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

Abstract

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 2 O 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 2 O 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 ¼ 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.electrocatalysis | platinum nanoparticle H ere we report on the effect of 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 , respectively). The experiments were carried out in the dual-electrode microelectrochemical device shown in Fig. 1 (1). This configuration enables the study of the ORR under high k t conditions with simultaneous detection of H 2 O 2 . This is important because the methods currently used to study ORR at high k t , such as ultramicroelectrodes (UMEs) (2), nanoelectrodes (3), and microjet electrodes (4) do not incorporate H 2 O 2 detection. Accordingly, ORR data is generally interpreted based on steady-state, mass-transfer-limited currents alone. Using the microelectrochemical device, two principal effects are evident as k t is increased. First, the ORR current becomes significantly smaller than predicted by the Levich equation (5). Second, the effective number of electrons involved in the ORR, n eff , remains constant over the entire k t range, for both Pt 147 and Pt 55 . We draw three conclusions from these observations: (i) The decrease in ORR current at high k t is caused by kinetic effects; (ii) increasing k t does not affect the mechanism of the ORR catalyzed by Pt DENs; and (iii) there is a difference in n eff between Pt 147 and Pt 55 that cannot be attributed to masstransfer effects.The electrochemical ORR has been the subject of extensive research and has been reviewed on several occasions (6-10). The generally accepted mechanism for the ORR at a metallic surface is a multielectron reaction with intermediates as shown in Scheme 1 (2,7,9,11). Specifically, at the electrode surface, O 2 can be reduced directly to H 2 O or H 2 O 2 in...

Electrochemistry of Nanoparticles

“…The same effect was also demonstrated for other reactions that feature Figure 6. [217] A constant H 2 O 2 yield with flow rate was also observed by Behm and co-workers for extended polycrystalline Pt surfaces (see Figure 5 c in reference [48]), consistent with the results of Dumitrescu and Crooks. As the inter-NP distance is decreased, the diffusion spheres start to overlap.…”

“…[48] The amount of hydrogen peroxide detected downstream diminished as the NP density was increased and/or the flow rate was reduced (i.e. [216,217] A key attribute to this type of cell is that the hydrodynamics are very well defined and transport can be varied and controlled over a wide range. The same effect was also demonstrated for other reactions that feature Figure 6.…”

Oxygen Reduction Reaction Catalyzed by Noble Metal Clusters

“…Interestingly, with a series of Pt nanoparticles containing an average of 55, 100, 147, 200 and 240 Pt atoms, they found that the largest particles exhibited the highest specific activities [47]. In a following research effort, the effect of mass transfer on ORR catalyzed by Pt 55 and Pt 147 dendrimer encapsulated particles was also investigated by them [48].…”

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“…If n = 4, which is reasonable with mass transport of this magnitude [39], we obtain k t = 0.06 cm s −1 , although it should be noted that the effective n may be lower. This is a relatively high mass transport rate and provides a means of determining ORR kinetics [39][40][41]. Fig.…”