High-Temperature Molten Alkaline Water Electrolysis
Authors: Patil Kailash , Sweet Andrew , Greene Winfield , Xu Hui 
Published: February 2020

The rapid development and deployment of fuel cell vehicles require large numbers of hydrogen (H2) refueling stations. Hydrogen production for mobile and energy storage applications from water electrolysis is attractive due to its high efficiency, fast ramp rates, and high-pressure capability. However, current hydrogen production from electrolysis comprises only a small fraction of the global hydrogen market due to the high cost associated with expensive stack materials (membrane, catalyst, and bipolar plates) and electricity consumption of the commercially available electrolysis systems [1]. There are currently two main types of electrolyzers, both operating at moderate temperatures (50–80 ˚C): proton exchange membrane (PEM) and anion exchange membrane (AEM). Alkaline membrane electrolyzer is a promising technology it suffers from poor AEM stability, particularly at elevated temperature (> 80 ℃).
Giner has developed a high-temperature alkaline water electrolysis, molten alkaline electrolyte (MAE). In this technology, the high-temperature molten alkaline electrolyte employs lithium, sodium, or potassium hydroxide impregnated into a porous alumina or zirconia matrix as the electrolyte membrane [2, 3]. The operating temperature can vary from 200 to 550 °C, depending on the category and ratio of individual electrolyte.
In this work, we will present detailed configuration of high-temperature alkaline water electrolysis focusing on the alkaline composite electrolyte and non-precious metal electrodes, e.g., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts using a single cell electrolyzer set-up. We have developed a component assembly process utilizing a tape-casting method and a unique facility has been designed and constructed.
The OH– ion conductivity of single or binary hydroxide has been investigated, at a temperature between 300 to 550 °C. The temperature dependencies of conductivity for palletized composite single/binary hydroxide electrolyte are depicted in Figure 1. The binary hydroxide conductivity (LiOH-NaOH-melt-50) was higher compared to the single hydroxide (LiOH-50). The single or binary hydroxides went through premelting process before testing. This process was conducted to limit the evaporation, and the eventual loss, of the electrolyte during the testing by allowing the electrolyte to evaporate before testing began and not affect measurements and calculations conducted during testing. Design of experimental techniques have also been applied to optimize the ceramic membrane properties and to select non-precious HER/OER catalysts. The single cellperformance using selected molten alkaline electrolyte will also be discussed.

Figure 1 Temperature-dependence of OH– ion conductivity of composite single/binary hydroxide electrolyte at a temperature between 300 °C to 550 °C.

References

Anthony, J. Rand, and R. Dell, Hydrogen Energy Challenges and Prospects (RSC Energy Series) DOI: 10.1039/9781847558022 (2008).

Huang, C. Yuh, US Patent 5,869,203 (1999).

Patil, S. Yoon, J. Han, T. Lim, S. Nam, I. Oh, Int. J. Hydrogen Energy, 36, 6237-6247 (2011).

Figure 1