Ultrathin and Ion-Selective Janus Membranes for High-Performance

Jun 11, 2017 - The osmotic energy existing in fluids is recognized as a promising “blue” energy source that can help solve the global issues of en...
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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 100190, PR China ‡ School of Chemistry and Environment, Beihang University, Beijing 100191, PR China § Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ∥ University of Chinese Academy of Sciences, Beijing 100049, PR China S Supporting Information *

ABSTRACT: The osmotic energy existing in fluids is recognized as a promising “blue” energy source that can help solve the global issues of energy shortage and environmental pollution. Recently, nanofluidic channels have shown great potential for capturing this worldwide energy because of their novel transport properties contributed by nanoconfinement. However, with respect to membrane-scale porous systems, high resistance and undesirable ion selectivity remain bottlenecks, impeding their applications. The development of thinner, low-resistance membranes, meanwhile promoting their ion selectivity, is a necessity. Here, we engineered ultrathin and ion-selective Janus membranes prepared via the phase separation of two block copolymers, which enable osmotic energy conversion with power densities of approximately 2.04 W/m2 by mixing natural seawater and river water. Both experiments and continuum simulation help us to understand the mechanism for how membrane thickness and channel structure dominate the ion transport process and overall device performance, which can serve as a general guiding principle for the future design of nanochannel membranes for high-energy concentration cells.

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Nanofluidics, a field that has evolved considerably over the past decade, is the study of mass transport in confined fluidic systems whose characteristic length scale is in the nanometer range.12−16 Similar to the biological protein channels,17 the combination of strong confinement and surface charge effects at the nanoscale contributes to novel transport properties,18,19 including excellent ion selectivity and high ionic throughput,20,21 which may lead to technological breakthroughs in many areas, including biosensing, separation, energy conversion, and water desalination.22−27 Recently, nanofluidic channel systems have been proposed as new candidates to harness the osmotic energy and relevant researches are just emerging. Siria et al. investigated the osmotically induced electric currents generated by salinity gradients using a single boron nitride nanotube and obtained a maximum power output of approximately 20 pW.28 By reducing the nanochannel length to 0.65 nm using a single monolayer MoS2 nanopore, Radenovic’s group attained a substantial enhancement in power output.29 From a fundamental point of view, these single-channel-based systems are a groundbreaking develop-

INTRODUCTION The exploitation of renewable and sustainable energy from ambient environment has become paramount to the human civilization because of the ongoing depletion of fossil fuels and ever-growing energy demands.1,2 The osmotic energy existing in fluids is widely recognized as a vital energy source due to its large reserves and easy accessibility.3−5 Over the past few decades, membrane based reverse electrodialysis has been developed as a promising technology to capture this worldwide energy from natural water.6 When two fluidic systems with different salinities are separated by an ion selective membrane, the Gibbs free energy of mixing, which forces selective ion diffusion, can be harvested directly in the form of electrical power.7 Traditional reverse electrodialysis systems are generally based on ion exchange membranes and have achieved a maximum power density approximately 2.2 W/m2 using tandem stacks.8,9 To push the industrial development of the membrane-based reverse electrodialysis technology, the power density should reach a benchmark of at least 5.0 W/m2 for the sea-river water system,10 which is quite challenging as most ion exchange membranes suffer from inadequate mass transportation caused by large steric hindrance of their sub-1 nm pores.11 © 2017 American Chemical Society

Received: March 21, 2017 Published: June 11, 2017 8905

DOI: 10.1021/jacs.7b02794 J. Am. Chem. Soc. 2017, 139, 8905−8914

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Journal of the American Chemical Society Scheme 1. Two Block Copolymers Used in This Worka

a

(a) A synthetic block copolymer BCP-1: PEO12.5k-hv-PMA(Chal)44.7k that contains a photocleavable unit and a photo-cross-linkable unit. After PEO extraction, the remaining PMA(Chal), termed Molecule-1 (M-1), can serve as a matrix to construct nanoporous membranes. (b) A commercially available block copolymer BCP-2: PS48.4k-b-P4VP21.3k, also termed Molecule-2 (M-2).

Figure 1. Fabrication processes for the ultrathin and ion-selective Janus membranes based on the block copolymer. (i) Spin-coating water-soluble poly(styrenesulfonate) (PSS, gray) onto a silicon wafer. (ii) Spin-coating a BCP-1 layer onto the sacrificial PSS layer. (iii) Thermal annealing under vacuum. (iv) Degrading the photocleavable o-nitrobenzyl ester in the polymer junction and cross-linking the chalcone mesogens in the side chain upon UV irradiation. (v) Spin-coating an M-2 layer onto the BCP-1 layer. (vi) Exfoliating the Janus membrane from the silicon substrate and dissolving the cleaved PEO segments.

heterostructures rectifies the ionic current and preserves charge-governed ionic transport in a broad concentration range. Cyclic voltammetry analysis and dye permeation tests both indicate that the transmembrane ion transport is highly anion selective. The system is capable of high-performance osmotic energy conversion with a power density of approximately 2.04 W/m2 by mixing natural seawater and river water. A power output of more than 10 W/m2 is envisaged by increasing the utilization rate of the pores in the functional layer. Both experiments and continuum model simulation help us to understand the mechanism for how membrane thickness and channel structure dominate the selective ion transport process and overall device performance. This work should provide inspiration for designing a cation-selective counterpart and constructing tandem stacks, which would satisfy the large energy consumption requirements of real-world applications.

ment that can inspire scientists to construct macroscopic-scale porous membrane systems which will meet a wide range of requirements for different applications.30−33 For example, Kim and co-workers reported using a nanoporous polycarbonate track-etched membrane as a power generator with an energy density of approximately 0.058 W/m2.30 Our group designed a 14 μm-thick composite system based on track-etched membranes and achieved an increase in the power output to approximately 0.35 W/m2.31 Thus, far, great advancements have been made concerning nanofluidic reverse electrodialysis systems. However, the high membrane resistance and undesirable ion selectivity remain the main bottlenecks impeding their practical applications.34 Therefore, to strive for higher and more economically attractive power output, the development of thinner, low-resistance membranes, meanwhile promoting their ion selectivity, is highly demanded. Here, we demonstrate the use of an ultrathin and ionselective Janus membrane fabricated by hybridizing two block copolymer (BCP) membranes with different chemical compositions. The thickness of the tailor-made hybrid membrane is on the submicron-scale (∼500 nm), allowing low fluidic resistance and rapid mass transport.35,36 The membrane system with chemical, geometrical, and electrostatic

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RESULTS AND DISCUSSION Poly(ethylene oxide)-block-poly(methacrylate) (BCP-1), an amphiphilic liquid-crystalline BCP denoted as PEO-hv-PMA(Chal) and bearing chalcone (Chal) mesogens in the side chains of the PMA segments and o-nitrobenzyl ester (ONB) linkers in the polymer junction, was synthesized by atom8906

DOI: 10.1021/jacs.7b02794 J. Am. Chem. Soc. 2017, 139, 8905−8914

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Journal of the American Chemical Society transfer radical polymerization (see Experimental Section, and Figures S1−S3). The ONB linker between the two blocks can be degraded upon UV exposure,37 generating carboxyl groups on the end of the PMA chains. Additionally, the Chal mesogens can be photo-cross-linked under UV irradiation, forming a robust macromolecular network (Scheme 1a).38 Through film casting, thermal annealing, UV treatment, and PEO extraction, the remaining PMA(Chal), termed Molecule-1 (M-1), can form a free-standing thin membrane containing high-density straight nanochannels with carboxyl groups tethered on the inner walls. In this work, the functional M-1 membrane was adopted as the substrate for the coating of BCP-2 membrane (Scheme 1b), polystyrene-block-poly(4-vinylpyridine) (PS-bP4VP), also termed Molecule-2 (M-2), to construct the hybrid Janus nanochannel membrane. Figure 1 shows the fabrication processes for the BCP-based Janus membrane (details in Experimental Section). To prepare the hybrid membrane, poly(styrenesulfonate) (PSS) was spincoated onto a silicon wafer beforehand to form a sacrificial layer that could subsequently be dissolved with water. Then, the BCP-1 membrane was cast from a toluene solution onto the PSS layer, followed by vacuum thermal annealing to form hexagonally arranged straight PEO nanochannels. The thermally annealed BCP-1 membrane was then exposed to UV irradiation to trigger ONB degradation and Chal crosslinking. Next, a solution of BCP-2 in dioxane was spin-coated onto the UV-treated BCP-1 membrane and microphaseseparated into a M-2 membrane with hexagonally arranged P4VP nanochannels as the solvent evaporated.39 Finally, the sacrificial PSS layer and the cleaved PEO segments were dissolved via a subsequent water immersion process. A freestanding Janus membrane composed of a layer of PMA(Chal) (M-1) and a layer of PS-b-P4VP (M-2) was successfully fabricated. The thickness of the Janus membrane was imaged using atomic force microscopy (AFM). Figure 2a shows an AFM image of a section of an as-prepared Janus membrane. To expose a relatively observable interface, we prepared the sample by freeze-cracking the PSS-supported bilayer membrane and then exfoliating it onto a silicon wafer. The corresponding height profile reveals that the membrane has an ultrathin submicron-scale thickness of approximately 500 nm (Figure 2b). The bottom M-1 layer was approximately 400 nm thick, as determined using the same sectional AFM scanning method before the M-2 layer coating. The pore structure of the front and back sides of the Janus membrane was also investigated. The bottom M-1 layer displays hexagonally packed pores with a diameter of approximately 10 nm (Figure 2c, d). As observed using transmission electron microscopy (Figure S4), the nanochannels of the M-1 layer are perpendicularly oriented to the membrane surface. The top M-2 layer also exhibits a hexagonally packed pore structure but a relatively large pore diameter of approximately 17 nm (Figure 2e, f). Notably, the nanochannels in the M-2 layer also have a perpendicular orientation because of its sub-100 nm membrane thickness.40 Clearly, the hybridization of the two layers can provide a configured nanochannel with asymmetric geometry. In addition, the surface groups on the two sides are asymmetric. The inner walls of the M-1 nanochannels bear carboxyl groups (pKa ≈ 3.8) formed by UV degradation and subsequent PEO extraction, which was confirmed by zeta-potential measurements (Figure S5). The M-2 nanochannels are filled with endtethered P4VP chains containing pyridine groups (pKa ≈ 5.2).

Figure 2. Characterization of the free-standing ultrathin Janus membrane. (a, b) AFM height image of the section of an as-prepared Janus membrane transferred onto a silicon wafer (a) and the corresponding height profile (b), revealing its ultrathin submicronscale thickness of approximately 500 nm. The scale bar is 2 μm. (c, d) AFM height image of the back side of the Janus membrane (c) and the distribution of the pore diameter (d). The scale bar is 250 nm. (e, f) AFM height image of the front side of the Janus membrane (e) and the corresponding distribution of pore diameter (f). The scale bar is 250 nm.

The polymer chains exhibit a swollen and positively charged, state when the pH is less than 5.2.41,42 The ionic transport properties of the Janus membrane were examined by ionic current−voltage (I−V) measurements performed using an electrochemical device (Supporting Note 2 and Figure S6−S7).43 The ionic transport behavior of the M1 monolayer membrane was investigated first. As shown in Figure 3a, the naked M-1 membrane exhibits a linear I−V curve when a 0.1 M KCl solution with a pH of 4.3 is used as the electrolyte. Under the testing conditions, the naked M-1 membrane is negatively charged and bears symmetric channel structure and, hence, a symmetric electric potential distribution.44,45 The flow of dominant cations across the channel under different external biases is symmetric. Thus, the naked M-1 membrane behaves as an ohmic conductor with a rectification ratio (|I−2V|/|I+2V|) of approximately 1.0.46 However, the situation is different for the hybrid bilayer membrane. Figure 3b shows the I−V curve of the Janus membrane recorded in the same electrolyte, which exhibits an 8907

DOI: 10.1021/jacs.7b02794 J. Am. Chem. Soc. 2017, 139, 8905−8914

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Figure 3. Ionic transport properties of the Janus membrane. (a) I−V curve of the naked M-1 monolayer membrane recorded in 0.1 M KCl solution (pH ≈ 4.3). (b) I−V curve of the asymmetric bilayer Janus membrane recorded in the same electrolyte solution, showing a good ionic rectification effect. The anode is placed in the M-2 side. (c) Ionic conductance of the Janus membrane as a function of the electrolyte concentration; the pH value of the electrolyte solutions are all adjusted to approximately 4.3. (d) The calculated ionic concentration distribution of the three models confirms that the observed ionic rectification is caused by the ion depletion and accumulation effect induced by the different polarity of the external voltage.

accumulation effects (Figure S9). Furthermore, if the system is completely replaced by a separate M-2 channel, the concentration profile returns to a homogeneous state that is independent of the voltage polarity (Figure 3d, 3). Therefore, the formation of an asymmetric electrostatic heterojunction is the key reason for the experimentally observed rectification phenomenon.56 The system also exhibited excellent anion selectivity, which was investigated using the Janus membrane for controlling the transport of charged cargo. First, two electroactive redox probes, [Fe(CN)6]3− and [Ru(NH3)6]3+, which are commonly used in cyclic voltammetric analysis, were selected as the cargo. Both of these species have characteristic oxidation and reduction peaks upon potential scanning. The Janus membrane was formed on a bare ITO working electrode. Figure 4a shows cyclic voltammograms of the Janus membrane in the presence of 5 mM [Fe(CN)6]3− and [Ru(NH3)6]3+ using 0.1 M KCl solution (pH ≈ 4.3) as the supporting electrolyte. It exhibits a strong and highly selective electrochemical response for the transport of the anionic probe [Fe(CN)6]3− (blue trace) and a weak response for the cationic probe [Ru(NH3)6]3+ (red trace). The peak currents are approximately 100 μA and 1.5 μA, respectively (Figure 4a, inset). The contrast in voltammetric response reflects the difference in the specific probe diffusion ability because of the electrostatic environment of the hybrid nanochannel.57,58 Furthermore, two fluorophores with high quantum yields, rhodamine (Rh) 6G and sulforhodamine, which both have characteristic fluorescence emission spectra, were also chosen as the cargo. As shown in Figure 4b, they have a similar structure but opposite charges, i.e., + 1 and −1, under the experimental conditions (pH ≈ 4.3). Permeation experi-

obvious ionic rectification effect with a ratio of approximately 7.1. Under these conditions, the system bears an opposite charge distribution and an asymmetric structure formed by the hybridization, which result in asymmetric ionic flow under different biases.47,48 The conductance measurement in Figure 3c indicates that the ionic transport through the Janus membrane is fully governed by the surface charge.49 The transmembrane conductance deviates from bulk behavior (black dashed line) when the electrolyte concentration is less than 1 M.50,51 The ionic rectifying property of the hybrid membrane was also investigated by a numerical simulation based on the Poisson and Nernst−Planck (PNP) equations (details in Supporting Note 3 and Figure S8).52,53 The calculated ionic concentration profile inside the naked and negatively charged M-1 channel is homogeneous irrespective of the bias polarity (Figure 3d, 1), implying that it does not rectify. After a positively charged M-2 channel being coupled to form a heterojunction, remarkable ion depletion and ion accumulation effects are observed inside the hybrid channel (Figure 3d, 2), which contributes to the ionic rectification effect. Specifically, in the absence of an external bias, the cations (K+) and anions (Cl−) are predominantly enriched in the M-1 channel and M-2 channel respectively due to their different charge polarity. The negative bias drives both types of ions to migrate toward the bipolar junction, resulting in an ion accumulation zone evidenced by a high ion concentration in the junction, and thus a relatively high ionic current. In contrast, a positive bias will contribute to the formation of an ion depletion zone and thus a relatively low ionic current.54,55 The electrical potential distribution also helps us understand the ion depletion and 8908

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Figure 4. Controlling the transport of charged cargo. (a) Cyclic voltammograms of the ITO-supported Janus membrane using [Ru(NH3)6]3+ as a cationic electroactive probe (blue trace) and [Fe(CN)6]3− as an anionic electroactive probe (red trace). Scan rate: 200 mV/s, supporting electrolyte: 0.1 M KCl (pH ≈ 4.3) (inset: the corresponding peak currents of the electroactive probes). (b) Time−concentration curve of the permeation experiments using two oppositely charged dyes: rhodamine 6G (Rh (+), left) and sulforhodamine (Rh (−), right). The permeability rate of the negatively charged Rh (−) is much larger than that of the positively charged Rh (+). (c) The excellent anion selectivity of the Janus membrane is dominated by the BCP-2 part, whose nanochannels are filled with P4VP chains, showing stronger electrostatic interactions.

potential between the inlet and the outlet of the nanochannels.63 As a consequence, to maintain the electroneutrality of the solutions in the two reservoirs, electrochemical redox reactions will occur on the electrode surfaces and the electrons will be transferred to an external circuit.12,14 In this way, part of the Gibbs free energy existing in the salinity gradient can be harvested by the Janus membrane system.33 We evaluated the energy conversion performance by collecting I−V scans in the presence of a series of transmembrane concentration gradients. All of the electrolyte solutions were adjusted to pH 4.3 to maintain the charged state of the Janus membrane. From the intercepts on the current and voltage axes, we can directly obtain the corresponding short-circuit current (ISC) and open-circuit voltage (VOC). The measured VOC actually consists of two parts: the diffusion potential (Ediff) which is contributed by the power source and the redox potential (Eredox) which is generated by the unequal potential drop at the electrode− solution interface (see the equivalent circuit in Figure S11). In the subsequent discussions, the contribution of Eredox has already been subtracted via an electrode calibration process (Supporting Note 4).64 Two configurations can be adopted to arrange the concentration gradient. When the concentrated KCl solution is on the M-2 side, for example, cM‑1/cM‑2 = 10 μM/3 M, the ISC and the VOC are approximately −30 μA and +98 mV, respectively. Under a reversed concentration gradient from the M-1 side to the M-2 side (e.g., cM‑1/cM‑2 = 3 M/10 μM), the ISC decreases to about +21 μA and the VOC increases

ments were performed with a two-cell system separated by the hybrid membrane (details in Experimental Section). The feed solution (1.0 × 10−4 M) was in contact with the M-2 side, and the permeability was periodically monitored. The permeability rate of Rh (−) is much larger than that of Rh (+) (Figure 4b), consistent with the results from cyclic voltammetric analysis. These results indicate that the Janus membrane is highly anion selective, opposite to the naked M-1 membrane which behaves a certain degree of cation selectivity (Figure S10). In the testing solution with pH approximately 4.3, the P4VP chains are in the swollen and positively charged state, which will fill up the pores and make the pore diameter differ considerably from the dry state.59 Compared with the small amount of COO− in the M-1 membrane, electrostatic interactions between the traversing ion and the swollen, positively charged P4VP chains in the M-2 membrane are much stronger, resulting in excellent selectivity for anions (Figure 4c).60,61 Thus, the positively charged M-2 part plays a dominant role in the ion selectivity of the Janus membrane. The tailor-made Janus membrane shows high-performance osmotic energy conversion. The two compartments of the electrochemical cell are filled with concentrated and diluted solutions, respectively (Figure 5a). Because the hybrid system demonstrates excellent anion selectivity, Cl− mainly diffuses from the high-concentration side to the low-concentration side.62 This continuous asymmetric ion diffusion makes the low-concentration side negatively charged and leaves the highconcentration side positively charged, generating a diffusion 8909

DOI: 10.1021/jacs.7b02794 J. Am. Chem. Soc. 2017, 139, 8905−8914

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Figure 5. High-performance osmotic energy conversion. (a) Schematic of the energy harvesting process under a concentration gradient. (b) Two configurations are adopted to arrange the concentration gradient. With the dilute KCl solution on the M-2 side, the absolute value of ISC and VOC change from 30 μA and 98 mV to 21 μA and 120 mV, respectively, and the corresponding inner resistance (Rchannel) increases by approximately 75%. (c) As the concentration gradient increases, both ISC and VOC gradually increase and the maximum value can achieve 30 μA and 98 mV, respectively. The low-concentration (KCl) solution is placed in the M-1 side and is fixed at 10 μM. (d) The harvested energy can be transferred to supply an external resistance. The high-salinity (NaCl) solution is placed in the M-2 side and is fixed at 0.5 M. Under three salinity gradients, the measured current densities all gradually decrease with increasing external resistance. (e) The corresponding power output all achieve a maximum value at a moderate resistance. The values are 0.37 W/m2, 2.1 W/m2, and 3.8 W/m2 for the 5-fold, 50-fold, and 500-fold salinity gradients, respectively. (f) Comparison of power densities under different conditions: (1, 2) The power density of the pristine Janus membrane (P-membrane) under 50-fold salinity gradient at different pH values (1): pH 4.3, (2) pH 7.1. (3, 4) The power density of the quaternized Janus membrane (Q-membrane) under 50-fold salinity at pH 7.1 (3), and under natural salinity gradient: seawater (pH 7.4) and river water (pH 7.4) (4).

to about −120 mV, and both of them have opposite polarities because of the opposite diffusion direction (Figure 5b). The corresponding inner resistance (Rchannel), calculated as Rchannel = VOC/ISC, increases by approximately 75%, which largely suppresses the power generation. Thus, the following tests were all carried out using the former configuration. Figure 5c shows the corresponding ISC and VOC values under a series of concentration gradients. 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 10 μM to 3 M. Both the ISC and the VOC values gradually increase, and the maximum value of each that can be achieved are approximately 30 μA and 98 mV, respectively. Notably, the measured Voc gradually deviates from its theoretical value calculated from the Nernst potential, which can be ascribed to the decreased ion selectivity of M-2 side upon electrolyte concentration increasing.65 The large discrepancy of VOC at high concentration gradient is unfavorable to the osmotic power generation as the power output originates from the selective ion transport. The corresponding energy conversion efficiency decreases from 17.5% to 4.5% as the concentration gradient increases from 10 to 3 × 105 (Supporting Note 5 and Figure S12). The harvested electric power can also be transferred to an external circuit to supply an electrical load resistor (RL). A high salinity (NaCl) concentration was also placed on the M-2 side and was set at 0.5 M. As shown in Figure 5d, under three

salinity gradients, the current densities on the external circuits all decreased accordingly with increasing load resistance. The output power densities, calculated as Pmax = I2 × RL, all reached a maximum value at a moderate load resistance. The power density values were 0.37 W/m2, 2.1 W/m2, and 3.8 W/m2 with efficiencies approximately 46.7%, 24.3%, and 13.2% for the 5fold, 50-fold, and 500-fold salinity gradients, respectively (Figure 5e); these values are much larger than the power output values of the naked M-1 membrane under the same concentration gradients (Figure S13). Furthermore, the output power density scaled inversely with the membrane thickness. Under the 500-fold salinity gradient, the power density decreased from 3.8 W/m2 to approximately 1.1 W/m2 when the thickness of the Janus membrane was increased from 500 nm to 2.5 μm (Figure S14). As such, we believe more remarkable power output can be obtained by further reducing the membrane thickness. It is worthy to note that the power density of our membrane under 50-fold concentration is about 2.1 W/m2, which is comparable to that of traditional ion exchange membrane operating in reverse electrodialysis mode under a similar salinity gradient.8 The two layers of the Janus membrane both exhibit hexagonal packed pores, but with different pore size and different pore center-to-center distance. It appears that the alignment of the channels of the two layers is very critical to the functions of the system. Some of the channels will line up fully 8910

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Figure 6. Numerical simulation of the high-performance energy conversion process. (a, b) Counterion concentration profile under a salinity gradient (0.5 M/0.01 M) near the opening of a separate M-1 channel (a) and the hybrid nanochannel (b). (c) Relationship between the calculated diffusion currents of the BCP-1 nanochannel, and the M-1,2 Janus nanochannel and the total length of the simulated model. The net diffusion current scales inversely with membrane thickness.

polarization can be eliminated by the chemical design of the nanochannel structure. The presence of negatively charged M-1 channel on the low-concentration side will prevent the accumulation of counterions, which is conductive to the generation of net diffusion current under salinity gradient.31 The counterion concentration in the opening of the lowconcentration side of a naked M-1 channel and the hybrid channel under a concentration gradient (0.5 M/0.01 M) were calculated on the basis of PNP theory (Supporting Note 3). With regard to the negatively charged M-1 channel (Figure 6a), the counterion (i.e., K+) concentration is much larger than the bulk value, thus contributing to serve polarization. By contrast, the counterion (i.e., Cl) concentration in the orifice of the anion-selective hybrid channel is much lower than the bulk value, implying that the transmembrane counterions are not enriched on the low-concentration side (Figure 6b), which can be ascribed to their electrostatic interactions with the negatively charged M-1 channel.68 The polarization phenomenon can be eliminated by the asymmetric bipolar structure. Furthermore, the Janus nanochannel can also suppress the undesired mixing of the co-ions (Figure S16). As a result, the calculated net diffusion current through the Janus nanochannel is much larger than that through the naked M-1 nanochannel (Figure 6c), consistent with the experimental results (Figure S13). Most importantly, the ultrathin submicron-scale thickness plays a critical role in the energy conversion process. As shown in Figure 6c, the net diffusion current through the naked M-1 nanochannel and the M-1,2 Janus nanochannel both undergo a large decrease when the channel length is increased from 500 to 5000 nm. The submicron-scale thickness provides faster ion transfer through the membrane medium,69 which contributes to a low electrical resistance and, thus, to a high power output.

with 0% dead cross-section; some of the channels will partially overlap, which would lose some cross-section for ion flux; and some of the channels are dead-end with 100% dead crosssection, termed dead pores. The ratio of these three types of pores is approximately 7.2%, 46.8%, 46%, respectively (details in Figure S15). Therefore, the power density 2.1 W/m2 is quite considerable for the Janus membrane with such a low utilization rate of the pores (∼20%). By tuning pore size, center-to-center distance, and pore density through optimizing the chemical composition or the molecular weight of the synthesized BCP to increase the utilization of the pores in the functional layer, we believe that the final power density could achieve more than 10 W/m2. We also test the energy conversion performance at pH 7.1 under a 50-fold NaCl salinity gradient (i.e., cM‑1/cM‑2 = 0.01 M/ 0.5 M). The power density decreases substantially to 0.7 W/m2 (Figure 5f, 2), which is because the P4VP chains change into neutral state and lose ion selectivity. In order to make the Janus membrane be used in the neutral pH conditions that is relevant for natural waters (pH ∼ 6.5−8.5), the M-2 membrane was modified using a quaternization reaction66 (Experimental Section) and thus can maintain positively charged at neutral pH. As expected, the power density of the Janus membrane after quaternization modification increases largely to approximately 2.34 W/m2 (Figure 5f, 3). If we substitute real seawater and river water for the man-made electrolyte solutions, the power density can achieve about 2.04 W/m2 (Figure 5f, 4). Notably, the economic cost is an important parameter to evaluate if the BCP based membrane would be able to push the industrial applications. If the BCPs are synthesized from the monomers, not synthesized from the intermediates or commercial products, the cost is calculated to be approximately 7.5 USD($)/m2, which is below the international cost target (11 USD($)/m2) for the ion exchange membranes based reverse electrodialysis.4 The system exhibits excellent anion selectivity, which is essential for the generation of electric current under a concentration gradient. In general, continuously selective ion diffusion under a concentration gradient enriches the counterions on the low-concentration side, causing a rise of salinity concentration in the diffusion boundary layers. This polarization phenomenon would lower the effective concentration difference across the membrane and thus suppresses transmembrane ion transport.12,67 In our system, this concentration

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CONCLUSIONS In summary, our work demonstrates the use of a BCP-based ultrathin and ion-selective Janus nanochannel membrane for harvesting the Gibbs free energy inherently available in a fluid system. This tailor-made Janus membrane with chemical, geometrical, and electrostatic heterostructures is constructed via the phase separation of two functional BCPs and displays an ultrathin submicron-scale thickness of approximately 500 nm. Benefiting from the nanoscale pore size, the asymmetric membrane preserves the charge-governed ionic transport in a wide concentration range and exhibits excellent anion 8911

DOI: 10.1021/jacs.7b02794 J. Am. Chem. Soc. 2017, 139, 8905−8914

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Journal of the American Chemical Society

The BCP-1 layer was cast onto the PSS layer by spin-coating a 4 wt % solution of BCP-1 in toluene at a speed of 2000 rpm for 60 s, followed by vacuum annealing at 80 °C for 4 h. The thermally annealed sample was irradiated by 365 nm UV light with an intensity of 8 mW/cm2 for 15 min. Then, a 2 wt % solution of BCP-2 in dioxane was spin-coated onto the treated BCP-1 layer at 2000 rpm for 30 s to form the BCP-2 (M-2) layer. Finally, the Janus membrane was exfoliated from the substrate by immersing it in water. Notably, the casting order of BCP1 and BCP-2 membranes cannot be reversed because the BCP-2 membrane with a sub-100 nm thickness is not self-supporting. Electrical Measurements. The I−V measurements and subsequent energy conversion tests were performed with a Keithley 6487 semiconductor picoammeter (Keithley Instruments, Cleveland, OH). The Janus membrane was mounted between a two-compartment conductivity cell (Figure S6). The effective area for ion conduction was approximately 3 × 104 μm2 (Figure S7). A pair of homemade Ag/ AgCl electrodes was used to apply a transmembrane potential. The anode was placed in the M-2 side. The electrolyte (KCl or NaCl) solution was adjusted to the desired pH (∼4.3) and then injected into each compartment. The testing solutions were all prepared using degassed Milli-Q water (18.2 MΩ·cm). Cyclic Voltammetry Analysis. The cyclic voltammetry measurements were carried out using an electrochemical workstation (CHI660E, Chenhua, Shanghai) and a three-electrode system. The Janus membrane without a sacrificial PSS layer coating was prepared on a bare ITO electrode that was used as the working electrode. A platinum wire served as the counter electrode and an Ag/AgCl electrode was used as the reference electrode. Anionic [Fe(CN)6]3− and cationic [Ru(NH3)6]3+ probes were selected to determine the ion selectivity of the Janus membrane. Notably, the measurements were conducted at 23 °C. Permeation Experiments. Permeation experiments were performed using a two-cell system separated by the Janus membrane. As the fluorescence emission of rhodamine 6G is much stronger than that of sulforhodamine, the permeability rate was tested separately to avoid the interference. The M-2 side was facing the feed solution and the feed cell contained either 0.1 mM rhodamine 6G or 0.1 mM sulforhodamine solution. A transmembrane potential was applied to accelerate the permeation process. The permeability was monitored using a spectrofluorophotometer (RF-5301PC, SHIMADZU, Japan). Quaternization of the PS-b-P4VP Membrane. Before exfoliation, the Janus membrane on silicon wafer was immersed in solutions of 2-chloroacetamide at room temperature for approximately 6 h to make sure that the P4VP was fully quaternized.66 Next, the sample was transferred into Milli-Q water for complete exfoliation of the Janus membrane from the substrate. The quaternized membrane was then rinsed with Milli-Q water sufficiently to remove the remaining quaternization agent on the membrane surface. Numerical Simulation. The theoretical simulation was based on the coupled two-dimensional Poisson and Nernst−Planck (PNP) equations within the commercial finite element package COMSOL 5.1 script environment. The geometrical parameters of the numerical model were in agreement with the experimental values (Figure S8) and consisted of a 400 nm long M-1 channel (pore size: 10 nm) and a 100 nm long M-2 channel (pore size: 17 nm). Two electrolyte reservoirs were introduced in order to decrease the effect of entrance/ exit mass transfer resistances on the overall transport of ions. Notably, with regard to the M-2 channel, the charges of the polymer chains were strictly confined to the channel walls. Therefore, the surface charge density of M-2 channel (−0.08 C/m2) was set to be two times larger than the surface charge density of the M-1 channel (+0.24 C/ m2) (details in Supporting Note 3).

selectivity. The system shows high-performance osmotic energy conversion with a power density of approximately 2.1 W/m2. Through surface modification, the Janus membrane can also work in the neutral pH that is relevant for natural waters. By mixing natural river water and seawater, the power output can achieve about 2.04 W/m2. It is envisaged that the power density could achieve more than 10 W/m2 by increasing the utilization rate of the pores in the functional layer. The substantial power output can be attributed to its asymmetric bipolar structure and submicron-scale thickness. This work shows the potential of the design of ultrathin asymmetric membrane systems in salinity cells and other mass transfer processes, such as advanced separations and redox-flow batteries.70

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EXPERIMENTAL SECTION

Chemicals. Chlorobenzene (analytical purity, Beijing Chemical Reagents Co.) was distilled with CaH2 before use. Toluene and tetrahydrofuran (analytical purity, Beijing Chemical Reagents Co.) were distilled from Na/benzophenone under an N2 atmosphere. CuBr (analytical purity, Alfa Aesar), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99.5%, TCI), and methoxypolyethylene glycol azide (PEO-N3, Mn = 12 500 kg/mol, PDI = 1.08, Seebio) were used as received. 5-Propargylether-2-nitrobenzyl bromoisobutyrate was synthesized according to a method described in the literature.71 The methacrylate with chalcone was prepared according to the previously reported method.38 BCP-2 (M-2), or polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP, MnPS = 48.4 kg/mol, MnP4VP = 21.3 kg/mol, PDI = 1.13), was purchased from Polymer Source, Inc., Canada and used as received. Poly(styrenesulfonate) (PSS, Mw = 70 kg/mol) was purchased from Sigma-Aldrich. The quaternization agent 2-chloroacetamide was purchased from J&K Beijing Co., Ltd. Other chemicals were all analytical grade and acquired commercially. Synthesis of BCP-1. BCP-1, or PEO-hv-PMA(Chal) (MnPEO = 12.5 kg/mol, MnPMA(Chal) = 44.7 kg/mol, PDI = 1.25), was prepared via atom-transfer radical polymerization. The synthetic procedure is shown in Figure S1. In a typical “click” reaction procedure, 5.0 g of PEO-N3 (Mn = 12.5 kg/mol, PDI = 1.08, 0.4 mmol) and 1.0 g of 5propargylether-2-nitrobenzyl bromoisobutyrate (2.72 mmol) were added to a mixture of 40.0 mg of CuBr (0.278 mmol), 70.0 mg of PMDETA (0.405 mmol) in 20.0 mL of THF. The mixture was degassed by three freeze pump thaw cycles, and subsequently sealed under vacuum. The reaction tube was placed in an oil bath at 40 °C. After 120 h, the reaction was quenched by dipping the tube in ice and breaking it. The mixture was diluted with CH2Cl2 and passed through a basic alumina column; it was then precipitated into ether. The PEO macroinitiator was filtered and dried under vacuum (3.8 g, 63.3% yield). The PEO macroinitiator (0.5 g, Mn = 12.5 kg/mol, PDI = 1.08, 0.04 mmol) and 2.0 g of the chalcone-containing monomer (3.86 mmol) were added to a tube along with 8.0 mg of CuBr (0.0556 mmol), 20.0 mg of PMDETA (0.116 mmol), and 4 mL of chlorobenzene. The mixture was degassed by three freeze pump thaw cycles and subsequently sealed under vacuum. The reaction tube was placed in an oil bath at 90 °C. After 20 h, the reaction was quenched by dipping the tube in ice and breaking it. The mixture was diluted with CH2Cl2 and passed through basic alumina column; it was then precipitated into methanol. The block copolymer was filtered and dried under vacuum. The product was a canary-green solid (1.0 g, monomer conversion of 40%). The molecular weight was measured by gel permeation chromatography (Mn = 57.2 kg/mol, PDI = 1.25, Figure S2). The peak assignments from the 1H NMR spectrum in CDCl3 are given in Figure S3. Membrane Fabrication. Before membrane casting, the polymer solutions of BCP-1 and BCP-2 were both filtered through polytetrafluoroethylene (PTFE) filters (∼0.22 μm) to remove any large impurities and aggregates. The following is a typical protocol. A thin layer of sacrificial PSS film was first formed on a silicon substrate by spin-coating a 5 wt % PSS aqueous solution at 1600 rpm for 30 s.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02794. Experimental procedures and additional figures (PDF) 8912

DOI: 10.1021/jacs.7b02794 J. Am. Chem. Soc. 2017, 139, 8905−8914

Article

Journal of the American Chemical Society

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Lei Jiang: 0000-0003-4579-728X Author Contributions ⊥

Z.Z. and X.S. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2017YFA0206904, 2017YFA0206900), the National Natural Science Foundation (21625303, 51673206, 21434003, 91427303, 21421061), and the Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M03).

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DOI: 10.1021/jacs.7b02794 J. Am. Chem. Soc. 2017, 139, 8905−8914