This research project is a collaborative effort between the Department of Chemical Engineering and Materials Science (Professors Z. A. Munir and S. Kim) at the University of California, Davis and the Institute of Physical Chemistry (Professor M. Martin) at RWTH Aachen University, Germany. The goal of this research is to provide an understanding of the heretofore-unobserved proton and oxygen transport processes in dense, nanocrystalline oxides, as an example of the nanoscale effect in functional oxides. The investigators' ability to prepare oxides with ultra-small grain size (approaching 10 nm) has opened a window of investigation on this and on related nanometric effects. The aim is to prepare and investigate such materials in dense, bulk form with even smaller grain size ( 10 nm). Observation of the occurrence of low-temperature protonic conductivity in unhydrated yttria-stabilized zirconia and doped ceria, typical predominant oxygen ion conductors, opens a new door on the fundamental issue of a unique behavior of nanostructured electroceramics. Heretofore such a behavior has not been observed since prior attempts to prepare such materials in bulk form had not been successful. The researchers' success was made possible by their unique ability (through a novel pressure assisted field activated sintering method) to prepare highly dense (> 98%), bulk, nanometric oxides with a grain size of < 20 nm. The fundamental questions that arise from their observations include: what is the nature of this protonic conduction? What is the mechanism associated with protonic mobility? Since this phenomenon is only observed in materials with ultra-small grain size, how do grain boundaries play a role in mass and charge transport? In view of observation on thin films, are the role and nature of grain boundaries different when the grains are very small? What, if any, do dopant-generated point defects contribute to the process? The answers to these and related fundamental questions should provide a significant intellectual contribution to our understanding of the nanoscale effect in these functional oxides and stimulate new research in this important area.
The use of stable oxides with mechanical integrity as protonic conductors at low temperatures (even in water at room temperature) has an immense impact on application considerations for protonic conductors. Protonic conductors have an extensive field of application, including their use as hydrogen separators (when used as mixed conductors), or to produce power (when used in fuel cells). They can also be used in electrolysis for hydrogen production, and for reactions to hydrogenate and dehydrogenate organic compounds. Current solid oxide fuel cells require high temperatures (800 - 1000C), a condition that presents material degradation problems, as well as other technological complications and economic obstacles. The economic considerations alone make broad commercialization prohibitive. An effective way to reduce the cost is to reduce the operating temperature without scarifying fast electrode kinetics and high ionic conductivity of the electrolyte, which our results has demonstrated its feasibility. It is to be emphasized that the observed low-temperature protonic conductivity occurs at room temperature without the need to apply a catalyst. The results show that with optimization, viable power generation using water concentration cells at room temperature is a possible goal. An important aspect of the research is the participation and exchanges of graduate and undergraduate students and postdoctoral fellows. In addition to faculty exchange visits, an exchange program for students and postdoctoral fellows is planned. Each graduate student (both from Germany and the US) will go through the entire process from synthesis and consolidation and structural and electrical characterization (at UC Davis) to SIMS determinations (at RWTH Aachen University).
