Stabilizing High Density Cu Active Sites with ZrO2 Quantum Dots as Chemical Ligand in N-doped Porous Carbon Nanofibers for Efficient ORR.

The emerging transition metal-nitrogen-carbon (MNC) materials are considered as a promising oxygen reduction reaction (ORR) catalyst system to substitute expensive Pt/C catalysts due to their high surface area and potential high catalytic activity. However, MNC catalysts are easy to be attacked by the ORR byproducts that easily lead to the deactivation of metal active sites. Moreover, a high metal loading affects the mass transfer and stability, but a low loading delivers inferior catalytic activity. Here, a new strategy of designing ZrO2 quantum dots and N-complex as dual chemical ligands in N-doped bubble-like porous carbon nanofibers (N-BPCNFs) to stabilize copper (Cu) by forming CuZrO3-x /ZrO2 heterostructures and CuN ligands with a high loading of 40.5 wt.% is reported. While the highly porous architecture design of N-BPCNFs builds a large solidelectrolytegas phase interface and promotes mass transfer. The preliminary results show that the half-wave potential of the catalyst reaches 0.856 V, and only decreases 0.026 V after 10 000 cycles, exhibiting excellent stability. The proposed strategy of stabilizing metal active sites with both heterostructures and CuN ligands is feasible and scalable for developing high metal loading ORR catalyst.

Synthesizing Superior Flexible Oxide Perovskite Ceramic Nanofibers by Precisely Controlling Crystal Nucleation and Growth.

Oxide perovskite ceramics are cornerstone materials of electronic devices, but they easily break under bending. Such brittle failure phenomenon limits their applications in emerging flexible electronics. Here, the authors propose a scalable approach of sol-gel electrospinning to synthesize flexible perovskite Li0.35 La0.55 TiO3 (LLTO) nanofibers (NFs) by reducing grain sizes and pore defects in the NFs. The strategy is to precisely control crystal nucleation and growth by forming homogeneous nuclei in the sol before electrospinning and to construct soft twin and amorphous grain boundaries by controlling calcination temperatures. Ball-milling the sol promotes the formation of numerous LLTO nuclei and thus effectively refines grains, while using gradient calcination temperatures from 200 to 900 °C creates intricate soft grain boundaries. The individual LLTO NF shows ceramic toughness with an elastic modulus of ≈40 GPa and the NF film demonstrates silk-like softness with a large elastic strain of 1.51%. Moreover, the LLTO film shows superior fatigue resistance and it maintains structure well after repeated tensile-buckling cycles at 40% strain. The proposed strategy facilitates to develop flexible oxide perovskite ceramic films with appealing applications.

Facile Synthesis of Bimetallic Fluoride Heterojunctions on Defect-Enriched Porous Carbon Nanofibers for Efficient ORR Catalysts.

The development of efficient and stable catalysts for the oxygen reduction reaction (ORR) at low cost is crucial for realizing the large-scale application of metal-air batteries. Herein, we report an efficient ORR catalyst of bimetallic copper and cobalt fluoride heterojunctions, which are uniformly dispersed in nitrogen-fluorine-oxygen triply doped porous carbon nanofibers (PCNFs) that contain hierarchical macro-meso-micro pores. The composite catalyst materials are fabricated with a facile and green method of electrospinning with water as the solvent. By using poly(tetrafluoroethylene) as the pore inducer to anchor electropositive copper and cobalt salts in the electrospun hybrid nanofibers, bimetallic fluoride heterojunctions can be directly formed in PCNFs after calcination. The hierachical porous structures provide an effective way to transport matter, while the bimetallic fluorides expose abundant electroactive sites, both of which result in stable ORR activities with a high half-wave potential of 0.84 V. The study proposes a feasible strategy for the fabrication of nonprecious catalysts.

Low-Temperature Synthesis of Mesoporous Half-Metallic High-Entropy Spinel Oxide Nanofibers for Photocatalytic CO2 Reduction.

High-entropy oxides (HEOs) exhibit great prospects owing to their varied composition, chemical adaptability, adjustable light-absorption ability, and strong stability. In this study, we report a strategy to synthesize a series of porous high-entropy spinel oxide (HESO) nanofibers (NFs) at a low temperature of 400 °C by a sol-gel electrospinning technique. The key lies in selecting six acetylacetonate salt precursors with similar coordination abilities, maintaining a high-entropy disordered state during the transformation from stable sols to gel NFs. The as-synthesized HESO NFs of (NiCuMnCoZnFe)3O4 show a high specific surface area of 66.48 m2/g, a diverse elemental composition, a dual bandgap, half-metallicity property, and abundant defects. The diverse elements provide various synergistic catalytic sites, and oxygen vacancies act as active sites for electron-hole separation, while the half-metallicity and dual-bandgap structure offer excellent light absorption ability, thus expanding its applicability to a wide range of photocatalytic processes. As a result, the HESO NFs can efficiently convert CO2 into CH4 and CO with high yields of 8.03 and 15.89 µmol g-1 h-1, respectively, without using photosensitizers or sacrificial agents.

Fabrication of Flexible Mesoporous Black Nb2 O5 Nanofiber Films for Visible-Light-Driven Photocatalytic CO2 Reduction into CH4.

Achieving high selectivity and conversion efficiency simultaneously is a challenge for visible-light-driven photocatalytic CO2 reduction into CH4 . Here, a facile nanofiber synthesis method and a new defect control strategy at room-temperature are reported for the fabrication of flexible mesoporous black Nb2 O5 nanofiber catalysts that contain abundant oxygen-vacancies and unsaturated Nb dual-sites, which are efficient towards photocatalytic production of CH4 . The oxygen-vacancy decreases the bandgap width of Nb2 O5 from 3.01-2.25 eV, which broadens the light-absorption range from ultraviolet to visible-light, and the dual sites in the mesopores can easily adsorb CO2 , so that the intermediate product of CO* can be spontaneously changed into *CHO. The formation of a highly stable NbCHO* intermediate at the dual sites is proposed to be the key feature determining selectivity. The preliminary results show that without using sacrificial agents and photosensitizers, the nanofiber catalyst achieves 64.8% selectivity for CH4 production with a high evolution rate of 19.5 µmol g-1 h-1 under visible-light. Furthermore, the flexible catalyst film can be directly used in devices, showing appealing and broadly commercial applications.

Dynamic Regulation of Lithium Dendrite Growth with Electromechanical Coupling Effect of Soft BaTiO3 Ceramic Nanofiber Films.

Lithium (Li) metal batteries (LMBs) offer great opportunity for developing high-energy density energy-storage-systems, but the anodes suffer a severe problem of dendrite growth that hinders the practical applications of LMBs. Here, we report a soft BaTiO3 ceramic nanofiber film with excellent ferroelectricity and piezoelectricity that enables one to transverse the dense deposition of Li metal. During Li plating, the strong ferroelectricity reduces the Li-ion concentration gradient near the anode and thus facilitating their uniform deposition. Once squeezed by these Li deposits, the BaTiO3 film generates instantaneous piezo-effect to dynamically change the subsequent Li deposition from vertical to lateral. As a result, Li-Cu cells exhibit reversible plating-stripping processes for over 200 cycles with a high Coulombic efficiency of >98.3%. When pairing with high-voltage LiNi0.8Co0.15Al0.05O2 cathodes, the LMBs can retain >80% capacity in 300 cycles without forming dendrites even under challenging conditions including a high cathode loading of 7.2 mg/cm2, a lean electrolyte amount of 7 µL/mg, and high current rates. The findings point to a promising electromechanical coupling strategy to dynamically adjust dendrite growth for designing Li metal anodes.

Hierarchical Porous Carbon Nanofibers with Tunable Geometries and Porous Structures Fabricated by a Scalable Electrospinning Technique.

Porous carbon nanofibers (PCNFs) have rich channels for transporting ions, molecules, and nanoparticles, but the control over their porous structures is a challenge. Here, we report a scalable electrospinning technique by using poly(tetrafluoroethylene) as a pore template, boric acid as a cross-linking agent, and polyvinyl alcohol and polyurethane as dual carbon precursors to fabricate flexible PCNFs with tunable geometries and macro/meso/microporous structures. In the water solvent, the negatively charged template cross-links with the positively charged carbon precursors to form a stable sol for electrospinning. By varying the mass ratios of these precursors, the electrospun hybrid nanofibers are directly transformed into B-F-N-O doped PCNFs with tunable macro-, meso-, and micropores after carbonization. The porosity of an individual PCNF is as high as ∼85%, and the pore volume can be tuned from 0.23 to 0.58 cm3·g-1. When constructing high-sulfur-content (86 wt %) electrodes with the freestanding PCNF films, the porous structures with rich electroactive sites provide rapid pathways for poly-anions and have strong chemisorption of poly-sulfides, leading to a great electrochemical performance. The reported strategy offers a new perspective for synthesizing hierarchical PCNFs with appealing applications.

Direct Magnetic Reinforcement of Electrocatalytic ORR/OER with Electromagnetic Induction of Magnetic Catalysts.

Designing stable and efficient electrocatalysts for both oxygen reduction and evolution reactions (ORR/OER) at low-cost is challenging. Here, a carbon-based bifunctional catalyst of magnetic catalytic nanocages that can direct enhance the oxygen catalytic activity by simply applying a moderate (350 mT) magnetic field is reported. The catalysts, with high porosity of 90% and conductivity of 905 S m-1 , are created by in situ doping metallic cobalt nanodots (≈10 nm) into macroporous carbon nanofibers with a facile electrospinning method. An external magnetic field makes the cobalt magnetized into nanomagnets with high spin polarization, which promote the adsorption of oxygen-intermediates and electron transfer, significantly improving the catalytic efficiency. Impressively, the half wave-potential is increased by 20 mV for ORR, and the overpotential at 10 mA cm-2 is decreased by 15 mV for OER. Compared with the commercial Pt/C+IrO2 catalysts, the magnetic catalyzed Zn-air batteries deliver 2.5-fold of capacities and exhibit much longer durability over 155 h. The findings point out a very promising strategy of using electromagnetic induction to boost oxygen catalytic activity.

Superior Flexibility in Oxide Ceramic Crystal Nanofibers.

Oxide crystal ceramics are commonly hard and brittle, when they are bent they typically fracture. Such mechanical response limits the use of these materials in emerging fields like wearable electronics. Here, a polymer-induced assembly strategy is reported to construct orderly assembled TiO2 crystals into continuous nanofibers that are stretchable, bendable, and even knottable. Ball-milling the spinning sol and curved-drafting the electrospun nanofibers significantly improve the molecular structural order and reduce pore defects in the precursor nanofibers. Using this method, continuous TiO2 nanofibers, in which orderly assembled TiO2 nanocrystals (brick) are connected by twin grain boundaries or an amorphous region (mortar), are formed after sintering. Mechanical measurements and finite element analysis simulation indicate that the dislocation slip of "bricks" and the elastic deformation of "mortar" render the nanofibers with a small bending rigidity of ≈22 mN and a small elastic modulus of ≈20.8 Gpa, thus displaying properties associated with both soft and hard matter. More importantly, the reported approach can be easily extended to synthesize a wide range of soft, yet tough ceramic membranes, such as ZrO2 and SiO2 .

Electroless Deposition of Automatically Shedded Thin Copper Foils.

Electroless deposition (ELD) is a process widely used for the production of thin metal films, but stripping the films from the substrate remains challenging. Here, we report a low-cost ELD method for the large-scale production of freestanding copper (Cu) foils in a short time of 25-55 min. By atomizing a thin (<100 nm) sacrificial layer of chitosan with weak glycosyl bonds and a high degree of deacetylation on the glass substrate, the chitosan is completely decomposed in the process of Cu-deposition, producing automatically shedded Cu foils with varied thicknesses from 746 nm to 8.33 µm and high elastic modulus. When used as battery current collectors, the thin Cu foils with enhanced adhesive fastness and contact areas greatly enhance the capacity and rate capability of graphite anodes. Compared with the commercial Cu current collectors, both the battery capacity and energy density are increased by 429.6 and 484.1%, respectively. The reported approach can be extended for fabricating other metal foils such as nickel with properties appealing for applications.

Polymer Template Synthesis of Flexible SiO2 Nanofibers to Upgrade Composite Electrolytes.

Flexible oxide ceramic films offer prospects for revolutionizing diverse fields such as energy and electronics, but their fabrication methods are typically elaborate and cannot be expanded. Here, we report a scalable strategy to fabricate flexible and robust SiO2 nanofiber films with controllable morphology using a sol-gel electrospinning method followed by low-temperature calcination. When applied to composite polymer electrolytes (CPEs) for solid-state batteries by filling polyethylene oxide into porous ceramic films, SiO2 nanofibers with large surface areas (51 m2·g-1) demonstrate strong Lewis interfacial interactions and isotropic ionic transfer channels that mitigate polymer crystallinity and Li+-concentration polarization, imparting high conductivity (1.3 × 10-4 S·cm-1 at 30 °C) and structural stability to the electrolytes. As a result, all-solid-state LiFePO4||Li shows great rate capability and long cycling stability with high discharge capacities of 159 and 132 mA·h·g-1 at 0.5C under 60 and 45 °C, respectively, demonstrating broad commercial prospects for the scale production of efficient solid electrolytes.

Transformation of oxide ceramic textiles from insulation to conduction at room temperature.

Oxide ceramics are considered to be nonconductive brittle materials, which limits their applications in emerging fields such as conductive textiles. Here, we show a facile domino-cascade reduction method that enables rapid transformation of ceramic nanofiber textiles from insulation to conduction at room temperature. After putting dimethylacetamide-wetted textiles, including TiO2, SnO2, BaTiO3, and Li0.33La0.56TiO3, on lithium plates, the self-driven chemical reactions induce defects in oxides. These defects initiate an interfacial insulation-to-conductive phase transition, which triggers the domino-cascade reduction from the interface to the whole textile. Correspondingly, the conductivity of the textile sharply increased from 0 to 40 S/m over a period of 1 min. The modified oxide textiles exhibit enhanced electrochemical performance when substituting the metallic current collectors of lithium batteries. This room temperature reduction method can protect the nanostructures while inducing defects in oxide ceramic textiles, appealing for numerous applications.

Polymer Template Synthesis of Soft, Light, and Robust Oxide Ceramic Films.

Oxide ceramic materials underpin a wide variety of technologies. However, the inherent fragility of these materials limits their use in emerging fields like wearable electronics and soft energy storage devices. Here, we develop a sol-gel electrospinning technique followed by calcination to create a range of oxide ceramic nanofiber films that exhibit significant softness without fragility after various deformations. This approach causes the ceramic crystals to fuse together at a low temperature during their growth within the polymer nanofiber templates. All the synthesized ceramic films, from SiO2 to BaTiO3, Li0.33La0.56TiO3, and Li7La3Zr2O12, have silk-like softness of <31 mN, low density of <0.36 g/cm3 and robust fire resistance to 1,000°C. Fabricated separators based on these films display large electrolyte uptakes of >900% and high thermal insulation performance, enhancing the rate capability and safety of lithium batteries. The reported method allows scalable synthesis of soft oxide ceramic films with properties appealing for applications.