6. Summary

This paper has presented a brief overview of supercapacitors and a short review of recent developments. The structure and characteristics of these power systems has been described, while research in the physical implementation and the quantitative modeling of supercapacitors has been surveyed.

A hierarchy was presented of the physical implementations now under investigation. It was discussed how these supercapacitor implementations can be grouped into three distinct classes, according to their charge storage mechanism. The classes are: electrochemical double-layer capacitors, pseudocapacitors, and hybrid capacitors. In addition, it was discussed that each one of these classes has a number of subclasses, differentiated by electrode material. This hierarchy of operational principles and composition provides a very wide range of possible design and performance characteristics. These flexible characteristics can be adjusted to optimize supercapacitor power systems for a wide range of specific applications.

This wide range of possibilities and the expense of fabrication make quantitative modeling a critical step in supercapacitor optimization. Thus, the different approaches to the quantitative modeling and analysis of supercapacitor systems also were surveyed above, and the three main modeling approaches were identified and described:

equivalent circuit models, empirical relationships, and theoretical limits.

Finally, a prospectus for the future of supercapacitor R&D was presented. Based on research trends, the authors believe that efforts towards improving hybrid capacitors, reducing equivalent series resistance, optimizing electrolytes, and ameliorating self- discharge constitute the future of supercapacitor R&D.

7. Conclusions

Based upon the review of the literature described above, it seems unlikely that supercapacitors will replace batteries as the general solution for power storage. This is primarily because presently envisioned supercapacitor systems do not store as much energy as batteries.

Because of their flexibility, however, supercapacitors can be adapted to serve in roles for which electrochemical batteries are not as well suited. Also, supercapacitors have some intrinsic characteristics that make them ideally suited to specialized roles and applications that complement the strengths of batteries. In particular, supercapacitors have great potential for applications that require a combination of high power, short charging time, high cycling stability, and long shelf life.

Thus, supercapacitors may emerge as the solution for many application-specific power systems. Especially, there has been great interest in developing supercapacitors for electric vehicle hybrid power systems, pulse power applications, as well as back-up and emergency power supplies [1-3, 5].

Despite the advantages of supercapacitors in these niche areas, their production and implementation has been limited to date. There are a number of possible explanations for this lack of market penetration, including high cost, packaging problems, and self-discharge. Recent research suggests that at least some of these issues might be surmounted [2-3].

For all of these reasons, as the products of R&D efforts continue to mature, supercapacitors may become a realistic, widely available power solution for an increasing number of applications. It is hoped that this survey may further stimulate the R&D required for this outcome, as well as serve as a point of departure for developing future applications.

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