Catalysis Today, Vol.374, 102-116, 2021
Green catalytic synthesis of ammonia using solid oxide electrolysis cells composed of multicomponent materials
This study indicated that joining two different types of solid-state materials, co-doped lanthanum strontium chromium ferrite (LSCrF, La0.33Sr0.67Cr1-xFexO3-delta) perovskite oxide, and gadolinium-doped ceria (GDC, Ce0.9Gd0.1O2-delta) fluorite, yielded series of unique electrocatalytic composites, enabling a green production of ammonia (NH3, (g)) via nitrogen (N-2, (g)) and steam (H2O, (g)) reaction at the cathode compartment. An optimized ball milling approach was implemented to produce the cathodic composites with different formulations to respond to the environmental stimuli changes of operational temperature and applied voltage. The solid oxide electrolysis cells (SOECs) were then assembled and used for NH3 production with optimized synthesis and electrochemical measurement variables. As a result, these engineered cathodic catalysts maximized the efficacy of NH3 production due to the formation of oxygen vacancies, high interactivity between catalysts and reactants, and gas channeling through the macro-porous structure. To tune the macro-porous structure of the cathodic composites, the group invented an in-situ templating using a porogen (PMMA, polymethylmethacrylate, (C5O2H8)(n)) with a controllable size to facilitate gas diffusion and adsorption along with the interfaces of phase boundaries. Electrochemical analyses such as the measurement of impedance demonstrated that the macroporous structured (LSCrF-GDC) electrode exhibited superior electrocatalytic kinetics. Consequently, the formation rate of NH3 was found to achieve 3.0 x 10(-10) mol.s(-1).cm(-2) when the SOEC technology was implemented under an applied voltage of 1.8 V and operating temperature at 600 degrees C. The mechanistic study of electrochemical performance and interactivity between cathodic catalysts and reactants indicated that an optimized NH3 production pathway was carried out below the rmoneutral enthalpy to achieve high efficiency. This goal was achieved through adjustment of the ratio between the enthalpy flow of produced hydrogen and the input of electrical power. The contribution of this study lies in the optimized methodology to enhance interactive electrocatalysis applied for the green synthesis of ammonia.