Oxygen reduction at sparse arrays of platinum nanoparticles in aqueous acid: hydrogen peroxide as a liberated two electron intermediate

Electrodeposition methods are used to generate a sparse array of platinum nanoparticles on a glassy carbon electrode. Specifically electrodeposition from a 1 mM solution of H 2 PtCl 6 in 0.5 M H 2 SO 4 leads to surface coverages of 0.46% to 1.96% and nanoparticles of size 29 nm to 136 nm in diameter, using deposition times of 30 and 15 seconds. The reduction of oxygen at an array of 29 nm nanoparticles with a surface coverage of 0.46% showed voltammetric signals with a scan rate dependence consistent with a two electron reduction of O 2 to H 2 O 2 with the rate proportional to and formal potential ( E 0 f ) of −0.058 V vs. SHE, a standard electrochemical rate constant ( k 0 ) of ∼10 cm s −1 and a transfer coefficient ( α ) of 0.23. At higher Pt nanoparticle coverages, a scan rate dependence consistent with the partial further reduction of H 2 O 2 to water becomes evident. Diffusion is found to play a huge role in the oxygen reduction mechanism on platinum. At sufficiently low coverage, oxygen reduction on platinum is found to be a 2-electron process producing hydrogen peroxide.