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...

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.

Shape-controlled Pt nanoparticles (cubic, tetrahedral, and cuboctahedral) are synthesized using stabilizers and capping agents. The nanoparticles are cleaned thoroughly and electrochemically characterized in acidic (0.5 M H2SO4 and 0.1 M HClO4) and alkaline (0.1 M NaOH) electrolytes, and their features are compared to that of polycrystalline Pt. Even with less than 100% shape-selectivity and with the truncation at the edges and corners as shown by the ex-situ TEM analysis, the voltammetric features of the shape-controlled nanoparticles correlate very well with that of the respective single-crystal surfaces, particularly the voltammograms of shape-controlled nanoparticles of relatively larger size. Shape-controlled nanoparticles of smaller size show somewhat higher contributions from the other orientations as well because of the unavoidable contribution from the truncation at the edges and corners. The Cu stripping voltammograms qualitatively correlate with the TEM analysis and the voltammograms. The fractions of low-index crystallographic orientations are estimated through the irreversible adsorption of Ge and Bi. Pt-nanocubes with dominant {100} facets are the most active toward oxygen reduction reaction (ORR) in strongly adsorbing H2SO4 electrolytes, while Pt-tetrahedral with dominant {111} facets is the most active in 0.1 M HClO4 and 0.1 M NaOH electrolytes. The difference in ORR activity is attributed to both the structure-sensitivity of the catalyst and the inhibiting effect of the anions present in the electrolytes. Moreover, the percentage of peroxide generation is 1.5-5% in weakly adsorbing (0.1 M HClO4) electrolytes and 5-12% in strongly adsorbing (0.5 M H2SO4 and 0.1 M NaOH) electrolytes.

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Shape-controlled nanoparticles are of utmost scientific and technological importance because of their facet-dependent physical and chemical properties. Under long-term electrochemical conditions, little is known about the stability and fate of these nanoparticles with selected exposed crystallographic orientations (facets) of high surface energy, while it is generally accepted that the surface area decreases. Therefore, the reconstruction and dissolution of platinum nanocubes (Pt-NCs), platinum cuboctahedral (Pt-CO) and platinum polycrystalline (Pt-PC) nanoparticles are investigated using voltammetry and in situ irreversible adsorption of Bi and Ge; the cleanliness of the Pt nanoparticles and the purity of the electrolyte solution are established with systematic voltammetric analysis in a H2SO4 electrolyte of different concentrations (0.01, 0.05, 0.5 and 1 M). The voltammetric results suggest that the {100} terrace sites undergo reconstruction/dissolution at a much higher rate relative to that of the {111} ordered bi-dimensional terrace sites and the reconstruction leads to the formation of {110}/{100} step sites. Therefore, the stability of the Pt-NCs is lower than that of the Pt-CO nanoparticles. The gradual decrease in the Hupd area on prolonged cycling in the lower potential range (0.06-0.6 and 0.06-0.8 V) is attributed to the accumulation of oxy-anions from the electrolyte on the Pt surface. Moreover, dissolution of highly energetic Pt sites also contributes to the reduction in the Hupd area, unlike that observed with low index Pt single crystal surfaces. On cycling to higher potential limits (1.0 and 1.2 V), the adsorbed anions are replaced with the oxygenated species or oxide; the protective oxide layer helps to stabilize the electrochemical surface area (ESA) of the Pt nanoparticles. With cycling, both Pt-NCs and Pt-CO eventually get converted to Pt-PC. These results are supported with cyclic voltammograms, irreversible adsorption of Bi and Ge, and HR-TEM.