Beyond Randles-Sevcik Formalism: Towards Understanding Peak Currents of Nernstian Redox Systems in Square-Wave Voltammetry

In redox systems that obey the Nernst equation, where the surface and bulk concentrations remain in equilibrium during the potential sweep, the Randles-Sevcik equation is seen as a standard tool in both fundamental and applied linear scan voltammetry. As the Randles-Sevcik equation is seen as a key theoretical framework for interpreting voltammetric behavior in electrochemically reversible and diffusion-controlled redox systems considered under conditions of linear scan voltammetry, this foundational relationship becomes inapplicable when extended to pulse voltammetric techniques. Pulse voltammetric techniques differ fundamentally from linear scan voltammetric methods in both potential modulation and in current measurement protocols. The form of applied bias in pulse voltammetric techniques leads to conditions in which each applied pulse disrupts the diffusion profile of redox species of interest. Repeated disruption and compression of diffusion profiles in pulse voltammetric techniques introduce significant complexity into the current-potential behavior of redox species, thereby precluding the direct application of the Randles-Sevcik formalism. This study presents some basic theoretical insights into the limitations of applying Randles–Sevcik-type equations to square-wave voltammetry. In addition, a unifying parameter has been identified that governs the peak current response in square-wave voltammetry, which integrates the effects of potential step, frequency, square-wave amplitude, and temperature. At constant magnitude of the diffusion coefficient, this critical parameter is defined as χ = constant · (F/RT)·[Esw/(dE·f)^]1/2. This parameter "X" is discovered for the first time (and it also named as "JPG" parameter according to the initials of the authors (Janeva Pavlinka Gulaboski)) is seen as a foundation for developing more comprehensive models and analytical expressions describing peak current dependencies under square-wave voltammetric conditions.