Chengfei Yu

Lattice-strain control of the activity in dealloyed core–shell fuel cell catalysts

Electrocatalysis will play a key role in future energy conversion and storage technologies, such as water electrolysers, fuel cells and metal-air batteries. Molecular interactions between chemical reactants and the catalytic surface control the activity and efficiency, and hence need to be optimized; however, generalized experimental strategies to do so are scarce. Here we show how lattice strain can be used experimentally to tune the catalytic activity of dealloyed bimetallic nanoparticles for the oxygen-reduction reaction, a key barrier to the application of fuel cells and metal-air batteries. We demonstrate the core-shell structure of the catalyst and clarify the mechanistic origin of its activity. The platinum-rich shell exhibits compressive strain, which results in a shift of the electronic band structure of platinum and weakening chemisorption of oxygenated species. We combine synthesis, measurements and an understanding of strain from theory to generate a reactivity-strain relationship that provides guidelines for tuning electrocatalytic activity.

Effects of Composition and Annealing Conditions on Catalytic Activities of Dealloyed Pt–Cu Nanoparticle Electrocatalysts for PEMFC

Dealloyed Pt 25 Cu 75 bimetallic nanoparticle electrocatalysts exhibit up to six times higher oxygen reduction reaction activities than pure nanoparticle Pt catalysts at 0.9 V/ reversible hydrogen electrode (RHE). The active form of the catalyst is formed in situ from Pt-Cu precursor material using voltammetric dealloying. The effects of composition of precursors as well as effects of the annealing temperature and duration on the catalyst activity are studied. We vary the composition between Pt 25 Cu 75 and Pt 75 Cu 25 and change the annealing conditions from 600 to 950°C and for 7 and 14 h. X-ray diffraction and electrochemical analyses are used to obtain insight on the structural details of the catalyst samples. Information regarding the extent of alloying, atomic ordering, the Pt and Cu compositions, and distributions on the nanoparticles and particle (crystallite) sizes is correlated with the trends observed from mass and specific activities of the catalysts. It was found that an annealing duration of 14 h offers little or no benefit to catalytic activities compared to 7 h. Dealloyed Pt 25 Cu 75 annealed for 7 h, at 800°C yielded an optimal active material with respect to the extent of alloying and particle size growth and exhibited the highest Pt mass-based and favorable specific catalytic oxygen reduction reaction (ORR) activity. The occurrence and role of a noncubic Pt 50 Cu 50 Hongshiite phase is discussed.

Synthesis, Dealloying, and ORR Electrocatalysis of PDDA-Stabilized Cu-Rich Pt Alloy Nanoparticles

We report on the polymeric surfactant-assisted synthesis and characterization of highly dispersed, uniformly alloyed Cu-rich Pt-Cu bimetallic nanoparticles with Cu contents up to 75 atom % Cu for use as an oxygen reduction reaction (ORR) electrocatalyst at polymer electrolyte membrane fuel cells. Base-metal-rich Pt alloy particles are generally very difficult to prepare as a well-alloyed single-phase material with high dispersion using conventional liquid impregnation/reductive annealing routes. A comparison between the characteristics of Pt-Cu alloy particle obtained from a poly(dimethyl-diallyl ammonium) chloride (PDDA) assisted polyol process and a conventional impregnation method shows that the polyol process is able to form single-phase nanoparticles with a very narrow particle size at temperatures below 200°C. We further investigated the electrochemical behavior of Cu-rich electrocatalysts. We demonstrate the formation of highly active ORR Pt-Cu catalyst phases from a PtCu 3 precursor by selective electrochemical dissolution (dealloying) of Cu. The dealloyed catalyst yields ORR surface-area-specific activities rivaling those of the Pt-rich state-of-the-art Pt 3 Co electrocatalysts. Based on the severe depletion in Cu near the surface combined with moderate surface area increases, we propose geometric rather than electronic or surface-area effects as the origin of the observed activity enhancement.

Size and composition distribution dynamics of alloy nanoparticle electrocatalysts probed by anomalous small angle X-ray scattering (ASAXS)

Anomalous small angle X-ray scattering (ASAXS) is shown to be an ideal technique to investigate the particle size and particle composition dynamics of carbon-supported alloy nanoparticle electrocatalysts at the atomic scale. In this technique, SAXS data are obtained at different X-ray energies close to a metal absorption edge, where the metal scattering strength changes, providing element specificity. ASAXS is used to, first, establish relationships between annealing temperature and the resulting particle size distribution for Pt25Cu75 alloy nanoparticle electrocatalyst precursors. The Pt specific ASAXS profiles were fitted with log-normal distributions. High annealing temperatures during alloy synthesis caused a significant shift in the alloy particle size distribution towards larger particle diameters. Second, ASAXS was used to characterize electrochemical Cu dissolution and dealloying processes of a carbon-supported Pt25Cu75 electrocatalyst precursor in acidic electrolytes. By performing ASAXS at both the Pt and Cu absorption edges, the unique power of this technique is demonstrated for probing composition dynamics at the atomic scale. These ASAXS measurements provided detailed information on the changes in the size distribution function of the Pt atoms and Cu atoms. A shift in the Cu scattering profile towards larger scattering vectors indicated the removal of Cu atoms from the alloy particle surface suggesting the formation of a Pt enriched Pt shell surrounding a Pt-Cu core. Together with XRD and TEM, ASAXS is proposed to play an increasingly important role in the mechanistic study of degradation phenomena of alloy nanoparticle electrocatalysts at the atomic scale.

Growth Trajectories and Coarsening Mechanisms of Metal Nanoparticle Electrocatalysts

Functional ensembles of nanoscale dimension particles are of great importance in many areas of science and technology; for example, in the field of catalysis. The structural properties of an individual nanoparticle, such as particle size or shape, closely correlate with its catalytic activity. Within a particle ensemble, structural properties can be distributed over a range of values, making the observed catalytic activity a complex convolution of individual contributions. This complexity is why a fundamental understanding of structure–activity relationships of particle ensembles requires a statistically reliable and accurate measurement of the distribution of relevant particle properties. Understanding the underlying atomic-scale mechanisms and processes that cause performance degradation of supported, catalytically active Pt nanoparticles is of utmost importance for the design of durable electrocatalytic energy conversion devices, such as polymer electrolyte membrane fuel cells (PEMFCs). Electrode performance depends largely on the total available electrochemically active Pt particle surface area (ECSA), which in turn depends on the size of the catalytically active particles. Hence, an accurate experimental description of the time changes of the Pt particle size distribution (PSD) during electrochemical operation is a key requirement to better understand how and why distributed Pt particle electrodes loses active surface area and activity. A common method to measure PSDs of electrocatalysts involves direct space TEMderived histograms. TEM, however, is an invasive analytical method, which makes in situ observations of PSD evolutions difficult. Previous work has identified a small number of basic mechanisms that control active surface-area loss under electrochemical conditions: first, Pt dissolution off the conductive support (dissolution mass loss mechanism); second, Pt particle Ostwald ripening (Ostwald coarsening mechanism); third, active area decrease attributed to particle migration followed by particle coalescence (coalescence coarsening mechanism) or else, fourth, Pt surface area loss owing to support corrosion leading to the detachment of Pt particles (detachment mass loss mechanism). Accurate discrimination between competing particle growth mechanisms, such as Ostwald ripening and migration/coalescence has been of great interest early on. Wynblatt and Gjostein derived detailed kinetic expressions for Ostwald and coalescence growth, and showed differences in the expected kinetic exponent. Unfortunately, the experimental errors in the particle growth trajectories often limited a reliable conclusion as to the dominant growth mechanism. Granqvist and Buhrmann claimed that the shape of the PSD can provide clues as to the growth mechanism. A more detailed understanding of the role of PSD in ECSA loss and a clarification of which coarsening and loss mechanisms are mainly affecting the PSD in the course of electrochemical treatment has become a recent focus of research. Building on early approaches ever more sophisticated Pt coarsening models have been proposed to provide insight in experimental changes in PSD and concomitant ECSA losses. Since modeling of TEM-derived data is often limited to the initial and final PSD data, X-ray derived PSD data is an interesting alternative. However, combined experimental and modeling approaches have been missing. Small angle X-ray scattering (SAXS) can be used to noninvasive measure the time evolution of Pt nanoparticle PSDs. It can be applied in situ and provides statistically reliable average structural information of nanoscale objects in the 1– 100 nm range. Haubold et al. pioneered the use of SAXS to investigate growth and transformations of carbon-supported Pt nanoparticle electrocatalysts. Later, Stevens and Dahn used ex situ SAXS to determine Pt PSDs. More recently, anomalous SAXS was used to study structural details of bimetallic core-shell electrocatalysts, whereas in situ SAXS studies revealed a time-resolved look at Pt particle growth under potential cycling. Here, we present for the first time a comprehensive computational and experimental approach to investigate and understand the structural stability of Pt nanoparticle electrocatalysts through in situ time-resolved measurements and modeling. In situ SAXS is used to measure changes in the PSD (growth trajectories) under electrochemical conditioning mimicking a PEMFC cathode. The PSD trajectories are compared to computational predictions using particle dissolution/coarsening model. Variation of the initial PSD and the applied electrode [a] Dr. C. Yu Department of Chemical and Biomolecular Engineering University of Houston Houston, TX 77204 (USA) [b] Dr. E. F. Holby, Prof. D. Morgan Department of Materials Science and Engineering University of Wisconsin Madison, WI 53706 (USA) [c] Dr. R. Yang, Dr. M. F. Toney Stanford Institute of Materials and Energy Sciences and Stanford Synchrotron Radiation Lightsource SLAC National Accelerator Laboratory Menlo Park, CA 94025 (USA) [d] Prof. Dr. P. Strasser Department of Chemistry Chemical Engineering Division Technical University Berlin 10623 Berlin (Germany) E-mail : pstrasser@tu-berlin.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200090.

Effects of Annealing Conditions on Catalytic Activities of Pt-Cu Nanoparticle Electrocatalysts for PEM Fuel Cells

Dealloyed Pt25Cu75 bimetallic nanoparticle electrocatalysts exhibit up to 6 times higher oxygen reduction reaction activities than Pt-C catalysts. The effects of annealing temperature and duration on the catalyst activity are studied, with annealing temperature varied from 600 {degree sign}C to 950 {degree sign}C and for 7h and 14 h. XRD and electrochemical analyses is used to obtain insight to the structural details of the catalyst samples. Information regarding extend of alloying, Pt and Cu compositions and distributions on the nanoparticles and particle (crystallite) sizes is correlated with the trends observed from mass and specific activities of the catalysts. It is found that annealing duration of 14 h offers little or no benefit to catalytic activities compared to 7 h. Pt25Cu75 annealed for 7 h, at 800 {degree sign}C was found as an optimal compromise between the extend of alloying and particle size growth to give the highest catalytic activity.