Engineering 2D Nanofluidic Li-Ion Transport Channels for Superior Electrochemical Energy Storage

Nanoconfined electrostatic interaction for efficient anion sieving in graphene oxide membranes

Controllable ion transport and precise ion sieving are crucial for sustainable water treatment and resource recovery. 2D materials, including graphene oxide (GO) with tunable nanochannels, are emerging as ideal material platforms to develop ion sieving membranes. However, accurate ion sieving remains challenging due to the swollen and enlarged interlayer spacing of GO membranes in aqueous solution, resulting in the non-selective of small ions. Here, we reformed the GO nanosheets by physical reduction method and modified them with negatively charged molecule chains. The nanochannel sizes and electronegativity of the stacked 2D membranes were precisely controlled simultaneously. As a result, 2D nanochannel membrane with fast permeability, high efficiency and accurate Cl-/SO42- separation was constructed. The characterization and performance analysis further proved that the interlayer spacing and electrification of 2D nanochannels are strongly related to ion sieving. By precisely adjusting the synergy between the two, Cl-/SO42- selectivity up to 91.83 with Cl- permeation rate of 1.03 mol m-2h-1 was achieved, which is superior to state-of-the-art ion sieving membranes. Our study provides new insights into understanding ion separation mechanisms within nanochannels and enables the development for the precise construction of nanochannels to manipulate the selective transport of ions.

Exploring Ion Transmission Mechanisms in Clay-Based 2D Nanofluidics for Osmotic Energy Conversion

Clay-based 2D nanofluidics present a promising avenue for osmotic energy harvesting due to their low cost and straightforward large-scale preparation. However, a comprehensive understanding of ion transport mechanisms, and horizontal and vertical transmission, remains incomplete. By employing a multiscale approach in combination of first-principles calculations and molecular dynamics simulations, the issue of how transmission directions impact on the clay-based 2D nanofluidics on osmotic energy conversion is addressed. It is indicated that the selective and rapid hopping transport of cations in clay-based 2D nanofluidics is facilitated by the electrostatic field within charged nanochannels. Furthermore, horizontally transported nanofluidics exhibited stronger ion fluxes, higher ion transport efficiencies, and lower transmembrane energy barriers compared to vertically transported ones. Therefore, adjusting the ion transport pathways between artificial seawater and river water resulted in an increase in osmotic power output from 2.8 to 5.3 W m-2, surpassing the commercial benchmark (5 W m-2). This work enhanced the understanding of ion transport pathways in clay-based 2D nanofluidics, advancing the practical applications of osmotic energy harvesting.

Helical peptide structure improves conductivity and stability of solid electrolytes

Ion transport is essential to energy storage, cellular signalling and desalination. Polymers have been explored for decades as solid-state electrolytes by either adding salt to polar polymers or tethering ions to the backbone to create less flammable and more robust systems. New design paradigms are needed to advance the performance of solid polymer electrolytes beyond conventional systems. Here the role of a helical secondary structure is shown to greatly enhance the conductivity of solvent-free polymer electrolytes using cationic polypeptides with a mobile anion. Longer helices lead to higher conductivity, and random coil peptides show substantially lower conductivity. The macrodipole of the helix increases with peptide length, leading to larger dielectric constants. The hydrogen bonding of the helix also imparts thermal and electrochemical stability, while allowing for facile dissolution back to monomer in acid. Peptide polymer electrolytes present a promising platform for the design of next-generation ion-transporting materials.

Responsive Protection of Magnesium Alloys From Multicorrosive Media by Constructing Nanofluidic Channels in Self-Repairing Coatings

Low-density magnesium (Mg) alloys are excellent engineering materials, and can significantly reduce energy consumption by replacing existing steel and aluminum materials. However, Mg species are susceptible to corrosion, especially in harsh environments (high-temperature or acidic), severely limiting the range of practical applications. Here, 2D covalent organic framework (COF) is synthesized with pore diameters ranging from 1.5 to 2.9 nm to obtain ultrafast nanofluidic channels. Loaded with silver (Ag+) ions, 2-mercaptobenzimidazole (2-MB) inhibitors are immobilized in the COF channels through the silver bridges. Based on the strong metal-complexing capability, Ag+ ions precipitated with various corrosive media (Cl-, Br-, I-, SO3 2-, S2-, S2O3 2- SO4 2-, CO3 2-, PO4 3-); meanwhile, the 2-MB inhibitors are rapidly released through the nanofluidic channels, forming a passivation film as a corrosion barrier to protect the Mg substrate. After integration with commercial polyethersulfone (PES), the COF-based coating exhibits high repairing capability achieving 100% damage restoration within 7 h, outperforming all existing coatings of Mg alloys. Notably, the coating shows almost complete protection of Mg alloys after being treated in respective 473 K, acidic (pH ≈4.0), and alkaline (pH ≈10.0) environments.

Enhanced Ionic Power Generation via Light-Driven Active Ion Transport Across 2D Semiconductor Heterostructures

2D semiconductor heterostructures exhibit broad application prospects. However, regular nanochannels of heterostructures rarely caught the researcher's attention. Herein, a metal-organic framework (i.e., Cu3(HHTP)2) and transition metal dichalcogenides (i.e., MoS2)-based multilayer van der Waals heterostructure (i.e., Cu3(HHTP)2/MoS2) realized band alignment-dominated light-driven ion transport and further light-enhanced ionic energy generation. High-density channels of the heterostructure provide high-speed pathways for ion transmembrane transport. Upon light illumination, a net ionic flow occurs at a symmetric concentration, suggesting a directional cationic transport from Cu3(HHTP)2 to MoS2. This is because Cu3(HHTP)2/MoS2 heterostructures containing type-II band alignment can generate photovoltaic motive force through light-induced efficient charge separation to drive ion transport. After introducing into the ionic power generation system, the maximum power density under illumination can achieve notable improvement under different concentration differences. In addition to the photovoltaic motive force, type-II band alignment and material defect capture-induced surface charge increase also raise ion selectivity and flux, greatly facilitating ionic energy generation. This work demonstrates that 2D semiconductor heterostructures with rational band alignment can not only be a potential platform for optimizing light-enhanced ionic energy harvesting but also provide a new thought for biomimetic iontronic devices.

Built-in Electric Fields in Heterostructured Lamellar Membranes Enable Highly Efficient Rejection of Charged Mass

Separation membranes with homogeneous charge channels are the mainstream to reject charged mass by forming electrical double layer (EDL). However, the EDL often compresses effective solvent transport space and weakens channel-ion interaction. Here, built-in electric fields (BIEFs) are constructed in lamellar membranes by assembling the heterostructured nanosheets, which contain alternate positively-charged nanodomains and negatively-charged nanodomains. We demonstrate that the BIEFs are perpendicular to horizontal channel and the direction switches alternately, significantly weakening the EDL effect and forces ions to repeatedly collide with channel walls. Thus, highly efficient rejection for charged mass (salts, dyes, and organic acids/bases) and ultrafast water transport are achieved. Moreover, for desalination on four-stage filtration option, salt rejection reaches 99.9 % and water permeance reaches 19.2 L m-2 h-1 bar-1. Such mass transport behavior is quite different from that in homogeneous charge channels. Furthermore, the ion transport behavior in nanochannels is elucidated by validating horizontal projectile motion model.

Toward Enhancing Low Temperature Performances of Lithium-Ion Transport for Metal-Organic Framework-Based Solid-State Electrolyte: Nanostructure Engineering or Crystal Morphology Controlling

Metal-organic frameworks (MOFs) have emerged as attractive candidates for Li+ conducting electrolytes due to their regular channels and controllable morphology, making their presence prominent in the field of solid-state lithium batteries. However, most MOF-based electrolytes are researched at or near room temperature, which poses a challenge to their practical applications at low temperatures. Herein, a thin layer flower-shaped 2D Cu-MOF (CuBDC-10)-based solid-state electrolytes (SSEs) for lithium-ion batteries (LIBs) are developed for facilitating Li+ transport at lower temperatures, which achieve an ion conductivity of 10-4 S cm-1 at -30 °C. The CuBDC-10-based SSE exhibits outstanding ionic conductivity over a wide temperature range of -40 to 100 °C (0.073-3.68 × 10-3 S cm-1). This work provides strategies for exploring MOF-based SSEs with high ionic transport performances at low temperatures.

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

Bioinspired 2D nanofluidic membranes for energy applications

Bioinspired two-dimensional (2D) nanofluidic membranes have been explored for the creation of high-performance ion transport systems that can mimic the delicate transport functions of living organisms. Advanced energy devices made from these membranes show excellent energy storage and conversion capabilities. Further research and development in this area are essential to unlock the full potential of energy devices and facilitate the development of high-performance equipment toward real-world applications and a sustainable future. However, there has been minimal review and summarization of 2D nanofluidic membranes in recent years. Thus, it is necessary to carry out an extensive review to provide a survey library for researchers in related fields. In this review, the classification and the raw materials that are used to construct 2D nanofluidic membranes are first presented. Second, the top-down and bottom-up methods for constructing 2D membranes are introduced. Next, the applications of bioinspired 2D membranes in osmotic energy, hydraulic energy, mechanical energy, photoelectric conversion, lithium batteries, and flow batteries are discussed in detail. Finally, the opportunities and challenges that 2D nanofluidic membranes are likely to face in the future are envisioned. This review aims to provide a broad knowledge base for constructing high-performance bioinspired 2D nanofluidic membranes for advanced energy applications.

"Fast-Charging" Anode Materials for Lithium-Ion Batteries from Perspective of Ion Diffusion in Crystal Structure

"Fast-charging" lithium-ion batteries have gained a multitude of attention in recent years since they could be applied to energy storage areas like electric vehicles, grids, and subsea operations. Unfortunately, the excellent energy density could fail to sustain optimally while lithium-ion batteries are exposed to fast-charging conditions. In actuality, the crystal structure of electrode materials represents the critical factor for influencing the electrode performance. Accordingly, employing anode materials with low diffusion barrier could improve the "fast-charging" performance of the lithium-ion battery. In this Review, first, the "fast-charging" principle of lithium-ion battery and ion diffusion path in the crystal are briefly outlined. Next, the application prospects of "fast-charging" anode materials with various crystal structures are evaluated to search "fast-charging" anode materials with stable, safe, and long lifespan, solving the remaining challenges associated with high power and high safety. Finally, summarizing recent research advances for typical "fast-charging" anode materials, including preparation methods for advanced morphologies and the latest techniques for ameliorating performance. Furthermore, an outlook is given on the ongoing breakthroughs for "fast-charging" anode materials of lithium-ion batteries. Intercalated materials (niobium-based, carbon-based, titanium-based, vanadium-based) with favorable cycling stability are predominantly limited by undesired electronic conductivity and theoretical specific capacity. Accordingly, addressing the electrical conductivity of these materials constitutes an effective trend for realizing fast-charging. The conversion-type transition metal oxide and phosphorus-based materials with high theoretical specific capacity typically undergoes significant volume variation during charging and discharging. Consequently, alleviating the volume expansion could significantly fulfill the application of these materials in fast-charging batteries.

Engineered cellulose nanofibers membranes with oppositely charge characteristics for high-performance salinity gradient power generation by reverse electrodialysis

Reverse electrodialysis (RED) using nanofluidic ion-selective membrane may convert the salinity difference between seawater and river water into electricity. However, heterogeneous modification reactions of cellulose commonly leads to the inhomogeneous distribution of surface charges, thereby hampering the improvement of cellulose-based nanofluidic membranes for energy conversion. Herein, RED devices based on cellulose nanofibers (CNF) membranes with opposite charge characteristics were developed for the generation of salinity gradient power. Anion-CNF membrane (A-CNF) with varying negative charge densities was synthesized using 2,2,6,6-Tetramethylpiperidine 1-oxy radical (TEMPO) oxidation modification, whereas cation-CNF membrane (C-CNF) was prepared through etherification. By mixing artificial seawater and river water, the output power density of CNF RED device is up to 2.87 W m-2. The output voltage of 30 RED units connected in series may reach up to 3.11 V, which can be used to directly power tiny electronic devices viz. LED lamp, calculator, etc. The results of this work provide a feasible possibility for widespread application of ion exchange membranes for salinity gradient energy harvesting.

Uranium capture by a layered 2D/2D niobium phosphate/holey graphene architecture via an electro-adsorption and electrocatalytic reduction coupling process

As an energy-efficient and eco-friendly technique, capacitive deionization (CDI) has shown great potential for uranium (U(VI)) capture recently. However, extracting U(VI) with high kinetics, capacity and selectivity remains a major challenge due to the current surface active sites-based material and co-existing ions in aqueous solution. Here we rationally designed a layered 2D/2D niobium phosphate/holey graphene (HGNbP) electrode material, and originally demonstrated its efficient U(VI) capture ability via an electro-adsorption and electrocatalytic reduction coupling process. The less-accumulative loose layered architecture, open polycrystalline construction of niobium phosphate with active phosphate sites, and rich in-plane nano-pores on conductive graphene nanosheets endowed HGNbP with fast charge/ion transport, high electroconductivity and superior pseudocapacitance, which enabled U(VI) ions first to be electro-adsorbed, then physico-chemical adsorbed, and finally electrocatalysis reduced/deposited onto electrode surface without the limitation of active sites under a low potential of 1.2 V. Based on these virtues, the HGNbP exhibited a fast adsorption kinetics, with a high removal rate of 99.9% within 30 min in 50 mg L^-1 U(VI) solution, and a high adsorption capacity up to 1340 mg g^-1 in 1000 mg L^-1 U(VI) solution. Furthermore, the good recyclability and selectivity towards U(VI) were also realized.

Charged Nanochannels in Covalent Organic Framework Membranes Enabling Efficient Ion Exclusion

Controllable ion transport through nanochannels is crucial for biological and artificial membrane systems. Covalent organic frameworks (COFs) with regular and tunable nanochannels are emerging as an ideal material platform to develop synthetic membranes for ion transport. However, ion exclusion by COF membranes remains challenging because most COF materials have large-sized nanochannels leading to nonselective transport of small ions. Here we develop ionic COF membranes (iCOFMs) to control ion transport through charged framework nanochannels, the interior surfaces of which are covered with arrayed sulfonate groups to render superior charge density. The overlap of an electrical double layer in charged nanochannels blocks the entry of co-ions, narrows their passageways, and concomitantly restrains the permeation of counterions via the charge balance. These highly charged large-sized nanochannels within the iCOFM enable ion exclusion while maintaining intrinsically high water permeability. Our results reveal possibilities for controllable ion transport based on COF membranes for water purification, ionic separation, sensing, and energy conversion.

Spontaneous and Selective Potassium Transport through a Suspended Tailor-Cut Ti3C2Tx MXene Film

Biological ion pumps selectively transport target ions against the concentration gradient, a process that is crucial to maintaining the out-of-equilibrium states of cells. Building an ion pump with ion selectivity has been challenging. Here we show that a Ti₃C₂T_x MXene film suspended in air with a trapezoidal shape spontaneously pumps K⁺ ions from the base end to the tip end and exhibits a K⁺/Na⁺ selectivity of 4. Such a phenomenon is attributed to a range of properties of MXene. Thanks to the high stability of MXene in water and the dynamic equilibrium between evaporation and swelling, the film keeps a narrow interlayer spacing of ∼0.3 nm when its two ends are connected to reservoirs. Because of the polar electrical structure and hydrophilicity of the MXene nanosheet, K⁺ ions experience a low energy barrier of ∼4.6 k_BT when entering these narrow interlayer spacings. Through quantitative simulations and consistent experimental results, we further show that the narrow spacings exhibit a higher energy barrier to Na⁺, resulting in K⁺/Na⁺ selectivity. Finally, we show that the spontaneous ion transport is driven by the asymmetric evaporation of the interlayer water across the film, a mechanism that is similar to pressure driven streaming current. This work shows how ion transport properties can be facilely manipulated by tuning the macroscopic shape of nanofluidic materials, which may attract interest in the interface of kirigami technologies and nanofluidics and show potential in energy and separation applications.

A multifunctional protective layer with biomimetic ionic channel suppressing dendrite and side reactions on zinc metal anodes

A multifunctional graphitic carbon nitride (GCN) protective layer with bionic ion channels and high stability is prepared to inhibit dendrite growth and side reactions on zinc (Zn) metal anodes. The high electronegativity of the nitrogen-containing organic groups (NOGs) in the GCN layer can effectively promote the dissociation of solvated Zn²⁺ and its rapid transportation in bionic ion channels via a hopping mechanism. In addition, this GCN layer exhibits excellent mechanical strength to suppress the growth of Zn dendrites and the volume expansion of Zn metal anodes during the plating process. Consequently, the electrodeposited Zn presents a uniform and densely packed morphology with negligible side-product accumulation. As a result, the half-cell composed of the Cu-GCN anode can deliver a remarkable long-term cycling performance of 1000 h at 0.5 mA cm^-2 and 0.25 mAh cm^-2. A full cell assembled with MnO₂ cathode also displays improved long-term cycling performance (150 cycles at 200 mA g^-1) when the Cu-GCN@Zn composite anode is applied. This work deepens our understanding of the kinetics of ion migration in the interface layer and paves the way for next-generation high energy-density Zn-metal batteries (ZMBs).

Zinc vacancy modulated quaternary metallic oxynitride GeZn1.7ON1.8: as a high-performance anode for lithium-ion storage

Zn-defected GeZn1.7ON1.8 (GeZn1.7−xON1.8) was successfully synthesized by a simple ammoniation and acid etching method. This well-designed Zn cation-deficient GeZn1.7−xON1.8 anode shows enhanced lithium-ion storage performance.

Crystal Channel Engineering for Rapid Ion Transport: From Nature to Batteries

Ion transport behaviours through cell membranes are commonly identified in biological systems, which are crucial for sustaining life for organisms. Similarly, ion transport is significant for electrochemical ion storage in rechargeable batteries, which has attracted much attention in recent years. Rapid ion transport can be well achieved by crystal channels engineering, such as creating pores or tailoring interlayer spacing down to the nanometre or even sub-nanometre scale. Furthermore, some functional channels, such as ion selective channels and stimulus-responsive channels, are developed for smart ion storage applications. In this review, the typical ion transport phenomena in the biological systems, including ion channels and pumps, are first introduced, and then ion transport mechanisms in solid and liquid crystals are comprehensively reviewed, particularly for the widely studied porous inorganic/organic hybrid crystals and ultrathin inorganic materials. Subsequently, recent progress on the ion transport properties in electrodes and electrolytes is reviewed for rechargeable batteries. Finally, current challenges in the ion transport behaviours in rechargeable batteries are analysed and some potential research approaches, such as bioinspired ultrafast ion transport structures and membranes, are proposed for future studies. It is expected that this review can give a comprehensive understanding on the ion transport mechanisms within crystals and provide some novel design concepts on promoting electrochemical ion storage capability in rechargeable batteries.

Two-dimensional materials towards separator functionalization in advanced Li-S batteries

Functional separators have played important roles in improving the electrochemical performance of lithium-sulfur (Li-S) batteries by addressing the key issues of both the sulfur cathode and lithium anode. Compared with other materials that are used for separator functionalization, two-dimensional (2D) materials with atomic layer thickness and infinite lateral dimensions feature several advantages of ultra-thin laminate structure, remarkable physical properties and tunable surface chemistry, which show potential applications in separator functionalization towards addressing the issues of both the shuttle effect and formation of Li dendrites in Li-S batteries. In this review, the unique advantages of 2D materials for separator functionalization in Li-S batteries and their common construction methods are introduced. Then, recent progress and advances in the construction of 2D materials functional separators are summarized in detail towards inhibiting the shuttle effect of polysulfides and suppressing Li dendrite growth in Li-S batteries. Finally, some opportunities and challenges of 2D materials for constructing high-performance functional separators are proposed. We anticipate that this review will provide new insights into separator functionalization for developing advanced Li-S batteries.

Constructing Uranyl-Specific Nanofluidic Channels for Unipolar Ionic Transport to Realize Ultrafast Uranium Extraction

High-speed capturing of uranyl (UO₂²⁺) ions from seawater elicits unprecedented interest for the sustainable development of the nuclear energy industry. However, the ultralow concentration (∼3.3 μg L^-1) of uranium element leads to the slow ion diffusion inside the adsorbent particle, especially after the transfer paths are occupied by the coexisted interfering ions. Considering the geometric dimension of UO₂²⁺ ion (a maximum length of 6.04-6.84 Å), the interlayer spacing of graphene sheets was covalently pillared with phenyl-based units into twice the ionic length (13 Å) to obtain uranyl-specific nanofluidic channels. Applying a negative potential (-1.3 V), such a charge-governed region facilitates a unipolar ionic transport, where cations are greatly accelerated and co-ions are repelled. Notably, the resulting adsorbent gives the highest adsorption velocity among all reported materials. The adsorption capacity measured after 56 days of exposure in natural seawater is evaluated to be ∼16 mg g^-1. This novel concept with rapid adsorption, high capacity, and facile operating process shows great promise to implement in real-world uranium extraction.

Unusually High Ion Conductivity in Large-Scale Patternable Two-Dimensional MoS2 Film

The advancement of ion transport applications will require the development of functional materials with a high ionic conductivity that is stable, scalable, and micro-patternable. We report unusually high ionic conductivity of Li⁺, Na⁺, and K⁺ in 2D MoS₂ nanofilm exceeding 1 S/cm, which is more than 2 orders of magnitude higher when compared to that of conventional solid ionic materials. The high ion conductivity of different cations can be explained by the mitigated activation energy via percolative ion channels in 2H-MoS₂, including the 1D ion channel at the grain boundary, as confirmed by modeling and analysis. We obtain field-effect modulation of ion transport with a high on/off ratio. The ion channel is large-scale patternable by conventional lithography, and the thickness can be tuned down to a single atomic layer. The findings yield insight into the ion transport mechanism of van der Waals solid materials and guide the development of future ionic devices owing to the facile and scalable device fabrication with superionic conductivity.

Nanofluidic Membranes to Address the Challenges of Salinity Gradient Power Harvesting

Salinity gradient power (SGP) has been identified as a promising renewable energy source. Reverse electrodialysis (RED) and pressure retarded osmosis (PRO) are two membrane-based technologies for SGP harvesting. Developing nanopores and nanofluidic membranes with excellent water and/or ion transport properties for applications in those two membrane-based technologies is considered viable for improving power generation performance. Despite recent efforts to advance power generation by designing a variety of nanopores and nanofluidic membranes to enhance power density, the valid pathways toward large-scale power generation remain uncertain. In this review, we introduce the features of ion and water transport in nanofluidics that are potentially beneficial to power generation. Subsequently, we survey previous efforts on nanofluidic membrane synthesis to obtain high power density. We also discuss how the various membrane properties influence the power density in RED and PRO before moving on to other important aspects of the technologies, i.e., system energy efficiency and membrane fouling. We analyze the importance of system energy efficiency and illustrate how the delicately designed nanofluidic membranes can potentially enhance energy efficiency. Previous studies are reviewed on fabricating antifouling and antimicrobial membrane for power generation, and opportunities are presented that can lead to the design of nanofluidic membranes with superior antifouling properties using various materials. Finally, future research directions are presented on advancing membrane performance and scaling-up the system. We conclude this review by emphasizing the fact that SGP has the potential to become an important renewable energy source and that high-performance nanofluidic membranes can transform SGP harvesting from conceptual to large-scale applications.

Ultrafine MoP Nanoparticle Splotched Nitrogen-Doped Carbon Nanosheets Enabling High-Performance 3D-Printed Potassium-Ion Hybrid Capacitors

Size engineering is deemed to be an adoptable method to boost the electrochemical properties of potassium-ion storage; however, it remains a critical challenge to significantly reduce the nanoparticle size without compromising the uniformity. In this work, a series of MoP nanoparticle splotched nitrogen-doped carbon nanosheets (MoP@NC) is synthesized. Due to the coordinate and hydrogen bonds in the water-soluble polyacrylamide hydrogel, MoP is uniformly confined in a 3D porous NC to form ultrafine nanoparticles which facilitate the extreme exposure of abundant three-phase boundaries (MoP, NC, and electrolyte) for ionic binding and storage. Consequently, MoP@NC-1 delivers an excellent capacity performance (256.1 mAh g-1 at 0.1 A g-1) and long-term cycling durability (89.9% capacitance retention after 800 cycles). It is further confirmed via density functional theory calculations that the smaller the MoP nanoparticle, the larger the three-phase boundary achieved for favoring competitive binding energy toward potassium ions. Finally, MoP@NC-1 is applied as highly electroactive additive for 3D printing ink to fabricate 3D-printed potassium-ion hybrid capacitors, which delivers high gravimetric energy/power density of 69.7 Wh kg-1/2041.6 W kg-1, as well as favorable areal energy/power density of 0.34 mWh cm-2/9.97 mW cm-2.

Electrostatic-modulated interfacial polymerization toward ultra-permselective nanofiltration membranes

Interfacial polymerization (IP) is a platform technology for ultrathin membranes. However, most efforts in regulating the IP process have been focused on short-range H-bond interaction, often leading to low-permselective membranes. Herein, we report an electrostatic-modulated interfacial polymerization (eIP) via supercharged phosphate-rich substrates toward ultra-permselective polyamide membranes. Phytate, a natural strongly charged organophosphate, confers high-density long-range electrostatic attraction to aqueous monomers and affords tunable charge density by flexible metal-organophosphate coordination. The electrostatic attraction spatially enriches amine monomers and temporally decelerates their diffusion into organic phase to be polymerized with acyl chloride monomers, triggering membrane sealing and inhibiting membrane growth, thus generating polyamide membranes with reduced thickness and enhanced cross-linking. The optimized nearly 10-nm-thick and highly cross-linked polyamide membrane displays superior water permeance and ionic selectivity. This eIP approach is applicable to the majority of conventional IP processes and can be extended to fabricate a variety of advanced membranes from polymers, supermolecules, and organic framework materials.

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Electrostatic-modulated interfacial polymerization is proposed for the first time

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Electrostatic attraction regulates the spatial-temporal distribution of amine monomers

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Monomer regulation leads to reduced thickness and enhanced cross-linking of membrane

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Ultrathin and highly cross-linked polyamide membrane displays superior permselectivity

Stacked Cu2-xSe nanoplates with 2D nanochannels as high performance anode for lithium batteries

Transition metal chalcogenides are considered as promising alternative materials for lithium-ion batteries owing to their relatively high theoretical capacity. However, poor cycle stability combined with low rate capacity still hinders their practical applications. In this work, the Cu-N chemical bonding directed the stacking Cu_2-xSe nanoplates (DETA-Cu_2-xSe) is developed to solve this issue. Such unique structure with small nanochannels can enhance the reactive site, facilitate the Li-ion transport as well as inhibit the structural collapse. Benefitting of these advantages, the DETA-Cu_2-xSe exhibits high specific capacity, better rate capacity and long cyclability with the specific capacities of 565mAhg^-1 after 100 cycles at 200 mA g^-1 and 368mAhg^-1 after 500 cycles at 5000 mA g^-1. This novel DETA-Cu_2-xSe structure with nanochannels is promising for next generation energy storage and the synthetic process can be extended to fabricate other transition metal chalcogenides with similar structure.

Electrochromic Nanochannels for Visual Nanofluidic Manipulation in Integrated Ionic Circuits

Nanochannel system provides a promising platform to create nanofluidic components in large-scale integrated circuits for "lab-on-a-chip" applications. However, it is a big challenge to achieve in situ monitoring on microscopic nanofluidic manipulation of single nanofluidic components in the integrated ionic circuit. Herein, we present a simple approach to realize visual nanofluidic manipulation in asymmetric nanochannels by the functionalization of an electrochromic polyaniline coating, which demonstrates redox-tunable surface charge accompanied by a visible color variation. The electrochromic nanochannels present a green color when behaving as ionic diodes, while the color turns to light yellow in a manner of ionic resistor. Moreover, both ionic transport behavior and color transition could respond well with alternating switch between redox states, contributing to a reversible and stable visual nanofluidic manipulation of electrochromic nanochannels. This work will create new avenues on in situ characterizing nanofluidic manipulation of nanofluidic components in integrated ionic circuits for intelligent sensing and detection applications.

Gel Electrocatalysts: An Emerging Material Platform for Electrochemical Energy Conversion

Seeking sustainable and cost-effective energy sources is one of the significant challenges for the sustainable development of modern society. To date, considerable expectations have been held for technologies, such as fuel cells and electrolyzers, where the performance strongly depends on electrochemical conversion processes that can generate and store chemical energy through the breaking or formation of chemical bonds. However, those advanced technologies are severely limited by the efficiency, selectivity, and durability of electrocatalysis. Thanks to their hierarchically porous architecture, compositional and structural tunability, and ease of functionalization, the family of gel materials opens exciting opportunities for advanced energy technologies. Unique advances in gel materials based on controllable compositions and functions enable gel electrocatalysts to potentially break the limitations of current materials, enhancing the device performance of electrochemical energy. Here, recent developments and challenges for nanostructured gel-based materials for electrocatalysis applications are summarized. Future possibilities and challenges for gel electrocatalysts in terms of synthesis and applications are also discussed.

Nanochannels regulating ionic transport for boosting electrochemical energy storage and conversion: a review

Electrochemical power sources, as one of the most promising energy storage and conversion technologies, provide great opportunities for developing high energy density electrochemical devices and portable electronics. However, uncontrolled ionic transport in electrochemical energy conversion, typically undesired anion transfer, usually causes some issues degrading the performance of energy storage devices. Nanochannels offer an effective strategy to solve the ionic transport problems for boosting electrochemical energy storage and conversion. In this review, the advantages of nanochannels for electrochemical energy storage and conversion and the construction principle of nanochannels are introduced, including ion selectivity and ultrafast ion transmission of nanochannels, which are considered as two critical factors to achieve highly efficient energy conversion. Recent advances in applications of nanochannels in lithium secondary batteries (LSBs), electrokinetic energy conversion systems and concentration cells are summarized in detail. Nanochannels exist in the above systems in two typical forms: functional separator and electrode protective layer. Current research on nanochannel-based LSBs is still at the early stage, and deeper and broader applications are expected in the future. Finally, the remaining challenges of nanochannel fabrication, performance improvement, and intelligent construction are presented. It is envisioned that this paper will provide new insights for developing high-performance and versatile energy storage electronics based on nanochannels.

Solvation-Involved Nanoionics: New Opportunities from 2D Nanomaterial Laminar Membranes

Nanoporous laminar membranes composed of multilayered 2D nanomaterials (2D-NLMs) are increasingly being exploited as a unique material platform for understanding solvated ion transport under nanoconfinement and exploring novel nanoionics-related applications, such as ion sieving, energy storage and harvesting, and in other new ionic devices. Here, the fundamentals of solvation-involved nanoionics in terms of ionic interactions and their effect on ionic transport behaviors are discussed. This is followed by a summary of key requirements for materials that are being used for solvation-involved nanoionics research, culminating in a demonstration of unique features of 2D-NLMs. Selected examples of using 2D-NLMs to address the key scientific problems related to nanoconfined ion transport and storage are then presented to demonstrate their enormous potential and capabilities for nanoionics research and applications. To conclude, a personal perspective on the challenges and opportunities in this emerging field is presented.

2D Superlattices for Efficient Energy Storage and Conversion

2D genuine unilamellar nanosheets, that are, the elementary building blocks of their layered parent crystals, have gained increasing attention, owing to their unique physical and chemical properties, and 2D features. In parallel with the great efforts to isolate these atomic-thin crystals, a unique strategy to integrate them into 2D vertically stacked heterostuctures has enabled many functional applications. In particular, such 2D heterostructures have recently exhibited numerous exciting electrochemical performances for energy storage and conversion, especially the molecular-scale heteroassembled superlattices using diverse 2D unilamellar nanosheets as building blocks. Herein, the research progress in scalable synthesis of 2D superlattices with an emphasis on a facile solution-phase flocculation method is summarized. A particular focus is brought to the advantages of these 2D superlattices in applications of supercapacitors, rechargeable batteries, and water-splitting catalysis. The challenges and perspectives on this promising field are also outlined.

Selenium vacancy-rich and carbon-free VSe2 nanosheets for high-performance lithium storage

VSe2 is a typical transition metal dichalcogenide with metallic conductivity, which makes it a potentially promising electrode material for lithium-ion batteries (LIBs). However, further research into the VSe2 nanomaterial for electrochemical applications has been seriously impeded by the practical difficulty of synthesizing phase-pure VSe2. In this work, Se vacancy-rich VSe2 nanosheets were synthesized by a one-step solvothermal method with suitable reactants. Benefiting from the strong reduction ability of hydrazine hydrate, V4+ was partly reduced into V3+, resulting in abundant Se vacancies being generated in situ in the as-obtained VSe2 nanosheets. Positron annihilation lifetime spectroscopy, X-ray absorption spectroscopy and photoluminescence spectroscopy all confirmed the existence of Se vacancies. When applied as the anode material for LIBs, the VSe2 nanosheets can deliver a remarkable reversible capacity of 1020 mA h g-1 at 0.1 A g-1 after 100 cycles, and even at 2 A g-1 a high specific capacity of 430 mA h g-1 is reached. Electrochemical characterizations further reveal that the Se vacancies in the VSe2 nanosheets can significantly enhance lithium-ion diffusion kinetics and increase the number of electrochemical active sites, which are responsible for the good lithium-storage performance. This work may provide an alternative approach for rational design of other high-performance electrode materials for LIBs to satisfy demand for future sustainable development.

Crystal reconstruction of binary oxide hexagonal nanoplates: monocrystalline formation mechanism and high rate lithium-ion battery applications

Structural design and/or carbon modification are the most important strategies to improve the performance of materials in many applications, where metal (oxide)-based anode design attracts great attention in metal ion batteries due to their high capacities. However, achieving these two goals within one-step remains challenging due to the lower cost and higher efficiency needed to satisfy the demand in practical application. Herein, we report a new approach for the crystal reconstruction of metal oxides by acetylene treatment, in which a hierarchical binary oxide decorated with carbon (i.e., Mn2Mo3O8@C) is introduced. The mechanism of constructing unique monocrystalline hexagonal nanoplates and uniform carbon coating is discussed in detail. Benefiting from the uniqueness of structure and composition, the Mn2Mo3O8@C demonstrates an extremely high lithium storage capacity of 890 mA h g-1 and good rate capacities at 20 A g-1 over 1000 cycles. In addition, the high rate capabilities and long cycle lifespan are further confirmed when the Mn2Mo3O8@C anode is matched with the nickel-rich layered oxide cathode. This study not only introduces a new binary oxide anode with high performances in lithium (ion) batteries but also presents a convenient methodology to design more advanced functional materials.

Hierarchical Hollow-Nanocube Ni-Co Skeleton@MoO3 /MoS2 Hybrids for Improved-Performance Lithium-Ion Batteries

Improving the performance of anode materials for lithium-ion batteries (LIBs) is a hotly debated topic. Herein, hollow Ni-Co skeleton@MoS₂ /MoO₃ nanocubes (NCM-NCs), with an average size of about 193 nm, have been synthesized through a facile hydrothermal reaction. Specifically, MoO₃ /MoS₂ composites are grown on Ni-Co skeletons derived from nickel-cobalt Prussian blue analogue nanocubes (Ni-Co PBAs). The Ni-Co PBAs were synthesized through a precipitation method and utilized as self-templates that provided a larger specific surface area for the adhesion of MoO₃ /MoS₂ composites. According to Raman spectroscopy results, as-obtained defect-rich MoS₂ is confirmed to be a metallic 1T-phase MoS₂ . Furthermore, the average particle size of Ni-Co PBAs (≈43 nm) is only about one-tenth of the previously reported particle size (≈400 nm). If assessed as anodes of LIBs, the hollow NCM-NC hybrids deliver an excellent rate performance and superior cycling performance (with an initial discharge capacity of 1526.3 mAh g^-1 and up to 1720.6 mAh g^-1 after 317 cycles under a current density of 0.2 A g^-1 ). Meanwhile, ultralong cycling life is retained, even at high current densities (776.6 mAh g^-1 at 2 A g^-1 after 700 cycles and 584.8 mAh g^-1 at 5 A g^-1 after 800 cycles). Moreover, at a rate of 1 A g^-1 , the average specific capacity is maintained at 661 mAh g^-1 . Thus, the hierarchical hollow NCM-NC hybrids with excellent electrochemical performance are a promising anode material for LIBs.

A long-lifespan, flexible zinc-ion secondary battery using a paper-like cathode from single-atomic layer MnO2 nanosheets

Aqueous zinc ion secondary batteries (ZIBs) have recently attracted considerable attention and global interest due to their low cost, aqueous-based nature and great safety. Unfortunately, the intrinsic properties of poor cycle life, low energy density and uncontrolled dendrite growth during the charge/discharge process for metallic Zn anodes significantly hinder their practical application. In this work, we rationally designed two-dimensional (2D) δ-MnO₂ nanofluidic channels by the ordered restacking of exfoliated MnO₂ single atomic layers, which exhibited a high zinc ion transport coefficient (1.93 × 10^-14 cm² s^-1) owing to their appropriate d-spacing and the negative charge of the inner channel walls. More importantly, we found that Zn dendrite growth was prevented in the as-assembled ZIBs, resulting in superior stability compared with the bulk-MnO₂ sample. Our design sheds light on developing high-performance ZIBs from two-dimensional nanofluidic channels, and this strategy might be applicable to the storage of other metal ions (Mg²⁺, Ca²⁺, Al³⁺, etc.) in next-generation electrochemical energy storage devices.

A Flexible Film toward High-Performance Lithium Storage: Designing Nanosheet-Assembled Hollow Single-Hole Ni-Co-Mn-O Spheres with Oxygen Vacancy Embedded in 3D Carbon Nanotube/Graphene Network

Ternary transition metal oxides (TMOs) are highly potential electrode materials for lithium ion batteries (LIBs) due to abundant defects and synergistic effects with various metal elements in a single structure. However, low electronic/ionic conductivity and severe volume change hamper their practical application for lithium storage. Herein, nanosheet-assembled hollow single-hole Ni-Co-Mn oxide (NHSNCM) spheres with oxygen vacancies can be obtained through a facile hydrothermal reaction, which makes both ends of each nanosheet exposed to sufficient free space for volume variation, electrolyte for extra active surface area, and dual ion diffusion paths compared with airtight hollow structures. Furthermore, oxygen vacancies could improve ion/electronic transport and ion insertion/extraction process of NHSNCM spheres. Thus, oxygen-vacancy-rich NHSNCM spheres embedded into a 3D porous carbon nanotube/graphene network as the anode film ensure efficient electrolyte infiltration into both the exterior and interior of porous and open spheres for a high utilization of the active material, showing an excellent electrochemical performance for LIBs (1595 mAh g^-1 over 300 cycles at 2 A g^-1 , 441.6 mAh g^-1 over 4000 cycles at 10 A g^-1 ). Besides, this straightforward synthetic method opens an efficacious avenue for the construction of various nanosheet-assembled hollow single-hole TMO spheres for potential applications.

Rayleigh-Instability-Induced Bismuth Nanorod@Nitrogen-Doped Carbon Nanotubes as A Long Cycling and High Rate Anode for Sodium-Ion Batteries

Sodium-ion battery (SIB) as one of the most promising large-scale energy storage devices has drawn great attention in recent years. However, the development of SIBs is limited by the lacking of proper anodes with long cycling lifespans and large reversible capacities. Here we present rational synthesis of Rayleigh-instability-induced bismuth nanorods encapsulated in N-doped carbon nanotubes (Bi@N-C) using Bi₂S₃ nanobelts as the template for high-performance SIB. The Bi@N-C electrode delivers superior sodium storage performance in half cells, including a high specific capacity (410 mA h g^-1 at 50 mA g^-1), long cycling lifespan (1000 cycles), and superior rate capability (368 mA h g^-1 at 2 A g^-1). When coupled with homemade Na₃V₂(PO₄)₃/C in full cells, this electrode also exhibits excellent performances with high power density of 1190 W kg^-1 and energy density of 119 Wh kg^-1_total. The exceptional performance of Bi@N-C is ascribed to the unique nanorod@nanotube structure, which can accommodate volume expansion of Bi during cycling and stabilize the solid electrolyte interphase layer and improve the electronic conductivity.

Layered g-C3N4@Reduced Graphene Oxide Composites as Anodes with Improved Rate Performance for Lithium-Ion Batteries

As important anodes in lithium-ion batteries, graphene is always faced with the aggregation problem that makes most of the active sites lose their function at high current densities, resulting in low Li-ion intercalation capacity and poor rate performance. To address this issue, a layered g-C₃N₄@reduced graphene oxide composite (g-C₃N₄@RGO) was prepared via a scalable and easy strategy. The resultant g-C₃N₄@RGO composite possesses large interlayer distances, rich N-active sites, and a microporous structure, which largely improves Li storage performance. It shows excellent cycle stability (899.3 mA h g^-1 after 350 cycles under 500 mA g^-1) and remarkable rate performance (595.1 mA h g^-1 after 1000 cycles under 1000 mA g^-1). Moreover, the g-C₃N₄@RGO electrode exhibits desired capacity retention and relatively high initial Coulombic efficiency of 58.8%. Impressively, this result is better than that of RGO (29.1%) and most of RGO-based anode materials reported in the literature. Especially, the g-C₃N₄@RGO-based electrode is enough to power two tandem red-light-emitting diodes and run a digital watch. Interestingly, the electronic watch can work continuously for more than 20 days. This novel strategy shows the great potential of g-C₃N₄@RGO composites as energy-storage materials.

Muscle-Inspired Highly Anisotropic, Strong, Ion-Conductive Hydrogels

Biological tissues generally exhibit excellent anisotropic mechanical properties owing to their well-developed microstructures. Inspired by the aligned structure in muscles, a highly anisotropic, strong, and conductive wood hydrogel is developed by fully utilizing the high-tensile strength of natural wood, and the flexibility and high-water content of hydrogels. The wood hydrogel exhibits a high-tensile strength of 36 MPa along the longitudinal direction due to the strong bonding and cross-linking between the aligned cellulose nanofibers (CNFs) in wood and the polyacrylamide (PAM) polymer. The wood hydrogel is 5 times and 500 times stronger than the bacterial cellulose hydrogels (7.2 MPa) and the unmodified PAM hydrogel (0.072 MPa), respectively, representing one of the strongest hydrogels ever reported. Due to the negatively charged aligned CNF, the wood hydrogel is also an excellent nanofluidic conduit with an ionic conductivity of up to 5 × 10^-4 S cm^-1 at low concentrations for highly selective ion transport, akin to biological muscle tissue. The work offers a promising strategy to fabricate a wide variety of strong, anisotropic, flexible, and ionically conductive wood-based hydrogels for potential biomaterials and nanofluidic applications.

Significantly Improving Lithium-Ion Transport via Conjugated Anion Intercalation in Inorganic Layered Hosts

Layered hydroxides (LHs) have emerged as an important class of functional materials owing to their unusual physicochemical properties induced by various intercalated species. While both the electrochemistry and interlayer engineering of the materials have been reported, the role of interlayer engineering in improving the Li-ion storage of these materials remains unclear. Here, we rationally introduce pillar ions with conjugated anion dicarboxylate groups, cobalt oxalate ions ([CoOx₂]^2-), into the interlayers of Co(OH)₂ nanosheets [denoted as I-Co(OH)₂ NSs]. The pillar ion guarantees excellent structural stability, high electrical conductivity, and accelerated Li-ion diffusion. The structure delivers high-rate cycling performance for lithium-ion batteries. This work provides insights for the design of LH-based high-performance electrode materials by a rational interlayer-engineering strategy.

Structural Engineering of 2D Nanomaterials for Energy Storage and Catalysis

Research on 2D nanomaterials is rising to an unprecedented height and will continue to remain a very important topic in materials science. In parallel with the discovery of new candidate materials and exploration of their unique characteristics, there are intensive interests to rationally control and tune the properties of 2D nanomaterials in a predictable manner. Considerable attention is focused on modifying these materials structurally or engineering them into designed architectures to meet requirements for specific applications. Recent advances in such structural engineering strategies have demonstrated their ability to overcome current material limitations, showing great promise for promoting device performance to a new level in many energy-related applications. Existing in many forms, these strategies can be categorized based on how they intrinsically or extrinsically alter the pristine structure. Achieved through various synthetic routes and practiced in a range of different material systems, they usually share common descriptors that predestine them to be effective in certain circumstances. Therefore, understanding the underlying mechanism of these strategies to provide fundamental insights into structural design and property tailoring is of critical importance. Here, the most recent development of structural engineering of 2D nanomaterials and their significant effects in energy storage and catalysis technologies are addressed.

Wearable energy sources based on 2D materials

This review provides the most recent advances in wearable energy sources based on 2D materials, and highlights the crucial roles 2D materials play in the wearable energy sources.