Encapsulation of Co3 O4 Nanocone Arrays via Ultrathin NiO for Superior Performance Asymmetric Supercapacitors

On the epitaxial growth in ALD Co3O4- and NiO-based bilayers

NiO and Co3O4 are versatile materials studied for a plethora of applications, yet their performance for a specific application relies on the control of their crystallographic texture and corresponding surface facets. Achieving such control can be challenging, often requiring hetero-epitaxial growth on single-crystalline substrates, which are frequently incompatible with the requirements of the application. The combination of NiO and Co3O4 in heterostructures provides potential to control texture due to their similar crystal structures, whilst retaining the possibility to work with more versatile substrates. In this study, atomic layer deposited (ALD) thin films based on cyclopentadienyl precursors and an oxygen plasma are adopted to tailor the crystallographic texture of NiO from 〈100〉 to 〈111〉 using an ALD Co3O4 template layer, and similarly, to modify the Co3O4 texture from 〈111〉 to 〈100〉 on a NiO template. The films are shown to conform to the crystal orientation of the template material, whilst crystallizing directly in their own stable crystal structure with corresponding metal atom coordination. Further investigation includes ALD process parameters for NiO growth: the film texture is found to depend on the choice of co-reactant and the above-highlighted hetero-epitaxial relationship is stronger for plasma-based processes. In conclusion, these results demonstrate an original approach for application-oriented crystallographic engineering in thin films.

Gradient Nanostructures and Machine Learning Synergy for Robust Quantitative Surface-Enhanced Raman Scattering

Surface-Enhanced Raman Scattering (SERS) holds significant promise for trace-level molecular detection but faces challenges in achieving reliable quantitative analysis due to signal variability caused by non-uniform "hot spots" and external factors. To address these limitations, a novel SERS platform based on gradient nanostructures is developed using shadow sphere lithography, enabling the acquisition of diverse spectral features from a single analyte concentration under identical conditions. The gradient design minimizes fabrication variability and enhances spectral diversity, while the machine learning (ML) model trained on the multi-spectral dataset significantly outperformed traditional single-spectrum approaches, with the test Mean Squared Error (MSE) reduced by 84.8% and the coefficient of determination (R2) improved by 61.2%. This strategy captures subtle spectral variations, improving the precision, robustness, and reproducibility of SERS-based quantification, paving the way for its reliable application in real-world scenarios.

Triazine-containing Covalent Organic Polymer-derived Grid-Like Multilocular Spheres for Aqueous Supercapacitors

Triazine-containing covalent organic polymers (TCOPs) with unique structures and physicochemical properties are of great potential in energy storage and conversion applications, yet how to finely tune the morphology, and the accessible active sites, and to enhance capacitive activity remains a challenge. Here, the grid-like multilocular spheres derived from TCOP with abundant redox active sites and unique structures are fabricated via a molecular twist-induced regulation strategy, of which the number and size of cavities can be finely modulated by changing the conformers of the twisted unit and the Ostwald ripening time. The unique structure of the as-fabricated TCOP results in unprecedented high specific capacitance (8412 F g-1 at 1 A g-1) and enables the as-assembled supercapacitor with an ultra-high energy density of 675 Wh kg-1 in redox-active electrolyte (KI-mixed H2SO4), much better than all reported aqueous supercapacitors thus far. It is found that the high electro-activity is due to the synergistic effect of the enhanced accessibility of active sites and the enhanced interaction of the abundant active sites with the redox-active electrolytes. This approach may pave a new way to precise synthesis of COPs with tuned structure and properties for application-inspired cutting-edge electrochemical energy storage and beyond.

Development of Oleylamine-Directed Nano-OZIF-67 Induced Microporous Carbon Nanofiber Conjugated with Tiny Fibrous PEDOT Polymer for Hybrid-Capacitor Electrodes

Making nanostructured metal oxides intertwined with graphitic carbon materials is critically important to construct hybrid supercapacitors with high-performance, cost-affordable, and free-standing 1D hybrid electrode material and is of foremost importance to responding positively to the impending energy crisis. Fine-tuned nano-sized ZIF-67 particles of 5 - 25 nm width (OZIF-67) are achieved using oleylamine-involved synthesis and are seeded into fibrous poly-PAN-PVP. Porous N-doped carbon nanofibers impregnated with OZIF-67-derived Co3O4 (oz-Co3O4-PNCNF) are obtained by calcining the electrospun polymer. The surface area of oz-Co3O4-PNCNF is exceptionally high, 429 m2 g-1, with the pore size predominantly ≈2 - 5 nm. Thin conducting polymer films of poly-(3,4-ethylenedioxythiophene) are formed onto oz-Co3O4-PNCNF (oz-Co3O4-PNCNF/PEDOT). A porous, long nanofibrous structure (≈100 nm thick) of oz-Co3O4-PNCNF and a spongy needle-like network of PEDOT interconnecting outer layers are established from FESEM images. The material has shown excellent energy storage capability with the specific capacitance (CS) of 417 F g-1 (oz-Co3O4-PNCNF/PEDOT). Free-standing supercapacitor device delivers excellent performance in asymmetric assembly (oz-Co3O4-PNCNF/PEDOT//PNCNF) with 141.1 Wh kg-1 and 84% CS at the end of 5000 charge-discharge cycles. The excellent porous network, combined with finely-tuned nano-sized Co3O4 particles and an overlying thin electropolymer film, work together to establish high energy, efficient, and stable supercapacitors.

Review on recent advancements in the role of electrolytes and electrode materials on supercapacitor performances

Supercapacitors currently hold a prominent position in energy storage systems due to their exceptionally high power density, although they fall behind batteries and fuel cells in terms of energy density. This paper examines contemporary approaches aimed at enhancing the energy density of supercapacitors by adopting hybrid configurations, alongside considerations of their power density, rate capability, and cycle stability. Given that electrodes play a pivotal role in supercapacitor cells, this review focuses on the design of hybrid electrode structures with elevated specific capacitance, shedding light on the underlying mechanisms. Factors such as available surface area, porosity, and conductivity of the constituent materials significantly influence electrode performance, prompting the adoption of strategies such as nanostructuring. Additionally, the paper delves into the impact of novel bio-based hybrid electrolytes, drawing upon literature data to outline the fabrication of various hybrid electrode materials incorporating conducting polymers like polyaniline and polypyrrole, as well as metal oxides, carbon compounds, and hybrid electrolytes such as ionic liquids, gel polymers, aqueous, and solid polymer electrolytes. The discussion explores the contributions of different components and methodologies to overall capacitance, with a primary emphasis on the mechanisms of energy storage through non-faradic electrical double-layer capacitance and faradaic pseudo-capacitance. Furthermore, the paper addresses the electrochemical performance of hybrid components, examining their concentrations and functioning via diverse charge storage techniques.

Recent Development on Transition Metal Oxides-Based Core-Shell Structures for Boosted Energy Density Supercapacitors

In recent years, nanomaterials exploration and synthesis have played a crucial role in advancing energy storage research, particularly in supercapacitor development. Researchers have diversified materials, including metal oxides, chalcogenides, and composites, as well as carbon materials, to enhance energy and power density. Balancing energy density with electrochemical stability remains challenging, driving intensified efforts in advancing electrode materials. This review focuses on recent progress in designing and synthesizing core-shell materials tailored for supercapacitors. The core-shell architecture offers advantages such as increased surface area, redox active sites, electrical conductivity, ion diffusion kinetics, specific capacitance, and cyclability. The review explores the impact of core and shell materials, specifically transition metal oxides (TMOs), on supercapacitor electrochemical behavior. Metal oxide choices, such as cobalt oxide as a preferred core and manganese oxide as a shell, are discussed. The review also highlights characterization techniques for assessing structural, morphological, and electrochemical properties of core-shell materials. Overall, it provides a comprehensive overview of ongoing TMOs-based core-shell material research for supercapacitors, showcasing their potential to enhance energy storage for applications ranging from gadgets to electric vehicles. The review outlines existing challenges and future opportunities in evolving TMOs-based core-shell materials for supercapacitor advancements, holding promise for high-efficiency energy storage devices.

Engineering of Co3O4 electrode via Ni and Cu-doping for supercapacitor application

Although cobalt oxides show great promise as supercapacitor electrode materials, their slow kinetics and low conductivity make them unsuitable for widespread application. We developed Ni and Cu-doped Co3O4 nanoparticles (NPs) via a simple chemical co-precipitation method without the aid of a surfactant. The samples were analyzed for their composition, function group, band gap, structure/morphology, thermal property, surface area and electrochemical property using X-ray diffraction (XRD), ICP-OES, Fourier transform infrared (FTIR) spectroscopy, Ultraviolet-visible (UV-Vis), Scanning electron microscopy (SEM), Thermogravimetric analysis (TGA) and/or Differential thermal analysis (DTA), Brunauer-Emmett-Teller (BET), and Impedance Spectroscopy (EIS), Cyclic voltammetry (CV), respectively. Notably, for the prepared sample, the addition of Cu to Co3O4 NPs results in a 11.5-fold increase in specific surface area (573.78 m2 g-1) and a decrease in charge transfer resistance. As a result, the Ni doped Co3O4 electrode exhibits a high specific capacitance of 749 F g-1, 1.75 times greater than the pristine Co3O4 electrode's 426 F g-1. The electrode's enhanced surface area and electronic conductivity are credited with the significant improvement in electrochemical performance. The produced Ni doped Co3O4 electrode has the potential to be employed in supercapacitor systems, as the obtained findings amply demonstrated.

CoOx @NiMoN/Ti3 C2 Tx Interface for Stable High-Performance Electrochemical Energy Storage Devices

Transition metal nitrides (TMNs) are promising electrode materials for use in high-performance electrochemical energy storage devices due to their unique properties, which include a high conductivity, pseudocapacitance, and energy density. However, structural instability during electrochemical reactions has limited their practical deployment for energy storage devices. In this context, the present study fabricated a CoOx @NiMoN/Ti3 C2 Tx electrode via in situ growth on Ni foam using hydrothermal treatment with post-nitrogenization. The effect of atomic layer deposition (ALD) of CoOx on the TMN/Ti3 C2 Tx interface and the consequent electrochemical charge storage mechanisms are investigated in detail. The proposed CoOx @NiMoN/Ti3 C2 Tx electrode delivers an impressive specific capacity in a 2 m potassium hydroxide (KOH) electrolyte and is then employed in both a hybrid solid-state supercapacitor (HSSC) with reduced graphene oxide and a symmetric SC in a 2 m KOH + polyvinyl alcohol (PVA) gel electrolyte. Outstanding charge storage and high capacity retention during cyclic testing are observed for both energy storage devices. The exceptional electrochemical performance of the fabricated electrode is a result of its high conductivity and high number of active sites. Here a feasible new strategy is demonstrated for the fabrication of stable energy storage devices with a high energy density using TMNs and MXenes.

Atomic Layer Deposition-A Versatile Toolbox for Designing/Engineering Electrodes for Advanced Supercapacitors

Atomic layer deposition (ALD) has become the most widely used thin-film deposition technique in various fields due to its unique advantages, such as self-terminating growth, precise thickness control, and excellent deposition quality. In the energy storage domain, ALD has shown great potential for supercapacitors (SCs) by enabling the construction and surface engineering of novel electrode materials. This review aims to present a comprehensive outlook on the development, achievements, and design of advanced electrodes involving the application of ALD for realizing high-performance SCs to date, as organized in several sections of this paper. Specifically, this review focuses on understanding the influence of ALD parameters on the electrochemical performance and discusses the ALD of nanostructured electrochemically active electrode materials on various templates for SCs. It examines the influence of ALD parameters on electrochemical performance and highlights ALD's role in passivating electrodes and creating 3D nanoarchitectures. The relationship between synthesis procedures and SC properties is analyzed to guide future research in preparing materials for various applications. Finally, it is concluded by suggesting the directions and scope of future research and development to further leverage the unique advantages of ALD for fabricating new materials and harness the unexplored opportunities in the fabrication of advanced-generation SCs.

Bifunctional bimetallic oxide nanowires for high-efficiency electrosynthesis of 2,5-furandicarboxylic acid and ammonia

It is an appealing avenue for electrosyntheis of high-valued chemicals at both anode and cathode by coupling 5-hydroxymethylfurfural (HMF) oxidation and nitrate reduction reactions simultaneously, while the development such bifunctional electrocatalysts is still in its infancy with dissatisfied selectivity and low yield rate. Here, we first report that Zn-doped Co3O4 nanowires array can be served as an efficient and robust dual-functional catalyst for HMF oxidation and nitrate reduction at ambient conditions. Specifically, the catalyst shows a faradaic efficiency of 91 % and a yield rate of 241.2 μmol h-1 cm-2 for 2,5-furandicarboxylic acid formation together with a high conversion of nearly 100 % at a potential of 1.40 V. It also displays good cycling stability. Besides, the catalyst is capable of catalyzing the reduction of nitrate to NH3, giving a maximal faradaic efficiency of 92 % and a peak NH3 yield rate of 4.65 mg h-1 cm-2 at a potential of -0.70 V. These results surpass those obtained using pristine Co3O4 and are comparable to those of state-of-the-art electrocatalysts. Moreover, the catalyst is further employed as the cathode catalyst to assemble a Zn-nitrate battery, giving a peak power density of 5.24 mW cm-2 and a high yield rate of 0.72 mg h-1 cm-2. Theoretical simulations further reveal that Zn-doping favors the adsorption and dissociation of nitrate and HMF species and reduces the energy barrier as well. Our work demonstrates the potential interest of Co3O4-based materials for the highly selective production of valuable feedstocks via ambient electrolysis.

Prussian blue analogue-derived hollow metal oxide heterostructure for high-performance supercapacitors

Supercapacitors (SCs) have been the subject of considerable interest because of their distinct advantages. The performance of SCs is directly affected by the electrode materials. Metal oxides derived from Prussian blue analogues (PBAs) are often used as electrode materials for SCs. Herein, we developed a multi-step strategy to fabricate ternary hollow metal oxide (CuO/NiO/Co3O4) heterostructures. The core-shell structured PBA (NiHCC@CuHCC) with Ni-based PBA (NiHCC) as the core and Cu-based PBA (CuHCC) as the shell was prepared by a crystal seed method. The ternary metal oxide (CuO/NiO/Co3O4) with a hollow structure was obtained by calcinating NiHCC@CuHCC. The prepared CuO/NiO/Co3O4 demonstrates an excellent specific capacitance of 262.5 F g-1 at 1 A g-1, which is 27.4% and 16.2% higher than those of CuO/Co3O4 and NiO/Co3O4, respectively. In addition, the material showed outstanding cycling stability with a capacitance retention of 107.9% after 3000 cycles. The two-electrode system constructed with CuO/NiO/Co3O4 and nitrogen-doped graphene hydrogel (NDGH) demonstrates a stable and high energy density of 27.1 W h kg-1 at a power density of 1037.5 W kg-1. The capacitance retention rate was 100.7% after 4000 cycles. The reason for the excellent electrochemical properties could be the synergistic effect of the introduced heterojunction of CuO/NiO, the hollow structure, and various metal oxides. This strategy can greatly inspire the construction of SC electrodes.

Modulation of p- and n-Type Transitions in Co3- x Nix O4 Nanoparticles for Enhanced Supercapacitor Electrochemical Performance

The efficient storage of electrons and the type of conduction in semiconductor materials are important factors in determining their electrochemical performance. However, the interaction between these two factors is often overlooked by researchers. In this study, the effects of Ni doping at Co3- x Nix O4 nanoparticles on the electronic storage form of the material and resulting changes in the conduction p/n-type are reported. Theoretical calculations demonstrate that n-type conduction with high effective mass of electrons contributes significantly to the redox reaction of electrode materials and is beneficial for improving electrochemical performance. The specific capacitance of Co3- x Nix O4 (x = 0.67) electrode material is 10 times larger than that of Co3 O4 due to enhanced orbital hybridization caused by Ni atom doping. The findings provide new directions for exploring the mechanism of conductive type conversion of materials and offer insights beyond the traditional approach of considering doping content alone.

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.

Facile synthesis of Ni3Se4/Ni0.6Zn0.4O/ZnO nanoparticle as high-performance electrode materials for electrochemical energy storage device

To enhance the performance of transition metal chalcogenide composite electrode material, a key point is a composite design and preparation based on the synergistic effect between the oxide and selenide materials. With a facile 'one step template-annealing' step, Ni₃Se₄, Ni_0.6Zn_0.4O and ZnO are simultaneously synthesized, by 500 °C annealing. With the increase of annealing temperature from 350 °C to 600 °C, nickel selenides change from NiSe₂to Ni₃Se₄to NiSe. The charge storage capacity increases first and then decreases with the increase of annealing temperature, and the 500 °C annealing obtained three compound composite Ni₃Se₄/Ni_0.6Zn_0.4O/ZnO (NNZ-500) nanoparticle material displayed a high specific capacitance of 1089.2 F g^-1at 1 A g^-1, and excellent cycle stability of 99.8% capacitance retention after 2000 cycles at 5 A g^-1. Moreover, an asymmetric supercapacitor was assembled with NNZ-500 as the positive electrode material and activated carbon as the negative electrode material. This kind of asymmetric supercapacitor demonstrated a high energy density of 53.4 Wh kg^-1at 819.0 W kg^-1, and cycle stability with 98.6% capacitance retention after 2000 cycles. This material preparation approach provides great potential for the future development of high performance transition metal composite electrode materials in energy storage applications.

Nanostructured mixed transition metal oxide spinels for supercapacitor applications

There have been numerous applications of supercapacitors in day-to-day life. Along with batteries and fuel cells, supercapacitors play an essential role in supplementary electrochemical energy storage technologies. They are used as power sources in portable electronics, automobiles, power backup, medical equipment, etc. Among various working electrode materials explored for supercapacitors, nanostructured transition metal oxides containing mixed metals are highly specific and special, because of their stability, variable oxidation states of the constituted metal ions, possibility to tune the mixed metal combinations, and existence of new battery types and extrinsic pseudocapacitance. This review presents the key features and recent developments in the direction of synthesis and electrochemical energy storage behavior of some of the recent morphology-oriented transition metal oxide and mixed transition metal oxide nanoparticles. We also targeted the studies on a few of the recently developed flexible and bendable supercapacitor devices based on these mixed transition metal oxides.

Square-Facet Nanobar MOF-Derived Co3O4@Co/N-doped CNT Core-Shell-based Nanocomposites as Cathode Materials for High-Performance Supercapacitor Studies

The binary as well as ternary nanocomposites of the square-facet nanobar Co-MOF-derived Co₃O₄@Co/N-CNTs (N-CNTs: nitrogen-doped carbon nanotubes) with Ag NPs and rGO have been synthesized via an easy wet chemical route, and their supercapacitor behavior was then studied. At a controlled pH of the precursor solution, square-facet nanobars of Co-MOF were first synthesized by the solvothermal method and then pyrolyzed under a controlled nitrogen atmosphere to get a core-shell system of Co₃O₄@Co/N-CNTs. In the second step, different compositions of Co₃O₄@Co/N-CNT core-shell structures were formed by an ex-situ method with Ag NPs and rGO moieties. Among several bare, binary, and ternary compositions tested in 6 M aqueous KOH electrolyte, a ternary nanocomposite having a 7.0:1.5:1.5 stoichiometric ratio of Co₃O₄@Co/N-CNT, Ag NPs, and rGO, respectively, reported the highest specific capacitance (3393.8 F g^-1 at 5 mV s^-1). The optimized nanocomposite showed the energy density, power density, and Coulombic efficiency of 74.1 W h.kg^-1, 443.7 W.kg^-1, and 101.3%, respectively, with excellent electrochemical stability. After testing an asymmetrical supercapacitor with a Co₃O₄@Co/N-CNT/Ag NPs/rGO/nickel foam cathode and an activated carbon/nickel foam anode, it showed 4.9 W h.kg^-1 of energy density and 5000.0 W.kg^-1 of power density.

Oxygen vacancies refilling and potassium ions intercalation of δ-manganese dioxide with high structural stability toward 2.3 V high voltage asymmetric supercapacitors

Oxygen vacancies occupation and coordination environment modulation of the transition-metal based electrodes are effective strategies to improve the structural stability and electrochemical performance. In this work, the 2-methylimidazole (2-MI) doped manganese dioxide (MnO2) anchored on carbon cloth (CC) is fabricated via a simple method (MI-MnO2-x/CC), where the oxygen defects on/inside the K+ doped δ-MnO2 nanosheets are in-situ created by reductive ethanol/Mn2+ and occupied by 2-MI ligands. With the pre-embedded K+ ions and abundant ligand-refilled defects, the electronic coordination structure, structural stability and electron/ion diffusion efficiency can be effectively enhanced. Therefore, the MI-MnO2-x/CC reveals a remarkable specific capacitance of 721.2 mF cm-2 with excellent cycle durability (capacitance retention of 93.4% after 10,000 cycles) under 1.3 V operation potential window. In addition, an asymmetric supercapacitor assembled by MI-MnO2-x/CC and activated mechanical exfoliated graphene oxide yields a maximum energy density of 57.0 Wh kg-1 and a highest power density of 23.0 kW kg-1 under 2.3 V. This effective oxygen defect stabilization strategy by ligands refilling can be extended to various metal oxide-based electrodes for energy storage and conversion.

Ribbon-like Nickel Cobaltite with Layer-by-Layer-Assembled Ordered Nanocrystallites for Next-Generation All-Solid-State Hybrid Supercapatteries

In the context to develop ultra-efficient electrode materials with good physicoelectrochemical and electrostructural properties, for their application in high-performance supercapatteries, herein, a facile tartrate-mediated inhibited crystal growth method is reported to engineer thoroughly uniform ribbon-like nickel cobaltite (NiCo₂O₄) microstructure with unique layer-by-layer-assembled nanocrystallites. This material demonstrates significant kinetic reversibility, good rate efficiency and bulk diffusibility of the electroactive ions, and a predominant semi-infinite diffusion mechanism during the redox-based charge storage process. This material also shows bias-potential-independent equivalent series resistance, very low charge-transfer resistance, and diagonal Warburg profile, corresponding to the ion diffusion occurring during the electrochemical processes in supercapacitors and batteries. Further, the fabricated NiCo₂O₄-based all-solid-state supercapattery (NiCo₂O₄||N-rGO) delivers excellent rate-specific capacity, very low internal resistance, good electrochemical and electrostructural stability (∼94% capacity retention after 10,000 charge-discharge cycles), energy density (31 W h kg^-1) of a typical rechargeable battery, and power density (13,003 W kg^-1) of an ultra-supercapacitor. The ultimate performance of the supercapattery is ascribed to low-dimensional crystallites, ordered inter-crystallite and channel-type bulk and boundary porosity, multiple reactive equivalents, enhanced electronic conductivity, and "ion buffering pool" like behavior of ribbon-like NiCo₂O₄, supplemented with enhanced electronic and ionic conductivities of N-doped rGO (negative electrode) and PVA/KOH gel (electrolyte separator), respectively.

Recent Developments in Supercapacitor Electrodes: A Mini Review

The use of nonrenewable fossil fuels for energy has increased in recent decades, posing a serious threat to human life. As a result, it is critical to build environmentally friendly and low-cost reliable and renewable energy storage solutions. The supercapacitor is a future energy device because of its higher power density and outstanding cyclic stability with a quick charge and discharge process. Supercapacitors, on the other hand, have a lower energy density than regular batteries. It is well known that the electrochemical characteristic of supercapacitors is strongly dependent on electrode materials. The current review highlights advance in the TMOs for supercapacitor electrodes. In addition, the newly discovered hybrid/pseudo-supercapacitors have been discussed. Metal oxides that are employed as electrode materials are the focus of this study. The discovery of nanostructured electrode materials continues to be a major focus of supercapacitor research. To create high-performance electrode materials from a morphological standpoint, various efforts have been attempted. Lastly, we analyze the supercapacitor’s evolving trend and our perspective for the future generations of supercapacitors.

Mo-doped Co3O4 ultrathin nanosheet arrays anchored on nickel foam as a bi-functional electrode for supercapacitor and overall water splitting

Simple preparation, favorable price and environmental protection have been a long-term challenge in the field of electrochemistry. Herein, we studied and prepared a bifunctional Mo-doped Co₃O₄ ultrathin nanosheets, which has been validated as an effective binder-free electrode material for electrocatalytic water splitting and supercapacitors. The material has a large specific surface area, high electrical conductivity and exposure to more active sites, breaking down the limited performance and range of use of transition metal oxides. Benefiting from intriguing ultrathin property and conductivity, OER and HER process of 0.4Mo-Co₃O₄ have a small Tafel slope of 83.7 and 98 mV dec^-1, respectively. The current density at 10 mA cm^-2 show a low overpotential of 315 and 79 mV and significant stability. The water electrolytic device requires a potential of 1.64 V to reach 10 mA cm^-2, and the potential change is negligible after 12 h of continuous electrolysis. In addition, the manifest improved electrochemical performance of 0.3Mo-Co₃O₄ as supercapacitor electrode material shows high areal capacitance 2815 mF cm^-2 at 1 mA cm^-2, excellent rate performance (85% at 10 mA cm^-2) and retains 90% of the initial capacitance by cycling 5000 at a current density of 10 mA cm^-2. Moreover, 0.3Mo-Co₃O₄||0.3Mo-Co₃O₄ symmetrical supercapacitor has a maximum volumetric energy density of 1.25 mW h cm^-3 at a power density of 7.1 mW cm^-3 and superior cycle life. The influence of doping on electrochemical performance was studied by changing the content of doped metal ions, which is of great significance for the exploration of supercapacitor and electrocatalytic hydrolysis of bifunctional electrode materials.

Rationally designed NiMn LDH@NiCo2O4core-shell structures for high energy density supercapacitor and enzyme-free glucose sensor

Exploring high-efficiency and low-cost bifunctional electrodes for supercapacitors and sensors is significant but challenging. Most of the existing electrodes are mostly single-functional materials with simple structure. Herein, NiCo₂O₄nanowires as the core and NiMn layered double hydroxide (LDH) as the shell is directly grownin situon carbon cloth (CC) to form a heterostructure (NiMn LDH@NiCo₂O₄/CC). The performance in supercapacitors and enzyme-free glucose sensing has been systematically studied. Compared with a single NiCo₂O₄nanowire or NiMn LDH nanosheet, the heterogeneous interface produced by the unique core-shell structure has stronger electronic interaction and abundant active surface area, which shows excellent electrochemical performance. Electrochemical tests demonstrate that the NiMn LDH@NiCo₂O₄/CC core-shell electrode possesses an area specific capacitance of 2.40 F cm^-2and a rate capability of 76.22% at 20 mA cm^-2. Simultaneously, asymmetric supercapacitor is assembled with it as the positive electrode and NiFe LDH@NiCo₂O₄/CC as the negative electrode. The supercapacitor possesses an energy density of 47.74 Wh kg^-1when the power density is 175 W kg^-1, revealing excellent performance and maintains cycle stability of 93.48% after 6000 cycles at 10 mA cm^-2. Additionally, the electrode applied as enzyme-free glucose sensor electrode also displays outstanding sensitivity of 2139μA mM^-1cm^-2, wide detection range (2μM^-3mM and 4-8 mM) and low detection limit of 210 nM, representing good anti-interference performance. This work reveals the multi-metal synergy and rationally designed core-shell structure is critical to the electrochemical performance of bifunctional electrodes.

Facile assembly of 2D Ni-based coordination polymer nanosheets as battery-type electrodes for high-performance supercapacitors

Large-scale Ni-based nano-sized coordination polymers (Ni-nCPs) are facilely constructed by a self-assembled approach at room temperature and atmosphere pressure. In this strategy, we use only the environmentally friendly solvents of water and ethanol, and the synthesis of 2D Ni-nCPs via a self-assembly route appears close to the "green chemistry" concept. In addition, the morphologies of the Ni-nCPs can be easily adjusted by the water/ethanol ratio. Owing to its unique 2D ultrathin nature and large specific surface area, Ni-nCPs-1 achieves a great number of channels for the transport of electrons and ions and electrochemically redox active sites for a faradaic reaction. Therefore, battery-type Ni-nCPs-1 electrodes have a bright prospect in energy storage, and can reach an outstanding specific capacitance value as high as 1066.9 F g^-1 at 1 A g^-1. Additionally, the asymmetric supercapacitor (Ni-nCPs-1//active carbon) displays a high energy density of 47.9 W h kg^-1 at a power density of 440 W kg^-1 and an excellent long-term cycle stability. This work may open up a new path in advanced electrode materials for efficient and real-time energy storge applications.

Three-Dimensional Hierarchical Core/shell Electrodes Using Highly Conformal TiO2 and Co3O4 Thin Films for High-Performance Supercapattery Devices

The rational design and development of novel electrode materials with promising nanostructures is an effective technique to improve their supercapacitive performance. This work presents high-performance core/shell electrodes based on three-dimensional hierarchical nanostructures coated with conformal thin transition-metal oxide layers using atomic layer deposition (ALD). This effective interface engineering creates disorder in the electronic structure and coordination environment at the interface of the heteronanostructure, which provides many more reaction sites and rapid ion diffusion. At 3 A g^-1, the positive CuCo₂O₄/Ni₄Mo/MoO₂@ALD-Co₃O₄ electrode introduced here exhibits a specific capacity of 1029.1 C g^-1, and the fabricated negative Fe₃O₄@ALD-TiO₂ electrode significantly outperforms conventional carbon-based electrodes, with a maximum specific capacity of 372.6 C g^-1. The supercapattery cell assembled from these two interface- and surface-tailored electrodes exhibits a very high energy density of 110.4 W h kg^-1 with exceptional capacity retention over 20,000 cycles, demonstrating the immense potential of ALD for the next generation of supercapacitors.

Synthesis of the cathode and anode materials from discarded surgical masks for high-performance asymmetric supercapacitors

Advanced carbon-based electrode materials derived from wastes are essential to high-performance supercapacitors due to their abundance and sustainability. In this work, we fabricate novel cathodes and anodes based on discarded surgicalmask-derived carbon (DSM-C). Discarded surgicalmasks are good candidates for carbon-based electrode materials due to their unique fibrous structure and simple composition compared to conventional biomass sources. Benefiting from the excellent electrical conductivity of DSM-C and abundant redox reactions from nickel oxide (NiO), the electrochemical performances of NiO/DSM-C composites have been greatly improved. Specifically, the DSM-C and NiO/DSM-C electrodes show high specific capacitances of 240 F g^-1 and 496 F g^-1 at 1 A g^-1 respectively, and excellent rate capability. Moreover,asymmetric supercapacitors (ASCs) are assembled using DSM-C and NiO/DSM-C as anodes and cathodes, respectively. They deliver a high energy density of 57 Wh kg^-1 at a power density of 702 W kg^-1, accompanied by superior cycling stability (98.5% capacitance retention after 10,000 cycles). This work shows prospective applications of DSM-C as an electrode material for energy storage systems.

Transition Metal Oxide Electrode Materials for Supercapacitors: A Review of Recent Developments

In the past decades, the energy consumption of nonrenewable fossil fuels has been increasing, which severely threatens human life. Thus, it is very urgent to develop renewable and reliable energy storage devices with features of environmental harmlessness and low cost. High power density, excellent cycle stability, and a fast charge/discharge process make supercapacitors a promising energy device. However, the energy density of supercapacitors is still less than that of ordinary batteries. As is known to all, the electrochemical performance of supercapacitors is largely dependent on electrode materials. In this review, we firstly introduced six typical transition metal oxides (TMOs) for supercapacitor electrodes, including RuO₂, Co₃O₄, MnO₂, ZnO, XCo₂O₄ (X = Mn, Cu, Ni), and AMoO₄ (A = Co, Mn, Ni, Zn). Secondly, the problems of these TMOs in practical application are presented and the corresponding feasible solutions are clarified. Then, we summarize the latest developments of the six TMOs for supercapacitor electrodes. Finally, we discuss the developing trend of supercapacitors and give some recommendations for the future of supercapacitors.

All Si3 N4 Nanowires Membrane Based High-Performance Flexible Solid-State Asymmetric Supercapacitor

Recently, much attention has been drawn in the development of flexible energy storage devices due to the increasing demands for flexible/portable electronic devices with high energy density, low weight, and good flexibility. Herein, vertically oriented graphene nanosheets (VGNs) are in situ fabricated on the surface of free-standing and flexible Si₃ N₄ nanowires (NWs) membrane by plasma-enhanced chemical vapor deposition (PECVD), which are directly used as flexible nanoscale conductive substrates. NiCo₂ O₄ hollow nanospheres (HSs) and FeOOH amorphous nanorods (NRs) are finally prepared on Si₃ N_4NWs @VGNs, which are served as the positive and negative electrodes, respectively. Profiting from the structural merits, the synthesized Si₃ N_4NWs @VGNs@NiCo₂ O_4HSs and Si₃ N_4NWs @VGNs@FeOOH_NRs membrane electrodes exhibit remarkable electrochemical performance. Using Si₃ N_4NWs membrane as the separator, the assembled all Si₃ N_4NWs membrane-based flexible solid-state asymmetric supercapacitor (ASC) with a wide operating potential window of 1.8 V yields the outstanding energy density of 96.3 Wh kg^-1 , excellent cycling performance (91.7% after 6000 cycles), and good mechanical flexibility. More importantly, this work provides a rational design strategy for the preparation of flexible electrode materials and broadens the applications of Si₃ N_4NWs in the field of energy storage.

Mechanism orienting structure construction of electrodes for aqueous electrochemical energy storage systems: a review

Aqueous electrochemical energy storage systems (AEESS) are considered as the most promising energy storage devices for large-scale energy storage. AEESSs, including batteries and supercapacitors, have received extensive attention due to their low cost, eco-friendliness, and high safety. However, the insufficient energy densities of the state-of-the-art AEESSs limit their practical applications which are mainly dominated by the electrochemical performances of individual electrode materials. Understanding the underlying relationship between structures, reaction mechanisms, and performances can further lead to the design and optimization of structures of the electrodes instructively, thereby harvesting favorable performances. This review classified the intrinsic logic of structure-mechanism-performance by taking some prevailing mechanisms with some classical structures of materials as examples. Moreover, some problem-oriented structural engineering strategies are proposed aiming to optimize their performance. Finally, comprehensive structural design engineering and some suggestions for fine modifications of electrode materials at the atomic and molecular levels are proposed to combine the advantages of supercapacitor- and battery-type materials for designing excellent electrode materials for AEESSs.