Supporting Information. Ultrathin and Ion-selective Janus Membranes. for High-performance Osmotic Energy Conversion

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1 Supporting Information for Ultrathin and Ion-selective Janus Membranes for High-performance Osmotic Energy Conversion Zhen Zhang,,, Xin Sui,, Pei Li, Ganhua Xie,, Xiang-Yu Kong, Kai Xiao,, Longcheng Gao,*, Liping Wen,*,,, and Lei Jiang,, Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing , P. R. China School of Chemistry and Environment, Beihang University, Beijing , P. R. China Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing , P. R. China University of Chinese Academy of Sciences, Beijing , P. R. China S1

2 Supporting Methods Characterization : 1 H NMR measurments were carried out by Bruker AV-400 spectrometer. The molecular weight of the polymers was determined by gel permeation chromatography (Waters 2410 instrument equipped with a Waters 2410 refractive index detector), with THF as the eluent at a flow rate of 1.0 ml/min. Supporting Note 1 Characterization of the BCP-1 Molecule In this work, the BCP-1 molecule was chosen such that one block of the polymer was crosslinkable, allowing it to remain as the nanopore template. Meanwhile, the other block was sacrificial and was removable under mild conditions. The synthetic procedure is shown in Figure S1 and the experimental details are described in the main text (Experimental Section). Compared to the PEO macroinitiator, a remarkable unimodal shift to higher molecular weight is seen in Figure S2. The molecular weight of PEO macroinitiator is And the molecular weight of BCP is and a polydispersity index of From the 1 H NMR spectrum (Figure S3), the peaks between 6.5 and 8 represent aromatic protons of the chalcone groups, and the chemical shift at 3.6 is corresponding to the proton of PEO. From the integration of the corresponding peaks, we can get the weight fraction of PEO in the BCP, which is 12.6%. S2

3 Supporting Note 2 Electrical Measurements The electrical measurements were performed with a Keithley 6487 semiconductor picoammeter (Keithley Instruments, Cleveland, OH). As shown in Figure S6, the heterogeneous membrane was mounted between a two-compartment conductivity cell. A pair of home-made Ag/AgCl electrodes was used to apply a transmembrane potential. For the ionic transport test, the anode was placed in the M-2 side and the two chambers were filled with symmetric concentration electrolyte solutions from 10 μm to 3 M. For the subsequent energy conversion, the M-1 side faced the permeate champers, and the feed and permeate chambers were filled with a high-concentration salt solution and a low-concentration salt solution, respectively. All the testing solution was adjust to ph value about 4.3. Supporting Note 3 Numerical Simulation The ionic rectification phenomenon and the concentration gradient driven ionic transport are both theoretically investigated using a commercial finite-element software package COMSOL (version 5.1) Multiphysics using electrostatics (Poisson equation) and Nernst-Planck without Electroneutrality modules. The coupled governing Poisson and Nernst-Planck equations are shown below: 1,2 zifci Ji Di ( ci ) ciu (1) RT 2 F zc i i (2) J (3) i 0 The equation (1) is the Nernst Planck equation which defines the ionic flux contributed by the diffusion current due to concentration gradients and the drift current induced by potential gradients. The electric potential denoted by the applied voltage satisfies the equation (2). The flux should satisfy the time-independent continuity equation (3) when the system reaches a stationary regime. The physical quantities Ј i, D i, c i, φ, u, R, F, T, and ε refer to the ionic flux, diffusion coefficient, ion concentration, electrical potential, fluid velocity, universal gas constant, Faraday constant, absolute temperature, and dielectric constant of the S3

4 electrolyte solutions, respectively. The coupled equations can be solved by assuming appropriate boundary conditions. The simulation model is shown in Figure S8. It contains a 400 nm long M-1 channel (pore size: 10 nm) and a 100 nm long M-2 channel (pore size: 17 nm), which is in consistent with the experimental geometry values. We introduced two electrolyte reservoirs are introduced in order to decrease the effect of entrance/exit mass transfer resistances on the overall ionic transport. The external voltage is applied on the boundary W1 and the wall W2 offered the reference potential. The ion flux has the zero normal components at boundaries: nj (4) i 0 The boundary condition for the potential φ on the channel walls is: n (5) where σ represents the surface charge density. The surface charge density of M-1 channel is set to C/m 2. Notably, with regard to M-2 channel, as the channel are filled with P4VP chains, we simplified that the charges of polymer chains are strictly confined to the channel walls. Therefore, the surface charge density of M-2 channel (+0.24 C/m 2 ) is set to two times larger than the surface charge density of M-1 channel. The ionic current through the nanochannel can be calculated: ( pjp njn ) n d (6) s I F z z S For the ionic rectification simulation, the concentration of the electrolyte in the two reservoirs is both set to 0.1 M and the applied potential is 2 V. For the energy conversion simulation, a concentration gradient (c M-1 /c M-2 = 0.01 M/0.5 M) is applied and there is no external potential. The corresponding diffusion current can be calculated: Idiff In Ip (7) In Figure 6c, the total length of the naked BCP-1 nanochannel and the hybrid nanochannel is varied proportionally from 500 nm to 5000 nm, and a series of I diff can be obtained. The simulation parameters of Figure 6c are shown in Table S1. S4

5 Supporting Note 4 Electrode Calibration The energy conversion property is investigated by measuring the scanning I V cycles in the presence of a concentration gradient across the membrane. The sweeping voltages from -0.2 V to 0.2 V was applied with a step of 0.02 V. As shown in the equivalent circuit (Figure S11), the measured V OC actually consists of two parts: the diffusion potential (E diff ) which is contributed by the power source and the redox potential (E redox ) that is generated by the unequal potential drop at the electrode solution interface. The diffusion potential can be calculated as: Ediff Vapp Eredox The intercept on the voltage axis (V app ) is contributed by E redox and E diff. An experimental method was used to subtract the contribution of the redox potential on the electrodes. The electrode calibration was performed in the same electrochemical cell. The heterogeneous membrane was replaced by a nonselective silicon membrane containing a single 10-μm-width micro-window in which case the measured voltage was contributed solely by the E redox. This method could largely preclude the influence bought by other unexpected factors, such as electrolyte imperfection or contamination etc. 3 The obtained V app, E redox, and E diff are shown in Table S2. Supporting Note 5 Energy Conversion Efficiency For an anion-selective system, the energy conversion efficiency corresponding to maximum power generation, 1 max = (2t n 1) 2 max, can be calculated as: 4 where t n represent the anion transference number and can be given as: t n 1 Ediff ( 1) 2 RT c c H H In( ) zf c c L L where E diff, R, T, F, z, γ, and c refer to the diffusion potential, universal gas constant, absolute temperature, Faraday constant, charge number, activity coefficient of ions, ion concentration, respectively. The energy conversion efficiency under a series of S5

6 concentration gradient can be calculated (Figure S12). The energy conversion efficiency for our system operating in 5-fold, 50-fold, 500-fold concentration gradient is about 46.7%, 24.3%, 13.2%, respectively. S6

7 Figure S1. Synthetic procedure of the photocleavable diblock copolymer BCP-1. The block copolymer PEO 12.5k -hv-pma(chal) 44.7k was synthesized from PEO based macroinitiator containing photocleavable nitrobenzyl ester group by ATRP. The second block contains UV crosslinkable chalcone groups. After UV treatment, the degradation and crosslinking reaction occur at the same time. S7

8 Figure S2. The Gel Permeation Chromatography (GPC) curves. From the GPC curves of PEO macroinitiator and PEO 12.5k -hv-pma(chal) 44.7k, unimodal shift to higher molecular weight is seen, indicating successful polymerization. S8

9 Figure S3. 1 H NMR spectrum of BCP-1. From the spectrum, both the peaks from PEO and PMA(Chal) can be seen, indicating successful polymerization, as well as the content of the second block. S9

10 Figure S4. TEM image of the naked PEO-hv-PMA(Chal) film. The sample displays hexagonally packed pores oriented perpendicularly to the membrane surface. The film on the gird was thermally annealed at 80 C for 4 hours under vacuum. The PEO domains are stained dominantly with RuO 4 vapor to give a clear contrast. Figure S5. ZETA potential analysis of the naked thermally annealed PEO-hv-PMA(Chal) film after photo degradation and PEO extraction, indicating that the inner walls of the PEO-hv-PMA(Chal) nanochannels bear carboxyl groups (pk a ~ 3.8). S10

11 Figure S6. Schematic of the experimental setup. Figure S7. Optical image of the silicon wafer that contains an open window used to support the membrane for electrochemical measurement. The effective area testing area is about μm 2. S11

12 Figure S8. Numerical simulation model based on PNP theory. Figure S9. Theoretical electrical potential distribution. The electrical potential distribution along the axial line of the hybrid nanochannel, where x = 0 nm refers to the M-2 channel side, is calculated. Because the electric potential changed largely in the low ion density region due to the conservation of flux through the channel, the potential drop under positive bias across the M-1/M-2 interface was caused by the ion depletion effect. S12

13 Figure S10. Time-concentration curve of the permeation experiments using the naked M-1 membrane. Figure S11. The equivalent circuit diagram of the power source. The measure interpret on the voltage axis is composed of E diff and E redox, where only the E diff is contributed by the system. S13

14 Figure S12. The KCl concentration on the M-1 side is fixed at 10 μm, and the KCl concentration on the M-2 side is increased from 100 μm to 3 M. The corresponding energy conversion efficiency decreases from 17.5 % to 4.5 %. Figure S13. The power density values of the separate M-1 membrane. The density values are 0.09 W/m 2, 0.4 W/m 2, and 0.9 W/m 2 for the 5-fold, 50-fold, and 500-fold salinity gradient, respectively, which is much smaller than the energy output of the hybrid Janus membrane under the same concentration gradient. S14

15 Figure S14. Effect of the thickness of the Janus membrane on the power density. The output power density scales inversely with the membrane thickness. S15

16 Figure S15. Statistics analysis of the amount of the fully overlapped pores, partially overlapped pores, and dead pores. We first develop two arrays of pores mimicking the two polymer layers. The pores of the two layers are assumed to be perfect hexagonal packed distribution. The geometry parameters are set according to the AFM images. For the M-1 membrane, the pore size is set to 10 nm and the center-to-center distance is set to 31 nm. For the M-2 membrane, the pore size is set to 17 nm and the center-center distance is set to 42 nm. The ratio of these three types of pores is approximately 7.2 %, 46.8 %, 46 %, respectively. S16

17 Figure S16. Co-ion concentration profile under a salinity gradient (0.5 M/ 0.01 M) near the opening of the hybrid nanochannel. The concentration of co-ions (i.e. K + ) is higher than the bulk value, which decreases the effective concentration gradient of co-ions across the channel. Note that the high concentration in the boundaries of the channel walls is caused by strong ion absorption. The result indicates that our Janus membrane can not only promote the desired mixing of counter-ions, but also suppress the undesired mixing of the co-ions. S17

18 Table S1. The simulation parameters of Figure 6c. L M-1 (nm) L M-2 (nm) I p (μa/nm) I n (μa/nm) I diff (μa/nm) Table S2: The corresponding V app, E redox, and E diff. Concentration gradient (M/M) 10-5 / / / / / / /3 V app (mv) E redox (mv) E diff (mv) S18

19 Supporting References (1) Cervera, J.; Schiedt, B.; Neumann, R.; Mafé, S.; Ramírez, P. J. Chem. Phys. 2006, 124, (2) White, H. S.; Bund, A. Langmuir 2008, 24, (3) Ouyang, W.; Wang, W.; Zhang, H.; Wu, W.; Li, Z. Nanotechnology 2013, 24, (4) Kim, D. K.; Duan, C.; Chen, Y. F.; Majumdar, A. Microfluid. Nanofluid. 2010, 9, S19