Supporting Information. For

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1 Supporting Information For Engineered Asymmetric Heterogeneous Membrane: A Concentration-Gradient-Driven Energy Harvesting Device Zhen Zhang, Xiang-Yu Kong, Kai Xiao, Qian Liu, Ganhua Xie, Pei Li, Jie Ma, Ye Tian, Liping Wen,*, and Lei Jiang, Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing , P. R. China Laboratory of Bioinspired Smart Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing , P. R. China Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, Beijing Normal University, Beijing , P. R. China wlp@iccas.ac.cn; wen@mail.ipc.ac.cn Content of Supporting Information 1. Preparation of the porous PET membrane with conical nanochannels 2. BCP membrane formation process 3. Cryo-SEM characterization 4. Fluorescence characterization 5. Limited BCP solution penetration into PET pores 6. Electrical measurement 7. Rectification comparison 8. Numerical simulation 9. Effect of the BCP solution penetration 10. Electrode calibration 11. Reversed salinity gradient 12. Comparison with the commercial anion exchange membranes 13. Reference S1

2 1. Preparation of the porous PET membrane with conical nanochannels Poly(ethylene terephthalate) (PET, 12 μm thick) film was irradiated with single swift heavy ion (Au) of energy 11.4 MeV per nucleon at the UNILAC linear accelerator (GSI, Darmstadt, Germany). Before etching, each side of the PET membrane was treated by UV light for 1 hour. In order to fabricate the asymmetric conical nanochannel, the ion track polymer membrane was subsequently chemically etched at fixed temperature (about 303K) from one side with 9M NaOH, whereas the other side of the membrane was in contact with acidic stopping solution (1 M KCl/1 M HCOOH) to neutralize the etchant as soon as the pore opened. A voltage of 1 V was applied across the membrane to monitor the etching process (Figure S1). Figure S1. Schematic of the etching setup with the conductivity cell. S2

3 2. BCP membrane formation process During the spin-coating process, a gradient of solvent concentration (Φ S ) exists normal to the surface as a function of the depth r. The concentration of solvent at the surface is the lowest. As the solvent evaporates, the block copolymer molecules undergo microphase separation to form a layer with hexagonally packed cylindrical channels (~10 nm) perpendicular to the surface. While, in the interior of the membrane, the non-favorable interaction between the two components is mediated by the gradually increasing solvent concentration. This would promote the formation of the disordered network-like layer, in which the major component (PS) constituting the matrix (Figure S2). 1 Figure S2. The Cross-section SEM image of the as-prepared asymmetric BCP membrane and the schematic of the asymmetric membrane formation process, scale bar: 400 nm. S3

4 3. Cryo-SEM characterization The Cryo-scanning electron microscopy characterization (Cryo-SEM) technique was used to observe the ph responsive conformational changes of the P4VP chains. Firstly, the BCP membrane on silicon wafer was covered with an ultrathin layer of aqueous solutions with different ph values (Figure S3a). Secondly, the BCP membrane sample was loaded on the Cryo-specimen holder and treated in ultralow-temperature slush nitrogen (-210 ). Then the membrane sample was transferred under vacuum to a chamber (Quorum Technologies, PP300T, East Sussex, UK) attached to the microscope. To improve the contrast and sublimate any free-water on the membrane surface, the temperature of the sample was raised to approximately 90 C within 5 min. Then the temperature was decreased to about 130 C to stabilize the sample. The surface of the frozen sample surface was coated with platinum for 30 s at 10 ma to avoid charging problems and improve the signal-noise ratio. Finally, the sample coated with platinum was analyzed at a temperature of 180 C (FEI, Helios Nanolab 600i, Hillsboro, USA). When the BCP membrane was placed in solution with ph around 4.3, the porous morphology disappeared (Figure S3b) because the swollen P4VP chains filled up the pores. When the BCP membrane was placed in alkaline solution (ph~ 9.5), the porous morphology appeared (Figure S3c) as the P4VP chains changed into collapsed states. S4

5 Figure S3. Cryo-SEM characterization. (a) The experimental procedure of Cryo-SEM. (b,c) Cryo-SEM images of the BCP membrane immersed in solutions with different ph values (b: ph 4.3; c: ph 9.5), scale bar: 100 nm. S5

6 4. Fluorescence characterization The fluorescence characterization was used to study the inherent charged property of P4VP chains under different ph values. The selected fluorescent dye (Figure S4a) was the highly photostable sulfonated rhodamine (pk a ~ 1.8). When ph value of the solution was above 1.8, the dye became negatively charged and it could bind selectively to the positively charged P4VP chains through noncolvent electrostatic interaction, and not to the neutral P4VP chains. In the experiment, the BCP membrane was immersed in dye solutions with different ph values beforehand, and then the treated membrane sample was washed sufficiently with deionized water (~10 min). The fluorescence images of the BCP membrane treated by dye solutions with different ph values are shown in Figure S4. When the BCP membrane was treated with acidic solution (ph 4.3), the intense red fluorescence (Figure S4b) appeared, which indicated that the P4VP chains were positively charged. However, when the membrane was treated with alkaline solution (ph 9.5), the red fluorescence (Figure S4c) disappeared as the positively charged P4VP chains became neutral. Figure S4. Fluorescence characterization. (a) The structural formula of the photostable sulfonated rhodamine dye. (b, c) Fluorescence images of the BCP membrane treated by sulfonated rhodamine dye solution with different ph values (b: ph 4.3; c: ph 9.5), scale bar: 10 μm. S6

7 5. Limited BCP solution penetration into PET pores. Because the solvent evaporates within several seconds during spin coating, the BCP solution with its high viscosity only minimally penetrates into the small PET pores. From the SEM image of the BCP-PET interface (Figure S5), we can see obviously that the BCP slightly penetrates into the PET pore. In order to further confirm this observation, we did EDS analysis as an assisted proof. In our EDS spectra, small amount of N element can be detected (wt% = 1%) in zone 1, which value is lower than that of N element in our block copolymers (PS b-p4vp ) whose content is about 4%. This relatively small content is also reasonable because the PET material and penetrated BCP both exist in zone 1. Furthermore, in SEM-EDS measurement, the element with mass fraction above 0.1% can be used as guidance for qualitative analysis or semi-quantitative analysis. While in the zone 2, no N elemental concentration (wt% = 0%) can be detected because the zone 2 is composed of PET material absolutely where there is no N element. Based on these results, we conclude that the penetration of BCP into PET pores is very limited. The effect of the BCP solution penetration on the performance of the heterogeneous membrane is also investigated using numerical simulation in Figure S9. Figure S5. Cross-section SEM image of the BCP/PET interface and the corresponding EDS analysis of the zone 1 and zone 2. S7

8 6. Electrical measurement The ionic transport property of the heterogeneous membrane was studied by measuring the ionic current through the heterogeneous membrane. The ionic current was measured by a Keithley 6487 picoammeter (Keithley Instruments, Cleveland, OH). The heterogeneous membrane was mounted in between a two-compartment electrochemical cell. The Ag/AgCl electrodes were used to apply a transmembrane potential (Figure S6). Figure S6. Schematic of the electrochemical testing setup. S8

9 7. Rectification comparison. The rectification ratio of the separate PET membrane with conical nanochannels (tip: 50 nm) and the asymmetric BCP membrane exhibited slight ionic rectification (ratio ~ 3.0) due to their asymmetric geometries (Figure S7, 1, 2). Once the heterogeneous membrane was formed, ultrahigh ionic rectification with a ratio about 1075 was obtained which could be ascribed to the synergistic effect of large degree of geometry asymmetry and opposite charge distribution. If the PET membrane with conical nanochannels was modified with ethanediamine (positively charged), the rectification ratio of the membrane largely decreased to ~7.0 due to the loss of opposite charge distribution (Figure S7, 3). If the PET membrane with conical nanochannels was replaced by the PET membrane with cylindrical nanochannels (~ 50 nm), the rectification ratio decreased to ~ 230 due to the decreasing geometry asymmetry (Figure S7, 4). Figure S7. The rectification ratio comparison. (1) The separate PET membrane with conical nanochannels (tip: 50 nm). (2) The separate BCP membrane. (3,4) The heterogeneous membrane. (3) The PET membrane with conical nanochannels was modified with ethanediamine (positively charged). (4) The PET membrane with conical nanochannels was replaced by the PET membrane with cylindrical nanochannels (~50 nm). S9

10 8. Numerical simulation. Numerical simulation was carried out using commercial finite-element software package COMSOL (version 4.4) Multiphysics. The Poisson and Nernst-Planck (PNP) Equations is shown as below: 2 zifci ji Di ( ci ) RT F 2 zc i i (1) (2) j i 0 (3) To carry out the calculations, the Electrostatics (AC/DC Module) and Nernst Planck without Electroneutrality modules are used. Equation (1) is the flux equation for each ionic species (Nernst-Planck Equation) which physically describes the transport properties of a charged nanopore, where j i, D i, c i, z i and φ are the ionic flux, diffusion coefficient, ion concentration, valence number for each species i, and electrical potential, respectively. F, R, and T are the Faraday constant, universal gas constant, and absolute temperature, respectively. Equation (2) is the Poisson equation which describes the relationship between the electrical potential and ion concentrations, where ε represents the dielectric constant of the electrolyte solutions. The system is generally simplified by assuming steady-state conditions, and the flux should satisfy the time-independent continuity Equation (3) when the system reaches a stationary regime. The coupled Equation (1) to (3) must be solved for given geometry using appropriate boundary conditions. A sketch of the computation domain for the heterogeneous channel is shown in Figure S8. The simulation system contains a heterogeneous channel connected by two electrolyte reservoirs. Two surface (W1 and W2) correspond to the electrodes was used to apply a transmembrane potential and are held at constant potential. The heterogeneous channel contains a 12-μm-length conically shaped PET channel (tip: 50 nm; base: 500 nm) connected by a cylindrical BCP nanochannels array. As the inner surface of the BCP nanochannel are covered by the swollen P4VP chains, here we simplify the nanochannel of BCP S10

11 membrane to be a straight channel with a diameter of 3 nm which is less than the average value (8~12 nm) observed from AFM image. 3 The charges of polymer chains are strictly confined to the BCP channel walls. 4 The introduction of two electrolyte reservoirs helps to decrease the effect of entrance/exit mass transfer resistances on the overall transport of ions. 2 The ion flux has the zero normal components at boundaries, n j 0 (4) The boundary condition for the potential φ on the channel walls is, n (5) The parameter σ is the surface charge density of the channel walls. Then the ionic current can be calculated by I F( z j z j ) nds s p p n n (6) For ionic rectification simulations, the ionic concentration in both reservoirs is 0.01 M. The ionic current under +2 V and -2 V bias are calculated using equation (6). Then the rectification ratio can be obtained by I+2V Ratio I -2V (7) For ion selectivity and power generation calculations, the system is placed in a concentration gradient (c PET /c BCP = 0.01 M/0.5 M) without external bias. Under a concentration, the ion selectivity of the membrane will lead to the differences in the diffusion rates of the anions and cations, consequently results in diffusion potential (E diff ) which is contributed by the bipolar power source. The whole anion selectivity can be quantified via the calculated anion transference numbers. The corresponding anion transference number can be obtained by t n In I I n p (8) S11

12 The net diffusion current is the obtained by I I I diff n p (9) Figure S8. Numerical simulation model. The simulation system contains a heterogeneous channel connected by two electrolyte reservoirs. The heterogeneous channel contains a 12-μm-length conical PET channel (tip: 50 nm; base: 500 nm) connected by a cylindrical BCP nanochannels array. The introduction of two electrolyte reservoirs helps to decrease the effect of entrance/exit mass transfer resistances on the overall transport of ions. (Drawing not to scale.) S12

13 Table S1: The simulation parameters for ionic rectifying calculation. L BCP (nm) σ PET (C/m 2 ) σ BCP (C/m 2 ) I -2V (ma/m) I +2V (ma/m) Ratio Table S2: The simulation parameters for selectivity and power generation calculation. c PET c BCP σ PET σ BCP I p I n Diffusion current (mm) (mm) (C/m 2 ) (C/m 2 ) (μa/m) (μa/m) t n (μa/m) S13

14 9. Effect of the BCP solution penetration The effect of the BCP solution penetration on the performance of the heterogeneous membrane is also investigated using numerical simulation. The simulation model is shown in Figure S9a. The effect of solution penetration is simplified to be the charge inversion of the tip side. As the penetration length increases, the simulated rectification ratio slightly decreases and the anion transference number slightly increases (Figure S9b, c). When the penetration depth is 50 nm, the rectification ratio decreases about 20 and the anion transference increases about Thus, the effect of BCP penetration on the performance of the heterogeneous membrane is very weak and can be negligible. Figure S9. Effect of the BCP solution penetration. (a) Numerical simulation model of BCP solution penetration. The model is similar to the model in Figure S8. The effect of solution penetration is simplified to be the charge inversion of the tip side, and H represents the penetration depth. (Drawing not to scale.) (b) Effect of the BCP solution penetration on the rectification. (c) Effect of the BCP solution penetration on the ion selectivity. S14

15 Table S3: Effect of the BCP solution penetration on the rectifying property. H (nm) σ PET (C/m 2 ) σ BCP (C/m 2 ) I -2V (ma/m) I +2V (ma/m) Ratio Table S4: Effect of the BCP solution penetration on the ion selectivity. c PET c BCP H σ PET σ BCP I p I n (mm) (mm) (nm) (C/m 2 ) (C/m 2 ) (μa/m) (μa/m) t n S15

16 10. Electrode calibration The energy conversion properties were studied by applying sweeping voltages. Figure S10 shows the basic equivalent circuit of the heterogeneous membrane power source system under a concentration gradient. R channel represents internal resistance of the heterogeneous membrane. E redox, E diff and V app represent the redox potential on the electrodes, the diffusion potential of the bipolar power source, and the electrical potential measured from the sourcemeter testing apparatus, which satisfy the relationship below: 5 V E E app diff redox The observed intercept on the current axis represents short-circuit current which is the net current when no bias is applied. The intercept on the voltage axis (V app ) is contributed by E redox and E diff. E redox is produced at the electrodes by the unequal voltage drops. And the diffusion potential (E diff ) arises from the differences in the diffusion rates of the anions and cations, due to the ion selectivity of the bipolar membrane. Only the diffusion potential (E diff ) is contributed by the bipolar power source. To subtract the contribution of the redox potential on the electrodes, electrolyte calibration was performed using an experimental method which could largely preclude the influence bought by other unexpected factors, such as electrolyte imperfection or contamination etc. 6 We used a nonselective silicon membrane containing a single 10-μm-width micro-window to replace the heterogeneous membrane in which case the measured voltage was contributed solely by the redox reaction on the electrode/electrolyte interface (E redox ).Then E diff was calculated as: E V E diff app redox The corresponding V app, E redox, and E diff are shown in Table S5. S16

17 Figure S10. The basic equivalent circuit of the heterogeneous membrane power source under a concentration gradient. Table S5: The corresponding V app, E redox, and E diff. Concentration gradient 10-5 / / / / / / /3 0.01/0.5 (M/M) V app (mv) E redox (mv) E diff (mv) S17

18 11. Reversed salinity gradient. Figure S11. Under reversed concentration gradient from PET side to BCP side, for example, c PET /c BCP = 3 M/10 μm, the inner resistance of the bipolar membrane system under a concentration (r 0 = V OC / I SC ) largely increased about 50%. S18

19 12. Comparison with the commercial anion exchange membranes The energy conversion performance of three commonly used commercial reverse electrodialysis membranes 7 including Ionsep (Iontech, China), Neosepta (Tokuyama Corporation, Japan) and Qianqiu (Hangzhou QianQiu Industry Co., China) are also tested under identical experimental conditions. When 0.5 M NaCl is mixed with 0.01 M NaCl, the open-circuit voltage (V OC ) of our heterogeneous membrane is slightly lower than that of the commonly used reverse electrodialysis membranes, but the short-circuit current density (J SC ) is much larger (Table 6). Consequently, the overall output power density of our heterogeneous membrane is about two times higher. The commonly used anion exchange membranes have higher anion selectivity because the size of the ionic species is comparable to the channel width. However, the ion transport through such membrane channels with symmetric charge distribution would encounter great steric hindrance and severe concentration polarization, which would result in low ionic conductivity. With regard to our new designed heterogeneous membrane, the anion selectivity is not as high as those commercial anion exchange membranes due to its larger channel width, which results in a lower V OC. However, the wider channel is able to overlook the non-electrostatic steric hindrance and the asymmetric bipolar structure helps to eliminate the concentration polarization effect, in which case the transmembrane ionic conductivity can be greatly enhanced. The greatly enhanced ionic conductivity would result in a much larger J SC, which consequently promotes the power density. Table 6: Comparison with the commercial anion exchange membranes. Membrane type J SC (A/m 2 ) V OC (mv) P max (W/m 2 ) P L (W/m 2 ) Ionsep Neosepta Qianqiu Our membrane S19

20 13. Reference (1) Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. Adv. Mater. 2004, 16, 226. (2) White, H. S.; Bund, A. Langmuir 2008, 24, (3) Qiu, X. Y.; Yu, H. Z.; Karunakaran, M.; Pradeep, N.; Nunes, S. P.; Peinemann, K. V. ACS Nano 2013, 7, 768. (4) Zhang, H.; Hou, X.; Zeng, L.; Yang, F.; Li, L.; Yan, D.; Tian, Y.; Jiang, L. J. Am. Chem. Soc. 2013, 135, (5) Kim, D. K.; Duan, C.; Chen, Y. F.; Majumdar, A. Microfluid. Nanofluid. 2010, 9, (6) Gao, J.; Guo, W.; Feng, D.; Wang, H.; Zhao, D.; Jiang, L. J. Am. Chem. Soc. 2014, 136, (7) Veerman, J.; de Jong, R. M.; Saakes, M.; Metz, S. J.; Harmsen, G. J. J. Membrane Sci. 2009, 343, 7. S20