Ammonia Tolerance of Atomically Dispersed Single Metal Site Catalysts: Mechanistic Understanding and High-Performance Oxygen Reduction Electrocatalysis

The anion-exchange membrane direct ammonia fuel cell, as a carbon-free fuel cell type, has recently received increasing attention albeit suffering from high cost of using the platinum-group metal oxygen reduction reaction (ORR) catalysts. To pave the development of this promising power source, the atomically dispersed transition metal-nitrogen-carbon (M-N-C) materials with low cost and high ORR performance have allured to investigate their ammonia tolerance during the ORR. Herein, it is initially deconvoluted how compositional and structural elements of FeN₄ sites modulate catalyst's performance. Furthermore, ORR catalytic activities of the M-N-C (M = Fe, Co or Mn) and Pt/C catalysts are investigated in ammonia-containing electrolytes, showing that M-N-C catalysts have better ammonia tolerance than Pt/C. Among others, the Fe-N-C exhibits the best ammonia tolerance with only 4 mV negative shifts of half-wave potential, 2.7% decrease of current, and negligibly irreversible activity loss. The superior ammonia tolerance of MN₄ sites to Pt (111) surface is further confirmed by density functional theory calculations. The adsorption capacity of MN₄ for O₂ is higher than NH₃ and the bonding force between MN₄ and O₂ is stronger than NH₃, whereas opposite adsorption capacity and bonding force trends are observed on Pt (111) surface.     

Identification and Understanding of Active Sites of Non-Noble Iron-Nitrogen-Carbon Catalysts for Oxygen Reduction Electrocatalysis

Non-noble iron-nitrogen-carbon (Fe-N-C) catalysts have been explored as one type of the most promising alternatives of precious platinum (Pt) in catalyzing the oxygen reduction reaction (ORR). However, their catalytic ORR activity and stability still cannot meet the requirement of practical applications. Active sites in such catalysts are the key factors determining the catalytic performance. This review gives a critical overview on identification and understanding of active sties of non-pyrolytic and pyrolytic Fe-N-C catalysts in terms of design strategies, synthesis, characterization, functional mechanisms and performance validation. The diversity and complexity of active sites that greatly dominate the progress of Fe-N-C catalysts include Fe-containing sites (Fe-based nanoparticles and single-atom Fe-species) and metal-free sites (heteroatoms doping and defects). Meanwhile, synergistic effects are also discussed in this review with emphasis on the interaction among multiple active sites. Although substantial endeavors have been devoted to develop the efficient Fe-N-C catalysts, some challenges still remain. To facilitate further research on Fe-N-C catalysts toward practical applications, some research perspectives are prospected in the aspects of innovative synthesis methods, active-sites modulation strategies, high-resolution ex situ/in situ/operando characterization techniques, theoretical calculations, and so on. This review may provide a guideline for identifying and understanding active-sites for developing high-performance Fe-N-C catalysts.     

Coordination Shell Dependent Activity of CuCo Diatomic Catalysts for Oxygen Reduction,Oxygen Evolution,and Hydrogen Evolution Reaction

Challenges in rational designing dual-atom catalysts (DACs) give a strong motivation to construct coordination-activity correlations. Here, thorough coordination-activity correlations of DACs based on how the changes in coordination shells (CSs) of dual-atom Cu,Co centers influence their electrocatalytic activity in oxygen reduction reaction(ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is constructed. First, Cu,Co DACs with different CSs modifications are fabricated by using a controlled “precursors-preselection” approach. Three DACs with unique coordination environments are characterized as secondary S atoms that directly bond to Cu,Co-N₆ in lower CSs, indirectly bond in neighboring CSs, and are doped in higher CSs, respectively. Then, experimentally and theoretically, a coordination correlation resembling a planet-satellite system, where satellite coordinated atoms (heteroatom N, S) surround Cu-Co dual-atom entity in various orbitals CSs. By evaluating electrocatalytic activity indicators, differences are identified in electronic structure and electrocatalytic performance of Cu and Co centers in ORR, OER, and HER. Interestingly, initial CSs modifications for DACs may not always be advantageous for electrocatalysis. This work offers valuable insight for designing DACs for diverse applications.     

A Versatile Approach to Boost Oxygen Reduction of Fe-N4 Sites by Controllably Incorporating Sulfur Functionality

Although atomically dispersed Fe-N₄ on carbon materials (Fe-NC) have enormous potential for the oxygen reduction reaction (ORR), precise control over the electronic structure of Fe to enhance the catalytic performance and a full understanding of the catalytic mechanism remain elusive. Herein, a novel approach is designed to boost the kinetic activity of single Fe-N₄ centers by controlling S-doped content and species (namely, thiophene-like S and oxidized S). Due to confinement and catalysis effects, the innovative strategy of combining a Mg(OH)₂ template with KOH activation preferentially generates oxidized S and simultaneously constructs porous carbon with a high Fe loading (2.93 wt%) and hierarchical pores. Theoretical calculations suggest that neighboring S functionalities can affect the electronic configurations of Fe-N₄ sites and increase the electron density around Fe atoms, thereby optimizing the adsorption energy of intermediates and substantially accelerating reaction kinetics, following the trend: oxidized S doped > thiophene-like S doped > pristine Fe-N₄. Benefiting from high activity and accessibility of Fe-N₄ sites, the optimal FeNC-SN-2 electrode displays impressive ORR activity with large power density while maintaining outstanding durability in Zn-air batteries and microbial fuel cells. The work paves the way to prepare stable single-atom metal-N_x sites with heteroatom-doping for diverse high-performance applications.     

Zn Single Atom Catalyst for Highly Efficient Oxygen Reduction Reaction

The authors report first a new type of nitrogen‐triggered Zn single atom catalyst, demonstrating high catalytic activity and remarkable durability for the oxygen reduction reaction process. Both X‐ray absorption fine structure spectra and theoretical calculations suggest that the atomically dispersed Zn‐N₄ site is the main, as well as the most active, component with O adsorption as the rate‐limiting step at a low overpotential of 1.70 V. This work opens a new field for the exploration of high‐performance Pt‐free electrochemical oxygen reduction catalysts for fuel cells.     

Relay Catalysis of Multi-Sites Promotes Oxygen Reduction Reaction

The two-electron pathway to form hydrogen peroxide (H₂O₂) is undesirable for the oxygen reduction reaction (ORR) in iron and nitrogen doped carbon (Fe–N–C) material as it not only lowers the catalytic efficiency but also impairs the catalyst durability. In this study, a relay catalysis pathway is designed to minimize the two-electron selectivity of Fe–N–C catalyst. Such a design is achieved by introducing two other sites, that is, MnN₄ site and α-Fe(110) face. A combination of transmission electron microscopy image and X-ray absorption spectra verify the three site formation. Electrochemical test coupled with post-treatment confirm the improvement of MnN₄ site and α-Fe(110) face on catalyst performance. Theoretical calculation proposes a relay catalysis pathway of three sites, that is, H₂O₂ released from the FeN₄ site migrates to the MnN₄ site or α-Fe(110) face, on which the captive H₂O₂ is further reduced to H₂O. The relay catalysis pathway positioned the as-prepared catalyst among the best ORR catalysts in both aqueous electrode and alkaline direct methanol fuel cell test. This study examples an interesting relay catalysis pathway of multi-sites for the ORR, which offers insights into the design of efficient electrocatalysts for fuel cells or beyond.     

Dual 3D Ceramic Textile Electrodes: Fast Kinetics for Carbon Oxidation Reaction and Oxygen Reduction Reaction in Direct Carbon Fuel Cells at Reduced Temperatures

Direct carbon fuel cells (DCFCs) are an efficient energy‐conversion technology capable of generating electricity with carbon‐dioxide‐capture chemistry with solid carbon as fuels. The efficiency and performance of DCFCs depend on the kinetics of the carbon oxidation reactions (COR) and the oxygen reduction reactions (ORR), each occurring at anode and cathode, respectively. The limited active sites paired with reduced temperatures greatly decrease the efficiency of the electrochemical reactions. Ultraporous dual‐3D ceramic textiles (dual‐3DCT) are integrated into electrolyte‐supported DCFCs to enhance charge and mass transfer at the electrodes. Improved COR at the anode is achieved by the synergy between the 3DCT NiO–Ce_0.8Gd_0.2O_1.95 (GDC) structure and optimal carbon fuel choice. In a comparative study, DCFCs using graphitic carbon (GC) as fuel show the best COR performance when compared to DCFCs utilizing alternative fuels such as carbon black (CB) and activated carbon (AC). The 3DCT Sm_0.5Sr_0.5CoO_3‐δ–GDC (SSC–GDC) composite cathode shows electrochemical performance superior to that of the conventional screen‐printed SSC–GDC. A peak power density of 392 mW cm^?2 at 600 °C is obtained in a DCFC using the 3DCT‐anode/electrolyte/3DCT‐cathode configuration, an unprecedented value for any reported DCFC as of yet. This points toward promising applications of dual‐3DCT electrodes for reduced‐temperature DCFCs.     

Understanding the Synergistic Effects of Cobalt Single Atoms and Small Nanoparticles: Enhancing Oxygen Reduction Reaction Catalytic Activity and Stability for Zinc-Air Batteries

The development of earth-abundant oxygen reduction reaction (ORR) catalysts with high catalytic activity and good stability for practical metal-air batteries remains an enormous challenge. Herein, a highly efficient and durable ORR catalyst is reported, which consists of atomically dispersed Co single atoms (Co-SAs) in the form of Co-N4 moieties and small Co nanoparticles (Co-SNPs) co-anchored on nitrogen-doped porous carbon nanocage (Co-SAs/SNPs@NC). Benefiting from the synergistic effect of Co-SAs and Co-SNPs as well as the enhanced anticorrosion capability of the carbon matrix brought by its improved graphitization degree, the resultant Co-SAs/SNPs@NC catalyst exhibits outstanding ORR activity and remarkable stability in alkaline media, outperforming Co-SAs-based catalyst (Co-SAs@NC), and benchmark Pt/C catalyst. Density functional theory calculations reveal that the strong interaction between Co-SNPs and Co-N4 sites can increase the valence state of the active Co atoms in Co-SAs/SNPs@NC and moderate the adsorption free energy of ORR intermediates, thus facilitating the reduction of O₂. Moreover, the practical zinc-air battery assembled with Co-SAs/SNPs@NC catalyst demonstrates a maximum power density of 223.5 mW cm^–2, a high specific capacity of 742 W h kg^–1 at 50 mA cm^–2 and a superior cycling stability.     

Mesostructured Intermetallic Compounds of Platinum and Non‐Transition Metals for Enhanced Electrocatalysis of Oxygen Reduction Reaction

Alloying techniques show genuine potential to develop more effective catalysts than Pt for oxygen reduction reaction (ORR), which is the key challenge in many important electrochemical energy conversion and storage devices, such as fuel cells and metal‐air batteries. Tremendous efforts have been made to improve ORR activity by designing bimetallic nanocatalysts, which have been limited to only alloys of platinum and transition metals (TMs). The Pt‐TM alloys suffer from critical durability in acid‐media fuel cells. Here a new class of mesostructured Pt–Al catalysts is reported, consisting of atomic‐layer‐thick Pt skin and Pt₃Al or Pt₅Al intermetallic compound skeletons for the enhanced ORR performance. As a result of strong Pt–Al bonds that inhibit the evolution of Pt skin and produce ligand and compressive strain effects, the Pt₃Al and Pt₅Al mesoporous catalysts are exceptionally durable and ≈6.3‐ and ≈5.0‐fold more active than the state‐of‐the‐art Pt/C catalyst at 0.90 V, respectively. The high performance makes them promising candidates as cathode nanocatalysts in next‐generation fuel cells.     

Tunable Decoration of Reduced Graphene Oxide with Au Nanoparticles for the Oxygen Reduction Reaction

Reduced graphene oxide (rGO) films are decorated with non‐overlapping Au nanoparticles using diblock copolymer micelles that provide controllability over the number density as well as the diameter of the nanoparticles. This synthetic process produces a pure Au surface without extra layers. Further­more, the rGO film enables the transferability of the Au nanoparticles without deterioration of their arrays. Thus, the controllability of the Au nanoparticles and their transferability with rGO films allow the effective modification of electrochemical electrodes. With a glassy carbon electrode modified with an rGO film with Au nanoparticles, high electrochemical activity is observed in the oxygen reduction reaction (ORR). Furthermore, it is possible to identify a size‐dependent ORR mechanism, showing that Au nanoparticles with an average diameter of 8.6 nm exhibit a 4‐electron direct reduction of O₂ to H₂O.     

Hybrid of Iron Nitride and Nitrogen‐Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction

It is extremely desirable but challenging to create highly active, stable, and low‐cost catalysts towards oxygen reduction reaction to replace Pt‐based catalysts in order to perform the commercialization of fuel cells. Here, a novel iron nitride/nitrogen doped‐graphene aerogel hybrid, synthesized by a facile two‐step hydrothermal process, in which iron phthalocyanine is uniformly dispersed and anchored on graphene surface with the assist of π–π stacking and oxygen‐containing functional groups, is reported. As a result, there exist strong interactions between Fe _x N nanoparticles and graphene substrates, leading to a synergistic effect towards oxygen reduction reaction. It is worth noting that the onset potential and current density of the hybrid are significantly better and the charge transfer resistance is much lower than that of pure nitrogen‐doped graphene aerogel, free Fe _x N and their physical mixtures. The hybrid also exhibits comparable catalytic activity as commercial Pt/C at the same catalyst loading, while its stability and resistance to methanol crossover are superior. Interestingly, it is found that, apart from the active nature of the hybrid, the large surface area and porosity are responsible for its excellent onset potential and the high density of Fe–N–C sties and small size of Fe _x N particles boost charge transfer rate.     

Structurally Ordered Low‐Pt Intermetallic Electrocatalysts toward Durably High Oxygen Reduction Reaction Activity

Carbon‐supported low‐Pt ordered intermetallic nanoparticulate catalysts (PtM₃, M = Fe, Co, and Ni) are explored in order to enhance the oxygen reduction reaction (ORR) activity while achieving a high stability compared to previously reported Pt‐richer ordered intermetallics (Pt₃M and PtM) and low‐Pt disordered alloy catalysts. Upon high‐temperature thermal annealing, ordered PtCo₃ intermetallic nanoparticles are successfully prepared with minimum particle sintering. In contrast, the PtFe₃ catalyst, despite the formation of ordered structure, suffers from obvious particle sintering and detrimental metal–support interaction, while the PtNi₃ catalyst shows no structural ordering transition at all but significant particle sintering. The ordered PtCo₃ catalyst exhibits durably thin Pt shells with a uniform thickness below 0.6 nm (corresponding to 2–3 Pt atomic layers) and a high Co content inside the nanoparticles after 10 000 potential cycling, leading to a durably compressive Pt surface and thereby both high activity (fivefold vs a commercial Pt catalyst and 1.7‐fold vs an ordered PtCo intermetallic catalyst) and high durability (5 mV loss in half‐wave potential and 9% drop in mass activity). These results provide a new strategy toward highly active and durable ORR electrocatalysts by rational development of low‐Pt ordered intermetallics.     

Modulating Electronic Structure of Atomically Dispersed Nickel Sites through Boron and Nitrogen Dual Coordination Boosts Oxygen Reduction

Atomically dispersed 3D transitional metal active sites with nitrogen coordination anchored on carbon support have emerged as a kind of promising electrocatalyst toward oxygen reduction reaction (ORR) in the field of fuel cells and metal–air cells. However, it is still a challenge to accurately modulate the coordination structure of single-atom metal sites, especially first-shell coordination, as well as identify the relationship between the geometric/electronic structure and ORR performance. Herein, a carbon-supported single-atom nickel catalyst is fabricated with boron and nitrogen dual coordination (denoted as Ni-B/N-C). The hard X-ray absorption spectrum result reveals that atomically dispersed Ni active sites are coordinated with one B atom and three N atoms in the first shell (denoted as Ni-B₁N₃). The Ni-B/N-C catalyst exhibits a half-wave potential (E_1/2) of 0.87 V versus RHE, along with a distinguished long-term durability in alkaline media, which is superior to commercial Pt/C. Density functional theory calculations indicate that the Ni-B₁N₃ active sites are more favorable for the adsorption of ORR intermediates relative to Ni-N₄, leading to the reduction of thermodynamic barrier and the acceleration of reaction kinetics, which accounts for the increased intrinsic activity.     

Iron–Nitrogen‐Doped Vertically Aligned Carbon Nanotube Electrocatalyst for the Oxygen Reduction Reaction

A highly active iron–nitrogen‐doped carbon nanotube catalyst for the oxygen reduction reaction (ORR) is produced by employing vertically aligned carbon nanotubes (VA‐CNT) with a high specific surface area and iron(II) phthalocyanine (FePc) molecules. Pyrolyzing the composite easily transforms the adsorbed FePc molecules into a large number of iron coordinated nitrogen functionalized nanographene (Fe–N–C) structures, which serve as ORR active sites on the individual VA‐CNT surfaces. The catalyst exhibits a high ORR activity, with onset and half‐wave potentials of 0.97 and 0.79 V, respectively, versus reversible hydrogen electrode, a high selectivity of above 3.92 electron transfer number, and a high electrochemical durability, with a 17 mV negative shift of E _1/2 after 10 000 cycles in an oxygen‐saturated 0.5 m H₂SO₄ solution. The catalyst demonstrates one of the highest ORR performances in previously reported any‐nanotube‐based catalysts in acid media. The excellent ORR performance can be attributed to the formation of a greater number of catalytically active Fe–N–C centers and their dense immobilization on individual tubes, in addition to more efficient mass transport due to the mesoporous nature of the VA‐CNTs.     

Anchoring Single Copper Atoms to Microporous Carbon Spheres as High-Performance Electrocatalyst for Oxygen Reduction Reaction

Although the carbon-supported single-atom (SA) electrocatalysts (SAECs) have emerged as a new form of highly efficient oxygen reduction reaction (ORR) electrocatalysts, the preferable sites of carbon support for anchoring SAs are somewhat elusive. Here, a KOH activation approach is reported to create abundant defects/vacancies on the porous graphitic carbon nanosphere (CNS) with selective adsorption capability toward transition-metal (TM) ions and innovatively utilize the created defects/vacancies to controllably anchor TM–SAs on the activated CNS via TM N_x coordination bonds. The synthesized TM-based SAECs (TM-SAs@N-CNS, TM: Cu, Fe, Co, and Ni) possess superior ORR electrocatalytic activities. The Cu-SAs@N-CNS demonstrates excellent ORR and oxygen evolution reaction (OER) bifunctional electrocatalytic activities and is successfully applied as a highly efficient air cathode material for the Zn–air battery. Importantly, it is proposed and validated that the N-terminated vacancies on graphitic carbons are the preferable sites to anchor Cu-SAs via a Cu (N C₂)₃(N C) coordination configuration with an excellent promotional effect toward ORR. This synthetic approach exemplifies the expediency of suitable defects/vacancies creation for the fabrication of high-performance TM-based SAECs, which can be implemented for the synthesis of other carbon-supported SAECs.     

Opportunities and Challenges in Precise Synthesis of Transition Metal Single-Atom Supported by 2D Materials as Catalysts toward Oxygen Reduction Reaction

Transition metal single-atom catalysts (SACs) are currently a hot area of research in the field of electrocatalytic oxygen reduction reaction (ORR). In this review, the recent advances in transition metal single-atom supported by 2D materials as catalysts for ORR with high performance are reported. Due to their large surface area, uniformly exposed lattice plane, and adjustable electronic state, 2D materials are ideal supporting materials for exploring ORR active sites and surface reactions. The rational design principles and synthetic strategies of transition metal SACs supported by 2D materials are systematically introduced while the identification of active sites, their possible catalytic mechanisms as well as the perspectives on the future of transition metal SACs supported by 2D materials for ORR applications are discussed. Finally, according to the current development trend of ORR catalysts, the future opportunities and challenges of transition metal SACs supported by 2D materials are summarized.     

Shape Control of Mn3O4 Nanoparticles on Nitrogen‐Doped Graphene for Enhanced Oxygen Reduction Activity

Three kinds of Mn₃O₄ nanoparticles with different shapes (spheres, cubes, and ellipsoids) are selectively grown on nitrogen‐doped graphene sheets through a two‐step liquid‐phase procedure. These non‐precious hybrid materials display an excellent ORR activity and good durability. The mesoporous microstructure, nitrogen doping, and strong bonding between metal species and doped graphene are found to facilitate the ORR catalytic process. Among these three kinds of Mn₃O₄ particles, the ellipsoidal particles on nitrogen‐doped graphene exhibit the highest ORR activity with a more positive onset‐potential of –0.13 V (close to that of Pt/C, –0.09 V) and a higher kinetic limiting current density (J_K) of 11.69 mA cm^–2 at –0.60 V. It is found that the ORR performance of hybrid materials can be correlated to the shape of Mn₃O₄ nanocrystals, and specifically to the exposed crystalline facets associated with a given shape. The shape dependence of Mn₃O₄ nanoparticles integrated with nitrogen‐doped graphene on the ORR performance, reported here for the first time, may advance the development of fuel cells and metal‐air batteries.     

Iron,Nitrogen Co-Doped Carbon Spheres as Low Cost,Scalable Electrocatalysts for the Oxygen Reduction Reaction

Atomically dispersed transition metal-nitrogen-carbon catalysts are emerging as low-cost electrocatalysts for the oxygen reduction reaction in fuel cells. However, a cost-effective and scalable synthesis strategy for these catalysts is still required, as well as a greater understanding of their mechanisms. Herein, iron, nitrogen co-doped carbon spheres (Fe@NCS) have been prepared via hydrothermal carbonization and high-temperature post carbonization. It is determined that FeN₄ is the main form of iron existing in the obtained Fe@NCS. Two different precursors containing Fe²⁺ and Fe³⁺ are compared. Both chemical and structural differences have been observed in catalysts starting from Fe²⁺ and Fe³⁺ precursors. Fe²⁺@NCS-A (starting with Fe²⁺ precursor) shows better catalytic activity for the oxygen reduction reaction. This catalyst is studied in an anion exchange membrane fuel cell. The high open-circuit voltage demonstrates the potential approach for developing high-performance, low-cost fuel cell catalysts.     

Boosting Oxygen Dissociation over Bimetal Sites to Facilitate Oxygen Reduction Activity of Zinc-Air Battery

Zinc-air battery is of great interest but its wide-ranging application is impeded by the sluggish cathodic reactions, especially the oxygen reduction reaction. Despite blooming development in the past decades, achieving further breakthroughs in the activity improvement still appears challenging. Herein, the critical role of bimetal sites in boosting oxygen reduction activity is identified with the combination of theoretical calculations and electrochemical experiments. Density functional theory calculations suggest the elongation of O O bond over the dual-atom system, which is beneficial to its following dissociation and thus enhances the efficiency of the reaction. The proof-of-concept electrocatalyst experimentally delivers a half-wave potential of 0.92 V versus reversible hydrogen electrode and kinetic current density of 51.9 mA cm⁻², significantly outperforming the commercial Pt/C. Both aqueous and all-solid-state zinc-air battery assembled with such catalyst demonstrate superior durability with little performance fluctuation, confirming their potential feasibility in the practical applications.