Porous cobalt hydroxide nanosheets with excellent supercapacitive behavior

Role of synthetic process parameters of nano-sized cobalt/nickel oxide in controlling their structural characteristics and electrochemical energy performance as supercapacitor electrodes

The synthesis of nano-sized bimetallic Cobalt/Nickel oxides (Ni1.5Co1.5O4) with a 1:1 Co/Ni atomic ratio has been achieved using a surfactant-free co-precipitation/hydrothermal process. The growth mechanism of Cobalt/Nickel oxides Ni1.5Co1.5O4 is elucidated by tuning the synthesis process parameters, including co-precipitation pH and hydrothermal time. The formation of Cobalt/Nickel oxides Ni1.5Co1.5O4 oxide began with the nucleation of cobalt nickel hydroxide nanoplates through the co-precipitation process, followed by dissolution-recrystallization, stacked hexagonal nano-flakes, and a flower-like microstructure. The electrochemical performances of the oxides were evaluated, with the largest surface area observed at pH 9 being the main factor for the best super-capacitive performance. As hydrothermal time increased, the structural directing growth forward, resulted in the formation of a nano-flower structure with a larger surface area. The as-prepared cobalt nickel oxide exhibited a maximum specific capacitance value of 525.5 F g-1 at a current density of 1 A g-1 and energy and power densities of 88.2 WhKg-1 and 606 WKg-1, respectively.

A flexible solid-state asymmetric supercapacitor device comprising cobalt hydroxide and biomass-derived porous carbon

Development in the field of alternative and renewable energy sources is becoming necessary considering the current energy demands of the growing technologies. The main challenge associated with the produced energy is to store it for future use, such that it can be used when needed. Supercapacitors are among the electrochemical energy storage systems that provides higher power density, faster charging-discharging, high specific capacitance (C S), and long cycling life. Herein, the fabrication of a flexible solid-state asymmetric supercapacitor (ASC) device is reported, where Co(OH)2 hollow spheres and biomass-derived porous carbon (PC) are the cathode and anode, respectively. Co(OH)2 is a highly redox active material, whereas PC is an electric double-layer capacitive (EDLC) material. In this device, aqueous KOH solution (electrolyte) encapsulated in PVA gel (separator) was used to bind the electrodes. This Co(OH)2//PC ASC device exhibited a high C S of 260 F g-1 (at 2 A g-1). It retained ∼91% of the initial C S value (at 6 A g-1) till ∼5000 cycles. Electrochemical impedance spectroscopy (EIS) study confirmed low internal resistance (0.95 Ω) and charge transfer resistance (1.41 Ω) values of Co(OH)2//PC. These results indicate that the high electron transfer process in the electrode-electrolyte interface during the electrochemical reaction, which is responsible for the excellent performance of this ASC device. The high-performance Co(OH)2//PC ASC device exhibited an energy density of 76.7 W h kg-1 at a power density of 1416.9 W kg-1. To demonstrate its practical use, LED lights were illuminated using this Co(OH)2//PC ASC device.

Co-Ni Layered Double Hydroxide for the Electrocatalytic Oxidation of Organic Molecules: An Approach to Lowering the Overall Cell Voltage for the Water Splitting Process

Electrocatalytic oxidation of simple organic molecules offers a promising strategy to combat the sluggish kinetics of the water oxidation reaction (WOR). The low potential requirement, inhibition of the crossover of gases, and formation of value-added products at the anode are benefits of the electrocatalytic oxidation of organic molecules. Herein, we developed cobalt-nickel-based layered double hydroxide (LDH) as a robust material for the electrocatalytic oxidation of alcohols and urea at the anode, replacing the WOR. A facile synthesis protocol to form LDHs with different ratios of Co and Ni is adapted. It demonstrates that the reactants could be efficiently oxidized to concomitant chemical products at the anode. The half-cell study shows an onset potential of 1.30 V for benzyl alcohol oxidation reaction (BAOR), 1.36 V for glycerol oxidation reaction (GOR), 1.33 V for ethanol oxidation reaction (EOR), and 1.32 V for urea oxidation reaction (UOR) compared with 1.53 V for WOR. Notably, the hybrid electrolyzer in a full-cell configuration significantly reduces the overall cell voltage at a 20 mA cm^-2 current density by ∼15% while coupling with the BAOR, EOR, and GOR and ∼12% with the UOR as the anodic half-cell reaction. Furthermore, the efficiency of hydrogen generation remains unhampered with the types of oxidation reactions (alcohols and urea) occurring at the anode. This work demonstrates the prospects of lowering the overall cell voltage in the case of a water electrolyzer by integrating the hydrogen evolution reaction with suitable organic molecule oxidation.

Electrochemical Assessment of As-Deposited Co(OH)2 by Electrochemical Synthesis: The Effect of Synthesis Temperature on Performance

In this paper, the influence of an electrolytic temperature bath was used in the electrodeposition process on the size, color, and shape of the as-deposited Co(OH)2, and the electrochemical performance was investigated. Three different temperatures of 25, 60, and 95 °C were evaluated for the electrodeposition of Co(OH)2 on stainless steel plates (SSP). The electrochemical performance of the as-deposited electrodes (SSP) was measured in a symmetric electrochemical cell (EC) arrangement. XRD, SEM, and N2 physisorption analyses were carried out to evaluate the structure and morphological composition, along with the textural properties. Results showed that the hexagonal platelet micro-clusters of Co(OH)2 are formed in a mixed composition of both α-Co(OH)2 and β-Co(OH)2 phases, with the α-Co(OH)2 phase being the major phase formed in the electrodeposition process at temperatures below 95 °C, as suggested by the XRD analysis. Electrochemical cell performances were evaluated by galvanostatic cycling, results showed maximum areal capacity values of 1.97, 2.69, and 4.34 mA h cm–2 at a charge/discharge current of 6.25mA cm–2, for the as-deposited materials at 25, 60 and 95 °C, respectively. The specific power of the EC reached up to 19 kW kg–1 for the EC obtained material at 60 °C, with a specific energy of 2.8 W h kg–1. The maximum specific energy was reached at a current density of 6.25 mA cm−2, with a value of 10.79 W h kg–1 for the EC at 60 °C. These results offer some insight into how the morphology and composition of thin films can be tuned by the electrochemical synthesis temperature, yielding different electrochemical performances and areal capacity behaviors.

Metal-organic layer derived metal hydroxide nanosheets for highly efficient oxygen evolution

A series of metal hydroxide nanosheets have been fabricated from metal-organic layers (MOLs) through a conformal conversion process. Significantly, the MOL-derived Fe(OH)3 with an ultrathin layer structure exhibited enhanced alkaline oxygen evolution with a low overpotential of 271 mV at a current density of 10 mA cm-2.

Synthesis of Honeycomb-Like Co₃O₄ Nanosheets with Excellent Supercapacitive Performance by Morphological Controlling Derived from the Alkaline Source Ratio

Honeycomb-like Co₃O₄ nanosheets with high specific surface area were successfully synthesized on porous nickel foam by the facile hydrothermal method followed by an annealing treatment (300 °C), which were used as high-performance supercapacitor electrodes. The effects of the mole ratio of hexamethylenetetramine (HMT) and Co(NO₃)₂ (1:1, 2:1, 3:1, 4:1, 5:1 and 6:1) as the reactants on the morphological evolution and electrochemical performance of the electrodes were investigated in detail. X-ray diffractometry (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were applied to characterize the structure and morphology of the products. The electrochemical performance was measured by cyclic voltammetry (CV) and galvanostatic charge/discharge. The mole ratio of HMT and Co(NO₃)₂ produced a significant effect on the morphological evolution of Co₃O₄. The morphological evolution of Co₃O₄ with the increase in the mole ratio was followed: the nanosheets accompanied with a large number of spherical nanoparticles → the formation of some strip-like particles due to the agglomeration of spherical nanoparticles → the formation of new nanosheets resulting from the growth of strip-like particles → the formation of coarse flower-like particles owing to the connection among the nanosheets → the nanosheets gradually covered with flower-like particles. Accompanied with the change, the specific surface area was increased firstly, and then decreased. A maximum was obtained at a HMT and Co(NO₃)₂ mole ratio of 4:1. The evolution in morphology of Co₃O₄ was responsible for the change in electrochemical performance of the electrode. The specific capacitance value of the electrode prepared at a HMT and Co(NO₃)₂ mole ratio of 4:1 was highest (743.00 F·g⁻¹ at 1 A·g⁻¹ in the galvanostatic charge/discharge test). The similar result was also observed in the CV test with a scanning rate of 5 mV·s⁻¹. Moreover, the electrode also demonstrated an excellent cyclic performance, in which about 97% of the initial specific capacitance remained at 1 A·g⁻¹ for 500 cycles in the galvanostatic charge/discharge test. This excellent electrochemical performance was ascribed to high specific surface area of Co₃O₄ nanosheets that provide added channels and space for the ions transportation.

Graphene nanoflakes functionalized with cobalt/cobalt oxides formation during cobalt organic framework carbonization

In this contribution, we present the synthesis and carbonization mechanism of metal-organic frameworks (MOFs) based on cobalt and terephthalic acid, with detailed attention to the carbonization mechanism of cobalt-based organic frameworks. The evolution of the unique morphology of carbonized cobalt organic frameworks induced by temperature allows the synthesis of a hybrid of multi-layered carbon structures with metal and metal oxide nanoparticles placed between them. The formation of various phases and diameter distributions of cobalt nanoparticles resulted in the partial degradation of carbon structure and exfoliation. Presented data describe the connection between cobalt particle oxidation and carboreduction with the phenomenon of metal particle agglomeration. The presented study allows us to select carbonization conditions in order to obtain the desired cobalt crystalline structure on the graphene flakes from cobalt-based MOFs.

Pseudocapacitive performance of a solution-processed β-Co(OH)2 electrode monitored through its surface morphology and area

In the present study, beta-cobalt hydroxide (β-Co(OH)₂) electrodes of various nanostructures and surface areas, viz. nano-rhombuses (NRs), nano-plates (NPs), and nano-grass (NGs), have been synthesized directly onto a stainless-steel (SS) substrate using a simple, economical and binder-free chemical solution-process, utilizing three cobalt precursor salts, i.e. cobalt acetate, cobalt chloride, and cobalt nitrate, respectively. Structural elucidation proves the crystallite size, type and phase-purity of β-Co(OH)₂, whereas the surface morphology analysis supports the evolution of the above mentioned nanostructures of various surface areas. The electrochemical pseudocapacitor performance investigation demonstrates a specific capacitance (S_c) of 367 F g^-1 at 1 mA cm^-2 for the NP-type morphology, which is higher than that that displayed by the other morphologies. This change in S_c value is attributed to different charge transfer resistance values, which have been obtained from electrochemical impedance spectroscopy spectra. Finally, we attempt to correlate the relationship between the surface morphology, i.e. surface area, and the charge transfer resistance with the obtained specific capacitance value of the respective electrode.

MOF-derived NixCo1−x(OH)2 composite microspheres for high-performance supercapacitors

NixCo_1−x(OH)₂ composite microspheres are successfully synthesized with the assistance of alkali solution by employing a bimetallic Co–Ni–MOF as both the precursor and the self-sacrificing template, demonstrating excellent electrochemical performance.

Recent development of metal hydroxides as electrode material of electrochemical capacitors

Recent research on electrochemical capacitors using transition metal hydroxides as electrode materials is reviewed.

Dandelion-like cobalt hydroxide nanostructures: morphological evolution, soft template effect and supercapacitive application

A facile one-pot water bath method has been proposed to synthesize cobalt hydroxide (Co(OH)₂) nanodandelions with polyvinyl pyrrolidone (PVP) as soft template.