Anion Diffusion Research Articles

Progress and prospects of two-dimensional materials for membrane-based osmotic power generation

The electrical energy that can be harnessed from the salinity difference across the sea water and river water interface can be one of the sustainable and clean energy resources of the future. This energy can be harnessed via the nanofluidic channels that selectively permeate ions. The selective diffusion of cations and anions can produce electricity through reverse electrodialysis. Two-dimensional (2D) materials are a class of nanomaterials that hold great promise in this field. Several breakthrough works have been previously published which demonstrate the high electrical power densities of 2D membranes. The ion transportation can be either through the nano-sized in-plane pores or interlayer spacings of 2D materials. This review article highlights the progress in 2D materials for salinity gradient power generation. Several types of 2D membranes with various nano-architectures are discussed in this review article. These include atom-thick 2D membranes with nanopores, 2D lamellar membranes, 2D lamellar membranes with nanopores, 2D/one-dimensional (1D), and 2D/2D hybrid membranes. The fabrication techniques, physical characteristics, ion transportation properties, and the osmotic power generation of these 2D membranes are elaborated in this review article. Finally, we overview the future research direction in this area. It is envisioned that the research on 2D materials can make practical salinity gradient power generation one step closer to reality.

Relative Cation-Anion Diffusion in Alkyltriethylammonium-Based Ionic Liquids.

19F Nuclear Magnetic Resonance spin-lattice relaxation experiments have been performed for a series of ionic liquids including the same anion, bis(trifluoromethanesulfonyl)imide, and cations with alkyl chains of different lengths: triethylhexylammonium, triethyloctylammonium, decyltriethylammonium, dodecyltriethylammonium, decyltriethylammonium, and hexadecyltriethylammonium. The experiments have been carried out in a frequency range of 10 kHz to 10 MHz versus temperature. A thorough analysis of the relaxation data has led to the determination of the cation–anion as a relative translation diffusion coefficient. The diffusion coefficients have been compared with the corresponding cation–cation and anion–anion diffusion coefficients, revealing a correlation in the relative translation movement of the anion and the triethylhexylammonium, triethyloctylammonium, decyltriethylammonium, and dodecyltriethylammonium cations, whereas the relative translation diffusion between the anion and the cations with the longer alkyl chains, decyltriethylammonium and hexadecyltriethylammonium, remains rather uncorrelated (correlated to a much lesser extent).

Layer-by-layer growth of ferrocene decorated metal–organic framework thin films and studies of their electrochemical properties

As electroactive metal–organic frameworks (MOFs) become the centerpiece of many emerging applications in electrocatalysis, energy storage/conversion and electronics, the ability to grow high-quality of electroactive MOF thin films and understanding the mechanism of charge transport in the frameworks are crucial. Herein, we demonstrate that the use of ferrocene pendant biphenyl dicarboxylate (bpdc-Fc) ligand allows the successful growth of redox-active surface-attached MOFs (SURMOFs) [Cu2(bpdc-Fc)2dabco] SURMOF, on self-assembled monolayers (SAMs) functionalized surfaces by layer-by-layer (LBL) deposition method. The nature of the SAMs significantly affects the morphologies and electrochemical properties of [Cu2(bpdc-Fc)2dabco] SURMOFs owing to their different ability to coordinate Cu2 dimmer. The purine-based SAMs offer excellent anchoring sites for the nucleation of MOF crystals leading to dense films with good electrochemical properties. On the contrary, sparse MOF nanocrystals with poor electrochemical properties were obtained on pyridine-based SAMs. A redox hopping mechanism accompanied by anion diffusion was proposed to explain the different redox properties of [Cu2(bpdc-Fc)2dabco] SURMOFs on SAMs. This work has potential to contribute to rational design and growth of novel MOFs for the applications in ionic conductors, electrocatalysts, energy storage devices and electrochemical sensors.

Effect of Cationic/Anionic Diffusion Dominated Passive Film Growth on Tribocorrosion

Tribocorrosion behaviours of nickel (Ni) and niobium (Nb) in sodium sulfate (Na2SO4) solution under potentiodynamic and potentiostatic conditions were studied. Under the potentiodynamic condition, the passivation was early broken, accompanied by a sharp increase in frictional coefficient on Nb. The current was more fluctuant, and larger material loss appeared at the higher potential in the potentiostatic condition. However, these phenomena did not occur for Ni, and it even showed lower material loss at the higher potential in the potentiostatic tribocorrosion test. The differences in tribocorrosion behaviour had a close relationship to the passive film growth mechanism, which decided the passive film/metal interface structure. Nb with anionic diffusion dominated mechanism in passive growth would cause the accumulation of oxygen vacancies at the passive film/metal interface. This may weaken the adhesion between the metal and the passive film. However, with the cationic diffusion dominated passive film growth on Ni, cation vacancies concentrated at the passive film/tribo-film interface, and this did not affect the adhesion between metal and passive film. Ni or other passive elements with the cationic diffusion-dominated mechanism in passive film growth were recommended as the alloying element for improving the tribocorrosion resistance of alloys.

Capacitance spectroscopy of thin-film formamidinium lead iodide based perovskite solar cells

This work concerns the interpretation of capacitance spectroscopy results in perovskite-based solar cells. Based on the deep level transient spectroscopy and admittance spectroscopy results, we present arguments that the observable signals in perovskite-based solar cells come from anion migration rather than being a response from deep trap energy levels. The ion migration parameters, such as activation energy and ion concentration, are calculated and compared to theoretical values for different migration paths of ions in perovskites. Those parameters evolve with time, reflecting in the degradation of the cells, which we propose to link with a change in the anion migration path in perovskite. • The same signal is observed in DLTS and admittance spectroscopy. • Its activation energy is in the 0.3 eV–0.5 eV range and decays with time. • Time constants and the presence of large capacitance transients at +1 V point into the diffusion related origin of the signals. • Low activation energy, DLTS sign, comparison with other experiments and theoretical calculations point into anion diffusion. • The observed, dominating signal correlates with the increased shunt of the device.

Radio-frequency magnetron sputtered thin-film La0.5Sr0.5Co0.95Nb0.05O3-δ perovskite electrodes for intermediate temperature symmetric solid oxide fuel cell (IT-SSOFC)

The present work explores the application of La0.5Sr0.5Co0.95Nb0.05O3-δ (LSCNO) perovskite as electrode material for the symmetric solid oxide fuel cell. Symmetric solid oxide fuel cells of thin-film LSCNO electrodes were prepared to study the oxygen reduction reaction at intermediate temperature. The Rietveld refinement of synthesized material shows a hexagonal structure with the R-3c space group of the prepared perovskite material. Lattice parameter and fractional coordinates were utilized to calculate the oxygen ion diffusion coefficient for molecular dynamic simulation. At 973 K, the oxygen ion diffusion of LSCNO was 1.407 × 10−8 cm2 s−1 higher by order of one magnitude than that of the La0.5Sr0.5CoO3-δ (7.751 × 10−9 cm2 s−1). The results suggest that the Nb doping provide the structural stability which improves oxygen anion diffusion. The enhanced structural stability was analysed by the thermal expansion coefficient calculated experimentally and from molecular dynamics simulations. Furthermore, the density functional theory calculation revealed the role of Nb dopant for oxygen vacancy formation energy at Sr–O and La–O planes is lower than the undoped structure. To understand the rate-limiting process for sluggish oxygen diffusion kinetics, 80 nm and 40 nm thin films were fabricated using radio frequency magnetron sputtering on gadolinium doped ceria electrolyte substrate. The impedance was observed to increase with an increasing thickness, suggesting the bulk diffusion as a rate-limiting step for oxygen ion diffusion. The electrochemical performance was analysed for the thin-film symmetric solid oxide fuel cell, which achieved a peak power density of 390 mW cm−2 at 1.02 V in the presence of H2 fuel on the anode side and air on the cathode side.

Boosting the kinetics of PF6− into graphitic layers for the optimal cathode of dual-ion batteries: The rehearsal of pre-intercalating Li+

Large anions exhibit slow diffusion kinetics in graphite cathode of dual-ion batteries (DIBs); particularly at high current density, it suffers severely from the largely-reduced interlayer utilization of graphite cathode, which as a bottleneck limits the fast charge application of DIBs. To maximize interlayer utilization and achieve faster anion diffusion kinetics, a fast and uncrowded anion transport channel must be established. Herein, Li+ was pre-intercalated into the graphite paper (GP) cathode to increase the interlayer spacing, and then hosted for the PF6- anion storage. Combined with theoretical calculation, it shows that the local interlayer spacing enlargement and the residual Li+ reduce the anion intercalation energy and diffusion barrier, leading to better rate stability. The obtained GP with Li+ pre-intercalation (GP-Li) electrode exhibits a discharge capacity of 23.1 mAh g−1 at a high current of 1300 mA g−1. This work provides a facile method to efficiently improve the interlayer utilization of graphite cathode at large currents.

Diffusiophoresis of a highly charged dielectric fluid droplet induced by diffusion potential

Diffusiophoresis of a dielectric fluid droplet in electrolyte solutions is investigated theoretically, focusing on the electrophoresis component resulting from the induced diffusion potential in the electrolyte solution when the diffusivities of cations and anions there are different. The resultant electrokinetic equations are solved with a pseudo-spectral method based on the Chebyshev polynomials. We found, among other things, that the electrophoresis component dominates at a larger Debye length, whereas the chemiphoresis component at a smaller Debye length for a dielectric droplet of a constant surface charge density. The two components are of comparable magnitudes in the NaCl solution. The dual between the spinning electric driving force tangent to the droplet surface and the hydrodynamic drag force reinforced by the motion-deterring electrokinetic Maxwell traction from the surrounding exterior osmosis flow is crucial in the determination of the ultimate droplet motion. Unlike the chemiphoresis component, which is independent of the sign of charges carried by the droplet, the droplet moving direction as well as its magnitude in the electrophoresis component depends on the sign of charges carried by the droplet as well as the direction of the electric field induced by the diffusion potential. This gives the electrophoresis component excellent maneuverability in practical applications like drug delivery and enhanced oil recovery, where migration of droplets toward regions of higher solute concentrations is often desired.

A Theoretical Study of the Oxygen Release Mechanisms of a Cu-Based Oxygen Carrier during Chemical Looping with Oxygen Uncoupling

The Cu-based oxygen carrier is a promising material in the chemical looping with oxygen uncoupling (CLOU) process, while its performance in the CLOU is significantly dependent on the oxygen release properties. However, the study of oxygen release mechanisms in CLOU is not comprehensive enough. In this work, the detailed oxygen release mechanisms of CuO(110) and CuO(111) are researched at an atomic level using the density functional theory (DFT) method, including the formation of O2, the desorption of O2 and the diffusion of O anion, as well as the analysis of the density of states. The results show that (1) the most favorable pathway for O2 formation and desorption occurs on the CuO(110) surface of O-terminated with energy barriers of 1.89 eV and 3.22 eV, respectively; (2) the most favorable pathway for O anion diffusion occurs in the CuO(110) slab with the lowest energy barrier of 0.24 eV; and (3) the total density of states for the O atoms in the CuO(110) slab shifts to a lower energy after an O vacancy formation. All of the above results clearly demonstrate that the CuO(110) surface plays a significantly important role in the oxygen release reaction, and the oxygen vacancy defect should be conducive to the reactivity of oxygen release in a Cu-based oxygen carrier.

Sulfate corrosion and phosphate passivation of Ni-based alloy in supercritical water

Corrosion behavior of Ni-based alloy Inconel 625 in sulfate- and phosphate-containing SCW, and the mechanism of phosphate passivation acting as a corrosion inhibitor were investigated in this work. Sodium phosphate passivation treatment reduced the corrosion weight gains in oxygenated and pure SCW by 68.4% and 77.4%, respectively. The synergistic effect of high concentration of sulfate ions and oxygen on the alloy surface caused occurrence and expansion of pitting corrosion. NiCr2O4, NiO, Cr2O3, divalent and trivalent iron compounds could stably exist on the surface of sodium phosphate-passivated sample before and after exposure to oxygenated SCW. CrPO4, FePO4 and Ni3(PO4)2, which were closely and uniformly distributed on the alloy surface, stably existed in SCW and served as a barrier layer for the alloy matrix. These oxides could inhibit the diffusion of metal cations and oxygen anions near the interface to improve corrosion resistance.

High temperature oxidation resistance of an amorphous layer induced by ion implantation on the surface of Ni-based superalloy GH202

The high-temperature cyclic oxidation behavior of Ni-based superalloy GH202 was investigated before and after Y ion implantation at 1100 °C. The chemical composition and microstructure of the alloy surface treated by ion implantation were characterized using X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electron probe microanalysis, and scanning electron microscopy. The results showed that a 30-nm-thick amorphous layer formed on the surface of the superalloy. The collision of Y ions with the alloy atoms resulted in a large number of crystal defects, such as vacancies and dislocations. Upon increasing the implantation dose, the shock of the ions prompted the dislocations to move and form dislocation tangles and dislocation arrays, which provided short circuit diffusion channels for aluminum (Al) ions during high-temperature oxidation. The oxide film on the surface of the specimens treated by ion implantation remained intact after 40 oxidation cycles, and no internal oxidation occurred in the substrate. The Y ions provided heterogeneous nucleation particles, reduced the distance between the Al2O3 nucleation positions, and promoted the formation of continuous, dense films. The amorphous layer changed the oxidation mechanism from outward cation diffusion in the oxide films to inward anion diffusion. The growth rate of the oxide film was effectively inhabited, and the high-temperature oxidation resistance was significantly improved.

Features of aluminum electrodeposition from 1,3-dialkylimidazolium chloride chloroaluminate ionic liquids

The mechanism of aluminum electrodeposition from acidic 1-butyl-3-methylimidazolium chloride chloroaluminate ionic liquids ([BMIm]Cl ILs) is studied over a wide range of the molar ratio of AlCl3 to [BMIm]Cl (N) at 30 °C. The findings for AlCl3–[BMIm]Cl and AlCl3–1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) are analysed and compared in order to identify dependences common for 1,3-dialkylimidazolium chloride ILs. Limiting currents were detected on the cathodic polarization curves at the aluminum | ionic liquid interface. Limiting currents are caused by the Al2Cl7– anions diffusion to the electrode surface. The limiting current density values for AlCl3–[BMIm]Cl are lower (ilim, 1.1 ≤ N ≤ 2.0 = 1.17 – 12.36 mA∙cm−2) than for AlCl3–[EMIm]Cl (ilim, 1.1 ≤ N ≤ 2.0 = 1.81 – 25.37 mA·cm−2). It is shown that anion AlCl4– can be reduced to metallic aluminum at cathodic overpotentials above 1.5 V. The diffusion coefficient of Al2Cl7– in AlCl3–[BMIm]Cl (7.4∙10−7 cm2∙s−1) is lower than in AlCl3–[EMIm]Cl (9.3∙10−7 cm2∙s−1) and is unaffected by aluminum chloride concentration in either system.

Exploration of Ion Transport in Blends of an Ionic Liquid and a Polymerized Ionic Liquid Graft Copolymer.

In this study, we prepared a composite membrane consisting of a poly(1-butyl-3-vinylimidazolium-tetrafluoroborate) (poly([BVIM]-[BF4])) polymerized ionic liquid graft copolymer (PILGC) and a blend of PILGC and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]-[BF4]) ionic liquid (IL) to explore techniques for improving the conductivity of PILGCs, which is normally three orders of magnitude lower than that of ILs. PILGCs, which are environmentally friendly, have attracted much interest. To gain a better understanding of ion transport in composites, the mechanisms of ion transport in composite components should be explored. We investigated anion transport in ILs and PILGCs and were able to obtain the correct ion transport mechanisms in IL-PILGC blends based on a previous work. We performed molecular dynamics (MD) simulations, which are commonly used to investigate molecular mechanisms. According to the MD simulation results, in most IL-PILGC blends of various compositions, the contributions of cations are greater than those of anions. This is one reason that blends have higher conductivities than their component PILGCs. To the best of our knowledge, we are the first to identify ion transport mechanisms in PILGCs and their blends with ILs by exploring subdiffusive ion motion regimes. The ratio of the number of cages with more than three cationic branch chains in the blend with 50 wt % PILGC, the blend with 80 wt % PILGC, and the PILGC was 0.26:0.39:0.65. Therefore, the ratio of firm cages gets a promotion as the PILGC content increases. Because the ratio of fast ions decreases as the ratio of firm cages increases, the blend with 80 wt % PILGC has lower anion diffusivities than the blend with 50 wt % PILGC. It was inappropriate to probe ion transport in PILGCs (or IL-PILGC blends) solely via analyzing ion association interactions. Analysis of only ion association interactions led to the incorrect conclusion that the time scales of ion transport in PILGCs are given by the continuous ion association time, which is the time when the ion association remains paired rather than the time when an ion is caught inside a cage. Proper methods should be used to obtain more accurate theories.

Controlling Li+ transport in ionic liquid electrolytes through salt content and anion asymmetry: a mechanistic understanding gained from molecular dynamics simulations.

In this work, we report the results from molecular dynamics simulations of lithium salt-ionic liquid electrolytes (ILEs) based either on the symmetric bis[(trifluoromethyl)sulfonyl]imide (TFSI-) anion or its asymmetric analogue 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide (TFSAM-). Relating lithium's coordination environment to anion mean residence times and diffusion constants confirms the remarkable transport behaviour of the TFSAM--based ILEs that has been observed in recent experiments: for increased salt doping, the lithium ions must compete for the more attractive cyano over oxygen coordination and a fragmented landscape of solvation geometries emerges, in which lithium appears to be less strongly bound. We present a novel, yet statistically straightforward methodology to quantify the extent to which lithium and its solvation shell are dynamically coupled. By means of a Lithium Coupling Factor (LCF) we demonstrate that the shell anions do not constitute a stable lithium vehicle, which suggests for this electrolyte material the commonly termed "vehicular" lithium transport mechanism could be more aptly pictured as a correlated, flow-like motion of lithium and its neighbourhood. Our analysis elucidates two separate causes why lithium and shell dynamics progressively decouple with higher salt content: on the one hand, an increased sharing of anions between lithium limits the achievable LCF of individual lithium-anion pairs. On the other hand, weaker binding configurations naturally entail a lower dynamic stability of the lithium-anion complex, which is particularly relevant for the TFSAM--containing ILEs.

Understanding ion diffusion in anion exchange membranes; effects of morphology and mobility of pendant cationic groups

A new methodology to assess ion diffusion in side-chain modified AEMs reveals separate roles for functionality and side-chain length: the former determines hydrophilic pathway morphology and the latter regulates anion diffusion rate in the channels.

A Highly Efficient Ion and Electron Conductive Interlayer To Achieve Low Self-Discharge of Lithium-Sulfur Batteries.

The practical use of lithium-sulfur (Li-S) batteries is limited by serious self-discharge, fast capacity loss, and severe lithium anode erosion due to the shuttling of lithium polysulfides (LiPSs). Herein, we developed a highly efficient ion and electron conductive interlayer composed of Ti2(SO4)3/carbon composite layer-coated Li1.3Al0.3Ti1.7(PO4)3 (CLATP) and graphene to effectively block the diffusion of polysulfide anions but allow rapid Li ion transfer, therefore significantly inhibiting the self-discharge and boosting the cyclic stability of Li-S batteries. The Ti2(SO4)3/carbon thin protective layer endows an optimized adsorption ability toward LiPSs and avoids the side reactions between LATP and LiPSs. The high electronic conductivity of graphene and high ionic conductivity of CLATP ensures the hybrid interlayer rapid electron and fast Li ion transport. As a result, the Li-S battery with the hybrid interlayer shows a high discharge capacity of 671 mAh g-1 after 500 cycles with an extremely low capacity fading of 0.022% per cycle at 1 C. Moreover, the battery shows no self-discharge even after rest for 12 days. This work opens up a new way for the design of functional separators to significantly improve the electrochemical performance of Li-S batteries.

Self‐Activation Enables Cationic and Anionic Co‐Storage in Organic Frameworks

AbstractThe low capacity of current anion‐hosting electrodes makes it challenging to meet future energy requirements. Maintaining the high capacity of ions per unit volume and/or weight is always the ultimate goal for an optimal electrode design. Here, an organic framework is reported that can efficiently co‐store anions and cations in a single discharge/charge process, accompanied with a special cation‐assisted electrode activation process. The inserted cations not only facilitate anion diffusion kinetics, but also make a considerable contribution to the total capacity of the electrodes. This anion–cation co‐storage cathode thus can deliver a high capacity of 530 mAh g−1 at 3.4–1.0 V and a high energy density of 901 Wh kg−1 after 800 cycles (at 0.15 A g−1). The understanding of the self‐activation process is expected to inspire electrode design that utilizes anionic–cationic hybrid electrode chemistries beyond current Li‐ion batteries.

Growth of polycrystalline gallium oxide films in stagnant oxygen stream ambient

Structural, morphological, optical, and electrical characteristics of polycrystalline gallium oxide (Ga2O3) films subjected to different post-deposition annealing temperatures (400–1000 °C) in a stagnant oxygen stream ambient were systematically studied. The transformation from γ-Ga2O3 to β-Ga2O3 phase was perceived as the temperature was enhanced to/beyond 600 °C. An alleviation of oxygen related defects in Ga2O3 films was supported through a reduction in full-width half maximum (FWHM) for β-Ga2O3 peak oriented in (400) plane and dislocation density (δ), as well as an improvement in crystallinity when a higher temperature was employed. Nonetheless, the employment of the highest temperature has contributed to an excessive diffusion of oxygen anions to the interface contributing to the formation of a well-defined interfacial layer. Besides, the oxygen ions were also occupying the interstitial sites of Ga2O3 lattice at 1000 °C contributing to a sudden contraction of indirect band gap (Eg). These detrimental effects have suggested that 1000 °C was not a suitable annealing temperature for Ga2O3 films. Of these investigated films, Ga2O3 film annealed at 800 °C has demonstrated a better leakage current density characteristic than that of 400 and 600 °C due to acquisition of the lowest δ, the smallest FWHM, as well as the largest direct and indirect Eg.

Polypyrrole as an ultrafast organic cathode for dual-ion batteries

Organic electrode materials based on the chemical bond cleavage/recombination working principle usually produce unimpressive reaction kinetics and stability. In this work, polypyrrole (PPy) is investigated as an ultrafast (87% retention at 20 ​A ​g−1) and stable (83% retention across 3000 cycles) cathode material in PPy|graphite dual-ion batteries. The fast intrinsic reaction kinetics, coupled with a capacitance-dominated mechanism, enable PPy to bypass the sluggish chemical bond rearrangement process. Electrochemically induced secondary doping improves the ordered aggregation of polymer chains and thus has a profound impact on anion diffusion and electrical conductivity. The excellent rate capability presented here changes our understanding of organic electrode materials and could prove useful for designing ultrafast rechargeable electrochemical devices.

Strengthening of Calcite Assemblages Through Chemical Complexation Reactions

AbstractUtilization of subsurface reservoirs, including fluid extraction/injection, can induce stress changes and modify in‐situ chemical equilibrium causing subcritical fracturing and deformation. Here, we show that chemical complexation reactions at the newly created fracture surfaces within the crack tip limit fracturing during consolidation of granular calcite assemblages. Previously, we showed that analogous chemical complexation reactions can increase in‐situ fracture toughness in pre‐fractured single crystal calcite. To test the chemical complexation effect under subsurface conditions, we consolidated samples under increasing hydrostatic pressure to induce widespread intergranular fracturing in the presence of fluid containing common anions. Measured consolidation, acoustic emissions, and microfracturing demonstrate that deformation correlates with complexation affinity for calcium in calcite and aqueous anions. We observed that deformation is lowest in the presence of strongly complexing anions (e.g., sulphate), provided sufficient anion diffusion rates. This understanding is important for predicting reservoir effective strength in the presence of complex aqueous fluids.