Raman spectroscopy was subsequently utilized to capture the reaction intermediates on the Cu surface under equivalent reaction conditions (Fig. ). While the assignment of each peak is not entirely unambiguous, we base our interpretation on previously established works. As the potentially was systematically shifted negatively from open circuit conditions, Cu2O bands (220 and 417-670 cm -1 ) diminished, pointing to the transition to metallic Cu by -0.58VRHE. At the same time, the Cu-CO band (369 cm -1 ) appeared starting at -0.58VRHE. Similarly, weak features potentially stemming from *CObridge (1850-1900 cm -1 , 0.42 to -0.18 VRHE) were noted (Fig. ). In addition to CO, *CO2 -(700 cm -1 ), SO3 2-(983 cm -1 ), HCO3 -(1022 cm -1 ) and CO3 2- (1069 cm -1 ) were identified as (near)surface species. These bands, with the exception of SO3 2-, were similarly detected during electrolysis with CO2 only. Finally, a set of bands at 700, 1331-1371, and 1580-1620 cm -1 arose under reductive potentials. While there is debate as to their exact identity, previous works have attributed them to *COO -or *COOH species and this is our tentative assignment (Fig. ). In all, the main takeaways from the above experiments were that most oxide-like features disappeared from the catalyst under operating conditions, SO3 2-was continually present as a (near)surface species, and CO and CO2 -were the primary reaction intermediates on the Cu surface. Thus, CO is a likely serving principal intermediate in the reaction pathways towards higher order products. Given that the measured quantities of CO were very low, it is likely that all of the detected *CO reacted further on the catalyst surface via hydrogenation or C-C coupling. Hydrogenation would occur en route to CH4 production while a combination of C-C coupling and hydrogenation would terminate in the observed ethanol and acetate in our system. Because the CH4 is the primary product that is suppressed in the presence of SO3 2-, it would be reasonable to assume that a partially hydrogenated *CO (e.g. *CHO, CH2O…) in the CH4 pathway may be the main species coupling with SO3 2-en route to HMS formation. This is also consistent with the fact that C-H bands around 2800-2900 cm -1 are seen under C-S bond forming conditions, even as CH4 production is suppressed (though other C-H containing products like ethanol and acetate are still produced). The Raman and XRD experiments further point to Cu as the main phase during catalysis rather than oxide or sulfide phases of Cu that were not detected within the limits of our experimental sensitivity. A complementary investigation into the mechanism for heterogeneous C-S bond formation was performed with density functional theory (DFT) computations using the results from electrocatalytic and spectroscopic experiments as a starting point. Here, three slab structures for copper (100), ( ) and ( ) were constructed as the model catalyst to simulate the reaction pathway. Further computational details are given in the SI and elsewhere. Based on the observations in figures 2 and 3, CO was used as a starting intermediate as it is one featured in the CO2 reduction pathway to higher order products. To compute energy differences of elementary proton coupled electron transfer (PCET) steps, the computational hydrogen electrode (CHE) model 44 was used. Since an explicit negative charge(s) was added to the simulation for SO3 2-and HSO3 -, to calculate the energy difference of the adsorption step and coupling step, the following equations were used, respectively: