Oxygen Reduction Reactions on Single‐ or Few‐Atom Discrete Active Sites for Heterogeneous Catalysis

The oxygen reduction reaction (ORR) is of great importance in energy‐converting processes such as fuel cells and in metal–air batteries and is vital to facilitate the transition toward a nonfossil dependent society. The ORR has been associated with expensive noble metal catalysts that facilitate the O₂ adsorption, dissociation, and subsequent electron transfer. Single‐ or few‐atom motifs based on earth‐abundant transition metals, such as Fe, Co, and Mo, combined with nonmetallic elements, such as P, S, and N, embedded in a carbon‐based matrix represent one of the most promising alternatives. Often these are referred to as single atom catalysts; however, the coordination number of the metal atom as well as the type and nearest neighbor configuration has a strong influence on the function of the active sites, and a more adequate term to describe them is metal‐coordinated motifs. Despite intense research, their function and catalytic mechanism still puzzle researchers. They are not molecular systems with discrete energy states; neither can they fully be described by theories that are adapted for heterogeneous bulk catalysts. Here, recent results on single‐ and few‐atom electrocatalyst motifs are reviewed with an emphasis on reports discussing the function and the mechanism of the active sites.     

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.     

Facile Metal Coordination of Active Site Imprinted Nitrogen Doped Carbons for the Conservative Preparation of Non‐Noble Metal Oxygen Reduction Electrocatalysts

Iron‐ or cobalt‐coordinated heteroatom doped carbons are promising alternatives for Pt‐based cathode catalysts in polymer‐electrolyte fuel cells. Currently, these catalysts are obtained at high temperatures. The reaction conditions complicate the selective and concentrated formation of metal–nitrogen active sites. Herein a mild procedure is introduced, which is conservative toward the carbon support and leads to active‐site formation at low temperatures in a wet‐chemical metal‐coordination step. Active‐site imprinted nitrogen doped carbons are synthesized via ionothermal carbonization employing Lewis‐acidic Mg²⁺ salt. The obtained carbons with large tubular porosity and imprinted N₄ sites lead to very active catalysts with a half‐wave potential (E_1/2) of up to 0.76 V versus RHE in acidic electrolyte after coordination with iron. The catalyst shows 4e^? selectivity and exceptional stability with a half‐wave potential shift of only 5 mV after 1000 cycles. The X‐ray absorption fine structure as well as the X‐ray absorption near edge structure profiles of the most active catalyst closely match that of iron(II)phthalocyanine, proving the formation of active and stable FeN₄ sites at 80 °C. Metal‐coordination with other transition metals reveals that Zn–N_x sites are inactive, while cobalt gives rise to a strong performance increase even at very low concentrations.     

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.     

Metal‐Organic Framework‐Derived Non‐Precious Metal Nanocatalysts for Oxygen Reduction Reaction

By virtue of diverse structures and tunable properties, metal‐organic frameworks (MOFs) have presented extensive applications including gas capture, energy storage, and catalysis. Recently, synthesis of MOFs and their derived nanomaterials provide an opportunity to obtain competent oxygen reduction reaction (ORR) electrocatalysts due to their large surface area, controllable composition and pore structure. This review starts with the introduction of MOFs and current challenges of ORR, followed by the discussion of MOF‐based non‐precious metal nanocatalysts (metal‐free and metal/metal oxide‐based carbonaceous materials) and their application in ORR electrocatalysis. Current issues in MOF‐derived ORR catalysts and some corresponding strategies in terms of composition and morphology to enhance their electrocatalytic performance are highlighted. In the last section, a perspective for future development of MOFs and their derivatives as catalysts for ORR is discussed.     

Surface‐Plasmon‐Assisted Photoelectrochemical Reduction of CO2 and NO3− on Nanostructured Silver Electrodes

Electrochemical reduction of carbon dioxide (CO₂) typically suffers from low selectivity and poor reaction rates that necessitate high overpotentials, which impede its possible application for CO₂ capture, sequestration, or carbon‐based fuel production. New strategies to address these issues include the utilization of photoexcited charge carriers to overcome activation barriers for reactions that produce desirable products. This study demonstrates surface‐plasmon‐enhanced photoelectrochemical reduction of CO₂ and nitrate (NO₃^?) on silver nanostructured electrodes. The observed photocurrent likely originates from a resonant charge transfer between the photogenerated plasmonic hot electrons and the lowest unoccupied molecular orbital (MO) acceptor energy levels of adsorbed CO₂, NO₃^?, or their reductive intermediates. The observed differences in the resonant effects at the Ag electrode with respect to electrode potential and photon energy for CO₂ versus NO₃^? reduction suggest that plasmonic hot‐carriers interact selectively with specific MO acceptor energy levels of adsorbed surface species such as CO₂, NO₃^?, or their reductive intermediates. This unique plasmon‐assisted charge generation and transfer mechanism can be used to increase yield, efficiency, and selectivity of various photoelectrochemical processes.     

Engineering Electronic Structure of Stannous Sulfide by Amino‐Functionalized Carbon: Toward Efficient Electrocatalytic Reduction of CO2 to Formate

Engineering electronic structure to enhance the binding energies of reaction intermediates in order to achieve a high partial current density can lead to increased yield of target products. Herein, amino‐functionalized carbon is used to regulate the electronic structure of tin‐based catalysts to enhance activity of CO₂ electroreduction. The hollow nanotubes composed of SnS (stannous sulfide) nanosheets are modified with amino‐functionalized carbon layers, achieving a highest formate Faraday efficiency of 92.6% and a remarkable formate partial current density of 41.1 mA cm^?2 (a total current density of 52.1 mA cm^?2) at a moderate overpotential of 0.9 V versus reversible hydrogen electrode, as well as a good stability. Density functional theory calculations demonstrate that the superior activity is attributed to the synergistic effect among SnS and Aminated‐C in increasing the adsorption energies of the key intermediates and accelerating the charge transfer rate.     

Rational Design of Nanocatalysts with Nonmetal Species Modification for Electrochemical CO2 Reduction

Converting CO₂ to valuable carbonaceous fuels and chemicals via electrochemical CO₂ reduction by using renewable energy sources is considered to be a scalable strategy with substantial environmental and economic benefits. One of the challenges in this field is to develop nanocatalysts with superior electrocatalytic activity and selectivity for targeted products. Nonmetal species modification of nanocatalysts is of great significance for the construction of distinctive active sites to overcome the kinetic limitations of CO₂ reduction. These types of modification enable the efficient control of the selectivity and significantly decrease the reaction overpotential. Herein, a comprehensive review of the recent progress of nonmetal species modification of nanocatalysts for electrochemical CO₂ reduction is presented. After discussing some fundamental parameters and the basic principles of CO₂ reduction, including possible reaction pathways in light of theoretical modeling and experiments, the identification of active sites and elucidation of reaction mechanisms are emphasized for unraveling the role of nonmetal species modification, such as heteroatom incorporation, organic molecule decoration, electrolyte engineering, and single‐atom engineering. In the final section, future challenges and constructive perspectives are provided, facilitating the accelerated advancement of mechanism research and practical applications of green carbon cycling.     

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.     

Single‐Atom Catalysts: Emerging Multifunctional Materials in Heterogeneous Catalysis

Supported metal nanoparticles are the most widely investigated heterogeneous catalysts in catalysis community. The size of metal nanostructures is an important parameter in influencing the activity of constructed catalysts. Especially, as coordination unsaturated metal atoms always work as the catalytically active centers, decreasing the particle size of the catalyst can greatly boost the specific activity per metal atom. Single‐atom catalysts (SACs), containing single metal atoms anchored on supports, represent the utmost utilization of metallic catalysts and thus maximize the usage efficiency of metal atom. However, with the decreasing of particle size, the surface free energy increases obviously, and tends to aggregate into clusters or particles. Selection of an appropriate support is necessary to interact with isolated atoms strongly, and thus prevents the movement and aggregation of isolated atoms, creating stable, finely dispersed active sites. Furthermore, with uniform single‐atom dispersion and well‐defined configuration, SACs afford great space for optimizing high selectivity and activity. In this review, a detailed discussion of preparing, characterizing, and catalytically testing within this family is provided, including the theoretical understanding of key aspects of SACs materials. The main advantages of SACs as catalysts and the challenges faced for further improving catalytic performance are also highlighted.     

Degradation Mechanism and Enhancing Strategies of Oxygen Reduction Reaction Catalyzed by Carbon-Based Metal Free Catalysts in Acidic Solution

Carbon-based metal free catalysts (CMFCs) are far away from commercial availability mainly attributed to their poor oxygen reduction reaction (ORR) performance in acidic environment with the causes remaining obscure. By investigating the heteroatoms (N, B, P, S, Se, and Te)-doped reduced graphene oxides, the degradation mechanism of acidic ORR performance of CMFCs is found to be correlated with the oxygen-baring defects in the carbon sp² lattice, which exhibit overpotentials as low as 0.44 V but weak trapping capabilities for oxygen molecules. These findings not only revise the previously reported strategy of modeling the active sites in the basal plane of CMFCs but also highlight the connections between those active sites and the triple-coordinated VIA group elements (XC₃). Further calculations demonstrate that the XC₃ dimer can efficiently enhance the acidic ORR performance and the 2D trigonal carbon-chalcogenides C₆X (X = S, Se, and Te) are accordingly designed toward acidic ORR, which contain homogeneously distributed basal plane active sites and exhibit low overpotentials but strong trapping capabilities for oxygen molecules. This work will help to cease the debates on the active sites in CMFCs for ORR in both acidic and alkaline solutions and to open a new avenue to design CMFCs independent on doping strategy.     

Atomic‐Level Reactive Sites for Semiconductor‐Based Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction is an effective means to generate renewable energy. It involves redox reactions, reduction of CO₂ and oxidation of water, that leads to the production of solar fuel. Significant research effort has therefore been made to develop inexpensive and practically sustainable semiconductor‐based photocatalysts. The exploration of atomic‐level active sites on the surface of semiconductors can result in an improved understanding of the mechanism of CO₂ photoreduction. This can be applied to the design and synthesis of efficient photocatalysts. In this review, atomic‐level reactive sites are classified into four types: vacancies, single atoms, surface functional groups, and frustrated Lewis pairs (FLPs). These different photocatalytic reactive sites are shown to have varied affinity to reactants, intermediates, and products. This changes pathways for CO₂ reduction and significantly impacts catalytic activity and selectivity. The design of a photocatalyst from an atomic‐level perspective can therefore be used to maximize atomic utilization efficiency and lead to a high selectivity. The prospects for fabrication of effective photocatalysts based on an in‐depth understanding are highlighted.     

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.     

Controlling the Active Sites of Sulfur‐Doped Carbon Nanotube–Graphene Nanolobes for Highly Efficient Oxygen Evolution and Reduction Catalysis

Controlling active sites of metal‐free catalysts is an important strategy to enhance activity of the oxygen evolution reaction (OER). Many attempts have been made to develop metal‐free catalysts, but the lack of understanding of active‐sites at the atomic‐level has slowed the design of highly active and stable metal‐free catalysts. A sequential two‐step strategy to dope sulfur into carbon nanotube–graphene nanolobes is developed. This bidoping strategy introduces stable sulfur–carbon active‐sites. Fluorescence emission of the sulfur K‐edge by X‐ray absorption near edge spectroscopy (XANES) and scanning transmission electron microscopy electron energy loss spectroscopy (STEM‐EELS) mapping and spectra confirm that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs not only enhances OER activity with an overpotential of 350 mV at a current density of 10 mA cm^?2, but also retains 100% of stability after 75 h. The bidoped sulfur carbon nanotube–graphene nanolobes behave like the state‐of‐the‐art catalysts for OER but outperform those systems in terms of turnover frequency (TOF) which is two orders of magnitude greater than (20% Ir/C) at 400 mV overpotential with very high mass activity 1000 mA cm^?2 at 570 mV. Moreover, the sulfur bidoping strategy shows high catalytic activity for the oxygen reduction reaction (ORR). Stable bifunctional (ORR and OER) catalysts are low cost, and light‐weight bidoped sulfur carbon nanotubes are potential candidates for next‐generation metal‐free regenerative fuel cells.     

Structural Regulation and Support Coupling Effect of Single‐Atom Catalysts for Heterogeneous Catalysis

Single atom catalysts (SACs) that integrate the merits of homogeneous and heterogeneous catalysts have been attracting considerable attention in recent years. The individual metal atoms of SACs can be stabilized on supports through various unsaturated chemical sites or space confinement for achieving the maximized atom utilization efficiency. Aside from the development of strategies for preparing high loading and high purity SACs, another key challenge in this field is precisely manipulating the geometric and electronic structure of catalytically active single metal sites, thus rendering the catalysts exceptionally reactive, selective, and stabile compared to their bulk counterparts. This review summarizes recent advancements in SACs for heterogeneous catalysis from the perspective of local structural regulation and the synergistic coupling effect between metal species and supports. Special emphasis is placed on the elucidation of the catalytic structure‐performance relationship in terms of coordination environment, valence state and metal‐support interactions by advanced characterization and theoretical studies. Select in situ or operando characterization techniques for tracking the SACs’ structure evolution under realistic conditions are highlighted. Finally, the challenges and opportunities are discussed to offer insight into the rational design of more intriguing SACs with high activity and distinct chemoselectivity.     

A Review of Phosphide‐Based Materials for Electrocatalytic Hydrogen Evolution

Hydrogen evolution by means of electrocatalytic water‐splitting is pivotal for efficient and economical production of hydrogen, which relies on the development of inexpensive, highly active catalysts. In addition to sulfides, the search for non‐noble metal catalysts has been mainly directed at phosphides due to the superb activity of phosphides for hydrogen evolution reaction (HER) and their low‐cost considering the abundance of the non‐noble constituents of phosphides. Here, recent research focusing on phosphides is summarized based on their synthetic methodology. A comparative study of the catalytic activity of different phosphides towards HER is then conducted. The catalytic activity is evaluated by overpotentials at fixed current density, Tafel slope, turnover frequency, and the Gibbs free energy of hydrogen adsorption. Based on the methods discussed, perspectives for the various methods of phosphides synthesis are given, and the origins of the high activity and the role of phosphorus on the improved activity towards HER are discussed.     

Overpotentials and Faraday Efficiencies in CO2 Electrocatalysis–the Impact of 1‐Ethyl‐3‐Methylimidazolium Trifluoromethanesulfonate

The mixtures of room temperature ionic liquid 1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate ([EMIM]TFO) and water as electrolytes for reduction of CO₂ to CO are reported. Linear sweep voltammetry shows overpotentials for CO₂ reduction and the competing hydrogen evolution reaction (HER), both of which vary as a function of [EMIM]TFO concentration in the range from 4 × 10^?3m (0.006 mol%) to 4869 × 10^?3m (50 mol%). A steady lowering of overpotentials up to an optimum for 334 × 10?³m is identified. At 20 mol% and more of [EMIM]TFO, a significant CO₂ reduction plateau and inhibition of HER, which is limited by H₂O diffusion, is noted. Such a plateau in CO₂ reduction correlates to high CO Faraday efficiencies. In case of 50 mol% [EMIM]TFO, a broad plateau spanning over a potential range of 0.58 V evolves. At the same time, the overpotential for HER is increased by 1.20 V when compared to 334 × 10^?3m and, in turn, HER is largely inhibited. The Faraday efficiencies for CO and H₂ formation feature 95.6% ± 6.8% and 0.5% ± 0.3%, respectively, over a period of 3 h in a separator divided cell. Cathodic as well as anodic electrochemical stability of the electrolyte throughout this time period is corroborated in ¹H NMR spectroscopic measurements.     

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.