Fuel and Operation Dependence of Interfacial Nanostructure from the Anode and Cathode of Solid Oxide Fuel Cells

In Solid Oxide Fuel Cells (SOFCs), electrochemical reactions take place at the triple-phase boundaries (TPBs) between the electrolyte, the electrode and the gaseous reactant.  Small changes in TPB structure or chemistry, and the interface between electrode and electrolyte can drastically affect SOFC performance and lifetime. However, very limited experimental work has been reported on the nanostructure and chemistry evolution of the interface and TPBs in the SOFC upon cell operation. The present work employed Transmission Electron Microscopy (TEM) to investigate the nanostructure and chemistry of the grain boundaries and interfaces including TPB in both anode and cathodes of SOFC.

For the Ni/YSZ anodes of SOFC, the evolution of interface nanostructure and chemical composition were systematically examined with respect to the thermodynamic state of cell operation. The fuels evaluated in this present study include H₂, synthesis gas, and fuels with trace (ppm) phosphine. Previous study shows the existence of an interface ribbon phase of NiO in the Ni/YSZ interfaces, but not along the Ni/Ni grain boundaries. Previous study also indicates that the thickness of the NiO ribbon layer is dependent upon cell operation time, with thicker NiO ribbon phases developing at longer operation times. The present study further demonstrates that evolution of the NiO ribbon phase is also fuel dependent. For the cells operated for an equivalent duration of 196 hours, the thickness of the NiO layer is about 10 nm under 97.5% H₂ (with balance of N₂), and the thickness of the NiO layer is about 50 nm under 25% H₂ (balance N₂). The interaction of trace (ppm) phosphine with Ni/YSZ anode of commercial SOFCs has also been investigated and evaluated for both synthesis gas and hydrogen fuels. Experiments indicate that degradation rates and mechanisms are fuel dependent. Degradation of cells operated in synthesis gas (syngas) with phosphine is more severe than that from cells operated in hydrogen with phosphine. In the cell operated in syngas containing 10 ppm phosphine, significant microstructure degradation was observed within both the Ni phase and the YSZ phase. In addition to the formation of Ni-P phases on the outer layer of the anode, significant pitting corrosion was observed in the Ni grains. At the YSZ side, a previously undetected YPO₄ phase is observed at the YSZ/YSZ/Ni triple phase boundaries, and tetragonal YSZ and cubic YSZ domains with sizes of several tens of nanometers are observed along the Ni/YSZ interface. These observations contrast with data obtained for the cell operated in hydrogen with phosphine, where no YPO₄ phase is observed and the alternating tetragonal YSZ and cubic YSZ domains at the Ni/YSZ interface are smaller with typical sizes of 5-10 nm.

For the cathode of commercial cells, cathode infiltration of nanomaterials has been proven to be effective in introducing eletro-catalyst to the intrinsically functional electrodes and enhancing the performance of the as-prepared SOFCs.  However, limited fundamental research has focused on study of the stability of such infiltrated nano-particles upon long-term operation.  In the present study, the nanostructure and chemistry of the infiltrated La_0.6Sr_0.4CoO₃ electro-catalyst and its evolution upon operation were analyzed by TEM for cells that were electrochemically characterized under industrially relevant conditions.