Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis

Xianbao Fu, Valerie A. Niemann, Yuanyuan Zhou, Shaofeng Li, Ke Zhang, Jakob B. Pedersen, Mattia Saccoccio, Suzanne Z. Andersen, Kasper Enemark-Rasmussen, Peter Benedek, Aoni Xu, Niklas H. Diessler, Jon Bjarke Mygind, Adam C. Nielander, Jakob Kibsgaard, Peter C.K. Vesborg, Jens K. Norskov, Thomas F. Jaramillo, Ib Chorkendorff
Year of publication: 
Nature Materials

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.

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