Oxygen Reduction at Nanostructured Electrodes Assembled from Polyacrylate-Capped Pt Nanoparticles in Polyelectrolyte

Oxygen reduction and related processes are studied at nanostructured Pt electrodes assembled from polyacrylate-capped Pt nanoparticles (〈<i>d</i>〉 = 2.5 ± 0.6 nm) in poly(diallyldimethylammonium)chloride (PDDA) on indium tin oxide or glassy carbon with varying nanoparticle surface coverage. The nanoparticle density was varied laterally by varying the dipping time of PDDA-modified electrodes in the nanoparticles solution, or vertically with the number of nanoparticle/polyelectrolyte (bi)layers following a layer-by-layer assembly. TEM images revealed submonolayer coverage in one bilayer at 60 min dipping with a fractal distribution, and a significant surface coverage at four bilayers with evidence of multilayer assembly. Cyclic voltammetry in oxygen-containing electrolytes showed the assemblies to be electroactive for oxygen and hydrogen peroxide reduction, with a pH-dependent oxygen reduction peak shifting by −50 mV/pH unit. OH adsorption was found to be less favored occurring at more positive potential at the nanostructured electrode compared to polycrystalline Pt, while the oxide reduction peak was negatively shifted at the former electrode, in agreement with reports of increased oxophilicity with decreased particle size. The oxygen reduction peak potential shifted positively upon increasing Pt nanoparticles coverage, consistent with the catalytic activity of Pt for oxygen reduction. The active surface area of Pt nanoparticles was measured electrochemically from the charge of hydrogen underpotential deposition at the assemblies in H<sub>2</sub>SO<sub>4</sub>, and the diffusion-limited peak current for oxygen reduction measured per real Pt surface area is reported to decrease with increasing catalyst loading, as a result of reaching a limiting effective diffusion field.