Sulfur-Tuned Main-Group Sb-N-C Catalysts for Selective 2-Electron and 4-Electron Oxygen Reduction

The selective oxygen reduction reaction (ORR) is important for various energy conversion processes such as the fuel cells and metal-air batteries for the 4e- pathway and hydrogen peroxide (H2O2) electrosynthesis for the 2e- pathway. However, it remains a challenge to tune the ORR selectivity of a catalyst in a controllable manner. Herein, an efficient strategy for introducing sulfur dopants to regulate the ORR selectivity of main-group Sb-N-C single-atom catalysts  is reported. Significantly, Sb-N-C with the highest sulfur content follows a 2e- pathway with high H2O2 selectivity (96.8%) and remarkable mass activity (96.1 A g-1 at 0.65 V), while the sister catalyst with the lowest sulfur content directs a 4e- pathway with a half-wave potential (E1/2 = 0.89 V) that is more positive than commercial Pt/C. In addition, practical applications for these two 2e-/4e- ORR catalysts are demonstrated by bulk H2O2 electrosynthesis for the degradation of organic pollutants and a high-power zinc-air battery, respectively. Combined experimental and theoretical studies reveal that the excellent selectivity for the sulfurized Sb-N-Cs is attributed to the optimal adsorption-desorption of the ORR intermediates realized through the electronic structure modulation by the sulfur dopants.

Tuning Carbon Dioxide Reduction Reaction Selectivity of Bi Single-Atom Electrocatalysts with Controlled Coordination Environments

Control over product selectivity of the electrocatalytic CO2 reduction reaction (CO2RR) is a crucial challenge for the sustainable production of carbon-based chemical feedstocks. In this regard, single-atom catalysts (SACs) are promising materials due to their tunable coordination environments, which could enable tailored catalytic activities and selectivities, as well as new insights into structure-activity relationships. However, direct evidence for selectivity control via systematic tuning of the SAC coordination environment is scarce. In this work, we have synthesized two differently coordinated Bi SACs anchored to the same host material (carbon black) and characterized their CO2RR activities and selectivities. We find that oxophilic, oxygen-coordinated Bi atoms produce HCOOH, while nitrogen-coordinated Bi atoms generate CO. Importantly, use of the same support material assured that alternation of the coordination environment is the dominant factor for controlling the CO2RR product selectivity. Overall, this work demonstrates the structure-activity relationship of Bi SACs, which can be utilized to establish control over CO2RR product distributions, and highlights the promise for engineering atomic coordination environments of SACs to tune reaction pathways.

Modulating the Structure and Composition of Single-Atom Electrocatalysts for CO2 reduction

Electrochemical CO2 reduction reaction (eCO2 RR) is a promising strategy to achieve carbon cycling by converting CO2 into value-added products under mild reaction conditions. Recently, single-atom catalysts (SACs) have shown enormous potential in eCO2 RR due to their high utilization of metal atoms and flexible coordination structures. In this work, the recent progress in SACs for eCO2 RR is outlined, with detailed discussions on the interaction between active sites and CO2 , especially the adsorption/activation behavior of CO2 and the effects of the electronic structure of SACs on eCO2 RR. Three perspectives form the starting point: 1) Important factors of SACs for eCO2 RR; 2) Typical SACs for eCO2 RR; 3) eCO2 RR toward valuable products. First, how different modification strategies can change the electronic structure of SACs to improve catalytic performance is discussed; Second, SACs with diverse supports and how supports assist active sites to undergo catalytic reaction are introduced; Finally, according to various valuable products from eCO2 RR, the reaction mechanism and measures which can be taken to improve the selectivity of eCO2 RR are discussed. Hopefully, this work can provide a comprehensive understanding of SACs for eCO2 RR and spark innovative design and modification ideas to develop highly efficient SACs for CO2 conversion to various valuable fuels/chemicals.

Study of CuSb bimetallic flow-through gas diffusion electrodes for efficient electrochemical CO2 reduction to CO

Electrochemical CO2 reduction (eCO2R) to industrially important feedstock has received great attention, but it faces different challenges. Among them, the poor CO2 mass transport due to low intrinsic CO2 solubility significantly limits the rate of reduction reactions, leading to lower catalytic performance; thereby, commercially relevant current densities can't be achieved. Moreover, the poor activity and selectivity of high-cost monometallic catalysts, including Cu, Zn, Ag, and Au, undermine the efficiency of eCO2R. Flow-through gas diffusion electrodes (FTGDE), a newly developed class of GDEs, can potentially solve the issue of poor CO2 mass transport because they directly feed the CO2 to the catalyst layer. In addition, abundant surface area, porous structure, and improved triple-phase interface make them an excellent candidate for extremely high rate eCO2R. Antimony, a low-cost and abundant metalloid, can be effectively tuned with Cu to produce useful products such as CO, formate, and C2H4 through eCO2R. Herein, a series of porous binary CuSb FTGDEs with different Sb compositions are fabricated for the electrocatalytic reduction of CO2 to CO. The results show that the catalytic performance of CuSb FTGDEs improved with increasing Sb content up to a certain threshold, beyond which it started to decrease. The CuSb FTGDE with 5.4 g of antimony demonstrated higher current density (206.4 mA/cm2) and faradaic efficiency (72.82 %) at relatively lower overpotentials. Compared to gas diffusion configuration, the poor catalytic activity and selectivity achieved by CuSb FTGDE in non-gas diffusion configuration signifies the importance of improved local CO2 concentration and improved triple-phase interface formation in GDE configuration. The several hours stable operation of CuSb FTGDEs during eCO2R demonstrates its potential for efficient electrocatalytic conversion applications.

Quasi-Copper-Mers Enable High-Performance Catalysis for CO2 Reduction

As the atmospheric carbon dioxide (CO2 ) level keeps hitting the new record, humanity is facing an ever-daunting challenge to efficiently mitigate CO2 from the atmosphere. Though electrochemical CO2 reduction presents a promising pathway to convert CO2 to valuable fuels and chemicals, the general lack of suitable electrocatalysts with high activity and selectivity severely constrains this approach. Herein, a novel class of electrocatalysts is investigated, the quasi-copper-mers, in which the CuN4 rather than Cu atom itself serve as the basic building block. The respective quasi-copper-monomers, -dimers, and -trimers hosted in a graphene-like substrate are first synthesized and then performed both experimental characterization and density functional theory (DFT) calculations to examine their atomic structures, evaluate their electrocatalytical performance and understand their underlying mechanisms. The experimental results show that the quasi-copper-trimers not only outperform the quasi-copper-dimer and quasi-copper-monomer when catalyzing CO2 to CO, it also shows a superior selectivity against the competing hydrogen evolution reaction (HER). The DFT calculations not only support the experimental observations, but also reveal the volcano curve and the physical origin for the qausi-copper-trimer superiority. The present work thus presents a new strategy in the design of high-performance electrocatalysts with high activity and selectivity.

Rational design of atomic site catalysts for electrochemical CO2 reduction

Renewable-energy-powered electrochemical CO₂ reduction (ECR) is a promising way of transforming CO₂ to value-added products and achieving sustainable carbon recycling. By virtue of the extremely high exposure rate of active sites and excellent catalytic performance, atomic site catalysts (ASCs), including single-atomic site catalysts and diatomic site catalysts, have attracted considerable attention. In this feature article, we focus on the rational design strategies of ASCs developed in recent years for the ECR reaction. The influence of these strategies on the activity and selectivity of ASCs for ECR is further discussed in terms of electronic regulation, synergistic activation, microenvironmental regulation and tandem catalytic system construction. Finally, the challenges and future directions are indicated. We hope that this feature article will be helpful in the development of novel ASCs for ECR.

Highly selective CO2 electroreduction to CO by the synergy between Ni-N-C and encapsulated Ni nanoparticles

Efficient catalysts are highly desirable for the selective electrochemical CO₂ reduction reaction (CO₂RR). Ni single-atom catalysts are known as promising CO₂RR catalysts, while Ni NPs are expected to catalyze the competing HER. In this work, we have modified the Ni NPs by encapsulating them into porous Ni-N-C nanosheets (Ni@Ni-N-C), to boost the synergy between Ni NPs and dispersed Ni-N species towards CO₂RR. The CO faradaic efficiency (FE_CO) reached 96.4% at -0.9 V and retained over 90% in a wide potential window. More importantly, FE_CO values of over 94% have been obtained from -50 to -170 mA cm^-2 with a peak FE_CO of 99% in a flow cell. Our work demonstrates that the surface modification of Ni NPs can inhibit the unexpected HER and activate the surface sites, offering a practical design strategy for CO₂RR catalysts.

Highly dispersed Cu nanoparticles on ceria for enhanced ethylene selectivity during electrochemical reduction of CO2

Strong interaction between Cu and CeO2 contributes to forming highly effective active sites for electroreduction of CO2 to ethylene.

High-curvature Carbon Supported Ni Single Atom with Charge Polarization for High-efficient CO2 Reduction

High curvature carbon supported isolated NiN4 sites with ideal CO2 reduction reaction performance were obtained through a top-down strategy. The charge polarization induced by the axial asymmetry of carbon substrate...

Engineering CuO-HfO2 interface toward enhanced CO2 electroreduction to C2H4

We report significantly enhanced electrochemical CO2 reduction (ECR) to C2H4 by tuning the interface of a metal oxide composite (CuOx/HfO2), enabling a C2H4 faradaic efficiency as high as 62.6 +...

Designing electrode materials for the electrochemical reduction of carbon dioxide

Electrochemical reduction of carbon dioxide is a viable alternative for reducing fossil fuel consumption and reducing atmospheric CO₂ levels. Although, a wide variety of materials have been studied for electrochemical reduction of CO₂, the selective and efficient reduction of CO₂ is still not accomplished. Complex reaction mechanisms and the competing hydrogen evolution reaction further complicates the efficiency of materials. An extensive understanding of reaction mechanism is hence essential in designing an ideal electrocatalyst material. Therefore, in this review article we discuss the materials explored in the last decade with focus on their catalytic mechanism and methods to enhance their catalytic activity.

Efficient CO2 Electroreduction on Tin Modified Cuprous Oxide Synthesized via a One-Pot Microwave-Assisted Route

Bimetallic copper-tin catalysts are considered cost-effective and suitable for large-scale electrochemical conversion of CO2 to valuable products. In this work, a class of tin (Sn) modified cuprous oxide (Cu2O) is simply synthesized through a one-pot microwave-assisted solvothermal method and thoroughly characterized by various techniques. Sn is uniformly distributed on the Cu2O crystals showing a cube-within-cube structure, and CuSn alloy phase emerges at high Sn contents. The atomic ratio of Cu to Sn is found to be crucially important for the selectivity of the CO2 reduction reaction, and a ratio of 11.6 leads to the optimal selectivity for CO. This electrode shows a high current density of 47.2 mA cm−2 for CO formation at −1.0 V vs. the reversible hydrogen electrode and also displays good CO selectivity of 80–90% in a wide potential range. In particular, considerable CO selectivity of 72–81% is achieved at relatively low overpotentials from 240 mV to 340 mV. During the long-term tests, satisfactory stability is observed for the optimal electrode in terms of both electrode activity and CO selectivity. The relatively low price, the fast and scalable synthesis, and the encouraging performance of the proposed material implies its good potential to be implemented in large-scale CO2 electrolyzers.

Boosting carbon monoxide production during CO2 reduction reaction via Cu-Sb2O3 interface cooperation

Development of multiple-component catalyst materials is a new trend in electrochemical CO₂ reduction reaction (eCO₂RR). A new type of metal-oxide interaction is reported here to improve carbon monoxide production via synergistic effect between the CO₂-to hydrocarbon selective metal material and CO₂-to hydrogen generation oxide material. Cu/Sb₂O₃ material originates from the hetero-structured CuO/Sb₂O₃ by a facile two-step hydrolysis and precipitation method, cooperative to inhibit hydrogen evolution or methane product, achieving CO Faradaic efficiency to 92% in CO₂ saturated KCl electrolyte at -0.99 V with good stability. The formation of a stable *COOH intermediate by electronic and geometric effects via Cu and Sb₂O₃ are responsible to promote CO selectivity. Cu-Sb₂O₃ interface interaction also destabilizes the adsorption *H as well, an intermediate for H₂ evolution. This study proposes a versatile design strategy for construction and utilization of metal-oxide interface for eCO₂RR.

MOFs-Derived Carbon-Based Metal Catalysts for Energy-Related Electrocatalysis

Electrochemical devices, as renewable and clean energy systems, display a great potential to meet the sustainable development in the future. However, well-designed and highly efficient electrocatalysts are the technological dilemmas that retard their practical applications. Metal-organic frameworks (MOFs) derived electrocatalysts exhibit tunable structure and intriguing activity and have received intensive investigation in recent years. In this review, the recent progress of MOFs-derived carbon-based single atoms (SAs) and metal nanoparticles (NPs) catalysts for energy-related electrocatalysis is summarized. The effects of synthesis strategy, coordination environment, morphology, and composition on the catalytic activity are highlighted. Furthermore, these SAs and metal NPs catalysts for the applications of electrocatalysis (hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction) are overviewed. Finally, some current challenges and foresighted ideas for MOFs-derived carbon-based metal electrocatalysts are presented.

Single-atom catalysts for the oxygen evolution reaction: recent developments and future perspectives

Single-atom catalysts (SACs) possess the potential to achieve unique catalytic properties and remarkable catalytic mass activity by utilizing low-coordination and unsaturated active sites. However, smaller particles tend to aggregate into clusters or particles owing to their high surface energy. In addition, support materials that have strong interactions with isolated metal atoms, extremely large surface areas, and electrochemical stability are required. Therefore, sufficient information about these factors is needed to synthesize and utilize SACs. Herein, we review the recent investigations and advances in SACs for the oxygen evolution reaction (OER). We present not only the structural characterization of SACs, but also in situ/operando spectroscopic techniques and computational research for SACs to understand the mechanism and reveal the origin of their excellent OER activity. Furthermore, the OER catalytic activity and stability of SACs are summarized to evaluate the current level of SACs. Currently, research on single-atoms as OER catalysts is in the infant stage for synthesis, characterization and mechanism studies. We discuss some challenges for understanding the fundamentals of SACs and enhancing the catalytic performance of SACs for industrial applications.

Heterogeneous Single-Atom Catalysts for Electrochemical CO2 Reduction Reaction

The electrochemical CO₂ reduction reaction (CO₂ RR) is of great importance to tackle the rising CO₂ concentration in the atmosphere. The CO₂ RR can be driven by renewable energy sources, producing precious chemicals and fuels, with the implementation of this process largely relying on the development of low-cost and efficient electrocatalysts. Recently, a range of heterogeneous and potentially low-cost single-atom catalysts (SACs) containing non-precious metals coordinated to earth-abundant elements have emerged as promising candidates for the CO₂ RR. Unfortunately, the real catalytically active centers and the key factors that govern the catalytic performance of these SACs remain ambiguous. Here, this ambiguity is addressed by developing a fundamental understanding of the CO₂ RR-to-CO process on SACs, as CO accounts for the major product from CO₂ RR on SACs. The reaction mechanism, the rate-determining steps, and the key factors that control the activity and selectivity are analyzed from both experimental and theoretical studies. Then, the synthesis, characterization, and the CO₂ RR performance of SACs are discussed. Finally, the challenges and future pathways are highlighted in the hope of guiding the design of the SACs to promote and understand the CO₂ RR on SACs.

Single yttrium sites on carbon-coated TiO2 for efficient electrocatalytic N2 reduction

We report a facile synthesis of single yttrium sites anchored on carbon-coated TiO₂ for efficient and stable electrocatalytic N₂ fixation.