equivalent circuit model includes solution resistance R S, charge transfer resistance R CT representing the electrode kinetics, and Warburg element CPEW representing the resistance encountered in diffusion and access of ions within nanoporous electrode structure. The inclusion of the constant phase element CPEdl instead of the conventional purely capacitive element C dl is to account for the BIBW2992 price dispersive behavior of the capacitance arising from the charge accumulation layer at the ZnO nanorods exposed to the electrolyte through pores in the PPy sheath and nanostructure of the electrode. Similarly, CPEnr is the capacitive element which characterizes the pseudocapacitance property of the nanotubular PPy-anion conjugation. The nanostructure resistance, R nr, is representative of the electron transport resistance due to narrow (approximately 60 nm diameter) vertically long (approximately 2.2 μm) ZnO nanorods and C nr its electrochemical capacitance Ensartinib . The continuous lines in the Nyquist plots in Figures 10 and 11 are the results of the fitting based on this model. Excellent fit is observed over the entire frequency range. Various electrical resistance and capacitive parameters estimated by fitting of Nyquist plots are summarized in Table 2. Figure 13 Equivalent electric circuit model used for simulation of Nyquist plots. Table 2 Characteristic
resistance and capacitive parameters estimated by fitting of Nyquist plots Components CPE dl (mMho, p) R ct (Ω) CPE w (mMho, p) R nr (Ω) CPE nr (mMho, p) ZnO nanorod core-PPy sheath Q = 0.025 p = 0.55 21.24 Q = 0.03 p = 0.61 6 Q = 0.012 p = 0.75 Narrow PPy nanotube (2-h etch) Q = 0.0006 p = 0.87 18 Q = 0.036 p = 0.74 28 Q = 0.065 p = 0.44 Open PPy nanotube (4-h etch) Q = 0.04 p = 0.61 16 Q = 0.04 p = 0.76 20 Q = 0.389 p = 0.42 The constant phase element (CPE) instead of the capacitor in the equivalent circuit above is justified in order to more appropriately account
for the heterogeneities including the surface roughness, porosity, and variation in the PPy thickness arising from the nanostructured nature of the ZnO-PPy electrode. The long, vertical, and dispersed 3-D ZnO nanorod core-PPy sheath (nanotube) nanostructure has a diverse aspect ratio selleck chemical relative to a flat 2-D electrode structure and therefore differently impacts the ion diffusion kinetics. This gives rise to the distributed time constants simulating the capacitance dispersion which is better represented by the RC network comprising of nanostructure resistance, R nr, and the constant phase element, CPEnr . The CPEnr impedance is given as, . (4) where exponent p represents dispersive nature of time constant, since with p = 1, the impedance Z″ is purely capacitive characterized by a single time constant and the parameter Q is equivalent to a capacitance, while for p < 1 parameter Q is basically a CPE with units Mho.cm-2.