Electrocatalysis by nanoparticles: Oxidation of formic acid at manganese oxide nanorods-modified Pt planar and nanohole-arrays

Abstract

The electro-oxidation of formic acid (an essential reaction in direct formic acid fuel cells) is a challenging process because of the deactivation of anodes by the adsorption of the poisoning intermediate carbon monoxide (CO). Pt electrodes in two geometries (planar and nanohole-array) were modified by the electrodeposition of manganese oxide nanorods (nano-MnOx). The modified Pt electrodes were then tested for their electrocatalytic activity through the electro-oxidation of formic acid in a solution of pH 3.45. Two oxidation peaks ($I_{\text{p}}^{\text{d}}$ and $I_{\text{p}}^{\text{ind}}$) were observed at 0.2 and 0.55 V, respectively; these were assigned to the direct and indirect oxidative pathways. A significant enhancement of the direct oxidation of formic acid to CO₂ was observed at the modified electrodes, while the formation of the poisoning intermediate CO was suppressed. $I_{\text{p}}^{\text{d}}$ increases with surface coverage (θ) of nano-MnOx with a concurrent depression of $I_{\text{p}}^{\text{ind}}$. An increase in the ratio $I_{\text{p}}^{\text{d}}$/ν^1/2 with decreasing potential scan rate (ν) indicates that the oxidation process proceeds via a catalytic mechanism. The modification of Pt anodes with manganese oxide nanorods results in a significant improvement of the electrocatalytic activity along with a higher tolerance to CO. Thus nano-MnOx plays a crucial role as a catalytic mediator which facilitates the charge transfer during the direct oxidation of formic acid to CO₂.