Xiaozhong Wu

Reduced graphene oxide aerogel with high-rate supercapacitive performance in aqueous electrolytes

Reduced graphene oxide aerogel (RGOA) is synthesized successfully through a simultaneous self-assembly and reduction process using hypophosphorous acid and I2 as reductant. Nitrogen sorption analysis shows that the Brunauer-Emmett-Teller surface area of RGOA could reach as high as 830 m2 g−1, which is the largest value ever reported for graphene-based aerogels obtained through the simultaneous self-assembly and reduction strategy. The as-prepared RGOA is characterized by a variety of means such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Electrochemical tests show that RGOA exhibits a high-rate supercapacitive performance in aqueous electrolytes. The specific capacitance of RGOA is calculated to be 211.8 and 278.6 F g−1 in KOH and H2SO4 electrolytes, respectively. The perfect supercapacitive performance of RGOA is ascribed to its three-dimensional structure and the existence of oxygen-containing groups.

On the origin of the high capacitance of carbon derived from seaweed with an apparently low surface area

Low surface area carbon materials, derived from pyrolyzing biomass or polymers, often possess high areal capacitances. However, the well-accepted pseudocapacitance introduced by heteroatoms could not explain this phenomenon without doubt. In order to explore the nature of the energy storage mechanism in these low surface area carbon materials, we prepared a series of laver-based carbon materials by regulating the heteroatom contents and investigated their electrochemical performance. Combining the results of advanced pore structure analyses and electrochemical measurements, we disclose that the presence of ultramicropores, which could not be probed by adsorbates such as nitrogen gas or argon, but are accessible to carbon dioxide or electrolyte ions, plays a most dominant role in the high capacitance of low surface area carbon materials. In this contribution, the previously accepted viewpoint that the capacitance is mainly derived from heteroatoms undergoing Faradaic reactions is challenged.

Remarkable supercapacitor performance of petal-like LDHs vertically grown on graphene/polypyrrole nanoflakes

A 3D hybrid nanostructure, in which petal-like ultrathin nickel–aluminum layered double hydroxides (LDHs) were vertically grown on a conductive graphene/polypyrrole (GP) substrate, was designed and fabricated by a facile hydrothermal method. SEM and TEM observations confirmed the successful synthesis of this specially designed nanostructure, in which the conductive substrate ensures very fast electron transfer during the charge–discharge process, whereas the 3D hierarchical structure facilitates rapid ion transfer. The ultrathin LDH nanoflakes (3–5 nm) expose their abundant active sites to the electrolyte, thus generating huge pseudocapacitance. Combining the abovementioned features, this specially designed 3D nanostructured hybrid possesses an exceptional specific capacitance (2395 F g−1 at 1 A g−1), excellent rate performance (retaining 71.8% of capacitance at the current density of 20 A g−1), and remarkable cycling stability (99.6% retention after 10 000 cycles). Moreover, the assembled asymmetric supercapacitor obtained using GP@LDH as a positive electrode and GP-derived carbon as a negative electrode exhibits an ultrahigh energy density of 94.4 W h kg−1 at the power density of 463.1 W kg−1, making GP@LDH very attractive as an electrode material for high performance and low-cost supercapacitors.

Fe3O4@Ti3C2 MXene hybrids with ultrahigh volumetric capacity as an anode material for lithium-ion batteries

The volumetric capacity of lithium-ion batteries is becoming an increasingly important parameter restricting their practical applications in limited space, such as in portable electronic products and electric vehicles. Therefore, novel electrode materials with high volumetric capacities are urgently desirable. Aiming to pursue such kind of electrode materials, a new Fe3O4@Ti3C2 MXene hybrid is fabricated through a simple ultrasonication of Ti3C2 MXene and Fe3O4 nanoparticles. Multi-layered Ti3C2 MXene in the prepared hybrids acts as a superior host to load Fe3O4 nanoparticles due to its open two dimensional structure, favorable electrical conductivity and low Li+ diffusion barrier. X-ray diffraction and scanning electron microscopy analysis show that the Ti3C2 MXene could be homogeneously covered by Fe3O4 nanoparticles at a mass ratio of 5 : 2. As an anode material, the Fe3O4@Ti3C2-2:5 hybrid exhibits high reversible capacities of 747.4 mA h g−1 at 1C after 1000 cycles and 278.3 mA h g−1 at 5C after 800 cycles, which indicate its long cycle lifetime and excellent stability. More importantly, the hybrid material possesses an outstanding volumetric capacity up to 2038 mA h cm−3 at 1C due to the high compact density of the electrode of the prepared hybrid. This study provides further insight into the application of transition metal oxides@MXene hybrids as high volumetric performance anode electrodes for lithium-ion batteries.