Current at a Rotating Disk Electrode under Electrosorption and Surface Diffusion Limitation

Abstract

Following a model for the current limited by adsorption and surface diffusion at a rotating disk electrode (rde), the case is analyzed when surface diffusion is preceded by adsorption with charge transfer. The electrode surface consists of a distribution of reaction sites embedded in a supporting phase where electrosorption and surface diffusion take place. Electrosorption is described by five parameters, an equilibrium potential (E 0 ad ), a microscopic partial charge transfer (z ad ), a charge transfer coefficient (β ad ), and the rate constants for desorption and adsorption (k 1 and k 2 , respectively). Analytical expressions for the current -overpotential (j vs η ad ) and the Koutecký -Levich (KL) relation (1/j vs 1/ω 1/2 ) are provided as a function of electrosorption parameters and surface diffusivity (D s ). The KL relation is linear, with the same slope as for a classical rde, whereas the intercept (1/ j K ) depends on electrosorption parameters and D s . Tafel relations for the intercept current (log j K vs. η) are provided for the general and limiting cases. Effects of the radial convective transport in the electrolyte over reaction sites, and of the overlapping of surface diffusion areas are included in the model. The analytical expressions can be used to determine surface diffusion kinetics parameters from rde results. Reactivity at electrode surfaces is in most cases localized on active sites. On single crystalline surfaces, active sites favor the interaction with electrolyte species, 1-4 or expose appropriate electronic orbitals that enhance reactivity. 5,6 Localized reactivity is more remarkable on composite material surfaces, like supported catalysts and partially blocked surfaces, and when the reactions are mediated by an interaction with the electrode surface, like electrocatalyzed reactions, 1,3 electroadsorption, 7 and electrodeposition. 4,8 Unlocalized reactivity is less usual and requires the participation of non-interacting reactants. 9Areas surrounding active sites, called supporting or inactive areas, may participate in the electrochemical reaction in different ways. If adsorption is favored, the inactive areas concentrate reactants and become a parallel path for the supply of species to active sites. Predominance of the adsorption path over the direct electrolyte path is very probable in this case, even at low adsorbabilities, by a pure concentration effect.10 After adsorption, the species diffuse toward the actives sites leading to adsorption-surface diffusion mediated mechanisms, like the spillover, 11-18 and the bifunctional mechanisms. 19,20 Surface diffusion mediated reactions require an optimal degree of interaction of the adsorbed species with inactive areas, which should lead to a 'volcano' dependence of reaction rate with the adsorption energy like the encountered for many catalytic reactions. 21The rotating disk electrode (rde) technique is very appropriate for the study of surface kinetics in electrochemical reactions because it provides controlled and well defined hydrodynam...

Ordered Array Electrodes Fabricated by a Mask‐Assisted Electron‐Beam Method as Platforms for Studying Kinetic and Mass‐Transport Phenomena on Electrocatalysts

“…Such changes were not observed on GC/Rh arrays, which indicates that there are no such kinetic interactions between Rh and GC. Identification of the actual phenomena that cause the enhancement verified on Au/Rh electrodes requires a much deeper analysis on a comprehensive collection of experimental data involving variations of the junction lengths and microelectrode geometries, which should be supported by proper hor models that must include adsorption, surface diffusion and spillover processes around the junctions . In addition, other techniques with high spatial resolution, such as scanning electrochemical microscopy, could be used to analyze the adsorption and surface diffusion behavior of H ad over the bimetallic regions.…”

“…Identification of the actual phenomena that cause the enhancement verified on Au/Rh electrodes requires a much deeper analysis on a comprehensive collection of experimental data involving variations of the junction lengths and micro- 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 electrode geometries, which should be supported by proper hor models that must include adsorption, surface diffusion and spillover processes around the junctions. [22][23][24] In addition, other techniques with high spatial resolution, such as scanning electrochemical microscopy, [35] could be used to analyze the adsorption and surface diffusion behavior of H ad [36] over the bimetallic regions.…”

“…[6,20] Well-ordered binary arrays are useful tools that can be used as model platforms to quantify these localized kinetic phenomena. For example, recent reports from A.M. Chaparro and co-workers [21][22][23][24] showed the usefulness of rotating disk electrode (RDE) arrays for measuring adsorption and surface diffusion processes induced by the bicomponent junctions. Moreover, another approach to tackle this type of study recently reported by A.C. Chialvo and co-workers is based on the use of the so-called excess current, [11,19] which can be measured on well-ordered bimetallic ensembles.…”