All the electrochemical urea synthesis experiments were conducted in a custom 3D-printed flow cell. Pt was used as the counter electrode, Ag/AgCl/sat. KCl was used as the reference electrode and 25 % HNO3 was used as the anolyte. The catholyte and anolyte were separated by using Nafion 117 membrane. Nafion 117 membrane was pre-treated by soaking it in 3 % H2O2 at 80 °C for 1 h, followed by deionized (DI) H2O at 80 °C for 2 h, and followed by 0.5 M H2SO4 at 80 °C for 1 h. The membrane is rinsed between each step using DI H2O and the final treated membrane is stored in the DI H2O. The anolyte volume used was 5 ml and it was kept stationary in the anolyte side. The catholyte volume used was 30 ml and it flowed from a reservoir to the catholyte side using a peristaltic pump at a rate of 40 ml/min. The electrodes were polished before experiments for 10 mins using a finer polishing pad (Pike Technologies -Crystal Polishing Kit #162-400) by using a ceria-based polishing compound (Pike Technologies -Polishing Compound #162-4014) followed by sonicating the electrodes in DI water for 10 mins after which they are dried in Ar.