Bridgid N. Wanjala

Nanostructured catalysts in fuel cells

One of the most important challenges for the ultimate commercialization of fuel cells is the preparation of active, robust, and low-cost catalysts. This review highlights some findings of our investigations in the last few years in developing advanced approaches to nanostructured catalysts that address this challenge. Emphasis is placed on nanoengineering-based fabrication, processing, and characterization of multimetallic nanoparticles with controllable size (1-10 nm), shape, composition (e.g. Ml(n)M2(100-n), M1(n)M2(m)M3(100-n-m), M1@M2, where M (1 or 2) = Pt, Co, Ni, V, Fe, Cu, Pd, W, Ag, Au etc) and morphology (e.g. alloy, core@shell etc). In addition to an overview of the fundamental issues and the recent progress in fuel cell catalysts, results from evaluations of the electrocatalytic performance of nanoengineered catalysts in fuel cell reactions are discussed. This approach differs from other traditional approaches to the preparation of supported catalysts in the ability to control the particle size, composition, phase, and surface properties. An understanding of how the nanoscale properties of the multimetallic nanoparticles differ from their bulk-scale counterparts, and how the interaction between the nanoparticles and the support materials relates to the size sintering or evolution in the thermal activation process, is also discussed. The fact that the bimetallic gold-platinum nanoparticle system displays a single-phase character different from the miscibility gap known for its bulk-scale counterpart serves as an important indication of the nanoscale manipulation of the structural properties, which is useful for refining the design and preparation of the bimetallic catalysts. The insight gained from probing how nanoparticle-nanoparticle and nanoparticle-substrate interactions relate to the size evolution in the activation process of nanoparticles on planar substrates serves as an important guiding principle in the control of nanoparticle sintering on different support materials. The fact that some of the trimetallic nanoparticle catalysts (e.g. PtVFe or PtNiFe) exhibit electrocatalytic activities in fuel cell reactions which are four-five times higher than in pure Pt catalysts constitutes the basis for further exploration of a variety of multimetallic combinations. The fundamental insights into the control of nanoscale alloy, composition, and core-shell structures have important implications in identifying nanostructured fuel cell catalysts with an optimized balance of catalytic activity and stability.

Fuel cell technology: nano-engineered multimetallic catalysts

Fuel cells represent an attractive technology for tomorrows energy vector because hydrogen is an efficient fuel and environmentally clean, but one of the important challenges for fuel cell commercialization is the preparation of active, robust and low-cost catalysts. The synthesis and processing of molecularly-capped multimetallic nanoparticles, as described in this report, serves as an intriguing way to address this challenge. Such nanoparticles are exploited as building blocks for engineering the nanoscale catalytic materials by taking advantage of diverse attributes, including monodispersity, processability, solubility, stability, capability in terms of size, shape, composition and surface properties. This article discusses recent findings of our investigations of the synthesis and processing of nanostructured catalysts with controlled size, composition, and surface properties by highlighting a few examples of bimetallic/trimetallic nanoparticles and supported catalysts for electrocatalytic oxygen reduction.

Gold-platinum nanoparticles: alloying and phase segregation

The ability to control nanoscale alloying and phase segregation properties is important for the exploration of multimetallic nanoparticles for the design of advanced functional materials and catalysts. This report highlights recent insights into the nanoscale phase properties of gold-platinum (AuPt) nanoparticles, which serves as an example to shine a light on the importance of changes in physical and chemical properties in which nanoscale multimetallic materials may differ from their bulk counterparts. In contrast to the wide miscibility gap well known for the bulk gold-platinum system, the bimetallic nanoparticles have been demonstrated to exist in phases ranging from alloy, partial alloy, to phase segregation depending on the preparation conditions, the bimetallic composition, and the supporting materials. For AuPt nanoparticles supported on carbon materials, the nanoscale alloying or phase segregation is shown to be controllable by thermal treatment temperatures, which is not only evidenced by detailed analysis of the phase and surface properties, but also supported by theoretical modeling based on thermodynamic and density function theory. The understanding of the nanoscale phase properties can be correlated with the electrocatalytic activities for fuel cell reactions such as methanol oxidation reaction and oxygen reduction reaction. Implications of the new insights to designing and nanoengineering the phase properties of multimetallic nanoparticles and catalysts are also briefly discussed.

Pt-Au alloying at the nanoscale.

The formation of nanosized alloys between a pair of elements, which are largely immiscible in bulk, is examined in the archetypical case of Pt and Au. Element specific resonant high-energy X-ray diffraction experiments coupled to atomic pair distribution functions analysis and computer simulations prove the formation of Pt-Au alloys in particles less than 10 nm in size. In the alloys, Au-Au and Pt-Pt bond lengths differing in 0.1 Å are present leading to extra structural distortions as compared to pure Pt and Au particles. The alloys are found to be stable over a wide range of Pt-Au compositions and temperatures contrary to what current theory predicts. The alloy-type structure of Pt-Au nanoparticles comes along with a high catalytic activity for electrooxidation of methanol making an excellent example of the synergistic effect of alloying at the nanoscale on functional properties.

Role of Support-Nanoalloy Interactions in the Atomic-Scale Structural and Chemical Ordering for Tuning Catalytic Sites

The understanding of the atomic-scale structural and chemical ordering in supported nanosized alloy particles is fundamental for achieving active catalysts by design. This report shows how such knowledge can be obtained by a combination of techniques including X-ray photoelectron spectroscopy and synchrotron radiation based X-ray fine structure absorption spectroscopy and high-energy X-ray diffraction coupled to atomic pair distribution function analysis, and how the support-nanoalloy interaction influences the catalytic activity of ternary nanoalloy (platinum-nickel-cobalt) particles on three different supports: carbon, silica, and titania. The reaction of carbon monoxide with oxygen is employed as a probe to the catalytic activity. The thermochemical processing of this ternary composition, in combination with the different support materials, is demonstrated to be capable of fine-tuning the catalytic activity and stability. The support-nanoalloy interaction is shown to influence structural and chemical ordering in the nanoparticles, leading to support-tunable active sites on the nanoalloys for oxygen activation in the catalytic oxidation of carbon monoxide. A nickel/cobalt-tuned catalytic site on the surface of nanoalloy is revealed for oxygen activation, which differs from the traditional oxygen-activation sites known for oxide-supported noble metal catalysts. The discovery of such support-nanoalloy interaction-enabled oxygen-activation sites introduces a very promising strategy for designing active catalysts in heterogeneous catalysis.

Correlation between Atomic Coordination Structure and Enhanced Electrocatalytic Activity for Trimetallic Alloy Catalysts

This Article describes findings of the correlation between the atomic scale structure and the electrocatalytic performance of nanoengineered PtNiFe/C catalysts treated at different temperatures for oxygen reduction reaction, aiming at providing a new fundamental insight into the role of the detailed atomic alloying and interaction structures of the catalysts in fuel cell reactions. Both mass and specific activities of the catalysts were determined using rotating disk electrode and proton exchange membrane fuel cell. The mass activities extracted from the kinetic regions in both measurements revealed a consistent trend of decreasing activity with increasing temperature. However, the specific activity data from RDE revealed an opposite trend, that is, increasing activity with increasing temperature. In addition to TEM, XRD, and XPS characterizations, a detailed XAFS analysis of the atomic scale coordination structures was carried out, revealing increased heteroatomic coordination with improved alloying structures for the catalyst treated at the elevated temperatures. XPS analysis has further revealed a reduced surface concentration of Pt for the catalyst for the high temperature treated catalyst. The higher mass activity for the lower temperature treated catalyst is due to Pt surface enrichment on the surface sites, whereas the higher specific activity for the higher temperature treated catalyst reflects an enhanced Pt-alloying surface sites. These findings have thus provided a new insight for assessing the structural correlation of the electrocatalytic activity with the fcc-type lattice change and the atomic scale alloying characteristics. Implications of these findings to the design of highly active alloy electrocatalysts are discussed, along with their enhanced electrocatalytic performance in the fuel cell.

Core-Shell Structured Magnetic Ternary Nanocubes

We report a novel core-shell-structured ternary nanocube of MnZn ferrite synthesized by controlling the reaction temperature and composition in the absence of conventionally used reducing agents. The highly monodispersed core-shell structure consists of an Fe(3)O(4) core and an MnZn Ferrite shell. The observation of a Moiré pattern indicates that the core and the shell are two highly crystalline materials with slightly different lattice constants that are rotated relative to each other by a small angle. The ternary core-shell nanocubes display magnetic properties regulated by a combination of the core-shell composition and exhibit an increased coercivity and field-cooled/zero-field-cooled characteristics drastically different from those of regular MnZn ferrite nanoparticles. The ability to engineer the spatial nanostructures of ternary magnetic nanoparticles in terms of shape and composition offers atomic-level versatility in fine-tuning the nanoscale magnetic properties.

Nanoscale alloying effect of gold–platinum nanoparticles as cathode catalysts on the performance of a rechargeable lithium–oxygen battery

The understanding of nanoscale alloying or the phase segregation effect of alloy nanoparticles on the catalytic properties is important for a rational design of the desired catalysts for a specific reaction. This paper describes findings of an investigation into this type of structural effect for carbon-supported bimetallic gold-platinum nanoparticles as cathode catalysts in a rechargeable lithium-oxygen battery. The nanoscale structural characteristics in terms of size, alloying and phase segregation were shown to affect the catalytic properties of the catalysts in the Li-O(2) battery. In addition to the composition effect, the catalysts with a fully alloyed phase structure were found to exhibit a smaller discharge-charge voltage difference and a higher discharge capacity than those with a partial phase segregation structure. This finding is significant for the design of alloy nanoparticles as air cathode catalysts in rechargeable lithium-air batteries, demonstrating the importance of the control of the nanoscale composition and phase properties.

Nanoengineered PtVFe/C Cathode Electrocatalysts in PEM Fuel Cells: Catalyst Activity and Stability

The understanding of factors controlling electrocatalytic activity and stability of carbon‐supported multimetallic catalysts is essential for advancing the design of fuel‐cell electrocatalysts for oxygen reduction. The structural and compositional changes of trimetallic PtVFe/C catalysts have been investigated by several techniques, including XRD, X‐ray absorption fine structure (XAFS), and inductively coupled plasma (ICP) analyses, in addition to electrochemical and fuel‐cell performance tests. The investigation aims at understanding whether the base metals incorporated into the multimetallic catalysts are stable and how the base metals in the Pt‐alloy are stabilized mechanistically. Changes are detected for the lattice properties and the composition of the base metals in the PtVFe nanoparticles after long‐term exposure to air, after thermal treatment at different temperatures, and after use in proton‐exchange membrane (PEM) fuel cells. These changes show certain correlations with the structure, activity, and stability of the catalysts. The fcc alloy phase is found to be predominant for catalysts treated at low temperature, but changes to a tetragonal‐type alloy phase when the catalysts are treated at a higher temperature. In the low‐temperature‐treated catalyst, Fe has more bonds with O atoms, whereas in the high‐temperature‐treated catalyst, Fe has fewer O neighbors and FeFe bonds are detected. Analysis of the catalysts after testing in fuel cells indicates that the percentage of iron oxide is reduced, reflecting the propensity of dissolution of Fe oxide in the electrolytes. The smaller lattice parameter for the higher‐temperature treated catalyst is considered to be an important factor in determining the catalyst stability under conditions related to fuel‐cell operation.

An in situ real-time x-ray diffraction study of phase segregation in Au–Pt nanoparticles

In situ real-time x-ray diffraction was used to study phase segregation and coarsening of Au-Pt nanoparticles supported on silica powder, and porous alumina membranes. Contrary to the expectations from the bulk phase diagram, silica supported Au-Pt nanoparticles have an alloyed structure that is preserved even after extensive annealing at temperatures as high at 700 degrees C. In stark contrast, alumina supported Au-Pt nanoparticles exhibit a rich phase behaviour that is sensitive to alloy composition and the details of the synthesis process. In particular, low-density as-prepared Au(41)Pt(59) nanoparticles exhibit the signature of incipient phase segregation that develops into full phase separation during annealing at high temperature.