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

The electrochemically-driven N 2 reduction reaction (N 2 RR) offers an alternative route to the traditional Haber-Bosch method for production of NH 3 . Electrifying NH 3 synthesis promises a route to the decarbonization and decentralization of ammonia production, which currently accounts for >1% of annual CO 2 emissions. While there have been many studies examining electrochemically-driven production of NH 3 in aqueous electrolytes at low-T (≤ 50 o C) and low-P (≤20 bar), lithium-mediated N 2 reduction (Li-N 2 R) in nonaqueous electrolytes at low-T and low-P has emerged as a promising method for obtaining reliable electroreduction of N 2 to NH 3 . 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-N 2 R reaction using solar illumination. Nonaqueous, photoelectrochemical Li-N 2 R 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-N 2 R reaction and the maximum attainable solar-to-NH 3 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-N 2 R 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 N 2 -to-NH 3 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 N 2 -to-NH 3 production.