Photoelectrochemical Nitrogen Fixation for Ammonia Synthesis Using Hybrid Plasmonic Nanostructures

Abstract

Cost-effective production of ammonia via (photo)electrochemical nitrogen reduction reaction (NRR) hinges on N2 electrolysis at high current densities with suitable selectivity and activity [1, 2]. 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 N2 molecules dissolved in an aqueous electrolyte and the hydration effects. The liquid cell demonstrated notable performance for electrocatalytic NRR, achieving an NH3 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 NH3 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 N2 electrolysis for ammonia synthesis. It also demonstrates the use of operando SERS as a powerful technique for unraveling reaction mechanisms for (photo)electrocatalytic phenomenon.[1] M. Nazemi, L. Soule, M. Liu, M. A. El-Sayed, “Ambient Ammonia Electrosynthesis from Nitrogen and Water by Incorporating Palladium in Bimetallic Gold-Silver Nanocages” Journal of The Electrochemical Society, 167(5), p. 054511, 2020.[2] M. Nazemi, M. A. El-Sayed, “Plasmon-Enhanced Photo(electro)chemical Nitrogen Fixation under Ambient Conditions Using Visible Light Responsive Hybrid Hollow Au−Ag2O Nanocages” Nano Energy, vol. 63, 103886, 2019.