Past, Present and Future of Electrochemical Capacitors: Pseudocapacitance, Aging Mechanisms and Service Life Estimation

The capacitance values printed on capacitor housings do not reflect the actual capacitance available for any technical application. The complex properties of the pseudocapacitance and the aging behavior of practical supercapacitors are reviewed with regard to the state-of-the-art and the latest findings. The long-term stability of various electrolyte systems and carbon materials is critically examined under phenomenological aspects that reflect the typical fault pattern of supercapacitors. Electrolyte resistance and voltammetric capacitance are reliable aging indicators. High temperatures have a greater impact on service life than high voltages, and overvoltages are worse than high currents. The anode more than the cathode suffers from a loss of pore volume, increase of nitrogen and fluorine compounds, and the unstable adhesive layer between active carbon and aluminum. Unwanted material changes and chemical reactions in the supercapacitor reflect the influence of water over several years in the life test. Credible reaction pathways are identified for the known aging processes. Fundamentals and materials are reviewed in a separate part.**