Synchrotron Investigation of Microporous Layer Thickness on Liquid Water Distribution in a PEM Fuel Cell

et al. 2017

J. Electrochem. Soc.

Self Cite

The presence of multiwall carbon nanotubes (MWCNTs) corresponded to a dispersion of carbon black particles in the microporous layer (MPL) of the polymer electrolyte membrane (PEM) fuel cell. The gas diffusion layer (GDL) with a MWCNT-based MPL exhibited larger pores and a higher porosity compared to a conventional GDL, and less MPL intrusion into the GDL substrate was observed with the MWCNTs-based MPL. The GDLs were evaluated in operando in a fuel cell that was customized for concurrent liquid water visualization (synchrotron X-ray radiography) and electrochemical characterization. The MWCNT-based fuel cell exhibited higher power densities and lower mass transport resistances compared to the fuel cell with the conventional GDL; however, a higher liquid water saturation was observed for the MWCNT-based GDL. Although the liquid water saturation in the MWCNTbased GDL was higher, its higher effective porosity led to superior performance compared to the conventional fuel cell. The use of the MWCNTs-based MPL resulted in improved oxygen transport in the fuel cell, particularly at high current densities. The achievement of effective water management in the polymer electrolyte membrane (PEM) fuel cell remains a challenge. Excess liquid water accumulation in the gas diffusion layer (GDL) hinders the transport of oxygen in the fuel cell. Reduced oxygen diffusion in the liquid water saturated GDL leads to efficiency losses and unstable performance.1,2 However, the commonly used bi-layered GDL (containing a carbon fiber substrate and a microporous layer (MPL)) has been shown to enhance liquid water management and the overall performance of the fuel cell. [3][4][5][6][7][8] In recent years, significant effort has been devoted to developing novel carbon fiber substrates and MPLs.9-18 Thomas et al. 9 developed a novel method for preparing hydrophobic GDLs via the electrochemical reduction of diazonium salts. GDLs were prepared with homogenous hydrophobicity distributions in the absence of pore structure modifications. Nguyen et al. 10 introduced the direct fluorination of the GDL for providing homogenous hydrophobicity. Less liquid water content was observed in these novel GDLs compared to the conventional GDL (SGL 24 BC) when visualized with soft X-rays. In the works of Forner-Cuenca et al.,11,12 a patterning of wettability was applied to a Toray GDL through radiation grafting. Hydrophilic regions facilitated preferential pathways for liquid water transport, while hydrophobic regions facilitated gas transport. Chevalier et al.13 created a novel GDL via electrospinning and radiation grafting. Ko et al.14 proposed a novel GDL through the dry deposition of hydrophobic silicone nanolayer on carbon fibers. The longterm operational stability and dynamic response of the GDL were improved.On the other hand, a number of works have focused on MPL additives for improving water management in fuel cells.19-29 While conventional MPLs consist of carbon black particles and a hydrophobic agent, such as polytetrafluoroethylene (PTFE), recent ...

Section: Experimental Methodologymentioning

confidence: 99%

“…Haußmann et al 30,31 visualized liquid water accumulation in the cracks and large holes of the MPL, and they suggested that these macro features were prone to liquid water accumulation. Lee et al 34 investigated the impact of MPL thickness on the water distribution in the GDL. They observed that thinner MPLs resulted in smaller quantities of liquid water.…”

mentioning

confidence: 99%

Multiwall Carbon Nanotube-Based Microporous Layers for Polymer Electrolyte Membrane Fuel Cells

Lee

Banerjee

et al. 2017

J. Electrochem. Soc.

Self Cite

“…Synchrotron X-ray radiography offers high spatial and temporal resolutions (less than 10 μm and less than 10 s, respectively) [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]. Though X-ray photons are not as sensitive as a neutron beam to the presence of liquid water, Chevalier et al [29] demonstrated that synchrotron X-ray radiography can achieve similar accuracy to neutron radiography with a high level of precision.…”

Section: Visualizations Of Liquid Water Saturationmentioning

confidence: 99%

Liquid water saturation and oxygen transport resistance in polymer electrolyte membrane fuel cell gas diffusion layers

Muirhead

Banerjee

et al. 2018

Electrochimica Acta

In this thesis, the relative humidity (RH) of the cathode reactant gas was investigated as a factor which influences gas diffusion layer (GDL) liquid water accumulation and mass transport-related efficiency losses over a range of operating current densities in a polymer electrolyte membrane (PEM) fuel cell. Limiting current measurements were used to characterize fuel cell oxygen transport resistance while simultaneous measurements of liquid water accumulation were conducted using synchrotron X-ray radiography. GDL porosity distributions were characterized with micro-computed tomography (μCT). The work presented here can be used by researchers to develop improved numerical models to predict GDL liquid water accumulation and to inform the design of next-generation GDL materials to mitigate mass transport-related efficiency losses. This work also contributes an extensive set of concurrent performance and liquid water visualization data to the PEM fuel cell field that can be used for validating multiphase transport models.iii

“…41 Specifically, an initial guess was assigned to F, and the vertical deviation of each experimentally measured data point from the fit curve, χ, was determined as follows: where the subscript "meas" indicates that the value was obtained from the experimental measurement, and the subscript "calc" indicates that the value was obtained from Equation 10 with an assigned value of F. For each water thickness value, the least squares criterion was defined as the sum of the squares of the vertical deviations:…”

Section: Methodsmentioning

confidence: 99%

Considering Photon Scattering and Harmonics for Synchrotron X-ray Radiographic Imaging of Polymer Electrolyte Membrane Fuel Cells

Gao

Lee

et al. 2017

J. Electrochem. Soc.

We predicted the attenuated, undesired secondary scattered, and undesired harmonic components of measured X-ray intensities from synchrotron X-ray radiographic visualizations of liquid water in an operating polymer electrolyte membrane (PEM) fuel cell. The undesired secondary scattered component of the measured intensity increased as a function of the liquid water thickness (traversed by the X-ray beam). This increase in the secondary scattered component led to a decrease in the calibrated attenuation coefficient for liquid water, decreasing the accuracy of water quantification. We recommend calibrating the attenuated coefficient with a range of water thicknesses defined by the maximum expected water thickness present in the PEM fuel cell. The undesired harmonic component of the measured intensity also increased as a function of liquid water thickness, which led to a decrease in the accuracy of the measured liquid water thickness. Operating the polymer electrolyte membrane (PEM) fuel cell at high current densities is a strategy for achieving effective catalyst utilization and decreasing the cost of power.1 However, high current density operation corresponds to high rates of water production, and excess liquid water can lead to the inhibition of reactant transport and poor overall fuel cell performance. Therefore, effective water management is an important design objective for the PEM fuel cell.Synchrotron X-ray radiography, with its high spatial and temporal resolutions, has been used as a diagnostic tool to directly visualize liquid water transport in PEM fuel cells in operando.2-21 Radiographic images of the operating fuel cell during open circuit voltage (OCV) are termed dry-state images because we assume that excess liquid water is not present during this operating regime. For all non-zero current densities, radiographic images are termed wet-state images because of the possible appearance of additional liquid water with respect to the reference dry-state images (obtained at OCV). The cumulative liquid water thickness traversed by the X-ray beam, t w [cm], is quantified for each pixel of a collected two-dimensional (2D) wet-state image using the following relationship 22 derived from the Beer-Lambert law:where μ at [cm −1 ] is the linear attenuation coefficient for a monoenergetic X-ray beam and is a function of the photon energy, and the subscript "w" identifies the material as water. Herein, the measured intensity of the X-ray beam after traversing the sample is referred to as the attenuated intensity. I m,dr y is the measured attenuated intensity of a reference dry-state image (in the absence of liquid water), and I m,wet is the measured attenuated intensity of a wet-state image.The accuracy of liquid water thickness quantities determined from Equation 1 corresponds to the accuracies of the measured attenuated * Electrochemical Society Student Member. 23 determined that the vertical position of the incident beam oscillated with a significant amplitude of 25 μm, which resulted in image artifacts. The auth...