Electrochemical reduction of CO2 in membrane electrode assembly (MEA) cells with gas diffusion electrodes (GDEs) offers a sustainable path to produce chemicals and fuels. Although these systems enable high-current densities and Cu electrocatalysts favor C2+ products like ethylene, the mechanisms controlling selectivity towards specific C2 products (ethylene, ethanol, and acetate) are not well understood. This study explores CO2 reduction selectivity trends and mechanisms on Cu-based electrocatalysts, emphasizing multicarbon product formation using different Cu alloys. In particular, CuSnx, Cu-P, and Cu-Se electrocatalysts were synthesized via a one-pot method and tested in MEA cells with alkaline electrolytes. Several ex situ characterization techniques (XRD, XPS, SEM, EDS, and ICP-OES) were employed to study the structural, compositional, and electronic properties of the electrocatalysts. Sn content in CuSnx alloys critically influenced the main CO2 reduction product, with high Sn (x ≥ 0.10) promoting formate. and low Sn (x ≤ 0.10) favoring CO, ethylene, and ethanol, depending on cell potential and pH. Comparing Cu, Cu-P0.065, Cu-Sn0.03, and Cu2Se electrocatalysts revealed the impact of dopants, alloys, and compounds on selectivity. In 0.1 M KHCO3, P-doped Cu showed higher ethylene FE (52%) than pure Cu (31%), while Cu-Sn and Cu-Se favored ethanol (24% FE) and acetate (32% FE), respectively. In 1 M KOH, Sn alloys promoted ethanol (48% FE), and Cu-Se achieved high acetate selectivity (40% FE) at 350 mA cm-2. The electrocatalysts demonstrated stability over 250 hours. A mechanism involving a common acetyl intermediate is proposed, with selectivity governed by the partial positive charge (δ+) on Cu sites calculated using density functional theory (DFT). Cu-P0.065 (Cuδ+ = 0.13) promotes ethylene, Cu-Sn0.03 (Cuδ+ = 0.27) favors ethanol, and Cu2Se (Cuδ+ = 0.47) enhances acetate selectivity.