Fuel Composition and Diluent Effect on Gas Transport and Performance of Anode-Supported SOFCs

Abstract

Anode-supported solid oxide fuel cells ͑SOFCs͒ with Niϩyttria-stabilized zirconia ͑YSZ͒ anode, YSZ-samaria-doped ceria ͑SDC͒ bilayer electrolyte, and Sr-doped LaCoO 3 ͑LSC͒ϩSDC cathode were fabricated. Fuel used consisted of H 2 diluted with He, N 2 , H 2 O, or CO 2 , mixtures of H 2 and CO, and mixtures of CO and CO 2 . Cell performance was measured at 800°C with the above-mentioned fuel gas mixtures and air as oxidant. For a given concentration of the diluent, cell performance was higher with He as the diluent than with N 2 as the diluent. Mass transport through porous Ni-YSZ anode for H 2 -H 2 O, CO-CO 2 binary systems, and H 2 -H 2 O-diluent gas ternary systems was analyzed using multicomponent gas diffusion theory. At high concentrations of diluent, the maximum achievable current density was limited by the anodic concentration polarization. From this measured limiting current density, the corresponding effective gas diffusivity was estimated. Highest effective diffusivity was estimated for fuel gas mixtures containing H 2 -H 2 O-He mixtures ͑ϳ0.55 cm 2 /s͒, and the lowest for CO-CO 2 mixtures ͑ϳ0.07 cm 2 /s͒. The lowest performance was observed with CO-CO 2 mixture as a fuel, which in part was attributed to the lowest effective diffusivity of the fuels tested and higher activation polarization. Recent work has demonstrated that anode-supported solid oxide fuel cells ͑SOFCs͒ exhibit high performance at intermediate temperatures. Maximum power densities as high as 1.8-1.9 W/cm 2 have been reported at 800°C for anode-supported single cells. [1][2][3][4] In a typical anode-supported SOFC, the anode support is a Ni-yttriastabilized zirconia ͑YSZ͒ cermet between ϳ0.5 and 2 mm thick. The electrolyte is a thin ͑ϳ10 m͒, dense YSZ film supported on a porous anode substrate. The cathode is usually a porous mixture of strontium-doped manganite, La 1Ϫx Sr x MnO 3Ϫ␦ ͑LSM͒, and YSZ, [1][2][3] or a porous mixture of strontium-doped cobaltite, La 1Ϫx Sr x CoO 3Ϫ␦ ͑LSC͒, and Sm-doped CeO 2 ͑SDC͒.4 In addition, single-phase, mixed ionic-electronic conductors ͑MIEC͒ materials have also been used in high-performance fuel cells. The use of thin electrolyte film results in a relatively low ohmic contribution to the total cell resistance, which makes it possible to operate anode-supported SOFCs at 800°C or lower and thereby realize the benefits of a lower temperature operation, such as the use of inexpensive metallic interconnect. One of the potential benefits of SOFCs over low-temperature fuel cells, such as proton exchange membranes ͑PEMs͒ is fuel flexibility, because SOFCs can potentially operate on various fuels including hydrogen, carbon monoxide, methane, and other hydrocarbon fuels without the problem of CO poisoning. Recent work has also shown that it may be possible to operate SOFCs directly on a number of hydrocarbon fuels, without the necessity of reforming.5-8 However, in anode-supported SOFCs, significant losses may occur due to the resistance to the transport of fuel gas through a relatively thick anode, and especial...

Comparison of the performance of Cu–CeO2–YSZ and Ni–YSZ composite SOFC anodes with H2, CO, and syngas

“…This result is consistent with that reported previously by Jiang and Virkar [4] who also observed poor performance for cells with Ni-YSZ cermet anodes when running on CO.…”

“…Since the diffusivity of a gas is only a weak function of temperature, this result further demonstrates that the change in slope in the V-I curve observed at 973 K is not due to diffusional limitations. The results obtained at 1073 K are in agreement with those reported previously for Jiang and Virkar who also found that other than lowering the OCV, the addition of water to H 2 fuel had little effect on the kinetics of the anode reaction for an SOFC with a Ni-YSZ anode [4]. Both of these studies, therefore, demonstrate that for operation on H 2 the addition of water has a relatively small effect on the kinetics of the oxidation reaction on the anode.…”

“…Since the V-I curves for these two fuel compositions are parallel to each other, this difference appears to be due primarily to a reduction in the OCV rather than inhibition of the rate of reaction on the anode due to the presence of CO 2 . It is useful to compare this result to that reported previously by Jiang & Virkar who also studied how dilution of CO in CO 2 affects the performance of an SOFC with a Ni-YSZ anode [4].…”

“…For operation on humidified H 2 , syngas, and CO the maximum power densities were 310, 360, and 370 mW cm -2 , respectively at 973 K. Thus, cell 3 exhibited higher performance when operating on CO compared to H 2 . It is noteworthy that the performance of the cell with the Cu-Co-CeO 2 -YSZ anode, even with a relatively thick electrolyte and operating at lower temperatures, exhibits a performance in CO that rivals that of some of the best Ni-YSZ cells operating on H 2 at 1073 K reported in the literature [4].…”

“…It is, therefore, important to understand how fuel composition and the presence of diluents such as N 2 and reaction products such as CO 2 and H 2 O affect SOFC performance. While most studies of SOFCs in the literature report performance characteristics for operation on humidified H 2 , there have been some studies that have looked at the effect of fuel composition [1][2][3][4]. For example, it has been reported that for SOFCs with conventional Ni-YSZ cermet anodes, the rate of the electrochemical oxidation of H 2 is 2-3 times greater than that of CO [1,2].…”