Comparison between gas phase and electrochemical nitridation of 8YSZ under nitrogen atmosphere to produce nitride conducting solid electrolytes

Abstract

Ammonia is one of the most used chemicals in the world. It is commonly synthesized by the Haber-Bosch process which requires high temperature (450–500 °C) and pressure (up to 300 bar). This process is thermodynamically limited and causes environmental problems due to CO₂ emissions caused by the production of H₂ required by this process from fossil fuels. Electrocatalytic processes using oxide and proton-conducting electrolytes are gaining interest for ammonia production to overcome these limitations. Although both methods overcome many of the problems associated with the Haber-Bosch process, due to strong NN triple bonds selectivity towards ammonia decreases. This is because the reaction occurs on the same side of the membrane electrode assembly, namely the cathode electrode, where nitrogen is fed in the gas phase and nitrogen bonds should be broken to react with hydrogen ions readily available on the electrolyte surface. Since NN bond cleavage requires very high energy, hydrogen ions generally recombine to form H₂ before the nitrogen can be ionized. Nitride conducting electrolytes can be an answer to this problem because in their use nitrogen ionization and ammonia synthesis reactions occur at different electrodes and nitrogen is fed to the reaction site in the ionic form which is more active for the reaction. This study focuses on two alternative methods for the production of nitride conducting solid electrolytes by nitridation of 8 % Yttria Stabilized Zirconia (8YSZ). Two different methods for nitridation were studied: gas phase powder nitridation and electrochemical nitridation of YSZ electrolytes. This study shows that although gas phase nitridation of YSZ powders at high temperatures under nitrogen is not efficient, electrochemical nitridation of YSZ electrolytes is a highly promising method to produce nitride conducting electrolytes.