The potential of zero total charge of Pt nanoparticles and polycrystalline electrodes with different surface structure: The role of anion adsorption in fundamental electrocatalysis

Scanning electrochemical probe microscopy: towards the characterization of micro- and nanostructured photocatalytic materials

Platinum-black (Pt-B) has been demonstrated to be an excellent electrocatalytic material for the electrochemical oxidation of hydrogen peroxide (H2O2). As Pt-B films can be deposited electrochemically, micro- and nano-sized conductive transducers can be modified with Pt-B. Here, we present the potential of Pt-B micro- and sub-micro-sized sensors for the detection and quantification of hydrogen (H2) in solution. Using these microsensors, no sampling step for H2 determination is required and e.g., in photocatalysis, the onset of H2 evolution can be monitored in situ. We present Pt-B-based H2 micro- and sub-micro-sized sensors based on different electrochemical transducers such as microelectrodes and atomic force microscopy (AFM)-scanning electrochemical microscopy (SECM) probes, which enable local measurements e.g., at heterogenized photocatalytically active samples. The microsensors are characterized in terms of limits of detection (LOD), which ranges from 4.0 μM to 30 μM depending on the size of the sensors and the experimental conditions such as type of electrolyte and pH. The sensors were tested for the in situ H2 evolution by light-driven water-splitting, i.e., using ascorbic acid or triethanolamine solutions, showing a wide linear concentration range, good reproducibility, and high sensitivity. Proof-of-principle experiments using Pt-B-modified cantilever-based sensors were performed using a model sample platinum substrate to map the electrochemical H2 evolution along with the topography using AFM-SECM.

Well-Defined PtCo@Pt Core-Shell Nanodendrite Electrocatalyst for Highly Durable Oxygen Reduction Reaction

The rational design of efficient electrocatalysts with controllable structure and composition is crucial for enhancing the lifetime and cost-effectiveness of oxygen reduction reaction (ORR). PtCo nanocrystals have gained attention due to their exceptional activity, yet suffer from stability issues in acidic media. Herein, an active and highly stable electrocatalyst is developed, namely 3D Pt7Co3@Pt core-shell nanodendrites (NDs), which are formed through the self-assembly of small Pt nanoparticles (≈6 nm). This unique structure significantly improves the ORR with an enhanced mass activity (MA) of 0.54 A mgPt -1, surpassing that of the commercial Pt/C (com-Pt/C) catalyst by three fold (0.17 A mgPt -1). The well-organized dendritic morphology, along with the Pt-rich shell, contributes significantly to the observed high catalytic activity and superior stability for acidic ORR, which exhibit a loss of 2.1% in MA and, impressively, an increase of 12.0% in specific activity (SA) after an accelerated durability test (ADT) of 40,000 potential-scanning cycles. This work offers insights for improving the design of highly stable Pt-based electrocatalysts for acidic ORR.

Rhenium-Based Electrocatalysts for Water Splitting

Due to the contamination and global warming problems, it is necessary to search for alternative environmentally friendly energy sources. In this area, hydrogen is a promising alternative. Hydrogen is even more promising, when it is obtained through water electrolysis operated with renewable energy sources. Among the possible devices to perform electrolysis, proton exchange membrane (PEM) electrolyzers appear as the most promising commercial systems for hydrogen production in the coming years. However, their massification is affected by the noble metals used as electrocatalysts in their electrodes, with high commercial value: Pt at the cathode where the hydrogen evolution reaction occurs (HER) and Ru/Ir at the anode where the oxygen evolution reaction (OER) happens. Therefore, to take full advantage of the PEM technology for green H2 production and build up a mature PEM market, it is imperative to search for more abundant, cheaper, and stable catalysts, reaching the highest possible activities at the lowest overpotential with the longest stability under the harsh acidic conditions of a PEM. In the search for new electrocatalysts and considering the predictions of a Trasatti volcano plot, rhenium appears to be a promising candidate for HER in acidic media. At the same time, recent studies provide evidence of its potential as an OER catalyst. However, some of these reports have focused on chemical and photochemical water splitting and have not always considered acidic media. This review summarizes rhenium-based electrocatalysts for water splitting under acidic conditions: i.e., potential candidates as cathode materials. In the various sections, we review the mechanism concepts of electrocatalysis, evaluation methods, and the different rhenium-based materials applied for the HER in acidic media. As rhenium is less common for the OER, we included a section about its use in chemical and photochemical water oxidation and as an electrocatalyst under basic conditions. Finally, concluding remarks and perspectives are given about rhenium for water splitting.

Imaging the strain evolution of a platinum nanoparticle under electrochemical control

Surface strain is widely employed in gas phase catalysis and electrocatalysis to control the binding energies of adsorbates on active sites. However, in situ or operando strain measurements are experimentally challenging, especially on nanomaterials. Here we exploit coherent diffraction at the new fourth-generation Extremely Brilliant Source of the European Synchrotron Radiation Facility to map and quantify strain within individual Pt catalyst nanoparticles under electrochemical control. Three-dimensional nanoresolution strain microscopy, together with density functional theory and atomistic simulations, show evidence of heterogeneous and potential-dependent strain distribution between highly coordinated ({100} and {111} facets) and undercoordinated atoms (edges and corners), as well as evidence of strain propagation from the surface to the bulk of the nanoparticle. These dynamic structural relationships directly inform the design of strain-engineered nanocatalysts for energy storage and conversion applications.

Ultrasmall, Coating-Free, Pyramidal Platinum Nanoparticles for High Stability Fuel Cell Oxygen Reduction

Ultrasmall (<5 nm diameter) noble metal nanoparticles with a high fraction of {111} surface domains are of fundamental and practical interest as electrocatalysts, especially in fuel cells; the nanomaterial surface structure dictates its catalytic properties, including kinetics and stability. However, the synthesis of size-controlled, pure Pt-shaped nanocatalysts has remained a formidable chemical challenge. There is an urgent need for an industrially scalable method for their production. Here, a one-step approach is presented for the preparation of single-crystal pyramidal nanocatalysts with a high fraction of {111} surface domains and a diameter below 4 nm. This is achieved by harnessing the shape-directing effect of citrate molecules, together with the strict control of oxidative etching while avoiding polymers, surfactants, and organic solvents. These catalysts exhibit significantly enhanced durability while, providing equivalent current and power densities to highly optimized commercial Pt/C catalysts at the beginning of life (BOL). This is even the case when they are tested in full polymer electrolyte membrane fuel cells (PEMFCs), as opposed to rotating disk experiments that artificially enhance electrode kinetics and minimize degradation. This demonstrates that the {111} surface domains in pyramidal Pt nanoparticles (as opposed to spherical Pt nanoparticles) can improve aggregation/corrosion resistance in realistic fuel cell conditions, leading to a significant improvement in membrane electrode assembly (MEA) stability and lifetime.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Unexpectedly High Capacitance of the Metal Nanoparticle/Water Interface: Molecular-Level Insights into the Electrical Double Layer

The electrical double-layer plays a key role in important interfacial electrochemical processes from catalysis to energy storage and corrosion. Therefore, understanding its structure is crucial for the progress of sustainable technologies. We extract new physico-chemical information on the capacitance and structure of the electrical double-layer of platinum and gold nanoparticles at the molecular level, employing single nanoparticle electrochemistry. The charge storage ability of the solid/liquid interface is larger by one order-of-magnitude than predicted by the traditional mean-field models of the double-layer such as the Gouy-Chapman-Stern model. Performing molecular dynamics simulations, we investigate the possible relationship between the measured high capacitance and adsorption strength of the water adlayer formed at the metal surface. These insights may launch the active tuning of solid-solvent and solvent-solvent interactions as an innovative design strategy to transform energy technologies towards superior performance and sustainability.

Electrolyte buffering species as oxygen donor shuttles in CO electrooxidation

Electrolyte buffering species have been shown to act as proton donors in the hydrogen evolution reaction (HER). Analogously, we study here whether these electrolyte species may participate in other reactions by investigating CO electrooxidation (COOR) on a gold rotating disk electrode. This model system, characterized by fast kinetics, exhibits a diffusion-limited regime, which helps in the identification of the species dictating the diffusion-limited current. Through a systematic concentration dependence study in a variety of buffers, we show that electrolyte buffering species act as oxygen donor shuttles in COOR, lowering the reaction overpotential. A similar correlation between electrolyte and electrocatalytic activity was observed for COOR on a different electrode material (Pt). Probing the electrode-electrolyte interface by attenuated total reflection infrared spectroscopy (ATR-FTIR) and modelling the surface speciation to include the effect of the solution reactions, we propose that the buffer conjugated base generates the oxygen donor (i.e. OH^-) through its acid-base reaction with water.

Understanding Cation Trends for Hydrogen Evolution on Platinum and Gold Electrodes in Alkaline Media

In this work, we study how the cation identity and concentration alter the kinetics of the hydrogen evolution reaction (HER) on platinum and gold electrodes. A previous work suggested an inverted activity trend as a function of alkali metal cation when comparing the performance of platinum and gold catalysts in alkaline media. We show that weakly hydrated cations (K⁺) favor HER on gold only at low overpotentials (or lower alkalinity), whereas in more alkaline pH (or high overpotentials), a higher activity is observed using electrolytes containing strongly hydrated cations (Li⁺). We find a similar trend for platinum; however, the inhibition of HER by weakly hydrated cations on platinum is observed already at lower alkalinity and lower cation concentrations, suggesting that platinum interacts more strongly with metal cations than gold. We propose that weakly hydrated cations stabilize the transition state of the water dissociation step more favorably due to their higher near-surface concentration in comparison to a strongly hydrated cation such as Li⁺. However, at high pH and consequently higher near-surface cation concentrations, the accumulation of these species at the outer Helmholtz plane inhibits HER. This is especially pronounced on platinum, where a change in the rate-determining step is observed at pH 13 when using a Li⁺- or K⁺-containing electrolyte.

In Situ Growth of Pt-Co Nanocrystals on Different Types of Carbon Supports and Their Electrochemical Performance toward Oxygen Reduction

Carbon-supported Pt-M (M = Co, Ni, and Fe) alloy nanocrystals are widely used as catalysts toward oxygen reduction, a reaction key to the operation of proton-exchange membrane fuel cells. Here we report a colloidal method for the in situ growth of Pt-Co nanocrystals on various commercial carbon supports. The use of different carbon supports resulted in not only variations in size and composition for the nanocrystals but also their catalytic activity and durability toward oxygen reduction in acidic media. Among the nanocrystals, those grown on Vulcan XC72 and Ketjenblack EC300J showed the highest specific and mass activities in the 0.1 M HClO4 and 0.05 M H2SO4 electrolytes, respectively. Additionally, the catalysts also showed different durability depending on the strength of the interaction between the nanocrystals and the carbon support. Our analysis demonstrated that the difference in catalytic performance could be ascribed to the distinct effects of carbon support on both the synthetic and catalytic processes.

Prudent Practices in ex situ Durability Analysis Using Cyclic Voltammetry for Platinum-based Electrocatalysts

Platinum (Pt)-based electrocatalysts are at the vanguard of research initiatives to meet activity and durability targets for promoting large-scale adoption of fuel cell vehicles. Ex situ characterization of electrocatalyst activity and durability using cyclic voltammetry (CV) has a steep learning curve. Thus, many researchers who do not receive formal training in electrochemistry are left unsure how to proceed. Herein, we identify and compile prudent practices for reliable assessment of ECSA values with examples from our research on nanoscale catalytic films formed by the self-terminating electrodeposition of Pt. Starting with a conceptual framework to understand typical features in the CV of reversible redox couples, we present prudent practices in acquiring CV data aimed at nonelectrochemists. We then highlight specific features related to ECSA computation from Pt CV. Finally, we suggest safeguards that help avoid missteps and achieve repeatable results while conducting ex situ durability tests that extend over days.

Improved Interface Charge Transfer and Redistribution in CuO-CoOOH p-n Heterojunction Nanoarray Electrocatalyst for Enhanced Oxygen Evolution Reaction

Electron density modulation is of great importance in an attempt to achieve highly active electrocatalysts for the oxygen evolution reaction (OER). Here, the successful construction of CuO@CoOOH p-n heterojunction (i.e., p-type CuO and n-type CoOOH) nanoarray electrocatalyst through an in situ anodic oxidation of CuO@CoSx on copper foam is reported. The p-n heterojunction can remarkably modify the electronic properties of the space-charge region and facilitate the electron transfer. Moreover, in situ Raman study reveals the generation of SO4 2- from CoSx oxidation, and electron cloud density distribution and density functional theory calculation suggest that surface-adsorbed SO4 2- can facilitate the OER process by enhancing the adsorption of OH- . The positively charged CoOOH in the space-charge region can significantly enhance the OER activity. As a result, the CuO@CoOOH p-n heterojunction shows significantly enhanced OER performance with a low overpotential of 186 mV to afford a current density of 10 mA cm-2 . The successful preparation of a large scale (14 × 25 cm2 ) sample demonstrates the possibility of promoting the catalyst to industrial-scale production. This study offers new insights into the design and fabrication of non-noble metal-based p-n heterojunction electrocatalysts as effective catalytic materials for energy storage and conversion.

Cation- and pH-Dependent Hydrogen Evolution and Oxidation Reaction Kinetics

The production of molecular hydrogen by catalyzing water splitting is central to achieving the decarbonization of sustainable fuels and chemical transformations. In this work, a series of structure-making/breaking cations in the electrolyte were investigated as spectator cations in hydrogen evolution and oxidation reactions (HER/HOR) in the pH range of 1 to 14, whose kinetics was found to be altered by up to 2 orders of magnitude by these cations. The exchange current density of HER/HOR was shown to increase with greater structure-making tendency of cations in the order of Cs⁺ < Rb⁺ < K⁺ < Na⁺ < Li⁺, which was accompanied by decreasing reorganization energy from the Marcus-Hush-Chidsey formalism and increasing reaction entropy. Invoking the Born model of reorganization energy and reaction entropy, the static dielectric constant of the electrolyte at the electrified interface was found to be significantly lower than that of bulk, decreasing with the structure-making tendency of cations at the negatively charged Pt surface. The physical origin of cation-dependent HER/HOR kinetics can be rationalized by an increase in concentration of cations on the negatively charged Pt surface, altering the interfacial water structure and the H-bonding network, which is supported by classical molecular dynamics simulation and surface-enhanced infrared absorption spectroscopy. This work highlights immense opportunities to control the reaction rates by tuning interfacial structures of cation and solvents.

Electrochemical Strain Dynamics in Noble Metal Nanocatalysts

The theoretical design of effective metal electrocatalysts for energy conversion and storage devices relies greatly on supposed unilateral effects of catalysts structure on electrocatalyzed reactions. Here, by using high-energy X-ray diffraction from the new Extremely Brilliant Source of the European Synchrotron Radiation Facility (ESRF-EBS) on device-relevant Pd and Pt nanocatalysts during cyclic voltammetry experiments in liquid electrolytes, we reveal the near ubiquitous feedback from various electrochemical processes on nanocatalyst strain. Beyond challenging and extending the current understanding of practical nanocatalysts behavior in electrochemical environment, the reported electrochemical strain provides experimental access to nanocatalysts absorption and adsorption trends (i.e., reactivity and stability descriptors) operando. The ease and power in monitoring such key catalyst properties at new and future beamlines is foreseen to provide a discovery platform toward the study of nanocatalysts encompassing a large variety of applications, from model environments to the device level.

How to appropriately assess the oxygen reduction reaction activity of platinum group metal catalysts with rotating disk electrode

The sluggish oxygen reduction reaction (ORR) has becoming the bottleneck of largescale implementation of proton exchange membrane fuel cells. However, when it comes to the ORR activity assessing of platinum group metals (PGMs) with rotating disk electrode, the corresponding potential conversion vs. reversible hydrogen electrode, test protocols, and activity calculation processes are still in chaos in many published literatures. In this work, two standard calculation processes for PGM ORR activities are demonstrated, followed by a specification for the usage of reference electrodes. Then a 4-fold discrepancy in ORR activities obtained via different test protocols is found for the same Pt/C, and an average adsorption model and the "coverage effects" are proposed to illustrate the hysteresis loop between negative and positive-going ORR polarization plots. Finally, four motions over appropriate assessment of PGM ORR activity are emphasized, hoping to bring a fair communication platform for researchers from different groups.

Linking Perfluorosulfonic Acid Ionomer Chemistry and High-Current Density Performance in Fuel-Cell Electrodes

Transport phenomena are key in controlling the performance of electrochemical energy-conversion technologies and can be highly complex, involving multiple length scales and materials/phases. Material designs optimized for one reactant species transport however may inhibit other transport processes. We explore such trade-offs in the context of polymer-electrolyte fuel-cell electrodes, where ionomer thin films provide the necessary proton conductivity but retard oxygen transport to the Pt reaction site and cause interfacial resistance due to sulfonate/Pt interactions. We examine the electrode overall gas-transport resistance and its components as a function of ionomer content and chemistry. Low-equivalent-weight ionomers allow better dissolved-gas and proton transport due to greater water uptake and low crystallinity but also cause significant interfacial resistance due to the high density of sulfonic acid groups. These effects of equivalent weight are also observed via in situ ionic conductivity and CO displacement measurements. Of critical importance, the results are supported by ex situ ellipsometry and X-ray scattering of model thin-film systems, thereby providing direct linkages and applicability of model studies to probe complex heterogeneous structures. Structural and resultant performance changes in the electrode are shown to occur above a threshold sulfonic-group loading, highlighting the significance of ink-based interactions. Our findings and methodologies are applicable to a variety of solid-state energy-conversion devices and material designs.

Atomic Regulation of PGM Electrocatalysts for the Oxygen Reduction Reaction

With the increasing enthusiasm for the hydrogen economy and zero-emission fuel cell technologies, intensive efforts have been dedicated to the development of high-performance electrocatalytic materials for the cathodic oxygen reduction reaction (ORR). Some major fundamental breakthroughs have been made in the past few years. Therefore, reviewing the most recent development of platinum-group-metal (PGM) ORR electrocatalysts is of great significance to pushing it forward. It is known that the ORR on the fuel cell electrode is a heterogeneous reaction occurring at the solid/liquid interface, wherein the electron reduces the oxygen along with species in the electrolyte. Therefore, the ORR kinetic is in close correlation with the electronic density of states and wave function, which are dominated by the localized atomic structure including the atomic distance and coordination number (CN). In this review, the recent development in the regulation over the localized state on the catalyst surface is narrowed down to the following structural factors whereby the corresponding strategies include: the crystallographic facet engineering, phase engineering, strain engineering, and defect engineering. Although these strategies show distinctive features, they are not entirely independent, because they all correlate with the atomic local structure. This review will be mainly divided into four parts with critical analyses and comparisons of breakthroughs. Meanwhile, each part is described with some more specific techniques as a methodological guideline. It is hoped that the review will enhance an insightful understanding on PGM catalysts of ORR with a visionary outlook.

The pure and representative types of disordered platinum nanoparticles from machine learning

The development of interpretable structure/property relationships is a cornerstone of nanoscience, but can be challenging when the structural diversity and complexity exceeds our ability to characterise it. This is often the case for imperfect, disordered and amorphous nanoparticles, where even the nomenclature can be unspecific. Disordered platinum nanoparticles have exhibited superior performance for some reactions, which makes a systematic way of describing them highly desirable. In this study we have used a diverse set of disorder platinum nanoparticles and machine learning to identify the pure and representative structures based on their similarity in 121 dimensions. We identify two prototypes that are representative of separable classes, and seven archetypes that are the pure structures on the convex hull with which all other possibilities can be described. Together these nine nanoparticles can explain all of the variance in the set, and can be described as either single crystal, twinned, spherical or branched; with or without roughened surfaces. This forms a robust sub-set of platinum nanoparticle upon which to base further work, and provides a theoretical basis for discussing structure/property relationships of platinum nanoparticles that are not geometrically ideal.

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.

Titrating Pt Surface with CO Molecules

Identification and quantification of the surface sites on Pt nanoparticles are essential for developing more active electrocatalysts for many practical devices such as fuel cells and electrochemical fuel generators. In this work, we studied CO adsorption from dissolved CO in an H₂SO₄ electrolyte solution on a polycrystalline Pt film electrode held at a constant potential in the underpotential hydrogen deposition region using in situ attenuated total reflectance-surface-enhanced IR absorption spectroscopy (ATR-SEIRAS). Slowing down the adsorption rate by limiting the CO addition rate to the solution allows the individual CO molecules arriving at the Pt surface to rearrange, move to, and occupy their most energetically favorable sites. By using ATR-SEIRAS spectroscopy to follow the stepwise CO adsorption process, one can identify and quantify the Pt surface sites along with uncovering the CO adsorption energetic sequence. This method of slow CO adsorption on the Pt surface is analogous to the chemical titrations used for quantitative chemical analyses.

Defect Dynamics at a Single Pt Nanoparticle during Catalytic Oxidation

Defects can affect all aspects of a material by altering its electronic properties and controlling its chemical reactivity. At defect sites, preferential adsorption of reactants and/or formation of chemical species at active sites are observed in heterogeneous catalysis. Understanding the structural response at defect sites during catalytic reactions provides a unique opportunity to exploit defect control of nanoparticle-based catalysts. However, it remains difficult to characterize the strain and defect evolution for a single nanocrystal catalyst in situ. Here, we report Bragg coherent X-ray diffraction imaging of defect dynamics in an individual Pt nanoparticle during catalytic methane oxidation. We observed that the initially tensile strained regions of the crystal became seed points for the development of further strain and subsequent disappearance of diffraction density during oxidation reactions. Our detailed understanding of the catalytically induced deformation at the defect sites and observed reversibility during the relevant steps of the catalytic oxidation process provide important insights of defect control and engineering of heterogeneous catalysts.

Recommended Practices and Benchmark Activity for Hydrogen and Oxygen Electrocatalysis in Water Splitting and Fuel Cells

Electrochemical energy storage by making H₂ an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state-of-the-art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic- and background-correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state-of-the-art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories.

Probing the Surface of Noble Metals Electrochemically by Underpotential Deposition of Transition Metals

The advances in material science have led to the development of novel and various materials as nanoparticles or thin films. Underpotential deposition (upd) of transition metals appears to be a very sensitive method for probing the surfaces of noble metals, which is a parameter that has an important effect on the activity in heterogeneous catalysis. Underpotential deposition as a surface characterization tool permits researchers to precisely determine the crystallographic orientations of nanoparticles or the real surface area of various surfaces. Among all the work dealing with upd, this review focuses specifically on the main upd systems used to probe surfaces of noble metals in electrocatalysis, from poly‒ and single-crystalline surfaces to nanoparticles. Cuupd is reported as a tool to determine the active surface area of gold‒ and platinum‒based bimetallic electrode materials. Pbupd is the most used system to assess the crystallographic orientations on nanoparticles’ surface. In the case of platinum, Bi and Ge adsorptions are singled out for probing (1 1 1) and (1 0 0) facets, respectively.

Determination of Specific Electrocatalytic Sites in the Oxidation of Small Molecules on Crystalline Metal Surfaces

The identification of active sites in electrocatalytic reactions is part of the elucidation of mechanisms of catalyzed reactions on solid surfaces. However, this is not an easy task, even for apparently simple reactions, as we sometimes think the oxidation of adsorbed CO is. For surfaces consisting of non-equivalent sites, the recognition of specific active sites must consider the influence that facets, as is the steps/defect on the surface of the catalyst, cause in its neighbors; one has to consider the electrochemical environment under which the "active sites" lie on the surface, meaning that defects/steps on the surface do not partake in chemistry by themselves. In this paper, we outline the recent efforts in understanding the close relationships between site-specific and the overall rate and/or selectivity of electrocatalytic reactions. We analyze hydrogen adsorption/desorption, and electro-oxidation of CO, methanol, and ammonia. The classical topic of asymmetric electrocatalysis on kinked surfaces is also addressed for glucose electro-oxidation. The article takes into account selected existing data combined with our original works.

Citrate-Coated, Size-Tunable Octahedral Platinum Nanocrystals: A Novel Route for Advanced Electrocatalysts

The development of green and scalable syntheses for the preparation of size- and shape-controlled metal nanocrystals is of high interest in many areas, including catalysis, electrocatalysis, nanomedicine, and electronics. In this work, a new synthetic approach based on the synergistic action of physical parameters and reagents produces size-tunable octahedral Pt nanocrystals, without the use of catalyst-poisoning reagents and/or difficult-to-remove coatings. The synthesis requires sodium citrate, ascorbic acid, and fine control of the reduction rate in aqueous environment. Pt octahedral nanocrystals with particle size as low as 7 nm and highly developed {111} facets have been achieved, as demonstrated by transmission electron microscopy, X-ray diffraction, and electrochemical methods. The absence of sticky molecules together with the high quality of the surface makes these nanocrystals ideal candidates in electrocatalysis. Notably, 7 nm bismuth-decorated octahedral nanocrystals exhibit superior performance for the electrooxidation of formic acid in terms of both specific and mass activities.

Correlating anisotropy and disorder with the surface structure of platinum nanoparticles

Due to the competition between numerous physicochemical variables during formation and processing, platinum nanocatalysts typically contain a mixture of shapes, distributions of sizes, and a considerable degree of surface imperfection. Structural imperfection and sample polydispersivity are inevitable at scale, but accepting bulk and surface diversity as legitimate design features provides new opportunities for nanoparticle design. In recent years disorder and anisotropy have been embraced as useful design parameters but predicting the impact of uncontrollable imperfection a priori is challenging. In the present work we have created an ensemble of uniquely imperfect nanoparticles extracted from classical molecular dynamics trajectories and applied statistical filters to restrict the ensemble in ways that reflect common industrial design principles. We find that targeting different sizes and size distributions may be an effective way of promoting or suppressing internal disorder or crystallinity (as required), but the degree of anisotropy of the particle as a whole has a greater impact on the population of different types of surface ordering and active sites. These results indicate that tuning of disordered and anisotropic Pt nanoparticles is possible, but it is not as straightforward as geometrically ideal nanoparticles with a high degree of crystallinity. It is unlikely that a synthesis strategy could eliminate this diversity entirely, or ensure this type of structural complexity does not develop post-synthesis under operational conditions, but it may be possible to bias the formation of specific bulk structures and the surface anisotropy.

Effects of heat treatment atmosphere on the structure and activity of Pt3Sn nanoparticle electrocatalysts: a characterisation case study

In this work, a variation in heat treatment atmosphere approach is taken to provide a series of related PtSn catalysts with the same nominal composition of Pt₃Sn, but with different surface compositions.

The influence of water content in a proton-conducting ionic liquid on the double layer properties of the Pt/PIL interface

The influence of the water content of 2-sulfoethylmethylammonium trifluoromethanesulfonate [2-Sema][TfO] on the double layer properties of the interface of platinum and the proton conducting ionic liquid (PIL) is investigated by means of impedance spectroscopy and cyclic voltammetry. By fitting the impedance spectra as complex capacitances, up to four differential double layer capacitances and corresponding time constants are obtained, depending on the potential (U = 0-1.6 V/RHE), water content (0.7-6.1 wt%) and temperature (T = 70-110 °C). Within the whole potential range investigated, a high frequency capacitance, C₁, and a low frequency capacitance, C₂, can be calculated. In the potential region of hydrogen underpotential deposition (H_UPD), C₁ can be separated into two parts, C_1a and C_1b. Whereas the high frequency capacitive processes can mainly be attributed to ion transport processes in the double layer, the low frequency process is ascribed to changes in the interfacial layer, including ad-/desorption and Faradaic processes. Alternative interpretations regarding the reorientation of ions, reconstruction of the metal surface and partial electron transfer between anions and Pt are considered.

Tunable Adsorption and Film Formation of Mussel Adhesive Protein by Potential Control

Mussel adhesive proteins are of great interest in many applications because of their outstanding adhesive property and film-forming ability. Understanding and controlling the film formation and its performance is crucial for the effective use of such proteins. In this study, we focus on the potential controlled film formation and compaction of one mussel adhesive protein, Mefp-1. The adsorption and film-forming behavior of Mefp-1 on a platinum (Pt) substrate under applied potentials were investigated by cyclic voltammetry, potential-controlled electrochemical impedance spectroscopy (EIS), and quartz crystal microbalance with dissipation monitoring (QCM-D). Moreover, microfriction measurements were performed to evaluate the mechanical properties of the Mefp-1 films formed at selected potentials. The results led to the conclusion that Mefp-1 adsorbs on the Pt substrate through both electrostatic and nonelectrostatic interactions and shows an effective blocking effect for the electroactive sites on the substrate. The properties of the adsorbed Mefp-1 film vary with the applied potential, and the compactness of the adsorbed Mefp-1 film can be reversibly tuned by the applied potential.