ELSEVIER Colloids and Surfaces A: Physicochemical and Engineering Aspects 93 ( 1994) 21 I-219 COLLOIDS AND SURFACES zyxwvutsrq A Control of reactivity at carbon electrode surfaces Richard L. McCreery *, Kristin Kneten Cline I, Christie Allred McDermott 2, Mark T. McDermott ’ Department of Chemistry. The Ohio Sttrte Unirersit_v, 120 W. 18th Arrnw. Columbus. OH 43,710. USA Received 29 November 1993: accepted IS March 1994 Abstract Electron transfer kinetics and adsorption were examined at several carbon surfaces. including basal plane highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC). The objective was an understanding of how surface structure affects reactivity. HOPG basal plane exhibited much slower electron transfer rates than GC for all systems studied. and the observed rate constants were several orders of magnitude lower than those predicted from Marcus theory. In addition. HOPG did not adsorb several quinoncs which showed strong adsorption to GC under similar conditions. Any disturbance to the ordered basal plane (such as near defects) grcally increased electron transfer rates and adsorption. GC exhibited both fast kinetics and strong adsorption. provided the surface was clean, For the cast of aquated Eu2*/3C, Fez+134 and V2+IJc, mild oxidation of fhc GC surface prcatly incrcascs the observed rate. apparently via an inner-sphcrc catalytic route involving surface oxides. The results Icad to the conclusion that both adsorption and electron transfer are suppressed at HOPG owing to the low density of clcctronic stauzs near the Fermi level. Disorder at dcfccts or in GC increases the density of states and causes the carbon to bchavc more like a metal. Activation of Eu’ ‘/‘+. Fe’ “” and V’ ‘1.) ’ by oxidation is consistent with formation of a surface oxide complex via displacement of ligated water. Ke~~or~/s: Carbon adsorption; Electrocatalysis; Electron transfer; Glassy caibon; Graphite 1. Introduction A central theme of electrochemistry, or for that matter chemistry as a whole, is the relationship between structure and reactivity. The study of electrode kinetics is motivated both by fundamen- tal questions about the factors or phenomena which control electron transfer rates and by major practical issues in the areas of electrosynthesis and energy conversion [I]. Recent environmental con- l Corresponding author. ’ Present address: Willenbcrg University, SpringkId. 011. USA. z Prcscnt address: Iowa Stale University, Ames. IA. USA. 0927-7757;94507.00’0 1994 Elscvicr Scicncc B.V. All rights rcscrved SSDI 09?7-7757( 94)01899-1 ccrns have incrcnsed the importance of batteries and fuel cells, both of which rely on efficient heterogeneous electron transfer kinetics. In eco- nomic terms, approximately $35 billion of the annual US gross national product is derived directly from heterogeneous electron transfer t-23, a significant fraction of which occurs at carbon electrodes. Events at electrodes share some similar- ities with those of heterogeneous catalysis. since both often involve adsorption. Surface structure can drastically affect reactivity in both cases, and surface preparation often profoundly affects beha- vior. Electron transfer between an electrode and a solution or adsorbed component has the added complexities of an electric field (often greater than