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Kinetic and Product Composition Studies of 9,10-Anthraquinone-2,7-Disulfonic Acid: Correlating Transient/Steady-State Modeling with Experimental Analysis

Organic-based quinones have been studied for use as an electrolyte for redox-based flow batteries due to their good chemical stability, high solubility, and low cost. In biological applications, quinones can be more advantageous than other charge carriers, such as viologens, due to all of the aforementioned reasons as well as their lower toxicity. One such quinone, 9,10-anthraquinone-2,7-disulfonic acid (AQDS), is reported to have lower redox potentials [1], but the reaction pathways to form desired AQDS products are more highly pH-dependent [2], which can be problematic in biological applications with restrictive pH ranges to maintain biocatalyst reactivity. Competition with dimerization has also been reported to be an issue, inhibiting the production of desired charged species for use in catalysts [3]. Consequently, there is a need to reliably predict the state of the charge carrier, requiring more robust theoretical models.
 The kinetics of AQDS are first studied here experimentally in a single-cell configuration with a buffer selected to maintain a neutral pH condition (pH = 7). Cyclic voltammetry (CV) studies revealed highly reversible kinetics for AQDS, indicating an advantage in its choice as a charge carrier over viologens, which tend to have certain additional irreversible redox reactions. Furthermore, a Randles-Sevcik analysis of the peak current vs. scan rate revealed strong equivalence in the reduction and oxidation reactions. Diffusion coefficients of 1-2 x 10-9 m2/s are consistent with previously reported values [4]. Additionally, kinetic constants were determined using the Kochi equation [5] and were found to be on the order of 10-3 to 10-5 cm/s, indicating more sluggish kinetics than would otherwise be anticipated.
 Chronoamperometry studies were conducted in an H-cell reactor with 10 mM AQDS in the cathode chamber with a pH buffer. One set of experiments was run using CO2 as the purge gas (used in biological applications to convert to usable organic products, decreasing pH to 6.3), and the other set was run using N2 (to serve as an inert gas and help maintain near neutral pH). Charging with -0.75 V vs. Ag/AgCl over a duration of 1.5 hours, the AQDS displayed high initial currents relative to a blank sample; however, longer steady-state times suggested slower reaction kinetics consistent with the CV study. Under the more pH neutral conditions, initial AQDS reduction was further delayed, indicating greater product favorability in more acidic environments. This knowledge is useful for battery applications where inert gases are typically used to purge but poses a challenge when catalysts such as enzymes are highly sensitive to pH.
 Finally, a transient-based equilibrium model is developed for AQDS in a batch reactor. The model is simulated using Butler-Volmer kinetics until steady-state is reached, using the same conditions studied for the experimental chronoamperometry. AQDS displays a lower charge input than other charge carriers, such as viologens, at -0.5 V vs. Ag/AgCl but is limited by slower kinetics. An extended transient study revealed AQDS kinetics taking nearly 10 hours to reach final equilibrium concentrations. The product composition of AQDS depends heavily on pH as well as voltage, with four unique product composition ranges. The use of a steady-state equilibrium model to connect with the transient model for the complete range of product compositions is reported here at various pH and voltage values. Designing charge carrier reduction experiments around such conditions predicted theoretically, rather than only -0.75 V, may enable more rapid and reliable charge carrier production to assist in applications such as fuel cells, batteries, and catalytic CO2 conversion to value-added products. 
 
 Forster RJ, O'Kelly JP. Protonation reactions of anthraquinone-2, 7-disulphonic acid in solution and within monolayers. Journal of Electroanalytical Chemistry. 2001 Feb 16;498(1-2):127-35.
 
 
 Batchelor-McAuley C, Li Q, Dapin SM, Compton RG. Voltammetric characterization of DNA intercalators across the full pH range: Anthraquinone-2, 6-disulfonate and anthraquinone-2-sulfonate. The Journal of Physical Chemistry B. 2010 Mar 25;114(11):4094-100.
 
 
 Wiberg C, Carney TJ, Brushett F, Ahlberg E, Wang E. Dimerization of 9, 10-anthraquinone-2, 7-Disulfonic acid (AQDS). Electrochimica Acta. 2019 Sep 10;317:478-85.
 
 
 Huskinson B, Marshak MP, Suh C, Er S, Gerhardt MR, Galvin CJ, Chen X, Aspuru-Guzik A, Gordon RG, Aziz MJ. A metal-free organic–inorganic aqueous flow battery. Nature. 2014 Jan;505(7482):195-8.
 
 
 Klingler RJ, Kochi JK. Electron-transfer kinetics from cyclic voltammetry. Quantitative description of electrochemical reversibility. The Journal of Physical Chemistry. 1981 Jun;85(12):1731-41.
 
 
 
 
 
 
 
 
 Figure 1
