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. The equations derived are obeyed only at low to moderately high scan rates (v) depending on the particular experimental system. The treatment used was applied on the assumption that the nucleation rate for the process is a function only of the concentration of nuclei of a critical size which depends on the overpotential. This approximation holds whether nucleation at a constant potential is instantaneous or progressive. Consequently, neither has physical significance under variable-potential 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 potential (DELTAE(p)) and v(1-x), were found. Analytical criteria based on such relationships are put forward and the way they are affected by the number of molecules forming critically sized nuclei is discussed. While the effect is complex, exponent x in the above relationships increases with increase in the number of such molecules.