Atomically dispersed transition metal active sites have emerged as one of the most important fields of study because they display promising performance in catalysis and have the potential to serve as ideal models for fundamental understanding. However, both the preparation and determination of such active sites remain a challenge. The structural engineering of carbon-and nitrogencoordinated metal sites (M−N−C, M = Fe, Co, Ni, Mn, Cu, etc.) via employing new heteroatoms, e.g., P and S, remains challenging. In this study, carbon nanosheets embedded with nitrogen and phosphorus dual-coordinated iron active sites (denoted as Fe-N/P-C) were developed and determined using cutting edge techniques. Both experimental and theoretical results suggested that the N and P dual-coordinated iron sites were favorable for oxygen intermediate adsorption/desorption, resulting in accelerated reaction kinetics and promising catalytic oxygen reduction activity. This work not only provides efficient way to prepare well-defined single-atom active sites to boost catalytic performance but also paves the way to identify the dual-coordinated single metal atom sites.

Conspectus Recent decades have witnessed the rapid development of catalytic science, especially after Taylor and Armstrong proposed the notion of the “active site” in 1925. By optimizing reaction paths and reducing the activation energies of reactions, catalysts appear in more than 90% of chemical production reactions, involving homogeneous catalysis, heterogeneous catalysis, and enzyme catalysis. Because of the 100% efficiency of active atom utilization and the adjustable microenvironment of metal centers, single-atom catalysts (SACs) shine in various catalytic fields for enhancing the rate, conversion, and selectivity of chemical reactions. Nevertheless, a solo active site determines a fixed adsorption mode, and the adsorption energies of intermediates from multistep reactions linking with a solo metal site are related to each other. For a specific multistep reaction, it is almost impossible to optimally adjust the adsorption of every intermediate on the solo site simultaneously. This phenomenon is termed the scaling relationship limit (SRL) and is an unavoidable obstacle in the development of pure SACs. Dual-atom catalysts (DACs), perfectly inheriting the advantages of SACs, can exhibit better catalytic performance than simple SACs and thus have gradually gained researchers’ attention. Depending on the dual-metal structure, dual-metal sites (DMSs) in DACs can be divided into two separated heterometal sites, two linked homometal sites, and two linked heterometal sites. Two separated heterometal sites prescribe a distance limit between two metal sites for electron interaction. Currently, the active origins of DACs can be summarized in the following three points: (1) electronic effect, in which only one metal center serves as active site and the other plays an electronic regulatory role; (2) synergistic effect, in which two metal centers separately catalyze different core steps to improve catalytic performance together; (3) adsorption effect, in which offering additional sites changes the adsorption structures to break the SRL based on SACs. Among the three active origins, optimizing the adsorption structure of intermediates upon DMSs is one of the most effective technologies to boost the catalytic property of DACs on the basis of SACs. To date, few contributions have focused on the development of DACs in various heterogeneous catalysis environments, including O2 reduction reaction, O2 evolution reaction, H2 evolution reaction, CO2 reduction reaction, N2 reduction reaction, and other conversion reactions. In this Account, a summary of recent progress regarding DACs in heterogeneous catalysis will be presented. First, the evolution of DACs from an unpopular discovery to research hot spot is illustrated through a timeline. In the next section, the DACs are divided into three categories, and the potential active origins of DACs are revealed by comparison with SACs. In addition, the techniques for constructing DACs are systematically summarized, including preparation of carbonous, pyrolysis-free, noncarbon-supported,...

The development of rechargeable metal–air batteries and water electrolyzers are highly constrained by electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). However, the construction of efficient trifunctional electrocatalysts for ORR/OER/HER are highly desirable yet challenging. Herein, hollow carbon nanotubes integrated single cobalt atoms with Co9S8 nanoparticles (CoSA + Co9S8/HCNT) are fabricated by a straightforward in situ self‐sacrificing strategy. The structure of the CoSA + Co9S8/HCNT are verified by X‐ray absorption spectroscopy and aberration‐corrected scanning transmission electron microscopy. Theoretical calculations and experimental results embrace the synergistic effects between Co9S8 nanoparticles and single cobalt atoms through optimizing the electronic configuration of the CoN4 active sites to lower the reaction barrier and facilitating the ORR, OER, and HER simultaneously. Consequently, rechargeable liquid and all‐solid‐state flexible Zn–air batteries based on CoSA + Co9S8/HCNT exhibit remarkable stability and excellent power density of 177.33 and 51.85 mW cm−2, respectively, better than Pt/C + RuO2 counterparts. Moreover, the as‐fabricated Zn–air batteries can drive an overall water splitting device assembled with CoSA + Co9S8/HCNT and achieve a current density of 10 mA cm−2 at a low voltage of 1.59 V, also superior to Pt/C + RuO2. Therefore, this work presents a promising approach to an efficient trifunctional electrocatalyst toward practical applications.

Oxygen reduction reaction (ORR) is an essential process for sustainable energy supply and sufficient chemical production in modern society. Singleatom catalysts (SACs) exhibit great potential on maximum atomic efficiency, high ORR activity, and stability, making them attractive candidates for pursuing next-generation catalysts. Despite substantial efforts being made on building diversiform single-atom active sites (SAASs), the performance of the obtained catalysts is still unsatisfactory. Fortunately, microenvironment regulation of SACs provides opportunities to improve activity and selectivity for ORR. In this review, first, ORR mechanism pathways on N-coordinated SAAS, electrochemical evaluation, and characterization of SAAS are displayed. In addition, recent developments in tuning microenvironment of SACs are systematically summarized, especially, strategies for microenvironment modulation are introduced in detail for boosting the intrinsic 4e − /2e − ORR activity and selectivity. Theoretical calculations and cutting-edge characterization techniques are united and discussed for fundamental understanding of the synthesis-construction-performance correlations. Furthermore, the techniques for building SAAS and tuning their microenvironment are comprehensively overviewed to acquire outstanding SACs. Lastly, by proposing perspectives for the remaining challenges of SACs and infant microenvironment engineering, the future directions of ORR SACs and other analogous procedures are pointed out.

Zinc ion capacitors (ZICs) hold great promise in large-scale energy storage by inheriting the superiorities of zinc ion batteries and supercapacitors. However, the mismatch of kinetics and capacity between a Zn anode and a capacitive-type cathode is still the Achilles' heel of this technology. Herein, porous carbons are fabricated by using tetra-alkali metal pyromellitic acid salts as precursors through a carbonization/ self-activation procedure for enhancing zinc ion storage. The optimized rubidium-activated porous carbon (RbPC) is verified to hold immense surface area, suitable porosity structure, massive lattice defects, and luxuriant oxygen functional groups. These structural and compositional merits endow RbPC with the promoted zinc ion storage capability and more matchable kinetics and capacity with a Zn anode. Consequently, RbPC-based ZIC delivers a high specific energy of 178.2 W h kg −1 and a peak power density of 72.3 kW kg −1 . A systematic ex situ characterization analysis coupled with in situ electrochemical quartz crystal microbalance tests reveal that the preeminent zinc ion storage properties are ascribed to the synergistic effect of the dual-ion adsorption and reversible chemical adsorption of RbPC. This work provides an efficient strategy to the rational design and construction of high-performance electrodes for ZICs and furthers the fundamental understanding of their charge storage mechanisms or extends the understanding toward other electrochemical energy storage devices.

Single‐atom catalysts (SACs) are attractive candidates for oxygen reduction reaction (ORR). The catalytic performances of SACs are mainly determined by the surrounding microenvironment of single metal sites. Microenvironment engineering of SACs and understanding of the structure–activity relationship is critical, which remains challenging. Herein, a self‐sacrificing strategy is developed to synthesize asymmetric N,S‐coordinated single‐atom Fe with axial fifth hydroxy (OH) coordination (Fe−N3S1OH) embedded in N,S codoped porous carbon nanospheres (FeN/SC). Such unique penta‐coordination microenvironment is determined by cutting‐edge techonologies aiding of systematic simulations. The as‐obtained FeN/SC exhibits superior catalytic ORR activity, and showcases a half‐wave potential of 0.882 V surpassing the benchmark Pt/C. Moreover, theoretical calculations confirmed the axial OH in FeN3S1OH can optimize 3d orbitals of Fe center to strengthen O2 adsorption and enhance O2 activation on Fe site, thus reducing the ORR barrier and accelerating ORR dynamics. Furthermore, FeN/SC containing H2O2 fuel cell performs a high peak power density of 512 mW cm−2, and FeN/SC based Znair batteries show the peak power density of 203 and 49 mW cm−2 in liquid and flexible all‐solid‐state configurations, respectively. This study offers a new platform for fundamentally understand the axial fifth coordination in asymmetrical planar single‐atom metal sites for electrocatalysis.

surface of MXene. It is generally obtained by etching the MAX phase (Ti 3 AlC 2) in a HF solution or a solution that generates HF in situ. Similar to other 2D materials (such as graphene, MoS 2 , and black phosphorus, etc.), however, MXene flakes are susceptible to stacking and agglomeration due to the strong interlayer van der Waals force. This problem greatly hindered the dispersion of MXene and significantly reduce their specific surface area, thus limiting the efficient use of interface and seriously affects its performance. [6] To address this issue, a series of strategies have been adopted. For example, Gogotsi and co-workers reported that Ti 3 C 2 T x layer intercalated with cation to suppress the restacking of MXene nanosheets, thus offers high volumetric capacitance. [7] Yu and co-workers reported an MXene@polystyrene nanocomposites constructed by electrostatic assembly MXene flakes on polystyrene microspheres then compressing molding for highly efficient EMI. [8] A 3D MXene hydrogel was fabricated by using metal ions to break electrostatic repulsion force between the MXene nanosheets and serving as connectors to link the nanosheets together, which make the restacking problem of MXene be restrained and effectively increases the surface utilization of MXene. [9] These strategies can effectively inhibit the stacking of MXene and increase the layer spacing. Nevertheless, most of above methods are suffering from complex and harsh manufacturing process limitations. The poor oxidation stability is another major drawback for MXene. Due to the interaction of water and oxygen under ambient surroundings, MXene flakes are unstable and apt to oxidize. [10] It has been reported that the introduction of polyanionic salts [11] or antioxidants sodium l-ascorbate [12] in the MXene solution enables the anions to encapsulate the positively charged MXene flake edges and prevent MXene from interacting with water molecules, thereby achieving the purpose of inhibiting MXene oxidation. A stable MXene organic dispersion was reported based on simultaneous interfacial chemical grafting and phase transfer method, which has strong antioxidant properties. [13] Although the above methods can effectively suppress the oxidation of MXene, on the other hand, MXene is "contaminated" due to the introduction of other substances. Conjugated microporous polymers (CMPs) are a unique class of organic porous materials constructed by rigid conjugated 2D MXenes have attracted wide attention due to their unique chemical and physical properties. However, MXene nanosheets suffer from restacking and are susceptible to oxidation and consequently lose their functional properties which limits their applications. Thus, it is desirable to explore strategies to preserve MXene nanosheets and avoid oxidation. Herein, an effective strategy to produce MXene-based conjugated microporous polymers (M-CMPs) by covalently sandwiching MXene between CMPs using p-iodophenyl functionalized MXene as templates is demonstrated. The as-prepared M-CMPs inherit the 2D architec...

2D transition metal carbides/nitrides (MXenes) have excellent physicalchemical properties, which makes them promising for electrochemical energy storage devices. However, because of their inherent self-stacking and narrow interlayer spacing, it is rarely used in multivalent ion energy storage systems. In this study, fatty diamines and aromatic diamines with different molecular sizes are inserted between MXene interlayers as pillars through a one-step amination process to inhibit the self-stacking and obtain different expanded interlayer spacings with improved antioxidant stability. X-ray diffraction results show that interlayer spacing of MXene increases from 1.23 to 1.40 nm. The p-phenylenediamine-intercalated MXene (PDA-MXene) exhibits better matching interlayer spacing (1.38 nm) and pore structure for improved electrolyte-accessible surface area, enhanced charge-transport properties, and promoted Zn 2+ ions storage. Therefore, zinc-ion hybrid supercapacitor (ZHSC) using PDA-MXene as cathode exhibits higher specific capacitance (124.4 F g −1 at 0.2 A g −1 ) in 2 m ZnSO 4 electrolyte together with outstanding cycling stability (85% capacity retention after 10 000 cycles at 1 A g −1 ). This study provides a route for precise control of MXene interlayer spacing by small organic molecules, which can be used to observe efficient charge storage in MXene-based electrochemical energy storage devices by optimizing interlayer spacing.