Solid state NMR study of protonic conductors for applications as electrolyte materials in Solid Oxide Fuel Cells

Abstract

In the context of the current energy crisis, we need to develop cleaner and more efficient energy conversion devices. The Solid Oxide Fuel Cell is one of the most promising conversion devices currently under study: it creates electricity by reduction of either fossil fuels or biofuels with an efficiency of up to 60% (compared to 30% for a combustion engine). In order to render this application viable for commercialization, we need to reduce the operating temperature of the device. This will decrease the fabrication cost and considerably extend the lifetime of the device, however the ionic conduction has to be kept as high as possible. Electrolyte materials used in the first commercialized devices were based on oxygen conduction and are active for temperatures above 700¶øC. These materials can be replaced by protonic conductors, which show similar conductivities but in the range of 200 to 400¶øC. This dissertation aims at explaining the structural differences and protonic conduction pathways between three of the most promising candidates: BaZr_1-xSc_xO_{3- y}(OH)_y, BaSn_1-xY<sub>x</>O_3-y(OH)_y and BaZr_1-xY_xO_{3- y}(OH)_y. The combination of substitution level, cationic arrangement and protonic distribution strongly influences the protonic conduction of each. In BaZr_1-xSc_xO_{3- y}(OH)_y, the scandium substitution is limited by the instability of the Sc-O-Sc environments, which exist but in a limited amount. These environments act as energetic traps for protons: the charge carriers are strongly bonded to these Sc-O-Sc environments considerably reducing their mobility. In BaSn_1-xY<sub>x</>O_3-y(OH)_y, Y<super>3+</super> and Zr<super>4+</super> tend to alternate in the structure for high substitution levels (50%) leading to the presence of mostly one oxygen site, Sn-O-Y. This should facilitate the conduction of protons; however some unordered regions lead to the trapping of protons on Sn-O-Sn sites, reducing the protonic conductivity. Finally in BaZr_1-xY_xO_{3- y}(OH)_y, the yttrium cations are distributed randomly in the structure and the larger size of Y<super>3+</super> combined with its smaller electronegativity - as compared to Sc<super>3+</super> - prevents the protons from being trapped on Y-O-Y environments. Our study explains why BaZr_0.80Y_0.20O_2.90-y(OH)_y is the best protonic conductor as reported by impedance spectroscopy.