Oxidation of carbon deposits on anode material Ni–YSZ in solid oxide fuel cells

Abstract

Solid oxide fuel cells (SOFCs) are electrochemical devices that convert the chemical energy of fuels directly into electricity with high conversion efficiency. A major advantage of SOFC systems compared to low-temperature fuel cells is that SOFCs can use humidified hydrocarbons as fuel. However, the use of hydrocarbons may lead to an undesirable carbon deposition as hydrocarbons are oxidized at the anode side in SOFCs. Such carbon deposits on the nickel–yttrium stabilized zirconia (Ni–YSZ) anode material are the most probable mode of deactivation of SOFCs. Therefore, commercially available NiO–YSZ anode material for high-temperature fuel cells was activated by a temperature-programmed reduction in a hydrogen atmosphere. In the next step, carbon was deliberately deposited on the reduced samples by isothermal deposition in a methane–argon atmosphere. The carbon deposits were then burned off with a temperature-programmed oxidation (TPO) in an oxygen–argon atmosphere at different heating rates using thermo-analytical equipment. The TPO was followed by TG, DTG, DTA and QMS measurements. The easiest way to distinguish among various types of deposited carbon was to follow the QMS curves. The obtained QMS curves were processed with Netzsch Peak Separation software in order to extract several peaks corresponding to various forms of carbon deposits. It was found that high amorphous and low amorphous carbon are burned off at relatively low temperatures (up to 650 °C), fibrous carbon is oxidized up to 750 °C, graphite is burned out up to 830 °C and finally, carbon diluted in nickel leaves the system in the 900–1000 °C range, depending on the oxidizing conditions. The isoconversional method was used to calculate activation energies of various carbon oxidation processes. The absolute values of the activation energies increase from amorphous carbon via fibrous carbon to graphite.