Double-layer capacitance electrolysis

In chemical terms, it may be proposed that the it electrons of the first graphite layer at the surface are localized at states which are separate from the bands of the bulk. Significant contribution of these states to the double layer capacitance necessitates an overpotential which is beyond the applied potential range (limited by water electrolysis). In terms of semiconductor theory, it may be assumed that the charge carrier density at the first and probably the second graphite layer is much lower than the bulk charge density value of 6 x 1018 carriers per cm3. 

As the potential that is applied across the electrodes is increased, the ionic velocities increase. Thus, the detector signal is proportional to the applied potential. This potential can be held to a constant value or it can oscillate to a sinusoidal or pulsed (square) wave. Cell current is easily measured however, the cell conductance (or reciprocal resistance) is determined by knowing the potential to which the ions are reacting. This is not a trivial task. Ionic behavior can cause the effective potential that is applied to a cell to decrease as the potential is applied. Besides electrolytic resistance that is to be measured, Faradaic electrolysis impedance may occur at the cell electrodes resulting in a double layer capacitance. Formation of the double layer capacitance lowers the effective potential applied to the bulk electrolyte. 

A quick method to estimate the safe charge-injeetion limit of the electrode is to measure the electrode potential as a function of the injected charge [21]. At low charge-injection levels, the electrode potential increases linearly with increasing charge injection as the double-layer capacitance charges. However, with the onset of electrolysis, any additional charge is consumed by hydrolysis and the electrode potential reaches a plateau. 

---