Introduction

Replicating the high selectivity and high permeability of biological ion channels is an enduring challenge for membrane scientists1,2,3. Beyond the generally accepted mechanisms of size exclusion and Coulombic repulsion, it is argued that the subtle interactions between ions and channel walls at atomic-scale confinement play a crucial role. These interactions were not clearly elucidated until the fabrication of angstrom-scale slits/capillaries/channels with atomic-scale precision4,5.

The spatial confinement of ion transport down to molecular-sized ion channels magnifies the impact of channel wall interactions and gives rise to exotic transport behavior. For example, hysteretic ion conduction occurs, resulting in an ion memory effect6,7, while the formation of Bjerrum ion pairs causes ionic Coulombic blockade8. These atypical ion motions are intimately related to the dramatically enhanced material-dependent interactions between hydrated ions and the confining channel walls (e.g., electrostatic, adsorption/desorption)9. For chemically inert and atomically smooth graphite channel walls, K+ demonstrates a mobility close to that of the value in bulk solutions10. By applying a voltage bias on the graphite channel, the streaming mobility of K+ is increased by up to 20 times11 and this may be ascribed to the electronic structure change under an external voltage bias12. It has also been demonstrated that by introducing Li+-coordinating functionality within the shape-persistent free volume elements of microporous polymer membranes, Li+ diffusivity can be greatly enhanced13. Similar improvements to Na+ transport have also been achieved by exploiting the synergy between micropore confinement and ion-membrane interactions14.

Despite the considerable improvements in cation transport due to the confinement effect, it is notable that chloride (Cl-) mobility experiences obvious suppression under confinement, also known as charge asymmetry effect10. The mobility of Cl- under confinement is three times less than that of K+, even though Cl- and K+ have similar mobilities in bulk water (7.58 × 10−8 vs. 7.86 × 10−8 m2 V−1 s−1) and hydrated diameters (6.64 vs. 6.62 Å)15. Similarly, Cs+ and Cl- exhibit similar ion-core sizes and hydrated diameters, but Cl- exhibits more than three times lower mobility under Å-scale confinement (1.7 × 10−8 vs. 6.0 × 10−8 m2 V−1 s−1)16. For chloride salts of high valency cations, the overall Cl- mobility decreases to almost zero in single-digit-sized nanopores17. A decrease in the mobility of other anions under confinement has also been observed16. This phenomenon is echoed by the relatively high energy barrier associated with anion exchange membranes that transport chloride ions (see Supplementary Table 1).

The transport and selectivity of anions are of critical relevance to applications such as direct seawater electrolysis18, solid-state batteries19 and redox flow batteries20,21,22,23,24,25. Understanding and overcoming the charge asymmetry effect for anion transport under confinement is therefore essential for enabling these technologies. Here we report the rational design and synthesis of a series of positively charged (bearing quaternary ammonium cations) covalent triazine framework (QCTF) membranes with nearly the same density of rigid micropores with almost identical pore size distributions. The QCTF membranes exhibit Coulombic repulsion-induced anion selectivity, with a chloride transference number t- of 0.95, and size exclusion-induced rejection of BTMAP-Vi (bis(3-trimethylammonio) propyl viologen tetrachloride) and FcNCl ((ferrocenylmethyl) trimethylammonium chloride), the representative redox-active organic flow battery electrolytes. The cross-membrane BTMAP-Vi permeability coefficient at 2.93 × 10−11 cm2 s−1 is over 20 times lower than that of the Selemion® DSV membrane which is a commonly used commercial membrane for pH-neutral aqueous organic redox flow batteries (AORFBs). We demonstrate that through on-membrane modification, the charge distribution of the pristine QCTF membrane framework can be regulated by protonation (affording P-QCTF) and methylation (affording M-QCTF), which dramatically alters the interactions between anions and the membrane framework and helps lower the energy barrier for anion transport. The cross-membrane Cl- conductivity increased twofold from 13.2 mS cm−1 for QCTF to 26.0 mS cm−1 for M-QCTF at 30 °C, and the activation energy for Cl- conduction decreased from 20.6 kJ mol−1 to 13.0 kJ mol−1, lower than values reported in the literature (see Supplementary Table 1). 19F PFG-NMR revealed an increase in the F- diffusion coefficient from 0.63 × 10−9 m2 s−1 for QCTF and 0.93 × 10−9 m2 s−1 for P-QCTF, to 1.1 × 10−9 m2 s−1 for M-QCTF which is close to the value in bulk water (1.2 × 10−9 m2 s−1). The greater anion conductivity can dramatically improve device performance as exemplified here in an BTMAP-Vi- and FcNCl-based AORFB in pH-neutral solutions. The BTMAP-Vi/FcNCl cell configured with the M-QCTF membrane exhibited a high-frequency area-specific resistance (ASR) as low as 0.23 Ω·cm2, which enabled charging and discharging of the BTMAP-Vi/FcNCl cell at a current density of >500 mA cm−2. The prolonged galvanostatic cell cycling at 400 mA cm−2 maintained a Coulombic efficiency of >99% and a stable energy efficiency of around 60% over the course of 1000 cycles. Notably, the achieved capacity utilization and efficiency with M-QCTF approaches similar values to those of alkaline AORFBs that leverage K+ as charge-carrying ions, while in otherwise identical cells assembled with QCTF or P-QCTF, an almost 20% lower energy efficiency was observed. This is notable and can be attributed to a dramatic reduction in the contribution of membrane resistance to whole–cell resistance, e.g., from >70% for the Selemion® AMV membrane and >50% for the Selemion® DSV membrane to ~25% for M-QCTF (Supplementary Tables 2 and 3). The above results imply a breakthrough in the charge asymmetry effect and will contribute a dramatic decrease in capital costs for the presented pH-neutral AORFBs and beyond.

Results

Covalent triazine framework membranes with tunable pore chemistry

Covalent triazine framework chemistry gives rise to a wide variety of microporous materials and offers enormous diversity in pore chemistry. We thus synthesized a stand-alone triazine framework membrane from 4,4’-biphenyldicarbonitrile and a derivative of 3-hydroxy-[1,1’-biphenyl]-4,4’-dicarbonitrile bearing a quaternary ammonium moiety via a superacid-catalyzed organic sol‒gel procedure (Fig. 1a and Supplementary Figs. 1–4)26. The process yields a free-standing membrane (namely, QCTF) with a Young’s modulus and tensile strength of 0.91 GPa and 32.0 MPa, respectively (Supplementary Fig. 5). The skeletal triazine rings of QCTF were subsequently protonated with HCl or methylated with CH3I, affording P-QCTF and M-QCTF, respectively. Overall, we constructed three covalent triazine framework polymers with similar molecular configurations and pore structures that can be processed into hydrophilic, uniform, and robust ion-selective membranes via an organo-sol‒gel procedure (Supplementary Figs. 6–8, Supplementary Table 4), but with slightly different and deliberately tailored pore chemistries.

Fig. 1: Synthesis and characterization of microporous covalent triazine framework membranes.
Fig. 1: Synthesis and characterization of microporous covalent triazine framework membranes.
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a Left panel: schematic showing the 3D interconnected micropore free volume for anion transport. Right panel: Molecular structure and synthesis of the covalent triazine framework membrane (QCTF) and subsequent protonation or methylation of the triazine ring skeleton, affording P-QCTF and M-QCTF. Coulombic/steric exclusion and intimate ion‒pore wall interactions enable selective and fast anion transport. Red and blue spheres: fixed functional groups or charged triazine rings; green spheres: counterions or charge carrier ions; lightning: ion‒pore wall interactions. b CO2 adsorption isotherms of QCTF, P-QCTF and M-QCTF at 273 K. c Pore size distributions of QCTF, P-QCTF and M-QCTF derived from CO2 adsorption isotherms through density functional theory (DFT) calculations. d XPS (N1s) spectra of covalent triazine framework (CTF) membranes: QCTF (top), protonated QCTF (P-QCTF, middle), and methylated QCTF (M-QCTF, bottom). Source data are provided as a Source data file.

Carbon dioxide (CO2) adsorption experiments and molecular simulations were conducted to probe the micropore structure of the covalent triazine framework polymers. CO2 sorption isotherms measured at 273 K revealed that powder samples of QCTF, P-QCTF, and M-QCTF had similar CO2 uptake capacities of 16, 15.2, and 14.7 cm3 g−1 STP, respectively (Fig. 1b). Notably, QCTF, P-QCTF, and M-QCTF exhibit almost identical pore size distributions, ranging from 0.3 nm to 0.9 nm, as derived from CO2 adsorption isotherms based on density functional theory (DFT) calculations (Fig. 1c). These experimental results are further supported by molecular simulations and pore free volume analysis of the 3D framework structure (Supplementary Figs. 9 and 10)27. This again indicates that QCTF, P-QCTF, and M-QCTF have similar framework structures, interconnected micropores, and pore size distributions.

The amount of charged functional groups (quaternary ammonium groups) within the pristine QCTF membrane, characterized by the ion exchange capacity (IEC, in mmol g−1), is 1.20 mmol g−1 for QCTF (as-designed IEC value is ~1.00 mmol g−1). During protonation, approximately 55% of the triazine rings were protonated and the same amount of triazine rings were methylated after methylation, as revealed by X-ray photoelectron spectroscopy (XPS, Fig. 1d). This suggests that P-QCTF and M-QCTF should have identical IEC values, which was confirmed by titration and zeta potential measurements (Supplementary Fig. 11).

Ion transport and selectivity

Despite the similar framework structure and almost identical pore size/size distributions, our experimental results reflect that cross-membrane ion transport is greatly affected by pore chemistry. We speculate that the difference is synergistically determined by Coulombic/steric effects and specific ion‒pore wall interactions, as shown in Fig. 2a. The current‒voltage (I‒V) curves across the membranes, as measured in a two-compartment diffusional H-cell under a 10-fold concentration gradient in KCl solutions (Fig. 2b), reveal a net anion flux, indicating anion selectivity. The anion transference number (t-) calculated for QCTF is 0.940, while the values for protonated QCTF (P-QCTF) and methylated QCTF (M-QCTF) are 0.947 and 0.953, respectively (Supplementary Fig. 12). These values suggest the good anion selectivity of the QCTF membranes. This result is reasonable considering the Coulombic repulsion of the <1 nm pore channel within the QCTF membranes.

Fig. 2: Ion selectivity and conductivity of microporous covalent triazine framework membranes.
Fig. 2: Ion selectivity and conductivity of microporous covalent triazine framework membranes.
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a Schematic showing the transport of anions across rigid micropores within positively charged covalent triazine framework (QCTF) membranes. Coulombic/steric exclusion and intimate ion‒pore wall interactions enable selective and fast anion transport. Red and blue spheres: fixed functional groups or charged triazine rings; green spheres: counterions or charge carrier ions; blue and gray spheres: positively charged ions with large or small hydrated diameters. The dashed lines indicate ion‒pore wall interactions, while the arrowed lines suggest rejection or transport of ions. b Current‒voltage (I‒V) curves of the M-QCTF, P–QCTF, QCTF, Selemion® DSV, Selemion® AMV and Sustainion® X37-50 membranes under a 10-fold concentration gradient in KCl solution. The intercept at 0 µA correlates to the transmembrane potential because of selective ion transport, from which the transference number t- can then be deduced. c The permeability coefficient of BTMAP-Vi and KCl across QCTF membranes and three commercial membranes, as determined from a two-compartment diffusional H-cell. d Cl- conductivity plotted as a function of hydration number for the M-QCTF, P-QCTF, QCTF, Selemion® DSV, Selemion® AMV and Sustainion® X37-50 membranes. The conductivity was measured via the four-probe EIS method. Each data point represents the Cl- conductivity at an individual temperature: from left to right (or from larger data points to smaller data points), 30‒80 °C, with a 10 °C increment. e A plot of activation energy versus hydration number (λ) at 30 °C for QCTF membranes, framework membranes, polymer of intrinsic microporosity (PIM) membranes and flexible polymer membranes. f Comparison on activation energy for QCTF membranes, representative commercial membranes and those reported previously. The detailed values can be found in Supplementary Table 1. Error bars are standard deviations derived from three parallel measurements based on separate membrane samples, while experimental errors of data from references were not shown. Source data are provided as a Source data file.

We measured the cross-membrane permeation rates for BTMAP-Vi (a redox-active organic cation) and KCl (Fig. 2c, Supplementary Figs. 13–15, Supplementary Tables 5 and 6), which are dramatically different in size. The QCTF membranes exhibited size-induced selectivity towards cations, allowing a small amount of K+ permeation while rejecting larger BTMAP-Vi cations. The permeability coefficients of BTMAP-Vi across the QCTF and the P-QCTF were determined to be 4.3 × 10−11 cm2 s−1 and 3.32 × 10−11 cm2 s−1, respectively. It further decreases to 2.93 × 10−11 cm2 s−1 for M-QCTF, a value that is much smaller than those of Selemion® DSV, Selemion® AMV, Sustainion® X37-50, Fumasep® FAA-3-PE-30, and some PIM-based membranes (Fig. 2c and Supplementary Fig. 14). The permeability coefficients of KCl (predominantly controlled by the diffusion of the minor co-ion (K+))28,29,30 through the QCTF and P-QCTF are 1.82 × 10−7 cm2 s−1 and 2.6 × 10−7 cm2 s−1, respectively and it is 3.1 × 10−7 cm2 s−1 for M-QCTF. The comparison between the chloride salt permeability coefficients and selectivity of the QCTF membranes versus representative commercial AEMs and previously reported membranes implies that these framework membranes maintain comparable charge selectivity, but deliver noticeably higher size-induced selectivity (Supplementary Fig. 16 and Supplementary Table 6). By contrast, the KCl permeability coefficient for the control Sustainion® X37-50 membrane (a commercial AEM) is 2.4 times that of the M-QCTF membrane, and the BTMAP-Vi permeability coefficient for Sustainion® X37-50 membrane is at least three orders of magnitude higher than that of the M-QCTF membrane because of severe water swelling (54.6% vs. 1%) and excessive water uptake (172% vs. 8.5%) at 30 °C.

We observed fast Cl- transport across the triazine framework membranes, as evidenced by the membrane conductivity measurements. Compared with representative commercial AEMs, triazine framework membranes show high Cl- conductivity at relatively low hydration numbers (Fig. 2d, Supplementary Fig. 17 and Supplementary Tables 7 and 8). The Cl- conductivity of QCTF, as measured by four-point electrochemical impedance spectroscopy (EIS), is 13.2 mS cm−1 at 30.0 °C and approaches 42.8 mS cm−1 at 80 °C with low hydration numbers (3.5 at 30 °C, 4.4 at 80 °C). In comparison, the Cl- conductivity of P-QCTF is 20.3 mS cm−1 at 30 °C and increases to 48.6 mS cm−1 at 80 °C. We find that the Cl- conductivity of M-QCTF is 26.0 at 30.0 °C, which is nearly twice that of QCTF, and reaches 52.7 mS cm−1 at 80 °C. A comparison of the Cl- conductivity versus water uptake and selectivity for QCTF membranes, control commercial AEMs and previously reported membranes implies that these framework membranes exhibit high conductivity at low water uptake, and simultaneously deliver fast ion transport and high selectivity, overcoming the usual tradeoff observed for many ion exchange membranes (Supplementary Fig. 18 and Supplementary Tables 1 and 6). The activation energy (Ea) for Cl- conduction across the QCTF membrane is 20.6 kJ mol−1, as derived from the conductivities at various temperatures (Fig. 2e and Supplementary Fig. 19), contrasting an Ea of 12.9 kJ mol−1 for K+ transport across an otherwise identical membrane with sulfonate functional groups14. Surprisingly, the Ea value for M-QCTF is as low as 13.0 kJ mol−1, which is nearly half that of QCTF and lower than values reported in the literature (Fig. 2f and Supplementary Table 1), implying that the methylation of triazine rings alters the transport energy barrier for Cl- ions.

Due to the aforementioned results, we conclude that electrostatic interactions alone cannot explain the differences in Cl- conductivity or activation energy for cross-membrane Cl- transport. To unravel why methylation of the triazine ring promotes fast Cl- conduction, compared to the protonated triazine ring in P-QCTF and the charge-neutral triazine ring in QCTF, the charge distribution and the Cl- transport routes within the matrices of the triazine framework membranes were portrayed based on molecular simulations, and the two-dimensional free-energy landscapes were computed according to current methodology13,14. Our calculations show that the charge distributions (as reflected by the RESP charges used in the force field, generated with the Multiwfn package31) of triazine framework membranes vary dramatically after protonation and methylation (Fig. 3a, Supplementary Fig. 20). The most even charge distribution is observed for M-QCTF. We speculate that the variation in charge distribution alters the interactions between anions and the membrane frameworks and helps establish low-energy-barrier pathways for anion transport. This is supported by free energy calculations for Cl- conduction (Fig. 3b). The simulation results showed that Cl- can interact with quaternary ammonium (QA) groups (Fig. 3c, Supplementary Figs. 21 and 22) and lowers the free energy, but an energy barrier must be overcome for Cl- ions to approach adjacent QA groups. The energy barrier for Cl- conduction is the highest for QCTF (Fig. 3b, left panel) and decreases when the triazine ring is protonated (Fig. 3b, middle panel), while methylation of the triazine ring in M-QCTF improves the diffusivity of Cl- within the framework and creates a Cl- diffusion pathway with the lowest energy barrier (Fig. 3b, right panel). We suspect that the synergy of electrostatic interactions between Cl- and the methylated triazine ring and the change in electron density along the Cl- diffusion path after methylation may account for the emergence of the low-energy-barrier diffusion pathway.

Fig. 3: Low barrier anion transport enabled by ion‒pore wall interactions under confinement.
Fig. 3: Low barrier anion transport enabled by ion‒pore wall interactions under confinement.
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a Charge distributions of QCTF (left), P-QCTF (middle), and M-QCTF (right) from restrained electrostatic potential (RESP). The charge values shown can be found in Supplementary Table 9. b Computed free energy map for the transport of Cl- ions within the QCTF (left), P-QCTF (middle), and M-QCTF (right) membrane matrices. The black or white lines denote the Cl- ion transport pathways (1-1 or 1-2-1) with the lowest free energy barrier. c Snapshots taken during simulation, demonstrating the interactions between Cl- and the M-QCTF membrane pore walls. Insets denote the specific interactions at positions 1 and 2. The parameters (r1 and r2) represent the distance between the Cl- ion and the geometric center of two quaternary ammonium (QA) groups. d 19F PFG-NMR spectra recorded for membrane samples of Selemion® DSV, QCTF, P-QCTF and M-QCTF immersed in 0.1 M KF solutions. B values (s m−2) is the total contribution of variables including γ, g, σ, and Δ (Supplementary methods). e F- diffusion coefficients derived from 19F PFG-NMR spectra (19F- is used instead of 35Cl- because of its higher NMR sensitivity). Error bars are standard deviations derived from three measurements based on three separate membrane samples. Source data are provided as a Source data file.

Molecular simulation results are supported by measurements of transmembrane F- diffusion coefficients via 19F pulsed-field gradient-stimulated-echo nuclear magnetic resonance (19F PFG-NMR; 19F was selected owing to its higher sensitivity compared with 35Cl). 19F PFG-NMR revealed two separate F- signals for Selemion® DSV and Selemion® AMV membranes (Fig. 3d and Supplementary Fig. 23), with the upfield signal corresponding to free F- in water (located at the same position as that in 0.1 M KF aqueous solution) and the downfield signal corresponding to associated F- within the membrane. By contrast, only the upfield signal was observed for all three triazine framework membranes (Fig. 3d), which is an indication of freely exchangeable F- within the membrane, with slight variations in the 19F chemical shifts. By fitting the echo profiles with the Stejskal‒Tanner equation (Supplementary Fig. 24), the derived F- diffusion coefficients within P-QCTF and QCTF are 0.93 × 10−9 m2 s−1 and 0.63 × 10−9 m2 s−1, respectively (Fig. 3e). The value reaches 1.1 × 10−9 m2 s−1 for M-QCTF, almost a twofold increase compared to that for QCTF. Notably, this value is 12.8 times that of Selemion® AMV and 10.8 times that of Selemion® DSV (Fig. 3e and Supplementary Fig. 24) and approaches the measured diffusion coefficient of F- in bulk water (1.2 × 10−9 m2 s−1; Supplementary Fig. 24). Overall, the above results confirm that by tailoring the pore chemistry of framework membranes, intimate ion‒pore wall interactions could provide a low-energy-barrier diffusion pathway for anions. Taken together with the Coulombic/steric exclusion by the charged framework micropores, the triazine framework membranes, particularly M-QCTF, will be of interest in applications demanding fast and highly selective transport of anions.

Triazine framework membrane powers fast-charging AORFBs

The fast and highly selective anion (particularly chloride ions) conduction through chemically tuned triazine framework membranes is desirable in electrochemical devices, such as redox flow batteries. As a proof of concept, we configured pH-neutral AORFBs with BTMAP-Vi/FcNCl as the redox-active organic electrolyte couple and triazine framework membranes as the ion-conducting membranes, while Cl- ions were transported back and forth as charge carriers (Fig. 4a). At an electrolyte concentration of 0.1 M, EIS of the BTMAP-Vi/FcNCl cells assembled with QCTF or P-QCTF showed area-specific membrane resistances (ASRs) of 0.63 Ω cm2 and 0.53 Ω cm2, respectively (Supplementary Figs. 25 and 26). An otherwise identical cell assembled with M-QCTF showed an ASR of 0.37 Ω cm2 (Supplementary Fig. 27), which is almost twofold lower than that of the QCTF membrane. This finding aligns with the high conductivity of M-QCTF (Figs. 2d and 3b), which enables charging of the BTMAP-Vi/FcNCl cells at high current densities. For example, at 200 mA cm−2, BTMAP-Vi/FcNCl with M-QCTF exhibited an energy efficiency (EE) of over 60% (Supplementary Fig. 27). In contrast, the control BTMAP-Vi/FcNCl cells assembled with Selemion® DSV or Selemion® AMV could not operate at this current density due to the immediate voltage cutoff. At lower current densities ranging from 20 to 80 mA cm−2, the reported energy efficiency for the control cells drops from 89.4% to 65.9% for Selemion® DSV or from 80.0% to 26.6% for Selemion® AMV32.

Fig. 4: Fast charging of pH-neutral AORFBs enabled by the M-QCTF membrane.
Fig. 4: Fast charging of pH-neutral AORFBs enabled by the M-QCTF membrane.
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a Schematic illustration of a pH-neutral BTMAP-Vi/FcNCl AORFB assembled with the M-QCTF membrane. b EIS spectra measured in cells assembled with M-QCTF, Selemion® DSV and Selemion® AMV membranes. A control EIS spectrum was recorded in a cell without a membrane. c Coulombic efficiency (CE), energy efficiency (EE), and capacity of cells assembled with the M-QCTF membrane at various current densities. d Galvanostatic cycling of the BTMAP-Vi/FcNCl cell assembled with the M-QCTF membrane over 1000 cycles (54.2 h) at 400 mA cm−2. The electrolyte compositions through (b) to (d): the negolyte comprised 5 mL of 0.5 M BTMAP-Vi in 2 M KCl, while the posolyte comprised 10 mL of 0.5 M FcNCl in 2 M KCl. Capacity utilization (e) and energy efficiency (f) of pH-neutral AORFBs assembled with P-QCTF, M-QCTF, PIM membranes, flexible polymer membranes are plotted as functions of the operating current density. Dashed lines and shades are visual guides. The detailed values can be found in Supplementary Table 10. Source data are provided as a Source data file.

At a higher electrolyte concentration of 0.5 M, BTMAP-Vi/FcNCl with M-QCTF demonstrated an even lower ASR of 0.23 Ω cm2 (Fig. 4b), a much lower value than that for Selemion® DSV or Selemion® AMV. The rate performance of the cell reveals an EE of 49.7% and a capacity utilization of 58.8% at a current density of 500 mA cm−2 (Fig. 4c). By contrast, the cell assembled with P-QCTF membrane cannot operate at 500 mA cm−2 and the cell performance at other current densities is inferior to that assembled with M-QCTF (Supplementary Figs. 28 and 29). Compared with Sustainion® X37-50 membrane, a recently commercialized AEM, the M-QCTF membrane achieved a much greater energy efficiency (59.1% vs. 42.9%) and capacity utilization (79.78% vs. 42.7%) at the same current density of 400 mA cm−2 (Supplementary Figs. 28 and 30). Compared with the most recent report of an AEM (MTCP-50 membrane, with the optimal ratio 1:1 of m-terphenyl to p-terphenyl) for pH-neutral AORFBs at 0.5 M21, M-QCTF achieved a much greater energy efficiency (76.9% vs. 60.1%) and capacity utilization (94.3% vs. 63.7%) at the same current density of 200 mA cm−2. It is worth mentioning that alkaline AORFBs that utilize K+ as charge-carrying ions assembled with a cation exchange membrane (SCTF-BP)14, which allows cation diffusion close to the value in bulk electrolyte, exhibit an EE of 50.4% and a capacity utilization of 62% at 500 mA cm−2. The current results demonstrate a similar efficiency for Cl- transport and therefore suggest a breakthrough in the charge asymmetry effect.

Robust and remarkable cell performance was observed during long-term galvanostatic cycling of over 2000 cycles at 200 mA cm−2 (0.1 M electrolyte concentration, Supplementary Fig. 27) and over 1000 cycles at 400 mA cm−2 (0.5 M electrolyte concentration, Fig. 4d). Comparisons of the EE and capacity utilization against the current density shows consistently better battery performance over multiple cell cycling experiments for the BTMAP-Vi/FcNCl cells with M-QCTF, compared to the pH-neutral AORFB with different membranes (Fig. 4e, f and Supplementary Table 10). The dramatically improved battery performance under high operating current density is critical in slashing costs for storing green energy33 and realizing the DoE cost target of $100 (kW h)−1.

Discussion

This work demonstrates that anions could diffuse within the M-QCTF membrane with a very low energy barrier, leading to impressive flow battery performance. The findings can be applied more broadly to designing anion exchange membranes for other technologies such as CO2 electrolysers34 and ion-capture electrodialysis35. Although the anion diffusion constants within the developed membranes are approaching the theoretical limit within the bulk aqueous solution, we expect further improvements in apparent ion conductivity and diffusion activation energy barrier to be achievable by eliminating micropore tortuosity and creating aligned micropore channels with monodispersed pore size distributions.

Methods

Materials

4,4’-Biphenyldicarbonitrile (98%) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, P.R. China). 3-Hydroxy-[1,1’-biphenyl]-4,4’-dicarbonitrile (97%) was purchased from Shanghai Adamas Reagent Co., Ltd. (Shanghai, P.R. China). 1,3-Dibromopropane (98%), trimethylamine (2 M solution in tetrahydrofuran), trimethylamine (4.2 M (33 wt.%) solution in ethanol), N,N-dimethylformamide (99.9%, DMF), N,N-dimethylacetamide (99.8%, DMAc), trifluoromethanesulfonic acid (98%, TFSA), 1,3-dichloropropane (98%), 4,4-bipyridine (98%), acetonitrile (99.99%, ACN) chloromethane solution (1.0 mol/L (ca.5.7%) in THF) were purchased from Energy Chemicals (Shanghai, P.R. China). Potassium carbonate (K2CO3), potassium hydroxide (KOH), sodium fluoride, hydrochloric acid (36–38%) (HCl), methanol, ethanol, methyl tert-butyl ether (MTBE), acetone, ether and 2-propanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P.R. China). Other chemicals were purchased from domestic chemical suppliers and used as received. Deionized water was used throughout. Selemion® DSV and AMV membranes were obtained from Asahi Glass CO., LTD (Japan). Sustainion® X37-50 Grade RT membrane was purchased from SCI Materials Hub, and was pretreated by soaking in 1 M KOH for 24 h according to a reported procedure36.

Monomers and electrolytes synthesis

Positively charged dicyano monomer, namely 3-((4,4’-dicyano-[1,1’-biphenyl]-3-yl)oxy)-N,N,N-trimethylpropan-1-aminium bromide, that was synthesized according to the literature14. 1,3-Dibromopropane (127.2 g, 630.0 mmol) was stirred with trimethylamine solution (4.2 M in ethanol, 50 mL, 210.0 mmol). After stirring at room temperature for 12 h, the reaction mixture was diluted with 500 mL of methyl tert-butyl ether (MTBE) and the suspended solid collected by vacuum filtration. The solid was rinsed with MTBE and dried in vacuo to obtain trimethyl(3-bromopropyl)ammonium bromide (48.2 g, 88.0% yield). 3-Hydroxy-[1,1’-biphenyl]-4,4’-dicarbonitrile (4.4 g, 20 mmol), trimethyl(3-bromopropyl)ammonium bromide (10.44 g, 40 mmol) and K2CO3(8.29 g, 60 mmol) were suspended in 20 mL of anhydrous DMAc and heated to 80 °C under argon. After heating for 24 h, the reaction mixture had partially solidified and had turned white. The reaction was cooled to room temperature and then poured into crushed ice. The solid was collected by vacuum filtration, rinsed with acetone, then finally dried in vacuo to obtain the product (6.8 g, 85% yield). This material was used without purification in the following step. 1H NMR (400 MHz, DMSO-d6) δ 8.02 (s, 4H), 7.91 (d, 1H), 7.59 (s, 1H), 7.45 (d, 1H), 4.37 (t, 2H), 3.50 (m, 2H), 3.14 (m, 9H), 2.30 (m, 2H).

1,1′-Bis[3-(trimethylammonio)propyl]-4,4′-bipyridinium tetrachloride (BTMAP-Vi) was synthesized according to the literature37. 1,3-Dichloropropane (22.6 g, 200 mmol) and trimethylamine solution (2 M in THF, 75 mL, 150 mmol) were added and then reacted at room temperature for 7 days. Then, the mixture was filtered and washed with ethyl acetate (250 mL) to obtain (3-chloro-propyl)-trimethylammonium chloride (19.3 g, 75% yield). Finally, (3-chloropropyl)-trimethylammonium chloride (10.3 g, 60 mmol) and 4,4’-bipyridine (3.1 g, 20 mmol) in anhydrous DMF (25 mL) were refluxed for 1 h under argon. Then, the mixture was cooled, filtered, and washed with 2-propanol (250 mL), then finally dried in vacuo to obtain BTMAP-Vi (4.2 g, 42% yield). 1H NMR (400 MHz, D2O) δ 9.12 (d, 4H), 8.63 (d, 4H), 4.87 (t, 4H), 3.61 (m, 4H), 3.22 (s, 18H), 2.72 (m, 4H).

(Ferrocenylmethyl)trimethylammonium chloride (FcNCl) was synthesized according to the literature24. (Ferrocenylmethyl)dimethylamine (20 g, 82.3 mmol) and chloromethane (1 M in THF, 90 mL, 90 mmol) were combined in 50 mL of anhydrous acetonitrile under N2. The reaction mixture was stirred at room temperature overnight. The red orange precipitate was formed and collected by filtration. Then, the mixture was filtered and washed with ether (200 mL), then finally dried in vacuo to obtain FcNCl (20 g, 93% yield). 1H NMR (400 MHz, D2O) δ 4.47 (d, 2H), 4.39 (s, 2H), 4.35 (s, 2H), 4.24 (s, 5H), 2.91 (s, 9H).

Preparation of covalent triazine framework membranes

Triazine framework membranes were synthesized via a similar organic sol‒gel procedure reported in the literature14,26. Briefly, trifluoromethanesulfonic acid (TFSA, 1 mL) was added to a mixture of 4,4’-biphenyldicarbonitrile (0.12 g, 0.59 mmol) and 3-((4,4’-dicyano-[1,1’-biphenyl]-3-yl)oxy)-N,N,N-trimethylpropan-1-aminium bromide (0.08 g, 0.20 mmol) at 0 °C under stirring. The resultant red solution was continuously stirred until the solution became viscous and then diluted to 3 mL with TFSA. This solution was transferred and cast on a clean glass plate and heated at 70 °C under nitrogen flow for 4 h, and finally at 100 °C until the membrane was completely dry. The membrane was then peeled off by immersion in water. The thickness of membranes could be controlled by the solution concentration and was kept at around 20 μm in this work. Excess TFSA within the membrane was then removed by immersing the membrane samples in KOH solution (0.1 M) for 24 h, affording QCTF membranes. Counter-ions of the QCTF membrane were exchanged to Cl- by immersion in KCl solution (1.0 M) and finally washed with deionized water at least five times during 24 h to remove remaining salts. The theoretical ion exchange capacity (IEC) of the QCTF membrane was designed to be 1.0 mmol g−1.

Protonation and methylation of the QCTF membrane samples were then conducted to alter the membrane framework chemistry. The QCTF membrane sample was immersed in HCl solution (1 M) for 24 h to protonate the triazine rings and then exchanged to the Cl- form according to the above procedure, affording P-QCTF membrane in the Cl- form. The QCTF membrane sample was immersed in a mixture of methyl iodide (2 mL) and methanol (20 mL) for 48 h to methylate the triazine rings and then exchanged to the Cl- form, yielding the M-QCTF membrane, in the Cl- form.

Characterization

1H-NMR spectra were recorded in deuterated solvents on a Bruker AscendTM 400 MHz spectrometer. 13C NMR spectra of the QCTF membrane samples were recorded with a 400 MHz WB solid-state nuclear magnetic resonance spectrometer (Bruker AVANCE III 400 WB). 19F NMR spectra of KF aqueous solution (0.1 M) with or without membrane samples were obtained with a 600 MHz WB solid-state NMR spectrometer (Bruker AVANCE NEO 600 WB). Fourier transform infrared spectra (FTIR) of the QCTF membrane samples were acquired on an attenuated total reflectance FTIR (ATR-FTIR, Thermo Nicolet FTIR spectrometer, USA). X-ray photoemission spectra (XPS) of the QCTF membrane samples were obtained on an X-ray photoemission spectroscopy instrument (ESCALAB 250Xi). The zeta potential of the QCTF membranes was measured in KCl (1 mM in water) on a SurPASS 3 (Anton Paar, Austria). Thermal stability of the QCTF membrane samples was measured with a TGA Q5000 thermogravimetric analyzer (TA Instruments, USA) under a nitrogen atmosphere from room temperature to 800 °C at a heating rate of 10 °C min−1. The tensile strength of the QCTF membrane samples was evaluated on a dynamic mechanical analyzer (DMA Q800). Water contact angles of the QCTF membrane samples were determined using static contact angle measurements on an SL200B instrument (Solon Tech Co., Ltd., China). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were collected on a GeminiSEM 500 instrument (Carl Zeiss AG). UV‒visible spectra (UV‒Vis) were obtained using a UV‒Vis spectrophotometer (Persee, TU-1901). CO2 adsorption/desorption isotherms of the powdered QCTF samples were measured at 273 K on ASAP 2020 (Micromeritics, USA). Prior to measurement, the powders were degassed under a high vacuum at 120 °C for 24 h.

Ion exchange capacity (IEC)

IEC (mmol g−1) values of QCTF membrane samples were determined through the titration method21. The membrane samples in the Cl- form were dried in a vacuum oven at 80 °C for 24 h to remove residual water, and the dry weight (Wdry, g) of the membrane samples was recorded. Then, the membrane samples were immersed in 0.5 M Na2SO4 for 6 h. This was repeated 4 times to allow the complete exchange of Cl- with SO42-. The exchanged Cl- in the solution was titrated with AgNO3 solution (0.01 M) utilizing 5 wt% K2CrO4 as the indicator, and the volume of consumed AgNO3 solution (VAgNO3, mL) was recorded. Titrated IEC values of QCTF membrane samples were calculated according to Eq. (1)21:

$${IEC}=\frac{0.01\times {V}_{{AgNO}3}}{{W}_{{dry}}}$$
(1)

Water uptake (WU), swelling ratio (SR), and hydration number (λ) measurements

Membrane samples (4×4 cm2, in the Cl- form) in a square shape were dried at 80 °C under vacuum for 24 h, while dry weight (Wdry) and dry length (Ldry) were recorded. These samples were then immersed in deionized water for 24 h at temperatures ranging from 30 °C to 80 °C. The weight (Wwet) and length (Lwet) of wet membrane samples were recorded after quickly wiping dry the membrane surface. The hydration number (λ) refers to the number of water molecules per mole of charged functional groups. The mean values and error bars are derived from three parallel measurements on three different samples. The WU, SR, and λ were calculated according to Eqs. (2–4)38:

$${WU}=\frac{{W}_{{wet}}-{W}_{{dry}}}{{W}_{{dry}}}\times 100\%$$
(2)
$${SR}=\frac{{L}_{{wet}}-{L}_{{dry}}}{{L}_{{dry}}}\times 100\%$$
(3)
$$\lambda=\frac{1000\times {WU}}{18\times {IEC}}$$
(4)

Transference number measurement

The transference number of Cl- (t-) across the membrane was measured by a laboratory-made two-compartment diffusion cell. One compartment was filled with 20 mL of 10 mM KCl solution, and the other was filled with 20 mL of 1 mM KCl solution. Transmembrane current–voltage (I–V) curves were measured using a pair of Ag/AgCl reference electrodes at a voltage range from −0.1 V to 0.1 V on a Keithley 6487 picoammeter. The potential value at zero current on the I–V curve is the membrane potential (Em). Prior to measurements, the membrane specimen was immersed in a 5.5 mM KCl solution for 24 h. The transference number of Cl- (t-) was calculated according to Eq. (5)39:

$${E}_{m}=\frac{{RT}}{F}\left(1-2{t}_{-}\right){{\mathrm{ln}}}\frac{{a}_{{cis}}}{{a}_{{trans}}}$$
(5)

where t-, R, T, and F represent transference number of Cl-, molar gas constant, absolute temperature, and Faraday constant, respectively, while acis and atrans are the activity of the ionic species in the cis and trans chamber, respectively. For low concentration solutions, as in our case, we used concentration instead of activity.

Ion permeation measurements

Ion permeation and redox active species crossover measurements were conducted on a laboratory-made two-compartment diffusion cell. During measurements, both the donating and receiving solutions were vigorously agitated to eliminate concentration polarization. For ion permeation measurements, 20 mL of 0.1 M KCl solution was added to the donating side, while the receiving compartment was filled with 20 mL of deionized water. The solution conductivity of the receiving compartment was measured constantly with a conductivity meter (DDS-307A from Shanghai INESA Scientific Instrument Co., Ltd.). The KCl concentration of the receiving solution was determined according to a pre-established relationship between solution conductivity and KCl concentration.

For redox active electrolyte crossover measurements, 20 mL of 0.1 M BTMAP-Vi (or FcNCl) in 1 M KCl aqueous solution was added to the donating compartment, and 20 mL of 1 M KCl aqueous solution was added to the receiving compartment. To note, 0.01 M BTMAP-Vi was used for the crossover measurement of Sustainion® X37-50 membrane because of fast permeation and detection limit of our apparatus. Changes in the concentration of BTMAP-Vi or FcNCl on the receiving side were quantitatively determined via the UV–Vis spectrophotometry and the concentration was obtained from a standard line according to the measured absorbance at the characteristic peak. All measurements were conducted at room temperature. Aliquots were taken from the receiving compartment at constant intervals.

The permeability coefficient (P, cm2 s−1) of KCl, BTMAP-Vi or FcNCl through the membrane was calculated from Eq. (6)40:

$$P=\frac{{{{\mathrm{ln}}}}\;(1-\frac{2{C}_{t}}{{C}_{0}})(-\frac{{Vl}}{2A})}{t}$$
(6)

Where Ct (mol L−1) is the concentration of electrolyte in the receiving compartment at time t, C0 (mol L−1) is the initial concentration of electrolyte in the donating compartment, V (mL) is the volume of solution, l (cm) is the thickness of the membrane, A is the effective membrane area (2 cm2), and t is time (s). The mean values and error bars are derived from three parallel measurements on three different samples.

The membrane selectivity S (i.e., the ratio of permeability coefficient of charge-carrying ions to that of redox-active electrolyte) can be obtained using Eq. (7)32:

$$S=\frac{{P}_{i}}{{P}_{R}}$$
(7)

where Pi and PR are the permeability coefficients of the charge-carrying ion and redox-active electrolyte, respectively.

Cl- conductivity measurement

We used the standard four-point probe technique (performed on an Autolab® PGSTAT 302 electrochemical workstation) that operated in galvanostatic mode at a set temperature with a frequency range from 1 MHz to 100 Hz. Membrane samples were placed in a Teflon conductivity cell, and immersed in a deionized water bath. The Cl- conductivity was calculated according to Eq. (8)14:

$$\sigma=\frac{L}{T\times W\times {R}_{0}}$$
(8)

where σ (mS cm−1) is the Cl- conductivity, L is the distance between potential sensing electrodes (herein 1 cm), R0 is the measured ohmic resistance from a Nyquist plot (Ω), T is the thickness of the membrane and W is its width. The mean values and error bars are derived from three parallel measurements on three different samples. The activation energy for Cl- conduction can be derived from conductivity measurements at various temperatures according to Arrhenius Eq. (9)41:

$${{{\mathrm{ln}}}}\;\sigma=-\frac{{E}_{a}}{{RT}}+{{\mathrm{ln}}}\;{\sigma }_{0}$$
(9)

where Ea (kJ mol−1) is the activation energy for Cl- conduction, σ (mS cm−1) is the Cl- conductivity, σ0 (mS cm−1) is Arrhenius constant, T (K) is absolute temperature, and R (J mol−1 K−1) is molar gas constant.

Model construction

A method based on the Polymatic algorithm was employed to initiate the trimeric polymerization of the positively charged covalent triazine framework (QCTF). The QCTF chains contained 63 monomers. The molar ratio of charged monomer to neutral monomer depended on the pre-designed ion exchange capacity (IEC) value. Polymerization steps were then performed sequentially. Triazine rings were added among three reactive atoms with lengths less than 8 Å. Energy minimization and dynamic steps to maintain a relaxed structure were conducted throughout the simulated polymerization. The polymerization cycles were repeated until no pairs meeting the bonding criteria were identified. Any unreacted end groups after polymerization were terminated appropriately.

To initiate the MD simulation, we first generated the GROMACS topology file of the oligomer model using Sobtop (http://sobereva.com/soft/Sobtop). The atomic charges of the oligomer were reparametrized by the RESP method from Gaussian 16’s checkpoint file obtained at the level of B3LYP/def2TZVP. We ran an energy minimization first to generate the initial configuration for MD. Then, we equilibrated the system with two consecutive steps, the first in NPT ensemble at P = 1 bar and T = 300 K for 100 ps, and the second in NVT ensemble at T = 300 K for another 100 ps. As to polymers in the wet state, a certain number of water molecules, which was based on experimental water uptake of corresponding membranes, was added.

Ion transport mechanism

We ran a production MD simulation in NVT ensemble (T = 300 K) for 1 ns and collected the trajectory for MSD analysis. In all MD simulations, long-range electrostatic interactions in the periodic system were treated using the particle-mesh Ewald (PME) method. The target pressure and temperature were maintained by Berendsen barostat and the V-rescale thermostat, respectively. The timestep was set to be 1 fs. We used a polymer fragment containing a triazine center and two quaternary ammonium side chains to investigate the free energy surface associated with anion transport between the two nearest cationic quaternary ammonium sites. The fragment was first optimized using the semi-empirical GFN2-xTB method42 and the generalized Born surface approximation (GBSA) implicit solvation model43 implemented in xTB version 6.5.144. Based on the relaxed structures of these fragments, their GROMACS-style topology files were generated with an online tool developed by Li (https://jerkwin.github.io/prog/gmxtop.html). The parameters of bonded and non-bonded interactions linked with the fragments were described by OPLS-AA force field45. The atomic point charges of every atom in the fragments were reparametrized using Kollman’s restrained electrostatic potential (RESP) method46. To do so, we first ran single-point calculations of the optimized fragments using B3LYP/6-311G*47 level of theory and Grimme’s D3 empirical dispersion correction scheme with the Beck‒Johnson damping function48 and the integral equation formalism for polarizable continuum model (IEFPCM)49 of water in the Gaussian 16 program, then processed the formed checkpoint files of the Gaussian 16 jobs to generate the RESP parameters using the Multiwfn version 3.8 package27,50.

The exploration of two-dimensional free energy surfaces was conducted with classical molecular dynamics simulation in GROMACS version 202051 in conjunction with metadynamics for enhanced sampling using the interfacing PLUMED version 2.7.352 plugin. Guided by the GROMACS tutorial written by Lemkul53, we placed the fragment in the center of the simulation box and at least 1.0 nm from the box edge, filled the box with thousands of TIP3P water as the solvent and an appropriate number of Cl- ions to maintain the charge neutrality of the system, and prepared the GROMACS input files for metadynamics simulation in three consecutive steps: energy minimization, pre-equilibrium in the NVT ensemble with a velocity rescaling thermostat54, and pre-equilibrium in NPT ensemble with velocity rescaling thermostat and Berendsen barostat. In the production runs, set up a 30-ns NPT simulation with a timestep of 1.0 fs for each system. To facilitate the simulation of a simplified short-range movement of Cl- ions, we applied position restraints (using the “-DPOSRES” option) on all fragment atoms and all Cl- ions except for the one migrating in water.

Like the previous study on the migration of potassium ions within negatively charged triazine framework membranes14, we define two collective variables of metadynamics through the linear combination of r1 and r2 (a = r1 – r2, b = r1 – r2), where r1 and r2 are the distances between the moving Cl- ion and the two N atoms in the quaternary ammonium of two side chains, respectively. With the same spirit as previous work, proper distance ranges were applied to a and b, respectively, to ensure efficient sampling of the configuration space involving potential paths of Cl- ion migration from one quaternary ammonium site to another. For the Gaussian hills in metadynamics, we set HEIGHT = 0.3 kJ mol−1, SIGMA = 0.2, and PACE = 500 as the height, width, and frequency of bias addition, respectively. The system temperature is set to be 600 K (TEMP = 600).

PFG NMR measurement

To measure the diffusion coefficient of ions in the bulk aqueous solution and under confined ion channels within these membrane samples, pulsed-field gradient NMR (PFG‒NMR) measurements were conducted. The diffusion coefficients of F- in 0.1 M KF solution were obtained using 19F PFG‒NMR with a gamma radiation of 25,172 rad (s×Gauss)−1. To measure the diffusion coefficient of F- within membranes, the membrane sample was placed in NMR tubes and immersed in 0.1 M KF solutions. All measurements were carried out on a 600 MHz NMR spectrometer (Bruker AVANCE NEO) equipped with a 5-mm BBO static probe, which can generate the maximum gradient strength of ~23 T m−1. PFG echo profiles with 16 equal steps as a function of gradient strength (g) were obtained using the bipolar gradient PFG sequence. The maximum gradient strengths were carefully chosen to observe a sufficient decay of the echo profiles. The diffusion coefficient of F- was derived from the echo profiles according to the Stejskal‒Tanner Eq. (10)14:

$$I={I}_{0}\cdot {e}^{-D\cdot {\gamma }^{2}\cdot {g}^{2}\cdot {\sigma }^{2}\cdot \left(\Delta -\frac{\sigma }{3}\right)}={I}_{0}\cdot {e}^{-D\cdot B}$$
(10)

where, I and I0 are the echo height at the gradient strength (g, Gauss m−1) of g and 0, respectively, D (m2 s−1) is the diffusion coefficient, γ is the gyromagnetic ratio of fluoride nucleus [25,172 rad (s×Gauss)−1], σ (s) is the gradient time, Δ (s) is the diffusion delay time, and B values (s m−2) is the total contribution of these variables (γ, g, σ, and Δ). The mean values and error bars are derived from three parallel measurements on three different samples.

Redox flow battery experiments

Cell hardware was purchased from Fuel Cell Tech (Albuquerque, USA) with two pieces of POCO single serpentine pattern graphite plates, which were used to assemble the flow batteries. A piece of membrane was sandwiched between the positive and negative electrodes (the effective geometric area is 5 cm2) composing a stack of three sheets of carbon paper (SGL 39AA). The electrolytes were fed to the cell at a pumping rate of 60 rpm utilizing a Masterflex L/S peristaltic pump (Cole-Parmer, Vernon Hills, IL). To acquire the electrochemical polarization curve, the cell was charged to a certain state of charge (SOC) and then polarized by linear sweep voltammetry. The corresponding power density at the desired SOC (e.g., 10, 50, and ~100%) was derived from the current–voltage curve. Potentio-controlled electrochemical impedance spectroscopy (PEIS) was collected at specific SOC (10, 50, and ~100%) at frequencies ranging from 1 Hz to 10 kHz. Other details regarding the assembly of pH-neutral organic flow batteries can be found in previous reports from our group and the Liu group24,32.

A pH-neutral AORFB was configured with either the QCTF, P-QCTF, or M-QCTF membrane (or commercial anion exchange membranes, including Sustainion® X37-50, Selemion® DSV and Selemion® AMV membranes) as separator and BTMAP-Vi/FcNCl as the electrolytes. Typically, the anolyte consisted of 5 mL of 0.1 M BTMAP-Vi in 2.0 M KCl, and the catholyte consisted of 10 mL of 0.1 M FcNCl in 2.0 M KCl. For high-concentration cells, the concentration of FcNCl and BTMAP-Vi was raised to 0.5 M, while the supporting electrolyte was 2.0 M KCl and the electrolyte volume remained the same. The cells were charged and discharged with cutoff voltages of 1.2 V and 0.3 V, respectively. All the experiments were performed once in a glovebox filled with argon at a room temperature of 20–25 °C.