Octane Steam Reforming and Electrochemical Reaction on Cu-GDC Cermet Anode of Internal Reforming Solid Oxide Fuel Cell

Abstract

A small and high-performance solid oxide fuel cell (SOFC) system is promising as a power source for many potential mobile robot applications. To realize the small SOFC system, the direct use of liquid hydrocarbon fuel, which has high energy density and is easy to storage and carry, is desirable. The optimization of type of fuel, operating conditions, and a fuel electrode (anode electrode) is essential. In the view point of the use at high temperatures in summer outdoor, octane having high boiling temperature is a good candidate as the fuel. Steam reforming is the first choice for the internal reforming method because it has been frequently-studied for the direct use of methane fuel. A porous cermet of Ni and yttria-stabilized zirconia (YSZ), which has widely been studied as an anode electrode for the internal reforming operation SOFC, is one of promising anode electrode materials, however, performance degradation due to sulfur poisoning and carbon deposition on the anode electrodes surface is concerned. A porous cermet of Cu and gadolinium oxide-doped ceria (Cu-GDC), which, respectively, have excellent sulfur tolerance and oxygen ion supply property, has high potential for the anode electrode of SOFC operated by internal reforming of methane. Therefore, in this study, the Cu-GDC cermet was focused as an alternative anode electrode for the octane-direct use SOFC. The aim of this study is to clarify the effects of operating conditions on the reforming reaction and electrochemical reaction on the Cu-GDC cermet anode electrode. In this study, the fuel-cracking reaction, which occurs at elevated temperature in the presence of excess steam, was estimated. In order to understand the effect of difference in branching of carbon chain, n-octane (linear alkane) and iso-octane (branched chain alkane) were used. Octane and steam were supplied to the SOFC cell by bubbling carrier gas (He or Ar) 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 a 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 700-850 oC. I-V and I-P characteristics, and stable power generation property by constant current were investigated to demonstrate the octane direct internal reforming operation of SOFC. The effects of S/C ratio and cell temperature on the open circuit voltage (OCV) and I-V and I-P characteristics were investigated. In order to identify the chemicals generated by the fuel cracking reaction, outlet gas compositions from the cell of open circuit condition and power generation condition were analyzed by gas chromatography. The following are the main findings that we have revealed in the present study. By raising the S/C ratio and/or the cell temperature, the reforming reaction is facilitated, but fluctuation of the OCV is increased. Chemical species and their ratio generated by the cracking reaction are changed by the difference in the carbon-chain of octane. Mainly H2, CO, CO2, CH4, and C2H6 are generated in the reforming reaction of n-octane, and C3H8, CH3OH, and C4H10 are also generated in addition to those in the reforming reaction of iso-octane. However, the main electrode reaction to generate electrical power is only the electrochemical oxidation of H2 and/or CO. By using the n-octane fuel, the carbon deposition is reduced. The rate-determining process is dissociation to H and CO.