Selective electrochemical synthesis of urea from nitrate and CO2 via relay catalysis on hybrid catalysts

The nitrogen cycle needed for scaled agriculture relies on energy- and carbon-intensive processes and generates nitrate-containing wastewater. Here we focus on an alternative approach-the electrified co- electrolysis of nitrate and CO2 to synthesize urea. When this is applied to industrial wastewater or agricultural runoff, the approach has the potential to enable low-carbon-intensity urea production while simultaneously providing wastewater denitrification. We report a strategy that increases selectivity to urea using a hybrid catalyst: two classes of site independently stabilize the key intermediates needed in urea formation, *CO2NO2 and *COOHNH2, via a relay catalysis mechanism. A Faradaic efficiency of 75% at wastewater-level nitrate concentrations (1,000 ppm NO3- [N]) is achieved on Zn/Cu catalysts. The resultant catalysts show a urea production rate of 16 mu mol h(-1) cm(-2). Life- cycle assessment indicates greenhouse gas emissions of 0.28 kg CO(2)e per kg urea for the electrochemical route, compared to 1.8 kg CO(2)e kg(-1) for the present-day route.