Contrasting Intermediate-Temperature La0.7Sr0.3VO3.86-⸹ (LSV) Porous Catalyst Elevated Sulfur Tolerance Using Gaseous and Liquid Fuel Explored through Experimental and Modeling Characterization Over Time
Authors: Burye Theodore E. , Sebastian Talia 
Published: September 2023

The U.S. Army is investigating Solid Oxide Fuel Cells (SOFCs), which can produce clean, secure, and sustainable energy to replace existing power sources. Today SOFCs are known to provide high density, high efficiency power with long term stability at low cost and environmental impact, utilizing a variety of naturally sourced fuels such as hydrogen, hydrocarbons, and alcohols. This fuel flexibility makes SOFCs attractive for use with multiple applications and helps bridge the gap between current hydrogen-fueled SOFC technologies and future hydrocarbon-fueled based use. Despite this potential, many technical hurdles remain, such as thermal stress cracking and coarsening, delamination, and catalyst contamination. Catalyst contamination is of particular concern for the U.S. Army, which operates on JP-8 kerosene-based fuel via AR-70 single fuel policy, as sulfur impurities commonly found in naturally sourced hydrocarbon-based fuels, can poison the catalyst, permanently degrading SOFC system performance, even at low concentrations. As the U.S. Army begins to investigate SOFCs to enable additional capabilities like silent watch, advanced radios, and exportable power in their fleet, catalyst poisoning from sulfur presents a concerning technology gap that must be addressed, especially since it is also possible that sulfur contaminated fuel, scavenged in theater, may be used, and could contain elevated levels (>300ppm) of sulfur.

While lower temperature SOFC operation is also actively being pursued to help alleviate physical material degradation issues, with the introduction of hydrocarbon-based fuels, catalyst development to reduce sulfur poisoning degradation also needs further exploration. This study continues to experimentally and theoretically investigate the response of sulfur tolerant anode catalyst La0.7Sr0.3VO3.86-⸹ (LSV) at currently targeted SOFC intermediate operating temperatures (400-600°C). We have previously investigated low and moderate hydrogen sulfide (H2S) concentrations (30ppm, 300ppm) in balance hydrogen and methane gas environments for up to 100 hours. In this work, LSV showed significant sulfur tolerance in hydrogen and methane balance gases when compared to the industry standard Ni-YSZ anode, where sulfur adsorption rates were 278-287x lower in some cases. The lowest rates occurred between 600-700°C, which is attributed to the cubic structure the material presents in this temperature range. Higher adsorption rates were observed in the monoclinic/tetragonal structure the material presents at temperatures between 400-500°C. Average adsorption rates were overall higher in methane. When compared against Ni-YSZ, which is known to accumulate sulfur from hydrogen sulfide via two-step dissociative adsorption, LSV was observed to accumulate sulfur through weak chemisorption (H2S*, 0.43 eV max) of molecular hydrogen sulfide on oxygen deficient surfaces and one-step dissociative adsorption (H2S*, 0.34 eV, max, HS*+H*, 8.34 eV, max) on oxygen sufficient surfaces. The weak chemisorption behavior on oxygen deficient surfaces is attributed to the reduced oxygen stoichiometry of the material, in which only vanadium V2+/3+ couples exist. Overall, molecular adsorption/dissociation was observed to occur near strontium defects on LaO/SrO terminations. The propensity of H2S adsorption on LSV surfaces was observed to be significantly less for oxygen deficient LSV, and significantly higher for oxygen sufficient LSV, when compared to H2S adsorption calculations on Ni (100) surfaces, which follow a two-step dissociative adsorption reaction (H2S* → HS*+H* → H*+S*), resulting in a strongly adsorbed S* species (5.96 eV, max).

Under the Army’s single fuel policy, it is more likely SOFCs would be fueled using hydrogen and/or methane, which are both gases, however SOFCs in the future could also be directly fueled using liquid alcohol fuels, or as scavenged from theater. Typical small alcohol molecules (methanol or ethanol) do not contain high concentrations of sulfur; however, alcohols can be contaminated with sulfur-containing hydrocarbons. This work will investigate sulfur adsorption using methanol contaminated with 300ppm thiophene using the same operating temperature and heating duration conditions with hydrogen and methane. These results will be contrasted against the 300ppm H2S hydrogen and methane results previously reported.

Figure 1