Cobalt-based Oxides Research Articles

Correction: Promoting nitrate electroreduction to ammonia over A-site deficient cobalt-based perovskite oxides

Correction for ‘Promoting nitrate electroreduction to ammonia over A-site deficient cobalt-based perovskite oxides’ by Fulong Liu et al., J. Mater. Chem. A, 2023, https://doi.org/10.1039/D3TA01063E.

NiFeOx and NiFeCoOx Catalysts for Anion Exchange Membrane Water Electrolysis

Hydrogen production using an Anion exchange membrane (AEM) electrolyzer allows the use of non-platinum group metal catalysts for oxygen evolution reaction (OER). Nickel and Cobalt-based oxides are active in an alkaline environment for OER and are relatively inexpensive compared to IrO2 catalysts used in Polymer electrolyte membrane (PEM) electrolysis. Mixed metal oxide catalysts NiFeOx and NiFeCoOx catalysts were synthesized by the coprecipitation method using NaOH. X-ray diffraction results showed mainly NiO diffraction peaks for the NiFeOx catalyst due to the low concentration of Fe, for the NiFeCoOx catalyst, NiCo2O4 diffraction peaks were observed. NiFeCoOx catalysts showed a higher Anion exchange membrane water electrolysis (AEMWE) performance compared to NiFeOx and commercial NiO, the highest current density at 2 V was 802 mA cm−2 at 70 °C using 1 M KOH as an electrolyte. The effect of electrolyte concentration was studied by using 0.01 M, 0.1 M and 1 M KOH concentrations in an electrolysis operation. Electrochemical Impedance spectroscopy was performed along with the equivalent circuit fitting to calculate ohmic and activation resistances, the results showed a decrease in ohmic and activation resistances with the increase in electrolyte concentration. Commercially available AEM (Fumasep FAA-3-50 and Sustainion dioxide membrane X-37-50 grade T) were tested at similar conditions and their performance was compared. EIS results showed that X-37-50 offered lower ohmic resistance than the FAA-3-50 membrane.

Anion-tuning of cobalt-based chalcogenides for efficient oxygen evolution in weakly alkaline seawater

Regulating the electronic structure is of great importance for improving the performance, stability, and selectivity of seawater splitting catalysts. Herein, cobalt-based oxide and chalcogenide (Co3X4, X = O, S, and Se) are synthesized to investigate the effect of anions on the electronic structure, catalytic performance and selectivity of the seawater oxidation catalysts. The theoretical calculations predict the possible active sites of oxygen evolution reaction and hypochlorite evolution reaction. Then the experimental characterization and calculation results demonstrate that S and Se substitution could regulate the electronic structure and d-band center of Co site, enhance the metallicity of catalysts. Moreover, the energy barrier of Cl* conversion to ClO- could be modulated for preventing the formation of ClO- by anions regulating. Consequently, Co3S4 could stably operate at a current density of 100 mA cm−2 in 0.1 M KOH + 0.6 M NaCl for 100 h and in neutral seawater for 20 h.

Co-MOFs as Emerging Pulse Modulators for Femtosecond Ultrafast Fiber Laser.

The metal organic framework (MOF) has attracted more and more attention due to its unique morphology, functional linkers, and orderly network structure. Zeolitio imidazolata frameworks (ZIFs), which are formed by bivalent transition metals (Zn, Co, etc.) and nitrogen-containing heterocyclic imidazole or purine organic ligands, are a very attractive subclass of MOFs. ZIF-67, obtained by the nucleation growth of dimethylimidazole and Co 2p, has been developed as a precursor for porous nanostructured cobalt-based metal oxides. During material preparation we add rGO because it can be used as a basic element to construct macroscopic three-dimensional carbon structural materials, which self-assemble into a 3D network structure with ZIF-67 through simple van der Waals forces or hydrogen bonds, and some samples contain specific functional groups that are added to the precursor. In this paper, we employ liquid-phase synthesis to generate rGO-ZIF-67 and calcine it at the temperature of 350 °C to obtain rGO-Co3O4. Then we fabricate rGO-Co3O4 and rGO-ZIF-67 modulators based on microfibers and test their nonlinear optical absorption in 1.5 μm range. The modulation depths of rGO-Co3O4 and rGO-ZIF-67 are measured as 10.41% and 6.61%, respectively. By using microfiber-based rGO-Co3O4 modulator, we have obtained a conventional soliton and a soliton molecule in Er3+-doped fiber lasers. The conventional soliton has a pulse width of 793.4 fs and a spectral width of 3.3 at 1558.9 nm, respectively. The obtained soliton molecule has a spectral modulation period of 1.65 nm and temporal separation of 4.94 ps at 1563.2 nm. By employing a microfiber-based rGO-ZIF-67 modulator, we obtain conventional solitons with a spectral width of 1.9 nm at the central wavelength of 1529.8 nm. Our research may expand the MOF-based materials for ultrafast photonics, blazing a new path for fiber laser, optical communications, and optoelectronics, etc.

CuCo2O4/Ti3C2Tx MXene hybrid electrocatalysts for oxygen evolution reaction of water splitting

Cobalt-based spinel oxides are promising candidates for oxygen evolution reaction (OER), however their limited electrical conductivity and low crystal stability may hinder further improvement. This report indroduces CuCo 2 O 4 nanoparticles/Ti 3 C 2 T x /nickel foam (NF) hybrid electrocatalyst as a robust electrode for OER, where the presence of the Ti 3 C 2 T x MXene structure could enhance the electrical conductivity of the electrocatalysts. The ultra-thin hybrid structure with large surface area can effectively increase the number of electrochemically active sites and enhance the electrocatalytic performance. Experimental findings indicated that CuCo 2 O 4 /Ti 3 C 2 T x hybrid structure on NF shows superior improved OER electrocatalytic activity compared to pristine Ti 3 C 2 T x . Accordingly, hybrid elecrocatalyst not only showed a low overpotential of 1.67 V at 100 mA.cm 2 , and a small Tafel slope of 49 mV dec −1 , but also exhebited an outstanding long-term durability. • CuCo 2 O 4 /GC, Ti 3 C 2 T x /GC, CuCo 2 O 4 / Ti 3 C 2 T x /GC, CuCo 2 O 4 / Ti 3 C 2 T x /NF hybrid electrocatalysts were synthesized by a facile solvothermal method. • Presence of the Ti 3 C 2 T x MXene structure enhanced the electrical conductivity of the electrocatalysts. • CuCo 2 O 4 / Ti 3 C 2 T x /NF exhibited excellent electrocatalytic activity for Oxygen evolution reaction for water splitting.

Controllable synthesis of 3D hierarchical cactus-like ZnCo2O4 films on nickel foam for high-performance asymmetric supercapacitors

High-performance ternary cobalt-based metal oxides, especially ZnCo2O4, have attracted increasing attention as promising electrode materials for supercapacitors. However, there are still great challenges for the as-reported ZnCo2O4 materials, such as self-aggregation during repeated discharging-charging process and low packing density. To tackle the above issues, it is envisaged as an efficient strategy to combine the advantages of both hierarchical porous micro/nanostructures and self-supporting electrode to fabricate a free-standing hierarchical micro/nanostructured ZnCo2O4 electrode. Herein we propose an efficient and simple synthetic strategy for the controllable synthesis of self-supporting hierarchical porous cactus-like ZnCo2O4 material directly grown on Ni-foam using a facile hydrothermal method with post-calcination treatment. Furthermore, a growth mechanism is also proposed. The as-prepared cactus-like ZnCo2O4 material possesses unique structural merits, such as hierarchical porous structures, large specific surface area, robust structural stability, and strong connections between the substrate and the ZnCo2O4 active material. The unique cactus-like ZnCo2O4 film electrode displayed excellent electrochemical performance, including a high specific capacity of 1115.7 F g−1 at 1 A g−1 and remarkable long cycle stability with 80.8% capacity retention after 30,000 cycles. A practical asymmetric supercapacitor (ASC) device was assembled using cactus-like ZnCo2O4 and active carbon, which showed an excellent energy density of 35.4 Wh kg−1 at high power density of 160.1 W kg−1 and superior cycle stability (62.5% capacity retention after 30,000 cycles at 1 A g−1). All these results show that such hierarchical porous ZnCo2O4 micro/nanostructured films are a promising candidate for high-performance supercapacitors for energy storage application.

Cobalt-Based Perovskite Electrodes for Solid Oxide Electrolysis Cells

Recently, many efforts and much attention has been paid to developing environmentally friendly energy. Solid oxide electrolyte cells (SOECs) process in reverse to solid oxide fuel cells (SOFCs) producing hydrogen gas as a green energy source. However, in this application, high-performance catalysts are usually required to overcome the sluggish oxygen evolution reactions (OER) during water decomposition. For this reason, discovery of catalysts with high performance is a crucial issue for the wide application of SOECs. Owning to their inherent activity and adequate stability in electrochemical conditions, perovskite oxides have been intensively employed in SOECs. In this mini review, we summarize the currently available studies concerning the applications of cobalt-based perovskite oxide catalysts in SOECs. Particularly, their structural properties and corresponding electronic structures are discussed based on their electrochemical performance, both experimentally and theoretically.

Defect structure regulation and mass transfer improvement of cobalt-based oxides for enhanced oxygen evolution reaction

Cobalt-based oxides have been widely studied as electrocatalysts for the oxygen evolution reaction (OER). Improving the intrinsic activity of the OER and increasing the number of active sites are two strategies used to enhance the OER performance. However, combining these two strategies to improve the catalytic activity of the OER for CoO catalysts has rarely been reported. Herein, we introduce a simple strategy to prepare CoO for the OER by fine tuning the calcination temperature of the Co(OH)2 precursor. The physical and chemical characterizations of these materials confirm the formation of oxygen vacancies and porous structures. Among the Co-based oxides prepared at different temperatures (300–600 °C), CoO prepared at 500 °C shows the best OER performance. The intrinsic catalytic activity and number of active sites of the CoO nanoflowers are also improved, which is attributed to the formation of oxygen vacancies and porous structures. This work introduces a simple strategy to improve the catalytic activity of the OER for transition metal-based oxides by improving their intrinsic catalytic activity and simultaneously increasing the number of active sites.

Flower-shaped and sea urchin-shaped structures coexisting in cobalt-based phosphate and oxides achieve efficient electrochemical overall water electrolysis

Active site engineering for electrocatalysts is an essential strategy to improve their intrinsic electrocatalytic capability for practical applications and it is of great significance to develop a new excellent electrocatalyst for overall water splitting. Here, Co3O4/nickel foam (NF) and Co2(P4O12)/NF electrocatalysts with flower-shaped and sea urchin-shaped structures are synthesized by a simple hydrothermal process and followed by a post-treatment method. Among them, Co2(P4O12)/NF shows good catalytic activity for hydrogen evolution reaction (HER), and at the current density of 10 mA cm−2, the overpotential is only 113 mV Co3O4/NF exhibits good catalytic activity for oxygen evolution reaction (OER), and the overpotential is 327 mV at 20 mA cm−2. An alkaline electrolyzer with Co3O4/NF and Co2(P4O12)/NF catalysts respectively as anode and cathode displays a current density of 10 mA cm−2 at a cell voltage of 1.59 V. This work provides a simple way to prepare high efficient, low cost and rich in content promising electrocatalysts for overall water splitting.

Manganese-doped cobalt tetroxide for formaldehyde oxidation

Cobalt-based oxides are appealing but less-well investigated catalytic materials for formaldehyde (HCHO) oxidation. We herein report a careful study of carbon-supported Co3−xMnxO4 catalyst, with special focuses on the incorporation of Mn ions into Co3O4 lattice and its effect on the catalytic activity for HCHO oxidation. The Co3−xMnxO4/C catalysts were fabricated using a co-precipitation method followed by annealing treatment. A combination of phase structure/chemical structure analyses and control experiments confirmed the substitutional doping of Mn ions in Co3O4 lattice, which resulted in increment of oxygen vacancy concentration. Benefiting from Mn-doping and the dispersion effect of carbon, the Co2.7Mn0.3O4/C catalyst prepared under optimal conditions exhibited an impressively high activity for HCHO oxidation, fairly good stability and excellent moisture tolerance. For instance, it enabled a 100% HCHO conversion at 90 °C with a ten-fold higher mass-specific activity compared to Co3O4/C. A possible mechanism describing the HCHO oxidation over Co3−xMnxO4/C catalyst was proposed based on the in situ diffused reflectance infrared spectroscopy results.

Steering Catalytic Activity and Selectivity of CO2 Photoreduction to Syngas with Hydroxy-Rich Cu2 S@ROH -NiCo2 O3 Double-Shelled Nanoboxes.

Simultaneous transformation of CO2 and H2 O into syngas (CO and H2 ) using solar power is desirable for industrial applications. Herein, an efficient photocatalyst based on double-shelled nanoboxes, with an outer shell of hydroxy-rich nickel cobaltite nanosheets and an inner shell of Cu2 S (Cu2 S@ROH -NiCo2 O3 ), is prepared via a multistep templating strategy. The high performance of Cu2 S@ROH -NiCo2 O3 (7.1 mmol g-1 h-1 for CO; 2.8 mmol g-1 h-1 for H2 ) is attributed to the hierarchical hollow geometry and p-n heterojunction to promote light absorption and charge separation. Spectroscopic and theoretical analyses elucidate that the ROH -NiCo2 O3 surface enhances *CO2 adsorption and lowers energy barriers for CO2 -to-CO. Therefore, modulating the hydroxy contents of ROH -NiCo2 O3 can achieve broad CO/H2 ratios from 0.51 to 1.24. This work offers in-depth insights into adjustable syngas photosynthesis and generalized concepts of selective heterogeneous CO2 photoreduction beyond cobalt-based oxides.

Urchin-like CoNiO2 microspheres supported on reduced graphene oxide with N and S co-doped for overall water splitting with trace load as the bifunctional electrocatalyst

The design and synthesis of bifunctional electrocatalysts with high catalytic activity and low cost are the key factors to efficient overall water splitting (OWS). In this work, CoNiO2 (CNO) microspheres with multiple microstructures are loaded on the N and S co-doped reduced graphene oxide (NSG) via an easy synthesis process to improve the conductivity and electrochemistry activity of CNO@NSG. The overpotentials of CNO@NSG are 109.6 mV and 287.4 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with the load mass of 0.1 mg/cm2 and the potential in OWS of CNO@NSG at the 10 mA/cm2 is 1.67 V. These results suggest that cobalt-based oxides with carbon-carrier bifunctional electrocatalyst provide a new practical method for OWS.

Optimizing the eg occupancy of magnesium cobalt spinel oxides via Fe substitution to promote oxygen evolution reaction

Cobalt-based spinel oxides are a promising class of electro-catalysts for oxygen evolution reaction (OER). Herein, a series of Mg(Co1−xFex)2O4 (x = 0.0–1.0) oxides are designed as OER catalysts. MgCo2O4 is selected to fix Co3+ in the octahedral position, which is active site for the OER. We elucidate the volcanic relationship between the OER activity and Fe substitution concentration in Mg(Co1−xFex)2O4, which shows an optimal activity at 20 at% Fe substitution. Superconducting Quantum Design (SQUID) technique reveals that Fe substitution tunes the eg orbital electronic configuration of Co3+ from low spin to high spin. The optimized eg occupancy of 0.91 is obtained in MgCo1.6Fe0.4O4. Density functional theory calculations verify that the improved OER performance stems from the enhanced Co-O covalence after Fe substitution, which promotes the transfer of electrons from oxygen, thus accelerates the catalytic oxygen evolution reaction. The formation energy of O* decrease as well by Fe substitution, and the conversion of OH* into O* is identified to be the rate-determining step. This work complements the optimizing electronic configuration of cobalt cations for OER and provides favorable principles for the rational design of OER electro-catalysts.

Nanostructuring Matters: Stabilization of Electrocatalytic Oxygen Evolution Reaction Activity of ZnCo2O4 by Zinc Leaching

Despite the enormous attention paid to cobalt oxide materials as efficient water splitting electrocatalysts, a deep understanding of their activity discrepancy is still elusive. In this work, we showed that stabilization of the internally generated oxygen evolution reaction (OER) active phase (oxyhydroxide) is crucial for ZnCo2O4 electrocatalysts. A systematic evaluation of the bulk and nanostructured ZnCo2O4 system concomitant with nanostructured Co3O4 showed that leaching of Zn is the driving force behind the near-surface transformation to the oxyhydroxide phase. The relative contribution to this near-surface reconstruction was found to be surface-sensitive. The electrochemical observations combined with Raman and impedance spectroscopy revealed that the good catalytic activity could be attributed to the formation of the cobalt oxyhydroxide phase, which was created by the dissolution of Zn from the nanostructured surface. Moreover, this study sheds light on previous contradicting postulates regarding the discrepancy of the OER activity of ZnCo2O4. Our finding regarding the formation of the OER active phase in spinel Zn-Co oxide will motivate researchers to focus more on the near-surface reconstruction behavior of cobalt-based oxide electrocatalysts in the future.

Cobalt-based metal oxide coated with ultrathin ALD-MoS2 as an electrode material for supercapacitors

A novel approach of preparing a composite supercapacitor electrode consisting of ternary metal oxide nanostructure decorated with transition metal sulfide is presented. High-surface area NiCo2O4 nano-sheets grown on Ni-foam are coated with an ultrathin layer of MoS2 by atomic layer deposition (ALD) and the hybrid structure reveals superior electrochemical performance. The composite is thoroughly studied with several characterizations like XRD, Raman, XPS, SEM and TEM. A comparative study with MoS2 coated Co3O4 electrodes not only ensures the advantage of the more conducting sulfide layer in enhancing the supercapacitor performance significantly but also establishes the superiority of the ternary oxide (NiCo2O4) for this application. The thinner and well-connected nano-sheets of NiCo2O4 provides the more interface between the electrode and the electrolyte whereas an optimal thickness of MoS2 helps to maximize the performance of the device. The high areal capacitance (2445 mF cm−2), enhanced rate capability of the optimized NiCo2O4-MoS2 composite is therefore ascribed to the synergistic effect of high-surface area of the oxide nano-sheets and better electronic conductivity of the sulfide. The current study thus opens a route to combine the conventional hydrothermal synthesis of ternary oxide nanostructures with ALD grown layered transition metal dichalcogenides to achieve a superior electrode for next-generation supercapacitor.

Magnesium as a Methanation Suppressor for Iron- and Cobalt-Based Oxide Catalysts during the Preferential Oxidation of Carbon Monoxide

The preferential oxidation of CO (CO-PrOx) to CO2 is an effective catalytic process for purifying the H2 utilized in proton-exchange membrane fuel cells for power generation. Our current work reports on the synthesis, characterization and CO-PrOx performance evaluation of unsubstituted and magnesium-substituted iron- and cobalt-based oxide catalysts (i.e., Fe3O4, Co3O4, MgFe2O4 and MgCo2O4). More specifically, the ability of Mg to stabilize the MgFe2O4 and MgCo2O4 structures, as well as suppress CH4 formation during CO-PrOx was of great importance in this study. The cobalt-based oxide catalysts achieved higher CO2 yields than the iron-based oxide catalysts below 225 °C. The highest CO2 yield (100%) was achieved over Co3O4 between 150 and 175 °C, however, undesired CH4 formation was only observed over this catalyst due to the formation of bulk fcc and hcp Co0 between 200 and 250 °C. The presence of Mg in MgCo2O4 suppressed CH4 formation, with the catalyst only reducing to a CoO-type phase (possibly containing Mg). The iron-based oxide catalysts did not undergo bulk reduction and did not produce CH4 under reaction conditions. In conclusion, our study has demonstrated the beneficial effect of Mg in stabilizing the active iron- and cobalt-based oxide structures, and in suppressing CH4 formation during CO-PrOx.

Direct ammonia solid oxide fuel cells based on spinel ACo2O4 (A=Zn, Fe, Ni) composite cathodes at intermediate temperature

ABSTRACT Direct ammonia solid oxide fuel cell (DA-SOFC) is a promising device to realize high-efficiency NH3-to-power. In this study, we fabricate a series of cobalt-based spinel ACo2O4 (ACO, A = Zn, Fe, Ni) oxide as DA-SOFC cathodes using the citric acid complexing method. We prepare three cobalt-based spinel oxides with different A-site elements, i.e., Zn, Fe, and Ni, to establish the relationships between the valence and active site of ACO spinel oxide. The structural analysis based on XRD, XPS, TEC, and UV-Vis diffuse reflectance confirms that NCO exhibits smaller band gap, higher electronic conductivity, and better compatibility with electrolyte. The electrochemical results indicate that the cathodic surface of NCO containing rich Co3+ and Oads leads to better oxygen reduction reaction (ORR) activity. The maximum power density of the DA-SOFC using NCO cathode reaches 1.06 W·cm−2 at 800°C, superior to those of using ZCO (0.83 W·cm−2) or FCO (0.92 W·cm−2) cathode. Kinetic analysis based on the distribution of relaxation time (DRT) and electrochemical impedance spectroscopy (EIS) indicates is rate-limiting.

Structural Changes of Spinel MCo2O4 (M = Mn, Fe, Co, Ni, and Zn) Electrocatalysts during the Oxygen Evolution Reaction Investigated by In Situ X-ray Absorption Spectroscopy

A comparison of the electrochemical and physicochemical behavior of cobalt-based oxides with spinel structure MCo2O4 (M = Mn, Fe, Co, Ni, and Zn) was conducted to investigate the effect of the oxidation state and cation distribution in the spinel on the electrocatalytic activity of the oxygen evolution reaction (OER) in an alkaline solution. Various spinel MCo2O4 electrocatalysts were synthesized by a facile microwave-assisted synthesis and low-temperature annealing. The overpotential of these MCo2O4 electrocatalysts for the OER is comparable to the reported overpotentials of catalysts based on cobalt oxides. From the findings, the catalytic activity of OER decreases in the order of ZnCo2O4 > NiCo2O4 > FeCo2O4 > Co3O4 > MnCo2O4. It was revealed that the active sites are controlled by the balance of M3+/M2+ cation distribution in octahedral and tetrahedral sites and by the bond strength between M and oxygen atoms at the catalyst surface from the direct combination of in situ X-ray absorption fine structure (XAFS) spectroscopy with the electrochemical experiments to track the oxidation state and the structural changes of electrocatalysts before and after the exposure to the OER conditions. This study provides insights into the effects of cation distributions on the OER activity and demonstrates a promising method for determining the fundamental mechanism of cation-substituted cobalt oxides for OER.

Giant magnetoresistance effect in Fe-doped SrCoO2.9-δF0.1 perovskites

Excellent magnetoresistive materials have attracted people's attention due to their wide applications. Unfortunately, few cobalt-based perovskite oxides can demonstrate good magnetoresistance (MR) properties. Herein, a new series of SrCo 1-x Fe x O 2.9-δ F 0.1 (x = 0, 0.1, 0.2, and 0.3) perovskites are prepared and studied as potential magnetoresistive materials. Room temperature X-ray diffraction (XRD) and Rietveld refinement confirm the simple cubic structure of all samples. The magnetization vs temperature measurement reveals that the magnetic transition temperature goes down with the increase of Fe content, suggesting the weakened ferromagnetism caused by Fe 4+ -O-Co 3+ , Fe 3+ -O-Fe 3+ and Fe 4+ -O-Fe 4+ interactions. And the resistivity data of all Fe doped samples can be better fitted using Mott's variable range hopping (VRH) model, suggesting its disordering semi-conduction behavior. Astonishingly, the SrCo 0.8 Fe 0.2 O 2.9-δ F 0.1 ceramic sample owns the giant negative magnetoresistance value of approximately 90% at 5 K in 90 kOe, which is almost the largest negative magnetoresistance in cobalt-based perovskite oxides, indicating that cobalt-based perovskites can be an underlying material system for the research of the potential material applying to magnetic sensors or other spintronic devices.

A low-cost and facile electrochemical sensor for the trace-level recognition of flutamide in biofluids using large-area bimetallic NiCo2O4 micro flowers

Nickel–cobalt-based bimetallic oxide compound (NiCo2O4) as a highly sensitive and selective platform for the detection of flutamide in biological fluids.