In this work, we present DFT simulations that demonstrate the ability of Cu to catalyze CO dimerization in CO2 and CO electroreduction. We describe a previously unreported CO dimer configuration that is uniquely stabilized by a charged water layer on both Cu(111) and Cu(100). Without this charged water layer at the metal surface, the formation of the CO dimer is prohibitively endergonic. Our calculations also demonstrate that dimerization should have a lower activation barrier on Cu(100) than Cu(111), which, along with a more exergonic adsorption energy and a corresponding higher coverage of CO, is consistent with experimental observations that Cu(100) has a high activity for C-C coupling at low overpotentials. We also demonstrate that this effect is present with cations other than H+, a finding that is consistent with the experimentally observed pH independence of C2 formation on Cu.