Open AccessCCS ChemistryRESEARCH ARTICLES22 Dec 2022Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries Xu Liu, Xuri Zhang, Chaoyu Bao, Zengrong Wang, Heng Zhang, Guoping Li, Ni Yan, Ming-Jia Li and Gang He Xu Liu Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049 Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710054 Google Scholar More articles by this author , Xuri Zhang Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710054 Google Scholar More articles by this author , Chaoyu Bao School of Materials Science and Engineering, Engineering Research Center of Transportation Materials, Ministry of Education, Chang’an University, Xi’an, Shaanxi Province 710064 Google Scholar More articles by this author , Zengrong Wang Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710054 Google Scholar More articles by this author , Heng Zhang Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710054 Google Scholar More articles by this author , Guoping Li Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049 Google Scholar More articles by this author , Ni Yan School of Materials Science and Engineering, Engineering Research Center of Transportation Materials, Ministry of Education, Chang’an University, Xi’an, Shaanxi Province 710064 Google Scholar More articles by this author , Ming-Jia Li Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049 Google Scholar More articles by this author and Gang He *Corresponding author: E-mail Address: [email protected] Key Laboratory of Thermo-Fluid Science and Engineering of Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049 Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710054 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202336 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Two-electron neutral aqueous organic redox flow batteries (AORFBs) hold more promising applications in the power grid than one-electron batteries because of their higher capacity. However, their development is strongly limited by the structural instability of the highly reduced species. By combining the extended π-conjugation structure of the anolytes and the enhanced aromaticity of the highly reduced species, we reported a series of highly conjugated and inexpensive arylene diimide derivatives ( NDI, PDI, and TPDI) as novel two-electron storage anolyte materials for ultrastable AORFBs. Matched with (ferrocenylmethyl)trimethylammonium chloride ( FcNCl) as catholyte, arylene diimide derivative-based AORFBs showed the highest stability in two-electron AORFBs to date. The NDI/ FcNCl-based AORFB delivered 98.44% capacity retention at 40 mA cm−2 for 350 cycles; TPDI/FcNCl-based AORFB also showed remarkable stability with 97.22% capacity retention at 20 mA cm−2 over 200 cycles. This finding lays the theoretical foundation and offers a reference for improving the stability of two-electron AORFBs. Download figure Download PowerPoint Introduction Currently, the imbalance between energy supply and demand has become increasingly prominent in the process of global economic development.1–3 Therefore, improving the storage and utilization efficiency of new energy (wind or solar) has attracted much attention.4–6 Flow batteries stand out in large-scale storage technology and have been applied to electricity grids due to the decoupled energy and power, scalability (up to MW/MWh), and security.7–9 Among them, neutral aqueous organic redox flow batteries (AORFBs) are expected to become the leader in next-generation energy storage and conversion devices, resulting from their high conductivity, safety, flexibility, environmental friendliness, and low cost.10–15 Electrolyte materials are the key points for neutral AORFBs.16,17 The energy source of existing electrolyte materials mainly relies on variable valence states such as C=O, C=N, N=O, and metals. The active materials, including arylene diimides,18,19 quinones,20,21 viologens,22–24 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO),25–28 ferrocenes,29–32 and so on,33–35 have excellent redox properties and high water solubility, rendering them very suitable for neutral AORFBs. Viologen has been widely used as an electrolyte material for neutral AORFBs.36–41 Viologens are mostly used for one-electron storage due to the lack of stability of highly reduced species.42 Therefore, improving the effective electron utilization and stability of viologen has become an important challenge in the application of neutral AORFBs.43 In recent years, some research groups have made many efforts to expand the conjugated structure and electron delocalization of viologen by introducing electron donor groups such as thiazolo[5,4-d]thiazole,44 phenyls,45 and thiophene.46 The results proved that this strategy effectively stabilized the highly reduced species. However, the capacity retention is still less than 90% in 300 cycles, which is due to the pyridinium rings changing from aromaticity [NICS(1) = −9.28] (NICS = nucleus-independent chemical shift) to antiaromaticity [NICS(1) = 2.73], resulting in the decline of stability. Therefore, it is necessary to develop a new electrolyte material with a more stable structure for highly reduced species to achieve efficient two-electron storage stability. Arylene diimide derivatives have more conjugated planar rigid structures than viologens, including strong π–π conjugation and weak p–π conjugation, beneficial for promoting redox activity and electron transfer.47–53 In different redox states, these derivatives maintain good aromaticity and high stability.54 Moreover, their excellent photoelectrochemical properties, such as two-electron transfer, strong visible light absorption, photostability, and thermal stability, make them widely used in organic batteries,55–58 pseudocapacitors,59 optoelectronic devices,60 biosensing,61 and other applications. Recently, arylene diimide derivatives have also been used as electrolyte materials for flow batteries.62–64 Jin and coworkers reported an all-polymer particulate slurry redox flow battery using a microsized and uniformly dispersed all-polymer particulate suspension. The discharge capacity retention after 300 cycles was 70% of the initial capacity, and the capacity utilization was only a poor value of 9.23%.65 Byon and coworkers demonstrated a potassium salt of N,N′-bis(glycinyl)naphthalene diimide [ K2-BNDI], which showed a poor solubility of 30 mM in water. 0.025 M [ K2-BNDI]/ 4-OH-TEMPO AORFB delivered 83.2% capacity retention at 5 mA cm−2 in 100 cycles.66 The results indicated that the existing arylene diimide-based electrolytes have certain two-electron transfer properties. However, they still have many challenges such as insufficient stability and poor water solubility. Thus, it could be envisioned that the extension of the π-conjugated structure and the decoration of more hydrophilic groups with large sizes into hydrophobic arylene diimide derivatives such as different numbers (2∼6) of hydrophilic ammonium cation groups, should significantly enhance the structural stability, as well as increase the solubility of the active materials. This method would improve the charge repulsion between the pendent ammonium groups, prevent the dimerization degradation process, and inhibit electrolyte penetration.67 Based on these considerations, a series of inexpensive and high-yield quaternary ammonium salt-containing arylene diimide derivatives ( NDI, PDI, and TPDI) with narrow bandgap, and stable π-conjugate structure were synthesized as anolytes for AORFBs. Density functional theory (DFT) calculations showed that the aromaticity of arylene diimide derivatives was further enhanced from oxidized to highly reduced species. The larger molecular size achieved zero penetration for 30 days. Using (ferrocenylmethyl)trimethylammonium chloride ( FcNCl) as the catholyte, this work delivered excellent ultrastability for two-electron storage with a low daily decay and Coulombic efficiency (CE) close to 100%. Compared with previous work, our current approach demonstrated much better battery performance and was the most stable system for two-electron storage. Experimental Methods NMR spectra were collected using a Bruker 400 MHz NMR spectrometer (Bruker, Zurich, Switzerland). UV–vis measurements were performed using a DH-2000-BAL Scan spectrophotometer (OceanOptics, Florida, USA). Cyclic voltammetry (CV) in solution was measured using a potentiostat model CHI660E B157216 (CH Instruments, Inc., Beijing, China). The Linear sweep voltammetry (LSV) was measured on a rotating disk electrode (RDE) device (Pine Instruments Co. (North Carolina, USA; 0.1963 cm2). The conductivity was measured using a conductivity meter (DDSJ-308A, Ningbo Hinotek Technology Co., Ltd., Zhejiang, China). Electrochemical impedance analysis (EIS) was performed using an Autolab electrochemical workstation (AUT86797-302N, Metrohm Instruments, Herisau, Switzerland). High-resolution mass spectra (HRMS) were collected on a Bruker maXis UHR-TOF mass spectrometer (Bruker Scientific Technology Co., Ltd., Beijing, China) in electrospray ionization (ESI) positive mode. Electron paramagnetic resonance (EPR) was measured using a Bruker A300-9.5/12 instrument (Bruker Scientific Technology Co., Ltd., Beijing, China) at room temperature in dry degassed methanol. The EPR parameters for the experiments are as follows: modulation frequency = 100 kHz, modulation amplitude = 1.0 G, time constant = 81.92 ms, conversion time = 80.00 m, center field = 3514.503 G, sweep width = 1000 G, microwave attenuation = 26.3 dB, microwave power = 0.00471 mW. All battery tests were conducted under an Ar atmosphere. The flow battery was tested at room temperature (RT) on a battery tester (NEWARE instrument, CT-4008T-5V12A-S1-F, Shenzhen, China). All photographs were taken using a Nikon D5100 digital camera. Both AMV and DSV anion exchange membrane were purchased from Wuhan Zhisheng New Energy Co., LTD. (Wuhan, China), with film thickness (130 μm, 100 μm), tensile strength (0.16 MPa, 0.14 MPa), pore diameter (1∼3 nm), and area-specific resistance (2.8 Ω cm2, 1.1 Ω cm2 for 0.5 M NaCl). Synthesis of NDI The cationic naphthalene diimide derivative NDI was synthesized following the previous literature procedure ( Supporting Information Scheme S1).68 Briefly, 1,4,5,8-naphthalene tetracarboxylic dianhydride ( NTDA) (1 g, 3.73 mmol) was dissolved in 120 mL dry toluene under N2 atmosphere and heated to 90 °C, and N,N-dimethyl-1,3-propanediamine (3 mL, 23.84 mmol) was added dropwise over 10 min. The reaction mixture was heated at 120 °C for 24 h. The crude mixture was concentrated on a rotary evaporator, and the yellow crystalline NDI-N was purified by recrystallizing from ethanol. Yield: 1 g (61%). A mixture of NDI-N (1 g, 2.29 mmol), 5 mL methyl chloride (1.0 mol L−1 in tetrahydrofuran [THF]), and 50 mL dry dimethylformamide (DMF) were added. The solution was sealed in a pressure vial with a Teflon bushing and heated at 85 °C for 12 h. The resulting suspension was cooled, collected by vacuum filtration, washed with DMF, acetone, and ether, and then dried in a vacuum at 60 °C overnight to give a gray solid NDI product. Yield: 1.1 g (89%). 1H NMR (400 MHz, D2O, δ): 8.60 (S, J = 2.4 Hz, 4H), 4.23 (t, J = 6.9 Hz, 4H), 3.57–3.48 (m, 4H), 3.14 (s, 18H), 2.32–2.22 (m, 4H). 13C NMR (101 MHz, D2O, δ): 163.47, 131.00, 125.57, 125.46, 63.95, 52.96, 37.70, 21.38. HRMS (ESI) m/z: [M−2Cl]2+ calcd for C26H34N4O4, 233.1285; found, 233.12779. Synthesis of PDI The cationic perylene diimide derivative PDI was synthesized following the literature procedure ( Supporting Information Scheme S2).69 3,4,9,10-Perylenetetracarboxylic dianhydride ( PTCDA) (2 g, 5.10 mmol) and N,N-dimethyl-1,3-propanediamine (6 mL, 47.68 mmol) were dissolved in 80 mL dry isobutanol and heated at 90 °C for 24 h with stirring under N2 atmosphere. The crude product was filtered and washed with deionized water and ethanol. The obtained residue was treated with 5% aqueous NaOH solution at 90 °C for 30 min to remove the unreacted raw material. The suspended mixture was filtered, washed with water and ethanol, and dried under vacuum to give the product a red solid PDI-N. Yield: 2.6 g (90%). To a mixture of PDI-N (2 g, 3.57 mmol), 8 mL methyl chloride (1.0 mol L−1 in THF), and 150 mL dry toluene were added. The solution was sealed in a pressure vial with a Teflon bushing and heated at 105 °C for 12 h. The resulting suspension was cooled, collected by vacuum filtration, washed with toluene, and ether, and then dried in a vacuum at 60 °C overnight to give a brownish-red PDI product. Yield: 2.2 g (93%). 1H NMR (400 MHz, CF3COOD, δ): 8.94 (s, 8H), 4.62 (s, 4H), 3.81 (s, 4H), 3.37 (s, 18H), 2.61 (s, 4H). 13C NMR (101 MHz, CF3COOD, δ): 165.93 (s), 136.37 (s), 133.24 (s), 129.41 (s), 126.44 (s), 124.48 (s), 121.85 (s), 64.88 (s), 53.22 (s), 37.92 (s), 21.84 (s). HRMS (ESI) [M−2Cl]2+ calcd for C36H38N4O4, 295.1441; found, 295.14421. Synthesis of TPDI Compound TPDI was synthesized according to a previous procedure in the literature ( Supporting Information Scheme S3).70 PTCDA (1.62 g, 4.13 mmol) and N,N-dimethyl-1,3-propanediamine (30 mL, 201 mmol) were added, mixed, and heated at 100 °C for 28 h, then the temperature was gradually increased to 170 °C for over 4 h. Then the mixture was cooled to room temperature and a mixture of ethanol and diethyl ether (1:3) was added. The resulting precipitate was collected by suction filtration, washed with diethyl ether, and dried under a vacuum to obtain a red solid TPDI-N. Yield: 2.4 g (90%). To a mixture of TPDI-N (1.38 g, 2.13 mmol), water (6 mL), 85% formic acid (6.4 mL), and 30% formaldehyde (4.4 mL) were added. The solution was stirred at room temperature for 1 h and then heated at 120 °C for 16 h. Caution: During this time, the mixture produced a lot of carbon dioxide, so slow deflation was required after the reaction. The solution was cooled to room temperature, then precipitated with ethyl ether, and centrifuged (8000 rpm for 5 min at 25 °C). The residue was dried under a vacuum, obtaining their tertiary amine analogue TPDI-MN, a red solid. Yield: 1.9 g (86%). A mixture of TPDI-MN (1.5 g, 1.45 mmol), dry MeOH (60 mL), and Na2CO3 (1 g) was stirred at room temperature for 12 h, then added methyl iodide (3 mL, 48.19 mmol), and heated at 60 °C for 12 h. The mixture was cooled to room temperature, then precipitated with ethyl ether. The resulting precipitate was collected by suction filtration, washed with diethyl ether, and dried under a vacuum. The product was exchanged for chloride by column anion exchange with Amberlite® IRA-900 chloride from anion exchange resin to give a red solid TPDI. Yield: 1.39 g (90%). 1H NMR (400 MHz, CF3COOD, δ): 8.95 (dd, J = 8.1 Hz, 8H), 5.10 (s, 4H), 4.56 (s, 20H), 4.35 (s, 8H), 3.60 (s, 36H). 13C NMR (101 MHz, CF3COOD, δ): 167.80 (s), 136.82 (s), 133.39 (s), 129.63 (s), 126.73 (s), 124.66 (s), 121.57 (s), 54.43 (s), 53.64 (s), 52.02 (s), 49.62 (s), 44.23 (s), 35.56 (s). HRMS (ESI) [M−3Cl]2+ calcd for C50H74N8O4, 283.5272; found, 283.52952. Synthesis of [(NPr)2V]Cl4 Compound [(NPr)2V]Cl4 was synthesized according to the literature.22 In a 250 mL N2 purged Schlenk flask, 4,4′-bipyridine (2.0 g, 12.8 mmol) was combined with (3-bromopropyl)trimethylammonium bromide (10 g, 38.3 mmol) in 15 mL of dimethyl sulfoxide (DMSO) and stirred at 100 °C for 3 h. The resulting precipitate was collected by suction filtration and washed with cold DMSO and acetonitrile. The product was exchanged for chloride by column anion exchange with Amberlite® IRA-900 chloride form anion exchange resin to give a white solid [(NPr)2V]Cl4. Yield: 4.48 g (70%). 1H NMR (400 MHz, D2O, δ): 9.09 (s, 2H), 8.51 (s, 2H), 4.74 (s, 2H), 3.47 (d, J = 4.4 Hz, 2H), 3.08 (d, J = 2.3 Hz, 9H), 2.57 (s, 2H). Synthesis of FcNCl FcNCl was synthesized according to a reported method.71 (Ferrocenylmethyl)dimethylamine (10 g, 41.2 mmol), methyl chloride (49.4 mL, 445.3 mmol), and 25 mL dry CH3CN were added to a round-bottom flask, which was stirred at RT overnight. The product was collected by filtration, washed three times with 10 mL ether, and dried under a vacuum to obtain a yellow solid FcNCl product. Yield: 10.9 g (90%). 1H NMR (400 MHz, D2O, δ): 4.49 (t, J = 1.8 Hz, 2H), 4.40 (t, J = 1.8 Hz, 2H), 4.37 (s, 2H), 4.25 (s, 5H), 2.92 (s, 9H). Computational methods We performed simulations of the experiments to confirm the conformation of the oxidized products via predictions of UV-vis spectroscopy of the compounds in water to resolve their properties. We employed the Polarizable Continuum Model (PCM) as a self-consistent reaction field (SCRF) for the calculation of their equilibrium geometries, vibrational frequencies, vertical excitation energies and the corresponding absorption wavelengths. The geometries for the ground state of these compounds were optimized at the B3LYP hybrid functional and 6-311+G(d) basis set for all atoms.37 The structures of all stationary points were characterized as true minima on the potential energy surface from a vibrational frequencies analysis in which imaginary modes were absent. Using the optimized ground-state equilibrium geometries in water solution as starting points, the absorption wavelengths (λTD-DFT), oscillator strength (f), molecular orbitals (MOs) were calculated with the non-equilibrium time-dependent Density Functional Theory (TD-DFT) framework. The same functional and basis set was employed in the optimization calculations. All of the above DFT and TD-DFT calculations reported in this work were performed using the Gaussian 09 code.23,72 The volume was estimated using Marching Tetrahedron (MT) mothed, based on the vdW surface defined by ρ = 0.001 au isosurface, using the Multiwfn code.73 NMR chemical shifts for Nucleus-independent chemical shift (NICS)74,75 values were calculated at the points shown using the GIAO76 method. Result and Discussion Synthesis and structure characterization The designed arylene diimide derivatives, NDI, PDI, and TPDI, were synthesized according to the previous literature (Scheme 1).68,70 First, NTDA and PTCDA underwent a ring-opening reaction, then reacted with an amino group to form amides and subsequently, imides by the closed-loop reaction. Then methyl chloride was introduced to produce highly hydrophilic naphthalene diimide ( NDI) and perylene diimide ( PDI) via ionization. The methylation reaction of TPDI is different from NDI and PDI. The amino group reacted with excess formic acid and formaldehyde to obtain tertiary amines by Eschweiler–Clarke reaction, and further got quaternary ammonium salt product with methyl iodide. Among them, the solubility of NDI in 2 M NaCl solution is as high as 1.00 M, corresponding to a theoretical capacity of 53.6 Ah L−1. Expanding only the conjugation resulted in the solubility of PDI of 0.08 M. Further improvement of the solubility by introducing more hydrophilic ammonium cation groups to prepare TPDI, yielded a solubility of 0.14 M, approximately twice that of PDI ( Supporting Information Figure S1). The solubility and capacity information of NDI/ PDI/ TPDI are summarized in Supporting Information Table S1. The molecular structures were verified by NMR and HRMS. The experimental and calculated values of UV–vis absorption spectra were consistent ( Supporting Information Figures S6–S9). Compared with NDI (3.10 eV), the absorption of PDI (2.02 eV) and TPDI (2.01 eV) showed a narrower bandgap, an obvious redshift, and wider absorption in the visible region. Scheme 1 | The design route of arylene diimide derivatives. Download figure Download PowerPoint Electrochemical characterization The electrochemical properties of NDI/ PDI/ TPDI redox couples were characterized by CV (Figure 1a and Supporting Information Figure S2). Figure 1b shows the structural changes of the arylene diimide derivatives during the redox process. NDI shows an obvious two-electron platform at E11/2 = −0.09 V ( NDI2+ ↔ NDI1+•), E21/2 = −0.46 V ( NDI1+• ↔ NDI0). PDI also presents two quasi-reversible redox potentials at E11/2 = −0.13 V ( PDI2+ ↔ PDI1+•), E21/2 = −0.45 V ( PDI1+• ↔ PDI0). TPDI shows two one-electron oxidation peaks and one two-electron reduction peak with redox potentials of E11/2 = −0.17 V ( TPDI6+ ↔ TPDI5+•), E21/2 = −0.35 V ( TPDI5+• ↔ TPDI4+), respectively, due to the overlap of the 1st and 2nd electron reductions. The redox potentials of PDI and TPDI were significantly lower than that of NDI. The potential difference between the two redox peaks of NDI/ PDI/ TPDI gradually decreased from 0.37 to 0.32 to 0.18 V, respectively, which was attributed to the increased conjugation and more electron-withdrawing groups. The peak current intensity of NDI was significantly higher than that of PDI and TPDI, which was caused by the solubility difference. The better the solubility of the electrolyte, the lower the viscosity; thus, the conductivity was enhanced, resulting in an increased number of effective charge transfers. The peak currents of redox processes of NDI, PDI, and TPDI showed a linear relationship with the square root of scan rates, indicating that all redox couples were reversible and diffusion controlled. Paired with FcNCl as the catholyte (0.61 V), the NDI/ FcNCl-, PDI/ FcNCl-, TPDI/ FcNCl-based AORFBs delivered 1.07, 1.06, and 0.96 V battery voltage, respectively, making them ideal candidates as anolytes to improve the energy density of batteries in terms of output voltage. At different scanning rates, NDI/ PDI/ TPDI showed good CV curves, with a consistent trend of redox peaks, implying its excellent redox reversibility and stability. Figure 1 | (a) Cyclic voltammograms of 4.0 mM NDI, PDI, TPDI, and FcNCl with 0.1 V s−1 in 0.5 M NaCl solution. (b) The reaction structure of the arylene diimide derivatives. Download figure Download PowerPoint Electrochemical kinetic characterization LSV was used to study the electrochemical kinetic characteristics of NDI/ PDI/ TPDI ( Supporting Information Figures S3–S5). The diffusion coefficient (D) and electron transfer constant (k0) for the first reduction process are 6.18 × 10−6, 1.03 × 10−6, and 9.07 × 10−6 cm2 s−1 and 5.73 × 10−3, 3.93 × 10−4, and 2.42 × 10−3 cm s−1, respectively. For the second reduction process, D and k0 are 1.10 × 10−5, 8.67 × 10−6, 1.06 × 10−4 cm2 s−1 and 2.07 × 10−2, 1.05 × 10−3, 7.29 × 10−3 cm s−1. The transfer constant of NDI/ PDI/ TPDI was high enough to operate flow batteries with low concentrations and kinetic polarization losses. DFT calculations DFT calculations were finally implemented to confirm the conformation of the oxidized NDI 2+, PDI 2+, and TPDI 6+, and reduced NDI 0, PDI 0, and TPDI 4+ (Figure 2a). The oxidized NDI 2+, PDI 2+, and TPDI 6+ (eigenstate) demonstrated excellent aromaticity, with a strong π-conjugated rigid planar skeleton and narrow bandgaps of 3.55, 2.45, and 2.43 eV, respectively. After gaining two electrons, the electron transfer channel of the eigenstate was broken, and a new π–π conjugated plane was formed. The bandgaps were 2.59, 2.48, and 2.46 eV, respectively, which are conducive to the rapid electron transfer of the electrolytes. The results are shown in Supporting Information Figure S10, revealing that the spin densities were well distributed within the center conjugate system and not affected by the side chain (quaternary ammonium salt). It further proved that diimide derivatives are a kind of stable electrolyte material. One likely explanation for the unusual stability of arylene diimide derivatives is aromaticity. The NICS(1) results led to the same conclusions (Figure 2b). For 1,1′-bis[3-(trimethylammonio)-propyl]-4,4′-bipyridinium tetrachloride [(NPr)2V]Cl4, the two pyridinium rings of [(NPr)2V] 4+ are aromatic (negative NICS values), but they are nonaromatic in [(NPr)2V] 2+. This aromatic transition from −18.56 to 5.46 might be an important reason for the instability of [(NPr)2V]Cl4 during two-electron storage. In NDI 2+, the central naphthalene ring was aromatic [NICS(1) = −9.89], and the diimide rings were nonaromatic [NICS(1) = 0.67]. In NDI 0, all the rings became aromatic [NICS(1) = −2.22, −5.94]. In PDI 2+, the central ring was slightly antiaromatic [NICS(1) = 1.31], the peripheral rings that form the two naphthalene cores were aromatic [NICS(1) = −8.06], and the diimide rings were nonaromatic [NICS(1) = −0.09]. In PDI 0, all rings are aromatic [NICS(1) = −9.80, −4.45, −5.35]. The results for TPDI are very similar to those for PDI. The total NICS(1) values showed that the order of their aromaticity is TPDI ≈ PDI > NDI > [(NPr)2V]Cl4, corresponding to molecular stability. Moreover, their molecular volumes are calculated as 574.93, 712.74, and 1068.72 Å3, which is beneficial for inhibiting the penetration of electrolytes. The permeability of the electrolyte for the anion-exchange membrane (Selemion™ AMV, AGC Engineering Co., LTD, Chiba, Japan) was tested by H-tube for 30 days, and no obvious color and concentration changes were found in the blank solution, determined by UV–vis spectroscopy, indicating that no crossover occurred ( Supporting Information Figure S11). Another important reason may be the presence of intermolecular hydrogen bonding and intrinsic π–π stacking interactions, which further increased the electrostatic interactions between molecules, and thus, hindered the shuttling of electrolytes. Figure 2 | (a) Highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) plots and energy-level diagrams of NDI 2+, NDI 0, PDI 2+, PDI 0, TPDI 6+, and TPDI 4+ by DFT calculations. (b) Schematic diagram of aromaticity change of [(NPr)2V]Cl4, NDI, PDI, and TPDI from the oxidized to the highly reduced species. The red upward arrow represents enhanced aromaticity; the red downward arrow represents reduced aromaticity but still retains aromaticity; the blue upward arrow represents enhanced antiaromaticity. The sum of the individual rings is used to assess the overall aromaticity of a molecule and all NICS(1) values are averages. Download figure Download PowerPoint In situ UV–vis spectra The in situ UV–vis spectra were adopted to clarify the two-electron transfer process and switchable conjugation mechanism of these full batteries (Figure 3). By applying different voltages, the UV–vis spectra of different molecular states were recorded (Figure 3a). For NDI (300–700 nm), the initial absorption of NDI 2+ tended to be in the ultraviolet region, mainly at 362 and 381 nm. With an applied potential of 0.81 V, NDI 2+ gained an electron and rapidly formed the radical species NDI 1+•. The absorption of the visible light region was greatly redshifted and a wide single peak was generated at 448 nm. The color of the anolyte changed from colorless to yellow. When the voltage is increased continuously to 0.92 V, NDI 1+• gained the second electron, forming reduced NDI 0. The absorption was enhanced at 402, 526, and 568 nm, and the color of the anolyte became light-red. PDI and TPDI adopt the same test methods. Compared with NDI, the visible light absorption of PDI and TPDI showed an obvious redshift, which might have been caused by increased conjugation. The absorption of the oxidized state PDI 2+ (light red) in the visible region was mainly located at 488 and 553 nm. When the voltage increases to 0.93 V, the radical species PDI 1+• (blue–purple) redshifts to 543 and 623 nm. When the voltage increased to 1.05 V, the absorption of reduced PDI 0 (purplish red) increased sharply at 510, 532, and 601 nm. Compared with PDI, TPDI had slight changes in absorption in the visible region and color change due to the consistent structure. The absorption of the oxidized state TPDI 6+ (pink) is mainly
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