Evaluation of thin film fuel cells with Zr-rich BaZr_{: X}Ce_{0.8- x}Y_0.2O_{3- δ} electrolytes (x ≥ 0.4) fabricated by a single-step reactive sintering method

This paper reports a survey of power generation characteristics of anode-supported thin film fuel cells with Zr-rich BaZr_xCe_0.8-xY_0.2O_3-δ (x = 0.4, 0.6, 0.7, and 0.8) proton-conducting electrolytes, which were fabricated by single step co-firing with Zn(NO₃)₂ additives at a relatively low temperature (1400 °C). The grain sizes significantly increased to several μm for x = 0.4 and 0.6, whereas the grain sizes remained in the sub-μm ranges for x = 0.7 and 0.8, which resulted in large gaps of the fuel cell performances at x over and below 0.6. The cells for x = 0.4 and 0.6 exhibited efficient power generation, yielding peak powers of 279 and 336 mW cm^-2 at 600 °C, respectively, which were higher than those of the corresponding cells previously reported. However, the performances abruptly deteriorated with the increasing x to more than 0.7 because the electrolyte films were highly resistive due to the coarse-grained microstructures. Impedance spectroscopy for the dense sintered BaZr_xCe_0.8-xY_0.2O_3-δ discs confirmed that the total proton conductivity of BaZr_0.6Ce_0.2Y_0.2O_3-δ was higher than that of BaZr_0.4Ce_0.4Y_0.2O_3-δ at temperatures above 500 °C despite relatively small grain sizes. In addition, BaZr_0.6Ce_0.2Y_0.2O_3-δ cells could gain a stable current throughout a continuous run for a few days under CO₂-containing fuel supply, which was due to high fraction of thermodynamically stable BaZrO₃ matrices. It was demonstrated that BaZr_0.6Ce_0.2Y_0.2O_3-δ is a promising electrolyte for proton-conducting ceramic fuel cells with excellent proton conductivity and CO₂ tolerance at intermediate temperatures.