A majority of anion exchange membrane fuel cells (AEMFCs) reported in the literature have been unable to achieve high current or power. A recently proposed theory is that the achievable current is ...largely limited by poorly balanced water during cell operation. In this work, we present convincing experimental results – coupling operando electrochemical measurements and neutron imaging – supporting this theory and allowing the amount and distribution of water, and its impact on AEMFC performance, to be quantified for the first time. We also create new electrode compositions by systematically manipulating the ionomer and carbon content in the anode catalyst layer, which allowed us to alleviate the mass transport behavior limitations of H 2 /O 2 AEMFCs and achieve a new record-setting peak power density of 1.9 W cm −2 – a step-change to existing literature. Our efforts cast a new light on the design and optimization of AEMFCs – potentially changing the way that AEMFCs are constructed and operated.
There is a need to understand the water dynamics of alkaline membrane fuel cells under various operating conditions to create electrodes that enable high performance and stable, long-term operation. ...Here we show, via operando neutron imaging and operando micro X-ray computed tomography, visualizations of the spatial and temporal distribution of liquid water in operating cells. We provide direct evidence for liquid water accumulation at the anode, which causes severe ionomer swelling and performance loss, as well as cell dryout from undesirably low water content in the cathode. We observe that the operating conditions leading to the highest power density during polarization are not generally the conditions that allow for long-term stable operation. This observation leads to new catalyst layer designs and gas diffusion layers. This study reports alkaline membrane fuel cells that can be operated continuously for over 1000 h at 600 mA cm
with voltage decay rate of only 32-μV h
- the best-reported durability to date.
Enhancement of plant drought stress tolerance by plant growth‐promoting rhizobacteria (PGPR) has been increasingly documented in the literature. However, most studies to date have focused on ...PGPR‐root/plant interactions; very little is known about PGPR's role in mediating physiochemical and hydrological changes in the rhizospheric soil that may impact plant drought stress tolerance. Our study aimed to advance mechanistic understanding of PGPR‐mediated biophysical changes in the rhizospheric soil that may contribute to plant drought stress tolerance in addition to plant responses. We measured soil water retention characteristics, hydraulic conductivity, and water evaporation in soils with various textures (i.e., pure sand, sandy soil, and clay) as influenced by a representative PGPR (Bacillus subtilis strain UD1022) using the HYPROP system. We found that all PGPR‐treated soils held more water and had reduced hydraulic conductivity and accumulative evaporation, compared to their corresponding controls. We discuss three mechanisms, due to B. subtilis incubation or production of extracellular polymeric substances (EPS), that are potentially responsible for the changes in hydraulic properties and soil evaporation: (i) EPS have a large water holding capacity; (ii) EPS alter soil matrix structure and connectivity of pore space; (iii) EPS modify the physicochemical properties of water (surface tension and viscosity). These results clearly demonstrate PGPR's ability to increase water availability to plants by slowing down evaporation and by increasing the time available for plants to make metabolic adjustments to drought stress.
Plain Language Summary
PGPR is a group of beneficial bacteria known to improve plant growth by, e.g., reducing pathogenic infection and/or promoting drought/salt tolerance. Despite the important role PGPR could potentially play in reducing drought stress to plants, we lack a complete understanding on the mechanisms through which PGPR mediate plant tolerance to drought. This study aimed to advance mechanistic understanding of PGPR‐mediated biophysical changes in soil through microbe‐soil interactions, to complement better understanding gained from previous studies that focused on microbe‐plant interactions. Through laboratory measurements and imaging of water retention in soil, we show that a representative PGPR (B. subtilis UD1022) can increase soil water retention and reduce soil water evaporation. This effect is likely caused by the PGPR's ability to produce extracellular polymeric substances, which have high water holding capacity and can induce changes in soil physical properties. These changes lead to slower evaporation from soil, which can make more water available to plants as well as increase the time available for plants to make metabolic adjustments to drought stress. Our results provide scientific support to recent efforts in promoting application of rhizobacteria isolates as “underground resource” to contribute to solving globally challenging issues, e.g., water resource shortage and food security.
Key Points
Improved soil water holding capacity and reduced soil water evaporation were found for PGPR‐treated soil samples
EPS production was responsible for the changes in the observed hydraulic properties
Findings imply PGPR increase water availability, slow down the drying processes, and relieve the stress experienced by roots upon drought
There are conflicting views in the literature concerning the optimum moisture state for an existing substrate prior to the application of a repair material. Both saturated-surface-dry (SSD) and dry ...substrates have been found to be preferable in a variety of studies. One confounding factor is that some studies evaluate bonding of the repair material to the substrate via pull-off (direct tension) testing, while others have employed some form of shear specimens as their preferred testing configuration. Available evidence suggests that dry substrate specimens usually perform equivalently or better in shear testing, while SSD ones generally exhibit higher bond strengths when a pull-off test is performed, although exceptions to these trends have been observed. This paper applies a variety of microstructural characterization tools to investigate the interfacial microstructure that develops when a fresh repair material is applied to either a dry or SSD substrate. Simultaneous neutron and X-ray radiography are employed to observe the dynamic microstructural rearrangements that occur at this interface during the first 4 h of curing. Based on the differences in water movement and densification (particle compaction) that occur for the dry and SSD specimens, respectively, a hypothesis is formulated as to why different bond tests may favor one moisture state over the other, also dependent on their surface roughness. It is suggested that the compaction of particles at a dry substrate surface may increase the frictional resistance when tested under slant shear loading, but contribute relatively little to the bonding when the interface is submitted to pull-off forces. For maximizing bond performance, the fluidity of the repair material and the roughness and moisture state of the substrate must all be given adequate consideration.
We report on efforts to improve the achievable spatial resolution in neutron imaging by centroiding the scintillation light from gadolinium oxysulfide scintillators. The current state-of-the-art ...neutron imaging spatial resolution is about 10 μm, and many applications of neutron imaging would benefit from at least an order of magnitude improvement in the spatial resolution. The detector scheme that we have developed magnifies the scintillation light from a gadolinium oxysulfide scintillator, calculates the center of mass of the scintillation event, resulting in an event-based imaging detector with spatial resolution of about 2 μm.
•We describe an event based neutron imaging detector with spatial resolution of 2 μm.•Detector system is comprised of off-the-shelf components.•Future system optimization will improve image acquisition and reconstruction time.•Optimized detector on a cold neutron beam could realize 1 μm resolution.
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•Low to high porosity gradient in the anode PTL for high performance.•50% reduction in PTL gas with low to high porosity gradient.•Enhanced water permeability for low to high porosity ...gradient.
Decarbonizing society’s energy infrastructure is foundational for a sustainable future and can be realized by harnessing renewable energy for clean hydrogen and on-demand power with fuel cells. Here, we elucidate how graded porous transport layers (PTLs) are instrumental for high performance gas evolving electrochemical energy conversion devices, with an emphasis on polymer electrolyte membrane (PEM) electrolyzers. Spatially graded PTLs fabricated by vacuum plasma spraying are examined via in operando neutron imaging, electrochemical characterization, and pore network modelling. The results reveal the staggering benefits of positioning the lower porosity region adjacent to the catalyst layer and the higher porosity region adjacent to the flow field, which lead to current densities up to 4.5 A/cm2 with a 29 % reduction in cell potential, 38 % reduction in mass transport overpotential, and 50 % reduction in PTL gas saturation. The liquid water permeability of the PTL also enhances by an order of magnitude, with a drastic reduction in gas saturation adjacent to the catalyst layer. Custom graded PTLs have the potential to transform performance levels for a broad array of gas evolving electrochemical energy conversion devices.
Liquid water saturation profiles were determined using high resolution neutron radiography for commercially available fuel cell materials and hardware. Temperature, pressure, and relative humidity ...(concentration) gradients were imposed on the cell to determine individual influences on water content for each gradient. The asymmetric anode/cathode channel/land architecture used in this work results in significant water accumulation in the anode diffusion media with saturation values of up to ∼50%. Anode water content was found to change substantially with imposed pressure or concentration gradient, whereas the cathode saturation profile remained relatively consistent, indicating the channel/land ratio and thickness have a determinant role in diffusion media retention. The data generated in this work has been made publicly available through www.pemfcdata.org, and should be useful for computational modelers seeking validation data.
•High resolution neutron radiography used to determine through-plane water content.•Test conditions chosen to push limitations of multi-phase models.•Online database provides rich validation data for multiphase models.•Flow field geometry significantly influences diffusion media saturation levels.•Anode was more sensitive to water content with asymmetric flow field.
The primary removal of product water in proton exchange membrane (PEM) fuel cells is through the cathode gas diffusion layer (GDL) which necessitates the understanding of vapor and liquid transport ...of water through porous media. In this investigation, the effect of microporous layer (MPL) coatings, GDL thickness, and polytetrafluorethylene (PTFE) loading on the effective water vapor diffusion coefficient is studied. MRC Grafil, SGL Sigracet, and Toray TGP-H GDL samples are tested experimentally with and without MPL coatings and varying PTFE loadings. A dynamic diffusion test cell is developed to produce a water vapor concentration gradient across the GDL and induce diffusion mass transfer. Tests are conducted at ambient temperature and flow rates of 500, 625, and 750 sccm. MPL coatings and increasing levels of PTFE content introduce significant resistance to diffusion while thickness has negligible effects.
► An experimental apparatus for measuring effective water vapor diffusivity in a PEMFC gas diffusion layer is developed. ► Effective water vapor diffusivity in a PEMFC gas diffusion layer is measured for MRC Grafil, SGL Sigracet and Toray TGP-H samples at 25 °C. ► Effect of MPL and PTFE content on the effective water vapor diffusivity is measured. ► MPL coatings and increasing level of PTFE coating introduce significant resistance to diffusion, whereas the thickness has negligible effect for the samples investigated.
Neutron interferometry uniquely combines neutron imaging and scattering methods to enable characterization of multiple length scales from 1 nm to 10 µm. However, building, operating, and using such ...neutron imaging instruments poses constraints on the acquisition time and on the number of measured images per sample. Experiment time-constraints yield small quantities of measured images that are insufficient for automating image analyses using supervised artificial intelligence (AI) models. One approach alleviates this problem by supplementing annotated measured images with synthetic images. To this end, we create a data-driven simulation framework that supplements training data beyond typical data-driven augmentations by leveraging statistical intensity models, such as the Johnson family of probability density functions (PDFs). We follow the simulation framework steps for an image segmentation task including Estimate PDFs
Validate PDFs
Design Image Masks
Generate Intensities
Train AI Model for Segmentation. Our goal is to minimize the manual labor needed to execute the steps and maximize our confidence in simulations and segmentation accuracy. We report results for a set of nine known materials (calibration phantoms) that were imaged using a neutron interferometer acquiring four-dimensional images and segmented by AI models trained with synthetic and measured images and their masks.
Despite recent advancement in fuel cell technology, significant challenges remain in achieving high power density operation to meet the stringent targets of performance, durability and cost. This is ...due to the lack of fundamental understanding in interactive transports of oxygen, protons, heat, and water. In this study, we employed both experimental and analytical methods to study water onset condensation using Toray and Freudenberg diffusion media, which have different thermal and diffusion properties. Toray performs better under dry conditions, while Freudenberg has improved performance under wet conditions. The dry and wet effective diffusivities obtained using the in situ limiting current support the performance results. Neutron images show that liquid water exists throughout the layer of diffusion media for Toray material, but only under the land for Freudenberg keeping the area under the channel open for oxygen transport. To further understand this fundamental mechanism, we developed a 1-D model to simulate fuel cell performance. In addition, we identify the water condensation behavior is controlled by the product of thermal conductivity and the ratio of tortuosity to porosity. The findings provide new insights into improving material design and boosting energy conversion efficiency under a wide range of fuel cell operation conditions.
•Experiments and modeling are performed to study water condensation in a PEMFC.•Limiting current method is used to measure diffusivity of the diffusion media.•In situ liquid water distribution is visualized using neutron radiography.•A maximum of 45% liquid water saturation is detected in the diffusion media.•A parameter controlling the onset of water condensation location is identified.
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