Past, present and future of electrochemical capacitors: Technologies, performance and applications

Over the past decades, many research efforts are focused on electrochemical capacitors (ECs) along with materials utilized in them. The two generalstrategies for improving the specific energy (Wh/kg) or energy density (Wh/L) are to increase the cell voltage of the device and to increase the specific capacitance (F/g) of its electrodes. Herein, we provide an overview of the electrochemical performance of various electrode active materials includingnano-structured carbon materials, metal oxides, and conducting polymers. The focus of the paper is to evaluate from available research results the potential of the advanced electrode materials and ionic liquid electrolytes to improve EC performance in the case of complete cells. This is not a simple task because the research data are given in terms of F/g and A/g based on active weight of the electrode or cell. A method is included in the paper that uses the research data to determine inputs to a calculation of complete cell performance in terms of energy and pulsed power capability based on cell voltage and resistance. Considerable test data for commercially available hybrid ECs are included in the paper to assist in evaluating their present performance and future potential. The history of the development of modern ECs starting in about 1990 is also reviewed. This paper is a companion paper to the paper "Past, Present, and Future of Electrochemical Capacitors: Pseudocapacitance, Aging Mechanisms and service Life estimation", P. Kurzweil, J, Schottenhauer, and C, Schell. That paper treats in detail the science of electrochemical capacitors and this paper reviews the technology and development in recent years.