Active Sites and Mechanisms for Oxygen Reduction Reaction on Nitrogen-Doped Carbon Alloy Catalysts: Stone–Wales Defect and Curvature Effect

Bromine Atoms Decorated Pyene-based Covalent Organic Frameworks for Accelerated Photocatalytic H2 Production

Designing materials with low exciton binding energy is an efficient way of improving the hydrogen production performance of COFs（Covalent Organic Frameworks. Here, it is demonstrated that the strategy of decorating bromine atoms on Pyene-based COFs can achieve elevated photocatalytic H2 evolution rates (HER = 13.61 mmol g-1h-1). Low-temperature fluorescence and time-resolved fluorescence spectroscopy (TRPL) indicate that the introduction of bromine atoms can significantly suppress charge recombination. DFT (Density Functional Theory) calculation clarified that the C atoms adjacent to Br are the active sites with a reduced energy barrier in the process of formatting H intermediate species (H*). The modification strategy of Br atoms in COF furnishes a new medium for exploiting exquisite photocatalysts.

B-N Co-Doped Graphene: Stability and Catalytic Activity in Oxygen Reduction Reaction - A Theoretical Insight

We systematically investigated the stable configurations and catalytic activity in the Oxygen Reduction Reaction (ORR) of graphene co-doped with boron and nitrogen (B-N) using first-principles methods. Compared to single B/N doping, co-doping with BN is energetically favored. We found that intermediate species of ORR process adsorb on boron atoms, which act as catalytic sites. The presence of neighboring nitrogen atoms around boron plays a crucial role in modulating the catalytic activity of boron. For the same adsorption configuration, the adsorption energy of the adsorbate increases with the number of neighboring nitrogen atoms around boron and generally correlates positively with the number of electrons gained by the adsorbate. Regarding the catalytic activity of ORR, excessively strong adsorption of adsorbates impedes their hydrogenation. The best substrates for ORR catalytic activity are B-N-graphene and N-B2-graphene, with the rate-determining step being the hydrogenation of *OO and overpotentials of 0.49 V and 0.54 V, respectively.

First-principles studies of enhanced oxygen reduction reactions on graphene- and nitrogen-doped graphene-coated platinum surfaces

Developing innovative platinum-based electrocatalysts and enhancing their efficiency are crucial for advancing high-performance fuel cell technology. In this study, we employed DFT calculations to provide a theoretical basis for interpreting the impact of graphene coatings on various Pt surfaces on oxygen reduction reaction (ORR) catalytic activity, which are currently applied as protective layers in experiments. We comprehensively assess the geometric and electronic properties of Pt(100), Pt(110), and Pt(111) surfaces in comparison to their graphene-coated counterparts, revealing different adsorption behaviors of O2 across these surfaces. The ORR mechanisms on different Pt surfaces show distinct rate-determining steps, with Pt(111) showing the highest ORR activity, followed by Pt(110) and Pt(100). Graphene coatings play a key role in enhancing charge transfer from the surface, resulting in modifications of O2 adsorption. Despite influencing ORR kinetics, these graphene-coated surfaces demonstrate competitive catalytic activity compared to their bare counterparts. Notably, Pt(111) with a graphene coating exhibits the lowest activation energy among graphene-coated surfaces. Our calculations also suggest that the ORR can occur directly on non-defective Pt@graphene surfaces rather than being restricted to exposed Pt centers due to point defects on graphene. Furthermore, our work highlights the potential of nitrogen doping onto the Pt(111)@C surface to further enhance ORR activity. This finding positions nitrogen-doped Pt@C as a promising electrocatalyst for advancing electrochemical technologies.

Tailoring coordination environments of single-atom electrocatalysts for hydrogen evolution by topological heteroatom transfer

The rational design of carbon-supported transition-metal single-atom catalysts requires the precise arrangement of heteroatoms within the single-atom catalysts. However, achieving this design is challenging due to the collapse of the structure during the pyrolysis. Here, we introduce a topological heteroatom-transfer strategy to prevent the collapse and accurately control the P coordination in carbon-supported single-atom catalysts. As an illustration, we have prepared self-assembled helical fibers with encapsulated cavities. Within these cavities, adjustable functional groups can chelate metal ions (Nx···Mn+···Oy), facilitating the preservation of the structure during the pyrolysis based phosphidation. This process allows for the transfer of heteroatoms from the assembly into single-atom catalysts, resulting in the precise coordination tailoring. Notably, the Co-P2N2-C catalyst exhibits electrocatalytic performance as a non-noble metal single-atom catalyst for alkaline hydrogen evolution, attaining a current density of 100 mA cm-2 with an overpotential of only 131 mV.

Nitrogen sites prevail over textural properties in N-doped carbons for the oxygen reduction reaction

Nitrogen-doped carbon-based electrodes are among the most promising alternatives to platinum-based electrodes in the cathode of fuel cells and metal-air batteries, where the oxygen reduction reaction (ORR) takes place. Among the approaches for improving ORR activity, nitrogen functionalities and well-developed textural properties have proved very effective. Nonetheless, the question of which between nitrogen active sites or textural properties are more crucial in N-doped carbon materials remains unanswered. This work proposes a comparative and critical approach through the selective functionalization of four commercial activated carbons with different textural properties. This study highlights the greater importance of N-doping in relation to the textural properties of carbon materials, and provides fundamental insights for conclusively addressing the ongoing debate within the carbon community about the significance of these two factors in the context of ORR.

Atomic Distance Engineering in Metal Catalysts to Regulate Catalytic Performance

It is very important to understand the structure-performance relationship of metal catalysts by adjusting the microstructure of catalysts at the atomic scale. The atomic distance has an essential influence on the composition of the environment of active metal atom, which is a key factor for the design of targeted catalysts with desired function. In this review, we discuss and summarize strategies for changing the atomic distance from three aspects and relate their effects on the reactivity of catalysts. First, the effects of regulating bond length between metal and coordination atom at one single-atom site on the catalytic performance are introduced. The bond lengths are affected by the strain effect of the support and high-shell doping and can evolve during the reaction. Next, the influence of the distance between single-atom sites on the catalytic performance is discussed. Due to the space matching of adsorption and electron transport, the catalytic performance can be adjusted with the shortening of site distance. In addition, the effect of the arrangement spacing of the surface metal active atoms on the catalytic performance of metal nanocatalysts is studied. Finally, a comprehensive summary and outlook of the relationship between atomic distance and catalytic performance is given.

Realizing a high OER activity in new single-atom catalysts formed by introducing TMNx (x = 3 and 4) units into carbon nanotubes using high-throughput calculations

Exploring highly efficient electrocatalysts for the oxygen evolution reaction (OER) is of great significance for hydrogen production through water splitting. By means of high-throughput density functional theory (DFT) calculations, we investigated the OER catalytic activity of a series of one-dimensional carbon nanotube (CNT)-based systems containing TMN4 or TMN3 functional units. Through the screening of 3d/4d/5d transition metals (TMs) from Group IVB to Group VIII, eight newly obtained TMNx@CNT (x = 3 and 4) systems were found to exhibit excellent OER activity, with very low overpotentials in the range 0.29-0.51 V, where the Co, Rh, Ir, Ti, Fe, and Ru atoms could be used as active sites. It was found that under the framework of TMN3@CNTs, the pre-adsorption of some species from water dissociation on the relevant TM sites (TM = Ti, Fe, and Ru) could lead to a high OER catalytic activity, which was different from the general situation where OER reactions directly occur on the clean surfaces of the remaining systems with Co/Rh/Ir metal centers. Moreover, the catalytic mechanisms were analyzed in detail. This work can be conducive to obtaining low-cost and high-performance OER single-atom electrocatalysts based on excellent CNT nanomaterials.

Pyridine Functionalized Carbon Nanotubes: Unveiling the Role of External Pyridinic Nitrogen Sites for Oxygen Reduction Reaction

Pyridinic nitrogen has been recognized as the primary active site in nitrogen-doped carbon electrocatalysts for the oxygen reduction reaction (ORR), which is a critical process in many renewable energy devices. However, the preparation of nitrogen-doped carbon catalysts comprised of exclusively pyridinic nitrogen remains challenging, as well as understanding the precise ORR mechanisms on the catalyst. Herein, a novel process is developed using pyridyne reactive intermediates to functionalize carbon nanotubes (CNTs) exclusively with pyridine rings for ORR electrocatalysis. The relationship between the structure and ORR performance of the prepared materials is studied in combination with density functional theory calculations to probe the ORR mechanism on the catalyst. Pyridinic nitrogen can contribute to a more efficient 4-electron reaction pathway, while high level of pyridyne functionalization result in negative structural effects, such as poor electrical conductivity, reduced surface area, and small pore diameters, that suppressed the ORR performance. This study provides insights into pyridine-doped CNTs-functionalized for the first time via pyridyne intermediates-as applied in the ORR and is expected to serve as valuable inspiration in designing high-performance electrocatalysts for energy applications.

Construction of Single-Atom Catalysts for N, O Synergistic Coordination and Application to Electrocatalytic O2 Reduction

Replacing expensive platinum oxygen reduction reaction (ORR) catalysts with atomically dispersed single-atom catalysts is an effective way to improve the energy conversion efficiency of fuel cells. Herein, a series of single-atom catalysts, TM-N2O2Cx (TM=Sc-Zn) with TM-N2O2 active units, were designed, and their catalytic performance for electrocatalytic O2 reduction was investigated based on density functional theory. The results show that TM-N2O2Cx exhibits excellent catalytic activity and stability in acidic media. The eight catalysts (TM=Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) are all 4e- reaction paths, among which Sc-N2O2Cx, Ti-N2O2Cx, and V-N2O2Cx follow dissociative mechanisms and the rest are consistent with associative mechanisms. In particular, Co-N2O2Cx and Ni-N2O2Cx enable a smooth reduction in O2 at small overpotentials (0.44 V and 0.49 V, respectively). Furthermore, a linear relationship between the adsorption free energies of the ORR oxygen-containing intermediates was evident, leading to the development of a volcano plot for the purpose of screening exceptional catalysts for ORR. This research will offer a novel strategy for the design and fabrication of exceptionally efficient non-precious metal catalysts on an atomic scale.

Quantifying Carbon Active Sites Chemisorbing Hydrogen on Oxygen Containing Activated Carbons during Heat Treatment in Hydrogen Atmosphere

Carbon edge sites have been widely studied because of their importance in surface reactivity and electronic properties. The surface chemistry of the carbon edge sites is relevant to various reactions, and carbon active sites are key topics in many applications. Temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) techniques are used to clarify the fate of oxygen atoms present as CO-yielding functional groups on the activated carbon during heat treatment in hydrogen with an argon balance atmosphere. It has been elucidated that CO is decomposed, H2O is released by a reduction reaction with atmospheric H2, and CO2 is evolved by secondary reactions from the CO-yielding functional groups during TPR. Atmospheric H2 consumption during TPR is observed and its rate is characterized. The amounts of carbon active sites are quantified by determining the amount of H2 chemisorbed onto the carbon surfaces. Finally, it is quantitatively determined that the active sites that chemisorb hydrogen are generated after the decomposition of CO and CO2 caused by secondary reactions between ca. 700 and 1100 K from the CO-yielding functional groups. The origin of these CO-yielding functional groups is generally attributed to phenol/ether groups. In addition to these oxygen-containing functional group decompositions, some free sites on the edge sites are activated for H2 chemisorption by heat treatment between ca. 700 and 1100 K.

Extension of Non-alternant Nanographenes Containing Nitrogen-Doped Stone-Thrower-Wales Defects

Non-alternant topologies have attracted considerable attention due to their unique physiochemical characteristics in recent years. Here, three novel topological nanographenes molecular models of nitrogen-doped Stone-Thrower-Wales (S-T-W) defects were achieved through intramolecular direct arylation. Their chemical structures were unambiguously elucidated by single-crystal analysis. Among them, threefold intramolecular direct arylation compound (C42 H21 N) is the largest nanographene bearing a N-doped non-alternant topology to date, in which the non-benzenoid rings account for 83 % of the total molecular skeleton. The absorption maxima of this compound was located in the near-infrared region with a long tail up to 900 nm, which was much longer than those reported for similarly sized N-doped nanographene with six-membered rings (C40 H15 N). In addition, the electronic energy gaps of these series compounds clearly decreased with the introduction of non-alternant topologies (from 2.27 eV to 1.50 eV). It is noteworthy that C42 H21 N possesses such a low energy gap (Eg opt =1.40 eV; Eg cv =1.50 eV), yet is highly stable under ambient conditions. Our work reported herein demonstrates that the non-alternant topology could significantly influence the electronic configurations of nanocarbons, where the introduction of a non-alternanting topology may be an effective way to narrow the energy gap without extending the molecular π-conjugation.

Transition metal chalcogenides carbon-based as bifunctional cathode electrocatalysts for rechargeable zinc-air battery: An updated review

The rechargeable alkaline aqueous zinc-air batteries (ZABs) are prospective candidates to supply the energy demand for their high theoretical energy density, inherent safety, and environmental friendliness. However, their practical application is mainly restricted by the unsatisfactory efficiency of the air electrode, leading to an intense search for high-efficient oxygen electrocatalysts. In recent years, the composites of carbon materials and transition metal chalcogenides (TMC/C) have emerged as promising alternatives because of the unique properties of these single compounds and the synergistic effect between them. In this sense, this review presented the electrochemical properties of these composites and their effects on the ZAB performance. The operational fundamentals of the ZABs were described. After elucidating the role of the carbon matrix in the hybrid material, the latest developments in the ZAB performance of the monometallic structure and spinel of TMC/C were detailed. In addition, we report topics on doping and heterostructure due to the large number of studies involving these specific defects. Finally, a critical conclusion and a brief overview sought to contribute to the advancement of TMC/C in the ZABs.

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.

Co-based Catalysts for Selective H2 O2 Electroproduction via 2-electron Oxygen Reduction Reaction

Electrochemical production of hydrogen peroxide (H₂ O₂ ) via two-electron oxygen reduction reaction (ORR) process is emerging as a promising alternative method to the conventional anthraquinone process. To realize high-efficiency H₂ O₂ electrosynthesis, robust and low cost electrocatalysts have been intensively pursued, among which Co-based catalysts attract particular research interests due to the earth-abundance and high selectivity. Here, we provide a comprehensive review on the advancement of Co-based electrocatalyst for H₂ O₂ electroproduction. The fundamental chemistry of 2-electron ORR is discussed firstly for guiding the rational design of electrocatalysts. Subsequently, the development of Co-based electrocatalysts involving nanoparticles, compounds and single atom catalysts is summarized with the focus on active site identification, structure regulation and mechanism understanding. Moreover, the current challenges and future directions of the Co-based electrocatalysts are briefly summarized in this review.

Curvature effects on the bifunctional oxygen catalytic performance of single atom metal-N-C

Understanding the fundamental relationship between the structural information of electrocatalysts and their catalytic activities plays a key role in controlling many important electrochemical processes. Recently, single-atom catalysts (SACs) with the so-called MN₄ structure, consisting of a central transition metal quadruply bound to four pyridine nitrogen atoms all situated in an extended carbon-based matrix, have attracted intensive scientific attention owing to their exceptional catalytic performance. In this work, we perform the first-principles density functional theory (DFT) calculations to explore the curvature effects of the carbon matrix surfaces on the catalytic activities for two fundamental electrochemical processes, namely, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Our DFT results suggest that the curved surface structure can weaken the interaction between the metal atom and the N-doped carbon matrix, modify the electronic structure of the metal atom, and thus increase the adsorption strength of the reaction intermediates, resulting in enhanced OER and ORR catalytic activities of MN₄ catalysts. More importantly, a prediction model is developed to evaluate the bifunctional catalytic activities of such catalysts based on their directly obtained parameters including the surface curvature of the catalysts, the number of d electrons of the metal element, and the electronegativity of the metal atom and its coordination atoms in MN₄ catalysts. This prediction model not only provides some candidates, for example, FeN₄, CoN₄ and OsN₄ for the ORR; CoN₄, NiN₄, RuN₄, RhN₄ and IrN₄ for the OER; and CoN₄, RuN₄, IrN₄ and OsN₄ for the bifunctional ORR and OER, but also reasonably links the structure of catalysts with their catalytic performance, providing new possibilities for the quick design of high-performance catalysts.

Space-Confined One-Step Growth of 2D MoO2 /MoS2 Vertical Heterostructures for Superior Hydrogen Evolution in Alkaline Electrolytes

2D material-based heterostructures are constructed by stacking or spicing individual 2D layers to create an interface between them, which have exotic properties. Here, a new strategy for the in situ growth of large numbers of 2D heterostructures on the centimeter-scale substrate is developed. In the method, large numbers of 2D MoS₂ , MoO₂ , or their heterostructures of MoO₂ /MoS₂ are controllably grown in the same setup by simply tuning the gap distance between metal precursor and growth substrate, which changes the concentration of metal precursors feed. A lateral force microscope is used first to identify the locations of each material in the heterostructures, which have MoO₂ on the top of MoS₂ . Noteworthy, the creation of a clean interface between atomic thin MoO₂ (metallic) and MoS₂ (semiconducting) results in a different electronic structure compared with pure MoO₂ and MoS₂ . Theoretical calculations show that the charge redistribution at such an interface results in an improved HER performance on the MoO₂ /MoS₂ heterostructures, showing an overpotential of 60 mV at 10 mA cm^-2 and a Tafel slope of 47 mV dec^-1 . This work reports a new strategy for the in situ growth of heterostructures on large-scale substrates and provides platforms to exploit their applications.

Local Coordination Regulation through Tuning Atomic-Scale Cavities of Pd Metallene toward Efficient Oxygen Reduction Electrocatalysis

Moderate adsorption of oxygenated intermediates takes a significant role in rational design of high-efficiency oxygen reduction reaction (ORR) electrocatalysts. Long-serving as a reliable strategy to tune geometric structure of nanomaterials, defect engineering enjoys the great ability of adjusting the coordination environment of catalytic active sites, which enables dominant regulation of adsorption energy and kinetics of ORR catalysis. However, limited to controllable nanocrystals fabrication, inducing uniformly dispersed high-coordinated defects into ultrathin 2D nanosheets remains challenging. Herein, atomic-scale cavities (ASCs) are proposed as a new kind of high-coordinated active site and successfully introduced into suprathin Pd (111)-exposed metallene. Due to its atomic concave architecture, leading to elevated CN and moderately downshifted d-band center, the as-made Pd metallene with ASCs (c-Pd M) exhibits excellent ORR performance with mass activity of 2.76 A mg_Pd ^-1 at 0.9 V versus reversible hydrogen electrode (RHE) and half-wave potential as high as 0.947 V, which is 18.9 (2.7) times higher and 104 (46) mV larger than that of commercial Pt/C (Pd metallene without ASCs). Besides, the durability of c-Pd M exceeds its commercial counterpart with ≈30% loss after 5000 cycles. This work highlights a new-style mentality of designing fancy active sites toward efficient ORR electrocatalysis.

Atomistic Understanding of Two-dimensional Electrocatalysts from First Principles

Two-dimensional electrocatalysts have attracted great interest in recent years for renewable energy applications. However, the atomistic mechanisms are still under debate. Here we review the first-principles studies of the atomistic mechanisms of common 2D electrocatalysts. We first introduce the first-principles models for studying heterogeneous electrocatalysis then discuss the common 2D electrocatalysts with a focus on N doped graphene, single metal atoms in graphene, and transition metal dichalcogenides. The reactions include hydrogen evolution, oxygen evolution, oxygen reduction, and carbon dioxide reduction. Finally, we discuss the challenges and the future directions to improve the fundamental understanding of the 2D electrocatalyst at atomic level.

Modulating the Electronic Structure of FeCo Nanoparticles in N-Doped Mesoporous Carbon for Efficient Oxygen Reduction Reaction

The development of highly efficient and stable oxygen reduction electrocatalysts and revealing their underlying catalytic mechanism are crucial in expanding the applications of metal-air batteries. Herein, an excellent FeCo alloy nanoparticles (NPs)-decorated N-doped mesoporous carbon electrocatalyst (FeCo/NC) for oxygen reduction reaction, prepared through the pyrolysis of a dual metal containing metal-organic framework composite scaffold is reported. Benefiting from the highly exposed bimetal active sites and the carefully designed structure, the Fe_0.25 Co_0.75 /NC-800 catalyst exhibits a promising electrocatalytic activity and a superior durability, better than those of the state-of-the-art catalysts. Suggested by both the X-ray absorption fine structures and the density functional theoretical calculation, the outstanding catalytic performance is originated from the synergistic effects of the bimetallic loading in NC catalysts, where the electronic modulation of the Co active sites from the nearby Fe species leads to an optimized binding strength for reaction intermediates. This work demonstrates a class of highly active nonprecious metals electrocatalysts and provides valuable insights into investigating the structure-performance relationship of transition metal-based alloy catalysts.

Two-Dimensional Metal-Organic Frameworks as Ultrahigh-Performance Electrocatalysts for the Fuel Cell Cathode: A First-Principles Study

Except for metal-organic frameworks (MOFs) with traditional metal-nitrogen sites, MOFs with metal-oxygen sites may also possess good oxygen reduction reaction (ORR) catalytic activity due to their unique electronic structures. Herein, using density functional theory methods, the ORR performances of a series of M₃(HHTT)₂ (where M is a 3d, 4d, or 5d transition metal and HHTT is 2,3,7,8,12,13-hexahydroxytetraazanaphthotetraphene)) catalysts are explored. The binding energy (ΔE_species) results suggest that the binding energy of *OH (ΔE_*OH) shows a good linear relationship with the binding energies of *O and *OOH (ΔE_*O and ΔE_*OOH, respectively), indicating that ΔE_*OH can serve as a descriptor to reflect the catalytic activity of M₃(HHTT)₂. In addition, the volcano plot suggests that M₃(HHTT)₂ catalysts with a moderate binding strength of the intermediate *OH (0.6 eV < ΔE_*OH < 0.9 eV) show relatively high ORR activity. Therefore, four highly active ORR catalysts are screened out, namely, Fe₃(HHTT)₂, Co₃(HHTT)₂, Rh₃(HHTT)₂, and Ir₃(HHTT)₂, which possess very small overpotentials of 0.35, 0.24, 0.31, and 0.29 V, respectively. Their potential-determining step is the reduction of O₂ to the intermediate *OOH. It is encouraging that the theoretically lowest overpotential of this kind of catalyst is 0.21 V, which is superior to that on Pt(111). Moreover, Co₃(HHTT)₂ has excellent poisoning-tolerance ability for impurity gases (CO, NO, and SO₂) as well as fuel molecules (CH₃OH and HCOOH).

Ordered carbonaceous frameworks: a new class of carbon materials with molecular-level design

Ordered carbonaceous frameworks (OCFs) are a new class of carbon materials with a three-dimensional ordered structure synthesized by simple carbonization of metalloporphyrin crystals with polymerizable moieties. Carbonization via solid-state polymerization results in the formation of graphene-based ordered frameworks in which regularly aligned single-atomic metals are embedded. These unique structural features afford molecular-level designability like organic-based frameworks together with high electrical conductivity, thermal/chemical stability, and mechanical flexibility, towards a variety of applications including electrocatalysis and force-driven phase transition. This feature article summarizes the synthetic strategies and characteristics of OCFs in comparison with conventional organic-based frameworks and porous carbons, to discuss the potential applications and further development of the OCF family.

Size Optimization of a N-Doped Graphene Nanocluster for the Oxygen Reduction Reaction

N-Doped graphene nanoclusters (N-GNCs) are promising electrocatalysts for the oxygen reduction reaction (ORR) at the cathode of fuel cells. In this study, the dependence of the ORR activity on the size of N-GNCs was investigated using first-principles calculations based on density functional theory. The maximum electrode potential (U _Max) was estimated from the free energy of the reaction intermediates of the ORR. U _Max was predicted to show a volcanic trend with respect to the cluster size. The results suggest that C₂₁₅H₃₆N with a radius of 13.6 Å is the best candidate for ORRs and is better than platinum in terms of U _Max. The volcano-shaped plot of U _Max is attributed to the switch of the reaction step that determines U _Max, which is caused by the destabilization of reaction intermediates. Such changes in the stability of the intermediates can be explained by the decrease in the local density of states at the reaction site, which is due to the development of the so-called edge state at the zigzag edge. The establishment of experimental techniques to control the cluster size and doping position will be the key to superior catalyst preparation in the future.

Axial ligand engineering for highly efficient oxygen reduction catalyst in transition metal-N4 doped graphene

Developing highly efficient and stable electrocatalysts for oxygen reduction reaction (ORR) is a challenging task in energy conversion technologies. In this work, diverse axial ligands have been used to modify...

Cubic core-shell structure of NiCoSx/CoS2 as high-efficiency tri-functional catalyst for Zn-air battery and overall water splitting

Cubic core-shell NiCoSx/CoS2 composite catalyst was successfully prepared on the basis of K3[Co(CN)6]2. First, Ni2+ is substituted for K+ in the K3[Co(CN)6]2 to prepare the binary metal ion precursor of...

First principles study on electronic properties and oxygen evolution mechanism of 2D bimetallic N-doped graphene

Currently, the oxygen evolution reaction (OER) is constrained by complex four-electron transport, thus it is difficult to understand the catalytic mechanism. In this work, the electronic properties and catalytic performance of M₁M₂/NC (M = Mn, Fe, Co, Ni, Cu and Zn, random combination in pairs) is studied by density functional theory, the calculated results show that the overpotential of FeCu/NC is 0.88 V, which is used as the optimal catalyst to further study the OER reaction mechanism. Combined with the volcano map and the d-band center position, the low overpotential of FeCu/NC is because it has a more suitable position of d-band center -1.806 eV than other materials. Moreover, the calculation results show that the density of states (DOS) of iron-containing materials is stronger than that of other materials near the Fermi level, which can promote the catalytic reaction. In addition, O∗OH and O∗H, O∗H and O∗ linearly related theoretical equations are proposed, respectively. Furthermore, the analysis of the catalytic mechanism shows that the formation of the catalytic rate-determining step is affected by the movement of the d-band center, the distance of the transition state adsorption and the electric field.

Nanocarbon for Energy Material Applications: N2 Reduction Reaction

Nanocarbons are an important class of energy materials and one relevant application is for the nitrogen reduction reaction, i.e., the direct synthesis of NH₃ from N₂ and H₂ O via photo- and electrocatalytic approaches. Ammonia is also a valuable energy or hydrogen vector. This perspective paper analyses developments in the field, limiting discussion to nanocarbon-based electrodes. These aspects are discussed: i) active sites related to charge density differences on C atoms associated to defects/strains, ii) doping with heteroatoms, iii) introduction of isolated metal ions, iv) creation and in situ dynamics of metal oxide(hydroxide)/nanocarbon boundaries, and v) nanocarbon characteristics to control the interface. Discussion is focused on the performances and mechanistic aspects. Aim is not a systematic state-of-the-art report but to highlight the need to use a different perspective in studying this challenging reaction by using selected papers. Notwithstanding the large differences in the proposed nature of the active sites, fall all within a restricted range of performances, far from the targets. A holistic approach is emphasized to make a breakthrough advance.

Substrate strain tunes operando geometric distortion and oxygen reduction activity of CuN2C2 single-atom sites

Single-atom catalysts are becoming increasingly significant to numerous energy conversion reactions. However, their rational design and construction remain quite challenging due to the poorly understood structure–function relationship. Here we demonstrate the dynamic behavior of CuN₂C₂ site during operando oxygen reduction reaction, revealing a substrate-strain tuned geometry distortion of active sites and its correlation with the activity. Our best CuN₂C₂ site, on carbon nanotube with 8 nm diameter, delivers a sixfold activity promotion relative to graphene. Density functional theory and X-ray absorption spectroscopy reveal that reasonable substrate strain allows the optimized distortion, where Cu bonds strongly with the oxygen species while maintaining intimate coordination with C/N atoms. The optimized distortion facilitates the electron transfer from Cu to the adsorbed O, greatly boosting the oxygen reduction activity. This work uncovers the structure–function relationship of single-atom catalysts in terms of carbon substrate, and provides guidance to their future design and activity promotion.

The rational design of single-atom catalysts is challenging. This work reveals a substrate-strain tuned geometry distortion of CuN₂C₂ single-atom site, which greatly boosts oxygen reduction activity by facilitating electron transfer to adsorbed O.

Hexachloroethane dechlorination in sulfide-containing aqueous solutions catalyzed by nitrogen-doped carbon materials

This study demonstrated that nitrogen-doped carbon materials (NCMs) could effectively catalyze the chlorine elimination process in hexachloroethane (HCA) declorination in sulfide-containing environments for the first time. The k_obs values of HCA dechlorination by sulfide in the presence of 10 mg/L NCMs were higher than that of no mediator at pH 7.3 by one or two orders of magnitude. The catalytic capabilities of NCMs on HCA dechlorination were evident in common ranges of natural pH (5.3-8.9) and it could be accelerated by the increase of pH but be suppressed by the presence of dissolved humic acid. Moreover, NCMs exhibited much better catalytic capability on HCA dechlorination compared to the carbon materials, mainly owing to the combined contributions of pyridine N, including enhanced nucleophilic attack to HCA molecule by generating newborn C-S-S and activation of HCA molecule by elongating C-Cl bonds. The functions of pyridine N in micron-sized NCMs with mesopores were better than in nano-sized NCMs on HCA dechlorination. These findings displayed the potential of NCMs, when released into sulfide-containing environments, may significantly increase the dechlorination of chlorinated aliphatic hydrocarbons.

Highly active and selective oxygen reduction to H2O2 on boron-doped carbon for high production rates

Oxygen reduction reaction towards hydrogen peroxide (H₂O₂) provides a green alternative route for H₂O₂ production, but it lacks efficient catalysts to achieve high selectivity and activity simultaneously under industrial-relevant production rates. Here we report a boron-doped carbon (B-C) catalyst which can overcome this activity-selectivity dilemma. Compared to the state-of-the-art oxidized carbon catalyst, B-C catalyst presents enhanced activity (saving more than 210 mV overpotential) under industrial-relevant currents (up to 300 mA cm^-2) while maintaining high H₂O₂ selectivity (85-90%). Density-functional theory calculations reveal that the boron dopant site is responsible for high H₂O₂ activity and selectivity due to low thermodynamic and kinetic barriers. Employed in our porous solid electrolyte reactor, the B-C catalyst demonstrates a direct and continuous generation of pure H₂O₂ solutions with high selectivity (up to 95%) and high H₂O₂ partial currents (up to ~400 mA cm^-2), illustrating the catalyst's great potential for practical applications in the future.

Dual Inorganic Sacrificial Template Synthesis of Hierarchically Porous Carbon with Specific N Sites for Efficient Oxygen Reduction

It is still a challenge to achieve efficiently controlled preparation of functional oxygen reduction reaction (ORR) carbon electrocatalysts with multi-preferred structures (hierarchically porous networks and specific carbon-nitrogen bonds) from carbohydrate-containing small molecules via simple one-step pyrolysis. Based on the step-by-step spontaneous gas-foaming strategy, we successfully prepare 3D hierarchically porous networks with tunable N sites (N_P/N_G ≈ 1:1) by pyrolyzing diverse carbohydrates (glucose, maltose, and cyclodextrin) using nonmetal-metal dual inorganic sacrificial templates. In situ evaporation templates can simplify the procedure of the experiments and avoid the active site loss compared with traditional hard templates. Crucially, dual inorganic sacrificial templates can induce abundant defects and microscopic pore structures (the specific surface area increased from 922.403 to 1898.792 m²·g^-1) and tunable N sites compared with single nonmetal sacrificial templates. The regulatory mechanism of dual inorganic templates on N sites (N_P/N_G ≈ 1:1) is independent of the polymeric state of carbohydrate precursors or even the carbonization condition of the pyrolysis process. A series of carbon materials prepared by this strategy all have ORR-preferred structures and exhibit low ORR overpotentials compared with Pt/C. For instance, the Zn-air battery with βCD-DSC-950-1 exhibits an open-circuit potential of 1.51 V and a peak power density of 180.89 mW·cm^-2, higher than those of Pt/C (1.47 V, 174.94 mW·cm^-2). In general, the conversion of carbohydrate-containing small molecules to functional carbon materials provides a new strategy for the development of carbonaceous electrocatalysts.

Cobalt encapsulated in bamboo-like N-doped carbon nanotubes for highly sensitive electroanalysis of Pb(II): enhancement based on adsorption and catalysis

Carbon nanotubes (CNTs) are recognized as desirable candidates to fabricate electrochemical sensing interfaces owing to their high surface area and excellent electron conductivity. However, the poor catalytic properties of CNTs significantly hinder their further application in electrochemical detection. Herein, for the first time we combined defective CNTs with catalytically active cobalt nanoparticles (Co NPs) to give cobalt encapsulated in a bamboo-like N-doped carbon nanotube nanocomposite (Co/N-CNTs). The novel constructed Co/N-CNTs are used as a modifier on a bare glass carbon electrode for the electrochemical detection of Pb(ii). As a result, the positive effect of adsorption and catalysis on Co/N-CNT shows a significant improvement in the electroanalytical performance towards Pb(ii) with a sensitivity of 69.74 μA μM-1 and a limit of detection of 0.039 μM. Moreover, the stability and practical applications of Co/N-CNTs towards Pb(ii) in real water samples obtained from a mining subsidence area were also considered. This method shows great promise, achieving an outstanding electroanalysis efficiency with noble-metal-free nanocomposite sensors based on the combination of carbon and transition metals.

Oxygen Reduction Reaction on N-Doped Graphene: Effect of Positions and Scaling Relations of Adsorption Energies

The goal of this study is to provide insight into the mechanism of the oxygen reduction reaction (ORR) on N-doped graphene surfaces. Using density functional theory and a computational hydrogen electrode model, we studied the energetics of the ORR intermediates, the effect of the position of the reaction site, and the effect of the position of the N modification relative to the active site on model graphene surfaces containing one or two N atoms. We found that scaling relations can be derived for N-doped graphenes as well, but the multiplicity of the surface should be taken into account. On the basis of the scaling relations between intermediates OOH* and OH*, the minimal overpotential is 0.33 V. Analysis of the data showed that N atoms in the meta position usually decrease the adsorption energy, but those in the ortho position aid the adsorption. The outer position on the zigzag edge of the graphene sheet also promotes the adsorption of oxygenated species, while the inner position hinders it. Looking at the most effective active sites, our analysis shows that the minimal overpotential can be approached with various doping arrangements, which also explains the contradicting results in the literature. The dissociative pathway was also investigated, but we found only one possible active site; therefore, this pathway is not really viable. However, routes not preferred thermodynamically pose the possibility of breaking the theoretical limit of the overpotential of the associative pathway.

Oxygen Reduction Reaction Catalyzed by Carbon-Supported Platinum Few-Atom Clusters: Significant Enhancement by Doping of Atomic Cobalt

Oxygen reduction reaction (ORR) plays an important role in dictating the performance of various electrochemical energy technologies. As platinum nanoparticles have served as the catalysts of choice towards ORR, minimizing the cost of the catalysts by diminishing the platinum nanoparticle size has become a critical route to advancing the technological development. Herein, first-principle calculations show that carbon-supported Pt₉ clusters represent the threshold domain size, and the ORR activity can be significantly improved by doping of adjacent cobalt atoms. This is confirmed experimentally, where platinum and cobalt are dispersed in nitrogen-doped carbon nanowires in varied forms, single atoms, few-atom clusters, and nanoparticles, depending on the initial feeds. The sample consisting primarily of Pt_2~7 clusters doped with atomic Co species exhibits the best mass activity among the series, with a current density of 4.16 A mg_Pt ^-1 at +0.85 V vs. RHE that is almost 50 times higher than that of commercial Pt/C.

Defect and Doping Co-Engineered Non-Metal Nanocarbon ORR Electrocatalyst

Exploring low-cost and earth-abundant oxygen reduction reaction (ORR) electrocatalyst is essential for fuel cells and metal-air batteries. Among them, non-metal nanocarbon with multiple advantages of low cost, abundance, high conductivity, good durability, and competitive activity has attracted intense interest in recent years. The enhanced ORR activities of the nanocarbons are normally thought to originate from heteroatom (e.g., N, B, P, or S) doping or various induced defects. However, in practice, carbon-based materials usually contain both dopants and defects. In this regard, in terms of the co-engineering of heteroatom doping and defect inducing, we present an overview of recent advances in developing non-metal carbon-based electrocatalysts for the ORR. The characteristics, ORR performance, and the related mechanism of these functionalized nanocarbons by heteroatom doping, defect inducing, and in particular their synergistic promotion effect are emphatically analyzed and discussed. Finally, the current issues and perspectives in developing carbon-based electrocatalysts from both of heteroatom doping and defect engineering are proposed. This review will be beneficial for the rational design and manufacturing of highly efficient carbon-based materials for electrocatalysis.

Atomic Arrangement Modulation in CoFe Nanoparticles Encapsulated in N-Doped Carbon Nanostructures for Efficient Oxygen Reduction Reaction

The properties and, hence, the application of materials are dependent on the way their constituent atoms are arranged. Here, we report a facile approach to produce body-centered cubic (bcc) and face-centered cubic (fcc) phases of bimetallic FeCo crystalline nanoparticles embedded into nitrogen-doped carbon nanotubes (NCNTs) with equal loading and almost similar particle size for both crystalline phases by a rational selection of precursors. The two electrocatalysts with similar composition but different crystalline structures of the encapsulated nanoparticles have allowed us, for the first time, to account for the effect of crystal structure on the overall work function of electrocatalysts and the concomitant correlation with the oxygen reduction reaction (ORR). This study unveils that the electrocatalysts with lower work function show lower activation energy to facilitate the ORR. Importantly, the difference between the ORR activation energy on electrocatalysts and their respective work functions are found to be identical (∼0.2 eV). A notable decrease in the ORR activity after acid treatment indicates the significant role of encapsulated FeCo nanoparticles in influencing the oxygen electrochemistry by modulating the material property of overall electrocatalysts.

Amino-1H-tetrazole-regulated high-density nitrogen-doped hollow carbon nanospheres for long-life Zn-air batteries

High-density nitrogen-doped porous carbon catalysts have been regarded as promising alternatives to precious metals in proton-exchange membrane fuel cells (PEMFC) and metal-air batteries based on the oxygen reduction reaction (ORR). We herein synthesized high-density pyridinic and graphitic N-doped hollow carbon nanospheres (G&P N-HCS) using a high-yield amino-1H-tetrazole (ATTZ) via a self-sacrificial-template method. The synthesized G&P N-HCS shows a high N content (15.2 at%), in which pyridinic (Pr) and graphitic (Gr) N are highly reactive for the ORR catalysis. We found that the half-wave potential and limiting current density of G&P N-HCS are comparable to the state-of-the-art Pt/C, whereas its cyclic durability is much superior to that of Pt/C. Experimental results indicate that an optimal ratio (1 : 1) between Gr N and Pr N in G&P N-HCS exhibits the highest ORR performances, rather than Gr N-dominated N-HCS or Pr N-dominated N-HCS. Notably, N-HCS containing only Gr N and Pr N has poor catalytic performance for ORR in alkaline electrolytes. Density functional theory (DFT) simulations untangle the catalytic nature of Pr and Gr N and decipher the relations between the N type(s) and total N content required for the ORR catalysis. This study provides a new way to design efficient N-doped porous carbon-enriched active sites, and solves the cathode catalyst in the commercialization of PEMFC and metal-air batteries.

On the Origin of the Effect of pH in Oxygen Reduction Reaction for Nondoped and Edge-Type Quaternary N-Doped Metal-Free Carbon-Based Catalysts

Metal-free carbon-based catalysts have gained much attention during the last 15 years as an alternative toward the replacement of platinum-based catalysts for the oxygen reduction reaction (ORR). However, carbon-based catalysts only show promising catalytic activity in alkaline solution. Concurrently, the most optimized polymer electrolyte membrane fuel cells use proton exchange membranes. This means that the cathode electrode is surrounded by a protonic environment in which carbon materials show poor performance, with differences above 0.5 V in E_ONSET for nondoped carbon materials. Therefore, the search for highly active carbon-based catalysts is only possible if we first understand the origin of the poor electrocatalytic activity of this kind of catalysts in acidic conditions. We address this matter through a combined experimental and modeling study, which yields fundamental principles on the origin of the pH effects in ORR for carbon-based materials. This is relevant for the design of pH-independent metal-free carbon-based catalysts.

Atomically deviated Pd-Te nanoplates boost methanol-tolerant fuel cells

The methanol crossover effect in direct methanol fuel cells (DMFCs) can severely reduce cathodic oxygen reduction reaction (ORR) performance and fuel efficiency. As a result, developing efficient catalysts with simultaneously high ORR activity and excellent antipoisoning methanol capability remains challenging. Here, we report a class of Pd-Te hexagonal nanoplates (HPs) with a Pd20Te7 phase that simultaneously overcome the activity and methanol-tolerant issues in alkaline DMFC. Because of the specific arrangement of Pd atoms deviated from typical hexagonal close-packing, Pd-Te HPs/C displays extraordinary methanol tolerance with high ORR performance compared with commercial Pt/C. DFT calculations reveal that the high performance of Pd-Te HPs can be attributed to the breakthrough of the linear relationship between OOH* and OH* adsorption, which leaves sufficient room to improve the ORR activity but suppresses the methanol oxidation reaction. The concurrent high ORR activity and excellent methanol tolerance endow Pd-Te HPs as practical electrocatalysts for DMFC and beyond.

Iron Nanoparticles Encapsulated in S,N-Codoped Carbon: Sulfur Doping Enriches Surface Electron Density and Enhances Electrocatalytic Activity toward Oxygen Reduction

Development of highly efficient nonprecious metal (NPM) catalysts for oxygen reduction reaction (ORR) in acidic media is challenging but of great significance. Herein, an effective ORR catalyst based on Fe nanoparticles encapsulated by S,N-codoped few-layer defective carbon (Fe@S,N-DC) was synthesized via a microwave-assisted strategy. The obtained Fe@S,N-DC nanocomposite showed a remarkable electrocatalytic activity toward ORR in acidic media, with a half-wave potential (E_1/2) of +0.785 V versus reversible hydrogen electrode, which was 80 mV more positive than that of the sulfur-free counterpart (Fe@N-DC). Furthermore, due to the protection by the S,N-codoped carbon shell, the Fe@S,N-DC nanocomposite displayed apparent stability with only a 13 mV negative shift of E_1/2 after 10,000 cycles and excellent tolerance to methanol. X-ray absorption near-edge spectroscopy measurements confirmed the formation of multiple defective sites on the S,N-codoped carbon surface and strong interfacial electron transfer from the Fe core to the outer carbon surface, as compared to the sulfur-free counterpart. The enriched electron density on the defective carbon surface of Fe@S,N-DC, induced by the interfacial electron transfer, facilitated the reduction of O₂ to OOH*, leading to enhanced ORR performance. These results shed light on the significance of S doping in Fe-N-C catalysts in the design of high-performance NPM catalysts for ORR in acidic media.

Theoretical Study on a Nitrogen-Doped Graphene Nanoribbon with Edge Defects as the Electrocatalyst for Oxygen Reduction Reaction

Both theory and experiment show that sp2 carbon nanomaterials doped with N have great potential as high-efficiency catalysts for oxygen reduction reactions (ORR). At present, there are theoretical studies that believe that C-sites with positive charge or high-spin density values have higher adsorption capacity, but there are always some counter examples, such as the N-doped graphene nanoribbons with edge defects (ND-GNR) of this paper. In this study, the ORR mechanism of ND-GNR was studied by density functional theory (DFT) calculation, and then the carbon ring resonance energy was analyzed from the perspective of chemical graph theory to elucidate the cause and distribution of active sites in ND-GNR. Finally, it was found that the overpotential of the model can be adjusted by changing the width of the model or dopant atoms while still ensuring proper adsorption energy (between 0.5 and 2.0 eV). The minimum overpotential for these models is approximately 0.36 V. These findings could serve as guidelines for the construction of efficient ORR carbon nanomaterial catalysts.