Libo Gao

Flexible Fiber-Shaped Supercapacitor Based on Nickel-Cobalt Double Hydroxide and Pen Ink Electrodes on Metallized Carbon Fiber

Flexible fiber-shaped supercapacitors (FSSCs) are recently of extensive interest for portable and wearable electronic gadgets. Yet the lack of industrial-scale flexible fibers with high conductivity and capacitance and low cost greatly limits its practical engineering applications. To this end, we here present pristine twisted carbon fibers (CFs) coated with a thin metallic layer via electroless deposition route, which exhibits exceptional conductivity with ∼300% enhancement and superior mechanical strength (∼1.8 GPa). Subsequently, the commercially available conductive pen ink modified high conductive composite fibers, on which uniformly covered ultrathin nickel-cobalt double hydroxides (Ni-Co DHs) were introduced to fabricate flexible FSSCs. The synthesized functionalized hierarchical flexible fibers exhibit high specific capacitance up to 1.39 F·cm-2 in KOH aqueous electrolyte. The asymmetric solid-state FSSCs show maximum specific capacitance of 28.67 mF·cm-2 and energy density of 9.57 μWh·cm-2 at corresponding power density as high as 492.17 μW·cm-2 in PVA/KOH gel electrolyte, with demonstrated high flexibility during stretching, demonstrating their potential in flexible electronic devices and wearable energy systems.

Rationally designed nickel oxide ravines@iron cobalt-hydroxides with largely enhanced capacitive performance for asymmetric supercapacitors

Prominent energy and power densities play crucial roles in supercapacitor devices because of their potential practical application in various electronic devices. Herein, an asymmetric supercapacitor (ASC) with high capacitive performance was manufactured by combining rationally designed NiO@FeCo-layered double hydroxide (LDH), which has enhanced areal capacitance and rate capability, with commercially available pen ink composites as the positive and negative electrodes, respectively. Strikingly, the FeCo-LDHs with ultra-stable rate capability (retaining 94% from 4 to 25 mA cm−2) reported for the first time in this study can be employed to modify other transition metal oxides/hydroxides to achieve balanced performance. The constructed ASC surprisingly delivers an ultrahigh energy density of 64.1 W h kg−1 and a power density of 15 kW kg−1 as well as a robust cyclability (90% capacitance retention after 3000 cycles). In addition, the ASC is capable of readily driving patterned commercial light-emitting diodes (LEDs), motor propellers, and even a toy car, demonstrating its application potential in future nano-energy storage devices.

Graphene-Bridged Multifunctional Flexible Fiber Supercapacitor with High Energy Density

Portable fiber supercapacitors with high-energy storage capacity are in great demand to cater for the rapid development of flexible and deformable electronic devices. Hence, we employed a 3D cellular copper foam (CF) combined with the graphene sheets (GSs) as the support matrix to bridge the active material with nickel fiber (NF) current collector, significantly increasing surface area and decreasing the interface resistance. In comparison to the active material directly growing onto the NF in the absence of CF and GSs, our rationally designed architecture achieved a joint improvement in both capacity (0.217 mAh cm-2/1729.413 mF cm-2, 1200% enhancement) and rate capability (87.1% from 1 to 20 mA cm-2, 286% improvement), which has never been achieved before with other fiber supercapacitors. The in situ scanning electron microscope (SEM) microcompression test demonstrated its superior mechanical recoverability for the first time. Importantly, the assembled flexible and wearable device presented a superior energy density of 109.6 μWh cm-2 at a power density of 749.5 μW cm-2, and the device successfully coupled with a flexible strain sensor, solar cell, and nanogenerator. This rational design should shed light on the manufacturing of 3D cellular architectures as microcurrent collectors to realize high energy density for fiber-based energy storage devices.

In Situ Mechanical Characterization of Structural Bamboo Materials under Flexural Bending

Experimental mechanical characterization of structural biomaterials, coupled with advanced in situ microscopic imaging, is critical for understanding their deformation and failure mechanisms in engineering applications. Our earlier study suggested that bamboo materials, widely used as scaffolding in construction, exhibit superior and asymmetric bending flexural behavior, while their corresponding mechanisms for crack growth under bending are not fully understood due to the complicated hierarchical structure. Here, we developed in situ characterization techniques assisted with high-resolution macro telescope to directly observe the flexural responses of bamboo strips under different loading configurations. Our in situ results show that the hierarchical microstructure of bamboo plays a critical role in alternating the crack propagation behaviors as well as failure mechanisms. In addition, a finite element analysis (FEA) model mimicking bamboo’s functional graded (FG) structure has been developed to quantitatively investigate the origins of bamboo’s asymmetric characteristics, with a numerical model proposed for crack propagation. Our technique could offer microscopical insights in the flexural failures of structural bamboo materials under bending, which may be of help on the design of advanced FG cellular composites.

Mechanical Enhancement of Core-Shell Microlattices through High-Entropy Alloy Coating

Mechanical metamaterials such as microlattices are an emerging kind of new materials that utilize the combination of structural enhancement effect by geometrical modification and the intrinsic properties of its material constituents. Prior studies have reported the mechanical properties of ceramic or metal-coated composite lattices. However, the scalable synthesis and characterization of high-entropy alloy (HEA) as thin film coating for such cellular materials have not been studied previously. In this work, stereolithography was combined with Radio Frequency (RF) magnetron sputtering to conformally deposit a thin layer (~800 nm) of CrMnFeCoNi HEA film onto a polymer template to produce HEA-coated three-dimensional (3D) core-shell microlattice structures for the first time. The presented polymer/HEA hybrid microlattice exhibits high specific compressive strength (~0.018 MPa kg−1 m3) at a density well below 1000 kg m−3, significantly enhanced stiffness (>5 times), and superior elastic recoverability compared to its polymer counterpart due to its composite nature. The findings imply that this highly scalable and effective route to synthesizing HEA-coated microlattices have the potential to produce novel metamaterials with desirable properties to cater specialized engineering applications.

Size-dependent fracture behavior of silver nanowires

Silver (Ag) nanowires have great potential to be used in the flexible electronics industry for their applications in flexible, transparent conductors due to high conductivity and light reflectivity. Those applications always involve mechanical loading and deformations, which requires an in-depth understanding of their mechanical behavior and performance under loadings. However, current understanding on the mechanical properties of Ag nanowires is limited, especially on their size-dependent fracture behavior. In this work, mechanical properties of Ag nanowires with diameters ranging from 50 to 300 nm were systematically studied by in situ TEM tensile testing for the first time. The size effect was clearly found, with the increasing of the diameter of Ag nanowires, the ultimate tensile stress decreased. More importantly, the fracture behavior of Ag nanowire was studied and a brittle-to-ductile transition in fracture behavior was observed at the diameters around 100 nm which could be attributed to the dislocation activities within the geometry confinement. This work could give insights for understanding nanosized Ag wires and the design of Ag nanowire-based flexible devices and touchable panels.

Mechanically stable ternary heterogeneous electrodes for energy storage and conversion

Recently, solid asymmetric supercapacitor (ASC) has been deemed as an emerging portable power storage or backup device for harvesting natural resources. Here we rationally engineered a hierarchical, mechanically stable heterostructured FeCo@NiCo layered double hydroxide (LDH) with superior capacitive performance by a simple two-step electrodeposition route for energy storage and conversion. In situ scanning electron microscope (SEM) nanoindentation and electrochemical tests demonstrated the mechanical robustness and good conductivity of FeCo-LDH. This serves as a reliable backbone for supporting the NiCo-LDH nanosheets. When employed as the positive electrode in the solid ASC, the assembly presents high energy density of 36.6 W h kg-1 at a corresponding power density of 783 W kg-1 and durable cycling stability (87.3% after 5000 cycles) as well as robust mechanical stability without obvious capacitance fading when subjected to bending deformation. To demonstrate its promising capability for practical energy storage applications, the ASC has been employed as a portable energy source to power a commercially available digital watch, mini motor car, or household lamp bulb as well as an energy storage reservoir, coupled with a wind energy harvester to power patterned light-emitting diodes (LEDs).

Mechanical properties of nanostructured CoCrFeNiMn high-entropy alloy (HEA) coating

An equiatomic CoCrFeMnNi high-entropy alloy (HEA) thin film coating has been successfully developed by high-vacuum Radio Frequency (RF) magnetron sputtering. The deposition of a smooth and homogenous thin film with uniformly distributed equiaxed nanograins (grain size ~ 10 nm) was achieved through this technique. The thin film coating exhibits a high hardness of 6.8 ± 0.6 GPa, which is superior compared to its bulk counterpart owing to its nanocrystalline structure. Furthermore, it also shows good ductility through nanoindentation, which demonstrates its potential to serve as an alternative to traditional transition metal nitride or carbide coatings for applications in micro-fabrication and advanced coating technologies.