Ryan C. Sekol

Bulk Metallic Glass Nanowire Architecture for Electrochemical Applications

Electrochemical devices have the potential to pose powerful solutions in addressing rising energy demands and counteracting environmental problems. However, currently, these devices suffer from meager performance due to poor efficiency and durability of the catalysts. These suboptimal characteristics have hampered widespread commercialization. Here we report on Pt(57.5)Cu(14.7)Ni(5.3)P(22.5) bulk metallic glass (Pt-BMG) nanowires, whose novel architecture and outstanding durability circumvent the performance problems of electrochemical devices. We fabricate Pt-BMG nanowires using a facile and scalable nanoimprinting approach to create dealloyed high surface area nanowire catalysts with high conductivity and activity for methanol and ethanol oxidation. After 1000 cycles, these nanowires maintain 96% of their performance-2.4 times as much as conventional Pt/C catalysts. Their properties make them ideal candidates for widespread commercial use such as for energy conversion/storage and sensors.

Scalable Fabrication of Multifunctional Freestanding Carbon Nanotube/Polymer Composite Thin Films for Energy Conversion

Translating the unique properties of individual single-walled carbon nanotubes (SWNTs) to the macroscale while simultaneously incorporating additional functionalities into composites has been stymied by inadequate assembly methods. Here we describe a technique for developing multifunctional SWNT/polymer composite thin films that provides a fundamental engineering basis to bridge the gap between their nano- and macroscale properties. Selected polymers are infiltrated into a Mayer rod coated conductive SWNT network to fabricate solar cell transparent conductive electrodes (TCEs), fuel cell membrane electrode assemblies (MEAs), and lithium ion battery electrodes. Our TCEs have an outstanding optoelectronic figure of merit σ(dc)/σ(ac) of 19.4 and roughness of 3.8 nm yet are also mechanically robust enough to withstand delamination, a step toward scratch resistance necessary for flexible electronics. Our MEAs show platinum utilization as high as 1550 mW/mg(Pt), demonstrating our techniques ability to integrate ionic conductivity of the polymer with electrical conductivity of the SWNTs at the Pt surface. Our battery anodes, which show reversible capacity of ∼850 mAh/g after 15 cycles, demonstrate the integration of electrode and separator to simplify device architecture and decrease overall weight. Each of these applications demonstrates our techniques ability to maintain the conductivity of SWNT networks and their dispersion within a polymer matrix while concurrently optimizing key complementary properties of the composite. Here, we lay the foundation for the assembly of nanotubes and nanostructured components (rods, wires, particles, etc.) into macroscopic multifunctional materials using a low-cost and scalable solution-based processing technique.

Bulk Metallic Glass Micro Fuel Cell

Micro fuel cells (MFC) have been identifi ed as promising alternative power sources for portable electronics. Using noncorrosive electrolytes, they offer high theoretical power densities at low operating temperatures, with the potential for stable long-term operation. [ 1 ] Although these attributes make MFCs attractive for many portable device applications, [ 2 ] the primary design challenge is to identify the most effective lowcost materials and fabrication methods. [ 3 ] Here, we present a micro fuel cell in which the catalyst layer, gas diffusion layer, and fl ow fi elds are fabricated from bulk metallic glass (BMG) using thermoplastic forming (TPF). We show that TPF is a scalable and economical technique, for the fabrication of multi-scale BMG components of a MFC. BMGs have high electrical conductivity [ 4 ] and corrosion resistance, [ 5 ] and we demonstrate that end-plates with serpentine fl ow fi elds can be embossed into Zr 35 Ti 30 Cu 8.25 Be 26.75 (Zr-BMG) through a TPFbased process. The BMG fuel cell embodies the processing advantage of TPF into hierarchical structures involving length scales ranging from nanometers to centimeters, [ 6 ] and signifi es the fabrication of fuel cell components from a single material. We show that a hierarchical architecture fabricated through TPF-based embossing of Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 (Pt-BMG) can function as a high-surface area catalyst as well as a porous gas diffusion layer, which allows us to demonstrate the concept of a metallic glass MFC. The ability to create structures over a wide range of length scales combined with remarkable electrochemical properties, suggests applications beyond MFCs, including sensors, lab-on-a-chip platforms, micro-reactors, and heterogeneous catalysis. [ 7 ]

Realizing Comparable Oxidative and Cytotoxic Potential of Single- and Multiwalled Carbon Nanotubes through Annealing

The potential applications as well as the environmental and human health implications of carbon nanomaterials are well represented in the literature. There has been a recent focus on how specific physicochemical properties influence carbon nanotube (CNT) function as well as cytotoxicity. The ultimate goal is a better understanding of the causal relationship between fundamental physiochemical properties and cytotoxic mechanism in order to both advance functional design and to minimize unintended consequences of CNTs. This study provides characterization data on a series of multiwalled carbon nanotubes (MWNTs) that underwent acid treatment followed by annealing at increasing temperatures, ranging from 400 to 900 °C. These results show that MWNTs can be imparted with the same toxicity as single-walled carbon nanotubes (SWNTs) by acid treatment and annealing. Further, we were able to correlate this toxicity to the chemical reactivity of the MWNT suggesting that it is a chemical rather than physical hazard. This informs the design of MWNT to be less hazardous or enables their implementation in antimicrobial applications. Given the reduced cost and ready dispersivity of MWNTs as compared to SWNTs, there is a significant opportunity to pursue the use of MWNTs in novel applications previously thought reserved for SWNTs.

Guided Evolution of Bulk Metallic Glass Nanostructures: A Platform for Designing 3D Electrocatalytic Surfaces

Electrochemical devices such as fuel cells, electrolyzers, lithium-air batteries, and pseudocapacitors are expected to play a major role in energy conversion/storage in the near future. Here, it is demonstrated how desirable bulk metallic glass compositions can be obtained using a combinatorial approach and it is shown that these alloys can serve as a platform technology for a wide variety of electrochemical applications through several surface modification techniques.

Finite size effects in the crystallization of a bulk metallic glass

We explore finite size effects in the crystallization of a bulk metallic glass with nm-scale dimensions. Nanorods of Pt57.5Cu14.7Ni5.3P22.5 are produced by thermoplastic extrusion of supercooled liquid through a nanoporous template. The nanorods exhibit remarkable differences in their crystallization behavior above the glass transition. Crystallization for 100 and 200 nm diameter nanorods occurred at 6 and 24 °C lower, respectively, than the nominal crystallization temperature for bulk material while the glass transition temperatures were unchanged from the bulk value. Size dependent crystallization kinetics is discussed within a framework of classical nucleation theory, as well as possible shear and surface-induced effects.

Electrocatalysts: Guided Evolution of Bulk Metallic Glass Nanostructures: A Platform for Designing 3D Electrocatalytic Surfaces (Adv. Mater. 10/2016).

On page 1940, A. D. Taylor and co-workers demonstrate nanoporous bicontinuous structures using controlled structural evolution of metallic glass. By using techniques such as dealloying, galvanic replacement, and under-potential deposition, bulk-metallic-glass alloys can be pushed beyond their compositional limitations and tuned for a wide variety of interfacial and electrochemical reactions. Examples are illustrated for hydrogen and methanol oxidation, as well as oxygen reduction reactions.