Control of grain boundary defect structure in composite mixed ionic-electronic conductors for energy conversion applications (Brinkman)

Traditionally, advances in energy conversion devices have been based on compositional modifications. However, it is now realized that engineered interfaces (i.e., grain boundaries) and tailored defects should be a key focus area for the design and fabrication of next generation energy conversion systems. Materials which transport ions, electrons, and gas/fluid species play an essential role in a number of energy conversion systems including fuel cells, Li-air batteries, oxygen separation & permeation membranes for oxygen production, partial oxidation of methane, and clean coal production via oxy-combustion resulting in significant reductions in CO2 emissions from coal fired power plants. Ceramic membranes have the potential to be increasingly prominent as high temperature electrolysis cells for hydrogen production and state of the art combustion control sensors. The properties and function of these materials are sometimes controlled by mixed ionic and electronic conductivity (MIEC) in the respective material system.

The performance of MIEC materials in electrochemical devices are determined by a number of factors including the interfacial characteristics and morphology, and the volume fraction and distribution of constituent phases in dual phase systems. The establishment of non-equilibrium operating conditions may lead to changes in phase formation, interfacial characteristics and microstructure of the material system.  A new method to address the problem of grain boundary suppression of ionic conductivity is based control of the interfacial defect structure in ceramic composites. This has been demonstrated in a model system consisting of Ce0.8Gd0.2O2-δ (CGO) oxygen ionic conductor and CoFe2O4 (CFO) electronic conductor that were shown to form an emergent crystalline phase that served to reduce the aliovalent dopant concentration that typically segregate at the grain boundary. Microscopy studies revealed that new Gd- and Fe-rich oxides with high content of oxygen vacancies were formed in-situ during the sintering process.  Electron energy loss spectroscopy demonstrated that the oxygen vacancy concentrations are nearly constant across the CGO-CGO grain boundaries interface in the CGO-CFO composite leading to very high oxygen ion conductivities. These results confirm the potential of this method using an emergent crystalline phase to control the grain boundary defect concentration to develop next generation of high performance MIECs for energy conversion and storage applications.

The students project will consist of fabrication and testing of ceramic samples with varying volume fraction of constitutive components.  In order to accomplish this task, the students will perform calculations on the preparation of ceramic phases, followed by learning the basics of ceramic powder processing including; weighing, mixing, ball-milling, drying, calcining, and sintering pressed pellets.  After fabricating test samples, the students will work with graduate student and postdoctoral research in the lab in order to examine their samples by X-ray Diffraction to investigate the phase assemblages formed and the microstructure by Scanning Electron Microscopy.  Finally, the students will measure the conductivity of the samples at elevated temperature in varying gas environments by both AC impedance and DC 4 point probe methods.

 

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