USE OF CYCLIC VOLTAMMETRY FOR STUDYING 2-DIMENSIONAL PHASE-TRANSITIONS - BEHAVIOR AT LOW SCAN RATES

A theoretical study of two-dimensional phase transitions taking place in electrode processes was carried out by using cyclic voltammetry. Th e equations derived are obeyed only at low to moderately high scan rat es (v) depending on the particular experimental system. The treatment used was applied on the assumption that the nucleation rate for the pr ocess is a function only of the concentration of nuclei of a critical size which depends on the overpotential. This approximation holds whet her nucleation at a constant potential is instantaneous or progressive . Consequently, neither has physical significance under variable-poten tial conditions, at least at low scan rates. Theoretical relationships between the peak current (I(p)) and v(x) (x = 0.6), and also between both the peak half-width (W; the width, in mV, at the peak half-height ) and the difference between the reduction and oxidation peak potentia l (DELTAE(p)) and v(1-x), were found. Analytical criteria based on suc h relationships are put forward and the way they are affected by the n umber of molecules forming critically sized nuclei is discussed. While the effect is complex, exponent x in the above relationships increase s with increase in the number of such molecules.