Anomalous Mechanical and Electrical Interplay in a Covalent Organic Framework Monolayer Membrane

Enhancing Selective Ion Transport by Stacking Covalent Organic Framework Monolayers

Nanopore-based power generation represents an efficient way for harvesting salinity gradient energy. Due to its ultrahigh ion conductivity and moderate ion selectivity, the crystalline covalent organic framework (COF) monolayer demonstrates the record-high output power density by mixing river water and seawater. To further improve energy conversion performance, it is necessary to enhance ion selectivity while achieving high membrane permeability. Here, a layer-by-layer stacking approach is developed to notably enhance the selective ion transport of ultra-thin COF layers, offering advantageous in both conversion efficiency and scalability. Under a standard NaCl salinity gradient (0.5 M/0.1 M), the ratio of ionic mobility between Cl- and Na+ increases from 1.4 to 2.9 with stacking the anion-selective COF monolayer from one to ten layers, leading to a more than seven-fold enhancement in osmotic energy conversion efficiency. By maximizing selectivity and permeability, the output power can reach 411 pW by stacking three layers in a single device. This strategy provides an effective approach for the integration of atomically thin membranes in selective mass transport applications.

Diffusive to Barrier-Limited Transition in the Aqueous Ion Transport through Nanoporous 2D Materials

The interplay of interactions between aqueous ions and the confinement of subnanoscale pores in solid 2D membranes causes a range of barrier-limited phenomena, including selective ion trapping and permeation, mechanosensitive transport, and memristive effects. A clear understanding of the transition from diffusive to barrier-limited transport regime is lacking, however. Moreover, the limits of applicability for the analytical formalism widely used to relate measured transport data to the effective pore size are unclear. Here, with the goal of identifying the transition between regimes and determining the pore sizes below which the diffusive formalism fails, we present a computational study of water-dissociated alkali salt transport through 2D membranes featuring pores of various sizes. Triangular nitrogen-terminated multivacancies in hexagonal boron nitride are used as a simple yet illustrative example of uncharged locally dipolar pores with various degrees of cation selectivity. We find that cation-cation selectivity and high mechanosensitivity are the clearest indicators of the barrier-limited regime onset. We also show that for triangular pore geometries, the diffusion-based analytical formalism is expected to fail when the side of the triangle is below order ≈2 nm. For circular geometries, similar failure is expected for pore diameters below ≈1.2 nm. Because an extensive theoretical description of barrier-limited transport is a major challenge, detailed computer models currently remain the most accurate nonexperimental methods for investigating ion transport in the barrier-limited regime. Given how sensitively the permeation regime depends on the pore size, our results suggest that in addition to advances in fabrication, accurate theoretical interpretation of measured transport data is vital to harnessing the unique features of barrier-limited ionic and molecular transport in nanofluidic systems using nanoporous 2D materials.

Ion Concentration Gradient Induced Efficient Ion Migration in Hydrogen-Bonded Organic Frameworks for High-Performance, Self-Powered Humidity Sensing

As one of the driving forces of ion migration, ion concentration gradients have large untapped potential to improve the performance of humidity sensors. A self-powered flexible humidity sensor based on hydrogen-bonded organic framework electrolytes wherein Na+ concentration gradients induce efficient ion migration is presented that can be attributed to the reversible effect of ambient water molecules on the migration barrier of Na+. The sensor exhibits superior flexibility, rapid responsiveness, high sensitivity (0.17 µA/% relative humidity), rapid response time (1.06 s), and outstanding stability (>200 cycles). The humidity-responsive device based on the Na+ concentration gradient exhibited excellent self-powering capability, eliminating the need for an external power unit and demonstrating impressive humidity power generation potential, which achieves a high current density of up to 164 mA m-2 and power density of 5.625 mW m-2. This research presents a new paradigm for developing self-powered humidity sensors and demonstrates their exceptional performance in noncontact sensing applications.

Large-Area Ultrathin Covalent-Organic Framework Membranes for Surface-Enhanced Raman Scattering: Optimal Performance Through Thickness Control

Exploration and construction of novel π-conjugated organic semiconductors with low cost, small background interference, and excellent performance as surface-enhanced Raman scattering (SERS) substrates is one of the current focuses for the development of SERS technology. Based on precise control over synthesis conditions, a series of large-area tetraphenylporphyrin-based 2D covalent-organic framework membranes (2D-porphyrin-COFs) with high uniformity and precisely controllable thickness are constructed as SERS substrates. The delicate balance among the intensity of the substrate interference, the degree of π-conjugation extension, and the proportion of the edge-on channels within the total exposed region results in the optimal SERS performance of ultrathin multilayer 2D-porphyrin-COFs with the thickness between 5.0 to 9.0 nm toward MB, including the enhancement factor on the order of 105 and the experimental limit of detection down to 10-8 M, which are comparable to classic plasmonic metal substrates. This work highlights the powerful application potential of COFs in the SERS field and unveils thickness control as an effective strategy to facilitate the exploration of high-performance organic SERS substrates.

Vacancy engineering in tungsten oxide nanofluidic membranes for high-efficiency light-driven ion transport

Bioinspired light-driven ion transport has shown great potential in solar energy harvesting. To achieve efficiencies comparable to biological counterparts, effective coregulation of permselectivity and photoresponsivity is crucial. Herein, vacancy engineering has been proven to be a powerful strategy for considerably increasing the efficiency of light-driven ion transport in tungsten oxide (WO3-x) nanofluidic membranes by enhancing the negative surface charges and narrowing bandgaps. The enhancement in light-driven ion transport can be attributed to the efficient redistribution of surface charges due to the effective separation of photogenerated carriers. At an optimized vacancy concentration, WO2.66 membrane (WO2.66M) delivers an ionic photocurrent of 0.8 μA cm-2 in a 10-4 M KCl electrolyte, which is four times higher than that generated by the original WO2.85 membrane (WO2.85M). Following this strategy, uphill ion transport and photoenhanced osmotic energy conversion are successfully achieved in the WO3-x nanofluidic membrane system. This study shows that atomic vacancy engineering is an efficient approach to increase the light-driven ion transport dynamics of nanofluidics, providing an efficient strategy to enhance light-driven ion transport for potential applications in power harvesting and ion separation.

Emerging Advances around Nanofluidic Transport and Mass Separation under Confinement in Atomically Thin Nanoporous Graphene

Membrane separation stands as an environmentally friendly, high permeance and selectivity, low energy demand process that deserves scientific investigation and industrialization. To address intensive demand, seeking appropriate membrane materials to surpass trade-off between permeability and selectivity and improve stability is on the schedule. 2D materials offer transformational opportunities and a revolutionary platform for researching membrane separation process. Especially, the atomically thin graphene with controllable porosity and structure, as well as unique properties, is widely considered as a candidate for membrane materials aiming to provide extreme stability, exponentially large selectivity combined with high permeability. Currently, it has shown promising opportunities to develop separation membranes to tackle bottlenecks of traditional membranes, and it has been of great interest for tremendously versatile applications such as separation, energy harvesting, and sensing. In this review, starting from transport mechanisms of separation, the material selection bank is narrowed down to nanoporous graphene. The study presents an enlightening overview of very recent developments in the preparation of atomically thin nanoporous graphene and correlates surface properties of such 2D nanoporous materials to their performance in critical separation applications. Finally, challenges related to modulation and manufacturing as well as potential avenues for performance improvements are also pointed out.

Boosting Osmotic Energy Harvesting from Organic Solutions by Ultrathin Covalent Organic Framework Membranes

Extracting osmotic energy from waste organic solutions via reverse electrodialysis represents a promising approach to reuse such industrial wastes and helps to mitigate the ever-growing energy needs. Herein, a molecularly thin membrane of covalent organic frameworks is engineered via interfacial polymerization to investigate its ion transport behavior in organic solutions. Interestingly, a significant deviation from linearity between ion conductance and reciprocal viscosity is observed, attributed to the nanoscale confinement effect on intermolecular interactions. This finding suggests a potential strategy to modulate the influence of apprarent viscosity on transmembrane transport. The osmotic energy harvesting of the ultrathin membrane in organic systems was studied, achieving an unprecedented output power density of over 84.5 W m-2 at a 1000-fold salinity gradient with a benign conversion efficiency and excellent stability. These findings provide a meaningful stepping stone for future studies seeking to fully leverage the potentials of organic systems in energy harvesting applications.