The mass-transport-controlled regime of the HOR/HER In most of the accessible potential and pH-range, the HOR/HER current is governed by mass transport. The i-V curves in this regime are entirely determined by the limiting currents of the net reactants/products and the potential (Eq. 36a). This allows the prediction of local activities (Eq. 36b-c). It also predicts a slope of the i-V curve of about 59 mV/decade and the Nernstian relationship between local pH and potential (Eq. 36d), as previously pointed out by Katsounaros et. al. .