Stereoselectively Assembled Metal–Organic Framework (MOF) Host for Catalytic Synthesis of Carbon Hybrids for Alkaline‐Metal‐Ion Batteries

Hierarchical Nano-Electrocatalytic Reactor for High Performance Polysulfides Redox Flow Batteries

The aqueous polysulfides is an important Earth-abundant and multielectron redox couple to construct high capacity density and low-cost aqueous redox flow batteries (RFB) ; nevertheless, the sluggish conversion and kinetic behavior of S2-/Sx2- result in a low power density output and poor active material utilizations. Herein, we present nanoconfined self-assembled ordered hierarchical porous Co and N codoped carbon (OHP-Co/NC) as an electrocatalytic reactor to enhance the mass transfer and redox activity of aqueous polysulfides. Finite element method simulation proves that the OHP-Co/NC with interconnected macropores and mesopores exhibits an enhanced mass transfer and delivers a larger redox electrolyte utilization of 50.1% compared to 23.3% of conventional Co/NC. Notably, the OHP-Co/NC obtained at 850 °C delivers the smallest redox peak potential difference (ΔE = 99 mV). Comparison studies of in operando Raman for aqueous polysulfides in the redox electrolyte and in situ electrochemical Raman on the single OHP-Co/NC particle for the adsorbed polysulfides were carried out. And it confirms that the OHP-Co/NC-850 catalyst has a strong adsorption of S42- and can retard the strong disproportionation and hydrolysis behavior of polysulfides on the electrocatalyst interface. Therefore, the polysulfide/ferrocyanide RFB with an OHP-Co/NC-850 based membrane-electrode assembly (MEA) exhibited a high power density of 110 mW cm-2, as well as a steady capacity retention over 99.7% in 300 cycles.

Dual carbon-confined Sb2Se3 nanoparticles with pseudocapacitive properties for high-performance lithium-ion half/full batteries

Transition metal selenides have attracted enormous research attention as anodes for lithium-ion batteries (LIBs) due to their high theoretical specific capacities. Nevertheless, the low electronic conductivity and dramatic volume variation in electrochemical reaction processes result in rapid capacity fading and poor rate capability. Herein, a metal-organic framework is used as a template to in situ synthesize Sb₂Se₃ nanoparticles encapsulated in N-doped carbon nanotubes (N-CNTs) grafted on reduced graphene oxide (rGO) nanosheets. The synergistic effects of N-doped carbon nanotubes and reduced graphene oxide nanosheets are beneficial for providing good electrical conductivity and maintaining the structural stability of electrode materials, leading to stable cycling performance and superior rate performance. Kinetic analysis suggests that the electrochemical reaction kinetics is dominated by pseudocapacitive contribution. Notably, a high discharge capacity of 451.1 mA h g^-1 at a current density of 2.0 A g^-1 is delivered after 450 cycles. Even at a high current density of 10.0 A g^-1, a discharge capacity of 192.6 mA h g^-1 is maintained after 10 000 cycles. When coupled with a commercial LiFePO₄ cathode, the full batteries show an excellent discharge specific capacity of 534.5 mA h g^-1 at 0.2 A g^-1. This work provides an effective strategy for constructing high-performance anodes for Li⁺ storage.

Sulfur-Induced Growth of Coordination Polymer Derived-Straight Carbon Nanotubes on Carbon Nanofiber Network for Zn-Air Batteries

Low-cost heteroatom-doped carbon nanomaterials have been widely studied for efficient oxygen reduction reaction and energy storage and conversion in metal-air batteries. A Masson pine twigs-like 3-dimensional network construction of carbon nanofibers (CNFs) with abundant straight long Co, N, and S-doped carbon nanotubes (CNTs) is developed by thermal treatment of Co-based polymer coated onto polyacrylonitrile nanofiber network together with thiourea at 900 °C, denoted as CNFT-Co₉ S₈ -900. It is interesting to note that the introduction of a high concentration of sulfur does not lead to the complete toxicity of catalysts, but promotes the axial growth to selectively form straight CNTs instead of curly bamboo-like CNTs. The highly graphitized in-situ grown Co, N, S-doped CNTs and the 3-dimensional N-doped CNF network provide both active catalytic sites and highly conductive paths, which are beneficial for oxygen reduction reaction (ORR). Thus, the optimal CNFT-Co₉ S₈ -900 performs the excellent ORR catalytic activity with a half-wave potential of 0.84 V and a diffusion-limited current density of 5.49 mA cm^-2 . Furthermore, the CNFT-Co₉ S₈ -900-based Zn-air devices also possess a high power density of 136.9 mW cm^-2 better than commercial Pt/C.

Melamine-assisted synthesis of porous V2O3/N-doped carbon hollow nanospheres for efficient sodium-ion storage

Vanadium-based oxides with relatively high theoretical capacity have been regarded as promising electrode materials for boosting energy conversion and storage. However, their poor electrical conductivity usually leads to unsatisfied performance and poor cycling stability. Herein, uniform V₂O₃/N-doped carbon hollow nanospheres (V₂O₃/NC HSs) with mesoporous structures were successfully synthesized through a melamine-assisted simple hydrothermal reaction and carbonization treatment. We demonstrated that the introduction of melamine played an essential role in the construction of V₂O₃/NC HSs. Benefitting from the special mesoporous structure and large specific surface area, the as-obtained sample exhibited enhanced conductivity and structural stability. As a proof of concept, well-defined V₂O₃/NC HSs exhibited excellent cycling stability and rate performance for sodium-ion batteries, and achieved a discharge capacity of 263.8 mA h g^-1 at a current density of 1.0 A g^-1 after 1000 cycles, one of the best performances of V-based compounds. The enhanced performance could be attributed to the synergistic effect of the hollow structure and surface carbon coating. The present work describes the design of the morphology and structure of vanadium-based oxides for energy storage devices.

Interfacial Engineering of Bimetallic Carbide and Cobalt Encapsulated in Nitrogen-Doped Carbon Nanotubes for Electrocatalytic Oxygen Reduction

Heterojunction engineering is a fundamental strategy to develop efficient electrocatalysts for the oxygen reduction reaction by tuning electronic properties through interfacial cooperation. In this study, a heterojunction electrocatalyst consisting of bimetallic carbide Co₃ ZnC and cobalt encapsulated within N-doped carbon nanotubes (Co₃ ZnC/Co@NCNTs) is synthesized by a facile two-step ion exchange-thermolysis pathway. Co₃ ZnC/Co@NCNTs effectively promotes interfacial charge transport between the different components with optimizes adsorption and desorption of intermediate products at the heterointerface. In situ-grown N-doped carbon nanotubes (NCNTs) not only improve the electrical conductivity but also suppress the oxidation of transition metal nanoparticles in alkaline media. Moreover, the abundant nitrogen types (pyridinic N, Co-N_x , and graphitic nitrogen) in the carbon skeleton provide more active sites for oxygen adsorption. Benefitting from this optimized structure, Co₃ ZnC/Co@NCNTs hybrid not only demonstrates excellent oxygen reduction activity, with a half-wave potential of 0.83 V and fast mass transport with limited current density of 6.23 mA cm^-2 , but also exhibits superior stability and methanol tolerance, which surpass those of commercial Pt/C catalysts. This work provides an effective heterostructure for interfacial electronic modulation to improve electrocatalytic performance.

3D Rosa centifolia-like CeO2 encapsulated with N-doped carbon as an enhanced electrocatalyst for Zn-air batteries

Reasonable design and synthesis of high-efficiency rare earth oxides-based materials as alternatives to noble-metal catalysts are of great significance for oxygen electrocatalysis. Herein, we report three-dimension (3D) Rosa centifolia-like CeO₂ encapsulated with N-doped carbon (NC) composites (CeO₂@NC) for enhancing oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities. This synthetic method allows CeO₂ to tune the oxygen vacancy concentration and electronic structure of a series of CeO₂@NC catalysts due to its large oxygen-storage-capacity (OSC) property. Moreover, benefiting from the exposed active sites in NC as well as the synergy between CeO₂ and NC, among as-prepared samples, the resultant CeO₂@NC-900 delivers a half-wave potential (E_1/2) of 0.854 V, which is more positive compared with counterpart of NC-900 (0.806 V) and even comparable to that of commercial Pt/C catalyst (0.855 V). This indicates that the ORR electrocatalytic activity of CeO₂@NC-900 is significantly improved. Meanwhile, CeO₂@NC-900 exhibits satisfactory performance toward OER. For practical application, the CeO₂@NC-900 involved rechargeable Zn-air battery possesses excellent energy efficiency, superior stability, and large energy density (666.1 Wh kg_Zn^-1 at 5 mA cm^-2). This approach provides a valid way to develop advanced rare earth oxides-based materials for energy applications.

Rechargeable Zinc-Air Battery with Ultrahigh Power Density Based on Uniform N, Co Codoped Carbon Nanospheres

Highly efficient catalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are key to the commercialization of rechargeable zinc-air batteries (ZABs). In this work, a catalyst with uniform nanospherical morphology was prepared from cobalt nitrate, acetylacetone, and hydrazine hydrate. The final catalyst possesses high ORR and OER performances, with a half-wave potential of 0.911 V [vs reversible hydrogen electrode (RHE)] for ORR and a low potential of 1.57 V (vs RHE) at 10 mA cm^-2 for OER in 0.1 M KOH solution. Specially, a ZAB based on the catalyst demonstrates an ultrahigh power density of 479.1 mW cm^-2, as well as excellent stability, and potential in practical applications.

S-Edge-rich MoxSy arrays vertically grown on carbon aerogels as superior bifunctional HER/OER electrocatalysts

Molybdenum disulfide (MoS₂) is a potential earth-abundant electrocatalyst for the hydrogen evolution reaction (HER), but the lack of in-depth understanding of its intrinsic activity still impedes the further optimization and design of MoS₂-based electrocatalysts. Herein, we report a facile in situ hydrothermal synthetic method to prepare vertical Mo_xS_y arrays grown on guar gum-derived carbon aerogels (GCA), termed Mo_xS_y@GCA. The obtained well-assembled Mo_xS_y@GCA architectures consist of uniform, few-layered and S-edge-rich Mo_xS_y nanoflakes with a length of approximately 100 nm, which effectively prevent the inherent stacking among Mo_xS_y layers and connect the charge transfer path between interlayers, thus endowing Mo_xS_y@GCA with a huge number of active sites and high conductivity. Benefitting from all these advantages, the optimal Mo₄S₁₆@GCA exhibited extraordinary HER/OER performances, including a low onset potential for both the HER (24.28 mV) and OER (1.53 V), and a low overpotential at 10 mA cm^-2 for the HER (54.13 mV) and OER (370 mV), which are both extremely close to that of the noble Pt/C. Furthermore, a series of operando Raman spectroscopy measurements on Mo₄S₁₆@GCA were conducted to identify the intrinsic HER/OER-active sites during the HER and OER process. The results show that the S-H bond is generated simultaneously as HER/OER excitation, indicating the rich S-edge may be the intrinsic active site, which will accelerate the HER/OER kinetic process. Density functional theory (DFT) calculations revealed that the observed superb HER/OER activity can be attributed to the synergistic effect of rich S-edge of Mo_xS_y and confinement effect of GCA, which collaboratively promote the proton adsorption and electrocatalytic kinetics. Reasonably, this study will have profound guiding value for the rational tailoring of the microstructure and size of transition metal electrocatalysts via hierarchical porous carbon aerogels.

General and Scalable Fabrication of Core-Shell Metal Sulfides@C Anchored on 3D N-Doped Foam toward Flexible Sodium Ion Batteries

Flexible self-standing transitional metal sulfides (TMSs)/carbon nanoarchitectures have attracted widespread research interests for sodium ion batteries (SIBs), thanks to their enormous capability to address intrinsic issues of TMSs for SIBs applications. However, controllable synthesis of hierarchical hybrid structures is always laborious and involves complicated procedures. Herein, a simple yet general and scalable adsorption-annealing strategy is first devised to finely construct core-shell carbon-coated TMSs (TMSs@C, including Co₉ S₈ @C, FeS@C, Ni₃ S₂ @C, MnS@C, and ZnS@C) nanoparticles anchored on 3D N-doped carbon foam (3DNCF) via the coordination and hydrogen-bond adsorption. Benefiting from synergistic contributions from strong chemical affinity between nanodimensional TMSs and 3DNCF, efficient electronic/ionic transport channels, as well as a uniform carbon accommodating layer, the resulted self-standing TMSs@C/3DNCF electrodes exhibit distinguished sodium storage performances, including large reversible capacities, high rate behaviors, and exceptional long-span cycle stability in both half cells and flexible full devices. More significantly, the smart methodology developed holds huge promise for commercialization of binder-free TMSs@C/3DNCF anodes toward advanced flexible SIBs.