Low-temperature electrochemical ammonia synthesis: recent progress and comparison to Haber-Bosch in terms of energy efficiency

Abstract

The electrochemical synthesis of ammonia, in the literature often referred to as Electrochemical Nitrogen Reduction Reaction (ENRR), has gained attention as a milder alternative to the Haber-Bosch process, which requires high temperature and pressure. Recently, several breakthrough scientific papers were published showing a way to achieve nearly 100% selectivity of the ENRR over hydrogen reduction at very high production rates, which had been identified earlier as a main challenge. These results motivated the present study to benchmark the energy efficiencies of the most reliable ENRR studies against the Haber-Bosch process. ENRR studies historically suffered from reliability issues due to low ammonia production rates, which were often within the range of ambient ammonia contamination, potentially leading to false positive measurements. Consequently, rigorous experimental protocols have been published in the last four years. Here we first reviewed the protocols and established a methodology and a reliability indicator to assess the reliability of the results. Then we used the indicator to evaluate the studies reported in the literature from January 2019 to Aug 2022. Finally, we discussed and calculated the energy efficiency of ENRR for works that were assessed as reliable and probably reliable with the indicator. We identified and discussed several promising systems and strategies enabling higher energy efficiency. The energy efficiency had a maximum of 41%, which is within the efficiency range of current Haber-Bosch plants. However, ENRR energy efficiencies must improve to get closer to the 56% some of the state-of-the art Haber-Bosch plants achieve.