Heterogeneous Catalysts with Well‐Defined Active Metal Sites toward CO2 Electrocatalytic Reduction

The global atmospheric CO₂ concentration reached 147% of pre‐industrial levels in 2019, and is still increasing with an accelerated rate. A series of methods have been developed to convert CO₂ to other non‐greenhouse molecules. Elelctrocatalytic CO₂ reduction reaction (CO2RR) is one of the promising methods, since it could support renewable energy. Optimizing the CO2RR system requires finding highly efficient catalysts, as well as electrolysis systems. In this essay, the development of promising heterogeneous catalysts with well‐defined active metal sites is discussed. These catalysts could be prepared by immobilizing metal cations onto chemically well‐defined substrates, such as metal‐organic frameworks, covalent‐organic frameworks, polyoxometalates, or immobilizing well‐defined molecular catalysts on conducting substrates. A clear perspective on the catalyst's structures contributes to the understanding of structure‐reactivity correlations, which could, in turn, shed light on designing better catalysts. Some methods to assist the electrocatalysis process, such as coupling with solar or heat energy, are also briefly discussed.     

Local Steric Hindrance for CO2 Electroreduction at a Thermodynamic Potential and Wide Working Window

To improve the energy-conversion efficiency and adaptability between a CO₂ electroreduction system and intermittent renewable energy, small onset potentials, and wide working windows are highly important. Here, three indium metal–organic frameworks (In-MOFs) have been projected using different ligands to adjust the local steric hindrance and electronic structure of In nodes, manipulating the whole workflow of CO₂ during electroreduction including local CO₂ transport, adsorption, activation, hydrogenation, and product desorption. Significantly, a CO₂ electroreduction to formate process promoted by 2,5-TDC In-MOF shows an onset potential of −0.1 V versus RHE around the therymodynamic potential, over 90% FE_foramte in a wide current-density window from 0.1 to 0.9 A cm⁻². Driven by solar cells, the system displays a high solar-to-chemical efficiency of 17.39%. In depth mechanism study indicates that the local CO₂ transport and adsorption of all In-MOFs are thermodynamically and kinetically favorable, while the energy barrier of potential-determine step (*HCOOH desorption) is the lowest for 2,5-TDC In-MOF.     

Confined Synthesis of 2D Nanostructured Materials toward Electrocatalysis

2D nanostructured materials have shown great application prospects in energy conversion, owing to their unique structural features and fascinating physicochemical properties. Developing efficient approaches for the synthesis of well‐defined 2D nanostructured materials with controllable composition and morphology is critical. The emerging concept, confined synthesis, has been regarded as a promising strategy to design and synthesize novel 2D nanostructured materials. This review mainly summarizes the recent advances in confined synthesis of 2D nanostructured materials by using layered materials as host matrices (also denoted as “nanoreactors”). By virtue of the space‐ and surface‐confinement effects of these layered hosts, various well‐organized 2D nanostructured materials, including 2D metals, 2D metal compounds, 2D carbon materials, 2D polymers, 2D metal‐organic frameworks (MOFs) and covalent‐organic frameworks (COFs), as well as 2D carbon nitrides are successfully synthesized. The wide employment of these 2D materials in electrocatalytic applications (e.g., electrochemical oxygen/hydrogen evolution reactions, small molecule oxidation, and oxygen reduction reaction) is presented and discussed. In the final section, challenges and prospects in 2D confined synthesis from the viewpoint of designing new materials and exploring practical applications are commented, which would push this fast‐evolving field a step further toward greater success in both fundamental studies and ultimate industrialization.     

Multi-Shell Copper Catalysts for Selective Electroreduction of CO2 to Multicarbon Chemicals

Electrocatalytic CO₂ reduction (CO₂R) coupled with renewable electricity has been considered as a promising route for the sustainability transition of energy and chemical industries. However, the unsatisfactory yield of desired products, particularly multicarbon (C₂₊) products, has hindered the implementation of this technology. This work describes a strategy to enhance the yield of C₂₊ product formation in CO₂R by utilizing spatial confinement effects. The finite element simulation results suggest that increasing the number of shells in the catalyst wil lead to a high local concentration of *CO and promotes the formation of C₂₊ products. Inspired by this, Cu nanoparticles are synthesized with desired hollow multi-shell structures. The CO₂ reduction results confirm that as the number of shells increase, the hollow multi-shell copper catalysts exhibit improved selectivity toward C₂₊ products. Specifically, the Cu catalyst with 4.4-shell achieved a high selectivity of over 80% toward C₂₊ at a current density of 900 mA cm⁻². Evidence from in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy unveils that the multi-shell Cu catalyst exhibits an enhanced *CO_atop coverage and the stronger interaction with *CO_atop compared to commercial Cu, confirming the simulation results. Overall, the work promises an effective approach for boosting CO₂R selectivity toward value-added chemicals.     

CeNCl-CeO2 Heterojunction-Modified Ni Catalysts for Efficient Electroreduction of CO2 to CO

Renewable-electricity-powered electrochemical CO₂ reduction (CO₂RR) is considered one of the most promising ways to convert exhaust CO₂ into value-added chemicals and fuels. Among various CO₂RR products, CO is of great significance since it can be directly used as feedstock to produce chemical products through the Fischer–Tropsch process. However, the CO₂-to-CO electrocatalytic process is often accompanied by a kinetically competing side reaction: H₂ evolution reaction (HER). Designing electrocatalysts with tunable electronic structures is an attractive strategy to enhance CO selectivity. In this work, a CeNCl-CeO₂ heterojunction-modified Ni catalyst is successfully synthesized with high CO₂RR catalytic performance by the impregnation-calcination method. Benefiting from the strong electron interaction between the CeNCl-CeO₂ heterojunction and Ni nanoparticles (NPs), the catalytic performance is greatly improved. Maximal CO Faradaic efficiency (FE) is up to 90% at −0.8 V (vs RHE), plus good stability close to 12 h. Detailed electrochemical tests and density functional theory (DFT) calculation results reveal that the introduction of the CeNCl-CeO₂ heterojunction tunes the electronic structure of Ni NPs. The positively charged Ni center leads to an enhanced local electronic structure, thus promoting the activation of CO₂ and the adsorption of ^*COOH.     

Selective Etching of Nitrogen‐Doped Carbon by Steam for Enhanced Electrochemical CO2 Reduction

Nitrogen‐doped carbon structures have recently been demonstrated as a promising candidate for electrocatalytic CO₂ reduction, while in the meantime the pyridinic and graphitic nitrogen atoms also present high activities for electroreduction of water. Here, an etching strategy that uses hot water steam to preferentially bind to pyridinic and graphitic nitrogen atoms and subsequently etch them in carbon frameworks is reported. As a result, pyrrolic nitrogen atoms with low water affinity are retained after the steam etching, with a much increased level of among all nitrogen species from 22.1 to 55.9%. The steam‐etched nitrogen‐doped carbon catalyst enables excellent electrocatalytic CO₂ reduction performance but low hydrogen evolution reaction activity, suggesting a new approach for tuning electrocatalyst activity.     

Selective Electroreduction of CO2 toward Ethylene on Nano Dendritic Copper Catalysts at High Current Density

In situ deposited copper nanodendrites are herein proven to be a highly selective electrocatalyst which is capable of reducing CO₂ to ethylene by reaching a Faradaic efficiency of 57% at a current density of 170 mA cm^?2. It is found that the desired structures are formed in situ under acidic pH conditions at high electrode potentials more negative than ?2 V versus Ag/AgCl. Detailed investigations on the preparation, characterization, and advancement of electrode materials and of the electrolyte have been performed. Catalyst degradation effects are intensively followed by scanning electron microscopy (SEM) and high‐resolution transmission electron microscopy (HR‐TEM) characterization methods and found to be a major root course for selectivity losses.     

Intermediates Adsorption Engineering of CO2 Electroreduction Reaction in Highly Selective Heterostructure Cu‐Based Electrocatalysts for CO Production

The challenge in the artificial CO₂ reduction to fuel is achieving high selective electrocatalysts. Here, a highly selective Cu₂O/CuO heterostructure electrocatalyst is developed for CO₂ electroreduction. The Cu₂O/CuO nanowires modified by Ni nanoparticles exhibit superior catalytic performance with high faradic efficiency (95% for CO). Theoretical and experimental analyses show that the hybridization of Cu₂O/CuO nanowires and Ni nanoparticles can not only adjust the d‐band center of electrocatalysts to enhance the intrinsic catalytic activity but also improve the adsorption of COOH* intermediates and suppress the hydrogen evolution reaction to promote the CO conversion efficiency during CO₂ reduction reaction. An in situ Raman spectroscopic study further confirms the existence of COOH* species and the engineering intermediates adsorption. This work offers new insights for facile designing of nonprecious transition metal compound heterostructure for CO₂ reduction reaction through adjusting the reaction pathway.     

Electronic Structure Engineering of LiCoO2 toward Enhanced Oxygen Electrocatalysis

Developing low‐cost and efficient electrocatalysts for the oxygen evolution reaction and oxygen reduction reaction is of critical significance to the practical application of some emerging energy storage and conversion devices (e.g., metal–air batteries, water electrolyzers, and fuel cells). Lithium cobalt oxide is a promising nonprecious metal‐based electrocatalyst for oxygen electrocatalysis; its activity, however, is still far from the requirements of practical applications. Here, a new LiCoO₂‐based electrocatalyst with nanosheet morphology is developed by a combination of Mg doping and shear force‐assisted exfoliation strategies toward enhanced oxygen reduction and evolution reaction kinetics. It is demonstrated that the coupling effect of Mg doping and the exfoliation can effectively modulate the electronic structure of LiCoO₂, in which Co³⁺ can be partially oxidized to Co⁴⁺ and the Co–O covalency can be enhanced, which is closely associated with the improvement of intrinsic activity. Meanwhile, the unique nanosheet morphology also helps to expose more active Co species. This work offers new insights into deploying the electronic structure engineering strategy for the development of efficient and durable catalysts for energy applications.     

Spontaneously Self‐Assembly of a 2D/3D Heterostructure Enhances the Efficiency and Stability in Printed Perovskite Solar Cells

As perovskite solar cells (PSCs) are highly efficient, demonstration of high‐performance printed devices becomes important. 2D/3D heterostructures have recently emerged as an attractive way to relieving the film inhomogeneity and instability in perovskite devices. In this work, a 2D/3D ensemble with 2D perovskites self‐assembled atop 3D methylammonium lead triiodide (MAPbI₃) via a one‐step printing process is shown. A clean and flat interface is observed in the 2D/3D bilayer heterostructure for the first time. The 2D perovskite capping layer significantly suppresses nonradiative charge recombination, resulting in a marked increase in open‐circuit voltage (V_OC) of the devices by up to 100 mV. An ultrahigh V_OC of 1.20 V is achieved for MAPbI₃ PSCs, corresponding to 91% of the Shockley–Queisser limit. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of the 2D perovskites. These results suggest a viable approach for scalable fabrication of highly efficient perovskite solar cells with enhanced environmental stability.     

Surface Functionalization and Defect Construction of SnO2 with Amine Group for Enhanced Visible-Light-Driven Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction to hydrocarbon fuels through solar energy provides a feasible channel for reducing CO₂ emission and resource depletion. Nevertheless, severe charge recombination and high energy barrier limit the CO₂ reduction efficiency. Herein, a surface amine-functionalized SnO₂ with oxygen vacancies (A-Vo-SnO₂) is fabricated to achieve visible-light-driven photocatalytic CO₂ reduction. Specifically, amino groups modified onto the surface of the catalyst can provide more active sites to promote the adsorption of CO₂. Meanwhile, the synchronously induced oxygen defect level reduces the band-gap energy and expands the light-absorption region from UV light to visible light. The oxygen vacancies can modulate the electronic structure and work as the separation centers of spatial charges, thus promoting the interfacial charge transfer efficiency and providing more catalytic sites, as evidenced by experimental observation and theoretical calculation. As expected, this A-Vo-SnO₂ exhibits a CH₄ evolution rate of 17.27 µmol g⁻¹ h⁻¹ without adding sacrificial agent and co-catalyst, much higher than 5.98 µmol g⁻¹ h⁻¹ of pure SnO₂. This work can provide significant inspiration for the design of defect engineering based on visible-light-driven photocatalysts towards photocatalytic CO₂ conversion.     

Highly Efficient and Durable Pd Hydride Nanocubes Embedded in 2D Amorphous NiB Nanosheets for Oxygen Reduction Reaction

A highly efficient and durable electrocatalyst of Pd hydride nanocubes encapsulated within 2D amorphous Ni‐B nanosheets is reported. The PdH_0.43 nanocubes are first synthesized via a simple N ,N ‐dimethylformamide thermal treatment. The as‐synthesized PdH_0.43 nanocubes are then encapsulated in 2D amorphous NiB nanosheets by NaBH₄ reduction in the presence of nickel species. During the NaBH₄ treatment, the PdH_0.43 can be further transformed into PdH_0.706 due to the presence of endogenous H₂. Electrochemical studies demonstrate that the degree of hydride of Pd nanocubes (PdH_x ) plays an important role in the enhancement of their oxygen reduction reaction (ORR) activity. With increasing x value, both the activity and stability increase significantly. At 0.90 V versus reversible hydrogen electrode, the ORR activity of PdH_0.706 @Ni‐B reaches 1.05 A mg_Pd^?1, which is nearly five times higher than that of the state‐of‐the‐art Pt catalysts. Accelerated durability tests show that even after 10 000 potential cycles, there is negligible shift in their half‐wave potential and no shape and structure change occurs, indicating the incorporation of amorphous 2D Ni‐B nanosheets can greatly improve their stability without compromising their activity. The present study illustrates the importance of high degree of hydride and presence of amorphous Ni‐B nanosheets on the enhancement of ORR activity for Pd‐based electrocatalyst.     

Dramatically Enhanced Ambient Ammonia Electrosynthesis Performance by In‐Operando Created Li–S Interactions on MoS2 Electrocatalyst

The Haber‐Bosch process can be replaced by the ambient electrocatalytic N₂ reduction reaction (NRR) to produce NH₃ if suitable electrocatalysts can be developed. However, to develop high performance N₂ fixation electrocatalysts, a key issue to be resolved is to achieve efficient hydrogenation of N₂ without interference by the thermodynamically favored hydrogen evolution reaction (HER). Herein, in‐operando created strong Li–S interactions are reported to empower the S‐rich MoS₂ nanosheets with superior NRR catalytic activity and HER suppression ability. The Li⁺ interactions with S‐edge sites of MoS₂ can effectively suppress hydrogen evolution reaction by reducing H* adsorption free energy from 0.03 to 0.47 eV, facilitate N₂ adsorption by increasing N₂ adsorption free energy from –0.32 to –0.70 eV and enhance electrocatalytic N₂ reduction activity by decreasing the activation energy barrier of the reaction control step (*N₂ → *N₂H) from 0.84 to 0.42 eV. A NH₃ yield rate of 43.4 μg h^?1 mg^?1 MoS₂ with a faradaic efficiency (FE) of 9.81% can be achieved in presence of strong Li–S interactions, more than 8 and 18 times by the same electrocatalyst in the absence of Li–S interactions. This report opens a new way to design and develop catalysts and catalysis systems.     

Simultaneous Achieving of High Faradaic Efficiency and CO Partial Current Density for CO2 Reduction via Robust,Noble‐Metal‐Free Zn Nanosheets with Favorable Adsorption Energy

Electrocatalytic CO₂ reduction to fuels is considered a promising strategy for the sustainable carbon cycle. However, the improvement of the catalytic performance of CO₂ electrocatalysts still poses many challenges, especially achieving the large partial current density of product and high faradaic efficiency simultaneously, which are essential for future applications of the electrochemical CO₂ reduction reaction. In response, herein, an in situ porous Zn catalyst is prepared and exhibits high faradaic efficiency and large CO partial current density at the same time, benefiting from the porous architecture with increased exposure and accessibility of active sites. Furthermore, density functional theory calculations demonstrate that the high faradaic efficiency is attributed to the favorable adsorption energy of the key intermediate, which promotes CO₂ electroreduction to CO.     

Continuous 3D Titanium Nitride Nanoshell Structure for Solar‐Driven Unbiased Biocatalytic CO2 Reduction

Z‐scheme‐inspired tandem photoelectrochemical (PEC) cells have received attention as a sustainable platform for solar‐driven CO₂ reduction. Here, continuously 3D‐structured, electrically conductive titanium nitride nanoshells (3D TiN) for biocatalytic CO₂‐to‐formate conversion in a bias‐free tandem PEC system are reported. The 3D TiN exhibits a periodically porous network with high porosity (92.1%) and conductivity (6.72 × 10⁴ S m^?1), which allows for high enzyme loading and direct electron transfer (DET) to the immobilized enzyme. It is found that the W‐containing formate dehydrogenase from Clostridium ljungdahlii (ClFDH) on the 3D TiN nanoshell is electrically activated through DET for CO₂ reduction. At a low overpotential of 40 mV, the 3D TiN‐ClFDH stably converts CO₂ to formate at a rate of 0.34 µmol h^?1 cm^?2 and a faradaic efficiency (FE) of 93.5%. Compared to a flat TiN‐ClFDH, the 3D TiN‐ClFDH shows a 58 times higher formate production rate (1.74 µmol h^?1 cm^?2) at 240 mV of overpotential. Lastly, a bias‐free biocatalytic tandem PEC cell that converted CO₂ to formate at an average rate of 0.78 µmol h^?1 and an FE of 77.3% only using solar energy and water is successfully assembled.     

Directed Self‐Assembly of MOF‐Derived Nanoparticles toward Hierarchical Structures for Enhanced Catalytic Activity in CO Oxidation

The controllable self‐assembly of nanomaterials remains a great challenge in nanotechnological applications, especially for the hierarchical structure with high complexity. Herein, by taking the advantage of highly dispersed metal nodes and mild thermal stability of metal‐organic frameworks (MOFs), the self‐assembly of nanoparticles is directed from MOFs to construct CuO hierarchical structures, which have an inherited octahedral framework consisting of the microspheres, nanowires, and polyhedrons, respectively. Unlike the conventional self‐assembly in a solution media (such as solvent and molten solid), the assembly in this work is the first demonstration through a solution‐free approach. Moreover, compared to the general MOF‐derived CuO octahedron, the assembled hierarchical CuO structure exhibits much enhanced catalytic activity in CO oxidation thanks to the exposure of more active sites during the assembly.