Electrochemical reduction of CO2 on copper-based catalysts has become a promising strategy to mitigate greenhouse gas emissions and gain valuable chemicals and fuels. Unfortunately, however, the generally low product selectivity of the process decreases the industrial competitiveness compared to the established large-scale chemical processes. Here, we present random solid solution Cu1-xNix alloy catalysts that, due to their full miscibility, enable a systematic modulation of adsorption energies. In particular, we find that these catalysts lead to an increase of hydrogen evolution with the Ni content, which correlates with a significant increase of the selectivity for methane formation relative to C2 products such as ethylene and ethanol. From experimental and theoretical insights, we find the increased hydrogen atom coverage to facilitate Langmuir-Hinshelwood-like hydrogenation of surface intermediates, giving an impressive almost 2 orders of magnitude increase in the CH4 to C2H4 + C2H5OH selectivity on Cu0.87Ni0.13 at -300 mA cm-2. This study provides important insights and design concepts for the tunability of product selectivity for electrochemical CO2 reduction that will help to pave the way toward industrially competitive electrocatalyst materials.