Abstract
The electrochemical oxidation of VO2+ at planar glassy carbon electrodes is investigated via stationary and rotating linear sweep voltammetry as well as via chronoamperometry. It is demonstrated that introducing finite kinetic rate constants into the Butler-Volmer equation captures the experimentally observed concentration dependence of the ordinate intercept in Koutecky-Levich plots, that cannot be explained by using the classical model. This new concept leads to a three-term Koutecky-Levich equation considering mass transport limitations, Butler-Volmer kinetics as well as finite heterogeneous kinetics simultaneously. Based on these findings it is pointed out that stationary linear sweep voltammetry followed by an irreversible Randles-Sevcik analysis is not sufficient for deducing the electrode kinetics of the VO2+-oxidation. In contrast, it is verified experimentally and theoretically that a Tafel analysis will still provide reasonable values of k0 = 1.35 10-5 cm/s and a = 0.38, respectively. Finally, it is shown that introducing the concept of finite heterogeneous kinetics into the theory of stationary linear sweep voltammetry also explains the failure of the irreversible Randles-Sevcik relation leading to an extension of the classical model and providing insights into the electrochemical oxidation reaction of VO2+.
Highlights
In vanadium redox flow battery (VRFB) research, stationary1 linear sweep voltammetry (S-LSV) and stationary cyclic voltammetry (S-CV) are the most prevalent techniques used for the fast assessment of the kinetic performance of surface-modified carbon felt electrodes, and numerous studies have been published on that topic (Flox et al, 2013a,b; Gao et al, 2013; Hammer et al, 2014; Suárez et al, 2014; Liu et al, 2015; Park and Kim, 2015; He et al, 2016, 2018; Kim et al, 2016; Park et al, 2016; Ryu et al, 2016; Zhang et al, 2016; Zhou et al, 2016; González et al, 2017; Jiang et al, 2017; Ghimire et al, 2018; Xiang and Daoud, 2019; Yang et al, 2019)
By including the estimated maximum rate constant into the theory of stationary linear sweep voltammetry, we propose a model that captures the deviations of the experimental S-LSV data from the ideal irreversible Randles-Ševcík behavior
The kinetics of the VO2+/VO22+ redox couple was investigated at planar glassy carbon electrodes via rotating and stationary linear sweep voltammetry as well as stationary chronoamperometry
Summary
In vanadium redox flow battery (VRFB) research, stationary linear sweep voltammetry (S-LSV) and stationary cyclic voltammetry (S-CV) are the most prevalent techniques used for the fast assessment of the kinetic performance of surface-modified carbon felt electrodes, and numerous studies have been published on that topic (Flox et al, 2013a,b; Gao et al, 2013; Hammer et al, 2014; Suárez et al, 2014; Liu et al, 2015; Park and Kim, 2015; He et al, 2016, 2018; Kim et al, 2016; Park et al, 2016; Ryu et al, 2016; Zhang et al, 2016; Zhou et al, 2016; González et al, 2017; Jiang et al, 2017; Ghimire et al, 2018; Xiang and Daoud, 2019; Yang et al, 2019). It is shown that interpreting S-LSV data in terms of the irreversible Randles-Ševcík relation leads to values in the electron transfer coefficient α that contradict the findings from Tafel analysis of RDE data. We account for these two unexpected features simultaneously by introducing finite heterogeneous kinetic rate constants into the Butler-Volmer equation. A three-term Koutecký-Levich equation is derived, allowing for unraveling mass transport, Butler-Volmer electrode kinetics and finite heterogeneous electron transfer kinetics In this manner, the maximum heterogeneous rate constant for the oxidation reaction of VO2+ is found to be kmax = 2.6 · 10-2 cm/s. It is shown mathematically that the classical Tafel analysis of RDE data is unaffected by the limited electron transfer kinetics and should be preferred for kinetic characterization
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