Photoelectrochemical Nitrogen Fixation for Ammonia Synthesis Using Hybrid Plasmonic Nanostructures

摘要

Cost-effective production of ammonia via (photo)electrochemical nitrogen reduction reaction (NRR) hinges on N_2 electrolysis at high current densities with suitable selectivity and activity. In this talk, 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 μg cm~(-2) h~(-1) at a cathodic potential 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 an NH_3 yield rate of 19.4 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%. In addition, the photoelectrocatalytic activities of these hybrid plasmonic nanostructures under illumination and dark conditions will be explored and photocurrent and photovoltage responses will be reported. 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 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. It also demonstrates the use of operando SERS as a powerful technique for unraveling reaction mechanisms for (photo)electrocatalytic phenomenon.