(Battery Division Technology Award) Lead-Carbon Ultracapacitors: How, Why and Where is the Technology

Energy is both enabling and pervasive. Of all forms of the energy, electrical energy is the most convenient and cleanest to use. The excess electricity generated by any power plant needs to be stored to meet the peak-power demand. In recent times, multifunctionality is driving the change in the energy storage land-scape. The parameters required to characterize the energy-storage devices are: their energy density that emphasizes the weight and the size of the device, their power density that is the delivery rate at which the energy needs to be made available, their life for which the energy-storage devices will be used to store and withdraw energy, their response time, their safety, and last but not the least is the all important factor of their cost. The objectives of storage could be many. These could be to level energy supply that would involve large amounts of energy. Technologies suitable for such applications are classified as energy-management technologies. There could be a need for electrical energy of constant voltage and frequency. Another kind of demand is for a large surge of power for a short duration, for example, in the starting of a car. Technologies for these applications are classified as quality-power technologies. Broadly speaking, energy-management technologies are well established and are industrially in use. Quality-power technologies are relatively new and many are still under development. It is noteworthy that a mix of storage devices with different characteristics may prove to be more efficient than a single one. For the purpose of a broad description, the technologies are classified into two categories, namely the conventional and newer technologies. The well-established conventional technologies are compressed-air energy storage, pumped hydroelectric-energy-storage, and fly wheel. These technologies can be thought of as physical storage since energy is stored as potential, kinetic or pressure-volume. These technologies are, by and large, the energy management technologies. These are primarily being practiced in industrially-developed countries. The effectiveness of these technologies is well known to be recounted here. The newer technologies, such as storage batteries, flow batteries, ultracapacitors and superconducting magnetic-energy storage are meant to provide quality power. It is the storage batteries, flow batteries and ultracapacitors among the newer technologies that are chemical in nature and are particularly seminal to renewable energy so much so that the experts regard them as the missing link in the success of renewable energy. In recent years, we have developed and tested substrate-integrated lead-carbon ultracapacitors. The lead-carbon ultracapacitors have attractive features for energy storage and delivery. It is noteworthy that the problems faced with lead-acid batteries in various applications are mostly related to the negative plates as the negative plates in the lead-acid batteries cannot accept high charging-currents and batteries operating at partial-state-of-charge suffer from rapid sulfation of the negative plates. The most logical approach to mitigate the aforesaid problem lies in looking for alternatives to the conventional lead plates or to reduce their contribution to battery operation. We have expended our efforts to develop Lead-Carbon hybrid ultracapacitors with substrate-integrated positive plates and graphitic carbon as negatives so as to mitigate the problems faced with Lead-Acid batteries. These capacitors exhibit long cycle-life and, although the energy density of these ultracacpacitors remains limited to ca. 2 Wh/kg, they exhibit power-density values as high as 2.5 kW/kg at a response time of about 2 s. Various technical features of these lead-carbon ultracapacitors will be highlighted along with some of their applications.