Determination of effective water vapor diffusion coefficient in PEMFC gas diffusion layers

Proton exchange membrane fuel cells have emerged as one of the leaders for the replacement of fossil fuel powered internal combustion engines. Water removal from the cell is one of the top concerns regarding fuel cell performance for transportation applications. During lower power output or high temperature operation, water removal in the vapor phase can dominate. The rate of water vapor diffusion through the porous cathode gas diffusion layer (GDL) of the fuel cell is limited by the porosity and tortuosity formed by the solid fiber matrix. In this work an experimental apparatus is designed to measure the rate of water vapor diffusion across the GDL to determine an effective diffusion coefficient. The effects of microporous layer (MPL) coating, GDL thickness, and polytetrafluoroethylene (PTFE) loading on the diffusion coefficient is demonstrated. Commercially available diffusion media are tested and include Mitsubishi Rayon Corp. Grafil U-105 series, SGL Sigracet® 25, 35, and 10 series, and Toray TGP-H-120 series. Standard corrections, such as the Bruggeman correction, used in fuel cell literature are found to overpredict the effective diffusion coefficient for the GDL. The MPL was found to produce a significant resistance to water vapor diffusion due to its smaller pore diameters, lower porosity, and an increase in tortuosity. The GDL Grafil U-105 A produced a higher effective diffusion coefficient of 0.070 cm2/s compared to the SGL 25BC value of 0.063 cm2/s. Confocal scanning laser microscope images indicated that the MPL for the Grafil U-105 A sample is possibly thinner, thus explaining some of the reduction in diffusion resistance. Thickness was found to have no influence on the effective diffusion coefficient for samples without MPL. PTFE causes a rapid decrease in effective diffusion coefficient from 0.095 cm2/s for TGP-H-120 0% PTFE to 0.024 cm2/s for TGP-H-120 40% PTFE. Comparison to other studies from the literature show good agreement with the present work thus validating the dynamic method for use in diffusion coefficient measurements in fuel cell diffusion media

The Role of Channel-Land Architecture, Diffusion Media Transport Properties, and Aging Effects on Water Transport and Storage in Polymer Electrolyte Fuel Cells

Thermally driven transport of water vapor in polymer electrolyte fuel cells, also known as the heat-pipe effect or phase-change-induced flow, can transport several times the generated amount of water given enough temperature differentials. Understanding this transport process is necessary to properly engineer the water balance in the fuel cell to ensure high performance and long operational life. Channel-land architecture, diffusion media heat and mass transport properties, and operational age can all have an influence on thermally driven flow. High resolution neutron imaging was used to determine the steady-state water accumulation in various cell configurations to understand the influence of these parameters. A novel non-dimensional parameter was proposed to predict the influence of engineering parameters on water balance. The thermal transport number (TTN) compares the strength of anode and cathode thermally driven flow to determine a bias for transport to the anode. Channel-land architecture, specifically asymmetric flow field patterns with larger anode lands, was found to pump water to the anode and allow for large accumulations of water with saturation approaching 60%. Water transport was facilitated to the anode by a delta-T inversion caused by the insulating effect of the cathode gas channel located opposite of the center of the large anode land. To limit anode water accumulation, a new experimental high diffusion resistance anode diffusion media was evaluated. This material was found to be effective at reducing anode water accumulation primarily due to the increased tortuosity of the material but also due to the higher thermal conductivity reducing the thermal transport effect. Material age was shown to reduce cell water content due to increased hydrophilic nature and increased thermal conductivity that increased from inlet to outlet with greater effects on the anode. A parametric study was performed to determine what parameters have the strongest influence on thermally driven transport. It was found that porosity and tortuosity of the diffusion media, and channel-land architecture can be engineered to drive water balance in a favorable direction. Temperature and thermal conductivity primarily influence the strength of thermal transport. This work demonstrated methods to properly design thermal management for optimized and predictive water transport

A portable triaxial cell for beamline imaging of rocks under triaxial state of stress

Acknowledgements The development of the cell was supported by the Research and Teaching Excellence Fund of the School of Engineering, University of Aberdeen. Experiments at BT2, NCNR were supported by UK Engineering and Physical Sciences Research Council grant number EP/N021665/1, NIST and the Physical Measurement Laboratory. Experiments at IMAT were supported by the UK STFC, Experiment number: 1910331 (https://doi.org/10.5286/ISIS.E.RB1910331).Peer reviewedPublisher PD

Demonstration of Focusing Wolter Mirrors for Neutron Phase and Magnetic Imaging

Image-forming focusing mirrors were employed to demonstrate their applicability to two different modalities of neutron imaging, phase imaging with a far-field interferometer, and magnetic-field imaging through the manipulation of the neutron beam polarization. For the magnetic imaging, the rotation of the neutron polarization in the magnetic field was measured by placing a solenoid at the focus of the mirrors. The beam was polarized upstream of the solenoid, while the spin analyzer was situated between the solenoid and the mirrors. Such a polarized neutron microscope provides a path toward considerably improved spatial resolution in neutron imaging of magnetic materials. For the phase imaging, we show that the focusing mirrors preserve the beam coherence and the path-length differences that give rise to the far-field moiré pattern. We demonstrated that the visibility of the moiré pattern is modified by small angle scattering from a highly porous foam. This experiment demonstrates the feasibility of using Wolter optics to significantly improve the spatial resolution of the far-field interferometer. Keywords: neutron imaging; Wolter optics; polarized neutron imaging; far-field interferometerNational Institute of Standards and Technology (U.S.) (Award 60NANB15D361

The Wyoming Survey for H-alpha. I. Initial Results at z ~ 0.16 and 0.24

The Wyoming Survey for H-alpha, or WySH, is a large-area, ground-based, narrowband imaging survey for H-alpha-emitting galaxies over the latter half of the age of the Universe. The survey spans several square degrees in a set of fields of low Galactic cirrus emission. The observing program focuses on multiple dz~0.02 epochs from z~0.16 to z~0.81 down to a uniform (continuum+line) luminosity at each epoch of ~10^33 W uncorrected for extinction (3sigma for a 3" diameter aperture). First results are presented here for 98+208 galaxies observed over approximately 2 square degrees at redshifts z~0.16 and 0.24, including preliminary luminosity functions at these two epochs. These data clearly show an evolution with lookback time in the volume-averaged cosmic star formation rate. Integrals of Schechter fits to the extinction-corrected H-alpha luminosity functions indicate star formation rates per co-moving volume of 0.009 and 0.014 h_70 M_sun/yr/Mpc^3 at z~0.16 and 0.24, respectively. The formal uncertainties in the Schechter fits, based on this initial subset of the survey, correspond to uncertainties in the cosmic star formation rate density at the >~40% level; the tentative uncertainty due to cosmic variance is 25%, estimated from separately carrying out the analysis on data from the first two fields with substantial datasets.Comment: To appear in the Astronomical Journa

Pore Microstructure Impacts on Lithium Ion Transport and Rate Capability of Thick Sintered Electrodes

Increasing electrode thickness is one route to improve the energy density of lithium-ion battery cells. However, restricted Li+ transport in the electrolyte phase through the porous microstructure of thick electrodes limits the ability to achieve high current densities and rates of charge/discharge with these high energy cells. In this work, processing routes to mitigate transport restrictions were pursued. The electrodes used were comprised of only active material sintered together into a porous pellet. For one of the electrodes, comparisons were done between using ice-templating to provide directional porosity and using sacrificial particles during processing to match the geometric density without pore alignment. The ice-templated electrodes retained much greater discharge capacity at higher rates of cycling, which was attributed to improved transport properties provided by the processing. The electrodes were further characterized using an electrochemical model of the cells evaluated and neutron imaging of a cell containing the ice-templated pellet. The results indicate that significant improvements can be made to electrochemical cell properties via templating the electrode microstructure for situations where the rate limiting step includes ion transport limitations in the cell

Intensifying inequality? Gendered trends in commercializing and diversifying smallholder farming systems in East Africa

While the commercialization and diversification of agricultural and livestock systems have been identified as key global strategies for climate change adaptation and mitigation, less is known as to the large-scale gendered impacts that are implicated in these transformations among smallholder crop and livestock farmers. This study explores these gender impacts across different farming systems and gender-respondent-household typologies using data from the Rural Household Multiple Indicator Survey (RHoMIS) in 2,859 households in three East African countries – Ethiopia, Kenya, and Tanzania. Female control scores over incomes or foodstuffs produced through both on and off farm activities were highest in farming systems that had more land and more livestock. However, increasing commercialization – defined herein as the increasing importance of crop and livestock sales to farm households – resulted in an overall decline in female control across all farming systems and gender-respondent-household typologies. In contrast, crop and livestock diversification were positively associated with female control across gender-respondent-household typologies. Analysis of specific crops and livestock products across farming systems and respondent typologies revealed women have far greater control over decisions related to consumption than decisions related to sales, although the gap between the two were less pronounced in lesser-valued livestock products (chickens, eggs). However, the analyses suggest that as sale of crops and livestock increase, female control over these areas could likely diminish, regardless of specific activity. The authors conclude that approaches to adapt to or mitigate climate change that rely on increasing market orientation of smallholder production will likely intensify men's control over benefits from production, whereas diversification will likely have a more positive impact on female control. Thus, climate adaptation strategies promoting increased diversification will likely have a more positive impact on women smallholders than commercialization alone. The authors recommend that when commercialization is the target intervention, it must be accompanied by a gender differentiated analysis of trade-offs and risks to mitigate the potential negative consequences shown in this study

Three Phase-Grating Moire Neutron Interferometer for Large Interferometer Area Applications

We demonstrate a three phase-grating moire neutron interferometer in a highly intense neutron beam as a robust candidate for large area interferometry applications and for the characterization of materials. This novel far-field moire technique allows for broad wavelength acceptance and relaxed requirements related to fabrication and alignment, thus circumventing the main obstacles associated with perfect crystal neutron interferometry. We observed interference fringes with an interferometer length of 4 m and examined the effects of an aluminum 6061 alloy sample on the coherence of the system. Experiments to measure the autocorrelation length of samples and the universal gravitational constant are proposed and discussed.U.S. Department of CommerceNational Institute of Standards and TechnologyCanada Excellence Research Chairs, Government of CanadaNatural Sciences and Engineering Research Council of CanadaU.S. Department of EnergyCanada First Research Excellence Fun