Electrochemical Characterization of Aromatic Molecules with 1,4-Diaza Groups for Flow Battery Applications
Authors: Pasadakis-Kavounis Alexandros , Baj Vanessa , Hjelm Johan 
Published: December 2020

Redox flow batteries based on organic active materials can provide a cost effective, biosustainable method for storing energy [4]. Quinones have been the primary focus for RFB applications with numerous reports on the properties and effect of structural modification by substitution with different functional groups [2, 3, 5]. The present study was focused on aromatic diaza compounds for RFB applications, namely pyrazine and quinoxaline. Both compounds undergo two electron transfer on the pyrazinic ring [6, 7]. Their small size and high solubility makes them attractive for flow cell application as it can maximize energy density [3]. In this work we examine the electrochemical properties as a function of pH and substitution. An initial screening using cyclic voltammetry in alkaline environment was performed on pristine pyrazine, and on pyrazine substituted with 1-4 methyl or carboxylic acid groups. The electron-donating methyl groups reduced the average redox potential about 55 mV per methyl group, while the electron-withdrawing carboxylic groups increased it. Additionally, the degree of substitution influenced the heterogeneous electron transfer kinetics significantly, with the greatest negative effect observed in the carboxylic pyrazines. Quinoxalines in the same alkaline environment featured similar results in different extend. The position of the functional group in regards to the active pyrazinic ring altered the degree of the effect, allowing for fine tuning of properties. Quinoxaline and quinoxaline-2,3-diyldimethanesulfonate (2,3-DMSQUI) were further examined in different pH under buffered and unbuffered solutions. It revealed that different processes take place depending on the availability of protons. Similar phenomena have been observed in quinone systems as well [2]. In the unbuffered solution, quinoxaline’s average potential had been stable up around pH = 11 where a slope of 44 mV/pH appeared. The cathodic and anodic potential had different slopes (25 and 63 mV/pH) indicating an uneven transfer coefficient, different rate limiting step, unequal amount of protons or a combination of the previous. The sharper voltammograms appeared in the high pH range. In contrast, cyclic voltammetry of 2,3-DMSQUI revealed high heterogeneous rate constant at pH 2-3. While at higher pH the kinetics appeared to be sluggish. The slopes of average, anodic and cathodic peak potential of quinoxaline in buffered solution were the same in the pH range of 2 to 12. Assuming that in buffered solution protons are always available, the difference of the slopes in the unbuffered case was most likely to an uneven amount of protons involved in the oxidation and reduction processes resulting from different acid dissociation constants. Moreover, in the acidic region, a peak at higher potential appeared which it has been attributed to an irreversible oxidative addition of a hydroxyl group [1]. Whereas, the same irreversible process was not present in 2,3-DMSQUI which meant substituting the 2 and 3 position hindered this degradation. No difference of 2,3-DSMQUI in buffered solution was found with its unbuffered counterpart, besides that the most reversible voltammogram appeared at a slightly more acidic environment ( pH = 4). Thus 2,3-DSMQUI is a good candidate for an acidic flow battery. Lastly, kinetic parameters determined by steady state voltammetry with rotating disk electrode. Diffusion coefficient and heterogeneous rate constant for quinoxaline were 7.93(±0.06)∗10-6 cm2/s and 9.6∗10-5 cm/s respectively, and 2,3-DSMQUI are 4.02(±0.06)∗10-6 cm2/s and 10.8∗10-4 cm/s.
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