Stability of molecular radicals in organic non-aqueous redox flow batteries: A mini review

Abstract

The application of novel organic redox materials is a plausible pathway towards technoeconomic energy storage targets due to their low cost and sustainable design. Their operation in non-aqueous redox flow batteries affords researchers the opportunity to innovate, design and optimise these new chemistries towards practical energy densities. Despite this, the identification of high capacity organics which also display long-term stability is inherently challenging due to the high reactivity of molecular radicals.

Bio-inspired Molecular Redesign of a Multi-redox Catholyte for High-Energy Non-aqueous Organic Redox Flow Batteries

“…In contrast, it is noteworthy that the UV-vis spectra of BMEPZ + in the electrolyte were virtually unchanged over 24 h ( Figure S10), indicating that BMEPZ + has superior radical stability. 12,56 We next performed a demonstration of the flow cell under the near-saturation condition of BMEPZ to investigate the electrochemical behavior under practical cell conditions. In Figure 5A, additional charge-discharge curves for flow cells using 0.1 M and 0.4 M BMEPZ are presented.…”

Recent Advances in the Development of Organic and Organometallic Redox Shuttles for Lithium‐Ion Redox Flow Batteries

“…Unlike other n‐type and p‐type molecules, which have problems of stability owing to a large structural rearrangement upon electron transfer events, nitroxide‐based compounds have a stable structure in which electron density is localized in the N−O bond, resulting in only a slight change in conformation during oxidation/reduction. This implies a strong stability and durability of these materials for battery systems . One of the most well‐known nitroxide compounds is (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl or (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxidanyl (referred to as TEMPO).…”

Dithiolene Complexes of First‐Row Transition Metals for Symmetric Nonaqueous Redox Flow Batteries

“…However, owing to their intermittentn ature,e fficient and cost-effective grid-scale energy storage is required before solara nd wind energy can achieve widespread implementation. [6][7][8][9][10] Acetonitrile (MeCN) is an attractive solvent for nonaqueous RFBs, andi st he main solvento f choice here, owing to its wide ( % 5V)e lectrochemical window as well as low viscosity (0.34 vs. 0.89 MPa sfor water) and moderate dielectric constant(35.9 vs. 78.4 for water). [3][4][5] As energy is stored externally to the electrochemical reactor,t he capacity can be increased independently of the battery power.A tp resent,c ommercial RFBs utilize aqueous electrolyte solutions of inorganic metal salts, however, despite continualp rogress in powero utputs and efficiencies being made, the cell potentiali si nherently limitedb y the narrow (1.23 V) electrochemical window of water.I nstead, the developmento fn onaqueous RFBs, which use organic solvents with wide electrochemical windows,i sa nticipated to improve the voltage outputs.…”

“…[3][4][5] As energy is stored externally to the electrochemical reactor,t he capacity can be increased independently of the battery power.A tp resent,c ommercial RFBs utilize aqueous electrolyte solutions of inorganic metal salts, however, despite continualp rogress in powero utputs and efficiencies being made, the cell potentiali si nherently limitedb y the narrow (1.23 V) electrochemical window of water.I nstead, the developmento fn onaqueous RFBs, which use organic solvents with wide electrochemical windows,i sa nticipated to improve the voltage outputs. [6][7][8][9][10] Acetonitrile (MeCN) is an attractive solvent for nonaqueous RFBs, andi st he main solvento f choice here, owing to its wide ( % 5V)e lectrochemical window as well as low viscosity (0.34 vs. 0.89 MPa sfor water) and moderate dielectric constant(35.9 vs. 78.4 for water). [11] Metal-ligandc oordination complexes are good candidates for nonaqueous RFBelectrolytes as they can be stable in multiple oxidation states and have high solubility in organic solvents.F urthermore, careful choice of metal ion as well as modification of the ligand scaffold (e.g.,s olubilizing groups, denticity,d onorg roups) can allow for fine tuning of the desired properties for RFB applications.…”