Flexible supercapacitor based on three‐dimensional cellulose/graphite/polyaniline composite

Fast charge‐discharge rate and high areal capacitance, along with high mechanically stability, are the pre‐requisites for flexible supercapacitors to power flexible electronic devices. In this paper, we have used three‐dimensional polyacrylonitrile graphite foam as flexible current collector for electro‐deposition of polyaniline (PANI) nanowires. The graphite foam with PANI was then used to fabricate symmetric supercapacitor. The fabricated supercapacitor in the three‐electrode system shows a high specific capacitance (C_sp) of 357 F.g^?1 and areal capacitance (C_areal) of 7142 mF.cm^?2 in 1 M H₂SO₄ at current density of 80 mA.cm^?2, while using two‐electrode system, it shows C_sp of 256 F.g^?1 and C_areal of 5120 mF.cm^?2 in 1 M H₂SO₄ at current density of 100 mA.cm^?2. The current density of 100 mA.cm^?2 is up to 10 folds higher than reported current densities of many PANI‐based supercapacitors. The high capacitance can be attributed to the spongy network of PANI‐NWs on three‐dimensional graphite surface which provides an easy path for electrolyte ions in active electrode materials. The developed supercapacitor shows specific energy of 64.8 Whkg^?1 and a specific power of 6.1 kWkg^?1 with a marginally decrease of 1.6% in C_sp after 1000^th cycles, along with coulombic efficiency retention of 87% in polyvinyl alcohol/H₂SO₄ gel electrolyte. This flexible supercapacitor exhibits great potential for energy storage application.     

Sol-gel synthesis of ternary conducting polymer hydrogel for application in all-solid-state flexible supercapacitor

In this contribution, we reported the preparation of a novel conducting polymer hydrogel (CPH) by a sol-gel method, which was subsequently employed to fabricate a flexible all-solid-state supercapacitor device. Taking advantage of the synergistic effects of the different components in the conducting polymer hydrogel and the merits of the proposed synthesis strategies, the prepared supercapacitor device with CPH as electrode exhibited high area-normalized capacitance (2.2 F cm⁻²), high gravimetric capacitance (1573.6 F g⁻¹) as well as high energy density of 0.18 mWh cm⁻² (or 128.7 Wh Kg⁻¹) at 0.08  mW cm⁻² (or 55.1 W kg⁻¹). This study did not only represent a novel all-solid-state, high performance, flexible supercapacitor with potential applications in flexible energy-related devices, but also developed a new method for enhancing capacitances and mechanical stability of all-solid-state flexible supercapacitor.     

Nickel silicate hydroxide on hierarchically porous carbon derived from rice husks as high-performance electrode material for supercapacitors

Nickel silicate hydroxide on hierarchically porous carbon derived from rice husks is prepared as electrode material for supercapacitors. AAEMs¹ in rice husks and CO₂ promote the development of pores, which act as pore-forming agent and catalyst respectively. The rice husks carbon is used as the substrate and the SiO₂ in rice husks is converted into Ni–Si compound by loading Ni. The C/NiSi-600-1 shows remarkable electrochemical performance with 237.07 F/g at 0.5 A/g. The performance declines with crystalline SiO₂ formed above 900 °C. A high-performance asymmetric water-system supercapacitor device is fabricated by C/NiSi-600-1 and activated carbon. This device shows capacitance of 142 mF/cm² at 4 mA/cm², the energy density of 25.24 Wh/kg at 551.4 W/kg and great cycle stability with 90% after 10,000 cycles. This work provides new insights into the green application of rice husks and promotes the development of electrode materials for supercapacitors.     

Preparation of flexible conductive composite electrode film of PEDOT:PSS/Aramid nanofibers via vacuum-assisted filtration and acid post-treatment for efficient solid-state supercapacitor

A highly conductive and flexible composite film of Poly (3,4-ethylenedioxythiophene):polystyrene sulfonate and aramid nanofiber (PEDOT:PSS/ANFs) is prepared by vacuum-assisted filtration and post-treated by acid. The composite displays excellent mechanical integrity under bending together with flexibility, properties being attributed to the strong attachment of PEDOT:PSS onto the surface of the ANFs via hydrogen bonding and the ANF structure, respectively. The conductivity of the prepared composite is progressively enhanced by the post-treatment using sulfuric acids (1 M H₂SO₄ and 1.5 M H₂SO₄), reaching 20–25 times higher than that of untreated film. This enhancement is traced to the removal of the insulating PSS group together with an analyzable change in crystallization of the PEDOT:PSS component. However, excessive use of acid treatment is seen to reduce the mechanical strength, and, thus, ultimate loss of conductivity after frequent bending (up to 1000 times), only having 59% conductivity retention with high concentration acid treatment (1.5 M H₂SO₄) compared to a high conductivity retention of 95% with 1.0 M H₂SO_4. Adopting the relatively weaker acid enables a balance to be reached between these crucial factors of electrical conductivity versus mechanical integrity. The prepared film of PEDOT: PSS/ANFs treated by acid as an electrode of supercapacitor shows good electrochemical performances, including good volumetric specific capacitance (83.5 F/cm³ with 1.0 M H₂SO₄ and 75 F/cm³ with 1.5 M H₂SO₄ at 0.5 A/cm^?3), cycle stability and capacitance retention of 83.3% and 87.5% after 2000 cycles, respectively. Furthermore, a solid flexible supercapacitor is finally assembled by the post-treatment of relatively low concentration acid with 1.0 M H₂SO_4. The configured supercapacitor displays excellent volumetric energy density of 23.44 mW h/cm² (power density of 399.95 mW/cm²) at a very wide operating potential window of 0–1.6 V and cycle stability. Therefore, it is quite feasible method to fabricate a highly conductive and flexible composite film using PEDOT:PSS and ANFs by vacuum filtration and acid post-treatment, which expects to be a promising flexible composite electrode material applied in the preparation of energy storage devices.     

Binder free and high performance of sputtered tungsten nitride thin film electrode for supercapacitor device

Here, we demonstrates the fabrication of binder free and very efficient supercapacitor electrode based on tungsten nitride (W₂N) thin film on stainless steel (SS) substrate using reactive sputtering technique. W₂N thin film as a working electrode exhibits high specific capacitance (163 F g⁻¹ at 0.5 mA cm⁻² in 1 M H₂SO₄) along with excellent cycling stability. The binder free symmetric supercapacitor (W₂N||W₂N) device delivers a high specific capacitance (80 Fg^-1) and long life span (90.46% capacitance retention after 10,000 cycles) along with high energy (12.92 Whkg⁻¹) and power (∼674 kWkg⁻¹ at 9.36 Whkg⁻¹) densities. These observed excellent electrochemical performances of the present W₂N thin film based supercapacitor device, recommend it as a potential candidate for energy storage applications.     

Facile one pot sonochemical synthesis of CoFe2O4/MWCNTs hybrids with well-dispersed MWCNTs for asymmetric hybrid supercapacitor applications

In this study, a facile sonochemical strategy is used for the fabrication of CoFe₂O₄/MWCNTs hybrids as an electrode material for supercapacitor applications. FE-SEM image demonstrates the uniformly well-distributed MWCNTs as well as porous structures in the prepared CoFe₂O₄/MWCNTs hybrids, suggesting 3D network formation of conductive pathway, which can enhance the charge and mass transport properties between the electrodes and electrolytes during the faradic redox reactions. The as-fabricated CoFe₂O₄/MWCNTs hybrids with the MWCNTs concentration of 15 mg (CFC15) delivers maximum specific capacitance of 390 F g⁻¹ at a current density of 1 mA cm⁻², excellent rate capability (275 F g⁻¹ at 10 mA cm⁻²), and outstanding cycling stability (86.9% capacitance retention after 2000 cycles at 3 mA cm⁻²). Furthermore, the electrochemical performance of the CFC15 is superior to those of pure CoFe₂O₄ and other CoFe₂O₄/MWCNTs hybrids (CFC5, CFC10 and CFC20), indicating well-dispersion MWCNTs and uniform porous structures. Also, as-fabricated asymmetric supercapacitor device using the CoFe₂O₄/MWCNTs hybrids as the positive electrode and activated carbon as the negative electrode materials shows the outstanding supercapacitive performance (high specific capacitance, superior cycling stability and good rate capability) for energy storage devices. It delivers a capacitance value of 81 F g⁻¹ at 3 mA cm⁻², ca. 92% retention of its initial capacitance value after 2000 charge-discharge cycles and excellent energy density (26.67 W h kg⁻¹) at high power density (~319 W kg⁻¹).     

Microwave synthesis of Sb2Se3/polyacrylonitrile-based carbon fiber mat electrodes for high-performance flexible capacitors and hydrogen evolution reaction

The development of bifunctional electrodes with good capacitive performance and efficient hydrogen evolution reaction activity is one of the potential solutions to combat energy depletion. In this study, flexible polyacrylonitrile-based carbon fiber mat with nitrogen doping and oxygen-containing functional group carbon structure was selected as the flexible substrate, and binder-free flexible Sb₂Se₃/polyacrylonitrile-based carbon fiber mat composite electrode was successfully prepared within 120 s using microwave synthesis. The electrode not only has a capacitance of 478.0C g^?1 and retains 97.4% of the initial capacitance after 50,000 charge–discharge tests but also exhibits good HER activity of low overpotential (152 mV) and Tafel slope (78.4 mV/dec) in alkaline electrolyte. The performance of the assembled flexible asymmetric supercapacitor is almost unaffected by bending up to 180°. The device has an energy density of 21.3 Wh kg^?1 at a power density of 800.0 W kg^?1, indicating that the electrode has good prospects for portable energy storage applications.     

High-performance textile electrode enhanced by surface modifications of fiberglass cloth with polypyrrole tentacles for flexible supercapacitors

Textile-based flexible supercapacitors have various desirable advantages in practical applications due to their excellent flexibility, ease of large-scale production, and low cost. In this study, a flexible supercapacitor was designed and fabricated using a two-step polymerization method based on fiberglass cloth and unique morphology of polypyrrole (PPy). In this extraordinary nanostructure, not only do PPy tentacles provide high-speed channels for the transfer of electron and ion, but they also create a larger specific surface area, thus enhancing the energy storage. The fabricated PPy/CFC supercapacitor possesses an excellent area-specific capacitance of 549.6 mF cm⁻² and a remarkable energy density of 48.85 μWh cm⁻². Besides, it achieves the high capacitance retention of 92.4% after 10 000 charge and discharge cycles and 96.08% after 1000 bending cycles. Furthermore, it is demonstrated that the PPy/CFC supercapacitor is capable of ensuring a stable power supply for practical applications by driving an LCD electronic watch. The fiberglass cloth-based supercapacitors with PPy tentacles provide a new approach to the practical applications of wearable power supplies.     

Synthesis of three-dimensional graphene@Ni(OH)2 nanoflakes on Ni foam by RF magnetron sputtering for application in supercapacitor

Three-dimensional graphene@Ni(OH)₂ nanoflake array grown on Ni foam (G/Ni(OH)₂/NF) as a binder-free electrode of supercapacitor was prepared by combining a one-step hydrothermal approach and Radio frequency (RF) magnetron sputtering technique. Its electrochemical properties were further investigated by the cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectra. The G/Ni(OH)₂/NF showed high specific capacitance (4.0F/cm² at 1.0 mA/cm²), good rate charge-discharge capability and long cycling stability (ca. 90.6% of its initial value). This work provides a new method to prepare 3D porous electrode materials based on graphene for application in electrochemical energy storage.     

Microwave-synthesized self-supporting CoSe2/carbon fiber felt electrode for ultra–high cycling life flexible supercapacitors

Flexible electrodes are candidate for portable and wearable electronic storage devices. In this work, high-performance flexible self-supporting CoSe₂/carbon fiber felt (CoSe₂/CFF) electrode was prepared via the microwave method without any binder. The CoSe₂/CFF electrodes exhibited superior electrochemical performance (621 F g^?1 at 1 A g^?1) and an ultra-high cycling life (84.7% capacitance retention after 100,000 cycles). When the CoSe₂/CFF was used as the positive electrode for the flexible supercapacitor, the assembled device exhibited an outstanding energy density of 22.43 W h Kg^?1 at a power density of 823.12 W kg^?1. Because of the excellent mechanical stability of the device, it maintained 91.3% of its initial C after bending from 0° to 180°. The CoSe₂/CFF proposed in this work shows electrode promising applicability to a low-cost, small-sized wearable and portable energy storage device.     

Chicken nuggets-like core/shell nanosheet arrays as high performance flexible all-solid-state supercapacitor cathode and buckwheat shell as anode

The energy density of a flexible all-solid-state supercapacitor (ASC) requires new electrode material with special structure and morphology as a prerequisite for its secured improvement. In this paper, a new morphological exploration of chicken nuggets-like core/shell NiCo₂O₄/MnO₂ (NCM) nanosheet arrays on Ni foam was employed. The application of this special morphology aims to greatly improve the electrochemical performance of the cathode electrode. Additionally, Buckwheat Biochar (BBC) is utilized as the anode while the PVA/KOH thin film is prepared as the separator. The chicken nuggets-like core/shell NCM nanosheet arrays were obtained by a two-step hydrothermal method. A series of characterization methods were carried out to further support the core/shell's well-designed structure and precise composition. The tests exhibited excellent specific capacitance of 593.3 F g^?1 at 5 mA cm^?2 and outstanding cycling stability with a retention of 90% after 10000 cycles. Furthermore, the assembled NCM//BBC ASC device indicated a high specific capacitance (239 F g^?1 at the current density of 5 mA cm^?2), this is in due part of the unique architecture of NCM nanosheet arrays and interconnected special porous structure of the BBC and the thin film PVA/KOH. Hence, the assembled ASC device exhibited high energy density (an energy density of 58 Wh·kg^?1 at 3263 W kg^?1) and remarkable cycling stability.     

Facile SILAR preparation of Fe(OH)3/Ag/TiO2 nanotube arrays ternary hybrid for supercapacitor negative electrode

The ternary hybrid composite electrode of Fe(OH)₃/Ag/TNTA (where TNTA stands for TiO₂ nanotube arrays) was prepared by a simple successive ionic layer adsorption and reaction method. The effects of calcination temperature of Ag/TNTA, drying temperature of Fe(OH)₃/Ag/TNTA, and deposition amount of Ag and Fe(OH)₃ on the supercapacitor performance of the composite electrode were investigated, and the related reasons were discussed in detail. The results show that Ag modification can obviously improve the performance of Fe(OH)₃/TNTA composite electrode. Both the calcination temperature of Ag/TNTA and the deposition amount of Ag affect the particle size of Ag and the reaction resistance of the electrode. The deposition amount of Fe(OH)₃ also has influence on the reaction resistance of the electrode. Under the optimized conditions, the capacitance value of the Fe(OH)₃/Ag/TNTA composite electrode is as high as 84.67 mF cm^?2@5 mV s^?1（596.30 F g^?1@5 mV s^?1）, and the electrode has high rate performance and good cycle stability. The asymmetric supercapacitor assembled with Fe(OH)₃/Ag/TNTA as the negative electrode and activated carbon as the positive electrode can store energy stably under the potential window of 0–1.5 V. When the power density is 2.77 kW kg^?1 (50 mW cm^?3), the energy density can reach 18.34 Wh kg^?1 (0.33 mWh cm^?3).     

Self-healing and wide temperature tolerant flexible supercapacitor based on ternary-network organo-hydrogel electrolyte

A novel ternary network organo-hydrogel electrolyte is developed to improve the environmental adaptability of flexible supercapacitors. The ternary network electrolyte is composed of graphene/boric acid/polyvinyl alcohol matrix and methyl sulfoxide/water/H₂SO₄ mixture. A flexible supercapacitor with a sandwich structure is further assembled with the electrolyte and two polyaniline fiber/carbon cloth electrodes. The supercapacitor exhibits a high capacitance retention rate (90%) after 5 cutting/self-healing cycles. Furthermore, supercapacitors have high specific capacitance and capacitance retention on a wide temperature range of −65 °C–65 °C. The specific capacitance of supercapacitor is about 237.8 F/g and 152 F/g at 65 °C and −65 °C, in which the corresponding capacitance retention of supercapacitor is about 110.5% and 70.7%, respectively. The work provides an effective strategy to design and prepare flexible electrolyte with broad temperature adaptability, good self-healing ability and good mechanical flexibility for flexible energy storage devices.     

High-density oxygen-enriched graphene hydrogels for symmetric supercapacitors with ultrahigh gravimetric and volumetric performance

The contradiction between the porous structure and density of graphene materials makes it unable to meet the dual requirements of the next generation supercapacitors for gravimetric capacitance and volumetric capacitance. Herein, we successfully synthesized high-density oxygen-enriched graphene hydrogels (HOGHs) by a one-step hydrothermal method using high concentration graphene oxide (GO) solution and trometamol as precursors. The as-prepared HOGHs samples present a dense 3D network structure and moderate specific surface areas, which leads to a high packing density. In addition, the HOGHs samples also contain abundant oxygen-containing functional groups and some nitrogen-containing functional groups. These heteroatomic functional groups can provide pseudocapacitance for the electrode materials. Therefore, the HOGH-140 based symmetric supercapacitor shows ultrahigh gravimetric and volumetric specific capacitance (325.7 F g⁻¹, 377.8 F cm⁻³), excellent rate performance and cycling stability. Simultaneously, the symmetric binder-free supercapacitor exhibits high gravimetric specific energy density (11.3 Wh kg⁻¹) and volumetric specific energy density (13.1 Wh L⁻¹) in 6 M KOH, respectively. These outstanding properties make the material have a good application prospect in the field of compact energy storage devices.     

Polyaniline-Acetylene black-Copper cobaltite based ternary hybrid material with enhanced electrochemical properties and its use in supercapacitor electrodes

A new system based on Polyaniline-Acetylene black-Copper cobaltite composite has been prepared and affirmed by XRD, UV, SEM, FTIR, and EDS characterizations. The rod-like texture of ternary hybrid system offered excellent electrochemical activity in comparison to single and binary systems. CV and CD results revealed outstanding redox behavior of the ternary hybrid electrodes. Ternary electrode (PACC) presented the highest specific capacitance value of 690 F/g at 1 mA/cm² current density. PACC electrode based symmetric supercapacitor had a specific capacitance of 137.25 Fg^-1 at 1 mA/cm² of current density. PACC symmetric supercapacitor had the highest specific power and specific energy of 3308.85 W.kg^-1 and 19.064 Wh.kg^-1, respectively. The ternary system provides less charge transfer resistance values compared to all other systems. Thermal stability of the ternary composite is way better than polyaniline, which is due to the contribution of copper cobaltite and acetylene black. The overwhelming characteristics of the ternary hybrid composite bring it to the limelight as an excellent candidate in the field of supercapacitors.     

Phase specific α-MnO2 synthesis by microbial fuel cell for supercapacitor applications with simultaneous power generation

Phase specific α-MnO₂ is synthesized by reduction of KMnO₄ in microbial fuel cell. Electrochemical characterization of synthesized α-MnO₂ in aqueous 1M Li₂SO₄ electrolyte exhibit supercapacitor behaviour with initial specific capacitance of 558 mF/g and is 3.77 times higher than its theoretical capacitance. After 50 cycles, α-MnO₂ maintained specific capacitance of 250 mF/g which is 1.69 times higher than the theoretical capacitance of 148 mF/g. Decay in the specific capacitance is noticed after 2000 cycles upto 50 mF/g.     

Self-adaptive amorphous Co2P@Co2P/Co-polyoxometalate/nickel foam as an effective electrode for electrocatalytic water splitting in alkaline electrolyte

Noble-metal-free transition metal based phosphides (TMPs) display great potential as candidates to replace the state-of-the-art noble metal-based catalysts for electronic water splitting. In this study, amorphous Co₂P was decorated on Co-polyoxometalate (POM) and conductive cobalt phosphide forming integrated Co₂P@ Co₂P/Co-POM/NF electrode, through in suit growth, low-temperature phosphating and electrocatalytic self-adaption pathway by the stripping of superficial Co-POM when subjected to persistent bubbles. The fantastic design simultaneously offers excellent electrical conductivity for fast electron transfer, a large surface area with numerous active edge sites and a conductive current collector facilitating mass transfer and gas release. The electrode showed high catalytic activity, requiring overpotential of 130 mV for HER to achieve a current density of 50 mA/cm², 336 mV for OER to achieve a current density of 50 mA/cm², affording a water-splitting current density of 10 mA/cm² at a low cell voltage of 1.6 V. The results and facile synthesis method also offer an exciting avenue for the design of amorphous phase TMPs on a current collector with high specific area and excellent electrical conductivity for energy storage and conversion devices.     

Influence of the dispersion state of ionomer on the dispersion of catalyst ink and the construction of catalyst layer

The ionomer state in the catalyst ink of a proton exchange membrane fuel cell (PEMFC) plays a critical role in the formation of the catalyst/ionomer interface on the catalyst layer (CL). In this study, the effect of ionomer dispersion state on catalyst ink dispersion and the construction of a reasonable CL was investigated. The study of catalyst inks revealed that the dispersion of n-propanol (NPA) -ionomer dispersion or sonication could effectively reduce the catalyst particle size in inks. For shear-dispersion and homogenizer-dispersion inks, the catalyst particle size was reduced from 6.17 nm to 5.12 nm and from 5.12 nm to 4.67 nm, respectively. The ionomer dispersion was capable of significantly reducing the size of agglomerates in the ink, which resulted in a reduction in the particle size of agglomerates on the surface of the cathode CL and an improvement in its flatness. The pore size distributions of the MEA cathode catalyst layers showed that water bath ultrasonic treatment of the ionomer could result in a more reasonable pore structure for the catalyst layer. The single-cell test revealed that changing the ionomer's dispersion state could significantly increase the fuel cell's output voltage to 0.707 V at 1000 mA cm⁻², and the cell's power density to 1028 mW cm⁻² at 2000 mA cm⁻².     

A hybrid PEMFC/supercapacitor device with high energy and power densities based on reduced graphene oxide/Nafion/Pt electrode

Proton exchange membrane fuel cells (PEMFCs) possess high energy and low power densities, while supercapacitors are characterized by high power and low energy densities. A hybrid PEMFC/supercapacitor device (HPSD) with high energy and power densities was proposed and fabricated for the first time using a reduced graphene oxide/Nafion/Pt electrode in this study. The reduced graphene oxide (rGO) was a capacitive material, and Pt was used as the electrocatalyst. Nafion ionomers adsorbed onto the rGO sheets surface and connected the rGO sheets and the electrolyte (Nafion membrane), thus increasing the utilization rate and specific capacitance of rGO. During the half-cell tests, the rGO/Nafion/Pt electrode exhibited better pulse discharge and galvanostatic discharge performance than the conventional Nafion/Pt electrode. Due to the unique synergy of electrochemical reaction current and capacitance current during the discharge process, the HPSD exhibited a higher power density (26.2 kW kg⁻¹) than the PEMFC (23.9 kW kg⁻¹). The energy density (12.7 kWh kg⁻¹) exhibited by HPSD was close to that of the PEMFC (13.5 kWh kg⁻¹). Therefore, the concept of HPSD is to create a new method for developing next-generation electrochemical devices with high energy and power densities.     

A comparative study of various polyelectrolyte/nanocomposite electrode combinations in symmetric supercapacitors

A more practical, nontoxic and cheaper electrolyte, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) was used to construct supercapacitors with different nanocomposite electrodes. The flexible devices were fabricated including active carbon (AC) electrode and nanocomposites electrodes of AC/nano-silica (nano-SiO₂) and AC/multiwalled carbon nanotubes (MWCNTs) at various weight percentages. The symmetrical cell made from AC electrodes generated a maximum specific capacitance (Cs) of 315 F g⁻¹ at 0.5 A g⁻¹. The energy density of this device was 55.5 Wh kg⁻¹ at a power density of 690 W kg⁻¹. Excellent performance was achieved after 5000 charge-discharge cycles where the supercapacitor maintains 92% of its activity. The energy storage capability of the supercapacitors was also investigated with the addition of nano-SiO₂ and MWCNTs. The Cs of the supercapacitors made with the electrodes AC/nano-SiO₂ (5%, 10%, 25% and 50%) were 172, 228, 247 and 55 F g⁻¹, respectively. Similarly, the capacity of the device including the electrodes of AC/MWCNTs (5%, 10%, 25% and 50%) varied as 191, 244, 93 and 20 F g⁻¹ at 0.5 A g⁻¹. The maximum energy density of the devices having nano-SiO₂ and MWCNT were 44.4 Wh kg⁻¹ and 43.8 Wh kg⁻¹, respectively at a power density of 520 W kg⁻¹. A supercapacitor with certain dimension successfully operated a light-emitting diode (LED).