The use of electricity to generate hydrogen is considered potentially to be a key technology for the storage of surplus energy from renewable sources (e.g., Audi’s e-gas,); it could also play an important role in the transition of the energy system and the stabilization of grids.

In a project funded by the German Ministry of Economics, scientists at Siemens’ global research unit Corporate Technology (CT) have developed ceramic electrolytic cells for high-temperature electrolysis. This technology could be more efficient than the conventional approach, since electrolysis reactions require a much lower cell voltage at high temperatures.

Another interesting property of high-temperature electrolysis is that the flow of the electricity can be reversed, allowing users to switch back and forth between efficient electrolysis processes and fuel cell operation. Such a system could use natural gas, biogas, or hydrogen to generate electricity or produce combined heat and power.

A future high-temperature electrolyzer could also be coupled with a system for synthesizing chemicals such as methane (e.g., an e-gas system). The resulting waste heat could be used to generate the water vapor needed for high-temperature electrolysis. According to the researchers’ simulations, hydrogen generation and methane synthesis would each have an efficiency of about 75% relative to their respective calorific values. This already takes into account the compression of the gases to 80 bar.

In the project, the CT researchers worked together with the ceramics manufacturer Kerafol and Forschungszentrum Jülich to optimize electrochemical cells that use an oxygen ion-conducting electrolyte as a substrate. The main challenge was to prevent the oxygen electrode from becoming detached, which had previously caused aging effects.

The researchers improved the electrode’s stability by making it from a material that conducts electrons as well as oxygen ions. In a CT lab in Erlangen, ceramic electrolytic cells ran for more than 8,000 hours at 850 °C. The cells had a current density of 0.5 amperes per square centimeter and a cell voltage of up to 1.1 volts.

In this endurance test, the researchers noticed that the voltage-related aging amounted to only 0.2% per 1,000 hours of operation.

The researchers also demonstrated a concept for constructing the cell stacks. However, further development work is needed before larger cell stacks will have a sufficiently high level of long-term stability.