Metal Single-atom Catalysts Research Articles

Anchoring Bimetal Single Atoms and Alloys on N-Doping-Carbon Nanofiber Networks for an Efficient Oxygen Reduction Reaction and Zinc–Air Batteries

Electrocatalysts for the oxygen reduction reaction (ORR) play a central role in fuel cells and zinc-air batteries. Bimetal single atoms and nanoparticle hybrids are emerging ORR electrocatalysts, superior to the most exploited unary metal single-atom catalysts (SACs). Here, we report bimetal SAC-based nanofiber networks of Co3Fe7@Co/Fe-SAC for efficient ORR electrocatalysis and zinc-air batteries. A facile and easy-to-scale-up process is developed, and the versatility is validated in three hybrids. Strong electronic interaction is revealed between bimetal single atoms and alloy nanoparticles, leading to improved catalytic performances for ORR. Specifically, the Co3Fe7@Co/Fe-SAC hybrids exhibit a half-wave potential of 0.841 V in a basic electrolyte, comparable to the Pt/C electrocatalyst. Assembled in a zinc-air battery, a Co3Fe7@Co/Fe-SAC hybrid-based cell demonstrates a power density 1.8 times higher than the benchmark Pt/C-IrO2-based one, and it is stable for 150 cycles galvanostatic charge/discharge. The superior device performance is attributed to the appealing intrinsic activity, the carbon shielding effect for anti-leaching, and the hierarchical porous networks for large accessibility of active sites and favorable mass transport. Theoretical calculations suggest that alloy nanoparticles significantly improved the intrinsic catalytic activity of Fe single-atom sites at the expense of slightly lowering the activity of Co single-atom sites. This work presents a versatile process for the mass production of efficient composite electrocatalysts and highlights the power of bimetal single-atom-based hybrids and hierarchically porous structures for ORR device performances.

Molybdenum atomic sites embedded 1D carbon nitride nanotubes as highly efficient bifunctional photocatalyst for tetracycline degradation and hydrogen evolution

Metal single atom catalysts (MSACs) stabilized by nitrogen in carbon nitride are intriguing photocatalysts for energy and environmental applications. However, the synergistic effect of simultaneously tailoring the carbon nitride nanostructure and fabricating single metal atoms has not been explored for photocatalysis. Herein, we report the facile synthesis of Mo atomic sites on porous carbon nitride nanotubes (Mo1@CNNTs) obtained by template free polymerization of urea and melamine monomers. AC-HAADF-STEM image confirmed the existence of isolated Mo atoms, while XAS analysis revealed the formation of unique atomic Mo-N3 structures on CNNTs. Incorporation of Mo atomic sites into CNNTs modulated electronic and band structure and thus improved light harvesting, charge separation, and transfer. The synergy of 1D nanotube structure and distinct atomic Mo sites turned Mo1@CNNTs as a highly efficient bifunctional photocatalyst for hydrogen evolution and tetracycline degradation. On one side Mo1@CNNTs demonstrated hydrogen evolution rate of 4861 µmol.h−1.g−1, while on the other hand Mo1@CNNTs achieved 97.3% TC degradation under visible light irradiation. DFT investigations revealed that induction of unique Mo atomic sites not only tuned the electronic structure of CNNTs, but also localized photoexcited electrons around Mo atoms to activate O2(ads) and generate reactive species (.O2–/.OH) for TC degradation. We believe that our findings are promising and will open new opportunities for the design of novel photocatalyst for a sustainable future.

Promising approach for preparing metallic single-atom catalysts: electrochemical deposition

Promising approach for preparing metallic single-atom catalysts: electrochemical deposition

Thermally-stable single-atom catalysts and beyond: A perspective.

Single-atom catalysis is a research Frontier and has attracted extensive interests in catalysis. Significant progresses have been carried out in the synthesis and characterization of metal single-atom catalysts (SACs). However, the stability and catalytic reactivity of metal SAC at elevated temperatures are not well documented because single atoms sinter at elevated temperatures. Therefore, the development of stable and reactive SAC at high temperatures remains a formidable challenge. In this perspective, we summarize recent efforts on the preparation of the thermally-stable SACs synthesized at elevated temperature via the reverse-Ostwald ripening mechanism, including the approaches of atom trapping and vapor-phase self-assembly. The reducibility of lattice oxygen, the loading upper limit and the location of the metal single atom are discussed, combining experiments with simulations. In addition, we demonstrate that the coordination structure of the metal single atom can be tailored to address the relationship of structure and performances of the metal SAC in reactions. We expect that this perspective can provide some insights to guide the study for the rational design of thermally-stable and active single atom catalysts, which are especially suitable for high-temperature reactions.

Targeted Synthesis of Metal Dual Atom Electrocatalysts

Natural enzymes present within their structure active centres composed of earth-abundant metals in atomic proximity. Such active sites, dual atom catalysts, display a unique efficiency in catalytic processes such as the nitrogen conversion to ammonia, the production of ethylene through C-C coupling, or the oxygen reduction reaction in fuel cells amongst others.1,2 Theoretical calculations suggest that the high catalytic activity of dual atom catalysts arises from the different binding mode of reactant molecules to that of metal foils and single atom catalysts, which allows to break transition scaling relationships.3 Nevertheless, the experimental synthesis and characterisation of this class of materials is highly challenging.4 It is particularly difficult to avoid the formation of single atom catalysts or nanoparticles. In this work we show a general approach to fabricate bioinspired Fe dual atom catalysts in a nitrogen doped carbon support; we test the catalyst for the oxygen reduction reaction under acidic conditions. The catalyst exhibited an activity of 2.4 ± 0.3 A g-1 carbon at 0.8 V versus a reversible hydrogen electrode in acidic media, comparable to the most active in the literature. The two-step procedure leads to well defined Fe-based dimers. We characterised these materials by means of X-ray absorption spectroscopy (XAS) and scanning transmission electron microscopy. Our general approach providing a new towards targeted synthesis of dual atom electrocatalysts for energy-critical reactions.References(1) Chen, J. G.; Crooks, R. M.; Seefeldt, L. C.; Bren, K. L.; Bullock, R. M.; Darensbourg, M. Y.; Holland, P. L.; Hoffman, B.; Janik, M. J.; Jones, A. K.; Kanatzidis, M. G.; King, P.; Lancaster, K. M.; Lymar, S. V; Pfromm, P.; Schneider, W. F.; Schrock, R. R. Beyond Fossil Fuel–Driven Nitrogen Transformations. Science 2018, 360, eaar6611(2) Lee, C. C.; Hu, Y.; Ribbe, M. W. Vanadium Nitrogenase Reduces CO. Science 2010, 329, 642(3) Singh, A. R.; Montoya, J. H.; Rohr, B. A.; Tsai, C.; Vojvodic, A.; Nørskov, J. K. Computational Design of Active Site Structures with Improved Transition-State Scaling for Ammonia Synthesis. ACS Catal. 2018, 8, 4017–4024(4) Pedersen, A.; Barrio, J.; Li, A.; Jervis, R.; Brett, D. J. L.; Titirici, M. M.; Stephens, I. E. L. Dual-Metal Atom Electrocatalysts: Theory, Synthesis, Characterization, and Applications. Adv. Energy Mater. 2021, 2102715 Figure 1

Single-Atom Electrocatalyst for Engineered Cathode Interfaces in Sodium-Sulfur Batteries

The demand for portable rechargeable energy storage devices is ever increasing, especially because of the advent of electric vehicles and widespread usage of portable electronics. The lithium-ion batteries are currently leading the battery market; however, the high-cost and potentially depleting storage of lithium metals are stimulating the search for alternative technologies. Metal-sulfur batteries are deemed to be promising candidates to supplant the ubiquitously used lithium-ion batteries owing to their high energy density, specific capacity, low cost of sulfur, and environmental benignity. Room temperature sodium sulfur batteries (RT Na-S) is a technologically viable alternative candidate which possesses astounding advantages such as low cost (both sodium and sulfur), non–toxicity, natural abundance, and high theoretical energy density (1274 Whkg-1). However, the inevitable problems such as the solubility of higher order polysulfides to the electrolyte, known as shuttle effect, and the slow kinetics of electrochemical conversion reactions of intermediate sodium polysulfides (Na2Sn) significantly impede the practical realization of Na-S batteries. The conventionally used various forms of carbonaceous nanomaterials for cathode design have floundered to overcome the challenges because their nonpolar nature cannot produce adequate anchoring and enhanced polysulfides reaction kinetics. The polar anchoring materials (AM) have exhibited promising performance to improve sulfur chemistry. It is generally understood that catalytic performance is directly connected to the surface area of catalytic particles, and the single-atomic level provides the maximum surface area, resulting in the highest catalytic efficacy. Herein, we use first principles-based density functional theory (DFT) simulations to investigate the interfacial interactions between Na2Sn and novel transition metal (TM) single-atom catalysts (SACs) embedded on nitrogen doped graphene and various lattice sites of transition metal chalcogenides (TMDC) (chalcogenides- and Metal-substitution, Metal-top sites). For example, the pristine and Mo-sub sites of MoS2 are found to be ineffective for efficient confinement of the polysulfides within the cathode material. We demonstrate that SACs on both S-site and Mo-top sites of MoS2 and on nitrogen doped graphene possess strong adsorption strength with the Na2Sn which are superior to the commonly used ether electrolyte solvents, a requisite to prevent shuttle effect. We illustrate the influence of d-band center of SACs as an important descriptor in describing Na2Sn interactions with them. The underlying anchoring mechanism of polysulfide adsorption over AM is examined through Bader charge, charge density difference and projected density of states (PDOS) analysis. We also investigate the effect of SACs in improving the kinetics of sulfur reduction reactions (SRRs) and catalytic decomposition of short-chain polysulfides which are crucial for achieving excellent rate capability and longer cycle life. Overall, the unprecedented insights obtained on the role of SACs in tailoring the polysulfides redox chemistry at the interfaces and their relation to their TM’s d-band center is an important step towards rational design cathode materials for high-performance Na-S batteries, in particular, but metal-chalcogenide batteries, in general.

Metallomacrocycle-Derived Atomically Dispersed Metal Electrocatalysts with Controlled Selectivity for Small Molecule Conversion Reactions

Atomically dispersed metal catalysts or single-atom catalysts have recently emerged as a new frontier in the field of catalysis. They exhibit extreme atom efficiency and unusual catalytic activity and selectivity, which are hard to be attained with nanoparticle-based concentional catalysts. In this talk, we present metallomacrocyclic compound-driven syntheses of carbon supported atomically dispersed metal (M–N/C) catalysts toward selective small molecule conversion reactions, including oxygen reduction reaction reaction (ORR), CO2 reduction reaction (CO2RR), and chlorine evolution reaction (CER) [1-5]. The M–N/C catalysts provide unique catalytic selectivity in these reactions. For the ORR, tuning of metal type can direct the reaction pathway with Fe–N/C catalyzing the ORR via the four-electron (4e–) reduction to generate water (H2O) [1,2] whereas Co–N/C exhibiting preferred 2e– pathway to produce hydrogen peroxide (H2O2) [3,4]. In the CO2RR, Ni–N/C selectively reduces CO2 to CO while suppressing parasitic hydrogen evolution reaction [5]. For the CER, Pt–N/C catalyst is capable of catalyzing the CER with very high selectivity approaching 100% without competitive oxygen evolution reaction, even under low concentration of Cl– and neutral electrolyte, where OER is favored [6,7]. In contrast, traditional mixed metal oxide catalysts show substantially low CER selectivity. For the selective ORR and CER, in situ X-ray absorption spectroscopy combined with other spectroscopic techniques and density functional theory calculations have played a pivotal role in identifying the active sites and uncovering reaction kinetics and mechanisms.References J. Sa et al., J. Am. Chem. Soc. 138, 15046 (2016).Woo et al., Chem. Mater. 30, 6684 (2018).Ko et al., Nature Commun. 10, 5123 (2019).Ko et al., Nature Catal. Accepted for Publication (2021).J. Sa et al., ACS Catal. 10, 10920 (2020).Lim et al., Nature Commun. 11, 412 (2020).Lim et al., ACS Catal. 11, 12232 (2021).

Screening of transition metal single-atom catalysts doped on γ-graphyne-like BN sheet for efficient nitrogen reduction reaction

Ammonia is an important industrial raw material. To promote the production of ammonia, it is urgent to develop efficient catalysts for nitrogen reduction reactions (NRR). Here, we have reported a novel electrocatalyst: γ -graphyne-like BN sheet-supported single metal atom (M/ γ BN, M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Ru) for NRR and studied the effect of Hubbard U correction in single-atom catalysts. The results of adsorption show that Ti/ γ BN, V/ γ BN, and Ru/ γ BN have the best adsorption energy for nitrogen. A detailed analysis of the NRR mechanism indicates that V/ γ BN has the lowest energy barrier in the rate-determining step when it follows a distal mechanism. Further analysis shows that the superior catalytic performance in V/ γ BN sheet is mainly attributed to the electron donation and back-donation mechanism. More interestingly, V/ γ BN greatly inhibited HER selectivity. By analyzing the doping structure and adsorption system, it can be found that when considering Hubbard U correction, there will be an obvious correlation between energy and distance. This study not only provides a basis for understanding the mechanism of nitrogen reduction reaction catalyzed by single-atom catalysts but also provides a new design idea for the rational design of high-efficiency NRR catalysts. • A novel BN based single-atom catalyst, Metal/γ-graphyne-like BN as an NRR electrocatalyst. • V/γBN follows the distal mechanism with low overpotential (0.190 V) and barrier (0.412 eV). • Excellent NRR catalytic activity comes from electron donation and back - donation mechanism.

Orbital Dependence in Single-Atom Electrocatalytic Reactions.

Transition metal single-atom catalysts supported on N-doped graphene (TM-N-C) could serve as an ideal model for studying orbital dependence in electrocatalytic reactions because the atom on the catalytic active site has discrete single-atom-like orbitals. In this work, the catalytic efficiency of Fe-N-C for the oxygen evolution reaction (OER) under a small structural perturbation has been comprehensively investigated with density functional theory calculations. The results suggest that the subtle local environment of a single atom can significantly modulate the catalytic reactivity. Further analysis demonstrates that the energy level of the TM dz2 orbital center, rather than the d-band center, is responsible for the OER catalytic efficiency as the dz2 orbital participates mainly in the reactions. Essentially, the d-band theory can be extended to the sub-d orbital level, and a small perturbation of the crystal field, induced by lattice strain or z-direction displacement of the TM atom, can prominently change the sub-d orbital associated with the reaction and in turn affect the catalytic activity.

Non-noble metal single-atom catalyst with MXene support: Fe1/Ti2CO2 for CO oxidation

MXenes have attracted considerable attention owing to their versatile and excellent physicochemical properties. Especially, they have potential applications as robust support for single atom catalysts. Here, quantum chemical studies with density functional theory are carried out to systematically investigate the geometries, stability, electronic properties of oxygen functionalized Ti 2 C (Ti 2 CO 2 ) supported single-atom catalysts M 1 /Ti 2 CO 2 (M = Fe, Co, Ni, Cu Ru, Rh, Pd, Ag Os, Ir, Pt, Au). A new non-noble metal SAC Fe 1 /Ti 2 CO 2 has been found to show excellent catalytic performance for low-temperature CO oxidation after screening the group 8-11 transition metals. We find that O 2 and CO adsorption on Fe 1 atom of Fe 1 /Ti 2 CO 2 is favorable. Accordingly, five possible mechanisms for CO oxidation on this catalyst are evaluated, including Eley-Rideal, Langmuir-Hinshelwood, Mars–van Krevelen, Termolecular Eley-Rideal, and Termolecular Langmuir-Hinshelwood (TLH) mechanisms. Based on the calculated reaction energies for different pathways, Fe 1 /Ti 2 CO 2 shows excellent kinetics for CO oxidation via TLH mechanism, with distinct low-energy barrier (0.20 eV) for the rate-determining step. These results demonstrate that Fe 1 /Ti 2 CO 2 MXene is highly promising 2D materials for building robust non-noble metal catalysts. MXene supported non-noble metal single-atom catalyst Fe 1 /Ti 2 CO 2 is found to exhibit excellent activity for low-temperature CO oxidation through first-principles computational modelling.

Anchoring single Ni atoms on CeO2 nanospheres as an efficient catalyst for the hydrogenolysis of lignin to aromatic monomers

Lignin is the most abundant renewable aromatic resource in nature; thus, the development of an efficient and economical catalysts that can depolymerize lignin into value-added aromatic monomers is highly desirable. Here, we report a single-atom Ni catalyst (xNi/CeO2-S, where x represents the Ni loading), in which the single-atom Ni sites are anchored to the oxygen vacancies on CeO2 nanospheres, for the conversion of cellulolytic enzyme lignin (CEL) via depolymerization and hydrodeoxygenation. The 1.0Ni/CeO2-S catalyst performed the best, achieving a lignin oil (LO) and maximum monomer yield of 84.3 wt% and 32.6 wt%, respectively; a higher heat value of 31.64 MJ/Kg is achieved for LO compared to 23.03 MJ/Kg for CEL. Thus, single-atom nickel catalyst exhibited excellent activity and stability in promoting the depolymerization of lignin. These results validate the viability of single-atom metal catalysts for the conversion of lignin resources.

Direct Visualization of the Evolution of a Single-Atomic Cobalt Catalyst from Melting Nanoparticles with Carbon Dissolution.

Transition metal single‐atom catalysts (SACs) are of immense interest, but how exactly they are evolved upon pyrolysis of the corresponding precursors remains unclear as transition metal ions in the complex precursor undergo a series of morphological changes accompanied with changes in oxidation state as a result of the interactions with the carbon support. Herein, the authors record the complete evolution process of Co SAC during the pyrolysis a Co/Zn‐containing zeolitic imidazolate framework. Aberration‐corrected environmental TEM coupled with in‐situ EELS is used for direct visualization of the evolution process at 200–1000 °C. Dissolution of carbon into the nanoparticles of Co is found to be key to modulating the wetting behavior of nanoparticles on the carbon support; melting of Co nanoparticles and their motion within the zeolitic architecture leads to the etching of the framework structure, yielding porous C/N support onto which Co‐single atoms reside. This uniquely structured Co SAC is found to be effective for the oxidation of a series of aromatic alkanes to produce selective ketones among other possible products. The carbon dissolution and melting/sublimation‐driven structural dynamics of transition metal revealed here will expand the methodology in synthesizing SACs and other high‐temperature processes.

Energy Science & Engineering: 10 Years of excellence

This year celebrates 10 years of the journal, Energy Science & Engineering (ESE). In 2013, ESE was launched as a peer-reviewed, open-access journal devoted to publishing high-quality research in energy. Published by Wiley and the Society of Chemical Industry (SCI), the journal offers authors a fast route to publication and the ability to share their important research in the highest visible global forum. As a prestigious journal, it maintains the highest standards of peer review as evidenced by an average rejection rate of over 60%. Dr Tomas Kaberger was the first editor-in-chief of the ESE, and he played an important leading role in the journal development. In 2018, it was my honor to succeed Tomas Kaberger as the Editor-in-Chief of the journal.1 In his inaugural editorial on the first page of this journal, Kaerger said “We hope this journal will serve as an excellent platform for integrated research, education, and communication, particularly for those with results of immediate relevance in the energy industry.”2 On the 10th birthday of ESE, we can see that ESE has grown in all possible ways, including the number of annual submissions (1000+), the number of papers (up to 300), and issues (12) published annually, and the impact factor (4.170). It has achieved numerous milestones (see Table 1) and has published many high-impact papers (see Table 2). This is because of your participation and contributions as authors, reviewers, readers, staff, editorial advisory board members, and editors. To celebrate the 10th anniversary of ESE, we organized this special issue, which consists of 12 invited articles in various energy areas, including CO2 reduction, solar energy conversion, photovoltaics, batteries, electrolysis of water, and clean fuels. The first three articles are reviews on the application of non-thermal plasma in methanol synthesis from CO2 hydrogenation, recent progress in the electrochemical reduction of carbon dioxide on metal single-atom catalysts, and CO2 reduction into formic acid under hydrothermal conditions, respectively. The fourth and fifth review articles summarized solar energy conversion, including the application of ultrathin TiO2 layers in solar energy conversion devices and the phase change of salt hydrates for photovoltaics thermal management. The review article on solid-state lithium-ion batteries constitutes the sixth article, while the seventh article is a review on sodium-ion batteries. The eighth review article assessed the degradation issues and highlighted the stabilization strategies of protonic ceramic electrolysis cells for steam electrolysis. Among four research papers, the first one is the investigation of energy management for robots. Renewable hydrogen production and natural gas blending systems were developed and assessed in the second research article, while the third research paper explored subphthalocyanine–triangulene dyads for light-harvesting device applications. The final article has made contributions to Fischer–Tropsch synthesis by converting waste to sustainable aviation fuel. I wish to express my gratitude by thanking all the authors and reviewers for their contributions to this special issue and my two colleagues (ESE Deputy Editors-in-Chief at Wiley), Dr Sonia Ojo and Dr Costas Kouroupis-Agalou, for their great efforts in the peer-review process and the organization of this special issue. At this unique moment, I would like to say “Happy Birthday” to ESE, and extend my sincere gratitude to all of you for your enthusiasm in submitting high-quality manuscripts as authors, your important evaluating manuscripts as reviewers, your energetic discussions, and helpful suggestions as editorial advisory board members, and your great contributions as handling editors. I also appreciate the strong support and help from the publisher Wiley and the SCI. Prof. Yun Hang Hu is the Editor-in-Chief of Energy Science & Engineering.

Race on engineering noble metal single-atom electrocatalysts for water splitting

Constructing noble metal single-atom catalysts (NMSACs) is essential to boosting the water splitting electrocatalysis by virtue of their maximum atom utilization and distinctive electronic structure, which can concurrently reduce the usage of noble metal and maintain high catalytic activity. In the past few years, great progress has been achieved in the field of NMSACs, including the structure and local coordination environment modifications, which significantly promote the electrochemical performance toward water splitting and deepen the understandings of the underlying mechanisms. Herein, in this review, the recent advances of the NMSACs in the field of water splitting applications have been comprehensively summarized, with a special emphasis on the advantages, synthetic strategies, and characterizations of the NMSACs. Moreover, some representative examples regarding the applications of NMSACs toward water splitting are also manifested according to the theoretical and experimental results. Furthermore, the challenges and future potentials of the NMSACs are also manifested to offer guidance for the development of more advanced NMSACs.

Front Cover: Noble Metal Single‐Atom Catalysts for the Catalytic Oxidation of Volatile Organic Compounds (ChemSusChem 7/2022)

The Front Cover shows the application of metal single-atom catalysts (SACs) in the catalytic oxidation of volatile organic compounds (VOCs). The structures of the SACs and the reaction mechanisms are discussed in the catalytic oxidation of various VOC molecules. The deactivation and regeneration mechanisms of the metal SACs are also summarized. The understanding of the structure–performance relationship over SAC can aid the designing of highly efficient metal SACs for the catalytic oxidation of VOCs. More information can be found in the Review by L. Zhang et al.

A clicking confinement strategy to fabricate transition metal single-atom sites for bifunctional oxygen electrocatalysis.

Rechargeable zinc-air batteries call for high-performance bifunctional oxygen electrocatalysts. Transition metal single-atom catalysts constitute a promising candidate considering their maximum atom efficiency and high intrinsic activity. However, the fabrication of atomically dispersed transition metal sites is highly challenging, creating a need for for new design strategies and synthesis methods. Here, a clicking confinement strategy is proposed to efficiently predisperse transitional metal atoms in a precursor directed by click chemistry and ensure successful construction of abundant single-atom sites. Concretely, cobalt-coordinated porphyrin units are covalently clicked on the substrate for the confinement of the cobalt atoms and affording a Co-N-C electrocatalyst. The Co-N-C electrocatalyst exhibits impressive bifunctional oxygen electrocatalytic performances with an activity indicator ΔE of 0.79 V. This work extends the approach to prepare transition metal single-atom sites for efficient bifunctional oxygen electrocatalysis and inspires the methodology on precise synthesis of catalytic materials.

Surface functionalization of polyaniline and excellent electrocatalytic performance for oxygen reduction to produce hydrogen peroxide

We have developed the CNT@PANI-NO electrocatalyst that is significantly different from the conventional electrocatalyst previously reported. • Polyaniline (PANI) can be changed into PANI-NO with the functional group of –NO. • Introducing PANI-NO on the surface of CNT obtains the CNT@PANI-NO. • PANI-NO has promoted selectivity of H 2 O 2 than PANI. • CNT@PANI-NO exhibits excellent electrochemical performance for H 2 O 2 production. Electrochemical two-electron oxygen reduction is an environmental-friendly and promising method to generate H 2 O 2 . However, a great challenge remains to develop more economic and effective electrocatalysts for practical application. Here, we have developed a CNT@PANI-NO electrocatalyst that is significantly different from the conventional electrocatalyst previously reported (such as metal, alloy, carbon materials doped with heteroatoms, metal single-atom catalysts). We firstly find that the polyaniline (PANI) treated by dilute nitric acid can be changed into the PANI-NO product with the functional group of –NO. Then we introduce the PANI-NO on the surface of carboxylated carbon nanotubes to obtain the CNT@PANI-NO nanocomposites, which exhibit excellent electrochemical performance to H 2 O 2 production. The H 2 O 2 selectivity of the typical CNT@PANI-NO is between 87% and 94% at the potential of 0.2–0.68 V (vs. RHE), the faradaic efficiency is close to 95% and the current density is up to 2.9 mA/cm 2 (at 0.3 V) in alkaline condition. The DFT results give the origin of the catalytic activity. This study provides a new idea for the application of polymer in electrocatalysis and a new way to develop the high-performance 2e-ORR electrocatalyst.

A first-principles study on the electrochemical reaction activity of 3d transition metal single-atom catalysts in nitrogen-doped graphene: Trends and hints

Electrochemical reactions are essential in the processes of energy storage and conversion, and performance is tightly dependent on the electrocatalysts. Herein, we systematically investigate the activity of 3d transition metal embedded nitrogen-doped graphene (MNx-G) for single-atom catalysts (SACs) in the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The calculated volcano curves reveal the optimal SAC configuration for each reaction to be CoN3-G for the ORR, CoN4-G for the OER, and Ni/CuN3-G for the HER. Analysis based on the machine learning method suggests that high catalytic performance is dominated by the number of valence electrons occupying the d orbitals, the covalent radius, the electronegativity, the ratio of nearest-neighbor N and C atoms for the metal atoms, and the bond length between metal atoms and adsorbates. This work may shed some light on further studies of the ORR, OER, and HER with non-precious metal SACs.

Single-Atom Catalysts for the Electro-Reduction of CO2 to Syngas with a Tunable CO/H2 Ratio: A Review

Nowadays, transition towards green chemistry is becoming imperative. In this scenario, an attractive perspective consists in the generation of CO through the electrochemical reduction of CO2 under ambient conditions. This approach allows storage of the electrical energy from intermittent renewable sources in the form of chemical bonds, and simultaneously reduces greenhouse gas emissions, giving carbon a second chance of life. However, most catalysts adopted for this process, i.e., noble metal-based nanoparticles, still have several issues (high costs, low current densities, high overpotentials), and in the view of generating syngas through co-electrolysis of H2O and CO2, do not enable a widely tunable CO/H2 ratio. Single-atom catalysts with N-doped carbon supports have been recently introduced to face these challenges. The following review aims to answer the demand for an extended and exhaustive analysis of the metal single-atom catalysts thus far explored for the electro-reduction of CO2 in aqueous electrolyte solution. Moreover, focus will be placed on the objective of generating a syngas with a tunable CO/H2 ratio. Eventually, the advantages of single-atom catalysts over their noble metal-based nano-sized counterparts will be identified along with future perspectives, also in the view of a rapid and feasible scaling-up.

Transition metal single-atom embedded on N-doped carbon as a catalyst for peroxymonosulfate activation: A DFT study

Single-atom catalysts perform excellently in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs), for which the generation of reactive oxygen species (ROS) is essential to the degradation of emerging organic pollutants in water. However, the detailed PMS activation mechanisms remain elusive. Density functional theory (DFT) calculation as a powerful approach can overcome the limitations of the experimental studies, providing a molecular-level perspective of catalytic process. This study conducted DFT calculations to clarify the electronic structures and PMS adsorption and activation mechanisms of a series of transition metal single-atom catalysts. According to the DFT study, significant electronic interaction and negative formation energy make nitrogen-doped carbon (N@C) supports suitable for stabilizing metal atoms (Me) to form MeN@C catalysts. As the active site, single metal atom adsorbs the oxygen atoms of PMS by electrostatic and magnetic interactions, and transfer electrons from MeN@C to activate PMS. Different adsorption configurations and the subsequent PMS activation lead to the generation of various ROS, including the SO4•- radical, •OH radical, singlet oxygen (1O2), high-valent metal-oxo species, and surface-activated PMS*. Electron transfer mediated by surface-activated PMS* may dominate in all MeN@C/PMS systems. The generation of free radicals can be difficult for some systems. High-valent metal-oxo species are readily formed by FeN@C/PMS and MnN@C/PMS, whereas 1O2 tends to be produced by CoN@C/PMS, NiN@C/PMS, and CuN@C/PMS. The findings will provide a theoretical basis for the design and synthesis of effective MeN@C catalysts for the AOPs to remove emerging organic pollutants from water and wastewater.