This work demonstrates that the inclusion of both chemical and electrochemical steps, modeled here under explicit applied bias using GC-DFT, are necessary to capture the thermodynamic limitations for NRR on metals. Volcano plots derived from NRR scaling relations are truncated by the chemical NH3* desorption step, which limits the energetic span, and Φmax, on the highest activity surfaces and should be included in future studies to identify superior NRR electrocatalysts. Φmax is observed to scale nonlinearly with bias on surfaces where the energetic span switches with the applied potential. The results presented herein are based on Φmax, which provides a thermodynamic approximation of catalyst activity; therefore, the validation of these results and the quantitative prediction of NRR activity and selectivity requires follow-up investigations of NRR and HER kinetics that include transition-state calculations and microkinetic modeling.