Reduced Graphene Oxide‐Wrapped Co9–xFexS8/Co,Fe‐N‐C Composite as Bifunctional Electrocatalyst for Oxygen Reduction and Evolution

Searching for highly efficient bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) using nonnoble metal‐based catalysts is essential for the development of many energy conversion systems, including rechargeable fuel cells and metal–air batteries. Here, Co_9–xFe_xS₈/Co,Fe‐N‐C hybrids wrapped by reduced graphene oxide (rGO) (abbreviated as S‐Co_9–xFe_xS₈@rGO) are synthesized through a semivulcanization and calcination method using graphene oxide (GO) wrapped bimetallic zeolite imidazolate framework (ZIF) Co,Fe‐ZIF (CoFe‐ZIF@GO) as precursors. Benefiting from the synergistic effect of OER active CoFeS and ORR active Co,Fe‐N‐C in a single component, as well as high dispersity and enhanced conductivity derived from rGO coating and Fe‐doping, the obtained S‐Co_9–xFe_xS₈@rGO‐10 catalyst shows an ultrasmall overpotential of ≈0.29 V at 10 mA cm^?2 in OER and a half‐wave potential of 0.84 V in ORR, combining a superior oxygen electrode activity of ≈0.68 V in 0.1 m KOH.     

Zero‐Field Nernst Effect in a Ferromagnetic Kagome‐Lattice Weyl‐Semimetal Co3Sn2S2

The discovery of magnetic topological semimetals has recently attracted significant attention in the field of topology and thermoelectrics. In a thermoelectric device based on the Nernst geometry, an external magnet is required as an integral part. Reported is a zero‐field Nernst effect in a newly discovered hard‐ferromagnetic kagome‐lattice Weyl‐semimetal Co₃Sn₂S₂. A maximum Nernst thermopower of ≈3 µV K^?1 at 80 K in zero field is achieved in this magnetic Weyl‐semimetal. The results demonstrate the possibility of application of topological hard magnetic semimetals for low‐power thermoelectric devices based on the Nernst effect and are thus valuable for the comprehensive understanding of transport properties in this class of materials.     

ZIF‐8/ZIF‐67‐Derived Co‐Nx‐Embedded 1D Porous Carbon Nanofibers with Graphitic Carbon‐Encased Co Nanoparticles as an Efficient Bifunctional Electrocatalyst

Herein, an approach is reported for fabrication of Co‐N_x‐embedded 1D porous carbon nanofibers (CNFs) with graphitic carbon‐encased Co nanoparticles originated from metal–organic frameworks (MOFs), which is further explored as a bifunctional electrocatalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Electrochemical results reveal that the electrocatalyst prepared by pyrolysis at 1000 °C (CoNC‐CNF‐1000) exhibits excellent catalytic activity toward ORR that favors the four‐electron ORR process and outstanding long‐term stability with 86% current retention after 40 000 s. Meanwhile, it also shows superior electrocatalytic activity toward OER, reaching a lower potential of 1.68 V at 10 mA cm^?2 and a potential gap of 0.88 V between the OER potential (at 10 mA cm^?2) and the ORR half‐wave potential. The ORR and OER performance of CoNC‐CNF‐1000 have outperformed commercial Pt/C and most nonprecious‐metal catalysts reported to date. The remarkable ORR and OER catalytic performance can be mainly attributable to the unique 1D structure, such as higher graphitization degree beneficial for electronic mobility, hierarchical porosity facilitating the mass transport, and highly dispersed CoN_xC active sites functionalized carbon framework. This strategy will shed light on the development of other MOF‐based carbon nanofibers for energy storage and electrochemical devices.     

Nitrogen‐Doped Co3O4 Mesoporous Nanowire Arrays as an Additive‐Free Air‐Cathode for Flexible Solid‐State Zinc–Air Batteries

The kinetically sluggish rate of oxygen reduction reaction (ORR) on the cathode side is one of the main bottlenecks of zinc‐air batteries (ZABs), and thus the search for an efficient and cost‐effective catalyst for ORR is highly pursued. Co₃O₄ has received ever‐growing interest as a promising ORR catalyst due to the unique advantages of low‐cost, earth abundance and decent catalytic activity. However, owing to the poor conductivity as a result of its semiconducting nature, the ORR activity of the Co₃O₄ catalyst is still far below the expectation. Herein, we report a controllable N‐doping strategy to significantly improve the catalytic activity of Co₃O₄ for ORR and demonstrate these N doped Co₃O₄ nanowires as an additive‐free air‐cathode for flexible solid‐state zinc‐air batteries. The results of experiments and DFT calculations reveal that the catalytic activity is promoted by the N dopant through a combined set of factors, including enhanced electronic conductivity, increased O₂ adsorption strength and improved reaction kinetics. Finally, the assembly of all‐solid‐state ZABs based on the optimized cathode exhibit a high volumetric capacity of 98.1 mAh cm^‐3 and outstanding flexibility. The demonstration of such flexible ZABs provides valuable insights that point the way to the redesign of emerging portable electronics.     

Origin of Fracture‐Resistance to Large Volume Change in Cu‐Substituted Co3O4 Electrodes

The electrode materials conducive to conversion reactions undergo large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, an in situ and ex situ electron microscopy investigation of structural evolutions of Cu‐substituted Co₃O₄ supplemented by first‐principles calculations is reported to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube‐on‐cube orientation relationship with Li₂O. In charge, a portion of Cu metal is oxidized to CuO, which maintains a cube‐on‐cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li₂O in the discharge cycle and underpins the catalytic activity of Li₂O decomposition in the charge cycle.     

Graphene Oxide‐Assisted Synthesis of Pt–Co Alloy Nanocrystals with High‐Index Facets and Enhanced Electrocatalytic Properties

Metal nanocrystals (NCs) are grown directly on the surface of reduced graphene oxide (rGO), which can maximize the rGO‐NCs contact/interaction to achieve the enhanced catalytic activity. However, it is difficult to control the size and morphology of metal NCs by in situ method due to the effects of functional groups on the surface of GO, and as a result, the metal NCs/rGO hybrids are conventionally synthesized by two‐step method. Herein, one‐pot synthesis of Pt–Co alloy NCs is demonstrated with concave‐polyhedrons and concave‐nanocubes bounded by {hkl} and {hk0} high‐index facets (HIFs) distributed on rGO. GO can affect the geometry and electronic structure of Pt–Co NCs. Thanks to the synergy of the HIFs and the electronic effect of the intimate contact/interaction between Pt–Co alloy and rGO, these as‐prepared Pt–Co NCs/rGO hybrids presents enhanced catalytic properties for the electrooxidation of formic acid, as well as for the oxygen reduction reaction.     

Co(OH)2 Nanoparticle‐Encapsulating Conductive Nanowires Array: Room‐Temperature Electrochemical Preparation for High‐Performance Water Oxidation Electrocatalysis

It is highly desired but still remains challenging to design and develop a Co‐based nanoparticle‐encapsulated conductive nanoarray at room temperature for high‐performance water oxidation electrocatalysis. Here, it is reported that room‐temperature anodization of a Co(TCNQ)₂ (TCNQ = tetracyanoquinodimethane) nanowire array on copper foam at alkaline pH leads to in situ electrochemcial oxidation of TCNQ^? into water‐insoluable TCNQ nanoarray embedding Co(OH)₂ nanoparticles. Such Co(OH)₂‐TCNQ/CF shows superior catalytic activity for water oxidation and demands only a low overpotential of 276 mV to drive a geometrical current density of 25 mA cm^?2 in 1.0 m KOH. Notably, it also demonstrates strong long‐term electrochemical durability with its activity being retrained for at least 25 h, a high turnover frequency of 0.97 s^?1 at an overpotential of 450 mV and 100% Faradic efficiency. This study provides an exciting new method for the rational design and development of a conductive TCNQ‐based nanoarray as an interesting 3D material for advanced electrochemical applications.     

Metal–Organic Framework Hybrid‐Assisted Formation of Co3O4/Co‐Fe Oxide Double‐Shelled Nanoboxes for Enhanced Oxygen Evolution

Rational design of complex metal–organic framework (MOF) hybrid precursors offers a great opportunity to construct various functional nanostructures. Here, a novel MOF‐hybrid‐assisted strategy to synthesize Co₃O₄/Co‐Fe oxide double‐shelled nanoboxes is reported. In the first step, zeolitic imidazolate framework‐67 (ZIF‐67, a Co‐based MOF)/Co‐Fe Prussian blue analogue (PBA) yolk–shell nanocubes are formed via a facile anion‐exchange reaction between ZIF‐67 nanocube precursors and [Fe(CN)₆]^3? ions at room temperature. Subsequently, an annealing treatment is applied to prepare Co₃O₄/Co‐Fe oxide double‐shelled nanoboxes. Owing to the structural and compositional benefits, the as‐derived Co₃O₄/Co‐Fe oxide double‐shelled nanoboxes exhibit enhanced electrocatalytic performance for oxygen evolution reaction in alkaline solution.     

TiO2 and Co Nanoparticle‐Decorated Carbon Polyhedra as Efficient Sulfur Host for High‐Performance Lithium–Sulfur Batteries

Metal organic frameworks (MOFs)‐derived porous carbon is proposed as a promising candidate to develop novel, tailorable structures as polysulfides immobilizers for lithium–sulfur batteries because of their high‐efficiency electron conductive networks, open ion channels, and abundant central ions that can store a large amount of sulfur and trap the easily soluble polysulfides. However, most central ions in MOFs‐derived carbon framework are encapsulated in the carbon matrix so that their exposures as active sites to adsorb polysulfides are limited. To resolve this issue, highly dispersed TiO₂ nanoparticles are anchored into the cobalt‐containing carbon polyhedras that are converted from ZIF‐67. Such a type of TiO₂ and Co nanoparticles‐decorated carbon polyhedras (C? Co/TiO₂) provide more exposed active sites and much stronger chemical trapping for polysulfides, hence improving the sulfur utilization and enhancing reaction kinetics of sulfur‐containing cathode simultaneously. The sulfur‐containing carbon polyhedras decorated with TiO₂ nanoparticles (S@C? Co/TiO₂) show a significantly improved cycling stability and rate capability, and deliver a discharge capacity of 32.9% higher than that of TiO₂‐free S@C? Co cathode at 837.5 mA g^?1 after 200 cycles.     

MOF‐Derived Hollow Co9S8 Nanoparticles Embedded in Graphitic Carbon Nanocages with Superior Li‐Ion Storage

Novel electrode materials consisting of hollow cobalt sulfide nanoparticles embedded in graphitic carbon nanocages (HCSP?GCC) are facilely synthesized by a top‐down route applying room‐temperature synthesized Co‐based zeolitic imidazolate framework (ZIF‐67) as the template. Owing to the good mechanical flexibility and pronounced structure stability of carbon nanocages‐encapsulated Co₉S₈, the as‐obtained HCSP?GCC exhibit superior Li‐ion storage. Working in the voltage of 1.0?3.0 V, they display a very high energy density (707 Wh kg^?1), superior rate capability (reversible capabilities of 536, 489, 438, 393, 345, and 278 mA h g^?1 at 0.2, 0.5, 1, 2, 5, and 10C, respectively), and stable cycling performance (≈26% capacity loss after long 150 cycles at 1C with a capacity retention of 365 mA h g^?1). When the work voltage is extended into 0.01–3.0 V, a higher stable capacity of 1600 mA h g^?1 at a current density of 100 mA g^?1 is still achieved.     

Ultrafine Co Nanoparticles Encapsulated in Carbon‐Nanotubes‐Grafted Graphene Sheets as Advanced Electrocatalysts for the Hydrogen Evolution Reaction

The rational design of an efficient and inexpensive electrocatalyst based on earth‐abundant 3d transition metals (TMs) for the hydrogen evolution reaction still remains a significant challenge in the renewable energy area. Herein, a novel and effective approach is developed for synthesizing ultrafine Co nanoparticles encapsulated in nitrogen‐doped carbon nanotubes (N‐CNTs) grafted onto both sides of reduced graphene oxide (rGO) (Co@N‐CNTs@rGO) by direct annealing of GO‐wrapped core–shell bimetallic zeolite imidazolate frameworks. Benefiting from the uniform distribution of Co nanoparticles, the in‐situ‐formed highly graphitic N‐CNTs@rGO, the large surface area, and the abundant porosity, the as‐fabricated Co@N‐CNTs@rGO composites exhibit excellent electrocatalytic hydrogen evolution reaction (HER) activity. As demonstrated in electrochemical measurements, the composites can achieve 10 mA cm^?2 at low overpotential with only 108 and 87 mV in 1 m KOH and 0.5 m H₂SO₄, respectively, much better than most of the reported Co‐based electrocatalysts over a wide pH range. More importantly, the synthetic strategy is versatile and can be extended to prepare other binary or even ternary TMs@N‐CNTs@rGO (e.g., Co–Fe@N‐CNTs@rGO and Co–Ni–Cu@N‐CNTs@rGO). The strategy developed here may open a new avenue toward the development of nonprecious high‐performance HER catalysts.