MoO3−x-deposited TiO2 nanotubes for stable and high-capacitance supercapacitor electrodes

Here we report the supercapacitive properties of a novel MoO_3−x/TiO₂ nanotube composite prepared by a facile galvanostatic deposition technique and subsequently thermal treatment in an argon atmosphere between 350 °C and 550 °C. X-ray diffraction and X-ray photoelectron spectroscopy confirm the existence of MoO_3−x. The MoO_3−x/TiO₂ electrode prepared at 550 °C exhibits a high specific capacitance of 23.69 mF cm⁻² at a scan rate of 10 mV s⁻¹ and good cycling stability with capacitance retention of 86.6% after 1000 cycles in 1 M Na₂SO₄ aqueous solution. Our study reveals a feasible method for the fabrication of TiO₂ nanotubes modified with electroactive MoO_3−x as high-performance electrode materials for supercapacitors.

Here we report the supercapacitive properties of a novel MoO_3−x/TiO₂ nanotube composite prepared by a facile galvanostatic deposition technique and subsequently thermal treatment in an argon atmosphere between 350 °C and 550 °C.     

MoS2/polyaniline/functionalized carbon cloth electrode materials for excellent supercapacitor performance

In this study, molybdenum disulfide (MoS₂), polyaniline (PANI) and their composite (MoS₂/PANI) were facilely prepared via a liquid-phase method and in situ polymerization. An MoS₂/PANI/functionalized carbon cloth (MoS₂/PANI/FCC) was facilely constructed by a drop-casting method. MoS₂/PANI-10/FCC displays remarkable electrochemical performances, and its specific capacitances varied from 452.25 to 355.5 F g⁻¹ at current densities ranging from 0.2 to 4 A g⁻¹, which were higher than those of MoS₂/CC (from 56.525 to 7.5 F g⁻¹) and pure PANI/CC (319.5 to 248.5 F g⁻¹), respectively. More importantly, the MoS₂-10/PANI/FCC electrode has a long cycling life, and a capacity retention of 87% was obtained after 1000 cycles at a large current density of 10 A g⁻¹. Moreover, the MoS₂/PANI-10/FCC-based symmetric supercapacitor also exhibits excellent rate performance and good cycling stability. The specific capacitance based on the total mass of the two electrodes is 72.8 F g⁻¹ at a current density of 0.2 A g⁻¹ and the capacitance retention of 85% is obtained after 1000 cycles.

A MoS₂/PANI/functionalized carbon cloth (MoS₂/PANI/FCC) was constructed by a drop-casting method. Its specific capacitances were higher than those of MoS₂/CC and pure PANI/CC.     

Carboxymethylcellulose ammonium-derived nitrogen-doped carbon fiber/molybdenum disulfide hybrids for high-performance supercapacitor electrodes

In this paper, a new type of nitrogen-doped carbon fiber/molybdenum disulfide (N-CFs/MoS₂) hybrid electrode materials are prepared via a certain concentration in solvothermal synthesis followed by a high-temperature carbonization process and using the carboxymethylcellulose ammonium (CMC-NH₄) as a structure-directing agent for MoS₂ nanosheet growth during the solvothermal synthesis process. The addition of CMC-NH₄ effectively prevents the agglomeration of MoS₂ nanosheets to increase the specific surface area. Moreover, it not only serves as a carbon source to provide conductive pathways, but also introduces N atoms to improve the conductivity of the CFs and promote the transfer of electrons and ions. This ultimately increases the conductivity of the electrode materials. Thus, the as-prepared N-CFs/MoS₂ hybrids exhibit excellent electrochemical performance. The specific capacitance is up to 572.6 F g⁻¹ under a current density of 0.75 A g⁻¹ and the specific capacitance retained 98% of the initial capacitance after 5000 cycles of charge–discharge tests at a current density of 2.5 A g⁻¹. Moreover, the hybrids show a maximum energy density of 19.5 W h kg⁻¹ at a power density of 94 W kg⁻¹. Therefore, the as-prepared N-CFs/MoS₂ hybrids with remarkable electrochemical properties, low cost and environment protection show potential for practical application in the development of high-performance electrochemical energy storage devices.

Novel CMC-NH₄-derived nitrogen-doped CFs/MoS₂ hybrid electrode materials are prepared using CMC-NH₄ as a structure-directing agent for MoS₂ nanosheets.     

Nanocellulose/polypyrrole aerogel electrodes with higher conductivity via adding vapor grown nano-carbon fibers as conducting networks for supercapacitor application

Nanocellulose-based conductive materials have been widely used as supercapacitor electrodes. Herein, electrode materials with higher conductivity were prepared by in situ polymerization of polypyrrole (PPy) on cellulose nanofibrils (CNF) and vapor grown carbon fiber (VGCF) hybrid aerogels. With increase in VGCF content, the conductivities of CNF/VGCF aerogel films and CNF/VGCF/PPy aerogel films increased. The CNF/VGCF₂/PPy aerogel films exhibited a maximum value of 11.25 S cm⁻¹, which is beneficial for electron transfer and to reduce interior resistance. In addition, the capacitance of the electrode materials was improved because of synergistic effects between the double-layer capacitance of VGCF and pseudocapacitance of PPy in the CNF/VGCF/PPy aerogels. Therefore, the CNF/VGCF/PPy aerogel electrode showed capacitances of 8.61 F cm⁻² at 1 mV s⁻¹ (specific area capacitance) and 678.66 F g⁻¹ at 1.875 mA cm⁻² (specific gravimetric capacitance) and retained 91.38% of its initial capacitance after 2000 cycles. Furthermore, an all-solid-state supercapacitor fabricated by the above electrode materials exhibited maximum energy and power densities of 15.08 W h Kg⁻¹, respectively. These electrochemical properties provide great potential for supercapacitors or other electronic devices with good electrochemical properties.

The electrochemical performances of nanocellulose-based electrode materials were improved via building nano-carbon conducting networks.     

A SnO2QDs/GO/PPY ternary composite film as positive and graphene oxide/charcoal as negative electrodes assembled solid state asymmetric supercapacitor for high energy storage applications

The work demonstrates tin oxide quantum dots/graphene oxide/polypyrrole (SnO₂QDs/GO/PPY) ternary composite deposited on titanium foil as a positive electrode and graphene oxide (GO)/charcoal on titanium foil as negative electrode separated by polyvinyl alcohol/potassium hydroxide (PVA/KOH) gel-electrolyte as a solid-state asymmetric supercapacitor for high energy storage applications. Here, tin oxide quantum dots (SnO₂QDs) were successfully synthesized by a hydrothermal technique, and SnO₂QDs/GO/PPY ternary composite was synthesized by an in situ method with pyrrole monomer, SnO₂, and GO. A pH value controlled, which maintained the uniform size of SnO₂QDs dispersed on PPY, through GO ternary composite was used for fabricating the asymmetric supercapacitor electrode with the configuration (SnO₂QDs/GO/PPY)/GO/charcoal (85 : 10 : 5). The device achieved the highest specific capacitance of 1296 F g⁻¹, exhibited an energy density of 29.6 W h kg⁻¹ and the highest power density of 5310.26 W kg⁻¹ in the operating voltage from 0 to 1.2 V. The device also possessed excellent reliability and retained the capacitance of 90% after 11 000 GCD cycles. This ternary composite is a prominent material for potential applications in next-generation energy storage and portable electronic devices.

Representation of the synthesis steps of SnO₂QDs/GO/PPY ternary composites and SnO₂QDs/GO/PPY//GO/charcoal asymmetric supercapacitor device.     

Pulse electrochemical synthesis of polypyrrole/graphene oxide@graphene aerogel for high-performance supercapacitor

A novel electroactive polypyrrole/graphene oxide@graphene aerogel (PGO@GA) was synthesized for the first time by pulse electropolymerization. The off-time in this technique allows polypyrrole (PPy) to go through a more stable structural arrangement, meanwhile its electronic transmission performance is enhanced by immobilizing graphene oxide between PPy chains. Moreover, graphene aerogel provides a three-dimensional structure with high conductivity to protect PPy from swelling and shrinking during the capacitive testing. Under these synergistic effects, PGO@GA presents exceptional capacitive performances including high specific capacitance (625 F g⁻¹ at 1 A g⁻¹), excellent rate capability (keeping 478 F g⁻¹ at 15 A g⁻¹ with retention rate of 76.5%), and excellent cycling life (retaining 85.7% of its initial value when cycling 5000 times at 10 A g⁻¹). Therefore, the strategy adopted by this research provides a good reference for preparing other PPy-based electrode materials applied in the fields of catalysis, sensing, adsorption and energy storage.

The as-prepared polypyrrole/graphene oxide@graphene aerogel by pulse electropolymerization technique presents excellent capacitive performance.

Polypyrrole (PPy), a widely-used conducting polymer, holds huge application potential in sewage disposal,¹ extraction of precious metals,² catalysts,³ sensing⁴ and energy storage,⁵ owing to its advantages including superior biocompatibility, relatively high conductivity, high electrochemical activity and low preparation cost.⁶ Together with its excellent pseudocapacitive properties, PPy usually acts as an outstanding electrode material applied in the field of supercapacitors. However, there still exist some defects in pure PPy because its chains suffer from serious swelling and shrinking during the charge–discharge process, which in turn severely weakens the rate capability and cycling life of supercapacitors.⁷Pulse electropolymerization, compared with the method of constant potential⁸ or galvanostatic deposition,⁹ can regulate on-off time, pulse cycles and work potential to make pyrrole monomers continually polymerized into PPy on new active sites, and meanwhile to allow newly-formed PPy have enough time of experiencing structural rearrangement to improve its structural stability and electrochemical activity.¹⁰ For the sake of further overcoming the intrinsic flaws of pure PPy and developing its potentials, up to now, various PPy-based composites have been designed. For instance, some groups have realized the improvement of electronic transport property of PPy^11,12 by incorporating graphene oxide with large-size anion nature into PPy chains. Singu and Yoon developed a novel ternary composite (GO–PPy–Ag)¹³ exhibiting high specific capacitance (277.5 F g⁻¹ at 2 A g⁻¹) and superior cycle life (keeping 93% after cycling 5000 charge–discharge at 2 A g⁻¹). Apart from these cases, graphene, owing to its exceptional conductivity, large theoretical specific surface area and excellent chemical stability,¹⁴ has always been the most acceptable material employed to functionalize PPy for integrating their respective advantages. For instance, Ma et al.¹⁵ synthesized polypyrrole/bacterial cellulose/graphene by a combination of in situ polymerization and filtering method, and the obtained nanomaterial exhibited a high areal capacitance of 3.66 F cm⁻² at 1 mA cm⁻². Nevertheless, the inherent advantages of graphene are far from being full developed owing to serious restacking of graphene sheets caused by the enhanced π–π interaction during chemical or electrochemical reduction of GO.¹⁶Graphene aerogel, as one emerging material, is synthesized by self-assembly of graphene sheets in different crosslinking ways.¹⁷ The crosslinks not only restrain the restacking of graphene sheets efficiently but also initiate the formation of hierarchical pores that can shorten transport route of ions. Together with its excellent conductivity, graphene aerogel can serve as an excellent conductive matrix¹⁸ to support the electrochemical deposition of PPy.In this research, polypyrrole/graphene oxide@graphene aerogel (PGO@GA) nanocomposite was designed and constructed, which achieves a more enhanced capacitive performance than PPy owing to the modification of GO and graphene aerogel. The structure–activity relationship between components, morphologies and capacitive performances for this ternary composite was discussed on the basis of a series of structural characterization and electrochemical test.The synthesis route of PGO@GA is illustrated in Fig. 1. First, the working electrode is prepared by coating method. Then it is immersed in electrolyte containing pyrrole monomer, GO and KCl (acting as supporting electrolyte). As is well known, GO dissolved in aqueous solution always carries negative charge owning to ionization of carboxyl and phenolic hydroxyl groups on GO.¹⁹ At work potential, pyrrole monomers are oxidized into PPy whose chains carry positive electricity to attract negatively charged GO onto them. At the subsequent open circuit potential, the circulating current is cut off so that the oxidation polymerization reaction stops and meanwhile the relatively long off-time promotes diffusion of pyrrole monomers towards reactive boundary layer on electrode surface to form new active sites, where pyrrole monomers constantly grow into PPy at the next work potential, accompanied with the continuous doping of ionized GO sheets. Going through such a recurrent pulse electropolymerization process, PGO@GA is formed.Open in a separate windowFig. 1Pulse electropolymerization technique for synthesis of PGO@GA. Fig. 2 exhibits the FTIR spectra of GO, PPy, PGO and PGO@GA. For GO, the bands at 1731, 1610 and 1056 cm⁻¹ correspond to stretching vibration of C O, graphitic C C and C–O–C (alkoxy group), respectively.²⁰ As for PPy, the bands at 1541 and 1460 cm⁻¹ correspond to the symmetric and asymmetric stretching vibrations of pyrrole ring.²¹ The band located at 1295 cm⁻¹ is related with the in-plane vibration of C–H while that at 1037 cm⁻¹ is attributed to C–H deformation vibration.^22,23 The bands at 963 and 773 cm⁻¹ indicate the formation of polymerized pyrrole, meanwhile, the bands at 1187 and 910 cm⁻¹ imply the doping state of PPy.²⁴ It is worth noting that all the bands of PPy also appear in PGO and PGO@GA, proving the introduction of PPy. Apart from that, the band ascribed to the C O group of GO downshifts to 1700 cm⁻¹ for PGO and PGO@GA because of the π–π interaction and the hydrogen bonding produced between GO layers and aromatic polypyrrole rings,²⁵ proving that carboxyl group plays an important role when GO is immobilized into PPy chains.Open in a separate windowFig. 2FTIR spectra of GO, PPy, PGO and PGO@GA.It can be observed from SEM images, as shown in Fig. 3(a–c), that GA exhibits three-dimensional network structure consisting of graphene sheets with highly wrinkled and chiffon-like features. As shown in Fig. S1 and S2,^† the morphology and specific capacitance of PPy vary as a function of pulse cycle. It can be observed that PPy prepared at pulse cycles of 1000 obtains the highest specific capacitance (453 F g⁻¹), owing to its smooth micromorphology (Fig. 3d–f) that facilitates the electron mobility,²⁶ and the mass of PPy deposited on carbon cloth (19.2 mg) is about 4.8 mg. By contrast, the mass of PPy at pulse cycles of 500, 1500, 2000, 2500 and 3000 achieves 2.6, 9.6, 10.6, 14.9 and 17.8 mg respectively. Therefore, it can be evaluated that the PPy film gets much thicker as the increase of pulse cycles and pulse cycles are set as 1000 to prepare PGO and PGO@GA. Compared with pure PPy, PGO obtains a smoother microstructure (Fig. 3g–i) owing to the introduction of GO, which endows PGO with higher electrochemical activity than PPy. It can be observed from Fig. 3j–l that PGO@GA exhibits a coral-like structure, on which flat polymer layers are distributed.Open in a separate windowFig. 3SEM images of samples at different magnifications. (a–c) GA, (d–f) PPy, (g–i) PGO, (j–l) PGO@GA.At excitation wavelength of 532 nm, the G peak of single and double layers of graphene is located at 1614 and 1608 cm⁻¹ respectively.²⁷ Based on interpolation method,²⁸ the stacked graphene layers of PGO@GA, whose G peak is at 1584 cm⁻¹ (Fig. S3b^†), are about 6. As is well known, thickness of a single layer of graphene is around 1 nm,²⁹ therefore, the thickness of stacked graphene in void pore wall is ∼6 nm. It can be tested from Fig. 3l that thickness of void pore wall is ∼14 nm, accordingly, the thickness of PPy deposited on GA is about 8 nm.The structures of GO, PPy, PGO and PGO@GA were analyzed by XRD and Raman. Two peaks centered at 2θ = 10.9° and 21.7° correspond to the (001) plane and (002) plane of GO, respectively (Fig. S3a^†). The broad peaks located at 2θ = 25.3° (3.5 Å) for PPy, PGO and PGO@GA are associated with the closest distance between the planar aromatic rings of pyrrole, like face-to-face pyrrole rings.^30,31 Except for PPy, diffraction peak for (001) crystal face of GO at 2θ = 10.9° also appears on PGO and PGO@GA, indicating the introduction of GO into these two materials.In the Raman spectra (Fig. S3b^†), as for PPy, the peak located at 1571 cm⁻¹ is due to C C backbone stretching.³² The double peaks situated at 1230 and 1320 cm⁻¹ are attributed to the ring-stretching mode of PPy and another double peaks at 1041 and 979 cm⁻¹ mainly come down to the C–H in-plane deformation.³³ However, these peaks belonging to PPy disappears in PGO and PGO@GA composites except for the slightly up-shifted D band approaching the peak of PPy at 1320 cm⁻¹. It can be speculated that the immobilization of GO into PPy chains results in the change in molecular vibration and rotation of PPy.The elemental constituents of samples were analyzed using XPS (Fig. S4^†). It is worth noting that the atomic percentage content of oxygen increases successively from PPy, PGO to PGO@GA (Fig. S4a^†), which is attributed to the functionalization of PPy by GO and GA. The relatively high oxygen content in PPy mainly results from its molecular conformation optimization during the pulse electropolymerization process,²⁶ which facilitates the diffusion of water to form hydroxyl radical for nucleophilic attack towards PPy. What is more important is that the high oxygen contents endow these materials with excellent hydrophilicity favourable for lowering the transmission resistance of electrolyte and thus forming large reactive interface area. It is observed from Fig. S4b^† that only pyrrolic N³⁴ exists, accompanied with bonding energy differences among them, which can be explained by the generated electron transfer between PPy, GO and GA. The elemental mappings reveal that carbon, nitrogen, and oxygen elements coexist and are distributed over the sheets (Fig. S5^†), demonstrating the hybrid structure.The capacitive performances of PPy, PGO and PGO@GA were investigated in 1.0 M KCl by using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). It can be seen from Fig. 4a that CV curves of PPy, PGO and PGO@GA at 5 mV s⁻¹ all exhibit quasi-rectangular shapes, demonstrating ideal electrical double-layer capacitor (EDLC) behaviors at electrode–electrolyte interface.³⁵ Apart from that, the closure area of CV curve for PPy, PGO and PGO@GA increases successively, indicating elevated specific capacitance in turn. That is due to the following reasons: (1) the negatively charged GO immobilized into PPy chains must be balanced by cation ingression from electrolyte, which contributes to a higher current density in the negative potential region²⁵ and the involved reaction mechanism can be described as: PPy⁺/GO⁻ + K⁺ +e ↔ PPy⁰/GO⁻/K⁺;³⁶ (2) the coral-like network with superior conductivity for GA not only shortens transmission path of ions in electrolyte and accelerates electron transfer but also protects PPy from structural deformation.Open in a separate windowFig. 4Capacitive properties of samples in 1.0 M KCl: (a) CV curves at 5 mV s⁻¹; (b) GCD profiles at 1 A g⁻¹; (c) capacitance values at current densities ranging from 1 to 15 A g⁻¹; (d) Nyquist plots of PPy, PGO, PGO@GA, and the equivalent circuit used to fit EIS results (inset); (e) cycling stability of PPy, PGO, PGO@GA at a current density of 10 A g⁻¹.Similar tendency can be also seen from Fig. 4b. All GCD curves present quasi-isosceles triangle, indicating that the capacitance mainly comes from the EDLC properties of materials in neutral electrolyte. The capacitance value for PPy, PGO and PGO@GA at 1 A g⁻¹ achieves 453, 514 and 625 F g⁻¹, respectively.The capacitance for PPy, PGO and PGO@GA corresponding to the current densities ranging from 1 to 15 A g⁻¹ was investigated and the results are shown in Fig. 4c. It can be observed that specific capacitance of PGO@GA still achieves 478 F g⁻¹ even at high current density of 15 A g⁻¹ with a relatively high capacitance retention rate of 76.5%, which is obviously higher than those of PGO (58.7%) and PPy (25.2%), demonstrating that the synergistic effect among GO, PPy and GA promotes the rate capability of PGO@GA.EIS analysis of PPy, PGO and PGO@GA was conducted to detect their charge transfer resistance and ion diffusion rate, and the results are presented in Fig. 4d. Based on the equivalent circuit, the intercept with the real impedance axis in the high-frequency region represents equivalent series resistance R_s, including contact resistance, intrinsic resistance of electrode materials and solution resistance.³⁷ The diameter of the semicircle in medium-frequency region symbolizes charge-transfer resistance (R_ct). As proved in M. Deng et al.''s reports,¹¹ electron transmission ability of PPy is improved owing to the introduction of GO between PPy chains. Moreover, X. Du et al.''s research²⁶ also demonstrates that point based on the reduced R_ct. Therefore, similar conclusion can be drawn from the fitted results illustrated in Fig. 4d that R_s and R_ct values for PGO and PGO@GA get lower than those of PPy after PPy goes through the functional modification of GO and GA.The Warburg region refers to the section of curve with 45° slope, corresponding to the intermediate frequency range. The shorter the frequency range gets, the better ion diffusion capability the material exhibits. Therefore, it can be concluded from the fitted results in Fig. 4d that the modification of GO, GA and pulse regulation for PPy realizes the optimum ion diffusion for PGO@GA.In addition, compared with the curve of PPy and PGO in low-frequency region, that for PGO@GA is closer to parallel with imaginary axis, which implies its more ideal EDLC behaviors.It can be seen from the cyclic stability test (Fig. 4e) that the capacitance retention for PGO@GA achieves 85.7% even undergoing 5000 cycles at 10 A g⁻¹, higher than that for PPy (38.5%) and PGO (59.2%).Most of the reported materials listed in Table S1^† are prepared by non-pulse deposition techniques. These electrode materials fail to achieve high specific capacitance as well as excellent capacitance stability simultaneously, which is mainly attributed to the serious swelling and shrinking of polypyrrole during charge–discharge process. By contrast, a combination of pulse regulation, modification of GO and protection of GA towards PPy endows PGO@GA with smooth porous microstructure and enhances structural stability of PPy, which facilitates electron mobility and electrolyte transport. Therefore, PGO@GA enjoys higher capacitance stability even after 5000 charge–discharge cycles at the high current density of 10 A g⁻¹, besides, the material also obtains a higher mass specific capacitance at 1 A g⁻¹.It can be concluded from all the above electrochemical performance test results that PGO@GA electrode holds optimum capacitance performance, which is attributed to the following reasons: (1) during pulse electropolymerization, PPy experiences more stable structural rearrangement to endow it with improved electrochemical activity; (2) large-sized GO with anionic attribute is immobilized into PPy to optimize its electron transport property and structural stability; (3) GA provides three-dimensional network with good conductivity supporting anisotropic deposition of pyrrole monomer and GO, which not only generates large number of active sites of electrochemical reaction but shortens ion transport paths and inhibit the swelling and shrinking of polypyrrole also during charge–discharge process.In conclusion, the synthetic PGO@GA by pulse electropolymerization technique exhibits high specific capacitance (625 F g⁻¹) at 1 A g⁻¹ and still retains 478 F g⁻¹ even at a high current density of 15 A g⁻¹ with superior capacitance retention rate of 76.5%. Moreover, excellent cycling life (retaining 85.7% of its initial value when cycling 5000 times at 10 A g⁻¹) for PGO@GA is also obtained. The outstanding capacitance behavior benefits from multiplexed conduction channel from GA and the enhanced structural stability and electrochemical activity of PPy. It is expected that the improved modification strategy for PPy provided by this research will be a good reference to synthesize other types of PPy-based nanomaterial by regulating the categories of anions and conductive matrix to realize applications in established fields.     

Heteroatom-doped nanoporous carbon initiated from bimetallic molecular framework micro-rods for supercapacitor electrodes

We report herein that zinc and cobalt bimetallic-organic-framework (BMOF) crystalline micro-rods are able to be constructed instantly with the eco-friendly glutamate ligand and building unit of double metallic ions. After carbonization and acid leaching of these precursors, the resultant heteroatom-doped porous carbon occupies not only the enriched mesopore architectures but the ultrathin graphitic networks. Moreover, due to cyclizing dehydration reaction of the glutamate ligand upon thermal conversion, the predominant pyrrolic and pyridinic nitrogen atom sites within the carbon lattices are achieved. The supercapacitor electrodes from these carbonaceous materials without any conductive addictive deliver an impressive specific gravimetric capacitance of 230 F g⁻¹ and a specific areal capacitance of 50 μF cm⁻² at a current density of 1 A g⁻¹ in alkaline aqueous electrolyte.

Zinc and cobalt bimetallic-organic-framework (BMOF) crystalline micro-rods are able to be constructed instantly with the eco-friendly glutamate ligand and building unit of double metallic ions.     

Electrochemical analysis of Na–Ni bimetallic phosphate electrodes for supercapacitor applications

Bimetallic sodium–nickel phosphate/graphene foam composite (NaNi₄(PO₄)₃/GF) was successfully synthesized using a direct and simple precipitation method. The hierarchically structured composite material was observed to have demonstrated a synergistic effect between the conductive metallic cations and the graphene foam that made up the composite. The graphene served as a base-material for the growth of NaNi₄(PO₄)₃ particles, resulting in highly conductive composite material as compared to the pristine material. The NaNi₄(PO₄)₃/GF composite electrode measured in a 3-electrode system achieved a maximum specific capacity of 63.3 mA h g⁻¹ at a specific current of 1 A g⁻¹ in a wide potential range of 0.0–1.0 V using 2 M NaNO₃ aqueous electrolyte. A designed and fabricated hybrid NaNi₄(PO₄)₃/GF//AC device based on NaNi₄(PO₄)₃/GF as positive electrode and activated carbon (AC) selected as a negative electrode could operate well in an extended cell potential of 2.0 V. As an assessment, the hybrid NaNi₄(PO₄)₃/GF//AC device showed the highest energy and power densities of 19.5 W h kg⁻¹ and 570 W kg⁻¹, respectively at a specific current of 0.5 A g⁻¹. The fabricated device could retain an 89% of its initial capacity with a coulombic efficiency of about 94% over 5000 cycling test, which suggests the material''s potential for energy storage devices applications.

CV curve of the hybrid device as reflection of the conductive NaNi₄(PO₄)₃/GF composite and AC.     

One-step and room-temperature fabrication of carbon nanocomposites including Ni nanoparticles for supercapacitor electrodes

With the increasing importance of power storage devices, demand for the development of supercapacitors possessing both rapid reversible chargeability and high energy density is accelerating. Here we propose a simple process for the room temperature fabrication of pseudocapacitor electrodes consisting of a faradaic redox reaction layer on a metallic electrode with an enhanced surface area. As a model metallic electrode, an Au foil was irradiated with Ar⁺ ions with a simultaneous supply of C and Ni at room temperature, resulting in fine metallic Ni nanoparticles dispersed in the carbon matrix with local graphitization on the ion-induced roughened Au surface. A carbon layer including fine Ni nanoparticles acted as an excellent faradaic redox reaction layer and the roughened surface contributed to an increase in surface area. The fabricated electrode, which included only 14 μg cm⁻² of Ni, showed a stored charge ability three times as large as that of the bulky Ni foil. Thus, it is believed that a carbon layer including Ni nanoparticles fabricated on the charge collective electrode with an ion-irradiation method is promising for the development of supercapacitors from the viewpoints of the reduced use of rare metal and excellent supercapacitor performance.

The charge collective electrode with faradaic redox reactor consisting of carbon nanocomposite including Ni nanoparticles is promising for the supercapacitor electrode.     

Electrochemical properties of kenaf-based activated carbon monolith for supercapacitor electrode applications

Activated carbon monoliths of kenaf (ACMKs) were prepared by moulding kenaf fibers into a column-shape monolith and then carrying out pyrolysis at 500, 600, 700 or 800 °C, followed by activation with KOH at 700 °C. Then, the sample was characterized using thermogravimetric analyzer (TGA), field-emission scanning electron microscopy (FE-SEM), field-emission transmission electron microscopy (FE-TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD) and N₂ sorption instruments. The prepared ACMK was subjected to electrochemical property evaluation via cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). The GCD study using a three-electrode system showed that the specific capacitance decreased with higher pyrolysis temperature (PYT) with the ACMK pyrolyzed at 500 °C (ACMK-500) exhibiting the highest specific capacitance of 217 F g⁻¹. A two-electrode system provided 95.9% retention upon a 5000 cycle test as well as the specific capacitance of 212 F g⁻¹, being converted to an energy density of 6 W h kg⁻¹ at a power density of 215 W kg⁻¹.

Monolithic carbon from kenaf-based fiber for supercapacitor electrode application provided a specific capacitance of 212 F g⁻¹via GCD at 1 A g⁻¹, converting to an energy density of 6 W h kg⁻¹ at the power density of 215 W kg⁻¹ as well as 95.9% retention upon 5000 cycling test.     

Co9S8 nanoparticle-decorated carbon nanofibers as high-performance supercapacitor electrodes

This work reported Co₉S₈ nanoparticle-decorated carbon nanofibers (CNF) as a supercapacitor electrode. By using a mild ion-exchange method, the cobalt oxide-based precursor nanoparticles were transformed to Co₉S₈ nanoparticles in a microwave hydrothermal process, and these nanoparticles were decorated onto a carbon nanofiber backbone. The composition of the nanofibers can be readily tuned by varying the Co acetate content in the precursor. The porous carbon nanofibers offered a fast electron transfer pathway while the well dispersed Co₉S₈ nanoparticles acted as the redox center for energy storage. As a result, high specific capacitance of 718 F g⁻¹ at 1 A g⁻¹ can be achieved with optimized Co₉S₈ loading. The assembled asymmetric supercapacitor with Co₉S₈/CNF as the cathode showed a high energy density of 23.8 W h kg⁻¹ at a power density of 0.75 kW kg⁻¹ and good cycling stability (16.9% loss over 10 000 cycles).

Through electrospinning and the ion-exchange method, Co₉S₈ nanoparticle-decorated carbon nanofibers (Co₉S₈/CNF) have been fabricated, and exhibit good supercapacitor performance.     

Sakura-based activated carbon preparation and its performance in supercapacitor applications

3D porous carbonaceous materials were prepared by combining pre-carbonization and KOH activation with sakura petals as raw materials. The prepared porous sakura carbon (SAC-4) exhibits a high specific surface area, a suitable pore size distribution, a low proportion of oxygen-rich groups and N functional groups, and a partially graphitized phase, which are very beneficial for the electrochemical performance of the material as a supercapacitor electrode. In the activation step, when the mass ratio of KOH to sakura carbon (SC) is 4, a supercapacitor is prepared. A maximal specific capacitance of 265.8 F g⁻¹ is obtained when the current density is 0.2 A g⁻¹. When the current density is 1 A g⁻¹, after 2000 cycles in succession, the capacitance retention rate is excellent and the cycling stability can reach as high as 90.2%. The obtained results indicate that porous carbon prepared with sakura blossom as the raw material is an effective and environmentally friendly electrode material for energy storage.

3D porous carbonaceous materials were prepared by combining pre-carbonization and KOH activation with sakura petals as raw materials.     

Developments and applications of nanomaterial-based carbon paste electrodes

This review summarizes the progress that has been made in the past ten years in the field of electrochemical sensing using nanomaterial-based carbon paste electrodes. Following an introduction into the field, a first large section covers sensors for biological species and pharmaceutical compounds (with subsections on sensors for antioxidants, catecholamines and amino acids). The next section covers sensors for environmental pollutants (with subsections on sensors for pesticides and heavy metal ions). Several tables are presented that give an overview on the wealth of methods (differential pulse voltammetry, square wave voltammetry, amperometry, etc.) and different nanomaterials available. A concluding section summarizes the status, addresses future challenges, and gives an outlook on potential trends.

This review summarizes the progress that has been made in the past ten years in the field of electrochemical sensing using nanomaterial-based carbon paste electrodes.     

Self-template construction of nanoporous carbon nanorods from a metal–organic framework for supercapacitor electrodes

The morphologies and structures of nanostructured carbons generally influence their catalysis, electrochemical performance and adsorption properties. Metal–organic framework (MOF) nanocrystals usually have various morphologies, and can be considered as a template to construct nanostructured carbons with shaped nanocubes, nanorods, and hollow particles by thermal transformation. However, thermal carbonization of MOFs usually leads to collapse of MOF structures. Here, we report shape-preserved carbons (termed as CNRods) by thermal transformation of nickel catecholate framework (Ni-CAT) nanorods. Supercapacitors of CNRods treated at 800 °C were demonstrated to have enhanced performance due to their structural features that facilitate electron conduction and ion transport as well as abundant O content benefiting the wettability of the carbon materials. This may provide a potential way to explore novel carbon materials for supercapacitors with controllable morphologies and high capacitive performance.

The Ni-CAT-derived porous carbon materials at 800 °C remain regular with a rod-like morphology and exhibit enhanced capacitive performance.     

Phyto-synthesized facile Pd/NiOPdO ternary nanocomposite for electrochemical supercapacitor applications

Sustainable and effective electrochemical materials for supercapacitors are greatly needed for solving the global problems of energy storage. In this regard, a facile nanocomposite of Pd/NiOPdO was synthesized using foliar phyto eco-friendly agents and examined as an electrochemical electrode active material for supercapacitor application. The nanocomposite showed a mixed phase of a ternary nano metal oxide phase of rhombohedral NiO and tetragonal PdO confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and XPS (X-rays photoelectron spectroscopy). The optical (direct) energy value of the synthesized nanocomposite was 3.14 eV. The phyto-functionalized nanocomposite was studied for electrochemical supercapacitor properties and revealed a specific capacitance of 88 F g⁻¹ and low internal resistance of 0.8 Ω. The nanoscale and phyto organic species functionalized nanocomposite exhibited enhanced electrochemical properties for supercapacitor application.

The natural phyto bio-factories were successfully utilized for the cost-effective synthesis of facile Pd/NiOPdO ternary nanocomposite for energy storage application with enhanced electro-active site.     

Hierarchically porous N-doped carbon derived from supramolecular assembled polypyrrole as a high performance supercapacitor electrode material

Rationally designed precursors of N-doped carbon are crucial for high performance carbon materials of supercapacitor electrodes. Herein, we report a scalable preparation of hierarchically structured N-doped carbon of micro/meso porous nanofiber morphology by using a supramolecular assembled polypyrrole as the precursor. The influences of the dose of supramolecular dopant on final products after carbonization and sequential chemical activation were investigated. The interconnected nanofiber backbone allows better electron transport and the optimized hierarchically porous structure of the material exhibits a large specific surface area of 2113.2 m² g⁻¹. The N content of the carbon is as high as 6.49 atom%, which is favorable to improve the supercapacitive performance via additional reversible redox reaction over pure carbon. The hierarchically porous N-doped carbon electrode delivered an outstanding specific capacitance of 435.6 F g⁻¹ at 0.5 A g⁻¹, significantly higher than that of the control sample derived from undoped polypyrrole samples. Moreover, the capacitance retention is as high as 96.1% after 5000 cycles. This precursor''s structural control route is readily applicable to various conducting polymers, and provides a methodology to design carbon materials with advanced structure for developing high-performance supercapacitor electrode materials.

Hierarchically porous N-doped carbon with optimized morphology exhibits an enhanced specific capacitance of 435.6 F g⁻¹ at 0.5 A g⁻¹ and 96.1% capacitance retention after 5000 cycles in 1 M H₂SO₄.     

Low cost,high efficiency flexible supercapacitor electrodes made from areca nut husk nanocellulose and silver nanoparticle embedded polyaniline

Energy storage is a key aspect in the smooth functioning of the numerous gadgets that aid easy maneuvering through modern life. Supercapacitors that store energy faradaically have recently emerged as potential inventions for which mechanical flexibility is an absolute requirement for their future applications. Flexible supercapacitors based on nanocellulose extracted from easily available waste materials via low cost methods have recently garnered great attention. In the present work, we discuss the construction of flexible, binder-free supercapacitive electrodes using nanocellulose extracted from locally available areca nut husks and polyaniline embedded with silver nanoparticles. The prepared electrodes were characterized using SEM, TEM, XRD, FTIR, EDX and electrochemical characterization techniques such as CV, galvanostatic charge–discharge, chronoamperometry and EIS. A specific capacitance of 780 F g⁻¹ was obtained for the silver nanoparticle embedded polyaniline–nanocellulose (Ag–PANI–NC) substrate supported electrodes, which is ∼4.2 times greater than that of bare polyaniline–nanocellulose electrodes. We attributed this enhancement to a lowering of the activation energy barrier of correlated electron hopping among localized defect states in the composite matrix by the Ag nanoparticles. An energy density value of 15.64 W h kg⁻¹ and a power density of 244.8 W kg⁻¹ were obtained for the prepared electrodes. It was observed that the Ag–PANI–NC based electrode can retain ∼98% of its specific capacitance upon recovery from mechanical bending to extreme degrees.

Energy storage is a key aspect in the smooth functioning of the numerous gadgets that aid easy maneuvering through modern life. Utilization of waste materials for energy storage applications enables the sustainable development of energy field.     

3D nanorhombus nickel nitride as stable and cost-effective counter electrodes for dye-sensitized solar cells and supercapacitor applications

Transition metal nitride based materials have attracted significant interest owing to their excellent properties and multiple applications in the field of electrochemical energy conversion and storage devices. Herein we synthesize 3D nanorhombus nickel nitride (Ni₃N) thin films by adopting a reactive radio frequency magnetron sputtering process. The as-deposited 3D nano rhombus Ni₃N thin films were utilized as cost-effective electrodes in the fabrication of supercapacitors (SCs) and dye-sensitized solar cells (DSSCs). The structure, phase formation, surface morphology and elemental composition of the as-deposited Ni₃N thin films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM). The electrochemical supercapacitive performance of the Ni₃N thin films was examined by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) techniques, in 3 M KOH supporting electrolyte. The areal capacitance of the Ni₃N thin film electrode obtained from CV analysis was 319.5 mF cm⁻² at a lower scan rate of 10 mV s⁻¹. Meanwhile, the Ni₃N thin film showed an excellent cyclic stability and retained 93.7% efficiency of its initial capacitance after 2000 cycles at 100 mV s⁻¹. Interestingly, the DSSCs fabricated with a Ni₃N CE showed a notable power energy conversion efficiency of 2.88% and remarkable stability. The prominent performance of the Ni₃N thin film was ascribed mainly due to good conductivity, high electrochemically active sites with excellent 3D nano rhombus structures and high electrocatalytic activity. Overall, these results demonstrate that the Ni₃N electrode is capable of being considered for efficient SCs and DSSCs. This investigation also offers an essential directive for the advancement of energy storage and conversion devices.

Self-supported 3D nano-rhombus (nano-diamond) shaped Ni₃N coated on FTO glass which serves as a CE in DSSCs and supercapacitors. .     

Synthesis of coaxial carbon@NiMoO4 composite nanofibers for supercapacitor electrodes

This work reports the synthesis of coaxial carbon@NiMoO₄ nanofibers for supercapacitor electrode applications. Thin NiMoO₄ nanosheets are uniformly coated on the conductive electrospun carbon nanofibers by a microwave assisted hydrothermal method to form a hierarchical structure, which increases the porosity as well as the conductivity of the electrode. The thickness of the NiMoO₄ can be easily adjusted by varying the precursor concentrations. The high specific surface area (over 280 m² g⁻¹) and conductive carbon nanofiber backbone increase the utilization of the active pseudocapacitive NiMoO₄ phase, resulting a high specific capacitance of 1840 F g⁻¹.

This work reports the synthesis of coaxial carbon@NiMoO₄ nanofibers for supercapacitor electrode applications.     

Gamma-radiated biochar carbon for improved supercapacitor performance

Biochar carbon YP-50 exposed to gamma radiation at 50 kGy, 100 kGy, and 150 kGy was used as an electrode for an electric double-layer capacitor. The gamma radiation affected the pore structure and pore volume of the biochar electrodes. The optimized surface morphology, pore structure, and pore volume of the biochar with an irradiation dose of 100 kGy showed outstanding specific capacitance of 246.2 F g⁻¹ compared to the untreated biochar (115.3 F g⁻¹). The irradiation dose of 100 kGy exhibited higher specific power and specific energy of 0.1 kW kg⁻¹ and 34.2 W h kg⁻¹ respectively, with a capacity retention of above 96% after 10 000 cycles at a current density of 2 A g⁻¹. This improvement can be attributed to the decrease in average particle size, an increase in the porosity of biochar carbon. Besides, the charge transfer resistance of supercapacitor is significantly reduced from 21.7 Ω to 7.4 Ω after treating the biochar carbon with 100 kGy gamma radiation, which implies an increase in conductivity. This gamma radiation strategy to pretreat the carbon material for improving the properties of carbon materials can be promising for the development of high-performance supercapacitors for large-scale applications.

(a) Schematic of the biochar carbon YP-50 exposed to gamma radiation. (b) Cyclic Voltammetry of supercapacitor with untreated biochar and biochar treated with 50 kGy, 100 kGy, and 150 kGy gamma-radiation.