A fundamental study on cyclic voltammetry at porous carbon thin-film electrodes

Porous carbons represent an important group of electrode materials for electrochemical applications due to their high specific surface area, good conductivity, and intrinsic porosity. However, the often-neglected influence of the electrode porosity on the electrochemical results may lead to wrong interpretation of data. The aim of this work is therefore to provide comprehensive understanding of the effects of electrode porosity on cyclic voltammetry (CV) – a technique which is frequently used for initial characterization of novel electrode materials. Our approach consists of simulating experimental results, observed for the redox-reactions of [Fe(CN)6]3-/4− at porous CNT thin-film electrodes, with a mathematical model. The results demonstrate that the main difference between planar and porous electrode is the additional charge transferred in the porous electrode domain, which has the decisive contribution to the overall faradaic current at high scan rates. To further verify the applied model, the porosity of the porous CNT film is varied on purpose by changing the Nafion-Carbon-ratio (NCR). We are able to determine the electrochemically available pore volume (ECPV) of the electrode, which is strongly affected by the NCR, and correlate changes in the ECPV with changes in the scan-rate dependent CV. Moreover, we implement an approach for determination of reaction rate constants at porous electrodes, which was verified by simulated CV data. Evaluation of the experimental results shows that the reaction rate constant of the porous CNT material actually has the same order of magnitude as the planar glassy carbon substrate.