ABSTRACT
To characterize the correlation of the crystal structure and Al-ion storage behavior, we prepared various crystal structures of MoO3 (α-MoO3, ß-MoO3 and h-MoO3) electrode materials and studied them via in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The α-MoO3 electrode material possesses a specific capacitance of 575.4 F g-1 and a gravimetric capacity of 207.8 mA h g-1 at a current density of 1 A g-1. From the in situ XRD results, the crystal structures of α-MoO3 and ß-MoO3 show a significant distortion, whereas that of h-MoO3 is minorly affected during the insertion or extraction of Al3+ ions. Based on the in situ XAS results, the MoO6 octahedral structure and Mo ion valence of α-MoO3 and ß-MoO3 also exhibit a strong variation, whereas those of h-MoO3 are nearly unchanged during the insertion or extraction of Al3+ ions. Notably, in situ XRD and XAS also clearly show a possible phase of AlxMoO3 during the Al3+ insertion and extraction cycles in the α-MoO3 and ß-MoO3 electrode materials, which may play a crucial role in the behavior of the residue of Al3+ ions and poor cycling stability. We provide clear evidence that the Al-ion energy storage performance of various MoO3 electrode materials is strongly associated with the corresponding tunnel space and the stability of their crystal structures. This work also provides new insight into a strong correlation between ion-storage efficiency and the corresponding crystal structure, which is greatly helpful for the development and improvement of new electrode materials for Al-ion energy storage.
ABSTRACT
The specific energy of an aqueous carbon supercapacitor is generally small, resulting mainly from a narrow potential window of aqueous electrolytes. Here, we introduced agarose, an ecologically compatible polymer, as a novel binder to fabricate an activated carbon supercapacitor, enabling a wider potential window attributed to a high overpotential of the hydrogen-evolution reaction (HER) of agarose-bound activated carbons in sulfuric acid. Assembled symmetric aqueous cells can be galvanostatically cycled up to 1.8 V, attaining an enhanced energy density of 13.5 W h/kg (9.5 µW h/cm2) at 450 W/kg (315 µW/cm2). Furthermore, a great cycling behavior was obtained, with a 94.2% retention of capacitance after 10,000 cycles at 2 A/g. This work might guide the design of an alternative material for high-energy aqueous supercapacitors.
ABSTRACT
Binary transition metal oxide CoMoO4@NiWO4 core-shell nanoplates grown directly on a Ni foam substrate were synthesized via a facile two-step hydrothermal process. The core-shell nanoplates with high electrochemical surface area (2933 cm2) demonstrated excellent electrochemical properties (areal capacity as high as 0.464 mA h cm-2 at a current density of 5 mA cm-2) and great cycle stability (92.5% retention after 3000 cycles with a high current density of 40 mA cm-2). The mechanism of the electrochemical reactions based on the in situ X-ray absorption spectroscopy technique clearly shows that the Co and Ni elements simultaneously participate in the faradaic reactions with the electrolyte. These results indicate that the excellent electrochemical performance of CoMoO4@NiWO4 compared to that of CoMoO4 nanoplates is attributed to a large electrochemical surface area and synergistic effect between NiWO4 and CoMoO4. This combination of two binary transition metal oxides can hence provide an excellent route to develop a high-performance electrode material for supercapacitor applications.
