Screening Viologen Derivatives for Neutral Aqueous Organic Redox Flow Batteries

Abstract

Viologen derivatives have been developeda sn egative electrolyte for neutrala queous organic redox flow batteries (AOFBs), but the structure-performancer elationship remains unclear. Here, it was investigated how the structure of viologens impacts their electrochemical behavior and thereby the battery performance, by taking hydroxylated viologensa se xamples. Calculations of frontierm olecular orbitale nergy and molecular configuration promise to be an effective tool in predicting potential, kinetics, and stability,a nd may be broadly applicable. Specifically,amodified viologen derivative, BHOP-Vi, was provedt ob et he most favorable structure, enablingaconcentrated 2 m battery to exhibit ap owerd ensity of 110.87 mW cm À2 and an excellent capacityr etention rate of 99.953 %h À1 .Renewable energy plays an indispensable role in replacing fossil fuel energy and provides ap ossible solution for as ustainable future. [1] However,t he large-scale adoption of renewable energy,s uch as solar and wind energy,i si mpeded by their intermittent and fluctuating features. The fluctuation in renewable energy supply can be solved by electrochemical energystoraget echnology,w hich stores electricity in electrochemically active materials and provides stable electricity output when needed. [2] As an emerging energy-storage technology,a queous organic redoxf low batteries (AOFBs) exploit the reversible redox reaction of low-cost organic compounds dissolved in aqueous solutions. [3] AOFBs that operate under neutral conditions do not involveh ighly basic or acidic solutionsa nd are therefore safe to handle and have fewer requirementso nb attery components. [4] The electrolyte solutionso faneutralA OFB flow along opposite sides of an ion-selective membrane and are circulated between cell stacksa nd externalt anks. The tanks can be as large as possible to provide long-duration energy supply,a nd the powerc apability can be independently tuned. Powerc apability and cycle lifetime of an eutral AOFB can be effectively tailoredb ye ngineering the chemical structure of redox-active organic electrolytes.Viologens, the characteristicn egative electrolytes (negolytes) for neutral AOFBs, have attracted increasingr esearch interests because of their diverse structure and straightforward chemical modification. The simplest viologen,m ethyl viologen,i sc ommerciallya vailablea nd can also be readily synthesized on a large scale with high yield by reacting bipyridine with either methyli odide or chloroacetic acid. [5] When paired with 4-OH-TEMPO [TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl],i tc an deliver ab attery capacity of 13.4 Ah L À1 and ac apacity loss rate of 3.6 %p er day.T his high capacity loss rate is owing to the dimerization of the intermediate, the cationic viologen radical. [6] To prevent the dimerization, Aziz and co-workers took advantage of coulombic repulsion, [7] and after adding quaternary ammonium groups to the viologen core they reporteda much lower capacity loss rate of 0.03 %p er day for the BTMAP-Vi/BTMAP-Fc cell ...

A Computational Protocol Combining DFT and Cheminformatics for Prediction of pH-Dependent Redox Potentials

“…The main goal of this contribution is the establishment and validation of a standard procedure for the computational prediction of redox potentials of organic molecules undergoing a proton-coupled electron transfer. One of the main motivations for benchmarking such predictions is the computational screening of the vast chemical space of organic molecules to identify electrolytes for redox flow batteries (RFBs) [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. At some point in any computational workflow for materials discovery, one needs a reliable method to predict with reasonable accuracy and moderate computational cost the properties of an already pre-selected candidate pool.…”

Redox‐Active Organic Compounds for Future Sustainable Energy Storage System

“…Their work afforded the scope for developing organic-based redox flow batteries, which are inexpensive and exhibit high energy efficiencies at practical power densities. Subsequently, other organic redox-active molecules were successfully demonstrated in the redox flow batteries, employing both aqueous [67][68][69] and non-aqueous electrolytes, showing the great promises. [70,71] Analogous liquid-state organic electrodes were also explored in rechargeable non-flow battery systems, which contain solid electrolyte membranes that physically separate the two electrodes.…”

Organic Negative Electrode Materials for Metal‐Ion and Molecular‐Ion Batteries: Progress and Challenges from a Molecular Engineering Perspective

“…Different to n-type ONEMs discussed in previous sections, N,N' di-quaternized bipyridyl salts, also known as viologens, have emerged recently as promising candidates among ONEMs (Figure 12). Viologen derivatives have been extensively used as dissolved active materials mainly explored to date as negolyte in redox flow batteries [251][252][253][254] but also in aqueous organic batteries. [232,233] Their redox activity can evolve between three reversible redox states according to p-type redox mechanism, starting from the fully oxidized form (V 2+ ) to the intermediate radical cation (V •+ ) and lastly the fully reduced neutral form (V), accompanied with reversible anion extraction/uptake (Figure 12a).…”