Infiltration of Metal Catalysts for NH3 Synthesis in Protonic Ceramic Cells

Abstract

The electrochemical synthesis of green NH3 representes a promising alternative to the conventional Haber-Bosch process, which currently relies on H2 mainly sourced from fossil fuels. Additionally, it compensates for the thermodynamic limitations of the ammonia synthesis reaction via the utilization of high pressures of 150-300 bar, resulting in high energy consumption. The prospect of solid state ammonia synthesis (SSAS) in proton conducting electrolysis cells (PCEC) has been the subject of increased interest recently [1]. It features steam in place of H2 as a reactant and the electrochemical activation of N2 at the hydrogen electrode (cathode) through the use of a catalyst. Possible candidates are metallic Ru or Fe, analogous to the thermocatalytic Haber-Bosch process.The implementation of the catalyst in the electrochemical cell is critical to the performance of the system. In order to augment the kinetics, high amounts of catalyst near the electrode-electrolyte interface are desired. Therefore, the microstructure of the electrode is of primary interest, as the void fraction of the layer must be large enough to enable a well-distributed deposition of catalyst particles. This may be achieved via the use of a pore-former, e.g. graphite powder, in the wet-route elaboration of the hydrogen electrode (see Fig. 1). However, modifications must not decrease the mechanical properties of the layer.The effect of various parameters (sintering conditions, thickness of the electrode layer, effect of additives to enhance wettability) on the final electrode structure were investigated, as well as the influence of the infiltration protocol (evacuation time, number of infiltration steps, drying procedure). The findings along with preliminary electrochemical characterizations will be discussed.[1] V. Kyriakou, I. Garagounis, E. Vasileiou, A. Vourros, M. Stoukides, Progress in the Electrochemical Synthesis of Ammonia, Catalysis Today, 286 (2017) 2-13. Figure 1