Sagar Prabhudev

Strained Lattice with Persistent Atomic Order in Pt3Fe2 Intermetallic Core–Shell Nanocatalysts

Fine-tuning nanocatalysts to enhance their catalytic activity and durability is crucial to commercialize proton exchange membrane fuel cells. The structural ordering and time evolution of ordered Pt3Fe2 intermetallic core-shell nanocatalysts for the oxygen reduction reaction that exhibit increased mass activity (228%) and an enhanced catalytic activity (155%) compared to Pt/C has been quantified using aberration-corrected scanning transmission electron microscopy. These catalysts were found to exhibit a static core-dynamic shell regime wherein, despite treating over 10,000 cycles, there is negligible decrease (9%) in catalytic activity and the ordered Pt3Fe2 core remained virtually intact while the Pt shell suffered a continuous enrichment. The existence of this regime was further confirmed by X-ray diffraction and the compositional analyses using energy-dispersive spectroscopy. With atomic-scale two-dimensional (2-D) surface relaxation mapping, we demonstrate that the Pt atoms on the surface are slightly relaxed with respect to bulk. The cycled nanocatalysts were found to exhibit a greater surface relaxation compared to noncycled catalysts. With 2-D lattice strain mapping, we show that the particle was about -3% strained with respect to pure Pt. While the observed enhancement in their activity is ascribed to such a strained lattice, our findings on the degradation kinetics establish that their extended catalytic durability is attributable to a sustained atomic order.

Synthesis and structural evolution of Pt nanotubular skeletons: revealing the source of the instability of nanostructured electrocatalysts

The durability of Pt catalysts in polymer membrane electrolyte fuel cells (PEMFCs) is critical to successful implementation of this clean energy technology in transportation and stationary applications. Despite reports on a variety of Pt nanostructures with improved durability, a clear understanding of the structure–durability relationship is still missing as a rigorous correlative investigation is lacking. Using five-fold twinned Pd nanowires as templates, we have prepared Pt nanotubular skeleton structures which duplicate the crystal structure of the template. By comparing the durability of these structures with commercial Pt/C catalysts and Pt nanotubes, we found that grain boundaries in Pt nanocatalysts contribute most to the instability of the catalyst structures. These results not only highlight a strategy to prepare more durable fuel cell catalysts, but also provide a new method to prepare elongated Pt catalysts with controlled crystal structures, which together will perpetuate the commercial success of this green energy conversion technology.

Enhanced figure of merit in Mg2Si0.877Ge0.1Bi0.023/multi wall carbon nanotube nanocomposites

The effect of multi wall carbon nanotubes (CNT) on the thermoelectric properties of Mg2Si0.877Ge0.1Bi0.023 was examined. While introducing CNTs increases the electrical conductivity from around 450 Ω−1 cm−1 to 500 Ω−1 cm−1 at 323 K, the increase is neutralized at higher temperature, with the conductivity resulting to be 440 Ω−1 cm−1–470 Ω−1 cm−1 at 773 K. The Seebeck coefficient of all nanocomposites is enhanced at 773 K due to energy filtering that stems from the introduction of CNTs–Mg2Si0.877Ge0.1Bi0.023 interfaces. The combined effect of CNTs on Seebeck coefficient and electrical conductivity leads to an approximately 20% power factor improvement, with the best sample reaching a maximum value of ∼19 μW cm−1 K−2 at 773 K. The lattice thermal conductivity of the nanocomposites is reduced due to the phonon scattering by nanodomains and grain boundaries, particularly at medium temperatures, resulting in a slight reduction in total thermal conductivity. According to high resolution transmission electron microscopy studies, bismuth is homogenously distributed within the grains, while germanium is accumulated at the grain boundaries. All in all, the enhanced thermoelectric figure of merit of 0.67 at 773 K for the sample containing 0.5 weight% MWCNT as compared to 0.55 for the pristine sample, demonstrates the promising effect of CNTs on the thermoelectric properties of Mg2Si0.877Ge0.1Bi0.023.

Surface Segregation of Fe in Pt–Fe Alloy Nanoparticles: Its Precedence and Effect on the Ordered‐Phase Evolution during Thermal Annealing

Coupling electron microscopy techniques with in situ heating ability allows us to study phase transformations on the single‐nanoparticle level. We exploit this setup to study disorder‐to‐order transformation of Pt–Fe alloy nanoparticles, a material that is of great interest to fuel‐cell electrocatalysis and ultrahigh density information storage. In contrast to earlier reports, we show that Fe (instead of Pt) segregates towards the particle surface during annealing and forms a Fe‐rich FeOx outer shell over the alloy core. By combining both ex situ and in situ approaches to probe the interplay between ordering and surface‐segregation phenomena, we illustrate that the surface segregation of Fe precedes the ordering process and affects the ordered phase evolution dramatically. We show that the ordering initiates preferably at the pre‐existent Fe‐rich shell than the particle core. While the material‐specific findings from this study open interesting perspectives towards a controlled phase evolution of Pt–Fe nanoalloys, the characterization methodologies described are general and should prove useful to probing a wide‐range of nanomaterials.

Bulk Immiscibility at the Edge of the Nanoscale

In the quest to identify more effective catalyst nanoparticles for many industrially important applications, the Au-Pt system has gathered considerable attention. Despite considerable effort the interplay between phase equilibrium behavior and surface segregation in Au-Pt nanoparticles is still poorly understood. Here we investigate the phase equilibrium behavior of 20 nm Au-Pt nanoparticles using a combination of high-resolution scanning transmission electron microscopy and a hybrid Monte Carlo and molecular dynamics atomistic simulation technique. Our approach takes into account the effects of immiscibility, elastic strain, interfacial free energy, and surface segregation. This is used to explain two key phenomena taking place in these nanoparticles. The first is whether the binary system remains immiscible at the nanoscale, and if so what morphology would the secondary phase take. Our findings suggest that even at sizes of 20 nm, thermally equilibrated Au-Pt nanoparticles remain largely immiscible and behave thermodynamically as bulk-like systems. We explain why 20 nm Au-Pt nanoparticles phase separate into hemispheres as opposed to a thick-shelled core-shell structure. These insights are central to further optimization of Au-Pt nanoparticles toward enhanced catalytic activities. The phase-separated Janus particles observed in this study offer enhanced material functionality arising from the nonuniformity of their plasmonic, catalytic, and surface properties.

Microscopy and Spectroscopy of Catalysts and Energy Storage Materials

Electron microscopy has always played an important role in the development and the understanding of new materials. In the last ten years there have been significant advancements in instrumentation, enabling improved studies of materials at the nanoscale. In the area of catalysts and energy storage materials, detailed microscopy of material structure, composition and bonding at the nanometer length scale are needed to optimize material properties and performance. Here we summarize recent examples of work related to the study of nano-alloy catalysts used in proton exchange membrane fuel cells and commercial Li ion battery materials, illustrating the crucial role of imaging and spectroscopy for the characterization of these materials.

Atomic Resolution Imaging and Spectroscopy of Pt-alloy Electrocatalytic Nanoparticles

Platinum-alloy nanoparticles (Pt-alloy) are a system of great interest for the fuel cell electrocatalysis. The cathodic reaction (called the oxygen reduction reaction (ORR)) in a typical polymer electrolyte membrane (PEM) fuel cell is notoriously sluggish owing to its multi-electron pathway, requiring efficient catalysts to accelerate the kinetics. Traditionally, Pt was being used in this regard, but despite its exceptional catalytic activity it is now widely accepted to be an unrealistic choice due to depleting sources and exceedingly high costs. In an ongoing attempt to reduce the mass loading of Pt, there has been a tremendous research till date, constantly suggesting that a nanoscale alloying of Pt with 3d transition metals (TM) and the Pt group metals (PGM) is a more viable option. Fine– tuning the surface-structure during synthesis and phase transformations can enhance their catalytic activity and durability by manifolds. However, control and feedback on the synthesis demand characterization techniques that provide atomic-resolution chemical information. The recent developments in electron microscopy pertaining to its instrumentation (aberration correctors, ultrafast electron energy loss spectrometers, monochromators) and the innovation with respect to specimen holders (in situ liquid cell, in situ heating, tomography) have had significant benefits on the study of catalyst nanoparticles by providing chemical analysis and imaging on the atomic-scale [1]. This report describes an overview of recent examples and insightful novel findings related to the study of various catalyst nanoparticles, including Pt-Fe [2], Pt-Au and Pt-Au-Co [3] systems where aberration correction has played a major role in understanding the structures of these materials. Discussed below are two highlights central to our present work.

Machine-Learning Aided Evolution Studies of Nano-composite Electrodes and Nano-particle Catalysts for Fuel Cell Applications

Automotive vehicles powered by proton exchange membrane fuel cells are approaching commercial viability but further improvements are necessary for a practical technology that can be mass-produced cost effectively. The bottleneck limiting their commercialization is the notorious cathodic cell reaction, called the oxygen reduction reaction (ORR), which is a multi-electron pathway reaction with inherently sluggish kinetics. Platinum (Pt) has been used as a catalyst to accelerate the ORR, but despite its exceptional catalytic activity, it is cost prohibitive for commercialization. The search for more affordable fuel cell cathode materials has focused on controlling the surface structure and composition of novel multi-metallic catalytic nanoparticles on high surface area support membranes. However, such nano-structured heterogeneous systems are notoriously challenging to characterize. In addition to difficulties associated with interpreting spatially and spectrally overlapping analytical signals, samples can be beam-sensitive, so care must be taken to limit the beam dose.

Studying tomorrow's materials today: Insights with quantitative STEM, EELS

The development of aberration correctors for the scanning transmission electron microscope has revolutionized the field of electron microscopy and dramatically improved the analytical “toolkit” of materials scientists. In particular, when combined with electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM) makes it possible to detect compositional and spectroscopic changes at the atomic level that can be used to understand the structure, and ultimately the performance of materials. Here we present some examples of quantitative STEM and EELS as applied to the study of graphene-based materials, complex nanoparticles used in electrocatalysts for fuel cells, group IIInitride nanowires used for light emitting devices, and the defects generated in implanted Si.