Concentration Effects on the First Reduction Process of Methyl Viologens and Diquat Redox Flow Battery Electrolytes

Abstract

Analyte concentration effects on the first reduction process of methyl viologens and diquat redox flow battery electrolytes were examined by cyclic voltammetry in aqueous media. A simple one-electron transfer mechanism to form radical cations was detected for diquat, 4,4′-dimethyl diquat, and bis(3trimethylammonio)-propyl viologen compounds. The radical cations attach to the electrode surface when the source of their electrogeneration is methyl viologen molecules bearing PF 6 − ions as a counterpart. However, this inner sphere reduction mechanism was not observed in methyl viologen having an I − counterion. For the latter compound, as well as for 5,5′-dimethyl diquat and 1,1′bis(3-sulfonatopropyl)-4,4′-bipyridinium, a piece of experimental evidence for unexpected, fast, and reversible dimerization interactions between their electrogenerated radical cations is presented. To get information on these bimolecular interactions, a screening methodology (using different levels of theory) was employed in finding suitable dimeric structures and their related interaction energies. By using diquat as a reference system, a relationship between calculated interaction energies and the corresponding experimental dimerization constants was obtained. The examination of redox-active molecules using this experimental and theoretical approach will allow a better selection of redox flow battery electrolytes.

Understanding Electron Transfer Reactions using Constrained Density Functional Theory: Complications due to Surface Interactions

“…The barrierless ET reactions occur when molecules are strongly bound to graphene. Our results are validated by the existing experiments, 23,24 where strong adsorption of active molecules to electrodes has been confirmed for DMDQ +2/+1 , and Me-Vi +2/+1 reactions. In this class, the reactant and product states minima collapse into a deep single-minimum on the free energy surface (See Fig.…”

“…To help solve these problems, quantum mechanical modeling can be used both to identify molecular properties and to rationalize kinetic differences. [20][21][22] Recent studies by Martínez-González et al 23,24 have combined experiments with calculations to study the reduction reactions of a handful of organic molecules on the glassy carbon electrodes in different circumstances.To simulate ET processes at the electrode-electrolyte interface, electron density is separately localized on each counterpart. One group of atoms can be considered the electron acceptor while the other group is the electron-donor.…”

“…To help solve these problems, quantum mechanical modeling can be used both to identify molecular properties and to rationalize kinetic differences. [20][21][22] Recent studies by Martínez-González et al 23,24 have combined experiments with calculations to study the reduction reactions of a handful of organic molecules on the glassy carbon electrodes in different circumstances.…”

Reversible Redox Chemistry in a Phenoxazine-Based Organic Compound: A Two-Electron Storage Negolyte for Alkaline Flow Batteries

“…35 Inspired by the ability of gallocyanine (GAL) to act as a redox mediator (undergoing two reversible electron transfers) in enzymatic and biological reduction processes, 26,29 and because it is a commercially available PXZ-based derivative that is also decorated with OH and COOH groups around its structure, here we examined and discussed its use as a negolyte electrolyte in an alkaline flow cell, also presenting a set of chemical and electrochemical reactions for reducing GAL deprotonated species (GAL ox RFB electrolytes are typically characterized by cyclic voltammetry at low analyte concentrations (close to 0.001 mol L −1 ) and scan rates (e.g., 0.1 V s −1 ). Based on this approach, a two-electron reduction process for GAL compound dissolved in 1 mol L −1 KOH (Figure 1A, black line) was identified when comparing its voltammetric peak current intensities to the ones measured from the singleelectron reduction voltammetric wave (Figure 1A, gray line) of 1,1′-bis(3-sulfonatepropyl)-4,4′-bipyridinium structure used as a reference system, 5,32 and considering the reported information on similar PXZ-based structures. 26,27 The signal assigned to the reduction process of GAL ox 2− species exhibits an apparent half-wave potential, E 1/2, app , equal to −0.56 V (vs Ag/ AgCl), and a peak-to-peak potential distance (ΔE p ) of 97 mV, which is close to the minimum value adopted (ΔE p > 200/n mV) in RFB protocols for screening irreversible systems that do not continue to the next stage.…”

Photoinduced Electron Transfer and Changes in Surface Free Energy in Polythiophene-Polyviologen Bilayered Thin Films

“…Moreover, the dication (V 2+ ) is usually a pale-yellow color as its chloride salt, the radical cation (V •+ ) is dark purple, appearing almost black, and the diradical or neutral state (V 0 ) is reddish-orange as its chloride salt. These prominent redox-induced color changes make viologen-based materials ideal for electrochromic devices; − however, they have also been explored in supercapacitors, − redox-flow batteries, − and biological assays , and even as dopants in dye-sensitized solar cells , and quantum dots. , …”