Unprecedented High Oxygen Evolution Activity of Electrocatalysts Derived from Surface-Mounted Metal-Organic Frameworks.

The oxygen evolution reaction (OER) is a key process for renewable energy storage. However, developing non-noble metal OER electrocatalysts with high activity, long durability and scalability remains a major challenge. Herein, high OER activity and stability in alkaline solution were discovered for mixed nickel/cobalt hydroxide electrocatalysts, which were derived in one-step procedure from oriented surface-mounted metal-organic framework (SURMOF) thin films that had been directly grown layer-by-layer on macro- and microelectrode substrates. The obtained mass activity of ∼2.5 mA·µg-1 at the defined overpotential of 300 mV is 1 order of magnitude higher than that of the benchmarked IrO2 electrocatalyst and at least 3.5 times higher than the mass activity of any state-of-the-art NiFe-, FeCoW-, or NiCo-based electrocatalysts reported in the literature. The excellent morphology of the SURMOF-derived ultrathin electrocatalyst coating led to a high exposure of the most active Ni- and Co-based sites.

Ionic Conductivity Measurements-A Powerful Tool for Monitoring Polyol Reduction Reactions.

The reduction of metal precursors during the polyol synthesis of metal nanoparticles was monitored by ex situ ionic conductivity measurements. Using commonly used platinum precursors (K2PtCl6, H2PtCl6, and K2PtCl4) as well as iridium and ruthenium precursors (IrCl3 and RuCl3), we demonstrate that their reduction in ethylene glycol at elevated temperatures is accompanied by a predictable change in ionic conductivity, enabling a precise quantification of the onset temperature for their reduction. This method also allows detecting the onset temperature for the further reaction of ethylene glycol with HCl produced by the reduction of chloride-containing metal precursors (at ≈120 °C). On the basis of these findings, we show that the conversion of the metal precursor to reduced metal atoms/clusters can be precisely quantified, if the reaction occurs below 120 °C, which also enables a distinction between the stages of metal particle nucleation and growth. The latter is demonstrated by the reduction of H2PtCl6 in ethylene glycol, comparing ionic conductivity measurements with transmission electron microscopy analysis. In summary, ionic conductivity measurements are a simple and straightforward tool to quantify the reduction kinetics of commonly used metal precursors in the polyol synthesis.

Clarifying the role of Ru in methanol oxidation at Ru(core)@Pt(shell) nanoparticles.

The catalytic activity of Rucore@Ptshell nanoparticles (NPs) towards CO oxidation, a strongly adsorbed intermediate that compromises the performance of direct methanol fuel cells, is known to be significantly better than at Pt alone. However, a systematic study aimed at understanding the beneficial effect of Ru on Pt during the methanol oxidation reaction (MOR) has not been carried out as yet. Here, Rucore@Ptshell NPs, having a controlled Ptshell coverage of zero to two monolayers and two different Rucore sizes (2 and 3 nm), were synthesized using the simple polyol method to determine the precise role and impact of Ru on the MOR in 0.5 M H2SO4 + 1 M methanol at RT and 60 °C. Because the structure of our Rucore@Ptshell NPs is known with such certainty, we were able to show here that the rate of methanol adsorption/dehydrogenation can be accelerated either by compression of the Ptshell (by making the Rucore larger) when it is less than one monolayer in thickness, or by decreasing the electronic effect of the Rucore on the Ptshell (achieved by adding a second Pt layer to the Ptshell). At low overpotentials, decreasing the Ptshell thickness also helps in increasing the rate of the MOR by enhancing the rate of oxidation of adsorbed CO. Finally, it is shown that the bi-functional effect of Ru on the Ptshell plays only a minor role in the catalysis of the MOR, especially at large particles where CO surface diffusion is facilitated.

Gold nanoparticle array formation on dimpled Ta templates using pulsed laser-induced thin film dewetting.

Here we show that pulsed laser-induced dewetting (PLiD) of a thin Au metallic film on a nano-scale ordered dimpled tantalum (DT) surface results in the formation of a high quality Au nanoparticle (NP) array. In contrast to thermal dewetting, PLiD does not result in deformation of the substrate, even when the Au film is heated to above its melting point. PLiD causes local heating of only the metal film and thus thermal oxidation of the Ta substrate can be avoided, also because of the high vacuum (low pO2) environment employed. Therefore, this technique can potentially be used to fabricate NP arrays composed of high melting point metals, such as Pt, not previously possible using conventional thermal annealing methods. We also show that the Au NPs formed by PLiD are more spherical in shape than those formed by thermal dewetting, likely demonstrating a different dewetting mechanism in the two cases. As the metallic NPs formed on DT templates are electrochemically addressable, a longer-term objective of this work is to determine the effect of NP size and shape (formed by laser vs. thermal dewetting) on their electrocatalytic properties.

The impact of fabrication conditions on the quality of Au nanoparticle arrays on dimpled Ta templates.

Highly ordered dimpled Ta (DT) nanotemplates, prepared by electrochemical anodization of Ta, were recently reported to be ideally suited for the fabrication of a Au nanoparticle (NP) array using a Au thin film dewetting method. Here, we provide guidance and understanding of the effect of the DT fabrication and Au film deposition steps on the characteristics of the resulting NP array. Specifically, the optimum anodization time, voltage and solution composition are established, and the thickness of the sputter-deposited metal film is shown to be a very important parameter in achieving the desired single Au NP per dimple. The resulting high quality Au NP arrays are demonstrated to be electrochemically addressable, with the total Au surface area, measured electrochemically for large-scale samples, agreeing with the calculated area, based on scanning electron microscope determination of average particle shape and distribution. As the NP formation process proceeds via confined thin film dewetting, the protocol developed here should be applicable to the formation of NP arrays of a range of other metals and alloys.

The Gas Diffusion Electrode Setup as Straightforward Testing Device for Proton Exchange Membrane Water Electrolyzer Catalysts.

Hydrogen production from renewable resources and its reconversion into electricity are two important pillars toward a more sustainable energy use. The efficiency and viability of these technologies heavily rely on active and stable electrocatalysts. Basic research to develop superior electrocatalysts is commonly performed in conventional electrochemical setups such as a rotating disk electrode (RDE) configuration or H-type electrochemical cells. These experiments are easy to set up; however, there is a large gap to real electrochemical conversion devices such as fuel cells or electrolyzers. To close this gap, gas diffusion electrode (GDE) setups were recently presented as a straightforward technique for testing fuel cell catalysts under more realistic conditions. Here, we demonstrate for the first time a GDE setup for measuring the oxygen evolution reaction (OER) of catalysts for proton exchange membrane water electrolyzers (PEMWEs). Using a commercially available benchmark IrO2 catalyst deposited on a carbon gas diffusion layer (GDL), it is shown that key parameters such as the OER mass activity, the activation energy, and even reasonable estimates of the exchange current density can be extracted in a realistic range of catalyst loadings for PEMWEs. It is furthermore shown that the carbon-based GDL is not only suitable for activity determination but also short-term stability testing. Alternatively, the GDL can be replaced by Ti-based porous transport layers (PTLs) typically used in commercial PEMWEs. Here a simple preparation is shown involving the hot-pressing of a Nafion membrane onto a drop-cast glycerol-based ink on a Ti-PTL.

Controlled interconversion of nanoarray of ta dimples and high aspect ratio ta oxide nanotubes.

We report the controlled formation of either high-aspect-ratio Ta(2)O(5) nanotubes or an organized nanoarray of Ta dimples by Ta anodization in a single H(2)SO(4) + HF solution. Dimpled Ta is the stable surface morphology in the first few seconds, followed by the growth of dense and fully vertically aligned Ta(2)O(5) nanotubes (up to 2.5 microm long). After 2 min, the dimpled surface morphology reappears, related to the build-up of a resistive Ta fluoride surface layer.

Direct PtSn Alloy Formation by Pt Electrodeposition on Sn Surface.

Electrochemical deposition is a viable approach to develop novel catalyst structures, such as Pt thin films on conductive support materials. Most studies, reaching out to control electrochemical deposition of Pt to monolayer quantities focus on noble metal substrates (e.g., Au). In contrast, conductive oxides, such as antimony doped tin oxide (ATO), are considered as support material for different applications, e.g., as fuel cell catalysts. Herein, we investigate the deposition process of Pt on Sn, used as a model system for the electrochemical deposition of Pt on non-noble metal oxide supports. Doing so, we shade some light on the differences of a metallic Sn surface and surface oxide species in electrochemical deposition processes. With respect to a borate buffer solution, containing K2PtCl4 as Pt precursor, we report for the first time that surface oxides have the capability to fully inhibit the electrochemical deposition of Pt. Furthermore, direct alloying of the deposited Pt with the Sn support during the electrodeposition process yielded a catalyst with a high activity for the oxidation of CO.

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.

Silver Nanoparticles-Decorated Titanium Oxynitride Nanotube Arrays for Enhanced Solar Fuel Generation.

We demonstrate, for the first time, the synthesis of highly ordered titanium oxynitride nanotube arrays sensitized with Ag nanoparticles (Ag/TiON) as an attractive class of materials for visible-light-driven water splitting. The nanostructure topology of TiO2, TiON and Ag/TiON was investigated using FESEM and TEM. The X-ray photoelectron spectroscopy (XPS) and the energy dispersive X-ray spectroscopy (EDS) analyses confirm the formation of the oxynitride structure. Upon their use to split water photoelectrochemically under AM 1.5 G illumination (100 mW/cm2, 0.1 M KOH), the titanium oxynitride nanotube array films showed significant increase in the photocurrent (6 mA/cm2) compared to the TiO2 nanotubes counterpart (0.15 mA/cm2). Moreover, decorating the TiON nanotubes with Ag nanoparticles (13 ± 2 nm in size) resulted in exceptionally high photocurrent reaching 14 mA/cm2 at 1.0 VSCE. This enhancement in the photocurrent is related to the synergistic effects of Ag decoration, nitrogen doping, and the unique structural properties of the fabricated nanotube arrays.

Novel electrochemical fingerprinting methods for the precise determination of Pt(shell) coverage on Ru(core) nanoparticles.

The surface composition of nanoparticles is critical in defining their chemical and electrochemical properties. However, there are a limited number of tools that can rapidly and reliably establish these important characteristics at this small scale. In the present work, a series of Rucore@Ptshell nanoparticles (2 or 3 nm diameter Ru core, 0 to 2 monolayers of Pt in the shell layer) were synthesized and several novel electrochemical fingerprinting methods were developed to determine the Pt shell characteristics. These involved tracking the charge associated with the reduction of the oxide film formed on the exposed Rucore, as well as the potential and charge associated with COads stripping, giving the precise coverage of the first and second Pt monolayer, respectively.

New insights into the initial stages of Ta oxide nanotube formation on polycrystalline Ta electrodes.

Ta oxide nanotubes (NTs) were formed by the anodization of Ta at 15 V in a solution of concentrated sulfuric acid containing 0.8-1.0 M hydrofluoric acid. To study the initial stages of NT formation, FESEM images of samples anodized for very short times were obtained. The results contradict the existing explanation of the current-time data collected during anodization, which has persisted in the literature for more than two decades. In addition to providing a first-time morphological study of Ta oxide NT formation at very early stages of anodization, we also propose a new interpretation of the i-t response, showing that pores are already present in the first few milliseconds of anodization and that NTs are formed well before present models predict. This behaviour may also extend to the anodization of other valve metals, such as Al, Ti, Zr, W, and Nb.

Controlled growth and monitoring of tantalum oxide nanostructures.

Nanoporous metal oxide structures produced by the electrochemical anodization of valve metals, such as Zr, Ti, W, Nb, Al, and recently Ta, have attracted increasing interest because of their potential use as catalysts, waveguides, and three-dimensionally arranged Bragg-stack reflectors. Here we demonstrate the formation of either supported nanotubular Ta oxide films or free-standing Ta oxide membranes, produced by controlling the conditions of Ta anodization in organic-free aqueous HF/H(2)SO(4) solutions. The supported oxide nanotubes, which are at least 15 mum in length, are characterized by very good adhesion to the Ta substrate, and extremely smooth and homogeneous walls. It is also reported here, for the first time, that these nanotubular films can be removed as free-standing Ta oxide membranes that are easily transferable to other substrates, making them potentially useful in sensors, optics, and catalysis. We also show that, when the Ta oxide nanotubes detach to form the membranes, they leave behind an ordered array of dimples in the Ta surface, with the dimples having the identical distribution and size as the pores in the previously attached nanotubes. Finally, we demonstrate how the in situ electrochemical response during anodization can be used to determine which of these highly useful Ta surface morphologies (nanotubes vs. dimples) are formed, without the need for post factum microscopic analysis. Knowledge of the meaning of these in situ signals can now serve to accelerate the controlled formation of oxide nanotubes or dimpled surfaces using other combinations of metals and anodization conditions.