Electrochemical surface reordering of Pt(111): A quantification of the place-exchange process

Driving Force of the Initial Step in Electrochemical Pt(111) Oxidation

The first step of electrochemical surface oxidation is extraction of a metal atom from its lattice site to a location in a growing oxide. Here we show by fast simultaneous electrochemical and in situ high-energy surface X-ray diffraction measurements that the initial extraction of Pt atoms from Pt(111) is a fast, potential-driven process, whereas charge transfer for the related formation of adsorbed oxygen-containing species occurs on a much slower time scale and is evidently uncoupled from the extraction process. It is concluded that potential plays a key independent role in electrochemical surface oxidation.

Tailoring the active site for the oxygen evolution reaction on a Pt electrode

Highly active electrocatalysts for the oxygen evolution reaction (OER) are essential to improve the efficiency of water electrolysis. The properties of OER active sites on single-crystal Pt electrodes were examined herein. The OER is markedly enhanced by repeated oxidative and reductive potential cycles on the Pt(111) surface. The OER activity on Pt(111) is nine times higher in the third cycle than that before the potential cycles. OER activation by potential cycling depends on the (111) terrace width, with wider (111) terraces significantly enhancing the OER. The oxidation/reduction of the Pt(111) surface produces atomic-sized vacancies on the terraces that activate the OER. Structural analysis using X-ray diffraction reveals that the active sites formed by potential cycling are defects in the second subsurface Pt layer. Potential cycling induces the bowl-shaped roughening of the electrode surface, wherein high-coordination number Pt atoms at the bottom of the cavities activate the OER.

Mapping the Potential of Zero Charge and Electrocatalytic Activity of Metal-Electrolyte Interface via a Grain-by-Grain Approach

Potential of zero charge (PZC) is a fundamental quantity that dictates the structure of the electrical double layer. Studies using single crystals suggest a polycrystalline surface should display an inhomogeneous distribution of PZC and electric field, which directly affects the electrochemical energy storage and conversion processes occurring at the electrode-electrolyte interface. Herein, we demonstrate the direct mapping of local PZC using scanning electrochemical cell microscopy (SECCM). The potential-dependent charging current upon the formation of the microscopic electrode-electrolyte interface is used to determine the PZC. Using polycrystalline Pt as a model system, correlative SECCM and electron backscatter diffraction (EBSD) images show the dependence of PZC on the local crystal grain orientation. The electrocatalytic activity can be mapped from the same SECCM experiment via local voltammetry, which demonstrates the variation of hydrogen evolution reaction (HER) activity across Pt grains. The method reported here can be readily applied to study other electrochemical interfaces, providing rich correlative information on the surface property and electrocatalytic activities.

Atomic-Scale Identification of the Electrochemical Roughening of Platinum

Electrode degradation under oxidizing conditions is a major drawback for large-scale applications of platinum electrocatalysts. Subjecting Pt(111) to oxidation-reduction cycles is known to lead to the growth of nanoislands. We study this phenomenon using a combination of simultaneous in situ electrochemical scanning tunneling microscopy and cyclic voltammetry. Here, we present a detailed analysis of the formed islands, deriving the (evolution of the) average island growth shape. From the island shapes, we determine the densities of atomic-scale defect sites, e.g., steps and facets, which show an excellent correlation with the different voltammetric hydrogen adsorption peaks. Based on this combination of electrochemical scanning tunneling microscopy (EC-STM) and CV data, we derive a detailed atomistic picture of the nanoisland evolution during potential cycling, delivering new insights into the initial stages of platinum electrode degradation.

Effect of the Random Defects Generated on the Surface of Pt(111) on the Electro-oxidation of Ethanol: An Electrochemical Study

In the present work, the Pt(111) surface was disordered by controlling the density of {110}- and {100}-type defects. The cyclic voltammogram (CV) of a disordered surface in acid media consists of three contributions within the hydrogen adsorption/desorption region: one from the well-ordered Pt(111) symmetry and the other two transformed from the {111}-symmetry with contributions of {110}- and {100}-type surface defects. The ethanol oxidation reaction (EOR) was studied on these disordered surfaces. Electrochemical studies were performed in 0.1 M HClO₄ +0.1 M ethanol using cyclic voltammetry and chronoamperometry. Changes in current densities associated to the specific potentials at which each oxidation peak appears suggest that different surface domains of disordered platinum oxidize ethanol independently. Additionally, as the surface-defect density increases, the EOR is catalysed better. This tendency is directly observed from the CV parameters because the onset and peak potentials are shifted to less positive values and accompanied by increases in the oxidation-peak current on disordered surfaces. Similarly, the CO oxidation striping confirmed this same tendency. Chronoamperometric experiments showed two opposite behaviors at short oxidation times (0.1 s). The EOR was quickly catalyzed on the most disordered surface, Pt(111)-16, and was then rapidly deactivated. These results provide fundamental information on the EOR, which contributes to the atomic-level understanding of real catalysts.

Mechanistic Insights into Cyclic Voltammograms on Pt(111): Kinetics Simulations

A detailed understanding of the electrochemistry of platinum electrodes is of great importance for the electrochemical oxidation of fuels and electrochemical reduction of dioxygen in fuel cells. The Pt(111) facet is the most representative model mimicking Pt nanoparticles and polycrystals for fundamental studies. Herein, we propose a site-specific model accompanied with the typical elementary steps of the electrochemistry of Pt(111) in non-adsorbing electrolyte within the potential range between 0.05 and 1.15 V versus reversible hydrogen electrode. Simulations were conducted at different scanning rates based on the kinetics models. We reproduce all the anodic and cathodic peaks observed in the reported experimental curves. These results demonstrate the underlying mechanisms of the peak formation in different potential regions.

Preparation of complex model electrocatalysts in ultra-high vacuum and transfer into the electrolyte for electrochemical IR spectroscopy and other techniques

Model studies at complex, yet well-defined electrodes can provide a better understanding of electrocatalytic reactions. New experimental devices are required to prepare such model electrocatalysts with atomic-level control. In this work, we discuss the design of a new setup, which enables the preparation of well-defined electrocatalysts in ultra-high vacuum (UHV) using the full portfolio of surface science techniques. The setup allows for direct transfer of samples from UHV and the immersion into the electrolyte without contact to air. As a special feature, the single crystal sample is transferred without any sample holder, which makes the system easily compatible with most electrochemical in situ methods, specifically with electrochemical infrared reflection absorption spectroscopy, but also with other characterization methods such as single-crystal cyclic voltammetry, differential electrochemical mass spectrometry, or electrochemical scanning tunneling microscopy. We demonstrate the preparation in UHV, the transfer in inert atmosphere, and the immersion into the electrolyte for a complex model catalyst that requires surface science methods for preparation. Specifically, we study Pt nanoparticles supported on well-ordered Co₃O₄(111) films which are grown on an Ir(100) single crystal. In comparison with reference experiments on Pt(111), the model catalyst shows a remarkably different adsorption and reaction behavior during CO electrooxidation in alkaline environments.

Correlation of surface site formation to nanoisland growth in the electrochemical roughening of Pt(111)

Platinum plays a central role in a variety of electrochemical devices and its practical use depends on the prevention of electrode degradation. However, understanding the underlying atomic processes under conditions of repeated oxidation and reduction inducing irreversible surface structure changes has proved challenging. Here, we examine the correlation between the evolution of the electrochemical signal of Pt(111) and its surface roughening by simultaneously performing cyclic voltammetry and in situ electrochemical scanning tunnelling microscopy (EC-STM). We identify a 'nucleation and early growth' regime of nanoisland formation, and a 'late growth' regime after island coalescence, which continues up to at least 170 cycles. The correlation analysis shows that each step site that is created in the 'late growth' regime contributes equally strongly to both the electrochemical and the roughness evolution. In contrast, in the 'nucleation and early growth' regime, created step sites contribute to the roughness, but not to the electrochemical signal.

Probing the surface fine structure through electrochemical oscillations

In the course of (electro)catalytic reactions, reversible and irreversible changes, namely the formation of adsorbed poisons, catalyst degradation, surface roughening, etc., take place at distinct time-scales. Reading the transformations on the catalyst surface from the measurement of the reaction rates is greatly desirable but generally not feasible. Herein, we study the effect of random surface defects on Pt(100) electrodes toward the electro-oxidation of methanol in acidic media. The surface defects are gently generated in situ and their relative magnitudes are reproducibly controlled. The system was characterized under conventional conditions and investigated under an oscillatory regime. Oscillatory patterns were selected according to the presence of surface defects, and a continuous transition from large amplitude/low frequency oscillations (type L) on smooth surfaces to small amplitude/high frequency oscillations (type S) on disordered surfaces was observed. Importantly, self-organized potential oscillations were found to be much more sensitive to the surface structure than conventional electrochemical signatures or even other in situ characterization methods. As a consequence, we proved the possibility of following the surface fine structure in situ and in a non-invasive manner by monitoring the temporal evolution of oscillatory patterns. From a mechanistic point of view, we describe the role played by surface defects and of the adsorbed and partially oxidized, dissolved species on the oscillations of type S and L.

How do random superficial defects influence the electro-oxidation of glycerol on Pt(111) surfaces?

The glycerol electrooxidation reaction (GEOR) has attracted huge interest in the last decade due to the very low price and availability of this polyol. In this work, we studied the GEOR on Pt(111) electrodes by introducing different densities of random defects. Our results showed that the generation of defects on Pt(111) slightly modified the GEOR onset potential, however it generates changes in the voltammetric oxidation charges and also in the relative production of CO₂ to carbonyl containing compounds, C[double bond, length as m-dash]O. The voltammetric profiles in the forward scan show two oxidation peaks. FTIR data show that the first one is connected with the GlOH dissociative adsorption to form CO (and others intermediates) while the second one, at higher potentials, matches the onsets of the CO oxidation to CO₂ and the C[double bond, length as m-dash]O production. FTIR also confirms that the lower activity of defected electrodes at lower potentials is connected to a higher CO poisoning. DFT calculations show that the presence of CO molecules on a Pt defected surface keeps water and GlOH molecules far from the surface and linked by H bonds. This paper is the last of a series of three works where we explore the GEOR on an important number of different Pt surfaces. These works show that it is difficult to oxidize GlOH at potentials lower than 0.6 V (under our experimental conditions) without suffering an important electrode poisoning (mainly by CO). Since the structure of nanoparticles might be mimicked by defected single crystals, these sets of reports provide a considerable amount of information concerning the influence of such surfaces towards GlOH reaction in acidic media. Therefore, if the well-known "nano"-effect does not produce substantial changes in the activity of Pt materials, they are not useful to be applied in a Direct Glycerol Fuel Cell (DGFC). On the other hand, it is very interesting that the density of electrode defects permits us to tune the relative production of CO₂ to C[double bond, length as m-dash]O.

Some reflections on the understanding of the oxygen reduction reaction at Pt(111)

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.

New insights into the oxygen reduction reaction mechanism on Pt(111): a detailed electrochemical study

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.