A main advantage of solid oxide fuel cells (SOFCs) operating at high temperatures (> 600 C) is the flexibility of the fuel they use, specifically as they offer the possibility to utilize hydrocarbons (e.g. natural gas). This would enable near-term realization of efficiency advantages for fuel conversion into electricity, even in the absence of a hydrogen delivery infrastructure. Essential in related research is the development of high-performance and robust anodes. Ceria ($CeO_2$), either doped or undoped, has been a key component presumably due to its high stability against carbon deposition and its high catalytic activity in hydrocarbon environments. However, even with the simplest hydrocarbon molecule, $CH_4$, the mechanism of electrochemical oxidation on the ceria surface has not been clarified. In particular, in addition to the complicated processes of $CH_4$ oxidation, it is challenging to investigate targeted electrochemical reactions selectively from various chemical reactions, such as the steam reforming reaction, that occur simultaneously on typical ceria-based composite anodes.
To address this issue, through pulsed laser deposition and photolithographic lift-off processes, I fabricated polarizable, Sm-doped ceria (SDC) thin-film-based model electrochemical cells which enable selective monitoring of the direct-electro-oxidation of CH4 at the ceria/gas interface. Combined experimental (impedance spectroscopy and ambient-pressure x-ray photoelectron spectroscopy) and theoretical (density functional calculations) methods were utilized to collect information about the surface reactions. Both experimental results consistently indicated that the SDC surface catalyzes the C-H cleavage. In contrast, the presence of a hydroxyl group as a dominant intermediate on the SDC, where $CH_4$ oxidation takes place, confirmed that the overall electrode reaction rate is mainly limited by the $H_2O$ formation step. Furthermore, the theoretical calculations showed that the electron transfer process which takes place during the $H_2O$ formation step can be important.
In order to improve the electrocatalysis of the SDC surface, the application of active metal nanoparticles (NPs) was investigated. Particularly, I focused on developing an accurate analysis of the reactivity of oxide electrodes boosted by metal nanoparticles, with all particles participating in the reaction. Monodisperse particles, in this case Pt, Pd, Au and Co, 10 nm in size and stable at high temperatures (> 600 C), are uniformly distributed onto mixed-conducting oxide electrodes as a model electrochemical cell via self-assembled nanopatterning. Impedance and X-ray photoelectron spectroscopy results showed that $H_2O$ formation is still likely to be the rate-limiting step for Pt NP-SDC electrodes, whereas the electron transfer rate was significantly faster than that on the bare SDC. This demonstrated the important electrocatalytic effects of metal catalysts on the surface reaction kinetics. In addition to $CH_4$ electro-oxidation, a study of $H_2$ electro-oxidation through the proposed model system was successfully conducted. Regarding high-temperature electrocatalysis, experimental evidence of active reaction sites and the inherent reactivity of four different metals is reported for the first time.
Finally, as a means by which to apply the insights obtained above to a practical SOFC electrode, I investigated a cost-effective coating method, well known as cathodic electrochemical deposition (CELD), to design highly active and robust ceria-based SOFC anodes by creating ceria nanostructures with a high specific surface area. A fundamental understanding of the electrochemical formation of ceria nanostructures was achieved through chronoamperometry and with an electrochemical quartz micro-balance. I applied CELD to the Ni/YSZ model anodes as a rapid surface coating method and confirmed that ceria nanostructures dramatically enhanced the electrode performance. Moreover, the coated layer effectively improved the coking resistance of the Ni surface in a $CH_4$ environment. As ongoing work, two-step CELD will be discussed to synthesize metal NP-decorated ceria nanostructures to increase the activity further.