Synthesis of homo- and hetero-metallic cobalt and zinc nano oxide particles by a calcination process using coordination compounds: their characterization,DFT calculations and capacitance behavioural study

Nano cobalt and porous zinc–cobalt oxide particles were synthesized using the concept of coordination compounds of the type [M(ii)L,L′] (where M(ii) = Co(ii) & Zn(ii) L= 4-hydroxy benzaldehyde and L′ = piperazine) and were thoroughly characterized. Because the precursors are coordination compounds possessing specific geometry in the crystal lattice, uniform and appropriately sized homo- and heterometallic nanocrystals of Co₃O₄ and ZnO·Co₃O₄ were obtained after a thermal process. The homo and hetero composite particles were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), FT IR spectroscopy and electrochemistry. The paramagnetic chemical shift of the methyl protons in DMSO due to the nanoparticles was studied by NMR spectroscopy, which indicated that the cobalt particles were ferromagnetic. The structural design modification and surface area of Co₃O₄ was improved by adding the ZnO component. DFT calculations were done to validate the nano structure. Supercapacitance ability of the nanoparticles was studied by cyclic voltammetry, and electrochemical calculations were performed to determine the microelectronic characteristics of the material. The specific capacitance was estimated at 207.3 and 51.1 F g⁻¹ for the ZnO·Co₃O₄ and Co₃O₄ electrodes, respectively. Clearly, ZnO·Co₃O₄ exhibited a much higher specific capacitance than the Co₃O₄ nanocrystal, which was attributed to better conductivity and higher surface area. The capacitance activity showed multifold enhancement due to the porous nature of Zn oxide in the heterometallic nano ZnO·Co₃O₄ composite.

Pictorial depiction of appropriately sized homo and hetero nanocrystals of Co₃O₄ and ZnO·Co₃O₄ and the optimized structures of [Co₃O₄]₄ [ZnO]₄ DMSO adduct.     

Effect of Ni/Co mass ratio and NiO–Co3O4 loading on catalytic performance of NiO–Co3O4/Nb2O5–TiO2 for direct synthesis of 2-propylheptanol from n-valeraldehyde

In the direct synthesis of 2-propylheptanol (2-PH) from n-valeraldehyde, a second-metal oxide component Co₃O₄ was introduced into NiO/Nb₂O₅–TiO₂ catalyst to assist in the reduction of NiO. In order to optimize the catalytic performance of NiO–Co₃O₄/Nb₂O₅–TiO₂ catalyst, the effects of the Ni/Co mass ratio and NiO–Co₃O₄ loading were investigated. A series of NiO–Co₃O₄/Nb₂O₅–TiO₂ catalysts with different Ni/Co mass ratios were prepared by the co-precipitation method and their catalytic performances were evaluated. The result showed that NiO–Co₃O₄/Nb₂O₅–TiO₂ with a Ni/Co mass ratio of 8/3 demonstrated the best catalytic performance because the number of d-band holes in this catalyst was nearly equal to the number of electrons transferred in hydrogenation reaction. Subsequently, the NiO–Co₃O₄/Nb₂O₅–TiO₂ catalysts with different Ni/Co mass ratios were characterized by XRD and XPS and the results indicated that both an interaction of Ni with Co and formation of a Ni–Co alloy were the main reasons for the reduction of NiO–Co₃O₄/Nb₂O₅–TiO₂ catalyst in the reaction process. A higher NiO–Co₃O₄ loading could increase the catalytic activity but too high a loading resulted in incomplete reduction of NiO–Co₃O₄ in the reaction process. Thus the NiO–Co₃O₄/Nb₂O₅–TiO₂ catalyst with a Ni/Co mass ratio of 8/3 and a NiO–Co₃O₄ loading of 14 wt% showed the best catalytic performance; a 2-PH selectivity of 80.4% was achieved with complete conversion of n-valeraldehyde. Furthermore, the NiO–Co₃O₄/Nb₂O₅–TiO₂ catalyst showed good stability. This was ascribed to the interaction of Ni with Co, the formation of the Ni–Co alloy and further reservation of both in the process of reuse.

NiO–Co₃O₄/Nb₂O₅–TiO₂ catalyst with a Ni/Co mass ratio of 8/3 and NiO–Co₃O₄ loading of 14% shows the best catalytic performance.     

Co3O4–Ag photocatalysts for the efficient degradation of methyl orange

In this paper, a series of Co₃O₄–Ag photocatalysts with different Ag loadings were synthesized by facile hydrothermal and in situ photoreduction methods and fully characterized by XRD, SEM, TEM, FTIR spectroscopy, XPS, UV-vis and PL techniques. The catalysts were used for the degradation of methyl orange (MO). Compared with the pure Co₃O₄ catalyst, the Co₃O₄–Ag catalysts showed better activity; among these, the Co₃O₄–Ag-0.3 catalyst demonstrated the most efficient activity with 96.4% degradation efficiency after 30 h UV light irradiation and high degradation efficiency of 99.1% after 6 h visible light irradiation. According to the corresponding dynamics study under UV light irradiation, the photocatalytic efficiency of Co₃O₄–Ag-0.3 was 2.72 times higher than that of Co₃O₄ under identical reaction conditions. The excellent photocatalytic activity of Co₃O₄–Ag can be attributed to the synergistic effect of strong absorption under UV and visible light, reduced photoelectron and hole recombination rate, and decreased band gap due to Ag doping. Additionally, a possible reaction mechanism over the Co₃O₄–Ag photocatalysts was proposed and explained.

A novel Co₃O₄–Ag catalyst covered on the Ni foam substrate was synthesized via facile hydrothermal and in situ photoreduction methods for the efficient degradation of methyl orange.     

Mn substituted MnxZn1−xCo2O4 oxides synthesized by co-precipitation; effect of doping on the structural,electronic and magnetic properties

Mn substituted Mn_xZn_1−xCo₂O₄ (x = 0, 0.3, 0.5, 0.7, 1) oxides were synthesized by a facile co-precipitation method followed by calcination at 600 °C. The presence of manganese ions causes appreciable changes in the structural and magnetic properties of the Mn-substituted ZnCo₂O₄. The morphologies, structures, and electronic properties of Mn–Zn–Co oxide microspheres were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The X-ray diffraction and Fourier transform infrared spectroscopy results confirmed the formation of spinel Mn_xZn_1−xCo₂O₄. It was shown that the Mn–Zn–Co oxide microspheres increase in size and become regular in shape with increasing Mn concentration with the crystal size lying in the range from 19.1 nm to 51.3 nm. Magnetization measurements were carried out using a vibrating sample magnetometer at room temperature and 10 K. The saturation magnetization is observed to increase with increasing Mn concentration from x = 0 to x = 1.

Mn substituted Mn_xZn_1−xCo₂O₄ (x = 0, 0.3, 0.5, 0.7, 1) oxides were synthesized by a facile co-precipitation method followed by calcination at 600 °C.     

High acetone sensing properties of In2O3–NiO one-dimensional heterogeneous nanofibers based on electrospinning

Pure NiO nanofibers and the In₂O₃–NiO one-dimensional heterogeneous nanofibers were prepared by electrospinning, and the gas sensing properties to acetone were also investigated. Material characterization proved that the heterogeneous nanofibers were composed of In₂O₃ and NiO, and the nanofibers exhibited an enhanced sensitivity to acetone. At the optimal working temperature, the response of In₂O₃–NiO nanofibers to 50 ppm acetone was more than 10 times higher than that of pure NiO nanofibers. The minimum detection limit of the heterogeneous nanofibers reached 10 ppb, while the pure NiO nanofibers only reached 100 ppb. Among acetone and the comparison gases (methanol, ethanol, triethylamine, ethyl acetate, and benzene), the heterogeneous nanofibers achieved the highest response to acetone. In addition, the heterogeneous nanofibers exhibited an improved response–recovery rate and good long-term stability. These results indicated that the In₂O₃–NiO one-dimensional heterogeneous nanofibers have great potential in low-concentration acetone detection. Combined with the material properties, the mechanism of the enhanced sensing properties was discussed in detail for the In₂O₃–NiO heterogeneous nanofibers.

The In₂O₃–NiO nanofiber with p–n heterojunctions exhibited an enhanced acetone sensing performance, and the detection limit reached 10 ppb.     

Preparation of 3D hierarchical porous Co3O4 nanostructures with enhanced performance in lithium-ion batteries

Three-dimensional hierarchical Co₃O₄ microspheres assembled by well-aligned 1D porous nanorods have been synthesized by hydrothermal methods with the help of CTAB and subsequent heat treatment. The morphology and compositional characteristics of the hierarchical Co₃O₄ microspheres have been investigated using different techniques. Based on the SEM and TEM analyses, the growth direction of the nanorods is in the [110] direction. The hierarchical Co₃O₄ microspheres have a comparatively large Brunauer–Emmett–Teller surface area of about 50.2 m²g⁻¹, and pore size distribution is mainly concentrated at 12 nm. On the basis of the time tracking experiment, a possible growth mechanism has been proposed. It demonstrates that the overall mechanism includes nucleation, oriented growth and self-assembly processes. These hierarchical Co₃O₄ microspheres provide several favorable features for Li-ion battery applications: (1) large Brunauer–Emmett–Teller surface area, (2) porous structure, and (3) hierarchical structure. Therefore, measurement of the electrochemical properties indicates that the specific capacity can maintain a stable value of about 1942 mA h g⁻¹ at a current of 100 mA g⁻¹ within 100 cycles.

Three-dimensional hierarchical Co₃O₄ microspheres assembled by well-aligned 1D porous nanorods were successfully fabricated. The sample exhibited excellent electrochemical properties as anode materials for LIBs.     

Novel mesoporous Co3O4–Sb2O3–SnO2 active material in high-performance capacitive deionization

Capacitive deionization (CDI), as an emerging eco-friendly electrochemical brackish water deionization technology, has widely benefited from carbon/metal oxide composite electrodes. However, this technique still requires further development of the electrode materials to tackle the ion removal capacity/rate issues. In the present work, we introduce a novel active carbon (AC)/Co₃O₄–Sb₂O₃–SnO₂ active material for hybrid electrode capacitive deionization (HECDI) systems. The structure and morphology of the developed electrodes were determined using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Brunauer–Emmett–Teller (BET)/Barrett–Joyner–Halenda (BJH) techniques, as well as Fourier-transform infrared (FT-IR) spectroscopy. The electrochemical properties were also investigated by cyclic voltammetry (CV) and impedance spectroscopy (EIS). The CDI active materials AC/Co₃O₄ and AC/Co₃O₄–Sb₂O₃–SnO₂ showed a high specific capacity of 96 and 124 F g⁻¹ at the scan rate of 10 mV s⁻¹, respectively. In addition, the newly-developed electrode AC/Co₃O₄–Sb₂O₃–SnO₂ showed high capacity retention of 97.2% after 2000 cycles at 100 mV s⁻¹. Moreover, the electrode displayed excellent CDI performance with an ion removal capacity of 52 mg g⁻¹ at the applied voltage of 1.6 V and in a solution of potable water with initial electrical conductivity of 950 μs cm⁻¹. The electrode displayed a high ion removal rate of 7.1 mg g⁻¹ min⁻¹ with an excellent desalination–regeneration capability while retaining about 99.5% of its ion removal capacity even after 100 CDI cycles.

Capacitive deionization (CDI), as an emerging eco-friendly electrochemical brackish water deionization technology, has widely benefited from carbon/metal oxide composite electrodes.     

α(β)-PbO2 doped with Co3O4 and CNT porous composite materials with enhanced electrocatalytic activity for zinc electrowinning

The high energy consumption during zinc electrowinning is mainly caused by the high overpotential of the oxygen evolution for Pb–Ag alloys with strong polarization. The preparation of new active energy-saving materials has become a very active research field, depending on the synergistic effects of active particles and active oxides. In this research, a composite material, α(β)-PbO₂, doped with Co₃O₄ and CNTs on the porous Ti substrate was prepared via one-step electrochemical deposition and the corresponding electrochemical performance was investigated in simulated zinc electrowinning solution. The composite material showed a porous structure, finer grain size and larger electrochemical surface area (ECSA), which indicated excellent electrocatalytic activity. Compared with the Pb–0.76 wt% Ag alloy, the overpotential of oxygen evolution for the 3D-Ti/PbO₂/Co₃O₄–CNTs composite material was decreased by about 452 mV under the current density of 500 A m⁻² in the simulated zinc electrowinning solution. The decrease in the overpotential of oxygen evolution was mainly ascribed to the higher ECSA and lower charger transfer resistance. Moreover, it showed the lowest self-corrosion current density of 1.156 × 10⁻⁴ A cm⁻² and may be an ideal material for use in zinc electrowinning.

3D-Ti/PbO₂–Co₃O₄–CNTs composite electrode was fabricated through galvanostatic electrodepositon, which shows outstanding electrocatalytic activity to OER in harsh media (50 g L⁻¹ Zn²⁺ + 150 g L⁻¹ H₂SO₄).     

Activation of peroxymonosulfate by metal (Fe,Mn, Cu and Ni) doping ordered mesoporous Co3O4 for the degradation of enrofloxacin

Various transition metals (Fe, Mn, Cu and Ni) were doped into ordered mesoporous Co₃O₄ to synthesize Co₃O₄-composite spinels. Their formation was evidenced by transmission electronic microscopy (TEM), X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) analysis. It was found that Co₃O₄-composite spinels could efficiently activate peroxymonosulfate (PMS) to remove enrofloxacin (ENR) and the catalytic activity followed the order Co₃O₄–CuCo₂O₄ > Co₃O₄–CoMn₂O₄ > Co₃O₄–CoFe₂O₄ > Co₃O₄–NiCo₂O₄. Moreover, through the calculation of the specific apparent rate constant (k_sapp), it can be proved that the Co and Cu ions had the best synergistic effect for PMS activation. The Co₃O₄-composite spinels presented a wide pH range for the activation of PMS, but strong acidic and alkaline conditions were detrimental to ENR removal. Higher reaction temperature could promote the PMS activation process. Sulfate radical was identified as the dominating reactive species in Co₃O₄-composite spinel/PMS systems through radical quenching experiments. Meanwhile, the probable mechanisms concerning Co₃O₄-composite spinel activated PMS were proposed.

Various transition metals (Fe, Mn, Cu and Ni) were doped into ordered mesoporous Co₃O₄ to synthesize Co₃O₄-composite spinels.     

Preparation of porous Co3O4 and its response to ethanol with low energy consumption

Co₃O₄ is a promising p-type semiconductor for ethanol detection. In this work, ethanol detection sensors were fabricated with nanostructured Co₃O₄, which exhibited higher selectivity and lower operating temperature. The Co₃O₄ was synthesised using ZIF-67 as a sacrificial precursor. The T400-Co₃O₄ that was obtained by calcining ZIF-67 at 400 °C showed the best sensing performance. Its response to 100 ppm ethanol vapor was 221.99 at a low optimal operating temperature (200 °C). Moreover, T400-Co₃O₄ achieved a low detection limit (1 ppm), remarkable repeatability, and higher selectivity compared to ammonia, carbon monoxide, acetone, hydrogen, methane, methanol, and nitrogen dioxide. The enhanced sensing performance was mainly attributed to three factors: (1) the adsorption/desorption of active adsorbed oxygen molecules (e.g. O⁻ and O²⁻) and abundant oxygen vacancies, which increased the number of active sites; (2) the catalytic activity of Co³⁺, which greatly increased the reaction route and decreased the activation energy; and (3) the effective diffusion of gas molecules, which increased the effect of collisions between gas molecules and the material surface. This work provides an effective means to fabricate sensitive ethanol gas sensors with low energy consumption.

The fabricated porous Co₃O₄ showed remarkable ethanol sensing performance, which benefited from adsorbed oxygen, catalytic and structural effects.     

Investigation on structure,thermodynamic and multifunctional properties of Ni–Zn–Co ferrite for Gd3+ substitution

This study presents a modification of structure-dependent elastic, thermodynamic, magnetic, transport and magneto-dielectric properties of a Ni–Zn–Co ferrite tailored by Gd³⁺ substitution at the B-site replacing Fe³⁺ ions. The synthesized composition of Ni_0.7Zn_0.2Co_0.1Fe_2−xGd_xO₄ (0 ≤ x ≤ 0.12) crystallized with a single-phase cubic spinel structure that belongs to the Fd$\bar{3}$m space group. The average particle size decreases due to Gd³⁺ substitution at Fe³⁺. Raman and IR spectroscopy studies illustrate phase purity, lattice dynamics with cation disorders and thermodynamic conditions inside the studied samples at room temperature (RT = 300 K). Ferromagnetic to paramagnetic phase transition was observed in all samples where Curie temperature (T_C) decreases from 731 to 711 K for Gd³⁺ substitution in Ni–Zn–Co ferrite. In addition, Gd³⁺ substitution reinforces to decrease the A-B exchange interaction. Temperature-dependent DC electrical resistivity (ρ_DC) and temperature coefficient of resistance (TCR) have been surveyed with the variation of the grain size. The frequency-dependent dielectric properties and electric modulus at RT for all samples were observed from 20 Hz to 100 MHz and the conduction relaxation processes were found to spread over an extensive range of frequencies with the increase in the amount of Gd³⁺ in the Ni–Zn–Co ferrite. The RLC behavior separates the zone of frequencies ranging from resistive to capacitive regions in all the studied samples. Finally, the matching impedance (Z/η₀) for all samples was evaluated over an extensive range of frequencies for the possible miniaturizing application.

This study presents a modification of structure-dependent elastic, thermodynamic, magnetic, transport and magneto-dielectric properties of a Ni–Zn–Co ferrite tailored by Gd³⁺ substitution at the B-site replacing Fe³⁺ ions.     

Co3O4 composite nano-fibers doped with Mn4+ prepared by the electro-spinning method and their electrochemical properties

In this work, based on the electrospinning method, pure Co₃O₄, pure MnO₂, and Co₃O₄ composite nano-fiber materials doped with different ratios of Mn⁴⁺ were prepared. XRD, XPS, BET and SEM tests were used to characterize the composition, structure and morphology of the materials. An electrochemical workstation was used to test the electrochemical performance of the materials. The results showed that the material properties had greatly improved on doping Mn⁴⁺ in Co₃O₄ nano-fibers. The relationship between the amount of Mn⁴⁺ doped in the Co₃O₄ composite nano-fiber material and its electrochemical performance was also tested and is discussed in this report. The results show that when n_Co : n_Mn = 20 : 2, the Co₃O₄ composite nano-fiber material had a specific surface area of 68 m² g⁻¹. Under the current density of 1 A g⁻¹, the 20 : 2 sample had the maximum capacitance of 585 F g⁻¹, which was obviously larger than that of pure Co₃O₄ nano-fibers (416 F g⁻¹). After 2000 cycles of charging/discharging, the specific capacitance of the 20 : 2 sample was 85.9%, while that of the pure Co₃O₄ nano-fiber material was only 76.4%. The mechanism of performance improvement in the composite fibers was analyzed, which demonstrated concrete results.

In this work, based on the electrospinning method, pure Co₃O₄, pure MnO₂, and Co₃O₄ composite nano-fiber materials doped with different ratios of Mn⁴⁺ were prepared.     

Rationally designed hierarchical porous CNFs/Co3O4 nanofiber-based anode for realizing high lithium ion storage

To achieve a high power density of lithium-ion batteries, it is essential to develop anode materials with high capacity and excellent stability. Cobalt oxide (Co₃O₄) is a prospective anode material on account of its high energy density. However, the poor electrical conductivity and volumetric changes of the active material induce a dramatic decrease in capacity during cycling. Herein, a hierarchical porous hybrid nanofiber of ZIF-derived Co₃O₄ and continuous carbon nanofibers (CNFs) is rationally constructed and utilized as an anode material for lithium-ion batteries. The PAN/ZIF-67 heterostructure composite nanofibers were first synthesized using electrospinning technology followed by the in situ growth method, and then the CNFs/Co₃O₄ nanofibers were obtained by subsequent multi-step thermal treatment. The continuous porous conductive carbon backbone not only effectively provides a channel to expedite lithium ion diffusion and electrode transfer, but also accommodates volume change of Co₃O₄ during the charge–discharge cycling process. The electrode exhibits a high discharge capacity of 1352 mA h g⁻¹ after 500 cycles at a constant current density of 0.2 A g⁻¹. Additionally, the composites deliver a discharge capacity of 661 mA h g⁻¹ with a small capacity decay of 0.078% per cycle at a high current density of 2 A g⁻¹ after 500 cycles. This hierarchical porous structural design presents an effective strategy to develop a hybrid nanofiber for improving lithium ion storage.

Hierarchical porous CNFs/Co₃O₄ nanofiber is rationally designed and constructed as an anode for achieving high capacity and stable lithium ion batteries.     

The structure and photoluminescence of a ZnO phosphor synthesized by the sol gel method under praseodymium doping

This work outlines some interesting results regarding the effects of Pr³⁺ substitution on the structural and optical properties of (x = 0 and 0.02) samples. Our samples were synthesized using the Pechini sol–gel method. The structural study using Rietveld refinement of XRD patterns showed a hexagonal structure with the P63mc space group for all the samples and also the existence of a secondary phase attributed to the praseodymium oxide (Pr₆O₁₁) for 2% wt Pr-doped ZnO. The refinement results revealed that both the lattice parameter and the unit cell volume increase with the increase of Pr content. X-ray peak broadening analysis was used to evaluate the crystallite size and lattice strain by the Williamson–Hall (W–H) method and size–strain plot method (SSPM). The physical parameters such as strain, stress and energy density values were also calculated using the W–H method with different models, namely uniform deformation model (UDM), uniform stress deformation model (USDM) and uniform deformation energy model (UDEDM). The obtained results showed that the mean particle size of the ZnO and Pr_0.01Zn_0.97O estimated from W–H analysis and the SSPM method are highly intercorrelated. Shifting of the absorption edge to lower wavelength and blue shift of the band gap are observed in the UV-visible spectra of Pr-doped ZnO samples. Particular emphasis was put on the PL measurements of such composites. A noticeable decrease of the maximum intensity of PL response was found after adding Pr³⁺ to ZnO. This finding is discussed in terms of the photo excited limitation of electron–hole pairs in such nanocomposites.

This work outlines some interesting results regarding the effects of Pr³⁺ substitution on the structural and optical properties of (x = 0 and 0.02) samples.     

Hydrothermal-template synthesis and electrochemical properties of Co3O4/nitrogen-doped hemisphere-porous graphene composites with 3D heterogeneous structure

Despite the high capacity of Co₃O₄ employed in lithium-ion battery anodes, the reduced conductivity and grievous volume change of Co₃O₄ during long cycling of insertion/extraction of lithium-ions remain a challenge. Herein, an optimized nanocomposite, Co₃O₄/nitrogen-doped hemisphere-porous graphene composite (Co₃O₄/N-HPGC), is synthesized by a facile hydrothermal-template approach with polystyrene (PS) microspheres as a template. The characterization results demonstrate that Co₃O₄ nanoparticles are densely anchored onto graphene layers, nitrogen elements are successfully introduced by carbamide and the nanocomposites maintain the hemispherical porous structure. As an anode material for lithium-ion batteries, the composite material not only maintains a relatively high lithium storage capacity (the first discharge specific capacity can reach 2696 mA h g⁻¹), but also shows significantly improved rate performance (1188 mA h g⁻¹ at 0.1 A g⁻¹, 344 mA h g⁻¹ at 5 A g⁻¹) and enhanced cycling stability (683 mA h g⁻¹ after 500 cycles at 1 A g⁻¹). The enhanced electrochemical properties of Co₃O₄/N-HPGC nanocomposites can be ascribed to the synergistic effects of Co₃O₄ nanoparticles, novel hierarchical structure with hemisphere-pores and nitrogen-containing functional groups of the nanomaterials. Therefore, the developed strategy can be extended as a universal and scalable approach for integrating various metal oxides into graphene-based materials for energy storage and conversion applications.

The Co₃O₄/N-HPGC nanocomposites synthesized by a hydrothermal-template approach with polystyrene microspheres as the template possess excellent electrochemical performance.     

Chemical stability of Ca3Co4−xO9+δ/CaMnO3−δ p–n junction for oxide-based thermoelectric generators

An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p–n junction. Ca₃Co_4−xO_9+δ and CaMnO_3−δ exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 °C for 10 h in air) on the phase and element distribution within this p–n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a Ca₃Co_1+yMn_1−yO₆ interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca₃Co_1+yMn_1−yO₆ varies and is close to 1 (y = 0) at the Ca₃Co_1+yMn_1−yO₆–CaMnO_3−δ boundary. The existence of a wide homogeneity range of 0 ≤ y ≤ 1 for Ca₃Co_1+yMn_1−yO₆ is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.

The heat treatment beneficially affects the performance of an all-oxide thermoelectric generator through phase and element distribution within this p–n junction.     

Nanostructured RuO2–Co3O4@RuCo-EO with low Ru loading as a high-efficiency electrochemical oxygen evolution catalyst

Electrochemical water splitting technology is considered to be the most reliable method for converting renewable energy such as wind and solar energy into hydrogen. Here, a nanostructured RuO₂/Co₃O₄–RuCo-EO electrode is designed via magnetron sputtering combined with electrochemical oxidation for the oxygen evolution reaction (OER) in an alkaline medium. The optimized RuO₂/Co₃O₄–RuCo-EO electrode with a Ru loading of 0.064 mg cm⁻² exhibits excellent electrocatalytic performance with a low overpotential of 220 mV at the current density of 10 mA cm⁻² and a low Tafel slope of 59.9 mV dec⁻¹ for the OER. Compared with RuO₂ prepared by thermal decomposition, its overpotential is reduced by 82 mV. Meanwhile, compared with RuO₂ prepared by magnetron sputtering, the overpotential is also reduced by 74 mV. Furthermore, compared with the RuO₂/Ru with core–shell structure (η = 244 mV), the overpotential is still decreased by 24 mV. Therefore, the RuO₂/Co₃O₄–RuCo-EO electrode has excellent OER activity. There are two reasons for the improvement of the OER activity. On the one hand, the core–shell structure is conducive to electron transport, and on the other hand, the addition of Co adjusts the electronic structure of Ru.

The optimized RuO₂/Co₃O₄–RuCo-EO electrode with Ru loading of 0.064 mg cm⁻² exhibits the excellent oxygen evolution activity with an overpotential of 220 mV at the current density of 10 mA cm⁻² and a Tafel slope of 59.9 mV dec⁻¹.     

Facile synthesis of mesoporous NixCo9−xS8 hollow spheres for high-performance supercapacitors and aqueous Ni/Co–Zn batteries

Porous micro/nanostructure electrode materials have always contributed to outstanding electrochemical energy storage performances. Co₉S₈ is an ideal model electrode material with high theoretical specific capacity due to its intrinsic two crystallographic sites of cobalt ions. In order to improve the conductivity and specific capacitance of Co₉S₈, nickel ions were introduced to tune the electronic structure of Co₉S₈. The morphology design of the mesoporous hollow sphere structure guarantees cycle stability and ion diffusion. In this work, Ni_xCo_9−xS₈ mesoporous hollow spheres were synthesized via a facile partial ion-exchange of Co₉S₈ mesoporous hollow spheres without using a template, boosting the capacitance to 1300 F g⁻¹ at the current density of 1 A g⁻¹. Compared with the pure Co₉S₈ and Ni-Co₉S₈-30%, Ni-Co₉S₈-60% exhibited the best supercapacitor performance, which was ascribed to the maximum Ni ion doping with morphology and structure retention, enhanced conductivity and stabilization of Co³⁺ in the structure. Therefore, Ni/Co–Zn batteries were fabricated by using a Zn plate as the anode and Ni-Co₉S₈-60% as the cathode, which deliver a high energy density of 256.5 W h kg⁻¹ at the power density of 1.69 kW kg⁻¹. Furthermore, the Ni/Co–Zn batteries exhibit a stable cycling after 3000 repeated cycles with capacitance retention of 69% at 4 A g⁻¹. This encouranging result might provide a new perspective to optimize Co₉S₈-based electrodes with superior supercapacitor and Ni/Co–Zn battery performances.

Mesoporous NiCo₉S₈-0.6 hollow spheres as a high-performance supercapacitor and aqueous Ni/Co–Zn battery.     

Highly efficient hamburger-like nanostructure of a triadic Ag/Co3O4/BiVO4 photoanode for enhanced photoelectrochemical water oxidation

The combination of a semiconductor heterojunction and oxygen evolution cocatalyst (OEC) is an important strategy to improve photoelectrochemical (PEC) water oxidation. Herein, a novel hamburger-like nanostructure of a triadic photoanode composed of BiVO₄ nanobulks, Co₃O₄ nanosheets and Ag nanoparticles (NPs), that is, Ag/Co₃O₄/BiVO₄, was designed. In our study, an interlaced 2D ultrathin p-type Co₃O₄ OEC layer was introduced onto n-type BiVO₄ to form a p–n Co₃O₄/BiVO₄ heterojunction with an internal electric field (IEF) in order to facilitate charge transport. Then the modification with Ag NPs can significantly facilitate the separation and transport of photogenerated carriers through the surface plasma resonance (SPR) effect, inhibiting the electron–hole recombination. The resulting Ag/Co₃O₄/BiVO₄ photoanodes exhibit largely enhanced PEC water oxidation performance: the photocurrent density of the ternary photoanode reaches up to 1.84 mA cm⁻² at 1.23 V vs. RHE, which is 4.60 times higher than that of the pristine BiVO₄ photoanode. The IPCE value is 2.83 times higher than that of the pristine BiVO₄ at 400 nm and the onset potential has a significant cathodic shift of 550 mV for the ternary well-constructed photoanode.

A novel hamburger-like nanostructure of a triadic photoanode Ag/Co₃O₄/BiVO₄ was designed to enhance photoelectrochemical water splitting, providing a fascinating pathway to efficiently improve the PEC conversion efficiency.     

Designed formation of Co3O4@NiCo2O4 sheets-in-cage nanostructure as high-performance anode material for lithium-ion batteries

Structural and compositional control of functional nanoparticles is considered to be an efficient way to obtain enhanced chemical and physical properties. A unique Co₃O₄@NiCo₂O₄ sheets-in-cage nanostructure is fabricated via a facile conversion reaction, involving subsequent hydrolysis and annealing treatment. Such hollow nanoparticles provide an excellent property for Li storage.

Unique Co₃O₄@NiCo₂O₄ sheets-in-cage hybrid nanoparticles are successfully fabricated through a template-assisted method. When evaluated as an anode material, they exhibit highly enhanced electro-chemical properties for lithium storage.

Recently, the development of low-cost and environment-friendly renewable energy conversion and storage systems has become a research focus to address the increasingly serious energy crisis and environment issues.^1–3 Lithium-ion batteries (LIBs), as one of the most important energy storage technologies, have received tremendous attention in the past decades.^4–8 Among the various anode materials, Co-based transition metal oxides have been widely investigated owing to their higher theoretical capacity compared with the traditional anode electrode, graphite (372 mA h g⁻¹).^9–13 However, such kind of anodes usually suffer from the large volume changes during the long-term charge/discharge process, as well as the low conductivity, which are the main reasons for the fast fading in capacity and poor rate performance.One efficient way to solve the problem is the construction of complex hollow nanostructures instead of traditional solid ones.^14–17 The complex hollow nanostructures have some unique advantages, such as the larger specific surface area, well-defined inner hollow space and increased surface permeability. These factors are favourable for the fast mass/charge transfer, resulting in highly increased property for lithium storage. To date, many kinds of synthetic strategies have been successfully developed and reported. As examples, Wang and co-workers reported a facile synthesis of multi-shelled Co₃O₄ nanospheres with controllable shells.¹⁸ Carbon nanospheres are pre-synthesized and used as the hard templates to adsorb the Co²⁺ ions, followed by an annealing treatment in air for the final conversion of Co₃O₄ crystals. Lou''s group synthesized a series of Co-based complex hollow nanostructures.^19–22 Co₃O₄@NiCo₂O₄,¹⁹ Co₃O₄@Co–Fe mixed oxide²⁰ and Co₃O₄@Co₃V₂O₈ triple-shelled nanocages²¹ are successfully fabricated. Otherwise, Co₃O₄ bubbles/N doped carbon,²³ Co₃O₄ hollow nanospheres with ultra-thin nanosheets as building blocks,²⁴ and Co₃O₄/N doped carbon nanocubes²⁵ are also prepared. Benefiting from the unique structural features, these hollow materials always show highly enhanced electro-chemical properties. In addition, using heteroatoms to couple with Co metal to form the mixed metal compound is considered as another effective way to increase the LIB performance. Such mixed metal compounds are favourable for controlling the local chemical environment and modulating the synergistic effect between etch component. More importantly, the coupling of different kinds of metal ions together can bring richer redox reactions and higher electronic conductivity, resulting in significant enhancement of the LIB performance.^26–31 NiCo₂O₄ hollow spheres,^27,28 hollow polyhedrons,²⁹ nanoarrays,³⁰ and ZnCo₂O₄/carbon cloth hybrids³¹ are fabricated and used as high-performance anodes for LIBs. Therefore, a rational design and synthesis of Co-based functional nanomaterials with complex inner hollow nanostructure and multi-component-coupled outer surface is expected to yield the increased LIB performance.In the past two decades, metal–organic frameworks (MOFs) have been widely used as sacrificial templates for the synthesis of functional materials.^{19–21,32–39} The MOFs have some distinctive structural and compositional advantages, such as high porosity and easily controllable composition. Furthermore, the chemical and thermal instability makes the conversion reaction occur easily, that MOFs can transform to functional nanoparticles via a facile ion-exchange reaction or annealing treatment in a specific atmosphere. Among the variety of MOFs, Co-based zeolitic imidazolate framework-67 (ZIF-67) has received continues attention and been widely studied. As successful examples, 2-methylimidazole ligand in ZIF-67 can be easily replaced by S²⁻, Se²⁻ and MoO₄²⁻ to form the corresponding CoS,³⁷ CoSe,³⁸ and CoMoO₄ hollow cages.³⁹ Such MOF-assisted synthetic strategy provides more possibilities to obtain functional materials with improved physical and chemical properties.Following the above considering, a new sheets-in-cage nanostructure composed by Co₃O₄ nanosheets inside and NiCo₂O₄ layer as outer shell has been designed and synthesized. The whole synthesis involves: (i) conversion reaction to produce ZIF-67@NiCo-LDH yolk@shell nanoparticles; (ii) hydrolysis reaction between ZIF-67@NiCo-LDH and hexamethylenetetramine (HMT) to form thin Co(OH)₂ nanosheets in the NiCo-LDH nanocage; (iii) annealing treatment in air atmosphere at 300 °C for 2 hours for the fabrication of final Co₃O₄@NiCo₂O₄ sheets-in-cage hollow nanoparticles. In our design, three important structural advantages should be pointed out. First, strongly coupled NiCo₂O₄ as outer shell is expected to bring higher electro-conductivity. Second, the inner hollow space could accommodate the large volume changes during the lithiation/delithiation process. Last, the encapsulated Co₃O₄ nanosheets may provide more active sits for lithium storage. As expected, in the following LIB application, the as-obtained Co₃O₄@NiCo₂O₄ sheets-in-cage nanoparticles exhibit enhanced LIB performance.The ZIF-67 crystals are synthesized according to the previous report.¹⁹ Scanning electron microscopy (SEM) transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) are used as characterization methods. As shown in Fig. S1,^† the product is uniform polyhedron with a particle size of ≈800 nm in average. The corresponding XRD pattern in Fig. S2^† (black line) matches well with the previous reports,^19,37,38 indicating the product is in the pure phase of ZIF-67. Next, the above obtained ZIF-67 crystals are further reacted with Ni(NO₃)₂ in absolute ethanol. 40 min later, the products are collected and washed with ethanol for three times (details are shown in the ESI^†).With the data displayed in Fig. 1a–c, the as-obtained particles are uniform and monodisperse. Compared with the original ZIF-67 templates, they keep the dodecahedral morphology with the similar average particle size (≈800 nm). However, the outer surface becomes much rougher. Ultra-thin nanosheet as the building block can be clearly observed. A solid core can be found in each particle. And the clear gap is formed between the shell and solid core, indicating the formation of yolk@shell nanostructure. The corresponding XRD pattern is shown in Fig. S2^† (red line). The specific diffraction peaks can be assigned to ZIF-67 crystal. By referring to the report,⁴⁰ it is considered Co²⁺ is slowly dissolved from ZIF-67 in such synthesis, then oxidized to Co³⁺ by NO₃⁻. Further co-precipitation with Ni²⁺ causes the formation of NiCo-LDH. Along with the hydrolysis reaction, H⁺ is generated, and ZIF-67 is etched continuously. As a result, ZIF-67@NiCo-LDH yolk@shell nanostructure is obtained. Further prolonging the reaction time to three hours can cause the complete dissolution of the inner ZIF-67 core. As revealed by the TEM images in Fig. S3,^† single-shelled NiCo-LDH nanocages are successfully produced.Open in a separate windowFig. 1TEM images of ZIF-67@NiCo-LDH yolk@shell nanoparticles (a to c) and Co(OH)₂@NiCo-LDH sheets-in-cage nanoparticles (d to f).In next step, HMT is chosen as the weak alkali source. After heated at 90 °C for two hours, the color of the solution becomes from purple to yellow, indicating the complete conversion of inner ZIF-67 core. TEM images are first used to investigate the structure information. As shown in Fig. 1d–f, there is no obverse change in appearance. The overall structure is well maintained. The particles have the similar particle size, the outer surface is still rough and the thin nanosheets are clear to be observed. As expected, the inner solid cores are disappeared. Unexpected, large amount of thicker Co(OH)₂ sheets are formed and criss-cross in the nanocage. The formation mechanism could be understood as follow: OH⁻ ions are first generated from HMT at high temperature, and then reacted with ZIF-67 to form the Co(OH)₂ nanosheets. The hollow space inside is large enough and the outer NiCo-LDH layer is stable to limit the expanded growth of Co(OH)₂ nanosheets, thus, the unique sheets-in-cage hybrid nanostructure is successfully synthesized. The XRD pattern of the as-obtained product is investigated and shown in Fig. S4.^† The specific diffraction peaks of the solid ZIF-67 crystals disappeared completely, no clear diffraction peaks could be found, indicating the poor crystallization degree of the product.Some control experiments are carried out to get a deeper understand about the formation mechanism. First, we tried to study the solvent-effect towards the structure evolution. Pure ethanol is used to replace the ethanol/water mixture. Two hours later, the colour of the solution is still purple, indicating the remain of ZIF-67. Fig. S5^† displays the typical TEM images, that the change of the inner solid core is not obvious. Otherwise, there is no Co(OH)₂ nanosheets formed during the process. It reveals that water is necessary, which can participate in the decomposition of HMT to release OH⁻ ions. Then, we increase the volume ratio of H₂O/ethanol to 2/1. With the TEM images shown in Fig. S6,^† the sheets-in-cage hybrid nanostructure is also obtained. The similar phenomenon is also appeared when doubling the feeding amount of HMT (Fig. S7^†). In such two conditions, large amount of OH⁻ ions are generated, and the inner ZIF-67 cores can be dissolved completely. A control experiment is also done by using pure water as the solvent. In such condition, no precipitation is obtained. It is considered that OH⁻ is generated fast, which causes the seriously damage of the three-dimensional cages. Thus, ultra-thin Co/Ni hydroxide nanosheets are well dispersed in water and hard to be collected by centrifugation. Next, we tried to use ZIF-67 crystals to instead of ZIF-67@NiCo-LDH yolk@shell nanoparticles to react with HMT directly. Four hours later, the original purple solution becomes to pink. The TEM images in Fig. S8^† reveal that the as-obtained hollow particles show significant shrinkage with decreased size compared with the original ZIF-67 templates. No criss-crossed nanosheets formed inside the cages.Such phenomenon indicates the outer NiCo-LDH layer is important for the formation of such sheets-in-cage nanostructure. The difference of crystal structure between NiCo-LDH and Co(OH)₂ prevents the expanded growth of Co(OH)₂ nanosheets. Otherwise, we also study the alkali effect. NH₃H₂O is used to instead of HMT. The TEM images in Fig. S9^† reveal that the inner hollow space cannot be maintained in such condition. Small black particles are formed and loaded on the surface of the nanosheets. However, when urea is used, similar sheets-in-cage hybrids are successful synthesized. Urea is much similar with HMT, they can provide OH⁻ ions slowly, which is considered as the necessary condition for the formation of inner nanosheets.Finally, the pre-synthesized sample is annealed in air at 300 °C for the formation of crystallized product. SEM, TEM, XRD and XPS have been used to characterize the structural and composition information. The typical SEM image is displayed in Fig. 2a and b. The particles can withstand the annealing treatment. Compared with the un-heated precursor, the polyhedron morphology has been maintained well with rough surface. The inner nanostructure is further investigated by TEM images. With the data shown in Fig. 2d–f, the hollow cage is filled with thin Co₃O₄ nanosheets. XRD pattern of the annealed products are taken and shown in Fig. 2c. Three diffraction peaks appear at 2θ = 31.2°, 36.8°, 44.8°, 59.4° and 65.3° which can be indexed to the (220), (311), (400), (511) and (400) planes of Co₃O₄. In addition, the composition of the product is further determined by EDX analysis. As shown in Fig. S10,^† Co and Ni signals are clear observed. X-ray photoelectron spectroscopy (XPS) is further used to determine the chemical state of Co, Ni and O in Co₃O₄/NiCo₂O₄ sheets-in-cage sample. With the data shown in Fig. 3, there are two peaks appear at 779 eV and 795 eV, corresponding to the Co 2p_3/2 and 2p_1/2 spin orbit peaks, respectively. As shown in Fig. 3c, two strong peaks are observed at 854 and 872 eV, which could be assigned to the Ni 2p_3/2 and 2p_1/2 spin orbit, respectively. These results reveal that the presence of Co²⁺, Co³⁺ and Ni²⁺ in the final product. Otherwise, the contents of Co and Ni are also calculated based on the XPS analysis, that the molar ratio of Co/Ni is ≈23/6. For better comparison, NiCo-LDH cages and ZIF-67 are also annealed in air under the same condition. With the typical TEM images shown in Fig. S11 and S12,^† the single-shelled NiCo₂O₄ and Co₃O₄ nanocages are obtained.Open in a separate windowFig. 2SEM (a and b), TEM images (d to f) and XRD pattern (c) of Co₃O₄@NiCo₂O₄ sheets-in-cage hybrid nanoparticle.Open in a separate windowFig. 3XPS spectra of Co₃O₄@NiCo₂O₄ sheets-in-cage hybrid nanoparticles.Next, we further test the Li storage performance of the as-obtained Co₃O₄@NiCo₂O₄ sheets-in-cage nanoparticles. Fig. 4a displays the first four cyclic voltammogram (CV) curves of the Co₃O₄@NiCo₂O₄ electrode at a scan rate of 0.1 mV s⁻¹ in the potential window of 3 V to 0.01 V. There is a big difference between the first cycle and the others. It is clear two reduction peaks appeared at about 1.3 V and 0.8 V in the first discharge process, corresponding to the destruction and/or amorphization of their crystal structure and the reduction reaction between Co₃O₄/NiCo₂O₄ and Li, respectively.^41,42 During this process, irreversible solid electrolyte interface is also generated. For the first charge process, two wide peaks at 1.5 and 2.2 V are clearly observed, which could be attributed to the oxidation reaction between Co/Ni metal and Li_xO. Starting from the second cycle, there is an obverse shift of the main reduction peak from 0.8 to 1.0 V, which could be assign to the irreversible processes in first cycle. Importantly, the following CV curves overlap well, indicating the excellent reversibility and stability.Open in a separate windowFig. 4Cyclic voltammogram measurements at a scan rate of 0.1 mV s⁻¹ (a), charge–discharge voltage profiles in the first and second cycles at a current density of 100 mA g⁻¹ (b) and rate performance at various current densities (d) of the Co₃O₄@NiCo₂O₄ sheets-in-cage electrodes; cycling performances (c) of Co₃O₄@NiCo₂O₄, NiCo₂O₄ and Co₃O₄ nanocages at a current density of 100 mA g⁻¹ for 100 successfully cycles (discharged capacity). Fig. 4b is the charge–discharge voltage profiles of the Co₃O₄@NiCo₂O₄ sheets-in-cage nanocages at a current density of 100 mA g⁻¹ for 1st and 2nd cycle. In the first cycle, the discharge voltage plateaus related the reduction reaction of Co₃O₄/NiCo₂O₄ and the charge voltage about the corresponding oxidation reaction appears at 1.2 and 2.1 V, respectively. Starting from the second cycle, the discharge voltage plateau is shifted, indicating the changes of the structure during the charging and discharging process. These results match well with the CV measurements. Fig. 4c shows the cycling performance of Co₃O₄@NiCo₂O₄ sheets-in-cage nanocages in the current density of 100 mA g⁻¹. In the first discharge process, an ultra-high capacity of 1262 mA h g⁻¹ with a coulombic efficiency of 62% is achieved. After then, the second cycle delivers a capacity of 959 mA h g⁻¹. The capacity is slightly decreased in the following ten cycles, and then increased to 1083 mA h g⁻¹ after 100 cycles. The increase of the capacities could be attributed to the activation effect. The electrode material can become more and more accessible to host lithium ions during the charge/discharge process.^43–45 Furthermore, each cycle exhibits a high coulombic efficiency of around 99% excepted the first cycle. For comparison, single-shelled NiCo₂O₄ and Co₃O₄ nanocages are also used as anode materials for LIBs. As shown in Fig. 4c blue line, NiCo₂O₄ nanocage delivers a specific capacity of 882 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. And the capacity of Co₃O₄ nanocage fads fast, a capacity of only 626 mA h g⁻¹ is obtained after 100 cycles (Fig. 4c black line). In addition, the long-term cycling performance of Co₃O₄@NiCo₂O₄ sheets-in-cage nanocage at higher current density of 1 A g⁻¹ is also tested. With the data displayed in Fig. S13,^† a high capacity of 658 mA h g⁻¹ is remained after 200 successful cycles. We also collect the Co₃O₄@NiCo₂O₄ sheets-in-cage samples after 30 cycles at the current density of 100 mA g⁻¹. The corresponding TEM images are shown in Fig. S14.^† Although the shrink of the nanostructure is observed, the overall sheets-in-cage hollow nanostructure is maintained well, indicating the high structural stability.The rate performance is another important factor for anode materials. Fig. 4d shows the rate performance of the as-obtained Co₃O₄@NiCo₂O₄ sheets-in-cage nanocage by gradually increasing the current density from 0.1 to 5 A g⁻¹ and then returning it to 0.1 A g⁻¹. It delivers specific capacities of 942, 872, 786, 734, 668, and 578 mA h g⁻¹ at the current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g⁻¹, respectively. Impressively, when the current density returned to 100 mA g⁻¹, the capacity is fast increased to 890 mA h g⁻¹, indicating the excellent rate performance and structural stability. For comparison, the average discharge capacities of the as-obtained NiCo₂O₄ and Co₃O₄ nanocages are measured to be 926, 845, 760, 705, 642, 550, and 735, 587, 542, 503, 475, 311 mA h g⁻¹ at a current density of 0.1, 0.2, 0.5, 1, 2 and 5 A g⁻¹ respectively, which is much lower than the Co₃O₄@NiCo₂O₄ sheets-in-cage material (with the data shown in Fig. S15^†).Electrochemical impedance spectroscopy (EIS) is adopted to further investigate the electrochemical kinetics difference among the as-obtained three samples. With the data shown in Fig. S16,^† the Nyquist plots are consisted of a semicircle at the high to medium frequencies followed by a sloped line at the low frequencies. It is obvious that Co₃O₄@NiCo₂O₄ sheets-in-cage electrode has the lowest charge transfer resistance, which could be a big benefit to the LIB performance.Such enhanced LIB property of the Co₃O₄@NiCo₂O₄ sheets-in-cage nanocages could be attributed to the following features. First, the inner thin Co₃O₄ nanosheets have larger surface area, which can provide massive active sites for lithium storage. Second, the formation of hollow space in NiCo₂O₄ nanocages is considered as the efficient way to alleviate the volume change and buffer the induced strain during quick charge/discharge process. Thirdly, the outer surface is strongly coupled NiCo₂O₄ mixed metal oxide, which is expected to bring higher electro-conductivity compared with the Co₃O₄ component.