Quantifying the Surface Diffusion of Oxygen Adspecies on Gold By Nanoelectrode Voltammetry
Authors: Wang Wei , Liu Cheng , Oleinick Alexander , Han Lianhuan , Sartin Matthew , Svir Irina , Amatore Christian , Zhan Dongping , Tian Zhong-Qun 
Published: July 2021

Surface diffusion of oxygen adspecies on noble metals plays crucial roles in heterogeneous catalysis, e.g., the anti-poisoning mechanism in fuel cells. However, quantitative measurements of surface diffusion in a solid/liquid environment remains challenging. Nanoelectrode voltammetry provides an easy but quantitative method to study the surface electrochemical behaviors of electroactive adspecies on metallic catalysts, which is valuable for the understanding of electroactalytic mechanisms and the rational design of electrocatalysts.
In this work [1] a gold nanodisk electrode was employed to study surface diffusion of oxygen adspecies. Surprisingly, we found that when the size of electrode was decreased to nanometer scale cyclic voltammograms (CV) behavior were drastically different from those observed at macrodisks gold electrodes. Indeed, instead of expected bell shaped CV characteristic to surface controlled process a seemingly diffusion controlled peak was observed during the forward scan. On the contrary, a very sharp almost symmetrical peak was recorded during the backward scan. A quantitative check on the charge of the adspecies during deposition and stripping phases revealed their equality and linear dependence with the reciprocal of the square root of scan rate, again suggesting a mass transport control in contradiction with the expectations. As was suggested previously [2, 3] the adspecies may apparently diffuse from the electrode-solution interface towards the surface of the electrode shaft which is in contact with the insulator. In agreement with this view, the amount of the adspecies stored by the system is drastically increased in comparison to the quantity of adspecies which can be accommodated on the disk electrode-solution interface only. In order to check consistency of these views we resorted to modeling by assuming a diffusion-equivalent transport of adatoms through site hopping [4-6] in agreement with the experimental observation of an Arrhenius’ dependence of the apparent diffusion coefficient. In addition, due to the unconventional behavior during the desorption stage we incorporated potential driven changes into the kinetics of the site hopping (i.e., leading to a larger apparent surface diffusion coefficient) to account for an apparent faster diffusion in agreement with the increasing instability of the metal-adatom bonds in the increasingly cathodic region. This rather simple model provided an excellent qualitative and quantitative agreement between experimental data and theoretical predictions. This, in particular, allowed to estimate the apparent potential-dependent surface diffusion coefficient of oxygen adspecies on Au surface.

References:

[1] W. Wang, C. Liu, A. Oleinick, L.H. Han, M.M. Sartin, I. Svir, C. Amatore, D.P. Zhan, Z.Q. Tian. Manuscript in preparation.
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