Electrochemical Ammonia Synthesis Using a BZCYYb4411 Proton Conductor

Abstract

The Haber Bosch process is the dominant route to synthesizing ammonia. Ammonia is an important component in fertilizers, which are widely used across the agriculture community. Dependence on the Haber Bosch process is detrimental because: (1) this process operates at high temperatures (~450°C)1 and pressures (150-300 bar) and accounts for 2% of the worlds energy supply2, (2) contributes 1% of all CO2 emissions, and (3) is typically centralized which limits access in developing worlds. Electrochemical ammonia production has the potential to greatly reduce society’s carbon footprint and energy consumption, as well as increase ammonia production in developing worlds and rural communities. This type of ammonia synthesis method has been shown to operate with air and water sources as opposed to highly pure nitrogen and hydrogen reactants used in commercial plants. Solid state electrochemical systems are able to operate at elevated temperatures (300-600 °C) unlike ambient processes (low temperature fuel cells) and thus have the potential for higher yields. Solid state ceramic proton conductors show high ionic conductivities (1E-2 Scm-1 at 600°C)3 and are stable in this intermediate temperature range. Currently, the investigation of intermediate temperature ion conductors in solid state ammonia synthesis is nascent but growing in interest. Herein, we describe the synthesis and processing of a highly conductive proton conductor BaZr0.4Ce0.4Y0.1Yb0.1O3- δ (BZCYYb4411) (Fig. 1a,b) is characterized and implemented in a solid state fuel cell arrangement. BZCYYb4411 is synthesized using a solid-state reaction route and Ag-Pd catalyst is used to make a symmetric cell. In this one chamber reactor employing 3% H2/N2, the faradaic efficiency and ammonia production rate is explored at different temperatures and operating voltages to elucidate the optimal conditions for ammonia synthesis. (1) Kyriakou, V.; Skodra, A.; Stoukides, M.; Vasileiou, E.; Garagounis, I. Electrochemical Synthesis of Ammonia in Solid Electrolyte Cells. Front. Energy Res. 2014, 2 (January), 1–10. (2) Martín, A. J.; Shinagawa, T.; Pérez-Ramírez, J. Electrocatalytic Reduction of Nitrogen: From Haber-Bosch to Ammonia Artificial Leaf. Chem 2019, 5 (2), 263–283. (3) Choi, S.; Kucharczyk, C. J.; Liang, Y.; Zhang, X.; Takeuchi, I.; Ji, H. Il; Haile, S. M. Exceptional Power Density and Stability at Intermediate Temperatures in Protonic Ceramic Fuel Cells. Nat. Energy 2018, 3 (3), 202–210. Figure 1