Co-based Oxides Research Articles

Reduced spinel oxide ZnCo2O4 with tetrahedral Co2+ sites for electrochemical nitrate reduction to ammonia and energy conversion

Electrocatalytic NO3− reduction to ammonia (NRA) at ambient conditions emerges as an appealing solution for green ammonia synthesis. In this work, we prepared a reduced Co-based spinel oxide for efficient ammonia synthesis and energy conversion. The in-depth structural characterization and electrochemical evaluation revealed thatthe introduction of Zn and reduction treatment led to the formation of electron-deficient tetrahedrally coordinated Co2+ sites. The resulting R-ZnCo2O4 exhibited remarkable NH3 production capacity with high NH3 selectivity of 92.2 %, FE of 92.3 %, and yield rate of 5.35 mgNH3 h−1 cm−2 (ca. 26.75 mgNH3 h−1 mgcat−1) under a low potential of − 0.3 V vs. RHE. The NRA pathway on R-ZnCo2O4 was further elucidated through DFT calculations and in situ characterizations. The high NRA efficiency of R-ZnCo2O4 was further explored in a Zn − NO3− battery, which was specified by an open circuit voltage of 1.46 V and a peak-power density of 1.40 mW cm−2, demonstrating its promising application for sustainable energy supply and green ammonia synthesis.

Investigating the Effects of Anode Catalyst Conductivity and Loading on Performance for Anion Exchange Membrane Water Electrolysis

Water electrolysis (WE) is a promising route for carbon-free production of green hydrogen, which is increasingly considered to be an important energy carrier. Among the low-temperature WE technologies, anion exchange membrane water electrolysis (AEMWE) has the potential advantages of high operating current densities and low cost due to the use of zero-gap assembly and platinum group metal (PGM)-free components, respectively [1]. However, significant challenges remain for this technology, including low efficiency, insufficient durability, and poor performance in pure water operation. Recent efforts have yielded improvements in the individual components, particularly the oxygen evolution reaction (OER) catalysts and anion exchange polymers; however, to further improve the performance of the AEMWE, it is also necessary to understand and improve the integration of these materials into the membrane electrode assembly (MEA). Within the MEA, the anode and cathode catalyst layers (CLs) incorporate catalyst and ion-conducting and/or binding polymers, which are deposited onto either the membrane or the porous transport layer. The material properties of each component, as well as the loadings of the catalyst and ionomer and methods of dispersion and deposition, have significant impact on the catalyst layer morphology and uniformity [2]. These CL properties affect the transport of electrons, hydroxide ions, and evolved gases, ultimately controlling the accessibility and utilization of catalyst active sites, with significant consequences for both the performance and durability of AEMWEs.Most of the effort in catalyst development for AEMWE has focused on the OER, aiming to improve sluggish kinetics and improve stability at the highly oxidizing operating potentials. A number of PGM-free catalysts have shown promising activity in rotating disk electrode (RDE) tests, particularly oxides with varying ratios of Ni, Fe, and Co. The activity improvements with tuning the composition are often attributed to the increase in surface area and shift in redox potentials for the in-situ conversion to higher valence sites with higher intrinsic OER activity. However, these activity trends do not always translate into device-level performance because of differences in the operating environment, e.g. hydroxide concentration, reaction rates, etc. In the AEMWE, where typically an order of magnitude higher loadings and current densities are required, the challenges of conductivity and mass transport to allow for full site access are exacerbated. Electrochemical impedance spectroscopy (EIS) is a valuable tool for understanding the origin of voltage losses in the AEMWE, typically broken down into ohmic, kinetic, and mass transport losses. Recently, a transmission line model has been applied to non-Faradaic EIS to isolate another contribution to voltage loss: the catalyst layer resistance (CLR), which corresponds to in-plane and through-plane electronic and ionic resistances in the catalyst layer [3]. Using this approach, the roles of intrinsic catalyst activity and catalyst layer properties can be decoupled, helping to guide materials integration efforts.In this study, we investigate two types of OER catalyst materials, NiFe- and Co-based oxides. Within each composition, the catalysts have similar active sites but vary in electronic conductivity, crystal structure, and surface area. By comparing the AEMWE performance at different catalyst loadings, we are able to determine the role of catalyst properties in determining active site utilization and electronic resistance within the catalyst layer. Figure 1 shows the trends in performance and ohmic and catalyst layer resistance as a function of loading for high-conductivity (Co@Co3O4) and low-conductivity (Co3O4) catalysts. Despite nominally having the same surface chemistry and similar particle morphology, they display both different performance and different trends with loading. Using in-situ EIS diagnostics and ex-situ characterization of the catalyst layer morphology, the relationship between catalyst utilization, CLR, and the physical properties of the catalyst and catalyst layer is elucidated. This work provides insight into the catalyst properties that most strongly affect AEMWE performance, as well as how materials integration strategies must be tailored based on these properties.

Enhanced catalytic transfer hydrogenation of biomass-based furfural into furfuryl alcohol over Co3O4-based mixed oxide catalysts from hydrotalcite

Catalytic furfural (FF) into high value-added furfuryl alcohol (FOL) is a very important biomass improvement process and remains a challenging task. Herein, a series of Co-based composite oxide (CoMgAl-LDO) catalysts with highly dispersed Co3O4-based spinel oxide nanoparticles were synthesized by the topological transformation of Co-based ternary layered hydroxides (CoMgAl-LDH) under calcination conditions and tested for catalytic transfer hydrogenation of FF to FOL using isopropanol (2-PrOH) as hydrogen donor. The CoMgAl-LDO, especially Co1.5Mg1.5Al1-LDO calcinated at 500 °C (Co1.5Mg1.5Al1-LDO-500) offered the outstanding catalytic performance for the catalytic transfer hydrogenation of FF to FOL (99.7 % FF conversion and 95.9 % FOL selectivity), which was attributed to effectively dispersed active sites, abundant acid-base sites and mesoporous structure. Additionally, the Co1.5Mg1.5Al1-LDO-500 catalyst manifested good stability with wider universality to other carbonyl compounds, making the catalytic system extremely economical and practical in various biomass hydrogenation processes.

Co-Based Bimetallic Oxide Catalysts with Abundant Vacancy Defects for Enhanced Oxidation of Toluene

Co-Based Bimetallic Oxide Catalysts with Abundant Vacancy Defects for Enhanced Oxidation of Toluene

Enhancing Trifunctional Electrocatalytic Activities via Lanthanide Modulation in Fe–Co-Based Double Perovskite Oxides

Enhancing Trifunctional Electrocatalytic Activities via Lanthanide Modulation in Fe–Co-Based Double Perovskite Oxides

Experimental and Computational Insights into the Catalytic Mechanism of Y1-xBaxCoO3-δ Perovskite Oxides with a Controlled Crystal Structure.

The crystal structure of Co-based perovskite oxides (ACoO3) can be controlled by adjusting the A-site elements. In this study, we synthesized Y1-xBaxCoO3-δ (x = 0, 0.5, and 1.0) via a coprecipitation method and investigated their CO oxidation performances. YCoO3 (x = 0; cubic perovskite oxide; Pbnm) shows a higher catalytic performance than Y0.5Ba0.5CoO2.72 (x = 0.5; A-site-ordered double perovskite oxide; P4/nmm), which exhibits high oxygen nonstoichiometric properties, and BaCoO3 (x = 1.0; hexagonal perovskite oxide; P63/mmc), which contains high-valent Co4+ species. To elucidate the reaction mechanism, we conducted isotopic experiments with CO and 18O2. The CO oxidation reaction on YCoO3 proceeds via the Langmuir-Hinshelwood mechanism, which is a surface reaction of CO and O2 gas that does not utilize lattice oxygen. Because of the significantly smaller specific surface area of YCoO3 compared with that of the reference Pt/Al2O3, the bulk features of the crystal structures affect the catalytic reaction. When density functional theory is applied, YCoO3 clearly exhibits semiconducting properties in the ground state with the diamagnetic t2g6eg0 states, which can translate to a magnetic t2g5eg1 configuration upon excitation by a relatively low energy of 0.64 eV. We propose that the unique nature of YCoO3 activates oxygen in the gas phase, thereby enabling the smooth oxidation of CO. This study demonstrates that the bulk properties originating from the crystal structure contribute to the catalytic activity and reaction mechanism.

Unraveling the modulation essence of p bands in Co-based oxide stability on acidic oxygen evolution reaction

Unraveling the modulation essence of p bands in Co-based oxide stability on acidic oxygen evolution reaction

Enhanced OER electrocatalyst performance by sulfur doping trimetallic compounds hybrid catalyst supported on reduced graphene oxide

Hydrogen evolution by electrolysis of water is an effective method to solve the energy problem, but the catalyst of oxygen evolution reaction is an effective measure to solve the slow kinetics of hydrogen production from electrolytic water. Therefore, an efficient electrocatalyst for oxygen evolution reaction is urgently needed to find. The Co-based trimetallic oxides has been prepared by using isopropyl alcohol and propan-1,3-diol as hydrothermal reaction environment and annealing at high temperature can enrich the types of active core materials of the catalyst. The sulphuration process forms the core-shell structures of crystalline and amorphous phases, and the heterogeneous interface between the crystalline and amorphous structures optimizes the electronic structure. Meanwhile, due to its excellent physical and chemical properties, not only does rGO provide a conductive substrate and promote carrier migration and high current response, but also effectively inhibits the aggregation of nanoparticles, so that the catalyst nanoparticles are uniformly dispersed and the active center is fully exposed, providing a way for oxygen release in the OER process. When Fe0.3Ni1Co2/S-C has been used as electrocatalytic for OER, the overpotential and Tafel slope were as low as 276 mV and 52.2 mV·dec−1 at 10 mA·cm−2 current density. With many improvements on transition metal catalysts, transition metal catalysts may be quickly replaced precious metal catalysts for large-scale use in electrolytic water.

Maraging and reversion-driven passivation engineering of Fe68Ni10Mn10Co10Ti1.5Si0.5 medium entropy alloy

In this study, the relationship between the microstructural evolution, electrochemical behavior, and passivation capability in Fe68Ni10Mn10Co10Ti1.5Si0.5 medium entropy alloy (MEA) with different microstructure is investigated. The passive film formed on the selectively corroded dual-phase microstructure was characterized by an inner thin discontinuous Fe-, Mn-, Ni-, and Co-based oxides and hydroxides layer with a thinner pure Si-oxide layer partially grown on the TiO2/SiO2 layer. By contrast, a continuous oxide/hydroxide layer accompanied by a dense TiO2/SiO2 monolayer in the uniformly corroded austenitic microstructure exhibited the highest protective effect owning to the prevention of galvanic corrosion resulting in the best corrosion resistance.

Hierarchical and Nanocrystallite-Assembled Ultraporous NiO/MnCo2O4 for All Solid-State Hybrid Supercapacitors with Robust Energy Efficiency and Extended Operational Durability.

To address the growing demand of highly Ragone efficient electrochemical energy storage devices, we have innovated a synthetic design strategy and employed a tartrate-mediated kinetic precipitation process to fabricate an ultraporous ribbon-like hierarchical microstructure of Ni-, Mn-, and Co-based ternary oxide, i.e., NiO/MnCo2O4, and employed it as a battery-type positive electrode material to assemble an ASSHSC (all-solid-state hybrid supercapacitor) device with nitrogen-doped reduced graphene oxide (N-rGO) as the positive electrode material. NiO/MnCo2O4 exhibits distinct crystallographic phase, near-perfect elemental stoichiometry, evident bulk porosity, and rich nanocrystallite assembly in the randomly arranged microstructure of near-uniform size and shape. Thorough electrochemical studies corroborate that the battery-type NiO/MnCo2O4 exhibits remarkable electrochemical reversibility during charge transfer, high efficiency in supercapacitive charge storage, low charge transfer, series and diffusion resistance. The NiO/MnCo2O4||N-rGO ASSHSC device assembled with the PVA-KOH film as the separator electrolyte offers rich charge storage physiognomies, which accentuates excellent electromicrostructural compatibility between the electrode materials in the device. The NiO/MnCo2O4||N-rGO ASSHSC device shows low charge transfer and diffusion resistance, and it also delivers high mass and areal specific capacitance/capacity, energy and power density, Ragone efficiency (∼131 Wh kg-1 at ∼2134 W kg-1 and ∼31 Wh kg-1 at ∼5005 W kg-1), and extended operational durability (98.2% specific capacitance retention after 12,000 successive GCD cycles) under high-rate working conditions. The present optimized approach to design highly efficient multiple transition-metal-based oxides as battery-type electrode materials and fabricate rich Ragone-efficient ASSHSC devices can be widely adopted in the future development of high-performance hybrid supercapacitors, which may be largely integrated in various pioneering technologies.

Boosting N2O Catalytic Decomposition by the Synergistic Effect of Multiple Elements in Cobalt-Based High-Entropy Oxides.

Nitrous oxide (N2O) has a detrimental impact on the greenhouse effect, and its efficient catalytic decomposition at low temperatures remains challenging. Herein, the cobalt-based high-entropy oxide with a spinel-type structure (Co-HEO) is successfully fabricated via a facile coprecipitation method for N2O catalytic decomposition. The obtained Co-HEO catalyst displays more remarkable catalytic performance and higher thermal stability compared with single and binary Co-based oxides, as the temperature of 90% N2O decomposition (T90) is 356 °C. A series of characterization results reveal that the synergistic effect of multiple elements enhances the reducibility and augments oxygen vacancy in the high-entropy system, thus boosting the activity of the Co-HEO catalyst. Moreover, density functional theory (DFT) calculations and the temperature-programmed surface reaction (TPSR) with isotope labeling demonstrate that N2O decomposition on the Co-HEO catalyst follows the Langmuir-Hinshelwood (L-H) mechanism with the promotion of abundant oxygen vacancies. This work provides a fundamental understanding of the synergistic catalytic effect in N2O decomposition and paves the way for the novel environmental catalytic applications of HEO.

Cobalt-free layered perovskites RBaCuFeO5+δ (R = 4f lanthanide) as electrocatalysts for the oxygen evolution reaction.

Co-based perovskite oxides are intensively studied as promising catalysts for electrochemical water splitting in an alkaline environment. However, the increasing Co demand by the battery industry is pushing the search for Co-free alternatives. Here we report a systematic study of the Co-free layered perovskite family RBaCuFeO5+δ (R = 4f lanthanide), where we uncover the existence of clear correlations between electrochemical properties and several physicochemical descriptors. Using a combination of advanced neutron and X-ray synchrotron techniques with ab initio DFT calculations we demonstrate and rationalize the positive impact of a large R ionic radius in their oxygen evolution reaction (OER) activity. We also reveal that, in these materials, Fe3+ is the transition metal cation the most prone to donate electrons. We also show that similar R3+/Ba2+ ionic radii favor the incorporation and mobility of oxygen in the layered perovskite structure and increase the number of available O diffusion paths, which have an additional, positive impact on both, the electric conductivity and the OER process. An unexpected result is the observation of a clear surface reconstruction exclusively in oxygen-rich samples (δ > 0), a fact that could be related to their superior OER activity. The encouraging intrinsic OER values obtained for the most active electrocatalyst (LaBaCuFeO5.49), together with the possibility of industrially producing this material in nanocrystalline form should inspire the design of other Co-free oxide catalysts with optimal properties for electrochemical water splitting.

Spin configuration modulation of Co3O4 by Ru doping for boosting the overall water splitting and hydrazine oxidation reactions

Co-based spinel oxides with unique crystal structures have been identified as a class of promising candidates for alkaline water oxidation catalysis in oxide structural families.

Research progress of Co-based spinel oxide materials for direct methanol fuel cells

With decades of research, both the synthesis and applications of spinels have achieved greatly increased development. The preparation, regulation strategies, and their application in MOR of Co based spinels are reviewed in this paper.

Self-templated fabrication of P-doped CoMoO4-Co3O4 hollow nanocages for the efficient oxygen evolution reaction.

Finding reservoir-rich and efficient electrocatalysts for the alkaline oxygen evolution reaction (OER) is crucial for further sustainable energy development. Despite the advantages of high earth abundance, easy availability, and tunable composition, transition-metal oxides are typically considered poor electrocatalysts for the OER. In this study, a composite P-doped CoMoO4-Co3O4 hollow nanocage is deliberately synthesized through a cation-exchange, pyrolysis, and phosphorization approach using an innovative self-template strategy with ZIF-67 as the sacrificial template. Hollow nanocages provide large surface areas and abundant active sites, enhancing electron transfer. Hybridization with other components increases the number of electrochemically reactive sites and optimizes the advantages of different element components. As a result, the P-CoMoO4-Co3O4 hollow nanocage catalyst demonstrates high OER performance, with an overpotential of 279 mV at a current density of 10 mA cm-2. Additionally, P-CoMoO4-Co3O4 catalysts exhibit good dispersibility and excellent long-term stability. Experimental findings and density functional theory (DFT) calculations indicate that the phosphorus-doping effect in various aspects contributes significantly to the superior catalytic activity of P-CoMoO4-Co3O4. This work provides a valuable method for designing cost-effective P doped Co-based bimetal oxide catalysts with outstanding OER performance for industrial applications.

Assessing the Stability of Co3O4 Under Oxygen Evolution Reaction Conditions at Low and Mild pH

Water and CO2 electrolysis will play a crucial role in converting renewable energy into value-added chemicals and chemical fuels. To successfully employ and expand these technologies globally, ideally, one should develop Earth-abundant, cost-effective catalysts with high activity and stability. The requirements for the catalysts depend on the configuration of the electrolyzer, and the electrolyte used there. However, especially in CO2 electrolysis, the stability of the catalyst in a certain electrolyte does not guarantee its successful use on a long-term scale, as the pH in the electrolyzer may change during long-term experiments due to ion transfer through the membrane.1 The anode materials should, therefore, be stable across a broad pH range and/or in carbonate-rich electrolytes. Although the earth-abundant, non-noble transition metals, and their compounds are highly stable at higher pH, they are less stable than Ir or Pt at low and (near-)neutral pH.2 Interestingly, recent studies have shown that Co-based oxides exhibit promising stability even at low pH.3, 4 However, the nature of this stability as well as the mechanisms of non-noble metal oxides catalyzing OER at low or near-neutral pH, are yet to be discovered. In this work, we analyze the stability of cobalt oxide under various electrochemical conditions at low pH with the use of a scanning flow cell (SFC) coupled with inductively coupled plasma mass spectrometry (ICP-MS) and differential electrochemical mass spectrometry (DEMS). By using these techniques, we can follow the dissolution of Co online and find potential narrow stability windows before and during the oxygen evolution reaction (OER). Benefiting from the obtained results, mitigating strategies for minimizing Co dissolution are proposed and discussed.

Enhanced Catalytic Soot Oxidation over Co-Based Metal Oxides: Effects of Transition Metal Doping.

A series of Co-M (M = Fe, Cr, and Mn) catalysts were synthesized by the sol-gel method for soot oxidation in a loose contact mode. The Co-Fe catalyst exhibited the best catalytic activity among the tested samples, with the characteristic temperatures (T10, T50, and T90) of 470 °C, 557 °C, and 602 °C, respectively, which were 57 °C, 51 °C, and 51 °C lower than those of the CoOx catalyst. Catalyst characterizations of N2 adsorption-desorption, X-ray diffraction (XRD), X-ray photo-electron spectrometry (XPS), and the temperature programmed desorption of O2 (O2-TPD) were performed to gain insights into the relationships between the activity of catalytic soot oxidation and the catalyst properties. The content of Co2+ (68.6%) increased due to the interactions between Co and Fe, while the redox properties and the relative concentration of surface oxygen adsorption (51.7%) were all improved, which could significantly boost the activity of catalytic soot oxidation. The effects of NO and contact mode on soot oxidation were investigated over the Co-Fe catalyst. The addition of 1000 ppm of NO led to significant reductions in T10, T50, and T90 by 92 °C, 106 °C, and 104 °C, respectively, compared to the case without the NO addition. In the tight contact mode, the soot oxidation was accelerated over the Co-Fe catalyst, resulting in 46 °C, 50 °C, and 50 °C reductions in T10, T50, and T90 compared to the loose contact mode. The comparison between real soot and model Printex-U showed that the T50 value of real soot (455 °C) was 102 °C lower than the model Printex-U soot.

A simple surface phosphating treatment accompanied by the formation of Co-based multiphase heterostructure for efficient oxygen evolution reaction

Constructing heterostructures to increase the conductivity and enhance reaction kinetics of Co-based oxides is a fascinating yet challenging method for creating effective catalysts for oxygen evolution reaction. The controlled synthesis of multi-phase heterostructures comprising identical elements has been rarely reported in the literature. Herein, a simple and scalable strategy is proposed to prepare a multi-phase heterostructure of Co3O4 and CoO through an efficient hydrogen reduction assisted strategy and subsequent surface phosphating treatment. In the process of effectively doping phosphorus (P) atoms, spinel-type Co3O4 and rock-salt type CoO heterojunctions are cleverly formed, and an abundance of oxygen defects are also introduced. The spontaneous integration of multi-phase heterostructure of well-crystallized Co elements as well as the incorporation of P on catalyst surface, not only alters the electronic structure to foster the activation of oxygen and the disaggregation of water, but also causes a synergistic effect at the interface. Benefiting from these characteristics, the hybrid catalyst surpasses the RuO2 benchmark with outstanding OER performance, reaching 10 mA cm−2 at only 299 mV overpotential and excellent stability in alkaline electrolyte.

Reduced porous 2D Co3O4 enhanced peroxymonosulfate activation to form multi-reactive oxygen species: The key role of oxygen vacancies

The introduction of oxygen vacancies (Ov) into Co-based metal oxides has been considered a promising strategy for activating peroxymonosulfate (PMS) to effectively degrade refractory organic contaminants. However, the types of reactive oxygen species (ROS) generated in Ov-catalyst/PMS systems exhibited variability. Therefore, the effect of the interaction between Ov and PMS on ROS generation required further research. Herein, the Ov-tuned 2D porous Co3O4 catalysts were easily constructed by simply controlling the NaBH4 concentration. The as-prepared 2D Co3O4-1.6 had the highest Ov content (2.88 × 1014 spins/g, δ = 0.726), and its catalytic activity for benzotriazole (BTA) degradation (k = 0.032 min−1) was three times higher than that of pristine 2D Co3O4 (k = 0.011 min−1). A series of characterizations and DFT calculations demonstrated that the major active site was Co(II), with Ov serving as an localized electron-rich co-site. These dual sites synergistically improved the adsorption capacity for PMS, enabling rapidly electrons transfer to PMS while promoting the dissociation of O-O bond in PMS to form multiple ROS. Compared to the pristine 2D Co3O4/PMS system, which was dominated only by SO4−, the key ROS in the 2D Co3O4-1.6/PMS system were found to be 1O2 (3.56 µM) and SO4− (2.53 µM), while both O2– (0.72 µM) and OH (0.65 µM) were involved in the activation reaction, the contributions of these four ROS were as follows: 1O2 (36.8%) > SO4− (25.4%) >O2– (9.5%) >OH (6.3%). Finally, three possible degradation pathways of BTA were proposed, and the toxicity of BTA degradation decreased with time, which was consistent with the toxicity prediction results of eleven intermediates. This study provides new insights into the key role of Ov in the heterogeneous catalytic system of PMS-based advanced oxidation processes.

Yolk-shell composite oxides with binuclear Co(II) sites toward low-overpotential nitrate reduction to ammonia

Electroreduction of nitrate to ammonia, a potential route for producing ammonia, is still confronted with a great challenge on developing efficient electrocatalysts. In this work, a reduced yolk-shell Co-based oxides (R-Co3O4) electrocatalyst with oxygen vacancy-rich Co3O4 and CoO was designed for nitrate reduction. This catalyst presented a nearly 100 % Faradaic efficiency (FE) of ammonia in a low potential from −0.05 to −0.1 V vs. RHE, and a maximum ammonia production rate of 7.25 mg h−1 mgcat.-1 at −0.05 V vs. RHE in 0.1 M KOH + 1000 ppm KNO3 electrolyte. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) pattern proved that the content of Co2+ was dramatically increased in R-Co3O4. The kinetic correlation experiments and Operando Raman spectra preliminarily manifest that the potential limiting step is the reduction of adsorbed *NO3. The density functional theory (DFT) calculations consistently demonstrated that on both oxygen-rich Co3O4 and CoO, the adsorption of key intermediates (*NO3, *NO and *NH) are greatly enhanced on the bridge site between two Co2+, boosting the nitrate reduction at low over-potential. This work is helpful to design the catalytic material with rich and low-valence sites towards electroreduction of nitrate to ammonia.