Propylene Carbonate Research Articles

Oxidative carbonylation of propylene glycol to propylene carbonate by copper-based catalysts

BackgroundDevelopment of copper-based catalyst and associated reaction engineering for heterogeneous oxidative carbonylation of propylene glycol (PG) to optimize conversion and selectivity. Coupling reactions with reverse water-gas shift reaction is a good way of realizing CO2 utilization. MethodThe synthesis of propylene carbonate (PC) through the oxidative carbonylation of propylene glycol is carried out in a pressurized autoclave reactor using solid copper catalysts. Various types of copper and composite copper/ceria catalysts are prepared by coprecipitation or impregnation method. Significant findingsReaction temperature and the roles of sodium acetate (SA) and potassium iodide (KI) additives are systematically studied with pure copper to map out the optimum reaction condition, i.e., a 100 °C-reaction using 2.0 g of PG mixed with 8.0 g of solvent with 0.02 g of SA and 0.02 g of KI additives. Using this optimized condition, the copper and composite copper/ceria catalysts are investigated with the composite catalysts composed of metallic copper on ceria, showing the highest selectivity of 94.7% towards PC. These findings in this study should pave way for the development of practical industrial scale CO2 utilization processes.

Investigation of P3HT:WO3 hybrid electrochromic thin films prepared by solution blending doping

Although poly(3-hexylthiophene) (P3HT) is a promising p-type conjugating polymer to optimize optical and electrical properties, it is known to be chemically unstable. To overcome this unstability, WO3 incorporated P3HT hybrid thin films has been synthesized by very simple solution blending doping method. Different amounts of WO3 were added into the P3HT solution and the incorporation of WO3 particles was confirmed by scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy. The electrochemical reactions in 1 M lithium perchlorate (LiClO4)/propylene carbonate (PC) were studied by cyclic voltammetry and by electrochemical impedance spectroscopy. With the increase amount of WO3 in P3HT, the electrochromic efficiency increases first and then decreases. The optimum concentration was found as 30 wt% of WO3. The efficient interaction and well distribution between WO3 and P3HT improves the capacitive properties and electrochromic performance resulting a 110% increase in coloration efficiency (from 220 cm2/C to 464 cm2/C). Moreover, WO3 doped hybrid films shows long-term cyclability than undoped P3HT films. Structural and electrochemical investigations suggest that the optimum amount of WO3 doping is an alternative way to obtain high performance and environmentally stable electrochromic devices.

Polymer electrolyte based on Nafion plasticized with carbonates and their ternary mixtures for sodium-ion batteries

The physicochemical properties and ionic conductivity of Nafion polymer membrane in sodium form plasticized with such organic solvents as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and their ternary mixtures were studied. According to the results of gravimetry, simultaneous thermal analysis, IR spectroscopy, small-angle X-ray scattering, voltammetry, and impedance spectroscopy, it was shown that the optimal plasticizer for Na-Nafion membrane among the studied systems is a ternary mixture with the composition EC/PC/DMC = 0.45/0.45/0.1. The polymer electrolyte based on Na-Nafion, plasticized with this ternary mixture, despite the lower electrochemical stability compared to the membrane swollen with EC, has the highest swelling degree of 31% and the ionic conductivity of 0.08 mS cm−1 at 20 °C, which is maintained even at −60 °C (6 × 10−8 S cm−1). The obtained values are comparable with the conductivity of the lithium form of Nafion. The possibility of using a membrane based on the sodium form of Nafion as an electrolyte in sodium and sodium-ion batteries was shown.

Effect of Residual Solvents on Properties of Composite Solid Electrolytes

Solid-state electrolytes (SSEs) can stabilize lithium (Li) metal anodes and achieve high energy density. However, they often suffer from solvent residues during the preparation. The residual solvent will affect the performance of SSEs. In this paper, we take Li1.5Al0.5Ge1.5P3O12 (LAGP)/Poly(propylene carbonate) (PPC) composite solid electrolyte (CSE) as an example, the content of the solvent residue of CSE prepared by the solution casting method and its effect on performance are investigated. The result reveals that the solvent content decreases with the increase of drying time. It still contains more than 5 wt % solvent even after vacuum drying for 48 h. The presence of residual solvent promotes the binding of lithium ions (Li+) and carbonyls, which is conducive to the transfer of Li+. When the residual solvent content is high, SSE has high ionic conductivity, cycling capacity, and excellent cycling stability. When the solvent content decreases, the electrochemical window becomes wider. This result is of great significance for the future regulation of SSE solvent content.

1,3,5-Trifluorobenzene endorsed EC-free electrolyte for high-voltage and wide-temperature lithium-ion batteries

Ethylene carbonate (EC) is susceptible to the aggressive chemistry of nickel-rich cathodes, making it undesirable for high-voltage lithium-ion batteries (LIBs). The arbitrary elimination of EC leads to better oxidative tolerance but always incurs interfacial degradation and electrolyte decomposition. Herein, an EC-free electrolyte is deliberately developed based on gradient solvation by pairing solvation-protection agent (1,3,5-trifluorobenzene, F3B) with propylene carbonate (PC)/methyl ethyl carbonate (EMC) formulation. F3B keeps out of inner coordination shell but decomposes preferentially to construct robust interphase, inhibiting solvent decomposition and electrode corrosion. Thereby, the optimized electrolyte (1.1 M) with wide liquid range (−70–77 °C) conveys decent interfacial compatibility and high-voltage stability (4.6 V for LiNi0.6Mn0.2Co0.2O2, NCM622), qualifying reliable operation of practical NCM/graphite pouch cell (81.1% capacity retention over 600 cycles at 0.5 C). The solvation preservation and interface protection from F3B blaze a new avenue for developing high-voltage electrolytes in next-generation LIBs.

Thermodynamics of 2-alkanol + polar organic solvent mixtures. I. Systems with ketones, ethers or organic carbonates

The mixtures 2-propanol or 2-butanol + n-alkanone, or + acetophenone, or + linear monoether, or + cyclic ether, or + linear organic carbonate, or + propylene carbonate have been investigated using thermodynamic data, and in terms of the Flory theory, and the Kirkwood-Buff integrals. The data considered are: excess molar enthalpies (HmE), volumes, entropies, and the temperature dependence ofHmE. The enthalpy of the 2-alkanol-solvent interactions have been determined, and the different contributions to HmE discussed. It is shown that HmE values of the 2-alkanol (fixed) + n-alkanone, or + linear carbonate mixtures change in the same manner that for n-alkanone, or linear carbonate + n-alkane (fixed) systems. In contrast, HmE values of 2-alkanol (fixed) + linear monoether or + n-alkane mixtures change similarly. This set of results suggests that solvent–solvent interactions are determinant in systems with n-alkanone or linear carbonate, while interactions between alcohol molecules are determinant in mixtures with linear monoethers. According to the Flory model, orientational effects in systems with a given 2-alkanol become weaker in the sequence: linear monoether > linear organic carbonate > n-alkanone, and are stronger in solutions with a cyclic monoether than in those with cyclic diethers, and in systems with acetophenone or propylene carbonate than in the mixtures with the corresponding linear solvents. Results obtained from the Kirkwood-Buff integrals are consistent with these findings. The application of Flory model reveals that orientational effects are similar in systems with 1- or 2-alkanols, with the exception of solutions with linear monoethers, where such effects are stronger in mixtures containing 1-alkanols. For the examined systems, the formalism of the Kirkwood-Buff integrals makes no meaningful distinction between solutions with 1-alkanols or 2-alkanols.

Hydrodynamics of gas–liquid flow in a capillary at elevated pressure: Implications for the fixation of CO2 into propylene carbonate in microreaction systems

It is an effective method to capture and utilize carbon dioxide (CO2) by using a microreactor to fix CO2 into propylene carbonate (PC). It is important to investigate the hydrodynamics of gas–liquid flow in order to ensure that the CO2 fixation in microreactor is operated under suitable conditions to obtain efficient mass transfer performance. However, few studies have been concerned with PC-CO2 two-phase flow in microreactors, especially at elevated pressures. Therefore, a high-pressure microreactor observation platform is established in this work to explore the hydrodynamics of PC-CO2 two-phase flow at a pressure range of 0.5–3.0 MPa. Three typical patterns such as bubbly flow, slug flow, and slug-annular flow were observed, and the flow pattern diagram at different pressures was plotted using WeG and WeL. The influence of the pressure on flow pattern transition lines was further discussed. Moreover, the effects of pressure on bubble size, liquid film thickness, bubble frequency, and gas volume fraction were investigated, as well as the corresponding correlation models. This work may provide guidance for the operation of gas–liquid microreactor at elevated pressure, especially for the fixation of CO2 into PC.

Photo-Electrical Tandem of Modulated Polyvinyl Alcohol Based Plasticized Solid Polymer Electrolyte Nanocomposite Films: Effect of Propylene Carbonate Contents

Plasticized solid polymeric electrolyte nanocomposites based on polyvinyl alcohol (PVA) incorporating Cs2CuO2 nanoparticles (NPs), electrolyte salt (LiClO4) and plasticizer (PC) were prepared via solution casting technique. The dynamic light-scattering histogram revealed that the prepared Cs2CuO2 NPs have a size in the range of 80-120 nm. The interaction between different components in PVA/Cs2CuO2/LiClO4-PC films (plasticized PVA-SPEs) were probed by FTIR, while the surface and structure were evaluated by SEM and XRD, which indicate to amorphous nature of plasticized PVA-SPEs. The thermal behavior of films was measured via TGA, where a partial decrease in the thermal stability of films was noticed with an increase of PC content in the PVA-SPEs. The highest conductivity achieved is 9.56X10-5 S/cm for PVA-SPEs containing 8wt% PC at 298K. The plasticized PVA-SPEs exhibited higher specific capacitance by two folds and photovoltaic efficiency by three folds compared to pure PVA matrix.

Preparation of controllable micro-nano fiber bundle asymmetric porous PLA-PPC ethyl acetate driving film by pre-freezing treatment

The intelligent organic solvent actuator can effectively avoid direct contact in hazardous environment, which has important research significance in the fields of soft robots, intelligent devices and sensing. The structure of organic solvent actuators usually has asymmetrical structure, but the preparation process of such structure is complex and difficult to realize. Herein, based on the wettability difference of polymers, the asymmetric porous polylactic acid (PLA)- poly propylene carbonate (PPC) driving films with micro-nano fiber bundles were prepared by the simple pre-freezing treatment. The ultra-fast reversible intelligent deformation response behavior was triggered in ethyl acetate solvent. The deformation properties, surface morphology and pore size could be adjusted by the preparation process (pre-freezing temperature and PPC content). The PLA-PPC film was prepared at −7 °C with 0.85 wt% PPC content. After being soaked in solvent for 3 s, the maximum bending degree (360°) and curvature (1.9 cm−1) were obtained. The potential application scenarios of intelligent driving materials were provided, the driving film was designed as bionic flower, artificial butterfly, gripper driver and controllable circuit switch, which proved the feasibility of driving film in practical applications.

Synthesis of dimethyl carbonate from methanol and propylene carbonate over the Ca-Zr catalyst modified by transition metals

A series of Ca-Zr catalysts modified by different transition metals were prepared by the sol-gel method and their catalytic performance in the synthesis of dimethyl carbonate (DMC) from methanol and propylene carbonate (PC) by transesterification at low temperature was investigated. The results indicate that the selectivity to DMC of various transition metal-modified Ca-Zr catalysts follows the order of Co-Ca-Zr > Cu-Ca-Zr > Ca-Zr > Fe-Ca-Zr > Ni-Ca-Zr > Zn-Ca-Zr. For the transesterification over the Co-Ca-Zr catalyst, in particular, the conversion of PC reaches 84.3% with a selectivity of 94.5% to DMC after reaction for 2 h under 35 °C, a methanol/PC molar ratio of 15, and catalyst amount of 4%. Combining with the XRD, FT-IR, XPS and CO2-TPD results, it is revealed that increasing the strength of basic sites can raise the conversion of PC, whereas increasing the density of basic sites leads to a decrease of the selectivity to DMC. As a result, the Co-modified Ca-Zr (Co-Ca-Zr) catalyst, with the lowest density of surface basic sites but the highest fraction of strong basic sites, exhibits a high conversion of PC and a high selectivity to DMC for the transesterification of PC with methanol at a low temperature.

The effect of lignin-based polyols on the properties of polyurethane coatings

The aim of this research was to replace petroleum-based polyols with polyols made from lignin and nano-lignin in the production of polyurethane coatings. Lignin and nano-lignin were first reacted with either diethylenetriamine or ethylenediamine. Subsequently, they were modified with propylene carbonate. The lignin- and nano-lignin-derived polyols replaced the conventional acrylic polyol in the production of polyurethane coatings. The results of Fourier transform infrared analysis showed that urethane, isothiocyanate and strong C–N bonds were formed in nano-lignin-based polyurethanes. Moreover, the surface adhesion test of the lignin-based polyurethane coating showed that with increasing lignin content, the adhesion strength of the coating decreased. Mixing lignin nanoparticles into the polyurethane substrate caused the adhesion strength to rise from 1.67 to 4.52 MPa. An increase in the amino nano-lignin content of oil-based polyol of up to 7% improved the curing of the coating and increased the scratch resistance to more than 5 kg. The results showed that lignin from a mechanical pulping process has a direct effect on reducing the smoothness and the glossiness of polyurethane coatings.

Non-Aqueous Zn-Ion Hybrid Supercapacitors: Acetonitrile vs Propylene Carbonate Based Electrolyte

The development of high efficiency energy storage systems is increasingly important as these systems enable utilize energy from renewable sources and reduce greenhouse gas evolution caused by fuel combustion technologies at the same time. Electrochemical characteristics of Zn-ion hybrid supercapacitor (ZIHS) cells based on 1 M acetonitrile and propylene carbonate electrolytes in zinc tetrafluoroborate (Zn(BF4)2), zinc di[bis(trifluoromethylsulfonyl)imide] (Zn(TFSI)2) and zinc trifluoromethanesulfonate (Zn(OTf)2) have been studied using cyclic voltammetry, constant current charge/discharge and electrochemical impedance methods. The Ragone plots have been calculated from constant power measurement data. Very high energy and power densities (80 Wh kg−1 and 21.2 kW kg−1) have been calculated for 1 M Zn(BF4)2/AN based Zn-ion hybrid supercapacitor. Some assembled ZIHSs had shown excellent cycling and energy stability over 20000 cycles.

Influence of the Inclusion of Propylene Carbonate Electrolyte Solvent on the Microstructure and Thermal and Mechanical Stability of Poly(l-lactic acid) and Poly(vinylidene fluoride-co-hexafluoropropylene) Battery Separator Membranes.

The influence of the inclusion of the organic solvent propylene carbonate (PC) in microporous membranes based on poly(l-lactic acid) (PLLA) and poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) has been studied based on its relevance for the application of those separator membranes in lithium-ion batteries. The membranes have been produced through solvent casting and characterized with respect to the swelling ratio originated by the uptake of the organic solvent. The organic solvent uptake affects the porous microstructure and crystalline phase of both membrane types. The organic solvent uptake amount affects the crystal size of the membranes as a consequence of the interaction between the solvent and the polymer, since the presence of the solvent modifies the melting process of the polymer crystals due to a freezing temperature depression effect. It is also shown that the organic solvent partially penetrates into the amorphous phase of the polymer, leading to a mechanical plasticizing effect. Thus, the interaction between the organic solvent and the porous membrane is essential to properly tailor membrane properties, which in turn will affect lithium-ion battery performance.

Zinc‐Supported Nitrogen‐Rich Carbon Catalyst for Efficient Carbon Dioxide Cycloaddition with Epoxides under Solvent‐Free Conditions

AbstractDeveloping carbon‐based catalyst with both high efficiency and stability presents an enticing prospect for CO2 cycloaddition. In this work, zinc‐loaded nitrogen‐rich carbon catalyst (Zn−NC) was fabricated facilely through the wet impregnation and one‐pot pyrolysis method. The Zn−NC coupled with tetrabutylammonium bromide (TBAB) could achieve the high catalytic efficiency with 93% or 98.1% yield of propylene carbonate (PC) in solvent‐free system under mild condition or traditional heating conditions. Excellent recyclability and versatility of Zn−NC catalyst were also observed. Moreover, the influence of calcination temperature on catalyst structure, composition and catalytic performance were investigated, and the catalytic mechanism of CO2 cycloaddition with Zn−NC and TBAB was legitimately proposed. Herein, this work provided a promising and sustainable insights to prepare high‐efficient and low‐cost carbon‐based catalysts for CO2 conversion.

Degradation of Polymers and the Improvements of Polybutylene Terephthalate /Polyethylene Glycol Terephthalate Composite-material

Polybutylene terephthalate and polyethylene glycol terephthalate are subjected to thermal-oxidative degradation and thermomechanical degradation during the process of melt blending, which affect the polymer structure and properties. The effect of feed properties of polyethylene glycol terephthalate and the addition of modified nanoparticles on blends are a question worthy of discussion. This paper introduced the latest development of biodegradable plastics industry as well as the applications of biodegradable plastics in fibers, daily expenses of membranes and bags, agricultural products and automobile industrial products. These biodegradable plastics included poly (lactic acid), polyhydroxyalknoates, poly (butyl enes adipatecoterephthalate), and poly (propylene carbonate). In addition to biochemical effects, degradation also has biophysical effects, that is, after microorganisms erode the polymer, the polymer material is mechanically damaged due to the increase of cells. Therefore, biodegradation is not a single mechanism, but a compomise biophysical and biochemical synergy and mutual promotion of physical and chemical processes. So far, the mechanism of biodegradation has not been fully elucidated. This paper focuses on explaining the currently clear polymer degradation mechanism and the improvement in composition and structure, so as to offer some references for future studies.

Development of High Efficiency, Spray-Coated Perovskite Solar Cells and Modules Using Additive-Engineered Porous PbI2 Films.

The development of anti-solvent free, scalable, and printable perovskite film is crucial to realizing the low-cost roll-to-roll development of perovskite solar cells (PSCs). Herein, large-area perovskite film fabrication is explored using a spray-assisted sequential deposition technique. How propylene carbonate (PC) solvent additive affects the transformation of lead halide (PbI2 ) into perovskite at room temperature is investigated. The result shows that PC-modified perovskite films exhibit a uniform, pinhole-free morphology with oriented grains compared with pristine perovskite films. The PC-modified perovskite film also has a prolonged fluorescence lifetime that indicates lower carrier recombination. The champion PSC devices based on PC-modified perovskite film realize a power conversion efficiency (PCE) of 20.5% and 19.3% at an active area (A) of 0.09 cm2 and 1 cm2 , respectively. The fabricated PSCs are stable and demonstrate ≥85% PCE retention following 60 days of exposure to ambient conditions. Furthermore, perovskite solar modules (A ≈ 13 cm2 ) that yield a PCE of 15.8% are fabricated. These results are among the best reported for the state-of-art spray-coated PSCs. Spray deposition coupled with a PC additive is highly promising for economical and high-output preparation of PSCs.

Synthesis of sulfur-containing polycarbonate block copolymers via Salen-metal catalyzed copolymerization of CO2, propylene oxide, and phthalic thioanhydride

Sulfur-containing ABA-type polycarbonate triblock copolymers were synthesized via sequential copolymerization of propylene oxide (PO) and carbon dioxide (CO2), followed by propylene oxide (PO) and phthalic thioanhydride (PTA) in the presence of water and Salen-Metal catalysts. The reaction was catalyzed using Salen-Co(III)-Cl/[PPN]Cl and Salen-Cr(III)-Cl/[PPN]Cl catalyst systems. The content of thioester units in the copolymer chain could be adjusted by varying the feed ratios of PTA. These triblock polymers displayed a higher refractive index, reaching up to 1.561.

Anti-adsorption Mechanism of Photoresist by Pluronic Surfactants: An Insight into Their Adsorbed Structure.

Photoresist stripping is the final step in the photolithography process that forms fine patterns for electronic devices. Recently, a mixture of ethylene carbonate (EC) and propylene carbonate (PC) has attracted attention as a new stripper based on its eco-friendliness and anti-corrosiveness. However, the EC/PC mixture causes re-adsorption of the photoresist during a process of subsequent water rinsing. In this study, we characterized the adsorption/desorption of the photoresist and a triblock Pluronic surfactant [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)] as a blocking agent on an indium tin oxide (ITO) substrate. In addition, we evaluated the dispersion of photoresist particles. The photoresist polymer formed a thin and rigid adsorption layer on an ITO substrate in the EC/PC mixture. When water was injected into the EC/PC mixture and the photoresist solutions, the photoresist polymer aggregated and was then deposited on the substrate. In contrast, the addition of Pluronic surfactant F-68 (PEO79PPO30PEO79) into the EC/PC mixture remarkably decreased the residual amount of the photoresist on the ITO after water injection. This variation was attributed to the PEO blocks of F-68 extended to the solution phase, whereas the PPO blocks of F-68 functioned as anchors for adsorption onto the photoresist. Therefore, the F-68-adsorbed layer prevented interaction between the photoresist particles or the photoresist and the ITO surface, which provides potential for future applications as new stripping agents with high removal performance.

Morphological Features of SiO2 Nanofillers Address Poor Stability Issue in Gel Polymer Electrolyte-Based Electrochromic Devices.

Nanofillers' applicability in gel polymer electrolyte (GPE)-based devices skyrocketed in the last decade as soon as their remarkable benefits were realized. However, their applicability in GPE-based electrochromic devices (ECDs) has hardly seen any development due to challenges such as optical inhomogeneity brought by incompetent nanofiller sizes, transmittance drop due to higher filler loading (usually required), and poor methodologies of electrolyte fabrication. To address such issues, herein, we demonstrate a reinforced polymer electrolyte tailored through poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP),1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), and four types of mesoporous SiO2 nanofillers, porous (distinct morphologies) and nonporous, two each. The synthesized electrochromic species 1,1'-bis(4-fluorobenzyl)-4,4'-bipyridine-1,1'-diium tetrafluoroborate (BzV, 0.05 M), counter redox species ferrocene (Fc, 0.05 M), and supporting electrolyte (TBABF4, 0.5 M) were first dissolved in propylene carbonate (PC) and then immobilized in an electrospun PVDF-HFP/BMIMBF4/SiO2 host. We distinctly observed that spherical (SPHS) and hexagonal pore (MCMS) morphologies of fillers endowed higher transmittance change (ΔT) and coloration efficiency (CE) in utilized ECDs; particularly for the MCMS-incorporated ECD (GPE-MCMS/BzV-Fc ECD), ΔT reached ∼62.5% and CE soared to 276.3 cm2/C at 603 nm. The remarkable benefit of filler's hexagonal morphology was also seen in the GPE-MCMS/BzV-Fc ECD, which not only marked an astounding ionic conductivity (σ) of ∼13.5 × 10-3 S cm-1 at 25 °C, thus imitating the solution-type ECD's behavior, but also retained ∼77% of initial ΔT after 5000 switching cycles. The enhancement in ECD's performance resulted from merits brought by filler geometries such as the proliferation of Lewis acid-base interaction sites due to the high surface-to-volume ratio, the creation of percolating tunnels, and the emergence of capillary forces triggering facile ion transportation in the electrolyte matrix.

Glyoxylic-Acetal-Based Electrolytes for Sodium-Ion Batteries and Sodium-Ion Capacitors.

A comprehensive study on the properties and implementation of glyoxylic-acetals in sodium-ion energy storage systems is presented. Electrolytes containing 1,1,2,2-tetramethoxyethane (tetramethoxyglyoxal, TMG), 1,1,2,2-tetraethoxyethane (tetraethoxyglyoxal, TEG) and a mixture of the latter with propylene carbonate (PC) exhibit increased thermal stabilities and higher flash points compared to classical electrolytes based on carbonates as solvents. Due to its favorable properties, 1 m NaTFSI in TEG/PC (3 : 7), has been selected and used for sodium-ion energy storage systems based on a Prussian Blue (PB) positive electrode and a hard carbon (HC) negative electrode. Compared to conventional electrolyte (based on a 1 : 1 mixture of ethylene carbonate, EC, and dimethyl carbonate, DMC), this glyoxylic-acetal electrolyte provides competitive capacity and prolonged cycle life. Postmortem XPS analysis indicates that the electrode-electrolyte interphases formed in presence of TEG are thicker and presumably more protective, inhibiting typical degradation processes of the electrodes. Furthermore, it is demonstrated that the suitable properties of TEG on the cycling stability can also be exploited for the construction of highly stable sodium-ion capacitors.