Thermo Scientific Nexsa XPS System
Characterized by their high efficiency, low emissions and long-term stability, solid oxide fuel cells are electrochemical devices that are capable of converting a fuel directly into electricity. The fuel cell device evaluated in this study consists of both porous cathodic and anodic layers with a dense, solid oxide electrolyte between them. In this device, the cathodic layer reductively ionizes atmospheric oxygen and transports it through to the electrolyte to meet the fuel. The overall efficiency of the device is directly related to the cathode’s ability to promote the Oxygen Reduction Reaction.
The strontium-substituted lanthanum cobaltites are one type of materials that are of particular interest, as these perovskite materials can affect the catalytic activity as a result of its elemental composition and chemistry of the surface of the material. The chemical specificity of the Nexsa XPS System is an ideal instrument to analyze solid oxide fuel cells.
The XPS was used to study a candidate cathode material at two time points of before and after annealing in the air at high temperatures through the simulation of the thermal cycling of a real solid oxide fuel cell device. A lanthanum strontium cobaltite (LSC) layer, which was deposited onto an yttrium-stabilized zirconia (YSZ) substrate that had a gadolinium-doped ceria barrier layer between them, was analyzed before and after annealing at high temperature. The LSC layer acts as a cathode, whereas the YSZ is the dense electrolyte. The top surface of the LSC was non-destructively analyzed with XPS to obtain chemical and elemental information of the surface of cathode layer. The rate at which oxygen is absorbed from the air and converted into ions is dependent upon the chemistry and composition of the outermost surface of the LSC.
Figure 1. Schematic of operation of solid oxide fuel cell
Figure 2. Scanning Electron Microscope image of cross section of solid oxide fuel cell
A straightforward and non-destructive XPS analysis of the top surface of the LSC layer revealed significant changes in the lm composition that was caused by annealing. In both the as received and annealed lms, the cobalt concentration was determined to be significantly less than the expected value. Additionally, the ratio of lanthanum to strontium was not found to be optimal for both the received or annealed lms. The ratio changes between the as received and unannealed cases indicate that the lm is not stable under thermal cycling conditions. High-energy resolution XPS analysis of the carbon spectrum from the LSC top surface enable the precise identification of the carbon bonding, including C-C and C-O components that originate from adventitiously-deposited carbon, which is typically found on any sample that has been in the atmosphere for any substantial period of time. However, at higher binding energies, these components are attributed to inorganic carbonates. There is less carbonate on the annealed surface, therefore the difference in the width of the carbonate peak may indicate that the carbonate in the annealed sample forms on a more physically ordered surface.
Figure 3. Elemental quantification of surface composition of annealed and as received samples
The assignment of a carbonate component in the carbon spectrum is confirmed with the strontium high-resolution spectrum from the as received sample that shows two bonding states that can be assigned to strontium carbonate and strontium in the LSC lattice. The interaction between the orbital and spin angular momentum of ionized strontium results in two XPS peak components for each bonding state. Annealing in the air at high temperatures demonstrated a significant decrease in the amount of strontium carbonate as compared to the concentration of the lattice- bound strontium atoms. These carbonates can cause detrimental effects to the oxygen reduction reaction occurring at the top surface, as well as to the oxygen ion transport through the LSC layer. As a result of these effects, the overall performance of the solid oxide fuel cell can also be affected.
Figure 4. High resolution spectra of a) carbon and b) strontium
Figure 5. High resolution lanthanum spectra of the surface
The chemistry of lanthanum at the top of the LSC surface can be monitored using strong electron orbital-spin angular momentum interactions that cause a splitting of lanthanum XPS peaks. The magnitude of this splitting and the ratio of the split components are diagnostic of the chemical states that are present. Lanthanum oxide and carbonate, for example, have splittings that are different by 1 eV. On the as received surface, the splitting appeared to be similar to that which is expected for carbonate, whereas, after annealing, the splitting was shown to increase towards the value for lanthanum oxide, thereby indicating that the level of carbonate has dropped and does not show pure lanthanum oxide bonding. The data of this study was not consistent with a mixture of oxide and carbonate.
The non-destructive technique of angle resolved XPS analysis allows for the evaluation of the depth distribution of strontium carbonate in the top few nanometers of the LSC layer. The information depth of XPS can change upon collection of electrons from different photoemission angles. More specifically, while using photoemission angle normal to the sample surface the information depth, the presence of strontium is 0 to 6 nm into the surface. While using a shallow angle, a thinner layer of the surface, between 0 and 3 nm, is sampled. For this study, the relative proportions of strontium in the LSC lattice and carbonate states showed a significant change in which the carbonate was shown to be relatively stronger when sampling only the top 3 nm, thereby confirming that the carbonate is a surface species.
Figure 6. High resolution strontium spectra of the as received sample a) normal photoemission angle and b) shallow photoemission angle
Figure 7. a) Valence band spectra and b) High resolution cobalt spectra from as received and annealed LSC surfaces (0-6 nm)
The chemistry of the cobalt concentrations on the top surface of the LSC layer was investigated through looking at the core as well as the valence levels of cobalt atoms. A broad band between 0 and 6 eV was shown to be attributed to the hybridization of O2p and Co3d valence orbitals. The narrow band at 2.2 eV has previously been assigned to transition metals in a +3 oxidation state in a polyhedral arrangement with oxygen. Following annealing procedures, this band increased in intensity which demonstrated that the concentration of Co(III) at the LSC top surface increases with annealing. Similarly, valence band analysis also showed Co(III) on the LSC surface. However, high energy resolution XPS analysis indicated that Co(II) was also present at the surface. In LaCoO3, with no strontium atoms, it would be expected to only see Co(III), however the substitution of strontium into the lattice at the expense of lanthanum caused an increase in the number of oxygen vacancies and the resulting formation of the cobalt +2 oxidation state.
The ratio of the XPS satellite peaks that are diagnostic of Co(II) to the main peak is also a useful tool for the quantification of both the cobalt +2 and +3 oxidation states. The XPS core-level data showed that the amount of Co(III) in the LSC top surface increased following annealing in the air.
A lanthanum strontium cobaltite (LSC) layer was analyzed in this study both before and after annealing at high temperature to simulate the thermal cycling of a real solid oxide fuel cell device. It was determined that the amount of carbonate on the surface reduces during annealing, which can cause detrimental effects on the oxygen reduction reaction at the surface and the ability of oxygen ions to transport through the LSC layer. It was also found that the carbonates are located adjacent to 3 nm of the surface. The unique surface and chemical sensitivity of XPS makes this instrument an ideal tool for fuel cell analysis.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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