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

γ-Graphyne nanotubes as defect-free catalysts of the oxygen reduction reaction: a DFT investigation

γGyNTs as excellent metal-free ORR catalysts without any defects.

Quantum proton tunneling in multi-electron/-proton transfer electrode processes

Quantum proton tunneling in multi-electron/-proton transfer electrode processes were investigated in order to understand their possible microscopic mechanisms.

Theoretical research on the oxidation mechanism of doped carbon based catalysts for oxygen reduction reaction

To understand the essential reasons of poor durability and rapid initial performance loss of heteroatom doped graphene catalysts during the electrochemical oxygen reduction reaction (ORR) process, it is necessary to explore the detailed mechanism of carbon active site oxidation reaction (COR) at different electrode potentials, as it may greatly influence the ORR activity. Herein, density functional theory (DFT) calculation is used to investigate all possible COR mechanisms, including Direct-COR and Indirect-COR, on four typical doped-graphene, and understand the competing relation between COR and ORR from a thermodynamic point of view. Our systematic calculations found that the Direct-COR is affected directly by the structural stability of doped-graphene relative to pure graphite, and the Indirect-COR can be accelerated largely by the ORR process due to the ORR intermediate, such as O and OOH. The competition relation between COR and ORR is mainly influenced by the interaction between the doped-graphene and reaction species, stability of doped-structure, ORR mechanism, and electrode potential. For COR, the partial oxidation of doped-graphene is the dominant oxidation reaction compared to complete oxidation in the ORR potential range. More importantly, both partial and complete oxidation of doped-graphene can remarkably depress the ORR activity. Hence, COR should be one of the major contributors to the rapid initial performance loss of carbon based catalysts in stability testing. Our results provide a comprehensive and deep understanding of the oxidation of carbon active sites on doped-graphene surfaces and can guide the design of more robust doped-carbon based catalysts.

Atomic- and Molecular-Level Design of Functional Metal-Organic Frameworks (MOFs) and Derivatives for Energy and Environmental Applications

Continuing population growth and accelerated fossil-fuel consumption with recent technological advancements have engendered energy and environmental concerns, urging researchers to develop advanced functional materials to overcome the associated problems. Metal-organic frameworks (MOFs) have emerged as frontier materials due to their unique porous organic-inorganic hybrid periodic assembly and exceptional diversity in structural properties and chemical functionalities. In particular, the modular nature and modularity-dependent activity of MOFs and MOF derivatives have accentuated the delicate atomic- and molecular design and synthesis of MOFs, and their meticulous conversion into carbons and transition-metal-based materials. Synthetic control over framework architecture, content, and reactivity has led to unprecedented merits relevant to various energy and environmental applications. Herein, an overview of the atomic- and molecular-design strategies of MOFs to realize application-targeted properties is provided. Recent progress on the development of MOFs and MOF derivatives based on these strategies, along with their performance, is summarized with a special emphasis on design-structure and functionality-activity relationships. Next, the respective energy- and environmental-related applications of catalysis and energy storage, as well as gas storage-separation and water harvesting with close association to the energy-water-environment nexus are highlighted. Last, perspectives on current challenges and recommendations for further development of MOF-based materials are also discussed.

Rational Design of Carbon-Rich Materials for Energy Storage and Conversion

Carbon-rich materials have drawn tremendous attention toward a wide spectrum of energy applications due to their superior electronic mobility, good mechanical strength, ultrahigh surface area, and more importantly, abundant diversity in structure and components. Herein, rationally designed and bottom-up constructed carbon-rich materials for energy storage and conversion are discussed. The fundamental design principles are itemized for the targeted preparation of carbon-rich materials and the latest remarkable advances are summarized in terms of emerging dimensions including sp² carbon fragment manipulation, pore structure modulation, topological defect engineering, heteroatom incorporation, and edge chemical regulation. In this respect, the corresponding structure-property relationships of the resultant carbon-rich materials are comprehensively discussed. Finally, critical perspectives on future challenges of carbon-rich materials are presented. The progress highlighted here will provide meaningful guidance on the precise design and targeted synthesis of carbon-rich materials, which are of critical importance for the achievement of performance characteristics highly desirable for urgent energy deployment.

Untangling Cooperative Effects of Pyridinic and Graphitic Nitrogen Sites at Metal-Free N-Doped Carbon Electrocatalysts for the Oxygen Reduction Reaction

Metal-free carbon electrodes with well-defined composition and smooth topography are prepared via sputter deposition followed by thermal treatment with inert and reactive gases. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy show that three carbons of similar N/C content that differ in N-site composition are thus prepared: an electrode consisting of almost exclusively graphitic-N (N_G ), an electrode with predominantly pyridinic-N (N_P ), and one with ≈1:1 N_G :N_P composition. These materials are used as model systems to investigate the activity of N-doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity toward 4e-reduction of O₂ is strongly influenced by the N_G /N_P site composition, with the material possessing nearly uniform N_G /N_P composition being the only one yielding a 4e-reduction. Computational studies on model graphene clusters are carried out to elucidate the effect of N-site homogeneity on the reaction pathway. Calculations show that for pure N_G -doping or N_P -doping of model graphene clusters, adsorption of hydroperoxide and hydroperoxyl radical intermediates, respectively, is weak, thus favoring desorption prior to complete 4e-reduction to hydroxide. Clusters with mixed N_G /N_P sites display synergistic effects, suggesting that co-presence of these sites improves activity and selectivity by achieving high theoretical reduction potentials while facilitating retention of intermediates.

Stability of pyridine-like and pyridinium-like nitrogen in graphene

Identification of N configurations in N-doped graphene is important to improve its functionalities. In most experiments, graphite-like, pyridine-like, and pyrrole-like N was usually chosen to assign the spectral features of N-doped graphene. Based on the first-principles simulations, we find that since one of nitrogen's electrons occupies the LUMO state, stability of pyridinium-like N (pyridine-like N with hydrogen termination) in multi-benzenes systems increases with the HOMO-LUMO gap decreasing. Therefore, unlike benzene with a large HOMO-LUMO gap, pyridinium-like N is a stable configuration in graphene with zero band gap, and the pyridinium-like N should be considered during the assignments of N configurations even under the environments with higher temperatures and lower hydrogen partial pressure than ambient conditions.

Oxygen Functionalization of Hexagonal Boron Nitride on Ni(111)

The interaction of single-layer hexagonal boron nitride (h-BN) on Ni(111) with molecular oxygen from a supersonic molecular beam led to a covalently bonded molecular oxygen species, which was identified as being between a superoxide and a peroxide. This is a rare example of an activated adsorption process leading to a molecular adsorbate. The amount of oxygen functionalization depended on the kinetic energy of the molecular beam. For a kinetic energy of 0.7 eV, an oxygen coverage of 0.4 ML was found. Near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy revealed a stronger bond of h-BN to the Ni(111) substrate in the presence of the covalently bound oxygen species. Oxygen adsorption also led to a shift of the valence bands to lower binding energies. Subsequent temperature-programmed X-ray photoelectron spectroscopy revealed that the oxygen boron bonds are stable up to approximately 580 K, when desorption, and simultaneously, etching of h-BN set in. The experimental results were substantiated by density functional theory calculations, which provided insight to the adsorption geometry, the adsorption energy and the reaction pathway.

Structural transformations of graphene exposed to nitrogen plasma: quantum chemical molecular dynamics simulations

Nitrogen-doped graphene (N-graphene) has been intensively studied for tailoring the electronic property of the graphene, because different nitrogen configurations influence the electronic properties of N-graphene in different ways. However, atomically precise control of the nitrogen configurations during the doping process remains a challenge in the synthesis of N-graphene. Moreover, additional structural transformations of the graphene carbon network structure as a side-effect of plasma doping are little understood and are as of yet uncontrollable. Therefore, we theoretically investigated the nitrogen doping process of graphene for a range of nitrogen atom incident kinetic energies in nonequilibrium quantum chemical molecular dynamics (QM/MD) simulations. We observed and characterized prominent configurations of N-containing graphene. In analogy to similar, earlier studies of graphene plasma hydrogenation, we observed an Eley-Rideal associative desorption mechanism during the graphene plasma nitrogenation, producing molecular nitrogen. Especially for graphitic-N (Gr-N) and Stone-Wales-defect-N (SW-N) configurations, which are frequently observed in experimental studies, we discovered two typical chemical reaction mechanisms which were well categorized by two key processes: adsorption of primary nitrogen dopant and collision with a secondary nitrogen dopant. We discussed effects of the incident nitrogen energy on the formation mechanism, and propose a method to generate of Gr-N and SW-N configurations selectively by tuning the conditions with respect to the two key formation processes.

Bifunctional mechanism of N, P co-doped graphene for catalyzing oxygen reduction and evolution reactions

The development of bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly desirable for fuel cells and rechargeable metal-air batteries. Till now, it is still challenging to achieve both efficient activities on a single commercial noble-metal catalyst. Recently, N, P co-doped graphene has shown good bifunctional evidence. However, the atomic-scale understanding of the bifunctional mechanism is still lacking. Here, we show that the N and P atoms prefer to bond with each other, forming embedded N-P clusters in graphene. The catalytic performances of the N-P clusters are sensitive to their geometries, especially the N:P ratios. The N:P ratio of ∼2 is optimal for OER, while ∼3 is optimal for ORR. Through evaluating the ORR/OER potential gaps, we found that the N-P cluster designated as N_C ²P_C ¹ shows both the high performances of ORR and OER, responsible for the unique bifunctionality in the N, P co-doped graphene.

Carbon-Based Metal-Free ORR Electrocatalysts for Fuel Cells: Past, Present, and Future

Replacing precious platinum with earth-abundant materials for the oxygen reduction reaction (ORR) in fuel cells has been the objective worldwide for several decades. In the last 10 years, the fastest-growing branch in this area has been carbon-based metal-free ORR electrocatalysts. Great progress has been made in promoting the performance and understanding the underlying fundamentals. Here, a comprehensive review of this field is presented by emphasizing the emerging issues including the predictive design and controllable construction of porous structures and doping configurations, mechanistic understanding from the model catalysts, integrated experimental and theoretical studies, and performance evaluation in full cells. Centering on these topics, the most up-to-date results are presented, along with remarks and perspectives for the future development of carbon-based metal-free ORR electrocatalysts.

Defect Engineering and Surface Functionalization of Nanocarbons for Metal-Free Catalysis

With the advent of carbon nanotechnology, which initiated significant research efforts more than two decades ago, novel materials for energy harvesting and storage have emerged at an amazing pace. Nevertheless, some fundamental applications are still dominated by traditional materials, and it is especially evident in the case of catalysis, and environmental-related electrochemical reactions, where precious metals such as Pt and Ir are widely used. Several strategies are being explored for achieving competitive and feasible metal-free carbon nanomaterials, among which doping and defect engineering approaches within nanocarbons are recurrent and promising. Here, the most recent efforts regarding the control of doping and defects in carbon nanostructures for catalysis, and in particular for energy-related applications, are addressed. Finally, an overview of alternative proposals that can make a difference when enabling carbon nanomaterials as efficient and emerging catalysts is presented.

Effect of Water on the Manifestation of the Reaction Selectivity of Nitrogen-Doped Graphene Nanoclusters toward Oxygen Reduction Reaction

We investigated the selectivity of N-doped graphene nanoclusters (N-GNCs) toward the oxygen reduction reaction (ORR) using first-principles calculations within the density functional theory. The results show that the maximum electrode potentials (U _Max) for the four-electron (4e^-) pathway are higher than those for the two-electron (2e^-) pathway at almost all of the reaction sites. Thus, the N-GNCs exhibit high selectivity for the 4e^- pathway, that is, the 4e^- reduction proceeds preferentially over the 2e^- reduction. Such high selectivity results in high durability of the catalyst because H₂O₂, which corrodes the electrocatalyst, is not generated. For the doping sites near the edge of the cluster, the value of U _Max greatly depends on the reaction sites. However, for the doping sites around the center of the cluster, the reaction-site dependence is hardly observed. The GNC with a nitrogen atom around the center of the cluster exhibits higher ORR catalytic capability compared with the GNC with a nitrogen atom in the vicinity of the edge. The results also reveal that the water molecule generated by the ORR enhances the selectivity toward the 4e^- pathway because the reaction intermediates are significantly stabilized by water.

Enhancing hydrogen evolution activity by doping and tuning the curvature of manganese-embedded carbon nanotubes

Doping heteroatoms (Mn and N) and tuning the curvature of carbon nanotubes could efficiently elevate the C p-band center, lower the absolute electrode potential, and thus enhance the HER performance.

Fe2 N/S/N Codecorated Hierarchical Porous Carbon Nanosheets for Trifunctional Electrocatalysis

Construction of multifunctional highly active earth-abundant electrocatalysts on a large scale is a great challenge due to poor control over nanostructural features and limited active sites. Here, a simple methodology to tailor metal-organic frameworks (MOFs) to extract highly active multifunctional electrocatalysts on a large scale for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution reaction (HER) is presented. The N, S codoped Fe₂ N decorated highly porous and defect-rich carbon nanosheets are grown using MOF xerogels, melamine, and polyvinylpyrollidone. The resulting catalyst exhibits excellent activity for ORR with an onset (0.92 V) and half-wave (0.81 V) potential similar to state-of-the-art Pt/C catalysts. The catalyst also shows outstanding OER and HER activities with a small overpotential of 360 mV in 1 m KOH and -123 mV in 0.5 m H₂ SO₄ at a current density of 10 mA cm^-2 , respectively. Excellent catalytic properties are further supported by theoretical calculations where relevant models are built and various possible activation sites are identified by first-principles calculations. The results suggest that the carbon atoms adjacent to heteroatoms as well as Fe₂ -N sites present the active sites for improved catalytic response, which is in agreement with the experimental results.

Activity Origins in Nanocarbons for the Electrocatalytic Hydrogen Evolution Reaction

Sustainable hydrogen production is an essential prerequisite for realizing the future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the cornerstone of exploring the mechanism of water electrolysis, has attracted extensive interest in the past decades. Carbon-based materials with significant merits such as abundance, low cost, high conductivity, and tunable molecular structures, are considered as promising candidates for replacing the commercial noble metal electrocatalysts. To date, activity origins of these carbon-based electrocatalysts are mainly attributed to the dopants (e.g., N, B, P or S), whereas the contribution of intrinsic/induced carbon defects has recently been a hot research topic. In this Review, besides the development of heteroatoms doping strategies, the latest studies on defective carbon-based materials for HER electrocatalysis are summarized, especially for various approaches to prepare defective carbons and the detailed introduction regarding the defect catalysis mechanism. Finally, an outlook into the development of future defective carbon-based HER electrocatalysts is presented.

Modulating the oxygen reduction activity of heteroatom-doped carbon catalysts via the triple effect: charge, spin density and ligand effect

For metal-doped-G and dual-heteroatom-doped-G, the triple effect can activate double carbon sites, and lead to the ORR following the dissociative mechanism, which breaks through the activity limitation of the association mechanism, and further enhances the catalytic activity for the ORR.

To enhance the intrinsic activity of and increase the number of active sites in heteroatom-doped graphene (doped-G), it is necessary to recognize the origin of its catalytic activity, and to search for a universal definition and description of the different active sites. Herein, we report an evaluation of a series of heteroatom-doped graphene materials as oxygen reduction reaction (ORR) catalysts with the aid of density functional theory (DFT) calculations. The results of the DFT calculations indicate that the intrinsic catalytic activity and the ORR mechanism depend on the triple effect, that is, the charge, the spin density and the coordinate state (ligand effect) of the carbon sites. The contribution of the above effects towards increasing the binding energies of *OOH or *OH has the following order: negative charge effect < positive charge effect < low spin effect < ligand effect < high spin effect. For nonmetallic single-heteroatom-doped-G, the triple effect separately activates the carbon sites around the doped atom. On each single carbon active site, only the end-on adsorption of *OOH is preferred, leading to the ORR following the associative mechanism, which has an intrinsic limitation in its overpotential of 0.44 V. However, for metal-doped-G and dual-heteroatom-doped-G, the triple effect can activate double carbon sites, and lead to the ORR following the dissociative mechanism, which breaks through the activity limitation of the associative mechanism, and further enhances the catalytic activity for the ORR. With comprehensive theoretical calculation, we conclude that it is possible for the heteroatom-doped-G, if well-modulated via the triple effect, to exceed Pt-based materials in catalysis of the ORR.

Copper-surface-mediated synthesis of acetylenic carbon-rich nanofibers for active metal-free photocathodes

The engineering of acetylenic carbon-rich nanostructures has great potential in many applications, such as nanoelectronics, chemical sensors, energy storage, and conversion, etc. Here we show the synthesis of acetylenic carbon-rich nanofibers via copper-surface-mediated Glaser polycondensation of 1,3,5-triethynylbenzene on a variety of conducting (e.g., copper, graphite, fluorine-doped tin oxide, and titanium) and non-conducting (e.g., Kapton, glass, and silicon dioxide) substrates. The obtained nanofibers (with optical bandgap of 2.51 eV) exhibit photocatalytic activity in photoelectrochemical cells, yielding saturated cathodic photocurrent of ca. 10 µA cm-2 (0.3-0 V vs. reversible hydrogen electrode). By incorporating thieno[3,2-b]thiophene units into the nanofibers, a redshift (ca. 100 nm) of light absorption edge and twofold of the photocurrent are achieved, rivalling those of state-of-the-art metal-free photocathodes (e.g., graphitic carbon nitride of 0.1-1 µA cm-2). This work highlights the promise of utilizing acetylenic carbon-rich materials as efficient and sustainable photocathodes for water reduction.

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

Abstract

Carbon-based metal-free catalysts possess desirable properties such as high earth abundance, low cost, high electrical conductivity, structural tunability, good selectivity, strong stability in acidic/alkaline conditions, and environmental friendliness. Because of these properties, these catalysts have recently received increasing attention in energy and environmental applications. Subsequently, various carbon-based electrocatalysts have been developed to replace noble metal catalysts for low-cost renewable generation and storage of clean energy and environmental protection through metal-free electrocatalysis. This article provides an up-to-date review of this rapidly developing field by critically assessing recent advances in the mechanistic understanding, structure design, and material/device fabrication of metal-free carbon-based electrocatalysts for clean energy conversion/storage and environmental protection, along with discussions on current challenges and perspectives.

Graphical Abstract

Enhanced electrocatalytic activity of nitrogen-doped olympicene/graphene hybrids for the oxygen reduction reaction

Developing inexpensive and non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) is among the major goals in fuel cells. Herein, by using density-functional theory calculations, we show that N-doped olympicene/graphene hybrids exhibit unexpectedly high ORR catalytic activity-even comparable to that of the Pt(111) surface. Both graphitic-type and pyridine-type N-doped olympicene/graphene hybrids are highly active for the ORR and have good CO tolerance. The formation of the second H2O molecule is the rate-determining step for the ORR with the graphitic-type hybrid, whereas on the pyridine-type hybrid, it is the formation of OOH. Note that N-doped olympicene/graphene hybrid materials combine the high reactivity of olympicene and the high electrical conductivity of graphene, which allows them to be potentially used as low-cost and non-precious-metal ORR catalysts.

Hierarchically Designed 3D Holey C2N Aerogels as Bifunctional Oxygen Electrodes for Flexible and Rechargeable Zn-Air Batteries

The future of electrochemical energy storage spotlights on the designed formation of highly efficient and robust bifunctional oxygen electrocatalysts that facilitate advanced rechargeable metal-air batteries. We introduce a scalable facile strategy for the construction of a hierarchical three-dimensional sulfur-modulated holey C₂N aerogels (S-C₂NA) as bifunctional catalysts for Zn-air and Li-O₂ batteries. The S-C₂NA exhibited ultrahigh surface area (∼1943 m² g^-1) and superb electrocatalytic activities with lowest reversible oxygen electrode index ∼0.65 V, outperforms the highly active bifunctional and commercial (Pt/C and RuO₂) catalysts. Density functional theory and experimental results reveal that the favorable electronic structure and atomic coordination of holey C-N skeleton enable the reversible oxygen reactions. The resulting Zn-air batteries with liquid electrolytes and the solid-state batteries with S-C₂NA air cathodes exhibit superb energy densities (958 and 862 Wh kg^-1), low charge-discharge polarizations, excellent reversibility, and ultralong cycling lives (750 and 460 h) than the commercial Pt/C+RuO₂ catalysts, respectively. Notably, Li-O₂ batteries with S-C₂NA demonstrated an outstanding specific capacity of ∼648.7 mA h g^-1 and reversible charge-discharge potentials over 200 cycles, illustrating great potential for commercial next-generation rechargeable power sources of flexible electronics.

Mechanisms of the oxygen reduction reaction on B- and/or N-doped carbon nanomaterials with curvature and edge effects

Despite recent increased research interest in hetero-atom (B and/or N) doping effects on the oxygen reduction reaction (ORR) performance of carbon nanomaterials, microscopic understanding of active catalytic sites and effects of B and/or N doping has not been conclusively reached. Here, through comparative first-principles simulations between BN codoping and isolated B or N doping in both graphene nanoribbons (GNRs) and carbon nanotubes (CNTs), we not only identify active sites in these doped carbon nanomaterials, but elucidate the underlying mechanism of ORR processes. While the distortion of C-C bonds in CNTs leads to strong binding of O₂, spin density distribution along the edges plays a key role in enhancing the adsorption strength of O₂ on GNRs. The effective adsorption of O₂ facilitates the following elementary reduction reactions. Based on thermodynamic analyses, O₂ adsorption as well as the electron and proton transfer to O atom are identified as key rate-determining steps. For CNTs, a good linear scaling is found between the adsorption energies of key intermediate products and that of atomic O, and thus the latter serves as a good descriptor for ORR activities. Further, N-doped high-quality CNTs are shown to exhibit best performance. For GNRs, due to edge effects, the linear relationship is broken, which promotes the catalytic activities in the BN codoping case. These findings resolve the long-standing controversy on the synergetic effects of B and N codoping, which deepens our understanding of the reaction mechanism. This work might further facilitate the optimization of the doping strategies for high-efficiency carbon-based ORR catalysts.

Size effect of oxygen reduction reaction on nitrogen-doped graphene quantum dots

The adsorption strength of ORR intermediates, the reaction free energy of rate-determining step, and the overpotential increase with the increase of the size of N-doped graphene quantum dots.

Theoretical insight into the catalytic activities of oxygen reduction reaction on transition metal–N4 doped graphene

The ORR (OER) activities are expressed as the function of ΔG_ads(O) on the M–N₄-C composites and the best component can be predicted.

Catalysis by hybrid sp2/sp3nanodiamonds and their role in the design of advanced nanocarbon materials

Hybrid sp²/sp³nanocarbons, in particular sp³-hybridized ultra-dispersed nanodiamonds and derivative materials, such as the sp³/sp²-hybridized bucky nanodiamonds and sp²-hybridized onion-like carbons, represent a rather interesting class of catalysts still under consideration.

High-performance oxygen reduction catalysts in both alkaline and acidic fuel cells based on pre-treating carbon material and iron precursor

We demonstrate a new and simple method for pre-treating the carbon material and iron precursor to prepare oxygen reduction reaction (ORR) catalysts, which can produce super-high performance and stability in alkaline solution, with high performance in acid solution. This strategy using cheap materials is simply controllable. Moreover, it has achieved smaller uniform nanoparticles to exhibit high stability, and the synergetic effect of Fe and N offered much higher performance in ORR than commercial Pt/C, with high maximum power density in alkaline and acid fuel cell test. So it can make this kind of catalysts be the most promising alternatives of Pt-based catalysts with best performance/price.

Efficient and Highly Selective Solvent-Free Oxidation of Primary Alcohols to Aldehydes Using Bucky Nanodiamond

Selective oxidation of alcohols to aldehydes is widely applicable to the synthesis of various green chemicals. The poor chemoselectivity for complicated primary aldehydes over state-of-the-art metal-free or metal-based catalysts represents a major obstacle for industrial application. Bucky nanodiamond is a potential green catalyst that exhibits excellent chemoselectivity and cycling stability for the selective oxidation of primary alcohols in diverse structures (22 examples, including aromatic, substituted aromatic, unsaturated, heterocyclic, and linear chain alcohols) to their corresponding aldehydes. The results are comparable to reported transition-metal catalysts including conventional Pt/C and Ru/C catalysts for certain substrates under solvent-free conditions. The possible activation process of the oxidant and substrates by the surface oxygen groups and defect species are revealed with model catalysts, ex situ electrochemical measurements, and ex situ attenuated total reflectance. The zigzag edges of sp² carbon planes are shown to play a key role in these reactions.

Nitrogen-Mediated Graphene Oxide Enables Highly Efficient Proton Transfer

Two-dimensional (2D) graphene and graphene oxide (GO) offer great potential as a new type of cost-efficient proton-exchange membranes (PEM) for electrochemical devices. However, fundamental issues of proton transfer mechanism via 2D membranes are unclear and the transfer barrier for perfect graphene are too high for practical application. Using ab initio molecular dynamic simulations, we screened the proton transfer barrier for different un-doped and nitrogen doped GO membranes, and clarified the corresponding transfer mechanisms. More significantly, we further identify that N-mediated GO can be built into a highly efficient PEM with a proton transfer rate of seven orders of magnitude higher than an un-doped case via. a proton relay mechanism between a ketone-like oxygen and a pyridine-like nitrogen across the vacancy site. The N-doped 2D GO is also impermeable to small molecules, and hence a highly efficient PEM for practical applications.

DFT Study of the Oxygen Reduction Reaction Activity on Fe-N₄-Patched Carbon Nanotubes: The Influence of the Diameter and Length

The influences of diameter and length of the Fe−N₄-patched carbon nanotubes (Fe−N₄/CNTs) on oxygen reduction reaction (ORR) activity were investigated by density functional theory method using the BLYP/DZP basis set. The results indicate that the stability of the Fe−N₄ catalytic site in Fe−N₄/CNTs will be enhanced with a larger tube diameter, but reduced with shorter tube length. A tube with too small a diameter makes a Fe−N₄ site unstable in acid medium since Fe−N and C−N bonds must be significantly bent at smaller diameters due to hoop strain. The adsorption energy of the ORR intermediates, especially of the OH group, becomes weaker with the increase of the tube diameter. The OH adsorption energy of Fe−N₄/CNT with the largest tube diameter is close to that on Pt(111) surface, indicating that its catalytic property is similar to Pt. Electronic structure analysis shows that the OH adsorption energy is mainly controlled by the energy levels of Fe 3d orbital. The calculation results uncover that Fe−N₄/CNTs with larger tube diameters and shorter lengths will exhibit better ORR activity and stability.

Nanocarbon for Oxygen Reduction Electrocatalysis: Dopants, Edges, and Defects

The oxygen reduction reaction (ORR) is the cornerstone of various sustainable energy-conversion technologies. Metal-free nanocarbon electrocatalysts with competitive activity, enhanced durability, and satisfactory cost, have been proposed as the most promising substitute for precious-metal catalysts. However, their further development is still primarily based on trial-and-error approaches due to the controversial knowledge of critical active sites and mechanisms. Herein, the activity origins of nanocarbon-based ORR electro-catalysts are comprehensively reviewed and correlated, considering the dopants, edges, and defects. Analogously, they can effectively modify the charge/spin distribution on the sp² -conjugated carbon matrix, leading to optimized intermediate chemisorption and facilitated electron transfer. Specific doping at defective edges is expected to render practical applications for metal-free nanocarbon electrocatalysts.

The effect of defects on the catalytic activity of single Au atom supported carbon nanotubes and reaction mechanism for CO oxidation

The mechanism of CO oxidation by O₂ on a single Au atom supported on pristine, mono atom vacancy (m), di atom vacancy (di) and the Stone Wales defect (SW) on single walled carbon nanotube (SWCNT) surface is systematically investigated theoretically using density functional theory.

Aziridine-Functionalized Multiwalled Carbon Nanotubes: Robust and Versatile Catalysts for the Oxygen Reduction Reaction and Knoevenagel Condensation

This paper describes the exohedral N-decoration of multiwalled carbon nanotubes (MWCNTs) with NH-aziridine groups via [2 + 1] cycloaddition of a tert-butyl-oxycarbonyl nitrene followed by controlled thermal decomposition of the cyclization product. The chemical grafting with N-containing groups deeply modifies the properties of the starting MWCNTs, generating new surface microenvironments with specific base (Brønsted) and electronic properties. Both of these features translate into a highly versatile single-phase heterogeneous catalyst (MW@N^Az) with remarkable chemical and electrochemical performance. Its surface base character promotes the Knoevenagel condensation with activity superior to that of related state of the art N-doped and N-decorated carbon nanomaterials; the N-induced electronic surface redistribution drives the generation of high-energy surface "C" sites suitable for O₂ activation and its subsequent electrochemical reduction (ORR).