotubule array electrodes were observed to more closely approach ideal reversible behavior than a typical Pt black/Nafion fuel cell electrode or a standard polished Pt disc electrode. The apparent heterogeneous electron transfer coefficient was determined using the Nicholson method and found to be one to two orders of magnitude greater for the nanotubule array electrodes, depending on the diameter of the nanotubules, in comparison with these same two more traditional electrode structures.
Metal nanowires, rods, tubes, and other so-called “one-dimensional nanostructures” [
Standard redox reactions such as those of ferrocyanide or ferrocene have long been employed as benchmarks in evaluating various carbon [
The focus of this work is to examine electron transfer kinetics of the classic ferrocyanide reaction on a platinum nanotubule array electrode fabricated using a simple template wetting process. The ubiquity and long history of this quasi-reversible, single-electron redox system should enable more meaningful comparison with other electrode structures. The template wetting nanofabrication process used to fabricate the platinum nanotubule array electrodes has been previously demonstrated by Luo et al. [
A template wetting process, previously demonstrated by Luo et al. [
Template wetting nanofabrication of platinum nanotubules. (a) A porous alumina template is wetted with a solution of platinum (II) 2,4-pentanedionate (Pt(acac)2) and poly(D,L-lactide) (PDLLA) in chloroform. (b) After evaporation of the solvent, a solid Pt(acac)2/PDLLA film coats template pores. (c) Excess material on the outer surfaces is removed using a helium plasma etch, followed by (d) annealing in air to both reduce the Pt(acac)2 to Pt0 and oxidize and remove the PDLLA. (e) Finally, a 2 min etch in 25 wt% KOH(aq) is used to selectively etch the alumina template and expose the Pt nanotubules.
The Pt nanotubule array was attached to a polished glassy carbon electrode (CH Instruments, 3 mm in Kel-F) using an alcohol-based conductive graphite adhesive (Alfa Aesar product number 42466) to facilitate electrochemical evaluation. Performance of the Pt nanotubule array was compared to that of a polished polycrystalline platinum disc electrode (CH Instruments, 2 mm in Kel-F) and a platinum black-based electrode prepared using a commercial catalyst (HiSPEC 1000, Alfa Aesar). The Pt black-based electrode was prepared by pipetting an ink containing Pt black dispersed in DI water and Nafion (5 wt.%, Solution Technology, Inc.) to a final concentration of 2 mg·mL−1 Pt with 10% Nafion by mass, a composition typical of fuel cell electrodes, onto a polished glassy carbon electrode surface and allowing it to dry in air at room temperature for 24 hrs [
A Gamry Instruments PCI4 Potentiostat was used to perform cyclic voltammetry (CV) experiments in a traditional three-electrode cell consisting of the Pt nanotubule array, Pt disc, or Pt black working electrode described above, a platinum counter electrode, and a Ag/AgCl reference electrode (CH Instruments). Deaeration of solutions in the electrochemical cell was accomplished by bubbling N2 prior to experiments and maintained by subsequently blanketing the cell with N2 during the experimental procedure. Prior to analysis, the catalyst structures were electrochemically cleaned by immersing in a 0.5 M H2SO4(aq) (GFS Chemicals, Veritas Grade, double distilled in 18 MΩ deionized water) and cycling the potential between 1.5 V and 0.03 V versus SHE (standard hydrogen electrode) at a scan rate of 500 mV/s until a steady-state voltammogram was obtained (approximately 50 cycles) [
Figure
SEM images of (a) an individual 100 nm Pt nanotubule and (b) an array 100 nm Pt nanotubules.
The active surface areas of platinum in electrodes made from the nanotubules, as well as for a more traditional platinum black described above, were evaluated using cyclic voltammetry in 0.5 M sulfuric acid. Representative voltammograms are shown in Figure
Cyclic voltammograms collected in 0.5 M H2SO4(aq) at 15 mV/s of 100 nm and 200 nm Pt nanotubule electrodes and a Pt black/10 wt% Nafion electrode. Surface areas obtained from the highlighted hydrogen adsorption region are shown in the table below the voltammograms.
Figure
Cyclic voltammograms obtained in 0.001 M potassium ferrocyanide (K4Fe(CN)6) in a 0.1 M KCl supporting electrolyte at 10 mV/s with
Randles-Sevcik plot of peak current versus square root of scan rate.
Of particular interest is the separation between the anodic and cathodic peak potentials,
The key kinetic parameter, the apparent heterogeneous electron transfer rate coefficient,
Similar trends have been reported when comparing carbon nanotube- [
Template wetting nanofabrication was used to prepare Pt nanotubular catalyst structures in a porous alumina membrane. The resulting high surface area Pt nanostructures were characterized electrochemically using the ferro/ferricyanide reaction. Though common with both Pt and carbon electrodes, especially carbon nanotube electrodes, this standard reaction has not previously been used to characterize nanostructured Pt electrodes. The nanotubule electrodes more closely approached ideal, reversible behavior and exhibited a one to two order of magnitude greater apparent heterogeneous electron transfer coefficient than standard platinum disc and platinum black electrodes. While the source of this improvement is not clear at this time, this nanoarray structure shows promise for sensor or small fuel cell applications.
The authors have of this paper do not have any direct financial relation with the commercial identities mentioned in this paper that might lead to a conflict of interests.
This work was funded in part by the Louisiana Board of Regents/RCS Program, Award LEQSF (2006-09)-RD-A-21.