Transition Metal Single-atom Catalysts Research Articles

Asymmetric coordination engineering of active centers in MXene-based single atom catalysts for high-performance ECO2RR

Asymmetric coordination effect is an important concept in heterogeneous catalysis where both catalytic activity and product selectivity are serious influenced. However, this effect has rarely been understood because of the difficult in characterizing detailed coordination information. Here, by employing a theoretical study on electrocatalytic CO2 reduction reaction (ECO2RR) towards HCOOH over Ti3C2O2-based NS/NN co-coordinated transition metal single atom catalysts (TM-NS/NN-Ti3C2O2 SACs), we show that asymmetric coordination environments have great impact on the electronic structure of the active center. Both electron transferring ability and spin polarization of the active center are vital to optimize the adsorption and hydrogenation of key intermediates. As a result, both catalytic activity and product selectivity can be maximized. V-NN-Ti3C2O2 are predicted to promote high-throughput HCOOH generation process at operation applied potential of −0.32 with considerable electrochemical stability, surpassing most reported catalysts. This work demonstrates V-NN-Ti3C2O2 as high-performance ECO2RR catalysts, and highlights that the introduction of the concept of asymmetric coordination effect may offer a new insight into the rational design of more efficient SACs.

Highly dispersed carbon-encapsulated FeS/Fe3C nanoparticles distributed in Fe-N-C for enhanced oxygen electrocatalysis and Zn-air batteries

Transition metal single-atom catalysts (SACs) have been widely used in oxygen reduction reactions (ORR) and oxygen evolution reaction (OER) due to its greatest atomic utilization and low costs, which catalytic performance can be further enhanced by electron distribution adjustment. Herein, we synthesized a carbon-encapsulated FeS/Fe3C nanoparticles doped carbon-based Fe single atom catalyst from fluid catalytic cracking (FCC) slurry though a one-pot pyrolysis. The synergistic effect between FeS/Fe3C nanoparticles and Fe single atom structure (FeNx) promotes the ORR/OER processes, which may due to the reduction of the adsorption free energy of intermediates. Meanwhile, the polyaromatic hydrocarbon in FCC slurry enhances the graphitization of catalyst to facilitate charge transfer in electrocatalysis process, and the carbon-encapsulated nanoparticles sites possess higher stability and dispersion. As a result, the optimized catalyst (FeS/Fe3C@Fe-N-C) presents a high nanoparticles dispersion and graphitization level, which has a higher ORR catalytic ability (E1/2 = 0.91 V vs RHE) compared with commercial Pt/C (20 wt%, E1/2 = 0.879 V vs RHE) and a similar OER catalytic ability (E10 = 0.1.506 V vs RHE) compared with RuO2 (E10 = 1.518 V vs RHE). A liquid Zn-air battery assembled with FeS/Fe3C@Fe-N-C show a peak power density of 113 mW cm−2 and an open potential of 1.432 V. This work sheds light on a new method to design transition metal active sites carbon based single-atom catalyst for enhanced ORR and OER processes.

Theoretical design of bifunctional single-atom catalyst over g-C2N2 for oxygen evolution and reduction reactions

The two-dimensional graphitic carbon nitride (g-CxNy) is a promising support for electrochemistry catalysts owing to its stability and controllable chemical composition. Here, by density functional theory (DFT) a serial of transition metal single-atom catalysts (SAC) supported on the g-C2N2 with vacancy (denoted as TM1/C2N2, TM=Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir and Pt) were studied for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The non-noble Ni and Co single atom exhibited relatively strong binding energies over the defective g-C2N2 support. The d-band center of TM performed a good descriptor for adsorption free energies of intermediates. For OER, the overpotential of non-noble Co1/C2N2 was as low as 0.30 V, comparable to Rh1/C2N2. For ORR, both Co1/C2N2 and Rh1/C2N2 displayed excellent performance with overpotentials of around 0.50 V. The volcano plot revealed that the proper binding strength of O* and OH* on Co1/C2N2 rendered it an excellent bifunctional catalyst for both OER and ORR. Considering the superior performance of Co SAC over g-C2N2 than not only g-C3N4 but also defective graphene, we postulate that there could exist an optimized C/N ratio of carbon-rich g-CxNy support for improving the non-noble Co SAC activity on oxygen electrode.

Screening of transition metal dual-atom catalysts for hydrogen evolution reaction based on high-throughput calculation and searching surrogate prediction model using simple features

Dual-atom catalysts for hydrogen evolution reaction have received widespread attention, but precise screening and prediction of high-performance catalysts through simple methods remains a challenge. In this study, we perform high-throughput density functional theory (DFT) calculation and machine learning (ML) to screen and predict the transition metal dual-atom catalysts with N-doped graphene support (TMDACs) for acidic hydrogen evolution reaction (HER). The Fe_Zn and V_Fe DACs were proposed to be the most promising candidates for Pt-based catalyst toward acidic HER from 406 TMDACs, based on the characteristics of HER activity, formation, thermodynamic stability, abundance, environmental friendliness. The Fe_Zn and V_Fe DACs with excellent HER performance is due to the synergistic effect deriving from the interaction between H and dual metal atoms in TMDACs. By determining 6 different ML models with four kind of input features, we find the artificial neural networks (ANN) model can predict the HER performance of TMDACs most accurately only using simple input features, including one-hot-encoding of atomic number and Gibbs free energies of transition metal single-atom catalysts. This work not only proposed the potential TMDACs with high HER performance, but also verified that the ANN model can accurately predict the HER activity of diatomic catalysts with simple input features.

High-throughput screening of N-doped curved graphene-loaded transition metal single-atom catalysts for nitrogen reduction reaction

Nitrogen reduction reaction (NRR) is necessary as an environmentally friendly and sustainable method for producing NH3 in the context of the energy crisis and increasing environmental concerns. Efficient catalysts play an important role in the NRR reaction. In this work, a high-curvature tetracoordinate transition metal (TM) nitrogen-doped graphene single-atom catalyst was designed and five candidate catalysts with excellent performances were selected from 406 models through a combination of high-throughput screening and density functional theory calculations. Using seven different coordination types MC4, MC3N1, MC2N2-1, MC2N2-2, MC2N2-3, MC1N3 and MN4, of which VN4 (side-on) with four N-atom coordination has the lowest limiting potential of 0.31 V and the competing Hydrogen Evolution Reaction (HER) was suppressed. This work provides guidance for the development of efficient NRR catalysts.

Turning natural copper phthalocyanine into high-loading single-atom catalysts using an electrochemically-generated template and cationic substitution

Phthalocyanine (PC) has a unique N4-coordinated structure that offers an inherent advantage with respect to the accommodation of metal ions. This feature can help overcome the limitations of many single-atom electrocatalysts, i.e. low loading and poor stability. Here, we detail the development of a universal electrochemical template and a cationic substitution synthesis protocol for preparing various single-atom catalysts with high-loading (≌ 8.6 wt%) from commercial copper phthalocyanine (CuPC). Commercial CuPC is transformed into Cu NPs and vacant N4-sites are created during applied potential cycling. The generated vacant N4-sites, with strong negative charges, can take-up Pd2+ ions from a precursor solution to create single-atom catalysts with Pd high-loadings. The material’s structural transformation and cationic substitution mechanism were investigated by in situ X-ray absorption spectroscopy (XAS). We also demonstrate the viability of extending the proposed electrochemical template synthesis method to the development of other high-loading transition metal single-atom catalysts, e.g., Ni, Co, and Fe.

In situ Activation of Molecular Oxygen at Intermetallic Spacing-Optimized Iron Network-Like Sites for Boosting Electrocatalytic Oxygen Reduction.

The oxygen reduction reaction (ORR) catalyzed by transition-metal single-atom catalysts (SACs) is promising for practical applications in energy-conversion devices, but great challenges still remain due to the sluggish kinetics of O═O cleavage. Herein, a kind of high-density iron network-like sites catalysts are constructed with optimized intermetallic distances on an amino-functionalized carbon matrix (Fe-HDNSs). Quasi-in situ soft X-ray absorption spectroscopy and in situ synchrotron infrared characterizations demonstrate that the optimized intermetallic distances in Fe-HDNSs can in situ activate the molecular oxygen by fast electron compensation through the hybridized Fe 3d-O 2p, which efficiently facilitates the cleavage of the O═O bond to *O species and highly suppresses the side reactions for an accelerated kinetics of the 4e- ORR. As a result, the well-designed Fe-HDNSs catalysts exhibit superior performances with a half-wave potential of 0.89V versus reversible hydrogen electrode (RHE)and a kinetic current density of 72mAcm-2 @0.80V versus RHE, exceeding most of the noble-metal-free ORR catalysts. This work offers some new insights into the understanding of 4e- ORR kinetics and reaction pathways to boost electrochemical performances of SACs.

The recent progress of cathode materials for heterogeneous electro-Fenton reactions

As one of environmentally friendly and efficient oxidation techniques, the electro-Fenton (EF) processes allow to remove and mineralize organic pollutants. However, the transitional EF processes require an acidic environment, adding ferrous ions, treatment of iron sludge, and costly solution pH adjustment. Heterogeneous EF (hetero-EF) processes enable EF to function over a broad working pH region and recycle across multiple reaction rounds which reduces overall cost and displays great potential for practical engineering applications. Here, this work reviewed the reported cathode materials for the hetero-EF processes. We firstly described the mechanisms of homogeneous and heterogeneous EF reactions. Then, we summarized different types of cathode materials for hetero-EF reactions, including metal-free carbon materials, transition metal single-atom catalysts (SACs)/carbon materials, alloys catalysts/carbon materials, metal compounds/carbon materials and other materials. Finally, we overviewed the challenges and proposal strategies of hetero-EF technique in order to expand its wide-scale applicability and practical significance for advanced wastewater treatment.

Emerging electrospinning platform toward nanoparticle to single atom transformation for steering selectivity in ammonia synthesis.

The rising top-down synthetic methodologies for transition metal single-atom catalysts (SACs) require controlled movement of metal atoms through the substrates; however, their direct transportation towards the ideal carrier remains a huge challenge. Herein, we showed a "top down" strategy for Co nanoparticles (NPs) to Co SA transformation by employing electrospun carbon nanofibers (CNFs) as atom carriers. Under high-temperature conditions, the Co atoms migrate from the surfaces of Co NPs and are then anchored by the surrounding carbon to form a Co-C3O1 coordination structure. The synthesized Co SAs/CNF electrocatalyst exhibits excellent electrocatalytic nitrate reduction reaction (NO3RR) activity with an NH3 yield of 0.79 mmol h-1 cm-2 and Faraday efficiency (FE) of 91.3% at -0.7 V vs. RHE in 0.1 M KNO3 and 0.1 M K2SO4 electrolytes. The in situ electrochemical characterization suggests that the NOH pathway is preferred by Co SAs/CNFs, and *NO hydrogenation and deoxygenation easily occur on Co SAs due to the small adsorption energy between Co SAs and *NO, as calculated by theoretical calculations. It is revealed that a small energy barrier (0.45 eV) for the rate determining step (RDS) ranges from *NO to *NOH and a strong capability for inhibiting hydrogen evolution (HER) significantly promotes the NH3 selectivity and activity of Co SAs/CNFs.

Design strategies towards transition metal single atom catalysts for the oxygen reduction reaction – A review

The electrochemical oxygen reduction reaction (ORR) is pivotal in energy conversion <i>via</i> a 4e⁻ ORR pathway and green hydrogen peroxide production <i>via</i> 2e⁻ ORR pathway. Transition metal single atom catalysts (TM SACs) have attracted intense attention in recent years for ORR due to their high activity and near maximum metal atom utilization. The future development of TM SACs for ORR requires improved understanding of reaction pathways, since currently the true origin of activity remains contentious owing to the lack of qualitative/quantitative information about active sites. Knowledge-guided design is imperative for the optimization of TM SACs performance in terms of activity and selectivity. This review focuses on the latest progress in the design of TM SACs for ORR, placing particular attention on efforts to elucidate reaction mechanisms. Experimental evidence based on <i>in-situ/operando</i> characterization measurements, along with theoretical predictions, are summarized to deepen understanding of the structure-performance relationships at both atomic and molecular level. Finally, some perspectives are offered relating to the fundamental science needed for TM SACs to find practical application in energy storage and conversion devices. We hope this review will inspire the development of new synthetic routes towards high-performance ORR electrocatalysts for the energy sector.

Single-Atom Catalysts for Selective Oxygen Reduction: Transition Metals in Uniform Carbon Nanospheres with High Loadings.

Transition metal single-atom catalysts (SACs) in uniform carbon nanospheres have gained tremendous interest as electrocatalysts owing to their low cost, high activity, and excellent selectivity. However, their preparation typically involves complicated multistep processes that are not practical for industrial use. Herein, we report a facile one-pot method to produce atomically isolated metal atoms with high loadings in uniform carbon nanospheres without any templates or postsynthesis modifications. Specifically, we use a chemical confinement strategy to suppress the formation of metal nanoparticles by introducing ethylenediaminetetraacetic acid (EDTA) as a molecular barrier to spatially isolate the metal atoms and thus generate SACs. To demonstrate the versatility of this synthetic method, we produced SACs from multiple transition metals, including Fe, Co, Cu, and Ni, with loadings as high as 3.87 wt %. Among these catalytic materials, the Fe-based SACs showed remarkable catalytic activity toward the oxygen reduction reaction (ORR), achieving an onset and half-wave potential of 1.00 and 0.831 VRHE, respectively, comparable to that of commercial 20 wt % Pt/C. Significantly, we were able to steer the ORR selectivity toward either energy generation or hydrogen peroxide production by simply changing the transition metal in the EDTA-based precursor.

Triethylenediamine cobalt complex encapsulated in a metal–organic framework cage to prepare a cobalt single-atom catalyst with a high Co-N4 density for an efficient oxygen reduction reaction

Transition metal single atom catalysts (TM SACs) are the most promising oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs) and metal-air batteries. However, the low density of M-Nx active sites seriously hinders further improvement of the ORR electrocatalytic activity. Here, a strategy for encapsulating nitrogen-rich guest molecules (triethylenediamine cobalt complex, [Co(en)3]3+) was proposed to construct a high-performance cobalt single-atom catalyst (Co-encapsulated SAC/NC). With this strategy, the guest molecules are encapsulated into metal–organic framework (MOF) cages as an additional cobalt source to boost cobalt loading, while abundant nitrogen from guest molecules contributes to the formation of Co-N4 active sites. Remarkably, the resulting Co-encapsulated SAC/NC has a high cobalt loading amount of 4.03 wt%, and spherical aberration-corrected transmission electron microscopy (AC-TEM) has confirmed that most cobalt exists in a single-atom state. As a result, the Co-encapsulated SAC/NC exhibits excellent ORR catalytic performance with a half-wave potential of 0.88 V. Furthermore, Zn-air batteries employing Co-encapsulated SAC/NC as air cathode show high peak power density and excellent cycling stability. Density functional theory (DFT) calculations reveal that adjacent active sites have different rate-determining steps and lower reaction energy barriers than a single active site.

Defective SiC nanotube based single-atom catalysts for electrocatalytic nitrogen fixation with curvature effect

The production of fertilizers, explosives, and various other chemicals relies heavily on ammonia (NH3). Electrochemical reduction of nitrogen to ammonia, also known as the nitrogen reduction reaction (NRR), is a potential alternative to the Haber-Bosch process that can be carried out at ambient conditions. However, many NRR catalysts suffer from low selectivity and require a large overpotential to drive the reaction, limiting their practical application. In this study, we used density functional theory (DFT) calculations to investigate the mechanistic aspects of the NRR and the effect of curvature variation on the catalytic activity of silicon carbide nanotubes (SiCNTs) decorated with 3d-5d transition metal single-atom catalysts. Our results suggest that the Os@SiCNT catalyst exhibits promising activity as an eNRR catalyst with an overpotential of 0.47 V and high selectivity over hydrogen evolution reaction (HER) competition. The electronic “acceptance-donation” interaction between N2 and Os promotes the adsorption and activation of N2 molecules on the catalyst surface. Additionally, our findings demonstrate that as the nanotube diameter decreases, the energy barrier for the NRR reaction decreases as well. However, this decrease in diameter also leads to a weaker capacity for *N2 adsorption, which may ultimately reduce the selectivity of the NRR reaction. These findings offer valuable insights that can aid in the rational design of nanotube-based electrocatalysts for eNRR.

Integration of atomic Co-N5 sites with defective N-doped carbon for efficient zinc-air batteries

Transition metal single-atom catalysts have attracted considerable attention for their applicability in electrocatalytic oxygen reduction reactions (ORR), but it is still very challenging to regulate the interaction between the active sites and oxygen-containing intermediates. In this study, atomically dispersed Co-N5 sites integrated with defective N-doped carbon (Co-N5/DHC) were developed using a facile templating approach, followed by NH3 treatment. NH3 can simultaneously etch inactive amorphous substrates on the surfaces of metal sites and construct intrinsic carbon defects, thereby increasing the number of both metal and metal-free active sites. By bridging the intrinsic carbon defects and metal sites, the optimized Co-N5/DHC catalyst exhibited significantly improved electrocatalytic ORR performance compared to the pristine Co-Nx/HC sample (cobalt-nitrogen sites anchored on heteroatom-doped carbon). Density functional calculations revealed that the strong interaction between the Co-N5 sites and carbon defects modified the electronic localization, thus optimizing the binding energy with oxygen-containing intermediates and resulting in significantly improved ORR catalytic activity with a half-wave potential of 0.877 V. In addition, a zinc-air battery assembled with the Co-N5/DHC as the cathode achieves a maximum power density of 160.7 mW cm−2 and affords a specific capacity of 766.2 mA h gZn−1.

MXene-supported transition metal single-atom catalysts for nitrogen dissociation

Industrial ammonia production follows the Haber-Bosch process, whose rate-limiting step is the dissociation of the nitrogen molecule (N2), hence requiring suitable catalysts to break its triple bond. MXenes, a class of two-dimensional transition metal carbides and nitrides, have been proposed as very efficient catalysts for N2 dissociation. Here, by employing density functional theory-based calculations, we assess whether the deposition of one atom of a transition metal element (TM) on the Ti2C MXene surface further improves the catalytic potential of the MXene, serving as a single-atom catalyst. The results show that, for 21 of the 30 TMs considered, N2 can exothermically bind to the TM adatom, this bonding being favourable with respect to adsorption on the pristine Ti2C MXene surface for TMs of groups 3 to 6 of the Periodic Table. All the 21 TMs that successfully bind to N2 effectively reduce the N2 dissociation energy barrier when compared to the bulk Ti2C MXene by 18 to 84 %. Our results strongly indicate that doping the Ti2C MXene with atoms of transition metal elements significantly reduces the energy required to break the triple bond in N2, which may impact the nitrogen-to-ammonia process.

Co Single Atoms Anchored in N and P Co-doped Porous Carbon Fibers for Efficient Water Splitting.

Single atom catalysts (SACs) show potential application for highly efficient water splitting. Herein, we designed Co single atoms (SAs) dispersed on N and P co-doped porous carbon nanofibers as electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The configuration of Co SAs is proved to coordinate with 4 N/O atoms. The long-range interactions between doped P atoms and Co-N4 (O) sites could modulate the electronic structures of M-N4 (O) sites, which greatly reduce the adsorption energies of HER and OER intermediates at metal sites. Density Functional Theory calculations reveal that CoSA /CNFs exhibits optimized HER and OER kinetics when P coordinates with two N atoms. The atomically dispersed Co electrocatalyst exhibits the low overpotentials of 61, 89 and 390 mV for acidic HER, alkaline HER and OER at 10 mA cm-2 current density, along with the Tafel slope of 54, 143 and 74 mV dec-1 , respectively. This work demonstrates the prospect of utilizing di-heteroatom-doping transition metal SACs, and provides a novel and general strategy for the preparation of SACs.

Mimicking Metalloenzyme Microenvironments in the Transition Metal-Single Atom Catalysts for Electrochemical Hydrogen Peroxide Synthesis in an Acidic Medium.

Electrochemical reduction of oxygen into hydrogen peroxide in an acidic medium offers an energy-efficient and green H2 O2 synthesis as an alternative to the energy-intensive anthraquinone process. Unfortunately, high overpotential, low production rates, and fierce competition from traditional four-electron reduction limit it. In this study, a metalloenzyme-like active structure is mimicked in carbon-based single-atom electrocatalysts for oxygen reduction to H2 O2 . Using a carbonization strategy, the primary electronic structure of the metal center with nitrogen and oxygen coordination is modulated, followed by epoxy oxygen functionalities close to the metal active sites. In an acidic medium, CoNOC active structures proceed with greater than 98% H2 O2 selectivity (2e- /2H+ ) rather than CoNC active sites that are selective to H2 O (4e- /4H+ ). Among all MNOC (M=Fe, Co, Mn, and Ni) single-atom electrocatalysts, the CoNOC is the most selective (> 98%) for H2 O2 production, with a mass activity of 10Ag-1 at 0.60V vs. RHE. X-ray absorption spectroscopy is used to identify the formation of unsymmetrical MNOC active structures. Experimental results are also compared to density functional theory calculations, which revealed that the structure-activity relationship of the epoxy-surrounded CoNOC active structure reaches optimum (ΔG*OOH ) binding energies for high selectivity.

Transition Metal (Fe, Zn, Co, and Ni) Single-Atom Catalysts Anchored on N,S-Codoped Hybrid Nanocarbons for Oxygen Reduction Reaction

Atomically dispersed transition metal catalysts have served as promising candidates to substitute noble metals in energy conversion. A majority of carbon-supported transition metal single-atom catalysts (SACs) have been synthesized via the high-temperature sintering method, in which the complex heating and cooling processes are time- and energy-consuming. Herein, we have developed a simple and highly effective microwave-assisted heating method to prepare a series of isolated transition metal SACs anchored on a N,S-codoped nanocarbon matrix, which are denoted as M-NSC-MW, M = Fe, Zn, Co, and Ni. Among the various M-NSC-MW, Fe-NSC-MW possesses the highest ORR electrochemical activity in alkaline electrolytes. Its onset potential (Eonset) is 0.955 V, and its half-wave potential (E1/2) can reach 0.852 V, both of which are the same as those of commercial Pt/C. Moreover, Fe-NSC-MW also exhibits much superior ORR catalytic efficiency than conventional pyrolysis samples. It is worth noting that Zn-NSC–MW is prepared for the first time, and it also shows a high ORR performance with an Eonset of 0.945 V and an E1/2 of 0.832 V. This work has revealed the general application of the microwave-assisted heating method to prepare well-dispersed transition metal SACs anchored on the N,S-codoped graphene/carbon hybrid matrix.

Theoretical Insights into Single-Atom Catalysts Supported on N-Doped Defective Graphene for Fast Reaction Redox Kinetics in Lithium-Sulfur Batteries.

Single-atom catalysts are proven to be an effective strategy for suppressing shuttle effect at the source by accelerating the redox kinetics of intermediate polysulfides in lithium-sulfur (Li-S) batteries. However, only a few 3d transition metal single-atom catalysts (Ti, Fe, Co, Ni) are currently applied for sulfur reduction/oxidation reactions (SRR/SOR), which remains challenging for screening new efficient catalysts and understanding the relationship between structure-activity of catalysts. Herein, N-doped defective graphene (NG) supported 3d, 4d, and 5d transition metals are used as single-atom catalyst models to explore electrocatalytic SRR/SOR in Li-S batteries by using density functional theory calculations. The results show that M1 /NG (M1 = Ru, Rh, Ir, Os) exhibits lower free energy change of rate-determining step and Li2 S decomposition energy barrier, which significantly enhance the SRR and SOR activity compared to other single-atom catalysts. Furthermore, the study accurately predicts the by machine learning based on various descriptors and reveals the origin of the catalyst activity by analyzing the importance of the descriptors. This work provides great significance for understanding the relationships between the structure-activity of catalysts, and manifests that the employed machine learning approach is instructive for theoretical studies of single-atom catalytic reactions.

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism, Synthesis, Characterization, Application, and Prospects.

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO3 RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO3 RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO3 RR. However, the real active sites of TM-SACs applied to NO3 RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO3 RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO3 RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO3 RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.