Diminishing the Uncoordinated N Species in Co-N-C Catalysts toward Highly Efficient Electrochemical CO2 Reduction

Modulating the Coordination Environment of Atomically Dispersed Nickel for Efficient Electrocatalytic CO2 Reduction at Low Overpotentials and Industrial Current Densities

Electrocatalytic CO2-to-CO conversion with a high CO Faradaic efficiency (FECO) at low overpotentials and industrial-level current densities is highly desirable but a huge challenge over non-noble metal catalysts. Herein, graphitic N-rich porous carbons supporting atomically dispersed nickel (NiN4-O sites with an axial oxygen) were synthesized (denoted as O-Ni-Nx-GC) and applied as the cathode catalyst in a CO2RR flow cell. O-Ni-Nx-GC showed excellent selectivity with a FECO over 92% at low overpotentials ranging from 17 to 60 mV, and over 99% at 80 mV. The FECO was ∼100% at industrial-level current densities from 200 to 900 mA·cm-2. Impressively, O-Ni-Nx-GC delivered a state-of-the-art FECO of >96% at 1 A·cm-2 with a turnover frequency of 81.5 s-1 in a 1 M KOH electrolyte. O-Ni-Nx-GC offered excellent stability during long-term operation for 140 h at 100 mA·cm-2, maintaining a FECO > 99%. Mechanism studies revealed that the axial oxygen at the atomically dispersed nickel sites enhanced electron delocalization, with the graphitic N-rich porous carbon support lowering the CO2-to-CO energy barrier and inducing a negative shift in the Ni-3d d-band center, effectively promoting the formation of the *COOH intermediate while weakening the adsorption of the *CO intermediate, thus optimizing the catalytic activity/selectivity to CO under practical conditions.

Transition Metal-Nitrogen-Carbon Single-Atom Catalysts Enhanced CO2 Electroreduction Reaction: A Review

As the global energy crisis and environmental challenges worsen, CO2 conversion has emerged as a focal point in international research. CO2 electroreduction reaction (CO2ER) is a green and sustainable technology that converts CO2 into high-value chemicals, thereby achieving the recycling of carbon resources. However, the activity and selectivity are constrained by the performance of the catalyst. Although traditional N-doped carbon-based catalysts exhibit excellent performance toward CO2ER, the atomic utilization rate in these materials is far from 100 %. Single atom catalysts (SACs) can attain nearly 100 % atomic utilization efficiency because of the fully exposing metal atoms. Therefore, SACs have emerged as one of the hot research materials in the field of CO2ER. Recently, transition metal-nitrogen-carbon single-atom catalysts (TM-N-C SACs) have flourished because of their extraordinary catalytic activity, low cost, and excellent stability, demonstrating enormous application prospects in CO2ER. In this review, we concentrate on TM-N-C SACs that electrochemically reduce CO2 to high value products. A comprehensive and detailed discussion were conducted on the synthesis method, chemical structure, chemical characterization of TM-N-C SACs, as well as their catalytic performance, active sources, and mechanism exploration for CO2ER. Finally, challenges and prospects for commercial application of TM-N-C SACs catalysts suitable for CO2ER are proposed.

Graphene-based single-atom catalysts for electrochemical CO2 reduction: unraveling the roles of metals and dopants in tuning activity

Discovering electrocatalysts that can efficiently convert carbon dioxide (CO2) to valuable fuels and feedstocks using excess renewable electricity is an emergent carbon-neutral technology. A single metal atom embedded in doped graphene, i.e., single-atom catalyst (SAC), possesses high activity and selectivity for electrochemical CO2 reduction (CO2R) to CO, yet further reduction to hydrocarbons is challenging. Here, using density functional theory calculations, we investigate stability and reactivity of a broad SAC chemical space with various metal centers (3d transition metals) and dopants (2p dopants of B, N, O; 3p dopants of P, S) as electrocatalysts for CO2R to methane and methanol. We observe that the rigidities of these SACs depend on the type of dopants, with 3p-coordinating SACs exhibiting more severe out-of-plane distortion than 2p-coordinating SACs. Using CO adsorption energy as a descriptor for CO2R reactivity, we narrow down the candidates and identify seven SACs with near-optimal CO binding strength. We then elucidate full reaction mechanisms towards methane and methanol generation on these identified candidates and observe highly dopant-dependent activity and rate-limiting steps, divergent from conventional mechanistic understanding on metallic surfaces, calling into question whether previous design principles established on metals are directly transferrable to SACs. Consequently, we find that zinc embedded in boron-doped graphene (Zn-B-C) is a highly active catalyst for electrochemical CO2R to C1 hydrocarbons. Our work reveals the opportunities of tuning SAC reactivity via engineering dopants and metals and highlights the importance of re-elucidating CO2R reaction mechanisms on SACs towards unearthing new design principles for SAC chemistry.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Graphite conjugated nickel phthalocyanine for efficient CO2 electroreduction and Zn-CO2 batteries

The linking chemistry between molecular catalysts and substrates is a crucial challenge for enhancing electrocatalytic performance. Herein, we elucidate the influence of various immobilization methods of amino-substituted Ni phthalocyanine catalysts on their electrocatalytic CO2 reduction reaction (eCO2RR) activity. A graphite-conjugated Ni phthalocyanine, Ni(NH2)8Pc-GC, demonstrates remarkable electrocatalytic performance both in H-type and flow cells. In situ infrared spectroscopy and theoretical calculations reveal that the graphite conjugation, through strong electronic coupling, increases the electron density of the active site, reduces the adsorption energy barrier of *COOH, and enhances the catalytic performance. As the cathode catalyst, Ni(NH2)8Pc-GC also displays remarkable charge-discharge cycle stability of over 50 hours in a Zn-CO2 battery. These findings underscore the significance of immobilization methods and highlight the potential for further advancements in eCO2RR.

Structural Degradation of M-N-C (M=Co, Ni and Fe) Single-Atom Electrocatalysts at Industrial-Grade Current Density for Long-Term Reduction

M-N-C single-atom catalysts (SACs) are promising electrode materials for many electro-reduction reactions. However, their stability is far from practical applications, and their deactivation mechanism has been rarely investigated. Herein, we demonstrate the structural degradation of M-N-C (M=Co, Ni, and Fe) at industrial-grade current density for long-term electro-reduction. Both M-N and N-C bonds are broken, resulting in the gradual hydrogenation and dissolution of N in the form of ammonia. The residual M is finally converted to M-containing core-shell nanoparticles after sequential dissolution, redeposition, and electro-reduction. The destruction of the M-N-C structure and the formation of nanoparticles greatly affect the electrocatalytic performance. Our work highlights the structural degradation and deactivation mechanism of M-N-C-type SACs under strong reductive conditions and provides useful information for inspiring researchers to develop new strategies to improve the electrocatalytic stability of similar types of materials.

Highly efficient selective hydrogenation of adiponitrile to hexamethylene diamine over barium and melamine formaldehyde resin-modified nickel-cobalt-based zeolitic imidazolate framework-derived catalyst

In the present study, the catalyst modified with alkaline oxide can enhance the selectivity to primary amines. However, the addition of alkaline oxide inevitably reduces catalytic activity. In this study, NiCo-NC@BaO-MFC catalyst derived from zeolitic imidazolate framework-67, Ba(CH3COO)2, and melamine formaldehyde (MF) resin was prepared and used for the hydrogenation of adiponitrile (ADN) to hexamethylene diamine (HDMA). The carbon layer obtained from the MF resin effectively prevents the interaction between barium (Ba) and the active center, thus improving target product selectivity without decreasing catalytic activity. The results of the density functional theory (DFT) calculation and characterization indicated that the effect of synergy between nickel (Ni) and cobalt (Co) bimetals induces an electron density growth on the Ni surface, bringing the d-band center toward the Fermi surface. Meanwhile, the high electron density of the active center compensates for the electron-deficient state of the carbon atom in -CN, thus improving the catalytic activity. Furthermore, it was found that the introduction of Ba promotes the formation of nucleophilic hydrogen anions, which facilitates the hydrogenation of 6-aminohexylimine (AHIM) to HDMA and inhibits the intramolecular condensation of AHIM, hence improving the selectivity to HDMA. The NiCo-NC@BaO-MFC catalyst gives 98.6 % ADN conversion and 97.2 % selectivity to HDMA in an alkali-free system.

Computational screening of defective BC3-supported single-atom catalysts for electrochemical CO2 reduction

The electrochemical CO2 reduction reaction (eCO2RR) driven by renewable electricity offers a green and sustainable technology for synthesizing chemicals and managing global carbon balance. However, developing electrocatalysts with high activity and selectivity for producing C1 products (CO, HCOOH, CH3OH, and CH4) remains a daunting task. In this study, we conducted comprehensive first-principles calculations to investigate the eCO2RR mechanism using B-defective BC3-supported transition metal single-atom catalysts (TM@BC3 SACs). Initially, we evaluated the thermodynamic and electrochemical stability of the designed 26 TM@BC3 SACs by calculating the binding energy and dissolution potential of the anchored TM atoms. Subsequently, the selectivity of the eCO2RR and hydrogen evolution reaction (HER) on stable SACs was determined by comparing the free energy change (ΔG) for the first protonation of CO2 with the ΔG of *H formation. The stability and selectivity screening processes enabled us to narrow down the pool of SACs to the 14 promising ones. Finally, volcano plots for the eCO2RR towards different C1 products were established by using the adsorption energy descriptors of key intermediates, and three SACs were predicted to exhibit high activity and selectivity. The limiting potentials (UL) for HCOOH production on Pd@BC3 and Ag@BC3 are -0.11 V and -0.14 V. CH4 is a preferred product on Re@BC3 with UL of -0.22 V. Elaborate electronic structure calculations elucidate that the activity and selectivity originate from the sufficient activation of the C-O bond and the strong orbital hybridization between crucial intermediates and metal atoms. The proposed catalyst screening criteria, constructed volcano plots and predicted SACs may provide a theoretical foundation for the development of computationally guided catalyst designs for electrochemical CO2 conversion to C1 products.

Stabilizing the oxidation state of catalysts for effective electrochemical carbon dioxide conversion

In the electrocatalytic CO2 reduction reaction (CO2RR), metal catalysts with an oxidation state generally demonstrate more favorable catalytic activity and selectivity than their corresponding metallic counterparts. However, the persistence of oxidative metal sites under reductive potentials is challenging since the transition to metallic states inevitably leads to catalytic degradation. Herein, a thorough review of research on oxidation-state stabilization in the CO2RR is presented, starting from fundamental concepts and highlighting the importance of oxidation state stabilization while revealing the relevance of dynamic oxidation states in product distribution. Subsequently, the functional mechanisms of various oxidation-state protection strategies are explained in detail, and in situ detection techniques are discussed. Finally, the prevailing and prospective challenges associated with oxidation-state protection research are discussed, identifying innovative opportunities for mechanistic insights, technology upgrades, and industrial platforms to enable the commercialization of the CO2RR.

Guanine-derived carbon nanosheet encapsulated Ni nanoparticles for efficient CO2 electroreduction

Developing novel electrocatalysts for achieving high selectivity and faradaic efficiency in the carbon dioxide reduction reaction (CO2RR) poses a major challenge. In this study, a catalyst featuring a nitrogen-doped carbon shell-coated Ni nanoparticle structure is designed for efficient carbon dioxide (CO2) electroreduction to carbon monoxide (CO). The optimal Ni@NC-1000 catalyst exhibits remarkable CO faradaic efficiency (FECO) values exceeding 90% across a broad potential range of -0.55 to -0.9 V (vs. RHE), and attains the maximum FECO of 95.6% at -0.75 V (vs. RHE) in 0.5 M NaHCO3. This catalyst exhibits sustained carbon dioxide electroreduction activity with negligible decay after continuous electrolysis for 20 h. More encouragingly, a substantial current density of 200.3 mA cm-2 is achieved in a flow cell at -0.9 V (vs. RHE), reaching an industrial-level current density. In situ Fourier transform infrared spectroscopy and theoretical calculations demonstrate that its excellent catalytic performance is attributed to highly active pyrrolic nitrogen sites, promoting CO2 activation and significantly reducing the energy barrier for generating *COOH. To a considerable extent, this work presents an effective strategy for developing high-efficiency catalysts for electrochemical CO2 reduction across a wide potential window.

A Co-based nitrogen-doped lignin carbon catalyst with high stability and wide operating window for rapid degradation of antibiotics

Co-based catalysts play a crucial role in the activation of peroxymonosulfate (PMS) for degradation contaminants. However, the practical application of such catalysts is hindered by challenges like the self-aggregation of Co nanoparticles and leaching of Co2+. In this study, the Co-based catalyst Co-N/C@CL was synthesized from carboxymethylated lignin obtained by grafting abundant carboxymethyl groups into alkali lignin, in which the presence of these carboxymethyl groups enhanced its water solubility and allowed the formation of stable macromolecular complexes with Co2+. This catalyst exhibited a high specific surface area (521.8 m2·g-1) and a uniform distribution of Co nanoparticles. Consequently, the Co-N/C@CL/PMS system could completely remove 20 ppm tetracycline (TC) in 2 min at a rate of 2.404 min-1. Experimental results and DFT calculations revealed that the synergistic effect of lignin carbon and Co NPs accelerated the cleavage and electron transfer of OO bonds, thus promoting the formation of 1O2, OH and SO4-, with 1O2 emerging as the predominant contributor. Moreover, Co-N/C@CL displayed excellent cycling stability and low Co2+ leaching. This work not only provides a feasible strategy for the preparation of highly active and stable Co-based carbon materials but also offers a promising catalyst for the efficient degradation of TC.

Boron-Doped Nickel-Nitrogen-Carbon Single-Atom Catalyst for Boosting Electrochemical CO2 Reduction

Tuning the coordination environment of the metal center in metal-nitrogen-carbon (M-N-C) single-atom catalysts via heteroatom-doping (oxygen, phosphorus, sulfur, etc.) is effective for promoting electrocatalytic CO2 reduction reaction (CO2 RR). However, few studies are investigated establishing efficient CO2 reduction by introducing boron (B) atoms to regulate the M-N-C structure. Herein, a B-C3 N4 self-sacrifice strategy is developed to synthesize B, N co-coordinated Ni single atom catalyst (Ni-BNC). X-ray absorption spectroscopy and high-angle annular dark field scanning transmission electron microscopy confirm the structure (Ni-N3 B/C). The Ni-BNC catalyst presents a maximum CO Faradaic efficiency (FECO ) of 98.8% and a large CO current density (jCO ) of -62.9 mA cm-2 at -0.75 and -1.05 V versus reversible hydrogen electrode, respectively. Furthermore, FECO could be maintained above 95% in a wide range of potential windows from -0.65 to -1.05 V. In situ experiments and density functional theory calculations demonstrate the Ni-BNC catalyst with B atoms coordinated to the central Ni atoms could significantly reduce the energy barrier for the conversion of *CO2 to *COOH, leading to excellent CO2 RR performance.

Customization from Single to Dual Atomic Sites for Efficient Electrocatalytic CO2 Reduction to Value-added Chemicals

In recent years, single-atom catalysts (SACs) have received increasing attention in the field of electrochemical CO2 RR with their efficient atom utilization efficiency and excellent catalytic performance. However, their low metal loading and the presence of linear relationships for single active sites with simple structures possibly restrict their activity and practical applications. Active site tailoring at the atomic level is a visionary approach to break the existing limitations of SACs. This paper first briefly introduces the synthesis strategies of SACs and DACs. Then, combining previous experimental and theoretical studies, this paper introduces four optimization strategies, namely spin-state tuning engineering, axial functionalization engineering, ligand engineering, and substrate tuning engineering, for improving the catalytic performance of SACs in the electrochemical CO2 RR process by combining previous experimental and theoretical studies. Then it is introduced that DACs exhibit significant advantages over SACs in increasing metal atom loading, promoting the adsorption and activation of CO2 molecules, modulating intermediate adsorption, and promoting C-C coupling. At the end of this paper, we briefly and succinctly summarize the main challenges and application prospects of SACs and DACs in the field of electrochemical CO2 RR at present.

Asymmetric dinitrogen-coordinated nickel single-atomic sites for efficient CO2 electroreduction

Developing highly efficient, selective and low-overpotential electrocatalysts for carbon dioxide (CO₂) reduction is crucial. This study reports an efficient Ni single-atom catalyst coordinated with pyrrolic nitrogen and pyridinic nitrogen for CO₂ reduction to carbon monoxide (CO). In flow cell experiments, the catalyst achieves a CO partial current density of 20.1 mA cm_geo^-2 at -0.15 V vs. reversible hydrogen electrode (V_RHE). It exhibits a high turnover frequency of over 274,000 site^-1 h^-1 at -1.0 V_RHE and maintains high Faradaic efficiency of CO (FE_CO) exceeding 90% within -0.15 to -0.9 V_RHE. Operando synchrotron-based infrared and X-ray absorption spectra, and theoretical calculations reveal that mono CO-adsorbed Ni single sites formed during electrochemical processes contribute to the balance between key intermediates formation and CO desorption, providing insights into the catalyst's origin of catalytic activity. Overall, this work presents a Ni single-atom catalyst with good selectivity and activity for CO₂ reduction while shedding light on its underlying mechanism.

Ultrathin Nitrogen-Doped Carbon Encapsulated Ni Nanoparticles for Highly Efficient Electrochemical CO2 Reduction and Aqueous Zn-CO2 Batteries

Electrochemical CO₂ reduction reaction (CO₂ RR), powered by renewable electricity, has attracted great attention for producing high value-added fuels and chemicals, as well as feasibly mitigating CO₂ emission problem. Here, this work reports a facile hard template strategy to prepare the Ni@N-C catalyst with core-shell structure, where nickel nanoparticles (Ni NPs) are encapsulated by thin nitrogen-doped carbon shells (N-C shells). The Ni@N-C catalyst has demonstrated a promising industrial current density of 236.7 mA cm^-2 with the superb FE_CO of 97% at -1.1 V versus RHE. Moreover, Ni@N-C can drive the reversible Zn-CO₂ battery with the largest power density of 1.64 mW cm^-2 , and endure a tough cycling durability. These excellent performances are ascribed to the synergistic effect of Ni@N-C that Ni NPs can regulate the electronic microenvironment of N-doped carbon shells, which favor to enhance the CO₂ adsorption capacity and the electron transfer capacity. Density functional theory calculations prove that the binding configuration of N-C located on the top of Ni slabs (Top-Ni@N-C) is the most thermodynamically stable and possess a lowest thermodynamic barrier for the formation of COOH^* and the desorption of CO. This work may pioneer a new method on seeking high-efficiency and worthwhile electrocatalysts for CO₂ RR and Zn-CO₂ battery.

Emerging materials for electrochemical CO2 reduction: progress and optimization strategies of carbon-based single-atom catalysts

The electrochemical CO₂ reduction reaction can effectively convert CO₂ into promising fuels and chemicals, which is helpful in establishing a low-carbon emission economy. Compared with other types of electrocatalysts, single-atom catalysts (SACs) immobilized on carbon substrates are considered to be promising candidate catalysts. Atomically dispersed SACs exhibit excellent catalytic performance in CO₂RR due to their maximum atomic utilization, unique electronic structure, and coordination environment. In this paper, we first briefly introduce the synthetic strategies and characterization techniques of SACs. Then, we focus on the optimization strategies of the atomic structure of carbon-based SACs, including adjusting the coordination atoms and coordination numbers, constructing the axial chemical environment, and regulating the carbon substrate, focusing on exploring the structure-performance relationship of SACs in the CO₂RR process. In addition, this paper also briefly introduces the diatomic catalysts (DACs) as an extension of SACs. At the end of the paper, we summarize the article with an exciting outlook discussing the current challenges and prospects for research on the application of SACs in CO₂RR.

Proton Transfer Dynamics-Mediated CO2 Electroreduction

Electrochemical CO₂ reduction reaction (CO₂ RR) is crucial to addressing environmental crises and producing chemicals. Proton activation and transfer are essential in CO₂ RR. To date, few research reviews have focused on this process and its effect on catalytic performance. Recent studies have demonstrated ways to improve CO₂ RR by regulating proton transfer dynamics. This Concept highlights the use of regulating proton transfer dynamics to enhance CO₂ RR for the target product and discusses modulation strategies for proton transfer dynamics and operative mechanisms in typical systems, including single-atom catalysts, molecular catalysts, metal heterointerfaces, and organic-ligand modified metal catalysts. Characterization methods for proton transfer dynamics during CO₂ RR are also discussed, providing powerful tools for the hydrogen-involving electrochemical study. This Concept offers new insights into the CO₂ RR mechanism and guides the design of efficient CO₂ RR systems.

Transplanting Gold Active Sites into Non-Precious-Metal Nanoclusters for Efficient CO2-to-CO Electroreduction

Electrocatalytic CO₂ reduction reaction (CO₂RR) is greatly facilitated by Au surfaces. However, large fractions of underlying Au atoms are generally unused during the catalytic reaction, which limits mass activity. Herein, we report a strategy for preparing efficient electrocatalysts with high mass activities by the atomic-level transplantation of Au active sites into a Ni₄ nanocluster (NC). While the Ni₄ NC exclusively produces H₂, the Au-transplanted NC selectively produces CO over H₂. The origin of the contrasting selectivity observed for this NC is investigated by combining operando and theoretical studies, which reveal that while the Ni sites are almost completely blocked by the CO intermediate in both NCs, the Au sites act as active sites for CO₂-to-CO electroreduction. The Au-transplanted NC exhibits a remarkable turnover frequency and mass activity for CO production (206 mol_CO/mol_NC/s and 25,228 A/g_Au, respectively, at an overpotential of 0.32 V) and high durability toward the CO₂RR over 25 h.

Defect chemistry of electrocatalysts for CO2 reduction

Electrocatalytic CO2 reduction is a promising strategy for converting the greenhouse gas CO2 into high value-added products and achieving carbon neutrality. The rational design of electrocatalysts for CO2 reduction is of great significance. Defect chemistry is an important category for enhancing the intrinsic catalytic performance of electrocatalysts. Defect engineering breaks the catalytic inertia inherent in perfect structures by imparting unique electronic structures and physicochemical properties to electrocatalysts, thereby improving catalytic activity. Recently, various defective nanomaterials have been studied and show great potential in electrocatalytic CO2 reduction. There is an urgent need to gain insight into the effect of defects on catalytic performance. Here, we summarized the recent research advances on the design of various types of defects, including carbon-based materials (intrinsic defects, heteroatom doping and single-metal-atom sites) and metal compounds (vacancies, grain boundaries, and lattice defects). The major challenges and prospects of defect chemistry in electrocatalytic CO2 reduction are also proposed. This review is expected to be instructive in the development of defect engineering for CO2 reduction catalysts.