Exploring the Potential of Cold Sintering for Proton-Conducting Ceramics: A Review

Abstract

Proton-conducting ceramic materials have emerged as effective candidates for improving the performance of solid oxide cells (SOCs) and electrolyzers (SOEs) at intermediate temperatures. BaCeO₃ and BaZrO₃ perovskites doped with rare-earth elements such as Y₂O₃ (BCZY) are well known for their high proton conductivity, low operating temperature, and chemical stability, which lead to SOCs' improved performance. However, the high sintering temperature and extended processing time needed to obtain dense BCZY-type electrolytes (typically > 1350 °C) to be used as SOC electrolytes can cause severe barium evaporation, altering the stoichiometry of the system and consequently reducing the performance of the final device. The cold sintering process (CSP) is a novel sintering technique that allows a drastic reduction in the sintering temperature needed to obtain dense ceramics. Using the CSP, materials can be sintered in a short time using an appropriate amount of a liquid phase at temperatures < 300 °C under a few hundred MPa of uniaxial pressure. For these reasons, cold sintering is considered one of the most promising ways to obtain ceramic proton conductors in mild conditions. This review aims to collect novel insights into the application of the CSP with a focus on BCZY-type materials, highlighting the opportunities and challenges and giving a vision of future trends and perspectives.

Keywords: BCZY; BZY; CSP; ceramic proton conductors; cold sintering; hydrogen; low sintering temperature; water-assisted densification; yttrium-doped barium cerate-zirconate.

Figure 1

Schematic representation of the cold sintering process (a) and the thermocompression apparatus (b).

Figure 2

Schematic representation of mechanical–chemical effects during cold sintering process.

Figure 3

Parameters affecting the cold sintering process [127].

Figure 4

A schematic representation of the stages involved in the cold sintering process [152].

Figure 5

STEM micrographs and corresponding EDS maps (the scale bars are 5 nm) of a GB in a sample produced by cold sintering + PA (a) and conventionally sintered (b). Atomic Probe Tomography investigation of grain boundaries (scale bar is 50 nm) in the cold-sintered +PA sample (c) and the relative composition profile (orange arrow in (c)) of a random GB (d,e) [149].

Figure 6

Microstructures of the BZCY-BZCY/NiO half-cell after cold sintering and post-annealing (a,b) and after subsequent reduction treatment (c,d); detail 1 shows the reduced Ni metal structure and detail 2 the remaining intact BZCY network [203].