Ion Energy Research Articles

Argon admixture-driven enhanced ionization and performance of a 5 kW Hall thruster on krypton

Utilizing alternative propellants has been recognized as a strategy to reduce the total cost of propellants in electric propulsion-based missions. The aim of this study is to quantify the Hall effect thruster (HET) performance and operating characteristics using a Kr–Ar mixture to enable mission designers to evaluate the impact on mission and spacecraft design. We present the performance and plume plasma properties of the P5 5 kW-class HET operated with a Kr–Ar mixture with Ar volumetric flow rate fractions from 0 to 100%. The thruster is characterized at discharge power levels of 2.6 kW and 4.1 kW at constant discharge current and voltage over the range of Ar fractions. Despite higher ionization energy and lower mass of Ar, the thruster exhibited a similar level of thrust within 2% when comparing the pure Kr and 26%-Ar mixture cases. The derived ion energy distribution functions and analytical modeling suggest that the characteristic length for the ionization region is extended as the Ar fraction increases. The increased residence time of Kr at the extended ionization region and background energetic electrons from the ionized Ar neutrals are considered to cause this enhanced ionization of the injected Kr neutrals. This leads to a 6% higher Kr ion density at the 26%-Ar case even at the 16% less injected Kr neutral density than in the pure Kr case. The enhanced Kr ionization and generated Ar ions in the 26%-Ar case consequently led to a comparable thrust with that of the pure Kr case. The study indicates that mixing Ar with approximately 26% volumetrically with Kr can provide a similar or even higher thrust performance at the same discharge power. This will be particularly advantageous for various space missions that require high impulses by reducing the total cost of the propellant.

Achieving a High Thermally Conductive One Micron AlN Deposition by High Power Impulse Magnetron Sputtering plus Kick.

High-power impulse magnetron sputtering (HiPIMS) plus kick is a physical vapor deposition method that employs bipolar microsecond-scale voltage pulsing to precisely control the ion energy during sputter deposition. HiPIMS plus kick for AlN deposition is difficult since nitride deposition is challenged by low surface diffusion and high susceptibility to ion damage. In this current study, a systematic examination of the process parameters of HiPIMS plus kick was conducted. Under optimized main negative pulsing conditions, this study documented that a 25 V positive kick biasing for AlN deposition is ideal for optimizing a high quality film, as shown by X-ray diffraction and transmission electron microscopy as well as optimal thermal conductivity while increasing high speed deposition (25 nm/min) and obtaining ultrasmooth surfaces (rms roughness = 0.5 nm). HiPIMS plus kick was employed to deposit a single-texture 1 μm AlN film with a 7.4° rocking curve, indicating well oriented grains, which correlated with high thermal conductivity (121 W/m·K). The data are consistent with the optimal kick voltage enabling enhanced surface diffusion due to ion-substrate collisions without damaging the AlN grains.

Neutralized, intense-ion beams for fusion

A charge- and current-neutralized, intense-ion beam (IIB) will propagate undeflected in a magnetized plasma by the E×B , or diamagnetic, drift for a distance large compared to the beam-ion gyro-radius. This propagation occurs because of collective plasma effects when the beam-energy density is sufficient to sustain the polarization-electric field, or diamagnetic-screening currents. Our principal interest is the E×B drift, with β=Ebeam/Efield<1 . IIBs are characterized by parameters in the range: 0.1–2 MeV ion energy, 1–100 MA m−2 ion-current density, and 17–350 kW average power produced on repetitively-pulsed systems. Multi-MW average-beam power is possible with appropriate modifications to the power supply and ion-source/accelerator. IIBs can be focused geometrically, and/or magnetically, to spot sizes of the order of a few cm’s and their angular divergence is typically, ∼15 mRadians, suitable for long-range propagation in a vacuum beam-line. IIBs lose energy and momentum in the same manner as particles injected by a neutral-beam injector (NBIs), hence could be useful for fusion applications, including: heating, current drive, fueling, profile modifications, etc and such applications have yet to be thoroughly studied. Summarized here are the physical principles accounting for IIB propagation; ion-source designs used to produce the IIB; and pulsed-power methods for energizing the IIB accelerator. This technology base informs scalable metrics for the ion-source/accelerator (excluding the HV power supply and interconnects) used in a conceptual injector that provides the same ion energy and injected power (1 MeV, 17 MW) as NBIs used on ITER. A comparison indicates that the IIB injector would be much smaller in size, lower cost, and have much greater efficiency, while also providing for real-time modulation of the beam energy and intensity and the use of small-diameter injection ports that could minimize fuel contamination and magnetic-field leakage between the IIB injector and tokamak.

Effective combinations of features in predicting the range of incident ions using machine learning

Machine learning techniques were used to predict the range of ions in material, even when stopping power data is unknown. Simple features such as atomic and mass numbers were used in learning process. Two different machine learning models, Decision Tree (DT) and Kernel Ridge Regression (KRR), were used to assess each feature’s importance and the effect of combining them. The results showed that the chosen features were sufficient for accurate prediction. Furthermore, it was found that the combination and features are essential for accurate results. Analysis by KRR established a relationship between the range of the implanted ions and the energy of the incident ions, showing that the range is approximately proportional to the energy in the dataset used in this work.

Toward Selective Transport of Monovalent Metal Ions with High Permeability Based on Crown Ether-Encapsulated Metal-Organic Framework Sub-Nanochannels.

Achieving selective transport of monovalent metal ions with high precision and permeability analogues to biological protein ion channels has long been explored for fundamental research and various applications, such as ion sieving, mineral extraction, and energy harvesting and conversion. However, it still remains a significant challenge to construct artificial nanofluidic devices to realize the trade-off effects between selective ion transportation and high ion permeability. In this work, we report a bioinspired functional micropipet with in situ growth of crown ether-encapsulated metal-organic frameworks (MOFs) inside the tip and realize selective transport of monovalent metal ions. The functional ion-selective micropipet with sub-nanochannels was constructed by the interfacial growth method with the formation of composite MOFs consisting of ZIF-8 and 15-crown-5. The resulting micropipet device exhibited obvious monovalent ion selectivity and high flux of Li+ due to the synergistic effects of size sieving in subnanoconfined space and specific coordination of 15-crown-5 toward Na+. The selectivity of Li+/Na+, Li+/K+, Li+/Ca2+, and Li+/Mg2+ with 15-crown-5@ZIF-8-functionalized micropipet reached 3.9, 5.2, 105.8, and 122.4, respectively, which had an obvious enhancement compared to that with ZIF-8. Notably, the ion flux of Li+ can reach up to 93.8 ± 3.6 mol h-1·m-2 that is much higher than previously reported values. Furthermore, the functional micropipet with 15-crown-5@ZIF-8 sub-nanochannels exhibited stable Li+ selectivity under various conditions, such as different ion concentrations, pH values, and mixed ion solutions. This work not only provides new opportunities for the development of MOF-based nanofluidic devices for selective ion transport but also facilitates the promising practical applications in lithium extraction from salt-like brines, sewage treatment, and other related aspects.

Ultramicrotomy‐Assisted Fabrication of Nanochannels for Efficient Ion Transport and Energy Generation

AbstractNano‐ and angstrom‐scale channels fabricated from 2D‐layered materials provide a unique platform for studying fluidic behavior at atomic‐scale confinement, with applications in desalination, osmotic power generation, and fuel cells. While various fabrication methods exist, achieving precision, scalability, and minimal fabrication time is challenging. Ultramicrotomy‐based nanofabrication is shown here as an efficient approach to create nanofluidic membranes containing nanochannels with atomically flat walls. This approach is compatible with both bottom–up nanolaminates and top–down nanochannels, produces multiple devices from the same 2D assembly and allows swift variation of channel length. Integration of these membranes into macroscopic silicon fluidic‐chips is achieved maintaining the structural and functional properties. The robustness of the sliced membranes is demonstrated through restacked vermiculate laminates that sustain applied pressures and generate ionic streaming currents. Sliced pristine vermiculite channels exhibit charge‐selective ion transport, leading to osmotic power density (Posm/A) ranging from 9.2 to 300.4 Wm−2 under various KCl concentration gradients. Maximum conversion efficiency of 23.3% is obtained at a 100‐fold gradient, yielding Posm/A of 234.6 Wm−2. Short channel lengths sliced by ultramicrotomy contribute to high Posm/A, promising applications of miniature energy devices for pressure‐driven electricity generation and osmotic power generation.

Multi-pellet injection into the NBI-heated phase of TJ-II plasmas

A pellet-induced enhanced confinement (PiEC) phase, with general characteristics similar to those reported for the stellarator W7-X, is observed after single pellet injection (>1019 H atoms) into the neutral beam injection heated phase of plasmas in the mid-sized heliac-type stellarator TJ-II. In addition to a step-like increase in density, plasma diamagnetic energy content rises significantly with respect to that of reference discharges, energy confinement time is similarly enhanced when compared to International Stellarator Scaling law predictions (Yamada et al 2005 Nucl. Fusion 45 1684) renormalized for TJ-II, and the triple product, n e · T i · τ E, exhibits a clear bifurcation towards an improved confinement branch when compared to the branch product predicted by the same law. In this work, multiple pellets are injected in series into NBI-heated plasmas in the TJ-II and post-injection plasma performance is reported and discussed. For instance, a charge-exchange recombination spectroscopy diagnostic reveals significantly increased core ion temperatures after pellet injection compared to temperatures achieved in comparable reference plasmas, this pointing to increased ion energy content and improved ion energy confinement during a PiEC phase. It is also found that enhanced performance is independent of whether co- or counter-NBI heating beam is employed. Finally, record stored diamagnetic energy content and plasma beta values are achieved when the largest available pellets are employed. The results indicate that pellet injections extend the operational regime well beyond limits previously achieved in TJ-II without pellets.

Numerical characterization of capacitively coupled plasma driven by tailored frequency modulated radio frequency source

Capacitively coupled plasma (CCP) is widely used in plasma etching and deposition processes because of its low cost, simple structure, and easy generation of a uniform plasma in large areas. Conventional CCPs are operated under a fixed frequency power source; however, CCPs driven by a variable frequency power source are poorly understood. In this paper, numerical simulations of CCPs driven by frequency modulated (FM) radio frequency (RF) sources within the frequency range of 2 MHz–18 MHz are carried out with a particle-in-cell/Monte Carlo collision model. Our research indicates that the CCP driven by an FM RF source can maintain a stable glow discharge and form a time-dependent plasma. Plasma density, electron and ion current, energy and heating rate, ion flux, and energy on the electrodes fluctuate consistently with the FM period. The electron and ion energy distribution function can also be modulated by the frequency variation of the FM source. A multi-peak structure that varies and shifts with frequency variation is observed in the ion energy distribution function. In addition, by fixing the chirp period while varying the start or end frequency of the chirp signal (start frequency from 0.4 to 6 MHz, or end frequency from 18 to 48 MHz), effective modulations can be produced on the electron density, electron energy, and the shape of the EEPF and IEDF.

Emission of ions and electrons correlated with soft and hard x-rays evolution from thermal plasma

This paper presents a correlation between time evolution of ions and electrons with soft and hard x-rays emitted from argon plasma. The plasma setup comprises of two copper electrodes connected with a dc power supply. Faraday cups were used to monitor time evolution of ions, to extract their energy, temperature, and flux. Double Langmuir probe was employed to determine electron temperature, energy, and density. To explore time-resolved emission of soft x-rays, PIN photodiodes filtered with 24 μm aluminum, 90 μm Mylar, 80 μm copper, and 10 μm silver have been used. To evaluate temporal evolution of hard x-rays, a scintillator–photomultiplier system was utilized. The plasma was generated using argon gas at atmospheric pressure 760 Torr and constant flow rate 5 L/min at 7, 9, and 11 kV. The flux, energy, and temperature of Ar ions increase with the increase in the operating potential. Soft x-ray signals last much longer (3000 ns) than those of electrons' and ions' signals (300 ns). The plasma operated at 11 kV permits highest emission of ions, electrons, and x-rays. The ions, electrons, and hard and soft x-ray irradiation on silicon wafer exhibited the presence of damaged trails. Ion irradiation showed the presence of latent damaged trails. Electron irradiation caused more damage to the Si surface compared to ion irradiation due to higher flux and density. Soft x-rays had a lesser effect as compared to Si exposed to hard x-rays due to higher energy of the hard x-rays. In the case of hard x-rays irradiated Si, erupted volcano-like structure is formed.

Focus on lithiation kinetics of selenium-doped NiO@C electrodes with multistage pore structure

Nickel oxide (NiO) has received considerable attention as an anode for lithium-ion batteries (LIBs) due to its superior theoretical specific capacity and cycling stability. However, its low conductivity as a wide bandgap semiconductor and sluggish lithiation kinetics are still obstacles to its widespread commercial use. In this study, we have developed a Se-doped NiO@C microsphere with a multistage pore structure. The Se-doped NiO@C shows satisfactory specific capacity and cycling stability when used as an anode for LIBs. Specifically, it exhibits a specific capacity of 1622.2 mAh g-1 after 190 cycles at 0.1 A g-1, and a capacity retention of 79.3% after 1500 cycles at 2 A g-1. The Se-doped NiO@C exhibited improved lithiation reaction kinetics due to its hierarchical pore structure, as indicated by rate performance, pseudocapacitance contribution, and galvanostatic intermittent titration technique. Furthermore, DFT calculations revealed that the addition of selenium altered the band structure of NiO, resulting in a narrower forbidden band width and faster charge transfer. Meanwhile, the electrons provided by selenium enhance the density of states of elemental oxygen and improve the adsorption energy of lithium ions. This study provides insights into the structure and composition necessary for improving the hysteresis kinetics of oxide electrodes.

A One‐Dimensional Model of Atmospheric Sputtering at Io Driven by S++ and O+

AbstractIo, the closest of Jupiter's four Galilean moons, suffers from intense ion bombardment from Jupiter's magnetosphere. The constant atmospheric erosion by energetic ion precipitation, referred to atmospheric sputtering, serves as an important mechanism of Io's atmospheric escape. This study is devoted to a state‐of‐the‐art study of atmospheric sputtering at Io, with the aid of constantly accumulated understandings of Io's space environment and atmospheric photochemistry, as well as the updated laboratory measurements. A Monte Carlo model is constructed to track the energy degradation of incident S++ and O+ and atmospheric recoils from which the sputtering yields of different atmospheric species are determined. Our calculations suggest a total escape rate of 3 × 1029 atom s−1 on Io, and SO2 is the dominant sputtered species. Further investigations reveal that S++ is the most efficient species for atmospheric sputtering on Io, and sputtering yields increase substantially with increasing incident ion mass, energy, and incidence angle. The model sensitivity to different influence factors is also discussed, including scattering angle distribution, atmospheric column density, proton precipitation, inelastic process, and surface sputtering, of which the former two dominate.

Electrochemical UV/Ozone process with Ni-Sb-SnO2/SiOx anode for degradation of micropollutants in wastewater

We investigated the synergistic effects of ozone evolution reaction (OZER) coupled with UV irradiation for degradation of O3-refractory micro-pollutants in electrochemical (E)-UV/O3 process. A SiOx heterojunction layer over-coated on Ni-Sb-SnO2 remarkably enhanced the OZER across a wide pH range (0.4–10). The exceptional OZER efficiency was harnessed in E-UV/O3 to increase degradation rates (by 2.5–3.5-fold) and mineralization efficiencies (by 1.3–3.9-fold) for 2,6-dinitrotoluene (DNT) compared to the sums under dark electrolysis and photolysis. Clarifying DNT degradation intermediates and reactive oxygen species elucidated the promoting mechanism within the reaction network of O3 transformation to ∙OH, the principal oxidant for the supreme performance at near-neutral pH. Applicability of E-UV/O3 was demonstrated by minuscule halate ion generation, outstanding stability and energy efficiency, and elimination of N,N-diethyl-m-toluamide (DEET) in reverse osmosis concentrates (ROC). These comprehensive findings from mechanistic studies to real applications are expected to broaden the utilization of OZER-based water treatments.

Ion kinetic effects on the formation of intense laser-driven shock waves

The ion kinetic effect on the formation of intense laser-driven collisional shock waves is investigated via hybrid fluid-particle-in-cell simulations. It is found that the ion heat flux dominates the shock formation, which is considerably larger than the electron heat flux in the shock region. The rise of the temperature due to the laser energy deposition drives a heatwave into the overdense plasma, creating an electron–ion energy exchange zone between the critical surface and heat wave front. The heated ions, which are generated at the electron–ion energy exchange zone via the friction force, are found to travel to the high-density region and cause a tail distribution gain. Despite the small quantity, the heated tail ions contribute most of the ion heat flux during the shock formation. Additionally, as the electron heat flux decreases, the population of the heated tail ions is reduced, leading to a fall in the ion heat flux. This results in the delay or even suppression of the shock formation, because the ions are in a non-equilibrium state in the vicinity of the shock region, the ratio of the downstream ion temperature to the upstream ion temperature tends to a modestly decrease in comparison to the theory. The study provides a clear picture of the formation process of laser-driven shock waves.

Pulse-stretching out of the CANREB EBIS

The CANadian Rare isotope facility with Electron-Beam ion source (CANREB) at TRIUMF is set to deliver rare isotope beams in high charge states. In the Electron Beam Ion Source (EBIS) ions are charge-bred by collisions with an electron beam of up to 500 mA. A strong magnetic field (up to 6T) maximizes the overlap between ions and electron beam and increases the breeding efficiency. Ion confinement is maintained by a combination of an electrostatic field and the electron beam space-charge potential. Ions are released by lowering the trapping potential with a step function. The system is operated at a pulse repetition frequency up to 100 Hz. Due to the short trap length, this fast extraction scheme produces pulses shorter than 10 µs with high instantaneous rates that can saturate detectors in experiments. Stretching the pulse can be done using a slowly varying voltage function to modify trap electrode potentials instead of a step function. The ideal function produces a pulse with a flat top distribution and can be calculated by knowing the ion energy distribution inside the trap. The latest pulse-stretching results will be discussed including the latest pulse duration up to 1.4 ms that have been produced. The slow extraction scheme has also been used for a measurement of the effective energy distribution of the ions inside the trap.

Mechanisms of ion irradiation induced ordering in amorphous TiO2 nanotubes: Effects of ion mass and energy

Amorphous TiO2 nanotubes were irradiated in-situ in a transmission electron microscope (TEM) with Kr+ ions at energies of 46 keV, 150 keV, and 1 MeV and with 46 keV Xe+ ions, to investigate the structural and morphological evolution of the nanotubes under irradiation. At all irradiation conditions, amorphous TiO2 nanotubes exhibited significant morphological instability, and tended to undergo volumetric swelling with increasing ion counts, often until collapse of the original nanotube structure. Molecular dynamics (MD) simulations confirmed that irradiation-induced defects can explain the observed swelling. Structurally, nanotubes remain amorphous following all Kr+ irradiation conditions, but irradiation with 46 keV Xe+ leads to the formation of anatase nanocrystallites. Importantly, through systematically varying ion energy and ion species, we try to elucidate the influence of nuclear and electronic stopping power on ion irradiation induced changes. By contextualizing these results within the existing literature, we propose that the observed changes in TiO2 nanotube morphology and structure could be due to a competition between two mechanisms: (1) disorder-induced swelling and (2) irradiation-induced amorphous-to-crystalline transformation.

Diamond ∆E-E telescope with an ultra-thin ∆E-diamond detector

The limits of mechanical polishing and RIE/ICP etching process were investigated to reduce the thickness of detector-grade ultra-thin below 10 μm, preserving the high quality of starting single crystal diamond (SCD) material. Free standing electronic grade SCD films with sizes up 4.5 × 4.5 mm2 and thickness down to 5 μm with overall thickness uniformity of ±0.35 μm were fabricated. The ultra-thin films were flat demonstrating the low internal stress.Diamond ∆E − E telescope was made from 5 μm thick ΔE-detector and 500 μm thick E-detector. The telescope was installed at the MARS spectrometer (Cyclotron Institute, TAMU) and tested with 241Am and 228Th alpha sources and 20Ne ion-beam, demonstrating the good spectral resolution around 100 keV. Simulations with the LISE++ program indicate that the telescope will be suitable to measure low energy heavy ion beams at energies as low as 2–3 MeV/nu for Au and U heavy ions, and even lower energies for lighter ions. The diamond telescope will be superior to common Si radiation telescopes having a low life-time in intense heavy ion-beams.

Investigation of electrode-driven hydrogen plasma method for in situ cleaning of tin-based contamination

Abstract To prolong the service life of the optics, the feasibility of in situ cleaning of multilayer mirror (MLM) of tin and its oxidized contamination was investigated using hydrogen plasma at different powers. Granular tin-based contamination consisting of micro and macro particles was deposited on the silicon by physical vapor deposition (PVD). The electrode-driven hydrogen plasma at different powers was systematically diagnosed using the Langmuir probe and the retarding field ion energy analyzer (RFEA). Moreover, the magnitude of the self-biasing voltage was measured at different powers, and the peak ion energy was corrected for the difference between the RFEA measurements and the self-biasing voltage (ΕRFEA-eVself). XPS analysis of O 1s, Sn 3d peaks demonstrated the chemical reduction process after 1 W cleaning. Analysis of surface and cross-section morphology revealed the holes emerged on the upper part of the macroparticles while its bottom remained smooth. Hills and folds appeared on the upper part of the microparticles, confirming the top-down cleaning mode with hydrogen plasma. This study provides in situ electrode-driven hydrogen plasma etching process of tin-based contamination and is of meaningful guidance for understanding the chemical mechanism of reduction and etching.

Entropy Engineering Constrain Phase Transitions Enable Ultralong-life Prussian Blue Analogs Cathodes.

Prussian blue analogs (PBAs) are considered as one of the most potential electrode materials in capacitive deionization (CDI) due to their unique 3D framework structure. However, their practical applications suffer from low desalination capacity and poor cyclic stability. Here, an entropy engineering strategy is proposed that incorporates high-entropy (HE) concept into PBAs to address the unfavorable multistage phase transitions during CDI desalination. By introducing five or more metals, which share N coordination site, high-entropy hexacyanoferrate (HE-HCF) is constructed, thereby increasing the configurational entropy of the system to above 1.5R and placing it into the high-entropy category. As a result, the developed HE-HCF demonstrates remarkable cycling performance, with a capacity retention rate of over 97% after undergoing 350 ultralong-life cycles of adsorption/desorption. Additionally, it exhibits a high desalination capacity of 77.24mg g-1 at 1.2V. Structural characterization and theoretical calculation reveal that high configurational entropy not only helps to restrain phase transition and strengthen structural stability, but also optimizes Na+ ions diffusion path and energy barrier, accelerates reaction kinetics and thus improves performance. This research introduces a new approach for designing electrodes with high performance, low cost, and long-lasting durability for capacitive deionization applications.

Accelerating the discovery of direct bandgap doped-spinel photovoltaic materials: A target-driven approach using interpretable machine learning

The spinel structure, unifying tetrahedral and octahedral coordination within a single crystal lattice, emerges as a promising alternative to perovskite and traditional semiconductors, particularly in the context of photovoltaic applications. However, the majority of surveyed spinels exhibit an indirect bandgap, imposing substantial limitations on their utility. Consequently, doping is required to tailor indirect bandgap spinels into direct ones. In this study, leveraging machine learning (ML) algorithms, we design a target-driven framework to accelerate the ab initio predictions of unknown doped-spinels using elements from the periodic table. Utilizing this approach, we have introduced a novel machine learning classification model designed for the prediction of the direct-indirect bandgap nature in spinel materials. This pioneering model shows promising potential for achieving rapid and precise predictions, particularly in scenarios involving small dataset. Especially, a full list of potential 3449 (AxA′1-x)B2X4-type doped-spinels and 3809 A(BxB′2-x)X4-type doped-spinels with direct bandgaps is identified from the vast pool of unknown doped-spinel materials. Further, the application of interpretable ML extracts the first ionization energy of the B-site ion as the most important feature for the nature of bandgap, offering insightful design rules. This research provides a novel perspective on unraveling spinel materials, and the proposed design framework proves effective in identifying high-performance materials within a vast chemical space, all while minimizing computational costs.

Ionic Covalent Organic Frameworks in Adsorption and Catalysis

AbstractThe ion extraction and electro/photo catalysis are promising methods to address environmental and energy issues. Covalent organic frameworks (COFs) are a class of promising template to construct absorbents and catalysts because of their stable frameworks, high surface areas, controllable pore environments, and well‐defined catalytic sites. Among them, ionic COFs as unique class of crystalline porous materials, with charges in the frameworks or along the pore walls, have shown different properties and resulting performance in these applications with those from charge‐neutral COFs. In this review, current research progress based on the ionic COFs for ion extraction and energy conversion, including cationic/anionic materials and electro/photo catalysis is reviewed in terms of the synthesis strategy, modification methods, mechanisms of adsorption and catalysis, as well as applications. Finally, we demonstrated the current challenges and future development of ionic COFs in design strategies and applications.