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Proton exchange membrane fuel cells (PEMFCs) have the potential to become a vital part of sustainable energy systems, provided that cathode catalysts with higher activity and stability are developed. A very promising group of materials for the oxygen reduction reaction (ORR) is the group of alloys with platinum and rare earth elements.[1, 2]. We have recently showed that sputtered thin films of Pt3Y have specific activities for ORR up to seven times higher than polycrystalline Pt in rotating disk measurements, which is similar to that of bulk alloys.[3] In addition, sputtered thin films of Pt3Y with thicknesses about 3 nm show a mass activity more than an order of magnitude higher than state-of-the-art Pt nanoparticles. However, in order to become realistic alternative catalysts these materials need to prove an increased activity also under real fuel cell conditions. By sputtering thin films of Pt3Y on gas diffusion layers (GDLs) we are able to construct full membrane electrode assemblies (MEAs) and evaluate these in single cell measurements.[4] The fuel cell measurements show that Pt3Y has a specific activity up to 2.5 times higher than pure Pt. Detailed characterization reveals important materials properties of the thin films and indicates that the catalyst structure of the films on GDL is not optimal. This suggests that even higher activities should be possible in the fuel cell if the electrode and catalyst structure is improved. References Escudero-Escribano, M., et al., Tuning the activity of Pt alloy electrocatalysts by means of the lanthanide contraction. Science, 2016. 352(6281): p. 73-76. Greeley, J., et al., Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nature Chemistry, 2009. 1(7): p. 552-556. Lindahl, N., et al., High Specific and Mass Activity for the Oxygen Reduction Reaction for Thin Film Catalysts of Sputtered Pt3Y. Advanced Materials Interfaces, 2017. 4(13): p. 1700311. Lindahl, N., et al., Fuel cell measurements with cathode catalysts of sputtered Pt3Y thin films. ChemSusChem, 2018. http://dx.doi.org/10.1002/cssc.201800023. Caption. a) Electrochemical single cell characterization of Pt and Pt3Y sputtered catalysts in H2/O2, 80 °C, 1.5 bar absolute pressure, 100% RH, 20 mV s-1. Inset shows a schematic image of the atomic stucture of the Pt3Y. b) Bright-field TEM micrograph of as-sputtered Pt3Y thin film on GDL. The scale-bar is 50 nm. Figure 1

The role and contribution of transport processes in electrocatalytic reactions was investigated in model studies of the oxidation of CO (single-product reaction) and formaldehyde (dual-product reac ...

Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W 2/3 M 2 1/3 ) 2 AC, where M 2 = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two—with a monoclinic C2/c structure—are predicted to be stable: (W 2/3 Sc 1/3 ) 2 AlC and (W 2/3 Y 1/3 ) 2 AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W 1.33 C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.

The role of mesoscopic mass transport and re-adsorption effects in electrocatalytic reactions was investigated using the oxygen reduction reaction (ORR) as an example. The electrochemical measurements were performed on structurally well-defined nanostructured model electrodes under controlled transport conditions in a thin-layer flow cell. The electrodes consist of arrays of Pt ultra-microelectrodes (nanodisks) of defined size (diameter approximately 100 nm) separated on a planar glassy carbon (GC) substrate, which were fabricated employing hole-mask colloidal lithography (HCL). The measurements reveal a distinct variation in the ORR selectivity with Pt nanodisk density and with increasing electrolyte flow, showing a pronounced increase of the H2O2 yield, by up to 65%, when increasing the flow rate from 1 to 30 microL s(-1). These results are compared with previous findings and discussed in terms of a reaction model proposed recently (A. Schneider et al., Phys. Chem. Chem. Phys., 2008, 10, 1931), which includes (i) direct reduction to H2O on the Pt surface and (ii) additional H2O2 formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H2O2 molecules on the Pt surface. The potential of model studies on structurally defined catalyst surfaces and under well-defined mass transport conditions in combination with simulations for the description of electrocatalytic reactions is discussed.

A 2D axisymetric model of a NOx storage and reduction catalyst monolith channel combining mass transport with a detailed kinetic model was created to evaluate the importance of mass transport in a Pt/BaO/Al2O3 washcoat. Results show that there are small radial gradients in stored species concentration early during transients. The Sherwood number calculated during the transient storage phase will not be constant in time nor space as a film correlation would predict, but instead shows a region of higher Sherwood number propagating through the channel as the storage reaches completion. It is concluded that incorporating detailed mass transport provides a better spatially resolved picture of the dynamics of the proposed reaction mechanism and minimises the risk of arriving at false intrinsic kinetics during the development of a microkinetic model.

0-18nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nation 117 membranes, and used as thin spacer electrolyte layers between the Nation and a 3 nm Pt catalyst fi ...

We demonstrate a flexible combined electrochemistry and fiber optics-based in situ UV/vis spectroscopy setup to gain insight into the depth evolution of electrochemical hydride and oxide formation in Pd films with thicknesses of 20 and 100 nm. The thicknesses of our model systems are chosen such that the films are thinner or significantly thicker than the optical skin depth of Pd to create two distinctly different situations. Low power white light is irradiated on the sample and analyzed in three different configurations; transmittance through, and, reflectance from the front and the back side of the film. The obtained optical sensitivities correspond to fractions of a monolayer of adsorbed or absorbed hydrogen (H) and oxygen (O) on Pd. Moreover, a combined simultaneous readout obtained from the different optical measurement configurations provides mechanistic insights into the depth-evolution of the studied hydrogenation and oxidation processes.

Fuel cells have the potential to play an important role in sustainable energy systems, provided that catalysts with higher activity and stability are developed. In this work, it is found that thin alloy films of single-target cosputtered platinum-yttrium exhibit up to seven times higher specific activity (13.4 +/- 0.4 mA cm(-2)) for the oxygen reduction reaction (ORR) than poly-crystalline platinum, and up to one order of magnitude higher mass activity (3.5 +/- 0.3 A mg(-1)) than platinum nanoparticles. These alloys have the highest reported ORR activity for an as-deposited material, i.e., without any additional chemical or thermal treatment. The films show an improvement in stability over the same materials in nanoparticulate form. Physical characterization shows that the thin films form a platinum overlayer supported on an underlying alloy. The high activity is likely related to compressive strain in that overlayer. As sputtering can be used to mass-produce fuel cell electrodes, the results open new possibilities for the preparation of platinum-rare earth metal alloy catalysts in commercial devices.

Ethanol is an alternative fuel for direct alcohol fuel cells, in which the electrode materials are commonly based on Pt or Pd. Owing to the excellent promotion effect of Ni2 P that was found in methanol oxidation, we extended the catalyst system of Pt or Pd modified by Ni2 P in direct ethanol fuel cells. The Ni2 P-promoted catalysts were compared to commercial catalysts as well as to reference catalysts promoted with only Ni or only P. Among the studied catalysts, Pt/C and Pd/C modified by Ni2 P (30 wt %) showed both the highest activity and stability. Upon integration into the anode of a homemade direct ethanol fuel cell, the Pt-Ni2 P/C-30 % catalyst showed a maximum power density of 21 mW cm(-2) , which is approximately two times higher than that of a commercial Pt/C catalyst. The Pd-Ni2 P/C-30 % catalyst exhibited a maximum power density of 90 mW cm(-2) . This is approximately 1.5 times higher than that of a commercial Pd/C catalyst. The discharge stability on both two catalysts was also greatly improved over a 12 h discharge operation.

Functionalization of transition metal oxides using metallic nanoparticles is an interesting route towards efficient photoelectrochemical hydrogen production via water splitting. Although an enhanced photocurrent in photoanodes upon functionalization with metallic nanostructures has been observed in several studies, to the best of our knowledge no measurements of the Faradaic efficiency (FE) of the oxygen evolution reaction (OER) have been reported for such systems. This work characterizes the FE on a model system consisting of ultra-thin films of hematite (Fe2O3) sensitized with Ti/Au nanodisks. Compared to bare hematite references, sensitized samples showed significantly enhanced photocurrents as well as O2 evolution. Experimental evidence suggests that the observed enhancement was not due to photocatalytic activity of the nanodisks. The FE has been determined to be 100%, within the experimental errors, for both sensitized and reference samples. Also, this work demonstrates that the sensitized samples were stable for at least 16 hours photocurrent testing. The concepts shown in this work are generally applicable to any situation in which a semiconductor has its water splitting performance enhanced by metallic nanostructures.

Future generations require more efficient and localized processes for energy conversion and chemical synthesis. The continuous on-site production of hydrogen peroxide would provide an attractive alternative to the present state-of-the-art, which is based on the complex anthraquinone process. The electrochemical reduction of oxygen to hydrogen peroxide is a particularly promising means of achieving this aim. However, it would require active, selective and stable materials to catalyse the reaction. Although progress has been made in this respect, further improvements through the development of new electrocatalysts are needed. Using density functional theory calculations, we identify Pt-Hg as a promising candidate. Electrochemical measurements on Pt-Hg nanoparticles show more than an order of magnitude improvement in mass activity, that is, A g(-1) precious metal, for H2O2 production, over the best performing catalysts in the literature.

In current polymer electrolyte fuel cells (PEMFC), high amounts of platinum catalysts is required due to the sluggish kinetics of the oxygen reduction reaction (ORR). As such it is important to find ways to increase the activity of the ORR catalyst. Previous studies have shown that alloying platinum with yttrium results in a catalyst which is more than six times as active as sole Pt in half cell rotating disk electrode measurements [1]. The model electrodes in this study are prepared by sputtering a well-defined layer of alloy catalyst onto a gas diffusion layer (GDL). In this manner the composition and thickness of the film can be controlled. These electrodes are studied with single cell PEMFC measurements to characterize the materials, similar to previous work done with these types of electrodes [2]. Using these methods the electrochemical surface area (ECSA), ORR activity, and stability are evaluated. The effects of different MEA preparations will also be investigated. Scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis as well and X-ray photoelectron spectroscopy (XPS) are used to characterize the structure of the alloy catalyst as well as the bulk and surface compositions, i.e. platinum to yttrium ratio. This characterization is performed on both as-prepared samples and electrodes after being used in the PEMFC. The main focus is to determine the role of yttrium in platinum thin-film platinum electrodes, and if the excellent activity in half cell experiments will also be seen in real PEMFC measurements. References [1] J. Greeley, I. E. L. Stephens, A. S. Bondarenko, T. P. Johansson, H. A. Hansen, T. F. Jaramillo, J. Rossmeisl, I. Chorkendorff and J. K. Nørskov , Nature Chemistry, 2009, 1, 552-556 [2] M. Wesselmark, B.Wickman,C.Lagergren,G.Lindbergh, Electrochimica Acta, 2013, 111, 152-159

The Li-O2 battery is a promising chemistry suggesting very high energy densities. In the literature it is agreed upon that during the discharge process Li2O2 is the final product formed via the intermediate LiO2. However, the nature of the intermediate is not fully understood, e.g. how long is the lifetime and how does it proceed to form the final product (chemical or electrochemical reaction). This study sets out to shed light on the reaction mechanism of the Li-O2 battery by studying the effects of different concentrations of lithium and oxygen. Experimentally this is done using an electrochemical quartz crystal microbalance (EQCM) and a cylindrical microelectrode. The EQCM results show that the system is unable to return to its initial mass, i.e. there is a buildup of reaction product during cycling. Further, it is not possible to recover all the discharge electrons during recharge. The intermediate is believed to diffuse away from the electrode rendering it unavailable for recharge. This result is further strengthened by the measurements on microelectrodes where intermediate is believed to diffuse away from the electrode more easily. There is also a discrepancy when looking at the change in mass and the expected mass change from the corresponding current suggesting that both electrochemical and chemical processes occurs. In conclusion it can be said that this study provides additional knowledge to the surface confined processes in the Li-O2 battery. The EQCM provides information that otherwise would go unnoticed, i.e. solely measuring the current response will not provide detailed quantitative information of the mass response.

Aiming at the investigation of spillover and transport effects in electrocatalytic reactions on bimetallic catalyst electrodes, we have prepared novel, nanostructured electrodes consisting of arrays of homogeneously distributed pairs of Pt and Ru nanodisks of uniform size and with controlled separation on planar glassy carbon substrates. The nanodisk arrays (disk diameter approximate to 60 nm) were fabricated by hole-mask colloidal lithography; the separation between pairs of Pt and Ru disks was varied from -25 nm (overlapping) via +25 nm to +50 nm. Morphology and (surface) composition of the Pt/Ru nanodisk arrays Were characterized by scanning electron microscopy, energy dispersive X-ray analysis, and X-ray Photoelectron spectroscopy, the electrochemical/electrocatalytic properties were explored by cyclic voltammetry, COad monolayer oxidation ("COad stripping"), and potentiodynamic hydrogen oxidation. Detailed analysis of the 2 COad oxidation peaks revealed that on all bimetallic pairs these cannot be reproduced by superposition of the peaks obtained on electrodes with Pt/Pt or Ru/Ru pairs, pointing to effective Pt-Ru interactions even between rather distant pairs (50 nm). Possible reasons for this observation and its relevance for the understanding of previous reports of highly active catalysts with separate Pt and Ru nanoparticles are discussed. The results clearly demonstrate that this preparation method is perfectly suited for fabrication of planar model electrodes with well-defined arrays of bimetallic nanodisk pairs, which opens up new possibilities for model studies of electrochemical/electrocatalytic reactions.

Transport effects in the methanol oxidation reaction (MOR) were investigated using nanostructured Pt/glassy carbon (GC) electrodes and, for comparison, a polycrystalline Pt electrode. The nanostructured Pt/GC electrodes, consisting of a regular array of catalytically active cylindrical Pt nanostructures with 55 +/- 10 nm in diameter and different densities supported on a planar GC substrate, were fabricated employing hole-mask colloidal lithography (HCL). The MOR measurements were performed under controlled transport conditions in a thin-layer flow cell interfaced to a differential electrochemical mass spectrometry (DEMS) setup. The measurements reveal a distinct variation in the MOR activity and selectivity (product distribution) with Pt nanostructure density and with electrolyte flow rate, showing an increasing overall activity, reflected by a higher Faradaic reaction current, as well as a pronounced increase of the turnover frequency for CO(2) formation and of the CO(2) current efficiency with decreasing flow rate and increasing Pt coverage. These findings are discussed in terms of the "desorption-readsorption-reaction" model introduced recently (Seidel et al. Faraday Discuss. 2008, 140, 67). Finally, consequences for applications in direct methanol fuel cells are outlined.

The Lithium-Oxygen battery is one of the most promising battery technologies being researched, it is also one of the more challenging ones due to the various problems associated with the battery. One of these problems is the poor understanding of the reaction occurring during oxygen reduction and evolution. It is known that the intermediates of the reaction are highly reactive and take part in unwanted side reactions. It is also known that the oxygen reduction products are poorly soluble in the electrolyte. This study sets out to further examine these effects. Qualitatively these aspects are somewhat known but quantitatively the answer is less straight forward. By combining a Quartz Crystal Microbalance with electrochemical techniques the mass, current and voltage can be measured simultaneously to help shed further light on the species solubility and occurrence of side reactions. Obtaining this information about the chemical nature of the species formed will serve as a well needed compliment to the more frequently used surface characterization methods used when trying to determine the reactions occurring at the oxygen electrode.

Colloidal lithography (CL) is a generic name for a collection of nanolithographic techniques, based on using colloidal nanoparticles as pattern (mask)-defining entities to produce various nanostructures. A key step in CL processes is the deposition, usually by evaporation or sputtering, of the material that makes up the final nanostructures. We have for the first time combined a special version of CL, called hole-mask colloidal lithography (HCL), with electrodeposition. We demonstrate how electrodeposition of Pt onto Au and carbon substrates, through a lithographic mask, can be used to prepare well-defined nanostructured surfaces. The results are compared with evaporated structures and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and cyclic voltammetry. Specific results are: (i) electrodeposition generates structures with very good adhesion, (ii) due to differences in the deposition mechanism, structures with much larger aspect (height/width) ratio can be made with electrodeposition than with evaporation and (iii) the originally deposited polycrystalline nanoparticles can be annealed into single crystals, as demonstrated by electron diffraction, SEM and TEM, before and after annealing, which is of great value for fundamental (electro)catalysis studies.

Degradation of fuel cell model electrodes during accelerated aging was studied using electrochemical quartz crystal microbalance with dissipation monitoring. The model electrodes, consisting of Pt particles (5 nm) on planar carbon and Pt-only and carbon-only films, were prepared by thermal evaporation onto uncoated quartz crystal sensors. The characterization of electrode composition and morphology was performed by X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, and atomic force microscopy. The experiments were conducted in a flow cell with 0.5 M H(2)SO(4) at room temperature and up to 70 degrees C by repeated cycling between 0.02 and 1.4 V(RHE) (where RHE is reversible hydrogen electrode) at 50 mV s(-1). During cyclic corrosion, the Pt-only sample loses mass equivalent to 0.6% of a monolayer per cycle in a process that is not temperature-dependent. The experiments with the Pt particle layer on a carbon electrode show a mass loss that is almost 2 times larger than the Pt-only sample and exhibits an Arrhenius type of temperature dependence. The results suggest that the presence of Pt catalyzes carbon corrosion with an apparent activation energy of 0.33 eV. In all measurements, the carbon-only sample loses much less mass than the other samples.

The role of transport and re-adsorption processes on the oxygen reduction reaction (ORR), and in particular on its selectivity was studied using nanostructured model electrodes consisting of arrays of Pt nanostructures of well-defined size and separation on a planar glassy carbon (GC) substrate. The electrochemical measurements were performed under controlled transport conditions in a double-disk electrode thin-layer flow-cell configuration; the model electrodes were fabricated by colloidal lithography techniques, yielding Pt nanostructures of well defined and controlled size and density (diameter: 140 or 85 nm, height: 20 or 10 nm, separation: from 1-2 to more than 10 diameters). The nanostructured model electrodes were characterized by scanning electron microscopy and electrochemical probing of the active surface area (via the hydrogen adsorption charge). The electrocatalytic measurements revealed a pronounced variation of the hydrogen peroxide yield, which increases by up to two orders of magnitude with increasing separation and decreasing size of the Pt nanostructures. Similar, though less pronounced effects were observed upon varying the electrolyte flow and thus the mass transport characteristics. These effects are discussed in a reaction model which includes (i) direct reduction to H(2)O on the Pt surface and (ii) additional H(2)O(2) formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H(2)O(2) molecules on the Pt surface.

Nanostructured, glassy carbon GC supported Pt/GC electrodes, with Pt nanostructures nanodisks of controlled size 100–140 nm in diameter and separation homogeneously distributed on a planar GC substrate, were recently shown to be interesting model systems for electrocatalytic reaction studies M. Gustavsson, H. Fredriksson, B. Kasemo, Z. Jusys, C. Jun, and R. J. Behm, J. Electroanal. Chem., 568, 371 2004. We present here electron microscopy and electrochemical measurements which reveal that the fabrication of these nanostructured electrodes via colloidal lithography, in addition to the intended nanodisks, results in a dilute layer of much smaller Pt nanoparticles diameter 5n m on the GC surface in the areas between the Pt nanodisks. We further demonstrate that by using the developed, related method of hole-mask colloidal lithography HCLH. Fredriksson, Y. Alaverdyan, A. Dmitriev, C. Langhammer, D. S. Sutherland, M. Zach, and B. Kasemo, Adv. Mater. (Weinheim, Ger.), 19, 4297 2007, similar electrodes can be prepared which are free from these Pt nanoparticles. The effect of the additional small Pt nanoparticles on the electrochemical and electrocatalytic properties of these nanostructured electrodes, which is significant and can become dominant at low densities of the Pt nanodisks, is illustrated and discussed. These results leave HCL the preferred method for the fabrication of nanostructured Pt/GC electrodes, in particular, of low-density Pt/GC electrodes.

Transport effects may not only influence the rate of electrocatalytic reactions, but also the product distribution. The underlying effects were studied under well-defined reaction and transport conditions, using nanostructured Pt/glassy carbon electrodes, which consist of ordered arrays of electrocatalytically active Pt nanostructures on a planar glassy carbon substrate. The effect of varying the density of the Pt nanostructures or the electrolyte flow rate on the reaction characteristics and product distribution was investigated for apparently simple electrocatalytic reactions such as CO oxidation, methanol oxidation and oxygen reduction. It is shown that reducing the Pt coverage leads to an increase of the relative amount of the reaction intermediates. Similar effects are obtained for increasing the electrolyte flow rate. The results are discussed on a molecular scale in terms of the 'desorption - re-adsorption - reaction' model which we had introduced recently (Y.E. Seidel et al., Faraday Discuss. 140, 167 (2008)).