Abstract Detail

Abstract

In this work we prepared and studied activated carbon materials known as promising materials for electrochemical applications. We aim to use them in energy production and storage systems such as supercapacitors and fuel cells. We prepared our samples by using the sol-gel method to obtain Xerogels from optimized Pyrogallol/Formaldehyde mixtures in presence of HClO4 acid used here as catalysts of the condensation/gelation process. We doped our samples by various metals (K, Ca and Ni). The final product of activated carbon is obtained in the second step of the process by heat treatment of the Xerogels in nitrogen atmosphere at 600°C. Structural and morphological characterization were conducted using X-ray diffraction, Infrared absorption and specific area BET determination. We determined the Ac conductivity from impedance measurements conducted on pellets in the temperature range between 20°C and 200°C and the frequency range between 0.1 Hz and 1MHz. The Ac conductivity is characterized by a frequency independent plateau at low frequencies representing the Dc conductivity (sdc) followed by a power law behavior rising from a critical frequency w0 which depends on temperature. The exponent n of the power law depends on temperature and doesn’t lie between 0 and 1 but exceeds 1 for some temperatures. This behavior is characteristic of charge transport by hopping processes in disordered materials. The Dc conductivity (sdc) and w0 are both thermally activated with comparable activation energies Ea. By using the modulus formalism, we detect a relaxation peak in the imaginary part of dielectric modulus with a relaxation frequency fmax thermally activated with an activation energy Ea comparable to that of sdc and w0. This relaxation process is attributed to hopping processes of charge carriers.

Keywords: Sol-Gel, Polymers, Activated carbon, Ac Conductivity, Charge transport, dielectric relaxation

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