An Isolated Zinc–Cobalt Atomic Pair for Highly Active and Durable Oxygen Reduction

New Insight of Pyrrole-Like Nitrogen for Boosting Hydrogen Evolution Activity and Stability of Pt Single Atoms

Single atomic Pt catalysts exhibit particularly high hydrogen evolution reaction (HER) activity compared to conventional nanomaterial-based catalysts. However, the enhanced mechanisms between Pt and their coordination environment are not understood in detail. Hence, a systematic study examining the different types of N in the support is essential to clearly demonstrate the relationship between Pt single atoms and N-doped support. Herein, three types of carbon nanotubes with varying types of N (pyridine-like N, pyrrole-like N, and quaternary N) are used as carbon support for Pt single atom atomic layer deposition. The detailed coordination environment of the Pt single atom catalyst is carefully studied by electron microscope and X-ray absorption spectra (XAS). Interestingly, with the increase of pyrrole-like N in the CNT support, the HER activity of the Pt catalyst also improves. First principle calculations results indicate that the interaction between the dyz and s orbitals of H and sp³ hybrid orbital of N should be the origin of the superior HER performance of these Pt single atom catalysts (SACs).

Cooperativity in supported metal single atom catalysis

The discussion concerning cooperativity in supported single-atom (SA) catalysis is often limited to the metal-support interaction, which is certainly important, but which is not the only lever for modifying the catalytic performance. Indeed, if the interaction between the SA and the support, which can be seen as a solid ligand presenting its own specificities that fix the first coordination sphere of the metal, plays a central role as in homogeneous catalysis, other factors can strongly contribute to modification of the activity, selectivity and stability of SAs. Therefore, in this mini-review, we briefly summarize the importance of the support (oxide, carbon or a second metal) in SA photo- electro- and thermal-catalysis (support-assisted operation), and concentrate on other types of cooperativities that in some cases enable previously impossible reaction pathways on supported metal SAs. This includes topics that are not specific to SA catalysis, such as metal-ligand or heterobimetallic cooperativity, and cooperativity which is SA-specific such as nanoparticle-SA or mixed-valence SA cooperativity.

Regulating Fe-spin state by atomically dispersed Mn-N in Fe-N-C catalysts with high oxygen reduction activity

As low-cost electrocatalysts for oxygen reduction reaction applied to fuel cells and metal-air batteries, atomic-dispersed transition metal-nitrogen-carbon materials are emerging, but the genuine mechanism thereof is still arguable. Herein, by rational design and synthesis of dual-metal atomically dispersed Fe,Mn/N-C catalyst as model object, we unravel that the O₂ reduction preferentially takes place on Fe^III in the FeN₄ /C system with intermediate spin state which possesses one e_g electron (t_2g4e_g1) readily penetrating the antibonding π-orbital of oxygen. Both magnetic measurements and theoretical calculation reveal that the adjacent atomically dispersed Mn-N moieties can effectively activate the Fe^III sites by both spin-state transition and electronic modulation, rendering the excellent ORR performances of Fe,Mn/N-C in both alkaline and acidic media (halfwave positionals are 0.928 V in 0.1 M KOH, and 0.804 V in 0.1 M HClO₄), and good durability, which outperforms and has almost the same activity of commercial Pt/C, respectively. In addition, it presents a superior power density of 160.8 mW cm⁻² and long-term durability in reversible zinc–air batteries. The work brings new insight into the oxygen reduction reaction process on the metal-nitrogen-carbon active sites, undoubtedly leading the exploration towards high effective low-cost non-precious catalysts.

The working mechanism of several low-cost electrocatalyst materials is still arguable. Here the authors show a model Fe,Mn/N-C catalyst where the oxygen reduction preferentially takes place on Fe(III) sites with the intermediate spin state (t2g4 eg1) caused by the adjacent Mn-N moieties.

Synergistic Modulation of Carbon-Based, Precious-Metal-Free Electrocatalysts for Oxygen Reduction Reaction

Developing alternatives to noble-metal-based catalysts toward the oxygen reduction reaction (ORR) process plays a key role in the application of low-temperature fuel cells. Carbon-based, precious-metal-free electrocatalysts are of great interest due to their low cost, abundant sources, active catalytic performance, and long-term stability. They are also supposed to feature intrinsically high activity and highly dense catalytic sites along with their sufficient exposure, high conductivity, and high chemical stability, as well as effective mass transfer pathways. In this Review, we focus on carbon-based, precious-metal-free nanocatalysts with synergistic modulation of active-site species and their exposure, mass transfer, and charge transport during the electrochemical process. With this knowledge, perspectives on synergistic modulation strategies are proposed to push forward the development of Pt-free ORR catalysts and the wide application of fuel cells.

Construction of Dual-Active-Site Copper Catalyst Containing both CuN3 and CuN4 Sites

Clear recognition and rational construction of suitable active center for specific reaction is always of great significance in designing highly efficient catalysts. Herein, a dual-active-site copper catalyst (DAS-Cu) containing both CuN₃ and CuN₄ sites is reported. Such catalysts show extremely high catalytic performance (yield: up to 97%) toward oxyphosphorylation of alkenes, while catalysts with single active site (CuN₃ or CuN₄ ) are chemically inert in this reaction. Combined with theoretical and experimental results, the different roles of two different Cu active sites in this reaction are further identified. CuN₃ site captures the oxygen and trigger further oxidizing process, while CuN₄ site provides moderate adsorption sites for the protection of phosphonyl radicals. This work deeply discloses the significant cooperated role with two single-atomic sites in one catalytic active center and brings up a valuable clue for the rational design of better-performing heterogeneous catalyst.

Design of Local Atomic Environments in Single-Atom Electrocatalysts for Renewable Energy Conversions

Single-atom electrocatalysts (SAECs) have recently attracted tremendous research interest due to their often remarkable catalytic responses, unmatched by conventional catalysts. The electrocatalytic performance of SAECs is closely related to the specific metal species and their local atomic environments, including their coordination number, the determined structure of the coordination sites, and the chemical identity of nearest and second nearest neighboring atoms. The wide range of distinct chemical bonding configurations of a single-metal atom with its surrounding host atoms creates virtually limitless opportunities for the rational design and synthesis of SAECs with tunable local atomic environment for high-performance electrocatalysis. In this review, the authors first identify fundamental hurdles in electrochemical conversions and highlight the relevance of SAECs. They then critically examine the role of the local atomic structures, encompassing the first and second coordination spheres of the isolated metal atoms, on the design of high-performance SAECs. The relevance of single-atom dopants for host activation is also discussed. Insights into the correlation between local structures of SAECs and their catalytic response are analyzed and discussed. Finally, the authors summarize major challenges to be addressed in the field of SAECs and provide some perspectives in the rational construction of superior SAECs for a wide range of electrochemical conversions.

Single-Atom and Dual-Atom Electrocatalysts Derived from Metal Organic Frameworks: Current Progress and Perspectives

Single-atom catalysts (SACs) have attracted increasing research interests owing to their unique electronic structures, quantum size effects and maximum utilization rate of atoms. Metal organic frameworks (MOFs) are good candidates to prepare SACs owing to the atomically dispersed metal nodes in MOFs and abundant N and C species to stabilize the single atoms. In addition, the distance of adjacent metal atoms can be turned by adjusting the size of ligands and adding volatile metal centers to promote the formation of isolated metal atoms. Moreover, the diverse metal centers in MOFs can promote the preparation of dual-atom catalysts (DACs) to improve the metal loading and optimize the electronic structures of the catalysts. The applications of MOFs derived SACs and DACs for electrocatalysis, including oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, carbon dioxide reduction reaction and nitrogen reduction reaction are systematically summarized in this Review. The corresponding synthesis strategies, atomic structures and electrocatalytic performances of the catalysts are discussed to provide a deep understanding of MOFs-based atomic electrocatalysts. The catalytic mechanisms of the catalysts are presented, and the crucial challenges and perspectives are proposed to promote further design and applications of atomic electrocatalysts.

Transition Metal and Nitrogen Co-Doped Carbon-based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures

Considering the urgent requirement for clean and sustainable energy, fuel cells and metal-air batteries have emerged as promising energy storage and conversion devices to alleviate the worldwide energy challenges. The key step in accelerating the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is to develop cost-effective and high-efficiency non-precious metal catalysts, which can be used to replace expensive Pt-based catalysts. Recently, the transition metal and nitrogen co-doped carbon (M-N_x /C) materials with tailored morphology, tunable composition, and confined structure show great potential in both acidic and alkaline media. Herein, the mechanism of ORR is provided, followed by recent efforts to clarify the actual structures of active sites. Furthermore, the progress of optimizing the catalytic performance of M-N_x /C catalysts by modulating nitrogen-rich precursors and porous structure engineering is highlighted. The remaining challenges and development prospects of M-N_x /C catalysts are also outlined and evaluated.

Dual transition-metal atoms doping: an effective route to promote the ORR and OER activity on MoTe2

Compared to traditional single-atom doping method, the OER/ORR electrocatalytic activity of inert 2H-MoTe₂ can be efficiently triggered by properly dual TM atoms doping.

A "MOFs plus ZIFs" Strategy toward Ultrafine Co Nanodots Confined into Superficial N-Doped Carbon Nanowires for Efficient Oxygen Reduction

N-doped carbon-confined transition metal nanocatalysts display efficient oxygen reduction reaction (ORR) performance comparable to commercial Pt/C electrocatalysts because of their efficient charge transfer from metal atoms to active N sites. However, the sheathed active sites inside the electrocatalysts and relatively large-size confined metal particles greatly restrict their activity improvement. Here, we develop a facile and efficient "MOFs plus ZIFs" synthesis strategy to successfully construct ultrafine sub-5 nm Co nanodots confined into superficial N-doped carbon nanowires (Co@C@NC) via a well-designed synthesis process. The unique synthesis mechanism is based on low-pressure vapor superassembly of thin zeolitic imidazolate framework (ZIF) coatings on metal-organic framework substrates. During the successive pyrolysis, the preferential formation of the robust N-doped carbon shell from the ZIF-67 shell keeps the core morphology without shrinkage and limits the growth of Co nanodots. Benefiting from this architecture with accessible and rich active N sites on the surface, stable carbon confined architecture, and large surface area, the Co@C@NC exhibits excellent ORR performance, catching up to commercial Pt/C. Density functional theory demonstrates that the confined Co nanodots efficiently enhance the charge density of superficial active N sites by interfacial charge transfer, thus accelerating the ORR process.

Engineering Atomic Sites via Adjacent Dual-Metal Sub-Nanoclusters for Efficient Oxygen Reduction Reaction and Zn-Air Battery

N-coordinated transition-metal materials are crucial alternatives to design cost-effective, efficient, and highly durable catalysts for electrocatalytic oxygen reduction reaction. Herein, the synthesis of uniformly distributed Cu-Zn clusters on porous N-doped carbon, which are accompanied by Cu/Zn-N_x single sites, is demonstrated. X-ray absorption fine structure tests reveal the co-existence of M-N (M = Cu or Zn) and M-M bonds in the catalyst. The catalyst shows excellent oxygen reduction reaction (ORR) performance in an alkaline medium with a positive half-wave potential of 0.884 V, a superior kinetic current density of 36.42 mA cm^-2 at 0.85 V, and a Tafel slope of 45 mV dec^-1 , all of which are among the best-reported results. Furthermore, when employed as an air cathode in Zn-Air battery, it reveals a high open-cycle potential of 1.444 V and a peak power density of 164.3 mW cm^-2 . Comprehensive experiments and theoretical calculations approved that the high activity of the catalyst can be attributed to the collaboration of the Cu/Zn-N₄ sites with CuZn moieties on N-doped carbons.

Heterogeneous Atomic Catalysts Overcoming the Limitations of Single-Atom Catalysts

Recent advances in heterogeneous single-atom catalysts (SACs), which have isolated metal atoms dispersed on a support, have enabled a more precise control of their surface metal atomic structure. SACs could reduce the amount of metals used for the surface reaction and have often shown distinct selectivity, which the corresponding nanoparticles would not have. However, SACs typically have the limitations of low-metal content, poor stability, oxidic electronic states, and an absence of ensemble sites. In this review, various efforts to overcome these limitations have been discussed: The metal content in the SACs could increase up to over 10 wt %; highly durable SACs could be prepared by anchoring the metal atoms strongly on the defective support; metallic SACs are reported; and the ensemble catalysts, in which all the metal atoms are exposed at the surface like the SACs but the surface metal atoms are located nearby, are also reported. Metal atomic multimers with distinct catalytic properties have been also reported. Surface metal single-atoms could be decorated with organic ligands with interesting catalytic behavior. Heterogeneous atomic catalysts, whose structure is elaborately controlled and the surface reaction is better understood, can be a paradigm with higher catalytic activity, selectivity, and durability and used in industrial applications.

MOF-Derived 2D/3D Hierarchical N-Doped Graphene as Support for Advanced Pt Utilization in Ethanol Fuel Cell

Development of bifunctional catalysts with low platinum (Pt) content for the ethanol oxidation reaction (EOR) and the oxygen reduction reaction (ORR) is highly desirable, yet challenging. Herein, we present structural engineering of a series of two-dimensional/three-dimensional (2D/3D) hierarchical N-doped graphene-supported nanosized Pt₃Co alloys and Co clusters (PtCo@N-GNSs) via a hydrolysis-pyrolysis route. For the ORR, the optimal PtCo@N-GNS exhibits a high mass activity of 3.01 A mg_Pt^-1, which is comparable to the best Pt-based catalyst obtained through sophisticated synthesis. It also possesses excellent stability with minor decay after 50 000 cyclic voltammograms (CV) cycles in acidic medium. For the EOR, PtCo@N-GNS achieves the highest mass-specific and area-specific activities of 1.96 A mg_Pt^-1 and 5.75 mA cm^-2, respectively, among all of the reported EOR catalysts to date. The unique 2D/3D hierarchy, high Pt utilization, and valid encapsulation of nanosized Pt₃Co/Co synergistically contribute to the robust ORR and EOR activities of the present PtCo@N-GNS. A direct ethanol fuel cell based on PtCo@N-GNS delivers a high open-circuit potential of 0.9 V, a stable power density of 10.5 mW cm^-2, and an excellent rate performance, implying the feasibility of the bifunctional PtCo@N-GNS. This work offers a new strategy for designing an ultralow Pt loading yet highly active and durable catalyst for ethanol fuel cell application.

Rational prediction of multifunctional bilayer single atom catalysts for the hydrogen evolution, oxygen evolution and oxygen reduction reactions

Bimetallic atom catalysts (BACs), which can exhibit remarkable catalytic performance compared with single atom catalysts (SACs) due to their higher metal loading and the synergy between two metal atoms, have attracted great attention in research. Herein, by means of density functional theory calculations, novel BACs with a bilayer structure composed of monolayers FeN4 (Fe and nitrogen co-doped graphene) and MN4 (Fe/M, M represents transition metal atoms) as electrocatalysts for the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER) are investigated. Among these bilayer SACs, a series of highly efficient monofunctional, bifunctional, and even trifunctional electrocatalysts have been screened. For example, the overpotentials for the HER, ORR, and OER can reach -0.02 (Fe/Cu), 0.31 (Fe/Hg), and 0.27 V (Fe/Hf), respectively; Fe/Hf and Ir/Fe can serve as promising bifunctional catalysts for the ORR/OER and HER/OER, respectively and Fe/Rh is considered as an excellent trifunctional catalyst for the HER, OER, and ORR. This work not only provides a new idea for understanding and optimizing the active sites of BACs, but also proposes a new strategy for designing high-performance multifunctional electrocatalysts for fuel cells and metal-air batteries.

Metal-organic framework based bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries: current progress and prospects

Zinc-air batteries (ZABs) are regarded as ideal candidates for next-generation energy storage equipment due to their high energy density, non-toxicity, high safety, and environmental friendliness. However, the slow oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics on the air cathode limit their efficiency and the development of highly efficient, low cost and stable bifunctional electrocatalysts is still challenging. Metal-Organic Framework (MOF) based bifunctional oxygen electrocatalysts have been demonstrated as promising alternative catalysts due to the regular structure, tunable chemistry, high specific surface area, and simple and easy preparation of MOFs, and great progress has been made in this area. Herein, we summarize the latest research progress of MOF-based bifunctional oxygen electrocatalysts for ZABs, including pristine MOFs, derivatives of MOFs and MOF composites. The effects of the catalysts' composites, morphologies, specific surface areas and active sites on catalytic performances are specifically addressed to reveal the underlying mechanisms for different catalytic activity of MOF based catalysts. Finally, the main challenges and prospects for developing advanced MOF-based bifunctional electrocatalysts are proposed.

High-power lithium-selenium batteries enabled by atomic cobalt electrocatalyst in hollow carbon cathode

Selenium cathodes have attracted considerable attention due to high electronic conductivity and volumetric capacity comparable to sulphur cathodes. However, practical development of lithium-selenium batteries has been hindered by the low selenium reaction activity with lithium, high volume changes and rapid capacity fading caused by the shuttle effect of polyselenides. Recently, single atom catalysts have attracted extensive interests in electrochemical energy conversion and storage because of unique electronic and structural properties, maximum atom-utilization efficiency, and outstanding catalytic performances. In this work, we developed a facile route to synthesize cobalt single atoms/nitrogen-doped hollow porous carbon (Co_SA-HC). The cobalt single atoms can activate selenium reactivity and immobilize selenium and polyselenides. The as-prepared selenium-carbon (Se@Co_SA-HC) cathodes deliver a high discharge capacity, a superior rate capability, and excellent cycling stability with a Coulombic efficiency of ~100%. This work could open an avenue for achieving long cycle life and high-power lithium-selenium batteries.

Lithium selenium batteries are attractive energy storage systems, but they are hindered by low selenium reaction activity and rapid capacity fading. Herein, the authors report a selenium host with atomic cobalt electrocatalyst which exhibits superior performances in lithium-selenium batteries.

Nonprecious Bimetallic Sites Coordinated on N-Doped Carbons with Efficient and Durable Catalytic Activity for Oxygen Reduction

Developing efficient, inexpensive, and durable electrocatalysts for the oxygen reduction reaction (ORR) is important for the large-scale commercialization of fuel cells and metal-air batteries. Herein, a hierarchically porous bimetallic Fe/Co single-atom-coordinated N-doped carbon (Fe/Co-N_x -C) electrocatalyst for ORR is synthesized from Fe/Co-coordinated polyporphyrin using silica template-assisted and silica-protection synthetic strategies. In the synthesis, first silica nanoparticles-embedded, silica-protected Fe/Co-polyporphyrin is prepared. It is then pyrolyzed and treated with acidic solution. The resulting Fe/Co-N_x -C material has a large specific surface area, large electrochemically active surface area, good conductivity, and catalytically active Fe/Co-N_x sites. The material exhibits a very good electrocatalytic activity for the ORR in alkaline media, with a half-wave potential of 0.86 V versus reversible hydrogen electrode, which is better than that of Pt/C (20 wt%). Furthermore, it shows an outstanding operational stability and durability during the reaction. A zinc-air battery (ZAB) assembled using Fe/Co-N_x -C as an air-cathode electrocatalyst gives a high peak power density (152.0 mW cm^-2 ) and shows a good recovery property. Furthermore, the performance of the battery is better than a corresponding ZAB containing Pt/C as an electrocatalyst. The work also demonstrates a synthetic route to a highly active, stable, and scalable single-atom electrocatalyst for ORR in ZABs.

Confined Fe-Cu Clusters as Sub-Nanometer Reactors for Efficiently Regulating the Electrochemical Nitrogen Reduction Reaction

Electrochemical nitrogen reduction reaction (NRR) over nonprecious-metal and single-atom catalysts has received increasing attention as a sustainable strategy to synthesize ammonia. However, the atomic-scale regulation of such active sites for NRR catalysis remains challenging because of the large distance between them, which significantly weakens their cooperation. Herein, the utilization of regular surface cavities with unique microenvironment on graphitic carbon nitride as "subnano reactors" to precisely confine multiple Fe and Cu atoms for NRR electrocatalysis is reported. The synergy of Fe and Cu atoms in such confined subnano space provides significantly enhanced NRR performance, with nearly doubles ammonia yield and 54%-increased Faradic efficiency up to 34%, comparing with the single-metal counterparts. First principle simulation reveals this synergistic effect originates from the unique Fe-Cu coordination, which effectively modifies the N₂ absorption, improves electron transfer, and offers extra redox couples for NRR. This work thus provides new strategies of manipulating catalysts active centers at the sub-nanometer scale.

Coexisting Single-Atomic Fe and Ni Sites on Hierarchically Ordered Porous Carbon as a Highly Efficient ORR Electrocatalyst

The development of oxygen reduction reaction (ORR) electrocatalysts based on earth-abundant nonprecious materials is critically important for sustainable large-scale applications of fuel cells and metal-air batteries. Herein, a hetero-single-atom (h-SA) ORR electrocatalyst is presented, which has atomically dispersed Fe and Ni coanchored to a microsized nitrogen-doped graphitic carbon support with unique trimodal-porous structure configured by highly ordered macropores interconnected through mesopores. Extended X-ray absorption fine structure spectra confirm that Fe- and Ni-SAs are affixed to the carbon support via FeN₄ and NiN₄ coordination bonds. The resultant Fe/Ni h-SA electrocatalyst exhibits an outstanding ORR activity, outperforming SA electrocatalysts with only Fe- or Ni-SAs, and the benchmark Pt/C. The obtained experimental results indicate that the achieved outstanding ORR performance results from the synergetic enhancement induced by the coexisting FeN₄ and NiN₄ sites, and the superior mass-transfer capability promoted by the trimodal-porous-structured carbon support.

Single Ni atoms with higher positive charges induced by hydroxyls for electrocatalytic CO2 reduction

To promote the faradaic efficiency of the electrocatalytic CO2 reduction reaction (CO2RR) with low-cost catalysts, single Ni atoms with higher positive charges induced by hydroxyls were proposed to form an atomically dispersed Ni-N4 structure in a cheap honeycomb-like carbon matrix for electrocatalytic CO2 reduction. Extended X-ray absorption fine structure spectroscopy, aberration-corrected High-angle annular dark-field scanning transmission electron microscopy and X-ray photoelectron spectroscopy measurements confirmed that the active-center structure consists of single Ni atoms and the adjacent hydroxyl via hydrothermal treatment (H-Ni/NC). Density functional theory calculations indicated that the isolated Ni atoms with higher positive charges induced by the hydroxyl decreased the free energy of the rate-limiting step to 1.05 eV for the CO2RR. The faradaic efficiency (FE) of CO2 reduction into CO was ≥88.0% over the H-Ni/NC catalyst in the potential range of -0.5 to -0.9 V (vs. RHE). The peak CO FE reached 97% at -0.7 V due to the synergistic effect between the unsaturated Ni-N4 active sites and the hydroxyl species.

Recent Advances in Single-Atom Electrocatalysts for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) plays significant roles in electrochemical energy storage and conversion systems as well as clean synthesis of fine chemicals. However, the ORR process shows sluggish kinetics and requires platinum-group noble metal catalysts to accelerate the reaction. The high cost, rare reservation, and unsatisfied durability significantly impede large-scale commercialization of platinum-based catalysts. Single-atom electrocatalysts (SAECs) featuring with well-defined structure, high intrinsic activity, and maximum atom efficiency have emerged as a novel field in electrocatalytic science since it is promising to substitute expensive platinum-group noble metal catalysts. However, finely fabricating SAECs with uniform and highly dense active sites, fully maximizing the utilization efficiency of active sites, and maintaining the atomically isolated sites as single-atom centers under harsh electrocatalytic conditions remain urgent challenges. In this review, we summarized recent advances of SAECs in synthesis, characterization, oxygen reduction reaction (ORR) performance, and applications in ORR-related H₂O₂ production, metal-air batteries, and low-temperature fuel cells. Relevant progress on tailoring the coordination structure of isolated metal centers by doping other metals or ligands, enriching the concentration of single-atom sites by increasing metal loadings, and engineering the porosity and electronic structure of the support by optimizing the mass and electron transport are also reviewed. Moreover, general strategies to synthesize SAECs with high metal loadings on practical scale are highlighted, the deep learning algorithm for rational design of SAECs is introduced, and theoretical understanding of active-site structures of SAECs is discussed as well. Perspectives on future directions and remaining challenges of SAECs are presented.

Ternary Sn-Ti-O Electrocatalyst Boosts the Stability and Energy Efficiency of CO2 Reduction

Simultaneously improving energy efficiency (EE) and material stability in electrochemical CO₂ conversion remains an unsolved challenge. Among a series of ternary Sn-Ti-O electrocatalysts, 3D ordered mesoporous (3DOM) Sn_0.3 Ti_0.7 O₂ achieves a trade-off between active-site exposure and structural stability, demonstrating up to 71.5 % half-cell EE over 200 hours, and a 94.5 % Faradaic efficiency for CO at an overpotential as low as 430 mV. DFT and X-ray absorption fine structure analyses reveal an electron density reconfiguration in the Sn-Ti-O system. A downshift of the orbital band center of Sn and a charge depletion of Ti collectively facilitate the dissociative adsorption of the desired intermediate COOH* for CO formation. It is also beneficial in maintaining a local alkaline environment to suppress H₂ and formate formation, and in stabilizing oxygen atoms to prolong durability. These findings provide a new strategy in materials design for efficient CO₂ conversion and beyond.

Nitrogen-rich graphitic-carbon@graphene as a metal-free electrocatalyst for oxygen reduction reaction

The metal-free nitrogen-doped graphitic-carbon@graphene (Ng-C@G) is prepared from a composite of polyaniline and graphene by a facile polymerization following by pyrolysis for electrochemical oxygen reduction reaction (ORR). Pyrolysis creates a sponge-like with ant-cave-architecture in the polyaniline derived nitrogenous graphitic-carbon on graphene. The nitrogenous carbon is highly graphitized and most of the nitrogen atoms are in graphitic and pyridinic forms with less oxygenated is found when pyrolyzed at 800 °C. The electrocatalytic activity of Ng-C@G-800 is even better than the benchmarked Pt/C catalyst resulting in the higher half-wave potential (8 mV) and limiting current density (0.74 mA cm^-2) for ORR in alkaline medium. Higher catalytic performance is originated from the special porous structure at microscale level and the abundant graphitic- and pyridinic-N active sites at the nanoscale level on carbon-graphene matrix which are beneficial to the high O₂-mass transportation to those accessible sites. Also, it possesses a higher cycle stability resulting in the negligible potential shift and slight oxidation of pyridinic-N with better tolerance to the methanol.

Mass Transfer in a Co/N/C Catalyst Layer for the Anion Exchange Membrane Fuel Cell

Developing highly efficient non-noble metal catalysts for the cathode of fuel cells is an urgent requirement for reducing the cost. Although the intrinsic activity of non-noble metal materials has been greatly improved, the fuel cell performance is also determined by the mass transfer within the catalyst layer (CL), particularly at high current density. Electrochemical impedance spectroscopy (EIS) combined with rotating disk electrode (RDE) analysis is a powerful tool to quantitatively analyze the influence of the structural properties on CL performance. Here, Co/N/C CLs with gradient pore structures are constructed based on the controllable synthesis of zeolitic imidazolate framework (ZIF)-derived catalyst. The influences of the carbon support, active site, and catalyst loading are comprehensively studied by EIS in different regions (kinetic and mixed-diffusion). The results indicate that a high micro-/mesopore ratio is beneficial to increasing the density of active sites while reducing the mass-transfer efficiency. Inversely, abundant mesopores promote mass transfer, but they result in low active site density. By carefully adjusting the pore structure and chemical composition of the ZIF-derived catalyst, the Co/N/C CL shows a low mass-transfer resistance (95.5 Ω at 0.75 V vs RHE). This work demonstrates the importance of mass transfer within the fuel cell CL, beyond seeking only high activity.

Improving the Catalytic Activity of Carbon-Supported Single Atom Catalysts by Polynary Metal or Heteroatom Doping

Single atom catalysts (SACs) are widely researched in various chemical transformations due to the high atomic utilization and catalytic activity. Carbon-supported SACs are the largest class because of the many excellent properties of carbon derivatives. The single metal atoms are usually immobilized by doped N atoms and in some cases by C geometrical defects on carbon materials. To explore the catalytic mechanisms and improve the catalytic performance, many efforts have been devoted to modulating the electronic structure of metal single atomic sites. Doping with polynary metals and heteroatoms has been recently proposed to be a simple and effective strategy, derived from the modulating mechanisms of metal alloy structure for metal catalysts and from the donating/withdrawing heteroatom doping for carbon supports, respectively. Polynary metals SACs involve two types of metal with atomical dispersion. The bimetal atom pairs act as dual catalytic sites leading to higher catalytic activity and selectivity. Polynary heteroatoms generally have two types of heteroatoms in which N always couples with another heteroatom, including B, S, P, etc. In this Review, the recent progress of polynary metals and heteroatoms SACs is summarized. Finally, the barriers to tune the activity/selectivity of SACs are discussed and further perspectives presented.

In-situ X-ray techniques for non-noble electrocatalysts

Electrocatalysis offers an alternative solution for the energy crisis because it lowers the activation energy of reaction to produce economic fuels more accessible. Non-noble electrocatalysts have shown their capabilities to practical catalytic applications as compared to noble ones, whose scarcity and high price limit the development. However, the puzzling catalytic processes in non-noble electrocatalysts hinder their advancement. In-situ techniques allow us to unveil the mystery of electrocatalysis and boost the catalytic performances. Recently, various in-situ X-ray techniques have been rapidly developed, so that the whole picture of electrocatalysis becomes clear and explicit. In this review, the in-situ X-ray techniques exploring the structural evolution and chemical-state variation during electrocatalysis are summarized for mainly oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and carbon dioxide reduction reaction (CO2RR). These approaches include X-ray Absorption Spectroscopy (XAS), X-ray diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS). The information seized from these in-situ X-ray techniques can effectively decipher the electrocatalysis and thus provide promising strategies for advancing the electrocatalysts. It is expected that this review could be conducive to understanding these in-situ X-ray approaches and, accordingly, the catalytic mechanism to better the electrocatalysis.

Strong metal-support interaction promoted scalable production of thermally stable single-atom catalysts

Single-atom catalysts (SACs) have demonstrated superior catalytic performance in numerous heterogeneous reactions. However, producing thermally stable SACs, especially in a simple and scalable way, remains a formidable challenge. Here, we report the synthesis of Ru SACs from commercial RuO2 powders by physical mixing of sub-micron RuO2 aggregates with a MgAl1.2Fe0.8O4 spinel. Atomically dispersed Ru is confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption spectroscopy. Detailed studies reveal that the dispersion process does not arise from a gas atom trapping mechanism, but rather from anti-Ostwald ripening promoted by a strong covalent metal-support interaction. This synthetic strategy is simple and amenable to the large-scale manufacture of thermally stable SACs for industrial applications.

Revealing of Active Sites and Catalytic Mechanism in N-Coordinated Fe, Ni Dual-Doped Carbon with Superior Acidic Oxygen Reduction than Single-Atom Catalyst

Herein, we synthesized a Fe, Ni dual-metal embedded in porous nitrogen-doped carbon material to endow higher turnover frequency (TOF), lower H₂O₂ yield, and thus superior durability than for the single-atom catalyst for oxygen reduction in acid media. Quantitative X-ray absorption near edge structure (XANES) fitting and density functional theory (DFT) calculation were implemented to explore the active sites in the catalysts. The results suggest FeNi-N₆ with type I (each metal atom coordinated with four nitrogen atoms) instead of type II configuration (each metal atom coordinated with three nitrogen atoms) dominates the catalytic activity of the noble-metal free catalyst (NMFC). Further, theoretical calculation reveals that the oxygen reduction reaction (ORR) activity trend of different moieties was FeNi-N₆ (type I) > FeNi-N₆ (type II) > Fe-N₄ > Fe₂-N₆. Our research represents an important step for developing dual-metal doping NMFC for proton exchange membrane fuel cells (PEMFCs) by revealing its new structural configuration and correlation with catalytic activity.