Kunli Goh

Scalable synthesis of hierarchically structured carbon nanotube-graphene fibres for capacitive energy storage

Micro-supercapacitors are promising energy storage devices that can complement or even replace batteries in miniaturized portable electronics and microelectromechanical systems. Their main limitation, however, is the low volumetric energy density when compared with batteries. Here, we describe a hierarchically structured carbon microfibre made of an interconnected network of aligned single-walled carbon nanotubes with interposed nitrogen-doped reduced graphene oxide sheets. The nanomaterials form mesoporous structures of large specific surface area (396 m2 g−1) and high electrical conductivity (102 S cm−1). We develop a scalable method to continuously produce the fibres using a silica capillary column functioning as a hydrothermal microreactor. The resultant fibres show a specific volumetric capacity as high as 305 F cm−3 in sulphuric acid (measured at 73.5 mA cm−3 in a three-electrode cell) or 300 F cm−3 in polyvinyl alcohol (PVA)/H3PO4 electrolyte (measured at 26.7 mA cm−3 in a two-electrode cell). A full micro-supercapacitor with PVA/H3PO4 gel electrolyte, free from binder, current collector and separator, has a volumetric energy density of ∼6.3 mWh cm−3 (a value comparable to that of 4 V–500 µAh thin-film lithium batteries) while maintaining a power density more than two orders of magnitude higher than that of batteries, as well as a long cycle life. To demonstrate that our fibre-based, all-solid-state micro-supercapacitors can be easily integrated into miniaturized flexible devices, we use them to power an ultraviolet photodetector and a light-emitting diode. Hierarchical hybrid carbon fibres consisting of a network of nitrogen-doped reduced graphene oxide and single-walled carbon nanotubes are synthesized and subsequently used to make a supercapacitor with high volumetric energy density.

Controlled functionalization of carbonaceous fibers for asymmetric solid-state micro-supercapacitors with high volumetric energy density.

A 1.8 V asymmetric solid-state flexible micro-supercapacitor is designed with one MnO2 -coated reduced graphene oxide/single-walled carbon nanotube (rGO/SWCNT) composite fiber as positive electrode and one nitrogen-doped rGO/SWCNT fiber as negative electrode, which demonstrates ultrahigh volumetric energy density, comparable to some thin-film lithium batteries, along with high power density, long cycle life, and good flexibility.

Transforming Pristine Carbon Fiber Tows into High Performance Solid‐State Fiber Supercapacitors

A facile activation strategy can transform pristine carbon fiber tows into high-performance fiber electrodes with a specific capacitance of 14.2 F cm(-3) . The knottable fiber supercapacitor shows an energy density of 0.35 mW h cm(-3) , an ultrahigh power density of 3000 mW cm(-3) , and a remarkable capacitance retention of 68%, when the scan rate increases from 10 to 1000 mV s(-1) .

Space-confined assembly of all-carbon hybrid fibers for capacitive energy storage: realizing a built-to-order concept for micro-supercapacitors

Miniaturized portable and wearable electronics have diverse power requirements, ranging from one microwatt to several milliwatts. Fiber-based micro-supercapacitors are promising energy storage devices that can address these manifold power requirements. Here, we demonstrate a hydrothermal assembly method using space confinement fillers to control the formation of nitrogen doped reduced graphene oxide and multi-walled carbon nanotube hybrid fibers. Consequently, the all-carbon hybrid fibers have tunable geometries, while maintaining good electrical conductivity, high ion-accessible surface area and mechanical strength; this allows us to address two important issues in micro-supercapacitor research. First, we found a clear correlation between the geometry of the hybrid fibers and their capacitive energy storage properties. Thinner fibers (30 μm in diameter) have higher specific volumetric capacitance (281 F cm−3), superior rate capability, and better length dependent performance. In contrast, larger-diameter hybrid fibers (236 μm in diameter) can achieve much higher specific length capacitance (42 mF cm−1). Second, we realized the first built-to-order concept for micro-supercapacitors by using all-carbon hybrid fibers with diversified geometry as electrodes. The device energy can cover two orders of magnitude, from <0.1 μW h to nearly 10 μW h, and the device power can be tuned in four orders of magnitude, from 0.2 μW to 2000 μW. Furthermore, multiple mechanically flexible fiber-based micro-supercapacitors can be integrated into complex energy storage units with wider operation voltage windows, demonstrating broad application potentials in flexible devices.

A high-performance metal-free hydrogen-evolution reaction electrocatalyst from bacterium derived carbon

We report a sustainable approach to obtain carbon materials with nitrogen and phosphorus dual functionalities from a common bacterium strain (S. aureus) as a highly efficient hydrogen-evolution reaction (HER) catalyst. With mesoporous structure introduced by ZnCl2 salt and cathodic activation, it demonstrates an onset overpotential as low as 76 mV, a Tafel slope of 58.4 mV dec−1 and a large normalized exchange current density of 1.72 × 10−2 mA cm−2, which are comparable to those of hitherto best metal-free and well-fabricated metallic HER catalysts.

Multifunctional nitrogen-rich “brick-and-mortar” carbon as high performance supercapacitor electrodes and oxygen reduction electrocatalysts

Novel porous carbon materials with excellent electronic, chemical and structural properties and high nitrogen content are desirable for many applications. Here, we show the design and synthesis of a new multifunctional porous carbon material with a unique architecture through a simple but effective activation-free procedure. Porous carbon derived from a copolymer poly(vinylidene chloride-co-acrylonitrile) serves as the nitrogen-rich “mortar”. Reduced graphene oxide layers work as “bricks” with an aim to provide an open nanoscale scaffold and connect porous carbon, as well as modulate the ratio between mesopores and micropores. This new material has a large surface area (957 m2 g−1), high nitrogen content (6.6 at%), excellent conductivity (up to 5.1 S cm−1), and favorable hierarchical meso- and microporosity. Benefiting from these intriguing features, this material shows an ultrahigh specific capacitance of 361 F g−1 in an aqueous electrolyte. The as-assembled asymmetric supercapacitors with the designed carbon materials as negative electrodes and porous cobalt oxide nanorods as positive electrodes deliver a high-energy of 50.1 W h kg−1 with a cell voltage of up to 1.6 V. Further, this material shows high electrocatalytic activity for the oxygen reduction reaction in alkaline medium comparable with that of 20 wt% platinum–carbon electrodes, with better methanol tolerance and long-term durability. We expect that this uniquely designed nitrogen-rich porous carbon material with its simple and scalable synthesis method will have great potential for various applications in energy storage, energy conversion and catalysis.

Sandwich-Architectured Poly(lactic acid)–Graphene Composite Food Packaging Films

Biodegradable food packaging promises a more sustainable future. Among the many different biopolymers used, poly(lactic acid) (PLA) possesses the good mechanical property and cost-effectiveness necessary of a biodegradable food packaging. However, PLA food packaging suffers from poor water vapor and oxygen barrier properties compared to many petroleum-derived ones. A key challenge is, therefore, to simultaneously enhance both the water vapor and oxygen barrier properties of the PLA food packaging. To address this issue, we design a sandwich-architectured PLA-graphene composite film, which utilizes an impermeable reduced graphene oxide (rGO) as the core barrier and commercial PLA films as the outer protective encapsulation. The synergy between the barrier and the protective encapsulation results in a significant 87.6% reduction in the water vapor permeability. At the same time, the oxygen permeability is reduced by two orders of magnitude when evaluated under both dry and humid conditions. The excellent barrier properties can be attributed to the compact lamellar microstructure and the hydrophobicity of the rGO core barrier. Mechanistic analysis shows that the large rGO lateral dimension and the small interlayer spacing between the rGO sheets have created an extensive and tortuous diffusion pathway, which is up to 1450-times the thickness of the rGO barrier. In addition, the sandwiched architecture has imbued the PLA-rGO composite film with good processability, which increases the manageability of the film and its competency to be tailored. Simulations using the PLA-rGO composite food packaging film for edible oil and potato chips also exhibit at least eight-fold extension in the shelf life of these oxygen and moisture sensitive food products. Overall, these qualities have demonstrated the high potential of a sandwich-architectured PLA-graphene composite film for food packaging applications.

Synergism of Water Shock and a Biocompatible Block Copolymer Potentiates the Antibacterial Activity of Graphene Oxide

Graphene oxide (GO) is promising in the fight against pathogenic bacteria. However, the antibacterial activity of pristine GO is relatively low and concern over human cytotoxicity further limits its potential. This study demonstrates a general approach to address both issues. The developed approach synergistically combines the water shock treatment (i.e., a sudden decrease in environmental salinity) and the use of a biocompatible block copolymer (Pluronic F-127) as a synergist co-agent. Hypoosmotic stress induced by water shock makes gram-negative pathogens more susceptible to GO. Pluronic forms highly stable nanoassemblies with GO (Pluronic-GO) that can populate around bacterial envelopes favoring the interactions between GO and bacteria. The antibacterial activity of GO at a low concentration (50 μg mL(-1) ) increases from <30% to virtually complete killing (>99%) when complemented with water shock and Pluronic (5 mg mL(-1) ) at ≈2-2.5 h of exposure. Results suggest that the enhanced dispersion of GO and the osmotic pressure generated on bacterial envelopes by polymers together potentiate GO. Pluronic also significantly suppresses the toxicity of GO toward human fibroblast cells. Fundamentally, the results highlight the crucial role of physicochemical milieu in the antibacterial activity of GO. The demonstrated strategy has potentials for daily-life bacterial disinfection applications, as hypotonic Pluronic-GO mixture is both safe and effective.

Bacterial physiology is a key modulator of the antibacterial activity of graphene oxide

Carbon-based nanomaterials have a great potential as novel antibacterial agents; however, their interactions with bacteria are not fully understood. This study demonstrates that the antibacterial activity of graphene oxide (GO) depends on the physiological state of cells for both Gram-negative and -positive bacteria. GO susceptibility of bacteria is the highest in the exponential growth phase, which are in growing physiology, and stationary-phase (non-growing) cells are quite resistant against GO. Importantly, the order of GO susceptibility of E. coli with respect to the growth phases (exponential ≫ decline > stationary) correlates well with the changes in the envelope ultrastructures of the cells. Our findings are not only fundamentally important but also particularly critical for practical antimicrobial applications of carbon-based nanomaterials.

Sulfur-induced chirality changes in single-walled carbon nanotube synthesis by ethanol chemical vapor deposition on a Co/SiO2 catalyst

Selective synthesis of single-walled carbon nanotubes (SWCNTs) with different chiral structures is highly desirable for their potential applications ranging from electronics, photovoltaics to medicine. Here, we have shown that introducing sulfur-containing compounds into carbon feedstock may efficiently alter the chiral selectivity of a Co/SiO2 catalyst toward different chiral species. When carbon disulfide (0.0001 wt%) was added to ethanol, the carbon yield increased significantly from 4.8 to 14 wt% without chirality changes. The changes in chiral selectivity are correlated with the significant changes in the carbon yield. With further increase of carbon disulfide concentration, the chirality distribution shifted from smaller diameter chiral nanotubes, such as (7,6) at 0.9 nm, toward larger diameter chiral ones, such as (8,7) at 1 nm and (9,8) at 1.2 nm, in addition to a sharp decrease in the carbon yield. Further, when a different sulfur-containing compound was used, the chiral selectivity changed differently. Thiophene (0.1 wt%) in ethanol led to a good chiral selectivity toward (9,8) nanotubes with a relative abundance of 43.1% among all semiconducting nanotubes. We propose that sulfur may selectively block active sites on Co particles through dynamic interactions among sulfur, hydrogen, carbon and Co metal particles. Our results suggest that sulfur-containing compounds may be used as an efficient additive to tune the chiral selectivity of catalysts in SWCNT synthesis.