Imidazolium functionalized viologen derivative as well as preparation method and application thereof

Abstract

The invention discloses an imidazolium functionalized viologen derivative as well as a preparation method and application thereof, and belongs to the technical field of electrochemical energy storage materials. The core is that a 1, 2-dimethylimidazolium functional group is introduced to a viologen skeleton. According to the molecular design, through steric hindrance and an electrostatic shielding effect, intermolecular pi-pi accumulation and ionic association under high concentration are remarkably inhibited, so that high water solubility is realized, and meanwhile, the viscosity of the electrolyte is effectively controlled. The invention also provides a preparation method of the derivative. When being used as an active substance for an aqueous organic flow battery negative electrode electrolyte, the composite material shows high volume energy density, high power density, excellent rate capability and long cycle stability. According to the invention, the technical bottleneck that the viscosity of the traditional viologen derivative is greatly increased under high concentration is solved.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage materials and devices, specifically relating to an imidazolium-functionalized viologen derivative, its preparation method, and its application. Background Technology

[0002] Aqueous organic redox flow batteries have attracted much attention due to their customizable redox active materials through molecular design, their environmentally friendly chemical properties, and the flexible system configuration enabled by energy and power decoupling. Molecular engineering can systematically adjust key physicochemical parameters such as the solubility, redox potential, chemical stability, and membrane compatibility of active materials, thereby achieving safe and efficient energy conversion.

[0003] In neutral pH aqueous organic flow batteries (AORFBs), viologen derivatives (such as 4,4-bipyridinium salts) remain the benchmark anode electrolytes due to their reversible redox kinetics and modular synthesis advantages. However, the parent compound methyl viologen (MV) has inherent limitations at practical application concentrations: at high concentrations, it undergoes radical association (dimerization); deep reduction to form neutral or weakly solvated species exacerbates transmembrane permeation or self-discharge. To address these issues, molecular engineering strategies have primarily focused on two directions.

[0004] The first strategy involves permanently attaching charged side chains (such as quaternary ammonium salts or sulfonate groups) to the viologen core or introducing sterically hindered substituents. These modifications increase molecular size and maintain ionic properties across all redox states, thereby reducing membrane permeability and inhibiting π-π stacking. However, adding charge increases counterionic loading and enhances ionic cohesion, which typically compromises the solubility limit. Crucially, the resulting extensive hydrogen-bonded network and ionic bridging effects often lead to exponential viscosity increases at high ionic strengths (e.g., Dex-Vi reaches tens of mPa·s near 1.5 M), severely impeding mass transfer.

[0005] The second strategy employs neutral hydrophilic substituents, such as hydroxyl groups or polyethylene glycol / oligopolyethylene glycol chains, to maintain high water solubility and suppress free radical stacking through steric hindrance and an enhanced hydration layer. While these designs improve solubility and reduce self-aggregation, they may render the two-electron reduced state electrically neutral. In certain specific PEG-containing designs, operation in the high-charge state may induce undesirable gelation.

[0006] Therefore, despite significant progress in balancing solubility, stability, and membrane selectivity in previous studies, current viologen-based systems rarely maintain stable flow battery operation at active material concentrations above 2.0 M. This limitation stems not only from the upper limit of solubility but also from the fact that viscosity becomes the dominant bottleneck when high-soluble derivatives cycle under high loads. Viscosity is not merely an operational challenge increasing pumping costs; it fundamentally affects mass transfer kinetics, charge transfer resistance, and voltage polarization, thus directly determining achievable power density and energy efficiency. While numerous strategies exist to improve solubility, the molecular design principles that mitigate the sharp increase in viscosity with concentration, thereby enabling high energy density operation, remain largely unexplored. Summary of the Invention

[0007] To address the problems existing in the prior art, this invention provides an imidazolium-functionalized viologen derivative, its preparation method, and its applications. This invention synthesizes a methylimidazolium-functionalized viologen molecule that maintains extremely low viscosity while achieving multi-molar concentration operation, thus simultaneously meeting the dual requirements of high energy density and high power efficiency electrolytes. The low viscosity characteristic, coupled with a rapid electrochemical response, enables the battery to achieve a peak power density of 0.275 W / cm². -2 It is worth noting that this battery achieved 100 mA / cm² under high concentration operation at 2.5 M. -2 Even at the lowest current density, it still exhibits an excellent capacity utilization of 95.2%, which strongly demonstrates that the mass transfer bottleneck is effectively alleviated even under near-saturation conditions. Ultimately, this system achieved a current density of 65.0 Ah / L. -1 Volumetric capacity and 41.6Wh / L -1 The energy density of this battery surpasses that of existing viologen-based aqueous organic flow batteries. Mechanistic studies based on computational modeling and electrostatic potential mapping reveal that the charged methylimidazolium group suppresses ion pair and π-π stacking through steric hindrance and electrostatic shielding effects, thereby minimizing viscosity. This molecular engineering strategy effectively delays the viscosity accumulation phenomenon commonly found in concentrated electrolytes.

[0008] This invention is achieved through the following technical solution: An imidazolium-functionalized viologen derivative has the structures shown in formulas (I) and (II) below:

[0009] (MIm-Me-Vi)

[0010] (DiMIm-Vi).

[0011] The present invention also provides a method for preparing the imidazolium-functionalized viologen derivative, wherein the preparation method of DiMIm-Vi includes the following steps: dissolving 4,4'-bipyridine and 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazolium-3-onium bromide in acetonitrile, wherein the molar ratio of 4,4'-bipyridine to 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazolium-3-onium bromide is 1:3, stirring the reaction at 80°C for 48 hours, and after the reaction is completed, filtering and collecting the resulting yellow precipitate, washing with acetonitrile, and drying under vacuum; .

[0012] Furthermore, the preparation method of the 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazol-3-onium bromide includes the following steps: An ethyl acetate solution of 1,3-dibromopropane was added dropwise to an ethyl acetate solution of 1,2-dimethyl-1H-imidazolium. The mixture was kept at 40°C and stirred for 24 hours. The resulting white precipitate was collected by filtration, washed with ethyl acetate, and dried. .

[0013] Furthermore, the preparation method of the MIm-Me-Vi includes the following steps: at room temperature, an ethanol solution of iodomethane is added dropwise to an ethanol solution of 1-(3-(1,2-dimethyl-1H-imidazol-3-onthio-3-yl)propyl)-[4,4'-bipyridine]-1-onthio bromide under stirring; the reaction mixture is stirred for 24 hours; the resulting red solid is collected by filtration, washed with ethanol, and dried under vacuum; .

[0014] Furthermore, the preparation method of the 1-(3-(1,2-dimethyl-1H-imidazol-3-onthium-3-yl)propyl)-[4,4'-bipyridine]-1-onthium bromide includes the following steps:

[0015] An acetonitrile solution of 4,4'-bipyridine was added dropwise to an acetonitrile solution of 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazol-3-onium bromide. The mixture was stirred at 80°C for 36 hours. After the reaction was completed, the resulting yellow solid was collected by filtration, washed with acetonitrile, and dried under vacuum.

[0016] The present invention also provides an electrolyte comprising an imidazolium-functionalized viologen derivative as described in formula (I) or formula (II) as claimed in claim 1 or 2 as an active substance.

[0017] The present invention also provides the application of the electrolyte in an aqueous redox flow battery.

[0018] Beneficial technical effects of the present invention: (1) By targeting imidazolium functionalization, the aggregation of viologen molecules at high concentrations is effectively inhibited, so that electrolytes (such as DiMIm-Vi) can still maintain low viscosity (<10mPa·s) and good fluidity at concentrations up to 2.5M, overcoming the mass transfer limitation caused by the sharp increase in viscosity of traditional viologen derivatives (such as Dex-Vi).

[0019] (2) Thanks to its high solubility and low viscosity, the DiMIm-Vi-based flow battery achieves a high volumetric energy density (41.6 Wh / L). -1 ) and high area power density (peak 0.275 Wcm²) -2 ), while at high current density (100 mA / cm²) -2 It maintains high capacity utilization (95.2%) and high energy efficiency (72.0%).

[0020] (3) The steric and electrostatic shielding effects provided by the imidazolium group enhance chemical stability (especially under alkaline conditions) and redox reversibility. The battery did not show significant capacity decay during long-term cycling (e.g., more than 500 cycles), demonstrating excellent durability.

[0021] (4) The unique molecular design enables DiMIm-Vi to maintain solubility and structural stability during two-electron reduction, avoiding precipitation of insoluble substances (such as MV). 0 This unlocks the full capacity and ensures stable operation within a wide SOC window.

[0022] (5) Under high load and without supporting salt, DiMIm-Vi exhibits low areal resistivity (1.2 Ω·cm) due to its high ion mobility and weak ion correlation. 2 The use of weak ohmic polarization and weak ohmic polarization can help reduce energy consumption and simplify the system.

[0023] In summary, this invention, through molecular structure innovation, simultaneously optimizes the solubility, viscosity, stability, and electrochemical performance of the electrolyte, providing key technical support for the development of high-performance, low-cost aqueous organic flow batteries. Attached Figure Description

[0024] Figure 1 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazol-3-onium 1 HNMR spectrum.

[0025] Figure 2 DiMIm-Vi 1 HNMR spectrum.

[0026] Figure 3.1-(3-(1,2-dimethyl-1H-imidazol-3-onthium-3-yl)propyl)-[4,4'-bipyridine]-1-onthium bromide 1 HNMR spectrum.

[0027] Figure 4 MImVi-Me-Vi 1 HNMR spectrum.

[0028] Figure 5 A comparison chart of the solubility of DiMIm-Vi, MIM-Me-Vi, MV, Dex-Vi, and DiOH-Vi.

[0029] Figure 6 Trend graph of viscosity of DiMIm-Vi, MIIm-Me-Vi, Dex-Vi, DiOH-Vi, and MV as a function of their concentration.

[0030] Figure 7 Comparison of the evolution of chemical stability of MV, Dex-Vi, DiOH-Vi, DiMIm-Vi, and MIM-Me-Vi.

[0031] Figure 8 Comparison of half-wave potentials of MV, Dex-Vi, DiOH-Vi, DiMIm-Vi, and MIM-Me-Vi.

[0032] Figure 9 Cyclic voltammogram of DiMIm-Vi in 1.0M NaCl.

[0033] Figure 10 Voltammetric diagrams of a rotating disk electrode at different rotational speeds for DiMIm-Vi.

[0034] Figure 11 Viscosity-concentration relationship of DiMIm-Vi electrolyte at different temperatures. The figure shows the viscosity (in mPa·s) of DiMIm-Vi aqueous solutions at different concentrations (0.5, 1.0, 1.5 and 2.0 M) measured at various temperatures (30–60°C).

[0035] Figure 12 Cyclic voltammetry curves of DiMIm-Vi.

[0036] Figure 13 The cyclic voltammetry curve of MV.

[0037] Figure 14 Cyclic voltammetry curves of Dex-Vi.

[0038] Figure 15 Cyclic voltammetry curves of DiOH-Vi.

[0039] Figure 16 A schematic diagram of the full battery configuration and the corresponding redox mechanisms of DiMIm-Vi negative electrode electrolyte and TEMPO-based positive electrode electrolyte.

[0040] Figure 17 Cyclic voltammetry curves of DiMIm-Vi and TEMPO cathode electrolytes in 1.0 M NaCl.

[0041] Figure 18 Battery discharge curves at different current densities.

[0042] Figure 19 Battery cycle charge-discharge capacity diagram under different current densities.

[0043] Figure 20 Curves showing the changes in coulombic efficiency and energy efficiency under different current densities.

[0044] Figure 21 Cyclic performance of low-concentration flow batteries using 0.2M DM-Vi and 1.0M NaCl as positive electrolyte (capacity-limited side) and 0.1M N+N+TEMPO and 1.0M NaCl as negative electrolyte.

[0045] Figure 22 Cyclic voltammetry curves of DiMIm-Vi negative electrode electrolyte and N+N+TEMPO positive electrode electrolyte after long-term flow battery cycling using DSVN membrane.

[0046] Figure 23 Nyquist plots of 1.5M Dex-Vi, 1.5M DiOH-Vi, 2.5M MV and 2.5M DiMIm-Vi.

[0047] Figure 24 Viscosity curves of 1.5M Dex-Vi and 2.5M DiMIm-Vi as a function of SOC.

[0048] Figure 25 2.5M DiMIm-Vi / / 1.25M N+N+TEMPO flow cell at 100 mAcm -2 Long-term cycling stability at current density.

[0049] Figure 26 The relationship between capacity utilization and energy efficiency and current density.

[0050] Figure 27 Polarization and power density curves of MV, Dex-Vi and DiMIm-Vi cells at ~100% SOC.

[0051] Figure 28 Comparison of energy efficiency with electrolyte concentration between DiMIm-Vi and other representative high-energy-density viologen compounds.

[0052] Figure 29 .MV 2+ DiMIm-Vi 2+ and Dex-Vi 2+ The optimized structure and electrostatic potential diagram.

[0053] Figure 30 Frontier molecular orbitals (HOMO / LUMO) and their corresponding energy levels. Detailed Implementation

[0054] Example 1 Synthesis of 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazol-3-onium bromide

[0055] A solution of 1,3-dibromopropane (31.5 g, 156 mmol) in ethyl acetate (100 mL) was added dropwise to a solution of 1,2-dimethyl-1H-imidazole (10.0 g, 104 mmol) in ethyl acetate (100 mL). The mixture was kept at 40°C and stirred for 24 hours. The resulting white precipitate was collected by filtration, washed with ethyl acetate, and dried.

[0056] Yield: 90%; 1 HNMR (400MHz, D2O) δ 7.39 (d, J =2.2Hz, 1H), 7.33(d, J =2.2Hz, 1H), 4.29(t, J =6.9Hz,2H),3.46(t, J =6.2Hz,2H),2.61(s,3H),2.41–2.32(m,2H).

[0057] Example 2. Synthesis of 1'-bis(3-(1,2-dimethyl-1H-imidazol-3-onthium-3-yl)propyl)-[4,4'-bipyridine]-1,1'-dionthium bromide (DiMIm-Vi)

[0058] 4,4'-Bipyridine (4.8 g, 30.6 mmol) and 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazol-3-onium bromide (20.0 g, 91.7 mmol) were dissolved in acetonitrile (200 mL) and the mixture was stirred at 80°C for 48 hours. After the reaction was complete, the resulting yellow precipitate was collected by filtration, washed with acetonitrile, and dried under vacuum.

[0059] Yield: 87%; 1HNMR (400MHz, D2O) δ9.26 (d, J = 7.0Hz, 4H), 8.67 (d, J = 7.1Hz, 4H), 7.52 (d, J = 2.3Hz, 2 H),7.44(d,J=2.2Hz,2H),5.02–4.90(m,4H),4.44(t,J=7.4Hz,4H),3.85(s,6H),2.71(s,11H).

[0060] Example 3 Synthesis of -(3-(1,2-dimethyl-1H-imidazol-3-onthium-3-yl)propyl)-[4,4'-bipyridine]-1-onthium bromide

[0061] A solution of 4,4'-bipyridine (10.7 g, 68.8 mmol) in acetonitrile (100 mL) was added dropwise to a solution of 3-bromopropyl-1,2-dimethylimidazolium bromide (5.00 g, 22.9 mmol) in acetonitrile (100 mL). The mixture was stirred at 80°C for 36 hours. After the reaction was complete, the resulting yellow solid was collected by filtration, washed with acetonitrile, and dried (under vacuum).

[0062] Yield: 85%; 1HNMR (400MHz, D2O) δ9.07(d,J=7.1Hz,2H),8.72(d,J=6.4Hz,2H),8.44(d,J=7.0Hz,2H),7.89(d,J=6.4Hz,2H ),7.49(d,J=2.2Hz,1H),7.41(d,J=2.2Hz,1H),4.85(t,J=8.0Hz,2H),4.41(t,J=7.4Hz,2H),3.81(s,3H),2.68(s,5H).

[0063] Example 4: Synthesis of 3-(1,2-dimethyl-1H-imidazol-3-onthium-3-yl)propyl)-1′-methyl-[4,4′-bipyridine]-1,1′-dionthium bromide (MIm-Me-Vi)

[0064] At room temperature, a solution of 2.89 g (21.4 mmol) of iodomethane in 20 mL of ethanol was added dropwise to a solution of (3-(1,2-dimethyl-1H-imidazol-3-onthiol-3-yl)propyl)-[4,4'-bipyridine]-1-onthiol bromide in 100 mL of ethanol under stirring. The reaction mixture was stirred for 24 hours. The resulting red solid was collected by filtration, washed with ethanol, and dried under vacuum.

[0065] Yield: 92%; 1HNMR (400MHz, D2O) δ9.24(d,J=7.0Hz,2H),9.10(d,J=7.0Hz,2H),8.64(d,J=7.0Hz,2H),8.58(d,J=7.0Hz,2H ),7.50(s,1H),7.42(s,1H),4.92(t,J=8.1Hz,2H),4.54(s,3H),4.42(t,J=7.4Hz,2H),3.83(s,3H),2.78–2.66(m,5H).

[0066] Examples 1-4 yielded the 1H NMR spectra of the compounds as follows: Figure 1-4 As shown.

[0067] Methyl viologen (MV), Dex-Vi, and DiOH-Vi were used as controls. The solubility, viscosity, and electrochemical stability of compound DiMIm-Vi prepared in Example 2, compound MIm-Me-Vi prepared in Example 4, and the controls were determined.

[0068] The water solubility of DiMIm-Vi and its asymmetric analog MIm-Me-Vi, as determined by UV-Vis spectroscopy, were 2.8 M and 2.1 M, respectively. Figure 5 DiMIm-Vi exhibits extremely low viscosities at 1.0 M, 2.0 M, and 2.5 M, at 1.29, 5.75, and 8.06 mPa·s, respectively. Figure 6 The viscosity-concentration relationship curves of DiMIm-Vi at different temperatures are shown in the figure. Figure 11 These values ​​are significantly lower than the viscosities of representative quaternary ammonium or PEG / OEG-substituted viologens at the same concentration, indicating a weaker coupling relationship between viscosity and concentration in the DiMIm-Vi series.

[0069] In addition to its excellent transport properties, DiMIm-Vi also exhibits better chemical stability than methyl viologen. Figure 7 In alkaline retention tests, DiMIm-Vi degraded significantly slower than MV, while the asymmetric analog MIM-Me-Vi showed accelerated decay under the same conditions. This contrast is consistent with the ability of the imidazolium substituent to provide both steric and electrostatic shielding for reaction sites on the viologen backbone. Furthermore, the increased molecular size and additional permanent cation charge reduced membrane permeability, thus mitigating capacity loss due to cross-linking during full-cell operation. Consistent with previous reports that the imidazolium group interacts more readily with hydroxyl groups than the pyridine nitrogen atom, its side chain may act as a competitive binding site, thereby reducing hydroxyl attack on the viologen core.

[0070] The cyclic voltammogram of DiMIm-Vi in 1.0 M NaCl showed two distinct and reversible redox couples with midpoint potentials of -0.40 V and -0.75 V relative to NHE, respectively. Figure 8 In contrast, the redox couples of the asymmetric analog MIm-Me-Vi are located at -0.43 V and -0.81 V, while those of Dex-Vi and DiOH-Vi are at -0.35 / -0.72 V and -0.40 / -0.82 V, respectively. Detailed cyclic voltammograms for each compound are available in [link to cyclic voltammogram]. Figure 12-15 .

[0071] Under the same conditions, the initial reduction potential of MV is approximately -0.45 V vs NHE. Compared to MV, both redox couples of DiMIm-Vi exhibit anodic shift, consistent with the electron-withdrawing effect induced by the cationic imidazolium substituent. Compared to conventional quaternary ammonium-functionalized viologen, DiMIm-Vi displays a slightly lower reduction potential, reflecting a more moderate electronic perturbation of the bispyridinium core. From a device design perspective, the higher reduction potential of DiMIm-Vi compared to Dex-Vi allows for the assembly of higher-voltage aqueous redox flow batteries when paired with high-potential cathode electrolytes. Notably, the second reduction of DiMIm-Vi remains reversible in diluted electrochemical measurements and can be utilized during high-concentration flow cell operation without significant precipitation. This behavior contrasts with MV and is consistent with the improved solubility and stability of the deeply reduced state resulting from imidazolium functionalization. The retained solubility of the reduced state can be attributed to the additional cationic charge introduced by the imidazolium substituent, which maintains ionic properties throughout the redox window. Therefore, the capacity associated with the second electron transfer (which is often unavailable in unmodified MV due to precipitation) is efficiently released in imidazolium-functionalized viologen. In summary, these results indicate that DiMIm-Vi and MIM-Me-Vi support reliable and reversible redox chemistry under relevant aqueous conditions.

[0072] The scan rate-dependent cyclic voltammetry of DiMIm-Vi showed a range from 5 to 500 mV / s. -1 Within this range, the peak broadening is minimal, which is consistent with the diffusion-dominated redox behavior under these conditions. Figure 10 Therefore, a rotating disk electrode measurement is used to establish an intrinsic transport and interfacial dynamics baseline under dilute supported electrolyte conditions, such as... Figure 10 As shown. Steady-state current analysis yields a diffusion coefficient D of 8.53 × 10⁻⁶ for DiMIm-Vi. -6 cm 2 s -1 The heterogeneous electron transfer rate constant k0 = 1.02 × 10 -1 cms -1 ( Figure 9 The asymmetric analogue MIM-Me-Vi exhibits similar fast dynamics under the same conditions, D=9.20×10⁻⁶. -6 cm 2 s -1 k0 = 1.04 × 10 -1 cms -1 In contrast, the benchmark viologen Dex-Vi exhibits a significantly lower diffusion coefficient and a reduced electron transfer rate constant.

[0073] In summary, imidazolium-functionalized viologens (DiMIm-Vi and MIM-Me-Vi) exhibited significantly higher diffusion coefficients and electron transfer rate constants than Dex-Vi, and were comparable to or exceeded the values ​​reported for other cationic-functionalized viologen derivatives. These results indicate that imidazolium substitution did not significantly impede intrinsic interfacial electron transfer in dilute aqueous electrolytes.

[0074] Electrochemical impedance spectroscopy measurements in the supporting electrolyte (1.0 M NaCl) further confirmed that, at dilute concentrations (0.1 M), the interfacial charge transfer resistance introduced by DiMIm-Vi, Dex-Vi, and MV was the smallest and comparable across all systems, exhibiting a predominantly ohmic response. In multi-molar concentrations of salt-free electrolytes, additional factors, including viscosity, ion dependence, and porous electrode transport, dominated the effective transport parameters. However, the favorable intrinsic diffusivity and rapid interfacial kinetics observed here provide a necessary benchmark, supporting the conclusion that the superior rate performance of concentrated DiMIm-Vi electrolytes stems primarily from their suppressed viscosity growth, rather than from limitations imposed by molecular-scale electron transfer kinetics.

[0075] The Gibbs free energy change associated with the dimerization process was quantified using density functional theory calculations to elucidate the fundamental reason for the viscosity decrease. As shown in the figure, the calculated ΔG value is positive and increases from +11.8 kJ / mol in MV to +14.9 kJ / mol in DiMIm-Vi. This trend indicates that the dimerization process is thermodynamically more unfavorable to DiMIm-Vi, which is consistent with the steric hindrance and electrostatic shielding effects generated by the imidazolium group. Therefore, the higher aggregation barrier effectively suppresses short-range π-π association, which is the main driving factor for the viscosity increase in concentrated viologen electrolytes.

[0076] Given that DiMIm-Vi exhibits higher solubility and improved alkaline stability compared to MIM-Me-Vi, it was chosen for performance evaluation in flow batteries. First, a dilute concentration benchmark was established to assess its intrinsic cycle stability while minimizing transport limitations. A dual-cation N+N+TEMPO derivative was used as the positive electrode electrolyte due to its high potential, sufficient solubility, and low cross-linking tendency. To explore the feasibility of using DiMIm-Vi in AORFBs, its electrochemical behavior was first investigated using cyclic voltammetry. Figure 16 ).like Figure 17 As shown, the CV curve (blue curve) of DiMIm-Vi reveals two pairs of reversible redox couples, E 0 The values ​​are -0.40V and -0.75V respectively. When used with N+ TEMPO positive electrode electrolyte (red curve, E...) 0 When paired (electron transfer = -0.88V), the calculated theoretical battery voltages are: 1.28V for the first electron transfer and 1.63V for the second electron transfer. This wide potential window is the basis for high energy density. The full cell is assembled using a DSVN film. The negative electrode electrolyte is configured as the capacity-limited side, while the positive electrode electrolyte is 0.1MN+N+TEMPO with a volume of 5.5mL. Its galvanostatic charge-discharge curve shows a voltage plateau at ~1.20V. Figure 18 ).

[0077] First, at 20 to 100 mAcm -2 Within the current density range, constant current cycling tests were performed using 0.2MDiMIm-Vi and 0.1MN+N+TEMPO. Capacity utilization was only from 20 mA / cm². -2 The percentage of 94.4% at that time decreased slightly to 100 mAcm -2 The efficiency was 81.3% at that time, while the coulombic efficiency remained above 99.99% throughout the entire current range. Figure 19 Furthermore, as current density increases, energy efficiency decreases from 87.2% to 72.2%. Figure 20 This is consistent with the increase in polarization at higher current densities, while the overall efficiency remains high.

[0078] Further at 20mAcm -2 Long-term cycling performance was evaluated at a current density of [value missing]. After three weeks of continuous operation, no capacity decay was observed, indicating excellent durability under continuous operation. Figure 21 Post-cycling diagnostic analysis provided further evidence of the stability of the electrolyte and battery structure. The EIS measurements before and after extended cycling showed negligible changes, indicating that the DSVN film and electrode interface remained intact. The post-cycling CV spectra were consistent with those of the original electrolyte, confirming the structural integrity of the active material. Figure 22The results show that the DiMIm-Vi / / N+N+TEMPO system maintains excellent chemical and electrochemical stability during long-term cycling.

[0079] To evaluate the intrinsic transport performance of the electrolyte under salt-free, high-load conditions in practical applications, concentrated viologen electrolyte was tested using the same DSVN membrane and hardware configuration. Electrochemical impedance spectroscopy showed that the areal resistivity of 1.5MDex-Vi, 2.5MMV, 1.5MDiOH-Vi, and 2.5MDiMIm-Vi were 2.5, 2.7, 2.6, and 1.2 Ω•cm, respectively. 2 ( Figure 23 Despite operating at the highest molar load, DiMIm-Vi maintained the lowest ASR, indicating higher charge transport efficiency without the addition of a supporting salt. Since salt-free electrolytes primarily rely on intrinsic counterions for ion conduction, the number and mobility of effective charge carriers become crucial. In this context, the higher concentration of accessible counterions in concentrated DiMIm-Vi solutions, along with reduced ion association, enhances electrolyte conductivity and reduces ohmic losses across the membrane / electrolyte system. The imidazolium substituent can delocalize positive charges and create steric and electrostatic shielding effects on the viologen core, consistent with the observed reduced ion pair / aggregation tendency and the suppressed viscosity growth at key multimolar concentrations. Therefore, higher ion mobility in the bulk electrolyte and reduced polarization at the membrane interface jointly contribute to the lower ASR.

[0080] In contrast, the concentrations of Dex-Vi and DiOH-Vi are limited to lower levels (1.5 M) due to solubility and processability constraints. Under salt-free conditions, this lower molar concentration inherently limits the total number of counterions available as charge carriers, thus reducing bulk conductivity. Furthermore, these electrolytes exhibit stronger solution structuring and higher viscosity under concentrated operation, which suppresses ion mobility and exacerbates transport losses at the porous electrode and membrane interface, leading to increased measured ASR. While MV was also tested at 2.5 M, it is more prone to ion association and intermolecular interactions at high ionic strength, which similarly reduces the proportion of effectively migrating ions. In summary, these results demonstrate that DiMIm-Vi uniquely combines high intrinsic charge carrier availability with high ion mobility, thanks to its low viscosity and reduced ion correlation, thereby minimizing ohmic losses under high-load, salt-free operating conditions.

[0081] Rheological measurements performed under the same salt-free, high-load conditions further highlight the unique processability of DiMIm-Vi within the actual state-of-charge window. Figure 24For Dex-Vi, its viscosity is already high at 1.5 M and gradually increases during charging, exceeding 100 mPa·s near full charge. This significant SOC-dependent thickening is consistent with strong solution structuring and ion bridging effects, which are known to severely impede pore-scale mass transfer and electrolyte pumping capabilities in flow electrodes. In contrast, DiMIm-Vi consistently maintains a viscosity below 10 mPa·s over a wide SOC range of 2.5 M. The viscosity of DiMIm-Vi increases only slightly during charging, indicating that changes in redox state do not trigger extensive intermolecular association or network formation. This weak SOC dependence has important operational implications because flow batteries typically experience continuous SOC fluctuations during charge-discharge cycles, rather than operating at a constant composition.

[0082] The significantly different state-of-charge (SOC) and viscosity-response relationships among the various electrolytes highlight the molecular origins of their behavioral differences. For Dex-Vi, its large substituents and strong hydrogen-bonding / ion-linking interactions promote the formation of a solution structure that is enhanced with changes in redox state, leading to a significant increase in viscosity. In contrast, the imidazolium-functionalized DiMIm-Vi benefits from its delocalized charge distribution and steric / electrostatic shielding effects, which suppress ion pairing and aggregation even with increasing reduction. Therefore, high electrolyte loading can be maintained across the entire SOC window without significant viscosity-driven transport losses. From the perspective of practical flow battery applications, DiMIm-Vi's ability to maintain low and SOC-independent viscosity at multi-molar concentrations directly supports stable operation at high current densities, minimizes pumping losses, and mitigates mass transfer polarization under practical cycling conditions. These rheological advantages, coupled with the impedance reduction observed under salt-free conditions, explain the excellent high-rate performance of DiMIm-Vi-based flow batteries.

[0083] A flow battery using 2.5M DiMIm-Vi as the negative electrode electrolyte and 1.25M N+N+TEMPO as the positive electrode electrolyte was tested at 100 mA / cm². -2 It can be stably cycled for more than 500 times at a current density without any measurable capacity decay observed. Figure 25 This demonstrates robust operating performance under high load and salt-free conditions. Rate-dependent testing further confirms the system's robustness over the actual relevant current density range. As the current density increases from 40 to 100 mA / cm²... -2 The battery maintained a high capacity utilization rate, only decreasing from 98.1% to approximately 95.2%. Figure 26 Meanwhile, energy efficiency increased from 100 mA / cm². -2The percentage dropped from 72.0% to 150 mAcm -2 At 58.9%, this is consistent with the increase in polarization at higher current densities. Only a slight loss in capacity utilization occurs at high current densities, indicating that charge extraction remains essentially complete even under harsh operating conditions. This behavior contrasts sharply with high-viscosity viologen electrolytes, which often suffer severe capacity loss at high rates due to premature mass transfer limitations. Therefore, the observed efficiency decay with increasing current density is primarily attributed to voltage polarization rather than incomplete utilization, highlighting the effectiveness of DiMIm-Vi in maintaining active material accessibility at multi-molar concentrations.

[0084] Polarization measurements provided direct evidence for understanding the root cause of its improved rate performance. With the same battery configuration and near 100% state of charge, the DiMIm-Vi-based battery exhibited lower voltage drop over a wide current density range. Its peak power density reached 0.275 Wcm³. -2 This value surpasses the results obtained using MV and Dex-Vi under the same experimental conditions. The extended linear region and reduced slope exhibited by DiMIm-Vi indicate significantly lower ohmic polarization losses, consistent with its significantly lower viscosity and smaller areal resistivity under salt-free conditions. Compared to previously reported viologen-based aqueous organic flow batteries, DiMIm-Vi dominates the spectrum, achieving high capacity and relatively high energy efficiency at practical current densities. Figure 27-28 This balanced performance underscores the importance of mitigating transport losses driven by viscosity and resistance at high electrolyte concentrations. Combining the EIS and rheological analyses described above, these results establish a coherent transport-performance relationship: by reducing viscosity-concentration coupling and maintaining low impedance, DiMIm-Vi enables multi-molar concentration electrolytes to operate at high current densities while maintaining high utilization and improved power output.

[0085] Further investigation was conducted to explore the behavior of the electrolyte outside the single-electron window and to assess the full capacity of the negative electrode electrolyte. For DiMIm-Vi, the transition to the two-electron reaction region did not lead to an increase in viscosity. On the contrary, its viscosity profile remained remarkably stable throughout the entire SOC window. This stability is attributed to the terminal imidazolium groups, which retain their positive charge even when the bispyridinium core is completely reduced to its neutral quinone form. These permanent positive charges ensure adequate retention of counterions, thereby maintaining solubility and ionic conductivity. Simultaneously, the reduction in total molecular charge (from 4+ to 2+) mitigates the electroviscous effect, effectively preventing the viscosity buildup commonly seen in concentrated electrolytes. Therefore, the electrolyte remained fluid, with no precipitation or flow channel blockage observed, a stark contrast to MV—which is characterized by its neutral, insoluble reduction products (MV...0 Rapid deposition on the cathode surface leads to electrode contamination, making reliable two-electron cycling impossible at high concentrations.

[0086] The unique solubility, viscosity, and electrochemical behavior of DiMIm-Vi indicate that imidazolium functionalization effectively modulates intermolecular interactions and the local electronic environment. To qualitatively elucidate these effects, DFT calculations of electrostatic potential and leading molecular orbitals were performed. Figure 29 ESP spectra show that, unlike the localized positive potential in MV, DiMIm-Vi exhibits a significantly distributed positive electrostatic potential on the imidazolium group. This spatially extended positive electrostatic field significantly enhances intermolecular electrostatic repulsion, thereby effectively suppressing thermodynamically favorable π-π dimerization and enhancing water solubility. This provides a reasonable molecular-level explanation for the ion pair suppression observed in the experiment and the maintenance of low viscosity at high concentrations. Furthermore, frontier orbital analysis indicates that the imidazolium substitution has a relatively mild perturbation to the intrinsic electronic structure of the viologen framework. Figure 30 LUMO remains primarily associated with the bispyridinium unit, consistent with the reversible redox activity retained in the functionalized molecule.

[0087] In summary, this invention significantly alleviates the viscosity problem commonly encountered in aqueous organic redox flow batteries at high electrolyte concentrations through a targeted imidazolium functionalization strategy. At the molecular level, the imidazolium group acts as a bimode shielding layer, utilizing steric hindrance and electrostatic repulsion to suppress intermolecular aggregation. This molecular engineering produces an electrolyte with significant fluidity even at concentrations up to 2.5 M, exhibiting performance significantly superior to conventional viologen derivatives. By translating this rheological advantage into practical device performance, the system demonstrates reduced mass transfer limitations and achieves a competitive 41.6 Wh / L electrolyte. -1 The energy density was [value missing]. Furthermore, the reduced viscosity facilitated rapid kinetics, resulting in a peak power density of 0.275 W / cm³. -2 At the same time, at 100mAcm -2 It maintains a high energy efficiency of 72.0% and an excellent capacity utilization of 95.2% at current densities.

Claims

1. An imidazolyl-functionalized viologen derivative, characterized in that: The structure is shown in equations (I) and (II) below: Mim-Me-Vi DiMIm-Vi.

2. The imidazolium-functionalized viologen derivative according to claim 1, characterized in that: The water solubility of DiMIm-Vi is 2.8 M and that of M1m-Me-Vi is 2.1 M; the viscosities of DiMIm-Vi at 1.0 M, 2.0 M and 2.5 M are 1.29, 5.75 and 8.06 mPa·s, respectively.

3. A method for preparing an imidazolium-functionalized viologen derivative as described in claim 1 or 2, characterized in that: The preparation method of DiMIm-Vi includes the following steps: 4,4'-bipyridine and 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazolium-3-onium bromide are dissolved in acetonitrile, the molar ratio of 4,4'-bipyridine to 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazolium-3-onium bromide is 1:3, the reaction is stirred at 80°C for 48 hours, after the reaction is completed, the resulting yellow precipitate is collected by filtration, washed with acetonitrile, and dried under vacuum; 。 4. The method for preparing the imidazolyl functionalized viologen derivative according to claim 3, characterized in that: The preparation method of the 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazol-3-onium bromide includes the following steps: adding an ethyl acetate solution of 1,3-dibromopropane dropwise to an ethyl acetate solution of 1,2-dimethyl-1H-imidazol, maintaining the mixture at 40°C and stirring for 24 hours, filtering and collecting the resulting white precipitate, washing with ethyl acetate and drying; .

5. A method for preparing an imidazolium-functionalized viologen derivative as described in claim 1 or 2, characterized in that: The preparation method of the MIm-Me-Vi includes the following steps: at room temperature, an ethanol solution of iodomethane is added dropwise to an ethanol solution of 1-(3-(1,2-dimethyl-1H-imidazol-3-onthio-3-yl)propyl)-[4,4'-bipyridine]-1-onthio bromide under stirring; the reaction mixture is stirred for 24 hours; the resulting red solid is collected by filtration, washed with ethanol, and dried under vacuum. 。 6. The method for preparing the imidazolium-functionalized viologen derivative according to claim 5, characterized in that: The preparation method of the 1-(3-(1,2-dimethyl-1H-imidazol-3-onthium-3-yl)propyl)-[4,4'-bipyridine]-1-onthium bromide includes the following steps: An acetonitrile solution of 4,4'-bipyridine was added dropwise to an acetonitrile solution of 3-(3-bromopropyl)-1,2-dimethyl-1H-imidazol-3-onium bromide. The mixture was stirred at 80°C for 36 hours. After the reaction was completed, the resulting yellow solid was collected by filtration, washed with acetonitrile, and dried under vacuum. .

7. An electrolyte, characterized in that, The active substance comprises an imidazolium-functionalized viologen derivative as described in formula (I) or (II) as claimed in claim 1 or 2.

8. The application of the electrolyte as described in claim 7 in an aqueous redox flow battery.

Images