The discovery of the proton polaron
Laboratory for High Performance Ceramics
Empa. Swiss Federal Laboratories for Materials Science and Technology High Performance Ceramics
Abstract: In electrochemical energy storage and conversion, electrodes and electrolytes are the key components of batteries, fuel cells and capacitors. Electrons and ions are the relevant electric charge carriers. The key player in technology for the hydrogen economy is the proton - an elusive charge carrier which cannot be so easy detected . Ceramic proton conductors can be used as electrolyte membranes in solid state devices. Smart defect engineering makes that oxygen vacancies can be filled with oxygen ions from ambient vapor water molecules. The protons from the water molecule form intermediate OH bonds with proximate oxygen ions. Upon thermal activation, the OH bonds melt and the proton be-comes liberated as positive charge carrier. The transport properties of the yttrium doped barium cerate and barium zirconate electrolytes were investigated by thermodynamic parameterization of their structure with temperature 273 K - 773 K and pressure 0 - 6 GPa. The proton conductivity activation energy decreases linear with increasing lattice spacing, suggesting that epitaxial strained films should be promising future electrolyte membranes. The Raman modes increase with increasing pressure and get a slightly higher 'pitch' upon protonation. The OH bond breaking occurs at a characteristic temperature range which is accompanied by the onset of a lateral proton diffusivity which accounts for the macroscopic conductivity as measured with electroanalytical methods. At the microscopic scale, ambient pressure XPS and quasi elastic neutron scattering which were carried out operando parallel on the same samples with impedance spectroscopy, confirm that it is exactly this proton phonon coupling which switches the proton conductivity on. The quantitative analysis of the proton jumping frequencies showed that the Ce-O stretching mode is the effective propeller for the proton at work. Moreover, the temperature dependence of the proton jump frequency follows exactly the mathematical model for a Holstein polaron, rendering the proton conductivity process in ceramic proton conductors a genuine proton polaron .
 Q. Chen, A. Braun, MRS Energy & Sustainability, 4, E14 (2017).
 A. Braun, Q. Chen, Nature Communications, 8, 15830 (2017).
Research done in collaboration with Qianli Chen (University of Michigan – Shanghai Jiao Tong University Joint Institute, Shanghai, China)