Effect of boron addition on the differential capacitance of stress-annealed pyrolytic graphite

Summary The non-faradaic differential electrode capacity of the basal plane of boronated stress-annealed pyrolytic graphite with 0.3 weight % boron has been studied in aqueous solution using an a.c. impedance bridge and compared to results previously obtained on the non-boronated material. On boronated graphite the potential of the minimum in the capacity-potential curve shifts negative by about 0.5 V relative to that for the non-boronated material. The direction of this shift is in agreement with the semiconductor interpretation proposed previously for the capacitance of the basal plane of the non-boronated material. The magnitude of the shift, however, is much larger and the capacity of the minimum for the boronated material much lower than estimated theoretically from the carrier concentration for the space charge region in the electrode phase. The potential of the minimum is essentially independent of pH over 14 units of pH for both the boronated and non-boronated material but the capacity-potential curves for the former deviate substantially from the near parabolic shape found for the latter, particularly at potentials anodic to the capacity minimum. Boronation apparently introduces surface chemical groups which contribute to the observed capacity at these anodic potentials. With scans to potentials more anodic than 1 V vs . RHE, pronounced hysteresis effects are observed in the capacity-potential curves for the boronated material, probably because of oxidation of the electrode surface.