Yuriy V. Tolmachev

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Osmium nanoislands spontaneously deposited on a Pt(111) electrode: an XPS, STM and GIF-XAS study

Scanning tunneling microscopy (STM) characterized adlayers of spontaneously deposited osmium on a Pt(111) electrode were investigated using ex-situ X-ray photoemission spectroscopy (XPS) and in-situ grazing incidence fluorescence X-ray absorption spectroscopy (GIF-XAS). After a single spontaneous deposition, monoatomic (or nearly monoatomic) nanoislands of osmium are formed. The island diameter varies from 2 to 5 nm depending on the Os coverage, which in turn is adjusted by varying the concentration of the Os precursor salt (OsCl3) in the deposition bath and/or by the deposition time. XPS reveals three oxidation states: a metallic Os (the 4f7 /2 core level binding energy of 50.8 eV), Os(IV) (51.5 eV) and Os(VIII) (52.4 eV). The metallic osmium exists at potentials below 500 mV (vs. RHE) while above 500 mV osmium is oxidized to Os(IV). Electrodissolution of osmium begins above 900 mV and occurs simultaneously with platinum oxidation. At ca. 1200 mV V versus the RHE reference, the oxidation state of some small amounts of osmium that survive dissolution is the Os(VIII). We demonstrate, for the first time, that mixed or odd valencies of osmium exist on the platinum surface at potentials higher that 800 mV. In-situ GIF-XAS measurements of an Os LIII edge also reveal the presence of three Os oxidation states. Namely, below the electrode potential of 400 mV, the X-ray fluorescent energy at maximum absorption is 10.8765 keV, and is characteristic of the metallic Os. In the potential range between 500 and 1000 mV this energy is gradually shifted to higher values, assignable to higher valencies of osmium, like Os(IV). This tendency continues to higher potentials consistent with the third, highly oxidized osmium form present, most likely Os(VIII). The variation of the ‘‘raw edge jump height’’ of Os with the electrode potential, which is equivalent to a drop in osmium surface concentration, demonstrates that the electrochemical stripping of Os begins below 1.0 V versus RHE, as expected from voltammetry. Also, the observed intensity of the white line of Os in the 100/400 mV region is larger than the value reported for metallic bulk Os. This discrepancy may result from the difference in the electronic properties of the metallic Os layers on Pt(111) and the metallic bulk Os: in the potential region between 100 and 400 mV, the 5d electrons in Os and Pt form a mixed electronic band, and the density of electronic states near the Fermi level, the main factor determining the white line intensity, may not be the same as in metallic bulk. The presented results on osmium adlayers are much more comprehensive than those available in our previous work due to the combined STM, GIF-XAS and XPS investigations. A nearly perfect convergence of the in situ and ex situ data is one of the main research outcomes of this project. Finally, platinum XPS spectra taken in the context of Os electrooxidation from the electrode surface are also presented and conclusions are made, that up to 900 mV platinum remain metallic, irrespective of a significant osmium oxidation on its surface. # 2003 Elsevier B.V. All rights reserved.

In Situ Surface X-Ray Scattering Observation of Long-Range Ordered ( 19 × 19 ) R23.4 ° ­ 13 CO Structure on Pt(111) in Aqueous Electrolytes

Presented herein is the experimental observation of the long-range ordered (√19 × √19)R23.4°-13CO structure on Pt(111) in aqueous electrolytes by in situ surface X-ray scattering. The results confirmed the presence of two mirrored domains suggested earlier on the basis of scanning tunneling microscopy and infrared measurements. Based on the weak intensity of the second order adlattice reflections and earlier results obtained by other techniques, a further refinement of the (√19 × √19) structure with tilted CO molecules is proposed. The hystereses observed in transitions between (2 × 2 )- 3 CO and (√19 × √19)R23.4°-13CO phases, as well as in CO adsorption and stripping, with change in electrode potential are discussed.

Electrochemical Properties of Pt Coatings on Ni Prepared by Atomic Layer Deposition

Presented herein is an approach to fabrication of Pt coatings on non-noble metals with submonolayer thickness. The Pt coatings were prepared using atomic layer deposition ALD in which Ni-disk substrate is exposed to MeCpPtMe3 and H2 in alternating cycles. The structure and electrocatalytic activity of the coatings were characterized using elemental analysis and various electrochemical techniques. We found that the ALD results in micrometer-size Pt islands up to ca 3.7 monolayers of Pt loading and continues more uniformly at higher loadings. The islandlike growth at low Pt loadings is attributed to the presence of adsorbed O on atomically rough facets of polycrystalline Ni substrate. The thin films of Pt on Ni show enhancement factors in the oxygen reduction reaction similar to what has been reported for PtNi alloys. Potential applications of the methodology demonstrated in this work for the development of the third-generation polymer electrolyte fuel cell catalytic layers with ultralow Pt loading are briefly discussed.

Impurity Effects on Oxygen Reduction Electrocatalysis at Platinum Ultramicroelectrodes: A Critical Assessment

Impurity effects associated with an increase in the rate of mass transport induced by a decrease in the size of electrodes have been examined with emphasis on species capable of undergoing irreversible adsorption. Calculations show that for hemispherical ultramicroelectrodes 50 nm in radius, the flux of a solution-phase contaminant, such as a small organic molecule, present at ppb levels, is large enough to reach saturation coverage over times on the order of seconds. In the case of dioxygen reduction on Pt in aqueous electrolytes, such adsorbed species are believed to be responsible for marked changes in the mechanism, promoting a two-electron over a four-electrode reduction pathway. Such aspects must be considered in the analysis of results obtained with Pt ultramicroelectrodes, either as single particles or in dilute arrays.

Polarization-dependent resonant anomalous surface X-ray scattering of CO/Pt(111)

Polarization dependence of resonant anomalous surface X-ray scattering (RASXS) was studied for interfaces buried in electrolytes and in high-pressure gas. We find that RASXS exhibits strong polarization dependence when the surface is only slightly modified by adsorption of light elements such as carbon monoxide on platinum surfaces. σ- and π-polarization RASXS data were simulated with the latest version of ab initio multiple scattering calculations (FEFF8.2). Based on the simulation, we find that Pt layer to C distances in the (2 × 2)-3CO structure are ~ 2.0 A and ~ 1.6 A for atop CO and 3-fold CO, respectively. Some considerations are presented for the origin of the polarization dependence in RASXS.

Theoretical Aspects of Laminar Flow in a Channel‐Type Electrochemical Cell as Applied to In Situ Attenuated Total Reflection‐Infrared Spectroscopy

Analytic expressions are derived, within the Leveque approximation, for the steady-state concentration profile of a reactant or a stable product generated via first-order kinetics at an electrode in a channel-type electrochemical cell under fully developed laminar flow. It is shown that for cell geometries and other experimental conditions easily realizable in the laboratory, the concentration of species directly above the insulating flat surface adjacent to the downstream edge of the electrode c(x,y), where x is the direction of fluid flow and y is the distance normal to the electrode surface, differs only slightly (less than 5%) from c(x,0), the concentration on the surface at the prescribed x, for values of y on the order of microns. Implications of these results to the quantitative analysis of in situ attenuated total reflection-infrared spectroscopy in a channel-type spectroelectrochemical cell (Barbour et al. 6 ) are discussed.

In Situ Spectroscopy in the Presence of Convective Flow under Steady‐State Conditions: A Unified Mathematical Formalism

A generalized mathematical treatment is presented that enables a quantitative analysis of various in situ spectroscopi.c experiments involving detection of solution-phase species generated at the surface of rotating disk and ring-disk, channel, and tube-type electrodes under steady state. This theory is valid for experimental conditions easily realizable in the laboratory and is only applicable to first-order irreversible heterogeneous electron-transfer processes without complications derived from homogeneous phase reactions. Illustrations of the advantages of this formalism are provided, including a rederivation of equations obtained by other authors, an analytic route to the solution of the rotating disktransparent ring electrode, and various aspects of normal incidence reflection absorption spectroscopy at rotating ring and ring-disk electrodes. The results were in excellent agreement with those of other workers (when available), and in some cases, yielded better accuracy than solutions generated by digital simulation techniques reported in the literature.

Cyclic versus Staircase Voltammetry in Electrocatalysis: Theoretical Aspects

The transient response of a series RC equivalent circuit to a staircase potential perturbation, where R represents the resistance due to the electrolyte and C a voltage dependent interfacial capacitance, has been examined theoretically using, as a model system, capacitance data believed to be characteristic of the Pt(111) | 0.1 M HClO 4 interface. Solutions to the governing algebraic differential equation were obtained by numerical techniques, which were then used to generate current versus potential curves assuming various data sampling protocols. In general, the resulting curves were not only found to be sensitive to the time at which the current was sampled within each step, but, also, quite different than those obtained by scanning the potential at a constant rate, as in conventional linear scan voltammetry (LSV). Excellent agreement between the staircase voltammetry (SCV) and LSV results could be achieved, however, based on values of the average step currents, Q step /T, where Q step represents the total charge injected during the step and T the length of the step, as well as by the postfiltering technique introduced by He [Anal. Chem., 67, 986 (1995)]. Implications of these results for studies involving electrocatalytic systems are discussed.

Extensions of the Koutecky–Levich Equation to Channel Electrodes

An analytical solution is herein presented for the current at a channel-type electrode associated with a simple first-order (in the reactant) heterogeneous electron transfer process involving a solution phase species. Assuming a fixed cell geometry, electrolyte composition, and temperature, a series expansion of this solution revealed that in the limit of small values of k3/v o , where k is the first-order kinetic rate constant and v o is the fluid velocity in the center of the channel, the reciprocal of the measured current, 1/i = 1/i k ; + β/i lim . where i k and i lim are the kinetic and diffusion limited currents, respectively, and β = 27/[8Γ(2/3)Γ(1/3)] = 0.93036. This equation bears striking resemblance to that reported by Koutecky and Levich for a rotating disk electrode, except that βin the latter case is unity. Rather surprisingly, and in agreement with recent findings [Electrochim. Acta, 52, 4124 (2007)], a plot of 1 1/ivs k/v 0 1/3 was found to be close to linear, with a slope reaching values slightly higher than β as the magnitude of k/v 0 1/3 increased.