Estimation of the electrochemical active site density of a metal-free carbon-based catalyst using phosphomolybdate (PMo12) as an adsorbate.

A method to estimate the electrochemical active site density (SD) of carbon (C) and nitrogen-doped carbon (N/C-900) using phosphomolybdate (PMo12) as a probe molecule is proposed. The complete coverage of the active sites by the probe molecules is established irrespective of the adsorbate concentration (1, 5, or 10 mM), potential cycling (1 or 10 cycles) and cleaning time (2, 5, or 10 min). A conversion factor derived from a smooth and polished glassy carbon disk of known geometrical area is used to estimate the electrochemical active surface area (ECSA) of the carbon catalyst from the SD. The relatively higher SD values estimated from DC voltammetry than from large-amplitude Fourier-transform alternating-current voltammetry (FTacV) is indicative of the contribution of capacitive charge in the former. Adsorbed probe molecules (PMo12) can readily be desorbed from the catalyst surface by cycling the electrode to lower potentials. The active site density of N/C-900 (∼0.36 × 1019 sites g-1) is higher than that of C (∼0.17 × 1019 sites g-1).

Kinetics of the Oxygen Evolution Reaction (OER) on Amorphous and Crystalline Iridium Oxide Surfaces in Acidic Medium.

Amorphous and crystalline IrO2 catalysts are synthesized by the Adams method and characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The oxygen evolution reaction (OER) is investigated on both the catalyst surfaces in 0.5 M H2SO4 electrolyte. The Tafel slope estimated in the temperature range of 293-333 K on the two surfaces indicates a change in the rate-limiting steps. The data are also analyzed in terms of the Eyring equation to estimate the activation enthalpy (ΔH#) and pre-exponential factor (Af) as a function of overpotential and therefore the charge-transfer coefficient (α). The estimated α values suggest strong electrocatalysis on both the surfaces. While the ΔH# plays a decisive role in the electrocatalysis on the amorphous sample, the trend of Af indicates that an increase in the entropy on the crystalline surface is pivotal in reducing the reaction barrier.

Low-frequency inductive features in the electrochemical impedance spectra of mass-transport limited redox reactions.

Low-frequency (lf) inductive features in the impedance spectra of electron-transfer reactions at the electrode-electrolyte interface in acidic and alkaline media are investigated. The trend in lf equivalent circuit (EC) parameters (inductor (L) and series resistance (R0)) is analysed as a function of overpotential for the two widely investigated electrochemical reactions, viz., methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). The decreasing trend in the lf EC parameter values of the reactions not limited by mass-transport (such as the MOR) and that in the mixed kinetic-diffusion controlled region for mass-transport limited reactions (ORR) are attributed to the faster surface relaxation with applied overpotential, whereas the increase of the L and R0 values in the mass-transport limiting region in an acidic medium is shown to originate from the extra hindrance caused by the limited supply of reactant species. Such analysis of lf EC parameters with overpotential is important to understand the underlying physical processes and to establish equivalent circuit models.

Influence of Nitrogen Doping into Carbon on the Activation Barrier of ORR in Alkaline Medium: An Investigation Based on Eyring Analysis.

The oxygen reduction reaction (ORR) is investigated on metal-free carbon (Vulcan XC-72) and nitrogen-doped (∼≤1%) carbon (N/C-900) in 0.1 M KOH. The product distribution (O2 to OH- and HO2-) as a function of overpotential (η) in the temperature range of 293-323 K is analyzed using a rotating ring-disk electrode (RRDE) assembly. The kinetic current due to reduction of O2 to HO2- is estimated and used in the Eyring analysis to determine the change in enthalpy of activation (ΔH#). It is shown that doping of carbon with nitrogen (even with ≤1 wt %) causes substantial increase in the number of active sites (almost 2-fold) and reduction in ΔH# at any η. Moreover, ΔH# is a stronger function of η on N/C-900 as compared to that on the carbon surface.

Relationship between the electron-transfer coefficients of the oxygen reduction reaction estimated from the Gibbs free energy of activation and the Butler-Volmer equation.

The rate of electron-transfer reactions, irrespective of whether electrochemical or electrocatalytic, is universally explained on the basis of Butler-Volmer (B-V) theory. The charge-transfer coefficient (α) obtained is typically in the range of 0.0-1.0, and is 0.6 ± 0.1 for the oxygen reduction reaction (ORR) on Pt, which is the subject of the present investigation. Alternatively, α can be estimated from the derivative of the change in Gibbs free energy of activation (ΔG#) with respect to the overpotential (η) and has the unreasonably high value of 1.1 ± 0.2. The origin of the difference in the α values obtained from these two methods is investigated. The value of α greater than 1.0 stems from the alternative potential-dependent lower energy barrier path for the formation of the activated complex, offered by the electrified catalyst surface. For the electrocatalytic reaction, the α value derived from the ΔG# is the true kinetic parameter. The theoretical background of such processes is presented to justify our claims.

Kinetics of Hydrogen Evolution Reactions in Acidic Media on Pt, Pd, and MoS2.

Hydrogen evolution reaction (HER) are investigated on Pt, Pd, and MoS2 in a 0.5 M H2SO4 electrolyte in a rotating disk electrode (RDE) configuration in the temperature range of 285-335 K. The reaction is temperature-sensitive on all of the three catalyst surfaces at their respective overpotential ranges. The kinetic parameters (activation enthalpy (ΔH#), free energy of activation (ΔG#), and pre-exponential factor (Af)) toward HER are obtained from the Arrhenius and Eyring relations, and the overall kinetics on the catalyst surfaces is analyzed. ΔH# for HER is a strong function of the overpotential in the case of both Pt and Pd. On the other hand, the trend in Af suggests that the electrocatalysis of HER on MoS2 originates from an increase in entropy factor, perhaps due to the solvent-dipole interaction at the interface. Such analysis is pivotal to the investigation of electrocatalysis of HER, especially on surfaces for which determination of active-site density is not established.

Tubercular esophagocutaneous fistula.

Tubercular esophagocutaneous fistula is a rare entity with only about four cases reported so far. We report here a case in a young female who has a very long tract but responded well to antitubercular treatment.

Cut throat zone II neck injury and advantage of a feeding jejunostomy.

Penetrating neck injuries account for 5-10% of trauma cases and are potentially life threatening. We report a case of cut- throat zone II neck injury in a 45-year-old male extending up to posterior pharyngeal wall and exposing the underlying cervical vertebra. Tracheostomy was done and wound repair was started from the posterior aspect in layers using 3-0 Vicryl. Intraoperatively, a conscious decision was taken for a feeding jejunostomy for postoperative feeding, which was likely to be prolonged, in view of sensory-nerve damage along the transected pharynx. Prolonged use of Nasogastric tube for postoperative feeding was thus avoided and the discomfort, risk of aspiration and foreign body at injury site eliminated. One week postoperative, the patient experienced severe bouts of coughing and restlessness on oral intake; during this period enteral nutrition was maintained through feeding jejunostomy. At the time of discharge at 1 month, the patient was accepting normal diet orally and was detubated and vocalizing normally. We conclude that postoperative nutrition is an important area to be considered for deep neck wound with nerve injuries due to delayed tolerance to oral feeding till the regeneration of sensory nerves. A feeding jejunostomy or feeding gastrostomy performed simultaneously in such patients with nerve injuries is far superior over nasogastric-tube feeding when prolonged postoperative feeding is expected.