Surface Activity and Bulk Defect Chemistry of Solid Oxide Fuel Cell Cathodes

In the first half of this thesis, a new robotic instrument called a scanning impedance probe is presented that can acquire electrochemical impedance spectra in automated fashion from hundreds of thin film microelectrodes with systematically varied properties. Results from this instrument are presented for three catalyst compositions that are commonly considered for use in state-of-the-art solid oxide fuel cell cathodes. For (La_0.8Sr_0.2)_0.95Mn_O3+δ (LSM), the impedance spectra are well fit by a through-the-film reaction pathway. Transport rates are extracted, and the surface activity towards oxygen reduction is found to be correlated with the number of exposed grain boundary sites, suggesting that grain boundaries are more surface-active than grains. For La_0.5Sr_0.5Co_O3-δ (LSC), the surface activity degrades ~50x initially and then stabilizes at a comparable activity to that of previously measured Ba_0.5Sr_0.5Co_0.8Fe_0.2O3-δ films. For Sr_0.06Nb_0.06Bi_1.87O₃ (SNB), an example of a doped bismuth oxide, the activity of the metal-SNB boundary is measured.

In the second half of this thesis, SrCo_0.9Nb_0.1O_3-δ is selected as a case study of perovskites containing Sr and Co, which are the most active oxygen reduction catalysts known. Several bulk properties are measured, and synchrotron data are presented that provide strong evidence of substantial cobalt-oxygen covalency at high temperatures. This covalent bonding may be the underlying source of the high surface activity.

Mechanistic Dissection of the Cop9 Signalosome’s Deneddylation Activity on Cullin-RING Ligases

We set out to understand the precise mechanisms that regulate the activation and deactivation of Cullin-RING Ligases (CRLs). While a great deal of work has already gone into identifying the players involved in these pathways and the cellular consequences associated with the loss of each, the biochemical mechanisms regulating these steps have remained elusive. In this work we sought to gain a better understanding of the mechanisms behind these steps by teasing apart specific their biochemical reactions. By measuring the individual microscopic rate constants of the reactions we have shed light on both the proper sequence of events in the regulation of CRLs as well as how they are in fact controlled.

Prior to this work, it was believed that CSN deactivated CRLs by binding them and enzymatically removing the activating post-translation modification Nedd8. It was believed that CSN could not bind to CRLs while they were active due to the steric hindrance by the CRL substrates, and that they would remain bound to deneddylated CRLs as a sequestering agent until a new substrate could displace it. We now have some insight that substrates themselves cannot inhibit CSN very well, but that the active ubiquitination by an E2 enzyme precludes CSN binding and activity. When the substrate for a CRL becomes depleted, CSN then binds to the CRL in a low affinity, low activity conformation. This triggers a conformational change that pulls the autoinhibitory Ins-1 loop away from the active site in the catalytic subunit Csn5, resulting in a large increase in affinity and cleavage of the isopeptide bond between CRLs and Nedd8. Upon dissociation of Nedd8, CSN rapidly returns to the low affinity state and dissociates from the CRL, allowing it reenter its activation cycle.