Post-synthesis isomorphous substitution of layered Co-Mn hydroxide nanocones with graphene oxide as high-performance supercapacitor electrodes

Construction of Cu7KS4@NixCo1-x(OH)2 Nano-Core-Shell Structures with High Conductivity and Multi-Metal Synergistic Effect for Superior Hybrid Supercapacitors

Reasonable design of materials with complex nanostructures and diverse chemical compositions is of great significance in the field of energy storage. Cu₇KS₄ (CKS) is considered a potential electrode material for supercapacitors due to its superior electrical conductivity. Transition metal hydroxides are widely used as electrode materials for supercapacitors due to their high theoretical specific capacitance (C_s); however, single metal species with limited active sites restrict their further applications for energy storage. Herein, through a hydrothermal reaction, CKS nanorods were prepared, and then binary metal hydroxide Ni_xCo_1-x(OH)₂ nanosheets were generated directly on CKS nanorods through a one-step hydrothermal reaction to form a nano-core-shell structure (NCSS). By regulating the mole ratio of nickel nitrate to cobalt nitrate, the resulting Ni_0.75Co_0.25(OH)₂ nanosheets with the best electrochemical activity were prepared and supported on CKS nanorods to form a CKS@N_0.75C_0.25OH NCSS. The as-prepared CKS@N_0.75C_0.25OH NCSS has a larger specific surface area, which can provide more active sites, while the abundant metal species composition can generate abundant redox reactions to boost the pseudocapacitance. The prepared CKS@N_0.75C_0.25OH/NF electrode exhibits outstanding specific capacitance and cycle life. The assembled CKS@N_0.75C_0.25OH/NF//AC all-solid-state asymmetric supercapacitor achieves a high energy density of 88.7 Wh kg^-1 at a power density of 849.9 W kg^-1 with superior cycle life. Therefore, the use of polymetallic hydroxides to construct NCSS electrodes has great research significance and broad application prospects.

Ready-to-use binder-free Co(OH)2 plates@porous rGO layers/Ni foam electrode for high-performance supercapacitors

In this work, an outstanding nano-structured composite electrode is fabricated through the co-deposition of Co(OH)₂ nanoplates and porous reduced GO (p-rGO) nanosheets onto Ni foam (NF). Through field emission scanning electron microscopy and transmission electron microscopy observations, it was confirmed that porous reduced graphene oxide sheets are completely wrapped by uniform hexagonal Co(OH)₂ plates. Due to the unique architecture of both components of the prepared composite, a high surface area of 234.7 m² g⁻¹ and mean pore size of 3.65 nm were observed for the Co(OH)₂@p-rGO composite. The constructed Co(OH)₂@p-rGO/NF composite electrode shows higher energy storage capability compared to that of Co(OH)₂/NF and p-rGO/NF electrodes. The Co(OH)₂/NF electrode shows specific capacitances of 902 and 311 F g^–1 at 5 and 30 A g^–1, while the Co(OH)₂@p-rGO/NF electrode delivers 1688 and 1355 F g^–1 under the same current loads, respectively. Furthermore, when the current load was increased from 1 to 30 A g^–1, 74.5% capacitance retention was observed for the Co(OH)₂@p-rGO/NF electrode, indicating its outstanding high-power capability, while the Co(OH)₂/NF electrode retained only 38.5% of its initial capacitance. The fabricated Co(OH)₂@p-rGO/NF//rGO/NF ASC device shows an areal capacitance of 3.29 F cm⁻², cycling retention of 91.2% after 4500 cycles at 5 A g⁻¹ and energy density of 68.7 W h kg⁻¹ at a power density of 895 W kg⁻¹. The results of electrochemical tests prove that Co(OH)₂@p-rGO/NF exhibits good performance as a positive electrode for use in an asymmetric supercapacitor device. The prepared porous composite electrode is thus a promising candidate for use in supercapacitor applications.

Fabrication mechanism of a ready-to-use Co(OH)2@p-rGO/NF electrode: (a) base generation step, (b) electrophoretic deposition of rGO onto NF and (c) chemical formation of Co(OH)2 on rGO.

High performance flexible asymmetric supercapacitor constructed by cobalt aluminum layered double hydroxide @ nickel cobalt layered double hydroxide heterostructure grown in-situ on carbon cloth

HYPOTHESIS

Three-dimensional layered layered double hydroxide (LDH) nanostructure materials grow in-situ on excellent conductive and flexible carbon cloth (CC) substrate not only reduce the ability of binders in resisting ions transfer, but also make them to be quasi-vertically arranged well on substrates without aggregation. This would result in enough electroactive sites, to obtain superior electrochemical performance.

EXPERIMENTS

A hierarchical CoAl-LDH@NiCo-LDH composite was prepared on a surface-modified carbon cloth by a simple two-step hydrothermal process. In this process, CoAl-LDH nanosheets (NSs)/CC acting as the inner core were wrapped up in NiCo-LDH nanoneedle arrays (NNAs) evenly. Also, a flexible quasi-solid-state supercapacitor device was constructed using CoAl-LDH@NiCo-LDH/CC and activated carbon (AC) as a positive electrode and a negative electrode, respectively.

FINDINGS

The CoAl-LDH@NiCo-LDH/CC developed had an excellent specific capacitance (2633.6F/g at 1 A/g) with remarkable cyclic performance (92.5% retention of its incipient over 5000 cycles at 4 A/g). The flexible quasi-solid-state supercapacitor device CoAl-LDH@NiCo-LDH/CC//AC/CC yielded a splendid energy density of 57.8 Wh/kg at a power density of 0.81 kW/kg and a brilliant power density of 16.09 kW/kg at 38.0 Wh/kg in a broad potential window of 1.55 V. Furthermore, the exceptional cyclic stability and excellent flexibility of the device show it can be applied in flexible energy storage systems.

Structure-dependent electrochemical properties of cobalt (II) carbonate hydroxide nanocrystals in supercapacitors

In this work, we report the structure-dependent electrochemical performance of cobalt carbonate hydroxide (Co₂(OH)₂CO₃) nanocrystals by experimental investigation and theoretical simulation. Different Co₂(OH)₂CO₃ nanostructures including two-dimensional (2D) nanosheets (NSs) and one-dimensional (1D) nanowires (NWs), were synthesized on self-supported carbon cloth substrates by a facile hydrothermal method. Compared to 1D NWs, 2D Co₂(OH)₂CO₃ NSs provided a short ion transfer path, and low electron transfer resistance during the electrochemical reaction. At the current density of 2 mA cm^-2, 2D Co₂(OH)₂CO₃ NSs exhibited a higher area capacitance of 2.15F cm^-2 and better cycling performance (96.2% retention after 10,000 cycles) than that of 1D NWs (1.15F cm^-2 and 90.1% retention). First-principles density functional theory (DFT) calculations revealed that the band gap of the (120) facet in 2D NSs was 0.2 eV, far less than of the (200) facet in 1D NWs (1.04 eV). Electrochemical impedance spectroscopy (EIS) measurements further indicated that the electron transfer and reaction kinetics were more efficient in 2D NSs. This work can provide an important insight in understanding the mechanism of electrochemical energy storage.

Insights into the morphology and composition effects of one-dimensional CuPt nanostructures on the electrocatalytic activities and methanol oxidation mechanism by in situ FTIR

Morphology modulation and surface structure-controlled synthesis are two effective ways to tune the electrocatalytic activities of metal nanomaterials. Pt-based binary or ternary metal nanostructures have become a class of promising catalysts toward the oxygen reduction reaction (ORR) and the methanol oxidation reaction (MOR) for direct methanol fuel cells. Herein to reveal the morphology and surface structure effects of one-dimensional (1D) Pt-based nanostructures on their electrocatalytic properties, two types of 1D CuPt nanowires (CuPt NWs) and CuPt nanotubes (CuPt NTs) with tunable surface structures and compositions were fabricated using a convenient and easy strategy. It was found that among all the studied samples, CuPt2.22 NWs exhibited the highest efficiency catalytic performances for both the ORR and MOR in an acidic electrolyte. For the ORR, CuPt2.22 NWs exhibited an onset potential (Eonset) of 0.749 V and a half-wave potential (E1/2) of 0.577 V, which are more positive than those of the commercial Pt/C (0.668 V and 0.558 V). On the other hand, CuPt2.22 NWs show a specific activity of 20.76 mA cm-2 and a mass activity of 0.171 mA μgPt-1 for the MOR, which are 7.75 and 1.82 times, respectively, larger than those of Pt/C (2.679 mA cm-2 and 0.094 mA μgPt-1). Meanwhile, the reaction mechanism of the MOR on CuPt2.22 NWs was examined by in situ FTIR. From the enhanced IR absorption, the linear- and bridge-adsorbed CO intermediates can be determined during the methanol oxidation on CuPt2.22 NWs, from which the MOR proceeds through a dual reaction pathway. This work reveals that rationally tuning the electronic structures of 1D metal nanomaterials by well-controlling the composition and surface morphology on the nanoscale could greatly enhance the catalytic properties, which are very important for their application in fuel cells.

A review of the α-Fe2O3 (hematite) nanotube structure: recent advances in synthesis, characterization, and applications

α-Fe2O3 nanotubes are exceptional one-dimensional transition metal oxide materials with low density, large surface area, promising electrochemical and photoelectrochemical properties, which are widely investigated in lithium-ion batteries, photoelectrochemical devices, gas sensors, and catalysis. They have drawn significant attention to the fields of energy storage and conversion, and environmental sensing and remediation due to the increase in the global energy crisis and environmental pollution. Many efforts have been made toward controlling the morphology or impurity doping to improve the intrinsic properties of α-Fe2O3 nanotubes. In this review, we introduce the synthesis methods and physicochemical properties of α-Fe2O3 nanotubes. The fabrication conditions, which are important for the physicochemical properties of materials, are also listed to describe the synthesis processes. Furthermore, the development and breakthrough of various applications in batteries, supercapacitors, photoelectrochemical devices, environmental remediation, and sensors are systematically reviewed. Finally, some of the current challenges and future perspectives for α-Fe2O3 nanotubes are discussed. We believe that this timely and critical mini-review will stimulate extensive studies and attract more attention, further improving the development of the α-Fe2O3 (hematite) nanotube structure.

Hierarchical NiMoO4@Co3V2O8 hybrid nanorod/nanosphere clusters as advanced electrodes for high-performance electrochemical energy storage

Herein, a synergistic strategy to construct hierarchical NiMoO₄@Co₃V₂O₈ (denoted as NMO@CVO) hybrid nanorod/nanosphere clusters is proposed for the first time, where Co₃V₂O₈ nanospheres (denoted as CVO) have been uniformly immobilized on the surface of the NiMoO₄ nanorods (denoted as NMO) via a facile two-step hydrothermal method. Due to the surface recombination effect between NMO and CVO, the as-prepared NMO@CVO effectively avoids the aggregation of CVO nanosphere clusters. The unique hybrid architecture can make the most of the large interfacial area and abundant active sites for storing charge, which is greatly beneficial for the rapid diffusion of electrolyte ions and fast electron transport. The optimized NMO@CVO-8 composite nanostructure displays battery-like behavior with a maximum specific capacity of 357 C g^-1, excellent rate capability (77.8% retention with the current density increasing by 10 times) and remarkable cycling stability. In addition, an aqueous asymmetric energy storage device is assembled based on the NMO@CVO-8 hybrid nanorod/nanosphere clusters and activated carbon. The device shows an ultrahigh energy density of 48.5 W h kg^-1 at a power density of 839.1 W kg^-1, good rate capability (20.9 W h kg^-1 even at 7833.7 W kg^-1) and excellent cycling stability (83.5% capacitance retention after 5000 cycles). More notably, two charged devices in series can light up a red light-emitting diode (LED) for 20 min, demonstrating its potential in future energy storage applications. This work indicates that the hierarchical NiMoO₄@Co₃V₂O₈-8 hybrid nanorod/nanosphere clusters are promising energy storage materials for future practical applications and also provides a rational strategy for fabricating novel nanostructured materials for high-performance energy storage.

Electronegativity principles in metal oxides based supercapacitors

To meet growing demands for energy consumptions in modern society, it is necessary to develop different energy sources. Renewable energy such as wind and solar sources are intermittent, therefore, energy storage devices become more and more important to store energy for use when no wind or no light. Supercapacitors play a key role in energy storage, mainly due to their high power density and long cycling life. However, supercapacitors are facing the obstacle of low energy density, one of the most intensive approaches is to rationally design new electrode materials. In this review, we focus on metal oxides-based materials and present an electronegativity criterion for the design and appropriate selection of new electrode chemical compositions. Metal elements with proper electronegativity scale have the potential to transfer electron for energy storage. Suitable positive and negative electrodes matching can enhance many properties of supercapacitors, which may overcome many related obstacles. Furthermore, electronegativity scale may also help people to find novel metal oxides based supercapacitors.

A facile fabrication of 1D/2D nanohybrids composed of NiCo-hydroxide nanowires and reduced graphene oxide for high-performance asymmetric supercapacitors

1D/2D nanohybrids composed of NiCo-hydroxide nanowires and graphene have been successfully fabricated by a simple method.

Facile Fabrication of NiCoAl-Layered Metal Oxide/Graphene Nanosheets for Efficient Capacitive Deionization Defluorination

Capacitive deionization (CDI) has aroused extensive attention as a prospective technology for different ionic species removal from aqueous solutions. Traditional studies on the adsorption and desorption of fluoride from wastewater are energy-intensive and may have harmful effects on the environment. Herein, the feasibility of fluoride removal from wastewater by CDI has been investigated. NiCoAl-layered metal oxide (NiCoAl-LMO) nanosheets and reduced graphene oxide (rGO) composites (NiCoAl-LMO/rGO) were synthesized and used as CDI electrode materials for fluoride ion removal. The as-obtained NiCoAl-LMO/rGO with unique structure and high conductivity is beneficial to the adsorption of fluoride ions. In addition, the introduction of Co element in the laminate enhances the pseudocapacitive behavior of the electrode material. As expected, the CDI system with NiCoAl-LMO/rGO composites as anode and activated carbon treated by nitric acid (H-AC) as cathode exhibits outstanding defluorination performance. The maximum adsorption capacity of NiCoAl-LMO/rGO, 24.5 mg g^-1, can be reached when the initial NaF concentration is 500 mg L^-1 at 1.4 V applied voltage. The composites also show good cycle stability over 40 consecutive cycles of the CDI defluorination process. The excellent defluorination performance of NiCoAl-LMO/rGO makes it possible for its practical application in wastewater treatment.