The activity predicted by the PCET mechanism increases linearly with more reducing biases as shown in Figure . Because every step of the PCET mechanism includes an electron transfer that becomes more favorable at more reducing biases, there is no limit to NRR activity predicted by the PCET mechanism. Consequently, the peak of the PCET volcano reaches Φmax < 0 eV at a sufficiently reducing bias, indicating that all intermediate reaction steps are downhill. Furthermore, the PCET mechanism predicts a region of materials (shaded regions of Figure ) where the Φmax for NRR is lower than the Φmax for HER (see section 2.5 in the SI for HER pathways and energies). These results for the PCET pathway suggest that optimal electrocatalyst materials that obey the identified scaling relations can provide high activity and selectivity towards NRR. However, this is not observed experimentally. Instead, experimental observations have not identified any monometallic materials that provide high activity and selectivity towards NRR. Conversely, the volcano plot for the composite mechanism is truncated by the chemical NH3* desorption step. As the applied potential becomes more reducing, the NH3* desorption line widens and truncates a larger width of the volcano. This leads to a nonlinear dependence of Φmax on applied bias for materials that switch energetic spans at more reducing biases to the NH3* → NH3 chemical step (i.e., orange arrows in Figure ). The composite mechanism shows that materials at more reducing potentials are limited by the NH3* desorption step, which is less sensitive to applied potential than PCET steps. However, by using GC-DFT to study the NH3* chemical desorption step at each bias, we predict that the grand free energy for NH3* desorption is sensitive to the applied bias, resulting in a slight upward shift of the volcano's peak with more reducing potentials. This result would not be captured using a post-hoc bias correction approach that doesn't include dipole and charging corrections. For all studied materials, HER is calculated to have a lower Φmax (greater -Φmax) than NRR using the composite mechanism (Figure ). Consequently, the results based on the composite mechanism suggest that all materials that obey the identified scaling relations for metals will have lower activity toward NRR than HER, based on their Φmax. The direct calculation of NRR and HER activity was not performed here due to the significant additional computational expense required to perform transition state searches and microkinetic modeling for the 23 studied surfaces. The qualitative differences observed between the composite and PCET mechanisms highlight the importance of considering chemical, non-PCET steps in electrochemical reaction mechanisms, and that these steps should be modeled at discrete biases using grand-canonical modeling approaches such as GC-DFT.