Larry J. Markoski

Air-Breathing Laminar Flow-Based Direct Methanol Fuel Cell with Alkaline Electrolyte

Microfuel cells have the potential to achieve higher energy densities than batteries and have thus received intense investigation as a power source for a wide range of portable applications. Extensive research efforts are focused on the development and miniaturization of promising fuel cell technologies, including direct methanol fuel cells DMFCs and polymer electrolyte membrane-based fuel cells PEMFCs, operated with hydrogen/oxygen. 1-3 In most fuel cells, a polymer electrolyte membrane such as Nafion allows protons to diffuse from the anode to the cathode, while trying to prevent fuel molecules from diffusing across and mixing with oxygen at the cathode. Poor performance or a lack of selectivity by the membrane leads to a key performance-limiting process called fuel crossover that has plagued the PEM-based fuel cells. In addition to fuel crossover, cathode flooding and anode dry-out water management due to osmotic drag of water molecules associated with protons diffusing from the anode to the cathode, as well as due to the formation and consumption of water at the cathode and anode, respectively, impedes the performance and commercial implementation of these fuel cells. 4

Crosslinking chemistry for high-performance polymer networks☆

Abstract A new thermally reactive monomer has been designed and synthesized that brings novel crosslinking chemistry to high-performance polymers. This monomer (XTA) is a derivative of terephthalic acid and was based on the thermal chemistry of benzocyclobutene. Various model compounds have been synthesized to investigate substituent effects on benzocyclobutene reactivity. Irreversible reaction exotherms around 350°C were observed in these model compounds using differential scanning calorimetry. Based on these studies, polyaramid and poly(aryl ether ketone) XTA copolymers were synthesized. The formation of an insoluble network resulted after heat treatment of these polymers.

Thermally crosslinkable thermoplastic PET-co-XTA copolyesters

Abstract A series of thermally crosslinkable polyester copolymers were synthesized by incorporation of a benzocyclobutene-containing terephthalic acid derivative (XTA) into polyethylene terephthalate (PET). The cyclobutene moiety on the XTA monomer allows for reactive crosslinking at temperatures ∼350°C requiring no catalyst and causing no change in mass. Copolymers were synthesized containing 1, 5, 10, 20, 50, and 100 mol%o XTA. Crosslinking occurred above the melting temperature (∼250°C) yet below the degradation temperature (∼400°C), providing a window for melt processing of the copolymer. To demonstrate this point fibres were melt spun. The PET-co-XTA copolymers show systematic variations in the glass transition, recrystallization, melting and degradation temperatures as a function of benzocyclobutene content. The degradation and melting temperature both decrease slightly with increased XTA, while the recrystallization and glass transition temperature were relatively insensitive to XTA content. Thermal gravimetric analysis (TGA) indicated a decrease in the degradation temperature as higher amounts of XTA were incorporated, although an increase in the %char at 800°C was seen. This decrease in degradation temperature may be due to the generation of free radicals. Limiting Oxygen Index (LOI) measurements showed an increase in the oxygen content required to maintain a stable flame in copolymers with increasing amounts of XTA. LOI values ranged from 18 for neat PET to 35 for the copolymer containing 20 mol % XTA. Wide-angle X-ray scattering data showed little change in the crystalline structure, but decreasing crystallinity for PET for blends containing up to 20 mol% XTA. The 50 mol% XTA copolymer was amorphous, while the 100% XTA homopolymer (PEXTA) showed evidence of a new crystalline structure. Crystalline diffraction peaks showed reduced intensities in data recorded for heat treated samples, and there was evidence for new peaks in the copolymer containing 20 mol% XTA when heated near 300°C. Transmission electron microscopy of cross-sections through burned samples showed a highly crystalline char at the surface of XTA copolyesters. This crystalline char appeared to protect the underlying copolymer from further flame-induced degradation. Evidence for significantly increased adhesion of the copolymers to polyimide films was also obtained.

Membraneless Fuel Cell Based on Laminar Flow

An increasing societal demand for a wide range of small, often portable devices that can operate for an extended period of time without recharging has resulted in a surge of research in micropower sources. Most efforts in this area focus on downscaling of existing fuel cell technology such as the well-known proton exchange membrane (PEM) fuel cells. Here we study a novel concept for fuel cells: the use of laminar flow instead of a physical barrier such as a PEM to separate the fuel and oxidant streams. Laminar flow, i.e. low Reynolds number flow, is a property of fluid flow at the microscale: one or more liquid streams that are brought together under low Reynolds number conditions flow in parallel and contact with each other without turbulent mixing. Mass transport transverse to the direction of flow takes place by diffusion only. In our laminar flow-based fuel cell a fuel-containing stream and an oxidant-containing stream are brought together in laminar flow conditions with the electrodes placed on opposite walls within the channel. In un-optimized fuel cell configurations, current densities as high as 10 mA/cm2 are obtained at room temperature using different fuels such as methanol or formic acid vs. oxygen saturated solvents or other oxidants.© 2003 ASME

Effect of branching on the rheological properties of solutions of aromatic etherimide copolymers

A series of AB/AB2 etherimide copolymers of nearly constant weight-averaged molecular weight, synthesized from starting comonomer compositions ranging from 0 to 1 mole fraction AB (xAB), were characterized. Zero-shear viscosity of various concentration solutions in N-methyl pyrrolidinone showed a slight increase with xAB in the range of 0.00⩽xAB⩽0.80, followed by a sharp rise at higher xAB. Likewise, the solution flow birefringence showed negligible response for xAB < 0.80, followed by a rise with xAB at higher fractions. The dilute concentration, zero shear viscosity, and intrinsic viscosity correlated directly with the calculated distance between branches (lAB). The zero-shear viscosity, η0, exhibited a linear dependence on concentration in the dilute regime for the entire series of branched polymers and a power law dependence in the concentrated regime, with the coefficient increasing with xAB. The concentration dependence of η0 also scaled with the product of the concentration and intrinsic viscosity ...

Microfluidic fuel cells as microscale power sources and analytical platforms

A continuously growing need for high energy density miniaturized power sources for portable electronic applications has spurred the development of a variety of microscale fuel cells. For portable applications, membrane-based fuel cells using small organic fuels (i.e., methanol, formic acid) are among the most promising configurations as they benefit from the high energy density and easy storage of the liquid fuels. Unfortunately, the performance of these fuel cells is often hindered by membrane-related issues such as water management (i.e., electrode dry-out / flooding) and fuel crossover. Furthermore, high costs of, for example, catalysts and membranes as well as durability concerns still hinder commercialization efforts. To address these challenges we have developed membraneless laminar flow-based fuel cells (LFFCs), which exploit microscale transport phenomena (laminar flow) to compartmentalize streams within a single microchannel. The properties of various fuel and media flexible LFFCs will be presented and novel strategies for improving fuel utilization and power density will be discussed. Furthermore, the performance of a scaled-out 14-channel LFFC prototype is presented. We have also developed a microfluidic fuel cell as a powerful analytical platform to investigate and optimize the complex processes that govern the performance of catalysts and electrodes in an operating fuel cell. This platform bridges the gap between a conventional 3-electrode electrochemical cell and a fuel cell, as it allows for standard electrochemical analysis (e.g., CV, CA, EIS) as well as fuel cell analysis (e.g., IV curves).Copyright

Rapid synthesis of etherimides via catalytic arylation of silylated phenols

Abstract We report here a rapid and efficient method for the synthesis of aromatic etherimides from silylated phenols and 4-fluorophthalimides catalyzed by CsF. Reaction times are a few minutes and products are easily purified requiring no column chromatography.