Long-term continuous ammonia electrosynthesis.

Ammonia is crucial as a fertilizer and in the chemical industry and is considered to be a carbon-free fuel1. Ammonia electrosynthesis from nitrogen under ambient conditions offers an attractive alternative to the Haber-Bosch process2,3, and lithium-mediated nitrogen reduction represents a promising approach to continuous-flow ammonia electrosynthesis, coupling nitrogen reduction with hydrogen oxidation4. However, tetrahydrofuran, which is commonly used as a solvent, impedes long-term ammonia production owing to polymerization and volatility problems. Here we show that a chain-ether-based electrolyte enables long-term continuous ammonia synthesis. We find that a chain-ether-based solvent exhibits non-polymerization properties and a high boiling point (162 °C) and forms a compact solid-electrolyte interphase layer on the gas diffusion electrode, facilitating ammonia release in the gas phase and ensuring electrolyte stability. We demonstrate 300 h of continuous operation in a flow electrolyser with a 25 cm2 electrode at 1 bar pressure and room temperature, and achieve a current-to-ammonia efficiency of 64 ± 1% with a gas-phase ammonia content of approximately 98%. Our results highlight the crucial role of the solvent in long-term continuous ammonia synthesis.

Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis.

Ammonia (NH3) is a key commodity chemical for the agricultural, textile and pharmaceutical industries, but its production via the Haber-Bosch process is carbon-intensive and centralized. Alternatively, an electrochemical method could enable decentralized, ambient NH3 production that can be paired with renewable energy. The first verified electrochemical method for NH3 synthesis was a process mediated by lithium (Li) in organic electrolytes. So far, however, elements other than Li remain unexplored in this process for potential benefits in efficiency, reaction rates, device design, abundance and stability. In our demonstration of a Li-free system, we found that calcium can mediate the reduction of nitrogen for NH3 synthesis. We verified the calcium-mediated process using a rigorous protocol and achieved an NH3 Faradaic efficiency of 40 ± 2% using calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) as the electrolyte. Our results offer the possibility of using abundant materials for the electrochemical production of NH3, a critical chemical precursor and promising energy vector.

Phenol as proton shuttle and buffer for lithium-mediated ammonia electrosynthesis.

Ammonia is a crucial component in the production of fertilizers and various nitrogen-based compounds. Now, the lithium-mediated nitrogen reduction reaction (Li-NRR) has emerged as a promising approach for ammonia synthesis at ambient conditions. The proton shuttle plays a critical role in the proton transfer process during Li-NRR. However, the structure-activity relationship and design principles for effective proton shuttles have not yet been established in practical Li-NRR systems. Here, we propose a general procedure for verifying a true proton shuttle and established design principles for effective proton shuttles. We systematically evaluate several classes of proton shuttles in a continuous-flow reactor with hydrogen oxidation at the anode. Among the tested proton shuttles, phenol exhibits the highest Faradaic efficiency of 72 ± 3% towards ammonia, surpassing that of ethanol, which has been commonly used so far. Experimental investigations including operando isotope-labelled mass spectrometry proved the proton-shuttling capability of phenol. Further mass transport modeling sheds light on the mechanism.

Continuous-flow electrosynthesis of ammonia by nitrogen reduction and hydrogen oxidation.

Ammonia is a critical component in fertilizers, pharmaceuticals, and fine chemicals and is an ideal, carbon-free fuel. Recently, lithium-mediated nitrogen reduction has proven to be a promising route for electrochemical ammonia synthesis at ambient conditions. In this work, we report a continuous-flow electrolyzer equipped with 25-square centimeter-effective area gas diffusion electrodes wherein nitrogen reduction is coupled with hydrogen oxidation. We show that the classical catalyst platinum is not stable for hydrogen oxidation in the organic electrolyte, but a platinum-gold alloy lowers the anode potential and avoids the decremental decomposition of the organic electrolyte. At optimal operating conditions, we achieve, at 1 bar, a faradaic efficiency for ammonia production of up to 61 ± 1% and an energy efficiency of 13 ± 1% at a current density of -6 milliamperes per square centimeter.