Hierarchical Core-Shell Co2 N/CoP Embedded in N, P-doped Carbon Nanotubes as Efficient Oxygen Reduction Reaction Catalysts for Zn-air Batteries

Isolated p-Block Antimony Atoms Activated CuO@Co-CN Enable High Performances for Water Splitting and Zn-Air Batteries

This study reports an effective strategy for designing 3D electrocatalyst via the deposition of ZIF67-derived Co-CN shell layer over CuO nanoarrays to form a CuO@Co-CN hybrid, followed by incorporation with p-block Sb single atoms (CuO@Co-CN/Sb) to obtain highly activated catalytic behaviors. Inheriting both the excellent intrinsic catalytic activity of the components and their synergy, the CuO@Co-CN/Sb material serves as a high-efficiency multifunctional catalyst for overall water splitting and zinc (Zn)-air batteries. The material yields a current density of 10 mA cm-2 at a low overpotential of 72 and 250 mV for the hydrogen evolution reaction and oxygen evolution reaction, respectively. Furthermore, an electrolyzer based on CuO@Co-CN/Sb shows remarkable performance with a derived current density of 0.5 A cm-2 at low cell voltage of 2.67 V and good stability for 50 h continuous operation at a high current density of 0.5 A cm-2. Simultaneously, Zn-air battery using the CuO@Co-CN/Sb material as air cathode yields a high open circuit voltage of 1.455 V and a discharge power density of 131.07 mW cm-2.

Transition metal phosphide-based oxygen electrocatalysts for aqueous zinc-air batteries

Electrically rechargeable zinc-air batteries (ZABs) are emerging as promising energy storage devices in the post-lithium era, leveraging the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at the air cathodes. Efficient bifunctional oxygen electrocatalysts, capable of catalyzing both the ORR and OER, are essential for the operation of rechargeable ZABs. Traditional Pt- and RuO2/IrO2-based catalysts are not ideal, as they lack sufficient bifunctional ORR and OER activity, exhibit limited long-term durability, require high overpotentials and are expensive. In contrast, non-precious metal-based catalysts, including transition metal phosphides (TMPs), have gained significant attention for their promising bifunctional catalytic properties, making them attractive candidates for ZABs. Despite encouraging lab-scale achievements, translating these advancements into market-ready applications remains challenging due to suboptimal energy performance. Rationally engineered bifunctional TMPs hold great potential for overcoming these challenges and meeting the requirements of rechargeable ZABs. This feature article reviews recent progress in the development of TMP-based catalysts for ZABs, providing a comprehensive overview of ZAB fundamentals and strategies for catalyst design, synthesis, and engineering. A particular emphasis is placed on widely studied bifunctional Fe, Co, and Ni phosphides, along with approaches to enhance their catalytic performance. Key performance metrics are critically evaluated, including the potential gap (ΔE) between the ORR and the OER, specific capacity, peak power density, and charge-discharge cycling stability. Finally, this feature article discusses the challenges faced in TMP-based ZABs, proposes strategies to address these issues, and explores future directions for improving their rechargeability to meet the demands of commercial-scale energy storage technologies.

Bamboo-like nitrogen-doped carbon nanotubes directly grown from commercial carbon black for encapsulating FeCo nanoparticles as efficient oxygen reduction electrocatalysts

Efficient methods for preparing carbon nanotube (CNT)-confined metal catalysts are of great significance for electrocatalysis. Hence, in this study, Fe and Co promoters were added to the precursors of commercial carbon black and melamine to form N-doped CNTs confined bimetallic catalysts (FeCo@N-CNTs) via in situ pyrolysis. The FeCo@N-CNT catalysts exhibited a bamboo-like morphology with FeCo alloy nanoparticles encapsulated in the CNTs and high activity toward the oxygen reduction reaction, with a half-wave potential of 0.864 mV, higher than that of commercial Pt/C (0.827 mV) in alkaline solutions. The catalytic performance is attributable to the synergistic effects between the FeCo alloy and N-doped CNT structure. Moreover, the confinement of the FeCo nanoparticles inside the CNTs imparted the prepared catalysts with resistance to methanol poisoning and long-term stability. This versatile method of synthesizing CNTs directly from carbon black provides a new strategy for preparing high-performance non-precious-metal-based N-doped CNT catalysts for practical fuel cell applications.

N-regulated three-dimensional turf-like carbon nanosheet loaded with FeCoNi nanoalloys as bifunctional electrocatalysts for durable zinc-air batteries

N-regulated three-dimensional (3D) turf-like carbon material loaded with FeCoNi nanoalloys (F-CNS-CNT), composed of carbon nanotubes (CNT) grown in situ on carbon nanosheets(CNS), was synthesized using a low-temperature solution combustion method and organic compounds rich in pyridinic-N. This distinct structure significantly expands the effective electrochemical surface area, revealing an abundance of active sites and enhancing the mass transfer capability for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Both experimental observations and theoretical calculations corroborate that the synergy between the FeCoNi nanoalloy and the highly pyridinic N-doped carbon substrate optimizes the adsorption and desorption-free energy of oxygen intermediates, resulting in a remarkable improvement of intrinsic ORR/OER activity. Therefore, the derived F-CNS-CNT electrocatalyst can present a favorable half-wave potential of 0.85 V (ORR) and a lower overpotential of 260 mV (corresponding to a current density of 10 mA cm-2, OER) in alkaline media. Moreover, when employed in the air cathode of a flowable zinc-air battery, the electrocatalyst exhibits exceptional discharge and charge performance, including a high power density of 144.6 mW cm-2, a high specific capacity of 801 mAh g-1, and an impressive cycling stability of 600 cycles at a current density of 10 mA cm-2. Notably, these results markedly surpass those of the commercial catalyst Pt/C + IrO2.

Unveiling the Dynamic Evolution of Catalytic Sites in N-Doped Leaf-like Carbon Frames Embedded with Co Particles for Rechargeable Zn-Air Batteries

The advancement of cost-effective, high-performance catalysts for both electrochemical oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) is crucial for the widespread implementation of metal-air batteries. In this research, we fabricated leaf-like N-doped carbon frames embedded with Co nanoparticles by pyrolyzing a ZIF-L/carbon nanofiber (ZIF-L/CNF) composite. Consequently, the optimized ZIF-L/CNF-700 catalyst exhibit exceptional catalytic activities in both ORRs and OERs, comparable to the benchmark 20 wt% Pt/C and RuO2. Addressing the issue of diminished cycle performance in the Zn-air battery cycle process, further detailed investigations into the post-electrolytic composition reveal that both the carbon framework and Co nanoparticles undergo partial oxidation during both OERs and ORRs. Owing to the varying local pH on the catalyst surface due to the consumption and generation of OH- by OERs and ORRs, after OERs, the product is reduced-size Co particles, while after ORRs, the product is outer-layer Co(OH)2-enveloping Co particles.

Surface Plasmonic Core-Shell Nanostructures in Surface Enhanced Raman Scattering and Photocatalysis

Core-shell nanostructures are a typical material design. Usually, it consists of a core wrapped in a shell. It has attracted much attention due to its tunable structure and composition, high surface area, and high programmability. The properties and resonance frequency of their surface plasmons can be adjusted by regulating the shape, size, and composition of metal core-shell nanostructures. This interaction makes core-shell nanostructures an excellent platform for plasmon-enhanced optical effects. This Perspective explores the categories of core-shell nanostructures, their exchanges with excitons in two-dimensional materials, their spectrum-enhanced aspects, and prospects for future applications of core-shell nanostructures.

Carbon nanotube-encapsulated Co/Co3Fe7 heterojunctions as a highly-efficient bifunctional electrocatalyst for rechargeable zinc-air batteries

In oxygen electrocatalysis, how to rationally design low-cost catalysts with reasonable structure and long-term stability is a crucial issue. Here, an in-situ growth strategy is used to construct a shaped structure encapsulating a uniformly-dispersed Co/Co3Fe7 heterojunction in nitrogen-doped carbon nanotubes (Co/Co3Fe7@NCNTs). Hollow CoFe layered-double-hydroxide prisms act as sacrifices for in-situ growth of Co/Co3Fe7 nanoparticles, which also catalyze the growth of bamboo-like NCNTs. Tubular structure not only accelerates the charge transfer through the interactions between Co and Co3Fe7, but also limits the aggregation of the particles, thereby promoting the 4e- oxygen reduction/evolution reactions (ORR/OER) kinetics and stabilizing the bifunctional activity. Co/Co3Fe7@NCNTs-800 (pyrolyzed at 800 °C) shows exceptional ORR activity (half-wave potential of 0.89 V) and methanol tolerance. Meanwhile, Co/Co3Fe7@NCNTs-800 shows a small OER overpotential of 280 mV, which increases by only 9 mV after 1000 cyclic voltammetry (CV) cycles. The outstanding bifunctionality (potential gap of 0.62 V) is ascribed to the electronic structure modulation at the Co/Co3Fe7 heterointerface. Notably, it also has a high performance as an air-cathode for rechargeable zinc-air battery, showing high power density (165 mW cm-2) and specific capacity (770.5 m Ah kg-1). This work provides a new reference for promoting the development of alloy catalysts with heterogeneous interfaces.

Frankincense-Based Functionalized Multiwalled Carbon Nanotubes with Iron Oxide Composites for Efficient Removal of Crystal Violet: Kinetic and Equilibrium Analysis

In this study, novel adsorbents were developed by functionalizing multiwalled carbon nanotubes with frankincense (Fr-fMWCNT) and adding iron oxide (Fe3O4) to the adsorbent (Fr-fMWCNT-Fe3O4). The morphology, surface characteristics, and chemical nature of the synthesized samples were analyzed by using various characterization techniques. The prepared adsorbents were then applied for the elimination of the toxic dye, crystal violet (CV), from water-based solutions by employing a batch adsorption method. The effectiveness of materials for the adsorption of CV was investigated by tuning various effective experimental parameters (adsorbent dosage, dye quantity, pH, and contact time). In order to derive adsorption isotherms, the Langmuir and Freundlich adsorption models were investigated and compared. The Fr-fMWCNT and Fr-fMWCNT-Fe3O4 were found to remove 85 and 95% of the CV dye within 30 min of the adsorption experiment at pH 6, respectively. It was found that a pseudo-second-order reaction rate was consistent with the experimental adsorption kinetics. The equilibrium data demonstrated that the Langmuir model adequately explained the adsorption behavior of the CV dye on the Fr-fMWCNT and Fr-fMWCNT-Fe3O4 surfaces, respectively. According to the Langmuir study, the highest adsorption capacities of the dye are 434 mg/g for Fr-fMWCNT and 500 mg/g for Fr-fMWCNT-Fe3O4. Remediation of the CV dye using our novel composite materials has not been reported previously in the literature. The synthesized Fr-fMWCNT and Fr-fMWCNT-Fe3O4 adsorbents can be economical and green materials for the adsorptive elimination of CV dye from wastewater.

Superhydrophilicity boron-doped cobalt phosphide nanosheets decorated carbon nanotube arrays self-supported electrode for overall water splitting

Transition metal borides (TMBs) or phosphides (TMPs) have attracted great attention to the design of bifunctional electrocatalysts for energy storage. The superaerophobicity and superhydrophilicity of the catalytic electrode surface are crucial factors to determine the reaction process of the gas electrode. Herein, we report a self-supported electrode of carbon nanotube (CNTs) array grown on carbon cloth (CC) modulated together by boron-doped cobalt phosphide (CoP-B/CNTs/CC). The electrode requires the overpotential of 73.8 mV and 189.5 mV at the current density of ±10 mA cm-2 for hydrogen and oxygen evolution reactions in an alkaline electrolyte (1.0 M KOH), respectively, meanwhile maintaining outstanding long-term durability for more than 300 h. The excellent activity of CoP-B/CNTs/CC is attributed to boron doping regulating its electronic structure and further enriching active sites. The attractive stability of CoP-B/CNTs/CC is due to the unique geometric structure of the self-supported electrode. Furthermore, the superaerophobicity and superhydrophilicity of the electrode surface also accelerate the reaction process of the gas electrode. Expectedly, water splitting cells assembled using CoP-B/CNTs/CC electrodes as cathode and anode, respectively, require a cell voltage of 1.54 V at 10 mA cm-2, which is lower than that of the Pt/C/CC||RuO2/CC couple (1.69 V at 10 mA cm-2). Importantly, CoP-B/CNTs/CC||CoP-B/CNTs/CC achieve stable cell voltage under the step current changes (10 mA cm-2, 50 mA cm-2, and 100 mA cm-2) over 300 h. This work highlights a new path to understanding the effects of the static and dynamic behavior of bubbles on the surface of self-supporting electrodes on catalytic performance.

Structural Design Induced Electronic Optimization in Single-Phase MoCoP Nanocrystal for Boosting Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution

Structural and compositional design of multifunctional materials is critical for electrocatalysis, but their rational modulation and effective synthesis remain a challenge. Herein, a controllable one-pot synthesis for construction of trifunctional sites and preparation of porous structures is adopted for synthesizing dispersed MoCoP sites on N, P codoped carbonized substance. This tunable synthetic strategy also endorses the exploration of the electrochemical activities of Mo (Co)-based unitary, Mo/Co-based dual and MoCo-based binary metallic sites. Eventually benefiting from the structural regulation, MoCoP-NPC shows excellent oxygen reduction abilities with a half-wave potential of 0.880 V, and outstanding oxygen evolution and hydrogen evolution performance with an overpotential of 316 mV and 91 mV, respectively. MoCoP-NPC-based Zn-air battery achieves excellent cycle stability for 300 h and a high open-circuit voltage of 1.50 V. When assembled in a water-splitting device, MoCoP-NPC reaches 10 mA cm-2 at 1.65 V. Theoretical calculations demonstrate that the Co atom in the single-phase MoCoP has a low energy barrier for oxygen evolution reaction (OER) owing to the migration of Co 3d orbital toward the Fermi level. This work shows a simplified method for controllable preparation of prominent trifunctional catalysts.

Application of Conductive MOF in Zinc-Based Batteries

The use of conductive MOFs (c-MOFs) in zinc-based batteries has been a popular research direction. Zinc-based batteries are widely used with the advantages of high specific capacity and safety and stability, but they also face many problems. c-MOFs have excellent conductivity compared with other primitive MOFs, and therefore have better applications in zinc-based batteries. In this paper, the transfer mechanisms of the unique charges of c-MOFs: hop transport and band transport, respectively, are discussed and the way of electron transport is further addressed. Then, the various ways to prepare c-MOFs are introduced, among which solvothermal, interfacial synthesis, and postprocessing methods are widely used. In addition, the applications of c-MOFs are discussed in terms of their role and performance in different types of zinc-based batteries. Finally, the current problems of c-MOFs and the prospects for their future development are presented.

Highly-Exposed Co-CoO Derived from Nanosized ZIF-67 on N-Doped Porous Carbon Foam as Efficient Electrocatalyst for Zinc-Air Battery

Non-precious-metal based electrocatalysts with highly-exposed and well-dispersed active sites are crucially needed to achieve superior electrocatalytic performance for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) toward zinc-air battery (ZAB). Herein, Co-CoO heterostructures derived from nanosized ZIF-67 are densely-exposed and strongly-immobilized onto N-doped porous carbon foam (NPCF) through a self-sacrificial pyrolysis strategy. Benefited from the high exposure of Co-CoO heterostructures and the favorable mass and electron transfer ability of NPCF, the Co-CoO/NPCF electrocatalyst exhibits remarkable performance for both ORR (E1/2  = 0.843 V vs RHE) and OER (Ej = 10 mA cm-2  = 1.586 V vs RHE). Further application of Co-CoO/NPCF as the air-cathode in rechargeable ZAB achieves superior performance for liquid-state ZAB (214.1 mW cm-2 and 600 cycles) and flexible all-solid-state ZAB (93.1 mW cm-2 and 140 cycles). Results from DFT calculations demonstrate that the electronic metal-support interactions between Co-CoO and NPCF via abundant C-Nx sites is favorable for electronic structure modulation, accounting for the remarkable performance.

Phase Engineering of Molybdenum Carbide-Cobalt Heterostructures for Long-Lasting Zn-Air Batteries

Developing highly active and robust oxygen catalysts is of great significance for the commercialization of Zn-air batteries (ZABs) with long-life stability. Herein, heterostructured catalysts comprising molybdenum carbide and metallic Co are prepared by a simple dicyandiamide-assisted pyrolysis strategy. Importantly, the crystalline phase of molybdenum carbide in the catalysts can be carefully regulated by adjusting the CoMo-imidazole precursor and dicyandiamide ratio. The electronic configuration of Co and Mo centers as well as the phase-dependent oxygen reduction reaction performance of these heterostructures (β-Mo2C/Co, β-Mo2C/η-MoC/Co, and η-MoC/Co) was disclosed. A highly active η-MoC/Co cathode enables ZABs with outstanding long-term stability over 850 h with a low voltage decaying rate of 0.06 mV·h-1 and high peak power density of 162 mW·cm-2. This work provides a new idea for the rational design of efficient and stable cathode catalysts for ZABs.

A General "In Situ Etch-Adsorption-Phosphatization" Strategy for the Fabrication of Metal Phosphides/Hollow Carbon Composite for High Performance Liquid/Flexible Zn-Air Batteries

Benefiting from the admirable energy density (1086 Wh kg-1 ), overwhelming security, and low environmental impact, rechargeable zinc-air batteries (ZABs) are deemed to be attractive candidates for lithium-ion batteries. The exploration of novel oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional catalysts is the key to promoting the development of zinc-air batteries. Transitional metal phosphides (TMPs) especially Fe-based TMPs are deemed to be a rational type of catalyst, however, their catalytic performance still needs to be further improved. Considering Fe (heme) and Cu (copper terminal oxidases) are nature's options for ORR catalysis in many forms of life from bacteria to humans. Herein, a general "in situ etch-adsorption-phosphatization" strategy is designed for the fabrication of hollow FeP/Fe2 P/Cu3 P-N, P codoped carbon (FeP/Cu3 P-NPC) catalyst as the cathode of liquid and flexible ZABs. The liquid ZABs manifest a high peak power density of 158.5 mW cm-2 and outstanding long-term cycling performance (≈1100 cycles at 2 mA cm-2 ). Similarly, the flexible ZABs deliver superior cycling stability of 81 h at 2 mA cm-2 without bending and 26 h with different bending angles.

Enhancing Cu2+ Ion Removal: An Innovative Approach Utilizing Modified Frankincense Gum Combined with Multiwalled Carbon Tubes and Iron Oxide Nanoparticles as Adsorbent

Aquatic pollution, which includes organic debris and heavy metals, is a severe issue for living things. Copper pollution is hazardous to people, and there is a need to develop effective methods for eliminating it from the environment. To address this issue, a novel adsorbent composed of frankincense-modified multi-walled carbon nanotubes (Fr-MMWCNTs) and Fe3O4 [Fr-MWCNT-Fe3O4] was created and subjected to characterization. Batch adsorption tests showed that Fr-MWCNT-Fe3O4 had a maximum adsorption capacity of 250 mg/g at 308 K and could efficiently remove Cu2+ ions over a pH range of 6 to 8. The adsorption process followed the pseudo-second-order and Langmuir models, and its thermodynamics were identified as endothermic. Functional groups on the surface of modified MWCNTs improved their adsorption capacity, and a rise in temperature increased the adsorption efficiency. These results highlight the Fr-MWCNT-Fe3O4 composites' potential as an efficient adsorbent for removing Cu2+ ions from untreated natural water sources.

Interfacial Engineering of CoN/Co3 O4 Heterostructured Hollow Nanoparticles Embedded in N-Doped Carbon Nanowires as a Bifunctional Oxygen Electrocatalyst for Rechargeable Liquid and Flexible all-Solid-State Zn-Air Batteries

The design of economical, efficient, and robust bifunctional oxygen electrocatalysts is greatly imperative for the large-scale commercialization of rechargeable Zn-air battery (ZAB) technology. Herein, the neoteric design of an advanced bifunctional electrocatalyst composed of CoN/Co₃ O₄ heterojunction hollow nanoparticles in situ encapsulated in porous N-doped carbon nanowires (denoted as CoN/Co₃ O₄ HNPs@NCNWs hereafter) is reported. The simultaneous implementation of interfacial engineering, nanoscale hollowing design, and carbon-support hybridization renders the synthesized CoN/Co₃ O₄ HNPs@NCNWs with modified electronic structure, improved electric conductivity, enriched active sites, and shortened electron/reactant transport pathways. Density functional theory computations further demonstrate that the construction of a CoN/Co₃ O₄ heterojunction can optimize the reaction pathways and reduce the overall reaction barriers. Thanks to the composition and architectural superiorities, the CoN/Co₃ O₄ HNPs@NCNWs exhibit distinguished oxygen reduction reaction and oxygen evolution reaction performance with a low reversible overpotential of 0.725 V and outstanding stability in KOH medium. More encouragingly, the homemade rechargeable liquid and flexible all-solid-state ZABs utilizing CoN/Co₃ O₄ HNPs@NCNWs as the air-cathode deliver higher peak power densities, larger specific capacities, and robust cycling stability, exceeding the commercial Pt/C + RuO₂ benchmark counterparts. The concept of heterostructure-induced electronic modification herein may shed light on the rational design of advanced electrocatalysts for sustainable energy applications.

Atomic Fe/Zn anchored N, S co-doped nano-porous carbon for boosting oxygen reduction reaction

Dual-single-atom catalysts are well-known due to their excellent catalytic performance of oxygen reduction reaction (ORR) and the tunable coordination environment of the active sites. However, it is still challengable to finely modulate the electronic states of the metal atoms and facilely fabricate a catalyst with dual-single atoms homogeneously dispersed on conductive skeletons with good mass transport. Herein, atomic FeN_x/ZnN_x sites anchored N, S co-doped nano-porous carbon plates/nanotubes material (Fe_0.10ZnNSC) is rationally prepared via a facile room-temperature reaction and high-temperature pyrolysis. The as-prepared Fe_0.10ZnNSC catalyst exhibits a positive onset potential of 0.956 V, an impressive half-wave potential of 0.875 V, excellent long-term durability, and a high methanol resistance, outperforming the benchmark Pt/C. The outstanding ORR performance of Fe_0.10ZnNSC is due to its unique nanoarchitecture: a large specific surface area (1092.8 cm² g^-1) and well-developed nanopore structure ensure the high accessibility of active sites; the high conductivity of the carbon matrix guarantees a strong ability to transport electrons to the active sites; and the optimized electronic states of FeN_x and ZnN_x sites possess good oxygen intermediate adsorption/desorption capacity. This strategy can be extended to design and fabricate other non-precious dual-single-atom ORR catalysts.