Self‐Assembled Liquid Crystal Interphase Enabling Long‐Life All‐Iron Redox Flow Batteries

ABSTRACT

Aqueous all‐iron redox flow batteries (AIRFBs) are an attractive avenue for large‐scale energy storage due to the safety and cost‐effectiveness of iron. However, the performance and durability of AIRFBs are limited by nonuniform Fe plating, hydrogen evolution and corrosion reactions at the anodes. Herein, a non‐ionic surfactant, polyethylene glycol cetyl ether (Brij 56), is introduced into the electrolyte for stabilizing the anode, which can self‐assemble on the iron surface to form a dynamic liquid crystal interphase. This oriented interfacial layer not only inhibits H₂O‐induced side reactions by efficient Fe²⁺ ion desolvation at the anode interface, but also modulates Fe²⁺ ion transport owing to its anisotropic properties, acting as a soft template for the uniform iron deposition. Consequently, the liquid crystal interphase increases the anode reversibility of the AIRFB, achieving a high Coulombic efficiency (CE) of 99.4% and an energy efficiency of 74.5% over 300 h (230 cycles) at 20 mA cm⁻². Furthermore, the AIRFB with liquid crystal interphase retained a CE of 98.2% at a high current density of 60 mA cm⁻² and delivered a high capacity of 26 mAh cm⁻². These findings highlight the potential of interfacial engineering to promote performance for the next‐generation energy storage technologies.

Surfactant molecules (Brij 56) self‐assemble on iron anode to form a liquid crystal interphase in all‐iron redox flow batteries, which not only inhibits side reactions by efficient Fe²⁺ ion desolvation, but also modulates Fe²⁺ ion transport owing to its anisotropic properties, acting as a soft template for the uniform deposition.

1. Introduction

Aqueous redox flow batteries (RFBs) are promising technologies for grid‐scale energy storage owing to their inherent safety, modular design, and facile scalability arising from the decoupling of capacity and power [1, 2]. In addition, compared with static batteries, the continuous flow of electrolytes in RFBs effectively alleviates battery polarization and decreases irregular temperature variation [3]. Among them, all‐iron redox flow batteries (AIRFBs) feature low‐cost, natural abundance, environmental benignity, and manageable crossover, due to the use of iron species in both the anolyte and catholyte [4, 5]. The AIRFBs employ the plating/stripping pair of Fe²⁺/Fe⁰ at the anode and the pair of Fe²⁺/Fe³⁺ at the cathode (Figure 1a), which enable a favorable theoretical cell voltage of 1.21 V [6]. Besides, the two‐electron transfer reaction and the high density of iron result in a high theoretical capacity (960 mAh g⁻¹) [7]. However, the poor reversibility of the anode hinders the long‐term stability of AIRFBs, as in the following challenges (Figure 1b): (i) the acidic anolyte (pH < 3) accelerates iron corrosion, resulting in deterioration of Coulombic efficiency (CE) [4]; (ii) the standard potential of Fe²⁺/Fe⁰ (−0.44 V vs standard hydrogen electrode, SHE) is lower than that of hydrogen evolution reaction (HER, 0 V vs SHE), thereby HER often occurs during the Fe plating process [8]; (iii) the competitive HER process continuously generates OH⁻, which leads to the formation of iron hydroxides that can clog the pore network of the carbon felt electrode or the ion‐exchange membrane [9]; (iv) non‐uniform iron deposition exacerbates the growth of dendrites, which are prone to detachment in flow electrolytes, resulting in substantial drop of capacity [10].

FIGURE 1.

(a) Scheme of all‐iron redox flow battery with Fe²⁺/Fe⁰ anode and Fe²⁺/Fe³⁺ cathode. (b) Illustrating the deposition behavior on carbon felt electrode in the pristine electrolyte. (c) Molecular structure of Brij 56 surfactant (C₁₆H₃₃‐(OCH₂CH₂)₁₀OH). (d) Illustrating the deposition behavior in BLC electrolyte. The zoomed‐in region depicts aligned surfactant molecules forming lamellar liquid crystal at the interface.

In recent years, interfacial engineering has emerged as a versatile approach, not only for mitigating parasitic reactions, but also for regulating the deposition morphology. Functional molecules can form protective layers on the electrode surface via adsorption, which have been widely employed to enhance the performance of aqueous batteries [11, 12]. For example, ligand‐functionalized layers improve anode reversibility by homogenizing the Fe²⁺ ion flux at the interface [13]. Interestingly, certain interfacially adsorbed surfactant molecules can generate in situ liquid crystal phases with ordered nanostructures and anisotropic fluidity [14]. Moreover, these unique characteristics have demonstrated their theoretical feasibility as soft templates for regulating electrode deposition [15]. Specifically, liquid crystals create self‐assembled and well‐aligned ion channels at the interfaces, continuously guiding ion migration and optimizing local concentration gradients [16]. We hypothesize that the dynamic liquid crystal interphase with a rational design can modulate the anode reaction kinetics and surface morphology in AIRFBs.

In this work, liquid crystal interphase was introduced to AIRFBs by adding a non‐ionic surfactant, polyethylene glycol cetyl ether (Brij 56), into the electrolyte at low concentrations. Brij surfactants contain a hydrophobic alkyl chain and a hydrophilic head, functioning as lyotropic liquid crystal molecules (Figure 1c) [17, 18]. We discovered that Brij 56 molecules can self‐assemble on iron surface to form a stable lamellar liquid crystal interphase (Figure 1d). This interfacial layer, as a template for iron deposition, promoted ion desolvation, suppressed side reactions and optimized Fe²⁺ ion transport environment, achieving a flat and compact iron deposition. Benefiting from the Brij‐based liquid crystal (BLC) interphase, the AIRFB displayed a significantly extended life of 300 h (over 230 cycles) with a high average CE of 99.4% and energy efficiency (EE) of 74.5% at 20 mA cm⁻², whereas the AIRFB using the pristine electrolyte showed a short life of 50 h with an inferior CE of 93.4%. Moreover, the current density and operation temperature of AIRFB with BLC electrolyte could be extended to 60 mA cm⁻² and −10° C, respectively, exhibiting practical application potential. This dynamically adaptable liquid crystal interphase offers low‐cost and facile regulation approach for electrochemical deposition.

2. Results and Discussion

Surfactant molecules typically form micelles in water [19], and the adsorption of surfactants at the interface can influence the size of micelle in the bulk solution [14]. Dynamic laser scattering (DLS) measurements were carried out to investigate the micelle size variation in bulk solution with and without adding Fe metal fragments. As shown in Figure 2a, the micelle diameter increases from 484 nm to 1.65 µm after the introduction of iron fragments, and the reduced surfactant concentration usually results in larger and less dense micelles in water [20]. This indicates that the Brij 56 molecules can self‐assemble at the Fe interface, leading to the decreased concentration in the bulk phase.

FIGURE 2.

(a) DLS tests of the Brij aqueous solution without and with Fe metal. The insets illustrate the size change of micelle after introducing the Fe metal in the solution. POMs of Fe surfaces immersed in (b) pristine or (c) BLC electrolyte. (d) Galvanostatic cycling of Fe||Fe symmetric flow cells at 20 mA cm⁻² and 2 mAh cm⁻². (e) Raman spectra and (f, g) in‐depth C 1s XPS analysis of electrode interphase after cycling for 10 h. (h) HRTEM images of liquid crystal. (i) Cryo‐TEM image of the liquid crystal on Cu grid mesh in aqueous condition.

The 0.5 M FeSO₄−2 M NH₄Cl aqueous solution (pH 2.1) with 0.1 mM Brij 56 molecule is denoted as the BLC electrolyte, while the control group without surfactant incorporation is denoted as the pristine electrolyte. To further characterize the structure of molecule alignment at the interface, Fe foils after immersing in the electrolytes were observed by polarized optical microscopy (POM). As shown in Figure 2b, the Fe surface treated with the pristine electrolyte shows no interference colors, suggesting the absence of anisotropic crystal structures. In contrast, the sample treated with the BLC electrolyte exhibits rainbow‐like interference fringes (Figure 2c), corresponding to characteristics of liquid crystal phase at the interface [21]. These observations confirmed the formation of a self‐assembled liquid crystal interphase on the electrode surface.

To evaluate the effect of the liquid crystal layer during the iron plating/stripping, galvanostatic cycling tests were conducted in Fe||Fe symmetric flow cells under a current density of 20 mA cm⁻² and a capacity of 2 mAh cm⁻². As shown in Figures 2d and S1, the cell employing the pristine electrolyte failed by circuit break after only 20 h (101 cycles). In contrast, the lifespan of the cell using the BLC electrolyte increased to 276 h (1419 cycles) with no pronounced increase of voltage polarization during the cycling, which was superior to the recently reported Fe symmetric cells (Table S1) [7, 8, 13, 22, 23, 24], demonstrating that the liquid crystal interphase significantly improved the cycling stability.

To confirm the interfacial composition, Raman spectra analysis was performed on electrodes disassembled from the symmetric cells after cycling. As presented in Figure 2e, the distinct signatures at 1137, 1301, and 1442 cm⁻¹ are observed on the BLC electrode, attributed to the C−O−C stretching vibrations [25], the CH₂ twisting and the CH₂ scissoring of the Brij surfactant [26], respectively, which are absent in the pristine sample. Furthermore, X‐ray photoelectron spectroscopy (XPS) C 1s depth profiling reveals the components of the interfacial species (Figure 2f, g). As for the pristine electrode, the C−O bond intensity diminishes after deeper etching. In comparison, the intensity of C−O bond strengthens in the BLC electrode, implying the adsorption of Brij surfactants via their metallophilic hydroxyl and ether groups [27]. These characteristic features provide unequivocal evidence for the adsorption of liquid crystal molecules at the electrode/electrolyte interface.

Contact angle measurements (Figure S2) reveal improved iron interface compatibility of BLC electrolyte (29.3°) compared to the pristine electrolyte (38.3°). This reduced interfacial tension suggests the formation of bilayer structures at the interface with hydrophilic heads facing the electrolyte (Figure 1d) [28, 29]. Figure 2h shows high resolution transmission electron microscope (HRTEM) images of Brij 56 liquid crystal that display periodical lattice fringes arising from the microphase segregation of alkyl chains [30]. Furthermore, cryogenic transmission electron microscopy (cryo‐TEM) was employed to study the liquid crystal structure in the aqueous condition, and the Cu grid mesh used in cryo‐TEM can act as a substrate for the self‐assembly of surfactants. A thin liquid crystal film is observed in the pore of mesh (Figure 2i), which may be peeled from the Cu surface, confirming the existence of liquid crystal layer [31]. Density functional theory (DFT) calculations were conducted to elucidate the interfacial interaction behavior. As shown in Figure S3, the adsorption energy of Brij 56 molecule on the Fe interface (E_ab  = −0.91 eV) is stronger than H₂O molecule (E_ab  = −0.46 eV), indicating that the surfactant molecules are likely to create a local waterless region near the electrode. Besides, the surfactant prefers to vertically bond to Fe with the hydrophilic head. The adsorption‐induced high local concentration and vertical configuration can drive the self‐assembly of surfactants with packed lamellar arrangement, leading to the formation of liquid crystal interphase [32, 33].

To determine the reversibility of plating/stripping process, cyclic voltammetry (CV) tests were carried out (Figure 3a). The CV curves in both the BLC and pristine electrolytes revealed similar shapes, indicating that Brij surfactants were not redox active during Fe plating/stripping process. Meanwhile, the BLC electrolyte exhibited reduced redox peak separation, and an anodic/cathodic area ratio (1.05) closer to unity than that of the pristine electrolyte (1.26), suggesting that the liquid crystal interphase effectively enhanced electrochemical reversibility [34]. Furthermore, the nucleation behaviors of Fe²⁺ ions were evaluated by chronoamperometry (CA) at −250 mV overpotential. Figure 3b showed that the pristine electrolyte yielded a higher response current at the beginning with a continuous decline as the test time prolonged, indicating an uncontrollable diffusion process and dendrite formation [22]. By contrast, the BLC electrolyte exhibited a stable current, owing to the optimized Fe²⁺ concentration gradient achieved by the adsorbed liquid crystal at the interface, which favored a dense and smooth Fe deposition [35].

FIGURE 3.

(a) CV curves in BLC and pristine electrolytes at 10 mV s⁻¹. (b) CA curves in BLC and pristine electrolytes at −250 mV overpotential. (c) In situ GC monitoring for HER in symmetric flow cells using BLC and pristine electrolytes. (d) Tafel curves of Fe foils tested in BLC and pristine electrolytes at 1 mV s⁻¹. (e) Calculated activation energies and (f) Fe²⁺ transference numbers in BLC and pristine electrolytes.

To investigate the influence of liquid crystal interphase toward HER, linear sweep voltammetry (LSV) was performed in acidic supporting electrolyte solution (pH ≈ 2), where the Fe²⁺ species were removed to avoid interference from metal plating (Figure S4) [36]. After the addition of Brij, the onset potential of HER shifted from −1.12 to −1.25 V (vs saturated calomel electrode, SCE) and the response current decreased, indicating that the liquid crystal layer reduced the HER activity. Furthermore, in situ electrochemical experiments were performed to monitor the H₂ evolution in Fe||Fe symmetric flow cells cycled at 20 mA cm⁻² and 13.3 mAh cm⁻² (Figures 3c and S5). During the operation, the H₂ accumulated in the headspace of electrolyte tanks, and a N₂ flow with fixed flow rate carried the evolved H₂ to gas chromatography (GC) for further analysis. The H₂ signal of the pristine cell is strong, indicating that the Fe²⁺ reduction process was accompanied by continuous HER [9]. In contrast, little H₂ was detected in the cell with BLC electrolyte, demonstrating that the liquid crystal interphase effectively suppressed the HER side reaction during Fe plating/stripping process.

The iron corrosion was analyzed by Tafel plots seen in Figure 3d, and the corrosion currents were calculated by Tafel fit. The surfactant addition shifted the corrosion potential positively (from −0.580 to −0.559 V vs. SCE) and reduced the corrosion current from 0.82 to 0.31 mA cm⁻², confirming that liquid crystal interphase suppressed iron corrosion kinetics. Moreover, the variable‐temperature impedance spectra (Figures 3e and S6) of the Fe||Fe symmetric cells revealed that the Fe²⁺ activation energy dropped from 37.4 kJ·mol⁻¹ in the pristine system to 29.2 kJ·mol⁻¹ in the BLC system, indicating that the Brij surfactants accelerated the desolvation kinetics of Fe²⁺ and enhanced charge transfer at the electrode/electrolyte interface [37]. In addition, the Fe²⁺ transference number increased from 0.32 to 0.63 after introducing liquid crystal interphase (Figures 3f and S7), as the long‐chain molecular structure hindered bulky anion migration via steric hindrance [16]. Consequently, the self‐assembled lamellar liquid crystal interphase suppressed concentration polarization, optimized the interfacial kinetics and ion transport properties of the iron anode, ensuring favorable conditions for uniform electrodeposition.

Full‐cell AIRFBs (Fe²⁺/Fe⁰||Fe²⁺/Fe³⁺) were evaluated at 20 mA cm⁻² with a constant charging capacity (13.3 mAh cm⁻², charging 40 min). The AIRFB using BLC anolyte enabled significantly lower voltage polarization for over 300 h (Figure 4a), benefiting from enhanced reversibility of plating/stripping. As shown in Figure 4b, the AIRFB with BLC anolyte exhibited excellent stability of long‐term cycling, reaching high average CE of 99.4% and EE of 74.5% during 230 cycles. This result outperformed that of the AIRFB using the pristine anolyte, which exhibited a significantly reduced 39 cycles with average CE of 93.4% and EE of 69.6%, due to the excessive polarization. Notably, the rapid pH increases in the pristine system confirmed competitive HER (Figure S8), which promoted the formation of iron hydroxide on the electrode and ion exchange membrane, ultimately leading to charge‐transfer blockage and battery failure [13]. Furthermore, the BLC cell maintained stable plating/stripping plateaus and high discharge capacity of 13.3 mAh cm⁻² (Figure 4c), whereas the pristine cell underwent a sharp increase in overpotentials after 39 cycles (Figure 4d). The improved cyclability of the AIRFB with BLC anolyte was attributed to the liquid crystal interphase's ability to mitigate HER, suppress corrosion, and regulate deposition on the anode, which will be elucidated later.

FIGURE 4.

(a) Voltage‐time curves, (b) efficiencies and discharge capacities of AIRFBs using pristine and BLC anolytes at 20 mA cm⁻² with a charge capacity of 13.3 mAh cm⁻² at room temperature. Capacity‐voltage curves at different cycles using (c) BLC and (d) pristine anolytes. (e) XRD pattern evolutions of anodes at different cycles. (f) POM image of iron‐deposited carbon fiber after cycling in AIRFB using BLC anolyte. SEM images of iron‐deposited carbon fibers after cycling in AIRFBs using (g) pristine and (h) BLC anolytes.

Furthermore, the growth trends of anode crystal planes were analyzed by x‐ray diffraction (XRD). As displayed in Figure 4e, the intensity ratio of I₍₁₁₀₎/I₍₂₁₁₎ decreased from 4.0 to 3.0 after 39 cycles in the pristine electrolyte, and the increased high‐index plane (211) was conducive to HER [38]. In comparison, the Fe deposition in the BLC system preserved a high ratio of I₍₁₁₀₎/I₍₂₁₁₎ (from 4.5 to 4.6) after 150 cycles, demonstrating that the lamellar liquid crystal acted as a template that preferentially guided iron deposition along the (110) plane [39]. In addition, the impact of surfactant on the ion‐exchange membrane was also investigated. As shown in Figure S9, the POM image of Nafion 115 in BLC electrolyte exhibits the absence of liquid crystal. The uncharged feature of non‐ionic surfactant reduces the electrostatic interaction with charged groups in Nafion [40], thus avoiding the blockage of ion transport channels in the membrane. The POM image of iron‐deposited carbon fiber cycled with BLC anolyte still displays the interference colors (Figure 4f), because the orderly‐packed configuration endows liquid crystal interphase with exceptional rheological properties, which can withstand the shear force of the flowing electrolyte [41]. Therefore, the in situ formed dynamic liquid crystal layer surpasses the traditional approach via substrate‐controlled epitaxial growth [42], maintaining integrity and efficacy during long‐term tests.

The morphology evolution of iron‐deposited anodes during cycling was observed via scanning electron microscopy (SEM). In the initial deposition stage, small Fe dendrites were observed on the anode in pristine anolyte (Figure S10a), and the dendrites kept growing during the plating. The large specific surface area of Fe dendrites provided more reactive sites for H₂ evolution [43]. In addition, the micro‐bubbles generated from competitive HER disrupted the deposition process, resulting in uneven and loose structure [44]. Therefore, disordered dendrites and voids dominated on the anode after cycling in pristine anolyte (Figure 4g). In contrast, the Fe grains in BLC‐treated anode showed uniform growth tendency (Figure S10b), and remained a compact and smooth Fe surface even after cycling (Figure 4h), implying that the liquid crystal interphase effectively restrained the parasitic reactions and regulated the Fe²⁺ diffusion process. The morphologies of anode substrates after stripping were further analyzed. The carbon fiber in the pristine anolyte revealed abundant dendrite residues (Figure S11a). This indicated that the dendrites were easily oxidized and transformed into electrically isolated “dead iron”, compromising anode reversibility. In comparison, the BLC system showed minimal residues after stripping (Figure S11b). Consequently, the liquid crystal interphase effectively suppressed side reactions and ensured reversible iron plating/stripping.

The surfactant concentration also influences the performance of AIRFBs (Figure S12). Specifically, a higher Brij 56 concentration of 0.5 mM fostered an excess of surfactant micelles in the electrolyte that caused higher charge transfer resistance as evidenced by electrochemical impedance spectroscopy (Figure S13a) [45], hastening battery degradation with dropped average EE (71.8%) and battery life (147 cycles). When the surfactant concentration was as low as 0.02 mM, liquid crystals could not form a continuous layer at the interface, as presented in POM image (Figure S13b), which led to limited lifespan (50 cycles) and low average CE (94.4%), due to the corrosion and HER in the uncovered zones [36]. Therefore, the surfactant concentration plays an important role in achieving the appropriate liquid crystal structure. Interestingly, it seemed to be a general phenomenon that the non‐ionic surfactants formed liquid crystal interphase on electrodes under the right conditions. For instance, the POM image of Fe surface immersed in the Tween 80 surfactant‐based electrolyte confirmed the in situ formation of liquid crystals (Figure S14), and the AIRFB with Tween 80 anolyte delivered a high CE of 99.2% at 20 mA cm⁻² during the long‐term cycling. Hence, the formulation of liquid crystal interphase presented broader applicability.

Furthermore, the AIRFB with BLC anolyte showed improved rate performance at current density of 10−60 mA cm⁻² (Figure 5a), demonstrating the high reversibility and stability. To verify the positive impact of liquid crystal interphase on practical applications, higher current densities with a higher constant charging capacity (26.6 mAh cm⁻²) were applied to the long‐term cycling of AIRFBs. As shown in Figures 5b and S15, during 140 cycles (up to 185 h) at 40 mA cm⁻², the cell with BLC anolyte delivered an average CE of 98.7% and EE of 64.7% with a higher capacity output of 26.3 mAh cm⁻². In addition, a current density of 60 mA cm⁻² was applied to the cell (Figures 5b and S16), which delivered CEs of 98.2% with EEs of 60.1%. These superior performances of the AIRFBs using BLC anolyte were attributed to minimized side reactions and effective deposition regulation at the anode interface. Even after cycling with higher current densities, Fe deposits still show preferential tendency for uniform growth and form a compact surface without dendrites and by‐products (Figure S17). Compared with previously reported Fe²⁺/Fe⁰ anodes in AIRFBs (Figure 5c) [9, 22, 23, 46, 47], the battery with BLC electrolyte exhibited superior performance including increased energy efficiency, enhanced areal capacity, and high current density, proving the remarkable advantages of liquid crystal interphase strategy for enhancing performance.

FIGURE 5.

(a) Rate performance of AIRFBs using pristine and BLC anolytes. (b) Efficiencies and discharge capacities of AIRFBs with BLC anolytes at 40 and 60 mA cm⁻². (c) Comparison of AIRFBs using Fe²⁺/Fe⁰ anodes in published literature and this work. (d) Efficiencies and discharge capacities of AIRFBs with BLC anolytes at −10 °C and 20 mA cm⁻². Simulations of electric field for (e) pristine and (f) BLC interfaces, and simulations of current distribution for (g) pristine and (h) BLC interfaces during Fe deposition, where the white regions at the bottom represent electrodes and the gradational color regions represent electrolytes, and the dashed lines represent the initial surface of anodes. (i) Mechanism of the templated deposition process of Fe at liquid crystal interphase.

The AIRFB using the BLC anolyte demonstrated excellent performance in a wider range of temperatures (Figures 5d and S18), delivering a stable CE of 98.3% and EE of 69.5% for 188 cycles (246 h) at −10° C. Because it was difficult for water molecules to penetrate the hydrophobic tail area of liquid crystal layers, which prevented the formation of ice crystals, thus maintaining the continuity of ion transport at the interface [48]. In addition, at higher temperature (50 °C), the AIRFB using BLC anolyte showed better thermodynamic stability, which maintained a high CE of 99.0% and an EE of 75.8% (Figure S19), owing to enhanced corrosion resistance and reduced HER activity [14]. These properties of lamellar liquid crystal interphase endow AIRFBs with extended operational temperature range for practical applications.

Molecular dynamics (MD) simulations were performed to verify the influence of Brij 56 on the coordination structure of Fe²⁺ at electrode interface (Figure S20). As presented in Figure S21, the analysis of coordination numbers (CNs) reveals that Brij 56 molecules participate in the solvation structure of Fe²⁺ at the interface, decreasing the coordination number of Fe²⁺‐H₂O. This implies that the Brij 56 changes the solvation environment of Fe²⁺ ion by displacing active H₂O molecules [16, 36]. The interaction behavior among Fe²⁺ ions, surfactant and water molecules were further investigated by DFT calculations (Figure S22). The binding energy between the Fe²⁺ and H₂O molecule in primary solvation shell is −0.58 eV, while the hydroxyl group in the Brij 56 shows higher interaction intensity with Fe²⁺ ion (−0.78 eV). The stronger affinity guarantees that the oxygen species in Brij 56 are able to strip the H₂O molecules out of the Fe²⁺ solvation shell [27], consistent with the MD results, which contributes to reducing Fe²⁺ desolvation energy barrier and facilitating interfacial Fe²⁺ ion transport. Therefore, the liquid crystal interphase effectively modulates the solvation structure of the Fe²⁺and mitigates the side reactions, promoting reversible iron deposition.

Finite element modeling (FEM) was conducted to simulate the electric field, current density distribution, and dynamic variation of anode morphology during the Fe²⁺ deposition process, using convex nuclei as the initial anode morphology. In the pristine electrolyte, the potential gradient distorts around the protuberances due to the “tip effect”, where the electric field intensifies at sharp edges (Figure 5e) [49]. Once introducing surfactants, they are self‐assembled on the electrode surface and form evenly distributed interphase (Figure S23). Encouragingly, because of the desolvation interaction with Fe²⁺ ions and highly oriented nanostructures, the uniform liquid crystal layer effectively creates even electric field at the electrode/electrolyte interface (Figure 5f) [50]. Simultaneously, Fe²⁺ ion diffusion is investigated by the current distribution simulation. In the pristine system, the uneven electric field leads to localized current accumulation at the tips of protrusions, while adjacent “valleys” receive low current density. This tip effect drives dendrite growth (Figure 5g), compromising anode reversibility [22]. Conversely, the BLC system achieves a uniform current density distribution across the entire surface due to the even interfacial electric field (Figure 5h), which ensures a homogeneous iron deposition process, benefiting for promoting cell efficiency.

Based on these results, a comprehensive deposition mechanism of Fe²⁺ ions on the liquid‐crystal‐modified anode is proposed (Figure 5i). Initially, Brij 56 surfactant molecules are spontaneously assembled on the iron anode surface along a certain orientation, forming the lamellar liquid crystal with periodic structure. The high molecular weight of surfactant molecules provides steric hindrance protection of iron anode against corrosion by acidic electrolyte [51]. The anisotropic diffusion coefficients within the liquid crystal layer enhance transport kinetics, which minimizes local concentration polarization and optimizes the electric field distribution [52]. Meanwhile, the oxygen‐containing functional groups in surfactant reconfigure the Helmholtz layer by expelling active H₂O molecules from the solvated structure of [Fe(H₂O)₆]²⁺, thereby suppressing the undesired HER [53]. As soft templates, the liquid crystal molecules form anisotropic ion diffusion channels that restrict Fe²⁺ ion 2D diffusion on the surface of the anode [54], avoiding dendrite nucleation and inducing layer‐by‐layer Fe growth along desired orientation. These synergistic effects of liquid crystal interphase significantly enhance the durability and reversibility of anodes.

3. Conclusion

In summary, a novel soft‐templating strategy was developed to improve the anodic performance of all‐iron redox flow batteries. The characterizations confirmed that the Brij 56 surfactant molecules spontaneously self‐assemble at the anode interface, forming in situ lamellar liquid crystal bilayers. This mesomorphic phase not only effectively displaces active water in the Fe²⁺ solvation shell by O‐containing functional groups, reducing the desolvation energy barrier, but also suppresses hydrogen evolution and corrosion, acting as a protective layer. In addition, the highly oriented arrangement of the liquid crystal achieved a high ion transference number of 0.63 and induced homogeneous Fe²⁺ migration, which inhibited iron dendrite formation and enhanced the reversibility of iron plating/stripping in acidic media. Leveraging these advantages, the AIRFB using BLC anolyte delivered a high average CE of 99.4% and EE of 74.5% during 230 cycles (up to 300 h) at 20 mA cm⁻², superior to the cell using the pristine anolyte, which experienced inferior efficiencies and limited cycle life. Notably, the AIRFB with BLC anolyte maintained a CE of 98.2% with a high capacity over 26 mAh cm⁻² at a remarkable current density of 60 mA cm⁻². Even at a sub‐zero temperature of −10° C, the BLC system still exhibited notable stability with 69.5% EE over 240 h. Our results suggest that low‐cost and versatile liquid crystals are promising interfacial modification materials for advancing energy storage systems.

Author Contributions

Zhikun Liu: conceptualization, methodology, software, data curation, investigation, validation, visualization, writing – original draft, writing – review and editing, and formal analysis. Jing Cui: validation. Han Shi: validation. Jiaxin Sun: validation. Jing Hou: validation. Peng Kang: conceptualization, validation, supervision, funding acquisition, project administration, resources, and writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72884‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.