Supplemental Information. Enabling Graphene-Oxide-Based Membranes. for Large-Scale Energy Storage. by Controlling Hydrophilic Microstructures

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1 Chem, Volume 4 Supplemental Information Enabling Graphene-Oxide-Based Membranes for Large-Scale Energy Storage by Controlling Hydrophilic Microstructures Leyuan Zhang, Yu Ding, Changkun Zhang, Yangen Zhou, Xufeng Zhou, Zhaoping Liu, and Guihua Yu

2 Supplemental Information Experimental Chemicals and materials Nafion 212 membranes (50.8 μm thickness) was purchased from Fuel Cell Store, which was produced by DuPont. Most of chemicals, including potassium hexacyanoferrate(ii) (K 4 Fe(CN) 6 ) trihydrate (98.5%), methyl viologen dichloride hydrate (MV 98%), potassium hydroxide (99.99%), 4-hydroxy-tempo (4-HO- Tempo 97%) and potassium chloride (anhydrous 99%) were commercially purchased from Sigma-Aldrich. Riboflavin-5 -phosphate sodium salt (FMN-Na, >93%) and anthraquinone-2,7-disulfonic acid disodium salt (AQDS, >95%) were purchased from TCI America. The potassium bromide and lithium bromide (99+%) were obtained from ACROS Organics. The sulfuric acid (certified ACS plus) was purchased from Fisher Chemical. All chemicals were used as received without further purification. The dissolution of salts was carried at room temperature. Preparation of GO-based membranes Graphene oxide (GO) was synthesized from natural graphite powders using the modified Hummers method as reported previously, and then the water wash process was followed to remove the excess salts and acids. By adjusting the amount of oxidizing agent, two GO samples with different oxidation degrees were obtained, named as LGO and HGO. LGO was lowly oxidized with a black color while HGO was highly oxidized with a dark brown color. The aqueous dispersions of LGO (0.5 mg ml -1 ) and HGO (1 mg ml -1 ) were prepared in 80 ml and 40 ml batches, respectively. To make free-standing GO membranes, the LGO and HGO dispersions were filtered by a porous AAO filter film (Anodisc, 47 mm in diameter, 0.2 μm pore size, Millipore). Before vacuum filtration, the dispersion was treated in sonication for 10 min. Typically, it took 3-5 days to filter a 40 ml of HGO dispersion (1 mg ml -1 ) while 80 ml LGO (0.5 mg ml -1 ) only need 1-2 days. After dried in air, the GO membranes can be easily peeled off the filter film. The thicknesses of the LGO and HGO membranes were around μm and μm, respectively. Bacterial celluloses (BC) were provided by TianAn Biologic Materials Co., Ltd., China in form of hydrogel pellicles. After the purification process, the BC hydrogel was minced into microgel and freezedried for storage. For preparing HGO-BC composite membranes, the dried BC would be dispersed in formamide with sonication. Finally, a stable BC dispersion with concentration of 1 mg ml -1 was obtained. The HGO-BC composite membranes was consisted of three layers: two BC layers (bottom & top) and one GO/BC composite layer (middle). First, 5 ml BC dispersion was filtered on the surface of filter film and then followed by the filtration of 30 ml BC (1 mg ml -1 ) and 15 ml HGO (1 mg ml -1 ) mixed dispersions. When the vacuum filtration was almost finished, the last step was adding another 5 ml BC dispersion to form a thin layer of BC on the top of the membrane. The thicknesses of HGO-BC composite membranes were around μm. Similarly, the other HGO-BC membranes without the pure BC layer were also obtained by filtering the HGO and BC mixed solutions. Permeation test of membranes

3 The ion permeation tests were carried out in a made-to-order H-shaped device as shown in Fig. S5, which is consisted of two glass chambers and GO-based membranes or Nafion 212 membranes. The freestanding membrane was placed in the middle of the two chambers and clamped with a screw clamp. To further prevent the solution leakage, vacuum grease was used to seal around the junction. The exposed membrane diameter is 15.8mm. In a typical experiment, 40 or 50 ml solutions of redox species were injected into the left chamber as feed solutions and the same amounts of deionized water as the blank electrolyte was injected into the right chamber simultaneously. The possible diffusion of different redox species during the permeation experiment was monitored by measuring UV-vis spectra of the solution in the right chamber (permeate solution). The UV-vis spectra measurement was performed on a UV-vis spectrometer (Evolution 300, Thermo Scientific). Ionic conductivity measurement The ionic conductivities of three GO membranes and Nafion 212 in different electrolytes (acidic, alkaline and neutral) were measured by the electrochemical impedance spectra (EIS) using two steel spacers (diameter 15.8 mm). The thickness of Nafion 212 was about 50.8 μm and the thicknesses of three GO membranes were determined by micrometer caliper. Based on thickness of membranes, contact area and the resistance values from EIS tests, we could approximate the ionic conductivities of GO membranes and Nafion 212. To measure the ionic conductivities in acidic, alkaline and neutral electrolytes, the GO membranes and Nafion 212 were immersed in 1M H 2 SO 4, 2M KOH or 2M KCl aqueous solutions for over 30 mins. Since LGO membrane showed poor water stability, exception was made that the immersion time of LGO was 10 min. After the uptake of electrolytes, the EIS measurements were immediately operated. Characterizations of GO-based membranes The cross-sectional morphologies of GO-based membranes were observed by Scanning Electron Microscopy (SEM) (S5500, Hitachi) operating at SEM mode. The XRD patterns of GO-based membranes with/without hydration were characterized by X-ray diffractometer (XRD) (Rigaku MiniFlex 600) using Cu Kα radiation (λ = Å). The hydration of GO-based membranes was achieved by immersing GO membranes in water for over 30 minutes. After hydration process, the XRD measurements of GO membranes were immediately conducted. In the XRD tests, the GO membranes were fixed on the surface of Si substrate. To characterize the oxidation degrees or oxygen groups of three GO membranes, the TGA curves were measured by TGA/SDTA851e thermogravimetric analyzer under the nitrogen atmosphere from 30 to 500 C at a heating rate of 1 C min -1 and the XPS spectra were collected on a Kratos AXIS Ultra XPS spectrometer. All binding energies were referenced to the C1s peak of ev. Electrochemical measurements All electrochemical measurements were conducted on a BioLogic VMP3 potentiostat system at room temperature. Except for HGO-BC composite membranes, all the electrochemical tests were conducted in the static mode. A three-electrode system was used to determine the CV curves of various redox species. From the CV curves, the half-wave potentials were recorded as redox potentials of redox species. Generally, the standard three electrodes were glassy carbon electrode (WE), Ag/AgCl electrode (RE) and Pt wire (CE), respectively. However, in order to obtain better CV curves, the working electrode for FMN-Na and LiBr was changed to be Pt plate (1*1 cm 2 ).

4 Full-cell tests were carried out using a RFB cell comprised of polypropylene (PP) frame, conductive plastic plates without flow fields, copper current collector, and graphite felt electrodes with an active area of 4 cm 2. In alkaline electrolytes, galvanostatic experiments were conducted at current densities from 10 to 80 ma cm -2 and the potential range was from 0.1 to 2 V. GO-based membranes and Nafion 212 were used as separators in the charging and discharging test. For Nafion 212 membranes, the treatment for exchanging H + with K + or Na + was inevitable when used as ion-exchange membranes in alkaline electrolytes. This exchange procedure was carried out in 1 M KOH at 80 for 8 h and then it was washed with deionized water. In the flow mode, 6 or 8 ml catholytes and anolytes were pumped at a flow rate of 40 ml min -1 through a peristaltic pump. The compositions of catholyte and anolyte were 0.1 M K 4 Fe(CN) 6 in 1 M KOH aqueous electrolytes and 0.06 M FMN-Na in 1 M KOH aqueous electrolytes, respectively. The capacity density was calculated based on the total volume of catholytes and anolytes.

5 Table S1. The dimensions of candidate ions or molecules as active species Redox species Length / nm Width / nm Height / nm K 4 Fe(CN) 6 FMN Na 4 HO Tempo MV AQDS

6 Figure S1. The molecular structure of K 4 Fe(CN) 6 (RS-1), FMN-Na (RS-2), 4-HO-Tempo (RS-3), MV (RS- 4) and AQDS (RS-5) and the corresponding three-dimensional sizes.

7 Figure S2. The ion size, hydrated ion size and hydration energy of different cations and anions.1-5

8 Figure S3. The evolution of interlayer spacing of HGO-BC composite membranes (A) before and (B) after hydration process. The black curve is the XRD pattern of BC membrane. The red, orange, green, blue and magenta curves are 5HGO-15BC, 15HGO-15BC, 15HGO-10BC, 15HGO-7.5BC and 15HGO-5BC composite membranes, respectively. (The number represents the mass weight in the unit of mg.)

9 Figure S4. XPS results of LGO (black), HGO (red) and HGO-BC (blue) composite membranes.

10 Figure S5. The XPS spectra of BC. (A) XPS survey scan and (B) deconvoluted C1s spectra of BC.

11 Figure S6. Photographs of the (A) pure LGO, (B) HGO and (C) HGO-BC composite membranes before and after being soaked in water for a period of time, and a picture showing the breaking of LGO membranes after using a tweezers to touch it.

12 Figure S7. Pictures of the LGO, HGO and HGO-BC composite membranes before and after being soaked in 1M KCl, KOH and H 2 SO 4 solutions for a period of time. The pictures in the right show the states of GO membranes after using the tweezer to take them out.

13 Figure S8. H-shaped permeation device with pure HGO membranes. All the permeation tests are over 72h. The feed solutions for the permeation test are (A) 0.1M K 4 Fe(CN) 6, (B) 0.1M FMN-Na, (C) 0.1M 4-HO- Tempo, (D) 0.1M MV and (E) 0.1M AQDS.

14 Figure S9. H-shaped permeation device with (A) HGO-BC composite membrane or (B) Nafion 212. The feed solution for both is 0.1 M FMN-Na.

15 Figure S10. UV-vis absorption spectra of (A) K 4 Fe(CN) 6, (B) FMN-Na, (C) 4-HO-Tempo, (D) MV and (E) AQDS in the permeate side during the permeation test through HGO membranes. The initial feed solution concentrations of all five redox species are 0.1M. UV-vis absorption spectra of FMN-Na in the permeate side during the permeation test through (F) HGO-BC composite membrane and (G) Nafion 212. The initial concentration in feed side is 0.1M. During the permeation test of 0.1M MV, the permeate solution concentration exceeded the upper detection limit of UV-vis instrument, and the final permeate solution was diluted 10 folds (72h/10) to measure the UV-vis absorption spectra.

16 Figure S11. UV-vis absorption spectra of (A) K 4 Fe(CN) 6, (B) FMN-Na, (C) 4-HO-Tempo, (D) MV and (E) AQDS solutions in different concentrations and the corresponding calibration curves of (F) K 4 Fe(CN) 6, (G) FMN-Na, (H) 4-HO-Tempo, (I) MV and (J) AQDS.

17 Figure S12. The scheme of apparatus for measuring the ionic conductivity. To measure the ionic conductivity of GO membranes in different electrolytes, a two-electrode AC impedance method was used, as shown in Fig. S14. Two steel plates with a diameter of 15.8mm were used as current collectors. The GO membrane was placed in the middle of these two steel plates and then a pressure was applied to ensure the good contact between GO membranes and current collectors when measuring the impedance. During the EIS test, the GO membrane was filled with electrolytes. By obtaining the resistance value (R s ) from the EIS curve, we are able to calculate the conductivity by using the following equation, where σ is the ionic conductivity, A is the surface area of steel plate, R s is the ohmic resistance from the EIS data, and L is the thickness of GO membranes measured by a screw micrometer.

18 Figure S13. Electrochemical impedance spectroscopy of LGO, HGO, HGO-BC and Nafion 212 membranes in (A) 1M H 2 SO 4, (B) 2M KOH and (C) 2M KCl electrolytes.

19 Figure S14. Cyclic voltammograms of (A) K 4 Fe(CN) 6 (10mM catholyte in 1M KOH electrolyte), (B) FMN-Na (10mM anolyte in 1M KOH electrolyte), (C) 4-HO-Tempo (10mM catholyte in 0.5M KCl electrolyte), (D) MV (10mM anolyte in 0.5M KCl electrolyte), (E) LiBr ((50mM catholyte in 1M H 2 SO 4 electrolyte), (F) AQDS (2mM anolyte in 1M H 2 SO 4 electrolyte), and (G) AQDS (50mM anolyte in 0.5M KCl electrolyte), at various sweeping rates.

20 Figure S15. (A) The HGO dispersion before and after adding KOH. The concentration of KOH in the HGO dispersion is 1M. (B) The XRD patterns of HGO, HGO in KOH, HGO after KOH and KOH treated HGO membranes. HGO in KOH is the HGO membrane filled with 1M KOH solutions. HGO after KOH is the HGO in KOH after washed in water and dried in air. KOH treated HGO is the membrane filtered by the HGO dispersion with KOH. (C) The stability test of HGO membranes in 1M KOH solutions. HGO sheets experienced a color change after adding KOH into the HGO dispersion for 30 mins, as displayed in Fig. S15A. HGO dispersion was filtered into the HGO membrane (KOH treated HGO). Although the color change of HGO dispersion was obvious, the XRD pattern of KOH treated HGO membrane was consistent with the previous HGO membrane which was filtered by the HGO dispersion. Furthermore, we conducted the stability test of HGO membranes in 1M KOH solutions. As shown in Fig. S15C, the entire morphology of HGO membranes could be well maintained after 20 hours and it didn t break when using the tweezer to handle it, implying its good mechanical stability under alkaline conditions. Moreover, the XRD patterns of HGO membranes in KOH solutions (HGO in KOH) and after KOH (HGO after KOH) were also obtained, which was similar to the result in water (Fig. S15B). After the treatment of KOH, the HGO membrane was obtained by being washed in water and dried in air. Since the XRD patterns of HGO membrane after KOH were close to that of HGO membrane, it is believed that the performance of HGO membranes was not seriously harmed by the alkaline solution. Moreover, there are probably several reasons contributing to the good cycling stability in the alkaline medium, (1) In the static battery, the amount of added electrolytes is limited, which should not have a strong effect on HGO membranes at room temperature. (2) In the electrochemical test, carbon felt is used as the current collector, which can absorb electrolytes, preventing the direct contact of HGO membranes to all the electrolyte. (3) The ion transport in the KOH medium may be mainly provided by the potassium ions and water molecules, so it is difficult for OH - to penetrate the whole membrane to achieve the serious deoxygenation. In addition, as displayed in Fig. S16, the O/C ratio of HGO after the treatment of 1M KOH solutions for several hours does not show a serious decrease and most oxygen groups, especially hydroxyl group, still remain, implying that HGO membranes should have the reasonable stability in 1M KOH solutions at room temperature.

21 Figure S16. The XPS spectra of HGO membrane after the treatment of 1M KOH solutions. (A) XPS survey scan and (B) deconvoluted C1s spectra of HGO membrane.

22 Figure S17. The charge and discharge curves of the HGO membrane using 0.1 M 4-HO-Tempo and 0.1 M MV as positive and negative electrolytes (1M KCl electrolytes) at varied current densities.

23 Figure S18. (A) Representative charge and discharge profiles over time of HGO-BC membrane using 0.05M 4-HO-Tempo and 0.05M MV as positive and negative electrolytes (1M KCl) at the current density of 5 ma cm -2. (B) Representative charge and discharge profiles over time of HGO-BC membrane using 0.4M LiBr + 0.2M KBr and 0.1M AQDS as positive and negative electrolytes (1M H 2 SO 4 ) at the current density of 10 ma cm -2.

24 Figure S19. (A) The rate performance of LGO membrane. (B) The cycling performance of LGO membrane at the current density of 40 ma cm -2.

25 Figure S20. (A) The rate performance of Nafion 212 membrane. (B) The cycling performance of Nafion 212 membrane at the current density of 20 ma cm -2.

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