Atomically Dispersed Fe-Co Bimetallic Catalysts for the Promoted Electroreduction of Carbon Dioxide

Current Status and Perspectives of Dual-Atom Catalysts Towards Sustainable Energy Utilization

The exploration of sustainable energy utilization requires the implementation of advanced electrochemical devices for efficient energy conversion and storage, which are enabled by the usage of cost-effective, high-performance electrocatalysts. Currently, heterogeneous atomically dispersed catalysts are considered as potential candidates for a wide range of applications. Compared to conventional catalysts, atomically dispersed metal atoms in carbon-based catalysts have more unsaturated coordination sites, quantum size effect, and strong metal-support interactions, resulting in exceptional catalytic activity. Of these, dual-atomic catalysts (DACs) have attracted extensive attention due to the additional synergistic effect between two adjacent metal atoms. DACs have the advantages of full active site exposure, high selectivity, theoretical 100% atom utilization, and the ability to break the scaling relationship of adsorption free energy on active sites. In this review, we summarize recent research advancement of DACs, which includes (1) the comprehensive understanding of the synergy between atomic pairs; (2) the synthesis of DACs; (3) characterization methods, especially aberration-corrected scanning transmission electron microscopy and synchrotron spectroscopy; and (4) electrochemical energy-related applications. The last part focuses on great potential for the electrochemical catalysis of energy-related small molecules, such as oxygen reduction reaction, CO2 reduction reaction, hydrogen evolution reaction, and N2 reduction reaction. The future research challenges and opportunities are also raised in prospective section.

Elucidating the origin of catalytic activity of nitrogen-doped carbon coated nickel toward electrochemical reduction of CO2

Converting CO2 into valuable chemicals and fuels through clean and renewable energy electricity provides a way to achieve sustainable development for human societies. In this study, carbon coated nickel catalysts (Ni@NCT) were prepared by solvothermal and high-temperature pyrolysis methods. A series of Ni@NC-X catalysts were obtained by pickling with different kinds of acids for electrochemical CO2 reduction reaction (ECRR). The results show that Ni@NC-N treated with nitric acid has the highest selectivity but lower activity, Ni@NC-S treated with sulfuric acid has the lowest selectivity, and Ni@NC-Cl treated with hydrochloric acid shows the best activity and good selectivity. At -1.16 V, Ni@NC-Cl has a considerable CO yield of 472.9 μmol h-1 cm-2, which is significantly superior to Ni@NC-N (327.5), Ni@NC-S (295.6) and Ni@NC (270.8). The controlled experiments show that there is a synergistic effect between Ni and N. The chlorine adsorbed on the surface can promote the performance of ECRR. The poisoning experiments indicate that the contribution of surface Ni atoms to the ECRR is very small, and the increase of activity is mainly due to the nitrogen doped carbon coated Ni particles. The relationship between activity and selectivity of ECRR on different acid-washed catalysts was correlated by theoretical calculations for the first time, which is also in good agreement with the experimental results.

Promoting CO2 Electroreduction to Multi-Carbon Products by Hydrophobicity-Induced Electro-Kinetic Retardation

Advancing the performance of the Cu-catalyzed electrochemical CO2 reduction reaction (CO2 RR) is crucial for its practical applications. Still, the wettable pristine Cu surface often suffers from low exposure to CO2 , reducing the Faradaic efficiencies (FEs) and current densities for multi-carbon (C2+ ) products. Recent studies have proposed that increasing surface availability for CO2 by cation-exchange ionomers can enhance the C2+ product formation rates. However, due to the rapid formation and consumption of *CO, such promotion in reaction kinetics can shorten the residence of *CO whose adsorption determines C2+ selectivity, and thus the resulting C2+ FEs remain low. Herein, we discover that the electro-kinetic retardation caused by the strong hydrophobicity of quaternary ammonium group-functionalized polynorbornene ionomers can greatly prolong the *CO residence on Cu. This unconventional electro-kinetic effect is demonstrated by the increased Tafel slopes and the decreased sensitivity of *CO coverage change to potentials. As a result, the strongly hydrophobic Cu electrodes exhibit C2+ Faradaic efficiencies of ≈90 % at a partial current density of 223 mA cm-2 , more than twice of bare or hydrophilic Cu surfaces.

Modulating Metal-Nitrogen Coupling in Anti-Perovskite Nitride via Cation Doping for Efficient Reduction of Nitrate to Ammonia

The complexes of metal center and nitrogen ligands are the most representative systems for catalyzing hydrogenation reactions in small molecule conversion. Developing heterogeneous catalysts with similar active metal-nitrogen functional centers, nevertheless, still remains challenging. In this work, we demonstrate that the metal-nitrogen coupling in anti-perovskite Co4 N can be effective modulated by Cu doping to form Co3 CuN, leading to strongly promoted hydrogenation process during electrochemical reduction of nitrate (NO3 - RR) to ammonia. The combination of advanced spectroscopic techniques and density functional theory calculations reveal that Cu dopants strengthen the Co-N bond and upshifted the metal d-band towards the Fermi level, promoting the adsorption of NO3 - and *H and facilitating the transition from *NO2 /*NO to *NO2 H/*NOH. Consequently, the Co3 CuN delivers noticeably better NO3 - RR activity than the pristine Co4 N, with optimal Faradaic efficiency of 97 % and ammonia yield of 455.3 mmol h-1  cm-2 at -0.3 V vs. RHE. This work provides an effective strategy for developing high-performance heterogeneous catalyst for electrochemical synthesis.

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.

Bifunctional Single Atom Catalysts for Rechargeable Zinc-Air Batteries: From Dynamic Mechanism to Rational Design

Ever-growing demands for rechargeable zinc-air batteries (ZABs) call for efficient bifunctional electrocatalysts. Among various electrocatalysts, single atom catalysts (SACs) have received increasing attention due to the merits of high atom utilization, structural tunability, and remarkable activity. Rational design of bifunctional SACs relies heavily on an in-depth understanding of reaction mechanisms, especially dynamic evolution under electrochemical conditions. This requires a systematic study in dynamic mechanisms to replace current trial and error modes. Herein, fundamental understanding of dynamic oxygen reduction reaction and oxygen evolution reaction mechanisms for SACs is first presented combining in situ and/or operando characterizations and theoretical calculations. By highlighting structure-performance relationships, rational regulation strategies are particularly proposed to facilitate the design of efficient bifunctional SACs. Furthermore, future perspectives and challenges are discussed. This review provides a thorough understanding of dynamic mechanisms and regulation strategies for bifunctional SACs, which are expected to pave the avenue for exploring optimum single atom bifunctional oxygen catalysts and effective ZABs.

Metal-Organic Framework Derived Cu-Ag Interface for Selective Carbon Monoxide Electroreduction to Acetate

Electrochemical carbon monoxide reduction reaction (CORR) is a potential way to obtain high-value multi-carbon (C2+ ) products. However, achieving high selectivity to acetate is still a challenge. Herein, we develop a two-dimensional Ag-modified Cu metal-organic framework (Ag0.10 @CuMOF-74) that demonstrates Faradaic efficiency (FE) for C2+ products up to 90.4 % at 200 mA cm-2 and an acetate FE of 61.1 % with a partial current density of 122.2 mA cm-2 . Detailed investigations show that the introduction of Ag on CuMOF-74 favors the generation of abundant Cu-Ag interface sites. In situ attenuated total reflection surface enhanced infrared absorption spectroscopy confirms that these Cu-Ag interface sites improve the coverage of *CO and *CHO and the coupling between each other and stabilize key intermediates *OCCHO and *OCCH2 , thus significantly promoting to the acetate selectivity on Ag0.10 @CuMOF-74. This work provides a high-efficiency pathway for CORR to C2+ products.

Visible-Light Photocatalytic CO2-to-CO and H2O-to-H2O2 by g-C3N4/Cu2O-Pd S-Scheme Heterojunctions

Visible-light photocatalytic conversion of CO₂-to-fuels for green electricity is sustainably attractive for alleviating carbon emissions. Photocatalytic CO₂-to-CO frequently suffered from relatively low yields, mainly due to ineffective charge transfer rates. A new approach for photocatalytic CO₂-to-CO enhanced with effective H⁺ from H₂O-to-H₂O₂ through the water oxidation reaction (WOR) has been studied in the present work. Here, the nano palladium (9 wt %), serving as a cocatalyst, dispersed on the g-C₃N₄/Cu₂O heterojunctions (i.e., g-C₃N₄/Cu₂O-Pd) has been prepared to facilitate charge separation for the two-electron reduction of CO₂ to CO. Experimentally, the g-C₃N₄/Cu₂O-Pd heterojunctions have a higher photocatalytic H₂O-to-H₂O₂ yield than the g-C₃N₄/Cu₂O heterojunction by 5.3 times. The photocatalytic WOR provides sufficient electrons (e^-) and H⁺ (2H₂O → H₂O₂ + 2H⁺) for CO₂-to-CO (CO_2(aq) + 2H⁺ + 2e^- → CO_(g) + H₂O_(l)). Relatively high photocatalytic yields of H₂O₂ (34.0 μmol/mg) and CO (14.6 μmol/mg) affected by the heterojunctions can be achieved. Also, the heterojunctions have a high photostability with a photocatalytic generated CO/H₂ ratio of 1.75 approximately. This visible-light photocatalytic CO₂-to-CO and H₂O-to-H₂O₂ by the new g-C₃N₄/Cu₂O-Pd S-scheme heterojunctions demonstrates the feasibility of the zero carbon emission approach with additional green oxidant (H₂O₂) generation.

Role of the Support Effects in Single-Atom Catalysts

In recent years, single-atom catalysts (SACs) have received a significant amount of attention due to their high atomic utilization, low cost, high reaction activity, and selectivity for multiple catalytic reactions. Unfortunately, the high surface free energy of single atoms leads them easily migrated and aggregated. Therefore, support materials play an important role in the preparation and catalytic performance of SACs. Aiming at understanding the relationship between support materials and the catalytic performance of SACs, the support effects in SACs are introduced and reviewed herein. Moreover, special emphasis is placed on exploring the influence of the type and structure of supports on SAC catalytic performance through advanced characterization and theoretical research. Future research directions for support materials are also proposed, providing some insight into the design of SACs with high efficiency and high loading.

Atomic-Level Interface Engineering for Boosting Oxygen Electrocatalysis Performance of Single-Atom Catalysts: From Metal Active Center to the First Coordination Sphere

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the core reactions of a series of advanced modern energy and conversion technologies, such as fuel cells and metal-air cells. Among all kinds of oxygen electrocatalysts that have been reported, single-atom catalysts (SACs) offer great development potential because of their nearly 100% atomic utilization, unsaturated coordination environment, and tunable electronic structure. In recent years, numerous SACs with enriched active centers and asymmetric coordination have been successfully constructed by regulating their coordination environment and electronic structure, which has brought the development of atomic catalysts to a new level. This paper reviews the improvement of SACs brought by atom-level interface engineering. It starts with the introduction of advanced techniques for the characterizations of SACs. Subsequently, different design strategies that are applied to adjust the metal active center and first coordination sphere of SACs and then enhance their oxygen electrocatalysis performance are systematically illustrated. Finally, the future development of SACs toward ORR and OER is discussed and prospected.

CoN5 Sites Constructed by Anchoring Co Porphyrins on Vinylene-Linked Covalent Organic Frameworks for Electroreduction of Carbon Dioxide

Developing effective electrocatalysts for CO₂ reduction (CO₂ RR) is of critical importance for producing carbon-neutral fuels. Covalent organic frameworks (COFs) are an ideal platform for constructing catalysts toward CO₂ RR, because of their controllable skeletons and ordered pores. However, most of these COFs are synthesized from Co-porphyrins or phthalocyanines-based monomers, and the available building units and resulting catalytic centers in COFs are still limited. Herein, Co-N₅ sites are first developed through anchoring Co porphyrins on an olefin-linked COF, where the Co active sites are uniformly distributed in the hexagonal networks. The strong electronic coupling between Co porphyrins and COF is disclosed by various characterizations such as X-ray absorption spectroscopy (XAS) and density functional theory calculation (DFT). Thanks to the CoN₅ sites, the catalytic COF shows remarkable catalytic activity with Faraday efficiencies (FE_CO ) of 84.2-94.3% at applied potentials between -0.50 and -0.80 V (vs RHE), and achieves a turnover frequency of 4578 h^-1 at -1.0 V. Moreover, the theoretical calculation further reveals that the CoN₅ sites enable a decrease in the overpotential for the formation COOH*. This work provides a design strategy to employ COFs as scaffold for fabricating efficient CO₂ electrocatalysts.

Biomimetic FeCo@PDA nanozyme platform with Fenton catalytic activity as efficient antibacterial agent

The multidrug resistance of bacteria caused by the abuse of traditional antibiotics poses a great threat to public health security, so it is urgent to develop effective antibacterial agents to...