High Volumetric Specific Capacitance Research Articles

A Universal Strategy for Synthesis of Large-Area and Ultrathin Metal Oxide/rGO Film Towards Scalable Fabrication of High-Performance Wearable Microsupercapacitors.

High-performance wearable microsupercapacitor (MSC) as energy storage components is highly desirable for developing self-powering wearable electronics. However, synthesis of MSC electrode film concurrently possessing large area, ultrathin thickness, and high areal energy storage capability is still challenging. Herein, a universal strategy is reported to synthesize large-area and ultrathin metal oxide nanoparticles (MONPs)/reduced graphene oxide (rGO) hybrid-structured films by attaching self-assembled film of a wide range of MONPs onto self-assembled rGO film and subsequent carbonization. Combining a template-assisted patterning strategy and a floating film salvaging process, flexible symmetric and asymmetric MSCs based on MONPs@C/rGO films can be easily prepared in large areas. MONP agglomeration is avoided and its pseudocapacitive behavior is well utilized, thereby realizing a high areal specific capacitance of 9.32 mF cm-2 in Fe3O4@C/rGO-based symmetric MSC at a film thickness of only 275 nm, corresponding to a high volumetric specific capacitance of 338.9 F cm-3. Moreover, the MnO@C/rGO‖Fe3O4@C/rGO-based asymmetric MSC delivers a high volumetric energy density of 69.8 mWh cm-3. The MSCs also demonstrate their efficient power supply in wearable self-powering systems. This work provides a new route for scalable preparation of high-performance wearable MSCs, and also enables customizable fabrication and performance tuning of MSCs.

Dense graphene composite hydrogels as advanced electrode materials for high-performance supercapacitors

In this work, alanine functionalized nanocellulose/graphene composite hydrogels (NCGHalas) were prepared via an easy hydrothermal reaction using alanine, GO, and nanocellulose as reactant. Thanks to this synthesis strategy, NCGHalas exhibited large bulk density, abundant heteroatom functional groups, reasonable porous structure and moderate specific surface area. Based on the above characteristics, the aqueous symmetric supercapacitors (ASSC) assembled by NCGHala25 presented high gravimetric and volumetric specific capacitance of 276.5 F g−1 and 320.8 F cm−3 at 0.3 A g−1, excellent rate performance (77.9 % at 10 A g−1), and long working life of 95.4 % retention after 10,000 cycles at 10 A g−1. Furthermore, the NCGHala25 based ASSC also exhibited large gravimetric and volumetric specific energy densities of 9.6 Wh kg−1 and 11.1 Wh L−1. More importantly, the flexible solid-state supercapacitors (FSSC) fabricated by NCGHala25 and PVA-KOH electrolyte delivered high specific capacitance (178.2 F g−1, 0.3 A g−1), and this value can be maintained for 74.2 % even at 10 A g−1.

Application of N/P-rGO composite electrode materials and novel electrolytes in high-performance supercapacitors

Traditional graphene-based electrode materials often can not have excellent gravimetric and volumetric electrochemical properties at the same time. Furthermore, the gel electrolyte widely used at present has the disadvantages of poor water retention, easy solidification, low ionic conductivity, and narrow working temperature range during the working process. In this paper, nanocellulose/N, P co-doped reduced graphene oxide composites (NC-NPrGOs) were synthesized via a simple approach using graphene oxide (GO), ammonium dihydrogen phosphate, and nanocellulose (NC) as raw materials. Meanwhile, a new type of gel electrolyte (SNG) with renewable, wide working temperature range and ultra-high water retention properties was prepared using sodium alginate (SA), NC, GO, and potassium hydroxide as precursors. Benefiting from the unique physical and chemical properties of NC-NPrGOs, the assembled aqueous symmetric supercapacitors assembled by NC-NPrGOs present high gravimetric (303.9 F/g) and volumetric specific capacitance (416.5 F cm−3), large gravimetric (10.5 Wh kg−1) and volumetric energy density (14.6 Wh/L). Due to the multiple interactions of several components in SNG, the flexible solid-state supercapacitors (ASSC) assembled by NC-NPrGO55 and SNG deliver high specific capacitance (240.0 F/g), ultra-long cycling life, excellent bending resistance, a wide range of operating temperatures, and good regenerative performance. In summary, NC-NPrGOs and SNGs are ideal for the investigation of new energy storage devices.

Fabrication of Soft-Hard Heterostructure Porous Carbon with Enhanced Performance for High Mass-Loading Aqueous Supercapacitors.

With the increasing attention to energy and environmental issues, the high value-added utilization of biomass and pitch to functional carbon materials has become an important topic in science and technology. In this work, the soft-hard heterostructure porous carbon (NRP-HPC) is prepared by bio-template method, in which biomass and pitch are used as hard carbon and soft carbon precursors, respectively. The prepared NRP-HPC-4 shows high specific surface area (2293m2g-1), suitable pore size distribution, good conductivity (0.25Ωcm-1), and strong wettability. The synergistic effect of soft carbon and hard carbon ensures the composite material exhibiting excellent electrochemical performance for high mass loading (12.0mgcm-2) aqueous supercapacitor, i.e., high specific capacitance (304.69Fg-1 at 0.1Ag-1), high area capacitance (3.67Fcm-2 at 0.1Ag-1), high volumetric specific capacitance (202.74Fcm-3 at 0.1Ag-1), low open-circuit voltage attenuation rate (21.04mVh-1), good voltage retention (79.12%), and excellent cyclic stability (92.04% capacitance retention and 100% coulombic efficiency after 20000 cycles). The composite technology of soft carbon and hard carbon not only ensures the prepared porous carbon electrode materials with enhanced electrochemical performance, but also realizes the high value-added coupling utilization of biomass and pitch.

Sodium Dodecyl Sulfate-Adjusted Phase Composition of Hydrated Tungsten Oxides as Stable Self-Supporting Electrodes for Supercapacitors with High Volumetric Specific Capacitance.

High pseudocapacitive activity of hydrated tungsten oxides (WO3·xH2O, x = 1 or 2) makes them promising materials for supercapacitors (SCs). During their synthesis, additives such as complexing agents and surfactants generally can only affect the morphology and/or size of the products. Here, we demonstrate that not only morphology and size of WO3·xH2O were affected, its phase composition could also change from WO3·2H2O to WO3·H2O simply by increasing the amount of sodium dodecyl sulfate (SDS) during its anodization synthesis. To the best of our knowledge, such a phenomenon has not been reported before. In addition, SDS introduced a special structure to the products, i.e., WO3·xH2O nanoplatelets constructed from nanoparticle multilayers with abundant nanogaps between the multilayers, which further arranged into nanoflowers with increased amounts of SDS. Benefiting from such a structure, low internal resistance, enhanced stability, and fast redox kinetics, the as-obtained WO3·xH2O/W-3 self-supporting electrode showed a high volumetric specific capacitance of 1402.92 F cm-3 and good cycling stability (a capacity retention of 106% after 10 000 cycles). In addition, an all-solid-state asymmetric SC device based on WO3·xH2O/W-3 delivered high a volumetric energy density of 44.0 mW h cm-3 at 0.5 W cm-3. Our method demonstrates a potential way to fabricate excellent self-supporting electrodes for SCs.

High-performance supercapacitors based on compact graphene composite hydrogels

Because of its ultra-high specific surface area and outstanding electrochemical performance, graphene has become a study focus of electrode materials for supercapacitors. However, severe agglomeration and poor volumetric electrochemical properties greatly reduce its potential for practical applications. Here, we proposed a simple hydrothermal strategy to synthesize L-glutamic acid functionalized nanocellulose/reduced graphene oxide composite hydrogels (GNGHs) with compact porous structures and plentiful heteroatom functional groups. In the hydrothermal process, a high concentration of graphene oxide solution (GO) was employed to boost the packing density of the samples. Nanocellulose (NC) and the molecular chains of L-glutamic acid were used as spacers to modulate the porous structure of GNGHs. At the same time, NC and heteroatomic groups can also boost the surface wettability of GNGHs, thereby increasing their ion-available specific surface area. The GNGH-15 based symmetric aqueous supercapacitors (SASC) deliver high gravimetric (282.0 F g−1) and volumetric (329.9 F cm−3) specific capacitance, remarkable rate capacity, and good cycling durability. Moreover, the all-solid-state flexible supercapacitors (ASSC) assembled by GNGH-15 also exhibit prominent specific capacitance (154.7 F g−1) and good rate performance. These brilliant electrochemical properties make GNGHs a strong contender for electrode materials of energy storage devices.

Thermal instability-induced formation of hierarchically structured porous GaN/nitrogen-doped carbon with high performance for supercapacitor

Electrode materials with high specific surface area (SBET) and high stacking density face a dilemma, making it difficult for supercapacitor electrodes to simultaneously exhibit high areal discharge specific capacitance (Ca,dis) and high volumetric discharge specific capacitance (Cv,dis). By exploiting the high stacking density and thermal instability of metal (after group VIB) nitrides, an electrode material of GaN honeycombs/nitrogen-doped carbon is achieved that simultaneously exhibits high Ca,dis (730 mF cm−2) and Cv,dis (251 F cm−3) at 200 mF cm−2 and manifests a linear relationship between Ca,dis and electrode loading ranging from 3.4 to 7.6 mg cm−2. The GaN honeycomb-based symmetric supercapacitors can deliver high areal/volumetric specific energy of 21.8 μW h cm−2/3.12 mW h cm−3 at 5 mW cm−2/714.3 mW cm−3 after 20,500 cycles at varied current densities from 10 to 50 and then to 10 mA cm−2. The high performance of the GaN honeycomb-based electrodes is attributed to the synergistic effect of high SBET (378.3 m2 g−1), suitable pore sizes (concentrated at 1.4 nm), surface capacitive effect, and promoted kinetics of the faradaic reaction, due to the hierarchical structure combining the high SBET and fast kinetics of nanomaterials with the high stacking density of micron materials.

A high-performance and flexible electrode film based on bacterial cellulose/polypyrrole/nitrogen-doped graphene for supercapacitors

With the development and popularity of portable electronic devices, there is an urgent need for flexible energy storage devices suitable for mass production. We report freestanding paper electrodes for supercapacitors fabricated via a simple but efficient two-step method. Nitrogen-doped graphene (N-rGO) was first prepared via a hydrothermal method. This not only obtained nitrogen atom-doped nanoparticles but also formed reduced graphene oxide. Pyrrole (Py) was then deposited on the bacterial cellulose (BC) fibers as a polypyrrole (PPy) pseudo-capacitance conductive layer by in situ polymerization and filtered with nitrogen-doped graphene to prepare a self-standing flexible paper electrode with a controllable thickness. The synthesized BC/PPy/N15-rGO paper electrode has a remarkable mass specific capacitance of 441.9 F g−1, a long cycle life (96 % retention after 3000 cycles), and excellent rate performance. The BC/PPy/N15-rGO-based symmetric supercapacitor shows a high volumetric specific capacitance of 244 F cm−3 and a max energy density of 67.9 mWh cm−3 with a power density of 1.48 W cm−3, suggesting that they will be promising materials for flexible supercapacitors.

Two-dimensional transition metal carbide (Ti0.5V0.5)3C2Tx MXene as high performance electrode for flexible supercapacitor

MXenes have gained widespread interest in flexible supercapacitor due to their rich electrochemical activity and free-standing electrode structure. However, it has been a challenge to obtain an electrode with high (mass and volumetric) specific capacitance, high rate and long cycle life simultaneously. Herein, we have prepared a novel few-layer double transition metal carbide (Ti0.5V0.5)3C2Tx MXene. Multivalent V atoms with high electrochemical activity were constructed in stable M3C2-type MXene to obtain the (Ti0.5V0.5)3C2Tx electrode with excellent performance in flexible supercapacitors. The (Ti0.5V0.5)3C2Tx film has an excellent specific capacitance of 387F g−1 (1625 mF cm−3) at 1.0 A g−1, and 267 F g−1 (1121 mF cm−3) even at a high current density of 20.0 A g−1, demonstrating superior rate performance (69%). Moreover, the capacitance of the (Ti0.5V0.5)3C2Tx film remains stable during 100,000 cycles. The symmetric supercapacitor assembled using (Ti0.5V0.5)3C2Tx film has high energy and power densities, up to 5.6 Wh kg−1 and 5210.3 W kg−1. And the all-solid-state (Ti0.5V0.5)3C2Tx flexible SC maintains stable electrochemical performance after 200 bending cycles. This work shows the huge potential of (Ti0.5V0.5)3C2Tx in flexible supercapacitor, and provides a new idea for the design of high performance flexible electrodes.

Nanocellulose-based electrodes and separator toward sustainable and flexible all-solid-state supercapacitor

Nanocellulose, as the most abundant natural nanomaterial with sustainability, biodegradability, and excellent mechanical properties, has been widely applied in modern electronic systems, particularly, in the flexible electrochemical energy storage devices. Herein, a reduced graphene oxide (RGO)/cellulose nanocrystal/cellulose nanofiber (RCC) composite membrane was prepared by using a one-pot method. Compared to the pure RGO membranes, the RCC composite membranes exhibited better mechanical properties and hydrophilicity. Furthermore, due to the synergistic effect of nanocellulose and RGO sheets, the RCC composite membrane exhibited a specific capacitance as high as 171.3 F·cm−3. Consequently, a nanocellulose-based symmetric flexible all-solid-state supercapacitor (FASC) was constructed, in which two RCC composite membranes served as electrodes and a porous cellulose nanofiber membrane acted as separator. This fabricated FASC demonstrated a high volumetric specific capacitance of 164.3 F·cm−3 and a satisfactory energy density of 3.7 mW·h·cm−3, which exceeded that of many other FASCs ever reported. This work may open a new avenue in design of next-generation nanocellulose based, sustainable and flexible energy storage device.

Ultrasimple air-annealed pure graphene oxide film for high-performance supercapacitors

Realizing both high gravimetric and volumetric specific capacitances (noted as CW and CV, respectively) is an essential prerequisite for the next-generation, high performance supercapacitors. However, the need of electronic/ionic transport for electrochemical reactions causes a “trade-off” between compacted density and capacitance of electrode, thereby impairing gravimetric or volumetric specific capacitances. Herein, we report a high-performance, film-based supercapacitor via a thermal reduction of graphene oxide (GO) in air. The reduced, layer-structured graphene film ensures high electrode density and high electron conductivity, while the hierarchical channels generated from reduction-induced gas releasing process offer sufficient ion transport pathways. Note that the resultant graphene film is employed directly as electrodes without using any additives (binders and conductive agents). As expected, the as-prepared electrodes perform particularly well in both CW (420F g−1) and CV (360F cm−3) at a current density of 0.5 A g−1. Even at an ultrahigh current density of 50 A g−1, CW and CV maintain in 220F g−1 and 189F cm−3, respectively. Furthermore, the corresponding symmetric two-electrode supercapacitor achieves both high gravimetric energy density of 54 W h kg−1 and high gravimetric power density of 1080 W kg−1, corresponding to volumetric energy density of 46 W h L-1 and volumetric power density of 917 W L-1.

Interfacial synthesis of crystalline quasi-two-dimensional polyaniline thin films for high-performance flexible on-chip micro-supercapacitors

Quasi-two-dimensional (q2D) conducting polymer thin film synergizes the advantageous features of long-range molecular ordering and high intrinsic conductivity, which are promising for flexible thin film-based micro-supercapacitors (MSCs). Herein, we present the high-performance flexible MSCs based on highly ordered quasi-two-dimensional polyaniline (q2D-PANI) thin film using surfactant monolayer assisted interfacial synthesis (SMAIS). Owing to high electrical conductivity, rich redox chemistry, and thin-film morphology, the q2D-PANI MSCs show high volumetric specific capacitance (ca. 360 F/cm3) and energy density (17.9 mWh/cm3), which outperform the state-of-art PANI thin-film based MSCs and promise for future flexible electronics.

High performance stretchable fibrous supercapacitors and flexible strain sensors based on CNTs/MXene-TPU hybrid fibers

In order to improve the practicability of fibrous supercapacitors, stretchability is a significant characteristic for the devices, which puts forward high requirements for electrode materials. In this work, highly stretchable and conductive CNTs/MXene-TPU hybrid fiber electrodes with porous structure were prepared by wet spinning technique. The mechanical and electrical properties of the fiber electrodes remained stable during the tensile process when the amount of CNTs/MXene is 10 wt%. Meanwhile, the electrodes also showed a high volumetric specific capacitance of 3.9 F cm−3 even at the scan rate of 1 V s−1. The all-solid-state fibrous supercapacitors based on CNTs/MXene-TPU fiber electrodes also exhibited superior electrochemical performance and can be stretched by more than 50% strain. The specific capacitance of the devices changed with the tensile strain and reached 3.1 F cm−3 while the stain was 50%. Based on these characteristics, the fibrous supercapacitors were further applied to flexible strain sensors, which can convert action signals to electrochemical signals rapidly. The stretchable fibrous supercapacitor can be used not only for energy storage, but also as a strain sensor to monitor human movements, making it very promising for the next generation smart textiles.

Development of high performance alpha-Co(OH)2/reduced graphene oxide microfilm for flexible in-sandwich and planar micro-supercapacitors

Herein, a series of alpha-Co(OH)2/reduced graphene oxide (rGO) microfilms and film-based nanodevices were developed via a new scalable technique. Due to the unique hexagonal nanoplates of ultrathin alpha-Co(OH)2 and the intrinsically conductive nature of rGO sheets, such thin films not only can improve the conductivity of alpha-Co(OH)2 and prevent the re-stacking of alpha-Co(OH)2 and rGO sheets but also short the transport routes of electrons and ions between the electrode and the electrolyte. The optimized alpha-Co(OH)2/rGO flexible electrode presents high specific capacitance (273.86 mF/cm2 at 0.1 mA/cm2), advanced rate capability, and excellent coulombic efficiency. Simultaneously, in-sandwich symmetric and asymmetric supercapacitors assembled with polyvinyl alcohol-KOH gel as the solid-state electrolyte achieved high areal and volumetric specific capacitances. Furthermore, a self-assembled planar alpha-Co(OH)2/rGO micro-supercapacitor (MSC) delivers high specific area capacitance (130F/cm2 at 0.5 mA/cm2) and excellent energy density (20 mWh/cm3@56 mW/cm3), which are superior to most of the recently reported carbon-based and metal hydroxides/oxides/sulfides-based planar MSCs. Also, our planar MSC shows excellent cycling performance, good flexibility, and mechanical stability. This work promotes the syntheses of other two-dimensional metal hydroxides/rGO composite film for high-performance flexible micro-electronics.

Catechol-Coordinated Framework Film-based Micro-Supercapacitors with AC Line Filtering Performance.

Coordination polymer frameworks (CPFs) have broad applications due to their excellent features, including stable structure, intrinsic porosity, and others. However, preparation of thin-film CPFs for energy storage and conversion remains a challenge because of poor compatibility between conductive substrates and CPFs and crucial conditions for thin-film preparation. In this work, a CPF film was prepared by the coordination of the anisotropic four-armed ligand and CuII at the liquid-liquid interface. Such film-based micro-supercapacitors (MSCs) are fabricated through high-energy scribing and electrolytes soaking. As-fabricated MSCs displayed high volumetric specific capacitance of 121.45 F cm-3 . Besides, the volumetric energy density of MSCs reached 52.6 mWh cm-3 , which exceeds the electrochemical performance of most reported CPF-based MSCs. Especially, the device exhibited alternating current (AC) line filtering performance (-84.2° at 120 Hz) and a short resistance capacitance (RC) constant of 0.08 ms. This work not only provides a new CPF for MSCs with AC line filtering performance but also paves the way for thin-film CPFs preparation with versatile applications.

Improving the volumetric specific capacitance of flexible polyaniline electrode: solution casting method and effect of reduced graphene oxide sheets

High volumetric specific capacitance is essential for flexible supercapacitors. In this paper, a flexible composite electrode with ultrahigh volumetric specific capacitance and good rate performance is designed and prepared. The film electrode is composed of non-porous compact polyaniline as the matrix and a small amount of reduced graphene oxide sheets as the conductive filler. The film is prepared by a newly developed solution casting method, where the casting solution is obtained by self-assembly of polyaniline and graphene oxide in a blended solution. A systematical investigation on the effect of reduced graphene oxide sheets reveals that they serve as both the conductive filler and the diffusion barrier. With the optimized reduced graphene oxide content, a large volumetric specific capacitance of 1354 F cm−3 at 2.4 A cm−3 is achieved, and good rate performance is also obtained because of the good ionic conductivity of polyaniline. This work provides a high-performance electrode for flexible supercapacitors, and the solution casting method is also valuable in fabricating other organic electrode materials.

Effect of Ball Milling on the Electrochemical Performance of Activated Carbon with a Very High Specific Surface Area

Activated carbon (AC) with a very high specific surface area of >3000 m2 g−1 and a number of course particles (average size: 75 µm) was pulverized by means of planetary ball milling under different conditions to find its greatest performances as the active material of an electric double-layer capacitor (EDLC) using a nonaqueous electrolyte. The variations in textural properties and particle morphology of the AC during the ball milling were investigated. The electrochemical performance (specific capacitance, rate and cyclic stabilities, and Ragone plot, both from gravimetric and volumetric viewpoints) was also evaluated for the ACs milled with different particle size distributions. A trade-off relation between the pulverization and the porosity maintenance of the AC was observed within the limited milling time. However, prolonged milling led to a degeneration of pores within the AC and a saturation of pulverization degree. The appropriate milling time provided the AC a high volumetric specific capacitance, as well as the greatest maintenance of both the gravimetric and volumetric specific capacitance. A high volumetric energy density of 6.6 Wh L−1 was attained at the high-power density of 1 kW L−1, which was a 35% increment compared with the nonmilled AC. The electrode densification (decreased interparticle gap) and the enhanced ion-transportation within the AC pores, which were attributed to the pulverization, were responsible for those excellent performances. It was also shown that excessive milling could degrade the EDLC performances because of the lowered micro- and meso-porosity and the excessive electrode densification to restrict the ion-transportation within the pores.

Highly-conductive PEDOT:PSS hydrogel framework based hybrid fiber with high volumetric capacitance and excellent rate capability

Fiber supercapacitors (FSCs) are attractive energy storage devices for wearable and portable electronics. However, it still remains a great challenge to achieve high volumetric specific capacitance without sacrificing the rate capability of FSC. Here, we demonstrate a novel hierarchically nanostructured hybrid fiber with thin polyaniline (PANI) layer anchored on the highly-conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) hydrogel framework as electrodes for FSCs. The hybrid fiber was prepared via hydrothermal assembly of a commercial PEDOT:PSS dispersion followed by acid treatment and in-situ chemical polymerization of aniline. The incorporation of highly-electroactive PANI boosts the whole capacitance of hybrid fiber; the robust PEDOT hydrogel framework provides abundant ions diffusion channels and rapid electron transfer pathways, improving the utilization efficiency and the kinetics of pseudocapacitance reaction of PANI. Attributed to the synergistic effect, the PEDOT/PANI hybrid fiber in FSC exhibits high volumetric/areal specific capacitance (554.1 F cm−3 or 740.0 mF cm−2 at 5 A cm−3), excellent rate performance and good electrochemical stability. The flexible FSC device delivers a high volumetric energy density of 11.9 mWh cm−3 at 981.4 mW cm−3 and areal energy density of 15.8 μWh cm−2 at 1310.5 μW cm−2, outperforming most of the reported counterparts. The well-designed PEDOT/PANI hybrid fiber based FSC hold a great potential for powering the next-generation wearable and portable electronics.

Two-dimensional nitrogen and oxygen Co-doping porous carbon nanosheets for high volumetric performance supercapacitors

Abstract Two-dimensional (2D) porous carbon nanosheets possess large open flat layer with high surface area, short ion diffusion route and small diffusion resistance have attracted greatly attention as electrode materials for supercapacitors. However, developing a simple and widely available preparation method remains a major challenge. Herein, we demonstrate a new, widely available way to rationally design the nitrogen and oxygen co-doping porous carbon nanosheets (PCN) utilizing graphene oxide as template by an electrostatic self-assembly process with cationic polyacrylamide and subsequent KOH activation. Benefiting from the open and accessible surface, short ion diffusion route and rich nitrogen and oxygen functional groups, the optimized PCN-3 electrode shows a high volumetric specific capacitance of 325.7 F cm−3 at 0.5 A g −1 with a high capacitance retention of 66% at 50 A g−1 (215.3 F cm−3). Moreover, the PCN-3//PCN-3 symmetric supercapacitor shows a high volumetric energy density of 22.1 Wh L−1 and outstanding cycling stability in 1 M Na2SO4 aqueous elec1trolyte. Consequently, the work highlights a convenient and widely available preparation way to prepare heteroatom doped porous carbon nanosheets for supercapacitors.

Direct Inkjet Printing of Aqueous Inks to Flexible All-Solid-State Graphene Hybrid Micro-Supercapacitors.

In this article, the inkjet printing technique is demonstrated for the stacking of reduced graphene oxide (RGO) and molybdenum trioxide (MoO3) nanosheets for flexible all-solid-state micro-supercapacitors. The ammonium molybdate tetrahydrate/graphene oxide ((NH4)6Mo7O24·4H2O/GO) aqueous inks are facilely printed on polymide (PI) film and transformed to RGO/MoO3 hybrids via thermal treatments at air atmosphere. The compound inks are water-based, inkjet-printable, and nontoxic for inkjet printing to form two-dimensional crystal materials. The physical properties of aqueous inks are optimized within a printable range characterized by the Ohnesorge number of 1 < Z < 14. The inkjet-printed symmetric micro-supercapacitors (MSCs) with poly(vinyl alcohol) (PVA)-H2SO4 gel electrolyte possess a wide voltage window of 0-0.8 V, excellent flexibility, a high volumetric specific capacitance of 22.5 F cm-3 at 0.044 A cm-3, as well as good cyclic stability due to the synergistic effect of RGO and MoO3. Furthermore, the inkjet-printed composite MSCs delivered a maximum energy density of 2 mWh cm-3 and a power density of 0.018 W cm-3, and the capacity retention rate of inkjet-printed MSCs is still retained 82% even after 10 000 charge-discharge cycles, indicating good electrochemical properties. Above all, the as-designed inkjet printing technique shows potential for flexible and wearable energy storage electronics.