Analyzing the Composition of a Solid Oxide Fuel Cell During Thermal Cycling

High temperature solid oxide fuel cells (SOFCs) promise a technology to electrochemically generate electricity at high efficiencies. They provide several advantages over traditional energy conversion systems, such as high efficiency, reliability, modularity, fuel adaptability, and very low levels of NO and SO emissions.

Schematic of operation of solid oxide fuel cell

The fuel cell device consists of porous cathodic and anodic layers with a dense, solid oxide electrolyte between them; one of the materials of interest as fuel cell cathode layers is strontium-substituted lanthanum cobaltites, a type of perovskite material. With perovskites, the catalytic activity is affected by the elemental composition and chemistry of the surface.

Perovskites represent an entire class of materials that could be used to improve SOFCs, however, a rapid non-destructive and accurate method to determine the surface chemistry change during thermal cycling is imperative. By simulating thermal cycling by annealing the material, this application note uses X-ray photoelectron spectroscopy (XPS) to accurately and non-destructively analyze the effect of thermal cycling on the critical surface of a specific perovskite, namely lanthanum strontium cobaltite (LSC).

In this analysis, a lanthanum strontium cobaltite (LSC) layer was deposited onto an yttrium-stabilized zirconia (YSZ) substrate with a gadolinium-doped ceria barrier layer between them. The sample was then exposed to high temperature annealing in an ambient environment (air), and the top surface of the LSC was non-destructively analyzed using the Thermo Scientific™ Nexsa XPS system. The chemical specificity of XPS makes it the ideal technique for analyzing solid oxide fuel cells.

Significant changes in the chemistry of the top surface of the LSC occurred after annealing. Interestingly, the as-received (pre-anneal) samples of LSC contained significantly less cobalt than the expected value (20 atomic % versus the measured 6.38 At%). After annealing, the surface amount of oxygen decreased, while all other percentages increased.

Using angle resolved XPS analysis, a much thinner layer of the surface, between 0 and 3 nm, was sampled, and revealed that the relative proportions of strontium in the LSC lattice and carbonate states changed, with the carbonate relatively stronger when sampling only the top 3 nm which confirms that the carbonate is a surface species.

a) Valence band spectra and b) High resolution cobalt spectra from as received and annealed LSC surfaces (0-6 nm)

Cobalt chemistry at the top surface of the LSC layer was investigated using two methods, looking at the core and valence levels of cobalt atoms. The XPS core-level data showed that the amount of Co(III) in the LSC top surface increases after annealing in air.

Using the core capabilities of the XPS technique, both in terms of chemical specificity paired with XPS’s unique surface sensitivity, we performed a non-destructive analysis of a potential SOFC material’s critical surface for real-world applications.

Read the app note here.