Thomas E. Moylan

Solvothermal Synthesis of Platinum Alloy Nanoparticles for Oxygen Reduction Electrocatalysis

Platinum alloy nanoparticles show great promise as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes. We report here on the use of N,N-dimethylformamide (DMF) as both solvent and reductant in the solvothermal synthesis of Pt alloy nanoparticles (NPs), with a particular focus on Pt-Ni alloys. Well-faceted alloy nanocrystals were generated with this method, including predominantly cubic and cuboctahedral nanocrystals of Pt(3)Ni, and octahedral and truncated octahedral nanocrystals of PtNi. X-ray diffraction (XRD) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), coupled with energy dispersive spectroscopy (EDS), were used to characterize crystallite morphology and composition. ORR activities of the alloy nanoparticles were measured with a rotating disk electrode (RDE) technique. While some Pt(3)Ni alloy nanoparticle catalysts showed specific activities greater than 1000 μA/cm(2)(Pt), alloy catalysts prepared with a nominal composition of PtNi displayed activities close to 3000 μA/cm(2)(Pt), or almost 15 times that of a state-of-the-art Pt/carbon catalyst. XRD and EDS confirmed the presence of two NP compositions in this catalyst. HAADF-STEM examination of the PtNi nanoparticle catalyst after RDE testing revealed the development of hollows in a number of the nanoparticles due to nickel dissolution. Continued voltage cycling caused further nickel dissolution and void formation, but significant activity remained even after 20,000 cycles.

Hydrophilic versus hydrophobic coadsorption: Carbon monoxide and water on Rh(111) versus Pt(111)

Abstract Carbon monoxide is a major poison to anodic reactions in aqueous fuel cells. To ascertain the effects of a controlled aqueous environment on the adsorption of CO on electrocatalytically active metals, the coadsorption of CO and water on Rh(111) and Pt(111) surfaces at 100 K was studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). On Rh(111) low coverages of CO shift monolayer water desorption from 182 to 207 K, indicating net attractive COH 2 O interactions. Water shifts the CO stretching frequency from 2020 to 1620 cm −1 , suggesting a displacement of CO from atop to three-fold hollow sites. A site shift is corroborated by XPS data. Concurrent changes in water vibrational features suggest formation of a mixed phase in which CO and water occupy adjacent sites. It has been hypothesized that such an adsorption geometry would facilitate the normally slow electrooxidation of CO and of fuels such as methanol. On Pt(111), in contrast, all coverages of CO decrease the desorption temperature of water, indicating net repulsive COH 2 O interactions. Water splits the single 2110 cm −1 CO stretch of low coverages of CO into two peaks corresponding to the atop and bridging species also seen for higher coverages of CO on the water-free surface. This result and the lack of a change in water vibrational features suggest that on Pt(111) the coadsorbates separate into incompressible islands containing only water and compressible, internally repulsive, patches containing only CO. If the disparate behavior of Rh(111) and Pt(111) also occurs in room temperature aqueous solutions, comparison of their electrooxidation activity in the presence of CO could provide a conclusive test of whether or not rational provision of adjacent CO and water binding sites is likely to be a productive strategy for the development of improved electrooxidation catalysts.

A comparison between water adsorbed on Rh(111) and Pt(111), with and without predosed oxygen

The adsorption of water and its interactions with oxygen on Rh(111) were studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD) ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS), and low energy electron diffraction (LEED); and comparison was made with similar data for Pt(111). On Rh(111) water absorbs molecularly in hydrogen-bonded clusters; no evidence for dissociation was seen on the clean surface. Reaction of water with adsorbed oxygen on Rh(111) produces hydrated surface hydroxyls. While the gross features of adsorption and hydroxyl formation are similar to those previously reported on Pt(111), significant differences in detail were found. In particular, the complex librational and OH-stretching regions of the HREELS spectra for H 2 O/Rh(111), more closely resemble those for other noble metal surfaces than the sharp, single feature observed for Pt(111). HREELS peaks at 970, 1020 and 1950 cm −1 seen for H 2 O/Pt(111) were absent on Rh(111). The middle (3a 1 ) molecular orbital for molecular water on Rh(111) is shifted towards the Fermi level, while on Pt(111) the spacing between the three orbitals is the same as in water vapor. Comparison with spectral data for bulk phases suggests that water on Pt(111) exists primarily in a state with O-O nearest neighbor distances closer to those of liquid water than of ice, allowing better match with the Pt(111) surface mesh. Additional minority species account for the additional EELS peaks specific to Pt(111). Water on Rh(111) is a mixture of ice-like water and water similar to the majority species on Pt(111). The structural differences lead to different chemistry. On both surfaces adsorbed oxygen and water react to yield a surface phase which evolves water upon heating to 210 K. On Pt(111) this phase contains OH but no H 2 O. On Rh (111) this phase contains both OH and H 2 O in association. The differences in the interactions between water and the (111) surfaces of these two catalytically and electrochemically similar metals may help explain electrochemical effects peculiar to the (111) face of Pt.

Sol−Gel Synthesis, Electrochemical Characterization, and Stability Testing of Ti0.7W0.3O2 Nanoparticles for Catalyst Support Applications in Proton-Exchange Membrane Fuel Cells

The materials currently used in proton-exchange membrane fuel cells (PEMFCs) require complex control of operating conditions to make them sufficiently durable to permit commercial deployment. One of the major materials challenges to allow simplification of fuel cell operating strategies is the discovery of catalyst supports that are much more stable to oxidative decomposition than currently used carbon blacks. Here we report the synthesis and characterization of Ti(0.7)W(0.3)O2 nanoparticles (approximately 50 nm diameter), a promising doped metal oxide that is a candidate for such a durable catalyst support. The synthesized nanoparticles were platinized, characterized by electrochemical testing, and evaluated for stability under PEMFC and other oxidizing acidic conditions. Ti(0.7)W(0.3)O2 nanoparticles show no evidence of decomposition when heated in a Nafion solution for 3 weeks at 80 °C. In contrast, when heated in sulfuric, nitric, perchloric, or hydrochloric acid, the oxide reacts to form salts such as titanylsulfatehydrate from sulfuric acid. Electrochemical tests show that rates of hydrogen oxidation and oxygen reduction by platinum nanoparticles supported on Ti(0.7)W(0.3)O2 are comparable to those of commercial Pt on carbon black.

Generation of surface hydronium from water and hydrogen coadsorbed on Pt(111)

Abstract The effective electrode potential of an ultrahigh vacuum model of the electrode-electrolyte interface was controlled through coadsorption of varying amounts of hydrogen with water on Pt(111) at 100 K. High resolution electron energy loss spectroscopy (HREELS) demonstrated formation of surface H 3 O + ions upon heating > 20% saturation coverages of hydrogen with water to 150 K, indicating an effective electrode potential below the potential of zero charge (pzc). Comparison of these results with cyclic voltanunetry of well-ordered Pt(111) in weakly-adsorbing electrolytes suggests that the pzc lies around + 0.2 V RHE and that the breadth of the hydrogen regions of the voltammograms is due to potential dependent interactions between coadsorbed H and H 3 O + .

Identification of surface hydronium: Coadsorption of hydrogen fluoride and water on platinum (111)

The adsorption of HF and its coadsorption with water were studied on Pt(111) by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) as a step in the UHV modeling of the acidic aqueous electrolyte/electrode interface. Anhydrous HF adsorbs without dissociation. HF coadsorbed with water reacts to form several phases distinguishable by TPD. HREELS spectra show that the reaction forms the H3O+ ion. The stoichiometries of thermal desorption identify an acid monohydrate phase ([H3O+][F−]) and fully hydrated phases with stoichiometries of HF · 5H2O in the monolayer and HF · 8H2O in the multilayer. To the as-yet-unknown extent that low-temperature measurements are relevant to normal aqueous electrochemistry, these results indicate that even such classically “non-specifically” adsorbed ions as H+ and F− interact sufficiently strongly with Pt surfaces to displace some water from their inner solvation shells. These data also show that Bronsted acid-base chemistry can be carried out and spectroscopically observed in low temperature monolayers in UHV, and point the way towards UHV studies of such pH-dependent phenomena as corrosion and electrocatalysis.

Electrochemically and UHV‐Grown Passive Layers on Ni(100): A Comparison by AC Impedance, XPS, LEED, and HREELS

The electrochemical, spectroscopic, and structural properties of passive layers on Ni(100) grown either electrochemically in aqueous or by dosing with gaseous oxygen in ultrahigh vacuum (UHV) were compared using a UHV‐electrochemical transfer system equipped for x‐ray photoemission spectroscopy (XPS), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). The two films share a similar component of several layers of anhydrous , but the electrochemical layer also contains smaller amounts of additional forms of oxygen. The elements of structural order in the two films differ in epitaxial relationships with the substrate. Similar ac impedance behaviors and dc passive current densities are seen when the clean and UHV‐oxidized Ni(100) surfaces are contacted with electrolyte at the passive potential; but 6 mC/cm2 of anodic charge must be passed to reach a steady passive state on the clean surface, while 0.8 mC/cm2 suffices on the UHV‐oxidized surface. Comparison with the very well‐defined UHV oxide allows improved precision in the characterization of the passive layer.

Hydrogen chloride adsorption and coadsorption with hydrogen or water on platinum (111)

Abstract Aqueous chloride ions accelerate the corrosion of all metals, and chloride chemistry is essential to the preparation of noble metal catalysts. To improve the understanding of the interactions of chloride species with metals, the adsorption of anhydrous HCl on Pt(111) at 90 K and its coadsorption with hydrogen and water were studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Low coverages of HCl fully dissociate to form a disordered mixture of adsorbed H and adsorbed Cl. Higher exposures produce first a well-ordered 3 × 3 phase and then an increasingly disordered form which saturates just above the density of one layer of close-packed Cl; no multilayer of HCl ice can be grown. Coadsorption of HCl and water produces adsorbed H 3 O + . The thermal desorption of water indicates two types of water stabilization by HCl, but there is no evidence for water molecules bound directly to adsorbed Cl. The chemistry of HCl+H 2 O coadsorption, which is dominated by strong PtCl interactions, is contrasted with that of the previously-studied HF+H 2 O system, which is dominated by hydrogen bonding effects.

CO on Pt(335) and Pt(111): Vibrational stark effect and electron energy loss investigation

Abstract An earlier study of CO on the stepped Pt(335) surface found a surprisingly large difference between the Stark tuning rates of CO on the step edge and CO on the (111) terrace. We report results of two experiments designed to clarify the source of this difference. In one experiment IR Spectroscopy in the range 1820–2020 cm −1 was used to measure the Stark tuning rate of CO on the flat Pt(111) surface. The external-field tuning rate of on-top CO at saturation coverage is (2.56 ± 0.12) × 10 −7 cm −1 /(V/cm), a factor two smaller than that of edge CO on Pt(335) at low coverage. The external-field Stark tuning rate of bridge-bonded CO on Pt(111) is about 50% larger than that of on-top CO. If one assumes the static and IR fields are screened equally, the local-field Stark tuning rate of on-top CO is (3.4 ± 0.9) × 10 −7 cm −1 /(V/cm), a factor four smaller than for CO on Pt(111) at the aqueous double layer. One explanation for the smaller than expected Stark tuning rates of CO on Pt in UHV is excess screening of the static electric field. In another experiment, electron energy loss spectroscopy was used to study CO on Pt(335). The spectra show bridge-bonded CO on Pt(335) at saturation coverage and at 40% of saturation. This is the first observation of bridge-bonded CO on Pt(335). A combination overtone and double-loss peak was observed at saturation coverage, with intensity relative to the fundamental of 0.017 ± 0.003, about a factor two larger than observed by Steininger et al. on Pt(111).

A comparison of calorimetric methods applied to the electrolysis of heavy water on palladium cathodes

Abstract The thermal power output of a galvanostatic D 2 O electrolysis cell was determined simultaneously by two methods. The first, water-flow calorimetry, gave results rigorously independent of the nature of the heat source. The second, an isoperibolic method involving measurement of temperatures within the electrolysis cells, gave results in agreement with the first when all factors were accounted for. However, neglect of a drop in effective cell impedance accompanying operation of an in-cell calibration heater, or neglect of effects of the dropping electrolyte level, can produce spurious indications of excess heat. This work demonstrates the need for extreme care in application of isoperibolic methods to electrolytic cells.