Self‐Organized Graphene Nanosheets with Corrugated,Ordered Tip Structures for High‐Performance Flexible Field Emission

Patterned reduced graphene oxide (rGO) films with vertically aligned tip structures are fabricated by a straightforward self‐assembly method. The size, uniformity of the patterns, and alignment of the tips are successfully controlled according to the concentration of a GO/octadecylamine (ODA)‐dispersed solution. The surface energy difference between the GO/ODA solution and a self‐assembled water droplet is a critical parameter for determining the pattern structure. Numerous rGO nanosheets are formed so as to be vertically aligned with respect to the substrate during film fabrication at GO concentrations below 2.0 g/L. These samples provide high field‐emission characteristics. The patterned rGO arrays are highly flexible with preservation of the field emission properties, even at large bending angles. This is attributed to the high crystallinity, emitter density, and good chemical stability of the rGO arrays, as well as the strong interactions between the rGO arrays and the substrate.     

Porous Hybrid Network of Graphene and Metal Oxide Nanosheets as Useful Matrix for Improving the Electrode Performance of Layered Double Hydroxides

Mesoporous hybrid network of reduced graphene oxide (rG‐O) and layered MnO₂ nanosheets could act as an efficient immobilization matrix for improving the electrochemical activity of layered double hydroxide (LDH). The control of MnO₂/rG‐O ratio is crucial in optimizing the porous structure and electrical conductivity of the resulting hybrid structure. The immobilization of Co‐Al‐LDH on hybrid MnO₂/rG‐O network is more effective in enhancing its electrode activity compared with that of on pure rG‐O network. The Co‐Al‐LDH?rG‐O?MnO₂ nanohybrid deliveres a greater specific capacitance than does MnO₂‐free Co‐Al‐LDH?rG‐O nanohybrid. The beneficial effect of MnO₂ incorporation on the electrode performance of nanohybrid is more prominent for higher current density and faster scan rate, underscoring the significant enhancement of the electron transport of Co‐Al‐LDH?rG‐O. This is supported by electrochemical impedance spectroscopy. The present study clearly demonstrates the usefulness of the porously assembled hybrid network of graphene and metal oxide nanosheets as an effective platform for exploring efficient LDH‐based functional materials.     

3D Carbon/Cobalt‐Nickel Mixed‐Oxide Hybrid Nanostructured Arrays for Asymmetric Supercapacitors

The electrochemical performance of supercapacitors relies not only on the exploitation of high‐capacity active materials, but also on the rational design of superior electrode architectures. Herein, a novel supercapacitor electrode comprising 3D hierarchical mixed‐oxide nanostructured arrays (NAs) of C/CoNi₃O₄ is reported. The network‐like C/CoNi₃O₄ NAs exhibit a relatively high specific surface area; it is fabricated from ultra‐robust Co‐Ni hydroxide carbonate precursors through glucose‐coating and calcination processes. Thanks to their interconnected three‐dimensionally arrayed architecture and mesoporous nature, the C/CoNi₃O₄ NA electrode exhibits a large specific capacitance of 1299 F/g and a superior rate performance, demonstrating 78% capacity retention even when the discharge current jumps by 100 times. An optimized asymmetric supercapacitor with the C/CoNi₃O₄ NAs as the positive electrode is fabricated. This asymmetric supercapacitor can reversibly cycle at a high potential of 1.8 V, showing excellent cycling durability and also enabling a remarkable power density of ～13 kW/kg with a high energy density of ～19.2 W·h/kg. Two such supercapacitors linked in series can simultaneously power four distinct light‐emitting diode indicators; they can also drive the motor of remote‐controlled model planes. This work not only presents the potential of C/CoNi₃O₄ NAs in thin‐film supercapacitor applications, but it also demonstrates the superiority of electrodes with such a 3D hierarchical architecture.     

Generalized Synthesis of Ultrathin Cobalt‐Based Nanosheets from Metallophthalocyanine‐Modulated Self‐Assemblies for Complementary Water Electrolysis

The development of effective approaches for preparing large‐area, self‐standing, ultrathin metal‐based nanosheets, which have proved to be favorable for catalytic applications such as water electrolysis, is highly desirable but remains a great challenge. Reported herein is a simple and versatile strategy to synthesize ultrathin Co₃O₄ and CoP NSs consisting of close‐packed nanoparticles by pyrolyzing cobalt(II) phthalocyanine/graphene oxide (CoPc/GO) assemblies in air and subsequent topotactic phosphidation while preserving the graphene‐like morphology. The strong π–π stacking interactions between CoPc and GO, and the inhibiting effect of the tetrapyrrole‐derived macrocycle for grain growth during the catalytic carbon gasification contribute to the NSs forming. The resulting homologous Co₃O₄ and CoP NSs display outstanding catalytic activity in alkaline media toward the oxygen evolution reaction and the hydrogen evolution reaction, respectively, ascribed to the richly exposed active sites, and the expedited electrolyte/ion transmission path. The integrated asymmetrical two‐electrode configuration also presents a superior cell voltage of 1.63 V at 10 mA cm^?2 for overall water splitting, accompanied with the excellent durability during long‐term cycling. Further evidences validate that this strategy is appropriate to fabricate graphene‐like ultrathin NSs of many other metal oxides, such as Fe₂O₃, NiO, MoO₃, and mixed‐metal oxides, for various applications.     

Porous Cobalt–Nickel Hydroxide Nanosheets with Active Cobalt Ions for Overall Water Splitting

Low‐cost and high‐performance catalysts are of great significance for electrochemical water splitting. Here, it is reported that a laser‐synthesized catalyst, porous Co_0.75Ni_0.25(OH)₂ nanosheets, is highly active for catalyzing overall water splitting. The porous nanosheets exhibit low overpotentials for hydrogen evolution reaction (95 mV@10 mA cm^?2) and oxygen evolution reaction (235 mV@10 mA cm^?2). As both anode and cathode catalysts, the porous nanosheets achieve a current density of 10 mA cm^?2 at an external voltage of 1.56 V, which is much lower than that of commercial Ir/C‐Pt/C couple (1.62 V). Experimental and theoretical investigations reveal that numerous Co³⁺ ions are generated on the pore wall of nanosheets, and the unique atomic structure around Co³⁺ ions leads to appropriate electronic structure and adsorption energy of intermediates, thus accelerating hydrogen and oxygen evolution.     

High Volumetric Energy Density Asymmetric Supercapacitors Based on Well‐Balanced Graphene and Graphene‐MnO2 Electrodes with Densely Stacked Architectures

The well‐matched electrochemical parameters of positive and negative electrodes, such as specific capacitance, rate performance, and cycling stability, are important for obtaining high‐performance asymmetric supercapacitors. Herein, a facile and cost‐effective strategy is demonstrated for the fabrication of 3D densely stacked graphene (DSG) and graphene‐MnO₂ (G‐MnO₂) architectures as the electrode materials for asymmetric supercapacitors (ASCs) by using MnO₂‐intercalated graphite oxide (GO‐MnO₂) as the precursor. DSG has a stacked graphene structure with continuous ion transport network in‐between the sheets, resulting in a high volumetric capacitance of 366 F cm^–3, almost 2.5 times than that of reduced graphene oxide, as well as long cycle life (93% capacitance retention after 10 000 cycles). More importantly, almost similar electrochemical properties, such as specific capacitance, rate performance, and cycling stability, are obtained for DSG as the negative electrode and G‐MnO₂ as the positive electrode. As a result, the assembled ASC delivers both ultrahigh gravimetric and volumetric energy densities of 62.4 Wh kg^–1 and 54.4 Wh L^–1 (based on total volume of two electrodes) in 1 m Na₂SO₄ aqueous electrolyte, respectively, much higher than most of previously reported ASCs in aqueous electrolytes.