In-Situ Electrolyte for Electrosynthesis: Scalable Anodically-Enabled One-Pot Sequence from Aldehyde to Isoxazol(in)es.

Electrochemical transformations are considered a green alternative to classical redox chemistry as it eliminates the necessity for toxic and waste producing redox reagents. Typical electrochemical reactions require the addition of a so-called supporting electrolyte - a salt bridge - and other additives, such as hexafluorisopropanol, to enhance conductivity and reaction outcomes, respectively. However, this is often accompanied by an increase in the amount of produced waste. Here, we report an "in-situ electrolyte" concept for facile, transition-metal-free, additive-free one-pot electrochemical preparation of isoxazol(in)es, important scaffolds for biologically active natural and synthetic molecules, from the respective aldehydes. The protocol utilizes no halogenated solvents and no external oxidants, while salt side-products provide the ionic conductivity necessary for the electrosynthesis. The electrolysis is performed in an undivided cell, using the state-of-the-art electrodes for the chlor-alkali industry dimensionally stable and scalable mixed metal oxide anode and platinized titanium anode of high durability. The cascade transformation comprises the condensation of aldehyde to oxime followed by its anodic oxidation and subsequent intra- and/or intermolecular [3+2] cycloadditions with an appropriate dipolarophile. Chemical yields up to 97%, and good Faradaic efficiency, scalability, and stability are observed for most substrates in a broad scope.

Oxygen Reduction Reaction in Alkaline Media Causes Iron Leaching from Fe-N-C Electrocatalysts.

The electrochemical activity of modern Fe-N-C electrocatalysts in alkaline media is on par with that of platinum. For successful application in fuel cells (FCs), however, also high durability and longevity must be demonstrated. Currently, a limited understanding of degradation pathways, especially under operando conditions, hinders the design and synthesis of simultaneously active and stable Fe-N-C electrocatalysts. In this work, using a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry setup, Fe dissolution is studied under conditions close to those in FCs, that is, with a porous catalyst layer (CL) and at current densities up to -125 mA·cm-2. Varying the rate of the oxygen reduction reaction (ORR), we show a remarkable linear correlation between the Faradaic charge passed through the electrode and the amount of Fe dissolved from the electrode. This finding is rationalized assuming that oxygen reduction and Fe dissolution reactions are interlinked, likely through a common intermediate formed during the Fe redox transitions in Fe species involved in the ORR, such as FeNxCy and Fe3C@N-C. Moreover, such a linear correlation allows the application of a simple metric─S-number─to report the material's stability. Hence, in the current work, a powerful tool for a more applied stability screening of different electrocatalysts is introduced, which allows on the one hand fast performance investigations under more realistic conditions, and on the other hand a more advanced mechanistic understanding of Fe-N-C degradation in CLs.

Engineering stable electrocatalysts by synergistic stabilization between carbide cores and Pt shells.

Core-shell particles with earth-abundant cores represent an effective design strategy for improving the performance of noble metal catalysts, while simultaneously reducing the content of expensive noble metals1-4. However, the structural and catalytic stabilities of these materials often suffer during the harsh conditions encountered in important reactions, such as the oxygen reduction reaction (ORR)3-5. Here, we demonstrate that atomically thin Pt shells stabilize titanium tungsten carbide cores, even at highly oxidizing potentials. In situ, time-resolved experiments showed how the Pt coating protects the normally labile core against oxidation and dissolution, and detailed microscopy studies revealed the dynamics of partially and fully coated core-shell nanoparticles during potential cycling. Particles with complete Pt coverage precisely maintained their core-shell structure and atomic composition during accelerated electrochemical ageing studies consisting of over 10,000 potential cycles. The exceptional durability of fully coated materials highlights the potential of core-shell architectures using earth-abundant transition metal carbide (TMC) and nitride (TMN) cores for future catalytic applications.

Implementation of an enclosed ionization interface for the analysis of liquid sample streams with direct analysis in real time mass spectrometry.

RATIONALE: The development of an interface to analyze liquid sample streams with direct analysis in real time mass spectrometry (DART-MS) is of great interest for coupling various analytical techniques, using non-volatile salts, with MS. Therefore, we devised an enclosed ionization interface and a sample introduction system for the versatile analysis of liquid samples with DART-MS. METHODS: The sample introduction system consists of a nebulizer, a spray chamber and a transfer line, while the confined ionization interface is created by implementing a cross-shaped housing between ion source outlet and mass spectrometer inlet. Methodical studies of the effects of various setup parameters on signal intensity and peak shape were conducted, while its diverse applicability was demonstrated by coupling with high-performance liquid chromatography (HPLC) for the analysis of alcohols, organic acids and furanic compounds. RESULTS: The confinement of the ionization interface results in a robust setup design with a well-defined ionization region for focusing of the sprayed sample mist. Thereby, an increase in analyte signal intensity by three orders of magnitude and improved signal stability and reproducibility were obtained in comparison with a similar open ionization interface configuration. Additionally, the successful quantification of alcohols could be demonstrated as well as the compatibility of the setup with HPLC gradient elution. CONCLUSIONS: A versatile setup design for the analysis of liquid sample streams with DART-MS was devised for monitoring reactions or hyphenating analytics with MS. The design minimizes interferences from the laboratory surroundings as well as allows for safe handling of hazardous and toxic chemicals, which renders it suitable for a broad range of applications.

Platinum Dissolution in Realistic Fuel Cell Catalyst Layers.

Pt dissolution has already been intensively studied in aqueous model systems and many mechanistic insights have been gained. Nevertheless, transfer of new knowledge to real-world fuel cell systems is still a significant challenge. To close this gap, we present a novel in situ method combining a gas diffusion electrode (GDE) half-cell with inductively coupled plasma mass spectrometry (ICP-MS). With this setup, Pt dissolution in realistic catalyst layers and the transport of dissolved Pt species through Nafion membranes were evaluated directly. We observed that 1) specific Pt dissolution increased significantly with decreasing Pt loading, 2) in comparison to experiments on aqueous model systems with flow cells, the measured dissolution in GDE experiments was considerably lower, and 3) by adding a membrane onto the catalyst layer, Pt dissolution was reduced even further. All these phenomena are attributed to the varying mass transport conditions of dissolved Pt species, influencing re-deposition and equilibrium potential.

Atomistic Insights into the Stability of Pt Single-Atom Electrocatalysts.

Single-atom catalysts (SACs) have quickly emerged as a new class of catalytic materials. When confronted with classical carbon-supported nanoparticulated catalysts (Pt/C), SACs are often claimed to have superior electrocatalytic properties, e.g., stability. In this study, we critically assess this statement by investigating S-doped carbon-supported Pt SACs as a representative example of noble-metal-based SACs. We use a set of complementary techniques, which includes online inductively coupled plasma mass spectrometry (online ICP-MS), identical location transmission electron microscopy (IL-TEM), and X-ray photoelectron spectroscopy (XPS). It is shown by online ICP-MS that the dissolution behavior of as-synthesized Pt SACs is significantly different from that of metallic Pt/C. Moreover, Pt SACs are, indeed, confirmed to be more stable toward Pt dissolution. When cycled to potentials of up to 1.5 VRHE, however, the dissolution profiles of Pt SACs and Pt/C become similar. IL-TEM and XPS show that this transition is due to morphological and chemical changes caused by cycling. The latter, in turn, is a consequence of the relatively poor stability of S ligands. As monitored by online ICP-MS and XPS, significant amounts of sulfur leave the catalyst during oxidation. Hence, in case catalysts with improved stability in the anodic potential region are desired, more robust supports and ligands must be developed.

Fabrication of a Robust PEM Water Electrolyzer Based on Non-Noble Metal Cathode Catalyst: [Mo3 S13 ]2- Clusters Anchored to N-Doped Carbon Nanotubes.

High investment costs and a dependence on noble metal catalysts currently obstruct the large-scale implementation of proton exchange membrane water electrolyzers (PEMWEs) for converting fluctuating green electricity into chemical energy via water splitting. In this context, this work presents a high-performing and stable non-noble metal catalyst for the hydrogen evolution reaction (HER), consisting of [Mo3 S13 ]2- clusters supported on nitrogen doped carbon nanotubes (NCNTs). Strikingly, a significant electrochemically induced activation of the Mo3 S13 -NCNT catalyst at high current densities is observed in full cell configuration, enabling a remarkable current density of 4 A cm-2 at a cell voltage of 2.36 V. To the authors' knowledge, this is the highest reported value to date for a PEMWE full cell using a non-noble metal HER catalyst. Furthermore, only a minor degradation of 83 µV h-1 is observed during a stability test of 100 h constant current at 1 A cm-2 , with a nearly unchanged polarization behavior after the current hold. Catalyst stability and activity are additionally analyzed via online dissolution measurements. X-ray photoelectron spectroscopy examination of the catalyst before and after electrochemical application reveals a correlation between the electrochemical activation occurring via electrodissolution with changes in the molecular structure of the Mo3 S13 -NCNT catalyst.

Anisotropy of Pt nanoparticles on carbon- and oxide-support and their structural response to electrochemical oxidation probed by in situ techniques.

Identifying the structural response of nanoparticle-support ensembles to the reaction conditions is essential to determine their structure in the catalytically active state as well as to unravel the possible degradation pathways. In this work, we investigate the (electronic) structure of carbon- and oxide-supported Pt nanoparticles during electrochemical oxidation by in situ X-ray diffraction, absorption spectroscopy as well as the Pt dissolution rate by in situ mass spectrometry. We prepared ellipsoidal Pt nanoparticles by impregnation of the carbon and titanium-based oxide support as well as spherical Pt nanoparticles on an indium-based oxide support by a surfactant-assisted synthesis route. During electrochemical oxidation, we show that the oxide-supported Pt nanoparticles resist (bulk) oxide formation and Pt dissolution. The lattice of smaller Pt nanoparticles exhibits a size-induced lattice contraction in the as-prepared state with respect to bulk Pt but it expands reversibly during electrochemical oxidation. This expansion is suppressed for the Pt nanoparticles with a bulk-like relaxed lattice. We could correlate the formation of d-band vacancies in the metallic Pt with Pt lattice expansion. PtOx formation is strongest for platelet-like nanoparticles and we explain this with a higher fraction of exposed Pt(100) facets. Of all investigated nanoparticle-support ensembles, the structural response of RuO2/TiO2-supported Pt nanoparticles is the most promising with respect to their morphological and structural integrity under electrochemical reaction conditions.

Electrifying model catalysts for understanding electrocatalytic reactions in liquid electrolytes.

Electrocatalysis is at the heart of our future transition to a renewable energy system. Most energy storage and conversion technologies for renewables rely on electrocatalytic processes and, with increasing availability of cheap electrical energy from renewables, chemical production will witness electrification in the near future1-3. However, our fundamental understanding of electrocatalysis lags behind the field of classical heterogeneous catalysis that has been the dominating chemical technology for a long time. Here, we describe a new strategy to advance fundamental studies on electrocatalytic materials. We propose to 'electrify' complex oxide-based model catalysts made by surface science methods to explore electrocatalytic reactions in liquid electrolytes. We demonstrate the feasibility of this concept by transferring an atomically defined platinum/cobalt oxide model catalyst into the electrochemical environment while preserving its atomic surface structure. Using this approach, we explore particle size effects and identify hitherto unknown metal-support interactions that stabilize oxidized platinum at the nanoparticle interface. The metal-support interactions open a new synergistic reaction pathway that involves both metallic and oxidized platinum. Our results illustrate the potential of the concept, which makes available a systematic approach to build atomically defined model electrodes for fundamental electrocatalytic studies.

Dissolution of Platinum Single Crystals in Acidic Medium.

Platinum single crystal basal planes consisting of Pt(111), Pt(100), Pt(110) and reference polycrystalline platinum Pt(poly) were subjected to various potentiodynamic and potentiostatic electrochemical treatments in 0.1 M HClO4 . Using the scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC-ICP-MS) the transient dissolution was detected on-line. Clear trends in dissolution onset potentials and quantities emerged which can be related to the differences in the crystal plane surface structure energies and coordination. Pt(111) is observed to have a higher dissolution onset potential while the generalized trend in dissolution rates and quantities was found to be Pt(110)>P(100)≈Pt(poly)>Pt(111).

Electrochemical On-line ICP-MS in Electrocatalysis Research.

Electrocatalyst degradation due to dissolution is one of the major challenges in electrochemical energy conversion technologies such as fuel cells and electrolysers. While tendencies towards dissolution can be grasped considering available thermodynamic data, the kinetics of material's stability in real conditions is still difficult to predict and have to be measured experimentally, ideally in-situ and/or on-line. On-line inductively coupled plasma mass spectrometry (ICP-MS) is a technique developed recently to address exactly this issue. It allows time- and potential-resolved analysis of dissolution products in the electrolyte during the reaction under dynamic conditions. In this work, applications of on-line ICP-MS techniques in studies embracing dissolution of catalysts for oxygen reduction (ORR) and evolution (OER) as well as hydrogen oxidation (HOR) and evolution (HER) reactions are reviewed.

Alkaline manganese electrochemistry studied by in situ and operando spectroscopic methods - metal dissolution, oxide formation and oxygen evolution.

Manganese-based systems are considered as candidate electrocatalysts for the electrochemical oxygen evolution reaction (OER), because of their abundance in biochemical oxygen producing catalyst systems. In this work, the surface of metallic manganese was investigated in situ and operando in potentiodynamic cyclic voltammetry (CV) experiments and potentiostatic chronoamperometry (CA) experiments in NaOH. In both cases, the surfaces were initially reduced. At corresponding potentials, no oxide species can be detected by Raman spectroscopy, though electrochemical data and the absence of dissolution above the reversible potential for reactions of type Mn → MnII indicate that the material is passive. The CV shows anodic peaks at potentials in line with expectations on the basis of thermodynamic data for the oxidation to Mn3O4 and Mn2O3; the thickness of the surface layer increases by a few nm during these peaks, as evidenced by spectroscopic ellipsometry. Dissolution of Mn as evidenced by downstream electrolyte analysis by inductively coupled plasma mass spectrometry in a scanning flow cell (SFC-ICP-MS) of the electrolyte is negligible in the range of electrode potential vs. Ag|AgCl|3 M KCl, EAg|AgCl, up to 0.3 V. Remarkably, Raman spectra already show the occurrence of α-MnO2 at EAg|AgCl > -0.25 V, which is ca. 0.5 V below the potential at which oxidation to MnO2 is expected. This observation is attributed to disproportionation above a certain level of MnIII. For EAg|AgCl > 0.4 V, dissolution sets in, at a constant layer thickness. Above the onset potential of the OER, at EAg|AgCl≈ 0.6 V, SFC-ICP-MS analysis shows fast dissolution, and the oxide layer thickness is constant or increases. CA experiments during the OER show strong dissolution, and the re-formation of a strongly disordered, ß-MnO2-like oxide, which exists in a quasi-stationary state at the interface. Several CV cycles increase the dissolution per cycle and the fraction of α-MnO2 on the surface which cannot be reduced. The high dissolution currents show that metallic Mn is hardly suitable as an OER catalyst, however, at least the MnIV oxides remain stationarily present in the system.

Electrochemical Real-Time Mass Spectrometry (EC-RTMS): Monitoring Electrochemical Reaction Products in Real Time.

Methods that provide real-time information are essential to resolve transients occurring at dynamic interfaces. Now a powerful method is presented that enables the time- and potential-resolved characterization of liquid and gaseous products of electrochemical reactions shortly after their formation. To demonstrate its extraordinary potential, the electrochemical real time mass spectrometry (EC-RTMS) approach is used to determine the products of the CO2 reduction reaction (CO2 RR) during potential step or sweep experiments on pristine and in situ anodized copper. The enhanced formation of several C2+ products over C1 products is tracked directly after copper anodization, with unprecedented temporal resolution. This new technique creates exciting new opportunities for resolving processes that occur at short timescales and eventually for guiding the design of new, robust catalysts for selective electrosynthesis under dynamic operation.

Shape-Controlled Nanoparticles in Pore-Confined Space.

Increasing the catalyst's stability and activity are one of the main quests in catalysis. Tailoring crystal surfaces to a specific reaction has demonstrated to be a very effective way in increasing the catalyst's specific activity. Shape controlled nanoparticles with specific crystal facets are usually grown kinetically and are highly susceptible to morphological changes during the reaction due to agglomeration, metal dissolution, or Ostwald ripening. A strong interaction of the catalytic material to the support is thus crucial for successful stabilization. Taken both points into account, a general catalyst design is proposed, combining the enhanced activity of shape-controlled nanoparticles with a pore-confinement approach for high stability. Hollow graphitic spheres with narrow and uniform bimodal mesopores serve as model system and were used as support material. As catalyst, different kinds of particles, such as pure platinum (Pt), platinum/nickel (Pt3Ni) and Pt3Ni doped with molybdenum (Pt3Ni-Mo), have exemplarily been synthesized. The advantages, limits and challenges of the proposed concept are discussed and elaborated by means of time-resolved, in and ex situ measurements. It will be shown that during catalysis, the potential boundaries are crucial especially for the proposed catalyst design, resulting in either retention of the initial activity or drastic loss in shape, size and elemental composition. The synthesis and catalyst design can be adapted to a wide range of catalytic reactions where stabilization of shape-controlled particles is a focus.

Carbon Monoxide as a Promoter of Atomically Dispersed Platinum Catalyst in Electrochemical Hydrogen Evolution Reaction.

Carbon monoxide is widely known to poison Pt during heterogeneous catalysis owing to its strong donor-acceptor binding ability. Herein, we report a counterintuitive phenomenon of this general paradigm when the size of Pt decreases to an atomic level, namely, the CO-promoting Pt electrocatalysis toward hydrogen evolution reactions (HER). Compared to pristine atomic Pt catalyst, reduction current on a CO-modified catalyst increases significantly. Operando mass spectroscopy and electrochemical analyses demonstrate that the increased current arises due to enhanced H2 evolution, not additional CO reduction. Through structural identification of catalytic sites and computational analysis, we conclude that CO-ligation on the atomic Pt facilitates Hads formation via water dissociation. This counterintuitive effect exemplifies the fully distinct characteristics of atomic Pt catalysts from those of bulk Pt, and offers new insights for tuning the activity of similar classes of catalysts.

Atomically-defined model catalysts in ultrahigh vacuum and in liquid electrolytes: particle size-dependent CO adsorption on Pt nanoparticles on ordered Co3O4(111) films.

We have studied particle size effects on atomically-defined model catalysts both in ultrahigh vacuum (UHV) and under electrochemical (EC) conditions in liquid electrolytes. The model catalysts were prepared in UHV by physical vapour deposition (PVD) of Pt onto an ordered Co3O4(111) film on Ir(100), yielding nanoparticles (NPs) with an average size from 10 to 500 atoms per particle (0.8 to 3 nm). The model systems were characterized in UHV using surface science methods including scanning tunnelling microscopy (STM), before transferring them out of the UHV and into the electrolyte without contact to ambient conditions. By X-ray photoelectron spectroscopy (XPS) we show that the model surfaces are stable in the EC environment under the applied conditions (0.1 to 1 M phosphate buffer, pH 10, 0.33 to 1.03 VRHE). As a reference, we study Pt(111) under identical conditions. In UHV, we also investigated the adsorption of CO using infrared reflection absorption spectroscopy (IRRAS). Under EC conditions, we performed equivalent experiments using EC infrared reflection absorption spectroscopy (EC-IRRAS) in combination with cyclic voltammetry (CV). Characteristic differences were observed between the IR spectra under EC conditions and in UHV. Besides the red-shift induced by the interfacial electric field (Stark effect), the EC IR bands of CO on Pt(111) show a larger width (by a factor of 2) as a result of local variations in the CO environment and coupling to the electrolyte. The CO IR bands of the Pt NPs are even broader (by a factor of 5), which is attributed to local variations of the interfacial electric field at the NP surface. Further pronounced differences are observed between the spectra taken in UHV and in the electrolyte regarding the site occupation and its dependence on particle size. In UHV, adsorption at on-top sites is preferred on Pt(111) at low coverage and similar adsorption ratios of on-top and bridge-bonded CO are formed at saturation coverage. In sharp contrast, on-top adsorption of CO on Pt(111) is partially suppressed under EC conditions. This effect is attributed to the competitive adsorption of anions from the electrolyte and leads to a clear preference for bridge sites at higher potentials (>0.5 VRHE). For the Pt NPs, the situation is different and an increasing fraction of on-top CO is observed with decreasing particle size, both under EC conditions and in UHV. For the smallest particles (10-20 atoms) we do not detect any bridge-bonded CO. This change in site preference as a function of particle size is attributed to stronger on-top adsorption on low-coordinated Pt atoms of small Pt NPs. The effect leads to a clear preference for on-top adsorption in the electrolyte even at low CO coverage and over the full potential range studied.

The Common Intermediates of Oxygen Evolution and Dissolution Reactions during Water Electrolysis on Iridium.

Understanding the pathways of catalyst degradation during the oxygen evolution reaction is a cornerstone in the development of efficient and stable electrolyzers, since even for the most promising Ir based anodes the harsh reaction conditions are detrimental. The dissolution mechanism is complex and the correlation to the oxygen evolution reaction itself is still poorly understood. Here, by coupling a scanning flow cell with inductively coupled plasma and online electrochemical mass spectrometers, we monitor the oxygen evolution and degradation products of Ir and Ir oxides in situ. It is shown that at high anodic potentials several dissolution routes become possible, including formation of gaseous IrO3 . On the basis of experimental data, possible pathways are proposed for the oxygen-evolution-triggered dissolution of Ir and the role of common intermediates for these reactions is discussed.

Importance and Challenges of Electrochemical in Situ Liquid Cell Electron Microscopy for Energy Conversion Research.

The foreseeable worldwide energy and environmental challenges demand renewable alternative sources, energy conversion, and storage technologies. Therefore, electrochemical energy conversion devices like fuel cells, electrolyzes, and supercapacitors along with photoelectrochemical devices and batteries have high potential to become increasingly important in the near future. Catalytic performance in electrochemical energy conversion results from the tailored properties of complex nanometer-sized metal and metal oxide particles, as well as support nanostructures. Exposed facets, surface defects, and other structural and compositional features of the catalyst nanoparticles affect the electrocatalytic performance to varying degrees. The characterization of the nanometer-size and atomic regime of electrocatalysts and its evolution over time are therefore paramount for an improved understanding and significant optimization of such important technologies like electrolyzers or fuel cells. Transmission electron microscopy (TEM) and scanning transmission electron microscope (STEM) are to a great extent nondestructive characterization tools that provide structural, morphological, and compositional information with nanoscale or even atomic resolution. Due to recent marked advancement in electron microscopy equipment such as aberration corrections and monochromators, such insightful information is now accessible in many institutions around the world and provides huge benefit to everyone using electron microscopy characterization in general. Classical ex situ TEM characterization of random catalyst locations however suffers from two limitations regarding catalysis. First, the necessary low operation pressures in the range of 10(-6) to 10(-9) mbar for TEM are not in line with typical reaction conditions, especially considering electrocatalytic solid-liquid interfaces, so that the active state cannot be assessed. Second, and somewhat related, is the lack of time resolution for the evaluation of alterations of the usually highly heterogeneous nanomaterials. Two methods offer a solution to these shortcomings, namely, identical location TEM (IL-TEM) and electrochemical in situ liquid TEM. The former is already well established and has delivered novel insights particularly into degradation processes; however, characterization is still performed in vacuum. The latter circumvents this issue by using dedicated in situ TEM holders but introduces extremely demanding technical challenges. Although the introduction of revolutionizing thin SiN window cells, which elegantly confine the specimen from vacuum, has allowed demonstration of the potential of the in situ approach, the reproducibility and data interpretation is still limited predominately due to the strong interaction of the electron beam with the supporting electrolyte and electrode material. Because of the importance of understanding the nanoelectrochemical structure-function relationship, this Account aims to convey a timely perspective on the opportunities and particularly the challenges in electrochemical identical location TEM and in situ liquid cell TEM with a focus on electrochemical energy conversion.

Balanced work function as a driver for facile hydrogen evolution reaction - comprehension and experimental assessment of interfacial catalytic descriptor.

A major step in the development of (electro)catalysis would be the possibility to estimate accurately the energetics of adsorption processes related to reaction intermediates. Computational chemistry (e.g. using DFT) developed significantly in that direction and allowed the fast prediction of (electro)catalytic activity trends and improved the general understanding of adsorption at electrochemical interfaces. However, building a reliable and comprehensive picture of electrocatalytic reactions undoubtedly requires experimental assessment of adsorption energies. In this way, the results obtained by computational chemistry can be complemented or challenged, which often is a necessary pathway to further advance the understanding of electrochemical interfaces. In this work an interfacial descriptor of the electrocatalytic activity for hydrogen evolution reaction, analogue to the adsorption energy of the Had intermediate, is identified experimentally using in situ probing of the surface potentials of the metals, under conditions of continuous control of the humidity and the gas exposure. The derived activity trends give clear indication that the electrocatalytic activity for hydrogen evolution reaction is a consequence of an interplay between metal-hydrogen and metal-water interactions. In other words it is shown that the M-H bond formation strongly depends on the nature of the metal-water interaction. In fact, it seems that water dipoles at the metal/electrolyte interface play a critical role for electron and proton transfer in the double layer.

Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts.

Fe-N-C catalysts with high O2 reduction performance are crucial for displacing Pt in low-temperature fuel cells. However, insufficient understanding of which reaction steps are catalyzed by what sites limits their progress. The nature of sites were investigated that are active toward H2 O2 reduction, a key intermediate during indirect O2 reduction and a source of deactivation in fuel cells. Catalysts comprising different relative contents of FeNx Cy moieties and Fe particles encapsulated in N-doped carbon layers (0-100 %) show that both types of sites are active, although moderately, toward H2 O2 reduction. In contrast, N-doped carbons free of Fe and Fe particles exposed to the electrolyte are inactive. When catalyzing the ORR, FeNx Cy moieties are more selective than Fe particles encapsulated in N-doped carbon. These novel insights offer rational approaches for more selective and therefore more durable Fe-N-C catalysts.