Slow Diffusion Reveals the Intrinsic Electrochemical Activity of Basal Plane Highly Oriented Pyrolytic Graphite Electrodes

This paper reports a method for distinguishing the electroactivity of different types of sites on heterogeneous electrode surfaces, exemplified through studies of basal plane highly oriented pyrolytic graphite (HOPG) electrodes. By depositing a thin film of Nafion with incorporated redox species (i.e., tris(2-2'-bipyridyl)ruthenium(II), Ru(bpy)(3)(2+), and hexaaminoruthenium(III), [Ru(NH3)(6)](3+) onto HOPG, diffusion is greatly slowed down. On the time scale of cyclic voltammetry, one can then distinguish between different scenarios of electrode activity because sites on the electrode, with different activity, become diffusionally decoupled. In particular, we show that one can discriminate readily between limiting cases in which the basal plane of HOPG is considered to be either (i) completely active or (ii) inert (with only step edges active). Experimental measurements coupled to modeling show unequivocally that the basal plane of HOPG is electrochemically active. The methodology described and the results obtained have important implications for understanding the intrinsic activity of the basal plane and step edges of graphite electrodes and related carbon-based electrode materials.