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Oxygen reduction reaction (ORR) in acidic media is investigated at various potentials in a thin-film rotating disk electrode (TF-RDE) configuration using electrochemical impedance spectroscopy (EIS). The ionomer-free and ionomer-containing thin-film catalyst layers are composed of Pt black and carbon-supported Pt catalysts of different metal loadings (5 and 20 wt%). The simplest EI spectrum consisting of an arc or a semi-circle is obtained at high potentials with ionomer-free Pt catalyst layers. The most complex spectrum consisting of a high frequency (HF) arc and two semi-circles is observed in the mixed diffusion-controlled region of the ionomer-containing catalyst layer with high loading of carbon-supported Pt. The nature of the EI spectrum is decided by the constituents of the thin-film catalyst layer and by the operating potential. The evolution of the EI spectra with ionomer and carbon contents is underlined. The effect of rotation rate (rpm) of the electrode on the impedance spectrum is also investigated. A series of equivalent circuits is required to completely describe the EI spectra of ORR. The kinetic parameters and the electrochemical surface area of the catalysts are derived from the impedance spectrum. Oxygen reduction reaction (ORR) is one of the most important reactions at the cathode side in low-temperature fuel cells (e.g., polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs)) and metal-air batteries.1-14 Because of the sluggish ORR kinetics and the stability issues of the catalyst in the electrochemical environment, expensive precious metal catalysts are often used in these electrochemical devices to catalyze the ORR. 15 Conventionally, the performance of the catalyst is evaluated in an operating fuel cell mode using the DC methods. [16][17][18] The information gathered from a DC analysis usually provides the sum of various polarizations of the electrode, which is difficult to separate into individual contributions. 19 On the other hand, electrochemical impedance spectroscopy (EIS), one of the AC methods, is a sensitive tool to investigate electrode-electrolyte interface and it allows the simultaneous resolution of various charge-transfer and mass-transfer processes (kinetic, ohmic, and diffusion). It involves a small sinusoidal electrical perturbation around a steady-state value and measures the impedance along with the phase angle. However, the interpretation of the EI spectra is difficult. Often, simple fitting models based on equivalent circuit analogues and physical models are used to extract the parameters those represent the underlying cell processes. [20][21][22][23][24][25][26] Springer et al. proposed the theoretical impedance spectrum of ORR on porous gas-diffusion electrode using the flooded-agglomerate electrode model in series with a thin electrolyte film. 20,25 The model predicted by Raistrick shows three semi-circles in the spectrum attributed to the charge-transfer process (ORR); agglomerate diffusion (depletion of the oxygen concentration in the pores...

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Nitrogen-doped carbon (N/C) and graphene (N/G) were synthesized by the established conventional heat-treatment method, and the incorporation of nitrogen into the carbon matrix was confirmed by CHN analysis, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electrochemical impedance spectroscopy (EIS) of the prepared catalysts in argon-saturated 0.1 M KOH was performed in a three-electrode rotating disk electrode (RDE) configuration. The capacitance derived from the low-frequency region of the EIS patterns was used to estimate the effective density of states [D(E F)] of carbon and its nitrogen-doped counterparts. Moreover, the carrier concentrations (N D) and flat band potentials of the samples were obtained by Mott–Schottky analysis. The metal-free catalyst samples were tested for possible oxygen reduction reaction (ORR) activity in oxygen-saturated 0.1 M KOH electrolyte, and the origin of the activity improvement with nitrogen doping of carbon/graphene can be explained on the basis of the effective density of states [D(E F)], carrier concentration (N D), and flat band potential. The results suggest that N/C-900 has the highest carrier concentration and maximum flat band potential and, therefore, the highest activity for the ORR.