Electrolyte-Dependent Sodium Ion Transport Behaviors in Hard Carbon Anode.

A comprehensive study is conducted on hard carbon (HC) series samples by tuning the graphitic local microstructures systematically as an anode for SIBs in both carbonate- (CBE) and glyme-based electrolytes (GBE). The results reveal more detailed charge storage characters of HCs on the LVP section. 1) The LVP capacity is closely related to the prismatic surface area to the basal plane as well as the bulk density, regardless of electrolyte systems. 2) The glyme-sodium ion complex can facilitate sodium ion delivery into the internal closed pores of the HCs along with not well-ordered graphitic structures. 3) The glyme-mediated sodium ion-storage behavior causes significant decreases in both surface film resistance and charge transfer resistance, leading to enhanced rate capability. 4) The LVP originates from the formation of pseudo-metallic sodium nanoclusters, which are the same in a CBE and GBE. These results provide insight into the sodium ion-storage behaviors of HCs, particularly on the interrelationship between graphitic local microstructures and electrolyte systems. In addition, a high-performance HC anode with a plateau capacity of ≈300 mA h g-1 is designed based on the information, and its workability is demonstrated in a full-cell SIB device.

Anode-Free Sodium Metal Batteries Based on Nanohybrid Core-Shell Templates.

Anode-free sodium metal batteries (AF-SMBs) can deliver high energy and enormous power, but their cycle lives are still insufficient for them to be practical as a power source in modern electronic devices and/or grid systems. In this study, a nanohybrid template based on high aspect-ratio silver nanofibers and nitrogen-rich carbon thin layers as a core-shell structure is designed to improve the Coulombic efficiency (CE) and cycling performance of AF-SMBs. The catalytic nanohybrid templates dramatically reduce the voltage overshooting caused by metal nucleation to one-fifth that of a bare Al foil electrode (≈6 mV vs ≈30 mV), and high average CE values of >99% are achieved over a wide range of current rates from 0.2 to 8 mA cm-2 . Moreover, exceptionally long cycle lives for more than 1600 cycles and an additional 1500 cycles are achieved with a highly stable CE of >99.9%. These results show that AF-SMBs are feasible with the nanohybrid electrode system.

Pyrolytic Carbon Nanosheets for Ultrafast and Ultrastable Sodium-Ion Storage.

Na-ion cointercalation in the graphite host structure in a glyme-based electrolyte represents a new possibility for using carbon-based materials (CMs) as anodes for Na-ion storage. However, local microstructures and nanoscale morphological features in CMs affect their electrochemical performances; they require intensive studies to achieve high levels of Na-ion storage performances. Here, pyrolytic carbon nanosheets (PCNs) composed of multitudinous graphitic nanocrystals are prepared from renewable bioresources by heating. In particular, PCN-2800 prepared by heating at 2800 °C has a distinctive sp2 carbon bonding nature, crystalline domain size of ≈44.2 Å, and high electrical conductivity of ≈320 S cm-1 , presenting significantly high rate capability at 600 C (60 A g-1 ) and stable cycling behaviors over 40 000 cycles as an anode for Na-ion storage. The results of this study show the unusual graphitization behaviors of a char-type carbon precursor and exceptionally high rate and cycling performances of the resulting graphitic material, PCN-2800, even surpassing those of supercapacitors.

Hybrid Nano Flake-like Vanadium Diselenide Combined on Multi-Walled Carbon Nanotube as a Binder-Free Electrode for Sodium-Ion Batteries.

As the market for electric vehicles and portable electronic devices continues to grow rapidly, sodium-ion batteries (SIBs) have emerged as energy storage systems to replace lithium-ion batteries (LIBs). However, sodium-ion is heavier and larger than lithium-ion, resulting in volume expansion and slower ion transfer. It is necessary to find suitable anode materials with high capacity and stability. In addition, wearable electronics are starting to be commercialized, requiring a binder-free electrode used in flexible batteries. In this work, we synthesized nano flake-like VSe2 using organic precursor and combined it with MWCNT as carbonaceous material. VSe2@MWCNT was mixed homogenously using sonication and fabricated film electrodes without a binder and substrate via vacuum filter. The hybrid electrode exhibited high-rate capability and stable cycling performance with a discharge capacity of 469.1 mAhg-1 after 200 cycles. Furthermore, VSe2@MWCNT exhibited coulombic efficiency of ~99.7%, indicating good cycle stability. Additionally, VSe2@MWCNT showed a predominant 85.5% of capacitive contribution at a scan rate of 1 mVs-1 in sodiation/desodiation process. These results showed that VSe2@MWCNT is a suitable anode material for flexible SIBs.

Chemical and physical reinforcement of hydrophilic gelatin film with di-aldehyde nanocellulose.

Gelatin is a representative hydrophilic protein material with remarkable biocompatibility and biodegradability. From the aspect of materials processing, gelatin also has the advantage that its entire fabrication process can be performed in an aqueous solution. However, practical application of various gelatin materials-in particular gelatin films-has thus far been limited because of their weak mechanical properties and vulnerability under aqueous environments. To overcome these disadvantages, both physical reinforcement approaches and chemical cross-linking agents have been tested. However, little research has been done to make these two roles work at the same time. In this study, cellulose nanocrystals containing aldehyde groups were prepared via a periodate oxidation process and used for cross-linkable reinforcement of gelatin-based bio-composite films. The results revealed that the di-aldehyde cellulose nanocrystal (D-CNC) could react and covalently cross-link with the amine group of the gelatin molecules via Schiff base formation and compared with neat CNC. The gelatin bio-composite film reinforced with the prepared D-CNC exhibited excellent tensile properties and water resistance, and its mechanical and hydrophilic properties could be easily controlled by adjusting the D-CNC content and was greater than addition of same amount in CNC. Therefore, D-CNC will facilitate the widespread use of existing water-soluble polymers, especially natural hydrophilic proteins and can be used in conventional application fields such as the food, pharmaceutical, and biomedical industries.

High-toughness natural polymer nonwoven preforms inspired by silkworm cocoon structure.

As the interest in environmentally friendly materials and concerns regarding depletion of petroleum resources has increased, the research on natural polymers is being actively pursued. Among the various materials based on natural polymeric resources, the interest in using natural fibers in bio-composites has grown due to their lightweight, non-toxicity, low cost, and abundance. However, the lack of interfacial adhesion between filaments and poor water resistance make the use of natural fiber-based polymer composites less attractive. To overcome these drawbacks, formaldehyde-based synthetic binders have been used. However, this requires an additional synthesis of the binder, and potential toxicity problems exist. In this work, robust and rigid natural polymer nonwoven preforms were prepared by mixing jute fibers with silk sericin (SS). SS was employed as a natural facile binder and the strong binding between jute fibers and SS resulted in remarkable enhancements in tensile strength, elongation, and toughness, which increased up to 539.1, 385.7, and 1943.8%, respectively, compared with the pristine jute nonwoven. In addition, the dense and rigid structure obtained through SS coating ensured the structural stability of the nonwoven preforms in moisture environments. Silkworm cocoon-structured natural polymer nonwoven preforms with excellent mechanical strength and higher physical stability may have more potential utilization in the composite material fields.

Intensification of Pseudocapacitance by Nanopore Engineering on Waste-Bamboo-Derived Carbon as a Positive Electrode for Lithium-Ion Batteries.

Nanoporous carbon, including redox-active functional groups, can be a promising active electrode material (AEM) as a positive electrode for lithium-ion batteries owing to its high electrochemical performance originating from the host-free surface-driven charge storage process. This study examined the effects of the nanopore size on the pseudocapacitance of the nanoporous carbon materials using nanopore-engineered carbon-based AEMs (NE-C-AEMs). The pseudocapacitance of NE-C-AEMs was intensified, when the pore diameter was ≥2 nm in a voltage range of 1.0~4.8 V vs Li+/Li under the conventional carbonate-based electrolyte system, showing a high specific capacity of ~485 mA·h·g-1. In addition, the NE-C-AEMs exhibited high rate capabilities at current ranges from 0.2 to 4.0 A·g-1 as well as stable cycling behavior for more than 300 cycles. The high electrochemical performance of NE-C-AEMs was demonstrated by full-cell tests with a graphite nanosheet anode, where a high specific energy and power of ~345 Wh·kg-1 and ~6100 W·Kg-1, respectively, were achieved.

Catalytic Pyroprotein Seed Layers for Sodium Metal Anodes.

We report a pyroprotein seed layer (PSL, ∼100 nm in thickness)-coated Cu foil electrode (PSL-Cu) demonstrating highly reversible Na metal storage behavior with a mean Coulombic efficiency (CE) of ∼99.96% over 300 cycles in a glyme-based electrolyte. Via a synergistic effect with the electrolyte, the carbonaceous thin film containing numerous nucleophilic active sites guides the homogeneous Na metal deposition/stripping process with the formation of numerous catalytic seeds, resulting in remarkably stable cycling and a low Na metal nucleation overpotential of ∼10 mV. In addition, the CE deviation values of the PSL-Cu electrode were ∼0.43% in several cell tests, demonstrating its reliable cycling behavior with low cell-to-cell variation. The practicality of PSL-Cu was further demonstrated via full-cell experiments with a polyanion cathode, in which it achieved a high specific power density and energy density of 3,800 W kg-1 and ∼402 W h kg-1, respectively. This work provides a simple process for the fabrication of a Na metal anode.

Magnesiophilic Graphitic Carbon Nanosubstrate for Highly Efficient and Fast-Rechargeable Mg Metal Batteries.

The high volumetric energy density of rechargeable Mg batteries (RMBs) gives them a competitive advantage over current Li ion batteries, which originates from the high volumetric capacity (∼3833 mA h cm-3) of bivalent Mg metal anodes (MMAs). On the other hand, despite their importance, there are few reports on research strategies to improve the electrochemical performance of MMAs. This paper reports that catalytic carbon nanosubstrates rather than metal-based substrates, such as Mo, Cu, and stainless steel, are essential in MMAs to improve the electrochemical performance of RMBs. In particular, three-dimensional macroporous graphitic carbon nanosubstrates (GC-NSs) with high electrical conductivities can accommodate Mg metal with significantly higher rate capabilities and Coulombic efficiencies than metal substrates, resulting in a more stable and longer-term cycling performance over 1000 cycles. In addition, while metal-based substrates suffered from undesirable Mg peeling-off, homogeneous Mg metal deposition is well-guided in GC-NSs owing to the better affinity of the Mg2+ ion. These results are supported by density functional theory calculations and ex-situ characterization.

Sericin-derived activated carbon-loaded alginate bead: An effective and recyclable natural polymer-based adsorbent for methylene blue removal.

Activated carbon has been widely used as an effective adsorbent for removing contaminants from the water stream. Preparation of activated carbon using agricultural by-products is environmentally friendly and can greatly contribute to the virtuous cycle of natural polymers. In this study, highly porous activated carbon was prepared using silk sericin, a secondary protein of the sericulture industry. For easy processability and regeneration stability, the bead-type adsorbent was prepared using alginate (Alg) as a matrix. After that, methylene blue (MB) removal behavior of sericin-derived activated carbon (S-AC)/Alg beads was investigated. S-AC obtained by NaOH chemical activation had a larger BET surface area (2150.1 m2/g), and this porous structure of S-AC was well maintained after S-AC/Alg bead preparation (1215.4 m2/g). S-AC/Alg beads had an excellent MB adsorption capacity (502.5 mg/g) with stable regeneration stability, and 90.1% of the original removal efficiency was maintained after 5 cycles of the repeated adsorption-desorption process. These findings reveal that S-AC/Alg composite beads can be used as low-cost adsorbents for the removal of contaminants in aqueous solutions.

Nanocomposite Films of Poly(vinyl alcohol)-Grafted Graphene Oxide/Poly(vinyl alcohol) for Gas Barrier Film Applications.

Poly(vinyl alcohol) (PVA) composites containing graphene oxide (GO) functionalized with PVA were synthesized via the esterification of the carboxylic groups of GO. The presence of PVA-grafted GO (PVA-g-GO) in the PVA matrix induced strong interactions between the chains of the PVA matrix and allowed the PVA-g-GO to be uniformly dispersed throughout the matrix. The grafting of PVA to GO increased the gas barrier properties of the GO/PVA composites because of the increased compatibility between GO and PVA. The PVA-g-GO/PVA composites were used to coat the surface of poly(ethylene terephthalate) films. These coated films exhibited excellent gas barrier properties; the film containing 0.3 wt% of PVA-g-GO had an oxygen transmission rate (OTR) of 0.025 cc/(m2 · day) and an optical transmittance of 83.8%. As a result, PVA-g-GO/PVA composites that exhibited enhanced gas barrier properties were prepared with a solution mixing method.

Hierarchically porous carbon nanosheets from waste coffee grounds for supercapacitors.

The nanostructure design of porous carbon-based electrode materials is key to improving the electrochemical performance of supercapacitors. In this study, hierarchically porous carbon nanosheets (HP-CNSs) were fabricated using waste coffee grounds by in situ carbonization and activation processes using KOH. Despite the simple synthesis process, the HP-CNSs had a high aspect ratio nanostructure (∼20 nm thickness to several micrometers in lateral size), a high specific surface area of 1945.7 m(2) g(-1), numerous heteroatoms, and good electrical transport properties, as well as hierarchically porous characteristics (0.5-10 nm in size). HP-CNS-based supercapacitors showed a specific energy of 35.4 Wh kg(-1) at 11250 W kg(-1) and of 23 Wh kg(-1) for a 3 s charge/discharge current rate corresponding to a specific power of 30000 W kg(-1). Additionally, the HP-CNS supercapacitors demonstrated good cyclic performance over 5000 cycles.

Hierarchical porous carbon/MnO2 hybrids as supercapacitor electrodes.

Hybrid electrodes of hierarchical porous carbon (HPC) and manganese oxide (MnO2) were synthesized using a fast surface redox reaction of potassium permanganate under facile immersion methods. The HPC/MnO2 hybrids had a number of micropores and macropores and the MnO2 nanoparticles acted as a pseudocapacitive material. The synergistic effects of electric double-layer capacitor (EDLC)-induced capacitance and pseudocapacitance brought about a better electrochemical performance of the HPC/MnO2 hybrid electrodes compared to that obtained with a single component. The hybrids showed a specific capacitance of 228 F g(-1) and good cycle stability over 1000 cycles.