Structural and chemical degradation mechanisms of pure YSZ and its components ZrO₂and Y₂O₃in carbon-rich fuel gases

Abstract

Structural and chemical degradation mechanisms of metal-free yttria stabilized zirconia (YSZ-8, 8 mol% Y2O3 in ZrO2) in comparison to its pure oxidic components ZrO2 and Y2O3 have been studied in carbon-rich fuel gases with respect to coking/graphitization and (oxy)carbide formation. By combining operando electrochemical impedance spectroscopy (EIS), operando Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS), the removal and suppression of CH4- and CO-induced carbon deposits and of those generated in more realistic fuel gas mixtures (syngas, mixtures of CH4 or CO with CO2 and H2O) was examined under SOFC-relevant conditions up to 1273 K and ambient pressures. Surface-near carbidization is a major problem already on the "isolated" (i.e. Nickel-free) cermet components, leading to irreversible changes of the conduction properties. Graphitic carbon deposition takes place already on the "isolated" oxides under sufficiently fuel-rich conditions, most pronounced in the pure gases CH4 and CO, but also significantly in fuel gas mixtures containing H2O and CO2. For YSZ, a comparative quantification of the total amount of deposited carbon in all gases and mixtures is provided and thus yields favorable and detrimental experimental approaches to suppress the carbon formation. In addition, the effectivity and reversibility of removal of the coke/graphite layers was comparably studied in the pure oxidants O2, CO2 and H2O and their effective contribution upon addition to the pure fuel gases CO and CH4 verified.

Surface reactivity and cation non-stoichiometry in BaZr1−xYxO3−δ (x = 0–0.2) exposed to CO2 at elevated temperature

“…9 Nevertheless, a recent study by Duan et al showed excellent coking resistance and sulfur tolerance of BaZrO 3 -based PCFCs. 24 Structural and chemical degradation has been reported even in yttria stabilized zirconia (YSZ) in carbon-rich fuel gases such as CH 4 , CO or syngas, inducing coking/ graphitization and (oxy)carbide formation [25][26][27][28] that led to irreversible changes in the transport properties. 27,28 Thus, a thorough understanding of potential degradation processes at the surface of BZY materials is highly desirable, and could lead to valuable insights into the surface chemistry and structure of this important proton-conducting electrolyte.…”

CO2 Reduction on the Pre‐reduced Mixed Ionic–Electronic Conducting Perovskites La0.6Sr‐0.4FeO3‐δ and SrTi0.7Fe0.3O3‐δ

“…Since no literature data on strontium carbide can be found, no comparison can be drawn for the Sr 3d component of this species. It does, however, follow the trend exhibited by other metals, where the carbide peaks are found at significantly lower binding energies than those of the oxides, with the 3d 5/2 component of the carbide species being located at 130.6 eV as compared to the 132.1 eV of the bulk oxide. The carbon deposition is also corroborated by the Sr/C ratio: for the oxidized sample, it is 1.8 (owing to adventitious carbon), whereas it drops to 0.3 for the specimen after CO treatment.…”

Increasing Complexity Approach to the Fundamental Surface and Interface Chemistry on SOFC Anode Materials

“…Essentially important for the fundamental understanding of carbon-fuel−catalyst interaction at high temperatures, the carbon reactivity is exemplified by the interaction of methane with clean Y 2 O 3 and CO 2 with m-/t-ZrO 2 . 2,3,11,18 Figure 4A shows, via correlated in situ FT-IR and EIS measurements, the distinct change of the surface at 1023 K by forming carbon deposits on the pure oxide surface (a common feature for all studied oxides), as indicated by both a drastic increase in conductivity by forming percolated carbon islands and an associated total decrease in IR transmission. Structurally, the carbon deposits are graphitic in nature, encapsulating grains and forming rosette-like morphologies (Figure 4B).…”

“… Carbon reactivity on ZrO 2 and Y 2 O 3 : (A) Correlated in situ FT-IR and EIS data on Y 2 O 3 during heating in methane up to 1143 K. (B) TEM image of a carbon-covered Y 2 O 3 grain after exposure to methane at 1073 K. (C) Nyquist plot obtained during isothermal treatment of Y 2 O 3 in methane at 973 K. (D) Deconvoluted Zr 3d XP spectrum after treatment of t-ZrO 2 in CO 2 at 1273 K. (E) Quantification of the surface species on t-ZrO 2 after various treatments. Reproduced from ref ( 18 ) (A,C), ref ( 3 ) (B), and ref ( 19 ) (D, E). Copyright 2016 the Royal Chemical Society, 2016 American Chemical Society, and 2017 the Royal Chemical Society, respectively.…”