Tunable and Efficient Tin Modified Nitrogen‐Doped Carbon Nanofibers for Electrochemical Reduction of Aqueous Carbon Dioxide

Efficient and selective earth‐abundant catalysts are highly desirable to drive the electrochemical conversion of CO₂ into value‐added chemicals. In this work, a low‐cost Sn modified N‐doped carbon nanofiber hybrid catalyst is developed for switchable CO₂ electroreduction in aqueous medium via a straightforward electrospinning technique coupled with a pyrolysis process. The electrocatalytic performance can be tuned by the structure of Sn species on the N‐doped carbon nanofibers. Sn nanoparticles drive efficient formate formation with a high current density of 11 mA cm^?2 and a faradaic efficiency of 62% at a moderate overpotential of 690 mV. Atomically dispersed Sn species promote conversion of CO₂ to CO with a high faradaic efficiency of 91% at a low overpotential of 490 mV. The interaction between Sn species and pyridinic‐N may play an important role in tuning the catalytic activity and selectivity of these two materials.     

Active MnOx Electrocatalysts Prepared by Atomic Layer Deposition for Oxygen Evolution and Oxygen Reduction Reactions

The ability to deposit conformal catalytic thin films enables opportunities to achieve complex nanostructured designs for catalysis. Atomic layer deposition (ALD) is capable of creating conformal thin films over complex substrates. Here, ALD‐MnO_x on glassy carbon is investigated as a catalyst for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), two reactions that are of growing interest due to their many applications in alternative energy technologies. The films are characterized by X‐ray photoelectron spectroscopy, X‐ray diffraction, scanning electron microscopy, ellipsometry, and cyclic voltammetry. The as‐deposited films consist of Mn(II)O, which is shown to be a poor catalyst for the ORR, but highly active for the OER. By controllably annealing the samples, Mn₂O₃ catalysts with good activity for both the ORR and OER are synthesized. Hypotheses are presented to explain the large difference in the activity between the MnO and Mn₂O₃ catalysts for the ORR, but similar activity for the OER, including the effects of surface oxidation under experimental conditions. These catalysts synthesized though ALD compare favorably to the best MnO_x catalysts in the literature, demonstrating a viable way to produce highly active, conformal thin films from earth‐abundant materials for the ORR and the OER.     

The Effects of Catalyst Layer Deposition Methodology on Electrode Performance

The catalyst layer of the cathode is arguably the most critical component of low‐temperature fuel cells and carbon dioxide (CO₂) electrolysis cells because their performance is typically limited by slow oxygen (O₂) and CO₂ reduction kinetics. While significant efforts have focused on developing cathode catalysts with improved activity and stability, fewer efforts have focused on engineering the catalyst layer structure to maximize catalyst utilization and overall electrode and system performance. Here, we study the performance of cathodes for O₂ reduction and CO₂ reduction as a function of three common catalyst layer preparation methods: hand‐painting, air‐brushing, and screen‐printing. We employed ex‐situ X‐ray micro‐computed tomography (MicroCT) to visualize the catalyst layer structure and established data processing procedures to quantify catalyst uniformity. By coupling structural analysis with in‐situ electrochemical characterization, we directly correlate variation in catalyst layer morphology to electrode performance. MicroCT and SEM analyses indicate that, as expected, more uniform catalyst distribution and less particle agglomeration, lead to better performance. Most importantly, the analyses reported here allow for the observed differences over a large geometric volume as a function of preparation methods to be quantified and explained for the first time. Depositing catalyst layers via a fully‐automated air‐brushing method led to a 56% improvement in fuel cell performance and a significant reduction in electrode‐to‐electrode variability. Furthermore, air‐brushing catalyst layers for CO₂ reduction led to a 3‐fold increase in partial CO current density and enhanced product selectivity (94% CO) at similar cathode potential but a 10‐fold decrease in catalyst loading as compared to previous reports.     

Activation of Ni Particles into Single Ni–N Atoms for Efficient Electrochemical Reduction of CO2

Electrochemical reduction of carbon dioxide (CO₂) to fuels and value‐added industrial chemicals is a promising strategy for keeping a healthy balance between energy supply and net carbon emissions. Here, the facile transformation of residual Ni particle catalysts in carbon nanotubes into thermally stable single Ni atoms with a possible NiN₃ moiety is reported, surrounded with a porous N‐doped carbon sheath through a one‐step nanoconfined pyrolysis strategy. These structural changes are confirmed by X‐ray absorption fine structure analysis and density functional theory (DFT) calculations. The dispersed Ni single atoms facilitate highly efficient electrocatalytic CO₂ reduction at low overpotentials to yield CO, providing a CO faradaic efficiency exceeding 90%, turnover frequency approaching 12 000 h^?1, and metal mass activity reaching about 10 600 mA mg^?1, outperforming current state‐of‐the‐art single atom catalysts for CO₂ reduction to CO. DFT calculations suggest that the Ni@N₃ (pyrrolic) site favors *COOH formation with lower free energy than Ni@N₄, in addition to exothermic CO desorption, hence enhancing electrocatalytic CO₂ conversion. This finding provides a simple, scalable, and promising route for the preparation of low‐cost, abundant, and highly active single atom catalysts, benefiting future practical CO₂ electrolysis.     

A Disquisition on the Active Sites of Heterogeneous Catalysts for Electrochemical Reduction of CO2 to Value‐Added Chemicals and Fuel

Renewable‐electricity‐powered electrocatalytic CO₂ reduction reactions (CO₂RR) have been identified as an emerging technology to address the issue of rising CO₂ emissions in the atmosphere. While the CO₂RR has been demonstrated to be technically feasible, further improvements in catalyst performance through active sites engineering are a prerequisite to accelerate its commercial feasibility for utilization in large CO₂‐emitting industrial sources. Over the years, the improved understanding of the interaction of CO₂ with the active sites has allowed superior catalyst design and subsequent attainment of prominent CO₂RR activity in literature. This review tracks the evolution of the understanding of CO₂RR active sites on different electrocatalysts such as metals, metal‐oxides, single atoms, metal‐carbon, and subsequently metal‐free carbon‐based catalysts. Despite the tremendous research efforts in the field, many scientific questions on the role of various active sites in governing CO₂RR activity, selectivity, stability, and pathways are still unanswered. These gaps in knowledge are highlighted and a discussion is set forth on the merits of utilizing advanced in‐situ and operando characterization techniques and machine learning (ML). Using this technique, the underlying mechanisms can be discerned, and as a result new strategies for designing active sites may be uncovered. Finally, this review advocates an interdisciplinary approach to discover and design CO₂RR active sites (rather than focusing merely on catalyst activity) in a bid to stimulate practical research for industrial application.     

Uncovering Atomic‐Scale Stability and Reactivity in Engineered Zinc Oxide Electrocatalysts for Controllable Syngas Production

In this study, scalable, flame spray synthesis is utilized to develop defective ZnO nanomaterials for the concurrent generation of H₂ and CO during electrochemical CO₂ reduction reactions (CO₂RR). The designed ZnO achieves an H₂/CO ratio of ≈1 with a large current density (j) of 40 mA cm^?2 during long‐term continuous reaction at a cell voltage of 2.6 V. Through in situ atomic pair distribution function analysis, the remarkable stability of these ZnO structures is explored, addressing the knowledge gap in understanding the dynamics of oxide catalysts during CO₂RR. Through optimization of synthesis conditions, ZnO facets are modulated which are shown to affect reaction selectivity, in agreement with theoretical calculations. These findings and insights on synthetic manipulation of active sites in defective metal‐oxides can be used as guidelines to develop active catalysts for syngas production for renewable power‐to‐X to generate a range of fuels and chemicals.     

Highly Efficient Solar‐Driven Carbon Dioxide Reduction on Molybdenum Disulfide Catalyst Using Choline Chloride‐Based Electrolyte

Conversion of CO₂ to energy‐rich chemicals using renewable energy is of much interest to close the anthropogenic carbon cycle. However, the current photoelectrochemical systems are still far from being practically feasible. Here the successful demonstration of a continuous, energy efficient, and scalable solar‐driven CO₂ reduction process based on earth‐abundant molybdenum disulfide (MoS₂) catalyst, which works in synergy with an inexpensive hybrid electrolyte of choline chloride (a common food additive for livestock) and potassium hydroxide (KOH) is reported. The CO₂ saturated hybrid electrolyte utilized in this study also acts as a buffer solution (pH ≈ 7.6) to adjust pH during the reactions. This study reveals that this system can efficiently convert CO₂ to CO with solar‐to‐fuel and catalytic conversion efficiencies of 23% and 83%, respectively. Using density functional theory calculations, a new reaction mechanism in which the water molecules near the MoS₂ cathode act as proton donors to facilitate the CO₂ reduction process by MoS₂ catalyst is proposed. This demonstration of a continuous, cost‐effective, and energy efficient solar driven CO₂ conversion process is a key step toward the industrialization of this technology.     

Nanoporous Au Thin Films on Si Photoelectrodes for Selective and Efficient Photoelectrochemical CO2 Reduction

An Si photoelectrode with a nanoporous Au thin film for highly selective and efficient photoelectrochemical (PEC) CO₂ reduction reaction (CO₂RR) is presented. The nanoporous Au thin film is formed by electrochemical reduction of an anodized Au thin film. The electrochemical treatments of the Au thin film critically improve CO₂ reduction catalytic activity of Au catalysts and exhibit CO Faradaic efficiency of 96% at 480 mV of overpotential. To apply the electrochemical pretreatment of Au films for PEC CO₂RR, a new Si photoelectrode design with mesh‐type co‐catalysts independently wired at the front and the back of the photoelectrode is demonstrated. Due to the superior CO₂RR activity of the nanoporous Au mesh and high photovoltage from Si, the Si photoelectrode with the nanoporous Au thin film mesh shows conversion of CO₂ to CO with 91% Faradaic efficiency at positive potential than the CO₂/CO equilibrium potential.     

Electrochemical CO2 Reduction with Atomic Iron‐Dispersed on Nitrogen‐Doped Graphene

Electrochemical reduction of CO₂ provides an opportunity to reach a carbon‐neutral energy recycling regime, in which CO₂ emissions from fuel use are collected and converted back to fuels. The reduction of CO₂ to CO is the first step toward the synthesis of more complex carbon‐based fuels and chemicals. Therefore, understanding this step is crucial for the development of high‐performance electrocatalyst for CO₂ conversion to higher order products such as hydrocarbons. Here, atomic iron dispersed on nitrogen‐doped graphene (Fe/NG) is synthesized as an efficient electrocatalyst for CO₂ reduction to CO. Fe/NG has a low reduction overpotential with high Faradic efficiency up to 80%. The existence of nitrogen‐confined atomic Fe moieties on the nitrogen‐doped graphene layer is confirmed by aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy and X‐ray absorption fine structure analysis. The Fe/NG catalysts provide an ideal platform for comparative studies of the effect of the catalytic center on the electrocatalytic performance. The CO₂ reduction reaction mechanism on atomic Fe surrounded by four N atoms (Fe–N₄) embedded in nitrogen‐doped graphene is further investigated through density functional theory calculations, revealing a possible promotional effect of nitrogen doping on graphene.     

Modified Ceria for “Low‐Temperature” CO2 Utilization: A Chemical Looping Route to Exploit Industrial Waste Heat

Efficient CO₂ utilization is key to limit global climate change. Carbon monoxide, which is a crucial feedstock for chemical synthesis, can be produced by splitting CO₂. However, existing thermochemical routes are energy intensive requiring high operating temperatures. A hybrid redox process (HRP) involving CO₂‐to‐CO conversion using a lattice oxygen‐deprived redox catalyst at relatively low temperatures (<700 °C) is reported. The lattice oxygen of the redox catalyst, restored during CO₂‐splitting, is subsequently used to convert methane to syngas. Operated at temperatures significantly lower than a number of industrial waste heat sources, this cyclic redox process allows for efficient waste heat‐utilization to convert CO₂. To enable the low temperature operation, lanthanum modified ceria (1:1 Ce:La) promoted by rhodium (0.5 wt%) is reported as an effective redox catalyst. Near‐complete CO₂ conversion with a syngas yield of up to 83% at low temperatures is achieved using Rh‐promoted LaCeO_4?x. While La improves low‐temperature bulk redox properties of ceria, Rh considerably enhances the surface catalytic properties for methane activation. Density functional theory calculations further illustrate the underlying functions of La‐substitution. The highly effective redox catalyst and HRP scheme provide a potentially attractive route for chemical production using CO₂, industrial waste heat, and methane, with appreciably lowered CO₂ emissions.     

Recent Trends,Benchmarking, and Challenges of Electrochemical Reduction of CO2 by Molecular Catalysts

CO₂ reduction using molecular catalysts is a key area of study for achieving electrical‐to‐chemical energy storage and feedstock chemical synthesis. Compared to classical metallic solid‐state catalysts, these molecular catalysts often result in high performance and selectivity, even under unfavorable aqueous environments. This review considers the recent state‐of‐the‐art molecular catalysts for CO₂ electroreduction and explains the observed performance, therefore guiding the design principles for the next generation of molecules and material/molecule hybrid electrodes. The most recent advances related to these issues are discussed.     

Electrocatalytic Reduction of Gaseous CO2 to CO on Sn/Cu‐Nanofiber‐Based Gas Diffusion Electrodes

Earth‐abundant Sn/Cu catalysts are highly selective for the electrocatalytic reduction of CO₂ to CO in aqueous electrolytes. However, CO₂ mass transport limitations, resulting from the low solubility of CO₂ in water, so far limit the CO partial current density for Sn/Cu catalysts to about 10 mA cm^?2. Here, a freestanding gas diffusion electrode design based on Sn‐decorated Cu‐coated electrospun polyvinylidene fluoride nanofibers is demonstrated. The use of gaseous CO₂ as a feedstock alleviates mass transport limitations, resulting in high CO partial current densities above 100 mA cm^?2, while maintaining high CO faradaic efficiencies above 80%. These results represent an important step toward an economically viable pathway to CO₂ reduction.     

Solar‐Light‐Driven CO2 Reduction by CH4 on Silica‐Cluster‐Modified Ni Nanocrystals with a High Solar‐to‐Fuel Efficiency and Excellent Durability

Catalytic CO₂ reforming of CH₄ (CRM) to produce syngas (H₂ and CO) provides a promising approach to reducing global CO₂ emissions and the extensive utilization of natural gas resources. However, the rapid deactivation of the reported catalysts due to severe carbon deposition at high reaction temperatures and the large energy consumption of the process hinder its industrial application. Here, a method for almost completely preventing carbon deposition is reported by modifying the surface of Ni nanocrystals with silica clusters. The obtained catalyst exhibits excellent durability for CRM with almost no carbon deposition and deactivation after reaction for 700 h. Very importantly, it is found that CRM on the catalyst can be driven by focused solar light, thus providing a promising new approach to the conversion of renewable solar energy to fuel due to the highly endothermic characteristics of CRM. The reaction yields high production rates of H₂ and CO (17.1 and 19.9 mmol min^?1 g^?1, respectively) with a very high solar‐to‐fuel efficiency (η, 12.5%). Even under focused IR irradiation with a wavelength above 830 nm, the η of the catalyst remains as high as 3.1%. The highly efficient catalytic activity arises from the efficient solar‐light‐driven thermocatalytic CRM enhanced by a novel photoactivation effect.     

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.     

Edge‐Exposed Molybdenum Disulfide with N‐Doped Carbon Hybridization: A Hierarchical Hollow Electrocatalyst for Carbon Dioxide Reduction

Electrochemical CO₂ reduction (CO₂RR) is a promising technology to produce value‐added fuels and weaken the greenhouse effect. Plenty of efforts are devoted to exploring high‐efficiency electrocatalysts to tackle the issues that show poor intrinsic activity, low selectivity for target products, and short‐lived durability. Herein, density functional theory calculations are firstly utilized to demonstrate guidelines for design principles of electrocatalyst, maximum exposure of catalytic active sites for MoS₂ edges, and electron transfer from N‐doped carbon (NC) to MoS₂ edges. Based on the guidelines, a hierarchical hollow electrocatalyst comprised of edge‐exposed 2H MoS₂ hybridized with NC for CO₂RR is constructed. In situ atomic‐scale observation for catalyst growth is performed by using a specialized Si/SiN_x nanochip at a continuous temperature‐rise period, which reveals the growth mechanism. Abundant exposed edges of MoS₂ provide a large quantity of active centers, which leads to a low onset potential of ≈40 mV and a remarkable CO production rate of 34.31 mA cm^?2 with 92.68% of Faradaic efficiency at an overpotential of 590 mV. The long‐term stability shows negligible degradation for more than 24 h. This work provides fascinating insights into the construction of catalysts for efficient CO₂RR.     

Hydride‐Enhanced CO2 Methanation: Water‐Stable BaTiO2.4H0.6 as a New Support

Catalytic CO₂ hydrogenation to CH₄ provides a promising approach to producing natural gas, and reducing the emissions of global CO₂. However, the efficiency of catalytic CO₂ methanation is limited by slow kinetics at low temperatures. This study first demonstrates that an air‐ and water‐stable perovskite oxyhydride BaTiO_2.4H_0.6 could function as an active support material for Ni‐, Ru‐based catalysts for CO₂ methanation at 300–350 °C, a relatively lower temperature. With the oxyhydride support, the activity for Ni and Ru increases by a factor of 2–7 when compared to the BaTiO₃ oxide support. Kinetic analysis shows reduced H₂ poisoning probably due to spillover, implying that the activity change is due to the kinetics being influenced by hydride. Furthermore, the oxyhydride‐supported Ni catalyst is also durable with its catalytic performance preserved for at least 10 h under a humid environment at elevated temperatures. It is anticipated that these perovskite oxyhydrides will shed new light on the design of high‐efficiency metal‐based catalysts for water‐involved catalytic reactions.     

Effect of Change of Forest Carbon Storage on Net Carbon Dioxide Balance of Wood Use for Energy in Japan

This study analyzed the net carbon dioxide (CO₂) emission reductions between 2005 and 2050 by using wood for energy under various scenarios of forest management and energy conversion technology in Japan, considering both CO₂ emission reductions from replacement of fossil fuels and changes in carbon storage in forests. According to our model, wood production for energy results in a significant reduction of carbon storage levels in forests (by 46% to 77% in 2050 from the 2005 level). Thus, the net CO₂ emission reduction when wood is used for energy becomes drastically smaller. Conventional tree production for energy increases net CO₂ emissions relative to preserving forests, but fast‐growing tree production may reduce net CO₂ emissions more than preserving forests does. When wood from fast‐growing trees is used to generate electricity with gas turbines, displacing natural gas, the net CO₂ emission reduction from the combination of fast‐growing trees and electricity generation with gas turbines is about 58% of the CO₂ emission reduction from electricity generation from gas turbines alone in 2050, and an energy conversion efficiency of around 20% or more is required to obtain net reductions over the entire period until 2050. When wood is used to produce bioethanol, displacing gasoline, net reductions are realized after 2030, provided that heat energy is recovered from residues from ethanol production. These results show the importance of considering the change in carbon storage when estimating the net CO₂ emission reduction effect of the wood use for energy.     

2D Assembly of Confined Space toward Enhanced CO2 Electroreduction

Rational design of electrocatalysts toward efficient CO₂ electroreduction has the potential to reduce carbon emission and produce value‐added chemicals. In this work, a strategy of constructing 2D confined‐space as molecular reactors for enhanced electrocatalytic CO₂ reduction selectivity is demonstrated. Highly ordered 2D nanosheet lamella assemblies are achieved via weak molecular interaction of atomically thin titania nanosheets, a variety of cationic surfactants, and SnO₂ nanoparticles. The interlayer spacings can be tuned from 0.9 to 3.0 nm by using different surfactant molecules. These 2D assemblies of confined‐space catalysts exhibit a strong size dependence of CO₂ electroreduction selectivity, with a peak Faradaic efficiency of 73% for formate production and excellent electrochemical stability at an optimal interspacing of ≈2.0 nm. This work suggests great potential for constructing new molecular‐size reactors, for highly selective electrocatalytic CO₂ reduction.     

Metal‐Free Artificial Photosynthesis of Carbon Monoxide Using N‐Doped ZnTe Nanorod Photocathode Decorated with N‐Doped Carbon Electrocatalyst Layer

An artificial photosynthesis system based on N‐doped ZnTe nanorods decorated with an N‐doped carbon electrocatalyst layer is fabricated via an all‐solution process for the selective conversion of CO₂ to CO. Substitutional N‐doping into the ZnTe lattice decreases the bandgap slightly and improves the charge transfer characteristics, leading to enhanced photoelectrochemical activity. Remarkable N‐doping effects are also demonstrated by the N‐doped carbon layer that promotes selective CO₂‐to‐CO conversion instead of undesired water‐to‐H₂ reduction by providing active sites for CO₂ adsorption and activation, even in the absence of metallic redox centers. The photocathode shows promising performance in photocurrent generation (?1.21 mA cm^?2 at ?0.11 V_RHE), CO selectivity (dominant CO production of ≈72%), minor H₂ reduction (≈20%), and stability (corrosion suppression). The metal‐free electrocatalyst/photocatalyst combination prepared via a cost‐effective solution process exhibits high performance due to synergistic effects between them, and thus may find application in practical solar fuel production.