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 ...
Solvent extraction is used widely for chemical separations and environmental remediation. Although the kinetics and efficiency of this process rely upon the formation of ion–extractant complexes, it has proven challenging to identify the location of ion–extractant complexation within the solution and its impact on the separation. Here, we use tensiometry and X-ray scattering to characterize the surface of aqueous solutions of lanthanide chlorides and the water-soluble extractant bis(2-ethylhexyl) phosphoric acid (HDEHP), in the absence of a coexisting organic solvent. These studies restrict ion–extractant interactions to the aqueous phase and its liquid–vapor interface, allowing us to explore the consequences that one or the other is the location of ion–extractant complexation. Unexpectedly, we find that light lanthanides preferentially occupy the liquid–vapor interface. This contradicts our expectation that heavy lanthanides should have a higher interfacial density since they are preferentially extracted by HDEHP in solvent extraction processes. These results reveal the antagonistic role played by ion–extractant complexation within the aqueous phase and clarify the advantages of complexation at the interface. Extractants in common use are often soluble in water, in addition to their organic phase solubility, and similar effects to those described here are expected to be relevant to a variety of separations processes.
Improvements in energy-water systems will necessitate fabrication of high-performance separation membranes. To this end, interface engineering is a powerful tool for tailoring properties, and atomic layer deposition (ALD) has recently emerged as a promising and versatile approach. However, most non-polar polymeric membranes are not amenable to ALD processing due to the absence of nucleation sites. Here, a sensitization strategy for ALD-coating is presented, illustrated by membrane interface hydrophilization. Facile dip-coating with polyphenols effectively sensitizes hydrophobic polymer membranes to TiO 2 ALD coating. Tannic acid-sensitized ALD-coated membranes exhibit outstanding underwater crude oil repulsion and rigorous mechanical stability through bending and rinsing tests. As a result, these membranes demonstrate outstanding crude oil-inwater separation and reusability compared to untreated membranes or those treated with ALD without polyphenol pretreatment. A possible polyphenolsensitized ALD mechanism is proposed involving initial island nucleation followed by film intergrowth. This polyphenol sensitization strategy enriches the functionalization toolbox in material science, interface engineering, and environmental science.
The aqueous organic flow battery (AOFB) holds enormous potential as an energy storage device for fluctuating renewable electricity by exploiting the redox reactions of water‐soluble organic molecules. The current development is impeded by lack of organic molecules adequate as catholyte, yet how the catholyte structure impacts the battery lifetime remains unexplored. Here, six ferrocene derivatives with deliberately tuned chemical structure were devised. They underwent reversible redox reactions in water, and the redox potentials were inversely related to the lowest unoccupied molecular orbital (LUMO) energy of their energized forms. The stability of the ferrocene derivatives was evaluated in full flow cells and in symmetric cells. Density function theory calculations, along with experimental results, revealed that the localized LUMO density on Fe led to fast capacity fading. BQH−Fc, which has the lowest LUMO density on Fe, showed the highest stability. No capacity loss was observed for the BQH−Fc/BTMAP−Vi cell at 0.1 m, and a high capacity retention rate of 99.993 % h−1 was recorded at 1.5 m, which could be attributed to electrolyte crossover. To facilitate explorations of robust and high capacity catholytes, a method was established to predict the water solubility of ferrocene molecules, and calculations were in good accordance with measured values.
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