Cobalt Sulfide Nanowires Core Encapsulated by a N, S Codoped Graphitic Carbon Shell for Efficient Oxygen Reduction Reaction

Carbon encapsulated nanoparticles: materials science and energy applications

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

Switching Electric Double Layer Potential by Phase Structure Control for Advanced Oxygen Reduction Reaction of Cobalt@Nitrogen Doped Carbon Core-Shell

The key to design an advanced oxygen reduction reaction (ORR) electrocatalyst is a well-balance between the adsorption and desorption of oxygen intermediates. This study systematically evaluated the ORR activity of HCP and FCC cobalt core-shell cobalt/N-doped carbon (Cobalt@NC) catalyst via theoretical and experimental studies. The electronic structure calculations using density functional theory (DFT) calculations revealed that the ORR activity of carbon layer can be improved by 1) switching the electrostatic potential in the electrical double layer due to the polarization induced at the carbon-cobalt interface and 2) modulating the electron population in the bonding orbital in the C-O bonds in an ORR. The results revealed that an O atom is bounded stronger to the outer NC shell with FCC Cobalt than HCP Cobalt, which hindered the desorption steps of OH*. Experimentally, plasma-engineered HCP Cobalt@NC also showed remarkably advanced performance toward ORR compared to that FCC Cobalt@NC. The kinetic current density of HCP Cobalt@NC at 0.85 V versus RHE is calculated as 6.24 mA cm-2, which is six folds higher than FCC Cobalt@NC and even outperform 20 wt.% Pt/C. In a practical Aluminium-air battery, HCP Cobalt@NC also exhibited slightly higher peak power density (110.57 mW cm-2) compared to 20 wt.% Pt/C.

Insights on MOF-derived metal-carbon nanostructures for oxygen evolution

Electrochemical water splitting has been regarded a promising method for the production of green hydrogen, addressing the need for efficient energy conversion and storage. However, it is severely hindered by the oxygen evolution reaction (OER) because of its multi-step four-electron transfer pathway with sluggish reaction kinetics. Microporous metal-organic-frameworks (MOFs), by virtue of large specific surface area, high porosity, tunable composition and morphology, find widespread use as precursors of metal-carbon nanostructures. The resulting carbon nanomaterials can well inherit the characteristics and advantages of the crystalline MOF precursors, and exhibit versatile application prospects in the fields of environment and energy, particularly in OER. Herein, a meticulous overview of the synthesis strategy for MOF-derived metal-carbon nanostructures and the origins of their enhanced OER properties has been demonstrated. We comprehensively illustrate these aspects across three dimensions: MOF selection, metal introduction, and carbon structures. Finally, the challenges and future prospects for this emerging field will be presented.

Nanoreactor of Fe, N Co-Doped Hollow Carbon Spheres for Oxygen Reduction Catalysis

A simple template strategy was applied to prepare a Fe, N co-doped hollow carbon (Fe-NHC) nanoreactor for the oxygen reduction reaction (ORR) by coating Fe nanoparticles (Fe-NPs) with polydopamine (PDA), followed by high temperature pyrolysis and acid-leaching. With this method, Fe-NPs were used as both the template and the metal precursor, so that the nanoreactors can preserve the original spherical morphology and embed Fe single atoms on the inner walls. The carbonized PDA contained abundant N content, offering an ideal coordination environment for Fe atoms. By regulating the mass ratio of Fe-NPs and PDA, an optimal sample with a carbon layer thickness of 12 nm (Fe-NHC-3) was obtained. The hollow spherical structure of the nanoreactors and the atomically dispersed Fe were verified by various physical characterizations. As a result, Fe-NHC-3 performed well in ORR tests under alkaline conditions, with high catalytic activity, durability, and methanol resistance, demonstrating that the as-fabricated materials have the potential to be applied in the cathodic catalysis of fuel cells.

Double-Faced Atomic-Level Engineering of Hollow Carbon Nanofibers as Free-Standing Bifunctional Oxygen Electrocatalysts for Flexible Zn-Air Battery

Flexible solid-state zinc-air batteries (ZABs) with low cost, excellent safety, and high energy density has been considered as one of ideal power sources for portable and wearable electronic devices, while their practical applications are still hindered by the kinetically sluggish cathodic oxygen reduction and oxygen evolution reactions (ORR/OER). Herein, a Janus-structured flexible free-standing bifunctional oxygen electrocatalyst, with OER-active O, N co-coordinated Ni single atoms and ORR-active Co₃O₄@Co_1-xS nanosheet arrays being separately integrated at the inner and outer walls of flexible hollow carbon nanofibers (Ni-SAs/HCNFs/Co-NAs), is reported. Benefiting from the sophisticated topological structure and atomic-level-designed chemical compositions, Ni-SAs/HCNFs/Co-NAs exhibits outstanding bifunctional catalytic activity with the ΔE index of 0.65 V, representing the current state-of-the-art flexible free-standing bifunctional ORR/OER electrocatalyst. Impressively, the Ni-SAs/HCNFs/Co-NAs-based liquid ZAB show a high open-circuit potential (1.45 V), high capacity (808 mAh g^-1 Zn), and extremely long life (over 200 h at 10 mA cm^-2), and the assembled flexible all-solid-state ZABs have excellent cycle stability (over 80 h). This work provides an efficient strategy for developing high-performance bifunctional ORR/OER electrocatalysts for commercial applications.

Three-dimensional hierarchical conductive metal–organic frameworks/NiFe layered double hydroxide/carbon nanofibers: an efficient oxygen evolution reaction catalyst for Zn–air batteries

The structural and compositional modulation of inexpensive hydroxides is not only important, but also an ongoing challenge for the preparation of efficient oxygen evolution reaction (OER) electrocatalysts.

In situ encapsulation engineering boosts the electrochemical performance of highly graphitized N-doped porous carbon-based copper-cobalt selenides for bifunctional oxygen electrocatalysis

Transition-metal selenides are gaining prominence as promising electrode materials for energy storage applications owing to their low electronegativity and environment-friendliness compared with metal sulfides/oxides. Herein, a CuCoSe@NC nanocomposite with copper-cobalt selenides embedded in highly graphitized N-doped porous carbon was synthesized by an in situ encapsulation strategy with metal-organic framework crystals (CuCo-BDC) as templates followed by selenization, and used as a bifunctional electrocatalyst for Zn-air batteries in lye. The result shows that the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity of the optimal CuCoSe@NC-2 was enhanced, and the assembled Zn-air batteries exhibited a remarkable electrochemical performance with the use of the CuCoSe@NC-2 electrode, including a high power density (137.1 mW cm^-2) and excellent charge-discharge cycling stability, which were better than those of the Pt/C + RuO₂ electrocatalyst. Such improvement is attributed not only to the higher porosity and larger specific surface area (342 m² g^-1) of the carbon matrix, which increased the contact area with oxygen-containing species, but also the encapsulation effect of the highly graphitized N-doped carbon layer and the high content of pyridine-N species also further improved the conductivity of selenide composites. This work has introduced N-doped bimetallic selenides as an ideal candidate for bifunctional electrocatalysts.

Insights into the Interfacial Lewis Acid-Base Pairs in CeO2 -Loaded CoS2 Electrocatalysts for Alkaline Hydrogen Evolution

Despite the known efficacy of CeO₂ as a promoter in alkaline hydrogen evolution reaction (HER), the underlying mechanism of this effect remains unclear. CoS₂ , a pyrite-type alkaline HER electrocatalyst, suffers from sluggish HER kinetics and severe catalyst leaching due to its weak water dissociation kinetics and oxygen-related corrosion. Herein, it is demonstrated that the interfacial Lewis acid-base Ce∙∙∙S pairs in CeO₂ -loaded CoS₂ effectively improve the catalytic activity and durability. In CeO₂ -loaded CoS₂ nanowire array electrodes, these interfacial Lewis acid-base Ce∙∙∙S pairs with unique electronic and structural configurations efficiently activate water adsorptive dissociation and kinetically accelerate hydrogen evolution, delivering a low overpotential of 36 mV at 10 mA cm^-2 in alkaline media. Such Ce∙∙∙S pairs also weaken O₂ adsorption on CoS₂ , leading to undecayed activity over 1000 h. These findings are expected to provide guidance for the design of CeO₂ -based electrocatalysts as well as other hybrid electrocatalysts for water splitting.

Recent progress of electrocatalysts for oxygen reduction in fuel cells

Oxygen reduction reaction (ORR) has gradually been in the limelight in recent years because of its great application potential for fuel cells and rechargeable metal-air batteries. Therefore, significant issues are increasingly focused on developing effective and economical ORR electrocatalysts. This review begins with the reaction mechanisms and theoretical calculations of ORR in acidic and alkaline media. The latest reports and challenges in ORR electrocatalysis are traced. Most importantly, the latest advances in the development of ORR electrocatalysts are presented in detail, including platinum group metal (PGM), transition metal, and carbon-based electrocatalysts with various nanostructures. Furthermore, the development prospects and challenges of ORR electrocatalysts are speculated and discussed. These insights would help to formulate the design guidelines for highly-active ORR electrocatalysts and affect future research to obtain new knowledge for ORR mechanisms.

Integrating a metal framework with Co-confined carbon nanotubes as trifunctional electrocatalysts to boost electron and mass transfer approaching practical applications

A facile and large-scale construction of robust and inexpensive trifunctional self-supporting electrodes for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in metal-air batteries and water splitting is crucial but remains challenging. Herein, we report a direct and up-scalable all-solid-phase strategy for the synthesis of a porous three-dimensional electrode consisting of cobalt nanoparticles wrapped in nitrogen-doped carbon tubes (Co/N-CNTs), which are in situ planted onto the surface of a cobalt foam. The resultant Co/N-CNTs can directly serve as a self-supporting and adhesive-free electrode with excellent and durable catalytic performances for the ORR, OER and HER. The metal framework substrate with an open-pore architecture is favorable for electron and mass transfer and allows fast catalytic kinetics. More importantly, when used in Zn-air batteries and overall water splitting, the as-prepared Co/N-CNT electrode displays a remarkable performance, implying bright perspects for practical application.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO₂ reduction, and N₂ reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

A Co-MOF-derived flower-like CoS@S,N-doped carbon matrix for highly efficient overall water splitting

In this study, we constructed a highly effective, low-cost, non-noble-metal-based electrocatalyst to replace Pt catalysts, with a CoS@SNC catalyst being successfully synthesized. The obtained nanocatalyst was characterized via scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, powder X-ray diffraction studies, and X-ray photoelectron spectroscopy. Herein, an initially prepared N-containing Co MOF formed flower-like particles, which were obtained via a solvothermal method; further it was used for a sulfuration process as a template to achieve an S,N (heteroatom)-doped carbon electrocatalyst with embedded CoS (CoS@SNC). The synthesized flower-like CoS@SNC electrocatalyst derived from a novel MOF showed a uniform distribution of Co, S, N, and C at the molecular level in the MOF and it was rich in active sites, facilitating enhanced electrocatalytic performance. During the HER and OER in 0.1 M KOH solution, to reach a current density of 10 mA cm⁻², lower overpotentials of −65 mV and 265 mV, respectively, were required and Tafel slopes of 47 mV dec⁻¹ and 59.8 mV dec⁻¹, respectively, were seen. In addition, due to a synergistic effect between CoS and the S,N-doped carbon matrix, long-term durability and stability were obtained. This facile synthetic strategy, which is also environmentally favorable, produces a promising bifunctional electrocatalyst.

In this study, we constructed a highly effective, low-cost, non-noble-metal-based electrocatalyst to replace Pt catalysts, with a CoS@SNC catalyst being successfully synthesized.

Nonspherical Metal-Based Nanoarchitectures: Synthesis and Impact of Size, Shape, and Composition on Their Biological Activity

Metal-based nanoentities, apart from being indispensable research tools, have found extensive use in the industrial and biomedical arena. Because their biological impacts are governed by factors such as size, shape, and composition, such issues must be taken into account when these materials are incorporated into multi-component ensembles for clinical applications. The size and shape (rods, wires, sheets, tubes, and cages) of metallic nanostructures influence cell viability by virtue of their varied geometry and physicochemical interactions with mammalian cell membranes. The anisotropic properties of nonspherical metal-based nanoarchitectures render them exciting candidates for biomedical applications. Here, the size-, shape-, and composition-dependent properties of nonspherical metal-based nanoarchitectures are reviewed in the context of their potential applications in cancer diagnostics and therapeutics, as well as, in regenerative medicine. Strategies for the synthesis of nonspherical metal-based nanoarchitectures and their cytotoxicity and immunological profiles are also comprehensively appraised.

In situ synthesis of nitrogen site activated cobalt sulfide@N, S dual-doped carbon composite for a high-performance asymmetric supercapacitor

Cobalt sulfides with high theoretical capacity are considered as potential electrodes for supercapacitors (SCs). However, the insufficient reactive sites and low electrical conductivity of bulky cobalt sulfides restrict their applications. Here, we proposed an efficient approach for in situ formation of nitrogen site activated cobalt sulfide@N, S dual-doped carbon composite (CS@NSC) by vulcanizing the cobalt-glutamine complex (CG) precursor in a tube furnace. The effects of the molecular structure and calcination temperature of CG precursors on the morphology, structure and electrochemical performance of CS@NSC were studied. The designed CS@NSC-2 exhibited a specific capacity of 593 C g^-1 at the current density of 1 A g^-1 and good cyclic stability with 88.7% retention after 2000 cycles. Moreover, an asymmetric supercapacitor (ASC) was fabricated by CS@NSC-2 (positive electrode) and activated carbon (AC) (negative electrode), which delivered ultra-high energy density of 67.8 Wh kg^-1 at a power density of 400 W kg^-1 and possessed 83.1% capacitance retention after 5000 cycles. The eco-friendly method was also suitable for synthesizing nickel sulfide. This work may provide an innovative horizon for the in situ formation of active sites in electrode materials.

Zinc, sulfur and nitrogen co-doped carbon from sodium chloride/zinc chloride-assisted pyrolysis of thiourea/sucrose for highly efficient oxygen reduction reaction in both acidic and alkaline media

Zn and N co-doped carbon (Zn-N-C) shows encouraging catalytic stability for oxygen reduction reaction (ORR) because of the fulfilled d orbital of Zn, but its catalytic activity is not satisfactory. Herein, hierarchically porous Zn, S and N co-doped carbon (Zn-S-N-C) with large specific surface area (2433 m² g^-1) and pore volume (3.007 cm³ g^-1) is synthesized by using NaCl/ZnCl₂-assisted pyrolysis of sucrose and thiourea. The Zn-S-N-C catalyst exhibits superior ORR activity with half-wave potentials (E_1/2) up to 0.774 V in 0.1 M HClO₄ and 0.894 V in 0.1 M KOH, good ORR stability with 19- and 4-mV loss in E_1/2 values after 10,000 potential cycles in 0.1 M HClO₄ and 0.1 M KOH, respectively, and excellent methanol tolerance. The good ORR performance of Zn-S-N-C can be attributed to its enhanced intrinsic ORR activity resulting from the formation of S, N doped carbon and ZnS in Zn-S-N-C, its hierarchically porous structure resulting from the pore-forming roles played by ZnCl₂, NaCl and thiourea, and its improved graphitization degree resulting from the added ZnCl₂ during Zn-S-N-C synthesis. This work will provide a novel strategy for the synthesis of hierarchically porous Zn, S and N co-doped carbon materials for ORR.

Concurrently Realizing Geometric Confined Growth and Doping of Transition Metals within Graphene Hosts for Bifunctional Electrocatalysts toward a Solid-State Rechargeable Micro-Zn-Air Battery

The simultaneous realization of confined growth and doping of transition metals within carbon hosts promises to deliver unusual bifunctional catalytic activity but still remains challenging due to the difficulty in achieving synchronous nucleation and diffusion of metallic ions in a single synthesis step. Herein, we present a simple synthesis strategy capable of concurrently realizing geometric confined growth and doping of transition metals within graphene hosts, demonstrated in Co,N-codoped graphene-confined FeNi nanoparticles (Co,N-GN-FeNi). The obtained Co,N-GN-FeNi can take full advantage of the hierarchy of interactions between the confined-grown FeNi nanoparticles (for high oxygen evolution reaction (OER) activity) and the Co,N-codoped graphene hosts (for high oxygen reduction reaction (ORR) activity). The overall structure is a rationally designed synergy that simultaneously realizes (i) adequate exposure of electroactive sites, (ii) effective protection against corrosion/aggregation of FeNi nanoparticles, and (iii) rapid transport of ions/electrons between the interfaces. As a result, Co,N-GN-FeNi exhibits excellent bifunctional electrocatalytic activity relying on a low ORR/OER subtraction (ΔE = 0.81 V). Subsequent combination with a planar electrode configuration and a solid polymer electrolyte further demonstrates the utilization of Co,N-GN-FeNi as air cathode bifunctional electrocatalysts in a solid-state rechargeable micro-Zn-air battery (SR-MZAB), which exhibits a large open-circuit voltage of 1.39 V, a high power density/specific capacity of 62.3 mW cm^-2/763 mAh g^-1, excellent durability (126 cycles/42 h), and mechanical flexibility. This work demonstrates an effective synthesis strategy for concurrently realizing geometric confined growth and doping of transition metals within carbon hosts, for enhanced bifunctional catalytic activity toward novel SR-MZABs with high energy efficiency, security, and flexibility for wearable micropower sources.

Carbon Nanohorn-Based Electrocatalysts for Energy Conversion

In the context of even more growing energy demands, the investigation of alternative environmentally friendly solutions, like fuel cells, is essential. Given their outstanding properties, carbon nanohorns (CNHs) have come forth as promising electrocatalysts within the nanocarbon family. Carbon nanohorns are conical nanostructures made of sp2 carbon sheets that form aggregated superstructures during their synthesis. They require no metal catalyst during their preparation and they are inexpensively produced in industrial quantities, affording a favorable candidate for electrocatalytic reactions. The aim of this article is to provide a comprehensive overview regarding CNHs in the field of electrocatalysis and especially, in oxygen reduction, methanol oxidation, and hydrogen evolution, as well as oxygen evolution from water splitting, underlining the progress made so far, and pointing out the areas where significant improvement can be achieved.

Optimization of Sulfurization Process of Cobalt Sulfide and Nitrogen Doped Carbon Material for Boosting the Oxygen Reduction Reaction Catalytic Activity in Alkaline Medium

In order to reduce fuel cell material cost and promote its application, it is urgent to develop non-noble metal materials to replace platinum as the cathode catalysts in fuel cells. The cobalt sulfide and nitrogen co-doped carbon (S-Co-N/C) materials with metal-organic frameworks as precursors have shown good oxygen reduction reaction (ORR) catalytic activity. Benefiting from the protection of catalytic active sites by sulfur atoms, the stability and alcohol-tolerance of the S-Co-N/C catalyst can be significantly improved. In order to fully understand the effect of the sulfurization process on the properties of the material, zeolite imidazole frameworks (ZIF)-8, and ZIF-67 are used as precursors to prepare a novel material of S-Co-N/C by using a sulfurization-pyrolysis method. Another S-Co-N/C material by using a pyrolysis- sulfurization method is prepared for comparison. The effects of the sulfurization process in the preparation on the morphology, chemical structure, specific surface area, and ORR catalytic activity of the final material are investigated. The experimental results show that the surface of the S-Co-N/C material tends to be rough due to the sulfurization reaction of the metal elements. The porosity of the material is reduced to some extent due to the remaining Zn elements in the final product. Interestingly, some carbon nanotubes are found to be generated on the surface of the S-Co-N/C material because of the synergistic effect of Zn and Co on the carbon material during the pyrolysis process, which is beneficial to accelerate the adsorption of oxygen on the S-Co-N/C surface and the electron transportation during the oxygen reduction reaction. In addition, the generated CoS during the sulfurization process can further protect the Co elements from agglomeration, which can effectively increase the ORR catalytic active sites in the final material. The S-Co-N/C material prepared by the sulfurization-pyrolysis method performs a superior ORR catalytic activity to the one synthesized by the pyrolysis-sulfurization method.

Mo-Doped Zn, Co Zeolitic Imidazolate Framework-Derived Co9S8 Quantum Dots and MoS2 Embedded in Three-Dimensional Nitrogen-Doped Carbon Nanoflake Arrays as an Efficient Trifunctional Electrocatalysts for the Oxygen Reduction Reaction, Oxygen Evolution Reaction, and Hydrogen Evolution Reaction

Herein, we first propose a facile strategy to synthesize Co₉S₈ and MoS₂ nanocrystals embedded in porous carbon nanoflake arrays supported on carbon nanofibers (Co₉S₈-MoS₂/N-CNAs@CNFs) by the pyrolysis of Mo-doped Zn, Co zeolitic imidazolate framework grown on carbon nanofibers and subsequent sulfuration. The electrocatalyst shows high and stable electrocatalytic performance, with a half-wave potential of 0.82 V for oxygen reduction reaction and an overpotential at 10 mA cm^-2 for oxygen evolution reaction (0.34 V) and hydrogen evolution reaction (0.163 V), which outperform the metal-organic framework-derived transition metal sulfide catalysts reported so far. Furthermore, the Co₉S₈-MoS₂@N-CNAs@CNFs are employed as an air cathode in a liquid-state and all-solid-state zinc-air battery, presenting high power densities of 222 and 96 mW cm^-2, respectively. Such excellent catalytic activities are mainly owing to the unique three-dimensional structure and chemical compositions, optimal electronic conductivity, adequate surface area, and the abundance of active sites. Thus, this work provides an important method for designing other metal-organic framework-derived three-dimensional structural sulfide quantum dot multifunctional electrocatalysts for wider application in highly efficient catalysis and energy storage.

Simultaneously Integrating Single Atomic Cobalt Sites and Co9 S8 Nanoparticles into Hollow Carbon Nanotubes as Trifunctional Electrocatalysts for Zn-Air Batteries to Drive Water Splitting

The development of rechargeable metal-air batteries and water electrolyzers are highly constrained by electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). However, the construction of efficient trifunctional electrocatalysts for ORR/OER/HER are highly desirable yet challenging. Herein, hollow carbon nanotubes integrated single cobalt atoms with Co₉ S₈ nanoparticles (CoSA + Co₉ S₈ /HCNT) are fabricated by a straightforward in situ self-sacrificing strategy. The structure of the CoSA + Co₉ S₈ /HCNT are verified by X-ray absorption spectroscopy and aberration-corrected scanning transmission electron microscopy. Theoretical calculations and experimental results embrace the synergistic effects between Co₉ S₈ nanoparticles and single cobalt atoms through optimizing the electronic configuration of the CoN₄ active sites to lower the reaction barrier and facilitating the ORR, OER, and HER simultaneously. Consequently, rechargeable liquid and all-solid-state flexible Zn-air batteries based on CoSA + Co₉ S₈ /HCNT exhibit remarkable stability and excellent power density of 177.33 and 51.85 mW cm^-2 , respectively, better than Pt/C + RuO₂ counterparts. Moreover, the as-fabricated Zn-air batteries can drive an overall water splitting device assembled with CoSA + Co₉ S₈ /HCNT and achieve a current density of 10 mA cm^-2 at a low voltage of 1.59 V, also superior to Pt/C + RuO₂ . Therefore, this work presents a promising approach to an efficient trifunctional electrocatalyst toward practical applications.

In Situ-Formed Hollow Cobalt Sulfide Wrapped by Reduced Graphene Oxide as an Anode for High-Performance Lithium-Ion Batteries

Transition-metal sulfides have been considered as promising anode materials for lithium-ion batteries (LIBs) due to their high theoretical specific capacity and superior electrochemical performance. However, the large volume change during the discharge/charge process causes structural pulverization, resulting in rapid capacity decline and the loss of active materials. Herein, we report Co_1-xS hollow spheres formed by in situ growth on reduced graphene oxide layers. When evaluated as an anode material for LIBs, it delivers a specific capacity of 969.8 mAh·g^-1 with a high Coulombic efficiency of 96.49% after 90 cycles. Furthermore, a high reversible capacity of 527.2 mAh·g^-1 after the 107th cycle at a current density of 2.5 A g^-1 is still achieved. The results illustrate that in situ growth on the graphene layers can enhance conductivity and restrain volume expansion of cobalt sulfide compared with ex situ growth.

Facile Preparation of Carbon Shells-Coated O-Doped Molybdenum Carbide Nanoparticles as High Selective Electrocatalysts for Nitrogen Reduction Reaction under Ambient Conditions

Electrochemical nitrogen reduction reaction (NRR) has been considered as a promising alternative to the traditional Haber-Bosch process for the preparation of ammonia (NH₃) under ambient conditions. The development of cost-effective electrocatalysts with suppressive activity for hydrogen evolution reaction is critical for improving the efficiency of NRR. Herein, oxygen-containing molybdenum carbides (O-MoC) embedded in nitrogen-doped carbon layers (N-doped carbon) can be easily fabricated by pyrolyzing the chelate of dopamine and molybdate. A rate of NH₃ formation of 22.5 μg·h^-1·mg_cat^-1 is obtained at -0.35 V versus reversible hydrogen electrode with a high faradaic efficiency of 25.1% in 0.1 mM HCl + 0.5 M Li₂SO₄. Notably, the synthesized O-MoC@NC-800 also exhibits high selectivity (no formation of hydrazine) and electrochemical stability. The moderate electron structure induced by the interaction between O-MoC and N-doped carbon shells can effectively weaken the activity of hydrogen evolution reaction and increase the faradaic efficiency of NRR. Additionally, by applying the in situ Fourier transform infrared spectroscopy, an associative reaction pathway is proposed on O-MoC@NC-800. This work provides new insights into the rational design of carbon-encapsulated metal nanoparticles as efficient catalysts for NRR at ambient conditions.

Direct Growth of CNTs@CoSx Se2(1-x) on Carbon Cloth for Overall Water Splitting

Searching for low-cost, high-efficiency, bifunctional, non-noble-metal electrocatalysts for overall water splitting is crucial to renewable energy conversion. Herein, a series of component-controllable CC/CNTs@CoS_x Se_2(1-x) (CC: carbon cloth, CNT: carbon nanotube) with excellent bifunctional properties in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were obtained by chemical vapor deposition. In this strategy, the Zif-67 precursor served as a structural inducer, which was directly grown on CC and pyrolyzed with the assistance of melamine to form multi-walled CNT-encapsulated CoS_x Se_2(1-x) hierarchical nanostructures. Subsequently, the electrocatalytic properties of the as-prepared materials were optimized by adjusting the S/Se molar ratio. Of note is that the lattice distortion caused by the different radii of Se and S generated a polarized electric field for easy adsorption of the intermediate products. The CoOOH generated in situ on the surface of CoS_x Se_2(1-x) , as well as n- and p-type domains in carbon, synergistically resulted in abundant active sites to boost the electrocatalytic activity. CC/CNTs@CoS_0.74 Se_0.52 exhibited overpotentials for the HER and OER of 225 and 285 mV, respectively and attained a current density of 10 mA cm^-2 in alkaline solution. The as-prepared electrocatalysts could act as both cathode and anode in a water electrolyzer showing a cell voltage of 1.74 V and delivering 10 mA cm^-2 , comparable to those of noble-metal-based water electrolyzers.

Core–shell hybrid nanowires with Co nanoparticles wrapped in N-doped porous carbon for lightweight microwave absorption

One-dimensional core–shell structured nanowires with Co nanoparticles wrapped in N doped porous carbon were designed as a lightweight, thin and high-performance electromagnetic absorber.

Reactive template-induced core-shell FeCo@C microspheres as multifunctional electrocatalysts for rechargeable zinc-air batteries

Sluggish kinetics and thermodynamic unfavorability restrict electrocatalysis for energy storage and conversion reactions such as oxygen reduction/evolution and hydrogen evolution reactions. Herein, we report the synthesis and electrochemical performance of novel core-shell nanoparticles@porous carbon microspheres. A unique core-shell architecture of dual-phase FeCo-based nanoparticles@heteroatom-doped carbon microspheres (FeCo@C MS) has been prepared via a two-step carbonization process from a reactive multifunctional core-double shell template. With the advantages of heterogeneous composition and architectural structure, the obtained FeCo@C MS exhibits excellent performances for the electrochemical oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), which are comparable to those of commercial Pt/C catalyst. As an excellent cathode catalyst of the Zn-air battery (ZAB), FeCo@C MS exhibits high discharge voltage of 1.27 V, high specific capacity of 503 mA h gZn-1, an energy density of 639 W h kgZn-1, and better cycling durability than the battery having a mixture of 20 wt% Pt/C and RuO2. This approach provides a new way to design structures with controlled morphology and excellent multifunctional electrocatalytic activity.

Boosting Oxygen Reduction Catalysis with N-doped Carbon Coated Co9S8 Microtubes

Herein, nitrogen-doped carbon coated hollow Co₉S₈ microtubes (Co₉S₈@N-C microtubes) are prepared through a facile solvothermal procedure, followed by dopamine polymerization process together with a post-pyrolysis which present excellent electrocatalytic activity for oxygen reduction reaction (ORR). The Co₉S₈ within the hollow Co₉S₈@N-C microtubes presents a well-defined single-crystal structure with dominated (022) plane. To obtain desired electrocatalyst, the annealing temperature and the thickness of carbon layer tuned by changing the dopamine concentration are optimized systematically. The electrochemical results demonstrate that the coordination of the N-doped carbon layer, exposed (022) plane, and hollow architecture of Co₉S₈ microtubes calcined at 700 °C affords outstanding ORR performance to Co₉S₈@N-C microtubes. The moderate thickness of the carbon layer is crucial for improving ORR activity of Co₉S₈@N-C microtubes, while increasing or decreasing the thickness would result in activity decrease. More importantly, the N-doped carbon layer can protect inner Co₉S₈ from undergoing aggregation and dissolution effectively during the ORR, resulting in excellent electrocatalytic stability.