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Interface Design of Nanochannels for Energy Utilization Yinglin Zhu,†,⊥ Kan Zhan,†,‡,§,⊥ and Xu Hou*,†,‡,§,∥ †
State Key Laboratory of Physical Chemistry of Solid Surface, College of Chemistry and Chemical Engineering, ‡Research Institute for Soft Matter and Biomimetics, College of Physical Science and Technology, §Collaborative Innovation Center of Chemistry for Energy Materials, and ∥Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China ABSTRACT: Nanochannels offer a variety of significant advantages for innovative applications, such as biosensing, filtering, and energy utilization. In this Perspective, we highlight the interface design and applications of nanochannels for energy utilization and discuss further challenges in the development of nanochannels for energy conversion, energy conservation, and energy recovery.
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Pressure-driven energy conversion, which is termed the electrokinetic effect, describes the conversion of an external driving force (pressure) into electrical energy throughout a narrow channel carrying a net electrical charge.12 The electrokinetic effect is related to the electric double layer, which forms through electrostatic interactions between the charged surface of the nanochannel and ions in fluids. External pressure induces the ions to migrate through the nanochannel, which generates ionic current (see Figure 1, top left).12 Salinity-gradient power conversion converts osmotic pressure to electricity (see Figure 1, top right).6 Photoelectric conversion can be achieved by utilizing photoacid molecules for proton release and uptake under irradiation and then darkness to obtain proton-driven electrochemical potentials inside the nanochannels.9 Although energy-conversion-related nanochannels have been widely studied,6,12 most researchers have focused on the material structures and concentration of electrolytes.6,13 The properties at the interfaces of nanochannels play important roles in ion transport during energy conversion. Surface charge density6,14 and effective diameter15 can be regulated by modifying the physicochemical properties of the nanochannel’s inner surface. For example, the energy efficiency in a salinity-gradient power conversion system can be promoted by enhancing the surface charge density.6 At present, the surface charge density can be continuously regulatedthe surface charge can even be changed from negative to positive; that is, the surface polarity can be reversed.14 Nanochannels with tunable effective diameters can be obtained by grafting responsive molecules onto the inner walls of the channels. The responsive molecules change conformation
anochannel development and related applications are a burgeoning area of interest because the ability of nanochannels to regulate transported substances in confined spaces is especially significant for high-resolution sensing and high-efficiency energy applications. Previously, we described how bio-inspired nanochannels could be used to build novel, smart nanodevices for biosensor applications.1 In the intervening years, a great deal of research related to energyand sensor-based applications of nanochannels and nanopores has been reported.2,3 However, research on nanochannels in energy-related areas continues to face challenges such as low efficiencies, complex preparation processes, and high fabrication costs.4 Overcoming these challenges is an important and difficult task in the field of energy conversion, energy conservation, and energy recovery. Therefore, creating nanochannels for energy utilization that are efficient, low-cost, and easy to prepare would lead the way to the development of clean and renewable energy resources.5−7 The energy utilization process in nanochannels is affected by the interactions between the nanochannel’s inner surface and flowing fluids. Consequently, the physicochemical properties of the interface inside nanochannels have a profound influence on the flowing liquid, which thereby affects energy utilization. Thus, the design of nanochannel interfaces could be a key point in energy utilization including energy conversion,2,8,9 energy conservation,5,10 and energy recovery.7,11 Energy Conversion. Energy conversion from pressure, osmotic potential, or photo energy, etc. to electrical energy through nanochannel-based devicessuch as pressure-driven energy conversion,12 salinity-gradient power conversion,6 or photoelectric conversion,9 etc.has become a hotspot recently because of nanochannels’ green energy characteristics.6,9,12 © XXXX American Chemical Society
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DOI: 10.1021/acsnano.7b07923 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano
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Figure 1. Illustration of interface design in nanochannels for energy utilization in energy conversion, energy conservation, and energy recovery. (Top left) Pressure-driven flow through a nanochannel induces a flow of counter charges in the double layer at the channel walls, thus generating an electrical current, which converts mechanical energy into electrical energy. (Top right) With a concentration gradient, the ions diffuse spontaneously across the nanochannel, converting Gibbs free energy in the form of a salinity gradient into electricity. (Bottom left) The slip interface is formed at the inner wall of nanochannels by modifying the interface with an additive bilayer, and the total streaming current generation from nanochannels is related to the slip of additive molecules. (Bottom right) Optimized interface design could improve the energy recovery efficiency in a capacitive deionization system for water desalination.
exist for higher energy utilization efficiency.5 In addition, the adsorption of ionic surfactants on the negatively charged channel surface will form new interface layers. These layers may then slip under high-stress force (Figure 1, bottom left).17 Furthermore, the introduction of liquid-based gating into nanochannels shows great potential in energy utilization due to sustained antifouling behavior, which would ultimately contribute to energy conservation with long-term usage.10 With liquid-based gating, the nanochannel is filled with a capillarystabilized fluid, sealing the channel in the closed state, and the nanochannel reversibly reconfigures under pressure to create a nonfouling, fluid-lined, and controllable channel in the open state, which integrates tunable pressure with sustained antifouling behavior.18
in response to ambient stimuli, which enables smart control of ion transport in confined spaces.15 This idea can be introduced into energy conversion systems where the power output is intelligently controlled by the ion transport.
Interface properties of nanochannels play important roles in ion transport during energy conversion. Energy Conservation. The interface design of nanochannels can be used not only for energy conversion but also for energy conservation. For example, slip interfaces enable liquid flowing near the inner wall of a nanochannel to move relative to the inner surface of the nanochannel.16 This effect contributes to increases in the advection of counterions and decreases in dissipative loss at the interface between the liquid phase and the solid-state inner wall of the nanochannel. Increasing the slip length would, in theory, reduce fluid impedance and increase fluid conductance,16 and an optimal ratio of slip length to nanochannel height might
The introduction of liquid-based gating into nanochannels shows great potential in energy utilization due to sustained antifouling behavior. B
DOI: 10.1021/acsnano.7b07923 ACS Nano XXXX, XXX, XXX−XXX
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efficient ion transport and storage. There are many ways to design nanochannels with complicated structures, such as nanotube-based multiscale metallic metamaterial method,24 threedimensional printing utilizing semiconductor as photoinitiator,25 and symmetric/asymmetric modifications,10 etc. Collectively, current methods related to energy production based on nanochannels give rise to problems such as complex preparation processes, high fabrication costs, fouling, and low efficiency. Interface design of nanochannels is one of the most promising, economical ways for targeting high-energy efficiency for real-world applications such as pressure-driven energy devices, salinity-gradient power cells, and desalination apparatuses. As for the functionalization of the interfaces of nanochannels, emerging modification strategies such as bio-inspired design,2,3 dynamic liquid interface design,18 and symmetric/asymmetric design26 provide possible approaches to realize new avenues in energy utilization.
Current research on energy conservation of nanochannels has focused on the solid materials, which are subject to problems such as fouling, physical damage, and limited lifetime.19 These drawbacks give rise to energetic losses. Fabrication of the slip interface in nanochannels is an efficient approach for improving fluid power generation efficiency. Note that the effect of the liquid-based gating thresholds in nanochannels may be rather strong, but as compared with solid−liquid interfaces, it is feasible to utilize liquid−liquid interface designs10 to weaken this effect to realize energy savings. The functional gating liquid can dynamically seal the channel to achieve stimuli-responsive layers with controllable and reversible reconfiguration as well as antifouling properties. Additionally, the dynamic multiphase transport and separation under steady-state applied pressure has great benefits for membrane science but has not yet been realized. For the traditional system, most of the multiphase transport and separation is based on pressure changes. On one hand, degassing by reducing the pressure20 or liquid−liquid separation by increasing the pressure21 usually requires extra energy for the pressure change. Meanwhile, the pressure change also affects the entire system, not just the membrane itself, and thus the energy used on the other parts is wasted. Liquid gating elastomeric nanochannel systems can be used to provide an economical means to control channel size for tuning the dynamic interface of the nanochannels by simply applying mechanical forces, which would be useful for multiphase transport and separations under steady-state applied pressures.22 Energy Recovery. As we face the costs of fossil fuels and growing concerns for their impact on the environment, the development of energy recovery technologies has become an important area in energy-related research.7,23 For example, water purified from seawater by reverse osmosis can be used for chemical, industrial, medical, and pharmacological applications, but the purification process requires significant amounts of energy.23 Reverse osmosis consumes a lot of energy because fresh water is removed from the seawater, rather than removing ions from the seawater.11 In contrast, the nanoporous capacitive deionization system removes ions from seawater by applying voltages. Ions can be temporarily stored in the electric double layer that exists at the electrode−solution interface inside porous carbon materials (Figure 1, bottom right). Długołec̨ ki and van der Wal investigated energy recovery values in this desalination system, and the overall energy consumption in producing desalinated water is as low as 0.26 kWh/m3, less energy for the treatment of seawater than reverse osmosis.7 For the purpose of energy recovery, highly conductive electrodes and ion-exchange membranes have been developed because of their ability to reduce electrical resistance.7 However, the interface between the ion-exchange membrane consisting of nanopores and the salt solution also affects energy recovery because the physicochemical properties of the seawater− solid porous membrane interface influences the amount of energy stored for the next desalination process. Energy recovery efficiency can likely be varied by changing the properties of the nanopore surfaces. Thus, optimizing the interface design of the membranes could produce even higher energy recovery. Because nanochannels have aspect ratios (depth/diameter) greater than those of nanopores, they provide more flexibility in designing desalination membranes with higher energy recovery. The hybrid nanochannel/pore systems possess complicated structures, interpenetrating with channels/pores, for which the dynamic liquid−liquid interface could be used for more
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Xu Hou: 0000-0002-9615-9547 Author Contributions ⊥
Y.Z. and K.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21673197, 21621091, 51706191), Young Overseas High-Level Talents Introduction Plan, the 111 Project (B16029), and the Fundamental Research Funds for the Central Universities of China (20720170050). We would like to thank Dr. Miao Wang, Zhizhi Sheng, and Mr. Yunmao Zhang for fruitful discussions. REFERENCES (1) Hou, X.; Jiang, L. Learning from Nature: Building Bio-Inspired Smart Nanochannels. ACS Nano 2009, 3, 3339−3342. (2) Li, R.; Fan, X.; Liu, Z.; Zhai, J. Smart Bioinspired Nanochannels and Their Applications in Energy-Conversion Systems. Adv. Mater. 2017, 29, 1702983. (3) Tian, Y.; Wen, L.; Hou, X.; Hou, G.; Jiang, L. Bioinspired IonTransport Properties of Solid-State Single Nanochannels and Their Applications in Sensing. ChemPhysChem 2012, 13, 2455−2470. (4) Feng, J.; Graf, M.; Liu, K.; Ovchinnikov, D.; Dumcenco, D.; Heiranian, M.; Nandigana, V.; Aluru, N. R.; Kis, A.; Radenovic, A. Single-Layer MoS2 Nanopores as Nanopower Generators. Nature 2016, 536, 197−200. (5) Chang, C. C.; Yang, R. J. Electrokinetic Energy Conversion Efficiency in Ion-Selective Nanopores. Appl. Phys. Lett. 2011, 99, 083102. (6) Guo, W.; Cao, L.; Xia, J.; Nie, F. Q.; Ma, W.; Xue, J.; Song, Y.; Zhu, D.; Wang, Y.; Jiang, L. Energy Harvesting with Single-IonSelective Nanopores: A Concentration-Gradient-Driven Nanofluidic Power Source. Adv. Funct. Mater. 2010, 20, 1339−1344. (7) Długołęcki, P.; van der Wal, A. Energy Recovery in Membrane Capacitive Deionization. Environ. Sci. Technol. 2013, 47, 4904−4910. (8) Cheng, H.; Zhou, Y.; Feng, Y.; Geng, W.; Liu, Q.; Guo, W.; Jiang, L. Electrokinetic Energy Conversion in Self-Assembled 2D Nanofluidic Channels with Janus Nanobuilding Blocks. Adv. Mater. 2017, 29, 1700177. C
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DOI: 10.1021/acsnano.7b07923 ACS Nano XXXX, XXX, XXX−XXX
