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Understanding the molecular-level mechanisms involved in transmembrane ion selectivity is essential for optimizing membrane separation performance. In this study, we reveal our observations regarding the transmembrane behavior of Li+ and Mg2+ ions as a response to the changing pore solvation abilities of the covalent-organic-framework (COF) membranes. These abilities were manipulated by adjusting the lengths of the oligoether segments attached to the pore channels. Through comparative experiments, we were able to unravel the relationships between pore solvation ability and various ion transport properties, such as partitioning, conduction, and selectivity. We also emphasize the significance of the competition between Li+ and Mg2+ with the solvating segments in modulating selectivity. We found that increasing the length of the oligoether chain facilitated ion transport; however, it was the COF membrane with oligoether chains containing two ethylene oxide units that exhibited the most pronounced discrepancy in transmembrane energy barrier between Li+ and Mg2+, resulting in the highest separation factor among all the evaluated membranes. Remarkably, under electro-driven binary-salt conditions, this specific COF membrane achieved an exceptional Li+/Mg2+ selectivity of up to 1352, making it one of the most effective membranes available for Li+/Mg2+ separation. The insights gained from this study significantly contribute to advancing our understanding of selective ion transport within confined nanospaces and provide valuable design principles for developing highly selective COF membranes.
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ConspectusMembranes are pivotal in a myriad of energy production processes and modern separation techniques. They are essential in devices for energy generation, facilities for extracting energy elements, and plants for wastewater treatment, each of which hinges on effective ion separation. While biological ion channels show exceptional permeability and selectivity, designing synthetic membranes with defined pore architecture and chemistry on the (sub)nanometer scale has been challenging. Consequently, a typical trade-off emerges: highly permeable membranes often sacrifice selectivity and vice versa. To tackle this dilemma, a comprehensive understanding and modeling of synthetic membranes across various scales is imperative. This lays the foundation for establishing design criteria for advanced membrane materials. Key attributes for such materials encompass appropriately sized pores, a narrow pore size distribution, and finely tuned interactions between desired permeants and the membrane. The advent of covalent-organic-framework (COF) membranes offers promising solutions to the challenges faced by conventional membranes in selective ion separation within the water-energy nexus. COFs are molecular Legos, facilitating the precise integration of small organic structs into extended, porous, crystalline architectures through covalent linkage. This unique molecular architecture allows for precise control over pore sizes, shapes, and distributions within the membrane. Additionally, COFs offer the flexibility to modify their pore spaces with distinct functionalities. This adaptability not only enhances their permeability but also facilitates tailored interactions with specific ions. As a result, COF membranes are positioned as prime candidates to achieve both superior permeability and selectivity in ion separation processes.In this Account, we delineate our endeavors aimed at leveraging the distinctive attributes of COFs to augment ion separation processes, tackling fundamental inquiries while identifying avenues for further exploration. Our strategies for fabricating COF membranes with enhanced ion selectivity encompass the following: (1) crafting (sub)nanoscale ion channels to enhance permselectivity, thereby amplifying energy production; (2) implementing a multivariate (MTV) synthesis method to control charge density within nanochannels, optimizing ion transport efficiency; (3) modifying the pore environment within confined mass transfer channels to establish distinct pathways for ion transport. For each strategy, we expound on its chemical foundations and offer illustrative examples that underscore fundamental principles. Our efforts have culminated in the creation of groundbreaking membrane materials that surpass traditional counterparts, propelling advancements in sustainable energy conversion, waste heat utilization, energy element extraction, and pollutant removal. These innovations are poised to redefine energy systems and industrial wastewater management practices. In conclusion, we outline future research directions and highlight key challenges that need addressing to enhance the ion/molecular recognition capabilities and practical applications of COF membranes. Looking forward, we anticipate ongoing advancements in functionalization and fabrication techniques, leading to enhanced selectivity and permeability, ultimately rivaling the capabilities of biological membranes.
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The evolution of porous membranes has revitalized their potential application in sustainable osmotic-energy conversion. However, the performance of multiporous membranes deviates significantly from the linear extrapolation of single-pore membranes, primarily due to the occurrence of ion-concentration polarization (ICP). This study proposes a robust strategy to overcome this challenge by incorporating photoelectric responsiveness into permselective membranes. By introducing light-induced electric fields within the membrane, the transport of ions is accelerated, leading to a reduction in the diffusion boundary layer and effectively mitigating the detrimental effects of ICP. The developed photoelectric-responsive covalent-organic-framework membranes exhibit an impressive output power density of 69.6 W m-2 under illumination, surpassing the commercial viability threshold by ≈14-fold. This research uncovers a previously unexplored benefit of integrating optical electric conversion with reverse electrodialysis, thereby enhancing energy conversion efficiency.
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A vast amount of energy can be extracted from the untapped low-grade heat from sources below 100 °C and the Gibbs free energy from salinity gradients. Therefore, a process for simultaneous and direct conversion of these energies into electricity using permselective membranes was developed in this study. These membranes screen charges of ion flux driven by the combined salinity and temperature gradients to achieve thermo-osmotic energy conversion. Increasing the charge density in the pore channels enhanced the permselectivity and ion conductance, leading to a larger osmotic voltage and current. A 14-fold increase in power density was achieved by adjusting the ionic site population of covalent organic framework (COF) membranes. The optimal COF membrane was operated under simulated estuary conditions at a temperature difference of 60â K, which yielded a power density of ≈231â W m-2 , placing it among the best performing upscaled membranes. The developed system can pave the way to the utilization of the enormous supply of untapped osmotic power and low-grade heat energy, indicating the tremendous potential of using COF membranes for energy conversion applications.
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It has long been a challenge to fabricate angstrom-sized functional pores for mimicking the function of biological channels to afford selective transmembrane transport. In this study, we describe a facile strategy to incorporate ionic elements into angstrom-sized channels using de novo encapsulation of charged dye molecules during the interface polymerization of a three-dimensional covalent organic framework (3D COF). We demonstrate that this approach is tailorable as it enables control over both the type and content of the guest and thus allows manipulation of the membrane function. The resulting membranes exhibit excellent permselectivity and low membrane resistance, thereby indicating the potential for harvesting salinity gradient (blue) energy. As a proof-of-concept study, the reverse electrodialysis device coupled with positive and negative dye encapsulated COF membranes afforded a power density of up to 51.4 W m-2 by mixing the simulated seawater and river water, which far exceeds the commercialization benchmark (5 W m-2). We envision that this strategy will pave the way for constructing new multifunctional biomimetic systems.
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Acids are extensively used in contemporary industries. However, time-consuming and environmentally unfriendly processes hinder single-acid recovery from wastes containing various ionic species. Although membrane technology can overcome these challenges by efficiently extracting analytes of interest, the associated processes typically exhibit inadequate ion-specific selectivity. In this regard, we rationally designed a membrane with uniform angstrom-sized pore channels and built-in charge-assisted hydrogen bond donors that preferentially conducted HCl while exhibiting negligible conductance for other compounds. The selectivity originates from the size-screening ability of angstrom-sized channels between protons and other hydrated cations. The built-in charge-assisted hydrogen bond donor enables the screening of acids by exerting host-guest interactions to varying extents, thus acting as an anion filter. The resulting membrane exhibited exceptional permeation for protons over other cations and for Cl- over SO42- and HnPO4(3-n)- with selectivities up to 4334 and 183, respectively, demonstrating prospects for HCl extraction from waste streams. These findings will aid in designing advanced multifunctional membranes for sophisticated separation.
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Access to sustainable energy is paramount in today's world, with a significant emphasis on solar and water-based energy sources. Herein, we develop photo-responsive ionic dye-sensitized covalent organic framework membranes. These innovative membranes are designed to significantly enhance selective ion transport by exploiting the intricate interplay between photons, electrons, and ions. The nanofluidic devices engineered in our study showcase exceptional cation conductivity. Additionally, they can adeptly convert light into electrical signals due to photoexcitation-triggered ion movement. Combining the effects of salinity gradients with photo-induced ion movement, the efficiency of these devices is notably amplified. Specifically, under a salinity differential of 0.5/0.01 M NaCl and light exposure, the device reaches a peak power density of 129 W m-2, outperforming the current market standard by approximately 26-fold. Beyond introducing the idea of photoelectric activity in ionic membranes, our research highlights a potential pathway to cater to the escalating global energy needs.
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Increasing the charge density of ionic membranes is believed to be beneficial for generating high output osmotic energy. Herein, we systematically investigated how the membrane charge populations affect permselectivity by decoupling their effects from the impact of the pore structure using a multivariate strategy for constructing covalent-organic-framework membranes. The thermo-osmotic energy conversion efficiency is improved by increasing the membrane charge density, affording 210 W m-2 with a temperature gradient of 40 K. However, this enhancement occurs only within a narrow window, and subsequently, the efficiency plateaued beyond a threshold density (0.04 C m-2). The complex interplay between pore-pore interactions in response to charge variations for ion transport across the upscaled nanoporous membranes helps explain the obtained results. This study has far-reaching implications for the rational design of ionic membranes to augment energy extraction rather than intuitively focusing on achieving high densities.
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Thermal sensation, which is the conversion of a temperature stimulus into a biological response, is the basis of the fundamental physiological processes that occur ubiquitously in all organisms from bacteria to mammals. Significant efforts have been devoted to fabricating artificial membranes that can mimic the delicate functions of nature; however, the design of a bionic thermometer remains in its infancy. Herein, we report a nanofluidic membrane based on an ionic covalent organic framework (COF) that is capable of intelligently monitoring temperature variations and expressing it in the form of continuous potential differences. The high density of the charged sites present in the sub-nanochannels renders superior permselectivity to the resulting nanofluidic system, leading to a high thermosensation sensitivity of 1.27 mV K-1, thereby outperforming any known natural system. The potential applicability of the developed system is illustrated by its excellent tolerance toward a broad range of salt concentrations, wide working temperatures, synchronous response to temperature stimulation, and long-term ultrastability. Therefore, our study pioneers a way to explore COFs for mimicking the sophisticated signaling system observed in the nature.