Water Transport Phenomena Research Articles

Operando X-ray radiography of liquid water distribution in 100 mm polymer electrolyte fuel cell channels

Preventing the accumulation of water in the gas diffusion layer (GDL) proves effective in enhancing the performance of polymer electrolyte fuel cells (PEFCs). To understand the water transport phenomena in GDLs and channels of PEFCs, cell hardware for operando synchrotron X-ray radiography was developed with a 100 mm channel length, facilitating the separate quantification of liquid water in the cathode and anode GDLs. The presence of liquid water in the cathode and anode GDLs was confirmed during operation with a supply of dry gas in a counter-flow configuration. Furthermore, the amount of liquid water in the cathode and anode GDLs increased toward the cathode inlet, while the amount of water in the cathode and anode channel regions increased toward each outlet. The liquid water distribution in the GDLs along the channel direction can be attributed to water transport from cathode to anode (back-diffusion), decreasing toward the anode outlet. Therefore, conducting radiography experiments aligned parallel to the GDLs and perpendicular to the channel could provide valuable insights for a more comprehensive understanding of water transport in cells.

Computational evaluation of PEMFC performance based on bipolar plate material types

Materials used in bipolar plates are essential for addressing the challenges associated with the large-scale commercialization of Proton Exchange Membrane Fuel Cells (PEMFCs). Therefore, it is necessary to develop effective performance evaluation approaches to evaluate the performance of materials. This work has presented a high-fidelity mathematical model that includes electrochemical, fluid flow, energy, species transport, water formation, and transport aspects to simulate the conditions in a PEMFC system and to theoretically assess the performances of commercial bipolar plate materials. Accordingly, the effects of metallic (aluminum) and carbon-based (graphite and graphite composite) bipolar plate materials on the current collection performance, thermal management, water management, and overall efficiency of PEMFCs are analyzed. The findings suggest that the current density, power density, and operating voltage of each PEMFC are greatly affected by the bipolar plate material used. When compared to graphite bipolar plates, the aluminum and graphite composite bipolar plates have exhibited 16.5 % and 7.5 % higher power densities, respectively. The change in operating conditions and thermal environments caused by a change of bipolar plate materials gives rise to distinct water transport phenomena according to their respective capabilities in the balancing of liquid water movements across the membrane, which in turn affects the species distribution pattern. Specifically, the high imbalance between the electro-osmotic drag and back diffusion in the case of aluminum bipolar plate leads to a severe membrane dry-out on the anode side, limiting the hydrogen concentration in the region.

Spatial Distribution and Diffusion Characterisation of Water in Coal Samples: An Experimental Study

Comprehending the water absorption process inherent to coal, including the associated spatial distribution patterns of water, proves indispensable in the design and evaluation of coal pillar dams in underground water reservoirs. To better understand this process, a series of NMR (nuclear magnetic resonance) tests were carried out on cylindrically shaped coal samples immersed in water for varying durations, with the upper and lower surfaces of the samples sealed. A method involving image digital processing and finite element simulation was used to quantitatively characterise the water absorption process, as well as the spatial distribution of water in the samples. The results showed that NMR imaging colour brightness differences were positively correlated with water content and that the wetted ring gradually increased in width as the water immersion time increased. The expectation and sum of squared deviations of the pixel greyscale values of the NMR images, which were used to characterise the water saturation and spatial distribution of the coal samples, represented positive and negative exponential functions of the water immersion time, respectively. This indicated that the water saturation gradually increased and became more uniformly distributed. Furthermore, based on the set threshold value of the target variable rate of change, the limiting expectation of the pixel greyscale values was obtained, and the limiting water absorption time of the coal sample was predicted. The water diffusion equation was then used to characterise the water absorption process of the coal samples, and a water diffusion model was developed to accurately obtain the wet ring boundary data. A reasonable value of the diffusion coefficient was determined by comparing and correcting the results of the numerical simulation and physical experiments with full consideration of the non-homogeneity of the numerical model. This water diffusion model can better characterise the water transport phenomena in the macroscopic barrier zone of coal pillar dams. Finally, the application prospects in terms of practical engineering were investigated.

Recent Advancement in Solar‐Driven Interfacial Steam Generation for Desalination: A State‐of‐the‐Art Review

Solar energy is one of the most efficient origins of energy for a wide range of environmentally beneficial purposes. Water desalination by steam generation with the help of solar energy is not only an economical and straightforward approach, but it also utilizes free energy sources to solve the problem of increasing freshwater scarcity. Solar water evaporation is an essential component of the low‐energy method for generating fresh water, which is required for economic development and human health. Freshwater productivity determines how effectively the system captures incoming solar energy and transforms it into usable heat. Effective water distillation has recently gained a lot of attention. The photothermal conversion process is built on the performance of the evaporator. This review thoroughly examines the most recent developments in photothermal materials, structure design, and engineering strategies, including design principles for highly efficient photothermal conversion, thermal management, water transport phenomena, salt rejection behavior, and improved evaporation rate. The prospective applications of this technique in saline water desalination, waste water purification, and energy generation are highlighted. Furthermore, the most recent scientific advancements are utilized to demonstrate the potential, prospects, and challenges of solar‐driven evaporation in energy conversion.

Water vapor transport properties of bio-based multilayer materials determined by original and complementary methods

To enhance PLA gas barrier properties, multilayer designs with highly polar barrier layers, such as nanocelluloses, have shown promising results. However, the properties of these polar layers change with humidity. As a result, we investigated water transport phenomena in PLA films coated with nanometric layers of chitosan and nanocelluloses, utilizing a combination of techniques including dynamic vapor sorption (DVS) and long-term water vapor adsorption–diffusion experiments (back-face measurements) to understand the influence of each layer on the behavior of multilayer films. Surprisingly, nanometric coatings impacted PLA water vapor transport. Chitosan/nanocelluloses layers, representing less than 1 wt.% of the multilayer film, increased the water vapor uptake of the film by 14.6%. The nanometric chitosan coating appeared to have localized effects on PLA structure. Moreover, nanocelluloses coatings displayed varying impacts on sample properties depending on their interactions (hydrogen, ionic bonds) with chitosan. The negatively charged CNF TEMPO coating formed a dense network that demonstrated higher resistance to water sorption and diffusion compared to CNF and CNC coatings. This work also highlights the limitations of conventional water vapor permeability measurements, especially when dealing with materials containing ultrathin nanocelluloses layers. It shows the necessity of considering the synergistic effects between layers to accurately evaluate the transport properties.

Mass Transport in a Gas-Fed CO2 Electrolyzer with a Forward-Bias Bipolar Membrane

In the context of synthetic fuel and chemical production, CO2 electrolyzers using bipolar membranes (BPM) in forward bias have proven to overcome current limitations in state-of-the-art alkaline CO2 electrolyzers. These limitations are CO2 cross-over and carbonate deposition in the cathode catalyst layer, which limit the large-scale development of alkaline CO2 electrolyzers. Devices with a BPM in forward bias (Figure 1) solve these issues by setting the anode under acidic conditions while keeping the cathode alkaline. This configuration prevents carbonate ions to reach the anode and thus suppresses CO2 cross-over. Moreover, it allows pure water as an anolyte. Pure water eradicates unwanted cation cross-over and thus prevents carbonate deposition at the cathode. However, the use of a BPM for CO2 also has limitations. The most prominent ones are low stability and high over-voltage. These are partly caused by the complex and unexplored water transport.This work presents efforts to characterize and optimize different water transport phenomena. For this, we study cells of different complexity. It implies understanding first the water diffusion across the membrane, by setting a water concentration gradient between the electrodes and analyzing water transport by dew point measurement. The membrane hydration state can be determined with the simultaneously measured ion conductivity. Then, the water transport of a full cell is measured in parallel to its electrochemical performance. In this setup, a gas chromatograph and a mass spectrometer measure the selectivity of the cathode reaction.The optimal conditions at the cathode are a subtle balance between excess and scarcity of water. Water is a reactant in the CO2 reduction reaction and helps to humidify the membrane, ensuring good ionic conductivity. Additionally, water is formed by ion recombination at the BPM junction affecting the membrane hydration state. Still, we observe that the water supply at the cathode can be insufficient, especially at high current density. However, too much water can flood the cathode and block CO2 transport to the catalyst layer.In summary, we deliver a clearer picture of the water paths and demonstrate that optimized material and operating conditions help to overcome current limitations. Figure 1

Liquid water transport phenomena in the porous transport layer of proton exchange membrane fuel cell based on lattice Boltzmann simulation

The porous transport layer (PTL) of proton exchange membrane fuel cell (PEMFC) plays an important role in water management. The liquid water transport behavior with real density ratio and viscosity ratio is discussed by establishing a multi-relaxation time (MRT) multiphase flow lattice Boltzmann model. The effects of temperature, wettability and microporous layer (MPL) thickness on the water transport process and water distribution are included. As the temperature increased from 278.15 K to 353.15 K, the water saturation in the PTL decreased from 0.501 to 0.219. The breaking phenomenon of water clusters are captured at temperatures of 328.15 K and 353.15 K. The hydrophobic structure has less water content under the ribs and the whole, which is beneficial to the reaction gas transport and the performance improvement of PEMFC. Adding MPL with a thickness of 25 µm can significantly improve water management. However, excessively thick MPL will drive water to flow under the ribs and reduce the permeability of the structure. Appropriate MPL thickness design is a feasible solution to improve water management.

Water behavior at PEMFC triple phase boundary: Exploring ionomer and catalytic layer effects via molecular dynamic simulations and NMR experiments

Polymer electrolyte membrane fuel cells (PEMFCs) offer a clean and efficient energy generation technology. However, optimizing their performance in terms of water transport through the electrolyte membrane remains a challenge. In this study, we employ molecular dynamics simulation and nuclear magnetic resonance (NMR) experiments to investigate the influence of carbon materials and membrane thickness on water diffusion within the triple phase boundary of a PEMFC. Our theoretical investigation highlights the significant impact of Nafion content on water morphology within the system. The structural analysis reveals the intricate interplay between material properties and water behavior in confined environments. The formation of water channels facilitated by the ionomer enhances water mobility, while excessive ionomer content leads to reduced diffusion coefficients. Experimental measurements using NMR diffusion validate the correlation between our simulation results and real-world observations. Furthermore, our Molecular Dynamic (MD) and NMR studies identify the optimal performance range for ionomer content, with systems containing 12 to 24 oligomers (20–30 % w/w) exhibiting superior water diffusion within Vulcan Carbon (VC) structures. By shedding light on water transport phenomena in PEMFCs, our findings contribute to a better understanding of fuel cell performance and provide valuable insights for the design and optimization of fuel cell systems.

Understanding and optimizing water transport phenomena in the catalyst layer for anion exchange membrane fuel cells

Anion exchange membrane fuel cells (AEMFCs) are a promising power source due to zero emissions and high efficiency, and low precious metal loading. However, water management remains a significant issue affecting cell performance and durability. In this study, we develop a multi-physics model to investigate and optimize transport phenomena under various operational conditions and microstructure parameters. The agglomerate model is optimized to consider the influence of anode flooding and cathode drying in electrochemical reactions, and the simulation is validated with experimental results. Our findings indicate that an optimal anode inlet gas relative humidity of 80% achieved a tradeoff between anode flooding and ionic conductivity, resulting in a maximal power density of 758.9 mW/cm2. To alleviate anode flooding, we optimize the microstructure of the anode gas diffusion layer, significantly improving maximal power density to 833.4 mW/cm2 by increasing the contact angle to 130°. The optimal porosity achieves a balance between the transport of liquid water and electron. Large pore diameter and small thickness lead to a slighter anode flooding and higher maximal power density. In summary, our study provides insights into the transport behavior of multi-phase water, reactant gas, ion, and electron to guide the design of high-performance AEMFCs.

Mechanisms and models for water transport in reverse osmosis membranes: history, critical assessment, and recent developments.

Water scarcity is one of the greatest societal challenges facing humanity. Reverse osmosis (RO) desalination, a widely used membrane-based technology, has proven to be effective to augment water supply in water-stressed regions of our planet. However, progress in the design and development of RO membranes has been limited. To significantly enhance the performance of RO membranes, it is essential to acquire a deep understanding of the membrane separation and transport mechanisms. In this tutorial review, we cover the pivotal historical developments in RO technology, examine the chemical and physical properties of RO membrane materials, and critically review the models and mechanisms proposed for water transport in RO membranes. Based on recent experimental and computational findings, we conduct a thorough analysis of the key transport models-the solution-diffusion and pore-flow models-to assess their validity for accurately describing water transport in RO membranes. Our analysis involves examining the experimental evidence in favor of the solution-diffusion mechanism. Specifically, we explain whether the water content gradient within the membrane, cited as evidence for the key assumption in the solution-diffusion model, can drive a diffusive transport through RO membranes. Additionally, we review the recent molecular dynamics simulations which support the pore-flow mechanism for describing water transport in RO membranes. We conclude by providing future research directions aimed at addressing key knowledge gaps in water transport phenomena in RO membranes, with the goal of advancing the development of next-generation RO membranes.

A HYBRID FRACTIONAL-DERIVATIVE AND PERIDYNAMIC MODEL FOR WATER TRANSPORT IN UNSATURATED POROUS MEDIA

Richards’ equation is a classical differential equation describing water transport in unsaturated porous media, in which the moisture content and the soil matrix depend on the spatial derivative of hydraulic conductivity and hydraulic potential. This paper proposes a nonlocal model and the peridynamic formulation replace the temporal and spatial derivative terms. Peridynamic formulation utilizes a spatial integration to describe the path-dependency, so the fast diffusion process of water transport in unsaturated porous media can be captured, while the Caputo derivative accurately describes the sub-diffusion phenomenon caused by the fractal nature of heterogeneous media. A one-dimensional water transport problem with a constant permeability coefficient is first addressed. Convergence studies on the nonlocal parameters are carried out. The excellent agreement between the numerical and analytical solutions validates the proposed model for its accuracy and parameter stability. Subsequently, the wetting process in two porous building materials is simulated. The comparison of the numerical results with experimental observations further demonstrates the capability of the proposed model in describing water transport phenomena in unsaturated porous media.

Fuel Cell Imaging with a Wolter Optics Neutron Microscope

Wolter optics promise to radically transform neutron imaging by improving the time resolution of the method by at least a factor 1,000 while providing spatial resolution of order a few micrometers [1]. These optics will enable neutron tomography of operating fuel cells with a time resolution of about a few minutes, which will provide unique insight into the water transport phenomena in membrane electrode assemblies of fuel cells and electrolyzers.Neutron imaging has played a critical role in understanding water transport in proton exchange membrane fuel cells and electrolyzers [2]. The primary limitation of the method is the inherent neutron source strength, which is about a billion times weaker than X-ray synchrotron sources. Additionally, neutrons are difficult to focus using refractive lenses so that neutron imaging beamlines heretofore can be described as pinhole cameras. In a pinhole camera, the best achievable spatial resolution (λg) is determined by the diameter of the pinhole (D ~1 cm), the pinhole-to-detector distance (L ~10 m), and the sample-to-detector distance (Z ~1 cm):λg = Z D / (L – Z) ~ Z D / LReturning to the source strength, the neutron fluence rate at the sample position scales as (D / L)2. This means that higher spatial resolution for finite sized objects requires sacrificing neutron intensity and incurring long exposure times. Typical measurements require about 20 minutes for a single two-dimension image of the water content in fuel cell for a spatial resolution of about 10 µm. Such time requirements preclude the use of tomography, which is required to understand the water content in real fuel cell components since the MEA cannot be considered to be planar at the few micrometer level.We will describe Wolter optics, and the optical system that will be first available in 2022. After the NCNR cold source upgrade is anticipated to yield a time resolution gain of about 10,000 compared to a pinhole camera. We will discuss our expected image acquisition and analysis procedures guided by ray tracing simulations.Figure 1. Proof of concept Wolter optics image of a fuel cell test section.[1] D. S. Hussey et al., “Design of a neutron microscope based on Wolter mirrors,” Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip., vol. 987, no. October 2020, p. 164813, 2021.[2] T. A. Trabold, J. P. Owejan, J. J. Gagliardo, D. L. Jacobson, D. S. Hussey, and M. Arif, “Use of neutron imaging for proton exchange membrane fuel cell (PEMFC) performance analysis and design,” Handb. fuel cells, vol. 6, no. Table 1, 2010. Figure 1

Effects of MPL Pore Size Distribution on Components of Oxygen Transport Resistance Increased Due to Water Accumulation in a PEFC

To increase the power output of PEFC, management of generated water is extremely important. A gas diffusion layer (GDL) with a microporous layer (MPL) on the cathode side is thought to play important roles in drainage of product water and optimizing oxygen transport, but the details of the mechanism have not been elucidated. In this study, to reveal the water and oxygen transport phenomena, the components of oxygen transport resistance were evaluated using the limiting current analysis (1) for two types of MPL with different pore size distributions. Further, the water distributions in the cathode side MPL and catalyst layer (CL) were observed by a freezing method and a cryo-SEM (2). The active area of the cell used in this study was 1.8 cm2 (0.9 cm × 2.0 cm). The anode and cathode flow fields were parallel straight type with 1.0 mm wide ribs and channels. The anode side was supplied with hydrogen, and the cathode side was supplied with a mixed gas of oxygen and nitrogen. The gas flow rates were 500 sccm for the anode and 4000 sccm for the cathode. The relative humidity of the supplied gases was 90%. Experiments were conducted under two different cell temperature, 30°C (low temperature condition) and 80°C (high temperature condition). The catalyst coated membrane was produced by Japan Gore (PRIMEA). We used two types of MPL with different pore size distributions for the cathode GDL: MPLs with one peak (conventional type) and with two peaks. The MPLs are called MPL-1 and MPL-2, respectively. For the anode side, a GDL with an MPL (SGL, 28 BC) was used. This study used the limiting current analysis developed by the authors (1). The developed method extended the conventional method to the separation under flooded conditions, considering vapor transport and water distribution. Inside the CL, it was assumed that the water production rate dominates the water accumulation, which is proportional to current density. Outside the CL, it was assumed that the product of total pressure and current density determines the water accumulation (1). The experiments were conducted with different back pressures (110 kPa, 140 kPa, 180 kPa) and different oxygen concentrations (1%, 2%, 4%, 6%, 8%, 12%, 15%, 18%, 21%) for the cathode side. Figure 1 shows the total oxygen transport resistances at 110 kPa. There is a significant difference in the tendencies of the total oxygen transport resistance between the MPL-1 and the MPL-2 under the low temperature condition, while the tendencies are similar under the high temperature condition. This suggests that the effects of water accumulated in the cell are larger at lower temperature. Figure 2 shows the results separating the total oxygen transport resistance into two resistances. The I Lim is the limiting current density. The R P and R NP are the pressure-dependent and -independent oxygen transport resistances. These correspond to the oxygen transport resistances outside and inside the CL. Under the low temperature condition, the pressure-dependent oxygen transport resistances (corresponding to those outside the CL) are different between the MPL-1 and the MPL-2. The pressure-independent resistances (corresponding to those inside the CL) are similar. These mean that the smaller total oxygen transport resistances with the MPL-2 in Figure 1 are caused mainly by the smaller resistance outside the CL. Before the separation, we predicted that the increase in the total oxygen transport resistance is suppressed by the MPL-2 due to enhanced drainage of product water from the CL. However, Figures 2(a) and (c) revealed that the R P of the MPL-2 becomes smaller than that of the MPL-1 rather than the R NP. This may be considered because the larger pores in the MPL-2 form drainage pathway for product water at the low temperature condition, which significantly contributes to increasing the oxygen transport pathway through the smaller pores. As expected from Figure 1, there is no significant difference in the R P and the R NP between two types of the MPL under the high temperature condition (Figures 2(b) and (d)). The water distribution in the vicinity of MPL-2 was observed after discontinuing operation at 2.0 A/cm2 just before the limiting current condition. The experiment was conducted with 30°C, 180 kPa back pressure, and 21% oxygen concentration. Cryo-SEM images showed that there was ice at the MPL/CL interfaces. Additionally, ice crystals were observed at the larger pores in the MPL-2, supporting the above consideration, the drainage pathway for product water through the larger pores.Reference (1) Y. Iizuka, et al., ECS Transactions, 104(8), 83-92 (2021). (2) Y. Aoyama, et al., Electrochem. Commun., 41, 72-75 (2014). Figure 1

Visualization of Water Distribution in Fuel Cell Microporous and Catalyst Layers with 3D Nanoscale X-Ray Imaging

As an efficient green energy technology, polymer electrolyte membrane fuel cells (PEMFCs) are rapidly growing in a wide range of applications. Development of high power-density PEMFCs is under investigation to further reduce the overall cost and footprint. However, in such operational conditions, excessive water production could restrict the transport of reactants due to flooding, and subsequently degrade the performance of the PEMFC. This makes the water management a major challenge and necessitates a good understanding of water distribution and transport within the porous layers of the PEMFC [1, 2].3D X-ray computed tomography (XCT) is a strong tool for visualizing the water distribution within porous media. Micro-scale XCT has shown great promise in understanding and visualizing the water behavior within the gas diffusion layer (GDL) substrate region possessing ~10-100 µm pores [3–6]. This resolution, however, is not sufficient to capture water distribution in microporous and catalyst layers (MPL and CL), which are predominantly composed of nano-scale pores. This work aims to explore a method of water capture and visualization within the MPL and CL of a PEMFC using a lab-based nano-resolution XCT (NXCT) with a spatial resolution as high as 50 nm. Benefitting from the non-invasive NXCT imaging technique, the same spot of a dry/wet material has been studied in Zernike phase contrast mode using a custom-built enclosed fixture with high X-ray throughput to maintain a constant humidity (100% RH) over the tomographic acquisition period. Water domains have been identified after alignment of wet and dry image data sets followed by subtraction of dry from wet data set. Figure 1a shows a 2D projection of the MPL on AvCarb GDL after vacuum-assisted water infiltration. A gold microsphere is fixed on the sample within a thin layer of epoxy to allow for accurate identification of the region of interest. Figure 1b shows a selected segmented 2D projection of wet MPL illustrating how liquid water clusters are distributed within the pores and the 3D volume rendering of structural features is displayed in Figure 1c. Using the above-described method, 18% and ~7% volume fraction of liquid water has been identified within the wetted MPL and CL, respectively. The methodology discussed here is a step toward deep understanding of water transport phenomena in CL and MPL materials dictating the strategies for materials design to prevent flooding under high current densities. Moreover, the set-up and methodology can be further optimized to visualize water domains under controlled temperature and relative humidity, to simulate the real operating conditions. Figure 1. XCT imaging of a wet MPL: (a) A radiograph of MPL on AvCarb GDL after vacuum-assisted water infiltration; (b) a selected 2D projection of MPL showing the segmented solid (green), liquid water (blue), and unoccupied pores (black) domains; and (c) 3D volume rendering of a subdomain in the MPL showing the distribution of liquid water within the pores of the structure (voxel size 32 nm in b and c). Acknowledgement Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Ballard Power Systems, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Canada Research Chairs.

Electrospun nanofibrous membranes with asymmetric wettability for unidirectional moisture transport, efficient PM capture and bacteria inhibition

The performance of moisture management, PM capture, and bacteria inhibition are significant for air cleaning materials. Herein, a nanofiber composite PVDF/PAN-CTAB membrane with multiple functions was constructed via a facile electrospinning strategy. Incorporating cetyltrimethyl ammonium bromide (CTAB) in the windward PAN layer simultaneously improved membrane moisture transport rate, antibacterial performance, and PM capture ability. The enhanced moisture management performance was achieved by regulating the microstructure of the nanofibers and the thickness ratio of the asymmetric wetting layers. Hydrophobic-hydrophilic interface superimposed force on water droplets was measured to thoroughly investigate directional water transport behavior. Consequently, the prepared PVDF/PAN-CTAB membrane possesses a water vapor transport rate of >13 kg m−2 d−1, PM0.3 removal efficiency of ≥99% with a low pressure drop of ∼84 Pa, and an E. coli inhibition rate of 99.99%. This work would pave the way for engineering asymmetric wettability structure to improve the moisture management performance of antibacterial air cleaning membrane, and provides a novel scientific sight on directional water transport phenomenon based on force test and analysis.

Fuel Cell Imaging with a Neutron Microscope Based on Wolter Optics

Wolter optics promise to radically transform neutron imaging by improving the time resolution of the method by at least a factor 1,000 while providing spatial resolution of order a few micrometers [1]. These optics will enable neutron tomography of operating fuel cells with a time resolution of about a few minutes, which will provide unique insight into the water transport phenomena in membrane electrode assemblies of fuel cells and electrolyzers.Neutron imaging has played a critical role in understanding water transport in proton exchange membrane fuel cells and electrolyzers [2]. The primary limitation of the method is the inherent neutron source strength, which is about a billion times weaker than X-ray synchrotron sources. Additionally, neutrons are difficult to focus using refractive lenses so that neutron imaging beamlines heretofore can be described as pinhole cameras. In a pinhole camera, the best achievable spatial resolution (λg) is determined by the diameter of the pinhole (D ~1 cm), the pinhole-to-detector distance (L ~10 m), and the sample-to-detector distance (Z ~1 cm):λg = Z D / (L – Z) ~ Z D / LReturning to the source strength, the neutron fluence rate at the sample position scales as (D / L)2. This means that higher spatial resolution for finite sized objects requires sacrificing neutron intensity and incurring long exposure times. Typical measurements require about 20 minutes for a single two-dimension image of the water content in fuel cell for a spatial resolution of about 10 µm. Such time requirements preclude the use of tomography, which is required to understand the water content in real fuel cell components since the MEA cannot be considered to be planar at the few micrometer level.We will describe Wolter optics, and the optical system that will be first available in 2022. After the NCNR cold source upgrade is anticipated to yield a time resolution gain of about 10,000 compared to a pinhole camera. We will discuss our expected image acquisition and analysis procedures guided by ray tracing simulations.Figure 1: Proof of concept Wolter optics image of a fuel cell test section.

Graphene oxide proton permselective membrane for electrodialysis-based waste acid reclamation: Simulation and validation

Nowadays, the development of novel technology for economical and efficient reclamation of waste acid containing metal ions is still the top priority. The graphene oxide (GO)-based membrane has gained attention in the relevant membrane processes driven by pressure gradient, concentration difference and even Donnan potential because of its special two-dimension (2D) structure and abundant hydrophilic sites. However, its work performances for electrodialysis (ED)-based acid recovery process are still unknown. In this work, GO-based proton permselective membranes (PPM) are designed and applied in the ED-based acid recovery. Above all, the possibility of selective separating protons from metal cations by GO membrane under a direct electric field and the related influencing factors are comprehensively explored by molecular dynamic simulation. Furthermore, a series of GO-based PPMs with designed thickness, interlayer spacing and charged state are prepared by adjusting GO loading, employing different crosslinking agents and attaching the relevant polyelectrolytes. Therein, the work performances of GO-based PPMs in a typical ion substitution ED process, including the metal cation leakage, water transport phenomenon and nonideal migration of anions are investigated and compared with those of commercial membrane. Results show that GO membranes prepared and used according to the optimized conditions embrace sharp separating property between protons and metal ions which is hardly deteriorated along with the change of their relative content. On the other hand, it is noticed that the leakage of organic anions and water transport seem to be unavoidable due to the exploitation of the porous support. Obviously, this work can contribute to not only developing a novel PPM for ED but extending the applications of the 2D nanomaterial as well.

Air‐liquid water transport phenomena in a proton exchange membrane fuel cell cathode with a leaf‐like flow field design

Air‐liquid water transport phenomena in a proton exchange membrane fuel cell cathode with a leaf‐like flow field design

Dried Figs Quality Improvement and Process Energy Savings by Combinatory Application of Osmotic Pretreatment and Conventional Air Drying.

This study concerns the implementation of osmotic dehydration (OD) as a pre-treatment of air-drying in fig halves, aiming at drying acceleration, energy savings and product quality improvement. The effect of solid/liquid mass ratio, process temperature (25–45 °C) and duration (up to 300 min) on water activity (aw) and transport phenomena during OD, was modelled. The effective diffusion coefficients, drying time and energy consumption, were also calculated during air-drying at 50–70 °C. At optimum OD conditions (90 min, 45 °C), the highest water loss and solid gain ratio were achieved, while the aw (equal to an initial value 0.986) was decreased to 0.929. Air-drying time of OD- and control samples was estimated at 12 and 21 h, at 60 °C, respectively, decreasing the required energy by up to 31.1%. Quality of dried figs was systematically monitored during storage. OD-assisted air-drying led to a product of improved quality and extended shelf-life.

Analyzing and Modeling of Water Transport Phenomena in Open-Cathode Polymer Electrolyte Membrane Fuel Cell

Water management is one issue that must be surpassed to ensure high membrane proton conductivity and adequate reactant transport in the membrane-electrode assembly (MEA) simultaneously. A well-designed water management system is based on a comprehensive understanding of water transport in the inner part of the polymer electrolyte membrane (PEM) fuel cell. In this work, the water transport phenomena in the MEA PEM fuel cell are analyzed by using a mathematical model. The transport of diluted species interface is used to model the transport of water in the ionomer phase in the catalytic layer and the membrane domains. The molecular flux of water is defined using Nernst–Planck equations, including migration and Fickian diffusion using parameters obtained experimentally for diffusivity and mobility based on water drag for a fully humidified membrane. The proposed model 1D model includes anode gas channel, cathode gas channel, anode gas diffusion layer (GDL), cathode GDL, anode catalyst layer, cathode catalyst layer, and proton exchange membrane. Water activity, ionomer conductivity, and output voltage are predicted by changing the humidity on the anode side of the fuel cell.