Gas Composition Analysis on Ni-YSZ Anode in Direct Internal Reforming Solid Oxide Fuel Cell

Solid oxide fuel cell (SOFC) is one of the most promising power generation devices. In Japan, the residential SOFC cogeneration systems have been commercialized since 2011. In this system, the supplied methane-rich fuel is reformed in an external reactor and the pre-reformed gas (hydrogen-rich fuel) is introduced to the anode chamber. In contrast, high operating temperature of SOFCs provides the possibility of a direct internal reforming operation, in which the anode material plays a role in the promotion of fuel reforming and fuel oxidation. Because the supplied fuel is reformed over anode catalyst, the concentration distribution of reactants and products appears along the direction of gas flow. Moreover, under discharge operation, the exothermic reaction of electrochemical oxidation as well as the endothermic reaction in the fuel reforming proceed simultaneously at the anode. In this case, the temperature distribution is more complicated than that in a supply of hydrogen fuel. Up to now, there are few reports on the direct evaluation of gas composition and temperature distributions over the anode although the theoretical calculation have been conducted.

This study, therefore, aimed to investigate the methane steam reforming and electrochemical oxidation over Ni–YSZ anode in an anode-supported cell. The distribution of gas composition over the anode surface along the direction of gas flow was examined by gas chromatograph under the open circuit and discharge conditions in a direct internal reforming SOFC.

The effect of total flow rate on the reaction process was studied under the open circuit condition. Regardless of total flow rate, the methane conversion remarkably increased at the upstream part. The extent of conversion enhancement was more significant at slower flow rates. In contrast, the correlation between the obtained methane conversion and the gas-catalyst contacting time agreed well in all flow rate conditions. The temperatures of anode surface at all measurement positions were lower than that under supplying only hydrogen due to the endothermic reaction in methane reforming process. The temperature variation against the direction of gas flow corresponded to the methane conversion. The surface temperature decreased at the upstream parts and then gradually rose toward the downstream parts.

During discharge, the enhancement of methane conversion and the decrement of CO concentration were accomplished with an increase in current density. This result suggests that the steam reforming reaction and water gas shift reaction were promoted by the consumption of hydrogen and the formation of steam in the electrochemical hydrogen oxidation.