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Iron–nitrogen–carbon (Fe–N–C) materials have been identified as a promising class of platinum (Pt)-free catalysts for the oxygen reduction reaction (ORR). However, the dissolution and oxidation of Fe atoms severely restrict their long-term stability and performance. Modulating the active microstructure of Fe–N–C is a feasible strategy to enhance the ORR activity and stability. Compared with common 3d transition metals (Co, Ni, etc.), the 4d transition metal atom Nb has fewer d electrons and more unoccupied orbitals, which could potentially forge a more robust interaction with the Fe site to optimize the binding energy of the oxygen-containing intermediates while maintaining stability. Herein, an asymmetric Fe–Nb diatomic site catalyst (FeNb/c-SNC) was synthesized, which exhibited superior ORR performance and stability compared with those of Fe single-atom catalysts (SACs). The strong interaction within the Fe–Nb diatomic sites optimized the desorption energy of key intermediates (*OH), so that the adsorption energy of Fe–*OH approaches the apex of the volcano plot, thus exhibiting optimal ORR activity. More importantly, introducing Nb atoms could effectively strengthen the Fe–N bonding and suppress Fe demetalation, causing an outstanding stability. The zinc–air battery (ZAB) and hydroxide exchange membrane fuel cell (HEMFC) equipped with our FeNb/c-SNC could deliver high peak power densities of 314 mW cm–2and 1.18 W cm–2, respectively. Notably, the stable operation time for ZAB and HEMFC increased by 9.1 and 5.8 times compared to Fe SACs, respectively. This research offers further insights into developing stable Fe-based atomic-level catalytic materials for the energy conversion process.

Single-atom catalysts (SACs) with unitary active sites hold great promise for realizing high selectivity toward a single product in the CO2electroreduction reaction (CO2RR). However, achieving high Faradaic efficiency (FE) of multielectron products like methane on SACs is still challenging. Herein, we report a pressure-regulating strategy that achieves 83.5 ± 4% FE for the CO2-to-CH4conversion on the asymmetric Cu–N2sites, representing one of the best CO2-to-CH4performances. Elevated CO2pressure was demonstrated as an efficient way to inhibit the hydrogen evolution reaction via promoting the competing adsorption of reactant CO2, regardless of the nature of the active sites. Meanwhile, the asymmetric Cu–N2structure could endow the Cu sites with stronger electronic coupling with *CO, thus suppressing the desorption of *CO and facilitating the following hydrogenation of *CO to *CHO. This work provides a synergetic strategy of the pressure-induced reaction environment regulating and the electronic structure modulating for selective CO2RR toward targeted products.

Electrocatalytic CO2reduction to valuable chemicals with exceptional selectivity is highly desirable. However, it is still challenging to bypass the competitive hydrogen evolution reaction (HER) and branched pathways via regulating the structure of catalysts. Herein, we disclose an unconventional solvent regulation strategy to realize near-unity CO2-to-CO conversion across diverse metal catalysts including Pt, Fe, Co, Ni, Mo, W, Nb, Cu, Ag, Au, Zn, Sn, and In. The water content in the acetonitrile electrolyte is precisely controlled, and the ratio of hydrogen evolution shows a positive correlation to the hydrogen binding energy of metal elements and the concentration of water. Importantly, we find that the acetonitrile could impair the interaction between *CO and metal electrode and thus promote the desorption of *CO. As a result, the Cu electrodes display poor activity to produce hydrocarbons in acetonitrile, unlike that in aqueous solutions, and near 100% CO Faradaic efficiencies over a wide potential window (−1.9 to −2.4 V vs. Fc/Fc+) are achieved at a water concentration of 0.19 M. This work reveals the solvent effect on determining the CO2RR selectivity, which adds a new dimension to tailoring the final products besides catalyst optimization.

Diatomic catalysts featuring a tunable structure and synergetic effects hold great promise for various reactions. However, their precise construction with specific configurations and diverse metal combinations is still challenging. Here, a selective etching and metal ion adsorption strategy is proposed to accurately assign a second metal atom (M2) geminal to the single atom site (M1–Nx) for constructing diatomic sites (e.g., Fe–Pd, Fe–Pt, Fe–Ru, Fe–Zn, Co–Fe, Co–Ni, and Co–Cu). In this strategy, hydrogen peroxide selectively etches the positively charged carbon atoms near the M1–Nxmoiety (denoted as α-C) and produces vacancy, which could trap the M2at the subsequent adsorption step. These catalysts show optimized electronic structure and enhanced oxygen reduction activity compared to single-site counterparts, and the representative Fe–Pd–NC and Co–Fe–NC catalysts stand as the most active oxygen reduction reaction catalysts (half-wave potential of 0.92 and 0.91 V, respectively). The selective etching of α-C in single-atom catalysts reported here represents a new post-treatment strategy for the targeting synthesis of diatomic sites.

Single-atom catalysts (SACs) are emerging as the most promising catalysts for various electrochemical reactions. The isolated dispersion of metal atoms enables high density of active sites, and the simplified structure makes them ideal model systems to study the structure–performance relationships. However, the activity of SACs is still insufficient, and the stability of SACs is usually inferior but has received little attention, hindering their practical applications in real devices. Moreover, the catalytic mechanism on a single metal site is unclear, leading the development of SACs to rely on trial-and-error experiments. How can one break the current bottleneck of active sites density? How can one further increase the activity/stability of metal sites? In this Perspective, we discuss the underlying reasons for the current challenges and identify precisely controlled synthesis involving designed precursors and innovative heat-treatment techniques as the key for the development of high-performance SACs. In addition, advanced operando characterizations and theoretical simulations are essential for uncovering the true structure and electrocatalytic mechanism of an active site. Finally, future directions that may arise breakthroughs are discussed.

Single-atom catalysts have become the forefront and hotspot in catalysis research due to their unique structural characteristics such as extreme atomic utilization efficiency and uniformity of reaction centers. To break through the bottleneck on their intrinsic activity enhancement, arduous efforts have been made to regulate the metal single-atomic sites. Herein, with a special interest in interatomic interfaces and interactions, our review focuses on the atomic-level regulation strategies of single-atom catalysts, including nonmetal heteroatom doping and polymetallic active site construction, referring to the introduction of nonmetal or metal atoms, respectively, into primitive metal single-atomic sites. We summarize the progress of these two strategies in optimizing single-atom active sites from the perspectives of synthesis, application, and mechanism. We start from a systematic overview of the atomic-level regulation methods and corresponding fabrication routes, followed by a comprehensive summary of the current advances in some typical electrocatalytic reactions. Then we expound on the synergistic reaction mechanism and the intrinsic structure-activity relationship after regulation. In the final part, we discuss the challenges and opportunities of these two atomic-level regulation strategies for further development. Our review of the atomic-level regulation of single-atom catalysts gives significant insight into the rational design of atom-efficient catalysts and adds depth and breadth to the fundamental understanding of atomically precise catalysis by elucidating its intrinsic mechanisms.

Introducing a second metal species into atomically dispersed metal–nitrogen–carbon (M–N–C) catalysts to construct diatomic sites (DASs) is an effective strategy to elevate their activities and stabilities. However, the common pyrolysis-based method usually leads to substantial uncertainty for the formation of DASs, and the precise identification of the resulting DASs is also rather difficult. In this regard, we developed a two-step specific-adsorption strategy (pyrolysis-free) and constructed a DAS catalyst featuring FeCo “molecular heterostructures” (FeCo-MHs). In order to rule out the possibility of the two apparently neighboring (in the electron microscopy image) Fe/Co atoms being dispersed respectively on the top/bottom surfaces of the carbon support and thus forming “false” MHs, we conducted in situ rotation (by 8°, far above the critical angle of 5.3°) and directly identified the individual FeCo-MHs. The formation of FeCo-MHs could modulate the magnetic moments of the metal centers and increase the ratio of low-spin Fe(II)–N4moiety; thus the intrinsic activity could be optimized at the apex of the volcano-plot (a relationship as a function of magnetic moments of metal–phthalocyanine complexes and catalytic activities). The FeCo-MHs catalyst displays an exceptional ORR activity (E1/2= 0.95 V) and could be used to construct high-performance cathodes for hydroxide exchange membrane fuel cells and zinc–air batteries.

An Fe–N–C catalyst with an FeN4active moiety has gained ever-increasing attention for the oxygen reduction reaction (ORR); however, the catalytic performance is sluggish in acidic solutions and the regulation is still a challenge. Herein, Fe–Mo dual-metal sites were constructed to tune the ORR activity of a mononuclear Fe site embedded in porous nitrogen-doped carbon. The cracking of O–O bonds is much more facile on the Fe–Mo atomic pair site due to the preferred bridge-cis adsorption model of oxygen molecules. The downshift of the Fe d band center when an Mo atom is introduced to the FeNxconfiguration optimizes the absorption–desorption behavior of ORR intermediates in the FeMoN6active moiety, thus boosting the catalytic performance. The construction of dual-metal atom sites to regulate the catalytically active moiety paves the way for boosting the electrocatalytic performance of other similar non-precious-metal catalysts.