Electro-oxidation of formic acid catalyzed by FePt nanoparticles

The electrocatalytic oxidation of formic acid at a gold electrode functionalized with FePt nanoparticles was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a mixed solution of 0.1 M HCOOH and 0.1 M HClO4. The FePt bimetallic nanoparticles, with a mean diameter of 3 nm, were prepared by a chemical reduction method. The Au/FePt nanostructured electrode was prepared firstly by the deposition of FePt nanoparticles onto a clean Au electrode surface, followed by ultraviolet ozone treatment to remove the organic coating. In CV measurements, two well-defined anodic peaks were observed at +0.20 and +0.51 V (vs. a Ag/AgCl quasi-reference). The anodic peak at +0.20 V was attributed to the oxidation of HCOOH to CO2 on surface unblocked by CO, whereas the peak at +0.51 V was ascribed to the oxidation of surface-adsorbed CO (an intermediate product of HCOOH oxidation) and further oxidation of bulk HCOOH. From the onset potential and current density of the electro-oxidation of HCOOH, FePt nanoparticles exhibit excellent electrocatalytic activities as compared to Pt and other metal alloys. EIS measurements were carried out to further examine the reaction kinetics involved in the HCOOH electro-oxidation. The EIS responses were found to be strongly dependent on electrode potentials. At potentials more positive than -0.25 V (vs. Ag/AgCl), pseudo-inductive behavior was typically observed. At potentials between +0.3 and +0.5 V, the impedance response was found to reverse from the first quadrant to the second quadrant; such negative Faradaic impedance was indicative of the presence of an inductive component due to the oxidation of surface-adsorbed CO. The impedance responses returned to normal behavior at more positive potentials (+0.6 to +0.9 V). The mechanistic variation was attributed to the formation of different intermediates (CO or oxygen containing species) on the electrode surface in different potential regions. Two equivalent circuits were proposed to model these impedance behaviors.