2-Methoxyhydroquinone from Vanillin for Aqueous Redox-Flow Batteries

Abstract

We show the synthesis of a redox-active quinone, 2-methoxy-1,4-hydroquinone (MHQ), from a bio-based feedstock and its suitability as electrolyte in aqueous redox flow batteries. We identified semiquinone intermediates at insufficiently low pH and quinoid radicals as responsible for decomposition of MHQ under electrochemical conditions. Both can be avoided and/or stabilized, respectively, using H₃ PO₄ electrolyte, allowing for reversible cycling in a redox flow battery for hundreds of cycles.

Keywords: lignin; quinone; redox-flow batteries; vanillin.

Figure 1

Left: Oxidation of vanillin to MHQ, redox reaction to MQ and potential insoluble dimeric side‐products (MHQMHQ, MQMHQ, DMBMQ. Right: experimentally derived and simulated cw‐ESR spectra of the photoproducts of MQ at pH values of 7.4, 4.0 and 3.0. The hyperfine coupling constants are given in the Supporting Information.

Figure 2

a) Cyclic voltammograms of MHQ in 0.5 M H₃PO₄ at scan speeds of 50, 100, 150, 200, 500, 800, and 1000 mV s⁻¹ (dark blue to light blue), respectively. b) Plot of I _p vs. ν ^1/2 of MHQ/MQ (1 mM in 0.5 M H₃PO₄).

Figure 3

a) Potentiostatic cycling of a pumped RFB with 18.65 mg MQ and 25 mg pBQ as active materials in 0.5 M H₃PO₄ at ±0.75 V for 5400 s cycle⁻¹. b) Galvanostatic cycling of a pumped RFB containing 20.6 mg MHQ and 23.4 mg pBQ as active materials in 0.5 M H₃PO₄ at a current density of 1.9, 5.7 and 9.5 mA cm⁻². The theoretical capacity of the cells (a: 112.6, b. 101.9 mAh L⁻¹) is indicated by the dashed line.