Cracking Reaction of Octane Fuel and Electrochemical Reaction on Ni-YSZ Cermet Anode of Internal Steam Reforming Solid Oxide Fuel Cell
The purpose of this study is to clarify the cracking reaction of octane fuel and the electrochemical reaction of the Ni-YSZ anode electrode under various operating conditions. In this study, the fuel-cracking reaction on the Ni-YSZ anode electrode is estimated by gas chromatography with thermal conductivity detector. Anode electrode performance is studied by electrochemical measurement. Two types of octane (n-octane; linear alkane, iso-octane; branched chain alkane) are used in order to elucidate the effect of difference in carbon-chain structure. Octane and steam of equilibrium vapor pressure were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Temperature of the octane bubbler and the carrier gas flow rate was 30 oC and 10 ml/min, respectively. The carrier gas flow rate of the water bubbler was 83 ml/min. Steam/carbon ratio (S/C) was controlled over an S/C ratio from 0.5 to 5.5 by changing the temperature of the water bubbler from 30 to 80 oC. The cell temperature range was 700-950 oC.
The octane internal steam reforming operation of SOFC was demonstrated by I-V and I-P characteristics and stable power generation property by constant current. The effects of operating conditions (S/C ratio and cell temperature) on the open circuit voltage (OCV) and power generation property were investigated. The chemicals generated by the cracking reaction of octane at various operating conditions were analyzed by gas chromatography and compared. The difference in outlet gas compositions from the cell of open circuit condition and power generation condition revealed the main species consumed for the power generation reaction on the anode electrode. The electrochemical reaction resistance of the Ni-YSZ anode was measured by ac impedance method and the rate-determining process was identified.
In this study the following was revealed. When the S/C and/or the operating temperature are high, the reforming reaction rate is large. By the reforming reaction of n-octane, H2, CO, CO2, and CH4 is mainly produced. When the iso-octane is used as the fuel, small amount of C2H6 is also generated in addition to them. Only H2 and CO contribute to the power generation directly. Rate-determining process of the electrode reaction is a cracking reaction to H2 and CO.