Co-based Oxides Research Articles

Insights into Electrocatalyst Transformations Studied in Real Time with Electrochemical Liquid-Phase Transmission Electron Microscopy.

ConspectusThe value of operando and in situ characterization methodologies for understanding electrochemical systems under operation can be inferred from the upsurge of studies that have reported mechanistic insights into electrocatalytic processes based on such measurements. Despite the widespread availability of performing dynamic experiments nowadays, these techniques are in their infancy because the complexity of the experimental design and the collection and analysis of data remain challenging, effectively necessitating future developments. It is also due to their extensive use that a dedicated modus operandi for acquiring dynamic electrocatalytic information is imperative. In this Account, we focus on the work of our laboratory on electrochemical liquid-phase transmission electron microscopy (ec-LPTEM) to understand the transformation/activation of state-of-the-art nanocatalysts for the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and CO2 electroreduction (CO2ER). We begin by describing the development of electrochemical microcells for TEM studies, highlighting the importance of tailoring the system to each electrochemical process to obtain reliable results. Starting with the anodic OER for alkaline electrolyzers, we demonstrate the capability of real-time monitoring of the electrowetting behavior of Co-based oxide catalysts and detail the fascinating insights gained into solid-liquid interfaces for the reversible surface reconstruction of the catalystic surfaces and their degradation processes. Importantly, in the case of the OER, we report the exceptional capacity of ec-LPTEM to probe gaseous products and therefore resolve solid-liquid-gas phenomena. Moving toward the cathodic ORR for fuel cells, we summarize studies that pertain to the evaluation of the degradation mechanisms of Pt nanoparticles and discuss the issues with performing real-time measurements on realistic catalyst layers that are composed of the carbon support, ionomer network, and Pt nanocatalysts. For the most cathodic CO2ER, we first discuss the challenges of spatiotemporal data collection in microcells under these negative potentials. We then show that control over the electrochemical stimuli is critical for determining the mechanism of restructuring/dissolution of Cu nanospheres, either for focusing on the first stages of the reaction or for start/stop operation studies. Finally, we close this Account with the possible evolution in the way we visualize electrochemical processes with ec-LPTEM and emphasize the need for studies that bridge the scales with the ultimate goal of fully evaluating the impact of the insights obtained from the in situ-monitored processes on the operability of electrocatalytic devices.

Preparation of various Co/Cu and Co/Fe molar ratios and gel-modified nanomaterials as anodes for Li-ion batteries

Co-based binary metal oxides with higher theoretical capacity have attracted increasing attention as anodes for lithium-ion batteries (LIBs). In this study, Co-based bimetallic oxides were prepared by optimizing different molar ratios of Cu/Co and Fe/Co using an electroconversion method. The purity and microstructure of the synthesized powders were investigated, and their lithium storage properties were studied by assembling half-cells. Cu/Co oxide exhibited three different microstructures, while Fe/Co oxide showed a nanoparticle morphology. The electrochemical performance results revealed that the optimal molar ratio of Co: Cu was 2:1, and the optimal molar ratio of Fe: Cu was 5:1. Particularly, Fe: Cu = 5:1 exhibited the best electrochemical stability and higher specific capacitance. Therefore, Fe3O4/Co3O4 (Fe: Cu = 5:1) was used as the raw material for modification, and a hydrogel method was employed with sodium alginate (SA) serving as the fixing agent and carbon source to prepare microspheres. Taking advantage of the natural freezing conditions in the winter of Northeast China, the dried microspheres were subjected to calcination to obtain Fe3O4/Co3O4/SA with enhanced conductivity. Even at a current density of 2 A/g, Fe3O4/Co3O4/SA still maintained a discharge capacity of around 280 mAh/g, demonstrating excellent cycling stability and rate capability.

Reducibility of Al3+-Modified Co3O4: Influence of Aluminum Distribution.

The reduction of Co-based oxides doped with Al3+ ions has been studied using in situ XRD and TPR techniques. Al3+-modified Co3O4 oxides with the Al mole fraction Al/(Co + Al) = 1/6; 1/7.5 were prepared via coprecipitation, with further calcination at 500 and 850 °C. Using XRD and HAADF-STEM combined with EDS element mapping, the Al3+ cations were dissolved in the Co3O4 lattice; however, the cation distribution differed and depended on the calcination temperature. Heating at 500 °C led to the formation of an inhomogeneous (Co,Al)3O4 solid solution; further treatment at 850 °C provoked the partial decomposition of mixed Co-Al oxides and the formation of particles with an Al-depleted interior and Al-enriched surface. It has been shown that the reduction of cobalt oxide by hydrogen occurs via the following transformations: (Co,Al)3O4 → (Co,Al)O → Co. Depending on the Al distribution, the course of reduction changes. In the case of the inhomogeneous (Co,Al)3O4 solid solution, Al stabilizes intermediate Co(II)-Al(III) oxides during reduction. When Al3+ ions are predominantly on the surface of the Co3O4 particles, the intermediate compound consists of Al-depleted and Al-enriched Co(II)-Al(III) oxides, which are reduced independently. Different distributions of elemental Co and Al in mixed oxides simulate different types of the interaction phase in Co3O4/γ-Al2O3-supported catalysts. These changes in the reduction properties can significantly affect the state of an active component of the Co-based catalysts.

Al- and Cr-doped Co3O4/CoO redox materials for thermochemical energy storage in concentrated solar power plants

The redox system of Co3O4/CoO is very promising for the thermochemical energy storage systems coupled to concentrated solar power plants because of its high energy storage density and reversibility. Nevertheless, the practical application of Co3O4/CoO system is limited by thermal hysteresis of the redox reaction and material sintering. Here, we synthesized Co-based metal oxides doped with Al and Cr, and studied the influence of the doping of these two metal elements on the thermochemical properties and sintering resistance of the pure Co3O4/CoO system. Both Al and Cr doping narrows the thermal hysteresis of the Co3O4/CoO system. In particular, when the molar doping rate of Al is 10%, the thermal hysteresis of Co–Al oxides is only 4.02 °C, which is much narrower than that of pure Co3O4, and the increase of its oxidation temperature also means that a higher level of thermal energy can be released during oxidation. In the redox cycle, the addition of Al inhibits material sintering and the formation of a loose porous structure facilitates the oxidation, ensuring the reversibility of the redox reaction. The 10% Al–Co honeycomb shows stable conversion behavior with an average conversion of 77.75% in 15 redox cycles. Co–Al oxides exhibit excellent oxidative exothermic characteristics and long-term cycle stability, making them suitable for efficient thermochemical energy-storage processes.

Boosting specific capacitance of Mo-Co-(S,O) electrode via a synergistic optimization strategy

Low electrical conductivity and insufficient active sites of Co-based binary metal oxides or sulfides hamper their practical specific capacitance especially at high current densities. In the current work, we provide a novel synergistic optimization strategy including P doping and constant-current-charge activation (CA) to greatly improve the specific capacitance of sulfur and oxygen-containing Mo-Co binary compounds (Mo-Co-(S,O)) electrode. The synergistic effect of the P doping and activation not only increases the electrical conductivity but also improves the electroactive sites. As a result, the optimized Mo-Co-(S,O)/P-CA electrode shows exceptional specific capacitance (∼9.64 F cm−2/2931 F g−1 at 12.5 mA cm−2), approximately 20 times the pristine Mo-Co-(S,O). Moreover, an asymmetric supercapacitor prepared with Mo-Co-(S,O)/P-CA cathode and conductive carbon cloth achieves a high energy density of 0.61 mWh cm−2 at a power density of 4.25 mW cm−2, and outstanding cyclic performance with 121.6% of capacitance retention after 12,000 cycles.

Coordination-Controlled Catalytic Activity of Cobalt Oxides for Ozone Decomposition.

Nowadays, it is still elusive and challenging to discover the active sites of cobalt (Co) cations in different coordination structures, though Co-based oxides show their great potency in catalytic ozone elimination for air cleaning. Herein, different Co-based oxides are controllably synthesized including hexagonal wurtzite CoO-W with Co2+ in tetrahedral coordination (CoTd2+) and CoAl spinel with dominant CoTd2+, cubic rock salt CoO-R with Co2+ in octahedral coordination (CoOh2+), MgCo spinel with dominant Co3+ in octahedral coordination (CoOh3+), and Co3O4 with mixed CoTd2+ and CoOh3+. The valences are proved by X-ray photoelectron spectroscopy, and the coordinations are verified by X-ray absorption fine structure analysis. The ozone decomposition performances are CoOh3+ ∼ CoOh2+ ≫ CoTd2+, and CoOh3+ and CoOh2+ show a lower apparent activation energy of ∼42-44 kJ/mol than CoTd2+ (∼55 kJ/mol). In specific, MgCo shows the highest decomposition efficiency of 95% toward 100 ppm ozone at a high space velocity of 1,200,000 mL/gh, which still retains at 80% after a long-term running of 36 h at room temperature. The high activity is explained by the d-orbital splitting in the octahedral coordination, favoring the electron transfer in ozone decomposition reactions, which is also verified by the simulation. These results show the promising prospect of the coordination tuning of Co-based oxides for highly active ozone decomposition catalysts.

The porous hollow cobalt-based oxides encapsulated with bimetallic PdAu Nanoparticles of electrochemical biosensor for highly sensitive pesticides detection

Efficient and portable electrochemical biosensors are received to evaluation of pesticides in the environment, which can make great significance for food safety. In this study, the Co-based oxides with a kind of hierarchical porous hollow and nanocages were constructed, in which the materials (Co3O4-NC) were encapsulated with PdAu nanoparticles (NPs). Due to the unique porous structure, the changeable valence state of cobalt and the synergistic effect of bimetallic PdAuNPs, PdAu@Co3O4-NC possessed excellent electron pathways, and showed more exposed active sites. Accordingly, the porous Co-based oxides have been applied to construct an acetylcholinesterase (AChE) electrochemical biosensor, which showed good performance for organophosphorus pesticides (OPs) detection. The optimum biosensing platform based on nanocomposites was applied to exhibit highly sensitive determination of omethoate and chlorpyrifos, with the relative low detection limit of 6.125 × 10−15 M and 5.10 × 10−13 M, respectively. And a wide detection range of 6.125 × 10−15 ∼ 6.125 × 10−6 M and 5.10 × 10−13 ∼ 5.10 × 10−6 M for these two pesticides were achieved. Therefore, the PdAu@Co3O4-NC may represent a powerful tool for ultrasensitive sensing of OPs, and have great potential application.

Engineering Morphology and Ni Substitution of NixCo3–xO4 Spinel Oxides to Promote Catalytic Combustion of Ethane: Elucidating the Influence of Oxygen Defects

Transition metal oxides are potential alternatives to noble metal catalysts for oxidation reactions. Co-based spinel oxides, in particular, have attracted significant attention. Herein, NixCo3–xO4 catalysts were synthesized to elucidate the influence of oxygen vacancies on catalyst activities and reaction mechanisms for ethane combustion. A correlation between the activity and the population and properties of O defects was developed, with an increased number of O defects typically resulting in higher activity. Also, the shape-induced facet effect is related to the amount of Ni that is incorporated into the octahedral sites of Co oxide. The substituted Ni atoms altered the redox ability of NixCo3–xO4 by changing O vacancy formation and C–H bond dissociation. The NiCo2O4-TM catalyst (6.0 mmol mL–1 h–1) exhibits the highest activity for ethane oxidation compared with NiCo2O4-PC (0.5 mmol mL–1 h–1) and NiCo2O4-OL catalysts (1.3 mmol mL–1 h–1) at 330 °C, and its activation energy (Ea) is 70.9 kJ mol–1. No activity decay is observed after the initial transition stage of the reaction in a long-term stability test up to 500 h on the NiCo2O4-TM-coated monolith, either with or without water addition. A vacancy-mediated pathway was proposed according to in situ diffuse reflectance infrared Fourier transform (DRIFT) and density functional theory (DFT) calculations over the NiCo2O4(311) facet. Findings from this study expand our understanding of the facet-dependent catalytic behavior and ultimately enable the rational design of high-performance catalysts.

The Polysulfide-Cathode Binding Energy Landscape for Lithium Sulfide Growth in Lithium-Sulfur Batteries.

A cathode substrate with strong adsorption of lithium polysulfides (LiPSs) has been preferred for lithium-sulfur(Li-S) batteries. However, the recent finding that controlled growth of lithiumsulfides(Li2 S) during discharge is crucial forS utilization stimulates improvement of this preference. Here, the Li2 S growth and cell capacity in the LiPS binding energy landscape of cathode substrates are investigated. Specifically, Co-based ternary oxides are employed to obtain binding energies in the range of 4.0-7.4eV. Of these substrates, only the MnCo2 O4 substrate with moderate LiPS affinity exhibits 3D Li2 S growth. The MnCo2 O4 cells achieve high sulfur utilization up to 84% at 0.2C and excellent performance even under high sulfur loading/lean electrolyte conditions. In contrast, weak affinity substrates such as ZnCo2 O4 and strong affinity substrates such as NiCo2 O4 and CuCo2 O4 exhibit low discharge capacity with 2D Li2 S growth. For optimal LiPS affinity driving 3D growth, a balance between promoting LiPS adsorption and diffusion limitation in the LiPS adsorption layer is suggested.

Modulating the electronic configuration of Co species in MOF/MXene nanosheet derived Co-based mixed spinel oxides for an efficient oxygen evolution reaction

Mixed Co-based spinel oxides were synthesized via a simple MOF/MXene pyrolysis–reorganization strategy, which revealed that the coupling effect between Co3O4 and Co2TiO4 is crucial for improving OER activity.

Influence of Ni content on the oxidation behavior of alumina-forming Co-based oxide dispersion strengthened superalloys at 900 °C

This work systematically investigated the effect of Ni additions of 10, 20 and 30 at% on the oxidation behavior of oxide dispersion-strengthened (ODS) Co-20Cr-15Al (at%) superalloys at 900 °C in air. Alumina scale with a duplex structure consisting of outer θ-alumina and inner α-alumina was formed on all tested Co-based ODS superalloys. Ni addition reduced the oxidation rate by inhibiting the growth of the outer θ-alumina during initial oxidation and the inner α-alumina during prolonged exposure. Cr2O3 formation between the interface of the θ- and α-alumina layers was confirmed with the increase of Ni content.

Modulating Direct Growth of Copper Cobaltite Nanostructure on Copper Mesh as a Hierarchical Catalyst of Oxone Activation for Efficient Elimination of Azo Toxicant.

As cobalt (Co) has been the most useful element for activating Oxone to generate SO4•-, this study aims to develop a hierarchical catalyst with nanoscale functionality and macroscale convenience by decorating nanoscale Co-based oxides on macroscale supports. Specifically, a facile protocol is proposed by utilizing Cu mesh itself as a Cu source for fabricating CuCo2O4 on Cu mesh. By changing the dosages of the Co precursor and carbamide, various nanostructures of CuCo2O4 grown on a Cu mesh can be afforded, including nanoscale needles, flowers, and sheets. Even though the Cu mesh itself can be also transformed to a Cu-Oxide mesh, the growth of CuCo2O4 on the Cu mesh significantly improves its physical, chemical, and electrochemical properties, making these CuCo2O4@Cu meshes much more superior catalysts for activating Oxone to degrade the Azo toxicant, Acid Red 27. More interestingly, the flower-like CuCo2O4@Cu mesh exhibits a higher specific surface area and more superior electrochemical performance, enabling the flower-like CuCo2O4@Cu mesh to show the highest catalytic activity for Oxone activation to degrade Acid Red 27. The flower-like CuCo2O4@Cu mesh also exhibits a much lower Ea of Acid Red 27 degradation than the reported catalysts. These results demonstrate that CuCo2O4@Cu meshes are advantageous heterogeneous catalysts for Oxone activation, and especially, the flower-like CuCo2O4@Cu mesh appears as the most effective CuCo2O4@Cu mesh to eliminate the toxic Acid Red 27.

Tuning nanostructured CuCo2O4 on 3D macro-support for enhanced degradation of carbofuran via catalytic activation monopersulfate: Key roles of morphology and active species

While a few OH-based oxidation technologies have been used for eliminating the highly toxic pesticide Carbofuran (CFR), very few studies have ever assessed SO4−-based oxidation technologies for degrading CFR. Even though monopersulfate (MPS) is increasingly used for producing SO4− and cobalt (Co) is the most useful element for activating MPS, almost no study has ever been implemented to evaluate cobaltic catalysts for activating MPS to degrade CFR. Thus, the goal of this study is to develop Co-based catalysts for activating MPS to degrade CFR. In particular, a hierarchic composite which is fabricated based on macroscale Cu foam (CF) with the decoration of nanoscale Co-based oxide is fabricated to combine the macroscale practicability, and nanoscale functionality of Co-based oxide. The modification of CF with Co would grow CuCo2O4 directly on CF, leading to the formation of CuCo2O4@CF (CCO@CF). Moreover, different cobalt salts and dosages of urea might alter the formation of CuCo2O4, leading to various morphologies and nanostructures, such as CCO@CF with filament (CCF), CCOF@CF with thorn (CCT), and CCOF@CF with sheet (CCS). Interestingly, CCS exhibited the faster interfacial reaction rate, lower charge transfer resistance, and more abundant Co2+ sites, and oxygen vacancy, enabling CCS to show the highest catalytic activity for MPS activation to degrade CFR among these three CCO@CFs. CCS also exhibits a much lower Ea of CFR degradation than CCF, CCT, and CuOF, revealing its advantage over other catalysts. Moreover, the degradation mechanism of CFR by CCO@CF-activated MPS has been also elucidated using the tests of radical probes, and electron paramagnetic resonance for identifying contributions of OH, SO4−, and 1O2 to CFR degradation. These results validate that CCO@CFs are useful heterogeneous catalysts for MPS activation, and especially, CCS appears as the most favorable CCO@CF to eliminate the toxic CFR.

Interstitial nitrogen-induced efficiency alcohol oxidation over heterogeneous N–CoMn2O4 catalyst under visible-light

Developing highly efficient and recyclable photocatalysts through harvesting solar light as energy is crucial to oxidation for industrial implementation, especially for simple transition metal oxidic catalysts without precious/heavy/rare metal dopants. Herein, we like to report the use of nitrogen-doped CoMn2O4 oxide (N–CoMn2O4) as a heterogeneous catalyst for efficient oxidation of various alcohols such as p/m/o-methyl-substituted aromatic alcohols, p-substituted aromatic alcohols including electron-donating and electron-withdrawing substituents, heterocycle-based alcohols and secondary aromatic alcohols to the corresponding aldehydes/ketones, under visible light (>420 ​nm) illumination and mild condition of oxygen as oxidant and room temperature. The relation of various Co-based oxides to their catalytic performance was studied. It is shown that the Co2+ species in N–CoMn2O4, obviously increased by the doping of nitrogen, are acted as catalytic active species coupled with the synergistic effect between Co and Mn species for the enhanced visible-light selective oxidation of alcohols to aldehydes. A plausible catalytic mechanism is proposed basis of control experiments and published studies, which suggests that this oxidation process probably occurs on Co2+ sites via an ionic reactive oxygen species pathway and 1O2 and O2·− species are the reactive oxygen species. This simple transition metal oxide-catalyzed aerobic oxidation provides a green alternative for the manufacture of aldehydes/ketones from alcohols.

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.

Co2.8Mg0.2O4 as a promising thermochemical energy storage material with lower reduction onset temperature and higher energy density

Co-based oxides have been considered as one of the most promising materials for thermochemical energy storage (TCES) systems, however, the high operation temperature limits their applications. Specially, when Co3O4-based materials are used in concentrated solar power (CSP) system, a large mirror field area is required to meet the high endothermic temperature, which significantly increases the cost of a CSP system. In addition, the high cost of Co-based materials also limits its application. Improving the energy density can effectively reduce the use of the materials. Thus, in this work, to reduce the reduction onset temperature and increase the energy density of Co-based oxides, the TCES performance of Co3-xMgxO4 oxides is investigated. Co2.8Mg0.2O4 shows the best TCES performance. The reduction onset temperature of Co2.8Mg0.2O4 is ∼775.6 °C, which is 133.7 °C lower than that of the pure Co3O4 (∼909.3 °C). Mg-decoration can also improve the energy density of Co3O4 by 8.02 %. Furthermore, the in situ XRD spectra of the oxides indicate that precipitation of MgO from the Co3O4 lattice during the reduction reaction result in a metastable structure of the oxide, which is conducive to its reduction reaction. Density functional theory calculations indicate that Co2.8Mg0.2O4 possessed a higher defect formation energy and shorter chemical bonds than the pure cobalt oxide, which result in its lower reduction onset temperature and higher energy density. The cost-benefit analysis results showed that Mg-decoration could significantly reduce the area of mirror field and the use of energy storage material for a CSP system.

Insights into Boosting SO2 Tolerance for Catalytic Oxidation of Propane over Fe2O3-Promoted Co3O4/Halloysite Catalysts

In the catalytic oxidation of volatile organic compounds, Co-based oxide catalysts show excellent potential and draw more and more attention, but SO2 poisoning in actual applications is one of the key problems to be solved. In this work, a series of treated halloysite (HLS)-supported Fe-modified Co oxide catalysts were prepared. In the catalytic oxidation of propane, the catalysts with 15 wt % Co loading amounts showed the best catalytic activity, while 2 wt % Fe modifications showed the best promotional effects. The introduction of Fe not only improved the catalyst activity but also enhanced the tolerance to SO2 poisoning. Various characterizations of physical–chemical properties, in situ diffuse reflectance infrared transform spectroscopy, and density functional theory calculation results showed that the active components of Co/HLS catalysts reacted with SO2 to form sulfate species, which could not provide sufficient catalytic activity afterward. The Fe species in Fe–Co/HLS catalysts could reduce the strong adsorption of SO2 on Co oxides and inhibit the formation of stable sulfate species, thus effectively reducing the effect of SO2 poisoning on the catalysts. Furthermore, the adsorption of reactants and the formation of intermediates proceeded on the surface of Fe–Co/HLS catalysts with a greater efficiency, and SO2 on the surface mainly existed in the form of surface-adsorbed SO2 species. Only a small proportion of SO2 could form SO3 species and sulfate species, thus achieving the purpose of protecting the active sites and maintaining the catalytic activity. The present study of halloysite-supported Fe-modified Co oxide catalysts would provide help for the development of Co oxide catalysts with excellent catalytic performance and tolerance to SO2 poisoning.

Investigating Perovskite Oxide Catalysts As Bifunctional Oxygen Electrodes Using Operando XAS

With the increase in renewable energy usage comes the need for energy storage systems due to intermittency issues. Hydrogen storage systems have been identified as one solution. Unitized regenerative fuel cells (URFC) combine electrolyzers and fuel cells in one device, allowing electricity to be stored and used easily. However, the oxygen electrodes are still affected by high overpotentials and slow kinetics. Perovskite oxides have been identified as a class of materials, which are low-cost, tunable, and active for the oxygen reduction (ORR) and evolution (OER) reactions. Here, we investigate perovskites as bifunctional catalysts for ORR and OER in alkaline solution. We examine and compare two strategies for bifunctional catalysts: using one catalyst, which is able to perform OER and ORR vs. a combination of two catalysts, one active for ORR and one active for OER. Frequently, the catalysts’ performances for these two reactions are measured separately.1,2,3 Here, we investigate how these bifunctional catalysts respond to cycling between the OER and ORR regions.Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) is known to be a promising OER catalyst.4,5,6 However, without carbon, it lacks ORR activity.4 La(1-x)SrxMnO3 (LSM) is a promising ORR catalyst.3,7 However, without modification, it has been shown to have limited OER activity.3 Separately, these catalysts lack high performance for both reactions. Here, we combine the two catalysts into a BSCF/LSM/Carbon composite electrode and compare to electrodes prepared from the constituent single material components. In addition, we have synthesized single material perovskites containing both Co and Mn that to the best of our knowledge have never been tested as electrodes for ORR/OER. In order to understand the catalysts’ behaviors under OER and ORR conditions, X-ray adsorption spectroscopy (XAS) was measured continuously while performing cyclic voltammetry. We were able to monitor the continuous changes of the Co, Mn, and Fe oxidation states and local environment during OER and ORR with remarkably high time/applied potential resolution. Our findings illustrate the reversible and irreversible changes that can occur during OER and ORR and provide strategies for future bifunctional catalyst design.References Kirsanova, M. A.; Okatenko, V. D.; Aksyonov, D. A.; Forslund, R. P.; Mefford, J. T.; Stevenson, K. J.; Abakumov, A. M. Bifunctional OER/ORR Catalytic Activity in the Tetrahedral YBaCo 4 O 7.3 Oxide. Mater. Chem. A 2019, 7 (1), 330–341.Elumeeva, K.; Masa, J.; Sierau, J.; Tietz, F.; Muhler, M.; Schuhmann, W. Perovskite-Based Bifunctional Electrocatalysts for Oxygen Evolution and Oxygen Reduction in Alkaline Electrolytes. Acta 2016, 208, 25–32.Xu, W.; Apodaca, N.; Wang, H.; Yan, L.; Chen, G.; Zhou, M.; Ding, D.; Choudhury, P.; Luo, H. A-Site Excessive (La0.8Sr0.2)1+ XMnO3 Perovskite Oxides for Bifunctional Oxygen Catalyst in Alkaline Media. ACS Catal. 2019, 9 (6), 5074–5083.Fabbri, E.; Nachtegaal, M.; Cheng, X.; Schmidt, T. J. Superior Bifunctional Electrocatalytic Activity of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ /Carbon Composite Electrodes: Insight into the Local Electronic Structure. Energy Mater. 2015, 5 (17), 1402033.Fabbri, E.; Nachtegaal, M.; Binninger, T.; Cheng, X.; Kim, B.-J.; Durst, J.; Bozza, F.; Graule, T.; Schäublin, R.; Wiles, L.; Pertoso, M.; Danilovic, N.; Ayers, K. E.; Schmidt, T. J. Dynamic Surface Self-Reconstruction Is the Key of Highly Active Perovskite Nano-Electrocatalysts for Water Splitting. Mater. 2017, 16 (9), 925–931.Kim, B. J.; Fabbri, E.; Abbott, D. F.; Cheng, X.; Clark, A. H.; Nachtegaal, M.; Borlaf, M.; Castelli, I. E.; Graule, T.; Schmidt, T. J. Functional Role of Fe-Doping in Co-Based Perovskite Oxide Catalysts for Oxygen Evolution Reaction. Am. Chem. Soc. 2019, 141 (13), 5231–5240.Tulloch, J.; Donne, S. W. Activity of Perovskite La1−xSrxMnO3 Catalysts towards Oxygen Reduction in Alkaline Electrolytes. Power Sources 2009, 188 (2), 359–366.

Recent progress in the development of synthetic oxygen carriers for chemical looping combustion applications

Continuously rising concentrations of CO2 in the atmosphere has led to significant negative impacts on the environment which propelled research efforts to replace fossil fuel-based energy with clean energy sources. However, the reliance on fossil fuel as the main energy source is projected to stay for many years to come, and therefore developing cleaner pathways to utilize fossil fuels for energy generation is paramount. Chemical looping combustion (CLC) has emerged in recent years as a promising technology for CO2 capture in power plants and other CO2 intensive industries. A key cornerstone for CLC is the development of efficient oxygen carriers with high oxygen capacity, fast kinetics, cyclic stability, mechanical strength, and low cost. The research progress in this field is rapidly growing to tackle the major materials and operational challenges to advance the knowledge toward a commercial scale CLC application. This work provides an overview of the significant advancements achieved over the past seven years in the development of synthetic metal oxide-based oxygen carriers. A summary of key performance indicators used to evaluate oxygen carriers is presented in this study to enable for a systematic assessment of the oxygen carriers’ properties and performance. This work is focused on summarizing and critically reviewing the literature on the development of oxygen carriers such as Ni, Cu, Fe, Mn, Co-based metal oxides and combined/mixed oxygen carriers in the past seven years. Studies of naturally occurring materials and waste materials used as oxygen carriers are excluded from this review which is mainly focused on synthetically prepared materials. Significant efforts were dedicated to investigating the role of the supports, synthesis methods, promotors, and other factors affecting the CLC performance of synthetic oxygen carriers using solid, liquid, and gaseous fuels. This review also outlines the main challenges, research needs and opportunities for future progress to highlight potential pathways to develop synthetic oxygen carriers in an environmentally friendly, and cost-effective manner.

Hollow carbon sphere-supported Pt/CoOx hybrid with excellent hydrogen evolution activity and stability in acidic environment

Electrocatalytic hydrogen evolution reaction (HER) in acidic solution using Co-based oxides is extremely difficult due to their instability. Herein, we report a hollow carbon sphere-supported Pt/CoOx (Pt/CoOx-HCS) electrocatalyst for active HER in 0.5 M H2SO4 solution. Microstructural and electrochemical evaluations confirm the dissolution of major Co3O4 in Pt/Co3O4-loaded HCS (pre-Pt/Co3O4-HCS) and the steady existence of remaining CoOx in Pt/CoOx-HCS after 3000 cyclic voltammetry (CV) cycles (Pt/CoOx-HCS-3000). The Pt/CoOx-HCS-3000 exhibits a pretty high turnover frequency (TOF) of 24.87 H2 s−1 at 100 mV overpotential, a low overpotential of 28 mV at 10 mA cm−2, and a large mass activity of 3.59 A mgPt−1 at 30 mV overpotential. The robust stability of Pt/CoOx-HCS-3000 is attributed to the strong electronic interaction between Pt and CoOx, preventing the aggregation of the former and the thorough dissolution of the latter. This work provides insights into the design of Co based-oxide electrocatalysts with outstanding acidic performances.