Bioinspired Nanoporous Membrane for Salinity Gradient Energy Harvesting
2020; Elsevier BV; Volume: 4; Issue: 11 Linguagem: Inglês
10.1016/j.joule.2020.09.009
ISSN2542-4785
Autores Tópico(s)Energy Harvesting in Wireless Networks
ResumoYahong Zhou is an assistant Professor at TIPC. She received her bachelor's degree from Jilin University in 2009 and PhD from the Institute of Chemistry, Chinese Academy of Sciences in 2014 under the supervision of Prof. Lei Jiang. Her research interest mainly focuses on the nanoporous membrane and integrated as-prepared nanoporous membrane into energy devices.Graphical AbstractView Large Image Figure ViewerDownload Hi-res image Download (PPT)Lei Jiang is a professor at the TIPC and Beihang University. He is an academician of the Chinese Academy of Sciences, Academy of Sciences for the Developing World, and National Academy of Engineering, USA. He received his bachelor's and master's degrees from Jilin University and PhD from the University of Tokyo. He worked as a postdoctoral fellow with Prof. Akira Fujishima and then as a senior researcher in the Kanagawa Academy of Sciences and Technology with Prof. Kazuhito Hashimoto. His scientific interests focus on bio-inspired, smart, multi-scale interfacial materials with superwettability. Yahong Zhou is an assistant Professor at TIPC. She received her bachelor's degree from Jilin University in 2009 and PhD from the Institute of Chemistry, Chinese Academy of Sciences in 2014 under the supervision of Prof. Lei Jiang. Her research interest mainly focuses on the nanoporous membrane and integrated as-prepared nanoporous membrane into energy devices. Lei Jiang is a professor at the TIPC and Beihang University. He is an academician of the Chinese Academy of Sciences, Academy of Sciences for the Developing World, and National Academy of Engineering, USA. He received his bachelor's and master's degrees from Jilin University and PhD from the University of Tokyo. He worked as a postdoctoral fellow with Prof. Akira Fujishima and then as a senior researcher in the Kanagawa Academy of Sciences and Technology with Prof. Kazuhito Hashimoto. His scientific interests focus on bio-inspired, smart, multi-scale interfacial materials with superwettability. Harvesting clean and substantial energy from water as a substitute to fossil fuels has long been desired. In theory, the salinity gradient energy is estimated to be 0.8 kWh m−3 at the sea and river interface, and its global total is estimated to reach up to 30 terawatts (TW). In 1954, Pattle declared that a great amount of energy was lost in terms of the osmotic pressure while the sea meets the river. Until 1975, Loeb utilized this salinity gradient energy with a selectively permeable membrane.1Loeb S. Norman R.S. Osmotic power plants.Science. 1975; 189: 654-655Crossref PubMed Scopus (244) Google Scholar With the help of rapid development of nanotechnology and membrane science, salinity gradient energy as a type of the blue energy has inspired scientists to develop high-performance blue-energy harvester systems. Generally, two traditional approaches, pressure-retarded osmosis (PRO) and reversed electrodialysis (RED), are considered to be the most promising methods, and they are both based on the membrane science. When two solutions with different salinities are placed on either side of a porous membrane, the membrane in the PRO method allows water to move from the fresh water into the salt solution, whereas salt is not allowed to pass through. As for the RED method, a net current is generated for a permselective membrane, which only allows the ions with an opposite charge polarity to pass through the membrane. Therefore, the RED approach can be used directly convert the energy into electricity. In this paper, we mainly discuss the RED-based technology and science. Traditional membranes are based on the permeability membranes created by sub-nanometer pores, for example, ion exchange membranes. The output power density is quite low because the pore size and the internal resistance of the membrane are limited. The energy conversion efficiency is reduced when the ions are polarized across the membrane and its selectivity is suppressed. In recent years, two-dimensional materials with abundant surface charge (boron nitride [BN], MoS2, and graphene oxides [GO]) have shown great potential in terms of their output power density.2Feng 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: 197-200Crossref PubMed Scopus (489) Google Scholar,3Bocquet L. Nanofluidics coming of age.Nat. Mater. 2020; 19: 254-256Crossref PubMed Scopus (92) Google Scholar These molecularly thick sheets could enhance ionic flow and ionic selectivity owing to their high surface charge density (up to ∼1 C/m-2). Although a remarkable magnitude of output power density is reported and these materials provide an excellent model for theoretical studies, industry-scale production remains a challenge. There are several factors impacting the performance of the energy generator based on these materials. If a membrane is under a salinity gradient, the surface charge on the pore screens the counterions while the ions with the same charge polarity are blocked from passing through, thus enabling the generation of a net osmotic current. Therefore, selectivity is a fundamental factor in salinity gradient energy harvesting. Both the surface charge and pore size can affect the anion or cation selectivity. Generally, this selectivity is better in sub- or nanoscale channels with a large surface charge density. As mentioned earlier, membranes with sub-/nanoscale pores are preferred for the selectivity. However, permeability is scarified at this scale, which eventually impacts the output power density.4Ramon G.Z. Feinberg B.J. Hoek E.M.V. Membrane-based production of salinity-gradient power.Energy Environ. Sci. 2011; 4: 4423-4434Crossref Scopus (315) Google Scholar Selectivity and permeability somewhat compete against the performance of the generator. Selectivity determines the function of the membrane, whereas permeability limits the output power density which restricts the industrial application of the membrane. Achieving a tradeoff between permeability and selectivity has become a challenge. A state-of-the-art option is to prepare an ultrathin membrane with a thickness of less than 1 μm. Energy loss during the conversion process may hinder the application as well.5Siria A. Bocquet M.-L. Bocquet L. New avenues for the large-scale harvesting of blue energy.Nat. Rev. Chem. 2017; 1https://doi.org/10.1038/s41570-017-0091Crossref Scopus (174) Google Scholar The ionic concentration increases near the interface between the membrane and electrolyte, such that the surface charge is over-screened by the counterions. This polarization at the interface weakens the selectivity of the membrane, deteriorating its functioning as an energy harvester. Improvement of the energy efficiency has become desirable as well. To maintain economic relevance, the ultimate aim of the industry is to ensure the output power density is higher than 5.0 watts (W)/m2. For decades, scientists have explored a wide range of membrane-based energy harvesting technologies. Realizing a high value of power density is still a challenge to overcome. The electric eel is well known for generating up to 600 volts (V) and 1 ampere (A) of current in a shock. Previously, scientist have focused on the selectivity of the electrocyte cells and bioinspired selective membranes as blue-energy harvesters.6Wu Q. Wang C. Wang R. Chen C. Gao J. Dai J. Liu D. Lin Z. Hu L. Salinity-Gradient Power Generation with Ionized Wood Membranes.Adv. Energy Mater. 2019; 10: 1902590Crossref Scopus (34) Google Scholar However, the potassium channel embedded in such a cell has an asymmetric structure, which leads to an inwardly rectifying K+ current, as shown in Figure 1. Compared to the symmetric pore structure, this structure leads to non-linear ionic transport across the membrane. This type of ionic-diode-nanofluidic provides an option for novel RED-based energy harvesters. As discussed earlier, conventional designs have mainly focused on a single-layer membrane with a symmetric structure with ionic selectivity. However, during the energy conversion process, counterions are enriched on the dilute solution side, which screens selectivity and increases the salinity concentration in the diffusion boundary layers. Thus, effective ion transportation is suppressed, and the energy conversion efficiency is reduced further. Energy is lost within the membrane itself, dissipating osmotic energy as Joule heating in the nanopore. With respect to the Janus membrane, an opposite charge of the membrane on the lower concentration side can prevent the accumulation of counterions. Thus, the resistance of the asymmetrically structured membrane works as a Shockley diode, and the back current is blocked. This type of membrane can avoid power dissipation and eliminate the polarization phenomenon. Heterogeneous membranes with one-way ionic transport properties have been demonstrated to be effective for salinity gradient generators (Figure 2). A heterogeneous membrane was constructed using two composites of negative mesoporous carbon (with 7-nm pores) and microporous alumina (with 80-nm pores).7Gao J. Guo W. Feng D. Wang H. Zhao D. Jiang L. High-performance ionic diode membrane for salinity gradient power generation.J. Am. Chem. Soc. 2014; 136: 12265-12272Crossref PubMed Scopus (288) Google Scholar The heterojunction between the two faces rectified the current when a potential was applied across the membrane (Figure 2A). The experimental data demonstrated that the rectification ratio reached up to ca. 450; this rectification was maintained in high-concentration electrolytes. The salinity gradient energy generator based on this membrane outperformed some commercial ion-exchange membranes, achieving a power density of up to 3.46 W/m2. To further improve its performance, an ultrathin Janus membrane was further developed via the phase separation of two block copolymers (Figure 2B).8Zhang Z. Sui X. Li P. Xie G. Kong X.Y. Xiao K. Gao L. Wen L. Jiang L. Ultrathin and Ion-Selective Janus Membranes for High-Performance Osmotic Energy Conversion.J. Am. Chem. Soc. 2017; 139: 8905-8914Crossref PubMed Scopus (165) Google Scholar Considering the reduction in the resistance of the membrane, a submicron-scale (∼500 nm) tailor-made hybrid membrane was prepared. The power density reached ∼2.04 W/m2 by mixing natural seawater and river water. When ions move from the sea into the river across a selective membrane, they tend to be polarized at the interface. This polarization phenomenon may reduce the selectivity of the membrane and lead to energy loss. Ionic transport behaviors, at the interface between the electrolytes and pores, play an important role in the osmotic energy conversion process. Generally, the ionic transport behaviors (ionic rectification and selectivity) are highly dependent on the ionic concentration. The selectivity and conversion efficiency of conventional membrane-based generators decrease with increasing salinity gradient. To address this bottleneck, two ionomers with opposite charge polarities and tunable surface charge densities were obtained by an exquisite chemical design (Figure 2C).9Zhu X. Hao J. Bao B. Zhou Y. Zhang H. Pang J. Jiang Z. Jiang L. Unique ion rectification in hypersaline environment: A high-performance and sustainable power generator system.Sci. Adv. 2018; 4: eaau1665Crossref PubMed Scopus (87) Google Scholar Scaled-up Janus (two-faced) membranes with three-dimensional (3D) pores were obtained via a phase separation process. The nanofluidic behavior of this system membrane is unique in hypersaline solutions. The experimental data demonstrated that the highest rectification ratio (ca. 57.2) was achieved in 1 molar per liter (M) KCl solution. This indicates that the critical concentration peak (the concentration where the highest rectification ratio appears) shifted to the concentrated saline solution by at least one order of magnitude. This membrane maintained high selectivity and rectified current in the hypersaline solution, which benefitted effective energy conversion (35.7%) and high output power density (2.66 W/m2) when mixing seawater and river water. To further improve the energy conversion process, a 3D gel interface was developed to explore the energy-harvesting performance (Figure 2D).10Zhang Z. He L. Zhu C. Qian Y. Wen L. Jiang L. Improved osmotic energy conversion in heterogeneous membrane boosted by three-dimensional hydrogel interface.Nat. Commun. 2020; 11: 875Crossref PubMed Scopus (52) Google Scholar A layer of functional polyelectrolyte gel layer was cast onto a supporting porous aramid nanofiber membrane. The gel layer provided a charged 3D transport network, which significantly enhanced the interfacial transport efficiency. Thus, when mixing natural sea water and river water, the power output achieved a high value of ∼5.06 W/m−2, reaching the benchmark of 5.0 W/m2 for industrial requirements. To date, various heterogeneous membrane systems with diverse dimensions and materials have been investigated, including a 1D and 1D inorganic and organic system, 2D and 3D organic system, 3D organic hybrid system, and so on.11Zhu X. Zhou Y. Hao J. Bao B. Bian X. Jiang X. Pang J. Zhang H. Jiang Z. Jiang L. A Charge-Density-Tunable Three/Two-Dimensional Polymer/Graphene Oxide Heterogeneous Nanoporous Membrane for Ion Transport.ACS Nano. 2017; 11: 10816-10824Crossref PubMed Scopus (49) Google Scholar,12Xin W. Zhang Z. Huang X. Hu Y. Zhou T. Zhu C. Kong X.Y. Jiang L. Wen L. High-performance silk-based hybrid membranes employed for osmotic energy conversion.Nat. Commun. 2019; 10: 3876Crossref PubMed Scopus (104) Google Scholar In addition to the experimental data, the underlying nanofluidic mechanisms in these systems have been simulated by the Poisson and Nernst-Plank (PNP) equations using COMSOL Multiphysics. It was revealed that the Janus membrane possesses the following features that could benefit the performance of the salinity gradient harvesting generators: (1) asymmetric factors in the membranes (surface charge polarity, pore size, and thickness of each layer)—this determines the direction for the ion flow, (2) abundant surface charge density, which ensures permselectivity, and (3) suitable pore size, which provides ultrahigh ionic transmembrane conductivity. Despite recent remarkable achievements in nanofluidic devices, current energy conversion systems based on osmotic pressure still have a long way to go till they can employed in industrial applications. The goal is to develop nanoporous membrane materials for large-scale industrial applications, and the foundation is the underlying mechanism in nanofluids. Large-scale membranes for energy harvesting are becoming critical for balancing renewable energy production and consumption. Before reaching the industrial settings (the benchmark of output power density for application is 5.0 W/m2) on a large scale, several practical challenges need to be addressed (Figure 3). We have noticed that it has become necessary to address the mitigation of fouling and clogging. With respect to this, we draw some inspiration from biological organisms to design membranes with low water friction or a coated bioadhesive layer. The scaling-up and robustness of membranes also hinder their industrial application, which will largely decrease the cost of porous membranes in the long run. Materials science plays a key role in the translation of nanofluids into salinity gradient energy-harvesting technologies. However, high-performance energy harvesters rely on the contributions from other scientific fields, such as engineering, biology, chemistry, and physics. As for the trade-off between selectivity and permeability, atypical nanofluidic behaviors such as ultrafast flow of ions and water and quantum-confined superfluids even at room temperature, may settle both sides.13Hao Y. Zhang X. Jiang L. Quantum-confined superfluid.Nanoscale Horiz. 2019; 4: 1029-1036Crossref Google Scholar Scientists have utilized the selectivity and rectification of ionic transmembrane properties to harness the blue energy. However, one significant feature of the electric eel is that its ultrafast ion transportation speed is negligible. By manipulating millions of cells to work together, the electric eel can release 109 ions per second and generate a 1 A current at a 20 ms timescale. Recent progress has proved that in an ultra-confined environment, where the pore size is comparable to the ions or molecules, ultrafast transportation may occur while the membrane still holds high selectivity.14Lu J. Zhang H. Hou J. Li X. Hu X. Hu Y. Easton C.D. Li Q. Sun C. Thornton A.W. et al.Efficient metal ion sieving in rectifying subnanochannels enabled by metal-organic frameworks.Nat. Mater. 2020; 19: 767-774Crossref PubMed Scopus (83) Google Scholar Therefore, designing membranes with controlled molecular-level pores and diverse chemistry could render the membranes both highly selective and ultrafast-permeable. In this respect, the dilemma between the selectivity and permeability could be settled, thereby improving efficiency and power density. In addition to the industrial utilization, this RED-based method can also be used for small-sized energy suppliers, such as implant materials for pacemakers, smart wearable devices, and intelligent textiles. Furthermore, except in the sub-nano scale domain, pores in the membrane may be converted into micro- or nano-scale for the future membrane designs, such as hydrogels and composites. These types of membranes may also be used for DNA sequencing, biosensing, water purification and filtration, and salt desalination. The discovery and development of novel materials, particularly in terms of nanofabrication and ionic transportation mechanism at the liquid and membrane interface, would reveal unique nanofluidic behavior that would push technological translation into the industry. We believe that the development of bioinspired nanoporous membranes will boost novel insights in energy harvesting and the design of smart devices. Our contributions to this research were supported by the National Key R&D Program of China ( 2017YFA0206904 , 2017YFA0206900 ), National Science Foundation of China ( 21875270 ), Frontier Science Key Projects of CAS ( QYZDY-SSW-SLH014 ), and National Natural Science Foundation of China ( 21988102 ). We thank the Fantastic Color Co. (Beijing, China) for the graphic design assistance. Y.Z. and L.J. conceived the original idea and wrote the paper.
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