Temperature-Induced Low-Coordinate Ni Single-Atom Catalyst for Boosted CO2 Electroreduction Activity

Single-atom catalysts (SACs) exhibit remarkable potential for electrochemical reduction of CO₂ to value-added products. However, the commonly pursued methods for preparing SACs are hard to scale up, and sometimes, lack general applicability because of expensive raw materials and complex synthetic procedures. In addition, the fine tuning of coordination environment of SACs remains challenging due to their structural vulnerability. Herein, a simple and universal strategy is developed to fabricate Ni SACs with different nitrogen coordination numbers through one-step pyrolysis of melamine, Ni(NO₃)∙6H₂O, and polyvinylpyrrolidone at different temperatures. Experimental measurements and theoretical calculations reveal that the low-coordinate Ni SACs exhibit outstanding CO₂ reduction performance and stability, achieving a Faradic efficiency (FE_CO) of 98.5% at −0.76 V with CO current density of 24.6 mA cm⁻², and maintaining FE_CO of over 91.0% at all applied potential windows from −0.56 to −1.16 V, benefiting from its coordinatively unsaturated structure to afford high catalytic activity and low barrier for the formation of *COOH intermediate. No significant performance degradation is observed over 50 h of continuous operation. Additionally, several other metallic single-atom catalysts are successfully prepared by this synthetic method, demonstrating the universality of this strategy.     

Revealing the Kinetic Balance between Proton-Feeding and Hydrogenation in CO2 Electroreduction

Electrocatalytic reduction of CO₂ to high-value-added chemicals provides a feasible path for global carbon balance. Herein, the fabrication of Ni_NPx@Ni_SAy-NG (x,y = 1, 2, 3; NG = nitrogen-doped graphite) is reported, in which Ni single atom sites (Ni_SA) and Ni nanoparticles (Ni_NP) coexist. These Ni_NPx@Ni_SAy-NG presented a volcano-like trend for maximum CO Faradaic efficiency (FE_CO) with the highest point at Ni_NP2@Ni_SA2-NG in CO₂RR. Ni_NP2@Ni_SA2-NG exhibited ≈98% of maximum FE_CO and a large current density of −264 mA cm⁻² at −0.98 V (vs. RHE) in the flow cell. In situ experiment and density functional theory (DFT) calculations confirmed that the proper content of Ni_SA and Ni_NP balanced kinetic between proton-feeding and CO₂ hydrogenation. The Ni_NP in Ni_NP2@Ni_SA2-NG promoted the formation of H* and reduced the energy barrier of *CO₂ hydrogenation to *COOH, and CO desorption can be efficiently facilitated by Ni_SA sites, thereby resulting in enhanced CO₂RR performance.     

Single Ni Atoms Anchored on Porous Few‐Layer g‐C3N4 for Photocatalytic CO2 Reduction: The Role of Edge Confinement

It is greatly intriguing yet remains challenging to construct single‐atomic photocatalysts with stable surface free energy, favorable for well‐defined atomic coordination and photocatalytic carrier mobility during the photoredox process. Herein, an unsaturated edge confinement strategy is defined by coordinating single‐atomic‐site Ni on the bottom‐up synthesized porous few‐layer g‐C₃N₄ (namely, Ni₅‐CN) via a self‐limiting method. This Ni₅‐CN system with a few isolated Ni clusters distributed on the edge of g‐C₃N₄ is beneficial to immobilize the nonedged single‐atomic‐site Ni species, thus achieving a high single‐atomic active site density. Remarkably, the Ni₅‐CN system exhibits comparably high photocatalytic activity for CO₂ reduction, giving the CO generation rate of 8.6 µmol g^?1 h^?1 under visible‐light illumination, which is 7.8 times that of pure porous few‐layer g‐C₃N₄ (namely, CN, 1.1 µmol g^?1 h^?1). X‐ray absorption spectrometric analysis unveils that the cationic coordination environment of single‐atomic‐site Ni center, which is formed by Ni‐N doping‐intercalation the first coordination shell, motivates the superiority in synergistic N–Ni–N connection and interfacial carrier transfer. The photocatalytic mechanistic prediction confirms that the introduced unsaturated Ni‐N coordination favorably binds with CO₂, and enhances the rate‐determining step of intermediates for CO generation.     

Ni Single Atoms Embedded in Graphene Nanoribbon Sieves for High-Performance CO2 Reduction to CO

Ni single-atom catalysts (SACs) are appealing for electrochemical reduction CO₂ reduction (CO₂RR). However, regulating the balance between the activity and conductivity remains a challenge to Ni SACs due to the limitation of substrates structure. Herein, the intrinsic performance enhancement of Ni SACs anchored on quasi-one-dimensional graphene nanoribbons (GNRs) synthesized is demonstrated by longitudinal unzipping carbon nanotubes (CNTs). The abundant functional groups on GNRs can absorb Ni atoms to form rich Ni–N₄–C sites during the anchoring process, providing a high intrinsic activity. In addition, the GNRs, which maintain a quasi-one-dimensional structure and possess a high conductivity, interconnect with each other and form a conductive porous framework. The catalyst yields a 44 mA cm⁻² CO partial current density and 96% faradaic efficiency of CO (FE_CO) at −1.1 V vs RHE in an H-cell. By adopting a membrane electrode assembly (MEA) flow cell, a 95% FE_CO and 2.4 V cell voltage are achieved at 200 mA cm⁻² current density. This work provides a rational way to synthesize Ni SACs with a high Ni atom loading, porous morphology, and high conductivity with potential industrial applications.     

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

Electrochemical CO₂ reduction reaction (CO₂RR), 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 CO₂RR and Zn-CO₂ battery.     

Promoting CO2 Dynamic Activation via Micro-Engineering Technology for Enhancing Electrochemical CO2 Reduction

Optimizing the coordination structure and microscopic reaction environment of isolated metal sites is promising for boosting catalytic activity for electrocatalytic CO₂ reduction reaction (CO₂RR) but is still challenging to achieve. Herein, a newly electrostatic induced self-assembly strategy for encapsulating isolated Ni-C₃N₁ moiety into hollow nano-reactor as I-Ni SA/NHCRs is developed, which achieves FE_CO of 94.91% at −0.80 V, the CO partial current density of ≈−15.35 mA cm⁻², superior to that with outer Ni-C₂N₂ moiety (94.47%, ≈−12.06 mA cm⁻²), or without hollow structure (92.30%, ≈−5.39 mA cm⁻²), and high FE_CO of ≈98.41% at 100 mA cm⁻² in flow cell. COMSOL multiphysics finite-element method and density functional theory (DFT) calculation illustrate that the excellent activity for I-Ni SA/NHCRs should be attributed to the structure-enhanced kinetics process caused by its hollow nano-reactor structure and unique Ni-C₃N₁ moiety, which can enrich electron on Ni sites and positively shift d-band center to the Fermi level to accelerate the adsorption and activation of CO₂ molecule and *COOH formation. Meanwhile, this strategy also successfully steers the design of encapsulating isolated iron and cobalt sites into nano-reactor, while I-Ni SA/NHCRs-based zinc-CO₂ battery assembled with a peak power density of 2.54 mW cm−⁻² is achieved.     

Fe Isolated Single Atoms on S,N Codoped Carbon by Copolymer Pyrolysis Strategy for Highly Efficient Oxygen Reduction Reaction

Heteroatom‐doped Fe‐NC catalyst has emerged as one of the most promising candidates to replace noble metal‐based catalysts for highly efficient oxygen reduction reaction (ORR). However, delicate controls over their structure parameters to optimize the catalytic efficiency and molecular‐level understandings of the catalytic mechanism are still challenging. Herein, a novel pyrrole–thiophene copolymer pyrolysis strategy to synthesize Fe‐isolated single atoms on sulfur and nitrogen‐codoped carbon (Fe‐ISA/SNC) with controllable S, N doping is rationally designed. The catalytic efficiency of Fe‐ISA/SNC shows a volcano‐type curve with the increase of sulfur doping. The optimized Fe‐ISA/SNC exhibits a half‐wave potential of 0.896 V (vs reversible hydrogen electrode (RHE)), which is more positive than those of Fe‐isolated single atoms on nitrogen codoped carbon (Fe‐ISA/NC, 0.839 V), commercial Pt/C (0.841 V), and most reported nonprecious metal catalysts. Fe‐ISA/SNC is methanol tolerable and shows negligible activity decay in alkaline condition during 15 000 voltage cycles. X‐ray absorption fine structure analysis and density functional theory calculations reveal that the incorporated sulfur engineers the charges on N atoms surrounding the Fe reactive center. The enriched charge facilitates the rate‐limiting reductive release of OH* and therefore improved the overall ORR efficiency.     

Nanostructure Engineering of Sn-Based Catalysts for Efficient Electrochemical CO2 Reduction

Excessive anthropogenic CO₂ emission has caused a series of ecological and environmental issues, which threatens mankind's sustainable development. Mimicking the natural photosynthesis process (i.e., artificial photosynthesis) by electrochemically converting CO₂ into value-added products is a promising way to alleviate CO₂ emission and relieve the dependence on fossil fuels. Recently, Sn-based catalysts have attracted increasing research attentions due to the merits of low price, abundance, non-toxicity, and environmental benignancy. In this review, the paradigm of nanostructure engineering for efficient electrochemical CO₂ reduction (ECO₂R) on Sn-based catalysts is systematically summarized. First, the nanostructure engineering of size, composition, atomic structure, morphology, defect, surficial modification, catalyst/substrate interface, and single-atom structure, are systematically discussed. The influence of nanostructure engineering on the electronic structure and adsorption property of intermediates, as well as the performance of Sn-based catalysts for ECO₂R are highlighted. Second, the potential chemical state changes and the role of surface hydroxides on Sn-based catalysts during ECO₂R are introduced. Third, the challenges and opportunities of Sn-based catalysts for ECO₂R are proposed. It is expected that this review inspires the further development of highly efficient Sn-based catalysts, meanwhile offer protocols for the investigation of Sn-based catalysts.     

Coexisting Single-Atomic Fe and Ni Sites on Hierarchically Ordered Porous Carbon as a Highly Efficient ORR Electrocatalyst

The development of oxygen reduction reaction (ORR) electrocatalysts based on earth-abundant nonprecious materials is critically important for sustainable large-scale applications of fuel cells and metal–air batteries. Herein, a hetero-single-atom (h-SA) ORR electrocatalyst is presented, which has atomically dispersed Fe and Ni coanchored to a microsized nitrogen-doped graphitic carbon support with unique trimodal-porous structure configured by highly ordered macropores interconnected through mesopores. Extended X-ray absorption fine structure spectra confirm that Fe- and Ni-SAs are affixed to the carbon support via Fe N₄ and Ni N₄ coordination bonds. The resultant Fe/Ni h-SA electrocatalyst exhibits an outstanding ORR activity, outperforming SA electrocatalysts with only Fe- or Ni-SAs, and the benchmark Pt/C. The obtained experimental results indicate that the achieved outstanding ORR performance results from the synergetic enhancement induced by the coexisting Fe N₄ and Ni N₄ sites, and the superior mass-transfer capability promoted by the trimodal-porous-structured carbon support.     

Highly Active,Durable Ultrathin MoTe2 Layers for the Electroreduction of CO2 to CH4

The electroreduction of CO₂ to CH₄ is a highly desirable, challenging research topic. In this study, an electrocatalytic system comprising ultrathin MoTe₂ layers and an ionic liquid electrolyte for the reduction of CO₂ to methane is reported, efficiently affording methane with a faradaic efficiency of 83 ± 3% (similar to the best Cu‐based catalysts reported thus far) and a durable activity of greater than 45 h at a relatively high current density of 25.6 mA cm^?2 (?1.0 V_RHE). The results obtained can facilitate research on the design of other transition‐metal dichalcogenide electrocatalysts for the reduction of CO₂ to valuable fuels.     

Highly Efficient Electroreduction of CO2 on Nickel Single‐Atom Catalysts: Atom Trapping and Nitrogen Anchoring

Construction of single atom catalysts (SACs) with high activity toward electroreduction of CO₂ still remains a great challenge. A very simple and truly cost‐effective synthetic strategy is proposed to prepare SACs via a impregnation–pyrolysis method, through one‐step pyrolysis of graphene oxide aerogel. Compared with other traditional methods, this process is fast and free of repeated acid etching, and thus it has great potential for facile operation and large‐scale manufacturing. Both X‐ray absorption fine structure and high‐angle annular dark‐field scanning transmission electron microscopy images confirm the presence of isolated nickel atoms, with a high Ni loading of ≈2.6 wt%. The obtained 3D porous Ni‐ and N‐codoped graphene aerogel exhibits excellent activity toward electroreduction of CO₂ to CO, in particular exhibiting a remarkable CO Faradaic efficiency of 90.2%. Density functional theory calculations reveal that free energies for the formation of intermediate *COOH on coordinatively unsaturated Ni? N sites are significantly lower than that on Ni? N₄ site, suggesting the outstanding activities of CO₂ electroreduction originate from coordinatively unsaturated Ni? N sites in catalysts.     

Crystal Phase and Architecture Engineering of Lotus‐Thalamus‐Shaped Pt‐Ni Anisotropic Superstructures for Highly Efficient Electrochemical Hydrogen Evolution

The rational design and synthesis of anisotropic 3D nanostructures with specific composition, morphology, surface structure, and crystal phase is of significant importance for their diverse applications. Here, the synthesis of well‐crystalline lotus‐thalamus‐shaped Pt‐Ni anisotropic superstructures (ASs) via a facile one‐pot solvothermal method is reported. The Pt‐Ni ASs with Pt‐rich surface are composed of one Ni‐rich “core” with face‐centered cubic (fcc) phase, Ni‐rich “arms” with hexagonal close‐packed phase protruding from the core, and facet‐selectively grown Pt‐rich “lotus seeds” with fcc phase on the end surfaces of the “arms.” Impressively, these unique Pt‐Ni ASs exhibit superior electrocatalytic activity and stability toward the hydrogen evolution reaction under alkaline conditions compared to commercial Pt/C and previously reported electrocatalysts. The obtained overpotential is as low as 27.7 mV at current density of 10 mA cm^?2, and the turnover frequency reaches 18.63 H₂ s^?1 at the overpotential of 50 mV. This work provides a new strategy for the synthesis of highly anisotropic superstructures with a spatial heterogeneity to boost their promising application in catalytic reactions.     

Regulation of Electrocatalytic Behavior by Axial Oxygen Enhances the Catalytic Activity of CoN4 Sites for CO2 Reduction

Recent studies have found that the existence of oxygen around the active sites may be essential for efficient electrochemical CO₂-to-CO conversion. Hence, this work proposes the modulation of oxygen coordination and investigates the as-induced catalytic behavior in CO₂RR. It designs and synthesizes conjugated phthalocyanine frameworks catalysts (CPF-Co) with abundant CoN₄ centers as an active source, and subsequently modifies the electronic structure of CPF-Co by introducing graphene oxide (GO) with oxygen-rich functional groups. A systematic study reveals that the axial coordination between oxygen and the catalytic sites could form an optimized O-CoN₄ structure to break the electron distribution symmetry of Co, thus reducing the energy barrier to the activation of CO₂ to COOH*. Meanwhile, by adjusting the content of oxygen, the proper supports can also facilitate the charge transfer efficiency between the matrix layer and the catalytic sites. The optimized CPF-Co@LGO exhibits a high TOF value (2.81 s⁻¹), CO selectivity (97.6%) as well as stability (24 h) at 21 mA cm⁻² current density. This work reveals the modulation of oxygen during CO₂RR and provides a novel strategy for the design of efficient electrocatalysts, which may inspire new exploration and principles for CO₂RR.     

Engineering Single Cu Sites into Covalent Organic Framework for Selective Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into value-added chemicals is a promising route but remains challenging due to poor product selectivity. Covalent organic frameworks (COFs) as an emerging class of porous materials are considered as promising candidates for photocatalysis. Incorporating metallic sites into COF is a successful strategy to realize high photocatalytic activities. Herein, 2,2′-bipyridine-based COF bearing non-noble single Cu sites is fabricated by chelating coordination of dipyridyl units for photocatalytic CO₂ reduction. The coordinated single Cu sites not only significantly enhance light harvesting and accelerate electron–hole separation but also provide adsorption and activation sites for CO₂ molecules. As a proof of concept, the Cu-Bpy-COF as a representative catalyst exhibits superior photocatalytic activity for reducing CO₂ to CO and CH₄ without photosensitizer, and impressively, the product selectivity of CO and CH₄ can be readily modulated only by changing reaction media. Experimental and theoretical results reveal the crucial role of single Cu sites in promoting photoinduced charge separation and solvent effect in regulating product selectivity, which provides an important sight onto the design of COF photocatalysts for selective CO₂ photoreduction.     

Edge Coordination of Ni Single Atoms on Hard Carbon Promotes the Potassium Storage and Reversibility

Hard carbon is the most promising anode for potassium-ion batteries (PIBs) due to its low cost and abundance, but its limited storage capacity remains a major challenge. Herein, edge coordination of metal single atoms is proved to be an effective strategy for promoting potassium storage in hard carbon for the first time, taking B, N co-doped hard carbon nanotubes anchored by edge Ni-N₄-B atomic sites (Ni@BNHC) as an example. It is revealed that edge Ni-N₄-B can provide active sites for interlayer adsorption of K⁺ and that Ni atoms can facilitate the reversibility of K⁺ storage on N and B atoms. Furthermore, an unprecedentedly reversible K⁺ storage capacity of 694 mAh g⁻¹ at 0.05 A g⁻¹ is realized by introducing commercial carbon nanotubes. This work provides a new perspective for the application of single-atom engineering and the design of high-performance carbon anodes for PIBs.     

Novelty All-Inorganic Titanium-Based Halide Perovskite for Highly Efficient Photocatalytic CO2 Conversion

Lead halide perovskite materials have great potential for photocatalytic reaction due to their low fabrication cost, unique optical absorption coefficient, and suitable band structures. However, the main problems are the toxicity and instability of the lead halide perovskite materials. Therefore, a facile synthetic method is used to prepare lead-free environmentally friendly Cs₂TiX₆(X = Cl, Cl_0.5Br_0.5, Br) perovskite materials. Their structural and optical characteristics are systematically investigated. The band gaps of the produced samples are illustrated to be from 1.87 to 2.73 eV. Moreover, these materials can keep high stability in harsh environments such as illumination and heating, and the Cs₂Ti(Cl_0.5Br_0.5)₆ microcrystals demonstrate the yields of 176 µmol g⁻¹ for CO and 78.9 µmol g⁻¹ for CH₄ after light irradiation for 3 h, which is of the first report of Ti-based perovskite photocatalysts. This finding demonstrates that the Ti-based perovskites will create opportunities for photocatalytic applications, which may offer a new idea to construct low-cost, eco-friendly, and bio-friendly photocatalysts.     

Boosting the Loading of Metal Single Atoms via a Bioconcentration Strategy

Increasing the mass loading of transition metal single atoms coordinated with nitrogen in carbon‐based materials (M‐N‐C) is still challenging. Herein, inspired by the bioconcentration effect in the living body, a biochemistry strategy for the synthesis of Fe‐N‐C single atoms is demonstrated. Through introducing ferrous glycinate into the growth of fungus, the Fe atoms are bioconcentrated in hyphae. The highly dispersed Fe‐N‐C single atoms in hyphae‐derived carbon fibers (labeled as Fe‐N‐C SA/HCF) are prepared by the pyrolysis of Fe‐riched hyphae. In the bioconcentration process, the uptake of Fe ions by hyphae promotes the secretion of glutathione and ferritin, which provides additional coordination sites for Fe ions. Accordingly, the mass content of Fe in bioconcentrated Fe‐N‐C SA/HCF reaches 4.8%, which is 5.3 times larger than that of the sample prepared by the conventional pyrolysis process. The present bioconcentration strategy is further extended to the preparation of Co, Ni, and Mn single atoms. Owing to the high content of Fe‐N‐C single atoms, Fe‐N‐C SA/HCF shows the onset potential (E_onset) of 0.931 V versus reversible hydrogen electrode (RHE) and half‐wave potential (E_1/2) of 0.802 V versus RHE in oxygen reduction reaction measurements, which is comparable to the commercial Pt/C catalysts.     

2D Metal Oxyhalide‐Derived Catalysts for Efficient CO2 Electroreduction

Electrochemical reduction of CO₂ is a compelling route to store renewable electricity in the form of carbon‐based fuels. Efficient electrochemical reduction of CO₂ requires catalysts that combine high activity, high selectivity, and low overpotential. Extensive surface reconstruction of metal catalysts under high productivity operating conditions (high current densities, reducing potentials, and variable pH) renders the realization of tailored catalysts that maximize the exposure of the most favorable facets, the number of active sites, and the oxidation state all the more challenging. Earth‐abundant transition metals such as tin, bismuth, and lead have been proven stable and product‐specific, but exhibit limited partial current densities. Here, a strategy that employs bismuth oxyhalides as a template from which 2D bismuth‐based catalysts are derived is reported. The BiOBr‐templated catalyst exhibits a preferential exposure of highly active Bi () facets. Thereby, the CO₂ reduction reaction selectivity is increased to over 90% Faradaic efficiency and simultaneously stable current densities of up to 200 mA cm^?2 are achieved—more than a twofold increase in the production of the energy‐storage liquid formic acid compared to previous best Bi catalysts.