© 2018 Elsevier B.V.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.

© 2018 Elsevier B.V.In this work, a short revised analysis regarding the ORR mechanism at Pt-based surfaces has been performed and some common kinetic criteria have been reexamined in light of recent experimental results. In this sense, the production of H2O2 under high mass transport conditions and the lack of reduction currents at potentials higher than the experimental reaction onset, Eonset ORR, have been analyzed, considering the existence of a soluble intermediate species and an outer sphere reaction inside the ORR mechanism, besides the possible occurrence of a zero current cycle at Eonset ORR. Finally, the implications of these findings in the stability and durability of Pt-based catalysts have been also discussed.

© 2018 Elsevier Inc. All rights reserved.The oxygen reduction reaction (ORR) is one of the fundamental reactions in electrochemistry. Unfortunately, despite many years of research, the exact ORR mechanism is still unknown. In this work, a short revision of the ORR on platinum single crystals is summarized, including the effect of strongly adsorbing electrolytes and the water structure near the electrode surface on the ORR activity. Recent information from scan rate studies at stationary and rotating disk electrodes, suggesting the possible existence of a soluble intermediate species close to the reaction onset on Pt(111) is presented. In addition, the possible role of adsorbed hydroxyl, OHads; adsorbed oxygen, Oads; and Pt-oxides in the reaction dynamics is also discussed.

© 2017, Pleiades Publishing, Ltd.In this work, the reduction and oxidation of hydrogen peroxide on Au single crystals is studied in weakly adsorbing electrolytes. Results are discussed in terms of the potential of zero charge and the adsorption strength of different anions, which in turn depend on the crystallographic orientation of the electrode. Close to the reaction onset, both reactions follow the same activity trend with Au(100) and Au(111) being the most and the least active surface planes, respectively. At high potentials, gold oxides inhibit the oxidation of H2O2, which seems to be controlled by a surface process.

© 2017 Elsevier B.V.In this work, the effect of transition metals (TMs), such as Fe, Co, Ni and Cu, on the activity toward the hydrogen evolution reaction (HER) of modified molybdenum carbide (TM-Mo2C) catalysts has been evaluated. Catalysts were prepared by a temperature programmed reduction method in both an inert and a reductive atmosphere, and characterized by different physicochemical techniques. A high activity toward the HER is measured for all TM-Mo2C catalysts, with onset potentials lower than −0.06 V, as detected by on-line differential electrochemical mass spectrometry, and mass activities between 29 and 50 mA mg−1, which suggest them as promising non-precious electrocatalysts for this reaction. However, a decrease in the HER activity upon metal doping is measured, following an activity trend of α-Mo2C > Fe-Mo2C > Co-Mo2C > Ni-Mo2C > Cu-Mo2C. In situ near-edge X-ray adsorption analysis reveals a positive charge of the TM in the materials in the electrochemical environment, at the origin of the deleterious effect of Fe, Co, Ni and Cu, in terms of an electronic effect that modifies the d-electron configuration of α-Mo2C particles. Additionally, results also suggest that TM-Mo2C is more stable (lower catalyst dissolution) in acid media than α-Mo2C. Finally, because there is a catalyst deactivation toward the HER after the α-Mo2C component of the catalysts is oxidized at E > 0.7 V, the oxidation process of α-Mo2C is employed for estimating, as a first approximation, the number of surface active sites for the HER.

© 2016 Elsevier LtdIn the last years, transition metal carbides have appeared as novel materials with promising catalytic properties toward important practical reactions. In this work, the use of the thin porous electrodes for evaluating the hydrogen evolution reaction (HER) of hexagonal molybdenum carbides (α-Mo2C)-based materials is analyzed and the effect of catalyst's load, Lcat, and catalyst's dispersion are discussed in terms of kinetic parameters calculated by employing the electrode geometric area. Catalysts were characterized by X-ray diffraction, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, cyclic voltammetry and differential electrochemical mass spectrometry. Results have shown that, even for the same catalyst, mass activities and specific activities depend on Lcat. and catalyst's dispersion. In contrast, intrinsic kinetic parameters, calculated from double layer capacitance normalizations, can be considered rather constant. XPS analysis of samples under different electrochemical treatments reveals a surface enrichment of carbon terminated planes after the HER, suggesting a higher HER activity on these planes. An investigation of the electrochemical oxidation of α-Mo2Cand the catalyst's HER activity show a direct correlation between active sites for HER and active sites for catalyst oxidation. Therefore, this oxidation is also used to estimate HER intrinsic parameters. Finally, the activity of a composite sample, in which α-Mo2C is the only active component, is also evaluated.

© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimIn the last few years, transition metal carbides have emerged as novel materials with promising catalytic properties toward important practical reactions. In this work, cubic and hexagonal molybdenum carbides are synthesized and evaluated as carbon-supported catalysts and as support materials for Pt nanoparticles for the electrochemical oxygen reduction reaction (ORR). The catalysts are characterized by XRD, energy-dispersive X-ray spectroscopy, TEM, XPS, and cyclic voltammetry on stationary and rotating ring-disk electrodes. The results suggest different reactivity of the molybdenum carbide phases as both catalysts and supports for the ORR. Enhanced mass and specific ORR activities at 0.9 V are calculated for Pt–molybdenum carbide-derived composites compared to commercial Pt and Pt/C catalysts prepared by depositing Pt by the same method. The origin of the improved ORR activity is discussed in terms of the synergistic effect between Pt and the carbide-derived support and a decrease in the adsorption strength of oxygen-containing species on the Pt surface, similar to that proposed for Pt–metal alloys. Additionally, the possible formation of a Pt–Mo alloy on the catalyst surface is proposed.

© 2015 Elsevier B.V. All rights reserved.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 isotherm and the statistical thermodynamics description are applied to calculate formal entropies, enthalpies and Gibbs energies, ΔG¯i0, 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 on ΔG¯i0 and on the lateral interactions between adsorbed species, ωij, at terraces and steps is also determined. Calculated ΔG¯i0, entropies and enthalpies are almost temperature-independent, especially at steps, but they depend on the step orientation. In contrast, ΔG¯i0 and ωij at terraces depend on the step density, following a linear tendency for terrace lengths larger than 5 atoms. However, while ΔG¯i0 increases with the step density, ωij decreases. Results were explained by considering the modification in the energetic surface balance by hydrogen, Hads, and water, H2Oads, co-adsorption on the electrode, which in turn determines the whole adsorption processes on terraces and steps.

© 2015 Elsevier B.V.Most of electrocatalytic reactions occur in an aqueous environment. Understanding the influence of water structure on reaction dynamics is fundamental in electrocatalysis. In this work, the role of liquid water structure on the oxygen reduction at Pt(1 1 1) electrode is analyzed in methanesulfonic (MTSA) and perchloric acids. This is because these different anions can exert a different influence on liquid water structure. Results reveal a lower ORR electrode activity in MTSA than in HClO4 solutions and they are discussed in light of anion's influence on water structural ordering. From them, the existence of an outer-sphere, rate determining, step in the ORR mechanism is suggested.

© 2015, Springer-Verlag Berlin Heidelberg.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, OH<inf>ads</inf>, adsorbed oxygen, O<inf>ads</inf>, 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<inf>ORR</inf>, is detected. This peak shows a different electrochemical dynamics than that of Pt(111) oxides. The data analysis suggests that neither OH<inf>ads</inf> nor O<inf>ads</inf> 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.

© 2014 Elsevier B.V. All rights reserved.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.

A model for the differential capacitance of metal electrodes coated by solid polymer electrolyte membranes, with acid/base groups attached to the membrane backbone, and in contact with an electrolyte solution is developed. With proper model parameters, the model is able to predict a limit response, given by Mott-Schottky or Gouy-Chapman-Stern theories depending on the dissociation degree and the density of ionizable acid/base groups. The model is also valid for other ionic membranes with proton donor/acceptor molecules as membrane counterions. Results are discussed in light of the electron transfer rate at membrane-coated electrodes for electrochemical reactions that strongly depend on the double layer structure. In this sense, the model provides a tool towards the understanding of the electro-catalytic activity on modified electrodes. It is shown that local maxima and minima in the differential capacitance as a function of the electrode potential may occur as consequence of the dissociation of acid/base molecular species, in absence of specific adsorption of immobile polymer anions on the electrode surface. Although the model extends the conceptual framework for the interpretation of cyclic voltammograms for these systems and the general theory about electrified interfaces, structural features of real systems are more complex and so, presented results only are qualitatively compared with experiments. © 2014 Elsevier B.V.

The differential capacity and the potential distribution at electrode/solid polymer electrolyte membrane/solution interfaces are calculated through an analytical approach. The model considers coions' and counterions' permeation through the membrane from the solvent phase and the ions' partitioning equilibrium at the SPEM/solution interface. The latter effects are included by incorporating the Donnan equilibrium, the steric hindrance, the solvation energy change when ions move from water to membrane pores and ion electrostatic interactions. It is shown that capacitance maxima in capacitance-potential curves may appear because of the acid-base dissociation process inside the membrane and the change in the ions' total interaction energy with the applied potential. For low dielectric constants inside membrane pores, εp, sharp peaks can be obtained. These peaks broaden, decrease in magnitude and shift to positive potentials once εp is increased. Finally, model predictions are discussed in light of recent experimental data obtained on Nafion® covered Pt(111) electrodes, providing a theoretical framework for the qualitative electroanalysis of these systems. This journal is © 2014 the Owner Societies.

The electrochemical oxidation of CO on metallic surfaces following a lattice-gas model, including effective lateral interactions between one of the adsorbates and species diffusion, is studied. The reaction occurs through a Langmuir-Hinshelwood (LH) mechanism in which adsorbed CO reacts with adsorbed hydroxyl species. The mean field approximation and dynamic Monte Carlo simulations have been compared. The effect of the molecular distribution and surface mobility of reacting species on the potential dependence of the CO oxidation rate is analyzed. The inclusion of lateral interactions into the reaction mechanism reconciles different experimental observations, such as island formation and fast CO diffusion. Results highlight the importance of effective interactions in the reaction kinetics and suggest that they should be taken into account when interpreting experimental data. Simulations are useful for an improved qualitative understanding of the kinetics of CO oxidation and other electrochemical LH reactions. © 2014 American Chemical Society.

Oxygen reduction reaction (ORR) dynamics at platinum single crystal surfaces is reviewed, and experimental results in acid and alkaline solutions are discussed in the framework of theoretical studies. Special emphasis is devoted to point out the role of the surface charge, water structure and adsorbed oxygen containing species. Additionally, discussion about the possible relevance of hydrogen peroxide as an intermediate species is also included. It is shown that the ORR is a complex process affected by many different factors and so neither surface charge nor oxygen-containing species coverage alone are a determining factor of the electrode activity. Instead, adlayer structures and the relation between adsorbed water, H2Oads, water dissociation products, OHads or Oads, and PtO oxide species coverage affect the whole energetics of the adsorption processes and may determine the surface reactivity. Finally, if H2O2 is an intermediate product in the ORR mechanism, it would be crucial to find a suitable catalyst able to effectively reduce H2O2 at high potentials and inhibit its oxidation. This journal is © the Partner Organisations 2014.

The oxygen reduction reaction (ORR) is a pivotal process in electrochemistry. Unfortunately, after decades of intensive research, a fundamental knowledge about its reaction mechanism is still lacking. In this paper, a global and critical view on the most important experimental and theoretical results regarding the ORR on Pt(111) and its vicinal surfaces, in both acidic and alkaline media, is taken. Phenomena such as the ORR surface structure sensitivity and the lack of a reduction current at high potentials are discussed in the light of the surface oxidation and disordering processes and the possible relevance of the hydrogen peroxide reduction and oxidation reactions in the ORR mechanism. The necessity to build precise and realistic reaction models, which are deducted from reliable experimental results that need to be carefully taken under strict working conditions is shown. Therefore, progress in the understanding of this important reaction on a molecular level, and the choice of the right approach for the design of the electrocatalysts for fuel-cell cathodes is only possible through a cooperative approach between theory and experiments. © 2013 Gómez-Marín et al.

The mean field approximation to model the CO electrocatalytic oxidation on metal surfaces, including attractive and repulsive effective lateral interactions between the species, is employed. Adsorbed CO reacts with adsorbed OH from water dissociation through a Langmuir-Hinshelwood mechanism. Simulated results suggest alternative explanations to experimental observations during CO stripping oxidation on different electrodes by potential step chronoamperometry or voltammetry. Even though the model ignores higher order interactions, such as three-body or long-ranged interactions, the results extend the conceptual framework for transient kinetics and stripping voltammograms for CO oxidation on metallic surfaces or any other electrochemical Langmuir-Hinshelwood reaction. In spite of the high level of simplicity, the model is a promising tool for the understanding of the electrocatalytic activity and results were qualitatively compared with experimental results. © 2013 American Chemical Society.

The oxygen reduction reaction (ORR) is undoubtedly the most important fuel-cell cathodic reaction. In this work, a detailed electrochemical analysis of the ORR on Pt (111) in nonadsorbing electrolytes was performed, which included the high-potential region Eup=1.15 V while ensuring the electrode surface structure stability. Our results suggest that the reduction of a soluble intermediate species formed during the ORR is the rate-determining step in the whole reaction mechanism. This species does not undergo any other electrochemical reaction at E>0.9 V and may accumulate close to the electrode surface. Together with dissolved O2, this intermediate may modify the oxide-growth dynamics on Pt (111). Hence, both species interact with the electrode surface through complex catalytic networks. Under certain experimental conditions, oxygenated species from the oxidation of Pt (111) may enhance the overall ORR current. These results propose an alternative to explain the current state of the art for this fundamental process. A radical idea! The oxygen reduction reaction on Pt (111) in the high potential region is not inhibited by the initial oxidized surface states. Instead, the reduction of a soluble intermediate species apparently is the rate determining step. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Oxidation (HPOR) and reduction (HPRR) reactions of hydrogen peroxide were studied on platinum single crystalline surfaces. Experimental curves were fitted to a simple model in which the whole process is described by the sum of two independent, mass controlled reactions. It is shown that stepped surfaces are more catalytic than basal planes for both HPOR and HPRR reactions. On the contrary, on oxide covered surfaces, those having large {111} terraces are better electrocatalysts for HPRR, but worse for HPOR. These results shed light concerning H2O2 reactivity on platinum surfaces and may help to unveil oxygen reduction reaction mechanism. © 2013 Elsevier B.V. All rights reserved.

A significant number of electrocatalytic reactions take place in a potential region in which the surface of platinum is partly covered by oxygenated species. Hence, the mechanism of the electrochemical formation and reduction of surface oxides is fundamental to understand how surface composition can determine the reactivity of the catalyst. In this paper, combined potential step chronoamperometry and cyclic voltammetry were performed to involve oxide formation and its evolution on Pt(1 1 1), in the absence of anion specific adsorption and keeping surface stability (0.95 < E < 1.1 V). As a first approximation, a simple mean field model scheme is outlined to describe the dynamics of the initial states of the electrochemical oxide growth on Pt(1 1 1). Qualitatively, the whole oxidation process can be properly described by the sequential electrochemical oxidation of water, in such a way that the main features of the cyclic voltammogram and potential step profiles can be explained. The results corroborate the existence of at least two different oxidation states of the surface. The first state is formed through a nucleation and growth mechanism controlled by the rate at which the nuclei are formed, i.e. it follows a progressive nucleation mechanism. The second state is a further ageing process that depends on the potential at which the oxide layer is fully developed. The final oxide film slows down subsequent water dissociation. © 2012 Elsevier Ltd. All rights reserved.

A significant number of electrocatalytic reactions take place in a potential region in which the surface of platinum is partly covered by oxygenated species. Hence, the mechanism of the electrochemical formation and reduction of surface oxides is fundamental to understand how its properties can determine the reactivity of the catalyst. In this paper, the dynamic of the initial states of the electrochemical oxide growth on Pt(1 1 1), in the absence of anion specific adsorption and keeping surface stability, is studied by employing electrochemical techniques. Several species, such as chemisorbed hydroxide and oxygen, initial Pt oxide structures and even sub-surface oxygen, interconvert and interact. Their existences depend more strongly on the total surface coverage than on the electrode potential. Considering structural and thermodynamic data related to the identity of the electrochemical oxide film at platinum, the results suggest that Pt(1 1 1) oxide starts by water dissociation to OHads followed by its electrochemical oxidation to Oads at higher potentials. The latter process occurs as a phase transition through a nucleation and growth mechanism, until a stable adlayer is formed. In parallel, Oads transforms to an initial Pt oxide structure. Thus, there are at least two types of processes involved in the growth of the surface oxide layer (i.e. two states of oxide films), and the separation between anodic and cathodic j-E profiles reflects a true hysteresis arising from a change of state of the surface oxide film. © 2012 Elsevier B.V. All rights reserved.

A significant number of electro-catalytic reactions take place in a potential region in which the surface of platinum is partly covered by oxygenated species. In this respect, the initial oxidation of Pt surfaces is an important process that could determine the reactivity of this catalyst. The understanding of electrochemical Pt oxidation has been hindered by a lack of surface structural definition. In this work, the electro-oxidation of Pt(1 1 1) electrode in the absence and presence of weak and moderately strong specific anion adsorption, and the subsequent surface modification induced by oxygen adsorption are studied. Two different potential dependences are found for the surface reordering kinetics in perchloric acid solutions, at higher and lower potentials, whereas only one appears on sulphate containing solutions. Additionally, a dual role of sulphate anion is observed: at high sulphate concentrations the protective character of the ordered sulphate adlayer delays surface disordering while small concentrations of sulphate anions increase the rate of surface reordering. Water dissociation is at the origin of the double behaviour in HClO 4 and also explains the dual role of sulphate anions. It is concluded that platinum oxidation is a complex process that involves several adsorbed species that appear at increasing potentials. All of these process are influenced by anion adsorption and coexist during the initial stages of Pt(1 1 1) oxidation. © 2012 Elsevier Ltd.

Hydrogen peroxide is recognized as one of the most probable intermediate species during oxygen reduction reaction (ORR) on various metals. In this work, H 2 O 2 reduction and oxidation on Pt(111) have been studied in a non-adsorbing electrolyte by cyclic voltammetry and online electrochemical mass spectrometry. H 2 O 2 is oxidized and reduced into two different, but interrelated electron transfer processes. As the potential increases, the reduction of H 2 O 2 switches rapidly to its oxidation at ~ E > 0.9 V. The whole process exhibits a marked hysteresis in the mixed charge transfer-diffusion controlled potential region, together with current overshoots at E > 0.85 V and E < 0.27 V in quiescent solutions. At high potentials, Pt(111) oxides and O 2 evolution explain the current-potential characteristics, while at low potentials, hydrogen adsorption is at the origin of the current response. © 2012 Elsevier B.V.

Understanding the structure and molecular processes at the electrode/membrane interfaces constitutes an important topic in PEFC as well as in electrochemistry. In this work, the Pt(111)/Nafion model interface in HClO4 acid solutions is studied by IRRAS and cyclic voltammetry. It was found that the presence of an electric field mainly promotes deprotonation of sulfonic groups and structuring of water inside the membrane (polar molecules), especially near the electrode surface, with a sudden change of system optical properties at the Pt(111)/membrane interface at 0.9 V, possibly due to clustering within the polymer. Furthermore, the performance of the Pt(111)/Nafion in a typical electrochemical reaction as CO oxidation has been also analyzed. It is shown that there are notable differences between the characteristics of CO adsorption and oxidation at Pt(111) with and without polymer electrolyte membrane, like a continual wavenumber increase with the potential for the on-top CO band, even during CO oxidation, which proceeds at higher potentials at the electrode covered by the polymer. The spectroscopic features suggest enhanced proton mobility inside the membrane concomitantly with the deprotonation of sulfonic groups near the electrode surface and with higher potentials, possibly due to oriented morphologies inside the membrane induced by high fields. © 2010 American Chemical Society.

Among technologies for melanin commercial-scale production, microbial synthesis is an attractive option. In this study, a melanin producer, wild strain Bacillus subtilis, was isolated from the soil, and pigment from this bacterium was purified and characterised using different techniques. The use of tandem pyrolysis/gas chromatography/mass spectrometry allowed us to classify the pigment as a sulphur-containing bacterial melanin. Thermogravimetric and differential thermal analyses indicated that melanin might contain two fractions of water, a strongly bound fraction and a weakly-bound fraction. Scanning electron microscopy showed an amorphous structure without differential characteristics, and electronic conductivity measurements at normal atmospheric conditions and humidities suggest that melanin may be an insulator. However, more work is underway to clarify this point. © 2010 Elsevier B.V. All rights reserved.