It is important to note that the interface capacitance has been obtained using a constant phase element (CPE), as opposed to a capacitor, due to frequency dispersion of the process. The CPE is effectively a leaky capacitor and has parameters Q and ϕ, where Q is the value of the CPE and ϕ is an exponent describing the extent of deviation from ideal capacitive behavior. The value of ϕ is constrained between 0.8 and 1.0 during fitting. A CPE with an exponent below 0.8 does not reasonably represent a capacitor, whereas a CPE with an exponent of 1.0 is an ideal capacitor. The pure capacitance is estimated using: