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.