Research Article

Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle

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Science  11 Jun 2021:
Vol. 372, Issue 6547, pp. 1187-1191
DOI: 10.1126/science.abg2371

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Shuttling protons in ammonia synthesis

An electrochemical route to ammonia could substantially lower the greenhouse gas emissions associated with the current thermal Haber-Bosch process. One relatively promising option under study involves reductive formation of lithium nitride, which can be protonated to ammonia. However, the ethanol used to date as a local proton source in these studies may degrade under the reaction conditions. Suryanto et al. report the use of a tetraalkyl phosphonium salt in place of ethanol (see the Perspective by Westhead et al.). This cation can stably undergo deprotonation–reprotonation cycles and, as an added benefit, it enhances the ionic conductivity of the medium.

Science, abg2371, this issue p. 1187; see also abi8329, p. 1149

Abstract

Ammonia (NH3) is a globally important commodity for fertilizer production, but its synthesis by the Haber-Bosch process causes substantial emissions of carbon dioxide. Alternative, zero-carbon emission NH3 synthesis methods being explored include the promising electrochemical lithium-mediated nitrogen reduction reaction, which has nonetheless required sacrificial sources of protons. In this study, a phosphonium salt is introduced as a proton shuttle to help resolve this limitation. The salt also provides additional ionic conductivity, enabling high NH3 production rates of 53 ± 1 nanomoles per second per square centimeter at 69 ± 1% faradaic efficiency in 20-hour experiments under 0.5-bar hydrogen and 19.5-bar nitrogen. Continuous operation for more than 3 days is demonstrated.

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