Electrocatalytic activity of Pd-Co bimetallic mixtures for formic acid oxidation studied by scanning electrochemical microscopy.

The electrochemical oxidation of formic acid was studied by the tip generation-substrate collection (TG-SC) mode of scanning electrochemical microscopy (SECM), extending the number of applications of SECM in electrocatalysis. Formic acid was generated at a Hg on Au ultramicroelectrode (UME) tip by reduction of CO(2) in a 0.1 M KHCO(3) solution saturated with this gas. The electrocatalytic activity of different Pd-Co bimetallic compositions was evaluated using a Pd-Co electrocatalyst array formed by spots deposited onto glassy carbon (GC) as a SECM substrate. The SECM tip, which generated a constant formic acid flux, was scanned above the array and the oxidation current generated when formic acid was collected by active electrocatalytic spots was displayed as a function of tip position. This generated a SECM image that showed the electrocatalytic activity of each spot. SECM screening identified Pd(50)Co(50) (Pd/Co = 50:50, atomic ratio) as a better electrocatalyst toward the formic acid oxidation than pure Pd or Pt in 0.1 M KHCO(3) solution and this result was confirmed by cyclic voltammetry. Positive feedback was observed for the most active compositions of Pd-Co which suggests fast reaction kinetics and chemical reversibility during the oxidation of formic acid to CO(2). Moreover this feedback increases the contrast between active and non-active spots in this imaging mode.