Great enthusiasm in single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) has been aroused by the discovery of M−N X as a promising ORR catalysis center. However, the performance of SACs lags far behind that of stateof-the-art Pt due to the unsatisfactory adsorption−desorption behaviors of the reported catalytic centers. To address this issue, rational manipulation of the active site configuration toward a well-managed energy level and geometric structure is urgently desired, yet still remains a challenge. Herein, we report a novel strategy to accomplish this task through the construction of an Fe−Co dual-atom centered site. A spontaneously absorbed electron-withdrawing OH ligand was proposed to act proactively as an energy level modifier to empower easy intermediate desorption, while the triangular Fe−Co−OH coordination facilitates O−O bond scission. Benefiting from these attributes, the as-constructed FeCoN 5 −OH site enables an ORR onset potential and half-wave potential of up to 1.02 and 0.86 V (vs RHE), respectively, with an intrinsic activity over 20 times higher than the single-atom FeN 4 site. Our finding not only opens up a novel strategy to tailor the electronic structure of an atomic site toward boosted activity but also provides new insights into the fundamental understanding of diatomic sites for ORR electrocatalysis.

Developing highly efficient, low-cost oxygen reduction catalysts, especially in acidic medium, is of significance toward fuel cell commercialization. Although pyrolyzed Fe-N-C catalysts have been regarded as alternatives to platinumbased catalytic materials, further improvement requires precise control of the Fe-N x structure at the molecular level and a comprehensive understanding of catalytic site structure and the ORR mechanism on these materials. In this report, we present a microporous metal−organic-framework-confined strategy toward the preferable formation of single-atom dispersed catalysts. The onset potential for Fe-N-C is 0.92 V, comparable to that of Pt/C and outperforming most noble-metal-free catalysts ever reported. A high-spin Fe 3+ -N 4 configuration is revealed by the 57 Fe Mossbauer spectrum and X-ray absorption spectroscopy for Fe L-edge, which will convert to Fe 2+ -N 4 at low potential. The in situ reduced Fe 2+ -N 4 moiety from high-spin O x -Fe 3+ -N 4 contributes to most of the ORR activity due to its high turnover frequency (TOF) of ca. 1.71 e s −1 sites −1 .

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Single‐atom catalysts (SACs) are attracting widespread interest for the catalytic oxygen reduction reaction (ORR), with Fe−Nx SACs exhibiting the most promising activity. However, Fe‐based catalysts suffer serious stability issues as a result of oxidative corrosion through the Fenton reaction. Herein, using a metal‐organic framework as an anchoring matrix, we for the first time obtained pyrolyzed Cr/N/C SACs for the ORR, where the atomically dispersed Cr is confirmed to have a Cr−N4 coordination structure. The Cr/N/C catalyst exhibits excellent ORR activity with an optimal half‐wave potential of 0.773 V versus RHE. More excitingly, the Fenton reaction is substantially reduced and, thus, the final catalysts show superb stability. The innovative and robust active site for the ORR opens a new possibility to circumvent the stability issue of the non‐noble metal ORR catalysts.

Emerging as a new frontier in heterogeneous catalysis, single-atom site catalysts (SSCs) have sparked enormous attention and bring about new opportunities to oxygen reduction electrocatalysis. Despite considerable progress achieved recently, most of the reported SSCs suffer from either insufficient activity or unsatisfactory stability, which severely retards their practical application. Here, we demonstrate a novel Ru-SSC with appropriate adsorption free energy of OH* (ΔG OH*) to confer excellent activity and low Fenton reactivity to maintain long-term stability. The as-developed Ru-SSC exhibits encouraging oxygen reduction reaction turnover frequency of 4.99 e– s–1 sites–1, far exceeding the state-of-the-art Fe-SSC counterpart (0.816 e– s–1 sites–1), as a result of Ru energy level regulation via spontaneous OH binding. Furthermore, Ru-SSC exhibits greatly suppressed Fenton reactivity, with restrained generation of reactive oxygen species directly observed, thus endowing the Ru-SSC with much more superior stability (only 17 mV negative shift after 20 000 cycles) than the Fe-SSC counterpart (31 mV). The practical application of Ru-SSC is further validated by its excellent activity and stability in a real fuel cell device.

conversion system due to their high energy density, low cost, and zero emission. [3-6] However, the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during the charge and discharge process result in high overpotential and low specific energy, thereby impeding the commercialization of Zn-air batteries. [7-12] Motivated by this challenge, great endeavors have been paid to exploit oxygen electrocatalysts with accelerated kinetics, with noble-metalbased materials known as the mainstream catalysts (Pt for ORR, Ir and Ru for OER). These catalysts, however, are only function actively to one of the two essential reactions, behave inactively to the other, and are criticized for their expensive and scarce nature. Therefore, developing costeffective and efficient bifunctional oxygen electrocatalysts is regarded a cutting edge and essential research topic. [13-16] The emergence of single-atom catalysts (SACs) is bringing out new opportunities to oxygen electrocatalysis. SACs based M-N-C with varied metal centers (M represents Fe, Co, Cu, Mn, Ni, Zn, etc.) have been explored as oxygen electrocatalysts. [17-24] Among them, single-atom Fe-N-C catalysts with FeN 4 Single-atom FeN 4 sites at the edges of carbon substrates are considered more active for oxygen electrocatalysis than those in plane; however, the conventional high-temperature pyrolysis process does not allow for precisely engineering the location of the active site down to atomic level. Enlightened by theoretical prediction, herein, a self-sacrificed templating approach is developed to obtain edge-enriched FeN 4 sites integrated in the highly graphitic nanosheet architecture. The in situ formed Fe clusters are intentionally introduced to catalyze the growth of graphitic carbon, induce porous structure formation, and most importantly, facilitate the preferential anchoring of FeN 4 to its close approximation. Due to these attributes, the as-resulted catalyst (denoted as Fe/N-G-SAC) demonstrates unprecedented catalytic activity and stability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) by showing an impressive half-wave potential of 0.89 V for the ORR and a small overpotential of 370 mV at 10 mA cm −2 for the OER. Moreover, the Fe/N-G-SAC cathode displays encouraging performance in a rechargeable Zn-air battery prototype with a low charge-discharge voltage gap of 0.78 V and long-term cyclability for over 240 cycles, outperforming the noble metal benchmarks.

Rational design of electrocatalysts to replace the noble-metal-based materials for oxygen reactions is highly desirable but challenging for rechargeable metal–air batteries. Herein, we demonstrate a unique two stage encapsulation strategy to regulate the structure and performance of catalysts featured with thin graphene nanosheets coupling with full encapsulated ultrafine and high-loaded (∼25 wt %) transition metal nanoparticles (TMs@NCX) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). By optimizing the electronic modulation effect from suitable metal cores, the best NiFe@NCX catalyst exhibits high stability and activity with an onset potential of 1.03 V for ORR and an overpotential of only 0.23 V at 10 mA cm–2 for OER, which is superior to commercial Pt/C and IrO2 catalysts. Rechargeable Zn–air battery using NiFe@NCX catalyst exhibited an unprecedented small charge–discharge overpotential of 0.78 V at 50 mA cm–2, high reversibility, and stability, holding great promise for the practical implementation of rechargeable metal–air batteries.