Enabling Graphene-Oxide-Based Membranes for Large-Scale Energy Storage by Controlling Hydrophilic Microstructures
Graphene-based membranes have been explored in different energy and environmental applications. The 2D nanochannel structure and low frictional water flow inside micrometer-thick graphene oxide (GO) laminates make them attractive candidates for large-scale energy storage systems. Through the design of large size differences between charge carriers and redox species, GO membranes can achieve high rejection and high ionic conductivity in various solutions. Furthermore, tailoring the degree of oxidation or using bacterial cellulose as the hybrid component shows the flexibility to tune the microstructure and ionic transport of GO-based membranes. As a separator, GO membranes can achieve a rejection of >95% for a number of active species, and the ionic conductivity can reach 1.7 3 10 À2 S cm À1 in 1 M H 2 SO 4 electrolytes. Redox flow batteries with GO-based separators show a charge-discharge profile similar to that of commercial Nafion 212 and achieve a stable cycling performance with a high Coulombic efficiency of $98%.
INTRODUCTION<!>The Bigger Picture<!>RESULTS AND DISCUSSION<!>2À<!>Permeation Test and Ionic Conductivity<!>Electrochemical Performance<!>Conclusion<!>EXPERIMENTAL PROCEDURES
<p>Securing a sustainable and environmentally friendly energy future relies on efficient utilization of renewable sources, such as solar or wind energy. Considering the intermittent nature of renewable energy, large-scale energy storage technology plays a critical role in efficient storage of the electricity from these sustainable sources. Rechargeable batteries, especially redox flow batteries (RFBs), stand out as a promising candidate for large-scale energy storage because of the advantages of decoupled power and energy control, the minimal maintenance required, and non-flammable features in aqueous electrolytes. 1 Although much progress has been made in identifying redox species for potential large-scale RFBs, another key limitation lies in finding low-cost and high-performance separators for their further development. For RFB separators, the essential requirement is achieving high ionic conductivity with minimal crossover at low cost. 2 Generally, the membrane can be categorized into two major types: polymeric or inorganic solid electrolyte. [1][2][3][4][5][6][7][8] Nafion is a typical polymer-based membrane and has been widely used as a proton conductor in commercial RFBs given its high proton conductivity and chemical stability as well as reasonable mechanical strength. But Nafion can only conduct cations (proton) because of the electrostatic repulsion effect of sulfonate substituents. 5,[9][10][11] In addition, the production process of Nafion is fairly laborious and expensive, accounting for almost 40% of the stack cost in RFBs, and the crossover problem is sometimes inevitable in the case of cationic</p><!><p>The increasing development of large-scale energy storage technology is largely driven by the critical need for efficient storage of renewable energy generated from intermittent resources. The redox flow battery (RFB) is regarded as a promising technology for potential gridscale storage. However, membranes separating the catholyte and anolyte are still a limiting factor for further development of next-generation RFBs. Here, graphene oxide (GO)based membranes show potential as a new class of RFB separators because of their precise sizeexclusion properties, mechanical and chemical stability, and flexible functionalization.</p><p>This work systematically studied the ion-transport behavior of three types of GO-based membranes by controlling hydrophilic microstructures. Because of their precise ion selectivity and high ionic conductivity in aqueous solutions, GO-based membranes represent a new type of low-cost yet highperformance separators for largescale energy storage systems. electroactive species. 4 For inorganic solid electrolytes, the crossover of redox species can be prevented, but the low ionic conductivity limits the current density. [12][13][14][15] Therefore, alternative membranes with a facile production process and competitive ionic conductivity are desired for further development of RFBs.</p><p>Inspired by recent works using graphene oxide (GO) membranes in ionic and molecular sieving applications, [16][17][18] there may be a significant opportunity to develop a fundamentally new class of membranes for RFBs. First, long-term stability of GO membranes in aqueous solutions can be achieved by the crosslinking effect of multivalent cations (Al 3+ ) 19 or through the use of polymers, such as bacterial cellulose (BC), as a hybrid component. 20 Second, GO membranes have a tightly stacked lamellar structure with tailorable surface charges, so size exclusion and charge repulsion can play an important role in preventing the crossover of redox species. 21,22 Last, water molecules or hydrated ions allow fast transport along the hydrophobic region formed by tightly stacked GO sheets, which provides high ionic conductivity. 23 However, although the interlayer space is tunable with the assistance of functional groups or other components such as salts, molecules, or nanofibers, [24][25][26] efficient ion selectivity enabled by size screening of electroactive materials still needs to be further explored. Moreover, the influence of oxygen groups of GO sheets on membrane microstructure and ion transport is also unclear.</p><p>In this research, by taking advantage of the size exclusion and impeded permeation of water in GO membranes, we propose and investigate in detail a proof-of-concept GO-based membrane for RFBs. Regarding potentially green and cost-efficient energy storage, organic active materials have stood out as promising redox species in recent years. [27][28][29][30][31][32][33][34] Here, the large size difference between organic active materials and charge carriers could enable GO membranes to achieve high rejection of active species and high ionic conductivity simultaneously, as depicted in Figure 1A. Because GO sheets are chemically stable in various environments, the stacked GO membrane is potentially applicable for diverse redox molecular species in neutral, acidic, or alkaline solutions. Furthermore, by controlling the degree of oxidation of GOs, we are able to adjust the microstructures of GO-based membranes and the ion transport inside the membrane. Finally, we demonstrate a prototype RFB by using GO membranes as separators and utilizing riboflavin-5 0 -phosphate sodium salt (FMN-Na) and potassium ferrocyanide as redox species in alkaline electrolytes and achieve stable cycling with a capacity retention of >90% after 300 cycles and a Coulombic efficiency as high as $98%.</p><!><p>Design Principle and Characterization First, we take advantage of the size difference between redox species and charge carriers to achieve high ionic conductivity and high selectivity. As schematically illustrated in Figure 1A, small ions (cations such as H + , Li + , Na + , and K + or anions such as Cl À , OH À , and SO 4</p><!><p>) can transport through the membrane quickly; meanwhile, the large redox species (catholytes such as K 4 Fe(CN) 6 and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl [4-HO-TEMPO], or anolytes such as FMN-Na and methyl viologen [MV]) can be totally blocked by size exclusion. To show the size difference between candidate redox species and possible charge carriers, we summarize the size of various ions and molecules in Figures S1 and S2 and Table S1. Moreover, GO sheets have a significant number of oxygen groups on the surface, as shown in Figure 1B. Changing the amount or type of oxygen groups (epoxide, hydroxyl, carbonyl, or carboxylic) can influence the microstructure of GO membranes, which could further affect the ion-transport behavior and rejection efficiency of GO membranes. 35 In this work, we demonstrate the potential of GO membranes as separators in RFBs and reveal the relationship between the microstructure and ion transport. Three types of GO-based membranes are selected to highlight the influence of oxygen groups and polymers as components: LGO, HGO, and HGO-BC composite membranes. LGO is a lowly oxidized graphene oxide, HGO is a highly oxidized graphene oxide, and BC is a bacterial cellulose nanofiber, a typical biomass material that is organized by poly-glucan (C 6 H 10 O 5 ) n chains. 36 The reasons for choosing BC as the component to interact with GO are that (1) BC can form an interconnected nanofibrous network, (2) BC can be produced in large amounts via an industrial-scale microbial fermentation process, and (3) BC possesses abundant surface hydroxyl groups and could provide water-holding capacity and facilitate chemical modification. 36 Although a number of polymers, such as polyvinyl alcohol or chitosan, are used to enhance the mechanical strength and water durability of GO membranes, the advantage of BC is that it provides abundant hydroxyl groups to facilitate the hydration process and meanwhile achieves long-term stability. Figures 1C and 1D show the schematic and photo of a prototype RFB with GO membranes as separators using K 4 Fe(CN) 6 and FMN-Na as positive and negative electrolytes. As shown in Figure 2A, LGO is lowly oxidized, and its dispersion is black in color. HGO is highly oxidized and its dispersion appears to be dark brown. Because HGO has more oxygen groups, it shows better dispersion. After being placed in an ambient environment for 1 week, most LGO sheets sank to the bottom of the vial, whereas HGO still remained a good dispersion. Figures 2B, 2C, and 2D show the cross-sectional morphologies of LGO, HGO, and HGO-BC composite membranes, respectively. Clearly, we can see that a tightly packed GO sheet lamellar structure was formed in the HGO membrane, whereas the lamellar structure of the LGO membrane seemed to be more disordered and loose, which might reduce the mechanical stability and ion diffusion. For HGO-BC composite membranes, the stacked cascading structure was finely maintained with the interaction of BC fibers and GO sheets, further enhancing the strength of GO membranes.</p><p>To characterize the differences of these three membranes in terms of oxygen groups and microstructures, we measured thermogravimetric analysis (TGA) curves, X-ray diffraction (XRD) patterns, and X-ray photoelectron spectra (XPS), as displayed in Figures 2E, 2F, and 2G. TGA curves showed that the oxygen content in these three membranes had the trend HGO-BC > HGO > LGO, indicating that more oxygen groups existed in HGO than in LGO. For BC, almost 80% mass was lost above 350 C, which explains the highest O/C ratio of HGO-BC membranes. From the XRD measurements, it was found that LGO and HGO membranes showed different interlayer space in the dried state. With the higher O/C ratio, HGO ($0.9 nm) presented a larger interlayer space than LGO ($0.8 nm). However, HGO-BC membranes after drying in air had the largest interlayer space of $0.98 nm as a result of the influence of BC. The peaks labeled with an asterisk were related to the BC component. On the other hand, when GO membranes were immersed in water or aqueous solutions, the hydration process could increase the interlayer space. After immersion in deionized water for 30 min, the interlayer space of HGO membranes increased to approximately 1.3 nm. Because only one peak at $7 occurred in the XRD pattern, it was speculated that HGO membranes retained a homogeneous structure after hydration. LGO membranes also showed a homogeneous structure after hydration. The peak at 7.5 in the XRD pattern of LGO membranes after soaking in water corresponds to an interlayer space of $1.2 nm. Therefore, the peak situation of LGO and HGO after hydration was consistent with the original peaks in the dried state. This can be attributed to the fact that LGO samples have a smaller amount of oxygen groups, leading to the lower degree of hydration in LGO membranes. For HGO-BC membranes, the interlayer space after hydration was increased to $1.42 nm without any other peaks, which was in line with the HGO membranes. The interlayer space of HGO-BC membranes can be tuned within a certain range via adjustment of the mass ratio of HGO/BC (Figure S3), which enables control of the ion-transport behavior. We used XPS measurements to further investigate the O/C ratios and oxygen groups of different GO membranes, as displayed in Figure In accordance with our design, the HGO-BC composite membrane showed the highest O/C ratio because of the additional contribution of oxygen groups from the BC component (Figure S5). Intrinsically, LGO sheets were less oxidized, so the O/C ratio of the LGO membrane was lower than that of the HGO membrane, which was also confirmed by the XPS result. Furthermore, Figure 2G shows the C 1s XPS spectra of LGO, HGO, and HGO-BC, confirming that hydroxyl groups played an important role in determining the O/C ratio and hydrophilic microstructure.</p><p>In order for GO membranes to be used as separators in RFBs, another important issue is mechanical stability in aqueous electrolytes. As reported in the literature, GO membranes filtered by a porous anodized aluminum oxide filter were highly stable in water because of Al 3+ crosslinking. 19 Meanwhile, polymer components such as BC can also improve the mechanical stability of GO membranes in water. 20 To compare the mechanical stability of LGO, HGO, and HGO-BC membranes, we soaked them in water for different periods of time. As displayed in Figure S6, when immersed in water for 3 hr, all of them maintained intact membrane morphology in water, but the LGO membrane immediately broke into several small pieces when we used tweezers to take it out. In contrast, HGO and HGO-BC composite membranes could be easily handled with tweezers multiple times without any structural damage and remained stable for a long time because of the interactions between GO sheets and the Al 3+ or BC skeleton. Furthermore, the water durability and chemical stability of GO membranes in 1 M KCl, H 2 SO 4 , or KOH aqueous solutions were also investigated, as shown in Figure S7. Similar to the situation in water, LGO membranes broke into several small pieces when tweezers were used to handle them, whereas HGO and HGO-BC still maintained their integrity.</p><!><p>We further evaluated the permeation behavior and the ability of GO-based membranes to prevent the crossover of redox species. We selected five redox species candidates, which were usually used as catholytes or anolytes in neutral, acidic, or alkaline conditions, to conduct the permeation test for HGO membranes (Figure S8). The permeation experiments were conducted in an H-shaped apparatus. Five aqueous solutions (K 4 Fe(CN) 6 , FMN-Na, 4-HO-TEMPO, MV, and anthraquinone-2,7-disulfonic acid disodium [AQDS]) with certain concentrations were injected into the left chamber as feed solutions and the same amount of deionized water was injected into the right chamber as a blank electrolyte. For comparison, permeation experiments with HGO-BC and Nafion 212 membranes as controls were also conducted with aqueous FMN-Na solutions (Figure S9). During the permeation experiments, we found a slight color change in the permeate solutions, implying diffusion of redox species from left to right. The concentration change of redox species in the permeate side was monitored by ultraviolet-visible (UV-vis) absorption spectroscopy, as shown in Figure S10. To quantify the change in concentration from the absorption intensity, we obtained standard UV-vis absorption spectra of redox species and the corresponding calibration curves. Figure S11 shows the UV-vis absorption spectra and corresponding calibration curves of five selected active species. The concentrations of permeate solutions were determined on the basis of the calibration curve. By comparing the concentration changes in permeate and feed solutions, we calculated the rejection values of the redox species for these membranes, which can be used as a parameter to reflect the selectivity of membranes for different species. The formula for rejection is as follows: rejection = 1 À solute concentration in permeate solution solute concentration in feed solution :</p><p>The rejection tests for different species for HGO, HGO-BC, and Nafion 212 membranes are summarized in Figure 3A. For HGO membranes, all rejections for the redox species were >99%, which can be ascribed to the large size of organic molecules. In terms of rejection of FMN-Na, the HGO membrane was comparable with Nafion 212. The crossover of FMN-Na for HGO-BC membranes was more serious than for HGO because of the larger interlayer space, which was confirmed by the color change of the permeate solution, but the rejection of HGO-BC membranes also reached $99.5% after 72 hr.</p><p>To understand the ion-transport behavior and influence of the membranes' microstructure, we measured the ionic conductivities of LGO, HGO, HGO-BC, and Nafion 212 membranes in 1 M H 2 SO 4 , 2 M KOH, and 2 M KCl aqueous solutions (Figure 3B). The ionic conductivities were calculated on the basis of the thickness of the membrane (L), contact area (A), and ohmic resistance (R s ) from electrochemical impedance spectroscopy (EIS); detailed information can be found in Figure S12. The EIS results for various electrolytes are summarized in Figure S13. The ionic conductivity of Nafion 212 as a reference was measured in 1 M H 2 SO 4 , which reached $1.7 3 10 À2 S cm À1 . 37 Generally, the ionic conductivity increases with increase in the interlayer space, which was confirmed by the results for GO-based membranes. We found that the ionic conductivity followed the same trend in all three electrolytes: HGO-BC > HGO > LGO, which was consistent with the XRD results. Furthermore, the ionic conductivity of HGO and HGO-BC membranes followed this order: H 2 SO 4 > KOH > KCl. Because we made the HGO-BC composite membrane by mixing solutions of HGO and BC, it was not surprising to find similar ion-transport behavior in these two membranes. Protons with the smallest hydrated diameter showed the highest conductivity, whereas the lowest conductivity was obtained in KCl solutions. Because the concentrations of cations and anions were the same in KOH and KCl solutions, it is likely that the difference in conductivity was caused by the pH-related interlayer space, which was suggested in some reported works. 21,25 The high pH would lead to increased dissociation of hydroxyl and carboxyl groups on GO sheets, resulting in an increase in the negative surface charge density within the nanochannels. On the other hand, compared with water, high pH solutions could further increase the interlayer space, whereas KCl decreased the interlayer space. Thus, the conductivity of K + was higher in KOH solutions than in KCl solutions, and anions had decreased conductivity. However, because potassium ions dominate the ion transport, the conductivity of both HGO and HGO-BC membranes in KOH is higher than KCl. For LGO membranes, the small amount of oxygen groups make them less sensitive to the change in pH and salts, leading to the similar conductivities in KOH and KCl electrolytes.</p><!><p>We measured the electrochemical performance of GO-based membranes and summarize the reported redox species with relatively large molecule or ion size in Figure 3C according to the redox potentials and equivalent molecular weight (per transferred electron). 38,39 The redox potentials of different redox species were determined by the cyclic voltammetry curves (Figure S14). To further increase the size difference between active species and charge carriers, one could apply the polymerization strategy to some molecules, such as TEMPO and viologen polymers. 40 A number of redox species were available in alkaline electrolytes, among which K 4 Fe(CN) 6 and FMN-Na were selected as positive and negative species, respectively, because of their stable reversibility and large molecule size. 41 Figure 4 shows the electrochemical performance of pure HGO membrane in alkaline electrolytes using K 4 Fe(CN) 6 and FMN-Na as active species. As shown in Figure 4A, the ion transport of the GO membrane could be facilitated by the hydration process, resulting in the decrease in resistance in the EIS test. Figure 4B illustrates the charging and discharging curves at various current densities of 10, 20, 40, and 80 mA cm À2 . The reasonable rate capability was achieved at high current density because of the high ionic conductivity of HGO membranes in KOH electrolytes. The discharge capacities and efficiencies at current densities of 20 and 40 mA cm À2 are displayed in Figures 4C and 4D. HGO membranes exhibited good cycling performance with a high Coulombic efficiency of $98%. At 40 mA cm À2 , the capacity retention reached as high as 95% after 300 cycles. Although one previous work 42 reported that GO sheets experience deoxygenation in alkaline medium at moderate temperature, our experiment confirmed that HGO membranes had the capability to show reasonable stability in KOH electrolytes at room temperature (Figures S15 and S16), which was consistent with the cycling performance.</p><p>Furthermore, the HGO membrane was also applied as the separator in a 4-HO-TEMPO/MV system (neutral conditions) with 1 M KCl solutions as supporting electrolytes (Figure S17). We also tested the performance of HGO-BC composite membrane by using 0.1 M K 4 Fe(CN) 6 and 0.06 M FMN-Na in 1 M KOH electrolytes in both static and flow modes. At a current density of 7 mA cm À2 , the charge and discharge profiles of HGO-BC almost coincided with those of Nafion 212 (Figure 5A). And the excellent rate capability was achieved in alkaline electrolytes, which was comparable with the performance of Nafion 212. As shown in Figure 5B, from 10 to 50 mA cm À1 , the discharge capacity was nearly maintained without large voltage loss in static mode. Similarly, a reasonable rate performance was also achieved in flow mode. The BC component in HGO-BC membranes has many hydroxyl groups on the surface, which could help the hydration process and thus facilitate ion transport. However, because the BC component changed the tightly stacked microstructure and increased the interlayer space, the Coulombic efficiency at low current density might be limited. However, the cycling performance of HGO-BC membranes at a current density of 40 mA cm À2 , as shown in Figure 5C, was comparable with that of pure HGO membranes. After charging and discharging for 400 cycles, $91% of the initial capacity was retained with a Coulombic efficiency of $97%. In flow mode, because the flow of electrolytes accelerated the disintegration of GO membranes and exacerbate the crossover problem, the discharge capacity retention could only reach 70% after 100 cycles (Figure 5D). To further demonstrate the flexibility of GO-based membranes in RFBs, we also tested the performance of HGO-BC membranes in neutral or acidic electrolytes (Figure S18). With 4-HO-TEMPO and MV as positive and negative electrolytes, respectively, reasonable charging and discharging profiles could be obtained at a low current density (Figure S18A). Similarly, charging and discharging curves in an acidic profile were obtained in 1 M H 2 SO 4 solutions with KBr + LiBr and AQDS as positive and negative electrolytes, respectively (Figure S18B).</p><p>Because of the different interlayer spacing after hydration, LGO and HGO membranes showed discrepant rate capability and cycling performance in the same alkaline electrolyte. Because the interlayer space of the LGO membrane showed a limited increase after soaking in water and a limited water-holding capability, it significantly impeded the transport behavior of charge carriers. As a result, the</p><p>LGO membrane showed a worse rate capability (Figure S19A). For cycling performance, the LGO membrane experienced serious capacity fading under 40 mA cm À2 (Figure S19B), which is mainly ascribed to the poor stability in aqueous solutions. Figure S20 shows the rate performance and cycling stability of Nafion 212.</p><p>Although the ionic conductivity of Nafion 212 in KOH solutions was reasonable, the rate performance was worse than that of GO-based membranes. As Nafion 212 blocked the diffusion of active species in this system, it exhibited stable cycling at a current density of 20 mA cm À2 , although the Coulombic efficiency was not high.</p><p>For a desired membrane or separator for RFBs, the requirements are high conductivity, high selectivity, and long stability. Although GO membranes achieved reasonable ionic conductivity and high rejection in our designed systems, there are still some limitations for practical applications. The big challenge of GO-based membranes is their stability, including mechanical and chemical stability. Even though the long-term water stability of GO membranes has been confirmed, the mechanical strength is still not enough to tolerate the external stress and washout from flowing electrolytes in large-scale RFBs. Furthermore, the oxygen groups of GO sheets can be reduced during long-term operation as a result of reduction from the applied current, redox species, or supporting electrolytes. Because the oxygen groups are important for the ion transport, this will finally influence the performance. Therefore, significant efforts are still needed to further improve the stability before applying GO-based membranes in large-scale electrochemical energy storage.</p><!><p>In this work, we demonstrate a proof-of-concept GO membrane as the separator for large-scale energy storage technology RFBs. GO laminate membranes exhibit a cascading microstructure with tunable interlayer spacing. After immersion in water, the hydration process can further increase the interlayer space and still act as a molecular or ionic sieve to prevent the crossover of large-sized redox species. Because of the large size difference between redox species and small ions as charge carriers, GO membranes as RFB separators achieve a high rejection of large molecules or ions as active species and a high ionic conductivity at the same time. The fast permeation of small ions can be attributed to the capillary-like network formed by the hydration process, whereas blocking the diffusion of large redox species is attributed to size exclusion and charge repulsion. Moreover, changing the degree of oxidation or using BC as an additional filling component can further adjust the microstructure, mechanical stability, and iontransport behavior. HGO and HGO-BC membranes retain their structural stability and reliability under practical electrochemical conditions. Using K 4 Fe(CN) 6 and FMN-Na as active species in alkaline electrolytes, RFBs with GO membranes achieve charge and discharge curves similar to those of Nafion 212 and show stable cycling performance with a Coulombic efficiency of 98%. Although the stability and performance of GO membranes in flow mode still need to be further enhanced, this proof-of-concept demo using GO membranes with tunable interlayer space, versatile chemical modification, and rational composite design provides useful guidelines for the future development of next-generation functional separators for potentially large-scale energy storage systems.</p><!><p>The details of GO membrane preparation, permeation measurements, electrochemical tests, and other characterizations are given in the Supplemental Information.</p>