Mesoscopic mass transport effects in electrocatalytic processes

Abstract

The role of mesoscopic mass transport and re-adsorption effects in electrocatalytic reactions was investigated using the oxygen reduction reaction (ORR) as an example. The electrochemical measurements were performed on structurally well-defined nanostructured model electrodes under controlled transport conditions in a thin-layer flow cell. The electrodes consist of arrays of Pt ultra-microelectrodes (nanodisks) of defined size (diameter approximately 100 nm) separated on a planar glassy carbon (GC) substrate, which were fabricated employing hole-mask colloidal lithography (HCL). The measurements reveal a distinct variation in the ORR selectivity with Pt nanodisk density and with increasing electrolyte flow, showing a pronounced increase of the H2O2 yield, by up to 65%, when increasing the flow rate from 1 to 30 microL s(-1). These results are compared with previous findings and discussed in terms of a reaction model proposed recently (A. Schneider et al., Phys. Chem. Chem. Phys., 2008, 10, 1931), which includes (i) direct reduction to H2O on the Pt surface and (ii) additional H2O2 formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H2O2 molecules on the Pt surface. The potential of model studies on structurally defined catalyst surfaces and under well-defined mass transport conditions in combination with simulations for the description of electrocatalytic reactions is discussed.

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

“…First, we show that simultaneous measurement of the H 2 O 2 current is vital for correct interpretation of ORR data. Second, while others have previously shown that the ORR catalyzed by large Pt nanoparticles is affected by changes in k t (22,23) we show here that this does not apply for smaller Pt materials. Finally, we show that the size of very small Pt nanoparticles (diameters <2 nm) has an effect on the mechanism of the ORR, and hence its product distribution.…”

“…In most of these studies, k t is increased by decreasing the surface coverage of the catalyst (21-23, 27, 28). For example, Behm and coworkers studied the ORR using twodimensional arrays of 100-nm Pt islands immobilized on glassy carbon (GC) electrodes (22,23). The key finding was that the amount of electrogenerated H 2 O 2 increased with decreasing catalyst coverage.…”

“…The analytical tools used for this purpose include the rotating disk and rotating ringdisk electrodes (RDE and RRDE) (21,(26)(27)(28), UMEs (2, 29), nanoelectrodes (3), microjet electrodes (4), and microfluidic thin-layer cells (22,23). In most of these studies, k t is increased by decreasing the surface coverage of the catalyst (21-23, 27, 28).…”

Platinum and Non‐Platinum Nanomaterials for the Molecular Oxygen Reduction Reaction

“…This experimental fact on extended catalytic surfaces, deduced via the RRDE technique, apparently supports the "direct pathway". However, recent studies [55][56][57] using the effect of mass transport put in evidence that an important amount of H 2 O 2 , on mesoscopic particle size of platinum, are generated. Interestingly, both studies used different approaches, namely, the single particle [55] and particle arrays.…”

“…Interestingly, both studies used different approaches, namely, the single particle [55] and particle arrays. [56,57] These latter are produced by colloidal lithography. Whereas the first approach obtained a maximum production of 25 % H 2 O 2 under high mass transport conditions, the second one (by varying the Pt coverage from 4 to 40 % on a glassy carbon substrate) produced a 10 to 20-fold increase in the H 2 O 2 yield.…”