Measuring Electrode Kinetics Under Spatial Confinement. Application of Pulse Voltammetry to Different Mass Transport Modes Using Butler–Volmer and Marcus–Hush–Chidsey Frameworks

Spatial confinement arises in many electrochemical phenomena, such as the electrochemical characterization of ultrathin membranes; insertion of ionic species into porous matrices; charge transfer processes in porous electrodes or through porous matrices; or redox cycling devices with bipotentiostatic control. In all these situations the mass transport occurs in solution layers whose thickness is comparable to or smaller than that of the diffusion layer. Two mass transport modalities can be considered in this context of finite diffusion: one leading to the partial or complete depletion of the electroactive species and the other related to the faradaic regeneration of the redox species. Of special interest is the relationship between spatial confinement and kinetic influences on overall electrochemical responses. In the specific case of redox kinetics, there is a lack of theoretical frameworks developed specifically for these new experimental scenarios, in order to provide simple and accurate tools that enable us to characterize the kinetics at play. In fact, methodologies developed for semi-infinite diffusion (as, for example, the Nicholson method) are still being applied, even though they are clearly unsuitable in this case. Normal Pulse Voltammetry (NPV) is proposed as a simple and accurate technique to characterize electrochemical processes with slow kinetics. Unusual features arise in the current–potential responses due to the overall kinetic dependence, which combines the effects of mass transport and redox kinetics. This allows all the key parameters of the responses to be determined with great accuracy. The well-known Butler–Volmer (BV) and Marcus–Hush–Chidsey (MHC) formalisms have been considered. This technique was applied to analyze the quasi-reversible oxidation of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-carboxy-TEMPO) radicals in aqueous solutions using a boron-doped diamond electrode. MHC formalism was found to be more coherent when describing the electrochemical responses for both mass transport modes.