(Invited) Design Rules for (Li-mediated) Photoelectrochemical N2 Reduction to NH3

Abstract

The electrochemically-driven N2 reduction reaction (N2RR) offers an alternative route to the traditional Haber-Bosch method for production of NH3. Electrifying NH3 synthesis promises a route to the decarbonization and decentralization of ammonia production, which currently accounts for >1% of annual CO2 emissions. While there have been many studies examining electrochemically-driven production of NH3 in aqueous electrolytes at low-T (≤ 50o C) and low-P (≤20 bar), lithium-mediated N2 reduction (Li-N2R) in nonaqueous electrolytes at low-T and low-P has emerged as a promising method for obtaining reliable electroreduction of N2 to NH3.With this in mind, our team at SUNCAT and NREL have begun evaluating the opportunity and key design rules necessary for coupling photoelectrochemical methods to the drive the Li-N2R reaction using solar illumination. Nonaqueous, photoelectrochemical Li-N2R promises a route to stable operation that has been a key challenge associated with photoelectrode operation in aqueous electrolytes. Recognizing the energetic constraints associated with the Li-mediated process, we have modeled and mapped the bandgap characteristics necessary for a semiconducting light absorber to drive the Li-N2R reaction and the maximum attainable solar-to-NH3 conversion yields. We have identified III-V based photoelectrodes that meet the modeled requirements and evaluated the surface chemistry at the electrode/electrolyte interface under Li-N2R conditions to understand the how reactive Li-species influence photoelectrochemical performance (e.g., rate, selectivity, stability), how solid-electrolyte interface (SEI) formation and electrolyte selection are mutually influenced by illumination, and how the current density and cycling conditions enforced by solar operation dictate photoelectrochemical N2-to-NH3 performance. We have further extended a neutron-reflectometry technique used for in situ observation of interfacial layer formation to be compatible with light-driven reactions, and with these insights in hand continue our drive toward demonstrating unassisted photoelectrochemical N2-to-NH3 production.