Synthesis and characterization of precursor derived TiN@Si–Al–C–N ceramic nanocomposites for oxygen reduction reaction

The development of efficient and durable catalysts is critical for the commercialization of fuel cells, as the catalysts’ durability and reactivity dictate their ultimate lifetime and activity. In this work, amorphous silicon-based ceramics (Si–C–N and Si–Al–C–N) and TiN@Si–Al–C–N nanocomposites were developed using a precursor derived ceramics approach. In TiN@Si–Al–C–N nanocomposites, TiN nanocrystals (with sizes in the range of 5–12 nm) were effectively anchored on an amorphous Si–Al–C–N support. The nanocomposites were found to be mesoporous in nature and exhibited a surface area as high as 132 m²/g. The average pore size of the nanocomposites was found to increase with an increase in the pyrolysis temperature, and a subsequent graphitization of free carbon was observed as revealed from the Raman spectra. The ceramics were investigated for electrocatalytic activity toward the oxygen reduction reaction using the rotating disk electrode method. The TiN@Si–Al–C–N nanocomposites showed an onset potential of 0.7 V versus reversible hydrogen electrode for oxygen reduction, which seems to indicate a 4-electron pathway at the pyrolysis temperature of 1000°C in contrast to a 2-electron pathway exhibited by the nanocomposites pyrolyzed at 750°C via the Koutecky–Levich plot.     

Low-temperature phase and morphology transformations in noble metal nanocatalysts

In situ real-time x-ray diffraction was used to study temperature-induced structural changes of 1-5 nm Au, Pt, and AuPt nanocatalysts supported on silicon substrates. Synchrotron-based x-ray diffraction indicates that the as-synthesized Au and Au(64)Pt(36) nanoparticles have a non-crystalline structure, while the Pt nanoparticles have the expected cubic structure. The nanoparticles undergo dramatic structural changes at temperatures as low as 120?°C. During low-temperature annealing, the Au and AuPt nanoparticles first melt and then immediately coalesce to form 4-5 nm crystalline structures. The Pt nanoparticles also aggregate but with limited intermediate melting. The detailed mechanisms of nucleation and growth, though, are quite different for the three types of nanoparticles. Most interestingly, solidification of high-density AuPt nanoparticles involves an unusual transient morphological transformation that affects only the surface of the particles. AuPt nanoparticles on silicon undergo partial phase segregation only upon annealing at extremely high temperatures (800?°C).     

Proton exchange membrane fuel cells with nanoengineered AuPt catalysts at the cathode

The report describes the findings of an investigation of nanoengineered gold-platinum (AuPt) catalysts in proton exchange membrane fuel cells. The membrane electrode assembly was prepared using carbon-supported Au_nPt_100−n nanoparticles with controlled sizes and bimetallic compositions that were thermally treated under controlled temperature, atmosphere, and time. Examples shown in this report included Au₂₂Pt₇₈/C and Au₄₉Pt₅₁/C catalysts treated at 400-500 °C. The electrocatalytic performances of these catalysts in the fuel cells was found to be dependent on the bimetallic composition and the nanoscale phase properties which are controlled by the thermal treatment parameters (temperature and time). Excellent fuel cell performance was observed for the catalysts which are characteristic of an alloyed AuPt phase with a lattice parameter approaching that for an Pt-rich alloy phase. The results have also demonstrated excellent stability of the nanoengineered AuPt catalysts in fuel cells. The observed combination of high activity and high durability of the selected AuPt catalysts indicated that this nanoengineered bimetallic catalyst system, upon further refinement and optimization of the nanoscale phase properties and durability serve as a promising candidate of electrocatalysts for the practical application in Proton Exchange Membrane fuel cells.     

Electrocatalytic performance of Pt-based trimetallic alloy nanoparticle catalysts in proton exchange membrane fuel cells

The electrocatalytic performance of nanoengineered PtNiFe catalysts in proton exchange membrane fuel cells (PEMFC) is described in this report. The membrane electrode assembly was prepared using carbon-supported PtNiFe nanoparticles treated at two different temperatures as the cathode electrocatalysts in PEMFC. The PtNiFe/C catalysts were found to exhibit excellent fuel cell performance, much better than that of commercial Pt/C catalyst. In addition to assessing the mass activities in the kinetic current region, the fuel cell performance was also determined in the ohmic and mass transport regions. The electrocatalytic and fuel cell performance are shown to depend on the thermal treatment temperature of the trimetallic catalysts. The higher-temperature treated catalysts showed a higher power density than the lower-temperature treated catalysts. The results are also discussed in terms of the effect of lattice shrinking in the trimetallic alloy nanoparticles on the electrocatalytic activity.     

Limited grain growth and chemical ordering during high-temperature sintering of PtNiCo nanoparticle aggregates

High-temperature sintering of ternary Pt(x)Ni(100-x-y)Co(y) (x?=?28-44%, y?=?40-54%) nanoparticles of interest in catalysis was studied in situ and in real-time with synchrotron-based x-ray diffraction. For the first time we were able to experimentally capture the early stage of the thermal treatment, and found the nanoparticles to undergo an unusual two-step coalescence process that involves transient growth and restructuring of the nanoparticles. The coalescence process is accompanied by lattice contraction, likely due to composition evolution towards a random alloy. In the late stage of sintering, evidence was found for self-limited grain growth and L1(0) chemical ordering. The order-disorder transition temperature was found to be around 800?°C in all four ternary alloy compositions studied. Fitting of the experimental data with the model for grain growth with size-dependent impediment leads to an activation energy for mass transport of about 100?kJ?mol(-1), and may be used as a predictive tool to estimate particle size as a function of heat treatment temperature and duration.     

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.     

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 ? 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.