Reconstruction of pH-universal atomic FeNC catalysts towards oxygen reduction reaction

Constructing of single atom catalysts that can stably exist in various energy conversion and storage devices is still in its infancy. Herein, a geometrically optimized three-dimensional hierarchically architectural single atomic FeNC catalyst with fast mass transport and electron transfer is rationally developed by post-molecule pyrolysis assisted with silicon template and reconstructs by ammonia treating. The ammonia-assisted secondary pyrolysis not only compensates for the volatilization of nitrogen species contained in organic precursors but also optimizes the surface structure of FeNC catalyst, thus increasing the content of pyridinic nitrogen and boosting the density of active sites (FeN_x) in FeNC samples. In addition, the pyridinic nitrogen adjusts the electronic distribution in Fe 3d active center and promotes the catalytic performances. Therefore, this hollow spherical atomically dispersed FeNC catalyst delivers outstanding oxygen reduction reaction (ORR) activity in pH-universal electrolyte and surpasses the most reported values.

Insight into atomically dispersed porous M-N-C single-site catalysts for electrochemical CO2 reduction

Transition metal single-site catalysts have unique activities for electrochemical CO₂ reduction. However, the exact active center and reaction mechanism remain unclear due to a number of challenges in the controllable synthesis of single-atom catalysts (SACs) and defects in metal supports. Here we combine both experimental and theoretical calculations to systematically explore the mechanistic reaction path of selected transition metal single sites on nitrogen-doped porous carbon. Facile pyrolysis was employed to prepare a fullerene type carbon with 0.35 nm interlayer distances to support the family of M-N-C (M = Ni, Fe, Co and Cu). Experimentally, Ni and Fe outperform the other metals with high faradaic efficiency up to >97% and 86.8%, respectively. The theoretical calculations reveal that Ni-N-C exhibits optimum activity for CO₂ reduction to CO at a higher overpotential because of the moderate *CO binding energy at the Ni site, which accommodates *COOH formation and *CO desorption. Furthermore, the strong binding energy of *CO on the Fe site enables the catalyst to reduce CO₂ beyond CO. A remarkable current density of 17.6 mA cm^-2 has been achieved with the Ni-N-C catalyst and a record of 5.74 s^-1 TOF has been realized at -0.8 V vs. RHE for the Ni-N-C catalyst.

Recent Advances in Single-Atom Electrocatalysts for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) plays significant roles in electrochemical energy storage and conversion systems as well as clean synthesis of fine chemicals. However, the ORR process shows sluggish kinetics and requires platinum-group noble metal catalysts to accelerate the reaction. The high cost, rare reservation, and unsatisfied durability significantly impede large-scale commercialization of platinum-based catalysts. Single-atom electrocatalysts (SAECs) featuring with well-defined structure, high intrinsic activity, and maximum atom efficiency have emerged as a novel field in electrocatalytic science since it is promising to substitute expensive platinum-group noble metal catalysts. However, finely fabricating SAECs with uniform and highly dense active sites, fully maximizing the utilization efficiency of active sites, and maintaining the atomically isolated sites as single-atom centers under harsh electrocatalytic conditions remain urgent challenges. In this review, we summarized recent advances of SAECs in synthesis, characterization, oxygen reduction reaction (ORR) performance, and applications in ORR-related H₂O₂ production, metal-air batteries, and low-temperature fuel cells. Relevant progress on tailoring the coordination structure of isolated metal centers by doping other metals or ligands, enriching the concentration of single-atom sites by increasing metal loadings, and engineering the porosity and electronic structure of the support by optimizing the mass and electron transport are also reviewed. Moreover, general strategies to synthesize SAECs with high metal loadings on practical scale are highlighted, the deep learning algorithm for rational design of SAECs is introduced, and theoretical understanding of active-site structures of SAECs is discussed as well. Perspectives on future directions and remaining challenges of SAECs are presented.

Graphitic phosphorus coordinated single Fe atoms for hydrogenative transformations

Single-atom metal-nitrogen-carbon (M-N-C) catalysts have sparked intensive interests, however, the development of an atomically dispersed metal-phosphorus-carbon (M-P-C) catalyst has not been achieved, although molecular metal-phosphine complexes have found tremendous applications in homogeneous catalysis. Herein, we successfully construct graphitic phosphorus species coordinated single-atom Fe on P-doped carbon, which display outstanding catalytic performance and reaction generality in the heterogeneous hydrogenation of N-heterocycles, functionalized nitroarenes, and reductive amination reactions, while the corresponding atomically dispersed Fe atoms embedded on N-doped carbon are almost inactive under the same reaction conditions. Furthermore, we find that the catalytic activity of graphitic phosphorus coordinated single-atom Fe sharply decreased when Fe atoms were transformed to Fe clusters/nanoparticles by post-impregnation Fe species. This work can be of fundamental interest for the design of single-atom catalysts by utilizing P atoms as coordination sites as well as of practical use for the application of M-P-C catalysts in heterogeneous catalysis.

Synergistic effects of heteroatom-decorated MXene catalysts for CO reduction reactions

In this study, using the density functional theory calculations, we present a strategy to improve the activity and selectivity of electrocatalytic CO reduction reactions (CORRs) towards CH₄ production occurring on single transition metal (TM) atoms embedded in a defective MXene Mo_2-xTiC₂O_y with one oxygen vacancy. Owing to the unique geometric and electronic structures, the exposed TM-Mo-Mo triangle can serve as an active site, and the surrounding oxygen atoms can break the scaling relationships between the CORR intermediates via the steric hindrance. The synergistic effects result in an excellent catalytic performance for CORRs. Based on the extensive investigation of series of candidates, W-decorated MXene was identified as the most promising CORR electrocatalyst, with a high selective activity towards the CH₄ production and strong suppression of competing hydrogen evolution reactions (HERs). The adsorption free energy of *COH [ΔG_ads (*COH)] is proposed as a descriptor to establish a relationship with the catalytic activity. Our rational design principles and rapid screening methods may shed light on the development of other highly efficient CORR electrocatalysts, as well as the other electrochemical systems.

Porous Carbon Membrane-Supported Atomically Dispersed Pyrrole-Type FeN4 as Active Sites for Electrochemical Hydrazine Oxidation Reaction

The rational design of catalytically active sites in porous materials is essential in electrocatalysis. Herein, atomically dispersed Fe-N_x sites supported by hierarchically porous carbon membranes are designed to electrocatalyze the hydrazine oxidation reaction (HzOR), one of the key techniques in electrochemical nitrogen transformation. The high intrinsic catalytic activity of the Fe-N_x single-atom catalyst together with the uniquely mixed micro-/macroporous membrane support positions such an electrode among the best-known heteroatom-based carbon anodes for hydrazine fuel cells. Combined with advanced characterization techniques, electrochemical probe experiments, and density functional theory calculation, the pyrrole-type FeN₄ structure is identified as the real catalytic site in HzOR.

Heterogeneous Single-Atom Catalysts for Electrochemical CO2 Reduction Reaction

The electrochemical CO₂ reduction reaction (CO₂ RR) is of great importance to tackle the rising CO₂ concentration in the atmosphere. The CO₂ RR can be driven by renewable energy sources, producing precious chemicals and fuels, with the implementation of this process largely relying on the development of low-cost and efficient electrocatalysts. Recently, a range of heterogeneous and potentially low-cost single-atom catalysts (SACs) containing non-precious metals coordinated to earth-abundant elements have emerged as promising candidates for the CO₂ RR. Unfortunately, the real catalytically active centers and the key factors that govern the catalytic performance of these SACs remain ambiguous. Here, this ambiguity is addressed by developing a fundamental understanding of the CO₂ RR-to-CO process on SACs, as CO accounts for the major product from CO₂ RR on SACs. The reaction mechanism, the rate-determining steps, and the key factors that control the activity and selectivity are analyzed from both experimental and theoretical studies. Then, the synthesis, characterization, and the CO₂ RR performance of SACs are discussed. Finally, the challenges and future pathways are highlighted in the hope of guiding the design of the SACs to promote and understand the CO₂ RR on SACs.

Metal Phthalocyanine-Derived Single-Atom Catalysts for Selective CO2 Electroreduction under High Current Densities

Single-atom catalysts (SACs) with atomically dispersed metal sites in nitrogen-doped carbon matrices (M-N/C) have been identified as promising candidates for the electrocatalytic CO₂ reduction reaction (CO₂RR). However, recent studies aiming at economic viability have been inhibited by the low faradaic efficiency (FE) and instability under high current density. Herein, we report a series of SACs derived from cyano-substituted metal phthalocyanines (MePc-CN) in ZIFs (denoted as Me-SACs (Pc)). These phthalocyanine molecules enable the efficient construction of SACs, affording higher metal loading and less variation when compared with their counterparts from metal nitrates (denoted as Me-SACs (S)). Thus, Me-SACs (Pc) exhibit higher activities and selectivities than Me-SACs (S) in H-cell measurements. In gas-diffusion electrode (GDE) setups, the unstable Fe-SAC (Pc) shows only a 50% FE of CO (FEco) at -100 mA cm^-2. In contrast, Ni-SAC (Pc) exhibits a higher FEco of >96% at current densities from -10 to -200 mA cm^-2 and can stably operate for over 16 h at -200 mA cm^-2. The performances of Ni-SAC (Pc) are comparable to those of precious metal catalysts and the best SACs reported so far, representing a promising candidate for practical electrolyzer devices for CO₂RR.

Solid Nanoporosity Governs Catalytic CO2 and N2 Reduction

Global demand for green and clean energy is increasing day by day owing to ongoing developments by the human race that are changing the face of the earth at a rate faster than ever. Exploring alternative sources of energy to replace fossil fuel consumption has become even more vital to control the growing concentration of CO₂, and reduction of CO₂ into CO or other useful hydrocarbons (e.g., C₁ and C_≥2 products), as well as reduction of N₂ into ammonia, can greatly help in this regard. Various materials have been developed for the reduction of CO₂ and N₂. The introduction of pores in these materials by porosity engineering has been demonstrated to be highly effective in increasing the efficiency of the involved redox reactions, over 40% increment for CO₂ reduction to date, by providing an increased number of exposed facets, kinks, edges, and catalytically active sites of catalysts. By shaping the surface porous structure, the selectivity of the redox reaction can also be enhanced. In order to better understand this area benefiting rational design for future solutions, this review systematically summarizes and constructively discusses the porosity engineering in catalytic materials, including various synthesis methods, characterization of porous materials, and the effects of porosity on performance of CO₂ reduction and N₂ reduction.

Ternary Sn-Ti-O Electrocatalyst Boosts the Stability and Energy Efficiency of CO2 Reduction

Simultaneously improving energy efficiency (EE) and material stability in electrochemical CO₂ conversion remains an unsolved challenge. Among a series of ternary Sn-Ti-O electrocatalysts, 3D ordered mesoporous (3DOM) Sn_0.3 Ti_0.7 O₂ achieves a trade-off between active-site exposure and structural stability, demonstrating up to 71.5 % half-cell EE over 200 hours, and a 94.5 % Faradaic efficiency for CO at an overpotential as low as 430 mV. DFT and X-ray absorption fine structure analyses reveal an electron density reconfiguration in the Sn-Ti-O system. A downshift of the orbital band center of Sn and a charge depletion of Ti collectively facilitate the dissociative adsorption of the desired intermediate COOH* for CO formation. It is also beneficial in maintaining a local alkaline environment to suppress H₂ and formate formation, and in stabilizing oxygen atoms to prolong durability. These findings provide a new strategy in materials design for efficient CO₂ conversion and beyond.

Atomically Dispersed Iridium on Indium Tin Oxide Efficiently Catalyzes Water Oxidation

Heterogeneous catalysts in the form of atomically dispersed metals on a support provide the most efficient utilization of the active component, which is especially important for scarce and expensive late transition metals. These catalysts also enable unique opportunities to understand reaction pathways through detailed spectroscopic and computational studies. Here, we demonstrate that atomically dispersed iridium sites on indium tin oxide prepared via surface organometallic chemistry display exemplary catalytic activity in one of the most challenging electrochemical processes, the oxygen evolution reaction (OER). In situ X-ray absorption studies revealed the formation of IrV=O intermediate under OER conditions with an Ir-O distance of 1.83 Å. Modeling of the reaction mechanism indicates that IrV=O is likely a catalyst resting state, which is subsequently oxidized to IrVI enabling fast water nucleophilic attack and oxygen evolution. We anticipate that the applied strategy can be instrumental in preparing and studying a broad range of atomically dispersed transition metal catalysts on conductive oxides for (photo)electrochemical applications.

Electrochemical CO2 reduction to high-concentration pure formic acid solutions in an all-solid-state reactor

Electrochemical CO₂ reduction reaction (CO₂RR) to liquid fuels is currently challenged by low product concentrations, as well as their mixture with traditional liquid electrolytes, such as KHCO₃ solution. Here we report an all-solid-state electrochemical CO₂RR system for continuous generation of high-purity and high-concentration formic acid vapors and solutions. The cathode and anode were separated by a porous solid electrolyte (PSE) layer, where electrochemically generated formate and proton were recombined to form molecular formic acid. The generated formic acid can be efficiently removed in the form of vapors via inert gas stream flowing through the PSE layer. Coupling with a high activity (formate partial current densities ~450 mA cm⁻²), selectivity (maximal Faradaic efficiency ~97%), and stability (100 hours) grain boundary-enriched bismuth catalyst, we demonstrated ultra-high concentrations of pure formic acid solutions (up to nearly 100 wt.%) condensed from generated vapors via flexible tuning of the carrier gas stream.

Electrochemical CO₂ reduction to liquid fuels is limited by low product concentrations and formation of mixtures with traditional liquid electrolytes. Here the authors report an all-solid-state system for a continuous generation of high-purity and high-concentration formic acid vapors and solutions.

Operando time-resolved X-ray absorption spectroscopy reveals the chemical nature enabling highly selective CO2 reduction

Copper electrocatalysts have been shown to selectively reduce carbon dioxide to hydrocarbons. Nevertheless, the absence of a systematic study based on time-resolved spectroscopy renders the functional agent-either metallic or oxidative Copper-for the selectivity still undecidable. Herein, we develop an operando seconds-resolved X-ray absorption spectroscopy to uncover the chemical state evolution of working catalysts. An oxide-derived Copper electrocatalyst is employed as a model catalyst to offer scientific insights into the roles metal states serve in carbon dioxide reduction reaction (CO2RR). Using a potential switching approach, the model catalyst can achieve a steady chemical state of half-Cu(0)-and-half-Cu(I) and selectively produce asymmetric C2 products - C2H5OH. Furthermore, a theoretical analysis reveals that a surface composed of Cu-Cu(I) ensembles can have dual carbon monoxide molecules coupled asymmetrically, which potentially enhances the catalyst's CO2RR product selectivity toward C2 products. Our results offer understandings of the fundamental chemical states and insights to the establishment of selective CO2RR.

Cation Exchange Strategy to Single-Atom Noble-Metal Doped CuO Nanowire Arrays with Ultralow Overpotential for H2O Splitting

Single-atom site catalysts (SACs) have aroused enormous attention and brought about new opportunities for many applications. Herein, we report a versatile strategy to rhodium (Rh) SAC by a facile cation exchange reaction. Remarkably, the Rh SAC modified CuO nanowire arrays on copper foam (Rh SAC-CuO NAs/CF) show unprecedented alkaline oxygen evolution reaction (OER) activity with a high current density of 84.5 mA cm^-2@1.5 V vs reversible hydrogen electrode (RHE), 9.7 times that of Ir/C/CF. More strikingly, when used as an anode and a cathode for overall water splitting, the Rh SAC-CuO NAs/CF can achieve 10 mA cm^-2 at only 1.51 V. Density functional theory calculations reveal that the high OER and HER intrinsic catalytic activities result from moderate adsorption energy of intermediates on Rh SAC. Finally, we demonstrate the general synthesis of different single-atom noble-metal catalysts on CuO NAs (M SAC-CuO NAs/CF, where M = Ru, Ir, Os, and Au).

Unveiling hydrocerussite as an electrochemically stable active phase for efficient carbon dioxide electroreduction to formate

For most metal-containing CO2 reduction reaction (CO2RR) electrocatalysts, the unavoidable self-reduction to zero-valence metal will promote hydrogen evolution, hence lowering the CO2RR selectivity. Thus it is challenging to design a stable phase with resistance to electrochemical self-reduction as well as high CO2RR activity. Herein, we report a scenario to develop hydrocerussite as a stable and active electrocatalyst via in situ conversion of a complex precursor, tannin-lead(II) (TA-Pb) complex. A comprehensive characterization reveals the in situ transformation of TA-Pb to cerussite (PbCO3), and sequentially to hydrocerussite (Pb3(CO3)2(OH)2), which finally serves as a stable and active phase under CO2RR condition. Both experiments and theoretical calculations confirm the high activity and selectivity over hydrocerussite. This work not only offers a new approach of enhancing the selectivity in CO2RR by suppressing the self-reduction of electrode materials, but also provides a strategy for studying the reaction mechanism and active phases of electrocatalysts.

2D Boron Imidazolate Framework Nanosheets with Electrocatalytic Applications for Oxygen Evolution and Carbon Dioxide Reduction Reaction

Ultrathin 2D materials possess unique properties that translate to enhanced efficiency as electrocatalysts, stimulating research toward methodologies that support their preparation. Herein, a two-step strategy is reported that involves the preparation of the new boron imidazolate framework (BIF-73) which is subsequently utilized as a precursor to yield the crystalline 2D nanosheet material (Fe@BIF-73-NS) via post-synthetic modification. This new electrocatalytic material stabilizes ultra-small (Fe₂ O₃ ) fragments resulting in an excellent electrocatalytic performance for the oxygen evolution reaction (OER: lower overpotential with 291 mV at the current density of 10 mA cm^-2 ) and carbon dioxide reduction reaction (faradaic efficiency of CO reaching 88.6% at -1.8 V vs Ag/AgCl) without the need for noble metals. Additionally, theoretical calculations and microscopy reveal that the superior OER performance can be attributed to the increased exposure of binding sites within the material to which the catalytically active Fe³⁺ centers are bound through a post-synthetic modification procedure. A red-shift of the Fermi level around the valence band is observed and is proposed to be a result of the aforementioned interactions. This work opens an avenue toward the development of 2D functional metal organic framework nanosheets for energy conversion applications.

Gas Diffusion Strategy for Inserting Atomic Iron Sites into Graphitized Carbon Supports for Unusually High-Efficient CO2 Electroreduction and High-Performance Zn-CO2 Batteries

Emerging single-atom catalysts (SACs) hold great promise for CO₂ electroreduction (CO₂ ER)_, but the design of highly active and cost-efficient SACs is still challenging. Herein, a gas diffusion strategy, along with one-step thermal activation, for fabricating N-doped porous carbon polyhedrons with trace isolated Fe atoms (Fe₁ NC) is developed. The optimized Fe₁ NC/S₁ -1000 with atomic Fe-N₃ sites supported by N-doped graphitic carbons exhibits superior CO₂ ER performance with the CO Faradaic efficiency up to 96% at -0.5 V, turnover frequency of 2225 h^-1 , and outstanding stability, outperforming almost all previously reported SACs based on N-doped carbon supported nonprecious metals. The observed excellent CO₂ ER performance is attributed to the greatly enhanced accessibility and intrinsic activity of active centers due to the increased electrochemical surface area through size modulation and the redistribution of doped N species by thermal activation. Experimental observations and theoretical calculations reveal that the Fe-N₃ sites possess balanced adsorption energies of *COOH and *CO intermediates, facilitating CO formation. A universal gas diffusion strategy is used to exclusively yield a series of dimension-controlled carbon-supported SACs with single Fe atoms while a rechargeable Zn-CO₂ battery with Fe₁ NC/S₁ -1000 as cathode is developed to deliver a maximal power density of 0.6 mW cm^-2 .

Recent Advances of Precise Cu Nanoclusters in Microporous Materials

This minireview highlights some recent advances in the rational design of precise Cu nanoclusters supported on microporous materials, including zeolites and metal-organic frameworks. The development of comprehensive characterisation techniques enables scientists to elucidate the structure-activity relationship of these catalysts, which aids the subsequent engineering of more superior catalytic systems at an atomistic perspective.

Nitrogen-doped iron for selective catalytic reduction of nitrate to dinitrogen

Nitrate is the leading cause of eutrophication worldwide and is one of the most challenging pollutants for restoration of polluted surface waters such as lakes, rivers and reservoirs. We report herein a new architecture of iron nanoparticles for high-efficiency denitrification by selective reduction of nitrate (NO₃^-) to dinitrogen (N₂). The iron nanoparticles are doped with nitrogen (FeN) and encapsulated within a thin layer of nitride-carbon (NC). The nanoparticles have high pyrrolic N content (17.4 at.%) and large specific surface area (2040 m²/g). Laboratory experiments demonstrated high N₂ selectivity (91%) and nitrate removal capacity (6004 mg N/g Fe) for treatment of nitrate-containing water. This iron-based nanomaterial overcomes shortcomings of conventional catalysts by eliminating the use of precious and toxic heavy metals (e.g., Pd, Pt, Cu, Ni) and minimizing the generation of undesirable byproducts (e.g., ammonia) from the reactions with nanoscale zero-valent iron (nZVI). The multiple electron transfers process from NO₃^- to N₂ can be fine-tuned by adjusting the NC shell thickness. Superior electrocatalytic performance, low cost and minimal environmental impact of the iron-derived nanocatalyst offer promising prospects for water purification, waste treatment and environmental remediation.

Assisting Atomic Dispersion of Fe in N-Doped Carbon by Aerosil for High-Efficiency Oxygen Reduction

Utilizing Zn as a "fencing" agent has enabled the pyrolytic synthesis of atomically dispersed metal-nitrogen-carbon (AD-MNC) materials for broad electrocatalysis such as fuel cells, metal-air batteries, and water electrolyzers. Yet the Zn residue troubles the precise identification of the responsible sites in active service. Herein we developed a simple aerosil-assisted method for preparing AD-MNC materials to cautiously avoid the introduction of Zn. The combined analysis of extended X-ray absorption fine structure (EXAFS) and aberration-corrected high-resolution transition electron microscopy verified the atomic dispersion of Fe species in the as-made Fe-NC sample with a well-defined structure of Fe-N₄. Besides, the EXAFS studies indicated the formation of oxygenated Fe-N₄ moieties (O-Fe-N₄) after the removal of aerosil nanoparticles. Therefore, the immobilization of Fe atoms in the carbon substrate was attributed to the heavily doping N and rich oxygen dangling species at the aerosil surface. Electrochemical measurements revealed that the as-made Fe-NC material furnished with O-Fe-N₄ moieties exhibited excellent oxygen reduction reaction (ORR) performance, characterized by individually indicating ∼22 mV higher half-wave potentials, with respect to commercial Pt/C catalyst. Density functional theory (DFT) computations suggested that the dangling oxygen ligand on the Fe-N₄ moiety could significantly boost the cleavage of OOH* and the reductive release of *OH intermediates, leading to the enhancement of overall ORR performance.

Metal-Organic Framework-Derived Strategy for Improving Catalytic Performance of a Chromia-Based Catalyst in the Chlorine/Fluorine Exchange Reactions for Unsaturated Fluorocarbons

Hydrofluoroolefins (HFOs) and cyclic hydrofluorocarbons (c-HFCs) have been the most favored alternatives of the ozone depletion substances; however, because of the poor performance of the present chlorine/fluorine (Cl/F) exchange catalysts, the development and production of HFOs and c-HFCs are hindered. Here, we first report a novel and facile route to fabricate high-performance Cl/F exchange catalysts via a metal-organic framework (MOF) carbonization method. The MOF-derived catalyst not only has high selectivity but also can significantly lower the reaction temperature. Moreover, benefiting from the stable structure and coke-inhibiting ability, the MOF-derived catalyst has a long service life compared with the traditional precipitation method. Furthermore, the nanoscopic MOF-derived catalyst can greatly reduce the Cr dosage, which would help to minimize the risk of Cr contamination.

Nanocasting SiO2 into metal-organic frameworks imparts dual protection to high-loading Fe single-atom electrocatalysts

Single-atom catalysts (SACs) have sparked broad interest recently while the low metal loading poses a big challenge for further applications. Herein, a dual protection strategy has been developed to give high-content SACs by nanocasting SiO₂ into porphyrinic metal-organic frameworks (MOFs). The pyrolysis of SiO₂@MOF composite affords single-atom Fe implanted N-doped porous carbon (Fe_SA-N-C) with high Fe loading (3.46 wt%). The spatial isolation of Fe atoms centered in porphyrin linkers of MOF sets the first protective barrier to inhibit the Fe agglomeration during pyrolysis. The SiO₂ in MOF provides additional protection by creating thermally stable FeN₄/SiO₂ interfaces. Thanks to the high-density Fe_SA sites, Fe_SA-N-C demonstrates excellent oxygen reduction performance in both alkaline and acidic medias. Meanwhile, Fe_SA-N-C also exhibits encouraging performance in proton exchange membrane fuel cell, demonstrating great potential for practical application. More far-reaching, this work grants a general synthetic methodology toward high-content SACs (such as Fe_SA, Co_SA, Ni_SA).

Atomically Dispersed Iron-Nitrogen Sites on Hierarchically Mesoporous Carbon Nanotube and Graphene Nanoribbon Networks for CO2 Reduction

Atomically dispersed metal and nitrogen co-doped carbon (M-N/C) catalysts hold great promise for electrochemical CO₂ conversion. However, there is a lack of cost-effective synthesis approaches to meet the goal of economic mass production of single-atom M-N/C with desirable carbon support architecture for efficient CO₂ reduction. Herein, we report facile transformation of commercial carbon nanotube (CNT) into isolated Fe-N₄ sites anchored on carbon nanotube and graphene nanoribbon (GNR) networks (Fe-N/CNT@GNR). The oxidization-induced partial unzipping of CNT results in the generation of GNR nanolayers attached to the remaining fibrous CNT frameworks, which reticulates a hierarchically mesoporous complex and thus enables a high electrochemical active surface area and smooth mass transport. The Fe residues originating from CNT growth seeds serve as Fe sources to form isolated Fe-N₄ moieties located at the CNT and GNR basal plane and edges with high intrinsic capability of activating CO₂ and suppressing hydrogen evolution. The Fe-N/CNT@GNR delivers a stable CO Faradaic efficiency of 96% with a partial current density of 22.6 mA cm^-2 at a low overpotential of 650 mV, making it one of the most active M-N/C catalysts reported. This work presents an effective strategy to fabricate advanced atomistic catalysts and highlights the key roles of support architecture in single-atom electrocatalysis.

In-situ X-ray techniques for non-noble electrocatalysts

Electrocatalysis offers an alternative solution for the energy crisis because it lowers the activation energy of reaction to produce economic fuels more accessible. Non-noble electrocatalysts have shown their capabilities to practical catalytic applications as compared to noble ones, whose scarcity and high price limit the development. However, the puzzling catalytic processes in non-noble electrocatalysts hinder their advancement. In-situ techniques allow us to unveil the mystery of electrocatalysis and boost the catalytic performances. Recently, various in-situ X-ray techniques have been rapidly developed, so that the whole picture of electrocatalysis becomes clear and explicit. In this review, the in-situ X-ray techniques exploring the structural evolution and chemical-state variation during electrocatalysis are summarized for mainly oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and carbon dioxide reduction reaction (CO2RR). These approaches include X-ray Absorption Spectroscopy (XAS), X-ray diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS). The information seized from these in-situ X-ray techniques can effectively decipher the electrocatalysis and thus provide promising strategies for advancing the electrocatalysts. It is expected that this review could be conducive to understanding these in-situ X-ray approaches and, accordingly, the catalytic mechanism to better the electrocatalysis.

Atomically Dispersed Nickel(I) on an Alloy-Encapsulated Nitrogen-Doped Carbon Nanotube Array for High-Performance Electrochemical CO2 Reduction Reaction

Single-atom catalysts (SACs) show great promise for electrochemical CO₂ reduction reaction (CRR), but the low density of active sites and the poor electrical conduction and mass transport of the single-atom electrode greatly limit their performance. Herein, we prepared a nickel single-atom electrode consisting of isolated, high-density and low-valent nickel(I) sites anchored on a self-standing N-doped carbon nanotube array with nickel-copper alloy encapsulation on a carbon-fiber paper. The combination of single-atom nickel(I) sites and self-standing array structure gives rise to an excellent electrocatalytic CO₂ reduction performance. The introduction of copper tunes the d-band electron configuration and enhances the adsorption of hydrogen, which impedes the hydrogen evolution reaction. The single-nickel-atom electrode exhibits a specific current density of -32.87 mA cm^-2 and turnover frequency of 1962 h^-1 at a mild overpotential of 620 mV for CO formation with 97 % Faradic efficiency.

Reaction mechanism and kinetics for CO2 reduction on nickel single atom catalysts from quantum mechanics

Experiments have shown that graphene-supported Ni-single atom catalysts (Ni-SACs) provide a promising strategy for the electrochemical reduction of CO2 to CO, but the nature of the Ni sites (Ni-N2C2, Ni-N3C1, Ni-N4) in Ni-SACs has not been determined experimentally. Here, we apply the recently developed grand canonical potential kinetics (GCP-K) formulation of quantum mechanics to predict the kinetics as a function of applied potential (U) to determine faradic efficiency, turn over frequency, and Tafel slope for CO and H2 production for all three sites. We predict an onset potential (at 10 mA cm-2) Uonset = -0.84 V (vs. RHE) for Ni-N2C2 site and Uonset = -0.92 V for Ni-N3C1 site in agreement with experiments, and Uonset = -1.03 V for Ni-N4. We predict that the highest current is for Ni-N4, leading to 700 mA cm-2 at U = -1.12 V. To help determine the actual sites in the experiments, we predict the XPS binding energy shift and CO vibrational frequency for each site.

Efficient CO2 electroreduction to CO at low overpotentials using a surface-reconstructed and N-coordinated Zn electrocatalyst

In this work, an effective vapor ammonization-electroreduction strategy is developed to fabricate a surface-reconstructed and N-coordinated Zn electrocatalyst (Zn-N), which exhibits high electrocatalytic activity and stability for CO₂ reduction to CO with a faradaic efficiency (FE) of 85.6% and a CO partial current density of 11.2 mA cm^-2 at -0.91 V vs. reversible hydrogen electrode (RHE), greatly outperforming Zn foil and surface-reconstructed Zn (Zn-H). Moreover, Zn-N also maintains good selectivity for CO production during the long-term CO₂ electrolysis.

Tuning lithium-peroxide formation and decomposition routes with single-atom catalysts for lithium-oxygen batteries

Lithium-oxygen batteries with ultrahigh energy density have received considerable attention as of the future energy storage technologies. The development of effective electrocatalysts and a corresponding working mechanism during cycling are critically important for lithium-oxygen batteries. Here, a single cobalt atom electrocatalyst is synthesized for lithium-oxygen batteries by a polymer encapsulation strategy. The isolated moieties of single atom catalysts can effectively regulate the distribution of active sites to form micrometre-sized flower-like lithium peroxide and promote the decomposition of lithium peroxide by a one-electron pathway. The battery with single cobalt atoms can operate with high round-trip efficiency (86.2%) and long-term stability (218 days), which is superior to a commercial 5 wt% platinum/carbon catalyst. We reveal that the synergy between a single atom and the support endows the catalyst with excellent stability and durability. The promising results provide insights into the design of highly efficient catalysts for lithium-oxygen batteries and greatly expand the scope of future investigation.

Metal-Organic Framework-Based Catalysts with Single Metal Sites

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Chemical Synthesis of Single Atomic Site Catalysts

Manipulating metal atoms in a controllable way for the synthesis of materials with the desired structure and properties is the holy grail of chemical synthesis. The recent emergence of single atomic site catalysts (SASC) demonstrates that we are moving toward this goal. Owing to the maximum efficiency of atom-utilization and unique structures and properties, SASC have attracted extensive research attention and interest. The prerequisite for the scientific research and practical applications of SASC is to fabricate highly reactive and stable metal single atoms on appropriate supports. In this review, various synthetic strategies for the synthesis of SASC are summarized with concrete examples highlighting the key issues of the synthesis methods to stabilize single metal atoms on supports and to suppress their migration and agglomeration. Next, we discuss how synthesis conditions affect the structure and catalytic properties of SASC before ending this review by highlighting the prospects and challenges for the synthesis as well as further scientific researches and practical applications of SASC.

Intrinsic Role of Molecular Architectonics in Enhancing the Catalytic Activity of Lead in Glucose Hydrolysis

Lewis acidity plays a key role in the catalytic activity of lead ion (Pb^II) in the hydrolysis of glucose in solution under harsh synthetic conditions. We report a number of structurally similar d-gluconamide amphiphiles as functional organic ligands with active an -NH center capable of coordinating Pb^II (viz., Pb^II-N-C) in basic condition to enhance the catalytic efficiency through the scheme of molecular architectonics. Amphiphiles with different hydrophobic unit form assembly-architectures with a varying second coordination sphere around the active metal ion center. As a result, the active Pb^II center in each architecture exhibits substantially different efficiency toward catalyzing the glucose hydrolysis under ambient temperature. The catalytic performance of the dynamic and reversible gluconamide-Pb^II assembly-architectures are highly dependent on their chemical environments in solution. Further, the active Pb^II center of gluconamide-Pb^II complex in the assembly architecture and dispersed states exhibits distinct outcomes with the former being a superior catalyst than the latter as well as Pb^II alone. The current study demonstrates the potential of molecular architectonics that relies on the hydrophobic units of designer functional amphiphiles to enrich surface electron density with enhanced σ-donation ability through space which substantially improves the catalytic efficiency of Pb^II toward glucose hydrolysis at ambient temperature.

Protecting Copper Oxidation State via Intermediate Confinement for Selective CO2 Electroreduction to C2+ Fuels

Selective and efficient catalytic conversion of carbon dioxide (CO₂) into value-added fuels and feedstocks provides an ideal avenue to high-density renewable energy storage. An impediment to enabling deep CO₂ reduction to oxygenates and hydrocarbons (e.g., C₂₊ compounds) is the difficulty of coupling carbon-carbon bonds efficiently. Copper in the +1 oxidation state has been thought to be active for catalyzing C₂₊ formation, whereas it is prone to being reduced to Cu⁰ at cathodic potentials. Here we report that catalysts with nanocavities can confine carbon intermediates formed in situ, which in turn covers the local catalyst surface and thereby stabilizes Cu⁺ species. Experimental measurements on multihollow cuprous oxide catalyst exhibit a C₂₊ Faradaic efficiency of 75.2 ± 2.7% at a C₂₊ partial current density of 267 ± 13 mA cm^-2 and a large C₂₊-to-C₁ ratio of ∼7.2. Operando Raman spectra, in conjunction with X-ray absorption studies, confirm that Cu⁺ species in the as-designed catalyst are well retained during CO₂ reduction, which leads to the marked C₂₊ selectivity at a large conversion rate.