On the Role of Heterogeneous Nanopore Junction in Osmotic Power Generation

Osmotic power generated by mixing ionic solutions of different concentration is an underutilized clean energy resource that satisfy potentially the ever‐growing energy demand. For decades, substantial efforts are made to enhance the power density. Toward this goal, we once developed a heterogeneous nanoporous membrane comprising of heterojunctions between negatively charged mesoporous carbon and positively charged macroporous alumina to harvest electric power from salinity difference and achieved outstanding performance (J. Am. Chem. Soc. 2014 , 136, 12265). The heterogeneous nanopore junction effectively suppresses ion concentration polarization (ICP) at low concentration end, and consequently promotes the overall power density. However, to date, a systematic understanding of the role of the heterogeneous nanopore junction in osmotic energy conversion remains urgent and largely unexplored. Herein, we provide an in‐depth theoretical investigation on this issue with special emphasis on several influential factors, such as the ionic concentration, the surface charge density, and the geometry of heterogeneous part. To balance the suppression of ICP and maintenance of charge selectivity, we find that these influential factors in the heterogeneous part should be restricted to a specific range. These findings provide direct guidance for design and optimization of high‐performance nanofluidic power sources.     

Oppositely Charged Ti3C2Tx MXene Membranes with 2D Nanofluidic Channels for Osmotic Energy Harvesting

Membrane‐based reverse electrodialysis (RED) is considered as the most promising technique to harvest osmotic energy. However, the traditional membranes are limited by high internal resistance and low efficiency, resulting in undesirable power densities. Herein, we report the combination of oppositely charged Ti₃C₂T_x MXene membranes (MXMs) with confined 2D nanofluidic channels as high‐performance osmotic power generators. The negatively or positively charged 2D MXene nanochannels exhibit typical surface‐charge‐governed ion transport and show excellent cation or anion selectivity. By mixing the artificial sea water (0.5 m NaCl) and river water (0.01 m NaCl), we obtain a maximum power density of ca. 4.6 Wm^?2, higher than most of the state‐of‐the‐art membrane‐based osmotic power generators, and very close to the commercialization benchmark (5 Wm^?2). Through connecting ten tandem MXM‐RED stacks, the output voltage can reach up 1.66 V, which can directly power the electronic devices.     

Tailoring A Poly(ether sulfone) Bipolar Membrane: Osmotic‐Energy Generator with High Power Density

Osmotic energy, obtained through different concentrations of salt solutions, is recognized as a form of a sustainable energy source. In the past years, membranes derived from asymmetric aromatic compounds have attracted attention because of their low cost and high performance in osmotic energy conversion. The membrane formation process, charging state, functional groups, membrane thickness, and the ion‐exchange capacity of the membrane could affect the power generation performance. Among asymmetric membranes, a bipolar membrane could largely promote the ion transport. Here, two polymers with the same poly(ether sulfone) main chain but opposite charges were synthesized to prepare bipolar membranes by a nonsolvent‐induced phase separation (NIPS) and spin‐coating (SC) method. The maximum power density of the bipolar membrane reaches about 6.2 W m^?2 under a 50‐fold salinity gradient, and this result can serve as a reference for the design of bipolar membranes for osmotic energy conversion systems.     

Heterogeneous Electrospinning Nanofiber Membranes with pH-regulated Ion Gating for Tunable Osmotic Power Harvesting

Biological ion channels existing in organisms are critical for many biological processes. Inspired by biological ion channels, the heterogeneous electrospinning nanofiber membranes (HENM) with functional ion channels are constructed by electrospinning technology. The HENM successfully realizes ion-gating effects, which can be used for tunable energy conversions. Introduction of pyridine and carboxylic acid groups into the HENM plays an important role in generating unique and stable ion transport behaviors, in which gates become alternative states of open and close, responding to symmetric/asymmetric pH stimulations. Then we used the HENM to convert osmotic energy into electric energy which reach a maximum value up to 12.34 W m⁻² and the output power density of HENM-based system could be regulated by ion-gating effects. The properties of the HENM provide widespread potentials in application of smart nanofluidic devices, energy conversion, and water treatment.     

The Combination of 2D Layered Graphene Oxide and 3D Porous Cellulose Heterogeneous Membranes for Nanofluidic Osmotic Power Generation

Salinity gradient energy, as a type of blue energy, is a promising sustainable energy source. Its energy conversion efficiency is significantly determined by the selective membranes. Recently, nanofluidic membrane made by two-dimensional (2D) nanomaterials (e.g., graphene) with densely packed nanochannels has been considered as a high-efficient membrane in the osmotic power generation research field. Herein, the graphene oxide-cellulose acetate (GO–CA) heterogeneous membrane was assembled by combining a porous CA membrane and a layered GO membrane; the combination of 2D nanochannels and 3D porous structures make it show high surface-charge-governed property and excellent ion transport stability, resulting in an efficient osmotic power harvesting. A power density of about 0.13 W/m² is achieved for the sea–river mimicking system and up to 0.55 W/m² at a 500-fold salinity gradient. With different functions, the CA and GO membranes served as ion storage layer and ion selection layer, respectively. The GO–CA heterogeneous membrane open a promising avenue for fabrication of porous and layered platform for wide potential applications, such as sustainable power generation, water purification, and seawater desalination.     

Low‐voltage efficient electroosmotic pumps with ultrathin silica nanoporous membrane

In this work, an efficient electroosmotic pump (EOP) based on the ultrathin silica nanoporous membrane (u‐SNM), which can drive the motion of fluid under the operating voltage as low as 0.2 V, has been fabricated. Thanks to the ultrathin thickness of u‐SNM (～75 nm), the effective electric field strength across u‐SNM could be as high as 8.27 × 10⁵ V m^?1 in 0.4 M KCl when 1.0 V of voltage was applied. The maximum normalized electroosmotic flow (EOF) rate was as high as 172.90 mL/min/cm²/V, which was larger than most of other nanoporous membrane based EOPs. In addition to the ultrathin thickness, the high porosity of this membrane (with a pore density of 4 × 10¹² cm^?2, corresponding to a porosity of 16.7%) also contribute to such a high EOF rate. Moreover, the EOF rate was found to be proportional to both the applied voltage and the electrolyte concentration. Because of small electrokinetic radius of u‐SNM arising from its ultrasmall pore size (ca. 2.3 nm in diameter), the EOF rate increased with increasing the electrolyte concentration and reached the maximum at a concentration of 0.4 M. This dependence was rationalized by the variations of both zeta potential and electrokinetic radius with the electrolyte concentration.     

Crosslinked P(VDF‐CTFE)/PS‐COOH nanocomposites for high‐energy‐density capacitor application

High‐capacity or high‐power‐density capacitors are being actively investigated for portable electronics, electric vehicles, and electric power systems. The dielectric nanocomposite with a small loading of carboxylic polystyrene (PS‐COOH) nanoparticles in poly(vinylidene fluoride‐chlorotrifluoroethylene) [P(VDF‐CTFE)] matrix, followed by chemical crosslinking has been described. Combination of these two methods significantly improved the capacity of electric energy storage at low electric field. Specially, the nanocomposite with 2 wt % nanoparticles and 15 wt % crosslinking agent achieved a dielectric constant of 17.2 and a discharged energy density of 17.5 J/cm³ (4.9 Wh/L) at an electric field as high as 324 MV/m, while corresponding values for pristine P(VDF‐CTFE) are 9.6 and 13.3 J/cm³ (3.7 Wh/L), respectively. Fundamental physics underlying the enhancement in the performance of the nanocomposites with respect to P(VDF‐CTFE) is illustrated by solid‐state ¹⁹F nuclear magnetic resonance of direct excitation or ¹⁹F{¹H} cross polarization. It revealed different dynamics behavior between crystalline/amorphous regions, and PS‐COOH nanoparticles favored the formation of polar γ‐form crystals. Small‐angle X‐ray scattering studies revealed the contribution of the interface to the extraordinary storage of electric energies in the nanocomposites. This approach provided a facile and straightforward way to design or understand PVDF‐based polymers for their practical applications in high‐energy‐density capacitors. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1160–1169     

Laser-Assisted Scalable Pore Fabrication in Graphene Membranes for Blue-Energy Generation

The osmotic energy from a salinity gradient (i. e. blue energy) is identified as a promising non-intermittent renewable energy source for a sustainable technology. However, this membrane-based technology is facing major limitations for large-scale viability, primarily due to the poor membrane performance. An atomically thin 2D nanoporous material with high surface charge density resolves the bottleneck and leads to a new class of membrane material the salinity gradient energy. Although 2D nanoporous membranes show extremely high performance in terms of energy generation through the single pore, the fabrication and technical challenges such as ion concentration polarization make the nanoporous membrane a non-viable solution. On the other hand, the mesoporous and micro porous structures in the 2D membrane result in improved energy generation with very low fabrication complexity. In the present work, we report femtosecond (fs) laser-assisted scalable fabrication of μm to mm size pores on Graphene membrane for blue energy generation for the first time. A remarkable osmotic power in the order of μW has been achieved using mm size pores, which is about six orders of magnitudes higher compared to nanoporous membranes, which is mainly due to the diffusion-osmosis driven large ionic flux. Our work paves the way towards fs laser-assisted scalable pore creation in the 2D membrane for large-scale osmotic power generation.     

Optimizing Membranes for Osmotic Power Generation

The design of ion-selective membranes is the key towards efficient reverse electrodialysis-based osmotic power conversion. The tradeoff between ion selectivity (output voltage) and ion permeability (output current) in existing porous membranes, however, limits the upgradation of power generation efficiency for practical applications. Thus, we provide the simple guidelines based on fundamentals of ion transport in nanofluidics for promoting osmotic power conversion. In addition, we discuss strategies for optimizing membrane performance through analysis of various material parameters in membrane design, such as pore size, surface charge, pore density, membrane thickness, ion pathway, pore order, and ionic diode effect. Lastly, a perspective on the future directions of membrane design to further maximize the efficiency of osmotic power conversion is outlined.     

Synthesis of Hierarchically Porous Sandwich‐Like Carbon Materials for High‐Performance Supercapacitors

For the first time, hierarchically porous carbon materials with a sandwich‐like structure are synthesized through a facile and efficient tri‐template approach. The hierarchically porous microstructures consist of abundant macropores and numerous micropores embedded into the crosslinked mesoporous walls. As a result, the obtained carbon material with a unique sandwich‐like structure has a relatively high specific surface (1235 m² g^?1), large pore volume (1.30 cm³ g^?1), and appropriate pore size distribution. These merits lead to a comparably high specific capacitance of 274.8 F g^?1 at 0.2 A g^?1 and satisfying rate performance (87.7 % retention from 1 to 20 A g^?1). More importantly, the symmetric supercapacitor with two identical as‐prepared carbon samples shows a superior energy density of 18.47 Wh kg^?1 at a power density of 179.9 W kg^?1. The asymmetric supercapacitor based on as‐obtained carbon sample and its composite with manganese dioxide (MnO₂) can reach up to an energy density of 25.93 Wh kg^?1 at a power density of 199.9 W kg^?1. Therefore, these unique carbon material open a promising prospect for future development and utilization in the field of energy storage.     

Tailoring A Poly(ether sulfone) Bipolar Membrane: Osmotic-Energy Generator with High Power Density

Osmotic energy, obtained through different concentrations of salt solutions, is recognized as a form of a sustainable energy source. In the past years, membranes derived from asymmetric aromatic compounds have attracted attention because of their low cost and high performance in osmotic energy conversion. The membrane formation process, charging state, functional groups, membrane thickness, and the ion-exchange capacity of the membrane could affect the power generation performance. Among asymmetric membranes, a bipolar membrane could largely promote the ion transport. Here, two polymers with the same poly(ether sulfone) main chain but opposite charges were synthesized to prepare bipolar membranes by a nonsolvent-induced phase separation (NIPS) and spin-coating (SC) method. The maximum power density of the bipolar membrane reaches about 6.2 W m⁻² under a 50-fold salinity gradient, and this result can serve as a reference for the design of bipolar membranes for osmotic energy conversion systems.     

Toward Scalable Nanofluidic Osmotic Power Generation from Hypersaline Water Sources with a Metal–Organic Framework Membrane

Nanofluidic membranes have shown great promise in harvesting osmotic energy but its scalablity remains challenging since most studies only tested with a membrane area of ≈10⁻² mm² or smaller. We demonstrate that metal-organic-framework membranes with subnanometer pores can be used for scalable osmotic power generation from hypersaline water sources. Our membrane can be scaled up to a few mm², and the power density can be stabilized at 1.7 W m⁻². We reveal that the key is to improve the out-of-membrane conductance while keeping the membrane's charge selectivity, contradicting the previous conception that the ionic conductivity of the membrane plays the dominating role. We highlight that subnanometer pores are essential to ensure the charge selectivity in hypersaline water sources. Our results suggest the importance to engineer the interplay between the in-membrane and out-of-membrane ion transport properties for scalable osmotic power generation.     

Heteroatom‐Doped Porous Carbon Materials with Unprecedented High Volumetric Capacitive Performance

The design of carbon‐based materials with a high mass density and large porosity has always been a challenging goal, since they fulfill the demands of next‐generation supercapacitors and other electrochemical devices. We report a new class of high‐density heteroatom‐doped porous carbon that can be used as an aqueous‐based supercapacitor material. The material was synthesized by an in situ dehalogenation reaction between a halogenated conjugated diene and nitrogen‐containing nucleophiles. Under the given conditions, pyridinium salts can only continue to perform the dehalogenation if there is residue water remaining from the starting materials. The obtained carbon materials are highly doped by various heteroatoms, leading to high densities, abundant multimodal pores, and an excellent volumetric capacitive performance. Porous carbon tri‐doped with nitrogen, phosphorous, and oxygen exhibits a high packing density (2.13 g cm^?3) and an exceptional volumetric energy density (36.8 Wh L^?1) in alkaline electrolytes, making it competitive to even some Ni‐MH cells.     

Stabilizing Lithium into Cross‐Stacked Nanotube Sheets with an Ultra‐High Specific Capacity for Lithium Oxygen Batteries

Although lithium–oxygen batteries possess a high theoretical energy density and are considered as promising candidates for next‐generation power systems, the enhancement of safety and cycling efficiency of the lithium anodes while maintaining the high energy storage capability remains difficult. Here, we overcome this challenge by cross‐stacking aligned carbon nanotubes into porous networks for ultrahigh‐capacity lithium anodes to achieve high‐performance lithium–oxygen batteries. The novel anode shows a reversible specific capacity of 3656 mAh g^?1, approaching the theoretical capacity of 3861 mAh g^?1 of pure lithium. When this anode is employed in lithium–oxygen full batteries, the cycling stability is significantly enhanced, owing to the dendrite‐free morphology and stabilized solid–electrolyte interface. This work presents a new pathway to high performance lithium–oxygen batteries towards practical applications by designing cross‐stacked and aligned structures for one‐dimensional conducting nanomaterials.     

Characterization of the surface charge property and porosity of track-etched polymer membranes

As an important property of porous membranes, the surface charge property determines many ionic behaviors of nanopores, such as ionic conductance and selectivity. Based on the dependence of electric double layers on bulk concentrations, ionic conductance through nanopores at high and low concentrations is governed by the bulk conductance and surface charge density, respectively. Here, through the investigation of ionic conductance inside track-etched single polyethylene terephthalate (PET) nanopores under various concentrations, the surface charge density of PET membranes is extracted as ∼−0.021 C/m² at pH 10 over measurements with 40 PET nanopores. Simulations show that surface roughness can cause underestimation in surface charge density due to the inhibited electroosmotic flow. Then, the averaged pore size and porosity of track-etched multipore PET membranes are characterized by the developed ionic conductance method. Through coupled theoretical predictions in ionic conductance under high and low concentrations, the averaged pore size and porosity of porous membranes can be obtained simultaneously. Our method provides a simple and precise way to characterize the pore size and porosity of multipore membranes, especially for those with sub-100 nm pores and low porosities.     

A Long Cycle Life,Self‐Healing Zinc–Iodine Flow Battery with High Power Density

A zinc–iodine flow battery (ZIFB) with long cycle life, high energy, high power density, and self‐healing behavior is prepared. The long cycle life was achieved by employing a low‐cost porous polyolefin membrane and stable electrolytes. The pores in the membrane can be filled with a solution containing I₃⁻ that can react with zinc dendrite. Therefore, by consuming zinc dendrite, the battery can self‐recover from micro‐short‐circuiting resulting from overcharging. By using KI, ZnBr₂, and KCl as electrolytes and a high ion‐conductivity porous membrane, a very high power density can be achieved. As a result, a ZIFB exhibits an energy efficiency (EE) of 82 % at 80 mA cm⁻², which is 8 times higher than the currently reported ZIFBs. Furthermore, a stack with an output of 700 W was assembled and continuously run for more than 300 cycles. We believe this ZIFB can lead the way to development of new‐generation, high‐performance flow batteries.     

A Long Cycle Life,Self‐Healing Zinc–Iodine Flow Battery with High Power Density

A zinc–iodine flow battery (ZIFB) with long cycle life, high energy, high power density, and self‐healing behavior is prepared. The long cycle life was achieved by employing a low‐cost porous polyolefin membrane and stable electrolytes. The pores in the membrane can be filled with a solution containing I₃^? that can react with zinc dendrite. Therefore, by consuming zinc dendrite, the battery can self‐recover from micro‐short‐circuiting resulting from overcharging. By using KI, ZnBr₂, and KCl as electrolytes and a high ion‐conductivity porous membrane, a very high power density can be achieved. As a result, a ZIFB exhibits an energy efficiency (EE) of 82 % at 80 mA cm^?2, which is 8 times higher than the currently reported ZIFBs. Furthermore, a stack with an output of 700 W was assembled and continuously run for more than 300 cycles. We believe this ZIFB can lead the way to development of new‐generation, high‐performance flow batteries.     

KOH‐Activated Porous Carbons Derived from Chestnut Shell with Superior Capacitive Performance

Porous carbons (PC) were prepared from a waste biomass named chestnut shell via a two‐step method involving carbonization and KOH activation. The morphology, pore structure and surface chemical properties were investigated by scanning electron microscopy (SEM), N₂ sorption, Raman spectroscopy, X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). The carbons have been evaluated as the electrode materials for supercapacitors by a two‐electrode system in 6 mol/L KOH electrolyte. Benefiting from the porous texture, high surface area and high oxygen content, the PCs derived from chestnut shell have exhibited high specific capacitance of 198.2 (PC‐1), 217.2 (PC‐2) and 238.2 F·g^?1 (PC‐3) at a current density of 0.1 A·g^?1, good rate capability of 55.7%, 56.6% and 54.9% in a range of 0.1–20 A·g^?1 and high energy density of 5.6, 6.1 and 6.7 Wh·kg^?1, respectively. This is believed to be due to electric double layer capacitance induced by the abundant micropores and extra pseudo‐capacitance generated by oxygen‐containing groups. At a power density of 9000 Wh·kg^?1, the energy density is 3.1, 3.5 and 3.7 Wh·kg^?1 for PC‐1, PC‐2 and PC‐3, respectively, demonstrating the potential of the carbons derived from chestnut shells in energy storage devices.     

Large‐Scale Synthesis of MOF‐Derived Superporous Carbon Aerogels with Extraordinary Adsorption Capacity for Organic Solvents

Carbon aerogels (CAs) with 3D interconnected networks hold promise for application in areas such as pollutant treatment, energy storage, and electrocatalysis. In spite of this, it remains challenging to synthesize high‐performance CAs on a large scale in a simple and sustainable manner. We report an eco‐friendly method for the scalable synthesis of ultralight and superporous CAs by using cheap and widely available agarose (AG) biomass as the carbon precursor. Zeolitic imidazolate framework‐8 (ZIF‐8) with high porosity is introduced into the AG aerogels to increase the specific surface area and enable heteroatom doping. After pyrolysis under inert atmosphere, the ZIF‐8/AG‐derived nitrogen‐doped CAs show a highly interconnected porous mazelike structure with a low density of 24 mg cm^?3, a high specific surface area of 516 m² g^?1, and a large pore volume of 0.58 cm^?3 g^?1. The resulting CAs exhibit significant potential for application in the adsorption of organic pollutants.     

Fiber‐in‐Tube Design of Co9S8‐Carbon/Co9S8: Enabling Efficient Sodium Storage

The sodium‐ion battery is a promising battery technology owing to its low price and high abundance of sodium. However, the sluggish kinetics of sodium ion makes it hard to achieve high‐rate performance, therefore impairing the power density. In this work, a fiber‐in‐tube Co₉S₈‐carbon(C)/Co₉S₈ is designed with fast sodiation kinetics. The experimental and simulation analysis show that the dominating capacitance mechanism for the high Na‐ion storage performance is due to abundant grain boundaries, three exposed layer interfaces, and carbon wiring in the design. As a result, the fiber‐in‐tube hybrid anode shows a high specific capacity of 616 mAh g^?1 after 150 cycles at 0.5 A g^?1. At 1 A g^?1, a capacity of ca. 451 mAh g^?1 can be achieved after 500 cycles. More importantly, a high energy density of 779 Wh kg^?1 and power density of 7793 W kg^?1 can be obtained simultaneously.