The reaction free energy is observed to be positive (≈ +0.87eV) at 0 V vs. RHE, indicating that CO2 adsorption is unfavorable at lower cathodic potential, consistent with reports from previous studies suggesting that CO2 adsorption is rate-limiting during electrochemical CO2 reduction on Ag . The adsorption-free energy becomes more favorable as the potential becomes more reducing and becomes spontaneous at more cathodic potential (< -0.87V vs. RHE), indicating an increasing COOH coverage on the catalyst surface as the potential becomes more cathodic. Our calculations reveal that the second C-N bond formation will likely occur after a series of protonation steps (*CONH2 + *NO → *CONH2NO, ΔG < 0 ). We find that additional protonation of adsorbed *CONH2 to produce HCONH2 (formamide) is thermodynamically unfavorable (ΔG > 1.5 eV at 0 V vs. RHE), explaining the lack of formamide detected by experimental efforts. The second C-N bond formation barrier for this reaction on Ag (100) is also calculated. Our results suggest that the second C-N bond formation barrier, which we found to be 0.56eV -0.62 eV, is likely not rate determining. The free energy diagram (at 0V vs. RHE) shows that the energies of all the elementary steps in this reaction are exergonic except for the protonation of adsorbed CON (*CON + H + + e -→ *CONH). Since the protonation step is a reducing electrochemical step, it becomes more favorable at a more cathodic potential. From our DFT results, we hypothesize that urea synthesis from the co-reduction of CO2 and nitrate on Ag is limited by either i) CO2 adsorption on the catalyst surface, or ii) mass transport of reactants from the bulk electrolyte to the reaction plane. The urea current density is drastically improved when GDE configuration is used, which validates this hypothesis.