Table 2.5:      Anisotropic electrical conductivity of graphite and YBa₂Cu₃O_7-x.

Graphite is one the most studied electrode materials.^{43, 49-58}  There are four major types of graphite: glassy carbon, carbon black, carbon fiber, highly ordered pyrolytic graphite (HOPG), with HOPG being the most ordered.  Made of sp² bonded sheets of carbon atoms aligned in the same direction separated by van der Waals gaps, HOPG is a highly anisotropic material with regards to its electrical conductivity.  As seen in Table 2.5, a ratio of ~3800 for the c-axis resistivity versus the a-b plane resistivity indicates the c-axis resistivity is quite high compared with that of the a-b plane.  Defects due to impurities increase the conductivity along the c-axis, therefore reduce electrical anisotropy.⁵⁰ 

HOPG electrodes are oriented to expose either the edge plane or basal plane as the electrode surface.  Electrochemical measurements on HOPG also exhibit anisotropy.  Edge plane HOPG electrodes are often sealed in epoxy and polished, and frequently the edges of these electrodes fold over to expose the basal plane.  These electrodes have an increased electrode surface area when compared to the geometric surface area, because it is not possible to create an atomicly flat surface.  Basal plane oriented electrodes, on the other hand, can be cleaved to produce an atomically flat electrode surface.  Preparation of a fresh electrode surface is accomplished⁵⁰ by cleaving of the HOPG by pressing ordinary “Scotch” tape on the basal plane surface and removing the tape along with the attached graphite layers.  The exposed surface is highly susceptible to mechanical damage which changes the electrode’s electrochemical properties,⁵⁵ so care must be taken to prevent damage. 

One of the most dramatic attributes of HOPG is the anisotropic response of redox species such as Fe(CN)₆^-3/-4.^{49, 55, 57}  For example, the DE_p of Fe(CN)₆^-3/-4 is 700 mV at 200 mV/sec for a basal plane HOPG electrode.  An edge plane HOPG electrode has a DE_p for Fe(CN)₆^-3/-4 of 70 mV  at 200 mV/sec.  This dramatic difference can be explained by means of the slow electron transfer associated with the basal plane orientation.  Like many semiconductors which have slow electron transfer rates, HOPG has a low density of states and a corresponding low number of charge carriers.^{59, 60}  Also, carbon has the correct number of valence electrons with none of the dangling bonds that exist in edge plane HOPG or metals.  It has been noted that with the introduction of defects on basal plane HOPG that the charge transfer rate is significantly increased.^{49, 50, 55, 57}   The formation of defects also increases the density of states and creates reactive sites on the surface. 

Introduction of defects has been achieved in a couple of ways.   The first is laser activation of HOPG.  For example, 10 nsec pulses from a Nd:YAG laser with an intensity of 50-90 MW/cm² can decrease the DE_p of Fe(CN)₆^-3/-4  from ~700 mV to less than 100 mV.  Another way is oxidizing the surface by electrochemical pretreatment of basal plane HOPG which rearranges the bonding, thus exposing some edge planes and leading to comparable decreases in DE_p.  Low capacitance values are characteristic of basal plane HOPG.^{50-52, 60}  Even after defects are introduced, there continues to be a low background current.  This makes defect activated basal plane HOPG a useful electrode for amperometric detection.

Much like electrical anisotropy is an important factor in graphite, the anisotropy of YBa₂Cu₃O_7‑x plays a role in the electrochemical response.  The reactivity of YBa₂Cu₃O_7‑x electrodes also strongly affects the faradaic electron transfer across the electrode solution interface.  Overall, it appears that the behavior of YBa₂Cu₃O_7‑x  electrodes mirrors that found in edge plane and basal plane graphite.