A research team affiliated with Ulsan National Institute of Science and Technology (UNIST) in South Korea has introduced a new water-electrolysis system with the highest reported electrochemical performance in hydrogen production. The Hybrid-Solid-Oxide Electrolysis Cell (Hybrid-SOEC) system is a promising new option for cost-effective and highly-efficient hydrogen production.

The breakthrough was led by Professor Guntae Kim in the School of Energy and Chemical Engineering at UNIST in collaboration with Professor Tak-Hyoung Lim of Korea Institute of Energy Research (KIER) and Professor Jeeyoung Shin of Sookmyung Women's University.

Illustration of hydrogen and oxygen ion transport through the mixed ionic electrolyte of a hybrid solid-oxide electrolysis cell. Source: ElsevierIllustration of hydrogen and oxygen ion transport through the mixed ionic electrolyte of a hybrid solid-oxide electrolysis cell. Source: Elsevier

A solid-oxide electrolyzer cell (SOEC) consists of two solid-state electrodes and a solid-state electrolyte. They have a number of advantages that make them strong candidates for hydrogen production. With no need to replenish lost electrolytes, and negligible corrosion issues, they require little maintenance. In addition, SOECs operate at relatively high temperatures (700-1,000 degrees Celsius), which minimizes electrical energy consumption.

Professor Kim and his research team have been seeking ways to improve the energy efficiency of hydrogen production using SOEC. In the study, the research team has demonstrated the novel concept of Hybrid-SOEC based on a mixed ionic conducting electrolyte, allowing water electrolysis to occur at both the hydrogen and air electrodes.

Traditional SOEC electrolytes allow the transport of only one ion to a single electrode. For SOEC electrolytes that transport oxygen ions, water electrolysis occurs at the anode, resulting in the production of hydrogen. In contrast, SOEC electrolytes that transport hydrogen ions cause water electrolysis to occur at the cathode, resulting in the production of oxygen. Here, hydrogen travels through the electrolyte to the anode.

Theoretically, using an electrolyte capable of transporting both hydrogen and oxygen ions simultaneously would allow the production of two electrolysis products, hydrogen and oxygen, on both sides of the cell. This could improve the hydrogen production rate greatly.

It is this result that Professor Kim and his research team achieved with their water-electrolysis system. The Hybrid-SOEC features a mixed-ion conductor that can transport both oxygen and hydrogen ions at the same time.

In comparison to other SOECs and representative water-electrolysis devices reported in the literature, the proposed system demands less electricity for hydrogen production, while exhibiting outstanding stable electrochemical performance. Moreover, the Hybrid SOEC exhibits no observable degradation in performance for more than 60 hours of continuous operation, implying a robust system for hydrogen production.

"By controlling the driving environment of the hydrogen ion conductive electrolyte, a 'mixed ion conductive electrolyte' in which two ions pass can be realized," said Junyoung Kim, a candidate in the doctoral program of Energy and Chemical Engineering and the first author of the study. "In Hybrid-SOEC where this electrolyte was first introduced, water electrolysis occurred at both electrodes, which results in significant increase in total hydrogen production."

A layered perovskite with excellent electrochemical properties was used as the electrode of the Hybrid-SOEC. By combining an excellent electrode material with a mixed ionic conducting electrolyte, enhanced electrochemical performance was achieved. As a result, the corresponding yield of hydrogen production was 1.9 L per hour at a cell voltage of 1.5 V and a temperature of 700 degrees Celsius. This is four times the hydrogen production efficiency of existing high-efficiency water electrolytic cells.