Researchers at Idaho National Laboratory have developed a new electrode material for a protonic ceramic electrochemical cell (PCEC) that can efficiently convert excess electricity and water into hydrogen. The triple conducting oxide of PrNi_0.5Co_0.5O_3-δ perovskite was developed as an oxygen electrode, and presents superior electrochemical performance at 400~600 °C.

An open-access paper on their work is published in Nature Communications.

Water splitting reaction on oxygen electrode and PNC’s hydration. (a) Mixed oxygen-ion O²⁻ and electron conducting electrode. (b) Triple (H⁺, O²⁻, and electron) conducting electrode to extend reaction to entire electrode surface. (c) Chemical expansion of PNC perovskite structure observed in high-temperature X-ray diffraction at 600 °C when exposed to wet air. Ding et al.

The protonic ceramic electrochemical cell (PCEC) is a proton-conductor-based solid oxide cell that can serve in a reversible operation manner to store renewable energies using water electrolysis to produce hydrogen and then convert it back to electricity in fuel cell mode. The application of PCECs demonstrates the uniqueness of combining the bi-function of energy storage and distributed power generation by integrating PCEC and balance of the plant into one system.

As significant advances have been made in solid state proton conductors and related electrochemical cells (fuel cells and electrolyzers) in the past decade PCEC represents a promising technology for the purpose of achieving low-cost energy storage and conversion at reduced temperatures by offering attracting advantages such as high efficiency, longer system durability, and less expensive materials.

However, the large-scale deployment of PCECs still remains elusive by severe limitation on developing highly active and robust electrode due to sluggish electrode kinetic at intermediate temperatures and decayed lifetime of the material and interface, especially under high-steam concentration.

… In this article, we report our findings from the creation of a perovskite TCO electrode PrNi_0.5Co_0.5O_3−δ (PNC) in the PCECs with superior WOR and ORR activity. In addition to demonstrating excellent durability and thermal cycling capability, it shows outstanding electrochemical performances and self-sustainable and reversible PCEC at reduced temperature range (400~600 °C). PNC is inspired by PrCoO₃ (PCO) and lanthanide nickelates. PCO is a highly active oxygen electrode in oxygen-ion conducting fuel cells without alkaline earth metal (e.g., Sr or Ba) doping, which tends to mitigate segregation and resulting degradation during operation.

… Our experimental study and density function theory (DFT) calculation show that proper Ni replacement in the B-sites of PCO perovskite can surprisingly reduce the migration barrier for proton conduction when proton defects are readily induced by hydration reaction, thus enhancing proton conduction. The demonstrated triple conduction of this material facilitates WOR and ORR and promoted electrochemical performance of the cell in a self-sustainable and reversible operation at reduced temperatures.

—Ding et al.

Protonic ceramic electrochemical cells (PCECs) uses electricity to split steam into hydrogen and oxygen. These devices have had limitations, especially the fact that they operate at temperatures as high as 800 ˚C. The high temperatures require expensive materials and result in faster degradation, making the electrochemical cells cost prohibitive.

In the paper, Dong Ding and colleagues describe a new material for the oxygen electrode—the conductor that facilitates the water splitting and oxygen reduction reactions simultaneously. Unlike most electrochemical cells, this new material—an oxide of a compound called a perovskite—allows the cell to convert hydrogen and oxygen into electricity without additional hydrogen.

Previously, Ding and his colleagues developed a 3D meshlike architecture for the electrode that made more surface area available to split the water into hydrogen and oxygen. Together, the two technologies—the 3D mesh electrode and the new electrode material—allowed for self-sustainable, reversible operation at 400 to 600 degrees C.

We demonstrated the feasibility of reversible operation of the PCEC at such low temperatures to convert generated hydrogen in hydrolysis mode to electricity, without any external hydrogen supply, in a self-sustaining operation. It’s a big step for high temperature electrolysis.

—Dong Ding

While past oxygen electrodes conducted only electrons and oxygen ions, the new perovskite is “triple conducting,” Ding said, meaning it conducts electrons, oxygen ions and protons. In practical terms, the triple-conducting electrode means the reaction happens faster and more efficiently, so the operating temperature can be reduced while maintaining good performance.

For Ding and his colleagues, the trick was figuring out how to add the element to the perovskite electrode material that would give it the triple-conducting properties—a process called doping.

We successfully demonstrated an effective doping strategy to develop a good triple-conducting oxide, which enables good cell performance at reduced temperatures.

—Hanping Ding, a materials scientist and engineer for Idaho National Laboratory’s Chemical Processing Group

In the future, Dong Ding and his colleagues hope to continue improving the electrochemical cell by combining materials innovation with advanced manufacturing processes so the technology can be used at an industrial scale.

Resources

• Ding, H., Wu, W., Jiang, C. et al. (2020) “Self-sustainable protonic ceramic electrochemical cells using a triple conducting electrode for hydrogen and power production.” Nat Commun 11, 1907 doi: 10.1038/s41467-020-15677-z