The electrochemical behavior of polyacrylonitrile-based activated carbon cloth combined with a stainless steel current collector was examined using ac impedance spectroscopy. H2SO4, KOH, and KNO3 were employed as the electrolytes. The data presented in the impedance complex plane exhibit a semicircle at high frequencies followed by a vertical line at low frequencies. The high frequency data were found to be characteristic of the space charge region of the semiconducting oxide layer on the stainless steel, while the low frequency data depicted the double layer formation on the porous carbon. The double layer capacitance was found to decrease with the space charge resistance, which was potential dependent and a major contribution to the overall resistance of the carbon/stainless steel electrode. The electrolyte type affected the potential window employed in energy storage and thus the semiconducting behavior of the oxide layer. Both the n- and p-type semiconductors in depletion condition appeared within the potential window applied for the H2SO4 electrolyte, and this caused the presence of a peak capacitance. Only the n-type depletion condition was found in the KNO3 and KOH electrolytes with the p-type oxide situated in accumulation at the potentials applied, and thus, the capacitance was larger at more negative potentials.
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