Mass transfer to a nanoelectrocatalyst

Abstract

There are few studies of mass transfer to nanospheres (1 nm ≤ d_p ≤ 100 nm). We have experimentally investigated the electrocatalytic reduction of hexacyanoferrate (III) to hexacyanoferrate (II) on gold nanospheres. The surface flux is insensitive to particle sizes of d_p ≥ 30 nm and is essentially identical to that for a diffusion-limited system. However, the measured fluxes in the range 5 nm ≤ d_p ≤ 30 nm were one to three orders of magnitude smaller than predicted by a purely diffusion-limited model. Using mathematical modeling, we evaluated six mechanisms affecting mass transfer to a nanoparticle in our experimental system. Among potential acceleratory effects, the curvature effect sharply increased the surface flux by a factor of 20. Other acceleratory effects of Brownian advection and enhanced surface reactivity played negligible roles, the latter due to screening by a charged stabilizing layer. Deceleratory effects of increased tortuosity by stabilizing layers and particle aggregation also played negligible roles. Electrostatic repulsion dominated mass transfer for d_p ≤ 30 nm. This finding suggests tuning the charge and the tortuosity of the stabilizer layer to potentiate the flux will be useful in engineering nanosuspensions.