(562g) Electrochemical Reduction of N2 to NH3 Using Porous Bimetallic Pd-Ag Nanoparticles in Liquid and Gas Phase Systems

Despite recent remarkable progress in electrochemical nitrogen reduction reaction (NRR) for ammonia synthesis, demonstrating N₂ electrolysis at high current densities with decent selectivity and activity is crucial to decrease the capital cost, leading to the commercialization of electrochemical NRR. Here, we demonstrate electrochemical NRR for ammonia synthesis using porous bimetallic Pd-Ag nanoparticles in the fuel cell type electrochemical cell (gas phase) and conventional H-cell (liquid phase) at current densities above 1 mA cm^-2 under ambient conditions. While the conventional H-cell benefits from concentrated N₂ molecules dissolved in the electrolyte (0.5M LiClO₄ (aq.)) near the electrode surface and hydration effect, which results in higher selectivity and Faradaic efficiency, the gas phase system takes advantage of a substantial reduction in ohmic losses and higher energy efficiency compared to a liquid phase system. The electrocatalytic NRR activity (NH₃ yield rate = 45.6 µg cm^-2 h^-1, Faradaic efficiency = 19.6% @ 1.1 mA cm^-2) at 0.6 V vs. RHE was achieved in the liquid phase. This activity corresponds to the energy efficiency of 9.9%. This is compared to the electrocatalytic NRR activity (NH₃ yield rate = 19.4 µg cm^-2 h^-1, Faradaic efficiency = 7.9% @ 1.15 mA cm^-2) at only 0.07 V vs. RHE and 27.1% energy efficiency in the gas phase. Operando surface-enhanced Raman spectroscopy (SERS) is used to identify the intermediate species relevant to NRR at the solid-liquid (electrode-electrolyte) interface. This work highlights the unparalleled importance of design and optimization of cell configuration in addition to the catalyst for high-performance N₂ electrolysis for ammonia synthesis.