Self-regulation of surface chemistry in oxygen electrodes for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) offer direct conversion of hydrogen or hydrocarbon fuels to electricity with high energy conversion efficiency (>50%). In reverse electrolysis operation, powered by wind or solar generated... [ view full abstract ]

Solid oxide fuel cells (SOFCs) offer direct conversion of hydrogen or hydrocarbon fuels to electricity with high energy conversion efficiency (>50%). In reverse electrolysis operation, powered by wind or solar generated electricity, the same devices produce H₂ for later consumption, thus serving as a valuable means of energy storage. The electrode polarization related to electrochemical reactions at the gas/solid interface of the cathode is often the dominant flux limiting mechanism. Accumulating surface impurities (e.g. Si, Na, Al, Cr, and Sr) leads to reduction of long term durability. Identifying means for overcoming the detrimental impact of surface impurity accumulation on oxygen surface exchange kinetics could result in high payoffs in both improved performance and extended operating life.

Pr-doped ceria (PCO), a mixed ionic and electronic conductor with a high electrocatalytic activity toward the oxygen reduction reaction, has a strong advantage over state-of-the-art materials such as La_0.6Sr_0.4CoO_3-δ and La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ by not containing strontium, known to readily segregate to the solid gas interface, thereby limiting catalytic activity. Silicon contamination, however, remains a problem even with Sr-free materials, given external sources such as glass sealants, or as an impurity in precursor materials used to fabricate the devices.

This work focuses on Si contamination of PCO cathodes, observed to reduce electrode kinetics by as much as several orders of magnitude. By controlling the levels of Si contaminant applied to PCO thin films by wet chemical methods, it becomes possible to systematically study its impact on the oxygen reduction reaction by an in-situ optical transmission relaxation method. We also further investigate the feasibility of using lanthanum as an efficient Si getter [1] by studying its exsolution from La substituted PCO specimens, potentially leading to a self-cleaning electrode material, more resistant to silicon poisoning.

[1] L. Zhao et al., Chem. Mat. 27 (2015) 3065-3070