cost, environmental friendliness, and universal availability. [6][7][8][9][10][11][12][13][14][15][16][17][18] To improve the performance and broaden the applications of TENGs, numerous efforts have been made with focus on both enhancement of the surface charge density and development of new structures/modes. [ 2,4,7,9,13,[19][20][21][22][23][24][25] So far, the electric power output of TENGs has been improved up to ≈500 W m −2 by sophisticated design of device structure. [ 18,[26][27][28] Demonstrated TENGs have been working in four modes, that is, the contact-separation or contact mode in short, sliding mode, single-electrode mode, and free-standing triboelectric-layer mode. The performance of the TENGs in contact-mode is better than that of TENGs in the sliding mode, because a much higher maximum displacement of TENGs in the sliding mode is required than that of TENGs working in the contact-mode, in order to achieve the same level of open circuit voltage. [ 29 ] The TENGs working in the contact-mode are particularly interesting for applications like foot-mounted wearable electronic systems because of the short vertical displacement involved and ease of integration.The fundamental principle of TENGs is based on the coupled effects of triboelectrifi cation and electrostatic induction. A theoretical model has been proposed for the TENGs working in the contact mode by Wang and co-workers. By utilizing the derived governing equation, the real-time output characteristics and the relationship between the optimum external resistance and TENG para meters were numerically simulated. [ 30 ] However, the model was not validated by corresponding experiments with measured real-time output characteristics. Furthermore, it is known that the real-time output characteristics would be affected by the materials, device structural parameters, operation conditions, and equivalent resistance. Assumptions made on simple profi les of working velocity may infl uence the model's validity in real situations. For instance, during real human walking, the previous wearing trials of an intelligent footwear system illustrated that the velocity profi le was complex and varied signifi cantly with time for a foot-mounted device. [ 31,32 ] Therefore, it is highly desirable to develop a theoretical model that is fully verifi ed by corresponding experiments. Such an experimentally verifi ed model is essential for designing and optimizing TENGs for intended applications.In this paper, based on conservation of charges and zerovoltage for close-loop circuits, a theoretical model is presented Harvesting mechanical energy from human activities by triboelectric nanogenerators (TENGs) is an effective approach for sustainable, maintenance-free, and green power source for wireless, portable, and wearable electronics. A theoretical model for contact-mode triboelectric nanogenerators based on the principles of charge conservation and zero loop-voltage is illustrated. Explicit expressions for the output current, voltage, and power are presented for the TENGs ...

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Molybdenum trioxide (MoO 3 ) is known as a promising pseudocapacitive material, but low conductivity limits its applications. Hydrogenation is demonstrated to increase the conductivity of MoO 3 and hence improve its electrochemical performance. Hydrogenated MoO 3 (MoO 3 À x ) shows enhanced conductivity based on, both first principle calculations and single nanobelt measurements. Freestanding MoO 3 À x /carbon nanotubes (CNT) composite films have been fabricated and showed much improved electrochemical performance compared to composites of CNT and as-synthesized MoO 3 (MoO 3 /CNT). Electrodes showed a specific capacitance of 337 F/g (based on the mass of MoO 3 À x ) and a high volumetric capacitance of 291 F/cm 3 (based on the whole electrode) with excellent rate capability. Also we confirmed that the improved intercalation kinetics and the increased intercalation pseudocapacitance could be attributed to the higher electronic conductivity of MoO 3 À x , which results in better and faster intercalations http://dx.(J. Zhou). Nano Energy (2014) 9, 355-363 of Li + ions. This electrochemical behavior implies that MoO 3 À x can serve as a very good negative electrode with high capacitance at high mass loading levels.