Harvesting Salinity Gradient Power - ACS Publications

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Surfaces, Interfaces, and Applications

Sulfonated Subnano Channels in a Robust MOF Membrane: Harvesting Salinity Gradient Power Yi Guo, Hubiao Huang, Zhuoyi Li, Xiaobin Wang, Peipei Li, Zheng Deng, and Xinsheng Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b13617 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Sulfonated Subnano Channels in a Robust MOF Membrane: Harvesting Salinity Gradient Power Yi Guo,†§ Hubiao Huang,‡§ Zhuoyi Li,† Xiaobin Wang,† Peipei Li,† Zheng Deng,†,⊥ Xinsheng Peng†*

†State

Key Laboratory of Silicon Materials, Department of Materials Science and Engineering,

Zhejiang University, Hangzhou, Zheda Rd. 38, 310027, China

‡Department

of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1

Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. ⊥Shenzhen

Key Laboratory of Laser Engineering, College of Optoelectronic Engineering,

Shenzhen University, Shenzhen, Nanhai Road. 3688, 518060, China

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KEYWORDS: heparin threaded ZIF-8 membrane, rapid and selective ionic transport, salinity gradient power generation, low resistance, high output electric power

ABSTRACT: We developed a robust, crack-free ultrathin zeolite imidazole framework (ZIF-8) membrane in-built with sulfonate ions-containing polymer (ZIFHep) via a vapor-assisted in situ conversion process. The sulfonated subnano channels of the ZIFHep membrane afforded a rapid and selective transport for Li+ over counteranions and other alkali ions attributed to electrostatic repulsion and optimal transport kinetics of cation-sulfonate ion pairs. A salinity gradient power generator (SGPG) was built by using ZIFHep membrane as a cell separator coupled with a pair of Ag/AgCl porous membrane electrodes. At a salinity gradient of 105, such a power generator presented a significantly decreased internal resistance (25.6 Ω), three-order of magnitude lower than that reported previously, and an output power as high as 9.03 μW.


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1. INTRODUCTION

Ion channels that have aqueous pores are able to selectively gate ion flux, such as Na+, K+, Ca2+, across cell membrane for generating electric signals1-2. In living organisms, millions of open-state ion channels that span cross cell membranes are working synergistically to generate pronounced electric signals so as to accomplish a physiological task3-5. Inspired by such elaborate systems, a number of membrane-based artificial ion channels have been invented otherwise for electric energy harvesting (also known as salinity gradient power generation, SGPG)6-9. The primary considerations for designing of high-performance membrane-based SGPG are (1) high ion selectivity (2) large channel density and short channel length to realize rapid ion transport, and (3) small internal resistance. However, to construct a SGPG that meets all these requirements is still a challenge10-16. Metal-organic frameworks (MOF) with porous structure17-23 have demonstrated great potential in selective mass transport. Here we report an ultrathin yet robust, crack-free ZIFHep membrane, where the sulfonate ions of heparin are uniformly distributed in the ACS Paragon Plus Environment


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nanoporous framework, affording a highly selective nanochannels for transporting Li+ over counteranions and other alkali ions. A SGPG that is built upon such a ZIFHep membrane coupled with a pair of Ag/AgCl membrane electrodes shows a significantly decreased internal resistance (25.6 Ω), and maximum output power up to 9.03 μW, which is dramatically higher than that reported previously6, 12-14, 24. 2. EXPERIMENTAL SECTION Synthesis of ZIFHep|AAO membranes Zinc hydroxide nanostrands (ZHNs) were employed as the precursor for ZIF-8.

4 mM

Zn(NO3)2 water-ethanol solution (water/ethanol volume ration, 3:2) was mixed with equal volume of 1.6 mM 2-aminoethanol water-ethanol solution (water/ethanol volume ration, 3:2) quickly at room temperature. After aging for 30 min, the ZHNs were formed. 5 mL ZHNs solution was mixed with certain volumes of 0.01wt% heparin solution. Then the mixed solution was filtered on AAO substrate to prepare ZHNsHep hybrid thin film. Finally, the composite thin film was hanged above 2-methylimidazole (Hmim) powders in the Teflon container and heated at 120 ° C for 24 h. Then, the heparin threaded ZIF-8 layer coated AAO (ZIFHep|AAO) membrane was formed. When the volume of heparin solution in the above procedure was 0.2 mL, 0.4 mL, 0.6 mL,0.8 mL and 1.0 mL, the heparin contents of resulted ZIFHep membranes was 2%, 4%, 6%, 8% and 10%, respectively, named as ZIFHep2%, ZIFHep4%, ZIFHep6%, ZIFHep8%and ZIFHep10%. Electrochemical impedance spectra (EIS)


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The EIS was measured by electrochemical workstation (CHI 660D) in a frequency range of 1 MHz-1 Hz at AC amplitude of 10 mV. The power generators were assembled with a pair of home-made Ag/AgCl electrodes and ZIFHep membranes. The salinity gradient was fixed at 105 (1 μM/ 0.1 M). Ionic rectification A Keithley 6487 picoammeter (Keithley Instruments, Cleveland, OH) was employed to measure the ionic rectification of ZIFHep6% and AAO membranes.

The membranes were mounted

between a two-compartment electrochemical cell with a pair of homemade Ag/AgCl electrodes. The salinity gradient was fixed at 105. The voltage ranges from -2 V to 2 V with step of 0.1 V. Diffusion potential (Ediff) The asymmetric ZIFHep membrane was tested in a two-compartment conductivity cell. A pair of Ag/AgCl electrodes with saturated KCl bridge were employed to eliminate offset electrode potential and inserted in each side of the ZIFHep membrane. The LiCl concentration in the AAO size was fixed at 1 μM, while the ZIFHep side was filled with LiCl solution with concentration from 10 μM to 1 M. Then the I-V curve was recorded by electrochemical workstation (CHI 660D). The sweeping voltage from -0.4 V to 0.4 V was applied with a scan rate of 50 mV/s. The intercept on the voltage axis is equal to the transmembrane diffusion potential, which is equal to open ACS Paragon Plus Environment


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circuit voltage. Output power density The asymmetric ZIFHep|AAO membrane was test with a two-compartment conductivity cell. Transmembrane potential was applied by a pair of home-made Ag/AgCl membrane electrodes. The external resistance was adjusted from 10 Ω to 1 MΩ. A Keithley 6487 picoammeter (Keithley Instruments, Cleveland, OH) was used to measure the current in the circuit. Then the output power density (Poutput) was calculated by the formula of Poutput=I2R/S. The effective area membrane (S) of ZIFHep|AAO was 0.785 cm2. Energy conversion efficiency and lithium ion flux Theoretically, Li+ transported through ZIFHep6%|AAO membranes from the ZIFHep6% side to AAO side, while Cl- being blocked by the semipermeable membrane. Thus, the amount-of-substance of Li+ through the ZIFHep|AAO equals to that of electrons transport through the external circuit, when energy efficiency was 100%. In practice, because of the defects and low ion selectivity of the semipermeable membranes, the amount-of-substance of electrons through the external circuit is less than that of Li+ through ZIFHep. The energy conversion efficiency is calculated as: 𝑁𝑒

𝑄𝑒𝑥𝑡𝑒𝑟𝑎𝑛𝑙

η= 𝑁𝐿𝑖 = 𝑄𝑖𝑛𝑡𝑒𝑟𝑛𝑎𝑙

(1)

η: the energy conversion efficiency; ACS Paragon Plus Environment

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Ne: the amount-of-substance of electrons through the external circuit; NLi: the amount-of-substance of Li+ through the ZIFHep|AAO; Qexternal: the quantity of electric charge through external circuit; Qinteranl: the quantity of electric charge through internal circuit. The Qexternal was calculated from integration of I-t curve recorded by CHI 660D with external resistance of 50 Ω. After recording the I-t curve, the concentration of LiCl solution in the AAO side was measured immediately by electrical conductivity meter (Lei Ci, DDS-307A). The Qinteranl was calculated from the concentration increment of LiCl in the low concentration side solution. In addition, lithium ion flux was derived from the concentration increment of LiCl in the solution. 3. RESULTS AND DISCUSSIONS The ZIFHep membrane deposited on an anodic aluminum oxide film (ZIFHep|AAO) was fabricated by a vapor-assisted in situ conversion process25-28, involving three primary steps (Figure 1): (i) assembly positively charged zinc hydroxide nanostrands and negatively charged heparin into colloidal hybrid structures (ZHNsHep); (ii) filtrate the colloidal ZHNsHep on a porous AAO support (ZHNsHep|AAO) as a precursor film (Figure S1); (iii) bake ZHNsHep|AAO membrane in a saturated vapor of methyl imidazole at 120 °C for 24 h, affording a hierarchical-structured ZIFHep|AAO membrane (Figure 1). Powder X-ray diffraction (PXRD) characterizations of pulverized ZIFHep samples show a diffraction pattern similar to that of ZIF-8 (Figure 2a and Figure S2a), a type of MOFs known for its exceptional stability against harsh conditions29. This result demonstrates that ZHNs were readily converted into nanoporous crystalline ZIF-8 by reacting with methyl imidazole molecules in situ at an elevated temperature (Figure S3). In order to obtain a continuous crack-free ZIFHep membrane with high loading of pendant sulfonate ions, ACS Paragon Plus Environment


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ZIFHep membranes with varied weight ratio of heparin (ZIFHepx%) were prepared (Supporting Information). Scanning electron microscopic (SEM) observations show that a precursor ZHNsHep film with 6 wt% heparin could afford a continuous ZIFHep membrane (ZIFHep6%) without any noticeable cracks and its thickness was determined as 200 nm in average (Figure 2b, c and Figure S4). The composition analysis of the cross-section of ZIFHep6% membrane was conducted by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (Figure 2d). The sulfur element (Figure 2d, green) that is exclusively derived from sulfonate groups of heparin was evenly distributed cross the ZIFHep6% membrane (red and blue for N and Zn, respectively). Brunauer–Emmett–Teller (BET) analysis of the ZIFHep6% membrane shows a surface area of 914 m2 g-1 and pore volume of 0.48 cm3 g-1, which are slightly smaller than those of pristine ZIF-8, suggesting that the incorporation of heparin in the nanochannels of ZIFHep6% membrane didn’t compromise its porous nature (Figure S2b).

And the diameter of the

nanochannels in ZIFHep6% membrane is estimated as ~1 nm (Figure 2e). Fourier transform infrared spectroscopic (FTIR) analysis of ZIFHep6% shows two characteristic peaks at 1259 and 1238 cm-1 (Figure 2f and Figure S2c), indicative of free sulfonate ions in ZIFHep6%, which was further supported by X-ray photoelectron spectroscopic (XPS) (Figure S2d, e), where two characteristic peaks at 169.7 and 168.6 eV assigned for S2p1/2 and S2p3/2 of sulfonate ions, respectively, were found. All these results indicate that the subnano channels of ZIFHep6% membrane are highly negatively charged (Figure S2f). These properties made ZIFHep|AAO possible to selectively transport cations and block anions by size sieving and electrostatic interaction. The ion transport within the nanochannels of ZIFHep6%|AAO membranes was investigated by assembling the membrane into an electrochemical device coupled with a pair of home-made Ag/AgCl electrodes at the both sides of the membrane (Figure S5). The I-V curve of ZIFHep6%|AAO membrane that was recorded in 0.1 M LiCl solution exhibits an obvious ionic rectification effect with a ratio of 6.3 (defined as |I(+2V)/I(-2V)|) (Figure 3a), since the asymmetric charge distribution caused by positively charged AAO and negatively charged ZIFHep. It is favorable for fast and continuous cation diffusion from ZIFHep side to AAO side. Such an effect could not be observed if the supporting AAO membrane with a ACS Paragon Plus Environment

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pore size of 200 nm and porosity of 50% was tested. These observations suggest an asymmetric ion transport22 through ZIFHep6%|AAO membranes driven by external bias voltage.

Meanwhile, the

transmembrane conductance of the ZIFHep6%|AAO membrane was evaluated from I-V scans performed at a voltage from a range from −0.4 V to 0.4 V in LiCl solution with varied concentration (blue curve in Figure 3b). A nonlinear relationship between the transmembrane conductance and LiCl concentration, especially at the highly diluted conditions, was found. A similar tendency was also observed when NaCl, KCl and MgCl2 were used as saline solutions. It therefore indicates that the ion transport is governed by the surface charges of subnano channels in the ZIFHep6%|AAO membrane12-13. The ZIFHep6%|AAO membrane with an effective area of 0.785 cm2 was assembled into a SGPG as a cell separator, where a pair of home-made porous Ag/AgCl membrane electrodes were placed on the both sides of ZIFHep6%|AAO membrane (Figure 4a). Prior to the tests, the ionic diffusion potential (Ediff) of the ZIFHep6%|AAO membrane was evaluated under a salinity gradient ranging from 10 to 106 cross the membrane (Figure S6). We found that the Ediff value became comparable to each other when the salinity gradient was over 105. Based on this result, we tested the electrical properties at the salinity gradient value of 105, namely, by filling the ZIFHep6%-side compartment with 0.1 M saline solution and AAO-side one with 1 µM saline solution. Figure 4b shows the electrochemical impendence (EIS) of SGPG in varied saline solution. The calculated internal resistance of SGPG from EIS in LiCl solution was 25.6 Ω, lower than 103.7 Ω of ZIFHep0%|AAO (Figure S7) and far smaller than 30 kΩ of SGPG reported so far14. Such a low internal resistance of SGPG we have built may be attributed to a high density of negatively charged subnano channels and relatively short pathways for ion transporting. Notably, the internal resistance of SGPG varied when different saline solutions were used and it followed an order of LiCl

Harvesting Salinity Gradient Power - ACS Publications

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