Pt(111) surface disorder kinetics in perchloric acid solutions and the influence of specific anion adsorption

Electrolyte selection toward efficient photoelectrochemical glycerol oxidation on BiVO4

Glycerol, a primary by-product of biodiesel production, can be oxidized into various value-added chemicals, significantly enhancing the techno-economic value of photoelectrochemical (PEC) cells. Several studies have explored various photoelectrode materials and co-catalysts, but the influence of electrolytes on PEC glycerol oxidation has remained relatively unexplored despite its significance. Here, we explore the impact of various acidic (pH = 2) electrolytes, namely NaNO3, NaClO4, Na2SO4, K2SO4, and KPi, on PEC glycerol oxidation using nanoporous thin film BiVO4 as a model photoanode. Our experimental findings reveal that the choice of electrolyte anion and cation significantly affects the PEC performance (i.e., photocurrent, onset potential, stability, and selectivity towards value-added products) of BiVO4 for glycerol oxidation. To explain this interesting phenomenon, we correlate the observed performance trend with the ion specificity in the Hofmeister series as well as the buffering capacity of the electrolytes. Notably, NaNO3 is identified as the optimal electrolyte for PEC glycerol oxidation with BiVO4 when considering various factors such as stability and production rates for glycerol oxidation reaction (GOR) products, surpassing the previously favored Na2SO4. Glycolaldehyde emerges as the most dominant product with ∼50% selectivity in NaNO3. The general applicability of our findings is confirmed by similar observation in electrochemical (EC) GOR with a polycrystalline platinum anode. Overall, these results emphasize the critical role of electrolyte selection in enhancing the efficiency of EC/PEC glycerol oxidation.

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.

Electric Double Layer: The Good, the Bad, and the Beauty

The electric double layer (EDL) is the most important region for electrochemical and heterogeneous catalysis. Because of it, its modeling and investigation are something that can be found in the literature for a long time. However, nowadays, it is still a hot topic of investigation, mainly because of the improvement in simulation and experimental techniques. The present review aims to present the classical models for the EDL, as well as presenting how this region affects electrochemical data in everyday experimentation, how to obtain and interpret information about EDL, and, finally, how to obtain some molecular point of view insights on it.

Role of Pt and PtO2 in the Oxygen-Evolution Reaction in the Presence of Iron under Alkaline Conditions

The oxygen-evolution reaction (OER) through water oxidation is an inevitable reaction for water splitting toward storing energy. However, OER is a four-electron and slow reaction, which is also a bottleneck for water splitting. To find the role of Pt and PtO₂ on the OER in the presence of Fe, the electrochemistry of Pt foil and PtO₂ is investigated in the absence/presence of K₂FeO₄ as a soluble Fe salt at pH ≈ 13. After the addition of K₂FeO₄, a remarkable increase in the OER is recorded in the presence of Pt or PtO₂. The obtained catalysts were characterized by operando visible spectroscopy, high-resolution transmission electron microscopy, scanning electron microscopy, electron-spin resonance spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and electrochemical methods. KOH solutions usually contain Fe and/or Ni impurities. It is found that neither Pt nor PtO₂ is an OER catalyst in a Ni/Fe-free KOH, and even at an overpotential of 570 mV in purified KOH (pH ≈ 13), no clear OER was observed.

Correlating Surface Structures and Electrochemical Activity Using Shape-Controlled Single-Pt Nanoparticles

We report a method for synthesizing and studying shape-controlled, single Pt nanoparticles (NPs) supported on carbon nanoelectrodes. The key advance is that the synthetic method makes it possible to produce single, electrochemically active NPs with a vast range of crystal structures and sizes. Equally important, the NPs can be fully characterized, and, therefore, the electrochemical properties of the NPs can be directly correlated to the size and structure of a single shape. This makes it possible to directly correlate experimental results to first-principles theory. Because just one well-characterized NP is analyzed at a time, the difficulty of applying a theoretical analysis to an ensemble of NPs having different sizes and structures is avoided. In this article, we report on two specific Pt NP shapes having sizes on the order of 200 nm: concave hexoctahedral (HOH) and concave trapezohedral (TPH). The former has {15 6 1} facets and the latter {10 1 1} facets. The electrochemical properties of these single NPs for the formic acid oxidation (FAO) reaction are compared to those of a single, spherical polycrystalline Pt NP of the same size. Finally, density functional theory, performed prior to the electrochemical studies, were used to interpret the experimental results of the FAO experiments.

Oscillatory dynamics during the methanol electrooxidation reaction on Pt(111)

Despite several papers describing the oscillatory methanol electrooxidation reaction (OMOR) catalyzed by polycrystalline Pt, these dynamic instabilities are less explored on single crystalline surfaces. Herein, we observed and mapped for the first time the OMOR on Pt(111) in non-adsorbing anion solutions as well as in the presence of small amounts of sulfate anions. Period 1 oscillations with oscillation frequencies from 1.2 to 2.0 Hz were observed for methanol concentrations higher than 1.0 M, with no evolution to more complex patterns. These oscillations occur in the potential range in which PtOH is partially covering the surface without irreversible oxidation processes. Small changes in both the mean potential (E_m) and the poisoning rate along the time-series were observed, the so-called drift, and were explained in terms of the accumulation of intermediates at the interface. The presence of sulfate strongly inhibits the OMOR, and the results are discussed in terms of sulfate adlayer formation on {111} domains.

In Situ Surface-Enhanced Raman Spectroscopy Characterization of Electrocatalysis with Different Nanostructures

As energy demands increase, electrocatalysis serves as a vital tool in energy conversion. Elucidating electrocatalytic mechanisms using in situ spectroscopic characterization techniques can provide experimental guidance for preparing high-efficiency electrocatalysts. Surface-enhanced Raman spectroscopy (SERS) can provide rich spectral information for ultratrace surface species and is extremely well suited to studying their activity. To improve the material and morphological universalities, researchers have employed different kinds of nanostructures that have played important roles in the development of SERS technologies. Different strategies, such as so-called borrowing enhancement from shell-isolated modes and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)-satellite structures, have been proposed to obtain highly effective Raman enhancement, and these methods make it possible to apply SERS to various electrocatalytic systems. Here, we discuss the development of SERS technology, focusing on its applications in different electrocatalytic reactions (such as oxygen reduction reactions) and at different nanostructure surfaces, and give a brief outlook on its development.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Thermodynamics of the formation of surface PtO2 stripes on Pt(111) in the absence of subsurface oxygen

This paper examines the thermodynamics of PtO₂ stripes formed as intermediates of Pt(111) surface oxidation as a function of the degree of dilation parallel to the stripes, using density functional theory and atomistic thermodynamics. Internal energy calculations predict 7/8 and 8/9 stripe structures to dominate at standard temperature and pressure, which contain 7 or 8 elevated PtO₂ units per 8 or 9 supporting surface Pt atoms, respectively. Moreover, we found a thermodynamic optimum with respect to mean in-stripe Pt-Pt spacing close to that of α-PtO₂. Vibrational zero point energies, including bulk layer contributions, make a small but significant contribution to the stripe free energies, leading to the 6/7 stripe being most stable, although the 7/8 structure is still close in free energy. These findings correspond closely to experimental observations, providing insight into the driving force for oxide stripe formation and structure as the initial intermediate of platinum surface oxidation, and aiding our understanding of platinum catalysts and surface roughening under oxidative conditions.

A platinum-nickel bimetallic nanocluster ensemble-on-polyaniline nanofilm for enhanced electrocatalytic oxidation of dopamine

We report a new approach to design flexible functional material platforms based on electropolymerized polyaniline (PANI) polymer nanofilms modified with bimetallic nanoclusters (NCs) for efficient electro-oxidation of small organic molecules. Composition defined ligand free Pt_0.75Ni_0.25 NCs were synthesized in the gas phase using the Cluster Beam Deposition (CBD) technology and characterized using RToF, HAADF-STEM, XAFS and XPS. NCs were then directly deposited on PANI coated templates to construct electrodes. Dopamine (DP) molecules were used as a representative organic analyte and the influence of the NC-PANI hybrid atomistic structure on the electrochemical and electrocatalytic performance was investigated. The as prepared, nearly monodispersed, Pt_0.75Ni_0.25 NCs of ca. 2 nm diameter featuring a PtOx surface combined with a shallow platelet-like Ni-O(OH) phase formed a densely packed active surface on PANI at ultralow metal coverages. Electrochemical measurements (EIS and CV) show a 2.5 times decrease in charge transfer resistance and a remarkable 6-fold increase at lower potential in the mass activity for Pt_0.75Ni_0.25 NCs in comparison with their pure Pt counterparts. The enhanced electrochemical performance of the Pt_0.75Ni_0.25 NC hybrid's interface is ascribed to the formation of mixed Pt metal and Ni-O(OH) phases at the surface of the alloyed PtNi cores of the bimetallic NCs under electrochemical conditions combined with an efficient charge conduction pathway between NCs.

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.

Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO₂ reduction (CO₂R). There are variety of factors that impact CO₂R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO₂R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO₂R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO₂R.

An Integrated Single-Electrode Method Reveals the Template Roles of Atomic Steps: Disturb Interfacial Water Networks and Thus Affect the Reactivity of Electrocatalysts

A method enabling the accurate and precise correlation between structures and properties is critical to the development of efficient electrocatalysts. To this end, we developed an integrated single-electrode method (ISM) that intimately couples electrochemical rotating disk electrodes, in situ/operando X-ray absorption fine structures, and aberration-corrected transmission electron microscopy on identical electrodes. This all-in-one method allows for the one-to-one, in situ/operando, and atomic-scale correlation between structures of electrocatalysts with their electrochemical reactivities, distinct from common methods that adopt multisamples separately for electrochemical and physical characterizations. Because the atomic step is one of the most fundamentally structural elements in electrocatalysts, we demonstrated the feasibility of ISM by exploring the roles of atomic steps in the reactivity of electrocatalysts. In situ and atomic-scale evidence shows that low-coordinated atomic steps not only generate reactive species at low potentials and strengthen surface contraction but also act as templates to disturb interfacial water networks and thus affect the reactivity of electrocatalysts. This template role interprets the long-standing puzzle regarding why high-index facets are active for the oxygen reduction reaction in acidic media. The ISM as a fundamentally new method for workflows should aid the study of many other electrocatalysts regarding their nature of active sites and operative mechanisms.

Anisotropic etching of rhodium and gold as the onset of nanoparticle formation by cathodic corrosion

Cathodic corrosion is a phenomenon in which negatively polarized metal electrodes are degraded by cathodic etching and nanoparticle formation. Though these changes are dramatic and sometimes even visible by eye, the exact mechanisms underlying cathodic corrosion are still unclear. This work aims to improve the understanding of cathodic corrosion by studying its onset on rhodium and gold electrodes, which are subjected to various constant cathodic potentials in 10 M NaOH. After this polarization, the electrodes are studied using cyclic voltammetry and scanning electron microscopy, allowing a corrosion onset potential of -1.3 V vs. NHE for rhodium and -1.6 V vs. NHE for gold to be defined. The mildness of the potentials on both metals suggests that cathodic corrosion is less extreme and more ubiquitous than expected. Furthermore, we are able to observe well-defined rectangular etch pits on rhodium. Combined with rhodium cyclic voltammetry, this indicates a strong preference for forming (100) sites during corrosion. In contrast, a (111) preference is indicated on gold by voltammetry and the presence of well-oriented quasi-octahedral nanoparticles. This different etching behavior is suggested to be caused by preferential adsorption of sodium ions to surface defects, as is confirmed by density functional theory calculations.

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.

Electrochemical Stripping of Atomic Oxygen on Single-Crystalline Platinum: Bridging Gas-Phase and Electrochemical Oxidation

To understand the interaction between Pt and surface oxygenated species in electrocatalysis, this paper correlates the electrochemistry of atomic oxygen on Pt formed in the gas phase with electrochemically generated oxygen species on a variety of single-crystal platinum surfaces. The atomic oxygen adsorbed on single-crystalline Pt electrodes, made by thermal dissociation of molecular oxygen, is used for voltammetry measurements in acidic electrolytes (HClO₄ and H₂SO₄). The essential knowledge of coverage, binding energy, and surface construction of atomic oxygen is correlated with the charge, potential, and shape of voltammograms, respectively. The differences of the voltammograms between the oxide made by thermal dissociation of molecular oxygen and electrochemical oxidation imply that atomic oxygen is not an intermediate of the electrochemical oxidation of Pt(111). The reconstruction of (100) terrace and step and the low-potential stripping of atomic oxygen on (111) step site provide insight into the first stages of degradation of Pt-based electrocatalysts.

Decrease in the double layer capacitance by faradaic current

This study describes the reverse of the well-known double layer effects on charge transfer kinetics in the relationship between a cause and an effect.

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.

Anisotropic etching of platinum electrodes at the onset of cathodic corrosion

Cathodic corrosion is a process that etches metal electrodes under cathodic polarization. This process is presumed to occur through anionic metallic reaction intermediates, but the exact nature of these intermediates and the onset potential of their formation is unknown. Here we determine the onset potential of cathodic corrosion on platinum electrodes. Electrodes are characterized electrochemically before and after cathodic polarization in 10 M sodium hydroxide, revealing that changes in the electrode surface start at an electrode potential of −1.3 V versus the normal hydrogen electrode. The value of this onset potential rules out previous hypotheses regarding the nature of cathodic corrosion. Scanning electron microscopy shows the formation of well-defined etch pits with a specific orientation, which match the voltammetric data and indicate a remarkable anisotropy in the cathodic etching process, favouring the creation of (100) sites. Such anisotropy is hypothesized to be due to surface charge-induced adsorption of electrolyte cations.

The corrosion mechanism of metals at cathodic potentials is still poorly understood. Here the authors report the cathodic corrosion onset potential of platinum in concentrated sodium hydroxide, showing etching anisotropy, and present a framework to determine such characteristics for other metals/solutions.

Intermediate stages of electrochemical oxidation of single-crystalline platinum revealed by in situ Raman spectroscopy

Understanding the atomistic details of how platinum surfaces are oxidized under electrochemical conditions is of importance for many electrochemical devices such as fuel cells and electrolysers. Here we use in situ shell-isolated nanoparticle-enhanced Raman spectroscopy to identify the intermediate stages of the electrochemical oxidation of Pt(111) and Pt(100) single crystals in perchloric acid. Density functional theory calculations were carried out to assist in assigning the experimental Raman bands by simulating the vibrational frequencies of possible intermediates and products. The perchlorate anion is suggested to interact with hydroxyl phase formed on the surface. Peroxo-like and superoxo-like two-dimensional (2D) surface oxides and amorphous 3D α-PtO₂ are sequentially formed during the anodic polarization. Our measurements elucidate the process of the electrochemical oxidation of platinum single crystals by providing evidence for the structure-sensitive formation of a 2D platinum-(su)peroxide phase. These results may contribute towards a fundamental understanding of the mechanism of degradation of platinum electrocatalysts.

Understanding degradation of platinum catalysts during oxygen reduction is vital for improving proton-exchange membrane fuel cells. Here, the authors identify intermediate stages in the oxidation of Pt(111) and Pt(100) in perchloric acid using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy.

Platinum-based heterogeneous nanomaterials via wet-chemistry approaches toward electrocatalytic applications

The heterogeneously structured nanomaterials usually exhibit enhanced catalytic properties in comparison with each one of the constituent materials due to the synergistic effect among their different domains. Within the last decade, the development of wet-chemistry methods leads to the blossom of research in materials with heterogeneous nanostructures, which creates great opportunities also a tremendous challenge to apply these materials for highly efficient energy conversion. We herein would systematically introduce the latest research developments in Pt-based nanomaterials with heterogeneous structures, e.g. core-shell, hollow interiors, stellated/dendritic morphologies, dimeric, or composite construction, and their potential applications as electrocatalysts toward direct methanol fuel cell reactions, including methanol oxidation reaction and oxygen reduction reaction in acidic conditions, aiming at the summarization of the fundamentals and technical approaches in synthesis, fabrication and processing of heterogeneous nanomaterials so as to provide the readers a systematic and coherent picture of the filed. This review will focus on the intrinsic relationship between the catalytic properties and the physical or/and chemical effects in the heterogeneous nanomaterials, providing for technical bases for effectively developing novel electrocatalyts with low cost, enhanced activity and high selectivity.

Kinetically induced irreversibility in electro-oxidation and reduction of Pt surface

A mean field kinetic model was developed for electrochemical oxidations and reductions of Pt(111) on the basis of density functional theory calculations, and the reaction mechanisms were analyzed. The model reasonably describes asymmetric shapes of cyclic voltammograms and small Tafel slopes of relevant redox reactions observed in experiments without assuming any unphysical forms of rate equations. Simulations using the model indicate that the oxidation of Pt(111) proceeds via an electrochemical oxidation from Pt to PtOH and a disproportionation reaction from PtOH to PtO and Pt, while its reduction proceeds via two electrochemical reductions from PtO to PtOH and from PtOH to Pt.

Non-covalent interactions in water electrolysis: influence on the activity of Pt(111) and iridium oxide catalysts in acidic media

Electrolyte components, which are typically not considered to be directly involved in catalytic processes at solid–liquid electrified interfaces, often demonstrate a significant or even drastic influence on the activity, stability and selectivity of electrocatalysts.