<p>Boundary conditions for catalyst performance in the conversion of common precursors such as N<sub>2</sub>, O<sub>2</sub>, H<sub>2</sub>O, and CO<sub>2</sub> are governed by linear free energy and scaling relationships. Knowledge of these limits offers an impetus for designing strategies to alter reaction mechanisms to improve performance. Towards a more sustainable carbon economy, understanding the basis of catalytic selectivity for CO<sub>2</sub> conversion to chemical feedstocks/fuels is key. Herein, high-throughput experimentation on 14 bulk copper bimetallic alloys allowed for data-driven identification of a fundamental linear scaling relationship between methane and C<sub>2+</sub> products that constrains the Faradaic efficiency for C–C coupling. We have furthermore demonstrated that coating the electrodes with a molecular film breaks the scaling relationship to promote C<sub>2+</sub> product formation.</p>