The Fuel Cell Nano-Materials Group at Japan’s National Institute for Material Science has successfully developed two types of novel materials which its says satisfy all the three requirements for a solid oxide fuel cell (SOFC) electrolyte: ion conductivity, chemical stability and sinterability, at high levels.
The previous work of the group, published in the journal Nature Materials on 20 September, introduced high-performance materials that show very high proton conductivity at even lower temperatures, but it was fabricated by using a special technology called pulsed laser deposition. In these new studies, high-performance electrolyte materials were obtained by simple co-pressing and subsequent sintering in the air, which is suitable for mass-production. The results could accelerate the commercialization of SOFCs, the researchers said.
One is yttrium-doped barium zirconate with 10 mol% of praseodymium (BZPY). The addition of Pr improves the sinterability of BZY and dense samples are obtained after sintering at 1500°C for 8 hours. This material showed very high proton conductivity (above 0.01S/cm at 600°C), comparable to the proton conductivity of BCZY, now proposed for proton conductor electrolyte, but with significantly better chemical stability, thereby resulting in realistic applicability in fuel cell devices.
Reducing the operating temperature of SOFCs below 700 °C is needed for a wide practical application of these devices. Yttrium-doped barium zirconate (BZY) is now considered as an alternative to the oxygen-ion conductor electrolytes conventionally used in SOFCs due to its higher bulk proton conductivity at low temperatures. However, BZY has not been exploited until now despite its excellent chemical stability because, when prepared as a ceramic polycrystalline material, it suffers from difficult sintering and proton conductivity is decreased by grain boundaries, which have a blocking effect.
The other material is indium-doped barium zirconate (BZI) on a NiO-BZY anode substrate. During sintering at 1,450 °C, a dense electrolyte film is formed and simultaneously indium evaporates, being substituted by yttrium. The final result is the achievement of a dense BZY electrolyte film on a NiO-BZY anode, which cannot be obtained at the same temperature with direct processing.
The fuel cells using this electrolyte film showed the largest fuel cell performance, 0.169 W/cm2 at 600 °C, ever reported for BZY-based electrolytes. The BZY film made by this method shows excellent chemical stability, indicating its potential for long-term operation, according to the researchers.
These two materials are promising electrolyte materials for SOFC operating in the intermediate temperature range, 500 to 650 °C, which allows reducing SOFC fabrication and operation costs, and thus accelerating their commercialization.
Daniele Pergolesi, Emiliana Fabbri, Alessandra D’Epifanio, Elisabetta Di Bartolomeo, Antonello Tebano, Simone Sanna, Silvia Licoccia, Giuseppe Balestrino, Enrico Traversa (2010) High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition. Nature Materials 9, 846-852 doi: 10.1038/nmat2837