Matthias Arenz

The Particle Size Effect on the Oxygen Reduction Reaction Activity of Pt Catalysts: Influence of Electrolyte and Relation to Single Crystal Models

The influence of particle size on the oxygen reduction reaction (ORR) activity of Pt was examined in three different electrolytes: two acidic solutions, with varying anionic adsorption strength (HClO(4) < H(2)SO(4)); and an alkaline solution (KOH). The experiments show that the absolute ORR rate is dependent on the supporting electrolyte; however, the relationship between activity and particle size is rather independent of the supporting electrolyte. The specific activity (SA) toward the ORR rapidly decreases in the order of polycrystalline Pt > unsupported Pt black particles (~30 nm) > high surface area (HSA) carbon supported Pt nanoparticle catalysts (of various size between 1 and 5 nm). In contrast to previous work, it is highlighted that the difference in SA between the individual HSA carbon supported catalysts (1 to 5 nm) is rather trivial and that the main challenge is to understand the significant differences in SA between the polycrystalline Pt, unsupported Pt particles, and HSA carbon supported Pt catalysts. Finally, a comparison between measured and modeled activities (based on the distribution of surface planes and their SAs) for different particle sizes indicates that such simple models do not capture all aspects of the behavior of HSA carbon supported catalysts.

Adsorbate-Induced Surface Segregation for Core–Shell Nanocatalysts†

Coming to the surface: The surface composition of carbon-supported Pt(3)Co catalyst particles changes upon a CO-annealing treatment. Platinum atoms segregate to the particle surface so that nanoparticles with a platinum shell surrounding an alloy core are formed. This modified catalyst has a superior activity in the oxygen reduction reaction compared to both a plain platinum catalyst and the untreated alloy particles.

Oxygen electrocatalysis in alkaline electrolyte: Pt(hkl) Au(hkl) and the effect of Pd-modification

Abstract The kinetics of the oxygen reduction reaction (ORR) was studied in alkaline electrolyte at 293–333 K on bare and Pd modified Pt(hkl) and Au(hkl) surfaces. The rotating ring-disk electrode technique was used to study the ORR with solution phase peroxide detected at the ring electrode. Pd modification was either by electrodeposition (Pt) or by vapor deposition in vacuum (Au). The surface concentration of Pd was determined in vacuum using low energy ion scattering. In agreement to the structure sensitivity found at room temperature previously, on the bare Au(hkl) surfaces the ORR was found to be strongly structure sensitive in the temperature range from 293 to 333 K, with order of activity being (100)≫(110)>(111). The structure sensitivity for Pt(hkl) is much less and varies in the nearly the opposite order (111)>(100)>(110). The peroxide intermediate pathway is clearly operative on Au(hkl) surfaces. At elevated temperature, significantly smaller amounts of peroxide are formed. The kinetics of the ORR were significantly enhanced by modification of both Pt(hkl) and Au(hkl) surfaces with Pd. The catalytic effect is most pronounced on the surfaces that are less active surfaces in the unmodified state, with enhancement at least an order of magnitude faster kinetics. Pd modification of the Au(hkl) surfaces, therefore, significantly reduces the structure sensitivity of the ORR. Even on the highly active Pt(111) surface the kinetics can be improved by a factor of approximately two to four due to Pd modification. The catalytic enhancement can be achieved with as little as 18 at.% Pd in the Au(hkl) surface.

The effect of particle proximity on the oxygen reduction rate of size-selected platinum clusters

The diminished surface-area-normalized catalytic activity of highly dispersed Pt nanoparticles compared with bulk Pt is particularly intricate, and not yet understood. Here we report on the oxygen reduction reaction (ORR) activity of well-defined, size-selected Pt nanoclusters; a unique approach that allows precise control of both the cluster size and coverage, independently. Our investigations reveal that size-selected Pt nanoclusters can reach extraordinarily high ORR activities, especially in terms of mass-normalized activity, if deposited at high coverage on a glassy carbon substrate. It is observed that the Pt cluster coverage, and hence the interparticle distance, decisively influence the observed catalytic activity and that closely packed assemblies of Pt clusters approach the surface activity of bulk Pt. Our results open up new strategies for the design of catalyst materials that circumvent the detrimental dispersion effect, and may eventually allow the full electrocatalytic potential of Pt nanoclusters to be realized.

The electro-oxidation of formic acid on Pt–Pd single crystal bimetallic surfaces

The interrelationship between the macroscopic kinetic rate of HCOOH oxidation in 0.1 M HClO4 solution and the morphology/composition of the electrode is studied on Pt(111) modified by Pd (denoted hereafter as the Pt(111)–PdxML system, 0 < x < 1) and on Pt–Pd bulk single crystal alloy surfaces (denoted hereafter as the PtPdxat%(111) system, x = 6 and x = 25). The Pd surface composition of the Pt(111)–PdxML and PtPdxat%(111) electrodes was established previously ex-situ by low energy ion scattering (LEIS) measurements. The nature of adsorbed intermediates (COad) and the electrocatalytic properties (the onset of CO2 formation) at the Pt(111)–PdxML and the PtPdxat%(111) interface were studied by FTIR spectroscopy. The results show that Pd atoms either on the surface or in the surface have an unique catalytic activity for HCOOH oxidation, with Pd atoms being three (bulk alloys) or five times (Pd films) more active than Pt atoms at 0.4 V. FTIR spectra reveal that on Pt atoms adsorbed CO is produced from dehydration of HCOOH, whereas no CO adsorbed on Pd can be detected although a high production rate of CO2 is observed at low potentials, indicating that the reaction can proceed on Pd at low potentials without the Pt typical “poison” formation.

Degradation of Carbon-Supported Pt Bimetallic Nanoparticles by Surface Segregation

Surface segregation of the non-noble component of a Pt bimetallic core-shell catalyst can occur even at room temperature under typical fuel cell cathode application conditions. While in an alkaline environment the nanoparticles remain stable, and the alteration in the surface composition can be tracked in situ; in an acidic electrolyte, any non-noble alloying material at the surface would immediately dissolve into the electrolyte. Therefore, such catalysts are expected to degrade steadily during operation in an acidic fuel cell until only Pt is left.

Impact of Glass Corrosion on the Electrocatalysis on Pt Electrodes in Alkaline Electrolyte

The influence of glass corrosion on the electrocatalytic activity of fuel cell catalysts was studied. A Teflon electrochem. cell was designed for measurements in alk. electrolyte. The cell performance was tested and compared to a std. electrochem. glass cell by measuring the O redn. reaction and the H oxidn. reaction on polycryst. Pt in 0.1M KOH. In the Teflon cell the shape of the cyclic voltammogram as well as the activity for the reactions are reproducible and do not alter over a long period of time. By comparison, using the std. electrochem. cell made out of Duran glass, the expts. on polycryst. Pt electrodes in alk. electrolyte are insufficiently reproducible. The cyclic voltammograms alter over time, and the activities for H oxidn. as well as O redn. depend on the applied potential scan limits. This is due to the contamination of the electrolyte because of the etching of glass by KOH - anal. of the alk. electrolyte after usage in the resp. cell types by ICP-OES confirmed this suggestion. [on SciFinder(R)]

IL-TEM investigations on the degradation mechanism of Pt/C electrocatalysts with different carbon supports

Utilizing our recently developed method of identical location transmission electron microscopy (IL-TEM) in combination with electrochemical surface area determination, the degradation behavior of different carbon supported Pt catalysts for polymer electrolyte membrane fuel cells (PEMFCs) is investigated. Two different Pt based catalysts supported on a low surface area (LSA) carbon are compared to a Pt catalyst with standard high surface area (HSA) carbon support. One of the LSA carbon supports is of conventional type, while the other is modified by a transition metal. Relative to the standard, both catalysts with LSA carbon support exhibit improved degradation behavior in terms of loss in active surface area upon accelerated stress tests. The catalyst with transition metal modified carbon support thereby exhibits by far superior improvements. The characterization of the bare carbon supports indicates that the observed differences between both catalysts with LSA carbon support are not related to the resistance of the support to complete oxidation to carbon dioxide. Instead, the IL-TEM results reveal that the improved properties of the catalyst with transition metal modified support are due to a stabilization of the Pt particles attached to the support. Particle detachment thus can be drastically reduced and the degradation is limited to a migration and coalescence or sintering mechanism.

CO adsorption and kinetics on well-characterized Pd films on Pt(111) in alkaline solutions

The electrochemistry of CO on a bare Pt(111) electrode as well as a Pt(111) electrode modified with pseudomorphic thin palladium films has been studied in alkaline solution by means of Fourier transform infrared (FTIR) spectroscopy. First Pd films were prepared and well characterized in UHV and subsequently transferred into the electrochemical cell for the registration of the voltammetric profiles. The charge corresponding to the formation of underpotentially deposited hydrogen (H{sub upd}) on these Pt(111)-xPd surfaces was established in sulfuric acid solution as a function of x (0 {le} x {le} 1 Pd monolayer (ML)). All subsequent measurements were then performed on electrochemically deposited palladium films using the above H{sub upd}-charge vs. Pd coverage relationship to evaluate the amount of electrochemically deposited palladium. FTIR spectra for CO adsorbed on one monolayer and a submonolayer coverage are compared to those of the unmodified Pt(111) surface, all surfaces having identical 2D lattice structures. Infrared absorption bands of CO bound on either Pt(111) or Pt(111)-1ML Pd are clearly distinguished. Spectra of CO adsorbed on Pd submonolayers show characteristic features of both CO bound to Pt and to Pd, indicating that on Pt(111)-xPd surfaces there is no coupling between Pt-CO{sub ad} and Pd-CO{sub ad} molecules. The kinetics of CO oxidation on these surfaces is determined either by rotating disk electrode (RDE) measurements or by FTIR spectroscopy, monitoring the CO{sub 3}{sup 2-} production. The oxidation of CO{sub ad} on Pt(111) and on Pd modified platinum surfaces starts at the same potential, ca. at 0.2 V. The oxidation rate is, however, considerably lower on the Pt(111)-xPd surfaces than on the Pt(111) surface. The kinetics of CO oxidation appears to be determined by the nature of adsorbed hydroxyl anions (OH{sub ad}), which are more strongly (less active) adsorbed on the highly oxophilic Pd atoms.

Fuel cells: Log on for new catalysts

Improvements to the efficiency and lifetime of polymer electrolyte membrane fuel cells can be realized by finding more active and stable electrocatalytic cathode materials. A computational search has found two such alloys and confirmed their enhanced properties experimentally.