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A Charge-Density-Tunable Three/Two Dimension - Polymer/Graphene Oxide Heterogeneous Nanoporous Membrane for Ion Transport Xuanbo Zhu, Yahong Zhou, Junran Hao, Bin Bao, Xiujie Bian, Xiangyu Jiang, Jinhui Pang, Haibo Zhang, Zhenhua Jiang, and Lei Jiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b03576 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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A Charge-Density-Tunable Three/Two Dimension Polymer/Graphene Oxide Heterogeneous Nanoporous Membrane for Ion Transport Xuanbo Zhu,† , ‡ Yahong Zhou, ‡, * Junran Hao,∥ Bin Bao,∥ Xiujie Bian,‡ Xiangyu Jiang,‡ Jinhui Pang,† Haibo Zhang,† Zhenhua Jiang,† ,* and Lei Jiang, ‡,∥ †

National & Local Joint Engineering Laboratory for Synthetic Technology of High Performance

Polymer, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡

CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics

and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ∥

School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China

KEYWORDS: ion channels, 3D pores, polymeric membrane, rectification, gating property, wetting, self-assembly.

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ABSTRACT: Design and fabrication of robust nanoporous membrane in large-scale is still being a challenge and is of fundamental importance for practical applications. Here, a robust three/two dimension - polymer/graphene oxide heterogeneous nanoporous membrane in large-scale via the selfassembly approach is constructed, by chemically designing a robust charge-density-tunable nanoporous ionomer with uniform pore size. To obtain nanoporous polymer which maintains high mechanical strength and promotes multi-functionality, we design a series of amphiphilic copolymer by introducing a positively charged pyridine moiety into engineering polymer polyphenylsulphone (PPSU). The multiphysical-chemical properties of the membrane enable it to work as a nanogate switch with synergy between wettability and surface charge change in response to pH. Then we systematically studied the transmembrane ionic transport properties of this two-/three-dimensional porous system. By adjusting the charge density of the copolymer via chemical copolymerization through controlled design route, the rectifying ratio of this asymmetric membrane could be amplified four times. Furthermore, we equipped a concentration-gradient-driven energy harvesting device with this charge-density-tunable nanoporous membrane, and a maximum power of ≈ 0.76 W m-2 is obtained. We expect this methodology for construction of charge-density-tunable heterogeneous membrane by chemical design will shed light on the material design and this membrane may further be used in energy devices, biosensor, and smart gating nanofluidic devices.

Nanoporous membrane has drawn great attention due to its huge potential application at separation, bio/chemical-sensing, energy devices, nanofluidic devices and so on.1-5 In general, membranes can be classified into one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) porous membrane according to the pore structure. The membrane with 1D pore is a good sample to build theoretical model.6-9 Membrane with 2D pores, like molybdenum disulphide (MoS2) and graphene oxide (GO) made of stacked layers, offers great opportunities in sensing, ionic and molecular sieving, due to defined uniform narrow pore size.10-15 For example, GO nanosheets as porous membrane have developed significantly since Nguyen constructed GO paper.16, 17 Membranes with 3D pores, especially ACS Paragon Plus Environment

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the polymeric ones, are facile to fabricate and offer great possibilities for application. Generally, the porous membranes are made by ion beam sculpting, electron-beam lithography, heavy-ion-irradiated, electrochemical etching, sacrificial template method, chemical vapor deposition and so on.18-24 However, art-of-lab state restrict the practical application for huge cost, low ion conductance and fragility. Self-assembly of membrane with 2D or 3D pores provides an effective route to produce macroscopic porous membrane for practical application, where both the pore size and the area of the membrane could be controlled.25, 26 For example, GO could be self-assembled into 2D nanosheets in large-scale.10, 27 And the 2D configuration GO with negatively surface charge and high packing density of the lamellar nanochannels provides multiple ionic transport properties.28-30 Membrane with 3D pores could also be obtained by self-assembly of amphiphilic block copolymers as well. Ionomers are a typical kind of copolymers which contain amphiphilic groups along the polymer chain, where the balance of repulsive interactions between dissimilar two segments induced spheres, cylinders, or interpenetrating networklike nanopores.31-35 For example, sulfonated poly (ether ether ketone) (SPEEK), traditional ionomer utilized as proton conductor in fuel cells, was proven to be efficient ion translocator in organic solvent and this work reveals different insights into membrane with 3D pores.36, 37 Furthermore, comparing this self-assembly approach with solvent phase-separated approach, the self-assembly way results in more uniformed pore size, which is essential for ion transport and ionic selectivity.38 As for practical application, the need to engender the membrane with mechanical, thermal and chemical stability has been a recognition. However, conventional porous polymer membrane may suffer from poor tolerance to high temperature, oxidants and organic solvents. So designing a robust ionomer by self-assembly approach could simply handle this problem. 39, 40 Here, a robust 2D/3D-GO/polymer heterogeneous nanoporous membrane in large-scale via selfassembly approach is constructed, by chemically designing a robust charge-density-tunable nanoporous ionomer with uniform pore size (Figure 1). To obtain nanoporous polymer which maintains high mechanical strength and promotes multi-functionality, we designed a series of amphiphilic copolymer ACS Paragon Plus Environment

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by introducing positively charged pyridine moiety into engineering polymer polyphenylsulphone (PPSU) (Figure 1b). Then, the copolymer, spin-coated on the self-assembled GO nanosheets, self-assembled into 3D porous membrane via micro/nano-phase separation (Figure 1a). So a large-scale heterogeneous membrane with asymmetric 2D/3D structure (negatively charged with ca. 0.8 nm pores, positively charged with ca. 8 nm pores) was obtained (Figure 1b, 1c and 1d). The multi-physical-chemical properties of the membrane render it the function as a nanogate switch with synergy between wettability and surface charge change in response to pH (Figure 1e). Subsequently, we systematically studied the ionic transmembrane properties and ion current rectification performance in this 2D/3D-pores-structured system. And also we explored the function of membrane as an osmotic power generator. RESULTS AND DISSUSION Design and synthesis of 3D porous membrane. As discuss above, we need to design a polymer with high mechanical, thermal and chemical stability for broad potential applications. We design a series of amphiphilic copolymer by introducing positively charged pyridine moiety into engineering polymer polyphenylsulphone (PPSU). The charged hydrophilic pyridine pendants trend to self-assemble into well-defined mesoscopic scale cylindrical pores and these pores would connect each other into 3D interconnected nanochannels, while the hydrophobic backbone aggregates as one phase. This selfassembly phase-separated porous membrane features the thermos-stability (aromatic rings along the backbone ensures the thermos-ability), high mechanical strength and solvent resistance attributing to the rigid aromatic ring in the backbone (Figure S4). Figure 2 demonstrates the copolymerization design and route. In order to bring positive charge in the polymer, pyridine group (pKa ≈ 5.2) was brought into the monomer 2-(pyridin-4-yl)-1, 4-dimethoxybenzene (Py-OMe) by the well-known Suzuki cross-coupling reaction. Then the monomer 2-(pyridin-4-yl)-1, 4-benzenediol (Py-OH) was obtained after demethylated of Py-OMe in the dichloromethane solution of BBr3 (Figure 2a and S2). Then a series of polyphenylsuphone copolymer bearing tunable pyridine pendants termed as PPSU-Pyx (x represents the molar percentage of bisphenol monomer Py-OH) were synthesized by nucleophilic ACS Paragon Plus Environment

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aromatic polycondensation reaction (Figure 2b). The structures of the copolymers were confirmed by 1H NMR measurements (Figure 2c). The signal of Ha in the pyridine pendants appeared at 8.5 ppm, while that of Hb in the aromatic ring around sulfone appeared between 7.8 and 8.0 ppm. Then the theoretical values of x in PPSU-Pyx were 20% - 100% (from top to bottom), in accord with the experimental integral ration of 2a/b (see full spectrogram in Figure S3). The result revealed that the content of pyridine groups in the obtained PPSU copolymer could be controlled simply by adjusting the molar ration of two kinds of bisphenol monomer. And these results would provide a route to fabricate 3D porous membrane with tunable charge density (Figure 2d) which is a fundamental factor determines the ionic transmembrane properties.41, 42 Then we investigate the pore structures of the copolymer with different pyridine proportion at PPSUPyx by transmission electron microscope (TEM) as shown in Figure 3. In order to enhance the comparison between the hydrophobic backbone and the pyridine moiety, the polymer membrane was grafted with phosphotungstic acid (PTA) as precipitating agent labelling the pyridine pendants. So the dark areas in the morphologies indicate the inverted-micelles cylinder pores and the white areas represent the main hydrophobic backbone along the copolymer chain. Noted that, the porosity (here refined as the fraction of the total pore area over the total area of the membrane) of the polymer membrane enlarged over six times with the increasing pyridine proportion of the copolymer ranging from 20% to 100%. This porosity variation was also justified by the transmembrane ionic transport through monolayer polymer film as shown in Figure S11. Furthermore, the charge-density-tunable copolymer is with well-defined pore size (slightly shifts from 8.0 to 9.3 nm Figure 3d and S5), attributing to intramolecular interaction inside the amphiphilic copolymer.43 Inspired by the asymmetric structure, heterogeneous membrane composed of a thin layer of PPSU-Pyx (3D pores, positively charged, ca. 9 nm, 1 µm) on the top of GO nanosheets (2D pores, negatively charged, ca. 0.9 nm, thickness) was then fabricated via a facile way (see Methods and Figure 1b).


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Wettability dominates smart gating property of the heterogeneous membrane. Smart nanoporous membrane controls the ion/molecule transmembrane movement by gating property.44, 45. And in the sustainable life, wettability plays an essential role in the gating process.46 This heterogeneous membrane behaves reversibly smart gating phenomenon induced by wettability as well. Here, the heterogeneous membrane (GO/PPSU-Py60) was used as an example to observe the gating property in responsive to pH. Varying the pH value from 6 to 11, a sharp drop in conductance (about 30 times decrement) was obtained (Figure 4a and 4b). To confirm the gating mechanism (open in acid and close in alkaline), the I-V curves of the GO/PPSU-Py60 membrane was conducted in asymmetric pH solution (insets in Figure 4a and Figure 4b). In asymmetric pH condition (the pH value on PPSU-Py60 side was fixed at 6, the pH value on GO side was increased from 3 to 11), the conductance stayed almost the same. While the pH value on GO side was fixed at 6 and the pH value on PPSU-Py60 side was increased from 3 to 11, the current dropped sharply as well. Generally, the decreased conductance could be ascribed to two major factors, surface charge and wettability.47-51 Due to the π-π stacking structure of GO membrane stabilizing the carboxyl and hydroxyl groups, here the GO membrane works as a stable negative charge provider even in varying pH environment. As for the copolymer membrane, we testified the contact angles (CAs) and confocal images in different pH condition. The CAs (Figure 4c) confirmed the wettability change from hydrophilic to hydrophobic reversibly in responsive to acid and alkaline solution, attributing to the conformational switch of the pyridine pendants (Figure 1e). It was also proved by the confocal images (Figure 4d). To enable imaging and detailed wettability investigation, the water-soluble fluorescence marker sulfonated rhodamine (Figure S6) was conjugated with copolymer to visualize the pore location (where the pyridine groups self-assembly into 3D connected pores). As shown here (Figure 4d), in acid solution (pH=3) and hydrophilic state (CA=63.3o), the labelled copolymer was observed as bright spots; while in alkaline solution (pH=11) and hydrophobic state (CA=106.5o), the whole image was dark, indicating water could not get through the membrane. Besides, the change of surface charge density induced by pH also contributes to this gating property (Figure 4e). ACS Paragon Plus Environment

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The increasing charge density of pyridyl in acid solution caused the enriched Cl- ions into the channels under positive bias, and thus results in high conductance. In alkaline solution, wettability dominates the ionic transport property. Considering such a small pore size (ca. 9 nm) and hydrophobic surface, vapor is much more stable with respect to liquid, and thus results in such low conductance.52, 53 So, synergy of the surface charge and wettability change guarantees the effective and reversible nanogate property. Ionic rectification property of the heterogeneous membrane. As discussed above, the 2D/3DGO/PPSU-Pyx membrane rectifies ionic currents due to the asymmetric structure in a wide pH range. And a representative rectification I–V curve is shown in Figure 5a. Then, we testified the effect of asymmetric factors (length, surface charge) on rectification ratio. Note that, by fixing the length of PPSU-Py60 (1 µm), the length of GO membrane was modulated from 10 to 70 µm. The maximum rectification ratio was found when the thickness of GO membrane is ca. 30 µm (Figure S11 and S12). So all the data in this manuscript were obtained with the length asymmetric (copolymer with 1µm, GO with 30 µm). Theoretically, surface charge density difference effects the rectification property. As shown in Figure 5b, the rectification ratio increases four times significantly (from ca. 25 to ca. 97) with the proportion of the pyridine moiety varying from 20% to 100%, while the surface charge density of GO was fixed. Besides, the rectification ratio highly depends on the concentration of electrolyte (KCl) solution and reached maximum value at intermediate 0.01 M ranging from 1 µM to 1 M, while the reifying property is weakened in both low and high KCl concentrations (Figure 5c) which is consistent with previous theoretical predications and experimental data.4, 54-55 The effect of ionic concentrations on ionic conductance for the heterogeneous membranes (Figure 5d) shows a signature of the presence of surface charge on the nanopore.42 In a wide concentration range of electrolyte KCl from 1 µM to 1 M as shown in Figure 5d, the transmembrane ionic conductance deviated from bulk values (dashed line). Note that with the increasing of surface-charge-density, the ionic conductance deviated from the bulk much further (from down to top along with modulating the charged pyridine moiety in copolymer from 20 to 80%), which proved a fully surface charge governed transport.42 Attributing to the high surface charge density and narrow pore size, this heterogeneous membrane could rectify the ionic current even in highACS Paragon Plus Environment

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contention electrolyte (the rectification ratio f > 10 even in 1

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KCl), for where the Debye length is

comparable to pore size. Also, we would like to emphasize the robustness of the heterogeneous membrane as shown in the post-experiments (Figure S12). The membrane (immersed in KCl solution after prepared two months) still rectified current apparently (Figure S12a), and this also proved the stability of the porous polymeric membrane as expected (Figure S12b). Since the 2D/3D heterogeneous membrane shows high reification ratio with smart tunable charge density, we expect the membrane could be worked as ionic diode membrane to harvest electric power from salinity gradient. So the membrane was mounted between artificial seawater (0.5 M NaCl) (PPSUPyx side) and artificial river water (0.01 M NaCl) (GO side) as shown in Figure 6a. The power generated with the heterogeneous membrane could be output to external circuit to supply an electronic load. The power (PR) can be directly obtained by the equation PR = I2×RL. In our 2D/3D heterogeneous membrane system, the current density and output power of GO/PPSU-Py20, GO/PPSU-Py60, and GO/PPSUPy100 as functions of load resistance were conducted. With increasing load resistance, the current density decreases and the output power reaches maximum at about 1000 kΩ, 620 kΩ and 300 kΩ, respectively (Figure 6b). With the increasing surface charge density of copolymer, the output power reached up to the maximum value 0.76 W/m2 with the GO/PPSU-Py100 heterogeneous membrane (Figure 6c). The data is in accord with the tunable reification ratio by modulating the pyridine proportion of the copolymer, attributing to surface-charge-governed ion transport.42, 56 Compared to the 1D pores in inorganic system or organic system, we develop a robust heterogeneous membrane with 3D/2D pores.56-58 And tuning the surface charge by precisely chemical design could quantitatively regulate the rectification ratio and power generator. CONCLUSIONS In summary, by chemically designing a robust charge-density-tunable nanoporous ionomer with uniform pore size, a robust 2D/3D-GO/polymer heterogeneous nanoporous membrane in large-scale via self-assembly approach is constructed. To meet the desire of practical application for robustness, a series of amphiphilic copolymer were designed by introducing a positively charged pyridine moiety into ACS Paragon Plus Environment

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engineering polymer polyphenylsulphone (PPSU). The as-prepared heterogeneous membrane could work as a nanogate switch with synergy between wettability and surface charge change in response to pH. Meanwhile, the ionic transmembrane properties of the membrane could be tuned by adjusting the surface charge density of the ionomer through chemical copolymerization at controlled design route. Our post-experiment data successfully proved the robustness of the heterogeneous membrane. And this simple method may be further to be used to address practical applications. METHODS Synthesis of copolymer. In order to control the charge density, a monomer containing pyridine group is designed. PPSU is generally synthesized by an aromatic nucleophilic substitution reaction of activated aryl dihalides with biphenol in a dipolar aprotic solvent. In general, a strong electron with drawing group such as carbonyl, sulfone or phosphine oxide is necessary to activate the aromatic dihalides. Obviously, to modify the hydroquinol monomer is more efficient. The Suzuki cross-coupling reaction was employed to form the C-C bond between aromatic ring and pyridine. And during this process, methyl ethers had been used to protect phenols. Subsequently, starting with the pyridine monomer, the co-polymer was synthesized via nucleophilic aromatic polycondensation. K2CO3 was used as base to ensure the nucleophilic ability of alkali metal hydroxides. The PPSU with any proportion of pyridine group could be synthesized by controlling the rate of charge accurately (details in Supplementary Information). Membrane preparation. A mixture of GO was suspended in deionized water and then sonicated 1 h. Then GO self-assembled into nanosheets with gradient thickness by vacuum filtrated various concentration of the suspension. PPSU-Pyx solution in N-methyl pyrrolidone (NMP) (0.1 g mL-1) was spin-coated on the as-prepared GO membrane and dried at 60℃ in a vacuum oven for 12 h. And the copolymer self-assembled into 3D porous membrane via micro/nano-phase separation. Thus the asymmetric membrane with 2D/3D pores was obtained.


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Characterization. 1H NMR experiments were conducted using a Bruker 510 spectrometer (500 MHz for 1H) with CDCl3 or DMSO-d6 as the solvent. The internal reference was tetramethylsilane (TMS). The cross section images of the sample were obtained with a field-emission scanning electron microscope (SEM) (S-4800, Japan). The pore structure of the copolymer was investigated by TEM (JEM 1200 EX, Jeol, Japan). Diluted PPSU-Pyx solutions were cast on an ultrathin-carbon-coated copper grid. Contact angles were conducted with an OCA20 instrument (DataPhysics, Germany). Selective staining of the pyridine was accomplished by exposure of the thin sections to phosphotungstic acid (PTA) solution. The fluorescent images that the permeation of fluorescent dyes through the membrane from the perpendicular direction were obtained using a Nikon C2 confocal laser scanning microscope (Nikon Corp., Tokyo, Japan). Current measurement. The ionic current through the membrane was measured by a Keithley 6487 picoammeter (Keithley Instruments, Cleveland, OH). The heterogeneous membrane was mounted between two halves of electrochemical chambers.59, 60 The I-V curves were recorded with Ag/AgCl electrodes, and the measurement were conducted in room temperature. Associated Content Supporting information is available free of charge at http://pubs.acs.org. Detailed Synthesis and Characterizations of monomer and copolymer, Experimental setup, Porosity and Pore size distribution are provided (PDF). Corresponding Author * [email protected] * [email protected] Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Here, we give our acknowledgement to Dr. Zheng Chen, Wenke Li, Yang Sun, Lianjun Ding, Su Li, Yifan Liu, Yuntao Han, Yunji Xie and Zhaoyang Wang (National & Local Joint Engineering Laboratory for Synthetic Technology of High Performance Polymer, College of Chemistry, Jilin University) for discussion and materials support. We would like to give our special appreciation to Dr. Moyuan Cao (School of Chemical Engineering and Technology, Tianjin University) for proofreading, Jietao Yu for editing the graph, and Beijing Hanlei Technology Co. Ltd. for software supply of Keithley. All authors have given approval to the final version of the manuscript. This work is financially supported by National Science Foundation of China (21504097), Frontier Science Key Projects of CAS (QYZDY-SSW-SLH014),

National

High-tech

R&D

Program

of

China

(863

Program)

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BRIEFS. A robust 2D/3D-GO/polymer heterogeneous nanoporous membrane in large-scale via selfassembly approach is constructed, by chemical designing a robust charge-density-tunable nanoporous ionomer with uniform pore size. SYNOPSIS


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A robust 2D/3D-GO/polymer heterogeneous nanoporous membrane in large-scale via self-assembly approach is constructed, by chemical designing a robust charge-density-tunable nanoporous ionomer with uniform pore size. 97x42mm (300 x 300 DPI)


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Figure 1. Schematic depiction of the heterogeneous membrane with asymmetric structures. a) The fabrication process of the membrane, copolymer was spin-coated on the GO sheets which was pre-prepared by vacuum filtration. b) Left is TEM image of PPSU-Pyx which was marked with phosphotungstic acid (PTA); middle is the SEM observation on the cross section of the heterogeneous membrane; right is photo of the heterogeneous membrane GO/PPSU-Pyx. c) Schematic of the heterogeneous structures with 3D pores (selfassembly into interconnected pores by ionomers) and 2D pores (self-assembly into nanosheets by GO). d) Illustration of GO nanosheets with spacing of ca. 0.84 nm. e) Chemical structure of the PPSSU-Pyx (upper); and configuration change of the polymer in response to pH (down). 250x207mm (300 x 300 DPI)


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Figure 2. Chemical design and synthesis of the copolymer PPSU-Pyx. a) Synthesis route of 2-(pyridin-4-yl)1, 4-dimethoxybenzene (Py-OMe) by Suzuki cross-coupling reaction. Then the pyridine monomer 2-(pyridin4-yl)-1, 4-benzenediol (Py-OH) was obtained after demethylated of Py-OMe via dichloromethane solution of BBr3. b) Copolymerization of PPSU-Pyx by nucleophilic aromatic polycondensation reaction based on PPSU. c) 1H-NMR spectra (500 MHz, DMSO-d6, room temperature) of PPSU-Pyx with different proportion pyridine pendants (ranging from top to down are 20, 40, 60, 80, 100%, respectively). The white columns highlight the signal of Ha and Hb that are marked in Figure 2b. d) The tunable charge density of the polymer membrane by monitoring the charged monomer proportion of the copolymer. 108x75mm (300 x 300 DPI)


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ACS Nano

Figure 3. The pore structure and size distribution with different pyridine moiety proportion at PPSU-Pyx. ac), the TEM images of the PPSU-Pyx. Dark areas refer to pores and white areas represent the hydrophobic backbone along the copolymer chain. The polymer was marked with PTA which reacted with the pyridine moiety. d) The porosity and diameter of the copolymer membrane with different pyridine moiety proportion, indicating the improved porosity while the pore size stay almost the same. The statistics were obtained from the TEM images data. 127x87mm (300 x 300 DPI)


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Figure 4. Wettability dominates the nanogate property of the heterogeneous membrane induced by pH. a) The I-V curves of heterogeneous membranes in 0.01 M KCl solutions with different pH values when a sweep voltage ranging from -2.0 to 2.0 V is applied (insets were the I-V cures of GO and polymer respectively). The anode is on the GO side, and the cathode is on the PPSU-Pyx side. b) Histogram of the ionic conductance calculated from the I-V curves of the pH-stimuli. c) Surface contact angles of polymer membrane at different pH environment. d) Confocal fluorescence images of polymer membrane at different pH environment (upsides were the top images; and vertical cross-section images were undersides). The green dash lines indicate the membrane surface. e) Schematic depiction of the surface charge density of the heterogeneous membrane at different pH environment. 103x78mm (300 x 300 DPI)


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Figure 5. The transmembrane ionic rectification property of the heterogeneous membrane. a) Representative rectification I–V curve of the GO-Py100 membrane in 0.01 M KCl solution at pH 5 from −2 to 2 V. The membrane thickness is about 31 µm (GO is about 30 µm and polymer membrane is about 1 µm). The anode is on the GO side, and the cathode is on the PPSU-Pyx side. b) The effect of surface charge change on rectification ratio. With the increasing positive charge density (pyridine moiety proportion varies ranging from 20% to 100%), the ratios values increase as well. All the data were recorded in 0.01 M KCl solution at pH 5. c) The rectification ratio depends on the concentration of electrolyte (KCl) solution and reached maximum at 0.01 M KCl solution. d) Conductance versus electrolyte concentration for GO/PPSU-Pyx membrane (with the pyridine proportion ranging from 20% to 80%). The transmembrane ionic conductance deviated from bulk value (purple dashed line) below 1 M, representing the effect of surface-charge on ionic transport. 93x60mm (300 x 300 DPI)


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Figure 6. Application for energy conversion, the output power density depends on the surface charge density difference. a) Schematic illustration of concentration-gradient-driven energy harvesting devices. The polymeric side was faced to artificial seawater (0.5 M NaCl), while the GO side was faced to artificial river water (0.01 M NaCl). The collected power can be output to external circuit and supply an electronic load. b) The current density and output power of GO/PPSU-Py20, GO/PPSU-Py60, and GO/PPSU-Py100 as functions of load resistance. With increasing load resistance, the current density decreases and the output power reaches maximum at about 1000 kΩ, 620 kΩ and 300 kΩ, respectively. c) The maximal output power density goes up with increasing the proportion of pyridine groups. The maximum of GO/PPSU-Py100 is about 0.76 W/m2. 58x14mm (300 x 300 DPI)


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