UC Merced

In SOFCs, we observe that the porous electrodes tend to agglomerate and thus decrease the cell performance over time. The agglomeration (or sintering) of a porous oxide can be understood as the diffusion of the oxide atoms along the surface of a material to minimize the surface energy of the structure. Therefore, even an ultrathin overcoat is expected to deter the sintering process and thus maintain the porous geometry for longer period of time. In this thesis, the actual role of YSZ overcoat in the ORR process is presented through a series of electrochemical analyses. Without an overcoat, a nanoporous Pt is significantly agglomerated during a high-temperature operation, and ORR kinetics becomes limited by the availability of TPB. An ultrathin YSZ overcoat significantly suppressed the sintering kinetics and preserved the morphology of its underlying Pt layer. More importantly, the overcoat acts as an excellent facilitator of the atomic oxygen species-mediated chemical process(es), which used to be rate-limiting in the ORR of a non-coated Pt/YSZ system.

In the next step, to understand the impact of spincoated layer on the interface of cathode/electrolyte, EIS data obtained at different temperatures and oxygen partial pressures. Beside SEM imaging and XRD, to support the hypothesis of ORR kinetics, infiltration and ALD coating also performed on different samples. We found out that spin-coated layer has smaller nanoparticle sizes and 3~4 times smaller nanocrystalinity that increase the active cites for dissosiative adsorption on the cathode side, comparing to the available cites on bare LNF. EIS data along with ALD coating also strongly support the hypothesis that ion conducting pass through the LNFGDC FL playes an important role in decreasing the polarization resistance comparing the bare LNF cathode as well. It helps the available cites not limited to the vicinity of GDC interlayer but be available in entire FL and the ion transport to the GDC interlayer through GDC nanoparticles paths.

To sum up, a combination of nanoscale treatments including cathode infiltration, metal oxide ALD coating, and cathode-electrolyte interface spin-coating pursued to achieve a significantly enhanced performance and durability by addressing the issues. In addition, I also performed a systematic studies on the change in electrochemical kinetics and a possible shift of bottleneck process by the treatments to better understand the effect of each of this nano-functionalization on oxygen reduction catalysis process in SOFCs.