Langmuir-Blodgett Thin Films of Fe20Pt80 Nanoparticles for the Electrocatalytic Oxidation of Formic Acid

The electro-oxidation of formic acid catalyzed by Fe20Pt80 nanoparticles was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) where the particles were deposited onto a gold electrode surface by the Langmuir-Blodgett (LB) technique at varied film thickness/coverage. It was found that the particle assembly thickness strongly affected the electrocatalytic activity for HCOOH oxidation. For a single monolayer and two layers of FePt particles, extensive CO adsorption (poisoning) was observed in CV measurements, whereas with a four-layer assembly of the FePt particles, the tolerance to CO poisoning improved drastically. Furthermore, from the current density for formic acid oxidation, the electrodes functionalized with LB layers of nanoparticles exhibited higher activities than those with dropcast films of similar thickness, suggesting the importance of the ordering of the particle layers in the electrocatalytic performance. The reaction kinetics in the HCOOH oxidation on the three kinds of particle film-modified electrodes was then examined by EIS measurements. It was found that except within the potential range for CO oxidation, the impedance spectra behaved normally with the responses shown in the first quadrant for all three nanoparticle assemblies under study, indicative of only resistive and capacitive components in the electrochemical cell. In contrast, at potentials of CO electro-oxidation, the impedance spectra were found to migrate from the first quadrant to the second quadrant and then back to the first quadrant with increasing electrode potential, which suggests that the reaction kinetics evolve from resistive to pseudoinductive and then to inductive behaviors. The different EIS behaviors are ascribed to the different degree of tolerance to CO poisoning, consistent with the voltammetric results.