Selective and preferential deposition of Cr on segregated SrO on the surface of LSCF cathodes of solid oxide fuel cells.

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The relationship between the surface segregation, boron poisoning and surface exchange coefficients of La0.6Sr0.4Co0.2Fe0.8O3−δ, (LSCF) cathodes of solid oxide fuel cells (SOFCs) is studied on dense bar samples using SEM, SIMS and conductivity relaxation method. The SEM results clearly indicate that the segregation on the LSCF surface occurs after heat-treatment at 700–800°C for 48 h, forming isolated particles on the LSCF surface. The presence of volatile boron species accelerates grain growth of the segregated particles and reacts with LSCF. The depth of boron reaction layer after heat-treatment in the presence of E-glass at 700°C, 750°C and 800°C for 48 h was 2, 5 and 40 nm, respectively. The depth profiles analysis of SIMS indicates that there is segregation and enrichment of constituent elements of LSCF on the electrode surface, in line with the depth profile of boron species on the LSCF sample surface layer. Boron deposition and poisoning deteriorates the surface exchange and diffusion processes for the oxygen reduction reaction on LSCF. After exposed to boron at 800°C for 48 h, the surface exchange coefficient, Kchem is 6.0 × 10−5 cm s−1, more than one magnitude lower than 1.1 × 10−3 cm s−1 of as-prepared LSCF samples.

Quantitative characterization of uniform density, electrochemically inert films on electrodes is achieved by cyclic voltammetry (CV) of a redox probe that partitions from electrolyte into the film. Electrochemically inert films generate no faradaic current in the voltammetric window of the probe. In simulation models, probes pre-equilibrate into films, electrolyze at electrodes, diffuse in film and solution, and extract across film solution interfaces. Film thickness is ℓ. Diffusion length δ approximates distance from the electrode where voltammetry perturbs probe concentration; δ∝ν− 1/2 for scan rate ν. At high ν, δ < ℓ and voltammetric morphologies are typical of semi-infinite linear diffusion. As ν slows, δ ≳ ℓ and CV morphologies can change with relative probe flux in the film and solution. For higher solution flux, voltammograms assume sigmoidal (S-shaped) characteristics; higher film flux generates gaussian (thin layer CV) characteristics. For film and solution diffusion coefficients Df and Ds and κ the equilibrium ratio of probe concentration in film to solution, diagnostics yield κ√(Df/Ds) and ℓ2/Df. Because diagnostics apply for all ν, films are fully parameterized by CV alone. Without these diagnostics, full characterization requires a second, steady state voltammetric measurement. Diagnostics are vetted with [Ru(bpy)3]2 + (probe) in inert polymer films of Nafion and of poly(styrenesulfonate).

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