Frederick T. Wagner

Truncated Octahedral Pt3Ni Oxygen Reduction Reaction Electrocatalysts

This communication describes the preparation of carbon-supported truncated-octahedral Pt(3)Ni nanoparticle catalysts for the oxygen reduction reaction. Besides the composition, size, and shape controls, this work develops a new butylamine-based surface treatment approach for removing the long-alkane-chain capping agents used in the solution-phase synthesis. These Pt(3)Ni catalysts can have an area-specific activity as high as 850 muA/cm(2)(Pt) at 0.9 V, which is approximately 4 times better than the commercial Pt/C catalyst ( approximately 0.2 mA/cm(2)(Pt) at 0.9 V). The mass activity reached 0.53 A/mg(Pt) at 0.9 V, which is close to a factor of 4 increase in mass activity, the threshold value that allows fuel-cell power trains to become cost-competitive with their internal-combustion counterparts. Our results also show that the mass activities of these carbon-supported Pt(3)Ni nanoparticle catalysts strongly depend on the (111) surface fraction, which validates the results of studies based on Pt(3)Ni extended-single-crystal surfaces, suggesting that further development of catalysts with still higher mass activities is highly plausible.

The Impact of Carbon Stability on PEM Fuel Cell Startup and Shutdown Voltage Degradation

Conventional carbon MEAs and graphitized carbon MEAs were evaluated for the resistance to carbon corrosion and startup/shutdown durability in this paper. Graphitized carbon MEAs show higher resistance to carbon corrosion than conventional carbon MEAs by a factor of 35 at a point where 5% weight loss had occurred. A graphitized carbon MEA yielded lower degradation rate than that of a conventional carbon MEA by a factor of 5 after 1,000 startup/shutdown cycles. The kinetics of carbon corrosion over both conventional carbon MEAs and graphitized carbon MEAs were measured, and carbon corrosion during startup/shutdown was explained and modeled. The model results correlate to what we have measured from our startup/shutdown durability test. Overall, MEAs with corrosion resistant carbon supports are one of major materials approaches to mitigate cell voltage degradation due to fuel cell startup/shutdown. We believe that a combination of corrosion resistant materials and system operating mitigation strategies is the path to attain the strict automotive durability targets.

Atomic-Resolution Spectroscopic Imaging of Ensembles of Nanocatalyst Particles Across the Life of a Fuel Cell

The thousand-fold increase in data-collection speed enabled by aberration-corrected optics allows us to overcome an electron microscopy paradox: how to obtain atomic-resolution chemical structure in individual nanoparticles yet record a statistically significant sample from an inhomogeneous population. This allowed us to map hundreds of Pt-Co nanoparticles to show atomic-scale elemental distributions across different stages of the catalyst aging in a proton-exchange-membrane fuel cell, and relate Pt-shell thickness to treatment, particle size, surface orientation, and ordering.

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.

Artifacts in Measuring Electrode Catalyst Area of Fuel Cells through Cyclic Voltammetry

Cyclic Voltammetry (CV) is a well-established technique to measure the electrochemically active surface area (ECA) of a catalyst in an electrode through hydrogen adsorption/desorption (HAD)[1]. In the field of proton exchange memebrane fuel cells this method can be applied over a wide range of scales ranging from rotating disc electrodes (RDE) to single-cell and even multiplecell stacks where the working electrode is part of a membrane-electrode assembly (MEA). It is important, however, to recognize there are two convoluted processes occurring at low potential (ca. Evs RHE < 0.1 V): HAD and hydrogen evolution.

Catalyst Development Needs and Pathways for Automotive PEM Fuel Cells

Mass production of fuel cell light-duty vehicles at competitive cost requires cathode (oxygen reduction reaction [ORR]) kinetic mass activities 4-fold higher than those of current state-of-the-art platinum / carbon black catalysts. 1 A catalyst-related cell voltage loss less than 50 mV over the entire current density range is sought over a >10 year automotive lifetime including ~300,000 large load cycles and ~30,000 start-stop cycles. While the materials set used in current demonstration vehicles falls short of these goals, pathways to these challenging targets are visible via increased attention to the structural details of Pt-containing multicomponent catalysts and through development of catalyst and support as a single system.