Article pubs.acs.org/JACS
Engineered Asymmetric Heterogeneous Membrane: A Concentration-Gradient-Driven Energy Harvesting Device Zhen Zhang,† Xiang-Yu Kong,‡ Kai Xiao,† Qian Liu,§ Ganhua Xie,† Pei Li,† Jie Ma,‡ Ye Tian,† Liping Wen,*,‡ and Lei Jiang†,‡ †
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Laboratory of Bioinspired Smart Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, Beijing Normal University, Beijing 100875, P. R. China S Supporting Information *
ABSTRACT: Engineered asymmetric membranes for intelligent molecular and ionic transport control at the nanoscale have gained significant attention and offer prospects for broad application in nanofluidics, energy conversion, and biosensors. Therefore, it is desirable to construct a high-performance heterogeneous membrane capable of coordinating highly selective and rectified ionic transport with a simple, versatile, engineered method to mimic the delicate functionality of biological channels. Here, we demonstrate an engineered asymmetric heterogeneous membrane by combining a porous block copolymer (BCP) membrane, polystyrene-b-poly(4vinylpyridine) (PS48400-b-P4VP21300), with a track-etched asymmetric porous polyethylene terephthalate membrane. The introduction of chemical, geometrical, and electrostatic heterostructures provides our heterogeneous membrane with excellent anion selectivity and ultrahigh ionic rectification with a ratio of ca. 1075, which is considerably higher than that of existing ionic rectifying systems. This anionselective heterogeneous membrane was further developed into an energy conversion device to harvest the energy stored in an electrochemical concentration gradient. The concentration polarization phenomenon that commonly exists in traditional reverse electrodialysis can be eliminated with an asymmetric bipolar structure, which considerably increases the output power density. This work presents an important paradigm for the use of versatile BCPs in nanofluidic systems and opens new and promising routes to various breakthroughs in the fields of chemistry, materials science, bioscience, and nanotechnology.
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ification, plasma grafting, and ion sputtering technology.7−10 These challenging nanoscale functionalization processes seriously constrain device performance and future applications,11,12 which has motivated emerging research of heterogeneous membranes (i.e., hybrid membranes composed of two chemical compositions).13 Heterogeneous membranes with inherent functional groups are superior because they do not require artificial functionalization. Additionally, compared with homogeneous membranes, heterogeneous membranes are particularly attractive because they can be endowed with novel functionality not available when using two homogeneous membranes separately.14 For example, Wang et al. demonstrated a multifunctional nanowire-based inorganic heterogeneous membrane that can provide multifunctionality as a single membrane.15 Our group recently developed an inorganicsbased heterogeneous membrane that provides ionic rectifica-
INTRODUCTION
Living organisms make extensive use of nanosized asymmetric ion channels as highly selective filters and ionic rectifiers to control the movement of various ions that play critical roles in life processes, including mass delivery, energy conversion, and signal transmission.1−4 However, it is difficult to integrate these lipid-membrane-based nanochannels into devices because they are fragile and susceptible to deterioration in changing external environments such as pH, temperature, mechanical stress, etc.5 The efficiency of these nanochannels in coordinating ionic transport in a highly directional and selective way has been a source of inspiration for scientists seeking to mimic such processes using synthetic asymmetric nanochannel membranes. These synthetic membranes are expected to exhibit similar or stronger functionality compared to their biological counterparts, with superior stability and robustness.6 To date, many ingenious asymmetric membranes with homogeneous materials have been developed using various functionalization protocols, such as solution chemical mod© 2015 American Chemical Society
Received: September 21, 2015 Published: November 4, 2015 14765
DOI: 10.1021/jacs.5b09918 J. Am. Chem. Soc. 2015, 137, 14765−14772
Article
Journal of the American Chemical Society
Figure 1. Engineered asymmetric heterogeneous membrane. (a) Schematic of the engineered asymmetric heterogeneous membrane composed of a pH-responsive porous BCP membrane (top layer) and a pH-responsive porous PET membrane with conical nanochannels (bottom layer). (b, c) SEM images of the base (b) and the tip (c) of the conical nanochannels (pore density: 107 cm−2). (d) AFM image of the as-prepared BCP membrane atop the PET membrane and a histogram of pore size distribution (pore density: 1011 cm−2) with Gaussian fit (e). The scale bars are all 100 nm.
tion with a ratio of ca. 450.16 The reported results have shown that the fabrication process of inorganics-based heterogeneous membranes is complicated and that material selection is constrained.13,15−19 Therefore, with regard to mimicking the delicate design of biological channels, it would be desirable to construct a high-performance heterogeneous membrane capable of coordinating highly selective and rectified ionic transport with a simple, versatile, engineered method. Block copolymer (BCP) is a promising candidate for the construction of a heterogeneous membrane.20−22 One- and two-dimensional nanofluidic channels can be easily obtained by simply tailoring self-assembled morphologies of the BCP membrane, such as its cylindrical and lamellar structure.23−25 Because of the structural and functional richness of macromolecular building blocks, the material system is highly versatile, engineered, and scalable and provides well-defined pores.26−30 Additionally, compared with small molecules,31−33 end-tethered polymer brushes can provide more effective control over ionic transport because they can increase the spatial density of functional groups on the channel walls by extending the two-dimensional distribution of functional groups to three dimensions.34−38 In this article, we demonstrate an engineered asymmetric heterogeneous membrane that can coordinate highly selective and rectified ionic transport by directly coating an porous BCP membrane, polystyrene-b-poly(4-vinylpyridine) (PS48400-bP4VP21300), onto a robust and flexible porous polyethylene terephthalate (PET) substrate with conical nanochannels. The hybrid membrane, which features asymmetric chemical composition, geometry, and surface charge polarity, exhibits ultrahigh ionic rectification with a notably high ratio of ca. 1075 and excellent anion selectivity. We further developed this anion selective bipolar membrane into a concentration-gradientdriven energy harvesting device. The output power density can be considerably increased because the concentration polarization phenomenon that commonly exists in traditional reverse electrodialysis can be eliminated using the asymmetric bipolar structure. Our newly designed membrane offers several distinct advantages: (1) the heterogeneous membrane can be prepared using a rapid and simple hybridization technique; (2) the rectification ratio (∼1075) is impressivemore than twice
the highest ratio reported to date; and (3) the design strategy is versatile and engineered. Various BCPs with different predefined functionalities can be used to further enhance the performance of the heterogeneous membrane. Such a system can trigger further experimental and theoretical efforts to construct “smart” heterogeneous membrane systems for sustainable power generation, water purification, and desalination.39
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RESULTS AND DISCUSSION To fabricate the asymmetric heterogeneous membrane, a concentrated viscous solution of BCP in dioxane was spincoated on a porous PET membrane to form a membrane via microphase separation (Figure 1a).40 The porous PET membrane substrate (12 μm thick) with conical nanochannels was first prepared by the well-developed ion track etching technique (Figure S1). The large opening of the conical nanochannel is termed the base, and the small opening is termed the tip. In all cases, the diameters of the bases and the tips are approximately 500 and 50 nm, respectively (Figure 1b,c). The nanochannel is electrically neutral as a result of the carboxylate groups generated by the etching process when the membrane is exposed to a solution with a pH below the pKa (∼3.8). If the pH is above the pKa, the carboxylate groups deprotonate to produce negative charges on the nanochannel walls.41 The as-prepared BCP membrane (∼1.5 μm thick) is composed of a 100 nm-thick layer of hexagonally packed pores (Figure 1d, ∼10 nm) atop a disordered network-like layer. A cross-sectional image of the as-prepared BCP membrane and the corresponding formation mechanism is shown in Figure S2. As shown in Figure 1a, the green segments represent porous matrix formed by the major component, PS, and the blue segments represent channels formed by the minor component, P4VP. The conformational changes of P4VP chains on the inner pore walls and the ionization equilibrium of the pyridine groups (pKa ≈ 5.2) are also thermodynamically controlled by the pH value.36,37 The P4VP chains exhibit a swollen, positively charged, and hydrophilic state when the pH is below the pKa and a collapsed, neutral, and hydrophobic state when the pH is above the pKa.42 The conformational changes 14766
DOI: 10.1021/jacs.5b09918 J. Am. Chem. Soc. 2015, 137, 14765−14772
Article
Journal of the American Chemical Society
Figure 2. Ultrahigh ionic rectification. (a) I−V curve of the heterogeneous membrane recorded in 10 mM KCl (pH 4.3) when a sweep voltage ranging from −2 to 2 V is applied. The anode is on the PET side, and the cathode is on the BCP side (inset). (b) I−T curve of the heterogeneous membrane recorded in 10 mM KCl (pH 4.3) for an external bias alternating between +2 V/−2 V. (c) Influence of electrolyte concentration on ionic conductance, which displays two distinct regions divided by the red dashed line: a surface charge dominant zone (left) and a bulk concentration dominant zone (right). The transmembrane ionic conductance (black squares) deviates from the bulk value (black dashed line) when the electrolyte concentration is
