γ-Fe2O3 nanoparticles stabilized by holey reduced graphene oxide as a composite anode for lithium-ion batteries

Holey penta-hexagonal graphene: a promising anode material for Li-ion batteries

Carbon allotropes are widely used as anode materials in Li batteries, with graphite being commercially successful. However, the limited capacity and cycling stability of graphite impede further advancement and hinder the development of electric vehicles. Herein, through density functional theory (DFT) computations and ab initio molecular dynamics (AIMD) simulations, we proposed holey penta-hexagonal graphene (HPhG) as a potential anode material, achieved through active site designing. Due to the internal electron accumulation from the π-bond, HPhG follows a single-layer adsorption mechanism on each side of the nanosheet, enabling a high theoretical capacity of 1094 mA h g-1 without the risk of vertical dendrite growth. HPhG also exhibits a low open circuit voltage of 0.29 V and a low ion migration barrier of 0.32 eV. Notably, during the charge/discharge process, the lattice only expands slightly by 1.1%, indicating excellent structural stability. This work provides valuable insights into anode material design and presents HPhG as a promising two-dimensional material for energy storage applications.

Fabrication of Porous Reduced Graphene Oxide Encapsulated Cu(OH)2 Core-shell Structured Carbon Fiber-Based Electrodes for High-Performance Flexible Supercapacitors

To explore next-generation flexible supercapacitors, lightweight, superior conductivity, low cost, and excellent capacitance are the preconditions for practical use. However, subjected to unsatisfactory conductivity, limited surface areas, and poor porosity leading to long ion transport channels, carbon fiber (CF)-based flexible supercapacitors need to further boost the electrochemical properties. Hence, a porous reduced graphene oxide encapsulated Cu(OH)2 core-shell structured CF-based electrode was fabricated through a scalable approach. The inexpensive Cu(OH)2 nanoarrays were controllably grown in situ on a CF substrate, with residual Cu promoting conductivity. Porous graphene oxide (PrGO), which served as the shell, was realized by Ni nanoparticle etching, which not only provided more active sites for capacitance as well as shortened accessible pathways for the ion transport but also effectively alleviated the exfoliation of the internal active materials. Moreover, thanks to this distinctive core-shell architecture, the extra space between the outer PrGO layer and the internal ordered Cu(OH)2 nanoarrays provided increased space for capacitance storage. The assembled PrGO/Cu(OH)2/Cu@CF electrode exhibited an excellent areal capacitance, reaching up to 722 mF cm-2 at a current density of 0.5 mA cm-2, attributed to its superior structure and materials advantages. The resulting PrGO/Cu(OH)2/Cu@CF//AC//CF asymmetric flexible all-solid-state supercapacitor achieved a high energy density of 0.052 mWh cm-2 and exhibited long-term durability. This work proposes a low-cost and effective way to fabricate hierarchically structured electrodes for wearable CF-based supercapacitors.

Reduced graphene oxide (rGO) supported and p yrolytic carbon (PC) coated γ-Fe2O3/PC-rGO composite anode material with enhanced Li-storage performance

As a high-capacity anode material for lithium ion batteries, γ-Fe 2 O 3 is a promising alternative to conventional graphite among multifarious transition metal oxides owing to its high theoretical specific capacity (1007 mAh g -1 ), abundant reserves, good safety and low cost. However, improving the electrical conductivity and overcoming the morphological damage caused by the severe volume expansion during cycling are still the tricky problems to be solved. Herein, a three-dimensional heterostructure composite (γ-Fe 2 O 3 /PC-rGO 60 ) was prepared by a facile solvothermal reaction followed by heat treatment in inert atmosphere. This composite material exhibits a reversible charge specific capacity of 1035 mAh g -1 at the current density of 0.1 A g -1 . After 100 cycles at 0.2 A g -1 , the capacity is increased from 966.2 to 1091.1 mAhg -1 . Even cycled for 200 cycles at 1 A g -1 , the capacity is only decreased from 751.4 to 670.6 mAh g -1 , giving capacity retention of 89.3%. The rGO network supported flexible composite architecture is beneficial for accommodating the volume expansion of the γ-Fe 2 O 3 active material during the lithiation/delithiation process. Besides, the conductive rGO network and the in-situ formed pyrolytic carbon (PC) can provide a smooth electron transmission path and a favorable lithium ion transport channel.

An amorphous hierarchical MnO2/acetylene black composite with boosted rate performance as an anode for lithium-ion batteries

Amorphization is considered to be an effective way to enhance the electrochemical performances of electrode materials due to the existence of isotropy and numerous defects. Herein, an amorphous hierarchically structured MnO₂/acetylene black (a-MnO₂/AB) composite is successfully fabricated via a redox method and subsequent mechanical ball milling. The a-MnO₂/AB composite is composed of approximately 300 nm flower-like amorphous MnO₂ submicron spheres and acetylene black particles with a diameter of about 50 nm. The a-MnO₂/AB electrode exhibits an initial coulombic efficiency of 73.2%, excellent rate capabilities of 318 mA h g^-1 at 9.6 A g^-1, and high specific capacity retention of 1300 mA h g^-1 after 300 cycles at 1 A g^-1. The amorphous structure can provide more channels for rapid lithium-ion transmission due to the disorder and defects, and the ion-diffusion coefficient (∼5 × 10^-7 cm² s^-1) is higher than those of crystalline materials. Due to the strong interactions (Mn-O-C bonds) between MnO₂ and AB as a result of the ball milling, the composite shows low charge transport resistance and small volume changes during the discharging/charging process. This work provides a facile route for the construction of amorphous hierarchically structured Mn-based oxides as anodes for lithium-ion batteries (LIBs).

Morphology-Tunable Synthesis of Intrinsic Room-Temperature Ferromagnetic γ-Fe2O3 Nanoflakes

Intrinsic two-dimensional (2D) magnetic materials with room-temperature ferromagnetism and air stability are highly desirable for spintronic applications. However, the experimental observations of such 2D or ultrathin ferromagnetic materials are rarely reported owing to the scarcity of these materials in nature and for the intricacy in their synthesis. Here, we report a successful controllable growth of ultrathin γ-Fe₂O₃ nanoflakes with a variety of morphologies tunable by the growth temperature alone using a facile chemical vapor deposition method and demonstrate that all ultrathin nanoflakes still show intrinsic room-temperature ferromagnetism and a semiconducting nature. The γ-Fe₂O₃ nanoflakes epitaxially grown on α-Al₂O₃ substrates take a triangular shape at low temperature and develop gradually in lateral size, forming eventually a large-scale γ-Fe₂O₃ thin film as the growth time increases due to a thermodynamic control process. The morphology of the nanoflakes could be tuned from triangular to stellated, petaloid, and dendritic crystalloids in sequence with the rise of precursor temperature, revealing a growth process from thermodynamically to kinetically dominated control. Moreover, the petaloid and dendritic nanoflakes exhibit enhanced coercivity compared with the triangular and stellated nanoflakes, and all the nanoflakes with diverse shapes possess differing electrical conductivity. The findings of such ultrathin, air-stable, and room-temperature ferromagnetic γ-Fe₂O₃ nanoflakes with tunable shape and multifunctionality may offer guidance in synthesizing other non-layered magnetic materials for next-generation electronic and spintronic devices.

Retracting interphasial stored Li+ ions by transition metal/metal carbide nanoparticles for enhanced Li+ ion storage capacity

Herein, we report the synthesis of metal/metal carbide (Co, Ni, and Fe₃C) nanoparticles (NPs) encapsulated nitrogen-doped carbon nanotubes (NCNT) and its application as the anode materials for lithium-ion battery (LIB). The electron microscopy images confirm the encapsulation of metal NPs inside the carbon nanotubes, which can inhibit the NPs aggregations and offer long cycle life for LIB. The metal/metal carbide encapsulated NCNT as anode material exhibits higher specific capacity than pure NCNT. The cyclic voltammetry studies reveal that Co, Ni, and Fe₃C NPs can oxidize and reduce the solid electrolyte interphase (SEI) layer components of the anode. This offers the extra specific capacity to Fe₃C/NCNT, Co/NCNT, and Ni/NCNT anodes by retracting the interphasial stored Li⁺ ions. Moreover, in this study, the catalytic activity of Co, Ni, and Fe₃C NPs for tailoring the SEI components are compared for the first time, and it shows Fe₃C/NCNT anode has the highest catalytic activity than Co/NCNT and Ni/NCNT. Co/NCNT and Fe₃C/NCNT also exhibit good cycle life up to 1300 cycles at a current density of 1 A g^-1. Overall, this work demonstrates an effective strategy to improve the performance of LIB anode by retracting the interphasial stored Li⁺ ions.

Engineering zinc ferrite nanoparticles in a hierarchical graphene and carbon nanotube framework for improved lithium-ion storage

This work presents the successful fabrication of a composite made of multi-walled carbon nanotubes and reduced graphene oxide, with immobilized zinc ferrite nanoparticles (ZnFe₂O₄@CNT/RGO). Functionalized CNT (F-CNT) and few-layered graphene oxide (GO) not only works as a precursor for the hierarchical CNT/RGO skeleton, but also participates in the redox reactions with zinc and ferrous ions to synthesize the intermediate products ZnO@CNT and FeOOH@RGO, respectively. A ZnO@CNT/FeOOH@RGO composite is obtained by through the spontaneous assembly process between the above intermediate species, and the final ZnFe₂O₄@CNT/RGO composite is fabricated through a simple solid-state reaction. The ZnFe₂O₄@CNT/RGO composite delivers a reversible capacity of about 1250 mAh·g^-1 after 100 cycles at a low current of 200 mA·g^-1, about 1100 mAh·g^-1 after 300 cycles at a high current of 1000 mA·g^-1. It has been verified that an increase in battery performance can be attributed to the engineered hierarchical CNT/RGO supportive skeleton, the generation of smaller electrochemically active ZnO and Fe₂O₃ crystals, and pseudocapacitive behavior. The sample design and preparation method in this work are both economical and scalable, allowing further development and use in other applications.

Highly efficient Co3O4/CeO2 heterostructure as anode for lithium-ion batteries

Co₃O₄ has been extensively studied as an anode material for lithium-ion batteries (LIBs) because of its high theoretical capacity. However, during the charging-discharging processes, the issues of large volume change and low electric conductivity arise, which significantly limit the practical applications of Co₃O₄. To solve these issues, a Co₃O₄/CeO₂ heterostructure derived from metal-organic frameworks (MOFs) was designed and synthesized through one-step microwave synthesis. Benefiting from the mesoporous structure and presence of hetero-components, Co₃O₄/CeO₂ having the molar ratio of Co/Ce = 5:1 (denoted as 5Co₃O₄/CeO₂) exhibits high reversible capacity and excellent cycling stability when used as an anode material for LIBs. Specifically, compared to a single-phase Co₃O₄ anode, which shows a capacity of 538.6 mAh/g after 100 cycles, 5Co₃O₄/CeO₂ exhibits a higher capacity (1131.2 mAh/g at 100 mA/g). This study provides a novel strategy for using rare earth components to modify electrode materials.

Fluorine and nitrogen functionalized magnetic graphene as a novel adsorbent for extraction of perfluoroalkyl and polyfluoroalkyl substances from water and functional beverages followed by HPLC-Orbitrap HRMS determination

Most of the reported magnetic adsorbents are difficult to absorb multi-class of per- and polyfluoroalkyl substances (PFASs), especially the short-chain PFASs. In this work, a novel fluorine and nitrogen functionalized magnetic graphene (G-NH-FBC/Fe₂O₃) was first synthesized and characterized by scanning electron microscope (SEM), Fourier Transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The as-prepared G-NH-FBC/Fe₂O₃ was utilized as adsorbents for the magnetic solid-phase extraction (MSPE) of 19 PFASs from water and functional beverages and showed excellent adsorption capacity probably due to the hydrophobic interaction. Under the optimal pretreatment and instrumental conditions, a selective and sensitive high performance liquid chromatography Orbitrap high resolution mass spectrometry (HPLC-Orbitrap HRMS) method was developed for the determination of PFASs. Results indicated that the proposed method had favorable linearity (R² ≥ 0.994) within a wide range of concentrations. Limit of detection (LOD) and limit of quantification (LOQ) for the developed method ranged from 3 ng/L to 15 ng/L and 10 ng/L to 49 ng/L, respectively. Finally, the method was successfully applied to determine PFASs in drinking water, river water, tap water, factory drainage and functional beverages with recoveries ranging from 71.9% to 117.6% and relative standard deviation of <10%. The prepared G-NH-FBC/Fe₂O₃ was easy to recycle and could be reused for five times without significant decrease in extraction recoveries of PFASs. These results demonstrated that this novel magnetic G-NH-FBC/Fe₂O₃ could efficiently enrich PFASs and the proposed method is reliable and robust for the determination of PFASs in water and beverage samples.

Revisiting the Stability of the Cr4+/Cr3+ Redox Couple in Sodium Superionic Conductor Compounds

In this work, we revisited the stability of the Cr⁴⁺/Cr³⁺ redox couple in a sodium superionic conductor (NASICON)-type compound, Na₂TiCr(PO₄)₃. Experimental results showed that the Na₂TiCr(PO₄)₃ compound exhibited a specific capacity of 49.9 mA h g^-1 at 20 mA g^-1, about 80% of its theoretical capacity of 62.2 mA h g^-1 with one Na⁺ insertion/deinsertion per formula Na₂TiCr(PO₄)₃. The redox couple was found to be stable against cycling with some 90.3% capacity retention after 300 cycles within the voltage range between 2.5 and 4.7 V. With a wider voltage range between 2.5 and 5.0 V, the capacity retention was about 76.6% after 1000 cycles, indicating the redox couple is stable against overvoltage. In addition, the effect of Ti/Cr ratio on the reversibility of the redox couple was studied by varying x in Na_1+xTi_2-xCr_x(PO₄)₃ (where x = 0.6, 0.8, 1.0, 1.2, 1.4, 2.0). It was confirmed that x = 1 is optimal for balancing the electrode stability and the capacity. The obtained optimal content of Cr in the compound provides useful guidance for designing new Cr-based NASICON-type cathode materials. Furthermore, in situ X-ray diffraction (XRD) analysis of compound Na₂TiCr(PO₄)₃ indicated a two-phase sodium-ion storage mechanism, which is different from the previously reported one-phase mechanism. Rietveld refinement XRD analysis showed a small volume change of the compound during cycling (about 2.6%), indicating good structural stability.

Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zinc-air battery

As one type of bifunctional oxygen electrocatalyst for Zn-air battery, herein, FeNi alloy was successfully embedded into N-doped carbon with tailored architectures by integrating MOF precursor method and polymer coating/encapsulation strategy. The content of Fe in primary precursor has been proven to be able to obviously affect the morphology of the final catalyst. Benefiting from the mature active site (e.g. FeNi alloy) and the stable carbon matrix, a series of catalysts exhibited good performance towards ORR and OER. Of great significance, a particular ratio of Fe/Ni happened to be able to catalyze the growth of 1D bamboo-like carbon nanotubes, giving rise to a conductive network to diffuse ORR/OER-relevant species. Apparently, a low discharge-charge voltage gap (1.1 V) was acquired in a liquid Zn-air battery with 1.5FeNi@NCNT air cathode. Moreover, the solid-state Zn-air battery assembled on it also displayed a high open circuit voltage (1.38 V) and yielded a high power density of 81 mW cm^-2 at 0.83 V. This would leverage a choice to tailor carbon geometry of FeNi alloy-based active sites for ORR/OER and further serve for devices of practical significance.

Perforated Pd Nanosheets with Crystalline/Amorphous Heterostructures as a Highly Active Robust Catalyst toward Formic Acid Oxidation

Perforated ultrathin Pd nanosheets with crystalline/amorphous heterostructures are rationally synthesized to offer a large electrochemically active surface area of 172.6 m² g^-1 and deliver a 5.6 times higher apparent reaction rate in comparison to commercial Pd/C, thus offering a facile confined growth method to generate superior catalysts.

Yolk–shell structured metal oxide@carbon nanoring anode boosting performance of lithium-ion batteries

A high-performance anode of nanoring-like Fe₂O₃@carbon with a yolk–shell structure enables excellent capacity, rate capability, and cycle stability of lithium-ion batteries.