Chilan Ngo

Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum–Nickel Nanowires through Synthesis and Post-Treatment Optimization

For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 μA cmPt–2 at 0.9 V) and high surface areas (>90 m2 gPt–1). Platinum–nickel (Pt—Ni) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m2 gPt–1, a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochemical test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the Pt—Ni nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications.

N-Bromosuccinimide-based bromination and subsequent functionalization of hydrogen-terminated silicon quantum dots

We report a mild, effective, room-temperature method for brominating and functionalizing colloidal hydrogen-terminated silicon quantum dots (H-SiQDs) using N-bromosuccinimide (NBS) as the bromination reagent. This post-synthesis bromination overcomes a long-standing challenge of producing emissive SiQDs through the functionalization of directly synthesized halogen-terminated colloidal SiQDs.

Organometallic Complexes Anchored to Conductive Carbon for Electrocatalytic Oxidation of Methane at Low Temperature

Low-temperature direct methane fuel cells (DMEFCs) offer the opportunity to substantially improve the efficiency of energy production from natural gas. This study focuses on the development of well-defined platinum organometallic complexes covalently anchored to ordered mesoporous carbon (OMC) for electrochemical oxidation of methane in a proton exchange membrane fuel cell at 80 °C. A maximum normalized power of 403 μW/mg Pt was obtained, which was 5 times higher than the power obtained from a modern commercial catalyst and 2 orders of magnitude greater than that from a Pt black catalyst. The observed differences in catalytic activities for oxidation of methane are linked to the chemistry of the tethered catalysts, determined by X-ray photoelectron spectroscopy. The chemistry/activity relationships demonstrate a tangible path for the design of electrocatalytic systems for C-H bond activation that afford superior performance in DMEFC for potential commercial applications.

Study of Lithium Silicide Nanoparticles as Anode Materials for Advanced Lithium Ion Batteries

The development of high-performance silicon anodes for the next generation of lithium ion batteries (LIBs) evokes increasing interest in studying its lithiated counterpart-lithium silicide (LixSi). In this paper we report a systematic study of three thermodynamically stable phases of LixSi (x = 4.4, 3.75, and 2.33) plus nitride-protected Li4.4Si, which are synthesized via the high-energy ball-milling technique. All three LixSi phases show improved performance over that of unmodified Si, where Li4.4Si demonstrates optimum performance with a discharging capacity of 3306 (mA h)/g initially and maintains above 2100 (mA h)/g for over 30 cycles and above 1200 (mA h)/g for over 60 cycles at the current density of 358 mA/g of Si. A fundamental question studied is whether different electrochemical paradigms, that is, delithiation first or lithiation first, influence the electrode performance. No significant difference in electrode performance is observed. When a nitride layer (LixNySiz) is created on the surface of Li4.4Si, the cyclability is improved to retain the capacity above 1200 (mA h)/g for more than 80 cycles. By increasing the nitridation extent, the capacity retention is improved significantly from the average decrease of 1.06% per cycle to 0.15% per cycle, while the initial discharge capacity decreases due to the inactivity of Si in the LixNySiz layer. Moreover, the Coulombic efficiencies of all LixSi-based electrodes in the first cycle are significantly higher than that of a Si electrode (∼90% vs 40-70%).

Synthesis of high surface area CaxLa(1−x)Al(1−x)MnxO(3−δ) perovskite oxides for oxygen reduction electrocatalysis in alkaline media

A series of perovskite oxide type catalysts with composition CaxLa1−xAl1−xMnxO1−δ were synthesized using solid state reaction, hybrid sol–gel, and aerogel synthesis techniques. The prepared catalyst materials were characterized with a suite of characterization techniques to determine morphology and composition. Electrochemical measurements for oxygen reduction reaction (ORR) activity in alkaline solution were performed using rotating disk electrode (RDE). ORR mass activity increased with increasing Brunauer–Emmett–Teller (BET) surface area, following the trend of solid state reaction < hybrid sol–gel < aerogel, when maintaining equal calcination time and temperature among all samples. Results also indicate a strong correlation between ORR specific activity and compositional homogeneity observed through transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) mapping and X-ray photoelectron spectroscopy (XPS). Specifically, lower surface area materials produced by solid state reaction showed the highest compositional homogeneity and demonstrated highest specific activity. Performance tradeoffs are discussed relating surface area, compositional homogeneity at the oxide surface, and ORR activity.

Carbon Capture by Metal Oxides: Unleashing the Potential of the (111) Facet

Solid metal oxides for carbon capture exhibit reduced adsorption capacity following high-temperature exposure, due to surface area reduction by sintering. Furthermore, only low-coordinate corner/edge sites on the thermodynamically stable (100) facet display favorable binding toward CO2, providing inherently low capacity. The (111) facet, however, exhibits a high concentration of low-coordinate sites. In this work, MgO(111) nanosheets displayed high capacity for CO2, as well as a ∼65% increase in capacity despite a ∼30% reduction in surface area following sintering (0.77 mmol g-1 @ 227 m2 g-1 vs 1.28 mmol g-1 @ 154 m2 g-1). These results, unique to MgO(111), suggest intrinsic differences in the effects of sintering on basic site retention. Spectroscopic and computational investigations provided a new structure-activity insight: the importance of high-temperature activation to unleash the capacity of the polar (111) facet of MgO. In summary, we present the first example of a faceted sorbent for carbon capture and challenge the assumption that sintering is necessarily a negative process; here we leverage high-temperature conditions for facet-dependent surface activation.

Atomic layer deposition of TiO 2 for stabilization of Pt nanoparticle oxygen reduction reaction catalysts

Atomic layer deposition (ALD) was used to modify two different types of carbon black-based Pt oxygen reduction catalysts with protective TiO2 nanostructures to increase catalyst durability. Rates of ALD growth and the structure of deposited TiO2 were observed to be highly dependent on oxygen content of the catalyst substrate. Electrochemical durability was enhanced with the addition of TiO2 ALD nanostructures, with up to 70% retention in mass activity measured over accelerated durability testing. High-temperature treatment of the top-performing ALD catalyst, which was found to promote structural rearrangement of the TiO2 and Pt phases into hybrid nanoparticles, yielded a twofold increase in activity but was detrimental to durability.Graphical Abstract