Transferring Electrochemically Active Nanomaterials into a Flexible

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Transferring Electrochemical Active Nano-Materials into Flexible Membrane Electrode via Slow Phase Separation Method Induced by Water Vapor Wenju Dong, Xiaoqin Niu, Xiwei Ji, Yuhong Chen, Ling-Bin Kong, Long Kang, and Fen Ran ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06071 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Sustainable Chemistry & Engineering

Transferring Electrochemical Active Nano-Materials into Flexible Membrane Electrode via Slow Phase Separation Method Induced by Water Vapor Wenju Dong b, Xiaoqin Niu c, Xiwei Ji b, Yuhong Chen c, Ling-Bin Kong a,b, Long Kang a,b, Fen Ran a,b,* a

State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals,

Lanzhou University of Technology, Lanzhou 730050, P. R. China b

School of Material Science and Engineering, Lanzhou University of Technology,

Lanzhou 730050, Gansu, P. R. China c

College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou

730050, P.R. China *Corresponding author: Fen Ran ([email protected]; or [email protected])

Abstract: Nickel hydroxide flexible membrane electrode (Ni(OH)2-FME) is prepared by coagulation bath (water) directly induced phase inversion (WIPS-FME), and water vapor induced phase inversion (VIPS-FME) methods, respectively. Interestingly, symmetrical structure of section morphology, surface porosity, and electrolyte-affinity characteristics of flexible membrane are obtained by VIPS technology. The effects of relative humidity, exposure time, and temperatures of phase inversion environment on the electrochemical performance and structure of VIPS are investigated. The specific capacitance of VIPS-FME reaches 2154.29 F·g-1 at a current density of 1 A·g-1, which is better than WIPS-FME (1801.14 F·g-1). Furthermore, a hybrid supercapacitor by employing the VIPS-FME as the positive electrode and the active carbon flexible membrane (AC-FME) as negative electrode exhibits a high specific capacitance of 96.03 F·g-1 at 1 A·g-1, a maximum energy density of 30.01 Wh·kg-1 and maximum power density of 3750.95 W·kg-1 with the energy density of 16.4 Wh·kg-1. Key words: Membrane Electrode; Flexible Device; Phase Inversion; Symmetrical Structure; Supercapacitor

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Introduction Recently, ever-increasing energy demand and serious environmental pollution problems have prompted people to urgently develop a variety of energy storage conversion techniques1, 2. Among various energy storage technologies, supercapacitors with fast charging-discharging ability, high power density, and excellent cycling stability have been widely employed as powerful substitutions to back-up power devices and some other renewable energy systems3, 4. However, the practical application of supercapacitors is subjected to many limitations due to its low energy density. Accordingly, numerous efforts have been made to increase its energy density by increasing the specific capacitance of the active material or expanding the potential window according to the formula: E = 1/2 CV25. To the best of our knowledge, nickel hydroxide (Ni(OH)2) as a battery-type electrode material, has been widely researched for its high specific capacitance, low price, and easy synthesis6, 7. There are many methods for synthesizing Ni(OH)2 such as electrochemical deposition, chemical coprecipitation, and template approaches8. Nevertheless, conventional supercapacitors is not bendable in shape thereby limiting the development of wearable electronic devices. The development of flexible electrode is important for the preparation of flexible supercapacitors and the expansion of the flexible electronic device market9,

10.

Generally speaking, traditional electrodes are

always constructed by slurry coating method, where the active materials, conductive agent, and binder are coated together onto the metallic current collector, and consequently the active materials are easily detached from the current collector during the electrochemical reaction11. Therefore, the elastic substrate polyethersulfone (PES) and the active material Ni(OH)2 are combined to form a uniform casting solution, and the flexible membrane electrode (FME) is prepared by conventional water induced phase separation (WIPS). What is unsatisfactory is the membrane structure obtained by this method presents an asymmetric structure composed of micropores, macropores and finger holes, which is due to the rapid double diffusion between the solvent in the casting solution and the water in the coagulation bath. For example, in our previous research12, activated carbon and PES were formulated into homogeneous casting solution and then prepared into membrane electrode by WIPS. It can be clearly seen that the membrane section exhibited a characteristic asymmetric structure. As an electrode material participating in electrochemical reaction, this structure is not conducive to the effective exploitation of electrolyte ions, which is fundamentally 2


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caused by the difference in polymer concentration due to rapid phase separation. If phase separation occurs slowly, that is, reduce the phase separation rate, then it is possible to obtain a membrane electrode with a symmetrical structure. Ghayeni et al.13 fabricated asymmetric and symmetric membranes based on PES/PEG/DMAc by the vapor induced phase separation (VIPS) and non-solvent induced phase separation (NIPS). The effects of relative humidity and pore-forming agent on the morphology of the membrane were investigated. Chen and coworkers14 prepared a homogeneous microfiltration membrane based on cellulose nitrate membrane by VIPS and applied it to the adsorption of copper ions. On the basis of previous research, Menut et al.15 had also investigated some mechanisms in the process of membrane formation of VIPS, such as the formation of surface liquid layer. However, these previous works focused on the application of membrane materials in field such as water treatment and metal ion adsorption, and did not be involved in electrochemistry. Therefore, inspired by this, the flexible support material PES and active material Ni(OH)2 were blended together to prepare casting solution and changed the membrane structure by VIPS to apply it to the supercapacitor flexible electrode material. The relative humidity, exposure time, and temperatures in the vapor induced phase separation (VIPS) environment were regulated to give the membrane section a symmetrical structure compared with the asymmetric structure of the WIPS process. This symmetrical membrane structure facilitated the smooth transfer of electrolyte ions and improved ion transport efficiency, thereby contributing to improved electrochemical performance. To evaluate the practical potential of as-prepared FME, a hybrid supercapacitor was assembled with FME as the positive electrode, activated carbon flexible membrane electrode (AC-FME) as the negative electrode. The device exhibited the maximum energy density of 30.01 Wh·kg-1 and maximum power density of 3750.95 W·kg-1 with the energy density of 16.4 Wh·kg-1, demonstrating its excellent rate performance. Fundamentally, these excellent electrochemical properties benefit from the satisfactory symmetrical structure of the membrane electrode. Experimental Chemicals Polyethersulfone (PES, Ultrason E6020P) was obtained from BASF, Germany. NiCl2·6H2O was purchased from Tianjin Beichen Founder Reagent Factory and NH3·H2O was obtained from Tianjin BASF Chemical Co., Ltd. Graphite powder and 3



acetylene black were acquired from Shenyang Kejing Auto-instrument Co., Ltd. Dimethyl acetamide (DMAc) was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical grade and used without further purification. Synthesis of Nickel Hydroxide (Ni(OH)2) Ni(OH)2 nanoparticles were prepared by chemical coprecipitation method. Typicaly, 40 g of NiCl2·6H2O was dissolved in 141 mL deionized water with magnetic stirring at room temperature. Afterwards, a 5 wt% NH3·H2O was added dropwise to the stirred nickel chloride solution at a rate of 5 seconds until the pH of the solution was 9. The resulting solution was ripened for 3 h at room temperature, then washed with deionized water to obtain a green powder and dried at 60 °C for 6 h for later using. Preparation of Ni(OH)2/PES Flexible Membrane Electrode (Ni(OH)2-FME) by Different Phase Inversion Induction Methods Ni(OH)2-FME was prepared by coagulation bath (water) directly induced phase inversion (WIPS), and water vapor induced phase inversion (VIPS), respectively. The obtained flexible membrane electrodes were named WIPS-FME and VIPS-FME, respectively. In a typical procedure, casting solution was prepared by slowly adding 0.85 g PES, 1.70 g Ni(OH)2, and 0.30 g conductive agent (graphite powder, acetylene black) into moderate amount of DMAc solvent under magnetic stirring until a homogeneous solution formed. The casting solution was prepared into membrane electrode by spin coating method and immediately transferred to two different phase inversion environments, which were coagulation bath consisting of deionized water (WIPS-FME) and water vapor environment consisting of a certain humidity and temperatures (VIPS-FME). For the preparation process of VIPS-FME, the casting solution was firstly subjected to preliminary phase transformation in water vapor environment and then to a final phase inversion process in a coagulation bath. The humidity and temperatures of phase inversion environment were adjusted by a heating humidifier during the experiment. Different induced humidity, time and temperatures were utilized to explore the optimal electrochemical performance of the FMEs. For instance, the water vapor humidity was set to 60, 70, 80, and 90 % (temperature and time were 40 °C and 10 min, respectively), which were named as VIPS-FME-1, VIPSFME-2, VIPS-FME-3 and VIPS-FME-4, respectively. Those by putting 5, 20, and 40 min (humidity and temperature were 70 % and 40 °C, respectively) were named as VIPS-FME-5, VIPS-FME-6 and VIPS-FME-7, respectively. Similarly, these by 4


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placing into water vapor environment at the temperatures of 25, 30 and 35 °C (humidity and time were 70 % and 10 min, respectively) were named as VIPS-FME-8, VIPSFME-9 and VIPS-FME-10, respectively. In addition to Ni(OH)2-FME, polyaniline (PANI) and activated carbon (AC) were also used as active materials to prepare FMEs by two phase inversion methods, which were denoted as PANI-WIPS-FME, PANIVIPS-FME, AC-WIPS-FME and AC-VIPS-FME, respectively. Typically, the specific preparation process for PANI-FME was as follows: PES (0.40 g) was firstly dissolved in DMAc under magnetic stirring and ultrasonication to obtain a clear homogeneous solution. After the bubbles in solutions were removed by vacuum degassing, PANI (1.24 g) and graphite powder (as a conductive agent, 0.10 g) were added and stirred for 24 h. The casting solution was prepared as membrane by spin coating at 20 °C, which was immediately immersed in a coagulation bath of deionized water or water vapor environment. Then, the membrane electrode was transferred to a water bath for 24 h to remove the residual solvent. Finally, the PANI-FMEs were dried in vacuum oven at 60 °C for 24 h. Similarly, AC-FME was also prepared by the same preparation method as the PANI-FME. The only difference was that the casting solution of AC-FME was composed of 0.45 g PES and 1.24 g activated carbon as well as an appropriate amount of DMAc. Materials Characterization The microstructures and morphologies of FMEs were examined by field emission scanning electron microscope (SEM, JSM-6701F, JEOL, Japan) and transition electron microscope (TEM, JEM-2010, JEOL, Japan). Surface wettability of FMEs was evaluated using static water contact angles (WCA, PHS-3C, Precision science co., Shanghai, China). Some digital photos of FMEs were obtained by using a Canon camera. The crystal structures and compositions of the as-prepared FMEs and Ni(OH)2 were investigated by X-ray diffraction (XRD, D/max-2400, Rigaku). Preparation of the Electrodes and Electrochemical Configurations For the preparation of Ni(OH)2 electrode: Ni(OH)2 active material (4.0 mg), conducting graphite (0.375 mg), acetylene black (0.375 mg) and polytetrafluoroethylene emulsion (0.25 mg) were mixed in an agate mortar, which were coated on the foamed nickel (as current collector) of geometric area of probably 1 cm2 and pressed at 10 MPa to minimize the loss of electroactive materials during the electrochemical testing process. For the preparation of membrane working electrode: a thin membrane of 1 × 1 cm2 (the 5



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average thickness of the membrane was 84 μm, the average weight per square centimeter was 6.2 mg) was cut from a whole piece of membrane and pressed on two pieces of foamed nickel current collector at 10 MPa for electrochemical measurement (Similarly, AC-FMEs were also produced as such). Three-electrode configuration was prepared by employing saturated calomel electrode as reference electrode, a platinum plate as counter electrode and flexible membrane pressed on foamed nickel as working electrode (FMEs). Two electrode configurations were assembled using WIPS-FME and VIPS-FME, AC-FME as positive and negative electrodes, respectively. Electrochemical Measurements The measurements were carried out by standard two or three-electrode system. The approaches of the electrochemical characterization mainly included cyclic voltammetry (CV), galvanostatic charging-discharging (GCD) and electrochemical impedance spectroscopy (EIS), which were performed in a 6 M KOH aqueous solution. Cycle performance of the asymmetric supercapacitor (ASC) was measured by a land cell tester (LAND CT2001A). All the electrochemical experiments were carried out in a CHI660E electrochemical workstation (Shanghai, China). Regarding the Single Electrode Test (three-electrode system): CV of three-electrode system was recorded between -0.2 and 0.5 V at different scan rates from 5 to 50 mV·s-1. For GCD measurement, the current densities were varied from 1 to 5 A·g-1 within a potential window of 0-0.35 V. EIS spectra were acquired with a frequency ranged from 10-2 to 105 Hz. Regarding the ASCs Test (Two-electrode system): CV of ASCs was recorded between 0 and 1.5 V at different scan rates from 5 to 50 mV·s-1. For GCD measurement, the current densities were varied from 0.5 to 5 A·g-1 within the same potential range as in CV measurement. The cycle stability test was performed using LAND instrument at a current density of 1 A·g-1. The specific capacitance was calculated based on the weight of Ni(OH)2 in the FMEs from GCD curves according to the following formula: C= I△t/m△V

(1)

Where C (F·g-1) was the specific capacitance, I (A) was the constant discharge current,

△t (s) was the discharge time, m (g) was the mass of active materials, and △V (V) was the potential window. Positive and negative charge balance principles were used to assemble asymmetric device, which followed the relationship: 6


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Q+ = Q-

(2)

Where Q+ and Q- meant charge stored at the positive and negative electrodes, respectively. Also known was the charge (Q) equaled to the current (I) multiplied by the time (t), equation (1) can be rewritten as follows: Q = C×m×△V

(3)

According to the relationship between equations (2) and (3), then: m+/m- =C-/C+ × △V-/△V+

(4)

Energy density and power density of ASC were calculated from GCD curves according to the following equations: E = 1/2 CV2

(5)

P=E/t

(6)

Where E (Wh·Kg-1) was energy density, C (F·g-1) was the specific capacitance of asymmetric device, V was potential window, P (W·Kg-1) was power density, t (h) was the discharging time.

Results and Discussion

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Figure 1 (a) Schematic diagram of the preparation process of the membranes, (b) the front and back photos of the membranes and folded into different shapes, (c) water contact angle, (d and e) surface and (f and g) cross-sectional view of WIPS-FME and VIPS-FME

Figure 1a schematically illustrated the fabrication procedure of the flexible membrane electrodes (FMEs), which was involved water induced phase separation (WIPS) and water vapor induced phase separation (VIPS). The casting solution (I) containing electrochemical active materials was directly prepared as FMEs by water induced phase inversion (II), which was termed as WIPS-FME. When the casting solution was directly subjected to one-step immersion precipitation phase transformation, fast solvents diffusion and exchange between solvent (DMAc) and non-solvent (H2O) would be intense. The main phenomenon responsible for phase separation was a solvent outflow rather than a nonsolvent inflow during this process. It was precisely because of the more intense WIPS mass transfer process that the membrane electrode morphology was different, and most of the membranes obtained by this wet method had asymmetric architecture mainly composed of finger pores and macroporous. For the surface morphology of the membrane, the polymer concentration near the coagulation bath/polymer solution interface could be much higher than that in the bulk to explain the formation of denser surface structure, which also in line with the membrane structure prepared by wet forming as reported in the literatures12, 16, 17. However, this 8


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asymmetric pore structure was not conducive to the adsorption and desorption of electrolyte ions for membrane electrode involved in electrochemical reactions. Therefore, it was especially significant to acquire a simple and easy preparation method to design a symmetrical pore structure. Contrary to the WIPS process, VIPS provided a better temperate mass transfer kinetics process for the exchange between solvent and non-solvent; in other words, the main phenomenon responsible for phase separation was a nonsolvent inflow rather than a solvent outflow. As shown in step III in Figure 1a, the preliminary phase inversion process took place in a chamber with controlled water vapor temperatures and humidity. The mutual migration between solvent and water vapor that occurred during this process was relatively slow. The initial stage was that the inward migration of water vapor dominated, the solvent inside the casting solution continuously migrated to the surface under the action of the concentration gradient. As the phase inversion continued, the rate at which water vapor migrated inwardly decreased, and then solvent and water vapor volatilization on the surface of the membrane gradually dominated. After a long period of repeated migration, the casting solution finally solidified to form the nascent membrane. Afterward, the membrane was immersed in a coagulation bath containing deionized water to complete membrane formation (VIPS-FME) as shown in step IV in Figure 1a. In addition, it was found that in the process of forming the nascent membrane, there was a surface liquid layer between the gas phase and the liquid phase, which was as reported in the relevant literatures15, 18. This surface evolution process played an important role in forming the final morphology of the membrane. It was also because of this moderate phase separation process that the final membrane morphology exhibited a uniform symmetrical structure and the surface also had certain porosity. This symmetrical membrane structure greatly shortened the transport path of electrolyte ions during the electrochemical reaction process, effectively improving the cycle stability of the membrane electrode. Membranes obtained by directly immersion precipitation phase transformation generally possessed an asymmetric structure; this structure was generally macroscopically different from the front and backsides of the membrane. As shown in the digital photos on both sides of the membrane electrode on the left side of Figure 1b, it can be clearly found that the color of the front and back of WIPS-FME was somewhat different and the surface was also not uniform. In contrast, the two sides of the VIPS-FME were basically the same color and the surface was also flat and uniform. 9



Other than this, the flexibility of the electrode was essential for wearable electronic device. The digital photos on the right side of Figure 1b exhibited that the membrane electrode can be folded into a variety of shapes (such as pine, spring word, aircraft shape and windmill model). The moderate phase inversion process that occurred in the water vapor environment had a greater impact on the morphology of the membrane, including surface and crosssectional structures. The final morphology of the membrane was mainly determined by this step. The water vapor with a certain temperatures and humidity exhibited slower diffusion behavior on the surface of the casting solution during the mild phase separation process. The micro mass transfer occurred during this period, which gave the membrane surface sufficient time to cause pores. To the best of our knowledge, the porosity of the electrode material facilitated the adsorption and desorption of electrolyte ions, thereby promoting electrochemical performance. The surface wettability of the electrode was also affected by porosity to some extent. Therefore, static water contact angle (WCA) could be used as an efficient method to evaluate the hydrophilicity of membrane surface. The WCA of the WIPS-FME and VIPS-FME were presented in Figure 1c. The WCA of WIPS-FME was up to 137.4°, while the WCA of the membrane after water vapor induction decreased to 92.1°, which was mainly due to the surface porosity caused by water vapor induction. The WCA test results can be further confirmed by surface SEM of the membrane, which were shown in Figure 1d and e. As can be seen from Figure 1d that the surface of WIPS-FME had a dense top-layer and almost no active substance was exposed on the surface. This may be caused by the driving force generated by the difference in polymer concentration at the interface between the surface layer of the casting solution and the coagulation bath during the intense phase separation. In sharp contrast, the surface of VIPS-FME exhibited a relatively noticeable porosity and some active materials and conductive agent were also exposed on the surface, which in turn increased the surface wettability of the electrode and reduced the WCA. Two distinct asymmetric and symmetrical cross-sectional structures of WIPS-FME and VIPS-FME were presented in Figure 1f and g. It can be clearly seen from Figure 1f that the upper section of the membrane was dominated by microporous features, the middle part was mostly large holes, and the lower section was mainly composed of finger holes obtained by WIPS. For the VIPS process in Figure 1g, it was exactly because the diffusion rate of non-solvent (water vapor) through the entire polymer system was relatively slow, then the changes in its concentration over 10


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the whole cross-section of the matrix were minimal, leading to characteristic symmetric structures. This also in turn explained why the asymmetric structure caused by strong phase inversion kinetics in the coagulation bath.

Figure 2 Mechanism diagram of the surface and cross-section views of the (a) WIPS-FME and (b) VIPS-FME, and (c) CV (5 mV·s-1), (d) GCD (1 A·g-1), (e) and (f) Nyquist plots, and (g) comparisons of specific capacitances for Ni(OH)2, WIPS-FME, and VIPS-FME

Figure 2a and b were used to visually illustrate the surface and cross-sectional properties of the WIPS-FME and VIPS-FME. When the casting solution was immersed 11



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in the coagulation bath, the solvent phase in the casting solution and the aqueous phase in the coagulation bath rapidly undergone bidirectional diffusion behavior. This diffusion behavior also produced a driving force that increased the polymer concentration at the interface between the casting solution and the aqueous phase. Therefore, a dense top-layer appeared on the surface of the membrane. WCA and SEM characterization also demonstrated poor wettability and depleted active substance on the surface of the membrane. For the cross-sectional structure of the membrane, the difference in polymer concentration due to the driving force generated by the intense phase separation eventually caused the entire cross section of the membrane to exhibit an asymmetrical structure containing micropores, macropores, and finger holes. Such an asymmetric structure did not provide a shorter diffusion path for electrolyte ions when participating in an electrochemical reaction19. For the VIPS process, a surface liquid layer can be generated on the surface of the casting solution. According to related reports by Menut et al15, a polymer gradient was also formed with a lower concentration near the top surface. Coarsening of droplets then became easier near the surface as the viscosity of the casting solution was slightly lower than that in the deeper layers20. Therefore, the resulting kinetic difference was not obvious across the membrane, naturally led to the creation of a symmetrical structure. This symmetrical structure was more conducive to shorten the electrolyte ion transport path and provided an excellent passageway

for

enhanced

electrode/electrolyte

interfacial

contact

during

electrochemical process21, 22. In addition, Ni(OH)2 as an active material had also been characterized in supporting information. Figure S1 and 2 were used to demonstrate the morphology and crystal structure of the prepared Ni(OH)2. It can be seen from the SEM images in Figure S1a and b that the Ni(OH)2 nanoparticles were interconnected to form a shape similar to a flower bone. Also, one can see clearly from the TEM images in Figure S1c-e that the Ni(OH)2 nanoparticles were composed of convoluted nanosheets, and the extended nanosheets had a length of about 240 nm. Figure S1f exhibited the corresponding SAED pattern and it revealed the polycrystalline nature of Ni(OH)2. The X-ray diffraction (XRD) patterns of the Ni(OH)2, WIPS-FME, and VIPS-FME were shown in Figure S2. The characteristic peaks of Ni(OH)2 in accordance with the literature at 2θ=10.2°, 33.5°, 38.4°, and 60.1°, which corresponded to the (001), (100), (101), and (110) facets23, 24. The diffraction peaks of Ni(OH)2 can also be found in the patterns of WIPS-FME and VIPS-FME, except that the intensity of the peak was weakened or 12


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shifted. Apart from this, there were some other new diffraction peaks, such as peaks at 23° and 26.5°, which corresponded to polymers (PES) and conductive agents (graphite powder, acetylene black)25-27. It was also because of the incorporation of the polymer and the conductive agent that the diffraction peak of Ni(OH)2 in the membrane electrode changed slightly. The electrochemical performance was investigated to explore the potential of FMEs for supercapacitors. Figure 2c displayed the CV curves of pristine Ni(OH)2, WIPS-FME, and VIPS-FME within a potential window of -0.2 to 0.5 V at a constant scan rate of 5 mV·s-1. The current values were normalized based on the weight of the active substance in the FMEs. A pair of well-defined redox peaks was detected in the CV curves, indicating that the charge storage behavior of the battery type of Ni(OH)26. That can be attributed to the reversible redox reaction of Ni(II)↔Ni(III) and can be delineated as Ni(OH)2 + OH- ↔ NiOOH + H2O + e-28, 29. Since the specific capacitance was directly proportional to the area enclosed by the CV curves, VIPS-FME presented greater charge storage capacity than WIPS-FME (Ni(OH)2 electrode measured by coating method as a comparison). Besides, the location of the redox peaks of the FMEs were slightly shifted to the positive potential compared to pristine Ni(OH)2, which may be caused by the electrode containing a certain amount of polymer and conductive agent, or maybe originated from the distinctness in the polarization behavior and the ohmic resistance of the electrodes during the CV test30. Figure 2d described GCD curves of pristine Ni(OH)2, WIPS-FME, and VIPS-FME within the potential window of 0 to 0.35 V at a current density of 1 A·g-1. According to equation (1), the specific capacitance of Ni(OH)2, WIPS-FME, and VIPS-FME were calculated to be 2271.43, 1801.14, and 2154.29 F·g-1, respectively. It was worth noting that the specific capacitance of VIPS-FME was almost the same as that of Ni(OH)2 and it was generally higher than WIPS-FME, which suggested that VIPS-FME had better potential as an electrode material for supercapacitors. One of the reasons was that VIPSFME had a symmetrical membrane structure, which led to a more uniform distribution of Ni(OH)2 particles inside the membrane (Figure 1g). In addition, this symmetrical structure provided faster diffusion path for electrolyte ion transport to more fully complete the electrochemical reaction. Another reason was that the increase in surface roughness increased the wettability of the electrode surface to the electrolyte, which also heightened the number of electrochemically active sites and made electrolyte ions more accessible. Figure 2e revealed EIS curves of Ni(OH)2, WIPS-FME, and VIPS13



FME. The Nyquist curves ranging from 10-2 to 105 Hz exhibited that the spectra consisted of a compressed semicircle in the high-to-medium frequency region and a straight line in the low-frequency region31. Generally, a semicircle diameter was associated with the charging-transfer resistance (Rct) and diffusion-limiting process in electrochemical reaction; the slopes of straight lines displayed steeper symbolized relatively low diffusion resistances (Warburg impedance, W0), a type of impedance that had something to do with migration inside electrode materials and the electrolyte ions diffusion32. The intercept of the high frequency region of curves and X-axis represented electrolyte resistance and intrinsic impedance (Rs) of active materials33. As one can see from the enlarged view of Figure 2f that the intercept of all curves and X-axis were basically identical, indicating minor impedance (Rs). VIPS-FME had a smaller semicircle diameter than WIPS-FME, manifesting that the electrolyte ion can be transferred rapidly during the electrochemical reaction and owned a lower chargetransfer resistance (Rct) (Ni(OH)2 had the smallest semicircle diameter because it was measured by coating method.). Other than this, it can be found that VIPS-FME had a steeper straight line in the low frequency region, signifying the minor diffusion impedance (W0) and better electrochemical behavior. The corresponding circuit elements calculated from appropriate equivalent circuit was used to further analyze electrochemical impedance spectroscopy, which was shown in the inset of Figure 2f. These parameters (RS, Rct, and W0) calculated with Zview-Impedance software were presented in Table S1, all of which indicated the favorable electrochemical impedance performance of VIPS-FME. Figure 2g described the function relationship of mass specific capacitance of Ni(OH)2, WIPS-FME, and VIPS-FME versus discharging current density. It can be seen that VIPS-FME had better electrochemical capacitance and higher rate performance. When the current density was 1.0 A·g-1, the specific capacitance can achieve the maximum value of 2154.29 F·g-1. When the current density increased from 1.0 to 5.0 A·g-1, the specific capacitance of VIPS-FME still retained 1402.86 F·g-1. The good electrochemical performance mainly benefited from the better symmetrical membrane structure that can provide high interfacial area, short ion diffusion channels thereby taking full advantages of active materials. To the best of our knowledge, electrochemical performance was largely dependent on the morphology of the material. Therefore, Figure S3 presented a clearer comparison of the surface and cross-section of WIPS-FME and VIPS-FME at different 14


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magnifications. Figure S3a-c exhibited different magnification surface morphology of WIPS-FME, and it can be found that the surface of the membrane revealed almost a dense layer and there were no obvious particles. This severely hindered the surface accessibility of electrolyte ions and thus reduced electrochemical performance. A clear contrast with the VIPS-FME surface exhibited rich porosity and the presence of many particulate matter (Ni(OH)2, conductive agent) as shown in Figure S3d-f. The upper, middle, and lower regions of the cross-sectional morphology of the WIPS-FME were displayed in Figure S3g-i. It can be clearly seen that micropores, macropores, and finger holes coexist in the cross section of the membrane and this was a typical asymmetric structure obtained by WIPS. Figure S3j-l reflected the overall morphology of the VIPS-FME in section, and all of them exhibited a symmetrical and uniform structure from the three regions of the membrane. Correlated electrochemical data demonstrated that this structure was significantly better than the asymmetric structure of WIPS-FME in the electrochemical reaction process. Importantly, test conditions had a greater impact on the final morphology of the membrane, which can be broadly classified into two main categories-process parameters and formulation parameters. Process parameters mainly involved the physical and chemical factors of polymer solutions preparation, such as casting solution composition and viscosity, etc. Formulation parameters were mainly related to conditions that affected the final membrane structure during the casting process, for instance the relative humidity, the exposure time to water vapors, and temperature. Formulation parameters were only explored since the proportion of each component of the casting solution was fixed in this experiment.

15



Figure 3 (a-c) Cross-sectional view of VIPS-FME-1, VIPS-FME-2, and VIPS-FME-4 (d-f magnification of the middle section area), (g and h) cross-sectional view of VIPS-FME-5, and VIPSFME-7 (j and k magnification of the middle area; i and l magnification of the upper and lower parts of VIPS-FME-7)

The cross-sectional morphology of the membrane with humidity of 60 %, 70 %, and 90 % were illustrated in Figure 3a-c, named VIPS-FME-1, VIPS-FME-2, and VIPS-FME4, respectively. All the other parameters (exposure time, and temperature) were constant when investigating the effect of humidity on the membrane structure. It can be seen from the SEM images that the cross section of the membrane exhibited a symmetrical structure as long as certain humidity was applied. However, the crosssectional structure of the membrane undergone a dense-loose-dense structural evolution. For lower humidity (60 %), the driving force for water vapor diffusion was lower. Moreover, solvent volatilization played a certain role in the course of membrane formation. As a consequence, phase separation was slower and the concentration of the polymer on the surface hardly changed, which can lead to incomplete diffusion in a limited time, resulting in a denser structure of the membrane. When the humidity 16


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increased to 70 %, phase transition kinetics were further appropriately increased. The distribution of the polymer concentration in the membrane section was more uniform, resulting in a relatively loose porous structure. When the humidity was further increased (90 %), which meant that the phase inversion environment was filled with non-solvent phases. Therefore, the diffusion kinetics was further increased, which caused the solvent in the casting solution rapidly diffused into the water vapor. Therefore, the surface polymer concentration was slightly higher, which resulted in a dense membrane structure (Figure 3d-f were enlarged view of the corresponding section). In terms of humidity, it can be concluded that dense to porous membranes can be formed by increasing the activity of the water vapor. Further increase in humidity would hinder the growth of polymer lean phase, leading to dense structure. Therefore, 70 % humidity was selected as the optimum humidity throughout the study. The effect of all humidity on electrochemical performance was given in Figure S4, which also indicated that the electrochemical and impedance properties of the FMEs were better at humidity of 70 %. Taking into account all other factors affecting membrane formation as constant, the exposure time to water vapor affected the porosity and interconnectivity of membrane structure. The cross-sectional morphology of the membrane with exposure time of 5, 10, and 40 min were illustrated in Figure 3g, b, and h, named VIPS-FME-5, VIPSFME-2, and VIPS-FME-7, respectively. Too short exposure time made the membrane structure produce macropores similar to those formed by the WIPS process as shown in Figure 3j. This was mainly due to the lack of sufficient time in the water vapor phase to generate phase separation, which was then immersed in a coagulation bath to form such a structure. It also can be seen that some macropores existed and the interconnectivity was poor. Extending the exposure time (10 min) to water vapor given rise to VIPS typical structures, as presented in Figure 3b, and e, which was a relatively uniform and ideal symmetric membrane structure that was beneficial to electrochemical reactions. Longer phase transformation time (40 min) was used to examine membrane structure as shown in Figure 3h. It can be found that the membrane generally displayed a very dense structure. Moreover, its porosity and interconnectivity were poor compared with Figure 3b, which seriously impeded the permeability of ions and degraded the electrochemical performance. The relevant data in Figure S5 and Table S2 illustrated the difference in electrochemical performance caused by the distinctness in the evolution of this structure. The final results demonstrated that the structure formed (10 min exposure time) was more favorable for electrochemical reaction. 17



Page 18 of 27

In addition to the humidity and exposure time affecting the electrochemical performance of the FMEs, temperature of phase transition environment was also explored as a factor as shown in the electrochemical capacitance in Table 1. VIPSFME-8, VIPS-FME-9, VIPS-FME-10, and VIPS-FME-2 indicated temperatures of 25, 30, 35, and 40 °C, respectively. It was found that the overall morphology of the membrane curly with increasing temperature during the experiment. Therefore, considering the electrochemical properties and the physical appearance of the membrane, 40 °C was selected as the optimum phase inversion temperature. In addition, Figure S6 and Table S3 presented the effects on electrochemical performance at all temperatures, and the results also exhibited that the rate performance and impedance performance of VIPS-FME-2 were superior. The impacts of all experimental variables on electrochemical performance were listed in Table 1 for a clearer comparison. To sum up, the humidity of 70 %, exposure time of 10 min, and temperature of 40 °C were optimal conditions for excellent electrochemical performance after comprehensive consideration. Table 1 Specific capacitance values of Ni(OH)2, WIPS-FME, and VIPS-FME at different induction conditions Current density (A·g-1)

1

2

3

4

5

Specific capacitance (F·g-1)

Sample Ni(OH)2

2271.43

1981.71

1817.14

1694.86

1605.71

WIPS-FME

1801.14

1437.14

1197.43

1022.86

872.57

VIPS-FME-1

2251.43

1894.86

1666.29

1486.86

1322.86

VIPS-FME-2

2154.29

1872.00

1677.43

1523.43

1402.86

VIPS-FME-3

2265.71

1909.71

1684.29

1517.71

1380

VIPS-FME-4

2294.29

1925.14

1701.43

1528

1375.71

VIPS-FME-5

2108.57

1733.71

1507.71

1338.29

1207.43

VIPS-FME-6

2145.71

1790.29

1561.71

1390.86

1251.71

VIPS-FME-7

2091.43

1583.43

1201.71

852.57

616.14

18


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VIPS-FME-8

1962.86

1564.57

1293.43

1089.14

917.57

VIPS-FME-9

2180

1834.29

1593.43

1414.86

1270.43

VIPS-FME-10

2197.14

1853.14

1628.57

1453.71

1315

Figure 4 (a) CV (5-50 mV·s-1), (b) GCD (0.5-5 A·g-1) for WIPS-FME||AC-FME ASC device; (c) CV (5-50 mV·s-1), and (d) GCD (0.5-5 A·g-1) for VIPS-FME||AC-FME ASC device; and (e) and (f) Ragone plot and cycle life

To evaluate the practical potential of as-prepared FMEs, a hybrid supercapacitor was assembled with VIPS-FME and WIPS-FME as the positive electrode, activated carbon flexible membrane electrode (AC-FME) as the negative electrode as shown in Figure 4. Detailed electrochemical performance of AC-FME was shown in Figure S7. Figure 19



Page 20 of 27

4a presented CV curves of WIPS-FME||AC-FME hybrid supercapacitor at different scan rates from 5 to 50 mV·s-1 with a potential window of 0.0 to 1.5 V. An obvious redox peak can be clearly observed in the curves, which was contributed by the battery behavior of Ni(OH)2 in hybrid supercapacitors34,

35.

Figure 4b exhibited the GCD

curves at different current densities of 0.5-5.0 A·g-1 within the same potential range as in CV measurement. The corresponding specific capacitances calculated according to the discharge curve of GCD curves were shown in Table S4 and the highest specific capacitance of the device reached 69.57 F·g-1 at a current density of 0.5 A·g-1. The shape of electrochemical performance curves (CV, and GCD) of VIPS-FME||AC-FME were similar to WIPS-FME||AC-FME as presented in Figure 4c and Figure 4d. However, it can be seen from Figure 4c that the area enclosed by the CV curve of VIPS-FME||AC-FME was larger than those of WIPS-FME||AC-FME (It should be noted that the current values of CV curves were normalized based on the weight of the active substance in the FMEs). This can be explained VIPS-FME||AC-FME presented greater charge storage capacity, which also can be confirmed by the GCD curve in Figure 4d. After calculation, the specific capacitance of device can achieve 96.03 F·g-1 and the curve also had no significant voltage drop compared to Figure 4b. The Ragone plot revealed the relationship between the energy density and power density of the hybrid supercapacitor was exhibited in Figure 4e. The device revealed the maximum energy density of 30.01 Wh·kg-1 and maximum power density of 3750.95 W·kg-1 with the energy density of 16.4 Wh·kg-1, demonstrating its excellent rate performance. The specific electrochemical performance of the devices was shown in Figure S4. In addition to this, specific electrochemical performance values (based on the area and volume of the devices) of WIPS-FME||AC-FME and VIPS-FME||AC-FME ASC devices were also exhibited in Table S5. These results were superior to some asymmetric

supercapacitors

reported

previously,

for

example,

Graphene@Ni(OH)2||graphene@CNT (18 Wh·kg-1 at 850 W·kg-1)36, NixCo1-x LDHZTO||AC (23.7 Wh·kg-1 at 284.2 W·kg-1)37, Ni-Co hydroxide||CG (26.3 Wh·kg-1 at 320 W·kg-1)38, and Mn3O4/Ni(OH)2||AC (17.8 Wh·kg-1 at 162 W·kg-1)39, etc24, 40. (More comparisons were shown in Table S6). Cyclic performance was another important indicator for the electrode material of supercapacitors, which was shown from capacitance retention versus cycle number at the current density of 1 A g-1 in Figure 4f. The capacitance retention of VIPS-FME||AC-FME showed an upward trend in the first 1000 cycles, which may be attributed to the electrode material produced large 20


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volume changes after repeating charging/discharging process, and then transformed more open-structured and more accessible to the electrolyte. Consequently, more active material was involved and contributed to the capacitance. After this stage, the curve became more moderate and the capacitance retention reached 81 % at the end of the 7000 cycles, which also proved the excellent cycle stability of the material. This was mainly due to the symmetrical structure of the membrane material, which can provide a more convenient channel for the transport of electrolyte ions and improve the ion transport efficiency as shown in the illustration in Figure 4f41.

Figure 5 Physical form of FME composed of different active substances after ultrasonic oscillation. (a) PANI-WIPS-FME and PANI-VIPS-FME ； (b) AC-WIPS-FME and AC-VIPS-FME ； (c) Ni(OH)2-WIPS-FME and Ni(OH)2-VIPS-FME, and (d-f) mechanism diagram corresponding to three membranes

In addition to the Ni(OH)2-FME, other active materials (such as PANI, and AC) were also used to construct FMEs by WIPS and VIPS methods, named as PANI-WIPS-FME and PANI-VIPS-FME, AC-WIPS-FME and AC-VIPS-FME, respectively. All the FMEs prepared by the WIPS had not much changed in physical appearance after ultrasonic oscillation. However, the interesting phenomenon during the experiment was that the FMEs prepared by the VIPS method subjected to the same treatment, only the Ni(OH)2-FME remained intact. This intuitive difference was shown in Figure 5a-c, which can be clearly seen that PANI-VIPS-FME and AC-VIPS-FME had a certain amount of active substance, and conductive agent peeled off after ultrasonic oscillation. This may be related to the physical properties of the active materials, such as the volume of PANI and AC were much larger than that of Ni(OH)2 in the case of the same weight. This may cause the active materials to be partially entangled by the PES macromolecular chain. If it was Ni(OH)2, the composition of the casting solution can 21



be effectively wrapped by PES, and the whole system will be stable as illustrated in Figure 5-f. This also demonstrated that when constructing FMEs with VIPS method, Ni(OH)2 was more suitable as an active material. Conclusions In summary, the structurally symmetrical FMEs with excellent electrochemical performance were prepared by regulating the relative humidity, exposure time, temperatures in the VIPS. A symmetrical structure was obtained due to the slower kinetics induced by VIPS process, which increased the electrochemically active reaction sites and provided a convenient channel for the transport of electrolyte ions, thus making the electrochemical performance of the VIPS-FME much better than that prepared by WIPS. The VIPS-FME||AC-FME hybrid supercapacitor demonstrated a maximum energy density of 30.01 Wh·kg-1 at power density of 376.04 W·kg-1 and after 7000 charging-discharging cycles, and 81 % of capacitance can be maintained. Excellent electrochemical performance was mainly due to the symmetric porous structure of the membrane material and favorable wettability to the electrolyte. In addition, the Ni(OH)2-VIPS-FME remained stable after ultrasonic oscillation compared with PANI-VIPS-FME and AC-VIPS-FME, which indicated that Ni(OH)2-FME was suitable as a supercapacitor flexible electrode material. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acssuschemeng.XXXXXX. Additional SEM and TEM images of Ni(OH)2; electrochemical performance of ACFME; XRD spectra, electrochemical performance of FME, WIPS-FME||AC-FME, and VIPS-FME||AC-FME ASC devices. Acknowledgments This work was partly supported by the National Natural Science Foundation of China (51203071, 51363014, 51463012, and 51763014), China Postdoctoral Science Foundation (2014M552509, and 2015T81064), Natural Science Funds of the Gansu Province (1506RJZA098), and the Program for Hongliu Distinguished Young Scholars in Lanzhou University of Technology. References [1] Chen, G. F.; Li, X. X.; Zhang, L. Y.; Li, N.; Ma, T. Y.; Liu, Z. Q., A Porous Perchlorate-Doped Polypyrrole Nanocoating on Nickel Nanotube Arrays for Stable Wide-Potential-Window Supercapacitors. Adv. Mater. 2016, 28 (35), 7680-7687, 22


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For Table of Contents Use Only

26


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SYNOPSIS: Nickel hydroxide flexible membrane electrode (Ni(OH)2-FME) is prepared by water directly induced phase inversion (WIPS-FME), and water vapor induced phase inversion

(VIPS-FME)

methods,

respectively.

VIPS-FME

has

excellent

electrochemical properties due to its symmetrical pore structure and excellent electrolyte-affinity.

27


Transferring Electrochemically Active Nanomaterials into a Flexible

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