Differential capacitance study of stress-annealed pyrolytic graphite electrodes

Abstract : The non-faradaic capacity of electrodes prepared from high pressure stress-annealed pyrolytic graphite has been examined in aqueous solutions using an a.c. impedance bridge. SUCH MATERIALS HAVE A ROCKING ANGLE (x-ray diffraction) as small as 0.4 degrees and the properties of surfaces oriented parallel to the basal plane approach rather closely those of single crystal graphite. The differential capacity measured on this surface has a near parabolic dependence on electrode potential with no evidence of a hump and a minimum of about 3 microfarads/sq. cm in 0.9 M NaF. This low value is explained on the basis that a substantial fraction of the potential change between the electrode bulk and the solution bulk occurs across a space charge layer within the graphite.