Spinel Cobalt Oxide Research Articles

Hydroxylation of an ultrathin Co3O4(111) film on Ir(100) studied by in situ ambient pressure XPS and DFT

In the present work, we have studied the interaction of water with spinel cobalt oxide (Co3O4), an effect which has been considered a major cause of its catalytic deactivation. Employing a Co3O4(111) model thin film grown on Ir(100) in (ultra)high vacuum, and ambient pressure X-ray photoelectron spectroscopy (APXPS), hydroxylation in 0.5 mbar H2O vapor at room temperature was monitored in real time. The surface hydroxyl (OH) coverage was determined via two different models based (i) on the termination of a pristine and OH-covered Co3O4(111) surface as derived from density functional theory (DFT) calculations, and (ii) on a homogeneous cobalt oxyhydroxide (CoO(OH)) overlayer. Langmuir pseudo-second-order kinetics were applied to characterize the OH evolution with time, suggesting two regimes of chemisorption at the mosaic-like Co3O4(111) film: (i) plateaus, which were quickly saturated by OH, followed by (ii) slow hydroxylation in the “cracks” of the thin film. H2O dissociation and OH formation, blocking exposed Co2+ ions and additionally consuming surface lattice oxygen, respectively, may thus account for catalyst deactivation by H2O traces in reactive feeds.

Isolated Octahedral Pt-Induced Electron Transfer to Ultralow-Content Ruthenium-Doped Spinel Co3O4 for Enhanced Acidic Overall Water Splitting.

The development of a highly active and stable oxygen evolution reaction (OER) electrocatalyst is desirable for sustainable and efficient hydrogen production via proton exchange membrane water electrolysis (PEMWE) powered by renewable electricity yet challenging. Herein, we report a robust Pt/Ru-codoped spinel cobalt oxide (PtRu-Co3O4) electrocatalyst with an ultralow precious metal loading for acidic overall water splitting. PtRu-Co3O4 exhibits excellent catalytic activity (1.63 V at 100 mA cm-2) and outstanding stability without significant performance degradation for 100 h operation. Experimental analysis and theoretical calculations indicate that Pt doping can induce electron transfer to Ru-doped Co3O4, optimize the absorption energy of oxygen intermediates, and stabilize metal-oxygen bonds, thus enhancing the catalytic performance through an adsorbate-evolving mechanism. As a consequence, the PEM electrolyzer featuring PtRu-Co3O4 catalyst with low precious metal mass loading of 0.23 mg cm-2 can drive a current density of 1.0 A cm-2 at 1.83 V, revealing great promise for the application of noniridium-based catalysts with low contents of precious metal for hydrogen production.

Dual-capable spinel cobalt oxide nanoparticles for electrocatalytic oxygen evolution and water contaminant removal.

This study investigates the synthesis and electrocatalytic performance of cobalt oxide (Co3O4) nanoparticles for the oxygen evolution reaction (OER) and their role in water treatment as contaminant removal agents. Cobalt oxide nanoparticles are recognized as promising materials in electrocatalysis due to their tunable properties and nanoscale engineering potential. Here, fine cobalt oxide nanoparticles are synthesized using the sol-gel method followed by various sintering temperatures to achieve precise control over surface morphology, size, and shape. Characterization via high-resolution scanning electron microscopy (HRSEM) and high-resolution transmission electron microscopy (HRTEM) elucidates the impact of sintering temperature on nanoparticle properties. Thin film electrodes of cobalt oxide are fabricated using the doctor blade method and evaluated using linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Among the tested sintering temperatures, cobalt oxide electrodes sintered at 600°C exhibit superior catalytic activity, demonstrating an overpotential of 258mV (vs RHE) at 10mAcm-2 current density and a Tafel slope of 17.33mV dec-1. Furthermore, these electrodes demonstrate excellent stability, maintaining OER performance for 10h in 1M NaOH electrolyte. Additionally, the role of cobalt oxide nanoparticles in water treatment is explored using inductively coupled plasma atomic emission spectrometry (ICP-AES). Experimental results reveal that lower sintering temperatures enhance the electrocatalytic properties of cobalt oxide nanoparticles, highlighting their potential contribution to sustainable energy and water treatment technologies. This work underscores the significance of cobalt oxide nanoparticles as dual-functional materials for advancing electrocatalysis and water purification applications, thus paving the way for the development of efficient and environmentally friendly technologies.

Facet-Dependent Evolution of Active Components on Spinel Co3O4 for Electrochemical Ammonia Synthesis.

Spinel cobalt oxides (Co3O4) have emerged as a promising class of catalysts for the electrochemical nitrate reduction reaction (eNO3RR) to ammonia, offering advantages such as low cost, high activity, and selectivity. However, the specific role of crystallographic facets in determining the catalysts' performance remains elusive, impeding the development of efficient catalysts. In this study, we have synthesized various Co3O4 nanostructures with exposed facets of {100}, {111}, {110}, and {112}, aiming to investigate the dependence of the eNO3RR activity on the crystallographic facets. Among the catalysts tested, Co3O4 {111} shows the best performance, achieving an ammonia Faradaic efficiency of 99.1 ± 1.8% with a yield rate of 35.2 ± 0.6 mg h-1 cm-2 at -0.6 V vs RHE. Experimental and theoretical results reveal a transformation process in which the active phases evolve from Co3O4 to Co3O4-x with oxygen vacancy (Ov), followed by a Co3O4-x-Ov/Co(OH)2 hybrid, and finally Co(OH)2. This process is observed for all facets, but the formation of Ov and Co(OH)2 is the most rapid on the (111) surface. The presence of Ov significantly reduces the free energy of the *NH2 intermediate formation from 1.81 to -0.53 eV, and plentiful active sites on the densely reconstructed Co(OH)2 make Co3O4 {111} an ideal catalyst for ammonia synthesis via eNO3RR. This work provides insights into the understanding of the realistic active components, offers a strategy for developing highly efficient Co-based spinel catalysts for ammonia synthesis through tuning the exposed facets, and helps further advance the design and optimization of catalysts in the field of eNO3RR.

Pd-substitution impact in nickel–cobalt spinel oxide grown on Ni foam as very effectual electrocatalyst for green hydrogen production supported by DFT study

The hydrothermal preparation of catalytic NiPdxCo2-xO4 (x ≦ 0.08) nano-electrocatalysts on Ni foam (NF) was successfully accomplished. The morphology and chemical composition of the catalysts are confirmed by advanced analytical techniques XRD, SEM, EDX, TEM, HR-TEM, and XPS. The NiPdxCo2-xO4 (x = 0.06) NF nano-electrocatalyst demonstrated remarkable performance in the HER, with an overpotential of 191.6 mV, a Tafel slope of 84.7 mV/dec, and extraordinary stability over 24 h using chronopotentiometry methods. The surface and electrochemical analyses demonstrated that the sample, doping with a 6.0% Pd2+ concentration, had appreciably enhanced performance in the HER. The improvement may be attributed to a much larger electrochemical surface area and fast charge transfer kinetics at the semiconductor and electrolyte interface. The influence of Pd dopants on the HER performance of CNs is studied with the help of density functional theory. It elucidates the way in which hydrogen and water molecules bind to the surface of PCN slabs. The theoretical results suggest that the incorporation of Pd dopants enhances the electrocatalytic process and boosts the HER activity as the concentration of Pd increases. The density of state spectra reveals their effect on spin-dependent electronic structure characteristics. Our findings point to a practical path for creating distinctive electrocatalysts that will serve as better electrodes for a variety of forthcoming hydrogen fuel cell applications.

Combustion-assisted sintering for tunable densification of manganese cobalt oxide layers

Facile sintering of metal oxides and ceramics is essential for energy-efficient manufacturing processes of various engineering systems. It should satisfy the increasing density of the resulting layer packed on the substrate in a mild environment and improve the specific performance through structural changes in the internal grain boundaries. However, conventional sintering inevitably involves maintaining high-temperature conditions for a long duration, resulting in energy inefficiency and unintended element diffusion. Herein, we report a novel composite-fuel-assisted sintering strategy as an alternative to conventional sintering for enhanced densification of manganese cobalt oxide (MCO) spinel. MCO spinels, which are commonly used to form a protective layer for metallic interconnects, are electrophoretically deposited on Cr-containing stainless steel, while the initial porosity and thickness are controlled by regulating the deposition parameters. Then, 2,4,6-trinitrophenol combined with sodium azide as a composite fuel is dropped onto the pre-deposited MCO layer. During air drying, this composite fuel tends to self-assemble into an elongated microwire-shaped structure featuring π-stacked aromatic rings and provides a serial connection between the dried fuel particles, facilitating their penetration into the coated material. The composite fuel enables the implementation of a combustion wave passing through the MCO layer in a mild sintering environment, thereby providing instantaneous thermal energy for densifying the initial MCO layer. The densification efficiency achieved using the optimized composite-fuel-assisted sintering strategy is 93 % higher than that achieved conventional air sintering. This study will pave the way for the development of new hybrid sintering strategies based on versatile composite fuels to achieve high efficiencies under mild environments.

Distance effect of single atoms on stability of cobalt oxide catalysts for acidic oxygen evolution

Developing efficient and economical electrocatalysts for acidic oxygen evolution reaction (OER) is essential for proton exchange membrane water electrolyzers (PEMWE). Cobalt oxides are considered promising non-precious OER catalysts due to their high activities. However, the severe dissolution of Co atoms in acid media leads to the collapse of crystal structure, which impedes their application in PEMWE. Here, we report that introducing acid-resistant Ir single atoms into the lattice of spinel cobalt oxides can significantly suppress the Co dissolution and keep them highly stable during the acidic OER process. Combining theoretical and experimental studies, we reveal that the stabilizing effect induced by Ir heteroatoms exhibits a strong dependence on the distance of adjacent Ir single atoms, where the OER stability of cobalt oxides continuously improves with decreasing the distance. When the distance reduces to about 0.6 nm, the spinel cobalt oxides present no obvious degradation over a 60-h stability test for acidic OER, suggesting potential for practical applications.

Electrocatalytic Reduction of Cobalt Oxide Spinel for Oxygen Reduction Reaction in Alkaline Media

The cathode material were prepared by dispersing Co3O4 on vulcan carbon in situ and manually grinding in pestle and mortar. Co3O4 prepared by hydrazine based combustion method showed nanocrystalline nature. Surface area was found to increase on dispersing Co3O4 on vulcan carbon by manually grinding it in pestle and mortar. The kinetics of the oxygen reduction reaction (ORR) was studied on Co3O4/C electrodes in KOH electrolyte by using cyclic voltammetry and steady state polarization measurements using Hg/HgO as the reference electrode. The morphological features of the electrocatalyst was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) & BET surface area. The polarization results showed higher electrocatlytic activity for the ORR on Co3O4/C (G) cathode in comparison to Co3O4/C (P) prepared by dispering Co3O4 on a vulcan carbon. Electrocatalytic activity of N(G) & N(P) were compared with Pt/C electrodes.

Low-Temperature controlled synthesis of nanocast mixed metal oxide spinels for enhanced OER activity

The controlled cation substitution is an effective strategy for optimizing the density of states and enhancing the electrocatalytic activity of transition metal oxide catalysts for water splitting. However, achieving tailored mesoporosity while maintaining elemental homogeneity and phase purity remains a significant challenge, especially when aiming for complex multi-metal oxides. In this study, we utilized a one-step impregnation nanocasting method for synthesizing mesoporous Mn-, Fe-, and Ni-substituted cobalt spinel oxide (Mn0.1Fe0.1Ni0.3Co2.5O4, MFNCO) and demonstrate the benefits of low-temperature calcination within a semi-sealed container at 150–200 °C. The comprehensive discussion of calcination temperature effects on porosity, particle size, surface chemistry and catalytic performance for the alkaline oxygen evolution reaction (OER) highlights the importance of humidity, which was modulated by a pre-drying step. The catalyst calcined at 170 °C exhibited the lowest overpotential (335 mV at 10 mA cm−2), highest current density (433 mA cm−2 at 1.7 V vs. RHE, reversible hydrogen electrode) and further displayed excellent stability over 22 h (at 10 mA cm−2). Furthermore, we successfully adapted this method to utilize cheap, commercially available silica gel as a hard template, yielding comparable OER performance. Our results represent a significant progress in the cost-efficient large-scale preparation of complex multi-metal oxides for catalytic applications.

Ru Single Atoms Integrated into Cobalt Oxide Spinel Structure with Interstitial Carbon for Enhanced Electrocatalytic Water Oxidation.

Oxygen evolution reaction (OER) plays a critical role in energy conversion technologies. Significant progress has been made in alkaline conditions. In contrast, it remains a challenge to develop stable OER electrocatalysts in acidic conditions. Herein, a new strategy is reported to stabilize single atoms integrated into cobalt oxide spinel structure with interstitial carbon (Ru0.27Co2.73O4), where the optimized Ru0.27Co2.73O4 exhibits a low overpotential of 265, 326, and 367mV to reach a current density of 10, 50, and 100mA cm2, respectively. More importantly, Ru0.27Co2.73O4 has long-term stability of up to 100h, representing one of the most stable OER electrocatalysts. X-ray adsorption spectroscopy (XAS) characterization and density functional theory (DFT) calculations jointly demonstrate that the significant catalytic performance of Ru0.27Co2.73O4 is due to the synergistic effect between the Ru and Co sites and the bridging O ligands, as well as the significant reduction of the OER energy barrier. This work provides a new perspective for designing and constructing efficient non-noble metal-based electrocatalysts for water splitting.

Interfacial electron interactions governed photoactivity and selectivity evolution of carbon dioxide photoreduction with spinel cobalt oxide based hollow hetero-nanocubes

In this work, an efficient CO2 photoreduction catalyst based on Co3O4/ZnIn2S4 hollow hetero-nanocubes is precisely constructed via an in-situ transformation of cobalt-organic framework followed by a solvothermal reaction. Comprehensive in-situ spectroscopic analyses and theoretical calculations have revealed that the critical interfacial electron interactions (IEIs) effects on both photoactivity evolution and selectivity modulation in the Co3O4/ZnIn2S4 hetero-structure. As the content of ZnIn2S4 increases in the hetero-structure, the photoactivity exhibits a volcano-like evolution profile but the CH4 selectivity reduces monotonously. The improved photoactivity is attributed to the IEIs-promoted charge separation as well as the specific-surface-area effect in terms of electron unitization rate, and the electronic structure of Co3O4 is tuned and the energy barrier for the key reaction intermediate *CHO is reduced, leading to improved CH4 selection in comparison with bare Co3O4. The IEIs-mediated production selectivity is further verified by a Co3O4/CeO2 heterojunction, indicating a certain universality of the IEI effect.

High specific capacitance and NTC behavior of substituted cobalt oxide spinels for electrochemical supercapacitor and thermistor applications

Due to the growing importance of low-cost energy storage devices in smart device applications, finding an appropriate electrode material with the least self-discharging, high power, and energy densities is the need of the hour. In this paper, Cu and Mn-modified Co3O4 spinels in the form of CuCo2O4 and MnCo2O4 are reported as the most promising electrode materials for high-performance electrochemical supercapacitors. Modified Co3O4 spinels were prepared by the co-precipitation technique, and the X-ray diffraction (XRD) studies show a single-phase formation with cubic crystal symmetry. The average crystallite size estimated by considering the highest intensity peak of the diffraction pattern using Scherrer’s relation shows the average crystallite size below 30 nm for reported spinels. Scanning electron microscopic (SEM) images stabilize the fact that grains with nanosizes consisting of regular shapes and excellent morphological features. The dc-conductivity measurement establishes the essence of negative temperature coefficient of resistance (NTCR) behavior in nanostructured spinels. The specific capacitance (Cs) of CuCo2O4 and MnCo2O4 estimated using cyclic voltammetry (CV) was found to be 616.074 F g−1 and 1401.16 F g−1 for a scan rate of 10 mV s−1 with verified stability up to 1000 cycles. The low internal resistance value in the range of 0.1 Ω for both the spinels verified by the electrochemical impedance spectroscopy (EIS) graph confirms these spinels as encouraging electrode materials for electrochemical supercapacitors.

Tailoring cobalt spinel oxide with site-specific single atom incorporation for high-performance electrocatalysis

Ta5+ dopants tend to stabilize on the surface of Co3O4, significantly increasing the surface Co2+ species. This dopant-rich shell and the high density of surface Co2+ provide active and protective layers, resulting in high-performance in acidic OER.

Tungsten single atoms incorporated in cobalt spinel oxide for highly efficient electrocatalytic oxygen evolution in acid

Individual tungsten (W) atoms incorporated into the spinel lattice of Co3O4 (W–Co3O4) can present remarkable OER activity and stability. The PEMWE with W–Co3O4 as an anode catalyst can be stably operated at 1 A cm−2 for 240 hours.

Atomically dispersed silver atoms incorporated in spinel cobalt oxide (Co3O4) for boosting oxygen evolution reaction

Incorporating noble metal single atoms into lattice of spinel cobalt oxide (Co3O4) is an attractive way to fabricate oxygen evolution reaction (OER) electrocatalysts because of the high activity and economic benefit. The commonly used high valence noble metal dopants such as ruthenium, iridium and rhodium tend to supersede Co3+ at octahedral site of Co3O4 and result in great activity, the origins of admirable activity were also wildly investigated. However, bare explorations on doping noble metal single atom into tetrahedral site of Co3O4 to construct OER catalyst have been reported, corresponding catalytic activity and mechanism remain mystery. Here, a promising structure that tetrahedrally substituent Ag single atom embedded in Co3O4 nanoparticles on the surface of carbon nanotube (Ag-Co3O4/CNT) was presented, and its performance in OER was probed. The high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) demonstrate the successful embeddedness of atomical Ag atom in Co3O4 lattice, the resultant electronic interaction is conducive to promote charge transfer for OER. Theoretical calculations further disclose that atomical Ag dopant prefers to replace tetrahedral Co2+ rather than octahedral Co3+. The substitution Ag acts as the active site through Ag-Co bridge and facilitates the desorption process, which improves the turnover frequency (TOF) and boosts the intrinsic activity of Ag-Co3O4/CNT. Benefiting from the essentials above, Ag-Co3O4/CNT displays remarkable activity (236 mV@10 mA cm−2) and robust stability for alkaline OER. This finding offers a potential direction for the design of noble metal single atom involved Co3O4 based OER electrocatalysts.

Metal Doping in 2D Co3O4 for Enhanced ORR/OER Electrocatalytic Performance

Spinel cobalt oxide or Co3O4, has been proven to be a promising material as bifunctional electrocatalyst for ORR/OER. In this study, we show further improvement of the electrocatalytic activity via metal doping. Metal-doped Co3O4 was synthesized using a simple hydrothermal method at 120° C for 24 h, followed by calcination at 400° C for 1 h. A surfactant was used to control the morphology of the sample such that either 1D or 2D structure was obtained. The 2D structure shows better electrical conductivity with Rct of 70 Ohm vs. 86 Ohm for the 1D structure. Electrochemical test show that the 2D structure gives a ∆E of 870 mV vs. 949 mV for the 1D structure, due to primarily the excellent ORR activity of the 2D structure, i.e., E-3 = 711 mV for the 2D structure vs. E-3 = 667 mV for the 1D structure. The use of the 2D structure in zinc air battery is also demonstrated.

(Invited) Acid Stable Water Oxidation Catalysts for PEM Water Electrolysis

Water electrolysis is a promising method to produce hydrogen from renewable electricity. However, the development of 3d metal catalysts for the oxygen evolution reaction (OER) is a critical challenge, especially in the case of polymer electrolyte membrane (PEM) electrolyzers, which are considered to be one of the most efficient electrolysis systems. The scarcity of iridium required for these systems prevents their widespread deployment at the global terawatt scale, and therefore the development of alternative catalysts has been highlighted as a real bottleneck for water electrolysis.In this talk, I will report on how the incorporation of Mn into the Co3O4 spinel lattice can result in a two order of magnitude increase in stability without compromising activity (1, 2). I will also report on how the lifetime of Mn-based catalysts in acid can be extended by modulating the lattice oxygen structure (3). Finally, I will discuss the performance of Mn-based OER catalysis under the actual conditions of PEM water electrolysis (3).(1) A. Li, S. Kong, C. Guo, H. Ooka, K. Adachi, D. Hashizume, Q. Jiang, H. Han, J. Xiao, R. Nakamura, Enhancing the stability of cobalt spinel oxide towards sustainable oxygen evolution in acid, Nat. Catal., 2022, 5, 109-118, DOI: 10.1038/s41929-021-00732-9(2) A. Li, H. Ooka, N. Bonnet, T. Hayashi, Y. Sun, Q. Jiang, C. Li, H. Han, R. Nakamura, Stable potential windows for long-term electrocatalysis by manganese oxides under acidic conditions, Angew. Chem., Int. Ed., 2019, 58, 5054-5058. DOI: 10.1002/anie.201813361(3) S. Kong, et al (under review)

Unveiling the geometric site dependent activity of spinel Co3O4 for electrocatalytic chlorine evolution reaction

Spinel cobalt oxide (Co3O4), consisting of tetrahedral Co2+ (CoTd) and octahedral Co3+ (CoOh), is considered as promising earth-abundant electrocatalyst for chlorine evolution reaction (CER). Identifying the catalytic contribution of geometric Co site in the electrocatalytic CER plays a pivotal role to precisely modulate electronic configuration of active Co sites to boost CER. Herein, combining density functional theory calculations and experiment results assisted with operando analysis, we found that the CoOh site acts as the main active site for CER in spinel Co3O4, which shows better Cl− adsorption and more moderate intermediate adsorption toward CER than CoTd site, and does not undergo redox transition under CER condition at applied potentials. Guided by above findings, the oxygen vacancies were further introduced into the Co3O4 to precisely manipulate the electronic configuration of CoOh to boost Cl− adsorption and optimize the reaction path of CER and thus to enhance the intrinsic CER activity significantly. Our work figures out the importance of geometric configuration dependent CER activity, shedding light on the rational design of advanced electrocatalysts from geometric configuration optimization at the atomic level.

Structure and Electron Engineering for Nitrate Electrocatalysis to Ammonia: Identification and Modification of Active Sites in Spinel Oxides.

Cobalt spinel oxides, which consist of tetrahedral site (AO4) and octahedral site (BO6), are a potential group of transition metal oxides (TMO) for electrocatalytic nitrate reduction reactions to ammonia (NRA). Identifying the true active site in spinel oxides is crucial to designing advanced catalysts. This work reveals that the CoO6 site is the dominant site for NRA through the site substitution strategy. The suitable electronic configuration of Co at the octahedral site leads to a stronger interaction between the Co d-orbital and the O p-orbital in O-containing intermediates, resulting in a high-efficiency nitrate-to-ammonia reduction. Furthermore, the substitution of metallic elements at the AO4 site can affect the charge density at the BO6 site via the structure of A-O-B. Thereafter, Ni and Cu are introduced to replace the tetrahedral site in spinel oxides and optimize the electronic structure of CoO6. As a result, NiCo2O4 exhibits the best activity for NRA with an outstanding yield of NH3 (15.49mgcm-2h-1) and FE (99.89%). This study introduces a novel paradigm for identifying the active site and proposes an approach for constructing high-efficiency electrocatalysts for NRA.

Interfacial Effect-Induced Electrocatalytic Activity of Spinel Cobalt Oxide in Methanol Oxidation Reaction.

In this study, spinel cobalt oxide (Co3O4) nanoparticles without combining with any other metal atoms have been decorated through the influence of two hard templating agents, viz., zeolite-Y and carboxy-functionalized multiwalled carbon nanotubes (COOH-MWCNT). The adornment of the Co3O4 nanoparticles, through the combined impact of the aluminosilicate and carbon framework has resulted in quantum interference, causing the reversal of signatory Raman peaks of Co3O4. Apart from the construction of small Co3O4 nanoparticles at the interface of the two matrices, the particles were aligned along the direction of COOH-MWCNT. The catalyst Co3O4-Y-MWCNT exhibited excellent catalytic activity toward the methanol oxidation reaction (MOR) in comparison to Co3O4-Y, Co3O4-MWCNT, and bared Co3O4 with the current density of 0.92 A mg-1 at an onset potential of 1.33 V versus RHE. The material demonstrated persistent electrocatalytic activity up to 300 potential cycles and 20,000 s without substantial current density loss. High surface area of zeolite-Y in combination with the excellent conductivity of the COOH-MWCNT enhanced the electrocatalytic performance of the catalyst. The simplicity of synthesis, scale-up, and remarkable electrocatalytic activity of the catalyst Co3O4-Y-MWCNT provided an effective way toward the development of anode materials for direct methanol fuel cells.