Cycle stability of the electrochemical capacitors patterned with vertically aligned carbon nanotubes in an LiPF6-based electrolyte.

The miniature ultracapacitors, with interdigitated electrodes of vertically aligned carbon nanotubes (VACNTs) and an inter-electrode gap of 20 µm, have been prepared in the LiPF6 organic electrolyte with and without PVdF-HFP gel. PVdF-HFP between two opposing electrodes enhances the device reliability, but lessens its power performance because of the extra diffusion resistance. Also noteworthy are the gel influences on the cycle stability. When the applied voltage is 2.0 or 2.5 V, both the LiPF6 and the gel capacitors exhibit excellent stability, typified by a retention ratio of ≥95% after 10,000 cycles. Their coulombic efficiencies quickly rise up, and hold steady at 100%. Nonetheless, when the applied voltage is 3.5 or 4.0 V, the cycle stability deteriorates, since the negative electrode potential descends below 0.9 V (vs. Li), leading to electrolyte decomposition and SEI formation. For the LiPF6 capacitor, its retention ratio could be around 60% after 10,000 cycles and the coulombic efficiency of 100% is difficult to reach throughout its cycle life. On the other hand, the gel capacitor cycles energy with a much higher retention ratio, >80% after 10,000 cycles, and a better coulombic efficiency, even though electrolyte decomposition still occurs. We attribute the superior stability of the gel capacitor to its extra diffusion resistance which slows down the performance deterioration.

Miniature asymmetric ultracapacitor of patterned carbon nanotubes and hydrous ruthenium dioxide.

A symmetric ultracapacitor CNT_CNT and an asymmetric ultracapacitor CNT_hRuO(2) of mini size have been prepared with patterned carbon nanotubes (CNT) and hydrous ruthenium dioxide. Galvanostatic charge/discharge results indicate that CNT_hRuO(2) is the superior one in both power and energy densities. In a potential window 2.0 V, the CNT_hRuO(2) cell displays an energy density of 24.0 W h kg(-1) at a power density of 22.9 kW kg(-1). Its power density can be raised to 41.1 kW kg(-1) at the expense of the energy density, which drops to 6.8 W h kg(-1). On the other hand, CNT_CNT performs at a lower level, delivering 5.2 W h kg(-1) at 5.5 kW kg(-1). The favorable charge/discharge performance of CNT_hRuO(2) is attributed to hydrous RuO(2), whose pseudocapacitance drives the other electrode of the vertical CNT array to work harder and makes more use of its double-layer capacitance. The analysis of individual electrode capacitance indicates that the high capacitance of hRuO(2) also causes a disproportion in voltage partition, which restricts the low limit of cycling current in an extended potential window. On energy cycling, CNT_hRuO(2) demonstrates sufficient stability in 10,000 cycles, after an initial 13% drop in capacitance.