## 3 Status and Future Development of Electrochemical Capacitors  
### Present Performance of Supercapacitors  
A summary of the characteristics of electrochemical capacitors that have been tested at UC Davis is given in Table 2. Both EDLC and hybrid HSC have been tested. The EDLC devices using activated carbon have rated voltages of 2.7-3.0 V and energy densities of 5-6 Wh/kg. EDLCs that use composites of carbon and graphene have energy densities of 8-9 Wh/kg. The power capability (W/kg)\({}_{95\%}\) of most EDLCs is between 1000 and 2000, but there are devices with much higher power (>3000 W/kg). The RC time constants of most EDLCs are between 1 and 2 s, but high-power devices have time constants of <0.5 s. It is generally accepted that EDLCs using activated carbon have cycle lives of 500, 000 to 1 million deep discharge cycles at reasonable temperature (<35 \({}^{\circ}\)C).  
Hybrid supercapacitors (HSC) have also been tested at UC Davis, as shown in Table 2. The energy densities of the hybrid supercapacitors are significantly higher than those of the EDLCs, varying from 10 to 60 Wh/kg. The rated voltage of most of the HSCs is 3.8-4.0 V. Most of the available HSCs use lithiated graphite in the negative electrode and activated carbon in the positive electrode. Yunasko uses a mixture of carbon and a metal oxide in both electrodes. Aowei-China has not disclosed the chemistry of their devices [8]. The power capability of the HSCs seems to be about the same as that of the EDLC devices, even though their RC time constants are much higher. Both JRS Micro and Aowei-China claim the cycle lives of their devices are comparable to those of EDLC devices. That is unexpected, and has not been confirmed by third parties. As shown by the linear V vs. t discharge profile in Figure 3, the Aowei cells show typical hybrid supercapacitor discharge behavior that is very different from batteries.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Device** & **V** & **C** & **R** & **RC** & **W/kg** & **W/kg** & **W/kg** & **Weight** & **Vol.** \\  & **Rate** & **(F)** & **(mOhm)** & **c** & **(s)** & **W**/kg** & **(95\%)** & **Match.Imped** & **d** & **(kg)** & **(L)** \\ \hline
**EDLCs** & & & & & & & & & & \\ Maxwell & 2.7 & 2885 & 0.375 & 1.1 & 4.2 & 994 & 8836 & 0.55 & 0.414 \\ Ioxus & 2.85 & 3095 & 0.33 & 1.0 & 5.0 & 1355 & 12,065 & 0.51 & 0.41 \\ Skeleton Tech. & 2.85 & 3450 & 0.13 & 0.45 & 5.4 & 3353 & 29,809 & 0.52 & 0.39 \\ Skeleton Tech. & 3.4 & 3200 & 0.48 & 1.5 & 8.9 & 1730 & 15,400 & 0.40 & 0.096 \\ Skeleton Tech. & 3.0 & 3320 & 0.25 & 0.83 & 5.6 & 1878 & 17,310 & 0.54 & 0.39 \\ Skeleton Tech. & 3.0 & 1900 & 0.52 & 0.98 & 4.9 & 1430 & 13,520 & 0.34 & 0.22 \\ Skeleton Tech. & 2.85 & 4100 & 0.22 & 0.90 & 6.3 & 1956 & 19,230 & 0.53 & 0.39 \\ Skeleton Tech & 3.0 & 5000 & 0.25 & 1.25 & 8.4 & 1783 & 15,845 & 0.568 & 0.39 \\ Yunasko * & 2.75 & 1275 & 0.11 & 0.13 & 4.55 & 8791 & 78,125 & 0.22 & 0.15 \\ DAE-China & 2.7 & 1668 & 0.6 & 1.0 & 6.2 & 1734 & 15,420 & 0.197 & 0.15 \\ DAE-China & 2.7 & 990 & 1.8 & 1.78 & 4.9 & 801 & 6935 & 0.146 & 0.11 \\ DAE-China & 2.7 & 412 & 3.7 & 1.52 & 4.1 & 791 & 7037 & 0.07 & 0.054 \\ NessMaxwell & 3.0 & 3650 & 0.27 & 0.98 & 6.5 & 1875 & 16,666 & 0.50 & 0.394 \\ Ness & 2.7 & 3160 & 0.4 & 1.3 & 4.4 & 982 & 8728 & 0.522 & 0.379 \\ \hline \end{tabular}
\end{table}
Table 2: Summary of the characteristics of available electrochemical capacitors.