Future tasks in interfacial electrochemistry and surface reactivity

Interfacial electrochemistry is a main part of Physical Electrochemistry, which is developing in several directions. In this respect, this note will reflect my personal views in the “to do” list of topics related to well-defined interfacial electrochemistry and its application to the understanding of electrocatalytic reactivity, deliberately limited to single-crystal platinum model electrodes in water electrolyte solutions. Several points are related to ongoing research and others have been suggested in meetings. Because of that, the text has a taste of déjà vu. References are a limited selection of examples for the interested readers.Financial support from MICINN-FEDER (project CTQ2016-76221-P) is greatly acknowledged

On the activation energy of the formic acid oxidation reaction on platinum electrodes

A temperature dependent study on the formic acid oxidation reaction has been carried out in order to determine the activation energy of this reaction on different platinum single crystal electrodes, namely Pt(1 0 0), Pt(1 1 1), Pt(5 5 4) and Pt(5 4 4) surfaces. The chronoamperometric transients obtained with pulsed voltammetry have been analyzed to determine the current densities through the active intermediate and the CO formation rate. From the temperature dependency of those parameters, the activation energy for the direct reaction and the CO formation step have been calculated. For the active intermediate path, the activation energy are in the range of 50–60 kJ/mol. On the other hand, a large dependence on the electrode potential is found for the activation energy of the CO formation reaction on the Pt(1 0 0) electrode, and the activation energy values for this process range between 20 and 100 kJ/mol. These results have been explained using a reaction mechanism in which the oxidation of formic acid requires the presence of a pre-adsorbed anion on the electrode surface

Effects of the anion adsorption and pH on the formic acid oxidation reaction on Pt(111) electrodes

The effects of solution pH and anion adsorption for the formic acid oxidation reaction on the Pt(111) electrode have been examined using electrochemical techniques. Regarding the pH effects, it has been found that oxidation currents for this reaction increase with pH, which indicates that solution formate is involved in the reaction mechanism. Unexpectedly, the adsorption of sulfate on the Pt(111) electrode has a positive effect on the oxidation of formic acid, which also suggests that adsorbed anions are also involved in the mechanism. The activation energy calculated from temperature dependent measurements diminishes with the solution pH and also in the presence of adsorbed sulfate. These measurements corroborate the involvement of solution formate and anions in the oxidation mechanism. Using these results, a rate equation for the oxidation of formic acid is proposed. The current values calculated from this equation are in very good agreement with the experimental currents in perchloric acid solutions.This work has been financially supported by the MICINN (Spain)(project CTQ2010-16271) and Generalitat Valenciana (project PROMETEO/2009/045, FEDER)

Role of oxygen-containing species at Pt(111) on the oxygen reduction reaction in acid media

The oxygen reduction reaction (ORR) is one of the fundamental reactions in electrochemistry and has been widely studied, but the mechanistic details of ORR still remain elusive. In this work, the role of electrochemically oxygenated species, such as adsorbed hydroxide, OHads, adsorbed oxygen, Oads, and Pt(111) oxide, PtO, in the ORR dynamics is studied by employing electrochemical techniques, i.e., combining rotating disk mass-transport control with potential sweep rate perturbation. In this framework, a reduction peak at 0.85 V, E ORR, is detected. This peak shows a different electrochemical dynamics than that of Pt(111) oxides. The data analysis suggests that neither OHads nor Oads are the main bottleneck in the mechanism. Instead, results support the reduction of a soluble intermediate species as the rate determining step in the mechanism. On the other hand, PtO species, which are generated at relatively high potentials and are responsible of surface disordering, strongly inhibit the ORR as long as they are adsorbed in the electrode surface.This study has been carried out in the framework of the European Commission FP7 Initial Training Network “ELCAT,” Grant Agreement No. 214936–2. Supports from the Spanish MINECO though project CTQ2013–44083-P and GV through PROMETEOII/2014/013 (FEDER) are greatly acknowledged

Thermodynamic properties of hydrogen–water adsorption at terraces and steps of Pt(111) vicinal surface electrodes

In this work, the effect of temperature on the adsorption states of Pt(111) vicinal surface electrodes in perchloric acid is studied through a thermodynamic analysis. The method allows calculating thermodynamic properties of the interface. In this framework, the concept of the generalized isothermand the statistical thermodynamics description are applied to calculate formal entropies, enthalpies and Gibbs energies,ΔGi0, of the adsorption processes at two-dimensional terraces and one-dimensional steps. These values are compared with data from literature. Additionally, the effect of the step density onΔGi0 and on the lateral interactions between adsorbed species,ωij, at terraces and steps is also determined. Calculated ΔGi0 , entropies and enthalpies are almost temperature-independent, especially at steps, but they depend on the step orientation. In contrast,ΔGi0 and ωij at terraces depend on the step density, following a linear tendency for terrace lengths larger than 5 atoms. However, while ΔGi0 increases with the step density, ωij decreases. Results were explained by considering the modification in the energetic surface balance by hydrogen, Hads, andwater,H2Oads, co-adsorption on the electrode, which in turn determines the whole adsorption processes on terraces and steps.This study has been carried out in the framework of the European Commission FP7 Initial Training Network “ELCAT”, Grant Agreement No. 214936-2. Support from the Spanish MINECO though project CTQ2013-44083-P and GV through PROMETEOII/2014/013 (FEDER) are greatly acknowledged

Oxygen reduction on nanostructured platinum surfaces in acidic media: Promoting effect of surface steps and ideal response of Pt(1 1 1)

In this paper, the role of surface steps on the electrocatalytic activity of platinum nanostructured surfaces for the oxygen reduction reaction (ORR) in acidic medium is evaluated by using stepped surfaces of structure Pt(S)[n(1 1 1) × (1 1 1)] with large terraces (20 ≤ n ≤ 50). It is realized that the inclusion of an even low amount of surface steps enhances the ORR, and linear activity trends are measured when currents at constant potential are plotted vs. the step density. As a consequence, an ideal ORR curve for a defect-free Pt(1 1 1) surface is extrapolated at zero defect density from experimental data and can be compared to that of a quasi-perfect Pt(1 1 1) electrode. It is clearly shown that surface steps promote the electrode activity toward ORR in acid medium. Results are discussed in light of available theoretical and experimental data.This work has been financially supported by a grant from COLCIENCIAS and the Universidad Nacional de Colombia in the framework of the National Program of Formation in Innovation Leaders (Contract No. 472 of 2007). Support from the Spanish MICYNN though project CTQ2010-16271 and GV through PROM-ETEO/2009/045 (FEDER) are also greatly acknowledged

Oxygen reduction reaction on stepped platinum surfaces in alkaline media

The oxygen reduction reaction (ORR) in 0.1 M NaOH on platinum single crystal electrodes has been studied using hanging meniscus rotating disk electrode configuration. Basal planes and stepped surfaces with (111) and (100) terraces have been employed. The results indicate that the Pt(111) electrode has the highest electrocatalytic activity among all the studied surfaces. The addition of steps on this electrode surface significantly diminishes the reactivity of the surface towards the ORR. In fact, the reactivity of the steps on the surfaces with wide terraces can be considered negligible with respect to that measured for the terrace. On the other hand, Pt(100) and Pt(110) electrodes have much lower activity than the Pt(111) electrode. These results have been compared with those obtained in acid media to understand the effect of the pH and the adsorbed OH on the mechanism. It is proposed that the surface covered by adsorbed OH is active for the reduction of the oxygen molecules.This work has been financially supported by the MICINN (Spain) (project CTQ2010-16271-FEDER) and Generalitat Valenciana (project PROMETEO/2009/045, -FEDER)

Oxygen reduction at platinum electrodes: The interplay between surface and surroundings properties

In this work, recent progress in the understanding of the mechanism of the oxygen reduction reaction at Pt surfaces is shortly reviewed. Specifically, the presence of a soluble and short-lived intermediate different to H2O2 in the ORR reaction path and the interrelated effect between the surface arrangement, adsorption of oxygen-containing species and water structure in the ORR reactivity in acid environments are discussed. Besides, the influence of the proton concentration on the ORR product distribution, the existence of a chemical step and the possible role of the soluble intermediate as a bifurcation point in the mechanism are also analyzed.The authors would like to thank Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP - Procs. 2013/16930-7 and 2014/23486-9), Brazil (A.M.G-M) and the Spanish MINECO though project CTQ2016-76221-P (FEDER) and GV through PROMETEOII/2014/013 ( FEDER) (J.M.F.), for financial supports

Cu UPD at Pt(100) and stepped faces Pt(610), Pt(410) of platinum single crystal electrodes

Published in Russian in Elektrokhimiya, 2016, Vol. 52, No. 9, pp. 999–1010The processes of adsorption/desorption of copper adatoms on the basal Pt(100) face and stepped Pt(610), Pt(410) surfaces have been studied in perchloric acid solution by cyclic voltammetry. It has been shown that the positions of the Cu stripping peaks are determined by perfection of the adlayer. The “island” model is suggested to describe electrochemical behavior of the Pt(hkl)+Cuad system. Obtained results are important for target modification of shape-controlled nanoparticles that are used in electrocatalysis.Financial support from the Russian Foundation for Basic Research (project no. 14-03-00530), MINECO (CTQ2013-44083-P) and Generalitat Valenciana (PrometeoII/2014/013) (Feder, Spain) is gratefully acknowledged

New insights into the hydrogen peroxide reduction reaction and its comparison with the oxygen reduction reaction in alkaline media on well-defined platinum surfaces

The hydrogen peroxide reduction reaction (HPRR) is investigated at pH = 13 on the Pt basal planes and stepped surfaces with (1 1 1) terraces separated by either monatomic (1 0 0) or (1 1 0) steps. A quantitative analysis of the surface structure effect revealed that Pt(1 1 1) is the most active surface and its activity progressively decreases when steps are introduced. Additionally, inhibition of the HPRR is observed at low potentials, which onset potential is governed by the OHads and the point of maximum entropy (pme) of the interphase. Experiments with different rotation rates suggest the formation of an HPRR intermediate linked to the inhibition process, which is more strongly adsorbed on (1 1 0) than (1 0 0) steps. Finally, a comparison of the HPRR and ORR (oxygen reduction reaction) illustrated the important differences for both reactions, which are dependent on the step density. These divergences have been discussed based on adsorbed intermediates and O2 interactions with the Pt surface.Financial support from Ministerio de Ciencia e Innovación (Project PID2019-105653 GB-100) and Generalitat Valenciana (Project PROMETEO/2020/063) is acknowledged