Shell membrane-aided synthesis of 3D porous flower-like Co2(OH)3Cl-MnO2 hybrid composites for high performance supercapacitors

Three-dimensional (3D) flower-like Co₂(OH)₃Cl-MnO₂ nanostructures were fabricated inside eggshell through a facile and effective method. Inspired by semipermeable membranes, a shell membrane was selected as an interface for ion diffusion. In specific, an eggshell with shell membrane was employed as a multifunctional reactor to separate the components of the precursor solution. OH^- ion diffusion was performed through porous eggshell membrane. The electrochemical measurements demonstrated that the hybrid composite achieves high capacitance 3.709?F/cm² at 1?mA/cm² (2061?F/g with the mass loading of 1.8?mg/cm²) and an excellent cycling stability (71% specific capacitance retained after 5000 cycle numbers), exhibiting a superior electrochemical performance compared to pure Co₂(OH)₃Cl or MnO₂. Moreover, an asymmetric supercapacitor was assembled by Co₂(OH)₃Cl-MnO₂-2 and activated carbon as positive and negative electrode, respectively (Co₂(OH)₃Cl-MnO₂-2//AC ASC), which exhibits high capacitance (134.8?F/g at 0.2?A/g), excellent energy density (42.2?W?h/kg at 150.3?W/kg), and remarkable cycling stability (80% capacitance retention after consecutive 5000 cycle numbers).     

Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials

Manganese oxide was synthesized and dispersed on carbon nanotube (CNT) matrix by thermally decomposing manganese nitrates. CNTs used in this paper were grown directly on graphite disk by chemical vapor deposition technique. The capacitive behavior of manganese oxide/CNT composites was investigated by cyclic voltammetry and galvanostatic charge–discharge method in 1 M Na₂SO₄ aqueous solutions. When the loading mass of MnO₂ is 36.9 μg cm^− 2, the specific capacitance of manganese oxide/CNT composite (based on MnO₂) at the charge–discharge current density of 1 mA cm^− 2 equals 568 F g^− 1. Additionally, excellent charge–discharge cycle stability (ca. 88% value of specific capacitance remained after 2500 charge–discharge cycles) and power characteristics of the manganese oxide/CNT composite electrode can be observed. The effect of loading mass of MnO₂ on specific capacitance of the electrode has also been investigated.     

Preparation and electrochemical capacitance of RuO2/TiO2 nanotubes composites

In this paper, RuO₂/TiO₂ nanotubes composites were synthesized by loading various amounts of RuO₂ on TiO₂ nanotubes. The symmetric supercapacitors based on these nanocomposites were fabricated by using gel polymer PVA-H₃PO₄-H₂O as electrolyte. The electrochemical capacitance performance of the nanocomposites in these supercapacitors was investigated by current-potential responses, galvanostatic charge-discharge tests and electrochemical impedance spectroscopy. The results show that the three dimensional nanotube network of TiO₂ offers a solid support structure for active materials RuO₂, allows the active material to be readily accessible (available) for electrochemical reactions, and improves the efficiency of the active materials. A maximum specific capacitance of 1263 F/g was obtained for the RuO₂ which was loading on TiO₂ nanotubes.     

Cauliflower-like poly(3,4-ethylenedioxythipohene)/nanocrystalline cellulose/manganese oxide ternary nanocomposite for supercapacitor

A cauliflower-like ternary nanocomposite of poly(3,4-ethylenedioxythipohene)/nanocrystalline cellulose/manganese oxide (PEDOT/NCC/MnO₂) was synthesized using one-step electropolymerization technique. The effect of manganese (Mn) concentration on the supercapacitive performance was investigated. The structural and morphology studies were conducted using field emission scanning electron microscope, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction. The morphology of ternary nanocomposite at an optimized concentration of Mn resembles the cauliflower-like structure. The two-electrode electrochemical analysis of a ternary nanocomposite PEDOT/NCC/MnO₂ exhibited a higher specific capacitance of 144.69 F/g at 25 mV/s in 1.0 M potassium chloride compared to PEDOT/NCC(63.57 F/g). PEDOT/NCC/MnO₂ ternary nanocomposite was able to deliver a specific power of 494.9 W/kg and 10.3 Wh/kg of specific energy at 1 A g⁻¹ and retained 83% of initial capacitance after 2,000 cycles. These promising results from the incorporation of Mn displayed great prospective in developing PEDOT/NCC/MnO₂ as an electrode material for supercapacitor.     

Hydrothermal synthesis and characterization studies of α-Fe2O3/MnO2 nanocomposites for energy storage supercapacitor application

In this present study, semiconductor magnetic α-Fe₂O₃/MnO₂ nanocomposites (NCs) were prepared by a facile hydrothermal (HT) method. The crystallographic structure, morphology, chemical configuration and magnetic features were analysed by X-ray powder diffraction (XRD), high resolution scanning electron microscope (HR-SEM), energy dispersive X-ray analysis (EDX), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) analyses. The as-prepared NCs were used as an electrode in energy storing supercapacitor was systematically examined. The electrochemical deeds of α-Fe₂O₃/MnO₂ NCs was analysed by cyclic voltammetry (C–V) and galvanostatic charge–discharge (GCD) tests. The CV analysis of the NCs electrode showed a distinctive pseudocapacitive behaviour in 1 M KOH solution. The NCs electrode reveals enhanced specific capacitance compared to plain α-Fe₂O₃ and MnO₂ nanoparticles (NPs) and generates high specific capacitance of 216.35 Fg⁻¹. Pseudocapacitor obtains of energy density 135.42 Wh kg⁻¹ at power density of 6.399 kW kg⁻¹, indicating the as-prepared α-Fe₂O₃/MnO₂ NCs shows noteworthy high-energy, specific capacitance, power densities and long-standing cyclic stability with 89.2% of preliminary capacitance reserved at 1A g⁻¹ after 10000 cycles in judgement with the pure α-Fe₂O₃ and MnO₂ NPs electrode. The α-Fe₂O₃/MnO₂ NCs electrode having noteworthy electrochemical characteristics performance renders promising applications in energy storing systems.     

Flexible electrodes based on polypyrrole/manganese dioxide/polypropylene fibrous membrane composite for supercapacitor

The composites of polypyrrole/manganese dioxide/polypropylene fibrous films (PPy/MnO₂/PPF) have been prepared in situ through chemical oxidation polymerization by using the mixture of FeCl₃·6H₂O and MnO₂ adsorbed on PPF as oxidant in the atmosphere of pyrrole vapor at room temperature. The morphologies and structures of the composites are investigated by using scanning electron microscope and X-ray diffraction spectroscopy. The properties of the capacitor cells assembled by the composites of PPy/MnO₂/PPF are evaluated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy methods. The results reveal that the morphologies, conductivities and capacitance performance of the composites are influenced strongly by the content of MnO₂ in the solution of oxidant. The capacitors assembled by PPy/MnO₂/PPF exhibit the property of quick charge/discharge, and the highest specific capacitance of about 110 F g⁻¹ is obtained when the PPy/MnO₂ content in the composite is about 17.4%.     

Electrochemical method integrating exfoliation and in-situ growth to synthesize MoS2 nanosheets/MnO2 heterojunction for performance-enhanced supercapacitor

Two-dimensional (2D) molybdenum disulfide (MoS₂) nanomaterials have become one of the promising options for constructing excellent supercapacitors. However, the application of MoS₂ materials is limited by low energy density, and the difficulty of large-scale and low-cost preparation seriously hinders its practical application in the field of energy storage. Here, the exfoliation of the MoS₂ nanosheets and the loading of MnO₂ nanoparticles on the MoS₂ nanosheets are realized in one step by electrochemical method. A series of characterization methods have fully confirmed that the electrochemical method has successfully prepared the MoS₂ nanosheet/MnO₂ (MoS₂ NS/MnO₂) heterojunction. The experimental results show that the MoS₂ NS/MnO₂ heterojunction has better electrochemical performance than a single MoS₂ nanosheet. It has a good capacitance even in a neutral solution, and its specific capacitance is 275 F g^?1 at a current density of 2 A g^?1. In addition, a supercapacitor device based on MoS₂ NS/MnO₂ heterojunction was constructed, which not only exhibited excellent capacitive performance, but also exhibited 10,000 charge-discharge cycle stability under 10 A g^?1 conditions. This work provides an experimental basis for the preparation of 2D nanosheets and the large-scale preparation of functionalized 2D material heterojunctions by electrochemical methods.     

Hierarchical MnO2@NiCo2O4@Ti3SiC2/carbon cloth core-shell structure with superior electrochemical performance for all solid-state supercapacitors

Core-shell hierarchical structured composites have demonstrated great advantages in numerous energy storage devices. In particular, structured composites with rationally structural components and controllable morphology are the most effective in enhancing electrochemical properties. In this work, MnO₂@NiCo₂O₄@Ti₃SiC₂/CC (carbon cloth) core-shell hierarchical structured composites were designed and successfully synthesized via electrospinning followed by a two-step hydrothermal reaction. The Ti₃SiC₂/CC nanofibers and core-shell nanoarrays were able to improve the specific capacitance and cycling stability. In the three-electrode system, the specific capacitance of MnO₂@NiCo₂O₄@Ti₃SiC₂/CC was observed as 1938.2 F/g at a current density of 1 A/g, while the rate capability retention was observed as 81.7% between 1 and 10 A/g. Furthermore, a superior cycling stability was observed following 5000 cycles with a specific capacitance retention rate of 55.4%. Employing MnO₂@NiCo₂O₄@Ti₃SiC₂/CC as the all solid-state supercapacitor positive electrode exhibited a high energy density of 58.0 W h/kg at the power density of 800 W/kg. Results demonstrate the potential of the MnO₂@NiCo₂O₄@Ti₃SiC₂/CC as an electrode material with phenomenal electrochemical properties for supercapacitors.     

Hydrothermal synthesis of MnO<Subscript>2</Subscript>/CNT nanocomposite with a CNT core/porous MnO<Subscript>2</Subscript> sheath hierarchy architecture for supercapacitors

MnO₂/carbon nanotube [CNT] nanocomposites with a CNT core/porous MnO₂ sheath hierarchy architecture are synthesized by a simple hydrothermal treatment. X-ray diffraction and Raman spectroscopy analyses reveal that birnessite-type MnO₂ is produced through the hydrothermal synthesis. Morphological characterization reveals that three-dimensional hierarchy architecture is built with a highly porous layer consisting of interconnected MnO₂ nanoflakes uniformly coated on the CNT surface. The nanocomposite with a composition of 72 wt.% (K_0.2MnO₂·0.33 H₂O)/28 wt.% CNT has a large specific surface area of 237.8 m²/g. Electrochemical properties of the CNT, the pure MnO₂, and the MnO₂/CNT nanocomposite electrodes are investigated by cyclic voltammetry and electrochemical impedance spectroscopy measurements. The MnO₂/CNT nanocomposite electrode exhibits much larger specific capacitance compared with both the CNT electrode and the pure MnO₂ electrode and significantly improves rate capability compared to the pure MnO₂ electrode. The superior supercapacitive performance of the MnO₂/CNT nancomposite electrode is due to its high specific surface area and unique hierarchy architecture which facilitate fast electron and ion transport.     

Electrochemical fabrication of polyaniline/MnO2/graphite felt as free‐standing,flexible electrode for supercapacitors

Polyaniline/MnO₂/graphite felt (PMGF) composite, which can be used as a novel free‐standing, flexible electrode for supercapacitors, was fabricated via a facile electrochemical method. Polyaniline/graphite felt (PANI/GF) electrode was prepared by electropolymerization of PANI onto the GF. Subsequently, manganese dioxide (MnO₂) was electrodeposited on the surface of the PANI/GF electrode to prepare PMGF electrode. The microstructure and morphology of the as‐prepared samples were characterized by Fourier transform infrared spectra, X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. Specific surface area was examined using N₂ adsorption/desorption test. Cyclic voltammogram, chronopotentiometry techniques and electrochemical impedance spectroscopy were introduced to investigate the electrochemical performance of the composites. The PMGF electrode exhibited specific capacitance as high as about 630 F g⁻¹ at the current density of 0.5 A g⁻¹, which is much higher than that of PANI/MnO₂ composites reported previously. The high specific capacitance of PMGF may be attributed to the fact that the porous GF is a good conductive matrix for the dispersion of PANI/MnO₂ and it can facilitate easy access of electrolytes to the electrode, which results in enhancement of the electrochemical performance of the composite. Moreover, the specific capacitance of PMGF is much larger than that of MnO₂/GF (MGF), which may be ascribed to the participant of PANI, which contributes additional pseudocapacitance and electron transport path. POLYM. COMPOS., 34:819–824, 2013. © 2013 Society of Plastics Engineers     

Facile synthesis of manganese–copper–zirconium mixed oxide nanocomposites: A potential electrode material for high performance supercapacitors

Amorphous MnO₂/CuO/ZrO₂nanocompositeswere synthesized by facile hydrothermal method at 120 °C. The nanomaterialswere prepared from KMnO₄ with Mn, Cu and Zr aqueous salts. The mixed oxide active materials were produced through three different molar ratios. The synthesized nanocomposites demonstrated a phase mixer of tetragonal-MnO₂, monoclinic-CuO and tetragonal-ZrO₂. The ultrahigh specific capacitance of 1964F/g at a current density of 1A/g was achieved for the nanocomposite MnO₂/CuO /ZrO₂synthesized at a mole ratio 1:2:1. Based on the electrochemical performance; the nanomaterial synthesized at higher mole percentage of copper has shown a high-rate capability of 20.93 Wh/kg at constant power discharge of 4013.10 W/kg. These findings interrogated that Manganese Copper, Zirconium mixed nanocomposite is proved to be an excellent electrode material for energy storage devices.     

Influences of graphene oxide support on the electrochemical performances of graphene oxide-MnO2 nanocomposites

MnO₂ supported on graphene oxide (GO) made from different graphite materials has been synthesized and further investigated as electrode materials for supercapacitors. The structure and morphology of MnO₂-GO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Nitrogen adsorption-desorption. As demonstrated, the GO fabricated from commercial expanded graphite (denoted as GO(1)) possesses more functional groups and larger interplane gap compared to the GO from commercial graphite powder (denoted as GO(2)). The surface area and functionalities of GO have significant effects on the morphology and electrochemical activity of MnO₂, which lead to the fact that the loading amount of MnO₂ on GO(1) is much higher than that on GO(2). Elemental analysis performed via inductively coupled plasma optical emission spectroscopy confirmed higher amounts of MnO₂ loading on GO(1). As the electrode of supercapacitor, MnO₂-GO(1) nanocomposites show larger capacitance (307.7 F g^-1) and better electrochemical activity than MnO₂-GO(2) possibly due to the high loading, good uniformity, and homogeneous distribution of MnO₂ on GO(1) support.     

Mn3O4 nanoparticles embedded into graphene nanosheets: Preparation, characterization, and electrochemical properties for supercapacitors

Mn₃O₄/graphene nanocomposites were synthesized by mixing graphene suspension in ethylene glycol with MnO₂ organosol, followed by subsequent ultrasonication processing and heat treatment. The as-prepared product consists of nanosized Mn₃O₄ particles homogeneously distributed on graphene nanosheets, which has been confirmed by field emission scanning electron microscopy and transmission electron microscopy analysis. Atomic force microscope analysis further identified the distribution of dense Mn₃O₄ nanoparticles on graphene nanosheets. When used as electrode materials in supercapacitors, Mn₃O₄/graphene nanocomposites exhibited a high specific capacitance of 175 F g⁻¹ in 1 M Na₂SO₄ electrolyte and 256 F g⁻¹ in 6 M KOH electrolyte, respectively. The enhanced supercapacitance of Mn₃O₄/graphene nanocomposites could be ascribed to both electrochemical contributions of Mn₃O₄ nanoparticles, functional groups attached to graphene nanosheets, and significantly increased specific surface area.     

Manganese dioxide/cobalt tungstate/ nitrogen-doped carbon nano-onions nanocomposite as new supercapacitor electrode

Application of the electrochemical supercapacitors is one way to store energy, which has widely attracted scientists worldwide. In this regard, in the present work, a triple-segment nanocomposite of manganese dioxide, cobalt tungstate, and nitrogen-doped carbon nano onions (MnO₂/CoWO₄/NCNO) was prepared based on a facile chemical method. The synthesized nanomaterials were analyzed by X-ray diffraction, infrared, as well as scanning electron microscopy. The supercapacitive properties of the synthesized nanomaterials were checked using electrochemical impedance spectroscopy, galvanostatic charge-discharge and voltammetry methods. The value of specific capacitor for the prepared electrodes containing MnO₂, CoWO₄, NCNO, MnO₂/CoWO₄, and MnO₂/CoWO₄/NCNO in an alkaline electrolyte (KOH: 6 M) at a current density of 2 A/g have obtained as 209, 97, 170, 413, and 536 F/g, respectively (Three-electrode system). Also, the retained stability of the MnO₂/CoWO₄/NCNO electrode after 3000 consecutive charges and discharges was 96%. The supercapacitive behavior and excellent capacitance of the MnO₂/CoWO₄/NCNO electrode was due to the synergistic effects of MnO₂, CoWO₄, and NCNO. A two-electrode symmetric supercapacitor assembled device is fabricated and exhibits a high specific capacitance of 1036 F/g at 2 A/g. It shows that the MnO₂/CoWO₄/NCNO composite provides potential applications in energy storage.     

A Study of Carbon Nanofibers and Active Carbon as Symmetric Supercapacitor in Aqueous Electrolyte: A Comparative Study

Symmetric supercapacitors are fabricated by carbon nanofibers (CNF) and activated carbon (AC) using similar proportions of 7 wt% polyvinylidene fluoride (PVDF) polymer binder in an aqueous electrolyte. In this study, a comparison of porous texture and electrochemical performances between CNFs and AC based supercapacitors was carried out. Electrodes were assembled in the cell without a current collector. The prepared electrodes of CNFs and AC present Brunauer-Emmett-Teller (BET) surface area of 83 and 1042 m²/g, respectively. The dominant pore structure for CNFs is mesoporous while for AC is micropore. The results showed that AC provided higher specific capacitance retention up to very fast scan rate of 500 mV/s. AC carbon had a specific capacitance of 334 F/g, and CNFs had 52 F/g at scan rate 5 mV/s in aqueous solution. Also, the results indicate the superior conductivity of CNFs in contrast to AC counterparts. The measured equivalent series resistance (ESR) showed a very small value for CNFs (0.28 Ω) in comparison to AC that has an ESR resistance of (3.72 Ω). Moreover, CNF delivered higher specific power (1860 W/kg) than that for AC (450 W/kg). On the other hand, AC gave higher specific energy (18.1 Wh/kg) than that for CNFs (2 Wh/kg).This indicates that the AC is good for energy applications. Whereas, CNF is good for power application. Indeed, the higher surface area will lead to higher specific capacitance and hence higher energy density for AC. For CNF, lower ESR is responsible for having higher power density.Both CNF and AC supercapacitor exhibit an excellent charge-discharge stability up to 2500 cycles.     

Potentiostatically deposited polypyrrole/graphene decorated nano-manganese oxide ternary film for supercapacitors

A simple method based on potentiostatic polymerization was developed for the preparation of ternary manganese oxide-based nanocomposite films. The ternary nanocomposites, which were characterized using x-ray diffraction spectroscopy and x-ray photoelectron spectroscopy, showed that the manganese oxide within the film consisted of MnO₂ and Mn₂O₃. Electrochemical measurements showed that the ternary nanocomposite electrode exhibited high specific capacitance (up to 320.6 F/g), which was attributed to the morphology of a polypyrrole/graphene/manganese-oxide (PPy/GR/MnO_x) ternary nanocomposite. The experimental approach maximized the pseudocapacitive contribution from redox-active manganese oxide (MnO_x) and polypyrrole (PPy), as well as the electrochemical double layer capacitive (EDLC) characteristic from graphene (GR) sheets. Long cyclic measurements indicated that the specific capacitance of the ternary nanocomposite film could retain 93% of its initial value over 1000 charge/discharge cycles, in the potential range of −0.2 to 0.7 V versus silver/silver chloride electrode (Ag/AgCl).     

Self-standing manganese dioxide/graphene carbon nanotubes film electrode for symmetric supercapacitor with high energy density and superior long cycling stability

The low capacitance utilization and capacitance fading of manganese dioxide (MnO₂) is mainly due to poor electro-conductivity and irreversible phase transform. This work proposes a new method of designing hierarchical and binder-free electrode based on MnO₂ material for stable supercapacitor with high specific capacitance. Herein, we fabricated the self-standing electrode of MnO₂ on nitrogen-doped graphene and single wall carbon nanotubes (SWCNTs) self-standing film (NGCF) by electrochemical deposition. As a result, as-prepared MnO₂/NGCF cathode showed excellent electrochemical performance of 489.7 F g^-1 at 1 A g^-1. Assembled symmetric aqueous supercapacitor (SC) manifests high voltage of 2.4 V and presents excellent high energy density of 106.7 Wh kg^-1 at 1200 W kg^-1 and outstanding long-life stability without no decay after 10 000 charge-discharge circuits. This work proposes a new view of designing hierarchical and binder-free electrode with high energy density and long cycling stability based on MnO₂ material for stable symmetric supercapacitor.     

Polyaniline/MnO2 composite with high performance as supercapacitor electrode via pulse electrodeposition

A method of pulse electrodeposition was proposed to synthesize polyaniline (PANI)/MnO₂ composite in aniline, H₂SO₄, and MnSO₄ aqueous solution. The PANI/MnO₂ composite has rod‐like structure and MnO₂ particles are distributed on PANI uniformly. To evaluate the performance of the as‐prepared materials as supercapacitor electrodes, cyclic voltammetry, galvanostatic charge–discharge measurements, and electrochemical impedance spectroscopy were performed. The PANI/MnO₂ composite shows a higher specific capacitance (810 F g⁻¹) than pure PANI (662 F g⁻¹) at a current density of 0.5 A g⁻¹. The cycle life of the composite was also excellent. After 1,000 cycles, it maintained 86.3% of its initial capacitance. POLYM. COMPOS., 36:113–120, 2015. © 2014 Society of Plastics Engineers     

Polypyrrole/MnO2 composites and their enhanced electrochemical capacitance

The composites of polypyrrole (PPY) and MnO₂ have been prepared through chemical oxidation of pyrrole monomer and MnO₂ suspension with ammonium peroxysulfate at low temperature. The morphology and structure of materials were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), Transmission electron microscopy (TEM), thermal gravity analysis ‐ differential thermal gravity (TG‐DTG), and X‐ray diffraction (XRD) measurements. The electrochemical properties of the composite were investigated by galvanostatic charge–discharge and electrochemical impedance spectroscopy. The specific capacitance of the composite electrode is 352.8 F/g at a current of 8 mA/cm² in Na₂SO₄ electrolyte of 0.5 mol/L, which is much higher than that of 246.2 F/g and 103.5 F/g of PPY and MnO₂, respectively. A convenient and effective technique has been developed to fabricate composite materials of PPY and MnO₂ promising for designing new capacitors. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Graphene and nanostructured MnO2 composite electrodes for supercapacitors

Graphene-based materials are promising electrodes for supercapacitors, owing to their unique two-dimensional structure, high surface area, remarkable chemical stability, and electrical conductivity. In this paper, graphene is explored as a platform for energy storage devices by decorating graphenes with flower-like MnO₂ nanostructures fabricated by electrodeposition. The as-prepared graphene and MnO₂, which were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), have been assembled into an asymmetric supercapacitor. The specific capacitance of the graphene electrode reached 245 F/g at a charging current of 1 mA after an electro-activation process. This value is more than 60% larger than the one before electro-activation. The MnO₂ nano-flowers which consisted of tiny rods with a thickness of less than 10 nm were coated onto the graphene electrodes by electrodeposition. The specific capacitance after the MnO₂ deposition is 328 F/g at the charging current of 1 mA with an energy density of 11.4 Wh/kg and 25.8 kW/kg of power density. This work suggests that our graphene-based electrodes are a promising candidate for the high-performance energy storage devices.