Four-electron Mechanism Research Articles

In Situ Grown NiFe-Based MOF for Efficient Oxygen Evolution in Alkaline Seawater at High Current Densities

The oxygen evolution reaction (OER) characterized by four-electron transfer mechanism is inherently limited by significant overpotential requirements and sluggish kinetics. A water-stable NH2-MIL-88B (Fe2Ni) Metal-Organic Framework (MOF) was in-situ synthesized...

Monitoring the Spontaneous Growth of Nickel-Iron Layered Double Hydroxides during Alkaline Oxygen Evolution at Industrial Conditions

The imperative shift towards a sustainable energy infrastructure necessitates cost-effective production of green hydrogen, wherein enhancing the electrolysers' efficiency stands essential. Among the possible levers for reducing internal losses is the oxygen evolution reaction (OER), which, due to its complex four-electron reaction mechanism, causes a substantial overpotential. In alkaline water electrolysis, where nickel electrodes are conventionally employed, small additions of iron to the nickel surface can reduce the OER overpotential tremendously.1 However, while many studies have developed catalysts based on this elemental composition with exceptional OER activity, their long-term stability under harsh industrial conditions often remains untested, which might lead to false conclusions regarding the catalyst's commercial potential.2 The active structure of these catalysts has been identified as nickel-iron layered double hydroxides (NiFe-LDHs),3 and research has logically strived towards optimisation of their synthesis to minimise the OER overpotential. In this context, structural parameters such as the iron content, vacancies and defects, intercalated anions and edge-to-plane site ratio are often in focus.4 The chemical environment of the electrolyser is nonetheless very different from the environment where many optimised NiFe-LDHs are formed, and despite any robust kinetic stabilisation of the initial electrocatalyst, the electrolytic environment and the operational conditions are expected to shape the structure during the electrolyser's lifetime.5 Meanwhile, with iron present in the electrolyte, NiFe-LDHs are known to grow spontaneously during alkaline OER on unactivated nickel anodes, offering an alternative and potentially more scalable and cost-effective route to obtain an efficient OER catalyst.6 In this study, we aim to leverage the spontaneous formation of NiFe-LDH catalysts via electrolyte-mediated pathways to produce highly active alkaline OER anodes that can endure the chemical environment of industrial electrolysers. To this end, we employ in situ Raman spectroscopy's exceptional ability to distinguish metal oxides in aqueous environments alongside electrochemical impedance spectroscopy, XPS and FEG-SEM analysis, and we monitor the evolution of various NiFe catalysts under industrially relevant conditions (80oC, 8M KOH, 600 mA/cm2). By rationalising the interplay between catalyst structure and the electrolytic environment, this research strives to elucidate whether NiFe-LDH catalysts synthesised in situ can outperform their ex situ counterparts in long-term stability, offering crucial insights for advancing sustainable hydrogen production.(1) M. W. Louie, A. T. Bell, J. Am. Chem. Soc., 135, 12329 (2013)(2) J. C. Ehlers, A. A. Feidenhans'l, K. T. Therkildsen, G. O. Larrazábal, ACS Energy Lett. 8, 1502 (2023)(3) D. Friebel, M. W. Louie, M. Bajdich, K. E. Sanwald, Y. Cai, A. M. Wise, M.-J. Cheng, D. Sokaras, T.-C. Weng, R. Alonso-Mori, R. C. Davis, J. R. Bargar, J. K. Nørskov, A. Nilsson, A. T. Bell, J. Am. Chem. Soc, 137, 1305 (2015)(4) P. M. Bodhankar, P. B. Sarawade, G. Singh, A. Vinu, D. S. Dhawale, J. Mater. Chem. A, 9, 3180 (2021)(5) M. Etzi, C. Pascuzzi, A. J. W. Man, J. P. Hoffman, E. J. M. Hensen, Catal. Sci. Tech. 10, 5593 (2020)(6) R. A. Marquez, E. Kalokowski, M. Espinosa, J. T. Bender, Y. J. Son, K. Kawashima, C. E. Chukwuneke, L. A. Smith, H. Celio, A. Dolocan, X. Zhan, N. Miller, D. J. Milliron, J. Resasco, C. B. Mullins, Energy Environ. Sci., 17 2028 (2024)

Redox-mediated synthesis of Cu2O/CuO/Mn3O4/C quaternary nanocomposites as an efficient electrode material for oxygen reduction reaction and supercapacitor application

Earth-abundant transition metal oxides (TMOs) hold significant promise as electroactive materials in various electrochemical energy conversion and storage applications, offering economic and environmental advantages over their less abundant counterparts. In this work, a simple and cost-effective redox-mediated reaction strategy under hydrothermal conditions has been adopted to synthesize the quaternary nanocomposites Cu2O/CuO/Mn3O4/C with variable amounts of carbon. The synthesized nanocomposites have been characterized using various analysis techniques, including powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Brunauer–Emmett–Teller (BET). The synthesized COM5 nanocomposite (Cu2O/CuO/Mn3O4/C-50), when used as an electrocatalyst for the oxygen reduction reaction (ORR), demonstrates a good limiting current density (JL), half-wave potential (E1/2), and onset potential (Eonset) of −5.50 mA/cm2, 0.75 V, and 0.95 V, respectively, as per the polarization curve. The COM5 nanocomposite exhibits a four-electron transfer mechanism, calculated using the Koutecky-Levich (K-L) equation. In an O2-saturated 0.1M KOH solution, the COM5 nanocomposite has shown a superior relative current durability of 84.46% over 14,000 s compared to the commercially available 10 wt% Pt/C, indicating its potential application in the ORR. However, for supercapacitor applications, the COM3 nanocomposite (Cu2O/CuO/Mn3O4/C-20) as active electrode material in galvanic charge-discharge (GCD) study shows superior performance (201 F/g at 1 A/g) compared to the COM2 nanocomposite (117 F/g at 1 A/g) in neutral aqueous electrolyte, possibly due to the lower charge transfer resistance (Rct) of 24.04 Ω compared to COM2 (34.63 Ω). In two-electrode studies, the COM3 nanocomposite exhibits a good energy density of 24.19 Wh/kg at 1 A/g and a 4.7 kW/kg power density at 5 A/g. Additionally, the COM3 nanocomposite shows excellent long-term durability (74% capacitance retention) at the current density of 5 A/g for 8000 cycles. The electrochemical findings indicate that the COM3 nanocomposite stands out as a superior electrode material for supercapacitors, while the COM5 nanocomposite proves to be an effective electrocatalyst for the oxygen reduction reaction.

Exploring the catalytic characteristics of binuclear bimetallic FeM sites (M = Co, Ni, Pt) on nitrogen-doped graphene through density functional theory and ab initio molecular dynamics

Certain supported bimetallic catalysts exhibit high efficiency in both oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). However, the catalytic mechanism of double-atom catalysts and how different-atom active sites affect charge transfer efficiency have not been fully revealed. In this work, we provide density functional theory (DFT) simulations of ORR/OER on FeM-NC (M = Co, Ni, Pt) supported on nitrogen-doped graphene. The stability of structures, electrical properties, adsorption energies of intermediates, and catalytic activity of ORR/OER on FeM-NC have been investigated. The results indicate that the Fe-atom in stable FeM-NC structures is a catalytically active site and that the FeM-NC-1 structures may be more appropriate as catalysts. The overpotentials for ORR/OER on FeM-NC-1 (M = Co, Ni, Pt) are 0.42/0.29 V on FeCo-NC-1, 0.57/0.33 V on FeNi-NC-1, and 0.64/0.33 V on FePt-NC-1, which is significantly lower than those on Pt(111) surfaces. Ab initio molecular dynamics (AIMD) simulations suggest that ORR/OER may occur vis a typical four-electron mechanism on FeNi-NC-1 and FePt-NC-1 configurations, while on FeCo-NC-1 structures, it may involve two different four-electron pathways. This study increases our knowledge of the structural stability of FeM-NC-1 structures as bifunctional catalysts and the ORR/OER pathway in acidic solution, and it also shows that three FeM-NC-1 structures are suitable candidate catalysts for ORR/OER.

Dopamine-Carbonized Coating PtCo Catalyst with Enhanced Durability toward the Oxygen Reduction Reaction.

Stability is the main challenge for the application of PtCo catalysts because Co tends to leach during the electrochemical reaction. Herein, we immerse and adsorb dopamine to densely coat Pt0.8Co0.2 particles and subsequently thermally carbonize the coating into few-layer nitrogen-doped graphene to produce Pt0.8Co0.2@NC. This coating effectively hinders direct contact between Pt0.8Co0.2 particles and the electrolyte, thereby enhancing the stability of the catalyst by preventing Ostwald ripening and suppressing competitive adsorption of toxic species, while also bolstering its antipoisoning ability. Experimental results indicate that the thin coating does not compromise the oxygen reduction reaction activity of the catalyst, showcasing a half-wave potential of 0.81 V in alkaline electrolytes. Spectroscopic results suggest that a strong bonding interaction between Pt and the pyridinic N of N-doped graphene contributes to the generation of a dense coating. The coating layer does not affect the four-electron reaction mechanism of the Pt0.8Co0.2 alloy, and the coordinatively unsaturated carbon atoms on Pt0.8Co0.2@NC serve as active oxygen reduction reaction centers.

Oxygen Electroreduction Reaction on Iron, Nitrogen-doped Nanocarbons: Structure – Reactivity Relationship

Theoretical calculations at the revPBE0-D3(PCM)/def2-TZVP level were performed to investigate the mechanism of the oxygen reduction reaction (ORR) on the FeN4-doped NCMs (graphene, single-walled (6,6)-armchair and (12,0)-zigzag carbon nanotubes) as catalysts. The effect of the support and relative orientation of FeN4 fragment in the catalyst structure on the ORR activity and stability of the differently adsorbed successive intermediates (O2*, HO2*, O*, HO*, H2O2*, H2O*) is discussed. Special attention is given to the possible poisoning effect of CO. The relative catalytic activity of the metal center and the adjacent C2 site of the carbon support are analyzed depending on the structure of the support. The metal catalytic center on the armchair nanotube and graphene supports is prone to deactivation by poisoning with CO, whereas in the zigzag nanotube supported catalyst the metal center is tolerant to CO. The four-electron mechanism of ORR is shown to be preferable over the two-electron mechanism in both acidic and alkaline media, both on the metal and C2 centers.

Hollow-structured cobalt sulfide electrocatalyst for alkaline oxygen evolution reaction: Rational tuning of electronic structure using iron and fluorine dual-doping strategy

Utilizing renewable electricity for water electrolysis offers a promising way for generating high-purity hydrogen gases while mitigating the emission of environmental pollutants. To realize the water electrolysis, it is necessary to develop highly active and precious metal-free electrocatalyst for oxygen evolution reaction (OER) which incurs significant overpotential due to its complicated four-electron transfer mechanism. Hence, we propose a facile preparation method for hollow-structured Fe and F dual-doped CoS2 nanosphere (Fe-CoS2-F) as an efficient OER electrocatalyst. The uniform hollow and porous structure of Fe-CoS2-F enlarge the specific surface area and increase the number of exposed active sites. Furthermore, the Fe and F dual-dopants synergistically contributed to the adjustment of electronic structure, thereby promoting the adsorption/desorption of oxygen-containing reaction intermediates on active sites during the alkaline OER procedure. As a result, the prepared Fe-CoS2-F exhibits outstanding OER activity, characterized by a low overpotential of 298 mV to achieve a current density of 10 mA cm−2 and a Tafel slope as small as 46.0 mV dec−1. Based on computational theoretical calculations, the introduction of the dual-dopants into CoS2 structure reduce the excessively strong adsorption energy of reaction intermediate in the rate determining step, leading to effectively promoted electrocatalytic cycle for OER in alkaline environment. This study presents an effective strategy for preparing noble metal-free OER electrocatalysts with promising potential for large-scale industrial water electrolysis.

Unraveling the Activity and Mechanism of TM@g-C4N3 Electrocatalysts in the Oxygen Reduction Reaction.

In this work, a high-throughput screening strategy and density functional theory (DFT) are jointly employed to identify high-performance TM@g-C4N3 (TM = 3d, 4d, 5d transition metals) single-atom catalysts (SACs) for the oxygen reduction reaction (ORR). Comprehensive studies demonstrated that Cu@, Zn@, and Ag@g-C4N3 show high ORR catalytic activities under both acidic and alkaline conditions with favorable overpotentials (ηORR) of 0.70, 0.89, and 0.89 V, respectively; among them, Cu@g-C4N3 is the best candidate. The ORR follows a four-electron mechanism with the final product H2O/OH-. Cu@, Zn@, and Ag@g-C4N3 catalysts also exhibit good thermal (500 K) and electrochemical (0.93-3.14 V) stabilities. Cu@, Zn@, and Ag@g-C4N3 demonstrate superior activities with low ηORR due to its moderate adsorption strength of *OH. The ηORR and the Gibbs free energy changes of *OH (ΔG4(acidic)/ΔG4(alkaline)) resemble a volcano-type relationship under acidic/alkaline conditions, respectively. Additionally, the O-O bond length in *OOH emerged as an effective structural descriptor for rapidly identifying the promising electrocatalysts. This research provides valuable insights into the origin of the ORR activity on TM@g-C4N3 and offers useful guidance for the efficient exploration of high-performance catalyst candidates.

Electrochemically produced films based on individual porphyrins and bimetallic composites: Morphology and electrocatalytic properties in the reaction of oxygen electroreduction

In this work, we obtained polyporphyrin films based on Fe(III)Cl-5,10,15,20-tetrakis(4-pyridyl)porphyrin and Mn(III)acetate-5,10,15,20-tetrakis(4-pyridyl)porphyrin and composite films on their basis by electrooxidation-induced electrochemical polymerization. The obtained films produced uniform coatings. Spectral analysis showed that the composite films were enriched with iron complexes. Comparative analysis of the prepared films demonstrated differences between the morphologies of the composite film and films of individual metal complexes. The oxygen electroreduction process on the obtained materials proceeded at more positive values than that on carbositall. The effective number of transferred electrons demonstrated that replacing catalysts based on individual porphyrins with the bimetallic composite changed the process mechanism: while the four-electron mechanism competed with subsequent two-electron reactions on the Fe(III)Cl-5,10,15,20-tetrakis(4-pyridyl)porphyrin and Mn(III)acetate-5,10,15,20-tetrakis(4-pyridyl)porphyrin films, on the bimetallic composite the process mainly represented a four-electron reaction.

Silver Nanowires Cascaded Layered Double Hydroxides Nanocages with Enhanced Directional Electron Transport for Efficient Electrocatalytic Oxygen Evolution.

Designing and fabricating highly efficient oxygen evolution reaction (OER) electrocatalytic materials for water splitting is a promising and practical approach to green and sustainable low-carbon energy systems. Herein, a facile in situ growth self-template strategy by using ZIF-67 as a consumable layered double hydroxides (LDHs) template and silver nanowires (AgNWs) as 1D conductive cascaded substrate to controllably synthesize the target AgNWs@CoFe-LDH composites with unique hollow shell sugar gourd-like structure and enhanced directional electron transport effect is reported. The AgNWs exhibit the key functions of the close connection of CoFe-LDH nanocages and the support of the directional electron transport effect in the composite catalyst inducing electrons directionally moving from CoFe-LDH to AgNWs. Meanwhile, the CoFe-LDH nanocages with ultrathin nanosheets and hollow structural properties show abundant active sites for electrocatalytic oxygen generation. The versatile AgNWs@CoFe-LDH catalyst with optimized components, enhanced directional electron transport, and synergistic effect achieves high OER performance with the overpotential of 207 mV and long-term 50 h stability at 10 mA cm-2 in an alkaline medium. Moreover, in-depth insights into the microstructure, structure-activity relationships, identification of key intermediate species, and a proton-coupled four-electron OER mechanism based on experimental discovery and theoretical calculation are also demonstrated.

Rational electrolyte design and electrode regulation for boosting high-capacity Zn-iodine fiber-shaped batteries with four-electron redox reactions.

Aqueous Zn ion-based fiber-shaped batteries (AZFBs) with the merits of high flexibility and safety have received much attention for powering wearable electronic devices. However, the relatively low specific capacity provided by cathode materials limits their practical application. Herein, we first propose a simple strategy for fabricating high-capacity Zn-iodine fiber-shaped batteries with a high concentration electrolyte and a reduced graphene oxide fiber (GF) cathode. It was found that oxygen functional groups in the graphene sheet demonstrate strong interaction with polyiodides but hinder electron conductivity; thus, the optimal balance between the specific capacity and coulombic efficiency of the GF electrode can be a function of the surface properties at different hydrothermal temperatures. Besides, the regulated high concentration electrolyte effectively suppresses the diffusion of polyiodides, which is attributed to the constrained freedom of water. More importantly, a four-electron redox mechanism was experimentally revealed through in situ Raman spectra. As a result, this fiber-shaped battery delivers a superior high reversible capacity of 390 mA h cm-3 at 1 A cm-3, an excellent rate performance of 125.7 mA h cm-3 at a high current density of 8 A cm-3 and outstanding cycling life with 82% capacitance retention after 2500 cycles.

CVD Growth Graphene Foam Functionalized with Platinum for Fuel Cells Applications

The development of catalysts with high activity for the ORR is essential to proton exchange membrane fuel cells (PEMFCs), since the majority of activation losses occur at the cathode and it turns into an interesting research area. In the present study, it is investigated the hydrothermal platinum functionalization of the CVD grown three-dimensional graphene foam (3D-GrFoam) using three concentrations of dihydrogen hexachloroplatinate (IV) hydrate [1]. The platinum functionalized graphene foam determined from XPS was 0.2at %, 0.3 at % and 0.4 at %, respectively.The catalytic activity towards ORR was analyzed from linear sweep voltammetry (LSV) plots recorded with a scan rate of 5 mV s-1 in oxygen saturated 0.5 M H2SO4. From the LSV curves and the Koutecky-Levich (K-L) plots for 0.4PtGrFoam as current density (mA-1 cm2 geo) vs. w-1/2 (rad s-1)-1/2), for various rotation speeds (among 250–1500 rpm) and different potentials (0.1 V-0.8 V). From the fitted K-L plots it is noticed a fair linear relationship at all potentials, that confirms the electroreduction of platinum. The number of transferred electrons is between 3.22 to 3.59 indicating the preponderance of the four-electron transfer mechanism in the ORR corresponding to the directly reduction of O2 to H2O.1. Ion-Ebrasu, R.D. Andrei, S. Enache, S. Caprarescu, C. C. Negrila, C. Jianu, A. Enache, I. Boerasu, E. Carcadea, M. Varlam, B. S. Vasile, J. Ren, Materials 2021, 14, 4952.

Growth of surfactant-free Au-Co bimetallic atom-bomb cracker-like microstructures: A highly active morphological dependent hybrid electrocatalyst for dioxygen reduction

This paper reports the formation of atom-bomb fire cracker (ABC) like Au-Co heterostructures by attaching folic acid-capped AuNPs (FA-AuNPs) on a glassy carbon (GC) electrode followed by cobalt electrodeposition. The characteristic absorption band at 516 nm from the UV–visible spectrum confirms the AuNPs formation and their average size was ∼6 nm as evidenced from HR-TEM. The AuNPs were then assembled on a GC electrode via 1,6-hexadiamine linker followed by the electrodeposition of Co at various applied potentials and time. XPS spectra of Au-Co clearly revealed the zero valent nature of Au and Co and Co2+ in the bimetallic assembly. The SEM results exhibited the variations in the morphology of Au-Co with respect to the applied potential and time. It was found that the deposition of Co at -0.50 V for 600 s leads to the formation of ABC-like structure only at FA-AuNPs surface. The specific atom-bomb morphology of Au-Co was not obtained at bare GC and citrate-capped AuNPs surfaces, indicating the influence of FA-AuNPs. The Au-Co showed higher electrocatalytic activity towards dioxygen reduction by shifting the onset potential by 230 and 280 mV towards zero respectively than Co and AuNPs alone. The Koutecky-Levich plot confirms the four-electron transfer mechanism of oxygen reduction at Au-Co fabricated electrode.

Strategies to Improve the Oxygen Reduction Reaction Activity on Pt-Bi Bimetallic Catalysts: A Density Functional Theory Study.

Decreasing the level of use of Pt in proton exchange membrane fuel cells is of great research interest both academically and industrially. In this work, we systematically studied the oxygen reduction reaction (ORR) following the four-electron association mechanism at various Pt-Bi surfaces with density functional theory calculations. The results showed that the introduction of Bi changes the potential-determining step of ORR. Moreover, the hydroxy adsorption free energy (GOH*) can be used as a descriptor of ORR activity, and 0.74 eV is the ideal GOH* for it to reach its maximum. Notably, we also found that the tensile strain introduced by Bi and electron transfer between Pt and Bi synergize to modulate the d-band of Pt to contract, shift downward, and break the 5d96s1 valence electron configuration of Pt, and accordingly, PtBi(100), with the lowest d-band center, gives the best ORR activity, which is even slightly higher than that of Pt(111).

Conductive Ytterbium Metal–Organic Framework Composite: A Lanthanide-Based Complex ORR Catalyst

Extensive research work has been invested in the past decade in finding replacements for platinum-based electrocatalysts for the oxygen reduction reaction in fuel cells. The majority of these alternative electrocatalysts are based on transition-metal ions coordinated by organic ligands. Different from previously reported approaches for electrocatalysts, we describe here the synthesis, characterization, and oxygen reduction reaction activity of lanthanide complex electrocatalyst with ytterbium as the metal center. A metal–organic framework of Yb and benzene tricarboxylic acid as a ligand was synthesized on activated carbon (Yb(III)BTC@AC) to achieve electrical conductivity in a procedure similar to M-BTC@AC electrocatalysts with transition-metal centers. The Yb complex in activated carbon presents oxygen reduction reaction activity in alkaline solution with high onset potential relative to other nonpyrolyzed molecular electrocatalysts and proceeds mostly through the preferred four-electron reduction mechanism. This work presents a novel category of platinum group metal-free electrocatalysts as an alternative electrocatalyst for the oxygen reduction reaction.

The synergistic effect of “soft-hard template” to in situ regulate mass transfer and defective sites of doped-carbon nanostructures for catalysis of oxygen reduction

Nitrogen-doped carbon materials are intensively investigated as electrocatalysts toward oxygen reduction reaction (ORR). The activity of the catalysts can be effectively boosted by regulating the microstructure to construct defective three-phase interface. Herein, an N-doped carbon-based porous catalyst (DT-Fe-N-C) is fabricated by a dual-template strategy with SiO2 and pluronic F127 as the hard and soft templates. Benefited from the modulation, the mesopore-dominated DT-Fe-N-C exhibits remarkable specific surface area (990.19 m2 g-1), high Fe-Nx content and plenty of defects. Resultantly, DT-Fe-N-C shows outstanding catalytic activity to ORR with 0.853 V of half-wave potential, high stability and selectivity to the four-electron ORR mechanism. The Zn-air battery with DT-Fe-N-C outputs 219 mW cm-2 of maximum power density, 861 Wh kgZn−1 of specific energy density at 50 mA cm-2 and runs at a higher voltage than that of Pt/C-based battery. This work provides an avenue for the defect engineering in the design of non-precious metal ORR catalysts.

Ab Initio Exploration of Energetically and Kinetically Favorable ORR Activity on a 1T-ZrO2 Monolayer for a Nonaqueous Lithium-Oxygen Battery.

Herein, we explore the potential applications of the experimentally synthesized ZrO2 monolayer as the cathode catalyst for nonaqueous lithium-oxygen batteries. First, we show that a new peroxide-like adsorption geometry is the most stable configuration for LiO2, which is distinct from the previously known O-Li-O triangular geometry. The proposed most stable adsorption configuration is because the Zr atoms in the substrate play a critical role in stabilizing the LiO2 cluster. Second, our ab initio calculations indicate that both the ORR and OER catalytic activities are most likely to adopt the four-electron mechanism with a considerably low overpotential of only 0.44 and 0.76 V, respectively. Finally, we show that the adsorption energy of Li2O2 is a good descriptor for both ORR and OER catalytic activities, and weak Li2O2 adsorption behavior is positively related to low overpotentials and satisfactory catalytic performance.

Core-Shell ZIF-67@ZIF-8-derived multi-dimensional cobalt-nitrogen doped hierarchical carbon nanomaterial for efficient oxygen reduction reaction

Fuel cells have emerged as a charming candidate for next-generation energy conversion devices. However, it is highly desired but challenging to engineer advanced non-precious oxygen reduction catalysts with highly actives and stability due to the sluggish kinetics of oxygen reduction reaction (ORR) on fuel cell cathode. Herein, a novel hybrid architecture with Co nanoparticles embedded in N-doped carbon nanotubes and hollow nanocarbon polyhedron (Co@N-CNT-HC) was constructed from core-shell ZIF-67@ZIF-8 via a facile epitaxial growth-pyrolysis process. With the Co@N-CNT-HC as the catalyst, a remarkable ORR performance is achieved in terms of a high half-wave potential of 0.84 V, a large limiting current density of 4.70 mA/cm2, and excellent long-term durability ( 97.8% current retention after 8 h) in alkaline medium, which outperform commercial Pt/C catalyst. Further dynamic calculations indicated that ORR follows a four-electron reaction mechanism. The enhancement activity of Co@N-CNT-HC is mainly due to the effective integration of 0D Co nanoparticles, 1D carbon nanotubes and 3D hollow nanocarbon, which synergistically strengthen the interfacial reaction kinetics of oxygen and accelerate mass/charge transfer. The strategy reported in this work provides a new insight to synthesis of low-cost and well-designed carbon hybrid electrocatalyst for ORR.

Few-layer versus mono-layer N-doped graphenes in oxygen reduction reaction

Doped graphenes, nitrogen-doped (N-doped) in particular, are known to possess promising electrocatalytic performance in oxygen reduction reaction (ORR). We study the effect of the structure of N-doped few-layer (N-Gr) and mono-layer (N-gr) graphenes prepared within one low-temperature approach on their functional behavior as ORR electrocatalysts. Spectral studies reveal the nitrogen content in N-Gr being a quarter greater than in N-gr and nearly one third of all nitrogen atoms in N-Gr being within graphitic fragments, while about half of all nitrogen atoms of N-gr are pyrrolic ones; N-Gr particles are characterized by the presence of defects at the edges, while vacancies are also characteristic of N-gr. It is established that the structural differences of N-Gr and N-gr determine higher electrocatalytic performance of N-Gr in ORR and its four-electron mechanism, which are associated with a significant increase in the charge carrier density and shift of the flat-band potential to negative potential range. The significance of the work is directly related to practical application, since N-Gr could be prepared by fundamentally simpler method, its preparation is ecologically friendly, easily scalable and, therefore, cost effective.

Synthesis of carbon-based CoV electrocatalyst and its application in Zn-air battery devices

Precious metal catalysts are generally considered to be the best electrocatalysts for slow four-electron transfer mechanism in oxygen reduction and oxygen evolution reactions. However, its large-scale commercialization is limited due to its high cost, scarce resources and lack of stability. Therefore, under the same catalytic performance conditions, low cost and environmentally friendly non-noble metal electrocatalyst will become the focus of future electrocatalyst engineering. Dicyandiamine was used for carbon resource to prepare CoV-based carbon nanotube composites (named CoV-NC) by means of group coordination combined with freeze drying strategy and carbonization treatment. The morphology and structure of the sample was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and N2 adsorption-desorption curve. In 0.1 M KOH electrolyte, the Eonset potential of CoV-NC catalyst for ORR is 0.931 V, and the limiting current density is higher. The OER voltage is only 1.63 V at the current density of 10 mA·cm-2, demonstrating that CoV-NC exhibits good catalytic activity of ORR and OER. As for an air-cathode material to assemble primary Zn-air battery, it can discharge continuously for 166 h at a current density of 5 mA·cm-2, which is much better than commercial Pt/C catalyst.