To model the water dissociation catalyst, we treat the CL as a thin neutral region located between the AEL and CEL where the large field in between the two layers drives the dissociation of water by shifting the forward and reverse rate constants as per equations ( ) and (13). While more detailed models of water dissociation catalysis have been presented that explicitly consider catalyst surface effects, we choose a more simple model for water dissociation for computational efficiency. Furthermore, the interaction of carbon species with catalyst surfaces are still not well understood; because this work is more focused on modeling the impact of reactive carbon species on polarization behavior and mesoscale transport, the exact details of the catalyst surface are not crucial to capture. More details on the choice of model for the CL can be found in Section S1.4.