Phenol serves as an effective proton shuttle and buffer in lithium-mediated ammonia electrosynthesis. This study demonstrates that phenol achieves a high Faradaic efficiency (72 ± 3%) for ammonia production, surpassing ethanol, which is commonly used as a proton shuttle. Experimental investigations, including operando isotope-labeled mass spectrometry, confirm phenol's proton-shuttling capability. Mass transport modeling reveals the mechanism of proton shuttling in the Li-NRR process. The study establishes design principles for effective proton shuttles, emphasizing the importance of functional groups, pKa values, and electrochemical stability. Phenol's deprotonated form (PhO⁻) acts as the primary species responsible for proton transfer during the Li-NRR process. The findings contribute to the understanding of the mechanistic aspects and design principles for efficient proton shuttles in practical Li-NRR applications, paving the way for sustainable and environmentally friendly ammonia production methods. The study also highlights the role of phenol as a proton buffer, reducing the adverse effects of the competitive hydrogen evolution reaction (HER). The results demonstrate that phenol exhibits excellent proton transfer capabilities and stability in the Li-NRR process, making it a promising candidate for practical ammonia synthesis.Phenol serves as an effective proton shuttle and buffer in lithium-mediated ammonia electrosynthesis. This study demonstrates that phenol achieves a high Faradaic efficiency (72 ± 3%) for ammonia production, surpassing ethanol, which is commonly used as a proton shuttle. Experimental investigations, including operando isotope-labeled mass spectrometry, confirm phenol's proton-shuttling capability. Mass transport modeling reveals the mechanism of proton shuttling in the Li-NRR process. The study establishes design principles for effective proton shuttles, emphasizing the importance of functional groups, pKa values, and electrochemical stability. Phenol's deprotonated form (PhO⁻) acts as the primary species responsible for proton transfer during the Li-NRR process. The findings contribute to the understanding of the mechanistic aspects and design principles for efficient proton shuttles in practical Li-NRR applications, paving the way for sustainable and environmentally friendly ammonia production methods. The study also highlights the role of phenol as a proton buffer, reducing the adverse effects of the competitive hydrogen evolution reaction (HER). The results demonstrate that phenol exhibits excellent proton transfer capabilities and stability in the Li-NRR process, making it a promising candidate for practical ammonia synthesis.