Shirlaine Koh

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.

Dealloyed Pt Nanoparticle Fuel Cell Electrocatalysts: Stability and Aging Study of Catalyst Powders, Thin Films, and Inks

Dealloyed Pt-Cu alloy nanoparticles are active oxygen reduction electrocatalysts; they are formed from Cu-rich alloy precursors during a selective Cu atom dissolution (dealloying) process. The surface of Cu-rich particle precursors is prone to oxidation under ambient air conditions, which may critically affect the aging behavior of the precursors. Here, we present a systematic stability and aging study of a carbon-supported Pt 25 Cu 75 alloy nanoparticle catalyst precursor. We study the impact of the aging of the catalyst material on its electrocatalytic performance for the oxygen reduction reaction (ORR) after dealloying. We obtain a practical insight into the electrochemical behavior of the materials in the formats of powders, inks, and films. Our studies suggest that the Pt-Cu precursors show a stable catalytic performance when aged as dry powders in air. Ink samples, however, reach their maximum ORR activity of up to 1.3 A/mg Pt , with aging for 24-48 h after which they deteriorated in performance. Finally, catalyst thin films were the most sensitive to aging in air and generally deteriorated rapidly after just one day. Our results provide practical insights and guidelines regarding the stability and handling of the nanoparticle catalyst powder.

Complementarity between high-energy photoelectron and L-edge spectroscopy for probing the electronic structure of 5d transition metal catalysts

We demonstrate the successful use of hard X-ray photoelectron spectroscopy (HAXPES) for selectively probing the platinum partial d-density of states (DOS) in a Pt-Cu nanoparticle catalyst which shows activity superior to pure Pt towards the oxygen-reduction reaction (ORR). The information about occupied Pt d-band states was complemented by Pt L(2)-edge X-ray absorption near-edge spectroscopy (XANES), which probes unoccupied valence states. We found a significant electronic perturbation of the Pt projected d-DOS which was narrowed and shifted to higher binding energy compared to pure platinum. The effect of this electronic structure perturbation on the chemical properties of the nanoparticle surface is discussed in terms of the d-band model. We have thereby demonstrated that the combination of L-edge spectroscopy and HAXPES allows for an experimental derivation of the valence electronic structure in an element-specific way for 5d metal catalysts.

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.