Effect of mass transfer on the oxygen reduction reaction catalyzed by platinum dendrimer encapsulated nanoparticles

Here we report on the effect of the mass transfer rate (k ₜ) on the oxygen reduction reaction (ORR) catalyzed by Pt dendrimer-encapsulated nanoparticles (DENs) comprised of 147 and 55 atoms (Pt ₁₄₇ and Pt ₅₅). The experiments were carried out using a dual-electrode microelectrochemical device, which enables the study of the ORR under high k ₜ conditions with simultaneous detection of H ₂O ₂. At low k ₜ (0.02 to 0.12 cm s ⁻¹) the effective number of electrons involved in ORR, n ₑff, is 3.7 for Pt ₁₄₇ and 3.4 for Pt ₅₅. As k ₜ is increased, the mass-transfer-limited current for the ORR becomes significantly lower than the value predicted by the Levich equation for a 4-electron process regardless of catalyst size. However, the percentage of H ₂O ₂ detected remains constant, such that n ₑff barely changes over the entire k ₜ range explored (0.02 cm s ⁻¹). This suggests that mass transfer does not affect n ₑff, which has implications for the mechanism of the ORR on Pt nanoparticles. Interestingly, there is a significant difference in n ₑff for the two sizes of Pt DENs (n ₑff = 3.7 and 3.5 for Pt ₁₄₇ and Pt ₅₅, respectively) that cannot be assigned to mass transfer effects and that we therefore attribute to a particle size effect.