Nanoparticle Surface Coverage Controls the Speciation of Electrochemically Generated Chlorine

Cyclic voltammetry is used to investigate the oxidation of chloride on platinum nanoparticles. The electrochemical response of the nanoparticles drop-cast onto a glassy carbon electrode is compared to that recorded at a platinum macroelectrode. The use of "nano"- and "macro"-scale Pt reveals different ratios of the electrochemically formed products, chlorine (Cl-2) and the trichloride (Cl-3(-)) anion. This difference in the speciation is attributed to the chloride oxidation being a surface-reaction-limited process. For the situation in which there are a limited number of active sites available on the electrode, owing to low nanoparticle surface coverages, the sub-diffusion-limited currents result in higher chloride concentrations adjacent to the electrochemical interface. This excess chloride at the interface leads to the formation of the trichloride anion. The effect of surface oxide formation towards the chloride oxidation is also examined on both electrodes. Formation of platinum oxide serves to inhibit the rate of chloride oxidation.