Ultrahigh Conductivity Research Articles

Directional electronic tuning of Ni nanoparticles by interfacial oxygen bridging of support for catalyzing alkaline hydrogen oxidation.

Metallic nickel (Ni) is a promising candidate to substitute Pt-based catalysts for hydrogen oxidation reaction (HOR), but huge challenges still exist in precise modulation of the electronic structure to boost the electrocatalytic performances. Herein, we present the use of single-layer Ti3C2Tx MXene to deliberately tailor the electronic structure of Ni nanoparticles via interfacial oxygen bridges, which affords Ni/Ti3C2Tx electrocatalyst with exceptional performances for HOR in an alkaline medium. Remarkably, it shows a high kinetic current of 16.39 mA cmdisk-2 at the overpotential of 50 mV for HOR [78 and 2.7 times higher than that of metallic Ni and Pt/C (20%), respectively], also with good durability and CO antipoisoning ability (1,000 ppm) that are not available for conventional Pt/C (20%) catalyst. The ultrahigh conductivity of single-layer Ti3C2Tx provides fast transmission of electrons for Ni nanoparticles, of which the uniform and small sizes endow them with high-density active sites. Further, the terminated -O/-OH functional groups on Ti3C2Tx directionally capture electrons from Ni nanoparticles via interfacial Ni-O bridges, leading to obvious electronic polarization. This could enhance the Nids-O2p interaction and weaken Nids-H1s interaction of Ni sites in Ni/Ti3C2Txenabling a suitable H-/OH-binding energy and thus enhancing the HOR activity.

Highly conductive freestanding sulfur cathode based on low-defect graphene-PEDOT:PSS aerogel for Li-S batteries

Highly conductive freestanding cathode, featuring with a unique aerogel configuration with sulfur deposited on low-defect graphene and subsequent encapsulation in poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate), is designed and synthesized for lithium-sulfur batteries. The freestanding cathode exhibits an ultrahigh conductivity for fast ions/electrons transportation and realizes effective immobilization of polysulfides, thereby enhancing lithium storage performance. This research provides a potential for use in high-performance flexible lithium-sulfur batteries.

Enhanced electrical conductivity of copper by nitrogen-doped graphene

The demand for Cu with ultra-high conductivity has been growing across various industries. However, conventional approaches to enhance electrical conductivities, such as improving purity levels and reducing grain boundaries, have shown limited effectiveness. In this study, we synthesized a vapor-deposited nitrogen-doped graphene/Cu foil that exhibited a remarkable 14% improvement in electrical conductivity compared to the annealed Cu counterpart. The conductivity showed an initial increase and then a decrease with increasing nitrogen concentration, reaching a peak value at 2.7 at% nitrogen. Through Kelvin Probe Force Microscope and Density Function Theory calculations, we found that dual doping of copper-electrons and nitrogen significantly increased the carrier concentration, leading to enhanced surface charge potential differences. We believe that further improvement in conductivity can be achieved by carefully adjusting the doping level to balance carrier concentration and mobility.

High-Efficiency Electromagnetic Interference Shielding from Highly Aligned MXene Porous Composites via Controlled Directional Freezing.

Lightweight porous composite materials (PCMs) with outstanding electromagnetic interference (EMI) shielding performances are ideal for aerospace, artificial intelligence, military, and other fields. Herein, a three-dimensional Ti3C2Tx MXene/sodium alginate (SA)/carbon nanotubes (CNTs) (MSC) PCMs was prepared by a controlled directional freezing process. This method constructs a directionally ordered porous structure, which can make the incident electromagnetic waves reflect and scattered several times in the PCMs. The introduction of CNTs into the MSC PCMs can form three-dimensional conductive networks with MXene, thus improving the conductivity and further improving the electromagnetic shielding performance. Furthermore, the SA with abundant hydrogen bonding can strengthen the interlayer interaction between MXene and CNTs. Profiting from the controlled directional freezing and highly aligned porous structure, the MSC PCMs with 75 wt % CNTs exhibit ultrahigh conductivity of 1630 S m-1, an ultrahigh EMI shielding effectiveness of 48.0 dB in X-band for electromagnetic waves incident perpendicular to the hole growth direction, and compressive strength of 72.3 kPa. The as-prepared MSC PCMs show excellent EMI shielding and mechanical properties and have significant applications in the preparation of an entirely novel type of EMI shielding materials with an absorption-based mechanism.

Synergistic promotion between tolerant polymer electrode and super-conductive polyelectrolyte enables anti-damage transient wearable electronics

Manufacturing highly durable and transient materials is urgently needed to achieve optimal performances in wearable electronics. Here, a comprehensive methodology to enhance the durability and transience of wearable electronics by utilizing polypyrrole/vermiculite/polysaccharide (PPy/VMT/AF) electrodes and acid-assisted polyacrylamide (HPAM) electrolytes. The mechanical/thermal/electrochemical properties of integrated electrodes are boosting by introducing negatively charged vermiculite in conductive polymer-polysaccharide, which can be completely degrade after 90 days. Pre-gel drying shrinkage and rehydration were utilized to prepare polyacrylamide hydrogel with frost resistance and ultrahigh conductivity (increased 32.2 times as compared to traditional polyacrylamide). The synergism between PPy/VMT/AF and HPAM enables flexible solid-state supercapacitors with exceptional electrochemical stability even under serious damage such as puncturing, burning and freeze. In addition, the fabricated self-powered pressure sensor realizes ML-assisted human motion recognition with an accuracy of 95.7 %. This work opens up a new perspective on future directions for multifunctional platforms for wearable electronic devices.

Incorporating graphene oxide into COF membranes enables ultrahigh proton conductivity and ultralow H2 crossover

Proton exchange membranes (PEMs) are the core components of electrochemical hydrogen energy production, storage and conversion devices. It is a great challenge to develop PEMs with a combination of high proton conductivity, minimal H2 crossover and mechanical robustness for achieving high efficiency and long-term stability in these devices. Here, a co-assembled PEM composed of ionic covalent organic framework (iCOF) nanosheets and functionalized graphene oxide (GO) nanosheets was presented. The iCOF (TpPa-SO3H) nanosheets and polydopamine-modified graphene oxide (DGO) nanosheets were co-assembled into TpPa-SO3H/DGO mixed nanosheets with ultrahigh aspect ratio, which were readily further processed into TpPa-SO3H/DGO co-assembled membranes. The abundant and uniformly distributed –SO3H groups on TpPa-SO3H nanosheets and the acid-base pairs formed between TpPa-SO3H and DGO nanosheets promoted proton conduction significantly. Additionally, the dense structured DGO nanosheets inhibited H2 crossover remarkably, and the multiple interactions between TpPa-SO3H and DGO nanosheets enhanced mechanical strength. The resultant TpPa-SO3H/DGO co-assembled membrane exhibited an ultrahigh proton conductivity of 916.4 mS cm−1 (80 °C, 100% RH), an unprecedently low H2 crossover of 9.7 barrer and a sufficient tensile stress of 87.8 MPa. The enhanced proton conduction and H2 barrier property led to enhanced performance in both electrochemical hydrogen compression and single hydrogen fuel cells.

Monolayer TiC—A high-performance Dirac anode with ultralow diffusion barriers and high energy densities for Li-ion and Na-ion batteries

Two-dimensional Dirac materials have stimulated substantial research interest as binder-free anodes in metal-ion batteries, owing to their ultrahigh electronic conductivity, large specific area, and higher energy density. Here, using first-principles density functional theory calculations, we have investigated the feasibility of monolayer TiC as a potential anode material for Li/Na-ion batteries. The results indicate that monolayer TiC exhibits excellent dynamical and thermal stability. The electronic structure of monolayer TiC shows semimetallic characteristics with a Dirac cone at the M high symmetry point and the formation of Ti or C vacancies transforms the Dirac cone into a nodal loop or a nodal surface, respectively. Thus, monolayer TiC possesses superior electrical conductivity, which can be further enhanced by the formation of Ti or C vacancies in the material. Furthermore, the calculated adsorption energy values of -0.85 and -0.46 eV for Li-ion and Na-ion, respectively, indicate that Li/Na atom adsorption over monolayer TiC is a favorable process. The density of states plots show that after the adsorption of a single Li/Na atom, monolayer TiC maintains its metallic state, which is advantageous for the diffusion of stored electrons. Most remarkably, monolayer TiC exhibits energy densities of 2684 and 2015 mWh/g for Li and Na, respectively, which are significantly higher than commercial graphite and most other 2D anode materials. The fully loaded TiC anode exhibits excellent cycle stability with volume expansions as low as 0.13 and 0.11%, for Li and Na, respectively. Furthermore, an ultrafast diffusivity with low energy barriers of 0.02 and 0.10 eV is found in monolayer TiC for Li-ion and Na-ion, respectively, which suggests that it has an excellent charge/discharge capability. These exceptional properties make monolayer TiC an excellent candidate as an anode material for Li-ion and Na-ion batteries. Finally, SiC(111) has been proposed as a candidate substrate for monolayer TiC due to its minimal lattice mismatch.

Rapid Fabrication of Flexible Cu@Ag Flake/SAE Composites with Exceptional EMIS and Joule Heating Performance.

High electromagnetic interference shielding (EMIS) effectiveness and good thermal management properties are both required to meet the rapid development of integrated electronic components. However, it remains challenging to obtain environmentally friendly and flexible films with high EMIS and thermal management performance in an efficient and scalable way. In this paper, an environmentally friendly strategy is proposed to synthesize multifunctional waterborne Cu@Ag flake conductive films using water as the solvent and silicone-acrylic emulsion (SAE) as a matrix. The obtained films show high electrical conductivity and exceptional EMI SE and electrothermal conversion properties. The EMI SE in the X-band is higher than 76.31 dB at a thickness of 60 μm owing to the ultrahigh electrical conductivity of 1073.61 S cm-1. The film warms up quickly to 102.1 °C within 10 s under a low voltage of 2.0 V. In addition, the shielding coating is sufficiently flexible to retain a conductivity of 93.4% after 2000 bending-release cycles with a bending radius of 3 mm. This work presents an alternative strategy to produce high EMIS effectiveness and Joule heating films for highly integrated and flexible electronic components in a green, scalable, and highly efficient way.

Highly Conductive Covalent-Organic Framework Films.

Chemically inert organic networks exhibiting electrical conductivity comparable to metals can advance organic electronics, catalysis, and energy storage systems. Covalent-organic frameworks (COFs) have emerged as promising materials for those applications due to their high crystallinity, porosity, and tunable functionality. However, their low conductivity has limited their practical utilization. In this study, copper-coordinated-fluorinated-phthalocyanine and 2,3,6,7-tetrahydroxy-9,10-anthraquinone-based COF (CuPc-AQ-COF) films with ultrahigh conductivity are developed. The COF films exhibit an electrical conductivity of 1.53 × 103 S m-1 and a Hall mobility of 6.02 × 102 cm2 V-1 s-1 at 298K, reaching the level of metals. The films are constructed by linking phthalocyanines and anthraquinones through vapor-assisted synthesis. The high conductivity properties of the films are attributed to the molecular design of the CuPc-AQ-COFs and the generation of high-quality crystals via the vapor-assisted method. Density functional theory analysis reveals that an efficient donor-acceptor system between the copper-coordinated phthalocyanines and anthraquinones significantly promotes charge transfer. Overall, the CuPc-AQ-COF films set new records of COF conductivity and mobility and represent a significant step forward in the development of COFs for electronic, catalytic, and electrochemical applications.

Multifunctional Flexible MXene/AgNW Composite Thin Film with Ultrahigh Conductivity Enabled by a Sandwich-Structured Assembly Strategy.

Flexible composite films have attracted considerable attention due to great potential for healthcare, telecommunication, and aerospace. However, it is still challenging to achieve high conductivity and multifunctional integration, mainly due to poorly designed composite structures of these films. Herein, a novel sandwich-structured assembly strategy is proposed to fabricate flexible composite thin films made of Ag nanowire (AgNW) core and MXene layers by combination of spray coating and vacuum filtration process. In this case, ultrathin MXene layers play crucial roles in constructing compact composite structures strongly anchored to substrate with extensive hydrogen-bonding interactions. The resultant sandwich-structured MXene/AgNW composite thin films (SMAFs) exhibit ultrahigh electrical conductivity (up to 27193 S cm-1 ), resulting in exceptional electromagnetic interference shielding effectiveness of 16223.3 dB cm2 g-1 and impressive Joule heating performance with rapid heating rate of 10.4°C s-1 . Moreover, the uniform SMAFs can also be facilely cut into kirigami-patterned interconnects, which indicate superior strain-insensitive conductance even after long-term exposure to extreme temperatures. The demonstrated strategy offers a significant paradigm to construct multifunctional composite thin films for next-generation integrated flexible electronics with practical applications.

Bioinspired Heterogeneous Construction of Lignocellulose-Reinforced COF Membranes for Efficient Proton Conduction.

The exponential interest in covalent organic frameworks (COFs) arises from the direct correlation between their diverse and intriguing properties and the modular design principle. However, the insufficient interlamellar interaction among COF nanosheets greatly hinders the formation of defect-free membranes. Therefore, developing a methodology for the facile fabrication of these materials remains an enticing and highly desirable objective. Herein, ultrahigh proton conductivity and superior stability are achieved by taking advantage of COF composite membranes where 2D TB-COF nanosheets are linked by 1D lignocellulosic nanofibrils (LCNFs) through π-π and electrostatic interactions to form a robust and ordered structure. Notably, the high concentration of -SO3 H groups within the COF pores and the abundant proton transport paths at COFs-LCNFs interfaces impart composite membranes ultrahigh proton conductivity (0.348 S cm-1 at 80 °C and 100% RH). Moreover, the directional migration of protons along the stacked nanochannels of COFs is facilitated by oxygen atoms on the keto groups, as demonstrated by density functional theory (DFT) calculations. The simple design concept and reliable operation of the demonstrated mixed-dimensional composite membrane are expected to provide an ideal platform for next-generation conductive materials.

Durability of advanced low temperature lithium compound electrode ceramic fuel cell for transportation

In recent years, a ceramic fuel cell with lithium compound such as Ni0·8Co0·15Al0·05LiO2 (NCAL) as its electrode is reduced in H2 to produce lithium compounds containing molten salt and diffused into oxide electrolyte to form an “oxide-lithium compounds molten salt” composite electrolyte with ultra-high ionic conductivity, which made the cell have remarkable low-temperature power generation performance. It is found that the dynamic migration of lithium compounds produced by NCAL anode in the cell with Ce0.9Gd0.1O2-δ (GDC) as electrolyte during the constant current durability test is the main reason for the cell performance degradation. In this paper, we found that adding different mass ratios of NaFeO2 to the GDC electrolyte to construct GDC/NaFeO2 composite electrolyte can significantly affect the durability of the cell. Under the constant current density test conditions of 550 °C, 0.2 A cm−2, the performance of the cell with GDC/NaFeO2 composite with a mass ratios of 8/2 as electrolyte maintained relatively good durability in the constant current test of 50 h. The characterization results show that the NaFeO2 reacts with lithium compounds such as LiOH to generate LiFeO2 and NaOH, and NaFeO2 is reduced to Fe and NaOH by H2. A proper amount of NaFeO2 in the GDC/NaFeO2 composite electrolyte can produce sodium compound molten salt during the performance test to replace the role of lithium compound molten salt in improving the electrolyte ionic conductivity and the cell sealing, while slowing down the dynamic migration of the molten salt in the cell, thus improving the durability of the cell. The findings in this paper provide evidence and relevant theories for the performance degradation and durability improvement mechanism of this new type of lithium compound electrode ceramic fuel cell (LCCFCs), and propose new strategies for obtaining LCCFCs with better durability.

Aliovalent dual element co-assisted strategy to enhance ionic conductivity and stability of NASICON-type solid electrolyte for all-solid-state sodium batteries

The urgent need for high-performance sodium batteries has drawn great attention to the development of novel solid-state electrolyte systems. However, it is still a great challenge to obtain excellent ionic conductivity and interfacial stability performance for most solid-state electrolytes. In this study, a new NASICON-type solid-state electrolyte Na3.1+xZr2−xScxSi2.1P0.9O12 (donated as NZSSP, x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3) was successfully prepared by solid-phase method. Due to the aliovalent substitution of Sc3+/Si4+, the phase transition or unwanted rhombohedral phase is effectively suppressed, the Na+ transport channel bottlenecks are greatly enlarged, and the concentration of Na+ is largely increased, giving rise to the significant improvement of the ionic conductivity and stability of NZSSP solid-state electrolyte. The Na3.35Zr1.75Sc0.25Si2.1P0.9O12 solid-state electrolyte has an ultra-high ionic conductivity of 4.92 mS cm−1 at room temperature. Meanwhile, it shows an excellent sodium plating/stripping stability by the optimization of the Na+ transport channel. The critical current density of the Na//Na3.35Zr1.75Sc0.25Si2.1P0.9O12//Na symmetric battery is achieved up to 1.0 mA cm−2, and it can stably cycle for nearly 3000 h at 0.1 mA cm−2 or 1000 h at 0.5 mA cm−2 at room temperature. A detailed analysis of these aliovalent dual element substitution samples has been performed, and the mechanism of the conductivity enhancement is discussed systematically. This study can provide a feasible method to improve the ionic conductivity and electrochemical stability of NZSP materials and paves a way for the application of all-solid-state sodium batteries.

Water in PHI Nanopores: Modeling Adsorption, Solvent Structure, and Thermodynamics.

We modeled the uptake of water molecules into the nanopores of potassium-polyheptazineimide (K-PHI), a 2D covalent material that is one of the best water-splitting photocatalysts to date possessing experimentally reported strong water binding. In the current models, we find that first water molecules are bound with -94.5 kJ/mol, i.e., twice the cohesion energy of water and one of the highest adsorption enthalpies reported so far. This strong binding proceeds unexpectedly on a similar enthalpy level until the pore is filled, while the binding strength is passed through a conjugated water network. The tight binding is also expressed in calculated, strongly shortened O-O distances, which are on average about 5% shorter than in bulk water, which corresponds to a much higher water density, for a 2D structure above 1.1 g/ cm3. The H-bridges are strongly aligned in the direction perpendicular to the covalent planes, which could give reasons for the experimentally observed ultrahigh ion fluxes and conductivity of K-PHI membranes. Decomposition of the adsorption energy into components reveals an unexpectedly high charge transfer contribution, where the partly naked K+ ions play a key role. The latter fact not only offers a new structural lead motif for the design of more strongly, but reversibly binding adsorption materials involving metal ions on their surface but also puts cations as known cofactors in enzymes into a new light.

High Performance Anion Exchange Membranes with Confined Sub‐2‐nm Ion Channel

AbstractAs a key component of low‐cost anion exchange membrane fuel cells (AEMFCs), anion exchange membranes (AEMs) are far from commercial application, because of dissatisfactory alkaline stability and conductivity. Herein, a new insight is proposed to prepare high performance AEMs by constructing of confined ion channel. With an intermediate oligomer produced before the main copolymerization, novel poly(vinyl‐carbazolyl aryl piperidinium) AEMs with confined sub‐2‐nm ion channel are successfully prepared. The unique sub‐2‐nm ion channel enable membranes ultrahigh hydroxide conductivity of 261.6 mS cm−1, and the state‐of‐the‐art chemical stability over 5000 h. Moreover, the AEMs also exhibit good mechanical stability with lower water uptake and dimensional swelling. Based on the as‐prepared AEMs and ionomer, fuel cells exhibit outstanding peak power density of 1.8 and 0.2 W cm−2 with Pt‐based catalysts and completely non‐precious metal catalysts, respectively.

Dithiol cross-linked polynorbornene-based anion-exchange membranes with high hydroxide conductivity and alkaline stability

Vinylic-addition polynorbornenes possess high thermal stability, excellent mechanical integrity, and chemically inert backbones. Herein, we conveniently synthesize a series of vinylic-addition polynorbornene copolymers containing pendant alkenes from commercial vinyl-substituted norbornene (VNB) and alkyl-bromide-substituted norbornene (BrNB). Subsequently, the copolymers are photo-crosslinked with dithiol reagents through UV-initiated thiol-ene click reactions followed by quaternization to prepare the cross-linked anion-exchange membranes (AEMs). The extent of cross-linking and the hydrophobicity of the cross-linkers can be easily controlled, so that the absorption of excess water can be inhibited with enhanced membrane strength. The resultant AEMs have ultrahigh hydroxide conductivities (up to 199 mS/cm at 90 °C under humidified conditions) and excellent alkaline stability in 1 M KOH at 80 °C for 1200 h. The vinylic-addition AEMs are also successfully applied in alkaline anion-exchange membrane water electrolyzers with commercial noble-metal-free electrocatalysts.

Multifunctional Filler-Free PEDOT:PSS Hydrogels with Ultrahigh Electrical Conductivity Induced by Lewis-Acid-Promoted Ion Exchange.

Highly conductive hydrogels with biotissue-like mechanical properties are of great interest in the emerging field of hydrogel bioelectronics due to their good biocompatibility, deformability, and stability. Fully polymeric hydrogels may exhibit comparable Young's modulus to biotissues. However, most of these filler-free hydrogels have a low electrical conductivity of <10 Scm-1 , which limits their wide applications of them in digital circuits or bioelectronic devices. In this work, a series of metal-halides-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hydrogels with an ultrahigh electrical conductivity up to 547 Scm-1 is reported, which is 1.5 times to 104 times higher than previously reported filler-free polymeric hydrogels. Theoretical calculation demonstrated that the ion exchange between PEDOT:PSS and the metal halides played an important role to promote phase separation in the hydrogels, which thus leads to ultrahigh electrical conductivity. The high electrical conductivity resulted in multifunctional hydrogels with high performance in thermoelectrics, electromagnetic shielding, Joule heating, and sensing. Such flexible and stretchable hydrogels with ultrahigh electrical conductivity and stability upon various deformations are promising for soft bioelectronics devices and wearable electronics.

Highly Robust and Strain-Resilient Thin Film Conductors Featuring Brittle Materials.

Stretchable conductors with stable electrical conductivity under various deformations are essential for wearable electronics, soft robots, and biointegrated devices. However, brittle film-based conductors on elastomeric substrates often suffer from unexpected electrical disconnection due to the obvious mechanical incompatibility between stiff films and soft substrates. We proposed a novel out-of-plane crack control strategy to achieve the strain-insensitive electrical performance of thin-film-based conductors, featuring conductive brittle materials, including nanocrystalline metals (Cu, Ag, Mo) and transparent oxides (ITO). Our metal film-based conductors exhibit an ultrahigh initial conductivity (1.3 × 105 S cm-1) and negligible resistance change (R/R0 = 1.5) over wide strain range from 0 to 130%, enabled by film-induced substrate cracking and liquid metal-induced electrical self-repairing. They could function well under multimodal deformations (stretching, bending, and twisting) and severe mechanical damage (cutting and puncturing). We demonstrated the strain-resilient electrical functionality of metal film-based conductors in a flexible light-emitting diode display that shows high mechanical compliance.

Exploring the Potential of MBenes Supercapacitors: Fluorine-Free Synthesized MoAl1-xB with Ultrahigh Conductivity and Open Space.

The present study describes the synthesis of multilayered MBenes MoAl1-xB with different degrees of Al deintercalation using a mild, fluorine-free approach of dilute alkali to remove Al from MoAlB. We propose an etching route and compare it to conventional fluoride etching products. Additionally, the study explores the possible application and energy storage mechanism of MBenes in supercapacitors, marking the first investigation of its kind. At room temperature, 1/24-MoAl1-xB with terminal groups -OH exhibits ∼25% Al removal in 1 wt % NaOH for 24 h, outperforming traditional etching technology. Increasing the Al removal exposed more open space, resulting in a higher capacitance. Compared to LiF/HCl-MoAl1-xB (etched by LiF + HCl), 1/24-MoAl1-xB has a higher energy storage capability. The multilayered 1/24-MoAl1-xB film electrode exhibits ultrahigh conductivity with a rapid relaxation time of 0.97 s and high areal capacitance (2006.60 mF cm-2) while maintaining 80.2% capacitance after 5000 cycles. The MoAl1-xB all-solid-state supercapacitor (ASSS) delivers a high capacitance of 741.6 mF cm-2 at 1 mV s-1 for a single electrode and exhibits stable capacitance even at a 90° bending angle, highlighting its potential practical use. Our research represents an important step in the synthesis of MBenes and highlights their potential applications in supercapacitors.

Expanding the dimensionality of proton conduction enables ultrahigh anhydrous proton conductivity of phosphoric acid-doped covalent-organic frameworks

Expanding the dimensionality of proton conduction enables ultrahigh anhydrous proton conductivity of phosphoric acid-doped covalent-organic frameworks