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Sodium‐ion batteries (SIBs), which are an alternative to lithium‐ion batteries (LIBs), have attracted increasing attention due to their low cost of Na resources and similar Na storage mechanism to LIBs. Compared with anode materials and electrolytes, the development of cathode materials lags behind. Therefore, the key to improving the specific energy and promoting the application of SIBs is to develop high‐performance sodium intercalation cathode materials. Transition‐metal oxides are one of the most promising cathode materials for SIBs owing to their excellent energy density, high specific discharge capacity, and environmentally friendly nature. In the present work, the latest progress in the research of transition‐metal oxides is summarized. Moreover, the existing challenges are discussed, and a series of strategies are proposed to overcome these drawbacks. This review aims at providing guidance for the development of metal oxides in the next stage. [ABSTRACT FROM AUTHOR]

The accessibility to highly active oxygen reduction reaction (ORR) catalysts with low cost is a priority for the rapid industrialization of Zn‐Air batteries. Herein, we propose a facile engineering synthesis of high‐activity quasi‐spherical Fe−N−C ORR nanocatalysts by chemical self‐assmbly. The morphological and structural features of the materials have been fully characterized. The synchrotron radiation techniques reveal that the targeted Fe−N−PQS‐900 with highly exposed Fe−Nx active sites well dispersed on the N‐rich micro‐nano carbon framework, endowing them with efficient access to oxygeneous species. The electrochemical measurements show its superior ORR catalytic performance to the commercial Pt/C catalyst, including a more positive half‐wave potential, smaller Tafel slope, better methanol‐tolerance capability, and more sustainable cycloability as well. An assembled Zn‐Air battery using the Fe−N−PQS‐900 as the cathode catalyst shows a specific capacity of 798 mAh g−1 with a power density of 114 mW cm−2 and an energy density of 958 Wh kg−1, enabling this non‐precious metal electrocatalyst very competitive in metal air fuel cell applications. [ABSTRACT FROM AUTHOR]

Development of fuel cells is greatly hindered by their sluggish oxygen reduction reaction (ORR). Therefore, rational design and cost‐effective fabrication of ORR catalysts with high activity and durability are of paramount importance to the commercialization of fuel cells. In this work, we develop one‐step synthesis of nitrogen‐doped Fe/Fe3C‐encapsulated graphitic layers/carbon nanotubes (Fe/Fe3C@NGL‐NCNT) for ORR. The interpenetration of the two‐dimensional (2D) NGL and the one‐dimensional (1D) NCNT conduces to the construction of three‐dimensional (3D) network structure. Such heterostructure renders Fe/Fe3C@NGL‐NCNT with different catalytically active sites and improved mass transfer. Fuethermore, the inimitable NCNT with winding earthworm‐like structure and a small diameter of below 10 nm provides plentiful active defect sites. Consequently, the synergistic effect of unique 1D/2D/3D heterostructure endows Fe/Fe3C@NGL‐NCNT catalyst with a superior activity and durability compared to commercial Pt/C catalyst. This work provides worthy inspiration for engineering excellent ORR electrocatalysts by integrating 1D/2D/3D materials with different catalytically active conponents into one heterostructure via in situ synthesis. [ABSTRACT FROM AUTHOR]

The precisely manipulated π–π stacking of the hybrid catalyst favors the specific prediction of the ORR active sites, which are the iron atoms in the form of natural Fe-N 4 that bind preferentially to the oxygen. [Display omitted] Metal macrocycles with well-defined molecular structures are ideal platforms for the in-depth study of electrochemical oxygen reduction reaction (ORR). Structural integrity of metal macrocycles is vital but remain challenging since the commonly used high-temperature pyrolysis would cause severe structure damage and unidentifiable active sites. Herein, we propose a pyrolysis-free strategy to precisely manipulate the exfoliated 2D iron polyphthalocyanine (FePPc) anchored on reduced graphene oxide (rGO) via π–π stacking using facile high-energy ball milling. A delocalized electron shift caused by π–π interaction is firstly found to be the mechanism of facilitating the remarkable ORR activity of this hybrid catalyst. The optimal FePPc@rGO-HE achieves superior half-wave potential (0.90 V) than 20 % Pt/C. This study offers a new insight in designing stable and high-performance metal macrocycle catalysts with well-defined active sites. [ABSTRACT FROM AUTHOR]

Bifunctional catalyst was prepared by one-step pyrolysis of trimetallic ZIF. The obtained catalyst showed excellent charge–discharge stability in rechargeable Zn-air battery. • A facile and effect strategy is proposed for the synthesis of catalyst precursor. • This strategy reduces the adverse effects of the pyrolysis process on the catalyst. • The coexistence of Co NPs and CoFe-N x improves the ORR/OER activity. • The catalyst displays excellent durability as a cathode for Zn-air battery. Searching for high active, low cost and durable catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) remains challenge in developing metal-air battery cathodes. Herein, we proposed a novel bifunctional catalyst derived from the pyrolysis of Co/Fe/Zn trimetallic zeolitic imidazolite framework. The obtained Co@CoFe 0.01 N C catalyst displays desirable activity for both ORR and OER (E 1/2 = 0.844 V, E j=10 = 1.654 V). The Zn-air battery equipped with Co@CoFe 0.01 N C catalyst on the cathode exhibits a high peak power density of 174.1 mW cm−2, which is much superior than that of commercial 20% Pt/C (87.6 mW cm−2). Significantly, the designed Co@CoFe 0.01 N C presents an outstanding stability for over 100 h in rechargeable Zn-air battery. [ABSTRACT FROM AUTHOR]

Sluggish oxygen reduction reaction (ORR) kinetics is the biggest obstacle in the development of metal air batteries. High-efficiency ORR catalysts with low cost are in urgent needs. Herein, we propose a robust Fe–N–C ORR catalyst with highly dispersed Fe-N x active sites on graphitic-N dominated porous carbon. The synchrotron radiation analyses reveal the discrete Fe-N x coordination derived from the Prussian blue (PB) while the graphitic-N doped porous carbon formed by pyrolysis of the polyaniline (PANI). In virtue of their synergistic catalysis, the tailored Fe–N–C catalyst shows a comparable catalytic activity and a better methanol-tolerance than the commercial Pt/C catalyst. The assembled Zn-Air battery demonstrates a specific capacity of 783 mAh g−1 as well as energy density of 917 Wh kg−1 at 10 mA cm−2, and a maximum power density of 112 mW cm−2 at 191 mA cm−2. A robust Fe– N –C ORR catalyst derived from PB-anchored PANI with well-dispersed Fe-N x active sites on graphitic-N dominated porous carbon host shows comparable performance to the commercial Pt/C catalyst. Image 1 [ABSTRACT FROM AUTHOR]

Exploring efficient and durable non-precious metal catalysts for oxygen reduction reaction (ORR) has long been pursued in the field of metal-air batteries, fuel cells, and solar cells. Rational design and controllable synthesis of desirable catalysts are still a great challenge. In this work, a novel approach is developed to tune the morphologies and structures of Fe–N–C catalysts in combination with the dual nitrogen-containing carbon precursors and the gas-foaming agent. The tailored Fe–N 1 /N 2 –C-A catalyst presents gauze-like porous nanosheets with uniformly dispersed tiny nanoparticles. Such architectures exhibit abundant Fe-N x active sites and high-exposure surfaces. The Fe–N 1 /N 2 –C-A catalyst shows extremely high half-wave potential (E 1/2 , 0.916 V vs. RHE) and large limiting current density (6.3 mA cm−2), far beyond 20 wt% Pt/C catalyst for ORR. This work provides a facile morphological and structural regulation to increase the number and exposure of Fe-N x active sites. The tailored Fe–N–C catalyst presents plentiful Fe-N x active sites and highly exposed active surfaces, resulting in an ultra-high ORR activity far beyond the 20 wt% Pt/C catalyst. Image 1 • A novel approach is explored to tune morphology and structure of Fe–N–C catalyst. • The tailored catalyst presents abundant active sites and high exposure surfaces. • The catalyst shows ultrahigh ORR activity, far beyond the commercial Pt/C. [ABSTRACT FROM AUTHOR]

Transition metal- and N-codoped carbon catalysts with low cost are latent substitutes to Pt-based catalysts in oxygen reduction reaction. However, the variety of N species obtained by pyrolysis often makes the determination of the active site controversial. Here, we design catalyst structures with pyrrolic-N anchored dual metal centers, while the presence of pyrrolic-N avoids the complex and variable structure of the active sites. Theory calculations demonstrate that coupling of the Cu–N 4 single-atom sites is favorable to reduce the reaction energy barrier of pyrrolic-N-type Fe–N 4 active site during the reaction process. Subsequently, the high-purity pyrrolic-N-type iron and copper single-atom catalyst is successfully synthesized to verify the hypothesis. The obtained FeCu–N–C catalyst achieves a half-wave potential of 0.913 V, higher than the 0.839 V of Pt/C. This route may provide new insights for regulating catalytic active sites in high purity pyrrolic-N type catalysts. The introduction of Cu single-atom optimizes the pyrrolic-N-type Fe–N 4 active site of the FeCu–N–C, endowing the catalyst with remarkable ORR activity (E 1/2 = 0.913 V). [Display omitted] • A pyrrolic-N anchors dual metal centers structure is designed. • DFT calculations demonstrate Cu doping can improve the ORR activity of iron sites. • Catalysts with high purity pyrrolic-N are prepared to prove the DFT calculations. • The half-wave potential of the obtained FeCu–N catalyst reaches 0.913 V. • This strategy provides new insights for regulating active sites of ORR. [ABSTRACT FROM AUTHOR]

[Display omitted] • A Co/Ni-free low-cost P 2 layered sodium ions metal oxides cathode materials. • Highly reversibility and negligible phase transition with obvious anion reduction phenomenon. • XPS analysis proves that a large proportion of capacity above 4.3 V mainly originate form anion reduction. • DFT calculations suggest that the existence of weak Cu–O interaction stimulates oxygen-related activity. To improve the energy and power density of P 2-type layered cathode materials in sodium ion batteries, triggering oxygen-related activity is a promising strategy, which has attracted an amount of attention. However, stimulating lattice oxygen activity may inevitably yield adverse effects, such as detrimental structure distortion and irreversible voltage decay. Herein, a low-cost Co/Ni free layered P 2-type Na 0.67 Mn 0.6 Cu 0.3 Mg 0.1 O 2 cathode is designed to construct reversible anion reduction. Na 0.67 Mn 0.6 Cu 0.3 Mg 0.1 O 2 exhibits an obvious anion reduction electrochemical behavior, confirming that oxygen occurred redox reactions and participated in the capacity compensation process. The density functional theory calculation proves that the presence of the covalency between Cu and O and the lager electron delocalization areas increase the anions redox activity. Furthermore, this compound provides an excellent cyclic stability and high capacity retention of 86 % at 0.2C rate after 100 cycles. In-situ Raman and ex-situ X-ray diffraction absorption spectroscopy reveal the reversible structure evolution and robust P 2-type structure. The interaction between Cu and Mg plays a positive role in building the reversible anion reduction in terms of constructing electronic structure and regulating charge balance. Therefore, the Na 0.67 (Mn-Cu-Mg)O 2 system provides an insight into designing a highly stable cathode material with anion reduction for sodium ion batteries. [ABSTRACT FROM AUTHOR]