| Traditional energy sources are considered to be replaced by renewable clean energies, such as wind energy, solar energy, and tidal energy. But the unstable output energy must be stored to realize continuous and stable supply due to the timeliness and unstable output power of the renewable energies. Also, modern electronic equipments are becoming more and more miniaturized, portable, and bending. Thus the energy storage devices must be more portable and flexible. The electrochemical supercapacitors (SCs) are regarded as a novel energy storage device to meet these requirements. Compared to the lithium-ion battery and traditional capacitor, the electrochemical SCs possess the following advantages:(1) high power density and energy density; (2) long life expectancy; (3) wide operating temperatures range from-40 to 70℃; (4) high efficiency; (5) environment friendly. The electrode materials are the key to electrochemical performance of SCs. Graphene is an ideal electrode material for SCs due to its large specific surface area, high conductivity, and high mechanical strength. However, the low capacitance limits the application of graphene because of its electric double layer storage mechanism. The performance of graphene can be improved by combining with other pseudocapacitive materials. However, strong Van Der Waals force leads to that obvious stacking occurs easily between graphene nanosheets. Moreover, the pseudocapacitive materials in such composites have poorer active centers for electrochemical reactions.To address the problems above, our work has prepared nano-micro three-dimensionally porous nitrogen doped graphene nanostructures via high power ultrasound and hydro thermal methods. The nano-micro nanostructure improves mechanical stability and electrical conductivity. Furthermore, the stacking and agglomeration of graphene and hydroxides can be effectively avoided by inducing transition metal hydroxides with nanoshapes into 3D porous N-doped graphene. In addition, flexible asymmetric all-solid-state supercapacitor devices are designed and assembled. The main contents are summarized as follows:(1) Using high power direct contact type ultrasound, three-dimensionally porous nitrogen doped graphene (3DPNG) was achieved under hydrothermal conditions with the reduction of ethylenediamine. In the 3DPNG, the unique micro size large sheets with nano scale pore assembled into three-dimensional structure. The formation of nano-pores increases the contact area of the material and the electrolyte, and the nano-micro structure improves mechanical stability and electrical conductivity. The effects of ultrasound power on diameter, thickness, and porosity of the N-doped graphene are studied. The 3DPNG achieves a capacitance of 530.2F g-1 at 5A g-1 which is close to the theoretical value of graphene (550F g-1). Also, it has an excellent cycling performance with a capacitance retention of 98.3% after 10,000 charge/discharge cycles. The columbic efficiency is kept at 99.7%(nearly 100%) throughout the 10,000 cycles.(2) The 3DPNG-Co(OH)2 was prepared via one-pot hydrothermal method. In 3DPNG-Co(OH)2, single-crystalline Co(OH)2 plates are distributed homogeneously inside the conductive interconnected 3DPNG networks, which hinder the stack of nanoplates and 3DPNG. The capacitance of optimized 3DPNG-Co(OH)2 exceeds the recently reported 2D graphene/Co(OH)2 composites even at the fast discharge condition. Capacitance retention over 2,000 cycles is still high as 95%. The improvements are attributed to three aspects:(1) the regular shaped Co(OH)2 nanoplates; (2) the porosity of 3DPNG prevents the stacking; (3) the continuously connected pores and highly conductive networks facilitate the transportation of electron and ion.(3) The structure stability of graphene can be promoted by combining with one-dimensional nanowires and graphene. The synthesized 2CoCO3·3Co(OH)2 is a new pseudocapacitor material with rich active centers. The 3DPNG-CCH composite was obtained through a one-step hydrothermal method firstly. The optimal composite has a capacitance of 1690F g-1 at 1A g-1, and good cycling stability of 94.2% after 10,000 charge/discharge cycles. The excellent performance is attributed to the decresing stacking of graphene by the nanowires, and the increasing effective reaction active sites in the electrochemical reactions. The assembled asymmetric supercapacitor based on the optimal composite has a high areal capacitance of 153.5mF cm-2 (at 1mA cm-2), a high voltage of 1.9V, and thus provides superior energy and power densities (0.77Wh m-2 and 25.3W m-2). Also, the asymmetric supercapacitor has an excellent cycling stability (93.6% over 2,000 cycles). The asymmetric supercapacitors can light two LEDs for more than 15 min with three capacitors in series.(4) Flexible asymmetric SCs were assembled based on the obtained composite hydrogels. The optimal SC device has a high specific areal capacitance of 255mFcm"2 at lmAcm-2 and a very high output voltage of 2.5V, which leads to an energy density of 80mWh cm-2 even at a high power of 944mW cm-2, considerably higher than that reported for similar devices. The devices exhibit a high cycling stability (92%) and columbic efficiency (99.5%) over 10,000 charging cycles as well as excellent flexibility with no clear performance degradation under strong bending. We connected three such advanced asymmetric devices in series to power 38 LEDs connected in parallel (’NJU’ pattern), and the LEDs stayed lit for 39 min. |
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