Direct Growth of Oxygen Vacancy-Enriched Co3O4 Nanosheets on Carbon Nanotubes for High-Performance Supercapacitors

Structure and Morphological Properties of Cobalt-Oxide-Based (Co3O4) Materials as Electrodes for Supercapacitors: A Brief Review

Over the past 15 years, there has been a significant increase in the search for environmentally friendly energy sources, and transition-metal-based energy storage devices are leading the way in these new technologies. Supercapacitors are attractive in this regard due to their superior energy storage capabilities. Electrode materials, which are crucial components of supercapacitors, such as cobalt-oxide-based electrodes, have great qualities for achieving high supercapacitor performance. This brief review presents some basic concepts and recent findings on cobalt-based materials used to fabricate electrodes for supercapacitors. The text also clarifies how morphological characteristics typically influence certain properties. The inner surface of the electrode exhibits several properties that change to provide it a great boost in specific capacitance and charge storage. Porous structures with defined pore sizes and shapes and high surface areas are important features for improving electrochemical properties. Finally, we present some perspectives for the development of cobalt-oxide-based supercapacitors, focusing on their structure and properties.

Oxygen vacancies enhancing hierarchical NiCo2S4@MnO2 electrode for flexible asymmetric supercapacitors

The limited energy density of supercapacitors hampers their widespread application in electronic devices. Metal oxides, employed as electrode materials, suffer from low conductivity and stability, prompting extensive research in recent years to enhance their electrochemical properties. Among these efforts, the construction of core-shell heterostructures and the utilization of oxygen vacancy (VO) engineering have emerged as pivotal strategies for improving material stability and ion diffusion rates. Herein, core-shell composites comprising NiCo2S4 nanospheres and MnO2 nanosheets are grown in situ on carbon cloth (CC), forming nanoflower clusters while introducing VO defects through a chemical reduction method. Density functional theory (DFT) results proves that the existence of VO effectively enhances electronic and structural properties of MnO2, thereby enhancing capacitive properties. The electrochemical test results show that NiCo2S4@MnO2-V3 exhibits excellent 1376 F g-1 mass capacitance and 2.06 F cm-2 area capacitance at 1 A g-1. Moreover, NiCo2S4@MnO2-V3//activated carbon (AC) asymmetric supercapacitor (ASC) can achieve an energy density of 39.7 Wh kg-1 at a power density of 775 W kg-1, and maintains 15.5 Wh kg-1 even at 7749.77 W kg-1. Capacitance retention is 73.1 % after 10,000 cycles at 5 A g-1, and coulombic efficiency reaches 100 %, demonstrating satisfactory cycle stability. In addition, the device's excellent flexibility offers broad application prospects in wearable electronic applications.

Ultrathin, large area β-Ni(OH)2 crystalline nanosheet as bifunctional electrode material for charge storage and oxygen evolution reaction

Bifunctional electrode materials are highly desirable for meeting increasing global energy demands and mitigating environmental impact. However, improving the atom-efficiency, scalability, and cost-effectiveness of storage systems, as well as optimizing conversion processes to enhance overall energy utilization and sustainability, remains a significant challenge for their application. Herein, we devised an optimized, facile, economic, and scalable synthesis of large area (cm2), ultrathin (∼2.9 ± 0.3 nm) electroactive nanosheet of β-Ni(OH)2, which acted as bifunctional electrode material for charge storage and oxygen evolution reaction (OER). The β-Ni(OH)2 nanosheet electrode shows the volumetric capacity of 2.82 Ah.cm-3(0.82 µAh.cm-2) at the current density of 0.2 mA.cm-2. The device shows a high capacity of 820 mAh.cm-3 with an ultrahigh volumetric energy density of 0.33 Wh.cm-3 at 275.86 W.cm-3 along with promising stability (30,000 cycles). Furthermore, the OER activity of ultrathin β-Ni(OH)2 exhibits an overpotential (η10) of 308 mV and a Tafel value of 42 mV dec-1 suggesting fast reaction kinetics. The mechanistic studies are enlightened through density functional theory (DFT), which reveals that additional electronic states near the Fermi level enhance activity for both capacitance and OER.

ZIF-derived oxygen vacancy-rich Co3O4 for constructing an efficient Z-scheme heterojunction to boost photocatalytic water splitting

With ZIF-67 as the precursor, oxygen vacancy-rich Co3O4 nanoparticles were derived and anchored on the surface of 2D polyimide (PI) to construct a Z-scheme hybrid heterojunction (20ZP) through a simultaneous solvothermal in situ crystallization and polymerization strategy. XRD, XPS and EPR confirmed that both Co(III) and oxygen vacancies are formed during the low temperature conversion of ZIF-67 to Co3O4 nanoparticles that in turn accelerate the polymerization of PI. Synchronous crystallization makes the interfacial architecture intermetal and compact, inducing a strong interfacial electronic interaction between Co3O4 nanoparticles and PI. UV-vis DRS spectra and transient photocurrent response demonstrate that the incorporation of Co3O4 on polyimide not only extends the light absorption in the visible range, but also enhances the charge transfer rate. EIS, TRPL techniques and DFT calculations have confirmed that the photoinduced interfacial charge transfer pathway of this hybrid heterojunction characterized the Z-scheme in which the photoinduced electrons transfer from the conduction band of Co3O4 to the valence band of PI, significantly inhibiting the recombination of electrons and holes within PI. More importantly, the oxygen vacancies located below the conductor band of Co3O4 can deepen the band bending, improve the charge separation efficiency and accelerate electron transfer between Co3O4 and PI. This Z-scheme hybrid heterojunction structure can not only maintain the high reducing capacity of photoinduced electrons on the conductor band of PI, but also enhance the oxidative capacity of the heterojunction composite material, thus promoting the overall progress of the photocatalytic hydrogen release reaction.

Multiple synergies on cobalt-based spinel oxide nanowires for electrocatalytic oxygen evolution

Cobalt-based spinel oxides have excellent oxygen evolution reaction (OER) activities and are cheap to produce; however, they have limited commercial applications due to their poor electrical conductivities and weak stabilities. Herein, we soaked Co3-xNixO4 nanowires in NaBH4 solutions, which endowed Co3-xNixO4 with significant oxygen vacancy content and decorated BOx motifs outside the Co3-xNixO4 nanowires. X-ray photoelectron spectroscopy and in situ Raman data suggest that these evolutions improved the conductivity, hydrophilicity, and increased active sites of the spinel oxides, which synergistically boosted their overall OER performances. This improved performance made the optimized BOx-covered Co2.1Ni0.9O4 nanowires generate a current density of 10 mA cm-2 when used for the OER at an overpotential of only 307 mV, maintaining excellent stability at 50 mA cm-2 for 24 h. This study provides a facile method for designing cobalt-based spinel oxide OER catalysts.

Fabrication of CoSe2/CoP with rich selenium- and phosphorus-vacancies and heterogeneous interfaces for asymmetric supercapacitors

CoSe2/CoP with rich Se- and P-vacancies and heterogeneous interfaces (v-CoSe2/CoP) is grown on the surface of nickel foam via a two-step strategy: electrodeposition and NaBH4 reduction, which can be used as the cathode material in asymmetric supercapacitors. The SEM characterization reveals the honeycomb-like structure of the v-CoSe2/CoP, and the results of EPR, XPS and HRTEM reveal the existence of anionic vacancies and heterogeneous interfaces in the v-CoSe2/CoP. The as-fabricated v-CoSe2/CoP exhibits high specific capacitance (3206 mF cm-2 at 1.0 mA cm-2) and cyclic stability (91 % capacitance retention after 2000 cycles). An asymmetric supercapacitor is assembled by using the v-CoSe2/CoP and activated carbon (AC) as cathode and anode materials, respectively, which displays a high energy density of 40.6 Wh kg-1 at the power density of 211.5 W kg-1. The outstanding electrochemical performances of the v-CoSe2/CoP might be ascribed to the synergistic effects of Se- and P-vacancies and the heterogeneous interfaces in the v-CoSe2/CoP.

Special NaBH4 hydrolysis achieving multiple-surface-modifications promotes the high-throughput water oxidation of CoN nanowire arrays

Designing an excellent OER catalyst in an alkaline environment is severe yet essential for industrial H₂ application under the electrochemical technique. This study has achieved multiple modifications on CoN nanowires, the classic OER catalyst, via a facile room-temperature NaBH₄ spontaneous hydrolysis. This facile process simultaneously generates oxygen vacancies and robust BN species. It wraps hydrophilic BO_x motifs on the OER response CoN nanowires, producing OER active Co-N-B species, increasing active numbers and guaranteeing structural stability. It suggests that a low NaBH₄ concentration (0.1 mol L^-1) treatment endows CoN_NWAs/CC with excellent OER performance and robust structure, which can drive a current density of 50 mA cm^-2 with only 325 mV overpotentials with more than 24 hours' durability. Even, the catalyst can drive 1000 mA cm^-2 around 480 mV overpotential. This study allows a novel strategy for designing high-performance OER catalysts.

Superstructures of Zeolitic Imidazolate Frameworks to Single- and Multiatom Sites for Electrochemical Energy Conversion

The exploration of electrocatalysts with high catalytic activity and long-term stability for electrochemical energy conversion is significant yet remains challenging. Zeolitic imidazolate framework (ZIF)-derived superstructures are a source of atomic-site-containing electrocatalysts. These atomic sites anchor the guest encapsulation and self-assembly of aspheric polyhedral particles produced using microreactor fabrication. This review provides an overview of ZIF-derived superstructures by highlighting some of the key structural types, such as open carbon cages, 1D superstructures, hollow structures, and the interconversion of superstructures. The fundamentals and representative structures are outlined to demonstrate the role of superstructures in the construction of materials with atomic sites, such as single- and dual-atom materials. Then, the roles of ZIF-derived single-atom sites for the electroreduction of CO₂ and electrochemical synthesis of H₂ O₂ are discussed, and their electrochemical performance for energy conversion is outlined. Finally, the perspective on advancing single- and dual-atom electrode-based electrochemical processes with enhanced redox activity and a low-impedance charge-transfer pathway for cathodes is provided. The challenges associated with ZIF-derived superstructures for electrochemical energy conversion are discussed.

Hierarchical Nanocages Assembled by NiCo-Layered Double Hydroxide Nanosheets for a High-Performance Hybrid Supercapacitor

Layered double hydroxides (LDHs) have attracted broad attention as cathode materials for hybrid supercapacitors (HSCs) because of their ultrahigh theoretical specific capacitance, high compositional flexibility, and adjustable interlayer spacing. However, as reported, specific capacitance of LDHs is still far below the theoretical value, inspiring countless efforts to these ongoing challenges. Herein, a hierarchical nanocage structure assembled by NiCo-LDH nanosheet arrays was rationally designed and fabricated via a facile solvothermal method assisted by the ZIF-67 template. The transformation from the ZIF-67 template to this hollow structure is achieved by a synergistic effect involving the Kirkendall effect and the Ostwald ripening process. The enlarged specific surface area co-occurred with broadened interlayer spacing of LDH nanosheets by finely increasing the Ni concentration, leading to synchronous improvement of electron/ion transfer kinetics. The optimized NiCo-LDH-210 electrode displays a maximum specific capacitance of 2203.6 F g^-1 at 2 A g^-1, excellent rate capability, and satisfactory cycling stability because of the highly exposed active sites and shortened ion transport paths provided by vertically aligned LDH nanosheets together with the cavity. Furthermore, the assembled HSC device achieves a superior energy density of 57.3 Wh kg^-1 with prominent cycling stability. Impressively, the design concept of complex construction derived from metal-organic frameworks (MOF) derivatives shows tremendous potential for use in energy storage systems.

Realizing Superior Redox Kinetics of Hollow Bimetallic Sulfide Nanoarchitectures by Defect-Induced Manipulation toward Flexible Solid-State Supercapacitors

As a typical battery-type material, CuCo₂ S₄ is a promising candidate for supercapacitors due to the high theoretical specific capacity. However, its practical application is plagued by inherently sluggish ion diffusion kinetics and inferior electrical transport properties. Herein, sulfur vacancies are incorporated in CuCo₂ S₄ hollow nanoarchitectures (HNs) to accelerate redox reactivity. Experimental analyses and theoretical investigations uncover that the generated sulfur vacancies increase the active electron states, reduce the adsorption barriers of electrolyte ions, and enrich reactive redox species, thus achieving enhanced electrochemical performance. Consequently, the deficient CuCo₂ S₄ with optimized vacancy concentration presents a high specific capacity of 231 mAh g^-1 at 1 A g^-1 , a ≈1.78 times increase compared to that of pristine CuCo₂ S₄ , and exhibits a superior rate capability (73.8% capacity retention at 20 A g^-1 ). Furthermore, flexible solid-state asymmetric supercapacitor devices assembled with the deficient CuCo₂ S₄ HNs and VN nanosheets deliver a high energy density of 61.4 W h kg^-1 at 750 W kg^-1 . Under different bending states, the devices display exceptional mechanical flexibility with no obvious change in CV curves at 50 mV s^-1 . These findings provide insights for regulating electrode reactivity of battery-type materials through intentional nanoarchitectonics and vacancy engineering.

Reduced Mo-doped NiCo2O4 with rich oxygen vacancies as advanced electrode material in supercapacitors

Reduced Mo-doped NiCo2O4 (R-Mo-NiCo2O4) was facilely prepared through a dual-defect strategy. Mo-doped NiCo layered double hydroxide (Mo-NiCo-LDH) was used as the precursor and calcined in an air atmosphere, and the...

Co2P decorated Co3O4 nanocomposites supported on carbon cloth with enhanced electrochemical performance for asymmetric supercapacitors

An effective strategy is demonstrated to promote electrochemical performance by the combination of Co3O4 with Co2P to form a composite electrode.

Hierarchically urchin-like hollow NiCo2S4 prepared by a facile template-free method for high-performance supercapacitors

Hollow structures draw much attention for high energy density supercapacitors due to their large hollow cavities, high specific surface area, and low interfacial contact resistance. However, constructing hierarchical hollow structures remains a challenge. Herein, we reported a facile template-free method for a novel urchin-like hollow nickel cobalt sulfide (NiCo₂S₄). The hollow interior and urchin exterior remarkably improved the specific capacitance and accommodated structural collapse caused by electrochemical reactions. Owing to these features, the urchin-like hollow NiCo₂S₄ spheres exhibited an impressive capacitance of 1398F g^-1 at 1 A g^-1 and maintained 1110F g^-1 with a large current density of 10 A g^-1. The hybrid supercapacitor fabricated by NiCo₂S₄ and active carbon possesses an energy density of 39.3 Wh kg^-1 at a power density of 749.6 W kg^-1 and an outstanding cycling stability of 74.4% retention after 5000 cycles. Our work presents a facile method of constructing a hollow structure of binary sulfide materials and also makes progress on highly efficient 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.

Feasible Tuning of Surface OVs on (001) TiO2 for Superior Photocatalytic Nitrogen Fixation Activity

A feasible tuning method for oxygen vacancies was realized by annealing under 3 atm H₂ with (001)-exposed TiO₂ nanosheets. The colored TiO₂ sample exhibits an excellent N₂ photo-fixation rate owing to the abundant oxygen vacancies (OVs) thus demonstrating that annealing with high pressure H₂ is exceedingly efficient for tuning surface OVs.

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.

Synergistic effect of defects and porous structure in CoCCHH-CoSe heterogeneous-tube @PEDOT:PSS foam towards elastic supercapacitor with enhanced pseudocapacitances

It is still challenging to construct stable 3D energy storage materials at the nanoscale by precise pore structure control and reasonable surface modification. Herein, a novel interwoven porous Co(CO₃)_0.35Cl_0.20(OH)_1.10 (CoCCHH)-CoSe heterogeneous-tube @PEDOT:PSS 3D foam with abundant active sites is presented as supercapacitor electrodes. The electrochemical results indicated that the pore structure provides ample space for redox reaction, and increases the number of ion transport channels. Besides, rational surface modification brings about sufficient active sites for redox reaction. The stable, porous PEDOT:PSS foam with a 3D elastic frame exhibited excellent electrical conductivity. Thus, the CoCCHH-CoSe@PEDOT:PSS foam possessed excellent specific capacitance and energy density, due to the synergistic effect of the unique 3D structure and surface defects. The home-made supercapacitor with CoCCHH-CoSe@PEDOT:PSS foam as cathode materials showed high specific capacitance (440.6F g^-1 at 1 A g^-1) and excellent energy density (137.7 Wh kg^-1). This work provides a valuable strategy to develop potential materials for electrochemical energy storage.