W. Grover Coors

Differential resistance analysis of protonic ceramic fuel cells for measuring bulk conductivity

A technique, called differential resistance analysis, was developed to determine the bulk conductivity of protonic ceramic electrolyte in hydrogen/air fuel cells under load. For these experiments, identical specimens of the protonic ceramic BaCe0.9Y0.1O3−α (BCY10) were prepared with thicknesses of 240, 560, 790, and 1150 μm with thin film platinum electrodes. The current–voltage (I–V) characteristic curves for each specimen were obtained between 600 and 800 °C, and the slope of each I–V curve was determined in the ohmic region between 10 and 20 mA, giving the total effective area specific resistance (ASR cell) of the cell under load as a function of temperature. The bulk electrolyte resistivity was found by taking the difference in resistance of two cells divided by the difference in electrolyte thickness. The bulk conductivity of the electrolyte measured at 1000 K by this technique was 5 mS/cm or less, depending on the overall electrolyte thickness, much lower than the values obtained by impedance spectroscopy. Also, the activation energy for bulk conduction was found to be higher than expected for pure protonic transport. This paper attempts to correlate the two measurement techniques and explain the apparent discrepancies.

Fabrication of Yttrium-Doped Barium Zirconate for High Performance Protonic Ceramic Membranes

Barium zirconate has emerged as the leading candidate material for fabricating dense ce‐ ramic membranes for hydrogen separation. B-sites in the ABO3 perovskite are acceptordoped with a +3 cation – most commonly yttrium – charge-compensated by the formation of oxygen ion vacancies in the lattice. A minor fraction of B-sites can be filled with cerium to give BaZr0.9-xCexY0.1O3-d, x ≤ 0.2. Upon hydration at elevated temperatures, weaklybound protons are formed in the lattice. This produces a cubic perovskite ceramic proton conductor useful in diverse applications, such as protonic ceramic fuel cells, electrolysers, and catalytic membrane reactors operating at temperatures between 600 and 800 °C. A necessary requirement for fabricating thin ceramic membranes for proton diffusion is to maximize grain size while eliminating percolating porosity. However, high-density, large-grained barium zirconate is a very difficult material to prepare by traditional pow‐ der sintering methods. This chapter describes a new methodology for making protonic ceramic membranes with large grains and virtually no residual porosity. This discovery has the potential to have a profound impact on energy conversion efficiency of the vari‐ ous membrane devices envisioned for the coming hydrogen energy economy.