Molecular Engineering of Azobenzene‐Based Anolytes Towards High‐Capacity Aqueous Redox Flow Batteries

Abstract

Aqueous redoxflowbatteries (RFBs) are promising alternatives for large-scale energy storage.H owever,n ew organic redox-active molecules with good chemical stability and high solubility are still desired for high-performance aqueous RFBs due to their lowc rossover capability and high abundance.W er eport azobenzene-based molecules with hydrophilic groups as new active materials for aqueous RFBs by utilizing the reversible redox activity of azo groups.B y rationally tailoring the molecular structure of azobenzene,the solubility is favorably improved from near zero to 2Mdue to the highly charged asymmetric structure formed in alkaline environment. DFT simulations suggest that the concentrated solution stability can be enhanced by adding hydrotropic agent to form intermolecular hydrogen bonds.T he demonstrated RFB exhibits long cycling stability with acapacity retention of 99.95 %per cycle over 500 cycles.Itpresents aviable chemical design route towards advanced aqueous RFBs.

Hybrid Electrolyte Engineering Enables Safe and Wide‐Temperature Redox Flow Batteries

“…TheABmolecule was recently reported as apromising active material in DMF-based electrolytes for nonaqueous RFBs, [29] but the CV result in hybrid electrolyte is very different, accompanying with the change of reaction mechanism, which can be ascribed to the protonation effect of water. [30] Further, we explored the influence of adding acids on the hybrid electrolyte,b ecause many promising redox species in aqueous electrolytes require acidic conditions to achieve high solubility and reversibility.I ti sf ound that the hybrid electrolyte can keep chemically stable with the presence of acids,a nd supports the reversible chemistry of redox species.H owever, as the study of redox chemistry in hybrid electrolytes is still in the infant stage and different organic solvents are available to hybridize with water, more efforts are needed to have abetter understanding of this new electrolyte chemistry.W ea lso did the screening of various selected redox species in the hybrid electrolyte in terms of their solubility.A ss hown in Figure S8, some reported redox species are hardly dissolved in the hybrid electrolyte though they have the promising electrochemical properties.H owever,several promising redox species (i.e.K 4 Fe(CN) 6 ,A QDS and NaI) can still obtain the reasonable solubility in the Angewandte Chemie Forschungsartikel 15158 www.angewandte.de DMF-H 2 Os olvent. Since NaI could show am uch higher solubility,itisselected as the model material to demonstrate the role of hybrid electrolytes on designing wide-temperature RFBs.…”

“…We also tested the CV curves of several other redox species in the hybrid electrolyte including azobenzene (AB) and NaI (Figure S7). The AB molecule was recently reported as a promising active material in DMF‐based electrolytes for nonaqueous RFBs, [29] but the CV result in hybrid electrolyte is very different, accompanying with the change of reaction mechanism, which can be ascribed to the protonation effect of water [30] . Further, we explored the influence of adding acids on the hybrid electrolyte, because many promising redox species in aqueous electrolytes require acidic conditions to achieve high solubility and reversibility.…”

Biomimetic Amino Acid Functionalized Phenazine Flow Batteries with Long Lifetime at Near‐Neutral pH

“…[8] Different families of organic molecules have been reported, including quinones, [8,9] viologens, [10] aza-aromatics, [11] ferrocene, [10c, 12] nitroxide radical species, [13] and azobenzene. [14] Phenazine and its derivatives, [15] which have been widely used as broad-spectrum antibiotics, [16] are good candidates for negolytes in AORFBs,owing to their low redox potentials and capability to accommodate two-electron transfers.However,the reported research of aqueous soluble redox-active phenazines for AORFBs is rather limited. The pioneering work reported by Schubert et al investigated the combined phenazine-TEMPO molecule through triethylene linker that exhibited reversible redox property but poor solubility in water.…”

Azobenzene‐Based Photomechanical Biomaterials

“…Unlike most other photoswitchable molecules, azobenzene (azo‐) and its derivatives impart materials with special qualities based on the photoisomerization of azobenzene between the trans and cis states upon irradiation with different wavelengths of light ( Figure 1 ). [ 37–39 ] In particular, azobenzenes exhibit high photoisomerization efficiency and the two isomers exhibit significant geometric structural differences, which permit their use in the development of new smart materials. [ 40 ] For example, liquid‐crystal networks (LCNs) functionalized with azobenzene moieties can directly convert light energy into motions such as twisting, rotation, and oscillation, making them a promising material for building actuators.…”