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 (La0.8Sr0.2)0.95MnO3+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 La0.5Sr0.5CoO3-δ (LSC), the surface activity degrades ~50x initially and then stabilizes at a comparable activity to that of previously measured Ba0.5Sr0.5Co0.8Fe0.2O3-δ films. For Sr0.06Nb0.06Bi1.87O3 (SNB), an example of a doped bismuth oxide, the activity of the metal-SNB boundary is measured. In the second half of this thesis, SrCo0.9Nb0.1O3-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.</p

Bulk Properties of the Oxygen Reduction Catalyst SrCo_(0.9)Nb_(0.1)O_(3-δ)

The perovskite SrCo_(0.9)Nb_(0.1)O_(3−δ) (SCN) has excellent electrochemical activity toward oxygen reduction, and it is also valuable as a possible model material for other state-of-the-art perovskite catalysts based on strontium and cobalt, such as Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3−δ) (BSCF). Here we report thermogravimetric, conductivity, and diffraction measurements from SCN. We find that the thermodynamic stability limits of SCN are slightly more favorable than those reported for BSCF, although both materials exhibit a slow oxidative partial decomposition under likely operating conditions. In SCN, this decomposition is thermodynamically preferred when the average formal oxidation state of cobalt is greater than ∼3.0+, but due to sluggish kinetics, metastable SCN with higher cobalt valence can be observed. The oxygen stoichiometry 3−δ varies from 2.45 to 2.70 under the conditions studied, 500–1000 °C and 10^(–4)–1 bar O_2, which encompass both stable and metastable behavior. The electronic conductivity is p-type and thermally activated, with a value at 600 °C in air of 250 S cm^(–1), comparable to that of La_(0.8)Sr_(0.2)MnO_(3−δ). The polaron migration enthalpy decreases linearly from 0.30 to 0.05 eV as 3−δ increases from 2.52 to 2.64. Thermal and chemical expansivities are also reported

Probing the reaction pathway in (La_(0.8)Sr_(0.2))_(0.95)MnO_(3+δ) using libraries of thin film microelectrodes

Libraries of (La_(0.8)Sr_(0.2))_(0.95)MnO_(3+δ) (LSM) thin film microelectrodes with systematically varied thickness or growth temperature were prepared by pulsed laser deposition, and a novel robotic instrument was used to characterize these libraries in automated fashion by impedance spectroscopy. The measured impedance spectra are found to be described well by an electrochemical model based on a generalized transmission model for a mixed conducting oxide, and all trends are consistent with a reaction pathway involving oxygen reduction over the LSM surface followed by diffusion through the film and into the electrolyte substrate. The surface activity is found to be correlated with the number of exposed grain boundary sites, which decreases with either increasing film thickness (at constant growth temperature) or increasing film growth temperature (at constant thickness). These findings suggest that exposed grain boundaries in LSM films are more active than exposed grains towards the rate-limiting surface process, and that oxygen ion diffusion through polycrystalline LSM films is faster than many prior studies have concluded

Rover Low Gain Antenna Qualification for Deep Space Thermal Environments

A method to qualify the Rover Low Gain Antenna (RLGA) for use during the Mars Science Laboratory (MSL) mission has been devised. The RLGA antenna must survive all ground operations, plus the nominal 670 Martian sol mission that includes the summer and winter seasons of the Mars thermal environment. This qualification effort was performed to verify that the RLGA design, its bonding, and packaging processes are adequate. The qualification test was designed to demonstrate a survival life of three times more than all expected ground testing, plus a nominal 670 Martian sol missions. Baseline RF tests and a visual inspection were performed on the RLGA hardware before the start of the qualification test. Functional intermittent RF tests were performed during thermal chamber breaks over the course of the complete qualification test. For the return loss measurements, the RLGA antenna was moved to a test area. A vector network analyzer was calibrated over the operational frequency range of the antenna. For the RLGA, a simple return loss measurement was performed. A total of 2,010 (3 670 or 3 times mission thermal cycles) thermal cycles was performed. Visual inspection of the RLGA hardware did not show any anomalies due to the thermal cycling. The return loss measurement results of the RLGA antenna after the PQV (Package Qualification and Verification) test did not show any anomalies. The antenna pattern data taken before and after the PQV test at the uplink and downlink frequencies were unchanged. Therefore, the developed design of RLGA is qualified for a long-duration MSL mission

Platinum-decorated carbon nanotubes for hydrogen oxidation and proton reduction in solid acid electrochemical cells

Pt-decorated carbon nanotubes (Pt-CNTs) were used to enhance proton reduction and hydrogen evolution in solid acid electrochemical cells based on the proton-conducting electrolyte CsH_2PO_4. The carbon nanotubes served as interconnects to the current collector and as a platform for interaction between the Pt and CsH_2PO_4, ensuring minimal catalyst isolation and a large number density of active sites. Particle size matching was achieved by using electrospray deposition to form sub-micron to nanometric CsH_2PO_4. A porous composite electrode was fabricated from electrospray deposition of a solution of Pt-CNTs and CsH_2PO_4. Using AC impedance spectroscopy and cyclic voltammetry, the total electrode overpotential corresponding to proton reduction and hydrogen oxidation of the most active electrodes containing just 0.014 mg cm^(−1) of Pt was found to be 0.1 V (or 0.05 V per electrode) at a current density of 42 mA cm^(−2) for a measurement temperature of 240 °C and a hydrogen-steam atmosphere. The zero bias electrode impedance was 1.2 Ω cm2, corresponding to a Pt utilization of 61 S mg^(−1), a 3-fold improvement over state-of-the-art electrodes with a 50× decrease in Pt loading

X-ray induced luminescence in single crystals of pure potassium iodide

The luminescent decay of pure potassium iodide has been measured under various conditions of temperature, annealing and x-irradiation time. The decay curves are found to be approximately of the form I(t) = [formula omitted] where I(t) is the luminescent intensity at the time t. Using a method that has been developed for the analysis of decay curves, numerical values for the decay constants, λi , and the trap populations Pi, have been estimated. The decay constants are found to be simple temperature functions of the form λi = [formula omitted] with the activation energies, Ei, lying between 0.4 and 0.8 ev and escape probabilities, Si, between 10⁴ and 10⁸ sec.⁻¹. The temperature dependence of the Pi is complicated and indicates that radiationless transitions strongly contribute to the emptying of traps at high temperatures. An irreversible increase in luminescent output produced by repeated x-irradiation without intermediate annealing is discovered; it points to considerable deviation from a first order decay mechanism in the case of slowly decaying traps. The mechanism of the irreversible effect is discussed on a semi-quantitative basis.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Scanning impedance probe for high-​throughput electrochemical characterization of solid state electrodes

We have developed a robotic instrument that can measure the electrochem. impedance of hundreds of thin-film microelectrodes in automated fashion. By measuring electrodes with systematically varied area, thickness, surface decoration, and compn., it is possible to probe reaction pathways, decouple bulk and surface properties, and rapidly screen hundreds of chem. compns. to discover trends and identify new high performing catalysts. Here we introduce the capabilities of this new instrument by using it to measure geometrically graded microdot electrodes of the solid-oxide fuel-cell (SOFC) cathode material (La_(0.8)Sr_(0.2))0.95MnO3-δ (LSM). We collect A.C. impedance spectra from several hundred microdots with diams. ranging from 30 to 500 μm and thicknesses from 30 to 300 nm over the temp. and oxygen partial pressure ranges of 700 to 800 °C and 3.2 × 10^(-4) to 1 atm, resp. Automated data anal. using a phys. motivated equiv. circuit model yields phys. parameters for each dot at each measurement condition. The LSM surface reaction resistance and bulk ionic resistance both exhibited a power law dependence on dot diam. with an exponent close to -2, indicative of a surface reaction pathway that encompasses the entirety of the dot surface. The slight deviation from -2 is attributed to local cooling of the sample by the microprobe tip, which slightly increases the resistances for smaller diam. microelectrodes. A surprising increase in surface reaction resistance with microelectrode thickness was obsd., tentatively assigned to an obsd. increase in film roughness with thickness. The results set the stage for exploration of a wide range of gradient types, from compn. to growth temp. to catalyst coating, while the use of impedance spectroscopy implies that a broad range of properties, from ionic cond. to material nonstoichiometry, can be extd