Exploring the Stability of Direct Ethanol Solid Oxide Fuel Cells at Intermediate Temperature

Abstract

Direct ethanol SOFC have gained increasing attention in the last decade due to a series of attractive characteristic of such renewable fuel. Ethanol is possibly the most successful example of biofuel with well-developed industries in different continents. Countries with large production, such as Brazil and USA, have a wide distribution infrastructure that makes ethanol a readily available fuel. Ethanol SOFC has a great potential for several applications, including as a range extender for electric vehicles as recently demonstrated by Nissan. Nevertheless, to reduce the complexity and cost of fuel cell systems, several studies have aimed at internal reforming taking advantage of the SOFC operating temperature. Nonetheless, critical issues such as the stability of the anodes and thermal stress arising from the endothermic reforming reactions still impose great challenge for the internal reforming in SOFC. Previous studies have demonstrated direct ethanol SOFCs operating with great stability for more than 600 hours at 850 °C [1]. On the other hand, relatively few studies have investigated direct ethanol SOFCs at T< 800 °C. Intermediate operating temperatures facilitate material selection, inhibit thermally activated degradation processes, and allow a fast start of the system. However, critical issues such as the stability of the anode require careful development for stable operation at intermediate temperature (600 - 700 °C). Usually, ethanol steam reforming in this temperature range demands specific catalysts to avoid the formation of carbon deposits. Therefore, it is necessary to develop stable catalysts that are resistant to carbon formation at intermediate temperature to facilitate the commercialization of the direct ethanol SOFC technology. The aim of this study is to evaluate catalytic materials in the active layer deposited in commercial anode-supported SOFC at intermediate temperature (600 - 700 °C). Steam reforming reactions were combined with electrochemical single cell testing with different active layers containing ceria-based catalysts. The starting point was to evaluate the commercial fuel cells without an active layer under ethanol at intermediate temperature. The as-received fuel cell has reasonable stability under dry ethanol if enough water is produced by the electrochemical polarization of the fuel cell, as shown in Fig. 1. However, the Ni-based anode develops carbon deposits under operation on dry ethanol. The Ir/doped-ceria catalyst previously studied at 850 °C showing excellent durability was investigated at reduced temperature. The steam reforming results revealed that the studied catalyst deactivate at 600 °C. Increasing the measuring temperature to 700 °C revealed that the catalyst remains active, but displays small carbon deposits after the catalytic reaction, as inferred by Raman spectroscopy and thermal gravimetry analyses. The main results have shown that the catalytic layer has no significant effect on the performance of the fuel cell under hydrogen, as long the microstructural properties of the catalytic layer, such as porosity and good adhesion to the anode, are ensured. The use of a ceria-based catalytic layer has enhanced the stability of the fuel cell under dry ethanol, but small carbon deposits were still detected after ~150 hours of continuous operation.[1] Steil, M.C.; Nobrega, S.D.; Georges, S.; Gelin, P.; Uhlenbruck, S.; Fonseca, F.C. Durable direct ethanol anode-supported solid oxide fuel cell. Applied Energy, 199, 2017, p. 180-186. Figure 1