Scanning Electrochemical Microscopy of Carbon Nanomaterials and Graphite

Carbon materials are tremendously important as electrode materials in both fundamental and applied electrochemistry. Recently, significant attention has been given not only to traditional carbon materials, but also to carbon nanomaterials for various electrochemical applications in energy conversion and storage as well as sensing. Importantly, many of these applications require fast electron-transfer (ET) reactions between a carbon surface and a redox-active molecule in solution. It, however, has not been well understood how heterogeneous ET kinetics at a carbon/solution interface is affected by the electronic structure, defect, and contamination of the carbon surface. Problematically, it is highly challenging to measure the intrinsic electrochemical reactivity of a carbon surface, which is readily passivated by adventitious organic contaminants. This Account summarizes our recent studies of carbon nanomaterials and graphite by scanning electrochemical microscopy (SECM) not only to reveal the fast ET kinetics of simple ferrocene derivatives at their graphitic surfaces, but also to obtain mechanistic insights into their extraordinary electrochemical reactivity. Specifically, we implemented new principles and technologies to reliably and reproducibly enable nanoscale SECM measurements. We took advantage of a new SECM imaging principle to resolve the high reactivity of the sidewall of individual single walled carbon nanotubes. In addition, we developed SECM-based nanogap voltammetry to find that monolayer graphene grown by chemical vapor deposition yields an unprecedentedly high standard ET rate constant, k(0), of ≥25 cm/s, which was >1000 times higher than that reported in the literature. Remarkably, the nonideal asymmetry of paired nanogap voltammograms revealed that the high reactivity of graphitic surfaces is compromised by their contamination with airborne hydrocarbons. Most recently, we protected the clean surface of highly oriented pyrolytic graphite from the airborne contaminants during its exfoliation and handling by forming a water adlayer to obtain a reliable k(0) value of ≥12 cm/s from symmetric pairs of nanogap voltammograms. We envision that SECM of clean graphitic surfaces will enable us to reliably address not only effects of their electronic structures on their electrochemical reactivity, but also the activity of carbon-based or carbon-supported electrocatalysts for fuel cells and batteries.