Nanointerfacial Engineering of a Copper-Based Electrocatalyst for the Selective Electrogeneration of Ammonia from Nitrate Pollution

Anthropogenic activity has directly led to increasing levels of nitrate (NO 3 - ) pollution across the globe, generating great necessity for a sustainable remediation pathway for this pervasive contaminant. The implementation of the electrocatalytic reduction of nitrate (ERN) provides a chemical free and low energy pathway to convert NO 3 - into a highly profitable resource ammonia (NH 3 ) via an 8-electron reduction reaction. NH 3 is a vital chemical for numerous industrial and agricultural applications and has been identified as a next generation energy source for hydrogen driven technologies. While ERN is a promising technology to address such pollution, there are two major limitations barricading ERN from being implemented in real world applications. First, current electrocatalyst technologies rely on costly and rare platinum group metals (PGMs). Second, highly sensitive potentiostatic conditions require highly alkaline media, specialized reactions cells, and initial NO 3 - concentrations greatly superseding those found in natural waters. This study aims to address these limitations by constructing a cost-effective, green, and earth-abundant electrocatalyst without reliance on PGMs to enable ERN under galvanostatic operation. The surface of a three-dimensional copper foam cathode was modified through electrosynthesis followed by thermal treatment to generate nano dimension CuO and Cu 2 O structures, directly tailoring the ratios of Cu 2+ to (Cu 0 + Cu 1+ ) as well as concentration of lattice oxygen and oxygen vacancies. Catalyst designs which increase both the Cu 2+ as well as lattice oxygen were found to be the most efficient in the ERN in terms of ammonia generation and selectivity. The Cu 2+ character of the electrocatalyst can be associated with the higher Lewis acidity of Cu 2+ catalytic sites when compared to metallic Cu foam. The enhanced Lewis acidity of the electrocatalyst promotes stronger binding and activation of nitrate and nitrite, which greatly synergizes the electrochemical conversion into ammonia. The same character has been known to stabilize NO 2 and NO intermediate species, suppressing parasitic side reactions (HER), providing a selective reaction pathway toward NH 3 formation. It has been reported that lattice oxygen effectively mediate proton shuttling as an essential step in the hydrogenation mechanism of NO x intermediates, favoring NH 3 formation over N 2 (g). These two material properties were found to lower reaction energy barriers for 8-electron reduction of NO 3 - to NH 3 . The optimization of these material properties resulted in the development of a highly selective Cu-ET electrocatalyst that achieved 91% conversion of NO 3 - with 97.6% selectivity toward NH 3 electrogeneration within 1 h of ERN. Performance metrics of the Cu-ET electrocatalyst were directly compared to Cu foam (control) which showcased a two-fold improvement in selectivity towards ammonia formation. In addition, all electrocatalytic conversion/measurement metrics were validated through monitoring the mass balance of reactant, products, and intermediates. The Cu-ET electrocatalyst maintained extremely stable performance for twenty-five continuous ERN cycles. The design of the Cu-ET electrocatalyst demonstrated a reduction in material cost of over 5300x when compared to the average PGM electrocatalysts reported in the literature based on the cost of raw materials. The robust galvanostatic operation of this technology adds to the modular nature and compatibility with renewable energy sources, which maximizes the scalability of this ERN technology. The highly selective nature of the Cu-ET electrocatalyst for the electrogeneration of NH 3 , further reduces treatment costs and increases accessibility to such water treatment technologies that enabled resource recovery capacity.