Electrosynthesis of Ammonia Using Porous Bimetallic Pd–Ag Nanocatalysts in Liquid- and Gas-Phase Systems

Cost-effective production of ammonia via electrochemical nitrogen reduction reaction (NRR) hinges on N 2 electrolysis at high current densities with suitable selectivity and activity. Here, we report our findings in electrochemical NRR for ammonia synthesis using porous bimetallic Pd–Ag nanocatalysts in both gas-phase and liquid-phase electrochemical cells at current densities above 1 mA cm –2 under ambient conditions. While the gas-phase cell has lower Ohmic losses and higher energy efficiency, the liquid-phase cell achieved higher selectivity and Faradaic efficiency, attributed to the presence of concentrated N 2 molecules dissolved in an aqueous electrolyte and the hydration effects. The liquid cell demonstrated notable performance for electrocatalytic NRR, achieving an NH 3 production rate of 45.6 ± 3.7 μg cm –2 h –1 at a cell voltage of −0.6 V (vs RHE) and current density of 1.1 mA cm –2, corresponding to a Faradaic efficiency of ∼19.6% and an energy efficiency of ∼9.9%. Similarly, the gas-phase cell achieved a NH 3 yield rate of 19.4 ± 2.1 μg cm –2 h –1 at −0.07 V (vs RHE) and 1.15 mA cm –2 with a Faradaic efficiency of 7.9% and an energy efficiency of 27.1%. Further, operando surface-enhanced Raman spectroscopy and density functional theory (DFT) are used to identify intermediate species relevant to the NRR at the electrode–electrolyte interfaces to provide insights into the NRR mechanism on Pd–Ag nanoparticles. This work highlights the importance of design and optimization of cell configuration in addition to the modification of the catalyst to achieve high-performance N 2 electrolysis for ammonia synthesis.