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The microlesion question is investigated by monitoring the electrical potential difference across the endothelium of rat corneas in vitro before, during, and after irradiation. When the corneas were exposed to 1 Gy of Fe-56 ions (450 and 600 MeV/a.m.u.), no effect was detected on this parameter. These results suggest that direct physical damage to cell membranes, as predicted by the microlesion theory, does not take place.
H 2 /air fuel cell membrane-electrode-assemblies (MEAs) were fabricated with electrospun particle/polymer nanofiber mat cathodes (0.1 mg Pt /cm 2 or 0.2 mg Pt /cm 2 Pt/C, PtCo/C or PtNi/C) and anodes (0.1 mg/cm 2 Pt/C), where the binder was a mixture of acid-form perfluorosulfonic acid ionomer (e.g., Nafion™) and poly(acrylic acid) (PAA) carrier polymer or sodium-form PFSA with a carrier polymer of either PAA or polyethylene oxide (PEO). For the latter two cases, the water-soluble carrier was removed from the fibers after electrospinning. MEAs with Pt/C anode/cathode catalyst loadings of 0.1 mg Pt /cm2 each, an 1100 EW PFSA binder (Nafion™ dispersion), and a Nafion 211 membrane produced high power at both high and low relative humidity (RH) conditions, e.g., a maximum power density of 919 mW/cm 2 at 100% RH and 832 mW/cm 2 at 40% RH for a test at 80 °C and 200 kPa abs . The high power at low RH was attributed to nm-size pores within the fibers that trap water via capillary condensation thus maintaining a high proton conductivity of the PFSA binder in the cathode and especially the anode while minimizing/eliminating ionic isolation of catalyst particles in low water content, poorly conductive binder. At the same time, micro-porosity between fibers in the cathode allows for fast removal of electrogenerated water, thus minimizing cathode flooding. Nanofiber MEAs with Pt alloy catalyst cathodes also performed well, where a fibrous PtCo/C catalyst cathode at a loading of 0.1 mg/cm 2 produced 20% more power than a conventional powder cathode MEA, e.g., a maximum power density of 1,045 mW/cm 2 vs. 869 mW/cm 2 at 80 °C, 100% RH, and 200 kPa abs , and a PtNi/C fiber cathode MEAs prepared with Na+-form Nafion + PEO generated a maximum power of 820 mW/cm 2 at 40% RH. Fiber electrode MEAs with a neat Nafion binder (prepared from Na+-form Nafion + PEO or PAA) where the cathode Pt loading was ~0.1 mg/cm 2 exhibited a 25% loss in maximum power at 30,000 metal dissolution cycles, as compared to a 12% loss when the cathode binder was H+-form Nafion + PAA. The performance of a fiber mat electrode MEA with Pt/C catalyst (0.2 mg/cm 2 cathode loading and 0.1 mg/cm 2 anode loading) was excellent. At 80 oC and 200 kPa abs , the maximum power density was 1104 mW/cm 2 . The maximum power was independent of feed gas humidity for 40 < RH < 100%. The power loss after a metal dissolution AST (30,000 voltage cycles) was only 13%.
A new cell designed to mimic the photosynthetic processes of plants to convert carbon dioxide into carbonaceous products and oxygen at high efficiency, has an improved configuration using a polymer membrane electrolyte and an alkaline medium. This increases efficiency of the artificial photosynthetic process, achieves high conversion rates, permits the use of inexpensive catalysts, and widens the range of products generated by this type of process. The alkaline membrane electrolyte allows for the continuous generation of sodium formate without the need for any additional separation system. The electrolyte type, pH, electrocatalyst type, and cell voltage were found to have a strong effect on the efficiency of conversion of carbon dioxide to formate. Indium electrodes were found to have higher conversion efficiency compared to lead. Bicarbonate electrolyte offers higher conversion efficiency and higher rates than water solutions saturated with carbon dioxide. pH values between 8 and 9 lead to the maximum values of efficiency. The operating cell voltage of 2.5 V, or higher, ensures conversion of the carbon dioxide to formate, although the hydrogen evolution reaction begins to compete strongly with the formate production reaction at higher cell voltages. Formate is produced at indium and lead electrodes at a conversion efficiency of 48 mg of CO2/kilojoule of energy input. This efficiency is about eight times that of natural photosynthesis in green plants. The electrochemical method of artificial photosynthesis is a promising approach for the conversion, separation and sequestration of carbon dioxide for confined environments as in space habitats, and also for carbon dioxide management in the terrestrial context. The heart of the reactor is a membrane cell fabricated from an alkaline polymer electrolyte membrane and catalyst- coated electrodes. This cell is assembled and held in compression in gold-plated hardware. The cathode side of the cell is supplied with carbon dioxide-saturated water or bicarbonate solution. The anode side of the cell is supplied with sodium hydroxide solution. The solutions are circulated past the electrodes in the electrochemical cell using pumps. A regulated power supply provides the electrical energy required for the reactions. Photovoltaic cells can be used to better mimic the photosynthetic reaction. The current flowing through the electrochemical cell, and the cell voltage, are monitored during experimentation. The products of the electrochemical reduction of carbon dioxide are allowed to accumulate in the cathode reservoir. Samples of the cathode solution are withdrawn for product analysis. Oxygen is generated on the anode side and is allowed to vent out of the reservoir.
During operation, polymer-electrolyte fuel cells undergo mechanical and chemical degradation, which behave synergistically and lead to accelerated membrane deterioration over time. This study builds upon previous modeling work on mechanical degradation, as described by a pinhole in the membrane, and chemical degradation mitigation by using cerium. By combining these two models, analysis can be carried out on the coupled degradation methods and how the mitigation effects of cerium disrupt the degradation cycle. The model results show how the presence of a pinhole in the membrane changes the distribution of cerium in the cell and modifies chemical degradation rates. In addition, the model shows how cerium affects the rate of pinhole growth by slowing down the rate of change in mechanical properties due to chemical degradation. Finally, the model shows how cerium modifies the mechanical and chemical degradation rates of the membrane under humidity- and voltage-cycling conditions.
A 37-residue amino acid sequence corresponding to the segment encoded by exon-5 of murine ameloblastin (Ambn), AB2 (Y67-Q103), has been implicated with membrane association, ameloblastin self-assembly, and amelogenin-binding. Here, our aim was to characterize, at the residue level, the structural behavior of AB2 bound to chemical mimics of biological membranes using NMR spectroscopy. To better define the structure of AB2 using NMR-based methods, recombinant 13 C- and 15 N-labelled AB2 (*AB2) was prepared and data collected free in solution and with deuterated dodecylphosphocholine (dPC) micelles, deuterated bicelles, and both small and large unilamellar vesicles. Amide chemical shift and intensity perturbations observed in 1 H- 15 N HSQC spectra of *AB2 in the presence of bicelles and dPC micelles suggest that a region of *AB2, S6-E36 (murine Ambn S68 – E98), associates with the membrane biomimetics. A CSI-3 analysis of the NMR chemical shift assignments for *AB2 free in solution and bound to dPC micelles indicated the peptide remains disordered except for the adoption of a short, 12-residue α-helix, F10-G21 (murine Ambn F72-G83). In dPC micelles, the NOE NMR data was void of patterns characteristic of long-lived helical structure indicating this helix was transient in nature. A continuum of intrinsic disorder in the membrane-bound state may be responsible for ameloblastin’s ability to dynamically interact with multiple partners at the same site during amelogenesis.
The performance of a molten carbonate carbon dioxide concentrator (MCCDC) cell, as a device for removal of CO2 from manned spacecraft cabins without fuel expenditure, is investigated. The test system consists of an electrochemical cell (with an Li2CO3-38 mol pct K2CO3 membrane contained in a LiAlO2 matrix), a furnace, and a flow IR analyzer for monitoring CO2. Operation of the MCCDC-driven cell was found to be suitable for the task of CO2 removal: the cell performed at extremely low CO2 partial pressures (at or above 0.1 mm Hg); cathode CO2 efficiencies of 97 percent were achieved with 0.25 CO2 inlet concentration at 19 mA sq cm, at temperatures near 873 K. Anode concentrations of up to 5.8 percent were obtained. Simple cathode and anode performance equations applied to correlate cell performance agreed well with those measured experimentally. A flow diagram for the process is included.
This DOE-supported research grant focused on understanding the nature of proton transport in complex systems such as proton exchange membranes (PEMs). The most unique aspect of the research was the development and implementation of a novel multiscale reactive molecular dynamics (MS-RMD) methodology. In this approach, covalent bonds can dynamically break and form, allowing one to accurately treat the proton hopping process essential to capturing the physics of proton transport. The Voth group applied this method to proton exchange membrane systems, providing insight into their proton transport mechanism. They found that protons can diffuse most rapidly in the water-rich regions, but that protons actually spend so little time in such regions that transport along the hydrophobic – hydrophilic interface controls the membrane performance. In addition, the group worked to increase understanding of acidic solutions, developing methods for simulating and interpreting experimental infrared vibrational spectroscopy for excess protons (acidic solutions). The group also implemented novel tools for developing proton transport reactive MD models using a relative entropy minimization scheme.
Covisualizations with wide-field computational optical-sectioning microscopy of living epidermal cells of the onion bulb scale have evidenced two major new cellular features. First, a sheath of cytoskeletal elements clads the endomembrane system. Similar elements clad the inner faces of punctate plasmalemmal sites interpreted as plasmalemmal control centers. One component of the endomembrane sheath and plasmalemmal control center cladding is anti-genicity-recognized by two injected antibodies against animal spectrin. Immunoblots of separated epidermal protein also showed bands recognized by these antibodies. Injected phalloidin identified F-actin with the same cellular distribution pattern, as did antibodies against intermediate-filament protein and other cytoskeletal elements known from animal cells. Injection of general protein stains demonstrated the abundance of endomembrane sheath protein. Second, the endomembrane system, like the plasmalemmal puncta, contains antigen recognized by an anti-beta 1 integrin injected into the cytoplasm. Previously, immunoblots of separated epidermal protein were shown to have a major band recognized both by this antibody prepared against a peptide representing the cytosolic region of beta 1 integrin and an antibody against the matrix region of beta 1 integrin. The latter antiboby also identified puncta at the external face of protoplasts. It is proposed that integrin and associated transmembrane proteins secure the endomembrane sheath and transmit signals between it and the lumen or matrix of the endoplasmic reticulum and organellar matrices. This function is comparable to that proposed for such transmembrane linkers in the plasmalemmal control centers, which also appear to bind cytoskeleton and a host of related molecules and transmit signals between them and the wall matrix. It is at the plasmalemmal control centers that the endoplasmic reticulum, a major component of the endomembrane system, attaches to the plasma membrane.
Quantitative measurements of water content within a single cell are notoriously difficult. In this work, we introduce a single-shot optical method for tracking the intracellular water content, by mass and volume, of a single cell at video rate. We utilize quantitative phase imaging and a priori knowledge of a spherical cellular geometry, leveraging a two-component mixture model to compute the intracellular water content. We apply this technique to study CHO-K1 cells responding to a pulsed electric field, which induces membrane permeabilization and rapid water influx or efflux depending upon the osmotic environment. The effects of mercury and gadolinium on water uptake in Jurkat cells following electropermeabilization are also examined.
The rapid progress of proton exchange membrane fuel cells (PEMFCs) and alkaline exchange membrane fuel cells (AMFCs) has boosted the hydrogen economy concept via diverse energy applications in the past decades. For a holistic understanding of the development status of PEMFCs and AMFCs, recent advancements in electrocatalyst design and catalyst layer optimization, along with cell performance in terms of activity and durability in PEMFCs and AMFCs, are summarized in this report. The activity, stability, and fuel cell performance of different types of electrocatalysts for both oxygen reduction reaction and hydrogen oxidation reaction are discussed and compared. Research directions on the further development of active, stable, and low-cost electrocatalysts to meet the ultimate commercialization of PEMFCs and AMFCs are also discussed.
Organic compounds are synthesized in the interstellar medium and can be delivered to planetary surfaces such as the early Earth, where they mix with endogenous organic mixtures. Some of these compounds are amphiphilic, having polar and non-polar groups on the same molecule. Amphiphilic compounds spontaneously self-assembly into more complex structures such as bimolecular layers, which in turn form closed membranous vesicles. The first forms of cellular life required self-assembled membranes that were likely to be available on the prebiotic Earth. Laboratory simulations show that such vesicles readily encapsulate functional macromolecules, including nucleic acids and polymerases. A goal of future investigations is to fabricate artificial cells as models of the origin of life.
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In this work, we implement a calendering technique to flatten stray fibers within the gas diffusion media, thereby mitigating pin-hole formation in the hot-pressed MEAs. We have investigated the influence of calendering on the long-term durability for several types of gas diffusion electrodes (GDEs) using a combined chemical and mechanical accelerated stress test (AST). The calendered MEAs demonstrate an average AST lifetime improvement of 77% relative to the as-fabricated MEAs.