High Energy Density All Solid State Asymmetric Pseudocapacitors Based on Free Standing Reduced Graphene Oxide-Co₃O₄ Composite Aerogel Electrodes

Modern flexible consumer electronics require efficient energy storage devices with flexible free-standing electrodes. We report a simple and cost-effective route to a graphene-based composite aerogel encapsulating metal oxide nanoparticles for high energy density, free-standing, binder-free flexible pseudocapacitive electrodes. Hydrothermally synthesized Co₃O₄ nanoparticles are successfully housed inside the microporous graphene aerogel network during the room temperature interfacial gelation at the Zn surface. The resultant three-dimensional (3D) rGO-Co₃O₄ composite aerogel shows mesoporous quasiparallel layer stack morphology with a high loading of Co₃O₄, which offers numerous channels for ion transport and a 3D interconnected network for high electrical conductivity. All solid state asymmetric pseudocapacitors employing the composite aerogel electrodes have demonstrated high areal energy density of 35.92 μWh/cm² and power density of 17.79 mW/cm² accompanied by excellent cycle life

Large-Area Buckled MoS₂ Films on the Graphene Substrate

In this study, a novel buckled structure of edge-oriented MoS₂ films is fabricated for the first time by employing monolayer graphene as the substrate for MoS₂ film growth. Compared to typical buckling methods, our technique has several advantages: (1) external forces such as heat and mechanical strain are not applied; (2) uniform and controllable buckling over a large area is possible; and (3) films are able to be transferred to a desired substrate. Dual MoS₂ orientation was observed in the buckled film where horizontally aligned MoS₂ layers of 7 nm thickness were present near the bottom graphene surface and vertically aligned layers dominated the film toward the outer surface, in which the alignment structure was uniform across the entire film. The catalytic ability of the buckled MoS₂ films, measured by performing water-splitting tests in acidic environments, shows a reduced onset potential of −0.2 V versus reversible hydrogen electrode (RHE) compared to −0.32 V versus RHE for pristine MoS₂, indicating that the rough surface provided a higher catalytic activity. Our work presents a new method to generate a buckled MoS₂ structure, which may be extended to the formation of buckled structures in various 2D materials for future applications

Self-Size-Limiting Nanoscale Perforation of Graphene for Dense Heteroatom Doping

A scalable and controllable nanoscale perforation method for graphene is developed on the basis of the two-step thermal activation of a graphene aerogel. Different resistance to the thermal oxidation between graphitic and defective domains in the weakly reduced graphene oxide is exploited for the self-limiting nanoscale perforation in the graphene basal plane via selective thermal degradation of the defective domains. The resultant nanoporous graphene with a narrow pore-size distribution addresses the long-standing challenge for the high-level doping of graphene with lattice-mismatched large-size heteroatoms (S and P). Noticeably, this novel heteroatom doping strategy is demonstrated to be highly effective for oxygen reduction reaction (ORR) catalysis. Not only the higher level of heteroatom doping but also favorable spin and charge redistribution around the pore edges leads to a strong ORR activity as supported by density functional theory calculations

Omnidirectional Deformable Energy Textile for Human Joint Movement Compatible Energy Storage

Omnidirectional deformability is an unavoidable basic requirement for wearable devices to accommodate human daily motion particularly at human joints. We demonstrate omnidirectionally bendable and stretchable textile-based electrochemical capacitor that retains high power performance under complex mechanical deformation. Judicious synergistic hybrid structure of woven elastic polymer yarns with carbon nanotubes and conductive polymers offers reliable electrical and electrochemical activity even under repeated cycles of severe complex deformation modes. The textile-based electrochemical capacitors exhibit omnidirectional stretchability with 93% of capacitance retention under repeated 50% omnidirectional stretching condition while demonstrating excellent specific capacitance (412 mF cm^–2) and cycle stability (>2000 stretch). The wearable power source stably powers red LED under omnidirectional stretching that accompanies human elbow joint motion

Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction

Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoS_<i>x</i>) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoS_<i>x</i> layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoS_<i>x</i> surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm² current density, which is the highest HER activity of molybdenum sulfide-based catalyst ever reported thus far

Liquid Crystal Size Selection of Large-Size Graphene Oxide for Size-Dependent N‑Doping and Oxygen Reduction Catalysis

Graphene oxide (GO) is aqueous-dispersible oxygenated graphene, which shows colloidal discotic liquid crystallinity. Many properties of GO-based materials, including electrical conductivity and mechanical properties, are limited by the small flake size of GO. Unfortunately, typical sonochemical exfoliation of GO from graphite generally leads to a broad size and shape distribution. Here, we introduce a facile size selection of large-size GO exploiting liquid crystallinity and investigate the size-dependent N-doping and oxygen reduction catalysis. In the biphasic GO dispersion where both isotropic and liquid crystalline phases are equilibrated, large-size GO flakes (>20 μm) are spontaneously concentrated within the liquid crystalline phase. N-Doping and reduction of the size-selected GO exhibit that N-dopant type is highly dependent on GO flake size. Large-size GO demonstrates quaternary dominant N-doping and the lowest onset potential (−0.08 V) for oxygen reduction catalysis, signifying that quaternary N-dopants serve as principal catalytic sites in N-doped graphene

Interface-Confined High Crystalline Growth of Semiconducting Polymers at Graphene Fibers for High-Performance Wearable Supercapacitors

We report graphene@polymer core–shell fibers (G@PFs) composed of N and Cu codoped porous graphene fiber cores uniformly coated with semiconducting polymer shell layers with superb electrochemical characteristics. Aqueous/organic interface-confined polymerization method produced robust highly crystalline uniform semiconducting polymer shells with high electrical conductivity and redox activity. When the resultant core–shell fibers are utilized for fiber supercapacitor application, high areal/volume capacitance and energy densities are attained along with long-term cycle stability. Desirable combination of mechanical flexibility, electrochemical properties, and facile process scalability makes our G@PFs particularly promising for portable and wearable electronics

Omnidirectional Deformable Energy Textile for Human Joint Movement Compatible Energy Storage

Omnidirectional deformability is an unavoidable basic requirement for wearable devices to accommodate human daily motion particularly at human joints. We demonstrate omnidirectionally bendable and stretchable textile-based electrochemical capacitor that retains high power performance under complex mechanical deformation. Judicious synergistic hybrid structure of woven elastic polymer yarns with carbon nanotubes and conductive polymers offers reliable electrical and electrochemical activity even under repeated cycles of severe complex deformation modes. The textile-based electrochemical capacitors exhibit omnidirectional stretchability with 93% of capacitance retention under repeated 50% omnidirectional stretching condition while demonstrating excellent specific capacitance (412 mF cm^–2) and cycle stability (>2000 stretch). The wearable power source stably powers red LED under omnidirectional stretching that accompanies human elbow joint motion

Selective and Regenerative Carbon Dioxide Capture by Highly Polarizing Porous Carbon Nitride

Energy-efficient CO₂ capture is a stringent demand for green and sustainable energy supply. Strong adsorption is desirable for high capacity and selective capture at ambient conditions but unfavorable for regeneration of adsorbents by a simple pressure control process. Here we present highly regenerative and selective CO₂ capture by carbon nitride functionalized porous reduced graphene oxide aerogel surface. The resultant structure demonstrates large CO₂ adsorption capacity at ambient conditions (0.43 mmol·g^–1) and high CO₂ selectivity against N₂ yet retains regenerability to desorb 98% CO₂ by simple pressure swing. First-principles thermodynamics calculations revealed that microporous edges of graphitic carbon nitride offer the optimal CO₂ adsorption by induced dipole interaction and allows excellent CO₂ selectivity as well as facile regenerability. This work identifies a customized route to reversible gas capture using metal-free, two-dimensional carbonaceous materials, which can be extended to other useful applications