(Invited) High Temperature Alkaline Electrolyte for Energy Conversion

Abstract

Alkaline electrolyte can enable to use non-precious metal catalysts for a variety of energy conversions. However, conventional polymer-based alkaline membranes generally have poor stability, particularly at elevated temperatures (> 80 ℃), thus limiting their viable applications. On the other hand, operating electrochemical cells at higher temperature can improve the electrode kinetics, simplify heat management, and increase the system efficiency. We have developed a high temperature alkaline system that uses molten hydroxides (e.g., lithium, sodium, or potassium hydroxide) impregnated into porous metal oxides. The attained hybrid alkaline electrolyte has demonstrated very high hydroxide ion (OH-) conductivity (> 0.5 S/cm) in a wide operating range (300-500 °C), depending on the category and ratio of each individual electrolyte. A key factor that influences the success of this technology is the microstructures of the porous oxide matrices. Their thickness, porosity, pore size and structure largely determine whether they can successfully retain molten hydroxides in their pores, particularly over extended period of time. We have thereby successfully used high temperature alkaline electrolyte for water electrolysis, the water electrolysis efficiency has been improved to nearly 90% at 500 °C. The strategies to further lower the temperature while retaining a similar performance has been discovered. Mitigation strategies have also been found to reduce the hot corrosion. In addition to water electrolysis, this high temperature alkaline electrolyte can also be used for both electrochemical synthesis of ammonia and direct ammonia fuel cells.