Nonaqueous organic redox flow batteries (NAORFBs) show great promise for grid energy storage but are currently facing key challenges such as high electroactive material cost and low energy density. Herein, we report the electrochemical properties and the potential application of a series of cost-effective electroactive nitrobenzene molecules in NAORFBs. Pairing the low-cost miscible liquid nitrobenzene (NB) with 2,5-di-tert-butyl-1-methoxy-4-(20-methoxyethoxy)-benzene (DBMMB) resulted in a flow battery that provides a high theoretical cell voltage of 2.2 V and a calculated energy density of 192 Wh L −1 . In the charge−discharge testing, this battery delivers a stable cycling capacity retention of 99.5% per cycle over 100 cycles and a 70% energy efficiency at 40 mA cm −2 operation current density, verifying that liquid nitrobenzene is a promising low-cost electroactive anode molecule for NAORFBs.
Aqueous organic redox flow batteries (AORFBs) employing synthetically tailorable organic electroactive compounds have received significant attention for energy storage technologies. There have been many efforts in developing electroactive materials for AORFBs with anion-exchange membranes. On the contrary, electroactive compounds that are compatible with cationexchange membranes in AORFBs are less studied. Here, we report an electroactive 4-carboxylic-2,2,6,6-tetramethylpiperidin-N-oxyl (4-CO 2 Na-TEMPO) molecule for neutral AORFBs. The compound exhibits a good solubility of 1.5 M in an aqueous sodium-based solution, which is 3 times more than that of the pristine 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO). When paired with a 1,10-bis(3-sulfonatopropyl)-4,4′-bipyridinium (SPr) 2 V anolyte, the resulting RFB operating through a cation-exchange membrane achieved an open-circuit voltage of 1.19 V and a high energy density of 14.7 W h L −1 . In the long-term cycling study, the RFB features a stable capacity retention of 99.94% per cycle over 400 cycles with nearly 100% Coulombic efficiency.
We herein report the synthesis and characterization of a series of ruthenium-substituted Keggin-type heteropolytungstates containing {Ru(II)(NO)}, {Ru(III)(H2O)} or {Ru(IV)Cl} species. Although anionic [PW11O39Ru(II)(NO)](4-) (1) and [PW11O39Ru(III)(H2O)](4-) (2) are known, a new synthetic method for the preparation of (n-Bu4N)4[1] and (n-Bu4N)4[2] is developed in this paper. Treatment of (n-Bu4N)4[XW11O39(Ru[triple bond, length as m-dash]N)] with Me3NO afforded the ruthenium(ii) nitrosyl complex (n-Bu4N)4[1] in almost quantitative yield. Photolysis of (n-Bu4N)4[1] solution in CH3CN/H2O gives (n-Bu4N)4[2], which is readily oxidized by PhICl2 to yield the Ru(IV) complex (n-Bu4N)4[PW11O39Ru(IV)Cl] ((n-Bu4N)4[3]). These complexes are fully characterized by (1)H NMR and (31)P NMR spectroscopy, infrared spectroscopy, cyclic voltammetry, elemental analysis, thermogravimetric-differential thermal analysis, electrospray ionization mass spectrometry (ESI-MS) and X-ray photoelectron spectroscopy (XPS).
We synthesize a covalently linked bipolar molecule 1-(2-(4methoxy-2,5-dimethylphenoxy) ethyl)-1′-methyl-[4,4′-bipyridine]-1,1′-diium hexafluorophosphate (VIODAMB) and apply this new compound to a nonaqueous organic redox flow battery (NAORFB) as both the anolyte and catholyte. We demonstrate that this symmetrical electrolyte can mitigate the cross-contamination issue in the flow battery. The compound exhibits an enhanced solubility of 0.66 M in acetonitrile. The flow cell delivers a capacity retention rate of 80% and an energy efficiency of 85% over 35 cycles at a current density of 1.5 mA cm −2 in the cycling test.
The operating temperature of vanadium redox flow batteries (VRFBs) affects their performance and reliability. However, previous studies focused on evaluating the effects on the performance of lab-scale single cells, in which electrolyte flow rates and current densities are different from those in stack-scale VRFBs, leading to a lack of guidance for the design of stacks. In this work, we investigate thermal effects on the performance of stack-scale VRFBs. It is found that as the operating temperature increases from 25 to 50°C, the discharge capacity increases by 42%, whereas the energy efficiency increases by 10%, implying that the temperature has greater effects on the discharge capacity than that on the energy efficiency. Additionally, the enhancement effect of temperature on the energy efficiency is gradually weakened with increasing flow rate, while that on the discharge capacity is almost unchanged. Furthermore, the enhancement effect of temperature on energy efficiency increases with the operating current density. Notably, an optimum operating condition of the stack-scale VRFBs is identified with a critical flow rate (2.88 mL min-1 cm-2) at 40°C to achieve a high system efficiency. This work provides guidance for the design of stack-scale VRFBs with high performance and safety.
Recently, the electrospinning method has been employed to fabricate fibrous carbon electrodes in redox flow batteries, due to the large specific surface area of the nano-scale electrospun carbon fibers[1-3]. However, the poor transport properties of the densely packed electrospun carbon electrodes cause a large flow resistance, and hence a large concentration overpotential, which limited the battery’s performances [4,5]. The vanadium redox flow battery (VRFB) is one of the most-studied redox flow battery systems. However, the state-of-art VRFB that employed electrospun carbon fibers as the electrode can only be operated a realtively small current density (~ 100 mA cm-2), which is mainly hindered by the poor transport properties of the electrode material. Hence, the geometric structures of the electrospun carbon materials should be tailored so that the battery with the electrospun carbon fibers can be operated at a much elevated current density with high energy efficiency. Currently, several strategies have been proposed to improve the transport properties of the electrospun material, which include increasing the fiber diameter and expanding the pore sizes. However, the enhancement in the transport properties is always at the cost of sacrificing the active surface area[6]. Here we report an approach to effectively enhancing the transport of the electrolyte inside the porous media while ensuring a large specific surface area of the material. Aligned electrospun polymer fiber bundles are successfully produced by properly adjusting the electrospinning conditions. By applying self-etching methods, highly holey aligned fiber bundles were fabricated. In this structure, the aligned large fiber bundles provide the pathway for the electrolyte and the large surface area enabled by the holey fibers provides sufficient active sites for the redox reactions to take place. We compared the single-cell employed with the as-prepared highly holey aligned fibers on both sides with the battery assembled with conventional electrospun materials as well as thermal-treated commercial carbon felts. The results show that the novel structure can enable an energy efficiency of around 79.3% at the current density of 400 mA cm-2. Details of fabricating and characterization of the holey aligned electrospun electrodes and more data relating to the battery tests will be disclosed at the meeting. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) References [1] A.D. Blasi, C. Busacca, O.D. Blasi, N. Briguglio, V. Antonucci, Journal of The Electrochemical Society. 165 (2018) A147-A1485. [2] C. Busacca, O. Di Blasi, N. Briguglio, M. Ferraro, V. Antonucci, A. Di Blasi, Electrochimica Acta. 230 (2017) 174-180. [3] A. Di Blasi, C. Busaccaa, O. Di Blasia, N. Briguglioa, G. Squadritoa, V. Antonuccia, Appl.Energy. 190 (2017) 165-171. [4] S. Liu, M. Kok, Y. Kim, J.L. Barton, F.R. Brushett, J. Gostick, Journal of The Electrochemical Society. 164 (2017) A203-A2048. [5] A. Fetyan, I. Derr, M.K. Kayarkatte, J. Langner, D. Bernsmeier, R. Kraehnert, C. Roth, ChemElectroChem. 2 (2015) 2055-2060. [6] C. Xu, X. Li, T. Liu, H. Zhang, RSC Adv. 7 (2017) 45932-45937.
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