Cobalt-Doped Perovskite-Type Oxide LaMnO3 as Bifunctional Oxygen Catalysts for Hybrid Lithium-Oxygen Batteries

Development of In Situ Methods for Preparing La-Mn-Co-Based Compounds over Carbon Xerogel for Oxygen Reduction Reaction in an Alkaline Medium

Metal oxides containing La, Mn, and Co cations can catalyze oxygen reduction reactions (ORRs) in electrochemical processes. However, these materials require carbon support and optimal interactions between both compounds to be active. In this work, two approaches to prepare composites of La-Mn-Co-based compounds over carbon xerogel were developed. Using sol-gel methods, either the metal-based material was deposited on the existing carbon xerogel or vice versa. The metal oxide selected was the LaMn0.7Co0.3O3 perovskite, which has good catalytic behavior and selectivity towards direct ORRs. All the as-prepared composites were tested for ORRs in alkaline liquid electrolytes and characterized by diverse physicochemical techniques such as XRD, XPS, SEM, or N2 adsorption. Although the perovskite structure either decomposed or failed to form using those in situ methods, the materials exhibited great catalytic activity, which can be ascribed to the strengthening of the interactions between oxides and the carbon support via C-O-M covalent bonds and to the formation of new active sites such as the MnO/Co heterointerfaces. Moreover, Co-Nx-C species are formed during the synthesis of the metal compounds over the carbon xerogel. These species possess a strong catalytic activity towards ORR. Therefore, the composites formed by synthesizing metal compounds over the carbon xerogel exhibit the best performance in the ORR, which can be ascribed to the presence of the MnO/Co heterointerfaces and Co-Nx-C species and the strong interactions between both compounds. Moreover, the small nanoparticle size leads to a higher number of active sites available for the reaction.

Enhanced Photocatalytic Water Splitting of SrTiO3 Perovskite through Cobalt Doping: Experimental and Theoretical DFT Understanding

Throughout extensive research endeavors, SrTiO3 has emerged as a promising photocatalytic material for utilizing solar energy and facilitating hydrogen production via water splitting. Yet, the pursuit of enhanced efficiency and amplified hydrogen generation has prompted researchers to delve into the realm of advanced doping strategies. In this work, using experimental characteristics and DFT calculations, we studied the effect of cobalt substitution on the structural, electronic, optical, and magnetic properties as well as the photocatalytic activity of SrTi1-xCoxO3-δ (x = 0, 0.125, 0.25, 0.375, and 0.5) perovskites. The samples were successfully prepared by using the solid-state reaction method. Based on X-ray diffraction and the Rietveld refinement method, the elaborated samples were shown to preserve the absorption range up to the visible region. Moreover, the position of band edge levels after cobalt doping becomes more appropriate for water splitting. Our findings report that all cobalt-doped compounds exhibit good photocatalytic activities and could be used as suitable photocatalyst materials for hydrogen production.

Transition metal oxides with perovskite and spinel structures for electrochemical energy production applications

Transition metal oxide-based materials are an interesting alternative to substitute noble-metal based catalyst in energy conversion devices designed for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution reactions (HER). Perovskite (ABO₃) and spinel (AB₂O₄) oxides stand out against other structures due to the possibility of tailoring their chemical composition and, consequently, their properties. Particularly, the electrocatalytic performance of these materials depends on features such as chemical composition, crystal structure, nanostructure, cation substitution level, e_g orbital filling or oxygen vacancies. However, they suffer from low electrical conductivity and surface area, which affects the catalytic response. To mitigate these drawbacks, they have been combined with carbon materials (e.g. carbon black, carbon nanotubes, activated carbon, and graphene) that positively influence the overall catalytic activity. This review provides an overview on tunable perovskites (mainly lanthanum-based) and spinels featuring 3d metal cations such as Mn, Fe, Co, Ni and Cu on octahedral sites, which are known to be active for the electrochemical energy conversion.

Cu-doped CaFeO3 perovskite oxide as oxygen reduction catalyst in air cathode microbial fuel cells

Cathode electrocatalyst is quite critical to realize the application of microbial fuel cells (MFCs). Perovskite oxides have been considered as potential MFCs cathode catalysts to replace Pt/C. Herein, Cu-doped perovskite oxide with a stable porous structure and excellent conductivity was successfully prepared through a sol-gel method. Due to the incorporation of Cu, CaFe_0.9Cu_0.1O₃ has more micropores and a larger surface area, which are more conducive to contact with oxygen. Doping Cu resulted in more Fe³⁺ in B-site and thus enhanced its binding capability to oxygen molecules. The data from electrochemical test demonstrated that the as-prepared catalyst has good conductivity, high stability, and excellent ORR properties. Compared with Pt/C catalyst, CaFe_0.9Cu_0.1O₃ exhibits a lower overpotential, which had an onset potential of 0.195 V and a half-wave potential of -0.224 V, respectively. CaFe_0.9Cu_0.1O₃ displays an outstanding four-electron pathway for ORR mechanism and demonstrates superiors corrosion resistance and stability. The MFC with CaFe_0.9Cu_0.1O₃ has a greater maximum power density (1090 mW m^-3) rather than that of Pt/C cathode (970 mW m^-3). This work demonstrated CaFe_0.9Cu_0.1O₃ is an economic and efficient cathodic catalyst for MFCs.

Highly graphited carbon-coated FeTiO3 nanosheets in situ derived from MXene: an efficient bifunctional catalyst for Zn-air batteries

Developing high-efficiency and low-cost catalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is of great significance for the commercialization of rechargeable metal-air batteries. Herein, we demonstrated the construction of graphited carbon-coated FeTiO₃ (FeTiO₃@C) via in situ annealing Ti₃C₂T_x nanosheets in a rusted-reactor and its efficient bifunctional activity for rechargeable Zn-air batteries (RZABs). The electron-transport dynamics of FeTiO₃@C can be improved by using highly conductive graphited carbon derived from Ti₃C₂T_x. The FeTiO₃@C catalyst annealed at 500 °C exhibits excellent OER and ORR activities. Specifically, FeTiO₃@C shows a low overpotential of 323 mV at 10 mA cm^-2 and a small Tafel slope of 53 mV dec^-1 towards the alkaline OER. During the OER process, FeTiO₃@C can be partially converted into highly active iron oxyhydroxide via in situ electrochemical reconstruction, which serves as the active species. After being assembled to RZABs, it shows an open-circuit potential of 1.33 V, a high trip efficiency of 63.4% and long-time cycling stability. This work can provide a new avenue for developing bifunctional electrocatalysts for RZABs used in portable devices.

Electrocatalytic activity of calcined manganese ferrite solid nanospheres in the oxygen reduction reaction

In this study, we synthesized MnFe₂O₄ solid nanospheres (MSN) calcined at different temperatures (200-500 °C) and MSN-based materials mixed with carbon black, for their use as electrocatalysts in the oxygen reduction reaction (ORR) in alkaline medium (0.1 M KOH). It was demonstrated that the calcination temperature of MSN material determined its chemical surface composition and microstructure and it had an important effect on the electrocatalytic properties for ORR, which in turn was reflected in the performance of MSN/CB-based electrocatalysts. The study revealed that the presence of Mn species plays a key role in the ORR activity. Among tested, MSN200/CB and MSN350/CB exhibited the best electrochemical performances together with outstanding stability.

Ru-incorporated Co3O4 nanoparticles from self-sacrificial ZIF-67 template as efficient bifunctional electrocatalysts for rechargeable metal-air battery

Ru-incorporated Co₃O₄ nanoparticles have been synthesized from self-sacrificial ZIF-67 template and utilized as efficient electrocatalysts towards oxygen reduction and evolution reactions (ORR and OER). Amongst, Ru@Co₃O₄-1.0 exhibited the optimum electrocatalytic behavior with an ultra-low potential gap (0.84 V) between the OER potential (1.61 V at 10 mA cm^-2) and ORR half-wave potential (0.77 V). The zinc-air battery using Ru@Co₃O₄-1.0 as a cathode presented high specific capacity (788.1 mAh g^-1) and power density (101.2 mW cm^-2). Meanwhile, this battery possessed relatively lower voltage gap and higher cycling stability compared with the commercial Pt/C-based one. Ruthenium incorporation induced remarkable lattice expansion of Co₃O₄ and engineered more oxygen vacancies, promoting the lattice oxygen mobility from the subsurface/bulk phase onto surface. All these properties were recognized to be the crucial parameters for electrocatalytic activity improvement. This work provided a facile approach to design highly active metal oxide with broad potentiality for rechargeable metal-air batteries.

Recent advances in the heteroatom doping of perovskite oxides for efficient electrocatalytic reactions

Perovskite-type transition metal oxides have emerged as promising electrocatalysts for various electrocatalytic reactions owing to their low cost, compositional tunability and high stability. However, insufficient electrocatalytic activities of pristine perovskite oxides hinder their pathway towards real-world applications. The incorporation of heteroatoms into perovskite oxide structures has been regarded as an efficient way to improve the electrocatalytic performance. This minireview summarizes the recent advances in the heteroatom doping of perovskite oxides as efficient electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). These heteroatom doping strategies are classified based on various types of doping sites. The mechanisms of improved electrocatalytic activities are discussed in detail within different doping sites and various kinds of dopants. Finally, the remaining challenges and perspectives are outlined for future developments of perovskite oxide-based catalysts.

Influence of the Amount of Carbon during the Synthesis of LaFe0.8Co0.2O3/Carbon Hybrid Material in Oxygen Evolution Reaction

The oxygen evolution reaction (OER) and the hydrogen evolution reaction occurred at the anode and cathode, which depends on the electronic structure, morphology, electrochemically active surface area, and charge-transfer resistance of the electrocatalyst. Transition metals like cobalt, nickel, and iron have better OER and oxygen reduction reaction activities. At the same time, transition-metal oxide/carbon hybrid has several applications in electrochemical energy conversion reactions. The rich catalytic site of transition metals and the excellent conductivity of carbon material make these materials as a hopeful electrocatalyst in OER. Carbon-incorporated LaFe_0.8Co_0.2O₃ was prepared by a simple solution combustion method for the development of the best performance of the electrocatalyst. The catalyst can deliver 10 mA/cm² current density at an overpotential of 410 mV with better catalytic stability. The introduction of carbon material improves the dispersion ability of the catalyst and the electrical conductivity. The Tafel slope and onset potential of the best catalyst are 49.1 mV/dec and 1.55 V, respectively.

High-Efficiency of Bi-Functional-Based Perovskite Nanocomposite for Oxygen Evolution and Oxygen Reduction Reaction: An Overview

High efficient, low-cost and environmentally friendly-natured bi-functional-based perovskite electrode catalysts (BFPEC) are receiving increasing attention for oxygen reduction/oxygen evolution reaction (ORR/OER), playing an important role in the electrochemical energy conversion process using fuel cells and rechargeable batteries. Herein, we highlighted the different kinds of synthesis routes, morphological studies and electrode catalysts with A-site and B-site substitution co-substitution, generating oxygen vacancies studies for boosting ORR and OER activities. However, perovskite is a novel type of oxide family, which shows the state-of-art electrocatalytic performances in energy storage device applications. In this review article, we go through different types of BFPECs that have received massive appreciation and various strategies to promote their electrocatalytic activities (ORR/OER). Based on these various properties and their applications of BFPEC for ORR/OER, the general mechanism, catalytic performance and future outlook of these electrode catalysts have also been discussed.

Graphene Nanosheet-Wrapped Mesoporous La0.8Ce0.2Fe0.5Mn0.5O3 Perovskite Oxide Composite for Improved Oxygen Reaction Electro-Kinetics and Li-O2 Battery Application

A novel design and synthesis methodology is the most important consideration in the development of a superior electrocatalyst for improving the kinetics of oxygen electrode reactions, such as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in Li-O₂ battery application. Herein, we demonstrate a glycine-assisted hydrothermal and probe sonication method for the synthesis of a mesoporous spherical La_0.8Ce_0.2Fe_0.5Mn_0.5O₃ perovskite particle and embedded graphene nanosheet (LCFM(8255)-gly/GNS) composite and evaluate its bifunctional ORR/OER kinetics in Li-O₂ battery application. The physicochemical characterization confirms that the as-formed LCFM(8255)-gly perovskite catalyst has a highly crystalline structure and mesoporous morphology with a large specific surface area. The LCFM(8255)-gly/GNS composite hybrid structure exhibits an improved onset potential and high current density toward ORR/OER in both aqueous and non-aqueous electrolytes. The LCFM(8255)-gly/GNS composite cathode (ca. 8475 mAh g⁻¹) delivers a higher discharge capacity than the La_0.5Ce_0.5Fe_0.5Mn_0.5O₃-gly/GNS cathode (ca. 5796 mAh g⁻¹) in a Li-O₂ battery at a current density of 100 mA g⁻¹. Our results revealed that the composite’s high electrochemical activity comes from the synergism of highly abundant oxygen vacancies and redox-active sites due to the Ce and Fe dopant in LaMnO₃ and the excellent charge transfer characteristics of the graphene materials. The as-developed cathode catalyst performed appreciable cycle stability up to 55 cycles at a limited capacity of 1000 mAh g⁻¹ based on conventional glass fiber separators.

Carbon Material and Cobalt-Substitution Effects in the Electrochemical Behavior of LaMnO3 for ORR and OER

LaMn_1−xCo_xO₃ perovskites were synthesized by a modified sol-gel method which incorporates EDTA. These materials’ electrochemical activity towards both oxygen reduction (ORR) and oxygen evolution reactions (OER) was studied. The cobalt substitution level determines some physicochemical properties and, particularly, the surface concentration of Co and Mn’s different oxidation states. As a result, the electroactivity of perovskite materials can be tuned using their composition. The presence of cobalt at low concentration influences the catalytic activity positively, and better bifunctionality is attained. As in other perovskites, their low electrical conductivity limits their applicability in electrochemical devices. It was found that the electrochemical performance improved significantly by physically mixing with a mortar the active materials with two different carbon black materials. The existence of a synergistic effect between the electroactive component and the carbon material was interpreted in light of the strong carbon–oxygen–metal interaction. Some mixed samples are promising electrocatalysts towards both ORR and OER.

Bifunctional LaMn0.3Co0.7O3 Perovskite Oxide Catalyst for Oxygen Reduction and Evolution Reactions: The Optimized eg Electronic Structures by Manganese Dopant

Perovskite oxides as bifunctional electrocatalysts toward oxygen reduction (ORR) and oxygen evolution reactions (OER) have been investigated for decades because of the flexible and adjustable electronic structures. For example, by optimizing the strength of the Co-O bond, the ORR and OER activity of a typical perovskite oxide, LaCoO₃, can be improved, but they are still unsatisfying. The insufficient insights into the effects of secondary metal dopants at the B-site on the electronic structure and activity, especially for ORR, significantly limit the R&D of bifunctional perovskite oxide catalysts. In this work, a series of LaMn_xCo_1-xO₃ (x = 0, 0.25, 0.3, 0.35, 0.5, 1) catalysts are prepared by a polyol-assisted solvothermal method to investigate the structure-property relationships between the B-site metal substitution and the electrochemical performance of perovskite oxides catalysts. The optimized LaMn_0.3Co_0.7O₃ catalyst demonstrates an enhanced half-wave potential of 0.72 V for ORR, 52 mV higher than that of the pristine LaCoO₃ (0.668 V). Meanwhile, the OER overpotential of LaMn_0.3Co_0.7O₃ catalyst is 416 mV, which is reduced by 64 mV compared to LaCoO₃ (480 mV). It is revealed that the appropriate Mn dopant efficiently optimizes the covalency of Co-O bonds and significantly reduces the e_g orbit-filling electron from 1.23 of pristine LaCoO₃ to 1.02 in LaMn_0.3Co_0.7O₃ (very close to theoretical value 1). This work paves a new way to design and synthesize bifunctional perovskite oxide electrocatalyst for ORR and OER.

Rare-earth-containing perovskite nanomaterials: design, synthesis, properties and applications

As star material, perovskites have been widely used in the fields of optics, photovoltaics, electronics, magnetics, catalysis, sensing, etc. However, some inherent shortcomings, such as low efficiency (power conversion efficiency, external quantum efficiency, etc.) and poor stability (against water, oxygen, ultraviolet light, etc.), limit their practical applications. Downsizing the materials into nanostructures and incorporating rare earth (RE) ions are effective means to improve their properties and broaden their applications. This review will systematically summarize the key points in the design, synthesis, property improvements and application expansion of RE-containing (including both RE-based and RE-doped) halide and oxide perovskite nanomaterials (PNMs). The critical factors of incorporating RE elements into different perovskite structures and the rational design of functional materials will be discussed in detail. The advantages and disadvantages of different synthesis methods for PNMs will be reviewed. This paper will also summarize some practical experiences in selecting suitable RE elements and designing multi-functional materials according to the mechanisms and principles of REs promoting the properties of perovskites. At the end of this review, we will provide an outlook on the opportunities and challenges of RE-containing PNMs in various fields.

Revealing Energetics of Surface Oxygen Redox from Kinetic Fingerprint in Oxygen Electrocatalysis

The key step for rational catalyst design in heterogeneous electrocatalysis is to reveal the distinctive energy profile of redox reactions of a catalyst that give rise to specific activity. However, it is challenging to experimentally obtain the energetics of oxygen redox in oxygen electrocatalysis because of the liquid reaction environment. Here we develop a kinetic model that constructs a quantitative relation between the energy profile of oxygen redox and electrochemical kinetic fingerprints. The detailed study here demonstrates that the kinetic fingerprints observed from experiments can be well described by different energetics of oxygen redox. On the basis of the model, a feasible methodology is demonstrated to derive binding energies of the oxygen intermediates from electrochemical data. The surface property of different catalysts derived from our model well rationalizes the experimental trends and predicts potential directions for catalyst design.

Oxygen Reduction Reaction Electrocatalysis in Alkaline Electrolyte on Glassy-Carbon-Supported Nanostructured Pr6O11 Thin-Films

In this work, hierarchical nanostructured Pr6O11 thin-films of brain-like morphology were successfully prepared by electrostatic spray deposition (ESD) on glassy-carbon substrates. These surfaces were used as working electrodes in the rotating disk electrode (RDE) setup and characterized in alkaline electrolyte (0.1 M NaOH at 25 ± 2 °C) for the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) for their potential application in alkaline electrolyzers or in alkaline fuel cells. The electrochemical performances of these electrodes were investigated as a function of their crystallized state (amorphous versus crystalline). Although none of the materials display spectacular HER and OER activity, the results show interesting performances of the crystallized sample towards the ORR with regards to this class of non-Pt group metal (non-PGM) electrocatalysts, the activity being, however, still far from a benchmark Pt/C electrocatalyst.