Correlation between electroconductive and structural properties of proton conductive acceptor-doped barium zirconate

Various dopants were added to BaZrO[3] and the conductivities, the proton concentrations, the site occupancy of the dopants and the change in lattice volume as a result of chemical expansion were investigated. Lanthanide group dopants occupied both the Ba and Zr sites, but the amount of these dopants in the Ba site was too limited to significantly influence the conductivity. The samples doped with Yb, Tm, Er, Y and Ho showed both high proton concentrations and high conductivities, together with a relatively large lattice expansion as a result of hydration. We therefore suggest that, in most instances, the proton concentration, proton conductivity and lattice change as a result of chemical expansion were all correlated in proton conductive acceptor-doped BaZrO[3]. However, Sc-doped BaZrO[3] seemed to be different. Its proton concentration was high, but the conductivity and lattice change as a result of chemical expansion were relatively small. This indicates that the conductivity was strongly related to the lattice expansion resulting from hydration rather than simply the proton concentration

Detrimental Effect of Sintering Additives on Conducting Ceramics: Yttrium‐Doped Barium Zirconate

Y‐doped BaZrO3 (BZY) is currently the most promising proton‐conductive ceramic‐type electrolyte for application in electrochemical devices, including fuel cells and electrolyzer cells. However, owing to its refractory nature, sintering additives, such as NiO, CuO, or ZnO are commonly added to reduce its high sintering temperature from 1600 °C to approximately 1400 °C. Even without deliberately adding a sintering additive, the NiO anode substrate provides another source of the sintering additive; during the co‐sintering process, NiO diffuses from the anode into the BZY electrolyte layer. In this work, a systematic study of the effect of NiO, CuO, and ZnO on the electroconductive properties of BaZr0.8Y0.2O3−δ (BZY20) is conducted. The results revealed that the addition of NiO, CuO, or ZnO into BZY20 not only degraded the electrical conductivity but also resulted in enhancement of the hole conduction. Removal of these sintering additives can be realized by post‐annealing in hydrogen at a mild temperature of 700 °C, but it is kinetically very slow. Therefore, the addition of NiO, CuO, and ZnO is detrimental to the electroconductive properties of BZY20, and significantly restrict its application as an electrolyte. The development of new sintering additives, new anode catalysts, or new methods for preparing BZY electrolyte‐based cells is urgently needed

Transport properties of acceptor-doped barium zirconate by electromotive force measurements

In this work, a systematic work was performed to investigate the electrochemical transport properties of acceptor-doped BaZrO₃ by measuring electromotive force on various gas concentration cells. For the measurements in the wet oxidizing atmosphere, where significant hole conduction occurs, the transport numbers of the ionic conduction were corrected by taking the effect of electrode polarization into consideration. The results revealed that regardless of whether Sc, Y, In, Ho, Er, Tm or Yb was doped, proton conduction predominated in the reducing atmosphere with the transport number close to unit. However, the contribution of ionic conduction weakens, and the contribution of hole conduction enhances, when the samples are exposed to the moist oxidizing atmosphere. In addition, introducing Ba-deficiency results in degraded electrochemical conductivity, but the transport number in either the moist reducing or the moist oxidizing atmosphere does not change obviously

Strategy to improve phase compatibility between proton conductive BaZr0.8Y0.2O3-δ and nickel oxide

BaZr0.8Y0.2O3-δ (BZY20) is a promising candidate as an electrolyte in protonic ceramic fuel cells (PCFCs), and nickel (Ni) is known to show good electrode properties for the anode reaction. However, their compatibility seems to be questionable, since during the co-sintering process for cell fabrication, a second phase of BaY2NiO5 formed due to a reaction between BZY20 and NiO. The results in this work revealed that BaY2NiO5 was unstable against high temperature (1500 and 1600 °C), and could also be reduced in a hydrogen atmosphere at 600 °C. The products of these reactions may affect fuel cell performance. A systematic work was then performed to provide fundamental insight into the reactivity between BZY20 and NiO, which was found to be impacted significantly by the compositional homogeneity of the BZY20 powder used for cell fabrication, and also the BaO activity during the co-sintering process. It is concluded that improving the compositional homogeneity of BZY20, by elevating the final heating temperature for BZY20 from 1300 to 1600 °C in this work, and choosing a proper sintering strategy may improve effectively the phase purity of the cell

A high temperature reduction cleaning (HTRC) process: A novel method for conductivity recovery of yttrium-doped barium zirconate electrolytes

Proton conducting Y-doped BaZrO₃ (BZY) and nickel oxide (NiO) are currently the most promising electrolyte and anode catalyst for protonic ceramic fuel cells, respectively. However, during the co-sintering process to fabricate the fuel cells, Ni cations diffuse from the anode into the lattice of the BZY electrolyte, resulting in significant degradation of the electrolyte conductivity and fuel cell performance. With the aim to solve such a problem, in this work, we report a novel method, named as high temperature reduction cleaning (HTRC) process, which is composed of several sequential heat-treatments in controlled atmospheres. The most interesting point is that after heat-treating the NiO-contaminated BZY at 1400 °C in a Ti-deoxidized Ar atmosphere for 100 h, Ni cations were observed to be expulsed from the BZY lattice and segregated at the grain boundary as Ni metal particles. And the conductivity of the BZY electrolyte was recovered. However, delamination along the grain boundary of the BZY electrolyte was introduced when the segregated Ni metal particles were oxidized to NiO particles in an oxygen atmosphere. And a series of sequential heat-treatments were designed to solve such a problem