Electrochemical nitrogen reduction: a sustainable way to produce ammonia and fertilizers

Solar energy driven processes with H2O, CO2 and N2 as basic feedstocks can produce “solar fuels” that could substitute their fossil-based counterparts. These renewable fuels could reasonably play a key role in future energy systems, e.g. as a storage medium in the power sector, as an energy carrier in the transport sector, as fundamental building blocks for the chemical industry. While H2 is the most likely and realistic candidate for electricity storage in solar fuels, research on the electrochemical conversion of CO2 and N2 into C-based fuels and NH3 has intrigued electrochemists for decades and is currently undergoing a notable renaissance. In contrast to H2 production by H2O electrolysis, CO2 and N2 electrocatalytic reduction processes are still far from a mature technology. Significant hurdles regarding reaction selectivity, energy efficiency and overall conversion rate need to be overcome if these processes have to become a viable option for storing renewable electricity and permit the ecological transition, for which Italy has recently created a new Ministry. Among these technologies, electrochemical nitrogen reduction (E-NRR) has emerged as a challenging strategy for the replacement of the Haber-Bosch process for NH3 and fertilizers production worldwide. Even if many E-NRR electrocatalysts and electrolytes have been proposed, major challenges are still related to the low activity and selectivity, and the competitive hydrogen evolution reaction (HER) dominates (the majority of electrons and protons goes toward H2 evolution instead of reducing N2 to NH3). The present lecture will be focused on fundamentals of E-NRR, as well as most significant materials and chemical approaches developed in the last 5 years.