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.

The impact of overpotential on the enthalpy of activation and pre-exponential factor of electrochemical redox reactions.

The kinetics of the V5+/V4+ redox reaction is investigated in a three-electrode configuration on a Vulcan XC-72 modified glassy carbon rotating disk electrode at four different temperatures (25 to 40 °C, with 5 °C interval). The values of enthalpy of activation (ΔH#) and pre-exponential factor (Af) estimated using the Eyring equation are in the range of 0.25-0.53 eV (24-51 kJ mol-1) and -1.3 to 5, respectively. The Eyring plots tend to diverge with overpotential, causing an increase in the values of the estimated ΔH# and Af. This is perhaps due to the retarding effect of the precipitates/adsorbates on the electrode surface. The investigation of the kinetics suggests that the V5+/V4+ redox reaction is electrocatalysed through an increase in the entropy of activation (ΔS#).

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.

What contributes to the internal mass-transport resistance of redox species through porous thin-film electrodes?

Transport of redox species through porous thin-film electrodes is investigated using electrochemical impedance spectroscopy (EIS). Redox species of small size and fast electron-transfer kinetics show two arcs in the EIS pattern: a high frequency arc corresponding to the charge-transfer process (electron-transfer) and a low frequency arc corresponding to the mass-transport process (transport of the redox species from the bulk of the solution to the electrode interface). Often, the features of the Nyquist plot corresponding to the transport of the redox species through the porous electrode and that through the bulk of the electrolyte are not resolved. It is shown that the resolution of such features depends on the (1) composition of the porous thin-film, (2) electron-transfer kinetics, (3) interaction of the redox species with the electrode components, and (4) bulkiness of the redox species and (5) its concentration.

Plasmonic resonance of Ag nanoclusters diffused in soda-lime glasses.

Silver nanoclusters were prepared in a soda-lime glass matrix through the ion-exchange (Ag(+)↔ Na(+)) method followed by thermal annealing in an air atmosphere. The nanoscale patterning of Ag nanoclusters embedded in a soda lime glass matrix in an air atmosphere at different annealing temperatures has been investigated. During annealing, Ag(+) is reduced to Ag(0) and subsequently forms silver nanoparticles inside the glass matrix. A blue shift of 20 nm has been observed as a function of the post annealing temperature. The photoluminescence intensity is highest for an annealing temperature of 500 °C for 1 h and continuously decreases as annealing temperature increases up to 600 °C. The presence of spherical nanoparticles with a maximum particle size of 7.2 nm has been observed after annealing at 600 °C for 1 hour, which is consistent with Mie theory based results.

Polymorphic transformations and optical properties of graphene-based Ag-doped titania nanostructures.

TiO2 is the most studied semiconductor material for photovoltaics and photocatalyst applications, but due to a very large electron hole recombination process it is difficult to use it as a photovoltaics material. In this context graphene-decorated Ag-doped TiO2 nanostructures have been synthesized by a simple, cost effective chemical method. In this paper, we have studied the structural transformations and electronic band structure of Ag-doped TiO2 due to the incorporation of graphene oxide. Pure rutile and anatase-rutile mixed phases of TiO2 nanoparticles were obtained by Ag doping and annealing at 400 °C. A large red shift was observed in most of the graphene-decorated, doped TiO2 hybrid nanostructures, which is because of the electron transfer between the conduction bands of the doped TiO2 and the multilayer graphene. The Ag-doped TiO2 nanoparticles appear in the shape of a bunch of bananas (or rice-like) because of the jumbled collection of particles, which remain unaltered even after graphene decoration. The strong electrical coupling of Ag-doped TiO2 with reduced graphene oxide produces an advanced hybrid material useful for superior photovoltaics, photocatalytic activity and other applications.