Extraction and recycling technologies of cobalt from primary and secondary resources:A comprehensive review

Cobalt has excellent electrochemical,magnetic,and heat properties.As a strategic resource,it has been applied in many hightech products.However,the recent rapid growth of the battery industry has substantially depleted cobalt resources,leading to a crisis of cobalt resource supply.The paper examines cobalt ore reserves and distribution,and the recent development and consumption of cobalt resources are summarized as well.In addition,the principles,advantages and disadvantages,and research status of various methods are discussed comprehensively.It can be concluded that the use of diverse sources(Cu-Co ores,Ni-Co ores,zinc plant residues,and waste cobalt products)for cobalt production should be enhanced to meet developmental requirements.Furthermore,in recovery technology,the pyro-hydrometallurgical process employs pyrometallurgy as the pretreatment to modify the phase structure of cobalt minerals,enhancing its recovery in the hydrometallurgical stage and facilitating high-purity cobalt production.Consequently,it represents a promising technology for future cobalt recovery.Lastly,based on the above conclusions,the prospects for cobalt are assessed regarding cobalt ore processing and sustainable cobalt recycling,for which further study should be conducted.

Integration of morphology and electronic structure modulation on cobalt phosphide nanosheets to boost photocatalytic hydrogen evolution from ammonia borane hydrolysis

The controllable and safe hydrogen storage technologies are widely recognized as the main bottleneck for the accomplishment of sustainable hydrogen energy.Ammonia borane(AB)has regarded as a competitive candidate for chemical hydrogen storage.However,developing efficient yet high-performance catalysts towards hydrogen evolution from AB hydrolysis remains an enormous challenge.Herein,cobalt phosphide nanosheets are synthesized by a facile salt-assisted along with low-temperature phosphidation strategy for simultaneously modulating its morphology and electronic structure,and function as hydrogen evolution photocatalysts.Impressively,the Co_(2)P nanosheets display extraordinary performance with a record high turnover frequency of 44.9 min^(-1),outperforming most of the noble-metal-free catalysts reported to date.This remarkable performance is attributed to its desired nanosheets structure,featuring with high specific surface area,abundant exposed active sites,and short charge diffusion paths.Our findings provide a novel strategy for regulating metal phosphides with desired phase structure and morphology for energy-related applications and beyond.

Ba promoter effect on cobalt-catalyzed ammonia decomposition kinetics: A theoretical analysis

Ammonia decomposition is a key reaction in the context of hydrogen storage, transport, and release. This study combines density functional theory(DFT) calculations with microkinetic modeling to address the promotion mechanism of Ba species for ammonia decomposition on Co catalysts. The modified adsorption properties of Co upon the addition of metallic Ba or BaO suggest that the promoters play a role in alleviating the competitive adsorption of H. Calculating the full reaction pathway of ammonia decomposition shows that limiting the investigation to the N–N association step, as done previously, overlooks the effect of the promoter on the energy barriers of the NHxdehydrogenation steps. Challenges of modeling the ammonia decomposition reaction are addressed by understanding that the NH_(2) intermediate is stabilized on the step sites rather than the terrace sites. When the effect of H-coverage on the adsorption of NH_(3) is not considered in the microkinetic simulations, the results conflict with the experiments.However, accounting for the effect of H-coverage, as performed here, shows that BaO-doped Co has higher rates than pristine Co and Ba-doped Co at the reaction temperature of 723.15 K. When H is adsorbed on the Ba-doped Co, the adsorption of ammonia becomes significantly endergonic, which makes the rates relatively slow. The superiority of the BaO-promoted catalyst is attributed to a lower energy for the transition state of the rate-determining step, coupled with a reduced impact of the hydrogen coverage on weakening the ammonia adsorption. The kinetic analysis of the influence of Ba and BaO on the Co surface shows that BaO-doped Co aligns more closely with experimental observations than Badoped Co. This implies that Ba on the Co surface is likely to be in an oxide form under reaction conditions.Understanding the kinetics of the ammonia decomposition reaction provides a foundation for developing highly effective catalysts to accelerate the industrial utilization of ammonia as a sustainable hydrogen carrier.

The component-activity interrelationship of cobalt-based bifunctional electrocatalysts for overall water splitting:Strategies and performance

Cobalt-based electrocatalysts take advantage of potentially harmonizable microstructure and flexible coupling effects compared to commercial noble metal-based catalytic materials.However,conventional water electrolysis systems based on cobalt-based monofunctional hydrogen evolution reaction(HER)or oxygen evolution reaction(OER)catalysts have certain shortcomings in terms of resource utilization and universality.In contrast,cobalt-based bifunctional catalysts(CBCs)have attracted much attention in recent years for overall water splitting systems because of their practicality and reduced preparation cost of electrolyzer.This review aims to address the latest development in CBCs for total hydrolysis.The main modification strategies of CBCs are systematically classified in water electrolysis to provide an overview of how to regulate their morphology and electronic configuration.Then,the catalytic performance of CBCs in total-hydrolysis is summarized according to the types of cobalt-based phosphides,sulfides and oxides,and the mechanism of strengthened electrocatalytic ability is emphasized through combining experiments and theoretical calculations.Future efforts are finally suggested to focus on exploring the dynamic conversion of reaction intermediates and building near-industrial CBCs,designing advanced CBC materials through micro-modulation,and addressing commercial applications.

Modification,application and expansion of electrode materials based on cobalt telluride

Metal(Li,Na,K,Al)-ion batteries and lithium-sulfur and lithium-tellurium batteries are gaining recognition for their eco-friendly characteristics,substantial energy density,and sustainable attributes.However,the overall performance of rechargeable batteries heavily depends on their electrode materials.Transition metal tellurides have recently gained significant attention due to their high electrical conductivity and density.Cobalt telluride has received the most extensive research due to its catalytic activity,unique magnetic properties,and diverse composition and crystal structure.Nevertheless,its limited conductivity and significant volume variation contribute to electrode structural deterioration and rapid capacity decline.This review comprehensively summarizes recent advances in rational design and synthesis of modified cobalt telluride-based electrodes,encompassing defect engineering(Te vacancies,cation vacancies,heterointerfaces,and homogeneous interfaces)and composite engineering(derived carbon from precursors,carbon fibers,Mxene,graphene nanosheets,etc.).Particularly,the intricate evolution mechanisms of the conversion reaction process during cycling are elucidated.Furthermore,these modified strategies applied to other transitional metal tellurides,such as iron telluride,nickel telluride,zinc telluride,copper telluride,molybdenum telluride,etc.,are also thoroughly summarized.Additionally,their application extends to emerging aqueous zinc-ion batteries.Finally,potential challenges and prospects are discussed to further propel the development of transition metal tellurides electrode materials for next-generation rechargeable batteries.

High-Voltage and Fast-Charging Lithium Cobalt Oxide Cathodes: From Key Challenges and Strategies to Future Perspectives

Lithium-ion batteries(LIBs)with the“double-high”characteristics of high energy density and high power density are in urgent demand for facilitating the development of advanced portable electronics.However,the lithium ion(Li+)-storage performance of the most commercialized lithium cobalt oxide(LiCoO_(2),LCO)cathodes is still far from satisfactory in terms of high-voltage and fast-charging capabilities for reaching the double-high target.Herein,we systematically summarize and discuss high-voltage and fast-charging LCO cathodes,covering in depth the key fundamental challenges,latest advancements in modification strategies,and future perspectives in this field.Comprehensive and elaborated discussions are first presented on key fundamental challenges related to structural degradation,interfacial instability,the inhomogeneity reactions,and sluggish interfacial kinetics.We provide an instructive summary of deep insights into promising modification strategies and underlying mechanisms,categorized into element doping(Li-site,cobalt-/oxygen-site,and multi-site doping)for improved Li+diffusivity and bulkstructure stability;surface coating(dielectrics,ionic/electronic conductors,and their combination)for surface stability and conductivity;nanosizing;combinations of these strategies;and other strategies(i.e.,optimization of the electrolyte,binder,tortuosity of electrodes,charging protocols,and prelithiation methods).Finally,forward-looking perspectives and promising directions are sketched out and insightfully elucidated,providing constructive suggestions and instructions for designing and realizing high-voltage and fast-charging LCO cathodes for next-generation double-high LIBs.

Structural and magnetic properties of micropolycrystalline cobalt thin films fabricated by direct current magnetron sputtering

Pure cobalt(Co)thin films were fabricated by direct current magnetron sputtering,and the effects of sputtering power and pres-sure on the microstructure and electromagnetic properties of the films were investigated.As the sputtering power increases from 15 to 60 W,the Co thin films transition from an amorphous to a polycrystalline state,accompanied by an increase in the intercrystal pore width.Simultaneously,the resistivity decreases from 276 to 99μΩ·cm,coercivity increases from 162 to 293 Oe,and in-plane magnetic aniso-tropy disappears.As the sputtering pressure decreases from 1.6 to 0.2 Pa,grain size significantly increases,resistivity significantly de-creases,and the coercivity significantly increases(from 67 to 280 Oe),which can be attributed to the increase in defect width.Corres-pondingly,a quantitative model for the coercivity of Co thin films was formulated.The polycrystalline films sputtered under pressures of 0.2 and 0.4 Pa exhibit significant in-plane magnetic anisotropy,which is primarily attributable to increased microstress.

MOF-Derived Iron-Cobalt Phosphide Nanoframe as Bifunctional Electrocatalysts for Overall Water Splitting

Transition metal phosphides(TMPs)have emerged as an alternative to precious metals as efficient and low-cost catalysts for water electrolysis.Elemental doping and morphology control are effective approaches to further improve the performance of TMPs.Herein,Fe-doped CoP nanoframes(Fe-CoP NFs)with specific open cage configuration were designed and synthesized.The unique nano-framework structured Fe-CoP material shows overpotentials of only 255 and 122 mV at 10 mA cm^(−2)for oxygen evolution reaction(OER)and hydrogen evolution reaction(HER),respectively,overwhelming most transition metal phosphides.For overall water splitting,the cell voltage is 1.65 V for Fe-CoP NFs at a current density of 10 mA cm^(−2),much superior to what is observed for the classical nanocubic structures.Fe-CoP NFs show no activity degradation up to 100 h which contrasts sharply with the rapidly decaying performance of noble metal catalyst reference.The superior electrocatalytic performance of Fe-CoP NFs due to abundant accessible active sites,reduced kinetic energy barrier,and preferable*O-containing intermediate adsorption is demonstrated through experimental observations and theoretical calculations.Our findings could provide a potential method for the preparation of multifunctional material with hollow structures and offer more hopeful prospects for obtaining efficient earth-abundant catalysts for water splitting.

Nano-Au-decorated hierarchical porous cobalt sulfide derived from ZIF-67 toward optimized oxygen evolution catalysis:Important roles of microstructures and electronic modulation

Enhancing both the number of active sites available and the intrinsic activity of Co-based electrocatalysts simultaneously is a desirable goal.Herein,a ZIF-67-derived hierarchical porous cobalt sulfide decorated by Au nanoparticles(NPs)(denoted as HP-Au@CoxSy@ZIF-67)hybrid is synthesized by low-temperature sulfuration treatment.The well-defined macroporous-mesoporous-microporous structure is obtained based on the combination of polystyrene spheres,as-formed CoxSy nanosheets,and ZIF-67 frameworks.This novel three-dimensional hierarchical structure significantly enlarges the three-phase interfaces,accelerating the mass transfer and exposing the active centers for oxygen evolution reaction.The electronic structure of Co is modulated by Au through charge transfer,and a series of experiments,together with theoretical analysis,is performed to ascertain the electronic modulation of Co by Au.Meanwhile,HP-Au@CoxSy@ZIF-67 catalysts with different amounts of Au were synthesized,wherein Au and NaBH4 reductant result in an interesting“competition effect”to regulate the relative ratio of Co^(2+)/Co^(3+),and moderate Au assists the electrochemical performance to reach the highest value.Consequently,the optimized HP-Au@CoxSy@ZIF-67 exhibits a low overpotential of 340 mV at 10 mA cm^(-2)and a Tafel slope of 42 mV dec-1 for OER in 0.1 M aqueous KOH,enabling efficient water splitting and Zn-air battery performance.The work here highlights the pivotal roles of both microstructural and electronic modulation in enhancing electrocatalytic activity and presents a feasible strategy for designing and optimizing advanced electrocatalysts.

Cobalt–Nickel Vanadate Nanonest Colonies Deposited Carbon Fabric as a Bifunctional Electrode for Li‑Ion Batteries and Oxygen‑Evolution Reactions

Transition metal vanadates(TMVs)have attracted significant attention in various research fields owing to their advantageous features.Furthermore,synthesizing TMVs directly on current collectors at the nanoscale is a promising strategy for achieving better performance.Herein,cobalt–nickel vanadate(CoV_(2)O_(6)–Ni_(2)V_(2)O_(7),CNV)was directly grown on carbon fabric using a facile one-step hydrothermal method.In particular,the CNV sample prepared for 3 h(CNV-3)exhibited a benefit-enriched nanonest-colony morphology in which abundant nanowires(diameter:10 nm)were intertwined,providing sufficient space for electrolyte diffusion.All the CNV electrodes exhibited good cycling performance in the lithium-ion battery study.Espe-cially,the CNV-3 electrode retained higher discharge and charge capacities of 616 and 610 mAh g-1,respectively at the 100th cycle than the other two electrodes owing to several morphologic features.The electrocatalytic activity of all the CNV samples for the oxygen-evolution reaction(OER)was also explored in an alkaline electrolyte.Among these CNV catalysts,the CNV-3 displayed excellent OER performance and required an overpotential of only 270 mV to drive a current density of 10 mA cm^(-2).The Tafel slope of this catalyst was also found to be low(129 mV dec^(-1)).Moreover,the catalyst exhibited excellent durability in a 24 h stability test.These results indicate that the metal vanadates with favorable nanostructures are highly suitable for both energy storage and water-splitting applications.

Assessing the dynamics of O_(2) desorption from cobalt phthalocyanine by in situ electrochemical scanning tunneling microscopy

We report here the in situ electrochemical scanning tunneling microscopy(ECSTM) study of cobalt phthalocyanine(CoPc)-catalyzed O_(2) evolution reaction(OER) and the dynamics of CoPc-O_(2) dissociation.The self-assembled CoPc monolayer is fabricated on Au(111) substrate and resolved by ECSTM in 0.1 M KOH electrolyte.The OH^(-)adsorption on CoPc prior to OER is observed in ECSTM images.During OER,the generated O_(2) adsorbed on Co Pc is observed in the CoPc monolayer.Potential step experiment is employed to monitor the desorption of OER-generated O_(2) from CoPc,which results in the decreasing surface coverage of CoPc-O_(2) with time.The rate constant of O_(2) desorption is evaluated through data fitting.The insights into the dynamics of Co-O_(2) dissociation at the molecular level via in situ imaging help understand the role of Co-O_(2) in oxygen reduction reaction(ORR) and OER.

Recovery of cobalt from converter slag of Chambishi Copper Smelter using reduction smelting process

The reduction smelting process for cobalt recovery from converter slag of the Chambishi Copper Smelter in Zambia was studied. The effects of reducing agent dosage, smelting temperature and time and the addition of slag modifiers （CaO and TiO2） were investigated. In addition, the depleted slag and cobalt-bearing alloy were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Under the determined conditions, 94.02% Co, 95.76% Cu and less than 18% Fe in the converter slag were recovered. It was found that the main phases of depleted slag were fayalite and hercynite; and the cobalt-bearing alloy mainly contained metallic copper, Fe-Co-Cu alloys and a small amount of sulfide.

Theoretical investigation of carbon dioxide adsorption on MgH_(2) with a cobalt catalyst

This work presents a theoretical investigation of carbon dioxide(CO_(2))adsorption on MgH_(2) and its reaction(chemisorption)with cobalt doped MgH_(2).The focus of this study is the properties and mechanisms involved in CO_(2) adsorption on clean MgH_(2) surfaces and the role of Co in enhancing the adsorption process.Density functional theory(DFT)calculations were performed to examine different CO_(2) adsorption sites on the MgH_(2) surface along with the adsorption distances,binding energies,and geometric parameters.The results indicate that physical adsorption of CO_(2) occurs on MgH_(2) with similar adsorption energies at different adsorption sites.The coverage effect of CO_(2) molecules on MgH_(2) was also investigated,revealing an increased affinity of CO_(2) with higher surface coverage.However,excessive coverage led to a decrease in adsorption efficiency due to competing surface adsorption and intermolecular interactions.The orientation of adsorbed CO_(2) molecules shifted from parallel to quasiperpendicular arrangements upon adsorption,with notable deformations observed at higher coverage,which gives a hint of CO_(2) activation.Furthermore,the study explores the CO_(2) adsorption capacity of MgH_(2) in comparison to other materials reported in the literature,showcasing its medium to strong affinity for CO_(2).Additionally,the effectiveness of a single Co atom and Co clusters as catalysts for CO_(2) adsorption on MgH_(2) was examined.Overall,this theoretical investigation provides insights into the CO_(2) adsorption properties of MgH_(2) and highlights the potential of Co catalysts to enhance the efficiency of the methanation process.

Preparation of spherical cobalt carbonate from cobalt sulfate solution

A precursor of cobaltous dihydroxycarbonate was firstly prepared by precipitation reaction of cobalt sulfate solution and ammonium carbonate solution,and then a hydrothermal process for the precursor was conducted to obtain the spherical cobalt carbonate with low sulfur content.The experimental results show that the feeding method,final p H value of the precipitation reaction slurry and the concentration of the cobalt sulfate solution have obvious effects on the sulfur content,morphology and particle size distribution of the precursor.The sulfur content of the precursor is 0.0115 wt.%under the optimized conditions.The hydrothermal treatment with temperatures of 125-150℃can transform the precursor of cobaltous dihydroxycarbonate into spherical cobalt carbonate and decrease the sulfur content to 0.0030 wt.%in the obtained product.

Cobalt‐N_(4) macrocyclic complexes for heterogeneous electrocatalysis of the CO_(2) reduction reaction

Metal-N_(4)(M-N_(4))macrocyclic complexes are interesting electrocatalysts due to their well-defined structures and rich molecular tuning.Among them,metal phthalocyanines have been widely studied for the carbon dioxide reduction reaction(CO_(2)RR)in heterogeneous systems and demonstrated good electrocatalytic performance.However,other complexes like metal corroles and metal porphyrins are much less explored,and often show inferior performances.In this study,three cobalt macrocyclic complexes,cobalt phthalocyanine,cobalt meso-tetraphenylporphyrin,and cobalt meso-triphenylcorrole(CoPc,CoTPP and CoTPC)are investigated in heterogeneous electrocatalysis of CO_(2)RR.Although CoPc/carbon nanotube(CNT)hybrid exhibits high electrocatalytic activity,CNT hybridization does not work for CoTPC and CoTPP that hold weak interactions with CNTs.By the drop-dry method with a high molecular loading of 5.4×10^(–7) mol cm^(–2),CoTPC and CoTPP could deliver appreciable electrode activities.Poly(4-vinylpyridine)(PVP)introduction is further demonstrated as a facile method to afford enhanced activities for CoTPP at low molecular loadings through enhancing molecule-substrate interactions.The partial current density of carbon monoxide for CoTPP+CNT/PVP is around 8 times higher than the sample without PVP at–0.67 V versus reversible hydrogen electrode.This work provides solutions to enhance the electrode activities of molecular electrocatalysts with weak substrate interactions in heterogeneous systems.

Extraction of arsenic from arsenic-containing cobalt and nickel slag and preparation of arsenic-bearing compounds

The arsenic extraction from the arsenic-containing cobalt and nickel slag,which came from the purification process of zinc sulfate solution in a zinc smelting factory,was investigated.The alkaline leaching method was proposed according to the mode of occurrence of arsenic in the slag and its amphoteric characteristic.The leaching experiments were conducted in the alkaline aqueous medium,with bubbling of oxygen into the solution,and the optimal conditions for leaching arsenic were determined.The results showed that the extraction rate of arsenic was maximized at 99.10%under the optimal conditions of temperature 140 ℃,NaOH concentration 150 g/L,oxygen partial pressure 0.5 MPa,and a liquid-to-solid ratio 5：1.Based on the solubilities of As2O5,ZnO and PbO in NaOH solution at 25 ℃,a method for the separation of As in the form of sodium arsenate salt from the arsenic-rich leachate via cooling crystallization was established,and the reaction medium could be fully recycled.The crystallization rate was confirmed to reach 88.9%（calculated on the basis of Na3AsO4） upon a direct cooling of the hot leachate down to room temperature.On the basis of redox potentials,the sodium arsenate solution could be further reduced by sulfur dioxide（SO2） gas to arsenite,at a reduction yield of 92%under the suitable conditions.Arsenic trioxide with regular octahedron shape could be prepared successfully from the reduced solution,and further recycled to the purification process to purify the zinc sulfate solution.Also,sodium arsenite solution obtained after the reduction of arsenate could be directly used to purify the zinc sulfate solution.Therefore,the technical scheme of alkaline leaching with pressured oxygen,cooling crystallization,arsenate reduction by SO2 gas,and arsenic trioxide preparation,provides an attractive approach to realize the resource utilization of arsenic-containing cobalt and nickel slag.

Catalytic performance of cobalt oxide-supported gold-palladium nanocatalysts for the removal of toluene and o-xylene

Using the molten salt and polyvinyl alcohol-protected reduction method,we fabricated Co3O4 octahedron-supported Au-Pd（x（AuPdy）/Co3O4;x =（0.18,0.47,and 0.96） wt%;y（Pd/Au molar ratio） =1.85-1.97） nanocatalysts.The molten salt-derived Co3O4 sample possessed well-defined octahedral morphology,with an edge length of 300 nm.The Au-Pd nanoparticles,with sizes of 2.7-3.2 nm,were uniformly dispersed on the surface of Co3O4.The 0.96（AuPd1.92）/Co3O4 sample showed the highest catalytic activity for toluene and o-xylene oxidation,and the temperature required for achieving 90%conversion of toluene and o-xylene was 180 and 187 ℃,respectively,at a space velocity of 40000 mL/（g·h）.The high catalytic performance of Co3O4 octahedron-supported Au-Pd nanocatalysts was associated with the interaction between Au-Pd nanoparticles and Co3O4 and high concentration of adsorbed oxygen species.

Electrodeposition of cobalt in double-membrane three-compartment electrolytic reactor

The process parameters were optimized for the electrodeposition of cobalt from cobalt chloride solution in the membrane electrolytic reactor. Effects of parameters such as catholyte composition, current density and temperature on the current efficiency, specific power consumption and quality of deposition were studied. The catholyte was a mixed solution of cobalt chloride, the initial middle electrolyte consisted of diluted hydrochloric acid, and the anolyte was sulfuric acid. An anion exchange membrane separated the catholyte from the middle electrolyte, and a cation exchange membrane separated the anolyte from the middle electrolyte. The results showed that a maximum current efficiency of 97.5% was attained under the optimum experimental condition of an catholyte composition of 80 g/L Co^2＋, 20 g/L H3BO3, 3 g/L NaF and pH of 4, at a cathode current density of 250 A/m2 and a temperature of 50 ℃ HCl could be produced in the middle compartment electrochemically up to 0.45 mol/L.

Kinetics of cobalt(Ⅱ) extraction from sulfate aqueous solution by sodium salt of di-decylphosphinic acid (DDPA)

Kinetics of Co（II） extraction from sulfate aqueous solution by the sodium salt of di-decylphosphinic acid （Na-DDPA） was studied using a modified Lewis cell to disclose the mechanism of extraction. Parameters affecting the extraction rate, such as stirring speed, temperature, interfacial area, Na-DDPA concentration and Co（II） concentration, were investigated, respectively. The effect of diffusion resistance on extraction rate was negligible when the stirring operation was conducted in a plateau region of 95-110 r/min. Extraction rate increased with the increase in the temperature or specific interfacial area. The activation energy E was calculated to be 32.75 kJ/mol. These suggested that rate controlling mechanism of Co（II） extraction by Na-DDPA was chemical reaction regime and the rate-determining step of Co（II） extraction was chemical reaction at the interface. The initial extraction rate also increased with the increase in the concentrations of Na-DDPA and Co（II）. The extraction rate equation and mechanism of Co（II） extraction by Na-DDPA were proposed.

Pseudo-capacitive Behavior of Cobalt Hydroxide/Carbon Nanotubes Composite Prepared by Cathodic Deposition

A novel type of composite electrode based on nmltiwalled carbon nanotubes coated with sheet-like cobalt hydroxide particles was used in supercapacitors. Cobalt hydroxide cathodlcally deposited fiom Co（NO3）O2 solution with carbon nanotubes as matrix exhibited large pseudo-capacitance of 322 F/g in 1 mol/L KOH. To characterize the cobalt hydroxide nanocomposite electrode, a charge-discharge cycling test, cyclic voltammetry, and an impedance test were done. This cobalt hydroxide composite exhibiting excellent pseudo-capacitive behavior （i.c. high reversibility, high specific capacitance, low impedance）, was demonstrated to be a candidate for the application of electrochemical supercapacitors. A combined capacitor consisting of cobalt hydroxide composite as a cathode and activated carbon fiber as an anode was reported. The electrochemical pcrformance of the combined capacitor was characterized by cyclic voltammetry and a dc charge/discharge test. The combined capacitor showed ideal capacitor behavior with an extended operating voltage of 1.4 V. According to the extended operating voltage, the energy density of the combined capacitor at a current density of 100 mA/cm^2 was found to be 11 Wh/kg. The combined capacitor exhibited high-energy density and stable power characteristics,