Differential Capacitance Study on the Basal Plane of Stress-Annealed Pyrolytic Graphite

Summary The non-faradaic differential electrode capacity of the basal plane of high-pressure stress-annealed pyrolytic graphite has been examined in aqueous solution using an a.c. impedance bridge. The capacity has a near parabolic dependence on electrode potential with a minimum of ∼3 μF cm−2 in concentrated electrolytes and is pH independent. Studies in NaF solutions at concentrations in the range 0.9–10−5 M indicate that in the range of potential studied (+0.5 to −0.5 V vs. NHE) the minimum capacity is best attributed to the space charge region within the graphite and not to the diffuse ionic layer. Apparently the potential of zero charge of the electrolyte is outside the potential range of the measurements. The capacity value on the basal plane of the stress-annealed pyrolytic graphite compares favorably with that estimated from the carrier concentration using the presently available theory of the space charge layer in semiconductor electrodes.