Impedance Insight in Catalytic Properties of Long-Lived Free-Radical Oxidizer Based on Silver(II)
Authors: Polczynski Piotr , Jurczakowski Rafal Robert , Grochala Wojciech 
Published: February 2020

Novel functional materials are of significant relevance to the development of numerous industrial technologies such as electronics, renewable energy sources and environmental protection. Compounds of divalent silver are particularly interesting due to their unusual physicochemical properties. Recently we have found that the formal potential of the Ag(II)/Ag(I) redox couple can reach as much as 2.9V vs. NHE[1] in superacidic media, which makes this system the strongest fluorine free oxidizer known. Electrooxidation of silver(I) in sulfuric acid solutions proceeds according to EqCi’ mechanism, in which one-electron transfer leading to solvated Ag(II) species is followed by a homogeneous chemical step with regeneration of depolarizer. These results indicate that silver(I) oxidation in superacidic media leads to the formation of exceptionally strong oxidizing, and relatively long-lived, species containing Ag(II) [2]. These powerful free-radical Ag(II) oxidizers can be used for 1e– oxidation of various kinetically inert systems and in general for initialization of free radical reactions in superacidic media.

Here we report electrochemical impedance studies of this interesting system [3]. Typical impedance spectrum is shown in Fig. 1. For data analysis we developed analytical model for impedance of catalytic processes together with potential dependence of all impedance parameters derived for general case in rigorous treatment [4]. The model is valid for various systems regardless the kinetics of the heterogeneous and homogeneous reactions and also for system with unequal diffusion coefficient. Resistance of the charge transfer was found to be potential independent at far anodic or far cathodic overpotentials for electrocatalytical oxidation and reduction, respectively. Resistance of the charge transfer in silver(I) oxidation is shown in Fig. 2 (top). The equivalent circuit for catalytic systems, in which the kinetics of the homogenous chemical step can be described by Gerisher element is shown in the inset to Fig. 1 (bottom). By fitting impedance spectra both the heterogeneous and the homogeneous rate constants for silver electrooxidation were obtained in various experimental conditions. We have found that the half-life of the free-radical oxidizer based on silver(II) is the largest in 100% sulfuric acid solutions (Fig.2 bottom) and it substantially decreases both for diluted sulfuric acid and as well as for oleums. The influence of water and SO3 on the silver(II) chemistry will be discussed.

Fig. 1. Impedance spectra of Ag(I) oxidation in 18 M (95%) H2SO4. Green continuous line (−) represent the fit result of experimental points (◆) by using equivalent circuit shown in the inset.

Fig. 2. (Top) Potential dependence of the charge transfer resistance for Ag(I) oxidation in 18 M(95%) H2SO4. (Bottom) Rate constant of homogenous chemical reaction in function of sulphuric acid concentration.

References

[1] P. Połczyński, R. Jurczakowski, W. Grochala, ChemComm, 2013, 49, 7480.

[2] P. Połczyński, R. Jurczakowski, W. Grochala, The Journal of Physical Chemistry C,2013, 117, 20689−20696.

[3] P. Połczyński, R. Jurczakowski, W. Grochala, manuscript in preparation.

[4] P. Połczyński, R. Jurczakowski, manuscript in preparation.