Transition-Metal Oxynitride: A Facile Strategy for Improving Electrochemical Capacitor Storage

A Review of Carbon Nanofiber Materials for Dendrite-Free Lithium-Metal Anodes

Lithium metal is regarded as ideal anode material due to its high theoretical specific capacity and low electrode potential. However, the uncontrollable growth of lithium dendrites seriously hinders the practical application of lithium-metal batteries (LMBs). Among various strategies, carbon nanofiber materials have shown great potential in stabilizing the lithium-metal anode (LMA) due to their unique functional and structural characteristics. Here, the latest research progress on carbon nanofibers (CNFs) for LMA is systematically reviewed. Firstly, several common preparation techniques for CNFs are summarized. Then, the development prospects, strategies and the latest research progress on CNFs for dendrite-free LMA are emphatically introduced from the perspectives of neat CNFs and CNF-based composites. Finally, the current challenges and prospects of CNFs for stabilizing LMA are summarized and discussed. These discussions and proposed strategies provide new ideas for the development of high-performance LMBs.

Heterointerfaces: Unlocking Superior Capacity and Rapid Mass Transfer Dynamics in Energy Storage Electrodes

Heterogeneous electrode materials possess abundant heterointerfaces with a localized "space charge effect", which enhances capacity output and accelerates mass/charge transfer dynamics in energy storage devices (ESDs). These promising features open new possibilities for demanding applications such as electric vehicles, grid energy storage, and portable electronics. However, the fundamental principles and working mechanisms that govern heterointerfaces are not yet fully understood, impeding the rational design of electrode materials. In this study, the heterointerface evolution during charging and discharging process as well as the intricate interaction between heterointerfaces and charge/mass transport phenomena, is systematically discussed. Guidelines along with feasible strategies for engineering structural heterointerfaces to address specific challenges encountered in various application scenarios, are also provided. This review offers innovative solutions for the development of heterogeneous electrode materials, enabling more efficient energy storage beyond conventional electrochemistry. Furthermore, it provides fresh insights into the advancement of clean energy conversion and storage technologies. This review contributes to the knowledge and understanding of heterointerfaces, paving the way for the design and optimization of next-generation energy storage materials for a sustainable future.

NiRu-Mo2Ti2C3O2 as an efficient catalyst for alkaline hydrogen evolution reactions: the role of bimetallic site interactions in promoting Volmer-step kinetics

The Volmer step in alkaline hydrogen evolution reactions (HERs), which supplies H* to the following steps by cleaving H-O-H bonds, is considered the rate-determining step of the overall reaction. The Volmer step involves water dissociation and adsorbed hydroxyl (*OH) desorption; Ru-based catalysts display a compelling water dissociation process in an alkaline HER. Unfortunately, the strong affinity of Ru for *OH blocks the active sites, resulting in unsatisfactory performance during HER processes. Hence, this study investigates a series of key descriptors (ΔG*H2O, ΔG*H-OH, ΔG*H, and ΔG*OH) of TM (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, or Pt)-Ru/Mo2Ti2C3O2 to systematically explore the effects of bimetallic site interactions on the kinetics of the Volmer step. The results indicate that bimetallic catalysts effectively reduced the strong adsorption of *OH on Ru sites; especially, the NiRu diatomic state shows the highest electron-donating ability, which promoted the smooth migration of *OH from Ru sites to Ni sites. Therefore, Ru, Ni and MXenes are suitable to serve as water adsorption and dissociation sites, *OH desorption sites, and H2 release sites, respectively. Ultimately, NiRu/Mo2Ti2C3O2 promotes Volmer kinetics and has the potential to improve alkaline HERs. This work provides theoretical support for the construction of synergistic MXene-based diatomic catalysts and their wide application in the field of alkaline HERs.

Using tungsten oxide quantum-dot enhanced electrochemiluminescence to measure thrombin activity and screen its inhibitors

A thrombin-activity-based electrochemiluminescence (ECL) biosensor was successfully constructed using tungsten oxide quantum dots (WO3-x QDS) as the co-reactant and thrombin-cleavable peptides as the recognizer. Specifically, Ru(bpy)32+ were doped on silica nanoparticles (Ru@SiO2), which greatly enhanced the ECL potential. AuNPs@WO3-x QDs composite was then prepared to accelerate electron transfer and improve the ECL signal by 219 times. Under ideal conditions, the limit of detection for thrombin in serum was determined to be 0.28 μU/mL with a linear range from 1 μU/mL to 1 U/mL. In addition, the developed ECL biosensor was used to screen for thrombin inhibitors from 12 compounds in Artemisiae Argyi Folium. Among the compounds tested, it was observed that 100 μmol/L luteolin exhibited a significantly higher inhibition rate (exceeding 80%) compared to apigenin, isorhamnetin, naringin, or eriodictyol. In an in-vitro anticoagulation experiment, luteolin (100 μmol/L) prolonged APTT by 49%, and the molecular docking assay indicated that luteolin had binding sites of Gly219 and Asp189 in the active pockets of thrombin. This may have been the main reason underpinning luteolin's anticoagulation effects. Overall, the Ru@WO3-x QDS ECL biosensor provided a reliable strategy for thrombin activity assay and screening of anticoagulant agents.

Multi-layer core-shell metal oxide/nitride/carbon and its high-rate electroreduction of nitrate to ammonia

The electroreduction of nitrate to ammonia is both an alternative strategy to industrial Haber-Bosch ammonia synthesis and a prospective idea for changing waste (nitrate pollution of groundwater around the world) into valuable chemicals, but still hindered by its in-process strongly competitive hydrogen evolution reaction (HER), low ammonia conversion efficiency, and the absence of stability and sustainability. Considering the unique electronic structure of anti-perovskite structured Fe4N, a tandem disproportionation reaction and nitridation-carbonation route for building a multi-layer core-shell oxide/nitride/C catalyst, such as MoO2/Fe4N/C, is designed and executed, in which abundant Fe-N active sites and rich phase interfaces are in situ formed for both suppressing HER and fast transport of electrons and reaction intermediates. As a result, the sample's NO3RR conversion displays a very high NH3 yield rate of up to 11.10 molNH3 gcat.-1 h-1 (1.67 mmol cm-2 h-1) with a superior 99.3% faradaic efficiency and the highest half-cell energy efficiency of 30%, surpassing that of most previous reports. In addition, it is proved that the NO3RR assisted by the MoO2/Fe4N/C electrocatalyst can be carried out in 0.50-1.00 M KNO3 electrolyte at a pH value of 6-14 for a long time. These results guide the rational design of highly active, selective, and durable electrocatalysts based on anti-perovskite Fe4N for the NO3RR.

Electrocatalysts for Zinc-Air Batteries Featuring Single Molybdenum Atoms in a Nitrogen-Doped Carbon Framework

Bifunctional catalysts can facilitate two different electrochemical reactions with conflicting characteristics. Here, a highly reversible bifunctional electrocatalyst for rechargeable zinc-air batteries (ZABs) is reported featuring a "core-shell structure" in which N-doped graphene sheets wrap around vanadium molybdenum oxynitride nanoparticles. Single Mo atoms are released from the particle core during synthesis and anchored to electronegative N-dopant species in the graphitic shell. The resultant Mo single-atom catalysts excel as active oxygen evolution reaction (OER) sites in pyrrolic-N and as active oxygen reduction reaction (ORR) sites in pyridinic-N environments. ZABs with such bifunctional and multicomponent single-atom catalysts deliver high power density (≈376.4 mW cm-2 ) and long cycle life of over 630 h, outperforming noble-metal-based benchmarks. Flexible ZABs that can tolerate a wide range of temperatures (-20 to 80 °C) under severe mechanical deformation are also demonstrated.

Electrochemically Versatile Graphite Nanoplatelets Prepared by a Straightforward, Highly Efficient, and Scalable Route

Nanostructured carbon materials with tailor-made structures (e.g., morphology, topological defect, dopant, and surface area) are of significant interest for a variety of applications. However, the preparation method selected for obtaining these tailor-made structures determines the area of application, precluding their use in other technological areas of interest. Currently, there is a lack of simple and low-cost methodologies versatile enough for obtaining freestanding carbon nanostructures that can be used in either energy storage or chemical detection. Here, a novel methodology for the development of a versatile electrochemically active platform based on freestanding graphite nanoplatelets (GNP) has been developed by exploiting the interiors of hollow carbon nanofibers (CNF) comprising nanographene stacks using dry ball-milling. Even though ball-milling could be considered as a universal method for any carbonaceous material, often, it is not as simple (one step, no purification, and no solvents), efficient (just GNP without tubular structures), and quick (just 20 min) as the sustainable method developed in this work, free of surfactants and stabilizer agents. We demonstrate that the freestanding GNP developed in this work (with an average thickness of 3.2 nm), due to the selective edge functionalization with the minimal disruption of the basal plane, can act either as a supercapacitor or as a chemical sensor, showing both a dramatic improvement in the charge storage ability of more than 30 times and an enhanced detection of electrochemically active molecules such as ascorbic acid with a 236 mV potential shift with respect to CNF in both cases. As shown here, GNP stand as an excellent versatile alternative compared to the standard commercially available carbon-based materials. Overall, our approach paves the way for the discovery of new nanocarbon-based electrochemical active platforms with a wide electrochemical applicability.

Modulation of Structural, Electronic, and Optical Properties of Titanium Nitride Thin Films by Regulated In Situ Oxidation

Epitaxial titanium nitride (TiN) and titanium oxynitride (TiON) thin films have been grown on sapphire substrates using a pulsed laser deposition (PLD) method in high-vacuum conditions (base pressure <3 × 10^-6 T). This vacuum contains enough residual oxygen to allow a time-independent gas phase oxidation of the ablated species as well as a time-dependent regulated surface oxidation of TiN to TiON films. The time-dependent surface oxidation is controlled by means of film deposition time that, in turn, is controlled by changing the number of laser pulses impinging on the polycrystalline TiN target at a constant repetition rate. By changing the number of laser pulses from 150 to 5000, unoxidized (or negligibly oxidized) and oxidized TiN films have been obtained with the thickness in the range of four unit cells to 70 unit cells of TiN/TiON. X-ray photoelectron spectroscopy (XPS) investigations reveal higher oxygen content in TiON films prepared with a larger number of laser pulses. The oxidation of TiN films is achieved by precisely controlling the time of deposition, which affects the surface diffusion of oxygen to the TiN film lattice. The lattice constants of the TiON films obtained by x-ray diffraction (XRD) increase with the oxygen content in the film, as predicted by molecular dynamics (MD) simulations. The lattice constant increase is explained based on a larger electrostatic repulsive force due to unbalanced local charges in the vicinity of Ti vacancies and substitutional O. The bandgap of TiN and TiON films, measured using UV-visible spectroscopy, has an asymmetric V-shaped variation as a function of the number of pulses. The bandgap variation following the lower number of laser pulses (150-750) of the V-shaped curve is explained using the quantum confinement effect, while the bandgap variation following the higher number of laser pulses (1000-5000) is associated with the modification in the band structure due to hybridization of O_2p and N_2p energy levels.

High utilization efficiencies of alkylbenzokynones hybridized inside the pores of activated carbon for electrochemical capacitor electrodes

Benzoquinone derivatives (BQDs) are hybridized inside activated carbon (AC) pores via gas-phase adsorption to prepare electrochemical capacitor materials. In this study, 2 mmol of BQDs are hybridized with 1 g of AC. The hybridization of alkylbenzoquinones (ABQs) with AC enhances the volumetric capacitances of the hybrids from 117 to 201 F cm^-3 at 0.05 A g^-1 and the capacitances are retained up to 73% at 10 A g^-1. Meanwhile, the volumetric capacitances are increased to 163 F cm^-3 at 0.05 A g^-1 by the hybridization of halobenzoquinones (HBQs) and the capacitance retentions at 0.05 A g^-1 are ∼62%, which are higher than that of AC (46%). The results of electrochemical measurements suggest that HBQs exist as agglomerates while ABQs are finely dispersed inside the pores. The ABQs have good contact with the conductive carbon pore surface compared to the HBQs. Consequently, most of the ABQ molecules undergo reversible redox reactions (i.e., high utilization efficiencies), and a large contact area facilitates charge transfer at the large contact interface, thereby endowing the hybrids of ABQs with fast charging and discharging characteristics. HBQ molecules can be finely dispersed by liquid-phase adsorption, but the finely dispersed HBQ molecules are mobile inside the pores at room temperature and gradually form agglomerates. The difference in the existing form of BQDs is explained by the dominant interaction affecting the BQD molecules. ABQs have a strong interaction with the carbon pore surface while the intermolecular interaction is dominant for HBQs.

Rational Design of Porous Nanowall Arrays of Ultrafine Co4N Nanoparticles Confined in a La2O2CN2 Matrix on Carbon Cloth for a High-Performing Supercapacitor Electrode

Transition metal nitrides (TMNs) have received special concern as important energy storage materials, owing to their high conductibility, good mechanical strength, and superior corrosion resistance. However, their insufficient capacitance and poor cycling stability limit their practical applications for supercapacitors. Here, a novel three-dimensional (3D) self-supported integrated electrode consisted of porous nanowall arrays of ultrafine cobalt nitride (Co₄N) nanoparticles encapsulated in a lanthanum oxycyanamide (LOC) matrix on carbon cloth (Co₄N@LOC/CC) for outstanding electrochemical energy storage is rationally designed and fabricated. The 3D monolithic configuration of porous nanowall arrays facilitates the mass/charge transfer, the exposure of electroactive sites, and the enhancement of electrical conductivity. Meanwhile, the unique core-shell structure of Co₄N@LOC can prevent ultrafine Co₄N nanoparticles from sintering, agglomeration, and oxidation and promotes electron transfer dynamics during the redox reaction, meanwhile enhancing the stability of the electrode. Additionally, the synergy of Co₄N and LOC can result in an efficient electron/ion transport in the process of the charge-discharge. Because of these features, the Co₄N@LOC/CC electrode displays superior specific capacitance (895.6 mF cm^-2 or 613.4 F g^-1 at 1 mA cm^-2) and admirable cycling durability (87.9% capacitance reservation after 10 000 cycles), surpassing the majority of nitride-based electrodes reported thus far. Furthermore, after being assembled into an asymmetric supercapacitor using active carbon (AC) as an anode, the obtained Co₄N@LOC/CC//AC/CC device displays a high energy density of 41.7 Wh kg^-1 at the power density of 875.8 W kg^-1 with a high capacitance reservation of 87.6% after 5000 cycles at 2 mA cm^-2. This work offers an efficient approach of combining TMNs with rare earth compounds to enhance the capacitance and stability of TMNs for supercapacitor electrodes.

Water-Induced Surface Reconstruction of Co3O4 on the (111) Plane for High-Efficiency Li-O2 Batteries in a Hybrid Electrolyte

The crystal plane effect of cobalt oxide has attracted much attention in Li-O₂ batteries (LOBs) and other electrocatalytic fields. However, boosting the catalytic activity of a specific plane still faces significant challenges. Herein, a strategy of adding water into the electrolyte is developed to construct a LiOH-based Li-O₂ battery system using the (111) plane-exposed Co₃O₄ as a cathode catalyst. The electrochemical performance shows that on the (111) plane, in the presence of water, the overpotential is largely reduced from 1.5 to 1.0 V and the cycling performance is enhanced. It is confirmed that during the discharge process, water reacts to form LiOH and induce the phase transformation of Co₃O₄ to amorphous CoO_x(OH)_y. At the recharge stage, LiOH is first decomposed and then CoO_x(OH)_y is reduced to Co₃O₄. Compared with pristine (111), the newly formed Co₃O₄ surface exhibits more active sites, which accelerates the following oxygen reduction and oxygen evolution processes. This work not only reveals the reaction mechanism of water-induced reaction on the (111) plane of Co₃O₄ but also provides a new perspective for further design of hybrid Li-O₂ batteries with a low polarization and a longer cycle life.

The design and synthesis of spinel one-dimensional multi-shelled nanostructures for Li-ion batteries

The rational design, synthesis, and massive production of one-dimensional (1D) spinel composite oxides with multi-shelled nanostructures are critical for the realization of highly efficient energy conversion and storage. However, owing to the limitations of the synthetic methods, the 1D multi-shelled nanostructures, especially for multi-element oxides and binary-metal oxides, have been rarely fabricated. Herein, we design a facile and general method to fabricate 1D spinel composite oxides with complex architectures. It is found that the concentration of the precursor polymer PAN can control the structures of the products at optimal heating rate, including hollow nanofibers, wire-in-tube nanofibers, and tube-in-tube nanofibers. This technique could be extended to various inorganic multi-element oxides and binary-metal oxides. Moreover, numerous twin boundaries (TBs) are found to form in the Co_0.5Ni_0.5Fe₂O₄ tube-in-tube nanofibers. Benefiting from both large porosity and TBs structures, the tube-in-tube hollow nanostructures are measured to possess superior electrochemical performances with high energy and stability in lithium-ion storage.

Construction of Novel Bimetallic Oxyphosphide as Advanced Anode for Potassium Ion Hybrid Capacitor

Potassium ion hybrid capacitors (PIHCs) have attracted considerable interest due to their low cost, competitive power/energy densities, and ultra-long lifespan. However, the more sluggish insertion kinetics of battery-type anodes than capacitor-type cathodes in PIHCs seriously limits their practical application. Therefore, developing advanced anodes with high capacitor and suitable K⁺ intercalation is imperative and significant. A novel core-shell structure of NiCo oxide/NiCo oxyphosphide (NCOP) nanowires are designed and constructed in this study via efficient and facile strategy. Combining the merits of the core-shell structure and the massive active sites in the oxyphosphide layer, the as-prepared NCOP composites manifest highly reversible capacitors and outstanding rate capability. Meanwhile, the insertion and conversion potassium storage mechanisms of the NCOP are successfully revealed through in situ X-ray diffraction and density functional theory calculations, respectively. Furthermore, the PIHC was assembled with NCOP anode and borocarbonitride cathode, which displays a large energy density and high-power density, along with an exceptional capacity retention of ≈90% over 10 000 cycles at 1.0 A g^-1 . This work provides the anion regulation strategy for modifying the transition metal oxide and constructing the advancing electrode materials for next-generation energy storage and beyond.

High mass load of oxygen-enriched microporous hollow carbon spheres as electrode for supercapacitor with solar charging station application

Carbon materials modified with pores and heteroatoms have been pursued as promising electrode for supercapacitors due to the synergic storage of electric double-layer capacitance (EDLC) and pseudocapacitance. A vital problem that the actual effect of pores and heteroatoms on energy storage varies with the carbon matrix used presents in numerous carbon electrodes, but is ignored greatly, which limits their sufficient utilization. Moreover, most of modified carbon electrodes still suffer from severe capacitance degeneration under high mass load caused by the blocked surface and inaccessible bulk phase. Here, we shape an interconnected hollow carbon sphere (HCS) as the matrix by regulating and selectively-etching low molecular weight component in the inhomogeneous precursors, accompanied with the decoration of rich oxygen groups (15.9at%) and micropores (centering at 0.6-1.4 nm). Finite-element calculation and energy storage kinetics reveal the modified HCS electrode exposes accessible dual active surface with highly-matched electrons and ions for pores and oxygen groups to improve both EDLC and pseudocapacitance. Under a commercial-level load of 11.2 mg cm^-2, the HCS exhibits a high specific capacitance of 288.3 F g^-1 at 0.5 A g^-1, performing a retention of 91.8% relative to 314 F g^-1 under 2.8 mg cm^-2 load, applicable for solar charging station to efficiently drive portable electronics.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Lithium Storage Mechanism and Application of Micron-Sized Lattice-Reversible Binary Intermetallic Compounds as High-Performance Flexible Lithium-Ion Battery Anodes

A strategy of lattice-reversible binary intermetallic compounds of metallic elements is proposed for applications in flexible lithium-ion battery (LIB) anode with high capacity and cycling stability. First, the use of metallic elements can ensure excellent electronic conductivity and high capacity of the active anode substance. Second, binary intermetallic compounds possess a larger initial lattice volume than metallic monomers, so that the problem of volume expansion can be alleviated. Finally, the design of binary intermetallic compounds with lattice reversibility further improves the cycle stability. In this work, the feasibility of this strategy is verified using an indium antimonide (InSb) system. The volumetric expansion and lithium storage mechanism of InSb are investigated by in situ Raman characterization and theoretical calculations. The active material utilization is significantly improved and the growth of In whiskers is inhibited in the micron-sized ball-milled and carbon coated InSb (bInSb@C) anode, which exhibits a reversible capacity of 733.8 mAh g^-1 at 0.2 C, and provides a capacity of 411.5 mAh g^-1 after 200 cycles at 3 C with an average Coulombic efficiency of 99.95%. This strategy is validated in pouch cells, illustrating the great potential of lattice-reversible binary intermetallic compounds for use as commercial flexible LIB anodes.

Rational design of Ti3C2Cl2 MXenes nanodots-interspersed MXene@NiAl-layered double hydroxides for enhanced pseudocapacitor storage

Although electrodes based on two dimensional hybrids with interstratification-assemble have been widely studied for supercapacitors, the performance enhancement still remains challenge mainly due to the random dispersion of surface passivated two dimensional nanosheets. Herein, a new covalent surface functionalization of MXene-based Ti₃C₂Cl₂ nanodots-interspersed MXene@NiAl-layered double hydroxides (QD-Ti₃C₂Cl₂@NiAl-LDHs) hybrid electrode with superior pseudocapacitor storage performance has been elaborately designed by electrostatic-assembled. As a result, the QD-Ti₃C₂Cl₂@NiAl-LDHs electrode exhibits a super specific capacitance of 2010.8F g^-1 at 1.0 A g^-1 and high energy density of 100.5 Wh kg^-1 at a power density of 299.8 W kg^-1. In addition, 94.1% capacitance retention is achieved after cycling for 10,000 cycles at 1.0 A g^-1, outperforming previously reported of two dimensional hybrids electrode for supercapacitor. Furthermore, density functional theory (DFT) calculations show that the superior pseudocapacitor storage performance of the QD-Ti₃C₂Cl₂@NiAl-LDHs may be attributed to the creation of numerous electrochemical active sites and the enhancement of electrical conductivity by the QD-Ti₃C₂Cl₂ MXene. This work provides new strategy for developing excellent pseudocapacitor supercapacitor based on two dimensional hybrid electrode.

Stability-Enhanced α-Ni(OH)2 Pillared by Metaborate Anions for Pseudocapacitors

α-Ni(OH)₂ is an ideal candidate material for a supercapacitor except for its low conductivity and poor stability. In this work, BO₂^--intercalated α-Ni_xCo_(1-x)(OH)₂ is synthesized by a hydrothermal method at a low cost. The Co dopant can decrease the charge-transfer resistance and enhance the cyclic stability. The special unsaturated electronic state of BO₂^- enhances the bonding with metal ions and attracts water molecules. Thus, the BO₂^- ions support the hydroxide layers as pillars and create efficient paths for proton transportation, optimizing the utilization of α-Ni(OH)₂. The three-dimensional (3D) flowerlike morphology supplies an enormous number of active sites, and r-GO is added to improve the conductivity. As a result, the modified α-Ni(OH)₂ exhibits the specific capacitance of 2179, 1592, and 1423 F·g^-1 at 1, 20, and 40 A·g^-1, respectively, showing improved rate performance. Matching with the commercial activated carbon (AC) as an anode, the asymmetric capacitor delivers an energy density of 40.66 W·h·kg^-1 when its power density is 187.06 W·kg^-1. Meanwhile, it retains 81.5% capacitance of the initial cycle at 5 A·g^-1 after 3000 cycles. With conductivity enhanced and structure stabilized, the modified α-Ni(OH)₂ confronts broader fields of application.

Polyaniline-Derived N-Doped Ordered Mesoporous Carbon Thin Films: Efficient Catalysts towards Oxygen Reduction Reaction

One of the most challenging targets in oxygen reduction reaction (ORR) electrocatalysts based on N-doped carbon materials is the control of the pore structure and obtaining nanostructured thin films that can easily be incorporated on the current collector. The carbonization of nitrogen-containing polymers and the heat treatment of a mixture of carbon materials and nitrogen precursor are the most common methods for obtaining N-doped carbon materials. However, in this synthetic protocols, the surface area and pore distribution are not controlled. This work enables the preparation of 2D-ordered N-doped carbon materials through the carbonization of 2D polyaniline. For that purpose, aniline has been electropolymerized within the porous structure of two different templates (ordered mesoporous Silica and ordered mesoporous Titania thin films). Thus, aniline has been impregnated into the porous structure and subsequently electropolymerized by means of chronoamperometry at constant potential. The resultant samples were heat-treated at 900 °C with the aim of obtaining 2D N-doped carbon materials within the template structures. Polyaniline and polyaniline-derived carbon materials have been analyzed via XPS and TEM and characterized by electrochemical measurements. It is worth noting that the obtained 2D-ordered mesoporous N-doped carbon materials have proved to be highly active electrocatalysts for the ORR because of the formation of quaternary nitrogen species during the heat treatment.

Tungsten nitride-coated graphene fibers for high-performance wearable supercapacitors

Graphene-fiber (GF) supercapacitors have attracted significant research attention in the field of wearable devices. However, there is still a need for active materials with high energy density. Transition Metal Nitrides (TMNs) are promising candidates for this purpose compared with conventional Transition Metal Oxides (TMOs) or conducting polymers (CPs) owing to their higher electrical conductivity, stability and relevant electrochemical properties. We have successfully integrated Tungsten Nitride (WN) with reduced graphene oxide fibers (rGOF) and developed high-performance hybrid fiber (WN-rGOF) supercapacitors. These hybrid supercapacitors attained a high capacitance of 16.29 F cm-3 at 0.05 A cm-3 and an energy density of 1.448 mW h cm-3, which is 7.5 and 1.75 times higher than those of the pure rGOF supercapacitor and the Tungsten Oxide/rGO hybrid fiber (WO3-rGOF) supercapacitor, respectively. The energy density readily increased up to 2.896 mW h cm-3 when three WN-rGOF supercapacitors were connected in series. The WN-rGOF supercapacitor also showed high capacitance retention of 84.7% after 10 000 cycles along with appreciable performance under severe mechanical deformation.

In Situ Growing Chromium Oxynitride Nanoparticles on Carbon Nanofibers to Stabilize Lithium Deposition for Lithium Metal Anodes

To address the dendrite growth and interface instability of high-capacity Li metal anode, heterogeneous seed-decorated 3D host materials are expected to suppress the growth of Li dendrites. The physical stability and chemical reactivity of these nanoseeds are the decisive conditions for long cycling lithium metal batteries. Herein, carbon nanofibers decorated with uniform CrO0.78N0.48 nanoparticles (ACrCFs) are synthesized by a novel in situ growing method, where the size, composition, distribution, and migration behavior of these nanoparticles are controlled by the introduction of asphaltene. As the 3D host materials for Li anodes, ACrCFs exhibit an excellent lithiophilicity, a superior mixed ion-electron conductivity, and abundant electrochemical active sites. Thus, the ACrCF-modified Li anodes deliver a smooth Li morphology, low nucleation overpotential (10.4 mV), superior cyclic stability with 320 stable cycles (Coulombic efficiency, >98.0%) at 1 mA cm-2, and excellent plating/stripping stability over 1000 h. Notably, no obvious detachment or chalking of these nanoparticles occur during the cycling process. The full cell with LiFePO4 cathode also delivers a better rate capability with more stable cycling performance. The homogeneous CrO0.78N0.48 nanoparticles achieved by this in situ growing method also promise a facile method for the potential applications of transition-metal oxynitride for high energy density battery systems.

Miniaturized Energy Storage Devices Based on Two-Dimensional Materials

A growing demand for miniaturized biomedical sensors, microscale self-powered electronic systems, and many other portable, wearable, and integratable electronic devices is continually stimulating the rapid development of miniaturized energy storage devices (MESDs). Miniaturized batteries (MBs) and supercapacitors (MSCs) were considered to be suitable energy storage devices to power microelectronics uninterruptedly with reasonable energy and power densities. However, in addition to similar challenges encountered with electrode materials in conventional energy storage devices, their performances are also greatly affected by microfabrication technologies, as well as the challenges of how to realize stable and high-performance MESDs in such a limited footprint area. Benefiting from the unique architectural engineering of two-dimensional materials and the emergence of precise and controllable microfabrication techniques, the output electrochemical performances of MSCs and MBs are improving rapidly. This minireview summarizes recent advances in MSCs and MBs built from two-dimensional materials, including electrode/device configuration designs, material synthesis, microfabrication processes, smart function incorporations, and system integrations. An introduction to configurations of the MESDs, from linear fibrous shapes, planar sandwich thin-film or interdigital structures, to three-dimensional configurations, is presented. The fundamental influences of the electrode material and configuration designs on the exhibited MB/MSC electrochemical performances are also highlighted.

Hierarchical bimetallic hydroxide/chalcogenide core-sheath microarrays for freestanding ultrahigh rate supercapacitors

Transition metal compounds (TMCs) either crystalline or amorphous exhibit specific advantages in electrochemical energy storage. To integrate their merits into one electrode, we have herein developed hierarchical bimetallic hydroxide/chalcogenide core-sheath microarrays on nickel foam (NF) for freestanding high-efficiency supercapacitors, wherein interior crystalline metal chalcogenides serve as highly conductive pivots and exterior amorphous bimetallic hydroxides provide rich ion diffusion channels. With the synergic effect of the unique structure and bimetallic composition, the as-prepared Ni(OH)₂-Co(OH)₂/NiSe-Ni₃S₂/NF electrode displays an ultrahigh areal specific capacitance of 19.01 F cm^-2 at 15 mA cm^-2, which can be retained as 6.01 F cm^-2 even at 125 mA cm^-2. To the best of our knowledge, such excellent tolerance of ultrafast ion insertion/extraction at high current density is rare among NF-based free-standing electrodes. The asymmetric supercapacitor by assembling with activated carbon as the negative electrode delivers a volumetric capacitance of 3.93 F cm^-3 at 30 mA cm^-2, corresponding to an energy density of 13.9 mW h cm^-3 at a power density of 200 mW cm^-3. A capacitance retention of 82.5% was observed after 4000 cycles, together with an average 97% coulombic efficiency. This work may provide a facile strategy to construct hierarchical microarrays for efficient energy storage devices.

MOF-assisted construction of a Co9S8@Ni3S2/ZnS microplate array with ultrahigh areal specific capacity for advanced supercapattery

Transition metal sulfides are important candidates of battery-type electrode materials for advanced supercapatteries due to their high electric conductivity and electrochemical activity. The Co9S8@Ni3S2/ZnS composite microplate array was prepared by a metal-organic framework-assisted strategy because the electrochemical properties of composite arrays are governed by the synergistic effects of their diverse structures and compositions. As a battery-type material, the Co9S8@Ni3S2/ZnS electrode expressed an ultrahigh areal specific capacity of 8192 C cm-2 at the current density of 2 mA cm-2, and excellent cycling stability of 79.7% capacitance retention after 4000 cycles. An assembled supercapattery device using the Co9S8@Ni3S2/ZnS microplate array as a positive electrode and active carbon as the negative electrode delivered a high energy density of 0.377 mW h cm-2 at a high power density of 1.517 mW cm-2, and outstanding retention of 95.2% after 5000 cycles. As a result, the obtained Co9S8@Ni3S2/ZnS shows potential for applications in high-performance supercapattery.

N-Doped Modified Graphene/Fe2O3 Nanocomposites as High-Performance Anode Material for Sodium Ion Storage

Sodium-ion storage devices have received widespread attention because of their abundant sodium resources, low cost and high energy density, which verges on lithium-ion storage devices. Electrochemical redox reactions of metal oxides offer a new approach to construct high-capacity anodes for sodium-ion storage devices. However, the poor rate performance, low Coulombic efficiency, and undesirable cycle stability of the redox conversion anodes remain a huge challenge for the practical application of sodium ion energy storage devices due to sluggish kinetics and irreversible structural change of most conversion anodes during cycling. Herein, a nitrogen-doping graphene/Fe₂O₃ (N-GF-300) composite material was successfully prepared as a sodium-ion storage anode for sodium ion batteries and sodium ion supercapacitors through a water bath and an annealing process, where Fe₂O₃ nanoparticles with a homogenous size of about 30 nm were uniformly anchored on the graphene nanosheets. The nitrogen-doping graphene structure enhanced the connection between Fe₂O₃ nanoparticles with graphene nanosheets to improve electrical conductivity and buffer the volume change of the material for high capacity and stable cycle performance. The N-GF-300 anode material delivered a high reversible discharge capacity of 638 mAh g⁻¹ at a current density of 0.1 A g⁻¹ and retained 428.3 mAh g⁻¹ at 0.5 A g⁻¹ after 100 cycles, indicating a strong cyclability of the SIBs. The asymmetrical N-GF-300//graphene SIC exhibited a high energy density and power density with 58 Wh kg⁻¹ at 1365 W kg⁻¹ in organic solution. The experimental results from this work clearly illustrate that the nitrogen-doping graphene/Fe₂O₃ composite material N-GF-300 is a potential feasibility for sodium-ion storage devices, which further reveals that the nitrogen doping approach is an effective technique for modifying carbon matrix composites for high reaction kinetics during cycles in sodium-ion storage devices and even other electrochemical storage devices.

Redox Tuning in Crystalline and Electronic Structure of Bimetal-Organic Frameworks Derived Cobalt/Nickel Boride/Sulfide for Boosted Faradaic Capacitance

The development of efficient electrode materials is a cutting-edge approach for high-performance energy storage devices. Herein, an effective chemical redox approach is reported for tuning the crystalline and electronic structures of bimetallic cobalt/nickel-organic frameworks (Co-Ni MOFs) to boost faradaic redox reaction for high energy density. The as-obtained cobalt/nickel boride/sulfide exhibits a high specific capacitance (1281 F g^-1 at 1 A g^-1 ), remarkable rate performance (802.9 F g^-1 at 20 A g^-1 ), and outstanding cycling stability (92.1% retention after 10 000 cycles). An energy storage device fabricated with a cobalt/nickel boride/sulfide electrode exhibits a high energy density of 50.0 Wh kg^-1 at a power density of 857.7 W kg^-1 , and capacity retention of 87.7% (up to 5000 cycles at 12 A g^-1 ). Such an effective redox approach realizes the systematic electronic tuning that activates the fast faradaic reactions of the metal species in cobalt/nickel boride/sulfide which may shed substantial light on inspiring MOFs and their derivatives for energy storage devices.

Multiple Anionic Transition-Metal Oxycarbide for Better Lithium Storage and Facilitated Multielectron Reactions

As an important class of multielectron reaction materials, the applications of transition-metal oxides (TMOs) are impeded by volume expansion and poor electrochemical activity. To address these intrinsic limitations, the renewal of TMOs inspires research on incorporating an advanced interface layer with multiple anionic characteristics, which may add functionality to support properties inaccessible to a single-anion TMO electrode. Herein, a transition-metal oxycarbide (TMOC, M = Mo) with more than one anionic species was prepared as an interface layer on a corresponding oxide. A multiple anionic TMOC possesses advantages of structural stability, abundant active sites, and elevated metal cation valence states. Such merits mitigate volume changes and enhance multielectron reactions significantly. The TMOC nanocomposite has a well-maintained capacity after 1000 cycles at 2 A·g^-1 and fully resumed rate performance. In situ synchrotron X-ray powder diffraction (SXRPD) analysis unveils negligible volume expansions occurring upon oxycarbide layer coupling, with lattice spacing variation less than 1% during cycling. The lithium storage mechanism is further inspected by combined analysis of kinetics, SXRPD, and first-principles calculations. Superior to TMO, multielectron reactions of the TMOC electrode have been boosted due to easier rupture of the metal-oxygen bond. Such improvements underscore the importance of incorporating an oxycarbide configuration as a strategy to expand applications of TMOs.

Modulation of oxygen vacancy in tungsten oxide nanosheets for Vis-NIR light-enhanced electrocatalytic hydrogen production and anticancer photothermal therapy

Oxygen vacancy (OV) tuning was introduced into oxygen-deficient WO3 nanosheets to optimize the chemical and electronic properties. Enhanced electronic conduction, extended light absorption, enhanced HER reaction kinetics and benign photothermal performance were verified by density functional theory (DFT) calculations and experimental studies. Vis-NIR light-enhanced electrocatalytic HER was accomplished with a small overpotential of 52 mV (at 10 mA cm-2) and a low Tafel slope of 37 mV dec-1 and performed much more efficiently than that in darkness, comparable to the noble-metal catalysts (Pt, Pt/C). Moreover, the resultant WO3-OVs possess good photothermal conversion efficiency. The promising potential of the WO3-OVs for anticancer photothermal therapy has been demonstrated with a high photothermal conversion efficiency (∼41.6%) upon single wavelength near-infrared irradiation and an efficient tumor inhibition rate (∼96.8%). This design of photoelectronic/thermal materials paves an exciting new avenue for the conversion of well-developed metal oxides to be high-performance and multifunctional materials for energy and oncology applications.

Heterocarbides Reinforced Electrochemical Energy Storage

The feasibility of transition metal carbides (TMCs) as promising high-rate electrodes is still hindered by low specific capacity and sluggish charge transfer kinetics. Improving charge transport kinetics motivates research toward directions that would rely on heterostructures. In particular, heterocomposing with carbon-rich TMCs is highly promising for enhancing Li storage. However, due to limited synthesis methods to prepare carbon-rich TMCs, understanding the interfacial interaction effect on the high-rate performance of TMCs is often neglected. In this work, a novel strategy is proposed to construct a binary carbide heteroelectrode, i.e. incorporating the carbon-rich TMC (M=Mo) with its metal-rich TMC nanowires (nws) via an ingenious in situ disproportionation reaction. Results show that the as-prepared MoC-Mo₂ C-heteronanowires (hnws) electrode could fully recover its capacity after high-rates testing, and also possesses better lithium accommodation performance. Kinetic analysis verified that both electron and ion transfer in MoC-Mo₂ C-hnws are superior to those of its singular counterparts. Such improvements suggest that by taking utilization of the interfacial component interactions of stoichiometry tunable heterocarbides, the electrochemical performance, especially high-rate capability of carbides, could be significantly enhanced.

Highly Lithiophilic Cobalt Nitride Nanobrush as a Stable Host for High-Performance Lithium Metal Anodes

Lithium metal is considered to be a holy grail of the battery anode chemistry due to its large specific capacity. Nevertheless, the uncontrollable formation of lithium dendrites resulting from uneven lithium nucleation/growth and the associated safety risk and short cyclability severely impede the practical use of lithium metal anodes. Herein, we demonstrate a highly lithiophilic cobalt nitride nanobrush on a Ni foam (Co₃N/NF) current collector as a stable three-dimensional (3D) framework to inhibit the dendrite formation of lithium. The 3D Co₃N/NF nanobrush electrode could enable a low nucleation overpotential for homogeneous deposition of dendrite-free lithium. Compared to the CoO/NF counterpart and the bare Cu foil/Ni foam, the Co₃N/NF nanobrush host is of great benefit for enhanced Coulombic efficiencies and longer lifespan at various current densities. The present work will offer a new insight for the exploration of the highly lithiophilic scaffold based on metal nitrides toward high-performance lithium metal anodes.

A universal and controllable strategy of constructing transition-metal nitride heterostructures for highly enhanced bifunctional electrocatalysis

A family of transition-metal nitride heterostructures were synthesized by a universal and controllable method to remedy the drawbacks of ordinary bifunctional electrocatalysts.