Pseudocapacitive Energy Storage Research Articles

Energy storage performance of thin film nanocrystalline vanadium oxide with fluorinated tin oxide current carrier electrode for solid-state transparent supercapacitors based on ionic liquid gel electrolyte

Energy-power performance of transparent supercapacitors based on sol gel processed nanometer thin nanocrystalline vanadium oxide film composite with fluorinated tin oxide (FTO) thin film electrode as current collector is investigated. The symmetric supercapacitor device configured in the solid-state form using the BMIMBF4 ionic liquid with LiClO4 dopant gel electrolyte shows high areal capacitance of 310 mFcm−2 attributed to pseudocapacitive energy storage mechanism with minimal ion kinetics limitation. The vanadium oxide electrode forms in the stoichiometric V2O5 and in the multivalent phases depending on the annealing process in film synthesis which impacts the specific energy-power performance. The supercapacitor based on V2O5 with distorted [VO5] pyramids alongside phases with V4+ and mixed V oxidation states exhibited high specific energy 7.7 μWhcm−2 at corresponding specific power of 64.7 μWcm−2 and significantly large specific power of 363.8 μWcm−2 at 1.5 V. High capacitance and energy-power is attributed to simultaneous redox reactions storing charges in the specific potential range to contribute to the pseudocapacitance. The stoichiometric V2O5 films in V5+ valence state show minimum loss of specific energy (1.20–0.89 μWhcm−2) at high power (40–132 μWcm−2) and much stable 75 ± 2% charge-discharge performance for 2000 cycles. The single valent V2O5 electrodes show sequential V5+/V4+ and V4+/V3+ redox processes and lesser stored charge but high cyclic stability. The significantly improved capacitive and specific energy-power parameters pertaining to a high >77% transmittance at 590 nm show the applicability of the V2O5 solid-state ionic liquid electrolyte based supercapacitor energy storage devices in futuristic transparent electronics.

Tuning the Electrochemical Performance of Titanium Carbide MXene by Controllable In Situ Anodic Oxidation

AbstractMXenes are a class of two‐dimensional (2D) transition metal carbides, nitrides and carbonitrides that have shown promise for high‐rate pseudocapacitive energy storage. However, the effects that irreversible oxidation have on the surface chemistry and electrochemical properties of MXenes are still not understood. Here we report on a controlled anodic oxidation method which improves the rate performance of titanium carbide MXene (Ti3C2Tx, Tx refers to ‐F, =O, ‐Cl and ‐OH) electrodes in acidic electrolytes. The capacitance retention at 2000 mV s−1 (with respect to the lowest scan rate of 5 mV s−1) increases gradually from 38 % to 66 % by tuning the degree of anodic oxidation. At the same time, a loss in the redox behavior of Ti3C2Tx is evident at high anodic potentials after oxidation. Several analysis methods are employed to reveal changes in the structure and surface chemistry while simultaneously introducing defects, without compromising electrochemically active sites, are key factors for improving the rate performance of Ti3C2Tx. This study demonstrates improvement of the electrochemical performance of MXene electrodes by performing a controlled anodic oxidation.

Tuning the Electrochemical Performance of Titanium Carbide MXene by Controllable In Situ Anodic Oxidation.

MXenes are a class of two-dimensional (2D) transition metal carbides, nitrides and carbonitrides that have shown promise for high-rate pseudocapacitive energy storage. However, the effects that irreversible oxidation have on the surface chemistry and electrochemical properties of MXenes are still not understood. Here we report on a controlled anodic oxidation method which improves the rate performance of titanium carbide MXene (Ti3 C2 Tx, Tx refers to -F, =O, -Cl and -OH) electrodes in acidic electrolytes. The capacitance retention at 2000 mV s-1 (with respect to the lowest scan rate of 5 mV s-1 ) increases gradually from 38 % to 66 % by tuning the degree of anodic oxidation. At the same time, a loss in the redox behavior of Ti3 C2 Tx is evident at high anodic potentials after oxidation. Several analysis methods are employed to reveal changes in the structure and surface chemistry while simultaneously introducing defects, without compromising electrochemically active sites, are key factors for improving the rate performance of Ti3 C2 Tx . This study demonstrates improvement of the electrochemical performance of MXene electrodes by performing a controlled anodic oxidation.

Rare earth nanostructures based on PrOx/CNT composites as potential electrodes for an asymmetric pseudocapacitor cell

Rare earth metal oxides have been widely used as pseudocapacitor electrodes owing to their unique physical and electronic properties. The present paper reports the synthesis and pseudocapacitor applications of praseodymium oxides, owing to their unique physical properties. PrOx/unzipped carbonnanotubes were synthesized following a hydrothermal approach. Detailed morphological as well as electrochemical analyses were performed to elucidate how the unique properties of PrOx affect the charge storage ability. Special emphasis was given on the effect of anion intercalation due to the surface oxygen vacancies in PrOx which would contribute towards the pseudocapacitive energy storage. Oxygen intercalation was exploited for the first time in fluorite crystals for fast energy storage and a specific capacitance value as high as 1099 F/g was obtained with the electrodes. An asymmetric supercapacitor prototype was also fabricated with a V2O5/graphene counter electrode and the energy and power density values obtained are as high as 52.08 Wh/kg and 2.9 kW/kg, respectively.

Commercial-Level Energy Storage via Free-Standing Stacking Electrodes

Summary Pseudocapacitors with a fast faradic redox reaction during the electrochemical charge/discharge process hold great promise for delivering high power and high energy. Here, we used emerging oxygenated carbon nitride (OCN) as a model for nitrogen- and oxygen-enriched carbons and found an anion-selective charging behavior in aqueous electrolytes. The N- and O-mediated hydroxyl anion-selective charging originates from inbuilt surface-positive electrostatic potential localized at heptazine and quaternary N atom structural units. The carbon atoms in heptazine adjacent to pyridinic N act as the electron transfer active sites for faradic pseudocapacitance. The OCN free-standing films (FSFs) were developed as current collector-free electrodes, and stacking OCN FSFs can confer a cell with loading weight over a commercial level. For demonstration, a symmetrical cell with 17.7 mg cm−2 delivers an energy of 140.9 mW h at a high power of 2,499.9 mW, and almost 100% capacitance retention after 20,000 charge/discharge cycles at 50 mA.

(Invited) 2D MXenes As Building Blocks for Assembly of High-Performance Electrodes for Electrochemical Energy Storage

Two-dimensional (2D) transition metal carbides/nitrides (MXenes) have shown promise for a range of applications due to their unique physical and chemical properties. The high electrical conductivity, 2D and layered structure, and rich surface chemistry of MXenes have sparked great interest in their properties as electrode materials for energy storage devices. MXenes are particularly of high interest because they can store charge in the bulk of the electrode through ultrafast intercalation of ions and fast and reversible redox reactions. To date, about 30 different MXenes with a general formula of Mn+1XnTx (M is a transition metal, X is carbon and/or nitrogen, n=1,2, and 3, and Tx represents different functional groups present at their surface) are synthesized by selective etching of the A layer atoms (i.e. Al) from MAX phases (e. g. Ti2AlC), a large group of layered ternary carbides and nitrides.1 However, similar to other 2D materials, the applications of MXenes in batteries and supercapacitors is dependent on their assembly into electrode structures with high electrical and ionic conductivities. The main focus of this talk is our group’s recent research on the assembly of MXene flakes into electrode structures with high energy and power densities. We have found that the electrochemical properties of MXene electrodes are highly dependent on the size of MXene flakes used in their fabrications.2,3 Controlling the flake size in the electrode structures leads to significant improvements in their specific capacitance and rate capability. Furthermore, our recent studies on the assembly of multilayer MXenes films and particles using electrostatic assembly methods will be discussed. The electrodes fabricated by these methods show superior chemical and electrochemical stabilities as well as energy storage capability.4 Finally, our work on 3D assembly of MXenes through a versatile 3D printing process will be presented. References (1) Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y. 2D Metal Carbides and Nitrides (MXenes) for Energy Storage. Nat. Rev. Mater. 2017, 2 (2), 16098. https://doi.org/10.1038/natrevmats.2016.98. (2) Kayali, E.; VahidMohammadi, A.; Orangi, J.; Beidaghi, M. Controlling the Dimensions of 2D MXenes for Ultrahigh-Rate Pseudocapacitive Energy Storage. ACS Appl. Mater. Interfaces 2018, 10 (31), 25949–25954. https://doi.org/10.1021/acsami.8b07397. (3) VahidMohammadi, A.; Moncada, J.; Chen, H.; Kayali, E.; Orangi, J.; A. Carrero, C.; Beidaghi, M. Thick and Freestanding MXene/PANI Pseudocapacitive Electrodes with Ultrahigh Specific Capacitance. J. Mater. Chem. A 2018, 6 (44), 22123–22133. https://doi.org/10.1039/C8TA05807E. (4) VahidMohammadi, A.; Mojtabavi, M.; Caffrey, N. M.; Wanunu, M.; Beidaghi, M. Assembling 2D MXenes into Highly Stable Pseudocapacitive Electrodes with High Power and Energy Densities. Adv. Mater. 2019, 31 (8), 1806931. https://doi.org/10.1002/adma.201806931.

An experimental and computational study of enhanced charge storage capacity of chemical vapor deposited Ni3S2-reduced graphene oxide hybrids

In the present approach, we have successfully synthesized wormlike Ni3S2 and Ni3S2-RGO hybrids on nickel (Ni) foam by a facile and highly reproducible one-step chemical vapor deposition (CVD) method. We have demonstrated that Ni3S2 and Ni3S2-RGO hybrids can be grown directly on the current collector of energy storage devices without using any binder, which may lead to serve industry for large-scale production of the material. We have studied the pseudocapacitive energy storage performance of the CVD grown Ni3S2 and its hybrids with variable concentration of RGO. The Ni3S2-RGO hybrid with 0.5 mg GO showed a maximum areal specific capacitance of 1.4 F·cm−2 at a current density of 1 mA·cm−2 (approximately 1124 Fg−1 at a current density of 1 Ag−1). The enhanced supercapacitive performance of Ni3S2-RGO predominantly due to its high surface area and high conductivity as compared to bare Ni3S2. The electrochemical impedance spectroscopic analysis revealed Ni3S2-RGO possess enhanced charge-transfer characteristics as compared to bare Ni3S2. Furthermore, Density Functional Theory (DFT) simulations infer the strong hybridization between C p orbital and Ni d orbital lead to enhanced electrochemical property and Density of States (DOS) is crucially responsible for the improved charge storage performance of Ni3S2-RGO hybrids.

Organic-inorganic all-pseudocapacitive asymmetric energy storage devices

Two-dimensional transition metal carbides (MXenes) have shown extraordinary promise for pseudocapacitive energy storage under negative potential in aqueous electrolytes, yet they lack matching positive electrodes. Here, we report an organic compound namely 2,5-dihydroxy-1,4-benzoquinone (DBQ), adsorbed on reduced graphene oxide (rGO) sheets as a positive electrode, which can deliver high capacitance (~500 F/g, ~800 F/cm3 at 2 mV/s), rate performance (83 F/g or 133 F/cm3 at 10 V/s), and remarkable cycle life (83% after 100,000 cycles), which is the highest for any reported discrete organic molecule. First-principle calculations were used to further understand the charge storage mechanism, find the preferred orientation of the adsorbed molecules, and to pinpoint the origin of the high pseudocapacitance and long cycle life. Optimized compositions of DBQ@rGO were paired with pseudocapacitive Ti3C2Tx MXene to manufacture devices composed of two entirely different classes of materials, where they electrochemically complement each other to expand the voltage window (and thus energy density) in aqueous electrolytes. As manufactured devices delivered energy density of 40 W h/kg at a power density of 2.9 kW/kg and capacitance retention of over 90% after 10,000 cycles.

Stable high-voltage aqueous pseudocapacitive energy storage device with slow self-discharge

We demonstrate an asymmetric supercapacitor in a potassium acetate-based water-in-salt electrolyte, where 2-D titanium carbide MXene and manganese oxide were used as negative and positive electrode materials, respectively. Use of water-in-salt electrolyte enables the assembled asymmetric device to be operated up to a cell voltage of 2.2 V, which overcomes the limited cell voltage issue in aqueous pseudocapacitors (1.2 - 1.4 V). This cell shows excellent rate capability (~48%) between 5 and 100 mV s-1 and good stability (~93%) throughout 10,000 charge-discharge cycles (at 1 A g-1) and 25 h voltage-hold at 2.2 V, which is competitive when compared with the performance of known asymmetric supercapacitors designed with activated carbon electrodes and fluorinated-imide based water-in-salt electrolytes. Moreover, our device shows slower self-discharge and ~32% higher volumetric energy density than activated carbon-based supercapacitors and is promising for applications where volumetric energy density is critical.

Nanoscale Engineering of Graphene‐Viologen Based 3D Covalent Organic Polymer Interfaces Leading to Efficient Charge‐Transfer for Pseudocapacitive Energy Storage

AbstractViologen based covalent organic polymer (COP) interfaced with graphene at the nanoscale showed pseudocapacitive energy storage associated with redox‐active moieties. The positively charged viologen moieties endow charge storage as well as easy wetting of the electrode. The non‐conducting COP grown over conducting graphene enhances its performance by extensive interface formation. All the redox states of viologen moieties can be reversibly attained without significant polarization when it is interfaced with graphene. However, the COP (devoid of graphene) shows irreversible/pseudo‐reversible behavior due to lack of electron conducting pathways. The intimate and dense conducting pathways in graphene‐COP composites facilitates the charge transfer across interface leading to effective and reversible participation of redox moieties across the entire range over 1000 cycles. Also, we show that the strategy being universal in nature and aid in electron transfer in dense 3D networks which lacks traditional π‐π stacking of 2D crystalline networks (COF).

MXene Electrode Materials for Electrochemical Energy Storage: First-Principles and Grand Canonical Monte Carlo Simulations

MXenes are a novel class of two dimensional materials, discovered by Barsoum and Gogotsi [M. Naguib, J. Come, B. Dyatkin, V. Presser, P. Taberna, P. Simon, M. W. Barsoum, and Y. Gogotsi, Electrochemistry Communications 16, 61-64 (2012); B. Anasori, M. R. Lukatskaya, and Y. Gogotsi, Nature Reviews Materials vol. 2, 16098 (2017)]. Their large specific surface area and the tunability of their physicochemical properties as a function of the transition metal and surface terminal group make them a unique design platform for various applications, a primary example of which is pseudocapacitive energy storage. However, there is still incomplete understanding of how the transition metal chemistry and stoichiometry, and the surface termination affect charge storage mechanisms in MXene. In this study, we have performed systematic first-principles calculations for bulk MXene and found that the atomic charge of the metal cations, which is related to their valence, decreases across the d-electron metal series. Electronic-structure indicators of performance are examined to understand the energy storage behavior, whereby charges are stored between the terminal groups and adsorbing cations. Importantly, we found that the differential Bader charges show good agreement with theoretical capacitances and are useful in predicting charge storage trends in MXene-based pseudocapacitors. Furthermore, we have performed first-principles and grand canonical Monte Carlo calculations for the slab systems, finding that the solvent plays a critical role in enhancing the pseudocapacitive response.

Organometallic-Derived Carbon (ODC)-Metal Nano-Oxide Composites as Improved Electrode Materials for Supercapacitors.

In the search for the new generation of electrochemical energy storage materials, a novel and straightforward synthetic route for porous carbons and metal oxide nanoparticle composites based on the chlorination of the organometallic compounds Ni(C5H5)2 and Mn(C5H7O2)2 at moderate temperatures, followed by hydrothermal treatment, has been developed. Electrochemical measurements in a three-electrode configuration show that, in both composites NiO@ODC and Mn3O4@ODC, a synergistic effect between the capacitive and pseudocapacitive energy storage mechanisms is observed, thereby improving their electrochemical performance vs pure carbon materials. Electrochemical evaluation of symmetric cells gave gravimetric capacitances of 124 and 130 F g-1 for NiO@ODC and Mn3O4@ODC, respectively. However, the porous structure of the carbon matrix and the higher conductivity of Mn3O4, together, were found to be responsible for the superior electrochemical performance of Mn3O4@ODC.

Metal Sulfide/Mxene Based Nanostructured Electrode Materials for High-Performance Supercapacitors

2D transition metal carbides and nitrides, known as MXenes, are an emerging class of 2D materials with a wide spectrum of potential applications, due to their unique layered structure, high surface area, remarkable chemical stability, and electrical conductivity for energy storage. The hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions is the key for high-rate pseudocapacitive energy storage in MXene electrodes. However, the low capacity of MXene electrodes limits their further application in ECs. In this study we developed the electrode materials comprised of titanium carbide MXene (Ti3C2Tx) decorated with MWCNTs and Metal Sulfide, which are both 2D layered materials that contribute to the fast ion diffusion in the interdigitated electrode architecture. The nanocomposite samples have been characterized by XRD, SEM, TEM, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements. Due to the synergistic effect between Metal sulfide and Ti3C2 matrix, the nanocomposite exhibits a superior capacitance and shows excellent cycling stability with capacitance up to several hundred cycles. These results show that pseudocapacitive MXene based negative electrodes exhibits excellent properties in asymmetric electrochemical capacitors, leading to an increased energy density.

Proton-assisted creation of controllable volumetric oxygen vacancies in ultrathin CeO2\u2212x for pseudocapacitive energy storage applications

Two-dimensional metal oxide pseudocapacitors are promising candidates for size-sensitive applications. However, they exhibit limited energy densities and inferior power densities. Here, we present an electrodeposition technique by which ultrathin CeO2−x films with controllable volumetric oxygen vacancy concentrations can be produced. This technique offers a layer-by-layer fabrication route for ultrathin CeO2−x films that render Ce3+ concentrations as high as ~60 at% and a volumetric capacitance of 1873 F cm−3, which is among the highest reported to the best of our knowledge. This exceptional behaviour originates from both volumetric oxygen vacancies, which enhance electron conduction, and intercrystallite water, which promotes proton conduction. Consequently, simultaneous charging on the surface and in the bulk occur, leading to the observation of redox pseudocapacitive behaviour in CeO2−x. Thermodynamic investigations reveal that the energy required for oxygen vacancy formation can be reduced significantly by proton-assisted reactions. This cyclic deposition technique represents an efficient method to fabricate metal oxides of precisely controlled defect concentrations and thicknesses.

Pillaring-Effect Induced Ultrahigh-Rate Pseudocapacitive Energy Storage Based on Layered Double Hydroxide Nanoplate Arrays

Two-dimensional layered materials with large interlayer distance to guarantee facilitated electrolyte diffusion are regarded as good candidates for high-performance supercapacitors. In this work, NiAl-layered double hydroxide (LDH) nanoplate arrays with NO3–, pentanesulfonate (PS) and dodecanesulfonate (DS) ions in their interlayer were synthesized, and the effect of different pillaring anions on the electrochemical properties of LDH electrodes was investigated. Combined studies of experiments and theoretical calculations show that the pillaring-effect of long-chain molecules drastically reduces the ion transport resistance between the electrode and electrolyte. An enhanced specific capacitance (1125 F g–1 at 1 A g–1) and a ultrahigh-rate capability (72.8% retention at 200 A g–1) were achieved for the NiAl(DS)-LDH electrode. Such an electrode was further assembled into an all-solid-state supercapacitor, which exhibits a significantly improved energy and power densities as well as long-term stability. This...

Revealing the Pseudo‐Intercalation Charge Storage Mechanism of MXenes in Acidic Electrolyte

AbstractSince the discovery of Ti3C2Tx in 2011, the family of two‐dimensional transition metal carbides, carbonitrides, and nitrides (collectively known as MXenes) has quickly attracted the attention of those developing energy storage applications such as electrodes for supercapacitors with acidic aqueous electrolytes. The excellent performance of these MXenes is attributed to a pseudocapacitive energy storage mechanism, based on the non‐rectangular shape of cyclic voltammetry curves and changes in the titanium oxidation state detected by in situ X‐ray absorption spectroscopy. However, the pseudocapacitive mechanism is not well understood and no dimensional changes due to proton insertion have been reported. In this work, in situ X‐ray diffraction and density functional theory are used to investigate the charge storage mechanism of Ti3C2Tx in 1 m H2SO4. Results reveal that a 0.5 Å expansion and shrinkage of the c‐lattice parameter of Ti3C2Tx occur during cycling in a 0.9 V voltage window, showing that the charge storage mechanism is intercalation pseudocapacitance with implication for MXene use in energy storage and electrochemical actuators.

Enhanced pseudocapacitive energy storage properties of Nb 2 O 5 /C core‐shell structures with the surface modification

Pseudocapacitive energy storage is an attractive technology as it can achieve high energy density at high rate conditions. However, its practical application has been an issue because of low electrical conductivity of nanoscale electrode materials. Surface coating is an effective way to enhance the electrochemical properties of the electrochemical energy storage system by helping electron transfer between the electrode material and current collector. In order for surface coating technologies to be applied to pseudocapacitive energy storage field, providing fast ions and electrolyte transport through the coating layer as well as high electrical conductivity is essential because pseudocapacitors aim for high rate charge/discharge capability. In this paper, the Nb2O5/carbon core-shell structure is developed to meet these requirements. Simple microwave-assisted method is applied to create nanoscale (approximately 5 nm) conductive carbon layer on the surface of bare Nb2O5 nanoparticles, and the high power capability of Nb2O5/carbon core-shell is improved further by oxidation process providing open structure for electrolyte and ion diffusion. Thick electrode architecture containing oxidized Nb2O5/carbon core-shell shows superior high rate performance as capacities of 215 C g−1 are obtained at a 50 mV s−1 scan rate.

(Invited) Characterization of Mxene/Graphene Supercapacitor Yarns Using Time and Frequency Models

Supercapacitors (SC) turned out to be efficient devices for electrochemical energy storage thanks to their high energy density vs. classical surface capacitors- and high power density vs. batteries. In the absence of electrode components prone to react with the electrolyte, charges are stored in the pore space in a purely capacitive way under the form of electrical double-layer EDL at the solid-liquid interface [1, 2]. In several cases, additional pseudo-capacitive energy storage appears through redox reactions, underpotential deposition of metals or -to a lower extent- to ions intercalation [3]. This may also lead eventually to charge losses. The optimization of supercapacitors can be facilitated using electrode and electrolyte models that capture the physics at the local scale to better understand the charge and mass transfer phenomena governing their behavior. Indeed, SC characterization is still based on relatively simple models based on macroscopic parameters that are generally determined according to voltage evolution during constant current charge and discharge. The electrical equivalent circuit (EEC) often includes only an equivalent series resistance and a capacitance tied up to the average pore population governing their dynamic behavior [1, 2]. In order to account for pore size distribution within the electrodes, EEC consisting of two or more branches of resistances and capacitances in series are sometimes used [1, 2], although some parameters may appear to be correlated. In addition to the charge/discharge behavior, Electrochemical Impedance Spectroscopy (EIS) is another extensively used tool for characterizing electrochemical systems [2, 4]. Since single or multiple branches RC EECs based on surface reaction assumption are not adapted to the frequency domain, SC impedance spectra are usually modelled using Transmission Line Models (TLM) [2, 4] accounting for the volumetric character of the electrodes. TLM are however rarely applied for SC characterization in the time domain, and in the few cases that exist, not under the form of simple analytical expressions [2]. In order to yield a basis for SC modelling and characterization, we present a parameter study comparing different model approaches. They can be applied to purely capacitive and pseudo-capacitive SC with the ultimate objective to derive a model able to capture part of the physics at the local scale and to describe the SC behavior in both the time and frequency domains. As an example we compare the parameters identified in the time and frequency domains on different types of supercapacitor yarns developed by Gao’s group at NCSU [5], including pure double-layer capacitive reduced Graphene Oxide (rGO) yarn SCs and titanium carbide (MXene) – rGO yarn SCs presenting an additional pseudo-capacitive behavior and potentially ion intercalation [3]. In these flexible 2D material based devices, the active components, 2D flakes, are stacked together in distinct morphologies (either smoothly packed with grooves on fiber surfaces, or highly wrinkled via cation crosslinking), wherein different porosity and pore distributions exist. Experimental results as reported by Dr. Gao’s group exhibit dramatically different specific capacitances and rate capabilities. Figure Caption Left: Scheme of the MXene – rGO yarn SC structure; right: Nyquist plot of the MXene-rGO Sc impedance data and interpolation curves obtained with a purely capacitive (dashed line) and a pseudo-capacitive (full line) TLM model.

A polymer-direct-intercalation strategy for MoS2/carbon-derived heteroaerogels with ultrahigh pseudocapacitance

The intercalation strategy has become crucial for 2D layered materials to achieve desirable properties, however, the intercalated guests are often limited to metal ions or small molecules. Here, we develop a simple, mild and efficient polymer-direct-intercalation strategy that different polymers (polyethyleneimine and polyethylene glycol) can directly intercalate into the MoS2 interlayers, forming MoS2-polymer composites and interlayer-expanded MoS2/carbon heteroaerogels after carbonization. The polymer-direct-intercalation behavior has been investigated by substantial characterizations and molecular dynamic calculations. The resulting composite heteroaerogels possess 3D conductive MoS2/C frameworks, expanded MoS2 interlayers (0.98 nm), high MoS2 contents (up to 74%) and high Mo valence (+6), beneficial to fast and stable charge transport and enhanced pseudocapacitive energy storage. Consequently, the typical MoS2/N-doped carbon heteroaerogels exhibit outstanding supercapacitor performance, such as ultrahigh capacitance, remarkable rate capability and excellent cycling stability. This study offers a new intercalation strategy which may be generally applicable to 2D materials for promising energy applications.

Influences from solvents on charge storage in titanium carbide MXenes

Pseudocapacitive energy storage in supercapacitor electrodes differs significantly from the electrical double-layer mechanism of porous carbon materials, which requires a change from conventional thinking when choosing appropriate electrolytes. Here we show how simply changing the solvent of an electrolyte system can drastically influence the pseudocapacitive charge storage of the two-dimensional titanium carbide, Ti3C2 (a representative member of the MXene family). Measurements of the charge stored by Ti3C2 in lithium-containing electrolytes with nitrile-, carbonate- and sulfoxide-based solvents show that the use of a carbonate solvent doubles the charge stored by Ti3C2 when compared with the other solvent systems. We find that the chemical nature of the electrolyte solvent has a profound effect on the arrangement of molecules/ions in Ti3C2, which correlates directly to the total charge being stored. Having nearly completely desolvated lithium ions in Ti3C2 for the carbonate-based electrolyte leads to high volumetric capacitance at high charge–discharge rates, demonstrating the importance of considering all aspects of an electrochemical system during development.