The redox flow battery (RFB) is one of the most promising systems for large scale electrochemical energy storage applications. The development of redox-active materials is an essential part of RFB research. Commercial RFBs utilize redox-active inorganic ions, which have several issues such as expensive and toxic active materials, crossover of redox species, and high cost of the ion exchange membrane. The incorporation of redox-active polymeric materials is an intriguing solution because low-cost polymeric redox-active materials and size-exclusion porous membranes formed by commodity polymers could be applied to replace expensive inorganic redox-active materials and ion exchange membranes, respectively. The development of polymer-based redox-active materials in RFB application (PRFB) has advanced rapidly in the past few years. In this review, the recent progress of PRFB is summarized. Three major categories based on materials are discussed: the aqueous redox-active polymers, the nonaqueous redox-active polymers and oligomers, and polymer suspension systems. This review not only provides comprehensive information on synthetic strategy of polymeric redox-active materials and their properties such as redox potential and solubility in electrolyte but also reports the performance of recent PRFB cells, including cycling stability, cell voltage, and implementation of size-exclusion porous membranes. Finally, a short future perspective of PRFB development is provided.

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Aqueous organic redox flow battery represents a potential solution to significantly reduce the cost of flow batteries. An ideal organic redox active material for flow battery application shall possess good stability in supporting electrolyte, fast redox reaction speed, and low manufacturing cost. In this work, a commercially available dye material was modified and evaluated in aqueous organic redox flow battery as anolyte. The desalted Basic Red 5 dye (d-BR5) exhibited reasonable solubility in acidic supporting electrolyte and stable redox performance. Two aqueous organic redox flow batteries with d-BR5 as anolyte and two different catholytes (4,5dihydroxybenzene-1,3-disulfonic acid (BQDS) and cerium methanesulfonate (Ce(CH 3 SO 3 ) 3 )) were evaluated in this work. The later system reached satisfactory stability in long-term cycling experiments. The cell of d-BR5 and (Ce(CH 3 SO 3 ) 3 ) exhibited an operating voltage ∼ 1.4 V, with 99.9% capacity retention after 200 cycles. The results indicated that organic dye molecules could be potential low-cost active materials for aqueous organic redox flow batteries.

We have developed a new germanium-bridged heptacyclic arene, dithienogermolocarbazole (DTGC), in which two outer thiophene subunits are covalently fastened to the central 2,7-carbazole core by two dibutylgermanium bridges. The germole moieties embedded in the DTGC structure were successfully constructed by one-pot nucleophilic cyclization in a high yield of 88%. Because of the relatively lower polarity of carbon−germanium bonds, the DTGC unit is chemically stable under basic conditions, rendering its more versatile functionalization. Comparison of germanium-bridged DTGC with the carbon-bridged DTCC (dithienocyclopentacarbazole) and silicon-bridged DTSC (dithienosilolocarbazole) analogues reveals that the HOMO energy level of DTGC lies between those of DTCC and DTSC and so does the LUMO energy level of DTGC. Density functional theory (DFT) calculations suggest that DTSC and DTGC have more bent structures than DTCC, which plays an important role in determining their frontier orbital energies. The structural disparity could be amplified in their corresponding polymers. The DTGC unit was copolymerized with four different comonomers, including benzothiadiazole (BT), dithienylbenzothiadiazole (DTBT), difluorobenzothiadiazole (FBT), and dithienyldifluorobenzothiadiazole (DTFBT) to yield a series of new alternating donor−acceptor copolymers, poly(dithienogermolo-carbazole-alt-benzothiadiazole) (PDTGCBT), poly-(dithienogermolocarbazole-alt-dithienylbenzothiadiazole) (PDTGCDTBT), poly(dithienogermolocarbazole-alt-difluorobenzothiadiazole) (PDTGCFBT), and poly(dithienogermolocarbazole-alt-dithienyldifluorobenzothiadiazole) (PDTGCDTFBT). Because of the two additional thiophene rings in the repeating units on the backbone to facilitate π-electron delocalization, PDTGCFDTBT showed a lower optical band gap than PDTGCFBT. Furthermore, PDTGCDTFBT also showed the lowerlying LUMO and HOMO energy levels than PDTGCDTBT as a result of the electron-withdrawing fluorine atoms. Consequently, the bulk heterojunction solar cell incorporating PDTGCDTFBT delivered the highest performance with V oc of 0.84 V, J sc of 9.87 mA/cm 2 , FF of 48.8%, and PCE of 4.05%. By adding 3 vol % 1-chloronaphthalene to tailor the morphology, the solar cell using PDTGCDTFBT with higher molecular weight exhibited the improved efficiency of 4.50% with a V oc of 0.84 V, a J sc of 11.19 mA/cm 2 , and an FF of 47.7%.

Bis-adduct fullerenes surrounded by two insulating addends sterically attenuate intermolecular interaction and cause inferior electron transportation. In this research, we have designed and synthesized a new class of bis-adduct fullerene materials, methylphenylmethano-C60 bis-adduct (MPC60BA), methylthienylmethano-C60 bis-adduct (MTC60BA), methylphenylmethano-C70 bis-adduct (MPC70BA), and methylthienylmethano-C70 bis-adduct (MTC70BA), functionalized with two compact phenylmethylmethano and thienylmethylmethano addends via cyclopropyl linkages. These materials with much higher-lying lowest unoccupied molecular orbital (LUMO) energy levels successfully enhanced the Voc values of the P3HT-based solar cell devices. The compact phenylmethylmethano and thienylmethylmethano addends to promote fullerene intermolecular interactions result in aggregation-induced phase separation as observed by the atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the poly(3-hexylthiophene-2,5-diyl) (P3HT)/bis-adduct fullerene thin films. The device based on the P3HT/MTC60BA blend yielded a Voc of 0.72 V, a Jsc of 5.87 mA/cm(2), and a fill factor (FF) of 65.3%, resulting in a power conversion efficiency (PCE) of 2.76%. The unfavorable morphologies can be optimized by introducing a solvent additive to fine-tune the intermolecular interactions. 1-Chloronaphthalene (CN) having better ability to dissolve the bis-adduct fullerenes can homogeneously disperse the fullerene materials into the P3HT matrix. Consequently, the aggregated fullerene domains can be alleviated to reach a favorable morphology. With the assistance of CN additive, the P3HT/MTC60BA-based device exhibited enhanced characteristics (a Voc of 0.78 V, a Jsc of 9.04 mA/cm(2), and an FF of 69.8%), yielding a much higher PCE of 4.92%. More importantly, the additive-assisted morphological optimization is consistently effective to all four compact bis-adduct fullerenes regardless of the methylphenylmethano or methylthienylmethano scaffolds as well as C60 or C70 core structures. Through the extrinsic additive treatment, these bis-adduct fullerene materials with compact architectures show promise for high-performance polymer solar cells.