Carbon-based electrodes are nearly ubiquitous in the laboratory today because of their availability in various forms and shapes, and usefulness over a wide potential range. There are various compositions of carbon, ranging from highly ordered pyrolytic graphite to less ordered glassy carbon to boron doped diamond to carbon-paste to exquisitely small carbon nano-tubes. Rick McCreery has written a comprehensive report on carbon electrodes, their surface properties and applications [ref. 8]. The most commonly used carbon-based electrode in the analytical laboratory is glassy carbon (GC). It is made by pyrolyzing a carbon polymer, under carefully controlled conditions, to a high temperature like 2000 oC. An intertwining ribbon-like material results with retention of high conductivity, hardness and inertness. The electrochemistry is affected greatly by its surface chemistry of carbon-oxygen functionalities and its cleanliness; i.e., absence of adsorbed impurities. This latter has been shown by McCreery and coworkers who cleaned and stored GC in solutions containing highly adsorbing carbon particles that preferentially removed impurities [ref. 9].
We will focus on GC and discuss methods of surface activation and maintenance of surface cleanliness. For analytical purposes, Cypress/ESA offers GC disk electrodes of 3 mm diameter encased in glass or 10 um and 1 mm disks encased in PEEK polymer.
The most common method of activation is to polish the GC surface with micro-sized abrasives. Such treatment exposes fresh new surface. The maintenance of clean conditions during such polishing is critical. For instance, polishing on some types of abrasive containing pads can deactivate the surface [ref. 10]. After polishing, sonication is often required to remove carbon particles and remains of polishing material. These steps may be time consuming and repeated sonication may eventually destroy the electrode by compromising the seal between the carbon and the outer cladding material (e.g., glass or PEEK). Use of large agglomerated particles for polishing may leave large surface scratches and indentations. The best activation procedure may be vacuum heat treatment (VHT) [ref.11] although polishing with alumina in cyclohexane may yield comparable active GC electrodes [ref. 9]. An interesting feature about VHT and the use of an organic solvent for polishing, are the absence or low oxygen-containing functionalities on the carbon surface. Chen and McCreery [ref. 12] has nicely addressed how and when such functionalities affect electron transfer processes.
In this TechNote a simple method is described on how to activate a Cypress System glassy carbon (GC) electrodes that produces a smooth surface with a high degree of activation. In addition, we demonstrate a method to keep the electrodes active during storage or between experiments to minimize need for repolishing.