An experimental study of polymer electrolyte fuel cell operation at sub-freezing temperatures

The ability of polymer electrolyte fuel cells (PEFCs) to startup at subfreezing temperatures is governed by whether it is able to overcome the freezing point (0°C) before product ice prevents the electrochemical reactions. In this work, we experimentally investigated the coulombs of charge Qc transferred in PEFCs under subfreezing operation before the output voltage drops to 0.0V. PEFCs with various membranes and catalyst-layer thicknesses, ionomer-carbon ratios, operating current density, and initial hydration of PEFCs were studied, and their influences on cold-start performance and coulombs of charge were experimentally measured. We find that subfreezing temperature, ionomer-catalyst ratio, and catalyst-layer thickness, significantly affect the amount of charge transferred before operational failure, whereas the membrane thickness and initial hydration level have limited effect for the considered cases. © 2013 The Electrochemical Society

Subfreezing operation of polymer electrolyte fuel cells: Ice formation and cell performance loss

In this work, we investigate the cold-start operation of polymer electrolyte fuel cells (PEFCs) through high-resolution neutron radiography, experimental testing, theoretical evaluation, and comparison with model prediction. Ice formation location, voltage evolution, and loss of the electro-catalyst surface area (ECSA) are examined. A dimensionless parameter , characterizing the spatial variation of the reaction rate across the cathode catalyst layer, is discussed at subfreezing temperature using newly determined membrane ionic conductivity. The evaluation identifies the operating range that the reaction rate can be treated uniform across the catalyst layer, in which the model is valid. © 2012 Elsevier Ltd. All rights reserved

Asymmetric interlimb transfer of concurrent adaptation to opposing dynamic forces

Interlimb transfer of a novel dynamic force has been well documented. It has also been shown that unimanual adaptation to opposing novel environments is possible if they are associated with different workspaces. The main aim of this study was to test if adaptation to opposing velocity dependent viscous forces with one arm could improve the initial performance of the other arm. The study also examined whether this interlimb transfer occurred across an extrinsic, spatial, coordinative system or an intrinsic, joint based, coordinative system. Subjects initially adapted to opposing viscous forces separated by target location. Our measure of performance was the correlation between the speed profiles of each movement within a force condition and an ‘average’ trajectory within null force conditions. Adaptation to the opposing forces was seen during initial acquisition with a significantly improved coefficient in epoch eight compared to epoch one. We then tested interlimb transfer from the dominant to non-dominant arm (D → ND) and vice-versa (ND → D) across either an extrinsic or intrinsic coordinative system. Interlimb transfer was only seen from the dominant to the non-dominant limb across an intrinsic coordinative system. These results support previous studies involving adaptation to a single dynamic force but also indicate that interlimb transfer of multiple opposing states is possible. This suggests that the information available at the level of representation allowing interlimb transfer can be more intricate than a general movement goal or a single perceived directional error

Time-dependent density functional theory for open systems with a positivity-preserving decomposition scheme for environment spectral functions

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Agricultural origins and the isotopic identity of domestication in northern China

Stable isotope biochemistry (δ 13C and δ 15N) and radiocarbon dating of ancient human and animal bone document 2 distinct phases of plant and animal domestication at the Dadiwan site in northwest China. The first was brief and nonintensive: at various times between 7900 and 7200 calendar years before present (calBP) people harvested and stored enough broomcorn millet (Panicum miliaceum) to provision themselves and their hunting dogs (Canis sp.) throughout the year. The second, much more intensive phase was in place by 5900 calBP: during this time both broomcorn and foxtail (Setaria viridis spp. italica) millets were cultivated and made significant contributions to the diets of people, dogs, and pigs (Sus sp.). The systems represented in both phases developed elsewhere: the earlier, low-intensity domestic relationship emerged with hunter-gatherers in the arid north, while the more intensive, later one evolved further east and arrived at Dadiwan with the Yangshao Neolithic. The stable isotope methodology used here is probably the best means of detecting the symbiotic human-plantanimal linkages that develop during the very earliest phases of domestication and is thus applicable to the areas where these connections first emerged and are critical to explaining how and why agriculture began in East Asia

Minimal basilar membrane motion in low-frequency hearing

Low-frequency hearing is critically important for speech and music perception, but no mechanical measurements have previously been available from inner ears with intact low-frequency parts. These regions of the cochlea may function in ways different from the extensively studied high-frequency regions, where the sensory outer hair cells produce force that greatly increases the sound-evoked vibrations of the basilar membrane. We used laser interferometry in vitro and optical coherence tomography in vivo to study the low-frequency part of the guinea pig cochlea, and found that sound stimulation caused motion of a minimal portion of the basilar membrane. Outside the region of peak movement, an exponential decline in motion amplitude occurred across the basilar membrane. The moving region had different dependence on stimulus frequency than the vibrations measured near the mechanosensitive stereocilia. This behavior differs substantially from the behavior found in the extensively studied high-frequency regions of the cochlea

Pathological behaviour of the scalar graviton in Ho\v{r}ava-Lifshitz gravity

We confirm the recent claims that, in the infrared limit of Ho\v{r}ava-Lifshitz gravity, the scalar graviton becomes a ghost if the sound speed squared is positive on the flat de Sitter and Minkowski background. In order to avoid the ghost and tame the instability, the sound speed squared should be negative and very small, which means that the flow parameter $\lambda$ should be very close to its General Relativity (GR) value. We calculate the cubic interactions for the scalar graviton which are shown to have a similar structure with those of the curvature perturbation in k-inflation models. The higher order interactions become increasing important for a smaller sound speed squared, that is, when the theory approaches GR. This invalidates any linearized analysis and any predictability is lost in this limit as quantum corrections are not controllable. This pathological behaviour of the scalar graviton casts doubt on the validity of the projectable version of the theory.Comment: 7 pages, references added; v3: Typos corrected, minor changes to text and precise determination of the strong coupling scale. Replaced to match published version

Semiparametric Multivariate Accelerated Failure Time Model with Generalized Estimating Equations

The semiparametric accelerated failure time model is not as widely used as the Cox relative risk model mainly due to computational difficulties. Recent developments in least squares estimation and induced smoothing estimating equations provide promising tools to make the accelerate failure time models more attractive in practice. For semiparametric multivariate accelerated failure time models, we propose a generalized estimating equation approach to account for the multivariate dependence through working correlation structures. The marginal error distributions can be either identical as in sequential event settings or different as in parallel event settings. Some regression coefficients can be shared across margins as needed. The initial estimator is a rank-based estimator with Gehan's weight, but obtained from an induced smoothing approach with computation ease. The resulting estimator is consistent and asymptotically normal, with a variance estimated through a multiplier resampling method. In a simulation study, our estimator was up to three times as efficient as the initial estimator, especially with stronger multivariate dependence and heavier censoring percentage. Two real examples demonstrate the utility of the proposed method