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Single‐atom catalysts (SACs) are widely studied in various chemical transformations due to their high catalytic activity and atom utilization. However, modulating the catalytic performance of catalysts by adjusting the microenvironment remains a great challenge. In this paper, a novel master‐double guest vulcanization‐assisted strategy is reported for synthesizing Fe single‐atom catalysts on N,S codoped porous hollow leaf carbon (FeSA/N,S‐PHLC) with highly exposed FeN3SOH sites. Fe(mIm) (guest I) is loaded on the surface of ZIF‐L (host) and then trithiocyanuric acid (TCA, guest II) is bonded with Fe(mIm), and the resulting ZIF‐L@Fe(mIm)@TCA precursors can be converted to FeSA/N,S‐PHLC with controllable structures. In addition, XPS analysis yield an increase in pyridine N content in FeSA/N,S‐PHLC compared to FeSA/N‐PHLC. It can be demonstrated by theoretical calculations that the N,S‐coordinated axially hydroxy‐coordinated asymmetric Fe centers synergistically with the abundance of pyridinic nitrogen facilitate the adsorption and desorption of oxygen intermediates, and the 3d orbitals of the Fe active centers can be optimized. The prepared FeSA/N,S‐PHLC catalyst has a half‐wave potential (E1/2) of 0.91 V under alkaline conditions, and E1/2 = 0.75 V under acidic conditions. It shows excellent cycle stability (882 h) and power density (217 mW cm−2) in the assembly of zinc–air battery (ZABs) devices. [ABSTRACT FROM AUTHOR]

Atomically dispersed metal‐nitrogen‐carbon catalysts have been extensively explored for various sustainable energy‐related reactions. From a material perspective, these catalysts are likely to consist of a combination of single‐atom, dual‐atom and possibly even multi‐atom sites. However, pinpointing their true active sites has remained a challenging task. In this study, a model catalyst is introduced, Co/CoMn‐NC, featuring both Co single‐atom sites and CoMn dual‐atom sites on a nitrogen‐doped carbon substrate. By employing a combination of X‐ray adsorption spectroscopy and density functional theory calculations, the atomic configuration of Co/CoMn‐NC has been determined. Density functional theory calculations are also used to unequivocally identify Co‐atom within the CoMn dual‐atom motif as the predominate active site of the Co/CoMn‐NC model catalyst toward oxygen reduction reaction (ORR), which is further confirmed by in situ Raman spectroscopy. The cooperative interactions between Co single‐atom sites and CoMn dual‐atom sites can finely tune the d‐band center and ameliorate the adsorption and desorption behaviors of the intermediates, thereby facilitating ORR kinetic. Overall, the study introduces a systematic strategy to elucidate the structure and the superiority of the model system and provides new insights into atomically dispersed multi‐metal active sites, showcasing that enhanced catalytic performance extends beyond unified diatomic sites or monatomic sites. [ABSTRACT FROM AUTHOR]

Challenges in rational designing dual‐atom catalysts (DACs) give a strong motivation to construct coordination‐activity correlations. Here, thorough coordination‐activity correlations of DACs based on how the changes in coordination shells (CSs) of dual‐atom Cu,Co centers influence their electrocatalytic activity in oxygen reduction reaction(ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is constructed. First, Cu,Co DACs with different CSs modifications are fabricated by using a controlled "precursors‐preselection" approach. Three DACs with unique coordination environments are characterized as secondary S atoms that directly bond to Cu,Co‐N6 in lower CSs, indirectly bond in neighboring CSs, and are doped in higher CSs, respectively. Then, experimentally and theoretically, a coordination correlation resembling a planet‐satellite system, where satellite coordinated atoms (heteroatom N, S) surround Cu‐Co dual‐atom entity in various orbitals CSs. By evaluating electrocatalytic activity indicators, differences are identified in electronic structure and electrocatalytic performance of Cu and Co centers in ORR, OER, and HER. Interestingly, initial CSs modifications for DACs may not always be advantageous for electrocatalysis. This work offers valuable insight for designing DACs for diverse applications. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR) is the core reaction of fuel cell/metal–air battery at the cathode, and it is of great significance to develop high‐performance ORR catalyst. Generally speaking, ORR catalysts are classified according to the class of materials they use, which often makes it difficult for different types of catalysts to have a clear connection with some unified optimization mechanism and creates difficulties in designing improvement strategies. Here, this review proposes three major strategies that can affect the activity of ORR catalysts: designing morphology, defect control, and heteroatom doping. The core content of this review is to analyze the principles of various optimization strategies that affect the performance of catalysts individually or synergistically at the micro level. In addition, the problems and challenges still faced in the design of ORR catalysts are sorted out, and possible solutions are proposed. This review makes the research direction of ORR catalysts clearer, and provides theoretical support and new ideas for the design of high‐efficiency electrocatalysts. [ABSTRACT FROM AUTHOR]

Dual–single‐atom catalysts with synergistic effect of adjacent atomic metal sites show a great potential for oxygen reduction reaction (ORR). Herein, a dynamical synthetic strategy is demonstrated for the rational design of dual‐atom catalyst ((Zn, Cu)−NC) with non‐covalent Cu and Zn sites as nitrogen‐doped carbon as support. Owing to the non‐covalent interaction of Zn and Cu atomic pair sites, (Zn, Cu)−NC exhibits significant performances for ORR, surpassing the catalysts with individual Zn or Cu site. The theoretical calculations reveal that (Zn, Cu)−NC can highly activate the linear O2 molecule via the non‐covalent interaction between Zn and Cu pairs, providing the more effective overlap between the metal 3d orbitals and O 2p orbital. Therefore, the ORR activity is optimized with the improvement of the adsorption configuration and adsorption energy of O2. Further, both liquid and quasi‐solid zinc−air batteries with (Zn, Cu)−NC as air cathodes achieve remarkable energy density and stability. This research proposes a facile synthetic strategy to construct single‐atom catalysts and presents an insightful understanding of the non‐covalent interplay between heteronuclear metal atoms in dual‐atom catalysts. [ABSTRACT FROM AUTHOR]

Surging interests exist in double‐atom catalysts (DACs), which not only inherit the advantages of single‐atom catalysts (SACs) (e.g., ultimate atomic utilization, high activity, and selectivity) but also overcome the drawbacks of SACs (e.g., low loading and isolated active site). The design of DACs, however, remains cost‐prohibitive for both experimental and computational studies, due to their huge design space. Herein, by means of density functional theory (DFT) and topological information‐based machine‐learning (ML) algorithms, we present a data‐driven high‐throughput design principle to evaluate the stability and activity of 16 767 DACs for oxygen evolution (OER) and oxygen reduction (ORR) reactions. The rational design reveals 511 types of DACs with OER activity superior to IrO2 (110), 855 types of DACs with ORR activity superior to Pt (111), and 248 bifunctional DACs with high catalytic performance for both OER and ORR. An intrinsic descriptor is revealed to correlate the catalytic activity of a DAC with the electronic structures of the DAC and its bonding carbon surface structure. This data‐driven high‐throughput approach not only yields remarkable prediction precision (>0.926 R‐squared) but also enables a notable 144 000‐fold reduction of screening time compared with pure DFT calculations, holding promise to drastically accelerate the design of high‐performance DACs. [ABSTRACT FROM AUTHOR]

Single‐atom catalysts (SACs) have shown great potential in the electrochemical oxygen reduction reaction (ORR) toward hydrogen peroxide (H2O2) production. However, current studies are mainly focused on 3d transition‐metal SACs, and very little attention has been paid to 5d SACs. Here, a new kind of W SAC anchored on a porous O, N‐doped carbon nanosheet (W1/NO‐C) is designed and prepared via a simple coordination polymer‐pyrolysis method. A unique local structure of W SAC, terdentate W1N1O2 with the coordination of two O atoms and one N atom, is identified by the combination of aberration‐corrected scanning transmission electron microscopy, X‐ray photoelectron spectroscopy and X‐ray absorption fine structure spectroscopy. Remarkably, the as‐prepared W1/NO‐C catalyzes the ORR via a 2e– pathway with high onset potential, high H2O2 selectivity in the wide potential range, and excellent operation durability in 0.1 m KOH solution, superior to most of state‐of‐the‐art H2O2 electrocatalysts ever reported. Theoretical calculations reveal that the C atoms adjacent to O in the W1N1O2‐C moiety are the most active sites for the 2e– ORR to H2O2 with the optimal binding energy of the HOO* intermediate. This work opens up a new opportunity for the development of high‐performance W‐based catalysts for electrochemical H2O2 production. [ABSTRACT FROM AUTHOR]

Atomic transition‐metal‐nitrogen‐carbon catalysts (M‐N‐Cs) hold great promise as Pt‐group‐metal‐free candidates for electrochemical reactions, yet their rational design and controllable synthesis remain fundamental challenges. Here, the molten‐salts mediated pyrolysis is demonstrated to be an effective and facile strategy for simultaneous morphology and electronic structure modulation of prototypical Fe‐N‐C materials, which functions as efficient oxygen reduction electrocatalysts. Taking advantage of the strong polarity and salt templating effects, the as‐obtained Fe‐N/C‐single atom catalyst (SAC) possesses hierarchical porous nanosheet morphology with an impressive specific surface area of 2237 m2 g−1 and unique FeN4Cl moieties as isolated active centers. The Fe‐N/C‐SAC delivers remarkable alkaline oxygen reduction reaction (ORR) activity with a half‐wave potential of 0.91 V and record kinetic current density up to 55 mA cm−2, outperforming the benchmark Pt/C. By virtue of dechlorination treatment, it is experimentally identified that the enhanced ORR activities are essentially governed by the axially bound Cl. Theoretical calculations rationalize this finding and demonstrate that the well‐defined fivefold‐coordinated configuration accelerates 4e− pathway kinetics through near‐optimal adsorption of the *OH intermediates and tunes the potential determining step from *OH reduction to *OOH formation. This study provides fundamental insights into the coordination‐engineered strategy in single‐atom catalysis. [ABSTRACT FROM AUTHOR]

The decomposition, electron transfer, and protonation of oxygen molecules are typically assumed to be the rate‐limiting steps of the oxygen reduction reactions (ORR), and the activation energy barriers of these reactions can be surmounted using catalysts. In this study, the physical rate‐limiting step of the ORR consists of the adsorption of gaseous oxygen molecules at the liquid–solid phase boundary, indicating that the formation of a gas–liquid–solid triple‐phase boundary (TPB) is important for accelerating the ORR kinetics. This is experimentally confirmed by analyzing the ORR in aluminum–air batteries. Moreover, the formation of a TPB using the hierarchical pores of sparked reduced graphene oxide is demonstrated, which serve as the cathode, and the remarkable electrochemical performance of the fabricated battery is presented. These findings can be used to accelerate the ORR kinetics by maximizing the TPB, particularly in aluminum–air batteries. [ABSTRACT FROM AUTHOR]

One of the main bottlenecks that limit the performance of reversible protonic ceramic electrochemical cells (R‐PCECs) is the sluggish kinetics of the oxygen reduction and evolution reactions (ORR and OER). Here, the significantly enhanced ORR and OER kinetics and stability of a conventional La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) air electrode by an efficient catalyst coating of barium cobaltite (BCO) is reported. The polarization resistance of a BCO‐coated LSCF air electrode at 600 °C is 0.16 Ω cm2, about 30% of that of the bare LSCF air electrode under the same conditions. Further, an R‐PCEC with the BCO‐coated LSCF air electrode shows exceptional performance in both fuel cell (peak power density of 1.16 W cm−2 at 600 °C) and electrolysis (current density of 1.80 A cm−2 at 600 °C at 1.3 V) modes. The performance enhancement is attributed mainly to the facilitated rate of oxygen surface exchange. [ABSTRACT FROM AUTHOR]

Transition metal single‐atom catalysts (SACs) are currently a hot area of research in the field of electrocatalytic oxygen reduction reaction (ORR). In this review, the recent advances in transition metal single‐atom supported by 2D materials as catalysts for ORR with high performance are reported. Due to their large surface area, uniformly exposed lattice plane, and adjustable electronic state, 2D materials are ideal supporting materials for exploring ORR active sites and surface reactions. The rational design principles and synthetic strategies of transition metal SACs supported by 2D materials are systematically introduced while the identification of active sites, their possible catalytic mechanisms as well as the perspectives on the future of transition metal SACs supported by 2D materials for ORR applications are discussed. Finally, according to the current development trend of ORR catalysts, the future opportunities and challenges of transition metal SACs supported by 2D materials are summarized. [ABSTRACT FROM AUTHOR]

Although Pd is a potential substitution of Pt‐based catalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), the binding of *H and oxygenated (*O, *OOH, *OH) intermediates on Pd are stronger than on Pt, leading to its inferior activity for HER and ORR. In this work, CuPd/Pd core/shell nanoparticles with an ultrathin Pd shell (0.5 nm) are developed, which demonstrate the Pt‐like bifunctional activity for HER and ORR in acid electrolytes. The overpotential at 350 mA cm−2 for HER and the half‐wave potential for ORR on the optimal CuPd/Pd core/shell NPs are 76 mV and 0.854 V versus reversible hydrogen electrode (RHE), respectively, which are comparable to that of Pt and among the best of the reported Pd‐based catalysts. Density functional theory calculations indicate that the significantly enhanced HER/ORR activity on CuPd/Pd core/shell NPs with 0.5 nm Pd shell stem from the compressive strain induced downshift of d‐band center for Pd (by 2.0%), which weakens the binding strength of *H and oxygenated intermediates and promotes the reaction kinetics. [ABSTRACT FROM AUTHOR]

An Fe/N/C catalyst derived from the pyrolysis of metal–organic frameworks, for example, a zeolitic‐imidazolate‐framework‐8 (ZIF‐8), has been regarded as one of the most promising non‐precious metal catalysts toward oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, its ORR mass activity is still much inferior to that of Pt, partly because of the lack of general and efficient synthetic strategies. Herein, a general carboxylate‐assisted strategy that dramatically enhances the ORR mass activity of ZIF‐derived Fe/N/C catalysts is reported. The carboxylate is found to promote the formation of Fe/N/C catalysts with denser accessible active sites and entangled carbon nanotubes, as well as a higher mesoporosity. These structural advantages make the carboxylate‐assisted Fe/N/C catalysts show a 2–10 fold higher ORR mass activity than the common carboxylate‐free one in various cases. When applied in H2–O2 PEMFCs, the active acetate‐assisted Fe/N/C catalyst generates a peak power density of 1.33 W cm−2, a new record of peak power density for a H2–O2 PEMFC with non‐Pt ORR catalysts. [ABSTRACT FROM AUTHOR]

Precise electrochemical synthesis under ambient conditions has provided emerging opportunities for renewable energy utilization. Among many promising systems, the production of hydrogen peroxide (H2O2) from the cathodic oxygen reduction reaction (ORR) has attracted considerable interest in past decades due to the increasing market demands and the vital role of ORR in the electrocatalysis field. This work describes recent advances in cathodic materials for H2O2 synthesis from 2e- ORR. By using Pt as a stereotype, the tuning knobs are overviewed, including the intrinsic binding strength of oxygenated species, the intermediate diffusion path and the isolation of Pt–Pt ensembles that enable 2e- ORR pathway from 4e- total reduction. This knowledge is successfully applied to other transition metal systems and leads to the discovery of more efficient alloy catalysts with balanced improvement on both activity and selectivity. In addition, mesostructure engineering and heteroatoms doping strategies on carbon‐based materials, which significantly boost the H2O2 production efficiency as compared to intact carbon sites, are also reviewed. Finally, future directions and challenges of transferring developed catalysts from lab scale tests to pilot plant operations are briefly outlooked. [ABSTRACT FROM AUTHOR]

Au‐incorporation is a promising strategy to retard composition‐loss in Pt‐based catalyst. However, the unclear mechanism limits guided catalyst design and the performance optimization. Here, direct evidence is provided to validate the outward diffusion of Au atoms in Au‐core/Pt‐based‐shell structures. A Co interlayer is built between the Au‐core and PtCo‐based shell to exclude the possibility of atomic diffusion caused by interfacial alloying. In conjunction with the improved catalytic durability of the Au‐core@Pt‐based‐shell structure, it is reasonable to conclude that it is the subsurface segregated Au atoms rather than interfacial interaction that boosts the catalytic durability of Au‐core/Pt‐based‐shell structured catalysts towards oxygen reduction reaction. More importantly, by constructing Au‐core@Co‐interlayer@PtCoAu‐shell multilayer structure, the specific (1.730 mA cm−2) and mass (0.692 A mg−1Pt) activities are enhanced 7‐ and 4‐ fold relative to the commercial Pt/C. After 10 000 cycles of accelerated durability test, the mass activity loss for the multilayered catalyst is as low as 6.14% while the loss exceeds 35% for the commercial Pt/C catalyst. The improved catalytic performance of the Au@Co@PtCoAu multilayer structure can be ascribed to the finely modulated electronic structure and the compensated composition loss owing to the delicate structure and composition profile design. [ABSTRACT FROM AUTHOR]

A highly active iron-nitrogen-doped carbon nanotube catalyst for the oxygen reduction reaction (ORR) is produced by employing vertically aligned carbon nanotubes (VA-CNT) with a high specific surface area and iron(II) phthalocyanine (FePc) molecules. Pyrolyzing the composite easily transforms the adsorbed FePc molecules into a large number of iron coordinated nitrogen functionalized nanographene (Fe-N-C) structures, which serve as ORR active sites on the individual VA-CNT surfaces. The catalyst exhibits a high ORR activity, with onset and half-wave potentials of 0.97 and 0.79 V, respectively, versus reversible hydrogen electrode, a high selectivity of above 3.92 electron transfer number, and a high electrochemical durability, with a 17 mV negative shift of E1/2 after 10 000 cycles in an oxygen-saturated 0.5 m H2SO4 solution. The catalyst demonstrates one of the highest ORR performances in previously reported any-nanotube-based catalysts in acid media. The excellent ORR performance can be attributed to the formation of a greater number of catalytically active Fe-N-C centers and their dense immobilization on individual tubes, in addition to more efficient mass transport due to the mesoporous nature of the VA-CNTs. [ABSTRACT FROM AUTHOR]

Fe-N-C Catalysts: Direct Growth of Carbon Nanotubes Doped with Single Atomic Fe-N4 Active Sites and Neighboring Graphitic Nitrogen for Efficient and Stable Oxygen Reduction Electrocatalysis (Adv. Funct. In article number 1906174, Jia Li, Lin Gan, Feiyu Kang, and co-workers report the direct growth of carbon nanotubes doped with Fe-N SB 4 sb moieties for improved electrocatalytic stability in the cathodic oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells. By increasing the neighboring graphitic nitrogen, the Fe-N SB 4 sb moieties also show enhanced activity due to the modified spin state of the Fe center and thereby optimized binding energy with ORR intermediates. [Extracted from the article]

Carbon‐supported low‐Pt ordered intermetallic nanoparticulate catalysts (PtM3, M = Fe, Co, and Ni) are explored in order to enhance the oxygen reduction reaction (ORR) activity while achieving a high stability compared to previously reported Pt‐richer ordered intermetallics (Pt3M and PtM) and low‐Pt disordered alloy catalysts. Upon high‐temperature thermal annealing, ordered PtCo3 intermetallic nanoparticles are successfully prepared with minimum particle sintering. In contrast, the PtFe3 catalyst, despite the formation of ordered structure, suffers from obvious particle sintering and detrimental metal–support interaction, while the PtNi3 catalyst shows no structural ordering transition at all but significant particle sintering. The ordered PtCo3 catalyst exhibits durably thin Pt shells with a uniform thickness below 0.6 nm (corresponding to 2–3 Pt atomic layers) and a high Co content inside the nanoparticles after 10 000 potential cycling, leading to a durably compressive Pt surface and thereby both high activity (fivefold vs a commercial Pt catalyst and 1.7‐fold vs an ordered PtCo intermetallic catalyst) and high durability (5 mV loss in half‐wave potential and 9% drop in mass activity). These results provide a new strategy toward highly active and durable ORR electrocatalysts by rational development of low‐Pt ordered intermetallics. [ABSTRACT FROM AUTHOR]