Effect of GDL Compression

Figure 25 displays the anisotropic effective oxygen diffusivity variations with liquid water saturation in the GDL based on the evaluated pore blockage correlations. Furthermore, the impact of GDL compression on the pore blockage effect was also investigated.67... 

The use of LIF in fuel cells is less extensive than PIV, but has produced valuable results. Bazylak et al.101 used ex-situ testing to quantify the effects that cell compression have on GDL hydropho-bicity by damaging the GDL and PTFE layer. Lozano et al.102,103 and Barreras et al.5 conducted a series of LIF experiments on mock fuel cells with various flow field designs (diagonal, serpentine, and... 

There are only a few reported studies that examine mechanical degradation and review the effect of compression of gas-diffusion layers on the performance of fuel cells [133-135]. Lee and Merida [133] characterized some GDL properties after 300 h. of compression at constant temperature in an ex situ test and found that the dry gas-phase permeabilities remained roughly constant. 

Wang, Y. and Chen, K.S. (2011) Effect of spatially-varying GDL properties and land compression on water distribution in PEM fuel cells.. /. Bectrochem. Soc., 158 (11), B1292 B1299. 

In the past years advanced characterization techniques such as synchrotron-based X-ray tomography have allowed to accurately determine the structure of the GDL materials including the effects of compression in the cell [4]. In combination with electrochemical methods [5, 6], this has lead to accurate determination of the effective diffusion coefficients as defined in Eq. 2. in dry GDL, depending on the compression and the direction (in- or through-plane), is in the range between 0.6 and 0.2 times the diffusion coefficient in the free volume (see Fig. 3). 

In Fig. 17.12, an example of a three-dimensional tomographic image volume is presented. This is a model of a Celtec P2100 MEA, which has been analyzed post-mortem by p-CT investigations. It shows the GDL material that has been significantly deformed by the compression forces of the serpentine flow field, the imprint of the flow flelds (channel and land area) is clearly visible, also showing the irreversible effect of compression on the GDL under the land area. 

The carbon powder corrosion in the MPL can also occur in the environment of an operating PEMFC. Porosimetry measurements indicate that carbon is lost from the MPL dining operation. " Porosimetry measurements have also shown that the GDL pore stmcture changes during lifetime tests. Large pore (30-60 pm diameter) volume has decreased, while small-pore volume increases. The loss in large-pore volume is probably due to irreversible compression due to cell compression. However, there are ordy a few hterature papers that examine mechanical degradation and review the effect of compression of the GDL on PEMFC performance. ... 

FIGURE 5.3 Approximate stress-strain curves for GDL materials calculated from representative, published data. (Reprinted from the Electrochimica Acta, 55, Karimi, G., Li, X., and Teertstra, P. Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media, 1619-1625, Copyright (2010) Journal of Power Sources, 196, Corrected proof, Sadeghi, E., Djilali, N., and Bahrami, M. Effective thermal conductivity and thermal contact resistance of gas diffusion layers in PEM fuel cells. Part 1 Effect of compressive load, 246-254, Copyright (2011), with permission from Elsevier.)... 

Several studies have been done over the last 5-7 years on the effects of inhomogeneous electrical and thermal gradients in operating cells or stacks as a result of poor GDL compression distribution (Eckl et al. 2006 Hwang 2006). 

Fig. 12 Le/i Effect of the flow configuration and methane conversion fraction (PR) on the stress. Case of an anode-supported cell with LSM-YSZ cathode and compressive gaskets, a Temperature profile and b First principal stress in the anode. The MIC is displayed in transparency, c First principal stress in the cathode (insert alxtve the symmetry line), d Contact pressure on the cathode GDL and compressive gasket and e vertical displacement along the z-axis, with an amplification factor of 2,000. Right column effect of creep in a cell based on a LSCF cathode and a temperature distribution, on b the evolution of the first principal stress in the anode support in operation and c during thermal cycling to RT and d evolution of the first principal stress in the GDC compatibility layer after thermal cycling. The profiles above and below the symmetry axis refer to different operation time [88, 89]. Reproduced here with kind permission from Elsevier 2012...

Polymer Electrolyte Fuel Cells, Mass Transport, Fig. 3 Effective relative diffusivity e/x as a function of the porosity of plain Toray 060 GDL (without PTFE) for different compressions (expressed in porosity), ip in-plane, tp through-plane. The gray lines indicate iso-tortuosity levels... 

Zamel et al. presented a study on the estimation of effective thermal conductivity of carbon paper GDL stmctures based on the aforementioned DNS formalism (Eq. 9.12). The 3D carbon paper GDL microstractures were reconstracted using the stochastic method by Schulz et al. They investigated the influence of fiber orientation, anisotropy, compression and binder fraction. Figure 9.27 shows the representative effective thermal conductivity prediction along with experimental data available in the literatirre." " ... 

PM) model, based on the mathematical morphology analysis of the digital porous structure, was presented. The PM model is a fast direct simulation method that provides two-phase distribution in the reconstmcted microstmctures. The DNS model was used to predict the effective diflfusivity in the unsaturated and partially saturated non-woven uncompressed and compressed GDL microstmctures. Furthermore, the estimation of the effective thermal conductivity of the carbon paper GDL microstmctures was presented. Finally, it is important to emphasize that understanding the influence of the underlying microstmctures on the effective properties and transport behavior is of paramount importance toward the development of high-fidelity, predictive performance models for the PEFC. 

Through-plane measurements of total GDL conductivity with varying compressive loads have been measured with four-probe arrangements in axially symmetric configurations (see Section 5.3.3). Several efforts with multiple GDL samples in a stack have been reported. However, the separation of interfacial and effective bulk conductivities (oj in thin, highly conductive samples can be challenging. 

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