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The development of anode materials with high-rate capability is critical to high-power lithium batteries. T-Nb2O5 has been widely reported to exhibit pseudocapacitive behavior and fast lithium storage capability. However, the other polymorphs of Nb2O5 prepared at higher temperatures have the potential to achieve even higher specific capacity and tap density than T-Nb2O5, offering higher volumetric power and energy density. Here, micrometer-sized H-Nb2O5 with rich Wadsley planar defects (denoted as d-H-Nb2O5) is designed for fast lithium storage. The performance of H-Nb2O5 with local rearrangements of [NbO6] octahedra blocks surpasses that of T-Nb2O5 in terms of specific capacity, rate capability, and stability. A wide range variation in valence of niobium ions upon lithiation was observed for defective H-Nb2O5 via operando X-ray absorption spectroscopy. Operando extended X-ray absorption fine structure and ex-situ Raman spectroscopy reveals a large and reversible distortion of the structure in the two-phase region. Computation and ex-situ X-ray diffraction analysis reveals that the shear structure expands along major lithium diffusion pathways and contracts in the direction perpendicular to the shear plane. Planar defects relieve strain through perpendicular arrangements of blocks, minimizing volume change and enhancing structural stability. In addition, strong Li adsorption on planar defects enlarges intercalation capacity. Different from nanostructure engineering, our strategy to modify the planar defects in the bulk phase can effectively improve the intrinsic property. The findings in this work offer new insights into designing fast Li-ion storage materials in micrometer sizes through defect engineering, and the strategy is applicable to the material discovery for other energy-related applications.

O3-Type layered oxides are widely studied as cathodes for sodium-ion batteries (SIBs) due to their high theoretical capacities. However, their rate capability and durability are limited by tortuous Na

Fe-N-C single-atom catalysts (SACs) are emerging as a promising class of electrocatalysts for the oxygen reduction reaction (ORR) to replace Pt-based catalysts. However, due to the limited loading of Fe for SACs and the inaccessibility of internal active sites, only a small portion of the sites near the external surface are able to contribute to the ORR activity. Here, this work reports a metal-organic framework-derived Fe-N-C SAC with a hierarchically porous and concave nanoarchitecture prepared through a facile but effective strategy, which exhibits superior electrocatalytic ORR activity with a half-wave potential of 0.926 V (vs RHE) in alkaline media and 0.8 V (vs RHE) in acidic media while maintaining excellent stability. The superior ORR activity of the as-designed catalyst stems from the unique architecture, where the hierarchically porous architecture contains micropores as Fe SAC anchoring sites, meso-/macro-pores as accessible channels, and concave shell for increasing external surface area. The unique architecture has dramatically enhanced the utilization of previously blocked internal active sites, as confirmed by a high turnover frequency of 3.37 s

While numerous oxygen electrocatalysts have been reported to enhance zinc-air battery (ZAB) performance, highly efficient electrocatalysts for the oxygen electrocatalysis need to be developed for broader commercialization of ZABs. Furthermore, areal (instead of volumetric) power density has been used to benchmark the performance of ZABs, often causing ambiguities or confusions. Here, we propose a methodology for evaluating the performance of a ZAB using the volumetric (rather than the areal) power density by taking into consideration the air electrode thickness. A nitrogen and sulfur co-doped metal-free oxygen reduction electrocatalyst (N-S-PC) is used as a model catalyst for this new metric. The electrocatalyst exhibited a half-wave potential of 0.88 V, which is similar to that of the Pt/C electrocatalyst (0.89 V) due to the effects of co-doping and a highly mesoporous structure. In addition, the use of volumetric activity allows fair comparison among different types of air electrodes. The N-S-PC-loaded air electrode demonstrated a higher peak power density (5 W cm-3) than the carbon felt or paper electrode in the ZAB test under the same testing conditions.

One of the key challenges that hinders broad commercialization of proton exchange membrane fuel cells is the high cost and inadequate performance of the catalysts for the oxygen reduction reaction (ORR). Here we report a composite ORR catalyst consisting of ordered intermetallic Pt-alloy nanoparticles attached to an N-doped carbon substrate with atomically dispersed Fe–N–C sites, demonstrating substantially enhanced catalytic activity and durability, achieving a half-wave potential of 0.923 V (vs. RHE) and negligible activity loss after 5000 cycles of an accelerated durability test. The composite catalyst is prepared by deposition of Pt nanoparticles on an N-doped carbon substrate with atomically dispersed Fe–N–C sites derived from a metal–organic framework and subsequent thermal treatment. The latter results in the formation of core–shell structured Pt-alloy nanoparticles with ordered intermetallic Pt3M (M = Fe and Zn) as the core and Pt atoms on the shell surface, which is beneficial to both the ORR activity and stability. The presence of Fe in the porous Fe–N–C substrate not only provides more active sites for the ORR but also effectively enhances the durability of the composite catalyst. The observed enhancement in performance is attributed mainly to the unique structure of the composite catalyst, as confirmed by experimental measurements and computational analyses. Furthermore, a fuel cell constructed using the as-developed ORR catalyst demonstrates a peak power density of 1.31 W cm−2. The strategy developed in this work is applicable to the development of composite catalysts for other electrocatalytic reactions.

Various morphological nanoscale designs have come into the spotlight to address the failure in the mechanism of high-capacity Si anodes, i.e. severe volume expansion (∼300%). However, the nanostructured Si anodes designed still suffer mechanical degradation upon repeated cycling, and eventually become shredded and surrounded by accumulated solid electrolyte interphase (SEI) layers. Here, we introduce a highly homogenous phase design of Si with N by scalable gas phase synthesis, which tackles the intrinsic challenges of Si anodes, i.e. mechanical degradation and slow Li diffusion. Si-rich silicon nitride (SiN) nanoparticles are realized using a specially customized vertical furnace, where Si3N4 acts as not only a strong inactive matrix but also a Li ion conductor after lithiation. Owing to their stubborn and ionic conductive matrix, SiN nanoparticles exhibit superior rate performances and cycling stability while maintaining their dense structure. Accordingly, when combined with commercially viable graphite-blended system for the pouch-type 1 A h cell, SiN nanoparticles demonstrate high rate capability at 5C, as well as contributing much higher capacity than silicon nanoparticles by mitigating electrode swelling during cycling.

Replacing the commercial Pt/C with non-precious metal-based electrocatalysts in the proton exchange membrane fuel cells and metal-air batteries is still challenging. Herein, an electrocatalyst (described as Fe-N-C/Fe100-x-y-zPxOyNz/NPC, NPC is N, P co-doped carbon) composed of multiple active components, such as NPC, iron-based porous hollow spheres and Fe-N-C, is reported, which exhibits an excellent activity that is comparable to state-of-the-art Pt/C for half-cell and full zinc-air battery in alkaline media. This catalyst exhibits an excellent activity with a half-wave potential of 0.86 V for the ORR in alkaline media, which is 10 mV more positive than to that of Pt/C (0.85 V), and a gravimetric energy density for zinc-air battery is up to 675 Wh Kgzn−1. The excellent activity is attributed to the synergetic effect of active NPC, iron-based porous hollow spheres (Fe100-x-y-zPxOyNz) and Fe-N-C in its structure. In addition, phosphoric acid groups are partially remained in the structure for our catalyst that make the catalyst excellent hydrophilicity. This work adds a new member into family of non-precious metal-based ORR electrocatalysts.

Oxygen vacancies (OV) are native defects in transition metal (TM) oxides and their presence has a critical effect on the physicochemical properties of the oxide. Metal oxides are commonly used in lithium-ion battery (LIB) cathodes and there is still a lack of understanding of the role of OVs in LIB research field. Here, we report on the behavior of OVs in a single-crystal LIB cathode during the non-equilibrium states of charge and discharge. We found that microcrack evolution in a single crystal occurs due to OV condensation in specific crystallographic orientations generated by the continuous migration of OVs and TM ions. Moreover, understanding the effects of the presence and diffusion of OVs in metal oxides enables the elucidation of most of the conventional mechanisms of capacity fading in LIBs and provides new insights for new electrochemical applications.

Developing highly efficient and economically sustainable electrocatalysts with bifunctions for the oxygen reduction reaction and oxygen evolution reaction is crucially important in practical implementation of rechargeable zinc-air batteries. Herein, a novel Cu-based electrocatalyst composed of Cu97P3-x-yOxNy nanoparticles supported on N, P co-doped carbon is synthesized straightforward by the annealing of the Cu-phytic acid gel under an argon and ammonia gases atmosphere. The Cu97P3-x-yOxNy/N, P co-doped carbon catalyst displays the outstanding catalytic performance for both the oxygen evolution reaction and oxygen reduction reaction in alkaline solution, which is ascribed to the synergistic effect of Cu97P3-x-yOxNy and N, P co-doped carbon. Meanwhile, the primary and rechargeable zinc-air batteries assembled by the Cu97P3-x-yOxNy/N, P co-doped carbon catalyst exhibit a high specific capacity and remarkable charge-discharge cycle stability, which are comparable to that of the benchmark Pt/C and IrO2. The energy density of the catalyst reaches to 737 Wh kgZn−1. To the best of our knowledge, few Cu-based electrocatalysts is reported for rechargeable zinc-air batteries.

While lithium-ion batteries (LIBs) have been widely used for portable devices and electric vehicles, it is highly desirable to develop safer and less expensive batteries as alternative to LIBs. In this regard, zinc (Zn) batteries have attracted much attention because of their excellent safety and low cost. However, one of the challenges is to develop cost-effective and highly efficient cathode materials for Zn-ion batteries (ZIBs) based on transition metal oxides. It would be more economical to recycle transition metals in order to reduce the fabrication cost. Co-precipitation method is widely used for synthesis of LIBs cathode materials, and large amount of wastewater would be produced during co-precipitation and battery production process. In this presentation, we will report a facile and general process for fabrication of cathode materials for aqueous Zn-ion batteries (ZIBs) by reusing wastewater from co-precipitation method. We have selected manganese rich phases with different ratio of nickel to cobalt precursors from co-precipitation wastewater, followed by a simple ball milling process, resulting in metal-hydroxide cathode materials (Mn0.6Ni0.1Co0.3OxHy, Mn0.6Ni0.2Co0.2OxHy, and Mn0.6Ni0.3Co0.1OxHy). Among them, the Mn0.6Ni0.1Co0.3OxHy cathode (with mass loading of 15 mg cm-2) exhibits an initial capacity of 263 mAh g-1 at a current density of 0.1 A g-1, as evaluated in an mixture electrolyte (2M ZnSO4 and 0.1M MnSO4). Furthermore, operando X-ray absorption spectroscopy analysis has revealed the role of each transition metal ions during insertion and desertion of Zn ions. It is found that the ratio of Ni to Co significantly influences ZIBs performances, providing important insight into rational design of more efficient cathode materials for aqueous Zn-ion batteries. Figure 1

Niobium oxide-based materials are promising anodes for high-power lithium-ion batteries. The electrochemical performance of these oxides depends sensitively on the crystal and the defect structures. For example, a recent study revealed that the planar defects introduced to the H-phase of Nb2O5 shear structure greatly enhanced the rate capability and structural durability. Our recent study indicates that the M-phase with a tetragonal crystal structure displayed the highest electrochemical performance among all crystalline phases of Nb2O5 studied, due likely to its fast transport of lithium ions and electrons through the ‘block structures’. In this presentation, we will report our findings in the development of a composite electrode composed of a tetragonal and a monoclinic phase of niobium pentoxide with desired defect structures (denoted as d-H,M-Nb2O5), synthesized using a solvothermal process followed by annealing under controlled conditions. Microscopic analysis reveals that the d-H,M-Nb2O5 has NbO6 and NbO4 block structures of both the M-Nb2O5 and the H-Nb2O5. The electrode based on the d-H,M-Nb2O5 exhibits the highest rate capability (142 mAh g-1 at 20 A g-1) and excellent cycling stability (86% capacity retention after 2000 cycles at 6 A g-1) among recently reported niobium pentoxides. Further, detailed crystallographic and defect structures, the physicochemical characteristics, and the long-term cycling behavior will be discussed.

Solid oxide fuel cells (SOFCs) can efficiently generate electricity from a wide variety of fuels, offering unique advantages over the conventional power generation systems based on combustion. In particular, SOFCs can directly use low-cost and readily available hydrocarbon fuels as the energy source. To be commercially competitive, however, breakthroughs in materials development is required. For instance, a durable and electro-catalytically active anode material or catalyst is needed that can facilitate hydrocarbon reforming and avoiding carbon deposition (coking) for SOFCs to operate stably on hydrocarbon fuels, including natural gas, ethanol, and iso-octane. Hydrocarbon fuels, especially liquid ones, have several advantages over hydrogen fuel: low-cost, high volumetric energy density, and ease for transportation and storage. Currently, direct-hydrocarbon SOFCs suffer from severe performance degradation due to carbon deposition. In this presentation, we report a promising anode catalyst for SOFCs operated on various hydrocarbon fuels, from methane to ethanol and iso-octane. The catalyst has been successfully applied to both small button cells and tubular cells with an effective area of about 38 cm2. For both cases, our cells demonstrate stable performance for continuous operation of >1,000 hours.

High-performance electrocatalysts for oxygen reduction reaction (ORR) are vital to many chemical and energy transformation processes. To date, however, the role of atomic defects in nanocarbon (NC) supports for ORR electro-catalysts is still poorly understood. In this presentation, we report our findings in understanding the contribution of atomic defects in NC supports to a series of transition metals (TM) based electro-catalysts to enhance the ORR activity and stability in an alkaline medium. The defect structures and their local coordination are characterized using Operando X-ray absorption spectroscopy and ex-situ Raman spectroscopy. Moreover, the atomic level structure and morphology of the materials are revealed using high-resolution transmission electron microscopy. Further, density functional theory calculations are used to gain deeper insight into the mechanisms of electro-catalytic processes. Our studies not only result in a highly promising ORR electro-catalyst in an alkaline medium but also open a new avenue for rational design of more efficient catalysts for other chemical and energy transformation processes.

Electrification of transportation has spurred the development of fast-charge energy storage devices. High-power lithium-ion batteries require electrode materials that can store lithium quickly and reversibly. Herein, the design and construction of a Nb

Multifunctional electrocatalysts with high catalytic activity and durability are needed for environmentally clean energy technologies such as water-splitting devices and metal–air batteries. Herein, we investigate a new catalyst, P-3G, consisting of a cation-ordered perovskite (PrBa0.5Sr0.5)0.95Co1.5Fe0.5O5+δ (PBSCF) and 3D porous N-doped graphene (3DNG). This new type of composite electrocatalyst simultaneously exhibited outstanding multifunctional catalytic activities and excellent stabilities for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). A possible mechanism for the synergistic effects between perovskite oxides and 3DNG on ORR, OER and HER was firstly proposed by DFT calculations. The electrocatalytic activity of P-3G appeared to have great potential for a rechargeable Zn–air battery system. The operating voltage differences between the charge and discharge (Δη) of P-3G and Pt/C–IrO2 after 110 cycles were 0.63 V and 0.87 V, respectively, indicating the substantial durability of P-3G. Moreover, a water-splitting device using P-3G efficiently produced H2 and O2 gases at rates of 0.859 μL s−1 and 0.417 μL s−1, respectively. This study highlights extended applications of coupled perovskite oxides/carbon materials as versatile electrocatalysts for ORR, OER, and HER and unveils the cause of synergistic interactions between oxide and carbon by DFT calculation.

Zn–air batteries suffer from the slow kinetics of oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER). Thus, the bifunctional electrocatalysts are required for the practical application of rechargeable Zn–air batteries. In terms of the catalytic activity and structural stability, pyrochlore oxides (A2[B2–xAx]O7–y) have emerged as promising candidates. However, a limited use of A-site cations (e.g., lead or bismuth cations) of reported pyrochlore catalysts have hampered broad understanding of their catalytic effect and structure. More seriously, the catalytic origin of the pyrochlore structure was not clearly revealed yet. Here, we report the new nanocrystalline yttrium ruthenate (Y2[Ru2–xYx]O7–y) with pyrochlore structure. The prepared pyrochlore oxide demonstrates comparable catalytic activities in both ORR and OER, compared to that of previously reported metal oxide-based catalysts such as perovskite oxides. Notably, we first find that the catalytic activity of the Y2[Ru2–xYx]O7–y is ...

Oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) electrocatalysts including carbon-, non-precious metal-, metal alloy-, metal oxide-, and carbide/nitride-based materials are of great importance for energy conversion and storage technologies. Among them, metal oxides (e.g., perovskite and pyrochlore) are known to be promising candidates as electrocatalysts. Nevertheless, the intrinsic catalytic activities of pyrochlore oxides are still poorly understood because of the formation of undesirable phases derived from the synthesis processes. Herein, we present highly pure single crystalline pyrochlore nanoparticles with metallic conduction (Pb2Ru2O6.5) as an efficient bi-functional oxygen electrocatalyst. Notably, it has been experimentally shown that the covalency of Ru–O bonds affects the ORR and OER activities by comparing the X-ray absorption near edge structure (XANES) spectra of the metallic Pb2Ru2O6.5 and insulating Sm2Ru2O7 for the first time. Moreover, we followed the interatomic distance changes of Ru–O bonds using in situ X-ray absorption spectroscopy (XAS) to investigate the structural stabilities of the pyrochlore catalysts during electrocatalysis. The highly efficient metallic Pb2Ru2O6.5 exhibited outstanding bi-functional catalytic activities and stabilities for both ORR and OER in aqueous Zn–air batteries.

Li- and Mn-rich layered oxides (LMRs) have emerged as practically feasible cathode materials for high-energy-density Li-ion batteries due to their extra anionic redox behavior and market competitiveness. However, sluggish kinetics regions (

Replacing noble-metal-based oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts is the key to developing efficient Zn-air batteries (ZABs). Here, a homogeneous ternary Ni

A facile H2 O2 oxidation treatment to tune the properties of metal disulfides for oxygen evolution reaction (OER) activity enhancement is introduced. With this method, the degree of oxidation can be readily controlled and the effect of surface S residues in the resulted metal (oxy)hydroxides for the OER is revealed for the first time. The developed NiFe (oxy)hydroxide catalyst with residual S demonstrates an extraordinarily low OER overpotential of 190 mV at the current density of 10 mA cm-2 after coupling with carbon nanotubes, and outstanding performance in Zn-air battery tests. Theoretical calculation suggests that the surface S residues can significantly reduce the adsorption free energy difference between O* and OH* intermediates on the Fe sites, which should account for the high OER activity of NiFe (oxy)hydroxide catalysts. This work provides significant insight regarding the effect of surface heteroatom residues in OER electrocatalysis and offers a new strategy to design high-performance and cost-efficient OER catalysts.

Highly efficient bifunctional oxygen electrocatalysts are indispensable for the development of highly efficient regenerative fuel cells and rechargeable metal-air batteries, which could power future electric vehicles. Although perovskite oxides are known to have high intrinsic activity, large particle sizes rendered from traditional synthesis routes limit their practical use due to low mass activity. We report the synthesis of nano-sized perovskite particles with a nominal composition of Lax(Ba[subscript 0.5]Sr[subscript 0.5])[subscript 1−x]Co[subscript 0.8]Fe[subscript 0.2]O[subscript 3−δ] (BSCF), where lanthanum concentration and calcination temperature were controlled to influence oxide defect chemistry and particle growth. This approach produced bifunctional perovskite electrocatalysts ∼50 nm in size with supreme activity and stability for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalysts preferentially reduced oxygen to water (, MIT & Masdar Institute Cooperative Program (02/MI/MIT/CP/11/07633/GEN/G/00), MIT Skoltech Initiative

Understanding the interaction between a catalyst and oxygen has been a key step in designing better electrocatalysts for the oxygen reduction reaction (ORR) as well as applying them in metal-air batteries and fuel cells. Alloying has been studied to finely tune the catalysts' electronic structures to afford proper binding affinities for oxygen. Herein, we synthesized a noble-metal-free and nanosized transition metal CuFe alloy encapsulated with a graphitic carbon shell as a highly efficient and durable electrocatalyst for the ORR in alkaline solution. Theoretical models and experimental results demonstrated that the CuFe alloy has a more moderate binding strength for oxygen molecules as well as the final product, OH(-), thus facilitating the oxygen reduction process. Furthermore, the nitrogen-doped graphitic carbon-coated layer, formed catalytically under the influence of iron, affords enhanced charge transfer during the oxygen reduction process and superior durability. These benefits were successfully confirmed by realizing the catalyst application in a mechanically rechargeable Zn-air battery.

Of the various catalysts that have been developed to date for high performance and low cost, perovskite oxides have attracted attention due to their inherent catalytic activity as well as structural flexibility. In particular, high amounts of Pr substitution of the cation ordered perovskite oxide originating from the state-of-the-art Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) electrode could be a good electrode or catalyst because of its high oxygen kinetics, electrical conductivity, oxygen capacity, and structural stability. However, even though it has many favorable intrinsic properties, the conventional high-temperature treatment for perovskite synthesis, such as solid-state reaction and combustion process, leads to the particle size increase which gives rise to the decrease in surface area and the mass activity. Therefore, we prepared mesoporous nanofibers of various cation-ordered PrBa0.5Sr0.5Co2–xFexO5+δ (x = 0, 0.5, 1, 1.5, and 2) perovskites via electrospinning. The well-controlled B-site metal ratio and lar...

A cable-type flexible Zn-air battery with a spiral zinc anode, gel polymer electrolyte (GPE), and air cathode coated on a nonprecious metal catalyst is designed in order to extend its application area toward wearable electronic devices.

Zn-air batteries (ZABs) have attracted much attention for possible applications in portable electronics and electric vehicles because of several advantages such as low cost, excellent safety, and high energy density (ca. 1,084 Wh kg–1) in alkaline media. However, the development of ZABs has been hampered by low working voltage due to the sluggish rate of oxygen reduction reaction (ORR) and short lifespan under deep discharge conditions (over 30%) due to poor durability of the air electrode associated with limited stability of ORR and oxygen evolution reaction (OER) electrocatalysts. Thus, platinum group metals (e.g., Pt and Ir-based alloys) have been widely used for ORR and OER electrocatalysts, respectively. However, these materials not only are expensive but also show poor durability in practical ZABs. In this presentation, we will report some latest advances in the development of low-cost, highly efficient bi-functional oxygen electrocatalysts for ZABs. In particular, we will highlight a composite catalyst consisting of Ni46Co40Fe14 nanoparticles (with a size distribution of 30~60 nm) dispersed in a carbon matrix (denoted as C@NCF-900), produced via supercritical reaction and subsequent heat treatment at 900 °C. Among all the transition metal-based electrocatalysts, the C@NCF-900 exhibits the highest ORR performance in terms of half-wave potential (0.93 V) in 0.1 M KOH. Furthermore, C@NCF-900 exhibits negligible activity decay after 10,000 voltage cycles with minor reduction of half-wave potential (0.006 V). When evaluated in ZABs, the C@NCF-900 outperforms the mixture of 20 wt% Pt/C and IrO2, cycled over 100 h under 58 % depth of discharge condition. Furthermore, a plausible ORR/OER mechanism of C@NCF-900 is elucidated by density functional theory calculations and confirmed by operando X-ray absorption spectroscopy.

Chemically prepared λ-MnO2 has not been intensively studied as a material for metal-air batteries, fuel cells, or supercapacitors because of their relatively poor electrochemical properties compared to α- and δ-MnO2 . Herein, through the electrochemical removal of lithium from LiMn2 O4 , highly crystalline λ-MnO2 was prepared as an efficient electrocatalyst for the oxygen reduction reaction (ORR). The ORR activity of the material was further improved by introducing oxygen vacancies (OVs) that could be achieved by increasing the calcination temperature during LiMn2 O4 synthesis; a concentration of oxygen vacancies in LiMn2 O4 could be characterized by its voltage profile as the cathode in a lithiun-metal half-cell. λ-MnO2-z prepared with the highest OV exhibited the highest diffusion-limited ORR current (5.5 mA cm(-2) ) among a series of λ-MnO2-z electrocatalysts. Furthermore, the number of transferred electrons (n) involved in the ORR was3.8, indicating a dominant quasi-4-electron pathway. Interestingly, the catalytic performances of the samples were not a function of their surface areas, and instead depended on the concentration of OVs, indicating enhancement in the intrinsic catalytic activity of λ-MnO2 by the generation of OVs. This study demonstrates that differences in the electrochemical behavior of λ-MnO2 depend on the preparation method and provides a mechanism for a unique catalytic behavior of cubic λ-MnO2 .

Fabricating perovskite oxide/carbon material composite catalysts is a widely accepted strategy to enhance oxygen reduction reaction/oxygen evolution reaction (ORR and OER) catalytic activities. Herein, synthesized, porous, perovskite-type Sm0.5 Sr0.5 CoO3-δ hollow nanofibers (SSC-HF) are hybridized with cross-linked, 3D, N-doped graphene (3DNG). This rationally designed hybrid catalyst, SSC-HF-3DNG (SSC-HG), exhibits a remarkable enhancement in ORR/OER activity in alkaline media. The synergistic effects between SSC and 3DNG during their ORR and OER processes are firstly revealed by density functional theory calculations. It suggests that electron transport from 3DNG to O2 and SSC increases the activity of electrocatalytic reactions (ORR and OER) by activating O2 , increasing the covalent bonding of lattice oxygen. This electron transfer-accelerated catalysis behavior in SSC-HG will provide design guidelines for composites of perovskite and carbon with bifunctional catalysts.

Replacing noble-metal-based oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts is the key to developing efficient Zn-air batteries (ZABs). Here, a homogeneous ternary Ni46 Co40 Fe14 nanoalloy with a size distribution of 30-60 nm dispersed in a carbon matrix (denoted as C@NCF-900) as a highly efficient bifunctional electrocatalyst produced via supercritical reaction and subsequent heat treatment at 900 °C is reported. Among all the transition-metal-based electrocatalysts, the C@NCF-900 exhibits the highest ORR performance in terms of half-wave potential (0.93 V) in 0.1 m KOH. Moreover, C@NCF-900 exhibits negligible activity decay after 10 000 voltage cycles with minor reduction (0.006 V). In ZABs, C@NCF-900 outperforms the mixture of Pt/C 20 wt% and IrO2 , cycled over 100 h under 58% depth of discharge condition. Furthermore, density functional theory (DFT) calculations and in situ X-ray absorption spectroscopy strongly support the active sites and site-selective reaction as a plausible ORR/OER mechanism of C@NCF-900.

Electrocatalysts facilitating oxygen reduction reaction (ORR) are vital components in advanced fuel cells and metal-air batteries. Here we report Ketjenblack incorporated nitrogen-doped carbon sheets derived from gelatin and apply these easily scalable materials as metal-free electrocatalysts for ORR. These carbon nanosheets demonstrate highly comparable catalytic activity for ORR as well as better durability than commercial Vulcan carbon supported Pt catalysts in alkaline media. Physico-chemical characterization and theoretical calculations suggest that proper combination of graphitic and pyridinic nitrogen species with more exposed edge sites effectively facilitates a formation of superoxide, [O2(ad)](-), via one-electron transfer, thus increasing catalytic activities for ORR. Our results demonstrate a novel strategy to expose more nitrogen doped edge sites by irregular stacked small sheets in developing better electrocatalysts for Zn-air batteries. These desirable architectures are embodied by an amphiphlilic gelatin mediated compatible synthetic strategy between hydrophobic carbon and aqueous water.