Nitrogen‐doped graphene (NG) is a promising metal‐free catalyst for the oxygen‐reduction reaction (ORR). A facile and low‐cost synthesis of NG via the pyrolysis of graphene oxide and urea is reported. The N content in NG can be up to 7.86%, with a high percentage of graphitic N (≈24%), which gives rise to an excellent catalytic activity toward the ORR.

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Suppressing the Sn coarsening in the Li2O matrix enabled highly reversible conversion between Li2O and SnO2 and an initial Coulombic efficiency of ∼95.5% was achieved.

While manganese oxide (MnO2) has been extensively studied as an electrode material for pseudocapacitors, a clear understanding of its charge storage mechanism is still lacking. Here we report our findings in probing the structural changes of a thin-film model MnO2 electrode during cycling using in operando Raman spectroscopy. The spectral features (e.g., band position, intensity, and width) are correlated quantitatively with the size (Li+, Na+, and K+) of cations in different electrolytes and with the degree of discharge to gain better understanding of the cation-incorporation mechanism into the interlayers of pseudocapacitive MnO2. Also, theoretical calculations of phonon energy associated with the models of interlayer cation-incorporated MnO2 agree with the experimental observations of cation-size effect on the positions of Raman bands. Furthermore, the cation-size effects on spectral features at different potentials of MnO2 electrode are correlated quantitatively with the amount of charge stored in the MnO2 electrode. The understanding of the structural changes associated with charge storage gained from Raman spectroscopy provides valuable insights into the cation-size effects on the electrochemical performances of the MnO2 electrode.

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In this paper, we report the growth of ultrathin Ni(OH) 2 nanosheets on nickel foam at room temperature via a cost-effective and simple process, oxidizing fresh nickel foam in a wet environment followed by a morphology transformation in a mixed alkaline and oxidative solution without the need for any additional nickel sources, templates, or surfactant. When tested as electrode for a supercapacitor, the Ni(OH) 2 nanosheets grown on nickel foam displayed excellent performance, demonstrating specific capacitance of 2384.3 F g-1 at a charge and discharge current density of 1 A g-1 and 1288.1 F g-1 at 5 A g-1 with a good cycling ability (~75% of the initial specific capacitance remained after 3000 cycles). The excellent electrochemical performance is attributed to its unique nanostructures, which may facilitate rapid ion transport near electrode surfaces, while allowing facile redox reactions associated with charge storage by the nanosheets. The demonstrated high specific capacity and the remarkable rate performance of the Ni(OH) 2 nanosheets, together with the flexibility of the nickel foam substrate, make the three-dimensional nanostructured electrodes ideally suited for low-cost, high-performance supercapacitor applications.

A facile hydrothermal method is utilized to produce nanostructured NiCo 2 S 4 arrays on carbon fiber paper with controlled morphologies to study the effect of morphology on their electrochemical performance in supercapacitors. Specifically, NiCo 2 S 4 solid nanofiber, nanotube, and hollow nanoneedle of the same crystalline structure are synthesized by controlling the conditions of the hydrothermal synthesis. Among the three different morphologies studied, the hollow nanoneedle of NiCo 2 S 4 shows the highest capacity and the longest cycling life, demonstrating a specific capacitance of ~1154 F g-1 at a charge-discharge current density of 1 A g-1 and negligible capacity loss after 8,000 cycles (at a rate of 10 A g-1). This high performance is attributed to the unique nanostructure of the hollow nanoneedle, suggesting that the morphology of NiCo 2 S 4 plays a vital role in determining the electrochemical performance. Further, an asymmetric capacitor consisting of NiCo 2 S 4 hollow nanoneedle electrode and a tape-cast activated carbon film electrode achieves an energy density of ~17.3 Wh kg-1 at 1 A g-1 and a power density of ~3.2 kW kg-1 at 20 A g-1 in a voltage range of 0 and 1.5 V, implying that it has a great potential for a wide variety of practical applications.

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