Oxygen reduction reaction mechanism on nitrogen-doped graphene: A density functional theory study

Metal-Free Graphene-Based Derivatives as Oxygen Reduction Reaction Electrocatalysts in Energy Conversion and Storage Systems: An Overview

Oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy storage and conversion devices. To overcome the slow kinetics, minimize the overpotential, and make this reaction feasible, efficient, and stable, electrocatalysts are needed. Metal-free graphene-based systems are considered promising and cost-effective ORR catalysts with adjustable structures. This review is meant to give a rational overview of the graphene-based metal-free ORR electrocatalysts, illustrating the huge amount of related research developed particularly in the field of fuel cells and metal-air batteries, with particular attention to the synthesis procedures. The novelty of this review is that, beyond general aspects regarding the synthesis and characterization of graphene, above 90% of the various graphene (doped and undoped species, composites)-based ORR electrocatalysts have been reported, which represents an unprecedented thorough collection of both experimental and theoretical studies. Hundreds of references are included in the review; therefore, it can be considered as a vademecum in the field.

A charge calibration strategy for describing the charge transfer during the electrochemical elementary step

Constant potential modeling of electrocatalytic processes remains a significant challenge in the field of computational catalysis, primarily due to the difficulty in simultaneously considering the influence of constant potential conditions, explicit solvent environment, and the double-layer structure. In this work, we propose a charge calibration strategy for electrocatalytic processes. This strategy accounts for charge transfer in systems with explicit solvation and ions during constant-potential free energy modeling. In our strategy, interfacial counter-ions are employed to model the Helmholtz layer and determine the surface charge density, which defines the electrode potential. During the simulation of electrochemical reactions, extra charges are introduced/extracted to/from the system to compensate for electron transfer between the electrode and the reaction species and keep a constant surface charge density along the reaction profile. Our method showcases the impact of potential-dependent solvent reorganization on reaction kinetics and underscores the importance of constant potential kinetics. We anticipate that the strategy presented here will inspire further theoretical and experimental studies for electrochemistry interfaces.

Oxygen electrode catalysis in N-doped graphene: the role of nitrogen coordination and solvation effects

The efficiency of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in energy conversion devices is often hindered by sluggish kinetics and high overpotentials. The role of different nitrogen coordination groups including graphitic N (GN), pyridinic N (PdN), and pyrrolic N (PrN) groups in N-doped carbon materials for these processes is still under debate. Using density functional theory (DFT) calculations, we explored graphene structures doped with in-plane GN, PdN, and PrN as cost-effective electrocatalysts for oxygen electrode reactions, respectively. Our results highlight the importance of explicit solvents in accurately describing the binding behaviours of ORR/OER intermediates, contrasting with vacuum modelling which ignores the hydrogen bond formed between the adsorbates and water layer. Utilizing an explicit water layer, PdN-doped graphene is theoretically recognized as a bifunctional electrocatalyst for oxygen electrode catalysis. Moreover, based on the frontier molecular orbital theory, the Highest Occupied Molecular Orbital (HOMO) energy level of active sites is the underlying factor for the strong binding affinity to oxygen-containing intermediates, leading to the improved ORR/OER activity. Our work provides insight into the identification of active sites in N-doped graphene and provides a theoretical guidance for the rational design of effective carbon-based ORR/OER catalysts.

Carbon encapsulated nanoparticles: materials science and energy applications

The technological implementation of electrochemical energy conversion and storage necessitates the acquisition of high-performance electrocatalysts and electrodes. Carbon encapsulated nanoparticles have emerged as an exciting option owing to their unique advantages that strike a high-level activity-stability balance. Ever-growing attention to this unique type of material is partly attributed to the straightforward rationale of carbonizing ubiquitous organic species under energetic conditions. In addition, on-demand precursors pave the way for not only introducing dopants and surface functional groups into the carbon shell but also generating diverse metal-based nanoparticle cores. By controlling the synthetic parameters, both the carbon shell and the metallic core are facilely engineered in terms of structure, composition, and dimensions. Apart from multiple easy-to-understand superiorities, such as improved agglomeration, corrosion, oxidation, and pulverization resistance and charge conduction, afforded by the carbon encapsulation, potential core-shell synergistic interactions lead to the fine-tuning of the electronic structures of both components. These features collectively contribute to the emerging energy applications of these nanostructures as novel electrocatalysts and electrodes. Thus, a systematic and comprehensive review is urgently needed to summarize recent advancements and stimulate further efforts in this rapidly evolving research field.

Exploration of metal-free 2D electrocatalysts toward the oxygen electroreduction

The advancement of economical and readily available electrocatalysts for the oxygen reduction reaction (ORR) holds paramount importance in the advancement of fuel cells and metal-air batteries. Recently, 2D non-metallic materials have obtained substantial attention as viable alternatives for ORR catalysts due to their manifold advantages, encompassing low cost, ample availability, substantial surface-to-volume ratio, high conductivity, exceptional durability, and competitive activity. The augmented ORR performances observed in metal-free 2D materials typically arise from heteroatom doping, defects, or the formation of heterostructures. Here, the authors delve into the realm of electrocatalysts for the ORR, pivoting around metal-free 2D materials. Initially, the merits of metal-free 2D materials are explored and the reaction mechanism of the ORR is dissected. Subsequently, a comprehensive survey of diverse metal-free 2D materials is presented, tracing their evolutionary journey from fundamental concepts to pragmatic applications in the context of ORR. Substantial importance is given on the exploration of various strategies for enhancing metal-free 2D materials and assessing their impact on inherent material performance, including electronic properties. Finally, the challenges and future prospects that lie ahead for metal-free 2D materials are underscored, as they aspire to serve as efficient ORR electrocatalysts.

Carbon Corrosion Induced by Surface Defects Accelerates Degradation of Platinum/Graphene Catalysts in Oxygen Reduction Reaction

Graphene supported electrocatalysts have demonstrated remarkable catalytic performance for oxygen reduction reaction (ORR). However, their durability and cycling performance are greatly limited by Oswald ripening of platinum (Pt) and graphene support corrosion. Moreover, comprehensive studies on the mechanisms of catalysts degradation under 0.6-1.6 V versus RHE (Reversible Hydrogen Electrode) is still lacking. Herein, degradation mechanisms triggered by different defects on graphene supports are investigated by two cycling protocols. In the start-up/shutdown cycling (1.0-1.6 V vs. RHE), carbon oxidation reaction (COR) leads to shedding or swarm-like aggregation of Pt nanoparticles (NPs). Theoretical simulation results show that the expansion of vacancy defects promotes reaction kinetics of the decisive step in COR, reducing its reaction overpotential. While under the load cycling (0.6-1.0 V vs. RHE), oxygen containing defects lead to an elevated content of Pt in its oxidation state which intensifies Oswald ripening of Pt. The presence of vacancy defects can enhance the transfer of electrons from graphene to the Pt surface, reducing the d-band center of Pt and making it more difficult for the oxidation state of platinum to form in the cycling. This work will provide comprehensive understanding on Pt/Graphene catalysts degradation mechanisms.

Emerging Atomistic Modeling Methods for Heterogeneous Electrocatalysis

Heterogeneous electrocatalysis lies at the center of various technologies that could help enable a sustainable future. However, its complexity makes it challenging to accurately and efficiently model at an atomic level. Here, we review emerging atomistic methods to simulate the electrocatalytic interface with special attention devoted to the components/effects that have been challenging to model, such as solvation, electrolyte ions, electrode potential, reaction kinetics, and pH. Additionally, we review relevant computational spectroscopy methods. Then, we showcase several examples of applying these methods to understand and design catalysts relevant to green hydrogen. We also offer experimental views on how to bridge the gap between theory and experiments. Finally, we provide some perspectives on opportunities to advance the field.

Two-dimensional materials and their applications in fuel cells

In recent years, two-dimensional (2D) materials have been extensively studied and applied in the field of catalysis on account of their high specific surface areas, high exposure of metal active sites, and readily tunable structures. This article introduces various 2D materials (including materials composed of a few atomic layers) and the related synthesis methods and discusses their catalytic performances for hydrogen fuel cells, in particular, for oxygen reduction reaction and hydrogen oxidation reaction. At the end of this review, the advantages and current challenges of 2D materials are summarized, and the prospects of 2D electrocatalytic materials are proposed.

Theoretical investigation of electrocatalytic activity of Pt-free dual atom-doped graphene for O2 reduction in an alkaline solution

Non-precious electrocatalysts as the alternative to Pt have become a hot research area in the last decade due to the suitable catalytic activity in Oxygen reduction reaction (ORR) in electrochemical systems. In this work, the density functional theory calculations were investigated to explore the activity of Fe, Cu, and Fe-Cu atoms supported by N-doped graphene as the ORR electrocatalyst for Oxygen-depolarized cathodes (ODCs). To this end, the ORR mechanism was surveyed in detail in the gas and solvent phases. The results show that the solvent phase leads to a higher overpotential and thermodynamic limiting potential. According to the density of states curves, there are strong interactions between metal atom and substrate that can effectively tune the electronics of catalysts. Bader's analysis confirms that, in addition to the single metal atoms, nitrogen atoms have also played a critical role in charge transfer between substrates and oxygen molecules in ORR. It is also predicted that Fe-Cu@NC SAC exhibits the highest catalytic activity which is consistent with thermodynamic limiting potential and theoretical overpotential of  - 0.26 and  0.66 (V vs. SHE), respectively, indicating that this type of catalyst may be a suitable candidate instead of precious metals in oxygen-depolarized cathodes in electrochemical devices.

A new attempt to remove toluene using nickel-iron bimetallic particle electrode reactor

A new attempt of removing toluene waste gas using a three-dimensional electrode reaction device with nickel-iron bimetallic particle electrode is presented in this paper. The particle electrode was prepared by a simple liquid phase reduction method. Through bimetal modification, the particle electrode mass transfer rate is increased to 1.29 times, and the degradation efficiency of the reactor is increased by nearly 40%, which makes it possible to remove toluene waste gas by other electrochemical methods in addition to plasma method. The removal efficiency of the particle electrode can be stabilized at more than 80% after 5 cycles (50 h). At the same time, the relationship between independent working parameters and dependent variables is analyzed using the central composite design, and the operating parameters are optimized. Based on this study, the removal mechanism and possible degradation pathway of toluene were investigated. This study provides a supplement to the possibility and theoretical basis of new technology application for electrocatalytic oxidation removal of VOCs.

Well-dispersed Pt/Nb2O5on zeolitic imidazolate framework derived nitrogen-doped carbon for efficient oxygen reduction reaction

In this work, an advanced hybrid material was constructed by incorporating niobium pentoxide (Nb2O5) nanocrystals with nitrogen-doped carbon (NC) derived from ZIF-8 dodecahedrons, serving as a support, referred to as Nb2O5/NC. Pt nanocrystals were dispersed onto Nb2O5/NC using a simple impregnation reduction method. The obtained Pt/Nb2O5/NC electrocatalyst showed high oxygen reduction reaction (ORR) activity due to three-phase mutual contacting structure with well-dispersed Pt and Nb2O5NPs. In addition, the conductive NC benefits electron transfer, while the induced Nb2O5can regulate the electronic structure of Pt element and anchor Pt nanocrystals, thereby enhancing the ORR activity and stability. The half-wave potential (E1/2) for Pt/Nb2O5/NC is 0.886 V, which is higher than that of Pt/NC (E1/2= 0.826 V). The stability examinations demonstrated that Pt/Nb2O5/NC exhibited higher electrocatalytic durability than Pt/NC. Our work provides a new direction for synthesis and structural design of precious metal/oxides hybrid electrocatalysts.

Silica-Derived Nanostructured Electrode Materials for ORR, OER, HER, CO2RR Electrocatalysis, and Energy Storage Applications: A Review

Silica-derived nanostructured catalysts (SDNCs) are a class of materials synthesized using nanocasting and templating techniques, which involve the sacrificial removal of a silica template to generate highly porous nanostructured materials. The surface of these nanostructures is functionalized with a variety of electrocatalytically active metal and non-metal atoms. SDNCs have attracted considerable attention due to their unique physicochemical properties, tunable electronic configuration, and microstructure. These properties make them highly efficient catalysts and promising electrode materials for next generation electrocatalysis, energy conversion, and energy storage technologies. The continued development of SDNCs is likely to lead to new and improved electrocatalysts and electrode materials. This review article provides a comprehensive overview of the recent advances in the development of SDNCs for electrocatalysis and energy storage applications. It analyzes 337,061 research articles published in the Web of Science (WoS) database up to December 2022 using the keywords "silica", "electrocatalysts", "ORR", "OER", "HER", "HOR", "CO2RR", "batteries", and "supercapacitors". The review discusses the application of SDNCs for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO2RR), supercapacitors, lithium-ion batteries, and thermal energy storage applications. It concludes by discussing the advantages and limitations of SDNCs for energy applications.

The role of nitrogen sources and hydrogen adsorption on the dynamic stability of Fe-N-C catalysts in oxygen reduction reaction

Fe-N-C catalysts are promising alternatives to Pt-based electrocatalysts for the oxygen reduction reaction (ORR) in various electrochemical applications. However, their practical implementation is impeded by their instability during prolonged operation. Various degradation mechanisms have been proposed, yet the real origin of the intrinsic instability of Fe-N-C structures under ORR operations is still disputed. Herein, we observed a new type of protonation mechanism based on advanced first-principles simulations and experimental characterizations. The results revealed strong evidence of pyrrolic-N protonation in pyrrolic-type FeN4, which plays a vital role for the low kinetic barrier of Fe leaching. Conversely, the pyridinic-type FeN4 prefers protonation at the Fe site, contributing to the higher barrier of Fe leaching and relatively higher stability. The facile pyrrolic-N protonation is verified by various spectroscopy characterizations in the Nafion-treated FePc molecule. Crucially, the presence of oxygen-containing intermediates at the Fe site can further work synergistically with N protonation to promote conversion of iron atoms (Fe-N4) into ferric oxide under working potentials, and the more positive the electrode potential, the lower the kinetic barrier of Fe leaching. These findings serve as a foundation for future research endeavors on the stability issues of Fe-N-C catalysts and advancing their application in sustainable energy conversion technologies.

Revision of the oxygen reduction reaction on N-doped graphenes by grand-canonical DFT

Nitrogen-doped graphenes were among the first promising metal-free carbon-based catalysts for the oxygen reduction reaction (ORR). However, data on the most efficient catalytic centers and their catalytic mechanisms are still under debate. In this work, we study the associative mechanism of the ORR in an alkaline medium on graphene containing various types of nitrogen doping. The free energy profile of the reaction is constructed using grand-canonical DFT at a constant electrode potential in combination with an implicit electrolyte model. It is shown that the reaction mechanism differs from the generally accepted one and depends on the surface potential and doping type. In particular, as the potential decreases, coupled electron-proton transfer changes to sequential electron and proton transfer, and the potential at which this occurs depends on the doping type. It has been shown that oxygen chemisorption is the limiting step. The electrocatalytic mechanism of the nitrogen dopants involves reducing the oxygen chemisorption energy. Calculations predict that, at different potentials, different types of nitrogen impurities most effectively catalyze the ORR.

Metal Alloys-Structured Electrocatalysts: Metal-Metal Interactions, Coordination Microenvironments, and Structural Property-Reactivity Relationships

Metal alloys-structured electrocatalysts (MAECs) have made essential contributions to accelerating the practical applications of electrocatalytic devices in renewable energy systems. However, due to the complex atomic structures, varied electronic states, and abundant supports, precisely decoding the metal-metal interactions and structure-activity relationships of MAECs still confronts great challenges, which is critical to direct the future engineering and optimization of MAECs. Here, this timely review comprehensively summarizes the latest advances in creating the MAECs, including the metal-metal interactions, coordination microenvironments, and structure-activity relationships. First, the fundamental classification, design, characterization, and structural reconstruction of MAECs are outlined. Then, the electrocatalytic merits and modulation strategies of recent breakthroughs for noble and non-noble metal-structured MAECs are thoroughly discussed, such as solid solution alloys, intermetallic alloys, and single-atom alloys. Particularly, unique insights into the bond interactions, theoretical understanding, and operando techniques for mechanism disclosure are given. Thereafter, the current states of diverse MAECs with a unique focus on structural property-reactivity relationships, reaction pathways, and performance comparisons are discussed. Finally, the future challenges and perspectives for MAECs are systematically discussed. It is believed that this comprehensive review can offer a substantial impact on stimulating the widespread utilization of metal alloys-structured materials in electrocatalysis.

Realistic Modeling of the Electrocatalytic Process at Complex Solid-Liquid Interface

The rational design of electrocatalysis has emerged as one of the most thriving means for mitigating energy and environmental crises. The key to this effort is the understanding of the complex electrochemical interface, wherein the electrode potential as well as various internal factors such as H-bond network, adsorbate coverage, and dynamic behavior of the interface collectively contribute to the electrocatalytic activity and selectivity. In this context, the authors have reviewed recent theoretical advances, and especially, the contributions to modeling the realistic electrocatalytic processes at complex electrochemical interfaces,  and illustrated the challenges and fundamental problems in this field. Specifically, the significance of the inclusion of explicit solvation and electrode potential as well as the strategies toward the design of highly efficient electrocatalysts are discussed. The structure-activity relationships and their dynamic responses to the environment and catalytic functionality under working conditions are illustrated to be crucial factors for understanding the complexed interface and the electrocatalytic activities. It is hoped that this review can help spark new research passion and ultimately bring a step closer to a realistic and systematic modeling method for electrocatalysis.

-SR removal or -R removal? A mechanistic revisit on the puzzle of ligand etching of Au25(SR)18 nanoclusters during electrocatalysis

Accurate identification of active sites is highly desirable for elucidation of the reaction mechanism and development of efficient catalysts. Despite the promising catalytic performance of thiolated metal nanoclusters (NCs), their actual catalytic sites remain elusive. Traditional first-principles calculations and experimental observations suggested dealkylated S and dethiolated metal, respectively, to be the active centers. However, the real kinetic origin of thiolate etching during the electrocatalysis of NCs is still puzzling. Herein, we conducted advanced first-principles calculations and electrochemical/spectroscopic experiments to unravel the electrochemical etching kinetics of thiolate ligands in prototype Au25(SCH3)18 NC. The electrochemical processes are revealed to be spontaneously facilitated by dethiolation (i.e., desorption of -SCH3), forming the free HSCH3 molecule after explicitly including the solvent effect and electrode potential. Thus, exposed under-coordinated Au atoms, rather than the S atoms, serve as the real catalytic sites. The thermodynamically preferred Au-S bond cleavage arises from the selective attack of H from proton/H2O on the S atom under suitable electrochemical bias due to the spatial accessibility and the presence of S lone pair electrons. Decrease of reduction potential promotes the proton attack on S and significantly accelerates the kinetics of Au-S bond breakage irrespective of the pH of the medium. Our theoretical results are further verified by the experimental electrochemical and spectroscopic data. At more negative electrode potentials, the number of -SR ligands decreased with concomitant increase of the vibrational intensity of S-H bonds. These findings together clarify the atomic-level activation mechanism on the surface of Au25(SR)18 NCs.

Theoretical study of Mo2N supported transition metal single-atom catalyst for OER/ORR bifunctional electrocatalysis

The rational design and development of an efficient bifunctional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the key to developing new renewable energy storage and conversion technologies. Transition metal nitrides (TMNs) have shown excellent energy storage and electrochemistry potential due to their unique electronic structure and physicochemical properties. In this paper, based on the first-principles method of density functional theory (DFT), a series of efficient and stable bifunctional single-atom catalysts (SACs) were designed on Mo2N by introducing transition metal atoms as active sites, and the effects of different TM atoms on the catalytic performance of 2D-Mo2N (Two dimensional Mo2N) were evaluated. The calculation results show that TM@Mo2N exhibits excellent stability and good conductivity, which is conducive to electron transfer during the electrocatalytic reaction. Among these SACs, the Au@Mo2N single-atom catalyst has a very low OER overpotential (0.36 V), exhibiting high OER activity. Meanwhile, Au@Mo2N also exhibits excellent ORR performance with a low overpotential of 0.4 V, indicating that Au@Mo2N is the best OER/ORR bifunctional catalyst. This work provides a feasible solution for developing transition metal bifunctional electrocatalysts. Au@Mo2N is expected to replace traditional commercial Pt catalyst materials and become a catalyst with excellent performance in fuel cell modules.

Advances in Transition-Metal-Based Dual-Atom Oxygen Electrocatalysts

Oxygen electrocatalysis has aroused considerable interest over the past years because of the new energy technologies boom in hydrogen energy and metal-air battery. However, due to the sluggish kinetic of the four-electron transfer process in oxygen reduction reaction and oxygen evolution reaction, the electro-catalysts are urgently needed to accelerate the oxygen electrocatalysis. Benefit from the high atom utilization efficiency, unprecedentedly high catalytic activity, and selectivity, single-atom catalysts (SACs) are considered the most promising candidate to replace the traditional Pt-group-metal catalysts. Compared with SACs, the dual-atom catalysts (DACs) are attracting more attraction including higher metal loading, more versatile active sites, and excellent catalytic activity. Therefore, it is essential to explore the new universal methods approaching to the preparation, characterization, and to elucidate the catalytic mechanisms of the DACs. In this review, several general synthetic strategies and structural characterization methods of DACs are introduced and the involved oxygen catalytic mechanisms are discussed. Moreover, the state-of-the-art electrocatalytic applications including fuel cells, metal-air batteries, and water splitting have been sorted out at present. The authors hope this review has given some insights and inspiration to the researches about DACs in electro-catalysis.

Monolayer AsC5 as the Promising Hydrogen Storage Material for Clean Energy Applications

One of the critical techniques for developing hydrogen storage applications is the advanced research to build novel two-dimensional materials with significant capacity and effective reversibility. In this work, we perform first-principles unbiased structure search simulations to find a novel AsC5 monolayer with a variety of functionally advantageous characteristics. Based on theoretical simulations, the proposed AsC5 has been found to be energetically, dynamically, and thermally stable, supporting the viability of experiment. Since the coupling between H2 molecules and the AsC5 monolayer is quite weak due to physisorption, it is crucial to be enhanced by thoughtful material design. Hydrogen storage capacity can be greatly enhanced by decorating the AsC5 monolayer with Li atoms. Each Li atom on the AsC5 substrate is shown to be capable of adsorbing up to four H2 molecules with an advantageous average adsorption energy (Ead) of 0.19 eV/H2. The gravimetric density for hydrogen storage adsorption with 16Li and 64 H2 of a Li-decorated AsC5 monolayer is about 9.7 wt%, which is helpful for the possible application in hydrogen storage. It is discovered that the desorption temperature (TD) is much greater than the hydrogen critical point. Therefore, such crucial characteristics make AsC5-Li be a promising candidate for the experimental setup of hydrogen storage.

Cu - Nitrogen doped graphene (Cu-N/Gr) nanocomposite as cathode catalyst in fuel cells - DFT study

Novel Cu-nitrogen doped graphene nanocomposite catalysts are developed to investigate the Cu-nitrogen doped fuel cell cathode catalyst. Density functional theory calculations are performed using Gaussian 09w software to study the oxygen reduction reaction (ORR) on Cu-nitrogen doped graphene nanocomposite cathode catalyst in low-temperature fuel cells. Three different nanocomposite structures Cu₂-N₆/Gr, Cu₂-N₈/Gr and Cu-N₄/Gr were considered in the acidic medium under standard conditions (298.15 K, 1 atm) in order to explore the properties of the fuel cell. The results showed that all structures are stable at the potential range 0-5.87 V. Formation energy, Mulliken charge and HOMO-LUMO energy calculations showed that Cu₂-N₆/Gr and Cu₂-N₈/Gr are more stable structure-wise, while free energy calculations showed that only Cu₂-N₈/Gr and Cu-N₄/Gr structures support spontaneous ORR. The maximum cell potential under standard conditions was shown at 0.28 V and 0.49 V for Cu₂-N₈/Gr and Cu-N₄/Gr respectively. From the calculations, the Cu₂-N₆/Gr and Cu₂-N₈/Gr structures are less favorable in H₂O₂ generation; however, Cu-N₄/Gr showed the potential for H₂O₂ generation. In conclusion, Cu₂-N₈/Gr and Cu-N₄/Gr are more favorable to ORR than Cu₂-N₆/Gr.

A pyrolysis-free Ni/Fe bimetallic electrocatalyst for overall water splitting

Catalysts capable of electrochemical overall water splitting in acidic, neutral, and alkaline solution are important materials. This work develops bifunctional catalysts with single atom active sites through a pyrolysis-free route. Starting with a conjugated framework containing Fe sites, the addition of Ni atoms is used to weaken the adsorption of electrochemically generated intermediates, thus leading to more optimized energy level sand enhanced catalytic performance. The pyrolysis-free synthesis also ensured the formation of well-defined active sites within the framework structure, providing ideal platforms to understand the catalytic processes. The as-prepared catalyst exhibits efficient catalytic capability for electrochemical water splitting in both acidic and alkaline electrolytes. At a current density of 10 mA cm^-2, the overpotential for hydrogen evolution and oxygen evolution is 23/201 mV and 42/194 mV in 0.5 M H₂SO₄ and 1 M KOH, respectively. Our work not only develops a route towards efficient catalysts applicable across a wide range of pH values, it also provides a successful showcase of a model catalyst for in-depth mechanistic insight into electrochemical water splitting.

Insights into Nitrogen-doped Carbon for Oxygen Reduction: The Role of Graphitic and Pyridinic Nitrogen Species

Nitrogen-doped carbons (N/Cs) manifest good catalytic performance for oxygen reduction reaction (ORR) for fuel cell systems. However, to date, controversies remain on the role of active sites in N/Cs. In the present study, ORR test was conducted on three N/Cs in O₂ -saturated 0.1 M KOH aqueous solution, where apparent linear correlation between graphitic N contents and ORR activity was observed. Theoretical calculations demonstrated that graphitic N doping is energetically more favorable than that of pyridinic N doping for ORR and the pyridinic N leads to more preferential with 2 e^- ORR pathway. These results reveal that graphitic N plays a key role in N/Cs mediated ORR activity. This work lays a solid foundation on identifying the active sites in heteroatom-doped carbons and can be exploited for rational design and engineering of effective carbon-based ORR catalysts.

Steering Selectivity in the Four-Electron and Two-Electron Oxygen Reduction Reactions: On the Importance of the Volcano Slope

In the last decade, trends for competing electrocatalytic processes have been largely captured by volcano plots, which can be constructed by the analysis of adsorption free energies as derived from electronic structure theory in the density functional theory approximation. One prototypical example refers to the four-electron and two-electron oxygen reduction reactions (ORRs), resulting in the formation of water and hydrogen peroxide, respectively. The conventional thermodynamic volcano curve illustrates that the four-electron and two-electron ORRs reveal the same slopes at the volcano legs. This finding is related to two facts, namely, that only a single mechanistic description is considered in the model, and electrocatalytic activity is assessed by the concept of the limiting potential, a simple thermodynamic descriptor evaluated at the equilibrium potential. In the present contribution, the selectivity challenge of the four-electron and two-electron ORRs is analyzed, thereby accounting for two major expansions. First, different reaction mechanisms are included into the analysis, and second, G _max(U), a potential-dependent activity measure that factors overpotential and kinetic effects into the evaluation of adsorption free energies, is applied for approximation of electrocatalytic activity. It is illustrated that the slope of the four-electron ORR is not constant at the volcano legs but rather is prone to change as soon as another mechanistic pathway is energetically preferred or another elementary step becomes the limiting one. Due to the varying slope of the four-electron ORR volcano, a trade-off between activity and selectivity for hydrogen peroxide formation is observed. It is demonstrated that the two-electron ORR is energetically preferred at the left and right volcano legs, thus opening a new strategy for the selective formation of H₂O₂ by an environmentally benign route.

Quantitative Evidence to Challenge the Traditional Model in Heterogeneous Catalysis: Kinetic Modeling for Ethane Dehydrogenation over Fe/SAPO-34

The production of ethylene from ethane dehydrogenation (EDH) is of great importance in the chemical industry, where zeolites are reported to be promising catalysts and kinetic simulations using the energetics from quantum mechanical calculations might provide an effective approach to speed up the development. However, the kinetic simulations with rigorous considerations of the zeolite environment are not yet advanced. In this work, EDH over Fe/SAPO-34 is investigated using quantum mechanical calculations with kinetic simulations. We show that an excellent agreement between the reaction rates from the self-consistent kinetic simulations using the coverage-dependent kinetic model developed in this work and the experimental ones can be achieved. We demonstrate that the adsorbate-adsorbate interactions are of paramount importance to the accuracy of kinetic calculations for zeolite catalysts. Our self-consistent kinetic calculations illustrate that the CH₃CH₂• radical rather than CH₃CH₂* is a favored intermediate. Perhaps more importantly, we reveal that the traditional model to describe catalytic reactions in heterogeneous catalysis cannot be used for the kinetics of the system and it may not be appropriate for many real catalytic systems. This work not only builds a framework for accurate kinetic simulations in zeolites, but also emphasizes an important concept beyond the traditional model.

Carbon Surface-Influenced Heterogeneity of Ni and Co Catalytic Sites as a Factor Affecting the Efficiency of Oxygen Reduction Reaction

Highly porous carbon black and micro/mesoporous activated carbon were impregnated with cobalt and nickel nitrates, followed by heat treatment at 850 °C in nitrogen. Detailed information about chemistry and porosity was obtained using XPS, XRD, TEM/EDX, and nitrogen adsorption. The samples were used as ORR catalysts. Marked differences in the performance were found depending on the type of carbon. Differences in surface chemistry and porosity affected the chemistry of the deposited metal species that governed the O₂ reduction efficiency along with other features of the carbon supports, including electrical conductivity and porosity. While dissociating surface acidic groups promoted the high dispersion of small metal species, carbon reactivity with oxygen and acidity limited the formation of the most catalytically active Co₃O₄. Formation of Co₃O₄ on the highly conductive carbon black resulted in an excellent performance with four electrons transferred and a current density higher than that on Pt/C. When Co₃O₄ was not formed in a sufficient quantity, nickel metal nanoparticles promoted ORR on the Ni/Co-containing samples. The activity was also significantly enhanced by small pores that increased the ORR efficiency by strongly adsorbing oxygen, which led to its bond splitting, followed by the acceptance of four electrons.

N-doped graphene for electrocatalytic O2 and CO2 reduction

The electrocatalytic CO₂ reduction reaction (CO₂RR) and oxygen reduction reaction (ORR) are important approaches to realize energy conversion and sustainable development. However, sluggish reaction kinetics severely hinders the practical application of devices related to these reactions. N-doped graphene (NG) with unique properties exhibits great potential in catalyzing the CO₂RR and ORR, which is attributed to the electron redistribution. In this review, we start from the fundamental properties of NG, especially emphasizing the changes caused by N doping. Then the synthetic methods are summarized by classifying them into top-down strategies and bottom-up strategies. Subsequently, the applications of NG in the ORR and CO₂RR are discussed and the effects of electronic structure on the electrocatalytic activity are highlighted. Finally, we give our own perspective on the future research direction of NG in the applications of the ORR and CO₂RR.

Multilevel Computational Studies Reveal the Importance of Axial Ligand for Oxygen Reduction Reaction on Fe-N-C Materials

The systematic improvement of Fe-N-C materials for fuel cell applications has proven challenging, due in part to an incomplete atomistic understanding of the oxygen reduction reaction (ORR) under electrochemical conditions. Herein, a multilevel computational approach, which combines ab initio molecular dynamics simulations and constant potential density functional theory calculations, is used to assess proton-coupled electron transfer (PCET) processes and adsorption thermodynamics of key ORR intermediates. These calculations indicate that the potential-limiting step for ORR on Fe-N-C materials is the formation of the Fe^III-OOH intermediate. They also show that an active site model with a water molecule axially ligated to the iron center throughout the catalytic cycle produces results that are consistent with the experimental measurements. In particular, reliable prediction of the ORR onset potential and the Fe(III/II) redox potential associated with the conversion of Fe^III-OH to Fe^II and desorbed H₂O requires an axial H₂O co-adsorbed to the iron center. The observation of a five-coordinate rather than four-coordinate active site has significant implications for the thermodynamics and mechanism of ORR. These findings highlight the importance of solvent-substrate interactions and surface charge effects for understanding the PCET reaction mechanisms and transition-metal redox couples under realistic electrochemical conditions.

Various defects in graphene: a review

Pristine graphene has been considered one of the most promising materials because of its excellent physical and chemical properties. However, various defects in graphene produced during synthesis or fabrication hinder its performance for applications such as electronic devices, transparent electrodes, and spintronic devices. Due to its intrinsic bandgap and nonmagnetic nature, it cannot be used in nanoelectronics or spintronics. Intrinsic and extrinsic defects are ultimately introduced to tailor electronic and magnetic properties and take advantage of their hidden potential. This article emphasizes the current advancement of intrinsic and extrinsic defects in graphene for potential applications. We also discuss the limitations and outlook for such defects in graphene.

Quantum-Chemical Modeling of the Catalytic Activity of Graphene Doped with Metal Phthalocyanines in ORR

The active centers of carbon catalysts doped with cobalt, nickel, copper, manganese, zinc, and chromium were modeled by density functional theory methods. Likewise, the thermodynamics of the oxygen reduction reaction (ORR) on model catalysts were determined. The features of the chemical properties of chromium-containing material, namely its spontaneous oxidation into the hydroxo form, were revealed. In addition, it was established that among the studied catalysts, graphene doped with cobalt showed the best properties.

Pseudo-adsorption and long-range redox coupling during oxygen reduction reaction on single atom electrocatalyst

Fundamental understanding of the dynamic behaviors at the electrochemical interface is crucial for electrocatalyst design and optimization. Here, we revisit the oxygen reduction reaction mechanism on a series of transition metal (M = Fe, Co, Ni, Cu) single atom sites embedded in N-doped nanocarbon by ab initio molecular dynamics simulations with explicit solvation. We have identified the dissociative pathways and the thereby emerged solvated hydroxide species for all the proton-coupled electron transfer (PCET) steps at the electrochemical interface. Such hydroxide species can be dynamically confined in a "pseudo-adsorption" state at a few water layers away from the active site and respond to the redox event at the catalytic center in a coupled manner within timescale less than 1 ps. In the PCET steps, the proton species (in form of hydronium in neutral/acidic media or water in alkaline medium) can protonate the pseudo-adsorbed hydroxide without needing to travel to the direct catalyst surface. This, therefore, expands the reactive region beyond the direct catalyst surface, boosting the reaction kinetics via alleviating mass transfer limits. Our work implies that in catalysis the reaction species may not necessarily bind to the catalyst surface but be confined in an active region.

Two-Dimensional Dirac Nodal Line Carbon Nitride to Anchor Single-Atom Catalyst for Oxygen Reduction Reaction

Two-dimensional carbon nitride (2DCN) materials have emerged as an important class of 2D materials beyond graphene. However, 2DCN materials with nodal-line semimetal characteristic are rarely reported. In this work, a new nodal-line semimetal 2DCN with the stoichiometry C₄ N₄ is designed by using density functional theory (DFT) calculations and its application to anchor single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) is investigated. C₄ N₄ is a planar covalent network (sp² hybridization) with regular holes formed by the four N atoms, which is dynamically, thermodynamically, and mechanically stable. The nodal line is contributed by the p_z orbitals of C and p_x/y orbitals of N atoms. C₄ N₄ shows an anisotropic Fermi velocity and high electron mobility. Because of its porous structure, C₄ N₄ can anchor heteroatoms as SACs for electrocatalysis. C₄ N₄ anchored with Fe or Co is shown to be highly active for the ORR with a rather high half-wave potential of around 0.90 V, which is higher than those of SACs on other carbon nitrides. These findings may provide a new strategy to design novel substrates for SACs.

Density Functional Theory Study of the Oxygen Reduction Reaction Mechanism on Graphene Doped with Nitrogen and a Transition Metal

The active centers of carbon nonplatinum catalysts doped with cobalt, iron, nickel, and copper have been simulated by quantum-chemical density functional theory methods. The thermodynamics of the electrochemical oxygen reduction reaction (ORR) on model catalysts has been determined. It was found that among the studied catalysts, graphene doped with cobalt and iron showed the best properties. A two-state reactivity effect has been found on a cobalt-containing catalyst, and a more detailed reaction mechanism has been proposed, including the stages of charging by an extra electron and association with water. The proposed mechanism explains several effects that have arisen during the modeling in relation to the classical mechanism.

Imidazolium ionic Liquid-Regulated Sub-5-nm Pt(111) with a stable configuration anchored on hollow carbon nanoshells for efficient oxygen reduction

Here, N-doped hollow carbon sphere (NHCS)-supported (111)-plane-engineered sub-5-nm Pt (Pt-NHCS) catalysts regulated precisely by imidazolium ionic liquids were synthesized successfully and used to catalyze oxygen reduction. The (111)-plane engineered Pt nanocrystals with a diameter of 4.5 ± 0.5 nm were homogeneously deposited on the 3-dimensional spherical nanoshells. The resulting Pt nanocrystals anchored on the carbon skeleton exhibit a stable configuration in both alkaline and acid electrolytes with the help of imidazolium cations and pyrolysis. Among all as-prepared catalysts, the optimized Pt-NHCS shows remarkable long-term durability. Specifically, Pt-NHCS maintains 95.3% of the original current density after 10,000 potential cycles, while Pt/C benchmarks exhibit a retention of 78.5%. Accelerated durability test results indicate that Pt-NHCS exhibits a high efficiency of 96 % in comparison with initial current density, while a value of 86% for Pt/C. Density functional theory calculations demonstrate that reactive Pt(111) planes with well-defined Schottky defects and vacancies adsorb and activate oxygen molecule rapidly while desorbing the reaction intermediates.

Extreme Environmental Thermal Shock Induced Dislocation-Rich Pt Nanoparticles Boosting Hydrogen Evolution Reaction

Crystal structure engineering of nanomaterials is crucial for the design of electrocatalysts. Inducing dislocations is an efficient approach to generate strain effects in nanomaterials to optimize the crystal and electronic structures and improve the catalytic properties. However, it is almost impossible to produce and retain dislocations in commercial mainstream catalysts, such as single metal platinum (Pt) catalysts. In this work, a non-equilibrium high-temperature (>1400 K) thermal-shock method is reported to induce rich dislocations in Pt nanocrystals (Dr-Pt). The method is performed in an extreme environment (≈77 K) created by liquid nitrogen. The dislocations induced within milliseconds by thermal and structural stress during the crystallization process are kinetically frozen at an ultrafast cooling rate. The high-energy surface structures with dislocation-induced strain effects can prevent surface restructuring during catalysis. The findings indicate that a novel extreme environmental high-temperature thermal-shock method can successfully introduce rich dislocations in Pt nanoparticles and significantly boost its hydrogen evolution reaction performance.

Graphene supported single metal atom catalysts for the efficient hydrogen oxidation reaction in alkaline media

Single Pt and Ni atoms anchored on the divacancy graphene exhibit both high activity and superior antioxidant capacity for the hydrogen oxidation reaction in alkaline fuel cells.