## 5 Summary and Conclusions  
This paper is concerned with the development and performance of high-energy density electrochemical supercapacitors (ECCs) and their application in HEVs, PHEVs, and HFCVs. Detailed test data are shown for the Skeleton Technology 5000 F carbon/carbon EDLC device and the Aowei 9000 F hybrid (4 V) supercapacitor (SC). The EDLC device had an energy density of 8.4 Wh/kg, and the hybrid SC had an energy density between 30 and 65, depending on its rated voltage and the power of the discharge. These energy densities are significantly higher than previous electrochemical capacitors tested. They indicate that progress is being made in increasing the energy density of commercial ECCs. Further progress is expected as the advanced electrode materials being studied in the laboratory, which have high specific capacitance (F/g), begin to be used in commercial products. It is important that the high-energy density capacitors have a high power capability, which will be the case if EDLC devices have an RC time constant less than 1 s and the hybrid devices have an RC time constant less than 3-5 s.  
Vehicle applications of the advanced EECs were evaluated based on Advisor simulations on city and highway driving cycles. Simulations were made for six vehicle types ranging from compact passenger cars to Class 8 long haul trucks. The fuel economy was calculated for each vehicle type using a lithium battery, the EDLC Skeleton Technology capacitor and the two Aowei hybrid capacitors as energy storage in the powertrain. The energy storage units were sized such that their round-trip efficiencies on the simulated driving cycles were close to 90%. For all vehicle types, the weight and volume of the capacitors decreased in the expected order; EDLC, hybrid 3.8 V, hybrid 4.1 V. The 4.1 V hybrid capacitor in all cases was lighter and smaller than the lithium battery. The fuel economies of the HEVs on the FUDS cycle were significantly higher (30-50%) than that of the corresponding ICE vehicle, except for the long haul truck, for which the fuel economy improvement was 20%. The effect of hybridizing on fuel economy on the highway cycles was small, at most 10%, but it was much less in most cases. The effect on the highway fuel economy of the long haul truck was 4-6%. In most cases, the fuel economy improvement was highest when using the 4.1 V hybrid capacitor. The estimated cost of the 4.1 V hybrid capacitor was USD 210/kWh. Hence, since the capacitor stores less energy than the battery by a factor of 5-6, its cost will be less than that of the battery.  
Simulations were also run for fuel cell-powered vehicles using the same lithium battery and capacitors for energy storage to assist the fuel cell. The fuel consumption of the fuel cell vehicles is given in terms of equivalent gasoline consumption, which translates to (1/mi/gal) kgH2/mi. For the fuel cell vehicles, the fuel economies using the three energy storage technologies varied only slightly. For all the fuel cell vehicles simulated, the 4.1 V hybrid capacitor was the lightest and smallest of the energy storage options, and produced the best fuel economy. As in the case of HEVs, the hybrid capacitors appeared to be the best option for energy storage in vehicle applications.  
Conceptualization, A.F.B. and J.Z.; methodology, A.F.B.; software, A.F.B. and J.Z.; validation, A.F.B.; data curation, A.F.B. and J.Z.; writing--original draft preparation, A.F.B.; writing--review and editing, A.F.B. and J.Z.; project administration, A.F.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
The authors declare no conflict of interest.