We measured the entropy of hydrogen adsorbed on Pt NPs with 3 different shapes. The entropy depends on the local geometry of the particles and matches the entropy of hydrogen adsorbed on single crystals with the corresponding orientation.

The reasons for the sluggish kinetics of the hydrogen adsorption reaction in alkaline media remain a question still to be solved. This information is important to achieve a complete understanding of the mechanistic details that could lead to the production of key catalytic materials necessary for the development of a future hydrogen economy. For a better understanding of this reaction, it is important to acquire information about the thermodynamic parameters characteristic of the different steps in the reaction. Among these, the hydrogen adsorption is a key step in the process of hydrogen evolution. Although some debate still remains about the difference between adsorbed hydrogen in the underpotential deposition (UPD) region and at the overpotential deposition region, there is no doubt that understanding the former can help in the understanding of the latter. Making use of charge density measurements, we report on this paper a thermodynamic study of the hydrogen UPD process on Pt(111) in 0.05M NaOH over the range of temperatures from 283 ≤ T/K ≤ 313. The coulometric features corresponding to HUPD allow for the calculation of the hydrogen coverage and a fit to a Generalized Frumkin isotherm. From these values, different thermodynamic functions for the UPD reaction have been calculated: ΔGads, ΔSads, ΔHads, and the Pt-H bond energy. From extrapolation, a value of ΔSads◦=-7.5±4Jmol-1K-1 was found, which is very close to 0, much lower than previously reported measurements both in acid and in alkaline solutions. Such value has an effect on the enthalpy and bond energy calculations, the latter having a decreasing tendency with pH and coverage. This tendency is completely different from the acidic systems and implies that the change in the thermodynamic functions due to the formation of the double layer and the reorganization of interfacial water has a strong influence on the process in high pH solutions.

The hydrogen evolution reaction is one of the most studied processes in electrochemistry, and platinum is by far the best catalyst for this reaction. Despite the importance of this reaction on platinum, detailed and accurate kinetic measurements of the steps that lead to the main reaction are still lacking, particularly because of the fast rate of the reaction. Hydrogen adsorption on Pt(111) has been taken as a benchmark system in a large number of computational studies, but reliable experimental data to compare with the computational studies is very scarce. To gain further knowledge on this matter, a temperature study of the hydrogen adsorption reaction has been carried out to obtain kinetic information for this process on Pt(111) in alkaline solution. This was achieved by measuring electrochemical impedance spectra and cyclic voltammograms in the range of 278 <= T <= 318 (K) to obtain the corresponding surface coverage by adsorbed species and the faradaic charge transfer resistance. From this data, the standard rate constant has been extracted with a kinetic model assuming a Frumkin-type isotherm, resulting in values of 2.60 x 10(-7) <= k(0) <= 1.68 x 10(-6) (s(-1)). The Arrehnius plot gives an activation energy of 32 kJ mol(-1). Comparisons are made with values calculated by computational methods and reported values for the overall HER, giving a reference frame to support future studies on hydrogen catalysis.

Iron nanoparticles, FeNPs, were electrodeposited onto the HOPG electrode surface from Fe(III) ions dissolved in the choline chloride-urea eutectic mixture using potentiostatic current density transients. The morphology of the FeNPs, supported onto HOPG, was characterized by means of AFM and SEM. From these techniques it was found that most of the FeNPs were formed by nanostructured hemispheric particles, monodisperse in size (displaying diameters of (60 +/- 8) nm with 30 nm height), that were homogeneously distributed on the HOPG surface. Furthermore, from EDX and XPS it was determined that the iron electrodeposit was constituted by core-shell type particles with zero-valence iron as the core and a shell composed by a mixture of FeO, Fe2O3, and Fe(OH)(3). From analysis of experimental current density transients, it was found that the electrodeposition mechanism of FeNPs involves multiple 3D nucleation with diffusion controlled growth and that residual water reduction occurs on the growing surface of the FeNPs as the applied potential becomes more negative. These models involve contributions to the overall current due to: an adsorption process, iron 3D nucleation with diffusion-controlled growth and residual water reaction over the growing surfaces of the Fe nuclei. The proposed models help determining the charge percentage due to each individual contribution to the total process. (C) 2019 Elsevier B.V. All rights reserved.

Department of Electrical Engineering a Engineering, Aalto University, PO Box 135 laurila@aalto.; Tel: +358 50 341 4375 Department Chemistry and Materials Scien University, PO Box 16200, 00076 Aalto, Fin Department of Bioprocess and Biosystems University, PO Box 16300, 00076 Aalto, Fin Instituto de Electroqúımica, Universidad de † Electronic supplementary informa 10.1039/c8ra01703d Cite this: RSC Adv., 2018, 8, 12742

Application specific Pt-grown carbon nanofibers for H2O2 detection were characterized and the roles of dissolved oxygen and chloride ions on the electrochemical performance were assessed in detail.

This work showed that chemically-synthesized gold nanoparticles, AuNPs, supported onto a Si (111) wafer electrode, can be selectively modified with a copper adlayer through underpotential deposition (upd) conditions, using both: potentiodynamic or potentiostatic electrochemical means. From analysis of experimental potentiostatic current density transients, it is shown that Cu upd onto the AuNPs occurs by a mechanism involving the simultaneous presence of a Langmuir-type adsorption-desorption and an instantaneous two-dimensional, 2D, nucleation process. The influence of the applied potential on the Cu upd kinetics and on the extent of Cu atoms coverage over the AuNPs was also reported. Furthermore, it is shown that the Cu overpotential deposition, opd, onto these AuNPs, starting from a potential in the upd region where the AuNPs surface is free from Cu atoms, occurs through a 2D-3D mechanism, where the 3D nucleation is mass-transfer controlled. Notwithstanding, when Cu opd started at the equilibrium potential the mechanism solely involved 3D nucleation.

The nucleation of Ag onto vitreous carbon from aqueous 3 M NaCl or 0.6 M NaClO4 and deep eutectic solvent (DES) 1:2 M mixture of choline chloride:urea solutions containing Ag+, has been studied analyzing the chronoamperometric response to single potential steps. From the coordinates of the maxima observed in the current responses, the nucleation frequencies A (s− 1) and number densities of nucleation sites N0 (cm− 2) were obtained from the standard model of nucleation with diffusion-controlled three-dimensional growth. Analysis of the overpotential dependence of nucleation frequencies using the classical electrochemical nucleation theory allowed to calculate the Gibbs free energy of nucleation ΔG˜nc and critical nucleus size nc as well as the exchange current density j0, transfer coefficient α and surface tension σ of silver nuclei. The kinetics of Ag+ reduction is two orders of magnitude slower in DES compared to both aqueous systems studied, and values of α ≪ 0.5 where found in both aqueous and DES media, indicating either that the intermediate state for metal ion reduction is located close to the initial state, i.e., the solvated or complexed metal ion in solution, or that the metal ion is specifically adsorbed on the surface and the symmetry factor involved requires an alternative electron transfer formalism. The low ΔG˜nc and nc values observed indicate that the discharge of a single Ag ion on the surface already becomes a supercritical nucleus, involving a very low Gibbs energy barrier, characteristic of a non-activated process.

From potentiodynamic and potentiostatic experiments, the cobalt electrochemical nucleation and growth process, onto the glassy carbon electrode surfaces, from an aqueous solution containing 10 M-2 CoCl2 and (1) M NH4Cl (pH = 4.66) at different temperatures (15-60 degrees C) is reported. It was found that while the equilibrium potential moves to more negative values as the temperature of the system was increased, the contrary was observed for the nucleation overpotential, hnucleation, required for the onset of cobalt nucleation onto the surfaces of vitreous carbon. From Tafel plots recorded at the different temperatures considered, both: the transfer coefficient, a, and the exchange current density, j(0), associated with the Co (II)/Co(0) system were assessed, and from the Arrhenius plot (ln j(0) vs. T (1)) the activation energy for Co(II) reduction of (27.9 +/- 0.3) kJmol (1) was estimated. From analysis of the potentiostatic current density transients according with the formalism proposed by Palomar-Pardave et al. (Electrochim. Acta 50 (2005) 4736-4745), the total current density was de-convoluted into individual contributions, due to formation of multiple mass-transfer controlled 3D Co nuclei and that associated to the proton reduction occurring simultaneously on the growing surface of the Co nuclei. The latter contribution was practically null at 10 degrees C, however, it drastically increased as the temperature of the system did, provoking that the cathodic efficiency for Co deposition diminished. Furthermore, it was found that both the number density of active sites, N-0, and the nucleation rate, A, for Co eta nucleation depend exponentially with hnucleation, regardless of temperature, except at 60 degrees C where N-0 diminished linearly with hnucleation. From variations of the Co(II) ions diffusion coefficient with temperature, a value of (8.5 +/- 0.2) kJmol (1) was obtained for the activation energy for bulk diffusion. SEM analysis showed that the cobalt nuclei are bigger as the temperature increases; however, the coverage of the electrode surfaces becomes lower. (C) 2017 Elsevier Ltd. All rights reserved.

This work was conducted to determine the influence of temperature of the electrolyte consisting of choline chloride and urea at 1: 2 molar ratio, for silver electrodeposition. Cyclic voltammetry and chronoamperometry analyses were employed to carry out the silver electrodeposition on glassy carbon at 50, 60 and 70 °C. The current density for silver electrodeposition increases with increasing temperature. The description of experimental data through Scharifker and Mostany model, see Figure 1 (a), indicate that the Ag nucleation and growth processes on glassy carbon, regardless of T, are a diffusion controlled processes. The diffusion coefficient, D0, the nucleation rate constant, A, the density of active sites, N0, the work of forming the critical nucleus, ΔG*, and the size of the critical radius, n*, are calculated from experimental data for all temperatures. From SEM images, the morphology of deposited Ag was seen to correspond to a progressive nucleation mechanism, regardless of T, and that the silver concentration in the electrolyte has a significant impact on the nucleation mechanism. Figure 1