Phosphate Ion Functionalized Co3 O4 Ultrathin Nanosheets with Greatly Improved Surface Reactivity for High Performance Pseudocapacitors

Tailoring and understanding the lithium storage performance of triple-doped cobalt phosphide composites

Cobalt phosphide (CoP) with high theoretical capacity as well as ceramic-like and metal-like properties is considered as a promising anode for lithium-ion batteries (LIBs). However, the large volume change and sluggish kinetic response limit its practical application. The optimization of composition, structural control and performance regulation of CoP electrodes can be achieved by the bottom-up assembly technique of metal-organic frameworks (MOFs). Due to the effective electronic regulation and lithiophilicity brought by the multiple heteroatoms doping and the synergistic effect of the unique structure derived from MOFs, the N, O, P triple-doped carbon and CoP composites (ZCP@NOP) exhibited excellent rate capability (554.61 mAh g-1 at 2 A g-1) and cycling stability (806.7 mAh g-1 after 500 cycles at 0.5 A g-1). The essence and evolution of lithium storage mechanism in CoP electrodes are also confirmed by the ex-situ techniques. The synergistic benefits of heteroatom co-doping carbon and cobalt phosphide, such as the decrease of the diffusion energy barrier of Li-ions and the optimization of electronic structures, are highlighted in theoretical calculations. In conclusion, new thoughts and ideas for the creation of future battery anode are provided by the combination of the N, O, P co-doping and the adaptable structural adjustment technique.

Highly exposed metal atomic active sites in Al2O3/CoNC: Modify reaction pathways by coupling oxygen species

The catalytic oxidation of formaldehyde (HCHO) at ambient temperature is a highly efficient, cost-effective and environmentally friendly approach for formaldehyde removal. Reactive oxygen (O*) and reactive hydroxyl groups (OH*) are the main active species in the catalytic oxidation reaction of HCHO. Therefore, it is crucial to design catalysts that can simultaneously enhance the surface concentrations of O* and OH*, thereby improving their overall catalytic performance. The present study aimed to design an Al2O3/CoNC catalyst featuring layered carbon nitride coupled with metal oxides possessing domain-limited cobalt (Co) metal active sites, to efficiently remove HCHO (≈100 %, 100 ppm, RH=50 %, GSHV=20,000 mL/(g h)) and ensure stability (more than 90 % formaldehyde removal within 450 h) at ambient temperature. The characterization revealed that the interaction between Al2O3-supported metal and CoNC resulted in enhanced confinement of Co, leading to a higher abundance of edge structures exposing more active sites. Additionally, the presence of highly dispersed Co-NX active sites and increased oxygen vacancies effectively facilitated the adsorption and activation processes of HCHO and O2, as well as the adsorption and desorption dynamics of intermediates during the reaction. These factors collectively contributed to an improved catalytic activity. The results of in situ infrared spectroscopy revealed that the catalyst improved the adsorption and activation of O2 and H2O, leading to the rapid generation of substantial amounts of O* and OH*. This synergistic interaction between Al2O3 and CoNC plays a crucial role in the sustained production of O* and OH*, promoting efficient of intermediate decomposition, and ensuring excellent catalytic activity and stability for HCHO.

Electrical-Driven Directed-Evolution of Copper Nanowires Catalysts for Efficient Nitrate Reduction to Ammonia

The electrocatalytic conversion of nitrate (NO3 -) to NH3 (NO3RR) at ambient conditions offers a promising alternative to the Haber-Bosch process. The pivotal factors in optimizing the proficient conversion of NO3 - into NH3 include enhancing the adsorption capabilities of the intermediates on the catalyst surface and expediting the hydrogenation steps. Herein, the Cu/Cu2O/Pi NWs catalyst is designed based on the directed-evolution strategy to achieve an efficient reduction of NO3‾. Benefiting from the synergistic effect of the OV-enriched Cu2O phase developed during the directed-evolution process and the pristine Cu phase, the catalyst exhibits improved adsorption performance for diverse NO3RR intermediates. Additionally, the phosphate group anchored on the catalyst's surface during the directed-evolution process facilitates water electrolysis, thereby generating Hads on the catalyst surface and promoting the hydrogenation step of NO3RR. As a result, the Cu/Cu2O/Pi NWs catalyst shows an excellent FE for NH3 (96.6%) and super-high NH3 yield rate of 1.2 mol h-1 gcat. -1 in 1 m KOH and 0.1 m KNO3 solution at -0.5 V versus RHE. Moreover, the catalyst's stability is enhanced by the stabilizing influence of the phosphate group on the Cu2O phase. This work highlights the promise of a directed-evolution approach in designing catalysts for NO3RR.

Bi-Interlayer Strategy for Modulating NiCoP-Based Heterostructure toward High-Performance Aqueous Energy Storage Devices

Nickel-cobalt (NiCo) phosphides (NCPs) possess high electrochemical activity, which makes them promising candidates for electrode materials in aqueous energy storage devices, such as supercapacitors and zinc (Zn) batteries. However, the actual specific capacitance and rate capability of NCPs require further improvement, which can be achieved through reasonable heterostructural design and loading conditions of active materials on substrates. Herein, novel hierarchical Bi-NCP heterogeneous structures with built-in electric fields consisting of bismuth (Bi) interlayers (electrodeposited on carbon cloth (CC)) are designed and fabricated to ensure the formation of uniform high-load layered active materials for efficient charge and ion transport. The resulting CC/Bi-NCP electrodes show a uniform, continuous, and high mass loading (>3.5 mg) with a superior capacitance reaching 1200 F g-1 at 1 A g-1 and 4129 mF cm-2 at 1 mA cm-2 combined with high-rate capability and durable cyclic stability. Moreover, assembled hybrid supercapacitors (HSCs), supercapatteries, and alkaline Zn-ion (AZBs) batteries constructed using these electrodes deliver high energy densities of 64.4, 81.8, and 319.1 Wh kg-1, respectively. Overall, the constructed NCPs with excellent aqueous energy storage performance have the potential for the development of novel transition metal-based heterostructure electrodes for advanced energy devices.

Nucleophilic hydrolysis enables HF-etched MXene kilofarad specific capacitance and excellent rate performance

Here, an unusual MXene with a high ratio of oxygen functional groups was prepared by hydrothermal treatment of HF-etched MXene in aqueous KOH solution. The prepared MXene (H-220) exhibits ultrahigh specific capacitance (1030 F g-1 in a potential window of 0.85 V), and excellent rate and cycling performance simultaneously in a sulfuric acid electrolyte, and can act as an anode material of proton batteries.

High-efficiency activated phosphorus-doped Ni2S3/Co3S4/ZnS nanowire/nanosheet arrays for energy storage of supercapacitors

Transition metal sulfides (TMS) have been considered as a promising group of electrode materials for supercapacitors as a result of their strong redox activity, but high volumetric strain of the materials during electrochemical reactions causes rapid structural collapse and severe capacity loss. Herein, we have synthesized phosphorus-doped (P-doped) Ni2S3/Co3S4/ZnS battery-type nanowire/nanosheet arrays as an advanced cathode for supercapacitor through a two-step process of hydrothermal and annealing treatments. The material has a one-dimensional nanowire/two-dimensional nanosheet-like coexisting microscopic morphology, which facilitates the exposure of abundant active centers and promotes the transport and migration of ions in the electrolyte, while the doping of P significantly enhances the conductivity of the electrode material. Simultaneously, the element phosphorus with similar atomic radii and electronegativity to sulfur may act as electron donors to regulate the electron distribution, thus providing more effective electrochemically active sites. In gratitude to the synergistic effect of microstructure optimization and electronic structure regulation induced by the doing of P, the P-Ni2S3/Co3S4/ZnS nanoarrays provide a superior capacity of 2716 F g-1 at 1 A/g, while the assembled P-Ni2S3/Co3S4/ZnS//AC asymmetric supercapacitor exhibits a high energy density of 48.2 Wh kg-1 at a power density of 800 W kg-1 with the capacity retention of 89 % after 9000 cycles. This work reveals a possible method for developing high-performance transition metal sulfide-based battery-like electrode materials for supercapacitors through microstructure optimization and electronic structure regulation.

Heterointerface manipulation in the architecture of Co-Mo2C@NC boosts water electrolysis

Heterostructures with tunable electronic properties have shown great potential in water electrolysis for the replacement of current benchmark precious metals. However, constructing heterostructures with sufficient interfaces to strengthen the synergistic effect of multiple species still remains a challenge due to phase separation. Herein, an efficient electrocatalyst composed of a nanosized cobalt/Mo2C heterostructure anchored on N-doped carbon (Co-Mo2C@NC) was achieved by in situ topotactic phase transformation. With the merits of high conductivity, hierarchical pores, and strong electronic interaction between Co and Mo2C, the Co-Mo2C@5NC-4 catalyst shows excellent activity with a low overpotential for the hydrogen evolution reaction (HER, 89 mV@10 mA cm-2 in alkaline medium; 143 mV@10 mA cm-2 in acidic medium) and oxygen evolution reaction (OER, 356 mV@10 mA cm-2 in alkaline medium), as well as high stability. Furthermore, this catalyst in an electrolyzer shows efficient activity for overall water splitting and long-term durability. Theoretical calculations reveal the optimized adsorption-desorption behaviour of hydrogen intermediates on the generated cobalt layered hydroxide (Co LDH)/Mo2C interfaces, resulting in boosting alkaline water electrolysis. This work proposes a new interface-engineering perspective for the construction of high-activity heterostructures for electrochemical conversion.

Effect of synthesis methods on the activity of NiO/Co3O4 as an electrode material for supercapacitor: in the light of X-ray diffraction study

The formation of heterostructures by combining individual components (NiO and Co3O4) is a preferred approach to enhance electrochemical performance as it leads to improved charge transfer and surface reaction kinetics. In the present work, a NiO/Co3O4 composite was prepared by two methods. First, neat NiO and Co3O4 were prepared by adopting the hydrothermal method followed by the formation of the composite (i) by a hydrothermal route (NC-Hydro) and (ii) by a calcination route (NC-Cal). NC-Hydro composite shows a specific capacity of 176 C g-1 at 1 A g-1 of current density in the three-electrode system in a 2 M KOH solution as an electrolyte with 90% cyclic retention after 5000 cycles at 4 A g-1. NC-Cal shows a specific capacity of 111 C g-1 at 1 A g-1 with 75% cyclic retention. The coulombic efficiency of NC-Hydro was 86.3% while for NC-Cal it was 42.3%. The reason behind the superior electrochemical performance of NC-Hydro in comparison to NC-Cal may be the large interlayer spacing and lattice parameters of the former, which provide large space for redox reactions. The unit cell volume of the composites was more than that of the constituents. This study reveals that the composites prepared by the hydrothermal method have superior electrochemical properties in comparison to composites prepared by the calcination method.

Defect engineering with N-doped carbon hybrid cobalt-molybdenum phosphide nanosheets wrapped molybdenum oxide nanorods for alkaline hydrogen evolution reaction

Regulating the electrocatalytic hydrogen evolution reaction (HER) performance through defect engineering of the surface of the catalysts is an effective pathway. Herein, cobalt-molybdenum phosphide (CoMoP) nanosheets wrapped molybdenum oxide (MoO3) core-shell nanorods (MoO3@CoMoP), as alkaline electrocatalysts with ligand-derived N-doped carbon hybrid and oxygen-vacancies, were synthesized via solvothermal approaches and followed by phosphorization. As expected, the MoO3@MoCoP affords efficient HER with a low overpotential (η) of 84.2 ± 0.4 mV at 10 mA cm-2. After phosphorization, not only the MoCoP active species are incorporated into the catalyst, but also the defects sites are achieved. Impressively, the metal-ligand-derived MoCoP are distributed uniformly in the N-doped carbon hybrid matrix, exhibiting well-exposed active sites. Benefiting from the synergy effect of MoCoP active species and oxygen-vacancy, the MoO3@MoCoP showed increased conductivity and stability, which can deliver a current density of 10 mA cm-2 over 40 h. MoO3@MoCoP exhibits an optimal electronic structure on the surface by charge redistribution at the interface, thereby optimizing the hydrogen adsorption energy and accelerating the hydrogen evolution kinetics. This work paves the way for the design of transition metal electrocatalysts with desirable properties through a promising strategy in the field of energy conversion.

Electronic Structure Regulation and Surface Reconstruction of Iron Diselenide for Enhanced Oxygen Evolution Activity

Regulating the electronic structure of active sites and monitoring the evolution of the active component is essential to improve the intrinsic activity of catalysts for electrochemical reactions. Herein, a highly efficient pre-electrocatalyst of iron diselenide with rich Se vacancies achieved by phosphorus doping (denoted as P-FeSe2 ) for oxygen evolution reaction (OER) is reported. Systematically experimental and theoretical results show that the formed Se vacancies with phosphorus doping can synergistically modulate the electronic structure of FeSe2 and facilitate OER kinetics with the resulting enhanced electrical conductivity and electrochemical surface area. Importantly, the in situ formed FeOOH species on the surface of the P-FeSe2 nanorods (denoted as P-FeOOH(Se)) during the OER process acts as an active component to efficiently catalyze OER and exhibits a low overpotential of 217 mV to reach 10 mA cm-2 with good durability. Promisingly, an alkaline electrolyzer assembled with P-FeOOH(Se) and Pt/C electrodes requires an ultra-low cell voltage of 1.50 V at 10 mA cm-2 for overall water splitting, which is superior to the RuO2 || Pt/C counterpart and most of the state-of-the-art electrolyzers, demonstrating the high potential of the fabricated electrocatalyst by P doping strategy to explore more highly efficient selenide-based catalysts for various reactions.

Surface Adsorption of Amorphous Phosphate on RuNi-Doped Molybdate for the Hydrogen Evolution Reaction

Developing highly active and cost-effective electrocatalysts is critical for enhancing the intrinsic performance of electrocatalytic water splitting. Oxoanion-based compounds, such as phosphates and molybdates, have emerged as promising electrocatalysts owing to their advantageous properties of nontoxicity, low price, and strong water adsorption ability. However, their relatively inferior activity has impeded extensive investigation into electrochemical applications. Herein, an amorphous phosphate-adsorbed and RuNi-doped molybdate (RuNiMo-P) composite is synthesized on nickel foam (NF) support by using a simple two-step method. Significantly, an acidic solution of phosphomolybdic acid (PMo12), containing a low concentration of Ru, can etch the NF, contributing to the in situ growth of the RuNi-doped molybdate precursor. Subsequent phosphating ensures the surface formation of the amorphous phosphate layer due to abundant oxygen in the precursor. The strong structural interaction between RuNi-doped molybdate and amorphous phosphate in RuNiMo-P prompts an enhanced hydrogen evolution reaction (HER) performance, delivering an overpotential of 38 mV at a current density of -10 mA cm-2, a Tafel slope of 53 mV dec-1, and good stability in an alkaline medium. Characterizations after HER reveal that RuNi doping, partial dissolution of phosphate and molybdate species, and newly formed NiOOH nanosheets can expose active sites, facilitate charge transfer, and modify electronic structures, thereby improving the HER performance effectively.

RGO decorated N-doped NiCo2O4 hollow microspheres onto activated carbon cloth for high-performance non-enzymatic electrochemical glucose detection

This paper reports the first effective fabrication of a high-performance non-enzymatic glucose sensor based on activated carbon cloth (ACC) coated with reduced graphene oxide (RGO) decorated N-doped urchin-like nickel cobaltite (NiCo₂O₄) hollow microspheres. Hierarchically mesoporous N-doped NiCo₂O₄ hollow microspheres were synthesized using a facile solvothermal method, followed by thermal treatment in a nitrogen (N₂) atmosphere. Subsequently, they were hydrothermally decorated with RGO nanoflakes. The resulting composite was dip-coated onto ACC, and its electrochemical and glucose sensing performances were investigated using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometric measurements in a three-electrode system. The composite electrode sensor demonstrates admirable sensitivity (6122 μM mM^-1 cm^-2) with an ultralow detection limit (5 nM, S/N = 3), and it performs well within a substantial linear range (0.5-1.450 mM). Additionally, it exhibits good long-term response stability and outstanding anti-interference performance. These outstanding results can be attributed to the synergistic effects of the highly electrically conductive ACC with multiple channels, the enhanced catalytic activity of highly porous N-doped NiCo₂O₄ hollow microspheres, and the large electroactive sites provided by its well-developed hierarchical nanostructure and RGO nanoflakes. The findings highlight the enormous potential of the ACC/N-doped NiCo₂O₄@RGO electrode for non-enzymatic glucose sensing.

Vanadate ion promoting the transformation of α-phase molybdenum trioxide (α-MoO3) to h-phase MoO3 (h-MoO3) for boosted Zn-ion storage

Molybdenum trioxide (MoO₃) has been widely studied in the energy storage field due to its various phase states and unique structural advantages. Among them, lamellar α-phase MoO₃ (α-MoO₃) and tunnel-like h-phase MoO₃ (h-MoO₃) have attracted much attention. In this study, we demonstrate that vanadate ion (VO₃^-) can transform α-MoO₃ (a thermodynamically stable phase) to h-MoO₃ (a metastable phase) by altering the connection of [MoO₆] octahedra configurations. h-MoO₃ with VO₃^- inserted (referred to as h-MoO₃-V) as the cathode material for aqueous zinc ion batteries (AZIBs) exhibits excellent Zn²⁺ storage performances. The improvement in electrochemical properties is attributed to the open tunneling structure of the h-MoO₃-V, which offers more active sites for Zn²⁺ (de)intercalation and diffusion. As expected, the Zn//h-MoO₃-V battery delivers specific capacity of 250 mAh·g^-1 at 0.1 A·g^-1 and rate capability (73% retention from 0.1 to 1 A·g^-1, 80 cycles), well exceeding those of Zn//h-MoO₃ and Zn//α-MoO₃ batteries. This study demonstrates that the tunneling structure of h-MoO₃ can be modulated by VO₃^- to enhance the electrochemical properties for AZIBs. Furthermore, it provides valuable insights for the synthesis, development and future applications of h-MoO₃.

A Mixed Protonic-Electronic Conductor Base on the Host-Guest Architecture of 2D Metal-Organic Layers and Inorganic Layers

The key to designing and fabricating highly efficient mixed protonic-electronic conductors materials (MPECs) is to integrate the mixed conductive active sites into a single structure, to break through the shortcomings of traditional physical blending. Herein, based on the host-guest interaction, an MPEC is consisted of 2D metal-organic layers and hydrogen-bonded inorganic layers by the assembly methods of layered intercalation. Noticeably, the 2D intercalated materials (≈1.3 nm) exhibit the proton conductivity and electron conductivity, which are 2.02 × 10^-5 and 3.84 × 10^-4 S cm^-1 at 100 °C and 99% relative humidity, much higher than these of pure 2D metal-organic layers (>>1.0 × 10^-10 and 2.01×10^-8 S cm^-1 ), respectively. Furthermore, combining accurate structural information and theoretical calculations reveals that the inserted hydrogen-bonded inorganic layers provide the proton source and a networks of hydrogen-bonds leading to efficient proton transport, meanwhile reducing the bandgap of hybrid architecture and increasing the band electron delocalization of the metal-organic layer to greatly elevate the electron transport of intrinsic 2D metal-organic frameworks.

Carbon-Hybridized Hydroxides for Energy Conversion and Storage: Interface Chemistry and Manufacturing

Carbon-hybridized hydroxides (CHHs) have been intensively investigated for uses in the energy conversion/storage fields. Nevertheless, the intrinsic structure-activity relationships between carbon and hydroxides within CHHs are still blurry, which hinders the fine modulation of CHHs in terms of practical applications to some degree. This review aims to figure out the intrinsic role of carbon materials in CHHs with a focus on the interface chemistry and the engineering strategy in-between two components. The fundamental effects of the carbon materials in enhancing the charge/mass transfer kinetics are first analyzed, particularly the extra electron pathways for fast charge transfer and the anchoring sites for boosting the mass transfer. Subsequently, the surface-guided/confined effects of carbon materials in CHHs to modify the morphology and tailor the hydroxides, and functional heterojunction for regulating the inner electronic structure are decoupled. The methods to efficiently construct a stable yet robust solid-solid heterointerface are summarized, including oxygen functional groups engrafting, topological defective sites construction and heteroatom incorporation to activate the inert carbon surface. The smart CHHs in some typical energy applications are demonstrated. Additionally, the methodologies that can reveal the hybridization electron configuration between two components are summed up. At last, the perspective and challenges faced by the CHHs for energy-related applications are outlined.

Enhancing pseudocapacitive properties of cobalt oxide hierarchical nanostructures via iron doping

Through co-precipitation and post-heat processing, nanostructured Fe-doped Co₃O₄ nanoparticles (NPs) were developed. Using the SEM, XRD, BET, FTIR, TGA/DTA, UV-Vis, and techniques were examined. The XRD analysis presented that Co₃O₄ and Co₃O₄ nanoparticles that had been doped with 0.25 M Fe formed single cubic phase Co₃O₄ NPs with average crystallite sizes of 19.37 nm and 14.09 nm, respectively. The as prepared NPs have porous architectures via SEM analyses. The BET surface areas of Co₃O₄ and 0.25 M Fe-doped Co₃O₄ NPs were 53.06 m²/g and 351.56 m²/g, respectively. Co₃O₄ NPs have a band gap energy of 2.96 eV and an extra sub-band gap energy of 1.95 eV. Fe-doped Co₃O₄ NPs were also found to have band gap energies between 2.54 and 1.46 eV. FTIR spectroscopy was used to determine whether M-O bonds (M = Co, Fe) were present. The doping impact of iron results in the doped Co₃O₄ samples having better thermal characteristics. The highest specific capacitance was achieved using 0.25 M Fe-doped Co₃O₄ NPs at 5 mV/s, which corresponding to 588.5 F/g via CV analysis. Additionally, 0.25 M Fe-doped Co₃O₄ NPs had energy and power densities of 9.17 W h/kg and 472.1 W/kg, correspondingly.

Emerging Phosphate-Functionalized Co3O4/Kaolinite Composites for Enhanced Activation of Peroxymonosulfate

The Fenton-like reaction, as one of the most efficient strategies to generate radical species for the degradation of environmental pollutants, has attracted considerable attention. However, engineering low-cost catalysts with excellent activity by phosphate surface functionalization has seldom been used for the activation of peroxymonosulfate (PMS). Herein, emerging phosphate-functionalized Co₃O₄/kaolinite (P-Co₃O₄/Kaol) catalysts have been prepared by hydrothermal and phosphorization. Kaolinite nanoclay with rich hydroxyl groups plays a vital role in realizing phosphate functionalization. The results indicate that P-Co₃O₄/Kaol shows superior catalytic performance and excellent stability to the degradation of Orange II, which could be attributed to the existence of phosphate that promotes the adsorption of PMS and the electron transfer of Co²⁺/Co³⁺ cycles. Furthermore, the ^•OH radical was identified as the dominating reactive species for the degradation of Orange II compared to the SO₄^•- radical. This work could offer a novel preparation strategy for emerging functionalized nanoclay-based catalysts for effective pollutant degradation.

Fabrication of 3D HierarchicalSphericalHoneycomb-Like Nd2O3/Co3O4/Graphene/Nickel Foam Composite Electrode Material for High-Performance Supercapacitors

A 3D hierarchical spherical honeycomb-like composite electrode materialof neodymium oxide (Nd₂O₃), cobalt tetraoxide (Co₃O₄), and reduced graphene oxide (rGO) on nickel foam (named as Nd₂O₃/Co₃O₄/rGO/NF) were successfully fabricated by combining the hydrothermal synthesis method and the annealing process. Nickel foam with a three-dimensional spatial structure was used as the growth substrate without the use of any adhesives. The Nd₂O₃/Co₃O₄/rGO/NF composite has outstanding electrochemical performance and can be used directly as an electrode material for supercapacitors (SCs). By taking advantage of the large specific surface area of the electrode material, it effectively slows down the volume expansion of the active material caused by repeated charging and discharging processes, improves the electrode performance in terms of electrical conductivity, and significantly shortens the electron and ion transport paths. At a 1 A/g current density, the specific capacitance reaches a maximum value of 3359.6 F/g. A specific capacitance of 440.4 F/g with a current density of 0.5A/g is still possible from the built symmetric SCs. The capacitance retention rate is still 95.7% after 30,000 cycles of testing at a high current density of 10 A/g, and the energy density is 88.1 Wh/kg at a power density of 300 W/kg. The outcomes of the experiment demonstrate the significant potential and opportunity for this composite material to be used as an electrode material for SCs.

Rare Earth Doping Engineering Tailoring Advanced Oxygen-Vacancy Co3 O4 with Tunable Structures for High-Efficiency Energy Storage

Co₃ O₄  with high theoretical capacitance is a promising electrode material for high-end energy applications, yet the unexcited bulk electrochemical activity, low conductivity, and poor kinetics of Co₃ O₄  lead to unsatisfactory charge storage capacity. For boosting its energy storage capability, rare earth (RE)-doped Co₃ O₄  nanostructures with abundant oxygen vacancies are constructed by simple, economical, and universal chemical precipitation. By changing different types of RE (RE = La, Yb, Y, Ce, Er, Ho, Nd, Eu) as dopants, the RE-doped Co₃ O₄  nanostructures can be well transformed from large nanosheets to coiled tiny nanosheets and finally to ultrafine nanoparticles, meanwhile, their specific surface area, pore distribution, the ratio of Co²⁺ /Co³⁺ , oxygen vacancy content, crystalline phase, microstrain parameter, and the capacitance performance are regularly affected. Notably, Eu-doped Co₃ O₄  nanoparticles with good cycle stability show a maximum specific capacitance of 1021.3 F g^-1 (90.78 mAh g^-1 ) at 2 A g^-1 , higher than 388 F g^-1 (34.49 mAh g^-1 ) of pristine Co₃ O₄  nanosheets. The assembling asymmetric supercapacitor delivers a high energy density of 48.23 Wh kg^-1  at high power density of 1.2 kW kg^-1 . These findings denote the significance and great potential of RE-doped Co₃ O₄  in the development of high-efficiency energy storage.

Investigation of High-Performance Electrode Materials: Processing and Storage Mechanism

The scope of the Special Issue entitled "Investigation of High-Performance Electrode Materials: Processing and Storage Mechanism" includes the research on electrodes of high-performance electrochemical energy storage and conversion devices (metal ion batteries, non-metallic ion batteries, metal-air batteries, supercapacitors, photocatalysis, electrocatalysis, etc [...].

Vanadium oxide nanorods as an electrode material for solid state supercapacitor

The electrochemical properties of metal oxides are very attractive and fascinating in general, making them a potential candidate for supercapacitor application. Vanadium oxide is of particular interest because it possesses a variety of valence states and is also cost effective with low toxicity and a wide voltage window. In the present study, vanadium oxide nanorods were synthesized using a modified sol-gel technique at low temperature. Surface morphology and crystallinity studies were carried out by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy analysis. To the best of our knowledge, the as-prepared nanorods were tested with magnesium ion based polymer gel electrolyte for the first time. The prepared supercapacitor cell exhibits high capacitance values of the order of ~ 141.8 F g^-1 with power density of ~ 2.3 kW kg^-1 and energy density of ~ 19.1 Wh kg^-1. The cells show excellent rate capability and good cycling stability.

Effect of Solvothermal Temperature on Morphology and Supercapacitor Performance of Ni-MOF

A series of Ni-MOF materials were synthesized via a simple hydrothermal method and can be employed as electrodes for supercapacitors (SCs). Different temperatures were selected to unveil the effect of temperature on the formation, structure, and electrochemical performance of Ni-MOF-x (x = 60, 80, 100, and 120). Ni-MOF-80 possessed a larger specific surface area with a cross-network structure formed on its surface. The synthesized Ni-MOF electrode delivered a specific capacity of 30.89 mA h g^-1 when the current density reached 1 A g^-1 in a three-electrode system. The as-fabricated Ni-MOF materials could be further designed and are expected to deliver satisfactory performance in practice.

A 3D structure C/Si/ZnCo2O4/CC anode for flexible lithium-ion batteries with high capacity and fast charging ability

ZnCo₂O₄ has attracted extensive attention as a bimetallic transition metal oxide anode material for lithium-ion batteries (LIBs) with high capacity. However, there is still a long way to go to meet the increasing demand for commercial batteries due to their modest conductivity and unobtrusive cycling stability. The use of finely controlled nanostructures and combination with other anode materials are the two main ways to improve the battery performance of ZnCo₂O₄. Herein, ZnCo₂O₄ (ZCO) nanosheets were in situ grown on carbon cloth (CC) through a facile solution method. Si was coated onto the ZCO nanosheet arrays by the magnetron sputtering method (SCZO/CC) to acheive the capacity increase. A layer of C was further coated onto SZCO/CC to improve the electrical conductivity of the whole electrode and to protect the SZCO nanostructure. The obtained CSZCO/CC electrode exhibits a high reversible areal capacity of 1.16 mA h cm^-2 at 5 mA cm^-2 after 500 cycles. At an ultra-high current density of 10 mA cm^-2, the CSZCO/CC electrode can still present a capacity of 0.38 mA h cm^-2 and maintain a capacity retention of 88.4% for 2000 cycles. In situ Raman spectroscopy was used to study the relationship between the electrochemical performance and structure of the electrode materials. The carbon cloth was found to have contributed a nonnegligible part of the capacity of the electrode.

3D hierarchical cobalt vanadate nanosheet arrays on Ni foam coupled with redox additive for enhanced supercapacitor performance

Room-temperature synthesized 3D hierarchical cobalt vanadate (Co₃V₂O₈) nanosheet arrays on Ni foam for use as supercapacitor electrode is presented. In a 3 M KOH electrolyte, the electrode exhibits a capacitance of 109.9 mA h g^-1 (878.9 F g^-1) at a current density of 1 A g^-1. The capacitance is enhanced to 198.1 mA h g^-1 (1584.5 F g^-1) at 1 A g^-1 through the addition of 0.05 M redox-additive K₃[Fe(CN)₆] into the KOH electrolyte. Furthermore, the Co₃V₂O₈/activated carbon asymmetric supercapacitor cell with the advanced electrolyte outperforms most reported Co₃V₂O₈-based electrodes with a remarkable energy density of 55.5 W h kg^-1 at an 800 W kg^-1 power density. Combining a facile synthetic strategy and excellent electrochemical performance, the obtained Co₃V₂O₈ exhibits potential for practical application.

A Novel Hybrid Point Defect of Oxygen Vacancy and Phosphorus Doping in TiO2 Anode for High-Performance Sodium Ion Capacitor

Although sodium ion capacitors (SICs) are considered as one of the most promising electrochemical energy storage devices (organic electrolyte batteries, aqueous batteries and supercapacitor, etc.) due to the combined merits of battery and capacitor, the slow reaction kinetics and low specific capacity of anode materials are the main challenges. Point defects including vacancies and heteroatoms doping have been widely used to improve the kinetics behavior and capacity of anode materials. However, the interaction between vacancies and heteroatoms doping have been seldomly investigated. In this study, a hybrid point defects (HPD) engineering has been proposed to synthesize TiO₂ with both oxygen vacancies (OVs) and P-dopants (TiO₂/C-HPD). In comparison with sole OVs or P-doping treatments, the synergistic effects of HPD on its electrical conductivity and sodium storage performance have been clarified through the density functional theory calculation and sodium storage characterization. As expected, the kinetics and electronic conductivity of TiO₂/C-HPD3 are significantly improved, resulting in excellent rate performance and outstanding cycle stability. Moreover, the SICs assembled from TiO₂/C-HPD3 anode and nitrogen-doped porous carbon cathode show outstanding power/energy density, ultra-long life with good capacity retention. This work provides a novel point defect engineering perspective for the development of high-performance SICs electrode materials.

Self-Assembly Vertical Graphene-Based MoO3 Nanosheets for High Performance Supercapacitors

Supercapacitors have been extensively studied due to their advantages of fast-charging and discharging, high-power density, long-cycling life, low cost, etc. Exploring novel nanomaterial schemes for high-performance electrode materials is of great significance. Herein, a strategy to combine vertical graphene (VG) with MoO₃ nanosheets to form a composite VG/MoO₃ nanostructure is proposed. VGs as transition layers supply rich active sites for the growth of MoO₃ nanosheets with increasing specific surface areas. The VG transition layer further improves the electric contact and adhesion of the MoO₃ electrode, simultaneously stabilizing its volume and crystal structure during repeated redox reactions. Thus, the prepared VG/MoO₃ nanosheets have been demonstrated to exhibit excellent electrochemical properties, such as high reversible capacitance, better cycling performance, and high-rate capability.

Redox-Active Metaphosphate-Like Terminals Enable High-Capacity MXene Anodes for Ultrafast Na-Ion Storage

2D transition metal carbides and/or nitrides, so-called MXenes, are noted as ideal fast-charging cation-intercalation electrode materials, which nevertheless suffer from limited specific capacities. Herein, it is reported that constructing redox-active phosphorus-oxygen terminals can be an attractive strategy for Nb₄ C₃ MXenes to remarkably boost their specific capacities for ultrafast Na⁺ storage. As revealed, redox-active terminals with a stoichiometric formula of PO₂ - display a metaphosphate-like configuration with each P atom sustaining three PO bonds and one PO dangling bond. Compared with conventional O-terminals, metaphosphate-like terminals empower Nb₄ C₃ (denoted PO₂ -Nb₄ C₃ ) with considerably enriched carrier density (fourfold), improved conductivity (12.3-fold at 300 K), additional redox-active sites, boosted Nb redox depth, nondeclined Na⁺ -diffusion capability, and buffered internal stress during Na⁺ intercalation/de-intercalation. Consequently, compared with O-terminated Nb₄ C₃ , PO₂ -Nb₄ C₃ exhibits a doubled Na⁺ -storage capacity (221.0 mAh g^-1 ), well-retained fast-charging capability (4.9 min at 80% capacity retention), significantly promoted cycle life (nondegraded capacity over 2000 cycles), and justified feasibility for assembling energy-power-balanced Na-ion capacitors. This study unveils that the molecular-level design of MXene terminals provides opportunities for developing simultaneously high-capacity and fast-charging electrodes, alleviating the energy-power tradeoff typical for energy-storage devices.

Using Metal Cation to Control the Microstructure of Cobalt Oxide in Energy Conversion and Storage Applications

Herein, a facile and efficient synthesis of microstructured Co₃ O₄ for both supercapacitor and water-splitting applications is reported. Metal cations (Fe³⁺ , Cu²⁺ ) serve as structure-directing agents regulating the structure of Co compounds, which are subsequently annealed to yield Co₃ O₄ . Detailed characterizations and density functional theory (DFT) calculations reveal that the in situ Cl-doping introduces oxygen defects and provides abundant electroactive sites, and narrows the bandgap, which enhances the electron excitation of the as-formed Co₃ O₄ . The as-prepared Cl-doped Co₃ O₄ hierarchical nanospheres (Cl-Co₃ O₄ -h) display a high specific capacitance of 1629 F g^-1 at 1 A g^-1 as an electrode for supercapacitors, with excellent rate capability and cyclability. The Cl-Co₃ O₄ -h//activated carbon (AC) asymmetric supercapacitor (ASC) electrode achieves a specific capacitance of 237 F g^-1 at 1 A g^-1 , with an energy density of 74 Wh kg^-1 at a power density of 807 W kg^-1 and even maintains 47 Wh kg^-1 at the higher-power density of 24.2 kW kg^-1 . An integrated electrolyzer for water-splitting with Cl-Co₃ O₄ -h as both cathode and anode can be driven by Cl-Co₃ O₄ -h//AC ASC. The electrolyzer provides a high current density of 35 mA cm^-2 at a cell voltage of 1.6 V, with good current density retention over 50 h.

Self-Supported Transition Metal-Based Nanoarrays for Efficient Energy Storage

Rechargeable batteries and supercapacitors are currently considered as promising electrochemical energy storage (EES) systems to address the energy and environment issues. Self-supported transition metal (Ni, Co, Mn, Mo, Cu, V)-based materials are promising electrodes for EES devices, which offer highly efficient charge transfer kinetics. This review summarizes the latest development of transition metal-based materials with self-supported structures for EES systems. Special focus has been taken on the synthetic methods, the selection of substrates, architectures and chemical compositions of different self-supported nanoarrays in energy storage systems. Finally, the challenges and opportunities of these materials for future development in this field are briefly discussed. We believe that the advancement in self-supported electrode materials would pave the way towards next-generation EES.

Entropy-Assisted High-Entropy Oxide with a Spinel Structure toward High-Temperature Infrared Radiation Materials

Developing advanced materials with a high-entropy concept is one of the hot trends in materials science. The configurational entropy of high-entropy materials can be tuned by introducing different atomic species, which can also impart a result in excellent physical and chemical properties. In this work, we synthesized a solid-solution oxide (Cu, Mn, Fe, Cr)₃O₄ by a simple and scalable solid-phase synthesis method. We extensively investigated the microstructure and chemical composition, indicating that (Cu, Mn, Fe, Cr)₃O₄ has a single-phase spinel structure. Simultaneously, we reasonably evaluated the position occupied by the elements of (Cu, Mn, Fe, Cr)₃O₄ in a spinel structure as (Cu_0.75Fe_0.25)(Fe_0.25Cr_0.375Mn_0.375)₂O₄. Here, we first evaluated the infrared radiation performance of (Cu, Mn, Fe, Cr)₃O₄. The new, high-entropy oxide (HEO) (Cu, Mn, Fe, Cr)₃O₄ powder exhibits high infrared emissivity values of 0.879 and 0.848 in the wavelengths of 0.78-2.5 and 2.5-16 μm, respectively, and has excellent thermal stability. More importantly, the infrared emissivity values of as-prepared HEO coating reach 0.955 (0.78-2.5 μm) at room temperature and 0.936 (3-16 μm) at 800 °C. This work provides a viable strategy toward the laboratory mass production of this HEO for infrared radiation materials, which shows great potential in the energy-related applications.

Fabrication of an ultra-stable composite electrode material of La2O3/Co3O4/graphene on nickel foam for high-performance supercapacitors

A three-dimensional lily-like structure was constructed by the novel combination of La2O3, Co3O4, and graphene on nickel foam (LCGN) through hydrothermal synthesis and thermal annealing.

Co2O3/Co2N0.67 nanoparticles encased in honeycomb-like N, P, O-codoped carbon framework derived from corncob as efficient ORR electrocatalysts

It is essential to develop cost-effective rechargeable metal-air batteries, with high activity, stability, and efficiency, that use non-precious metals (NPMs)-based cathodic oxygen reduction reaction (ORR) catalysts. Here, by using earth-abundant corncob (CC) as the carbon source, Co(OH)₂, NaH₂PO₄, and melamine as the precursors, and KOH as the chemical activator, CoNP@bio-C-a is obtained and comparative studies are carried out with three other types of CC-derived carbon-based catalytic materials, namely, bio-C, CoP@bio-C, and CoNP@bio-C. Depending mainly on the formation of Co₂O₃/Co₂N_0.67 active sites (as p-n heterojunctions) and N, P, O-containing functional groups, the resultant CoNP@bio-C-a catalyst exhibits best electrocatalytic activity among the four types of catalysts; via a 4-electron pathway, it has good stability and good methanol tolerance. In addition, its unique honeycomb-like porous structure, high graphitization degree, and abundant oxygen-containing groups contribute to its excellent ORR activity. This study provides insights for exploring the application of heteroatom-doped biomass-derived carbon catalysts.

Hierarchical flower-like architecture of nickel phosphide anchored with nitrogen-doped carbon quantum dots and cobalt oxide for advanced hybrid supercapacitors

The exploitation of hybrid supercapacitors with excellent electrochemical properties is of great significance for energy storage systems. Herein, a three-dimensional hierarchical flower-like architecture of nickel phosphide (Ni₂P) decorated with nitrogen-doped carbon quantum dots (N-CQDs) and cobalt oxide (Co₃O₄) is constructed by an effective two-step hydrothermal strategy followed by in situ phosphorization process. Introducing N-CQDs with superior electrochemical characteristics can not only induce the formation of N-CQDs deposited nickel hydroxide (Ni(OH)₂) flower-like architecture but also significantly enhance the electrochemical features of Ni(OH)₂ nanosheets. After combination with Co₃O₄ nanoparticles and phosphorization treatment, an advanced cathode of Ni₂P/Co₃O₄/N-CQDs with enriched surface phosphate ions is obtained, which possesses an ultra-high capacity of 1044 C g^-1 (2088 F g^-1) at 1 A g^-1 with a splendid rate capacity of 876 C g^-1 (1752 F g^-1) at 20 A g^-1. Moreover, a device assembled by Ni₂P/Co₃O₄/N-CQDs hierarchical flower-like architecture and p-phenylenediamine functionalized reduced graphene oxide (PPD/rGO) nanosheets depicts a commendable energy density of 53.5 Wh kg^-1 at 772.9 W kg^-1. This work provides a novel hierarchical multi-component electrode material with decent electrochemical capacities for hybrid supercapacitors, which has a broad prospect in energy storage devices.

Electrolyte Dynamics Engineering for Flexible Fiber-Shaped Aqueous Zinc-Ion Battery with Ultralong Stability

Flexible aqueous zinc-ion batteries (ZIBs) are considered as promising energy storage devices for wearable electronics due to their cost-effectiveness, environmental friendliness, and high theoretical energy density. Herein, a flexible fiber-shaped aqueous ZIB is demonstrated by using a self-assembled Co₃O₄ nanosheet array on a carbon nanotube fiber as the cathode and Zn nanosheets deposited on a carbon nanotube fiber as the anode. The cycle life span of the fiber-shaped battery is largely enhanced by a simple electrolyte dynamics engineering strategy of preadding a trace amount of Co²⁺ cations in the mild aqueous electrolyte. The assembled fiber-shaped ZIB shows a high specific capacity (158.70 mAh g^-1 at 1 A g^-1), superior rate capacity, and excellent cycling life span (97.27% capacity retention after 10,000 cycles). Additionally, the fiber-shaped ZIB also shows superior flexibility, which can charge a smart watch under deformed states. This work provides new opportunities for the development of flexible, safe, and high-performance energy storage devices for wearable electronics.

Atomically Thin Materials for Next-Generation Rechargeable Batteries

Atomically thin materials (ATMs) with thicknesses in the atomic scale (typically <5 nm) offer inherent advantages of large specific surface areas, proper crystal lattice distortion, abundant surface dangling bonds, and strong in-plane chemical bonds, making them ideal 2D platforms to construct high-performance electrode materials for rechargeable metal-ion batteries, metal-sulfur batteries, and metal-air batteries. This work reviews the synthesis and electronic property tuning of state-of-the-art ATMs, including graphene and graphene derivatives (GE/GO/rGO), graphitic carbon nitride (g-C₃N₄), phosphorene, covalent organic frameworks (COFs), layered transition metal dichalcogenides (TMDs), transition metal carbides, carbonitrides, and nitrides (MXenes), transition metal oxides (TMOs), and metal-organic frameworks (MOFs) for constructing next-generation high-energy-density and high-power-density rechargeable batteries to meet the needs of the rapid developments in portable electronics, electric vehicles, and smart electricity grids. We also present our viewpoints on future challenges and opportunities of constructing efficient ATMs for next-generation rechargeable batteries.

Plant polyphenols induced the synthesis of rich oxygen vacancies Co3O4/Co@N-doped carbon hollow nanomaterials for electrochemical energy storage and conversion

Reasonable hollow structure design and oxygen vacancy defects control play an important role in the optimization of electrochemical energy storage and electrocatalytic properties. Herein, a plant polyphenol tannic acid was used to etch Co-based zeolitic imidazolate framework (ZIF-67) followed by calcination to prepare a porous Co₃O₄@Co/NC hollow nanoparticles (Co₃O₄@Co/NC-HN) with rich oxygen vacancy defects. Owing to the metal-phenolic networks (MPNs), rich oxygen vacancy defects and the synergistic effect between Co₃O₄ and Co/NC, the box-like Co₃O₄@Co/NC-HN nanomaterials with large specific surface areas exhibit excellent supercapacitor performance and electrocatalytic activity. As expected, Co₃O₄@Co/NC-HN shows high specific capacity (273.9 mAh g^-1 at 1 A g^-1) and remarkable rate performance. Moreover, the assembled Hybrid supercapacitor (HSC, Co₃O₄@Co/NC-HN//Active carbon) device obtained a maximum energy density of 57.8 Wh kg^-1 (800 W kg^-1) and exhibited superior cycle stability of 92.6% after 4000 cycles. Notably, as an electrocatalyst, the nanocomposites exhibit small overpotential and Tafel slope. These results strongly demonstrate that both unique hollow structure and abundant oxygen vacancies designed from plant polyphenols provide superiorities for the synthesis of efficient and green multifunctional electrode materials for energy storage and conversion.

Magnetic enhancement of oxygen evolution reaction performance of NiCo-spinel oxides

Low-cost and high-efficiency transition metal oxide catalysts are desired for high-efficiency water splitting technology. An applying magnetic field (MF) enhancement method is presented to improve the oxygen evolution reaction (OER) performance of NiCo-spinel magnetic catalysts, the enhancement of OER performance depends on the applied MF strength and magnetic properties of catalysts. The maximum enhanced current density percentage of about 90.6%, 93.7%, and 70.1% are obtained by applying 105 mT MF in NiCo₂O₄, Ni_1.5Co_1.5O₄, and Ni₂CoO₄, respectively. The enhanced performance originates from the improved intrinsic activity and facilitated mass transfer process. The MF decreases the activation energy, which then leads to the improvement of intrinsic activity. This work provides more basic data for further gaining into the enhanced mechanism by applying the MF, meanwhile, the strategy can be used to enhance the performances of other electrocatalysts.

A universal electrochemical lithiation-delithiation method to prepare low-crystalline metal oxides for high-performance hybrid supercapacitors

The electrochemical performance of transition metal oxides (TMOs) for hybrid supercapacitors has been optimized through various methods in previous reports. However, most previous research was mainly focused on well-crystalline TMOs. Herein, the electrochemical lithiation–delithiation method was performed to synthesise low-crystallinity TMOs for hybrid supercapacitors. It was found that the lithiation–delithiation process can significantly improve the electrochemical performance of “conversion-type” TMOs, such as CoO, NiO, etc. The as-prepared low-crystallinity CoO exhibits high specific capacitance of 2154.1 F g⁻¹ (299.2 mA h g⁻¹) at 0.8 A g⁻¹, outstanding rate capacitance retention of 63.9% even at 22.4 A g⁻¹ and excellent cycling stability with 90.5% retention even after 10 000 cycles. When assembled as hybrid supercapacitors using active carbon (AC) as the active material of the negative electrode, the devices show a high energy density of 50.9 W h kg⁻¹ at 0.73 kW kg⁻¹. Another low-crystallinity NiO prepared by the same method also possesses a much higher specific capacitance of 2317.6 F g⁻¹ (302.6 mA h g⁻¹) compared to that for pristine commercial NiO of 497.2 F g⁻¹ at 1 A g⁻¹. The improved energy storage performance of the low-crystallinity metal oxides can be ascribed to the disorder of as-prepared low-crystallinity metal oxides and interior 3D-connected channels originating from the lithiation–delithiation process. This method may open new opportunities for scalable and facile synthesis of low-crystallinity metal oxides for high-performance hybrid supercapacitors.

The electrochemical performance of transition metal oxides (TMOs) for hybrid supercapacitors has been optimized through various methods in previous reports.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.