Enhancing the rate and efficiency of electrochemical ammonia synthesis from N2 in concentrated divalent electrolytes

Abstract

The electrochemical nitrogen reduction reaction (ENRR) is a promising alternative to the Haber–Bosch process, offering an energy-efficient and carbon-free method for ammonia synthesis. In the last few years, many catalyst materials have been investigated for N₂ conversion to ammonia, where PtRu has shown relatively high activity, selectivity, and stability. This work describes a novel approach to increase the ENRR rate and efficiency by utilizing concentrated electrolytes (≥40 wt%) of common and nontoxic divalent CaBr₂ and MgBr₂, which have high hydration energy, resulting in low free water concentration. This results in high overpotential for the competing hydrogen evolution reaction and high N₂ solubility while maintaining high conductivity (120–160 mS cm⁻¹ at 30–60 wt%). These properties have been harnessed to improve the rate of ammonia formation and the faradaic efficiency (FE) of the kinetically sluggish ENRR. The highest NH₃ production rate of 94.01 µg h⁻¹ cm⁻² and a FE of 23.7% at −0.15 V vs. the reversible hydrogen electrode (RHE) are observed in 60% CaBr₂ solution using the PtRu alloy as the catalyst. This ammonia production rate is 2 times higher than that in 0.1 M KOH and 5 times higher than that in 0.25 M (4.75 wt%) CaBr₂ electrolyte.