At 0 V (Figure ), the shape of the Φmax volcano is similar to previously predicted NRR activity volcanos (i.e., overpotential vs. N*) for NRR on metals. However a key difference is that at 0 V a third region of the volcano emerges at the peak for materials with optimal N* binding and minimal Φmax. This region is limited by the chemical step of NH3* desorption from the catalyst surface (green scaling line). This step involves no proton-coupled electron transfers and is thus minimally dependent on the applied potential in contrast to other Φmax limiting energetic spans comprised of PCET steps. Consequently, as the applied potential becomes more reducing (Figures ), the y-intercept of the green scaling line associated with NH3* desorption increases at a lower rate than that of the other scaling lines that involve a PCET step. From 0.0 to -0.5 V, the y-intercept shifts from -0.62 to -0.31 eV for the chemical NH3* desorption step scaling line, whereas the NHNH2* to NH3 scaling line shifts from 0.41 to 2.07 eV and the NH3* to N2H* scaling line shifts from -1.13 to -0.48 eV. Thus, as the PCET steps become more favorable at more reducing biases, a greater number of materials become limited by the non-PCET step of NH3* desorption (green scaling line). At -0.5 V, the most reducing potential considered here, a broader range of N* adatom binding energies exists where NH3* desorption limits Φmax. This results in a truncation of the volcano's peak and the inability of any of the surfaces studied herein to reach Φmax ≤ 0 eV where all reaction steps are favorable. This agrees with experimental investigations of low-index transition metals, which are reported to have NH3 activities < 10 μg/h -1 -g-catalyst. The PCET pathway (Figure ) does not consider chemical steps in the catalytic cycle, specifically the adsorption of N2 or desorption of NH3*. To compare the composite mechanism to the PCET mechanism, -Φmax for both mechanisms is plotted in Figures and at each studied bias. In the PCET mechanism, the limiting energetic span steps for all materials and facets at 0 V are the reductive desorption (NH2* →NH3 + *) and reductive adsorption (N2 + * →N2H*) steps (Figure ), where the desorption and adsorption steps are each coupled with a PCET. These are the same scaling relations identified by previous works to limit NRR. However, using the composite mechanism that includes the chemical NRR steps, different energetic spans are identified to limit Φmax and thus NRR activity (Figure ).