Atomic metal coordinated to nitrogen-doped carbon electrocatalysts for proton exchange membrane fuel cells: a perspective on progress, pitfalls and prospectives.

Proton exchange membrane fuel cells require reduced construction costs to improve commercial viability, which can be fueled by elimination of platinum as the O2 reduction electrocatalyst. The past 10 years has seen significant developments in synthesis, characterisation, and electrocatalytic performance of the most promising alternative electrocatalyst; single metal atoms coordinated to nitrogen-doped carbon (M-N-C). In this Perspective we recap some of the important achievements of M-N-Cs in the last decade, as well as discussing current knowledge gaps and future research directions for the community. We provide a new outlook on M-N-C stability and atomistic understanding with a set of original density functional theory simulations.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches.

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

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.

Decoupling the Contributions of Different Instability Mechanisms to the PEMFC Performance Decay of Non-noble Metal O2-Reduction Catalysts.

Non-noble metal catalysts (NNMCs) hold the potential to replace the expensive Pt-based materials currently used to speed up the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes, but they feature poor durability that inhibits their implementation in commercial PEMFCs. This performance decay is commonly ascribed to the operative demetallation of their ORR-active sites, the electro-oxidation of the carbonaceous matrix that hosts these active centers, and/or the chemical degradation of the ionomer, active sites, and/or carbon support by radicals derived from the H2O2 produced as an ORR by-product. However, little is known regarding the relative contributions of these mechanisms to the overall PEMFC performance loss. With this motivation, in this study, we combined four degradation protocols entailing different cathode gas feeds (i.e., air vs N2), potential hold values, and durations to decouple the relative impact of the above deactivation mechanisms to the overall performance decay. Our results indicate that H2O2-related instability does not depend on the operative voltage but only on the ORR charge. Moreover, the electro-oxidation of the carbon matrix at high potentials (which for the catalyst tested herein triggers at 0.7 V) seems to be more detrimental to the NNMCs' activity than the demetallation occurring at low potentials.

FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta-Coordinated Sites.

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O2 ) reduction at the cathode of proton exchange membrane fuel cells are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O2  reduction, their controlled synthesis and stability for practical applications remain challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilization remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, this issue is addressed by coordinating Fe in a highly porous nitrogen-doped carbon support (≈3295 m2  g-1 ), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine and a Mg2+ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54 × 1019  sites gFeNC -1  and a record 52% FeNx electrochemical utilization based on in situ nitrite stripping are achieved. The Fe single atoms are characterized pre- and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which are further studied by density functional theory calculations.

Dengue in hospitalized children with sickle cell disease: A retrospective cohort study in the French departments of America.

BACKGROUND: To describe the characteristics of dengue in sickle cell children and try to identify risk factors of severity. METHODS: In this retrospective study, we describe the evolution according to genotype (SS or SC and controls) and severity. RESULTS AND CONCLUSIONS: From 2005 to 2013, 106 hospitalizations for dengue fever were recorded, 35 SS genotype, 35 SC and 36 without SCD or any other chronic disease. The clinical evolution was quite different. During hospitalization, SC patients were more likely to develop multiorgan failure (31.4% versus 25.7% for SS, and 0% for controls, p=0.001), or acute pulmonary complications than patients without SC sickle cell disease (14.3% versus 8.6% for SS, and 0% for controls, p=0.03). Level 3 analgesic treatment was more frequent in SC patients (22.9% versus 3% for SS, and 0% for controls, p<0.001). Patients with SC sickle cell disease had a higher proportion of severe forms of dengue (57.1% versus 37.1% for SS, and 0% for controls, p<0.001) than patients without SC sickle cell disease. Transfer in intensive care unit was required for most SC patients (22.9% versus 3% for SS, and 0% for controls, p=0.005).Fatal episodes were more frequent in SC patients than in patients without SC sickle cell disease (5 deaths versus 1 for SS and 0 for controls, p=0.02). Thirty-three patients (47.1%) were diagnosed as having severe dengue (13 SS and 20 SC). On univariate analysis, age >10 years, acute pulmonary complications, multiorgan failure, severe anemia requiring transfusion, use of antibiotic treatment, need for treatment with morphine, and longer hospital stay were statistically more frequent in severe dengue-associated cases. Multiple logistic regression analysis showed that HbSC genotype and acute pulmonary complications, were significantly associated with severe dengue. In the multivariate model, the area of the ROC curve was 0.831. Children with SC genotype, typically thought to have less severe disease, actually had a higher rate of severe dengue and death than those with SS genotype.

On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts.

Fe-N-C catalysts containing atomic FeNx sites are promising candidates as precious-metal-free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The durability of Fe-N-C catalysts in fuel cells has been extensively studied using accelerated stress tests (AST). Herein we reveal stronger degradation of the Fe-N-C structure and four-times higher ORR activity loss when performing load cycling AST in O2 - vs. Ar-saturated pH 1 electrolyte. Raman spectroscopy results show carbon corrosion after AST in O2 , even when cycling at low potentials, while no corrosion occurred after any load cycling AST in Ar. The load-cycling AST in O2 leads to loss of a significant fraction of FeNx sites, as shown by energy dispersive X-ray spectroscopy analyses, and to the formation of Fe oxides. The results support that the unexpected carbon corrosion occurring at such low potential in the presence of O2 is due to reactive oxygen species produced between H2 O2 and Fe sites via Fenton reactions.

Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: Spectrometric Evidence for Pt-Catalysed Decarboxylation at Anode-Relevant Potentials.

The carbon oxidation reaction (COR) is a critical issue in proton-exchange membrane fuel cells (PEMFCs), as carbon in various forms is the most used electrocatalyst support material. The COR is thermodynamically possible above the C/CO2 standard potential, but its rate becomes significantly important only at high overpotential (e. g. PEMFC cathode potential). Herein, using on-line differential electrochemical mass spectrometry, we show that oxygen-containing carbon surface groups present on high-surface aera carbon, Vulcan XC72 or reinforced graphite are oxidized at PEMFC anode-relevant potential (E=0.1 V vs. the reversible hydrogen electrode, RHE), but not at E=0.4 V vs. RHE. We rationalized our findings by considering a Pt-catalysed decarboxylation mechanism in which Pt nanoparticles provide adsorbed hydrogen species to the oxygen-containing carbon surface groups, eventually leading to evolution of carbon dioxide and carbon monoxide. These results shed fundamental light on an unexpected degradation mechanism and facilitate the understanding of the long-term stability of PEMFC anode nanocatalysts.

Top-Down Synthesis of Nanostructured Platinum-Lanthanide Alloy Oxygen Reduction Reaction Catalysts: Pt xPr/C as an Example.

The oxygen reduction reaction (ORR) is of great interest for future sustainable energy conversion and storage, especially concerning fuel cell applications. The preparation of active, affordable, and scalable electrocatalysts and their application in fuel cell engines of hydrogen cars is a prominent step toward the reduction of air pollution, especially in urban areas. Alloying nanostructured Pt with lanthanides is a promising approach to enhance its catalytic ORR activity, whereby the development of a simple synthetic route turned out to be a nontrivial endeavor. Herein, for the first time, we present a successful single-step, scalable top-down synthetic route for Pt-lanthanide alloy nanoparticles, as witnessed by the example of Pr-alloyed Pt nanoparticles. The catalyst was characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and photoelectron spectroscopy, and its electrocatalytic oxygen reduction activity was investigated using a rotating disk electrode technique. Pt xPr/C showed ∼3.5 times higher [1.96 mA/cm2Pt, 0.9 V vs reversible hydrogen electrode (RHE)] specific activity and ∼1.7 times higher (0.7 A/mgPt, 0.9 V vs RHE) mass activity compared to commercial Pt/C catalysts. On the basis of previous findings and characterization of the Pt xPr/C catalyst, the activity improvement over commercial Pt/C originates from a lattice strain introduced by the alloying process.

Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis.

Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.

A Review on Recent Developments and Prospects for the Oxygen Reduction Reaction on Hollow Pt-alloy Nanoparticles.

Due to their interesting electrocatalytic properties for the oxygen reduction reaction (ORR), hollow Pt-alloy nanoparticles (NPs) supported on high-surface-area carbon attract growing interest. However, the suitable synthesis methods and associated mechanisms of formation, the reasons for their enhanced specific activity for the ORR, and the nature of adequate alloying elements and carbon supports for this type of nanocatalysts remain open questions. This Review aims at shedding light on these topics with a special emphasis on hollow PtNi NPs supported onto Vulcan C (PtNi/C). We first show how hollow Pt-alloy/C NPs can be synthesized by a mechanism involving galvanic replacement and the nanoscale Kirkendall effect. Nickel, cobalt, copper, zinc, and iron (Ni, Co, Cu, Zn, and Fe, respectively) were tested for the formation of Pt-alloy/C hollow nanostructures. Our results indicate that metals with standard potential -0.4

Elucidating the Mechanisms Driving the Aging of Porous Hollow PtNi/C Nanoparticles by Means of COads Stripping.

The oxygen reduction reaction (ORR) activity of Pt-alloy electrocatalysts depends on (i) the strain/ligand effects induced by the non-noble metal (3d-transition metal or a rare-earth element) alloyed to Pt, (ii) the orientation of the catalytic surfaces, and (iii) the density of structural defects (SDs) (e.g., vacancies, voids, interconnections). These SDs influence the "generalized" coordination number of Pt atoms, the Pt-alloy lattice parameter, and thus the adsorption strength of the ORR intermediates (O*, OH*, OOH*). Here, we discuss a set of parameters derived from COads stripping measurements and the Rietveld refinement of X-ray diffraction (XRD) patterns, aiming to show how the leaching of the non-noble metal and the density of SDs influence the ORR activity of porous hollow PtNi/C nanoparticles (PH-PtNi/C NPs). PH-PtNi/C NPs were aged at T = 353 K in an Ar-saturated 0.1 M HClO4 electrolyte during 20 000 potential cycles between E = 0.6 and 1.0 V versus the reversible hydrogen electrode, with an intermediate characterization after 5000 cycles. The losses in the ORR specific activity were attributed to the dissolution of Ni atoms (modifying strain/ligand effects) and to the increase of the crystallite size (dXRD), resulting in a diminution of the density of grain boundaries. In agreement with the Gibbs-Thompson equation, the electrocatalysts that presented larger crystallites (dXRD > 3 nm) were far more stable than the ones with the smallest crystallites (dXRD < 2 nm). We also observed that performing intermediate characterizations (in an O2-saturated electrolyte) results in activity losses for the ORR.

Atomic-Scale Snapshots of the Formation and Growth of Hollow PtNi/C Nanocatalysts.

Determining the formation and growth mechanism of bimetallic nanoparticles (NPs) with atomic detail is fundamental to synthesize efficient "catalysts by design". However, an understanding of the elementary steps which take place during their synthesis remains elusive. Herein, we have exploited scanning transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy, operando wide angle and small-angle X-ray scattering, and electrochemistry to unveil the formation and growth mechanism of hollow PtNi/C NPs. Such NPs, composed of a PtNi shell surrounding a nanoscale void, catalyze efficiently and sustainably the oxygen reduction reaction (ORR) in an acidic electrolyte. Our step-by-step study reveals that (i) Ni-rich/C NPs form first, before being embedded in a NixByOz shell, (ii) the combined action of galvanic displacement and the nanoscale Kirkendall effect then results in the sequential formation of Ni-rich core@Pt-rich/C shell and ultimately hollow PtNi/C NPs. The electrocatalytic properties for the ORR and the stability of the different synthesis intermediates were tested and structure-activity-stability relationships established both in acidic and alkaline electrolytes. Beyond its interest for the ORR electrocatalysis, this study also presents a methodology that is capable to unravel the formation and growth mechanism of various nanomaterials including preferentially shaped metal NPs, core@shell NPs, onion-like NPs, Janus NPs, or a combination of several of these structures.

Reversibility of Pt-Skin and Pt-Skeleton Nanostructures in Acidic Media.

Following a well-defined series of acid and heat treatments on a benchmark Pt3Co/C sample, three different nanostructures of interest for the electrocatalysis of the oxygen reduction reaction were tailored. These nanostructures could be sorted into the "Pt-skin" structure, made of one pure Pt overlayer, and the "Pt-skeleton" structure, made of 2-3 Pt overlayers surrounding the Pt-Co alloy core. Using a unique combination of high-resolution aberration-corrected STEM-EELS, XRD, EXAFS, and XANES measurements, we provide atomically resolved pictures of these different nanostructures, including measurement of the Pt-shell thickness forming in acidic media and the resulting changes of the bulk and core chemical composition. It is shown that the Pt-skin is reverted toward the Pt-skeleton upon contact with acid electrolyte. This change in structure causes strong variations of the chemical composition.

Relationships between systemic vascular resistance, blood rheology and nitric oxide in children with sickle cell anemia or sickle cell-hemoglobin C disease.

Vascular function has been found to be impaired in patients with sickle cell disease (SCD). The present study investigated the determinants of systemic vascular resistance in two main SCD syndromes in children: sickle cell anemia (SCA) and sickle cell-hemoglobin C disease (SCC). Nitric oxide metabolites (NOx), hematological, hemorheological, and hemodynamical parameters were investigated in 61 children with SCA and 49 children with SCC. While mean arterial pressure was not different between SCA and SCC children, systemic vascular resistance (SVR) was greater in SCC children. Although SVR and blood viscosity (ηb) were not correlated in SCC children, the increase of ηb (+18%) in SCC children compared to SCA children results in a greater mean SVR in this former group. SVR was positively correlated with ηb, hemoglobin (Hb) level and RBC deformability, and negatively with NOx level in SCA children. Multivariate linear regression model showed that both NOx and Hb levels were independently associated with SVR in SCA children. In SCC children, only NOx level was associated with SVR. In conclusion, vascular function of SCC children seems to better cope with higher ηb compared to SCA children. Since the occurrence of vaso-occlusive like complications are less frequent in SCC than in SCA children, this finding suggests a pathophysiological link between the vascular function alteration and these clinical manifestations. In addition, our results suggested that nitric oxide metabolism plays a key role in the regulation of SVR, both in SCA and SCC.

Hematological and hemorheological determinants of the six-minute walk test performance in children with sickle cell anemia.

The six-minute walk test is a well-established submaximal exercise reflecting the functional status and the clinical severity of sickle cell patients. The aim of the present cross-sectional study was to investigate the biological determinants of the six-minute walk test performance in children with sickle cell anemia. Hematological and hemorheological parameters, pulmonary function and the six-minute walk test performance were determined in 42 children with sickle cell anemia at steady state. The performance during the six-minute walk test was normalized for age, sex and height and expressed as percentage of the predicted six-minute walk distance. We showed that a high level of anemia, a low fetal hemoglobin expression and low red blood cell deformability were independent predictors of a low six-minute walk test performance. This study describes for the first time the impact of blood rheology in the six-minute walk test performance in children with sickle cell anemia.

Hematologic and hemorheological determinants of resting and exercise-induced hemoglobin oxygen desaturation in children with sickle cell disease.

The aim of the study was to determine the factors associated with resting and exercise-induced hemoglobin oxygen desaturation. The well-established six-minute walk test was conducted in 107 sickle cell children (50 with sickle hemoglobin C disease and 57 with sickle cell anemia) at steady state. Hemoglobin oxygen saturation was measured before and immediately after the six-minute walk test. Blood samples were obtained on the same day to measure hematologic and hemorheological parameters. Exercise-induced hemoglobin oxygen desaturation was defined as a drop in hemoglobin oxygen saturation of 3% or more at the end of the six-minute walk test compared to resting levels. No children with sickle hemoglobin C disease, but approximately 50% of children with sickle cell anemia showed mild or moderate oxygen desaturation at rest, which was independently associated with the percentage of reticulocytes. Exercise-induced hemoglobin oxygen desaturation was observed in 18% of children with sickle hemoglobin C disease and 34% of children with sickle cell anemia, and was independently associated with the six-minute walk test, acute chest syndrome rate and the strength of red blood cell aggregates in children with sickle cell anemia. No association was found in children with sickle hemoglobin C disease between exercise-induced hemoglobin oxygen desaturation and the measured parameters. Hemoglobin oxygen desaturation at rest was common in children with sickle cell anemia but not in children with sickle hemoglobin C disease, and was mainly associated with greater hemolysis. Physiological strain during exercise and red blood cell aggregation properties may predict the occurrence of exercise-induced hemoglobin oxygen desaturation in children with sickle cell anemia.

Impact of metal cations on the electrocatalytic properties of Pt/C nanoparticles at multiple phase interfaces.

Proton-exchange membrane fuel cells (PEMFCs) use carbon-supported nanoparticles based on platinum and its alloys to accelerate the rate of the sluggish oxygen-reduction reaction (ORR). The most common metals alloyed to Pt include Co, Ni and Cu, and are thermodynamically unstable in the PEMFC environment. Their dissolution yields the formation and redistribution of metal cations (M(y+)) within the membrane electrode assembly (MEA). Metal cations can also contaminate the MEA when metallic bipolar plates are used as current collectors. In each case, the electrical performance of the PEMFC severely decreases, an effect that is commonly attributed to the poisoning of the sulfonic acid groups of the perfluorosulfonated membrane (PEM) and the resulting decrease of the proton transport properties. However, the impact of metal cations on the kinetics of electrochemical reactions involving adsorption/desorption and bond-breaking processes remains poorly understood. In this paper, we use model electrodes to highlight the effect of metal cations on Pt/C nanoparticles coated or not with a perfluorosulfonated ionomer for the CO electrooxidation reaction and the oxygen reduction reaction. We show that metal cations negatively impact the ORR kinetics and the mass-transport resistance of molecular oxygen. However, the specific adsorption of sulfonate groups of the Nafion® ionomer locally modifies the double layer structure and increases the tolerance to metal cations, even in the presence of sulphate ions in the electrolyte. The survey is extended by using an ultramicroelectrode with cavity and a solid state cell (SSC) specifically developed for this study.

Hemorheological risk factors of acute chest syndrome and painful vaso-occlusive crisis in children with sickle cell disease.

BACKGROUND: Little is known about the effects of blood rheology on the occurrence of acute chest syndrome and painful vaso-occlusive crises in children with sickle cell anemia and hemoglobin SC disease. DESIGN AND METHODS: To address this issue, steady-state hemorheological profiles (blood viscosity, red blood cell deformability, aggregation properties) and hematologic parameters were assessed in 44 children with sickle cell anemia and 49 children with hemoglobin SC disease (8-16 years old) followed since birth. Clinical charts were retrospectively reviewed to determine prior acute chest syndrome or vaso-occlusive episodes, and rates of these complications were calculated. RESULTS: Multivariate analysis revealed that: 1) a higher steady-state blood viscosity was associated with a higher rate of vaso-occlusive crises in children with sickle cell anemia, but not in children with hemoglobin SC disease; 2) a higher steady-state red blood cell disaggregation threshold was associated with previous history of acute chest syndrome in children with hemoglobin SC disease and boys with sickle cell anemia. CONCLUSIONS: Our results indicate for the first time that the red blood cell aggregation properties may play a role in the pathophysiology of acute chest syndrome in children with hemoglobin SC disease and boys with sickle cell anemia. In addition, whereas greater blood viscosity is associated with a higher rate of vaso-occlusive crises in children with sickle cell anemia, no association was found in children with hemoglobin SC disease, underscoring differences in the etiology of vaso-occlusive crises between sickle cell anemia and hemoglobin SC disease.

The role of the support in CO(ads) monolayer electrooxidation on Pt nanoparticles: Pt/WO(x)vs. Pt/C.

The electrocatalytic properties of home-made Pt nanoparticles supported onto WO(x) were determined for the electrooxidation of a CO(ads) monolayer and compared with that of a commercial Pt/C having the same Pt particle size. By combining electrochemical and spectroscopic techniques, we found that Pt/WO(x) nanoparticles exhibit a very high tolerance to CO at low electrode potentials (E = 0.1 V vs. RHE), which was never reported in the literature before. CO adsorption at E = 0.1 V vs. RHE on Pt/WO(x) yields CO(2) production as observed by Fourier-transform infrared spectroscopy (FTIR). When the gas bubbling in solution changes from CO to Ar, the current attenuates and the CO(2) production vanishes. This points towards a limited number of "active sites" and a slow step in the electrocatalytic process. When H(2) is used to purge the electrolyte from CO, a steep and continuous increase of the H(2) electrooxidation current is observed pointing towards continuous liberation of the Pt catalytic sites. The high tolerance to CO of Pt/WO(x) is discussed in terms of strong metal-support interaction (SMSI), which involves formation of a metal-oxide film partially covering the Pt nanoparticles (encapsulation) and creation of W-OH groups upon H(+) insertion at low electrode potentials.