where 𝐶 is the capacitance, 𝑒 = 1.6 × 10 -19 C is the electron charge, 𝑁 𝑑 the carrier concentration,  0 = 8.8510 -12 Fm -1 and  𝑟 are the vacuum and relative permittivity, respectively. Furthermore, 𝑆 is the sample surface in contact with the electrolyte (𝑆 = 0.5 cm 2 in our case), 𝑉 𝑓𝑏 the flat band, 𝑉 the applied voltage, 𝐾 𝐵 = 1.38 × 10 -23 J • K -1 is the Boltzmann constant, and 𝑇 is the temperature. A wide range of relative permittivity values for hematite are reported in the literature, depending on the morphology (nanostructures or not), annealing temperature, frequency, etc. This results in a significant difference of the predicted capacitance values. In this work, the value used for the relative permittivity is  𝑟 = 10.