Electrochemical impedance spectroscopy of metal fiber/activated carbon-fiber composite materials for electrochemical capacitors

Composite materials of metal fibers and carbon fibers are capable of combining properties of high conductivity and high surface area in a manner previously unattainable by standard packed powder bed technology. This combination of properties is attractive for the possibility of producing electrochemical capacitors (ELCC) with high conductivity. Low conductivity in an ELCC is detrimental in applications such as power filtering due to resistive heat generation during cycling.; The frequency response of metal fiber/activated carbon fiber composite electrodes was studied to elucidate the mechanisms responsible for the impedance of composite electrodes. After much model discrimination, an equivalent circuit representing the composite metal fiber/activated carbon electrodes was determined which represents the impedance over the entire frequency range studied, 5 mhertz to 10{dollar}sp5{dollar} hertz.; The cost of activated carbon fibers (ACF), ca. {dollar}100/lb., is a limiting factor in the economization of ELCC, so less expensive materials were tested for use in ELCC electrodes. Wood pulps were tested for utilization of the surface area of residual carbon from the sintering process as double layer capacitor material.; Different ACFs have varying properties and some may be more suitable for ELCC application. Four different ACF were tested for use in ELCC electrodes, and the parameters in the equivalent circuit model for the composite metal fiber/activated carbon fiber electrodes were compared with time of sintering. Some carbon-metal contacts are lost during the sintering process, and can be regained by electrodeposition of nickel metal. The variations of the model circuit parameters with electrodeposition for the four ACFs is also presented.; The fundamental design parameter for composite metal fiber/activated carbon fiber electrodes is the metal loading. Length of metal fiber in the mesh is actually of most importance, so experiments were performed with equivalent loadings of different diameter metal fibers. Also, a program to simulate a random distribution of metal and activated carbon fibers in non-overlapping layers was written. The effect of different access lengths, of the activated carbon fibers for the metal-carbon fiber contacts, on the utilization of the activated carbon fibers was included in the simulation.