High-Performance Ionic Diode Membrane for Salinity Gradient. Power Generation

Transcription

1 Supporting Information High-Performance Ionic Diode Membrane for Salinity Gradient Power Generation Jun Gao, Wei Guo,*,,ǁ Dan Feng, Huanting Wang, Dongyuan Zhao,*, 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. Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai , China. ǁ Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing , P. R. China Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia. * To whom correspondence should be addressed: Wei Guo (wguo@iccas.ac.cn), Dongyuan Zhao (dyzhao@fudan.edu.cn), and Lei Jiang (jianglei@iccas.ac.cn). Content: 1. Supporting Texts 1.1 Fabrication and characterization 1.2 Electrical measurement 1.3 ph-dependent ionic rectification 1.4 Energy conversion efficiency 1.5 Comparison with ion-exchange membranes 1.6 Model calculation 2. Supporting Figures S1-S11 and Tables S1-S4 3. References S1

2 1. Supporting Texts 1.1 Fabrication and characterization The fabrication procedure is summarized in Fig. S2. In the first step, a 60- m-thick macroporous alumina film (MacroA, pore width is 84 ± 16 nm, Heifei Puyuan Nano, China) was coated with 10 w% solution of PMMA (ca. 550, 000 in molecular weight, Alfa Aesar) in acetone (Sinopharm, China). After dried in room temperature for 2 h, it was heated at 200 C for 5 h to ensure that the PMMA entered into the alumina pores. Then the PMMA outside the MacroA was gently removed with sandpaper (500-mesh) to expose the PMMA filled nanopore area. Then, the film was cleaned with water and ethanol (Sinopharm, China) followed by drying in nitrogen gas. Afterwards, the film was coated with mesoporous carbon (MesoC, FDU-16) precursor solution. After 3 h in room temperature to trigger the evaporation-induced self-assembly (EISA), the film was heated at 120 C in an oven for 24 h to polymerize the precursor. In the next step, the precursor was carbonized at 450 C for 3 h in 500 sccm argon flow. The heating rate was 1 C/min. At such high temperature, the residual PMMA in the alumina pores was decomposed completely. The FDU-16 precursor was synthesized according to our previous work 1. Briefly, the phenolic resol was prepared by polymerizing phenol (Sinopharm, China) and formaldehyde (Sinopharm, China) with base catalyst. Resol and Pluronic F127 (Alfa Aesar) was dissolved in ethanol to prepare the FDU-16 precursor solution. The molar ratio of phenol: formaldehyde: NaOH: F127 was 1: 2: 0.1: The FDU-18 precursor was also prepared using reported methods 2. FDU-18 has larger pore size than FDU-16. Phenolic resol was dissolved in THF at the concentration of 20 w%. PS-b- PEO polymers of ca and molecular weight were synthesized via ATRP and also S2

3 dissolved in THF at the concentration of 2 w%. Then 1.5 g resol solution was mixed with 2.5 g PS-b-PEO solution and stirred for 3 h. The resultant FDU-18 has mean pore size of 23 nm and 33 nm, respectively, for PS-b-PEO polymers of the above mentioned molecular weights. Note that PMMA is soluble in THF, so the FDU-18 precursor was dried at 40 C instead of room temperature to accelerate the evaporation of THF. The construction of PMMA sacrificial layer in and onto the MacroA film is crucial for the fabrication of longitudinal heterojunction structure between the MesoC and MacroA layers. If the alumina pores are not blocked sufficiently, the MesoC precursor solution would enter the nanopores and would not form a film on top of the MacroA film. The concentration of the PMMA should not be too high; otherwise, viscous PMMA solution is difficult to enter the pores. Also, the concentration should not be too low, for that the PMMA would form nanotubes that do not block the nanopores. We prepared a 10 w% PMMA in acetone and successfully blocked the nanopores in MacroA film (Fig. S3). The surface area and pore size distribution of the MesoC are determined by nitrogen sorption experiment. The hysteresis loop and the condensation step prove the uniform large cage-like mesostructure (Fig. S3). The surface area is calculated to be 499 m 2 g -1 and the volume of pores is 0.45 cm 3 g -1 (FDU-16). The peak value of the pore size is 6.67 nm. After carbonization at 450 C, many functional groups are present on the MesoC surface due to incomplete pyrolyzation 3. To further characterize them, we measured the infrared spectra of the MesoC film (Fig. S3). The peak at 3419 cm -1 indicates the existence of phenolic hydroxyl groups and the peaks at 1591, 1433 and 742 cm -1 indicate the existence of carboxyl groups. These groups would be negatively charged in solution. S3

4 1.2 Electrical measurement The samples were mounted in between the two halves of a custom-designed electrochemical cell, which contains 1.0 ml KCl solution in each cell 4. The I-V properties were measured with a Keithley 6487 picoammeter/voltage source (Keithley Instruments). The working electrode was placed on the MesoC side and the reference electrode on the MacroA side if not particularly mentioned. To study the ionic rectification properties, sweeping voltages from -2 V to +2 V was applied on the working electrode with the step voltage of 0.2 V. The ion concentration on the MesoC side and the MacroA side was kept equal (0.1 M KCl), if not specially mentioned. To study the energy conversion properties, sweeping voltages from -0.4 V to 0.4 V was applied with a step of 0.02 V. The concentration of the solution on the MesoC side is higher than that on the MacroA side if not particularly mentioned. The observed intercept on the current axis indicates a net current flow is generated when no external bias is applied. The intercept on the voltage axis is contributed by the redox potential on the electrode and the diffusion potential from the MesoC/MacroA membrane. Fig. S6 illustrates the equivalent circuit of the testing system. Under a concentration gradient, a potential drop is generated by the redox reaction on the electrode electrolyte interface (E Red ). But only the diffusion potential (E Diff ) is contributed by the cation-selective IDM. Acoording to the previous works by us and others 5,6, E Diff is calculated as: E Diff = E Mea - E Red Electrode calibration was performed in the same electrochemical cell as that in the current recording experiments. The separator membrane between two half-cells was replaced by a nonselective silicon membrane containing a single micro-window (side length~10 μm). In this case, S4

5 the measured voltage on the external circuit was contributed solely by the asymmetric redox reactions on the electrodes. The measured voltage on the external circuit (E Mea ), redox potential (E Red ), and diffusion potential (E Diff ) are summarized in Table S1. The calibration of the potential drop on the electrode electrolyte interface using an experimental method largely preclude the influence brought by these unexpected factors, such as the contamination or electrode imperfection, if there are any. In this way, the measured power generation is indeed from the asymmetric ion diffusion through the IDM. 1.3 ph-dependent ionic rectification The current rectification properties are also measured in symmetric and asymmetric ph conditions. Firstly, we fix the ph on the MesoC side to 2, and change the ph on the MacroA side from 2 (symmetric condition) to 9 (asymmetric condition). From the I-V measurements, one can see that the rectification ratio (f) in symmetric condition (f = 130) is higher than that obtained in asymmetric condition (f = 81) (Fig. S4a). Then, we fix the ph on the MesoC side to 9, and change the ph on the MacroA side from 9 (symmetric condition) to 2 (asymmetric condition) (Fig. S4b). But in this case, f measured in asymmetric condition (f = 274) is remarkably higher than that obtained in symmetric condition (f = 90). MacroA are positively charged in acidic solutions, and the charge density is reduced with the increasing ph. In contrast, MesoC is negatively charged in neutral and basic solutions, and the charge density is increased with the increasing ph. In the former case (Fig. S4a), the MesoC is negatively charged. The increase in ph from 2 to 9 on the MacroA side reduces the charge density on the alumina surface. Therefore, f decreases in the asymmetric ph condition. Similarly, in the latter case (Fig. S4b), the decrease S5

6 in ph from 9 to 2 on the MacroA side increases the charge density on the alumina surface, resulting in an enhanced f in the asymmetric ph condition. Based on these results, we can conclude that the asymmetric surface charge distribution is account for the high rectification ratio. Increasing the surface charge density in both the MesoC and MacroA parts promotes the rectification ratio. The highest rectification ratio is 274 for asymmetric ph condition. Interestingly, f at all ph conditions remains high (f > 80). This observation suggests that the structural asymmetry also plays a role in the highly rectifying system 7. On the other hand, from these results, one can see that the surface charge on the MesoC part dominate the rectification properties of the entire channel. 1.4 Energy conversion efficiency The energy conversion efficiency corresponding to the maximum power generation is calculated as 6, maxw 2 (2t p 1) (1) 2 where t p represents the transference number for cations. It actually quantifies the selective ion transportation through the IDM. When the membrane material is perfectly cation selective, t p has the value of 1. The maximum efficiency evaluated by this equation is 50%. The value of t p is given by: 2t -1= p RT zf Ediff c c H H ln( ) c cl L (2) where R, T, z, F,, c H, and c L represent the gas constant, temperature, charge valent, Faraday constant, activity coefficient of ions, high and low ion concentrations, respectively. Then the S6

7 efficiency can be calculated for a range of concentration gradient from 10 to folds. The concentration gradient was built by setting the concentration on MacroA side to 1 M and changed the concentration on MesoC side from 10 M to 3 M. The experimental conditions are identical with that shown in Fig. 4c. Results are shown in Fig. S8. As the concentration gradient increases, the efficiency ( maxw ) decreases gradually from 26.4% to 5.7%. 1.5 Comparison with ion-exchange membranes The energy conversion performance of the IDM is further compared with that produced by commercial ion-exchange membranes (the commonly used membrane materials in reverse electrodialysis) 8. Firstly, 1.0 ml of NaCl solution (0.5 M) was injected into one reservoir and another 1.0 ml of NaCl solution (0.01 M) was injected into the other reservoir to simulate the mixing of seawater and river water. Both the ion-exchange membranes and the IDM were tested with the same electrochemical cell and using the same electrodes. Four kinds of commercial cation exchange membranes were tested, including Nafion-110 (Dupont, USA), Ionsep (Iontech, China), CMI (Membrane International, USA), and FKS (Fumtech, Germany). These membranes were immersed in 1 M NaCl solution for 24 h before use. The output power (P R ) is directly measured on the external resistor load (R L ) as P R =I 2 R L, where I is the electric current through the resistor. Parallel measurements are conducted with a series of resistor load as that typically shown in Fig. 4d. The maximum power output (P max ) for each membrane type is summarized in Table 1. S7

8 1.6 Model calculation The ionic rectification mechanism was systematically analyzed by a theoretical model based on Poisson and Nernst-Planck (PNP) equations with proper boundary conditions 9, 2 F zc i i zfc i i ji Di( ci ) RT j 0 i (1) where, c, D, and j are the electrical potential, ion concentration, ionic flux, diffusion coefficient, and current density of ionic species i (i=p or n), respectively. p and n represent positive ions and negative ions, respectively. is the dielectric constant of the electrolyte solution. The diffusion coefficients for cations and anions are m 2 /s (we use KCl electrolyte for simplicity). The boundary condition for potential on the channel wall is, n (2) where is the surface charge density. On the reservoir wall, =0. On the walls of C1 and C2, is the surface charge density of MesoC. On the walls of A1 and A2, is the surface charge density of MacroA. The ion flux has the zero normal components at boundaries, n j 0 (3) Then, the ionic current can be calculated by F I i jds i D( c i zici ) ds RT (4) The calculated region is illustrated in Fig. S5. For rectification calculations, the external potential is applied on the boundary w1, and w2 offers the reference potential. The ionic S8

9 concentration in both reservoirs is 0.1 M. The ionic current under +2V and -2V bias are calculated and the rectification ratio (f) is obtained by f =I +2 V /I -2 V. For power generation calculations, no external potential bias is applied between w1 and w2. The ionic concentration on w1 is 0.5 M and that on w2 is 0.01 M. The diffusion potential generated by the selective ion diffusion is estimated as, RT ch E diff ( t p tn) ln( ) F c (5) where c H and c L are the bulk concentration at the concentrated and diluted region, respectively. Here c H =0.5 M and c L =0.01 M. t is the transference number. t p is obtained by, L t p I P I p I n (6) The exact fluidic pathway inside the mesoporous carbon film is complex. To gain an affordable computation scale, we use a 4-nm-width cylindrical nanochannel array to simulate the mesochannels inside the MesoC film and a 60-nm-width nanochannel to simulate the case in MacroA. The geometric parameters are estimated from the SEM images shown in Fig. 2e and 2f. The calculation region is illustrated in Fig. S5. Two electrolyte reservoirs are connected by a two-segment nanochannel composed of the MesoC and MacroA parts. The outer and inner surface of the MesoC channels is negatively charged and that of the MacroA channels is positively charged. Each reservoir has a width of 500 nm and a height of 1000 nm. The length and charge density of MesoC and MacroA for all the calculations is listed in Table S2 and S3. To carry out the calculations, the Comsol Multiphysics was used with the electrostatics (Poisson equation) and Nernst-Planck without Electroneutrality modules. The stationary solver was generally used. But when it fails, the parametric solver was applied. For all the calculations, the accuracy is set to be less than S9

10 2. Supporting Figures S1-S11 and Tables S1-S4 Figure S1. Schematic illustration of the cation-selective ionic diode membrane for harvesting salinity gradient power. The IDM comprises structural and electrostatic heterojunctions between mesoporous carbon (negatively charged) and macroporous alumina (positively charged) layers, which selectively and preferentially facilitates cation transport that substantially promotes the power density. S10

11 Figure S2. Schematic illustration of the fabrication procedure. a, The thickness of the pristine MacroA substrate is 60 m. The mean pore size is ca. 80 nm. b, The MacroA film was coated with PMMA solution followed by drying in room temperature and heating at 200 C for 5 h. c, The PMMA outside the alumina pores was removed with sandpaper to expose the alumina surface containing nanopores. d, The PMMA filled MacroA film was coated with MesoC precursor solution. e, After evaporation-induced self-assembly (EISA) in room temperature for 3 h, regular mesostructure was formed on top of the MacroA film. f, The mesostructure was then carbonized at 450 C in argon flow, resulting suspended MesoC layer on top of the MacroA film. The residual PMMA was decomposed at the same time. S11

12 Figure S3. Characterization of the IDM. a, The nanopores in the MacroA film are thoroughly blocked by PMMA. A layer of PMMA can be also found on top of the MacroA surface. Scale bar: 10 m. b, Magnified view of (a). Scale bar: 1 m. c, The nitrogen adsorption isotherm indicate a large surface area of 499 m 2 g -1 and a large volume of pores of 0.45 cm 3 g -1. d, The pore size distribution of the MesoC. The peak value is found at 6.67 nm. e, Fourier transform infrared spectra of the MesoC film. The peak at 3419 cm -1 indicates the phenolic hydroxyl groups and the peaks at 1591, 1433 and 742 cm -1 indicate the carboxyl groups. S12

13 Figure S4. ph-dependent ionic rectification. a, The ph on MesoC side is fixed to 2, and the ph on MacroA side is changed from 2 to 9. b, The ph on MesoC side is fixed to 9, and the ph on MacroA side is changed from 9 to 2. S13

14 Figure S5. Calculation model of the MesoC/MacroA herterojunction. Two electrolyte reservoirs are connected by a two-segment nanochannel composed of the MesoC and MacroA parts. For simplicity, we use a 4-nm-width cylindrical nanochannel array to simulate the fluidic pathway inside the MesoC film and a 60-nm-width nanochannel to simulate the case in MacroA. Both the outer and inner surface of the nanochannels are charged. Details of the model parameters can be found in Tables S2-S4. S14

15 Figure S6. Equivalent circuit of the ion diffusion through the IDM. S15

16 Figure S7. Model calculation verifies the power generation from selective ion diffusion. The diffusion current and the cation transference number go up with the surface charge density. The charge selectivity of the IDM is far from ideal permselectivity for cations. All the tests shown here is to simulate the mixing of artificial seawater (0.5 M) and river water (0.01 M) on the two sides of the IDM. The length ratio of the MesoC/MacroA is 1:1. The length of MesoC is 500 and 2000 nm, respectively. The surface charge density varies from 0.04 to 0.08 C/m 2. S16

17 Figure S8. Energy conversion efficiency. The concentration on the MacroA side was 1 M and the concentrations on the MesoC side were changed from 10 M to 3 M. The efficiency decreases with the concentration gradient from 26.4% to 5.7%. S17

18 Figure S9. Power generation from IDM composed of different MesoC materials with varied pore size. The pore size of the MesoC is ca. 7 nm (FDU-16), 23 nm (FDU-18), and 33 nm (FDU-18), respectively. The electric power is obtained by mixing 0.5 M and 0.01 M KCl solutions. The maximum output power density decreases with the pore size of MesoC. This is because, for larger pore size, the surface-governed property is less dominant. S18

19 Figure S10. Permeation tests across the IDM with two oppositely charged fluorescent dyes, phycoerythrin (PE, negatively charged) and propidium iodide (PI, positively charged). (a) A piece of IDM is placed on a slide. A droplet of feed solution (PBS, ph 7.4) containing ionic fluorescent dyes (either PE, 0.3 μg/ml or PI, 12.5 μg/ml) is added onto the membrane from the MesoC side. A confocal fluorescent microscope (Olympus FV1000) is used to image the permeation of fluorescent dyes through the membrane from the perpendicular direction. 10 (b, c) The fluorescent images of the membrane surface show that only the positively charged dyes (PI) are permeable across the IDM. The negatively charged dyes (PE) are excluded from the membrane and cannot pass through. This observation justify that the bipolar membrane is cationselective, determined by the negative surface charge of MesoC. S19

20 Figure S11. Time-dependence of the diffusion current. Under the concentration gradient of 0.1 M/1 μm, the diffusion current through the IDM slowly decreases with time mainly due to the dissipated concentration gradient, and also some uncontrolled reasons, such as CO 2 dissolution. In our experiment, the electrical measurement of the short-circuit current and the open-circuit potential is accomplished within 5 minutes. Before each measurement, the testing electrolyte solutions are refreshed. In this case, the decrement of the diffusion current could be limited within 11.2% during the test. S20

21 Table S1. The measured potential, redox potential, and diffusion potential. Concentration gradient (M/M) Measured potential (mv) Redox potential (mv) Diffusion potential (mv) 10-6 / / / / / / /3 0.01/ S21

22 Table S2. List of model parameters for ionic rectification calculation. L C (nm) L A1 (nm) C (C/m 2 ) A (C/m 2 ) I +2 V (ma/m) I -2 V (ma/m) Rectification ratio (f) Data shown in the manuscript Fig. 5a Fig. 5a Fig. 5a Fig. 5d Fig. 5d Fig. 5d Fig. 5d Fig. 5c Fig. 5c Fig.5a and Fig. 5c Fig. 5c Fig. 5c Fig. 5c S22

23 Table S3. List of model parameters for energy conversion calculation. L C (nm) L A1 (nm) C (C/m 2 ) A (C/m 2 Data shown in ) I net (ma/m) I p (ma/m) t p the manuscript Fig. S Fig. S Fig. S Fig. S Fig. S Fig. S Fig. S Fig. S Fig. S Fig. S7 Table S4. List of model parameters in Figure 7. L C (nm) L A1 (nm) C (C/m 2 ) A (C/m 2 ) Data shown in the manuscript ~0.06 Fig. 7a ( ) ~ Fig. 7a ( ) ~0.07 Fig. 7b Fig. 7c (membrane 2#) 4000 / / Fig. 7c ( membrane 1#) S23

24 3. References 1 Feng, D.; Lv, Y.; Wu, Z.; Dou, Y.; Han, L.; Sun, Z.; Xia, Y.; Zheng, G.; Zhao, D. J. Am. Chem. Soc. 2011, 133, Deng, Y.; Cai, Y.; Sun Z.; Gu, D.; Wei, J.; Li, W.; Guo, X.; Yang, J.; Zhao, D. Adv. Funct. Mater. 2010, 20, Wu, Z.; Webley, P. A.; Zhao, D. Langmuir 2010, 26, Cao, L. X.; Guo, W.; Ma, W.; Wang, L.; Xia, F.; Wang, S. T.; Wang, Y. G.; Jiang, L.; Zhu, D. B. Energy Environ. Sci. 2011, 4, Guo, W.; Cao, L. X.; Xia, J. C.; Nie, F. Q.; Ma, W.; Xue, J. M.; Song, Y. L.; Zhu, D. B.; Wang, Y. G.; Jiang, L. Adv. Funct. Mater. 2010, 20, Kim, D. K.; Duan, C.; Chen, Y. F.; Majumdar, A. Microfluid. Nanofluid. 2010, 9, Cheng, L. J.; Guo, L. J. Chem. Soc. Rev. 2010, 39, Veerman, J.; de Jong, R. M.; Saakes, M.; Metz, S. J.; Harmsen, G. J. J. Membr. Sci. 2009, 343, 7. 9 Liu, Q.; Wang, Y.; Guo, W.; Ji, H.; Xue, J.; Ouyang, Q. Phys. Rev. E 2007, 75, Vlassiouk, I.; Apel, P.; Dmitriev, S.; Healy, K.; Siwy, Z.; Proc. Natl. Acad. Sci. USA 2009, 106, S24