(Invited) H2-Free, Highly-Selective Electrocatalytic Reduction of Nitrates to Ammonia with Solar-to-Ammonia Efficiency >5%

Abstract

Direct electrochemical reduction of dinitrogen to NH3 is not efficient due to higher mass transfer resistances (owing to lower solubility of N2) and the competing hydrogen evolution reaction (HER). However, electrochemical reduction of NO3 - to NH3 is less challenging than direct N2 reduction to NH3 due to lower dissociation energy of N = O bond (204 kJ/mol) as compared to N ≡ N (941 kJ/mol). Efficient conversion of NO3 - can aid in sustainable metallurgical, agricultural, and water purification applications. For instance, NO3 - is mined directly from saltpeter mines and it is an industrial waste byproduct from metal finishing facilities and uranium purification and processing its byproducts. NO3 - contamination in ground water by the agricultural runoff water is a major problem, whereas its contamination in drinking water can lead to methemoglobinemia in infants and several types of cancers in adults.Here we show theory-guided search of efficient transition metals to identify the most active catalyst for the electrochemical nitrate reduction to NH3. Experimental investigations corroborated with the theoretical findings, leading to the identification of transition metals that yielded highest NH3 Faradaic efficiency (FE). We will also show the effect of morphology and oxidation state of efficient catalyst on NH3 current density and FE. Effect of operating conditions such as pH, concentration of nitrates, and applied potential will be also discussed. A maximum NH3 FE of 68.13 % at -0.6 V vs RHE (pH = 14) and NH3 current density of 245.34 mA/cm2 was observed at -0.8 V vs RHE (pH = 14) with a lowest onset potential of 0.1 V vs RHE. The oxide-derived metal catalyst was found to be stable for the test period of 24 h with an average solar-to-ammonia efficiency of >5%. Control studies were also performed using N-15 nitrates to confirm that the source of NH3 produced.