Oxygen Reduction Evolution Research Articles

Recovery of LiNi0.5Mn0.3Co0.2O2 cathode material from spent lithium-ion batteries with oxygen evolution reduction in ammonium sulfate low-temperature molten salt

The advancement and extensive application of lithium-ion batteries (LIBs) as an energy storage device has led to an increase in the number of spent LIBs, and recycling spent LIBs is an urgent need to address their resource and environmental sustainability issues. Herein, an efficient and comprehensive recycling method for spent LiNi0.5Mn0.3Co0.2O2 cathode material (NMC 532), one of the most important materials for electric vehicles and grid storage applications, is developed based on ammonium sulfate low-temperature molten salt (LTMS) reduction with oxygen evolution, and the feasibility of the strategy is systematically investigated. The results demonstrate that (NH4)2SO4 LTMS can effectively realize the sulfate conversion of spent NMC 532, in which the sulfate conversion rates of Li, Ni, Mn, and Co all reach more than 90%. Through mechanism analysis, it is found that the induced superoxide ion (O2·-) and H+ are the keys to the reduction of transition metal oxides in LTMS. Meanwhile, O2·-, as a single electron reducing agent in the intermediate state, can be used to explain the reduction behavior of transition metal oxides in other ammonium salts, mineral acids, and H-containing salts in molten. Based on (NH4)2SO4 LTMS reduction, water leaching, thermodynamic analysis of coprecipitation, and a series of precipitation and separation experiments, this study provides an effective strategy for recycling spent cathode materials, which can realize the combination of spent material recycling and electrode material regeneration.

Metal-Doped C3B Monolayer as the Promising Electrocatalyst for Hydrogen/Oxygen Evolution Reaction: A Combined Density Functional Theory and Machine Learning Study.

The development of high-efficiency electrocatalysts for hydrogen evolution reduction (HER)/oxygen evolution reduction (OER) is highly desirable. In particular, metal borides have attracted much attention because of their excellent performances. In this study, we designed a series of metal borides by doping of a transition metal (TM) in a C3B monolayer and further explored their potential applications for HER/OER via density functional theory (DFT) calculations and machine learning (ML) analysis. Our results revealed that the |ΔG*H| values of Fe-, Ag-, Re-, and Ir-doped C3B are approximately 0.00 eV, indicating their excellent HER performances. On the other hand, among all the considered TM atoms, the Ni- and Pt-doped C3B exhibit excellent OER activities with the overpotentials smaller than 0.44 V. Together with their low overpotentials for HER (<0.16 V), we proposed that Ni/C3B and Pt/C3B could be the potential bifunctional electrocatalysts for water splitting. In addition, the ML method was employed to identify the important factors to affect the performance of the TM/C3B electrocatalyst. Interestingly, the results showed that the OER performance is closely related to the inherent properties of TM atoms, i.e., the number of d electrons, electronegativity, atomic radius, and first ionization energy; all these values could be directly obtained without DFT calculations. Our results not only proposed several promising electrocatalysts for HER/OER but also suggested a guidance to design the potential TM-boron (TM-B)-based electrocatalysts.

Interfacial Engineering of Leaf-like Bimetallic MOF-Based Co@NC Nanoarrays Coupled with Ultrathin CoFe-LDH Nanosheets for Rechargeable and Flexible Zn-Air Batteries.

Exploring high-efficiency, low-cost, and long-life bifunctional self-supporting electrocatalysts is of great significance for the practical application of advanced rechargeable Zn-air batteries (ZABs), especially flexible solid-state ZABs. Herein, ultrathin CoFe-layered double hydroxide (CoFe-LDH) nanosheets are strongly coupled on the surface of leaf-like bimetallic metal-organic frameworks (MOFs)-derived hybrid carbon (Co@NC) nanoflake nanoarrays supported by carbon cloth (CC) via a facile and scalable method for rechargeable and flexible ZABs. This interfacial engineering for CoFe-LDHs on Co@NC improves the electronic conductivity of CoFe-LDH nanosheets as well as achieves the balance of oxygen evolution reduction (OER) and oxygen reduction reaction (ORR) activity. The unique three-dimensional (3D) open interconnected hierarchical structure facilitates the transport of substances during the electrochemical process while ensuring adequate exposure of OER/ORR active centers. When applied as an additive-free air cathode in rechargeable liquid ZABs, CC/Co@NC/CoFe-LDH-700 demonstrates high open-circuit potential of 1.47 V, maximum power density of 129.3 mW cm-2, and satisfactory specific capacity of 710.7 mAh g-1Zn. Further, the flexible all-solid-state ZAB assembled by CC/Co@NC/CoFe-LDH-700 displays gratifying mechanical flexibility and stable cycling performance over 40 h. More significantly, the series-connected flexible ZAB is further verified as a chain power supply for LED strips and performs well throughout the bending process, showing great application prospects in portable and wearable electronics. This work sheds new light on the design of high-performance self-supporting non-precious metal bifunctional electrocatalysts for OER/ORR and air cathodes for rechargeable ZABs.

Electron spin modulation engineering in oxygen-involved electrocatalysis

Electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reduction (OER) are regarded as the key reactions via the sustainable system (fuel cell and water splitting), respectively. In OER, the transition from singlet oxygen species to triplet oxygen molecules is involved, meanwhile the ORR involves the transition from triplet oxygen molecules to singlet oxygen species. However, in these processes, the number of unpaired electrons is not conserved, which is not thermodynamically favorable and creates an additional energy barrier. Fortunately, regulating the electrocatalysis by spin-state modulation enables a unique effect on the catalytic performance, but the current understanding on spin-state engineering for electro-catalyzing ORR and OER is still insufficient. Herein, this review summarized the in-spin engineering for the state-of-the-art ORR and OER electrocatalysts. It began by introducing engineering of spin-state to eg filling for ORR and OER process, and then moved to spin polarization and spin-pinning effect for OER process. Various designed strategies focusing on how to regulate the spin-state of the active center have been summarized up. The connectivity of the structures of typical ORR (e.g. metal–nitrogen–carbon) and OER (e.g. design strategies oxides, metal organic frameworks) catalysts depending on the spin level is also discussed. Finally, we present the outlook from the aspects of template catalysts, characterization methods, regulation strategies, theoretical calculations, which will further expand the possibility of better electrocatalytic performance through spin-state modulation. This review concluded some open suggestions and prospects, which are worthy of the community’s future work.

Doping and Vacancy Engineering in a Sandwich‐like g‐C3N4/NiCo2O4 Heterostructure for Robust Oxygen Evolution

AbstractHerein, N doping and O vacancy were simultaneously introduced into a g‐C3N4/NiCo2O4 heterostructure by a simple reflux and calcination strategy. The constructed g‐C3N4 displayed a porous lamellar structure with N‐doped carbon. Furthermore, numerous ultrathin NiCo2O4 nanosheets with abundant O vacancies tend to grow vertically on the porous g‐C3N4 lamellar, forming a sandwich‐like heterostructure. The prepared sandwich‐like g‐C3N4/NiCo2O4 heterostructure exhibited excellent oxygen evolution reduction (OER) activity with a low overpotential of 294 mV at 10 mA cm−2. Meanwhile, the heterostructure presented superior long‐term stability, retaining a current density of 97.9% after a 50 h chronoamperometry test. The outstanding OER performance is ascribed to the synergistic effect of doping and vacancy engineering in the catalyst, the strong coupling interaction between g‐C3N4 and NiCo2O4, and the porous sandwich‐like structure with abundant active sites. Therefore, the synthetic strategy of g‐C3N4/NiCo2O4 heterostructure can be extended to fabricate high‐performance noble metal‐free electrocatalysts for water splitting.

Facile and green synthesis cobalt embedded in N-doped porous carbon under zeo-waste conditions as an efficient oxygen evolution reduction catalyst

A facile and green method is developed to synthesize cobalt species embedded in a nitrogen-doped carbon matrix (P–Co@NCs). The materials are synthesized using hard template precursors via single-step in-situ thermal treatment under free solvent conditions. The optimal P–Co@NC composite prepared through rational synthesis and synergic composition shows an efficient electrocatalytic performance for oxygen evolution reduction (OER). The P–Co@NC exhibits the OER overpotential of 350 mv at 10 mA cm −2 and low Tafel 73 mv·dec −1 in 0.1 M KOH electrolyte. The advantage of catalyst is the dispersion of Co species, which are in close interaction with nitrogen-doped carbon matrix. The Co-Nx plays a dominating role among Co species in P–Co@NC and is well protected by the nitrogen-doped carbon matrix layer. That resulted in efficient electron transport on CNTs (high conductivity) that improves the OER performance of P–Co@NC compared to the reference catalysts (IrO 2 and B–Co@NC). The P–Co@NC also showed excellent cycling performance from long-term chronoamperometry testing. Interestingly, the P–Co@NC catalyst could be synthesized straightforward via a short solvent-free single-step process that introduced it as a green and environmental friendly method, which can be potential to develop catalyst based on metal embedded N-doped carbon. • The single-step, rapid and effective “green” route of the IST method to synthesize cobalt species embedded in nitrogen-doped carbon (P–Co@NC). • The advantages of economic practicability, minimization of synthesis period, energy consumption without any waste formation could outweigh the catalyst for OER performance. • The advantage of Co dispersion and close interaction with nitrogen-doped carbon matrix showed efficient catalytic performance. • The efficient electron transport on CNTs (high conductivity) improved the OER performance of P–Co@NC over the reference catalysts (IrO 2 and B–Co@NC).

Electrosynthesis of highly efficient WO3-x/graphene (photo-)electrocatalyst by two-electrode electrolysis system for oxygen evolution reaction

Integration of non-noble transition metal oxides with graphene is known to construct high-activity electrocatalysts for oxygen evolution reduction (OER). In order to avoid the complexity of traditional synthesis process, a facile electrochemical method is elaborately designed to engineer efficient WO3-x/graphene (photo-)electrocatalyst for OER by a two-electrode electrolysis system, where graphite cathode is exfoliated into graphene and tungsten wire anode evolves into VO-rich WO3-x profiting from formed reductive electrolyte solution. Among as-prepared samples, WO3-x/G-2 shows the best electrocatalytic performance for OER with an overpotential of 320 mV (without iR compensation) at 10 mA/cm2, superior to commercial RuO2 (341 mV). With introduction of light illumination, the activity of WO3-x/G-2 is greatly enhanced and its overpotential decreases to 290 mV, benefiting from additional reaction path produced by photocurrent effect and extra active sites generated by photogenerated carriers (h+). Characterization results indicate that both VO-rich WO3-x and graphene contribute to the efficient OER performance. The activity of WO3-x for OER is decided by the synergistic effect between VO concentration and particle size. The graphene could not only disperse WO3-x nanoparticles, but also improve the holistic conductivity and promote electron transmission. This work supports a novel method for engineering WO3-x/graphene composite for highly efficient (photo-)electrocatalytic performance for OER.

Low-loading Ir decorated cobalt encapsulated N-doped carbon nanotubes/porous carbon sheet implements efficient hydrogen/oxygen trifunctional electrocatalysis

Development of cost-efficeint and high activity muli-functional catalysts for gas-involved reactions such as Oxygen reduction evolution (ORR), oxygen evolution reaction (OER) and hydrogen evolution recation reaction (HER) is desirable but challenging. Herein, low-loading of Iridium (Ir) (≈2.3 wt%) decorated cobalt nanoparticles embedded in nitrogen-doped carbon nanotube/porous carbon sheets (IrCo@NCNT/PC) is prepared to serve as the OER/HER/ORR trifunctional electrocatalyst. The resulting composites demonstrate excellent OER and HER activity in both acidic and alkaline electrolytes, including low overpotentials at 10 mA cm−2 (η10) for OER (about 291 and 240 mV) and HER (about 26.8 and 37 mV), respectively, which are superior to the benchmarking Pt/C and IrO2 catalysts. Hence, low cell voltages of 1.51 and 1.45 V are required at j = 10 mA cm−2 (j10) for overall water splitting (OWS) in the acidic and alkaline electrolytes, respectively. In addition, the as-prepared IrCo@NCNT/PC also shows efficient ORR performance with a high half-wave potential (E1/2) of 0.83 V vs. RHE. It is then used as the air cathode for rechargeable Zn-air battery (r-ZAB), and exhibits an outstanding specific capacity (810 mAh g−1), low charge-discharge polarization (ΔE = 0.896 V at j = 50 mA cm−2) and long cycling performance (increased only 0.15 V after 195 cycles).

The activation of inert NiFe Prussian Blue analogues to boost oxygen evolution reaction activity

The inert sites of Prussian Blue Analogue (PBA) seriously affected its electrocatalytic activity and application, how to activate inert sites in PBA to fulfill effective oxygen evolution reduction (OER) is a major challenge. Herein, Mo substituted Fe sites and S doped in inert PBA were designed and synthesized by hydrothermal method to enhance structural stability and OER activity. PBA-SMo/NF shows the optimum activity with a low overpotential of 252 and 294 mV for harvesting current density 20 and 100 mA cm−2, respectively, and exhibits excellent durability under high current density. Theoretical calculation of H2O adsorption energy and Bader charges reveals that Mo sites in PBA-SMo possess favourable H2O adsorption kinetics. More important, Gibbs free energy diagram and DOS show PBA-SMo have lower energy barriers for OER and better conductivity. This work provides a kind of guidance for the design and optimization of PBA for broad applications.

S, N co-doped carbon nanotube encased Co NPs as efficient bifunctional oxygen electrocatalysts for zinc-air batteries

For the practical application of metal-air batteries, developing high-active and low-cost bifunctional oxygen reduction reaction (ORR) and oxygen evolution reduction (OER) catalysts is crucial and challenging. This study designed a unique ordered three-dimensional (3D) core–shell nanostructure bifunctional ORR and OER electrocatalysts consisting of Co nanoparticles (NPs) embedded in S, N co-doped carbon nanotubes (S,N-Co@CNT). The prepared S,N-Co@CNT exhibited excellent bifunctional electrocatalytic performance, with a high half-wave potential of 0.874 V versus reversible hydrogen electrode (RHE) toward ORR, and a low overpotential of 276 mV at 10 mA·cm−2 for the OER. The unique core–shell nanostructure has delivered considerable durability and stability in the S,N-Co@CNT, demonstrating almost no change in the potential gap between the ORR and OER after more than 5000 cycles of the accelerated durability test. Moreover, the S,N-Co@CNT serves as a high-active and low-price air–cathode catalyst for a rechargeable zinc-air battery, with a high peak power density of 171 mW·cm−2, and a long continuous charging-discharging ability. Based on mechanistic studies, the high activity of the S,N-Co@CNT was derived from the suitable oxygen adsorption energy due to the effects from S, N co-doping. The outcomes from this study offer a novel strategy to prepare non-noble metal bifunctional oxygen catalysts for metal-air batteries.

2D Metal‐Organic Framework Derived Co3O4 for the Oxygen Evolution Reaction and High‐Performance Lithium‐Ion Batteries

AbstractRecently, MOF‐derived (metal‐organic framework) 2D materials, due to the special structure of the MOF inherited, have become a potential candidate for application in energy storage and conversion. In this work, a new 2D layered Co−MOF was synthesized as the precursor for fabricating Napoleon‐like Co3O4 (NL−Co3O4). The monolayer thickness of NL−Co3O4 is approximately 70 nm. The unique structure of layered porous of NL−Co3O4 derived from Co−MOF plays an important role in improving electrochemical and electro‐catalysis properties. As lithium ion battery anode material, NL−Co3O4 shows out‐bound cycling performance and rate capacity, which has an unprecedented high reversible capacity of 1120.5 mA h g−1 at 200 mA g−1 after 300 cycles and high rate capacities of 579.4 mA h g−1 under the current density of 6.4 A g−1. NL−Co3O4 still exhibits capacity of 679 and 576 mA h g−1 after 200 cycles at high current densities of 1 and 4 A g−1, respectively. Moreover, NL−Co3O4 has efficient oxygen evolution reduction (OER) catalytic performance (overpotential, ca. 420 mV), comparable with commercial RuO2. During the process of continuous catalysis of OER, NL−Co3O4 shows good durability for about 15 h.

Engineering of Electronic States on Co3 O4 Ultrathin Nanosheets by Cation Substitution and Anion Vacancies for Oxygen Evolution Reaction.

Due to the earth abundance and tunable electronic properties, etc., transition metal oxides (TMOs) show attractive attention in oxygen evolution reaction. O-vacancies (Vo ) play important roles in tailoring the local surface and electronic environment to lower the activation barriers. Herein, an effective strategy is shown to enhance the oxygen evolution reduction (OER) performance on Co3 O4 ultrathin nanosheets via combined cation substitution and anion vacancies. The oxygen-deficient Fe-Co-O nanosheets (3-4 nm thickness) display an overpotential of 260 mV@10 mA cm-2 and a Tafel slope of 53 mV dec-1 , outperforming those of the benchmark RuO2 in 1.0 m KOH. Further calculations demonstrate that the combined introduction of Fe cation and Vo with appropriate location and content finely tune the intermediate absorption, consequently lowering the rate-limiting activation energy from 0.82 to as low as 0.15 eV. The feasibility is also proved by oxygen-deficient Ni-Co-O nanosheets. This work not only establishes a clear atomic-level correlation between cation substitution, anion vacancies, and OER performance, but also provides valuable insights for the rational design of highly efficient catalysts for OER.

FeOOH-enhanced bifunctionality in Ni3N nanotube arrays for water splitting

Here we report the functional FeOOH as a cocatalyst to enhance excellent bifunctionality of the Ni3N catalyst, which demonstrate remarkable electrocatalytic activity for hydrogen evolution reaction (HER) and oxygen evolution reduction (OER) compared to the pristine Ni3N sample. Experimental data and theoretical calcinations demonstrate that the FeOOH facilitates surface adsorption and cleavage of OH−/H2O species and decrease the d-band center of active Ni species for optimizing the Gibbs free energy of intermediates. Furthermore, relatively vertical Ni3N nanotubes can promote the mass transport and electrons transfer as a main catalyst. The unusual synergistic effect in hybrid system with high density of heterointerface is contributed to the improvement of HER/OER catalytic performance. Specifically, the obtained FeOOH/Ni3N hybrid catalysts display excellent OER/HER performance with an overpotential of 244 mV/67 mV at 10 mA cm−2 in 1.0 M KOH, along with an applied potential of 1.58 V to boost overall water splitting at 10 mA cm−2. This simple and effective strategy may provide a new path to design the bifunctional catalysts and improve the catalytic performance for water splitting.

Engineering vanadium phosphide by iron doping as bifunctional electrocatalyst for overall water splitting

To develop efficiently bifunctional electrocatalysts for oxygen evolution reduction (OER) and hydrogen evolution reaction (HER) favors in the large-scale development of water splitting. Herein, for the first time, Fe doped vanadium phosphide functionalized porous carbon loading on nickel foam (FexVy-PC/NF) catalyst is designed from the bimetallic Fe/V metal-organic framework (FexVy-MOF) by a combination of solvothermal and phosphorization approach. Interestingly, it is noted that the foreign Fe is introduced to the vanadium phosphide crystal rather than forming two crystals. Thus, the morphology, electron state of vanadium, conductivity and facet exposure of vanadium phosphide are observed to be tuned through adjusting the ratio of Fe to V. Remarkably, the optimized electrocatalyst can deliver 10 mA cm−2 at low overpotentials of 66, 201 mV for HER and OER in 1.0 M KOH. As for the overall water splitting, it only needs a lower potential of 1.56 V to reach 10 mA cm−2 and displays desirable durability attributed to the electron interaction between Fe and V, the good conductivity, porous feature of mother MOF and the graphitic layer encasing the metal nanoparticles.

Facile Synthesis of Monodispersed α-Ni(OH)2 Microspheres Assembled by Ultrathin Nanosheets and Its Performance for Oxygen Evolution Reduction

Electrocatalytic water splitting is an efficient route to generate renewable energy sources, in which the noble metal materials are usually used as electrocatalysts. But the high price and scarcity of these catalysts impede their large-scale applications. Ni-based materials are considered to be the most suitable catalytic materials to substitute these noble metal materials. In this paper, the monodispersed α-Ni(OH)2 microspheres assembled by ultrathin nanosheets were synthesized by a facile solvothermal method. This method was surfactant-free and no any precipitator was used. The as-obtained products were well characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), thermal gravimetric (TG) analysis, and X-ray photoelectron spectroscopy (XPS). The results of N2 adsorption/desorption isotherm indicate that the BET surface area of the products was 169.94 m2g-1, and the pore-size distribution centered at 3.5 nm. Then, the electrochemical properties were evaluated by linear sweep voltammetry (LSV) in 1 M KOH. At a current density of 10 mA cm2, the overpotential for the α-Ni(OH)2-MS is 320mV, and the Tafel slope is 98.7 mV dec-1, indicating its excellent reaction kinetics for an efficient catalyst toward oxygen evolution reaction (OER).

Facile synthesis of ZnCo-ZIFs-derived ZnxCo3−xO4 hollow polyhedron for efficient oxygen evolution reduction

The construction of highly active and stable non-noble metal electrocatalysts for oxygen evolution reaction (OER) is a crucial requirement for green energy utilization. Herein, we report a novel hybrid nanostructure of Zn/Co bimetallic oxide (ZnxCo3−xO4) hollow polyhedron through a pyrolysis−oxidation strategy derived from bimetallic Zn, Co-zeolitic imidazolate frameworks (ZnCo-ZIFs) rhombic polyhedral. Benefiting from the synergistic effects between highly active metal oxide and the unique polyhedron structure, the ZnxCo3−xO4 hollow polyhedron hybrid displayed outstanding electrocatalytic performances in alkaline media. When the ZnxCo3−xO4 polyhedron was employed as electrocatalysts for OER, a potential as low as 1.66 V (vs. RHE) was required to deliver the current density of 10 mA cm−2 in 1 M KOH, and ZnxCo3−xO4 still presented superior activity after continuously working for cyclic voltammetric scanning of 1000 cycles.

A powerful Li[sbnd]O2 battery based on an efficient hollow Cu2O cathode catalyst with tailored crystal plane

Issues with the sluggish kinetics of oxygen reduction and oxygen evolution reduction hinder the advance of LiO2 batteries for next-generation energy storage. To alleviate these problems, Cu2O nanoparticles with different crystal planes have been synthesized via a simple solution reaction and characterized as a bi-functional catalyst for non-aqueous LiO2 battery. In this work, catalytic behaviour of Cu2O nanoparticles with different crystal planes are initially explored and it is found that the Cu2O nanoparticles exposing {111} crystal planes exhibit the most catalytic performance compared with other crystal planes, which is further confirmed by using first-principles computations. Inspired by selectively catalytic behaviour of crystal plane, porous Cu2O nanoframes exposed with eight {111} crystal planes and higher surface area derives from all-corner-truncated rhombic dodecahedron Cu2O nanoparticles are synthesized, which can act as highly activity three-dimensional cathode in a typical LiO2 battery after being directly combined with carbon nanotubes by electrostatic interaction. Thanks to the excellent catalytic activity of carbon nanotubes-Cu2O nanoframes and unique cathode structure, batteries with this cathode display an enhanced electrochemical performance including superior round-trip efficiency, excellent cycling performance and high specific capacity.

Effect of Sr doping on the electrochemical properties of bi-functional oxygen electrode PrBa1−Sr Co2O5+

Abstract Sr-doped double perovskite oxides PrBa 1− x Sr x Co 2 O 5+ δ ( x = 0.0–1.0) are prepared and characterized by X-ray diffraction, X-ray photoelectron spectra, cyclic voltammetry and linear sweep voltammetry, respectively. Sr 2+ doping is found to promote the electrocatalytic activity towards both oxygen evolution reduction and oxygen reduction reaction, due to the concentration enhancement of Co 4+ (e g = 1) on the oxide surface. The PrBa 0.25 Sr 0.75 Co 2 O 5.95 catalyst, with the highest concentration of Co 4+ , exhibits the superior electrocatalytic activity. The onset potential, Tafel slope and the overpotential at given current density of 10.0 mA·cm geom −2 for the OER are 1.52 V, 75.8 mV·dec −1 and 0.42 V, respectively, comparable to the currently reported high performance OER catalysts. At the same time, the onset potential, Tafel slope, limiting current density and electron transfer number of PrBa 0.25 Sr 0.75 Co 2 O 5.95 for ORR are 0.753 V, 58.3 mV·dec −1 , 4.081 mA·cm geom −2 and n = 3.92, respectively, implying much improved ORR activity.

Pomegranate-Inspired Design of Highly Active and Durable Bifunctional Electrocatalysts for Rechargeable Metal-Air Batteries

The fast depletion of fossil fuels and severe deterioration of ecology have stimulated extensive research on the utilization and storage of clean and sustainable energy. Among various energy storage technologies, rechargeable metal-air batteries possess the highest energy density, making them excellent candidate for next generation electrical vehicles (EVs) and hybrid electrical vehicles (HEVs). In metal-air batteries, a metal anode is coupled with an air-breathing cathode that utilizes oxygen from the atmosphere as the reactant for the electrochemical reactions. Discharging and charging processes occur driven by the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reduction (OER). The major challenge associated with the commercialization of metal-air batteries resides in the sluggish kinetics of the ORR and OER resulting in large overpotentials. Therefore, developing efficient electrocatalyst with high catalytic activities is of great importance for high performance metal-air batteries. Precious metal-based catalysts such as platinum (Pt), palladium (Pd), iridium (Ir) and alloys have been intensively studied showing superb catalytic properties and have been widely accepted as the most active electrocatalysts. Unfortunately, the scarcity and electrochemical instability of these catalysts have prevented their widespread use due to extremely high costs and unsatisfactory durability. Nonprecious transition metals (e.g. Fe, Co, Ni, Mn) and perovskite-based catalysts have been extensively explored. However, they suffer from inefficient catalytic activity due to self-accumulation and intrinsically poor electrical conductivity. To address these challenges, nanostructured materials with various morphologies (e.g. nanoparticles, nanowires, nanosheets, nanotubes) have been designed for effective exposure of catalytic planes and enhanced diffusion of reactive species, while conductive carbon materials (e.g. carbon nanotubes (CNTs) and graphene) are often introduced into catalysts to increase the conductivity and structural stability. For example, Co3O4 nanocrystals/reduced graphene oxide bi-functional catalyst exhibits unusual catalytic activity attributed to the synergetic chemical coupling effects. Despite numerous studies, rational design strategy and efficient development of desired high-performance electrocatalysts is yet limited. An efficient electrocatalyst is expected to (i) exhibit high catalytic activity with a large amount of active sites for ORR and OER processes; (ii) possess sufficient mass transfer pathways for fast electrode kinetics; and (iii) be chemically stable with robust material and/or electrode architecture for high durability. Herein, we demonstrate the development of pomegranate-like electrocatalysts based on transition metal oxide nanocrystals embedded nitrogen-doped partially graphitized carbon framework with excellent catalytic activity for ORR and OER and outstanding durability. Nitrogen-doped has been reported to remarkably accelerate OER process by participating in the electrocatalytic reaction. To demonstrate the design concept, cobalt oxide (Co3O4) is chosen as a model material due to its abundance and theoretically high electrocatalytic activity. The Co3O4 nanocrystals embedded nitrogen-doped partially graphitized carbon framework (Co3O4/NPGC) with unique pomegranate-like composite architecture provides several major advantages: (i) low dimension of highly active Co3O4 nanocrystals seeds possess active sites for electrochemical reactions; (ii) the pomegranate-like structure efficiently prevents the metal oxide from self-accumulation and provide the mass transfer pathways, which further maintains the catalytic activity; (iii) graphitized carbon shell and framework is not only highly conductive which significantly increases electrical conductivity, but also chemically stable and highly robust which enhances the catalyst durability. Benefiting from the unique pomegranate-like architecture, the Co3O4-based composite electrocatalyst exhibits a high half-wave potential of 0.842 V for ORR, and a low overpotential of only 450 mV at the current density of 10 mA cm-2 for OER. Single-cell zinc-air battery is also fabricated with superior durability, holding great promise in the practical implementation of rechargeable metal-air batteries. Figure 1

Elucidating dz2 orbital selective catalytic activity in brownmillerite Ca2Mn2O5

3d-orbital filling in transition metal oxide is crucial to govern the catalytic activity in oxygen evolution reduction, nevertheless, it is not fundamentally accessible why specific orbital occupation produces a highest catalytic performance. Here, we utilize brownmillerite Ca2Mn2O5 to clarify the orbital selective catalytic behavior due to the crystal field splitting and on-site coulomb interactions. Within density functional theory plus dynamical mean field theory, Ca2Mn2O5 shows a paramagnetic Mott insulating behavior at room temperature, consistent with optical adsorption spectra and magnetic susceptibility. As the center of the dz2 orbital locates in the lower Hubbard sub-band, the unit occupation on dz2 orbital provides a moderate bonding with external O* species to cause a high catalytic activity of Ca2Mn2O5 with a square pyramid crystal field. Such concept of unit occupation of dz2 near Fermi level could be extended to other crystal fields for future design of oxide catalysts.