A one-dimensional model was developed to describe pH distribu�on in the catalyst layer. This model encompasses a system of par�al differen�al equa�ons (Supplementary Informa�on) that account for the transport of Fe and protons in the electrolyte and the 60 µm thick catalyst layer, as well as the proton consump�on by the ORR and the dissolu�on/precipita�on of Fe ca�ons in the catalyst layer. The modelling is based on the following assump�ons: