Ammonia (NH3) is vital in the agriculture industry to produce fertilizer and more recently has emerged as a candidate Carbon-free solar fuel and hydrogen carrier. Approximately 2% of the world’s energy is used to produce NH3 using the Haber-Bosch process. The electrocatalytic nitrogen reduction reaction (NRR, N2 + 6H+ + 6e- → 2NH3) is a promising alternative to the energy-intensive Haber-Bosch process producing ammonia at ambient temperatures and pressures; however, at present, finding electrocatalysts with high activity and stability remains a challenge. Two-dimensional (2D) MoS2 is recognized as a promising catalyst for several important electrochemical reactions including NRR with its low-cost, high-surface area, tunable electronic properties such as its existence in a semiconducting (2H) and metallic (1T) phase. Still, catalysis of NRR is plagued by low efficiency and reaction rates due to competitive H2 generation, poor N2 solubility in aqueous solutions, and slow reaction kinetics. Overall, the field can benefit from a better mechanistic understanding of NRR to guide catalyst engineering. In this work, we combine theoretical calculations with ongoing experimental work to obtain mechanistic insight into the NRR on 2D MoS2 and other transition metal dichalcogenides. First, to gain an understanding of the elementary catalyst, we probe the activity and selectivity of distinct surface sites with a focus on the effects of phase and intrinsic defects. We perform these calculations with density functional theory (DFT) while considering the effects of more realistic conditions of solvent and applied potential. We use an ab initio joint-DFT framework (JDFTx) that employs continuum solvation models to handle solvent interfaces and grand-canonical DFT for fixed potential calculations, thereby more accurately describing the true behavior of electrochemical systems. Finally, in conjunction with experimental work, we functionalize the 1T-MoS2 surface with substituted phenyl diazonium salts to both stabilize the 1T-phase and further tune the energetics with electron donating and withdrawing functional groups. With improved understanding of how phase, defects, and functional groups impact NRR, the goal is to guide future catalyst design and generate competitive catalysts for NRR.

Ab initio mechanistic studies of the nitrogen reduction reaction probing the effect of phase, defects, and functional groups on the 2D catalyst MoS2 in real media

ACS Spring 2022

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