The mixed mass transport regime of the HOR/HER This regime spans between pH 9-11 where the mass transport of molecular hydrogen and proton acceptors is similar to the kinetics of the HOR/HER. The transition towards kinetic control is marked by a substantial decrease in uncertainty in the determination of the kinetic micropolarization exchange current density, as can been seen in Fig. . During the transition, the mass transport of both molecular hydrogen and of proton acceptors is influencing the i-V response. This makes it necessary to estimate the respective limiting currents to use them for the mass transport correction (Eq. 35d). While the limiting current of molecular hydrogen is obtained directly, the value of the proton acceptor limiting current for pH > 11 must be obtained through a linear regression from lower pH values, as displayed in Fig. . While the transition between the mass-transport-controlled and the mixed regime requires different mathematical descriptions, the transition to the kinetic controlled regime is only noticed by a substantial decrease in the uncertainty of the kinetic micropolarization exchange current density and we do not need to discuss it separately.