Figure shows the free energy of OD-Cu reduction as a function of pH. At low pH, OD-Cu tends to be fully reduced to generate pure copper. While at high pH values, which are typical of highly active and ethylene selective CO2RR electrolyzers, all the steps of the reduction are endergonic, and the perfect Cu2O is the most stable state. At intermediate pH, the partially reduced Cu2O system is the thermodynamic ground state. In electrochemical media, with KHCO3 buffer as electrolyte (pH 6.8-8) about ~15.7 at.% oxygen would remain in a system with a S.S.A. of 0.146 Å -1 at zero potential vs SHE. However, the reduction of the first 45.8% (22/48) of oxygen is endergonic, which indicates that starting the reduction requires a driving force. As expected, the driving force can be the electric potential, as shown in Figure , at more reductive potentials, the reduction to copper is favorable. Metallic copper is more favorable than all the partially reduced OD-Cu structures at potential as small as -0.20 VSHE (0.28 VRHE). However, the 'elementary step' with largest positive reaction energy was considered to be the potential limiting step and prevent further reduction. This results in a potential of -0.68 VSHE (-0.20 VRHE) to ensure that all steps are exothermic, indicating OD-Cu reduction is kinetically limited at weak reduction potentials.