Effect of cyclic compression on structure and properties of a Gas Diffusion Layer used in PEM fuel cells

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Abstract

Proton Exchange Membrane Fuel Cell (PEMFC) components are known to deteriorate during use, in time scales much shorter than that required for commercially successful deployment of this technology. Therefore, servicing operations such as identifying and replacing poorly performing components will likely be required to extend the operational lifetime of PEMFC stacks. During such servicing operations fuel cell components are subjected to cyclic compression and expansion due to opening and rebuilding of the fuel cell stack. This cyclic compression may further contribute to the deterioration of PEMFC components. There are several reports in the literature showing the effect of static compression on change in GDL properties, however the present work focuses on the effect of cyclic compression on GDL properties, an aspect that has not been reported in detail elsewhere. This paper focuses on the impact of cyclic compression on the Gas Diffusion Layer (GDL). The results indicate that cyclic compression causes significant and irreversible changes to the structure and properties of the GDL such as surface morphology, surface roughness, pore size, void fraction, thickness, electrical resistance, contact angle, water uptake and in-plane permeability. The implications of these results are considered.

Introduction

Proton Exchange Membrane Fuel Cells (PEMFC) have several advantages when compared with other types of fuel cells such as quick start up, higher specific power and relatively low operating temperatures. PEMFCs are therefore actively being considered for commercialization in the near future. However, there are several durability issues, which remain to be addressed to make these fuel cells commercially viable [1].

To be commercially successful, it is necessary to develop PEMFCs that have long operational lifetimes, which is about 5500 h for automotive applications and about 10,000 to 40,000 h for stationary applications [2]. The present state of the art materials and components used in PEMFCs are known to deteriorate in much shorter time scales. Further, such deterioration is not necessarily uniform across the active area of a single fuel cell or across the various cells in the fuel cell stack. Strategies therefore need to be developed to counter performance degradation during use. One such strategy will likely be to service PEMFC stacks in various ways after deployment. Such servicing operations will need to identify the specific cell or component that has deteriorated and either repair or replace the component thereby extending the lifetime of the overall stack. Barring possible in-situ repair schemes, these servicing operations, will inevitably involve opening and rebuilding of the fuel cell stack on a number of occasions over the course of the operational lifetime of the PEMFC stack, a process that will result in cyclic compression of all of the components of the PEMFC stack. Therefore, to enable extended lifetimes for PEMFC stacks, the impact of cyclic compression on fuel cell components needs to be examined and understood.

Whereas the impact of steady state compression on fuel cell components has been reported, and are discussed below, there are hardly any reports specifically focused on the unique additional impact of cyclic compression on fuel cell components [3]. In this work we systematically study the impact of cyclic compression on one of the PEMFC components, the Gas Diffusion Layer (GDL).

The GDL is a porous layer placed in between the flow field and catalyst layer in a fuel cell. The porous nature of this layer allows effective diffusion of reactant gases to the catalyst surface. The electronically conducting GDL also provides paths of reduced electrical resistance for the current produced at the catalyst layer to reach the flow distributing plates and eventually to the external circuit. Another important task of GDL is to manage water effectively to meet the opposing requirements of keeping the membrane humidified for better proton conductivity and to remove liquid water from the catalyst layer ensuring better mass transport in the fuel cell. Typically GDLs are made of either carbon paper or carbon cloth. GDLs are usually wet-proofed with Poly Tetra Fluoro Ethylene (PTFE) for efficient water management [3].

In a typical PEM fuel cell, the GDL undergoes steady state compression at the rib of the flow field channel. Numerous studies in the literature have focused on the impact of this steady state compression on the structure and properties of the GDL are discussed below. Compression at the rib is seen to cause the properties of the GDL to be position dependent [4]. Changing the clamping pressure, applied on the cell hardware, can alter the performance of the cell [5]. At lower levels of clamping pressure, the cell performance improves with increase in the clamping pressure. This improvement is attributed to reduced contact resistance between the GDL and flow field plates. Further increase in clamping pressure results in reduced cell performance because of GDL crushing, which manifests as higher mass transport resistance in the cell. Thus there is an optimal compression for a cell. This optimal compression is further dependent on the operating current density, since increased mass flow is required to support higher current densities.

Lin et al. [6] studied the effect of compression on fuel cell performance, for two different carbon cloth diffusion media, and identified optimum compression pressures for the two media. Bazylak et al. [7] visualized, using Fluorescent Microscopy, that regions of GDL under compression act as preferential pathways for liquid water transport. They have also indicated that there is a degradation of the PTFE coating due to the compression. Escribano et al. [8] concluded that knowledge of material property is vital to increase the performance of GDL and that the applied compressive stress on the GDL has to be minimized for better performance and durability. Lee et al. [9] studied the effect of bolt torque on cell performance and concluded that the combination of bolt torque applied, gasket used and compressibility of the GDL determined the clamping pressure acting in the cell and that determines the cell performance. Gostick et al. [10] experimentally determined the variation of in-plane and through plane gas permeabilities as a function of GDL compression and found that there exists in-plane anisotropy in permeability of the GDL. Ihonen et al. [11] conducted both in-situ and ex-situ experiments and concluded that clamping pressure affects both contact resistance and thermal impedance and plays a vital role in water management in a cell.

Recently Chi et al. [12] from their 3 dimensional computational model concluded that compression of GDL results in an oscillation of temperature, heat flux and , species concentration between the rib and channel. Amplitude of oscillations increases with compression ratio. Also the temperature under the rib is lesser causing saturation and results in flooding. Zhou and Wu [13] found that the deformation due to GDL compression is significant at higher operating current density, higher humidity and thicker catalyst layer. Su et al. [14] predicted from their numerical model that the porosity and permeability of the GDL determines the cross flow from channel to channel through the GDL. Ahmed et al. [15] developed a numerical model and inferred that the isotropic permeability of GDL can vary by orders of magnitude because of clamping pressure. Such variation in clamping permeability impacts the water and thermal management in PEMFC. Hottinen et al. [16]. reported that in-homogeneous compression of the GDL results in temperature gradients over the cell active area. This temperature gradient is because of reduced contact resistance and increased thermal conductivity of the GDL under compression [16], [17].

Most of the studies reported in the literature, as discussed above, focus on the impact of steady state compression on the structure and properties of the GDL. However, as indicated earlier, any servicing of a PEMFC stack necessary to extend its operational lifetime will likely subject fuel cell components to cyclic compression due to opening and rebuilding of the PEMFC stack. Properties of the GDL may be affected specifically due to the cyclic compression, over and above the impact caused due to the first cycle of compression during the initial assembly of the PEMFC stack. This paper focuses on the systematic study of the change in the overall structure and properties of the GDL when subjected to such cyclic compression. GDL samples are characterized for their surface morphology, surface roughness, pore size, void fraction, thickness, electrical resistance, contact angle, water uptake and in-plane permeability, specifically as a function of cycling compression. The results obtained and their implications are analyzed and discussed.

Section snippets

Experimental details

Commercially available carbon paper from Toray (TGP-H-120) with 30% PTFE was used in the experiments conducted. Choice of the GDL material for this study was based on the fact that Toray carbon paper is commonly reported in the literature [3], [5], [7], [8], [11], [18], [19], [20], [21], [22] for use as a GDL material in PEMFC. Some of the properties of the GDL used, as given by the manufacturer, are presented in Table 1. SEM micrographs of a fresh GDL sample used for this study are shown in

SEM observations

Fig. 4, Fig. 5 show micrographs of GDL samples subjected to cyclic compression at 1.7 MPa and 3.4 MPa respectively. Fig. 4, Fig. 5 show surface morphology and are to the same scale. Fig. 4, Fig. 5 show cross sectional morphology and are to the same scale. Fig. 4, Fig. 5 are after the first cycle of compression. Fig. 4, Fig. 5 are after the third cycle of compression, and Fig. 4, Fig. 5 are after the fifth cycle of compression. Comparison of SEM micrographs of fresh GDL samples and GDL samples

Conclusions

In the present work a systematic study was carried out to understand the impact of cyclic compression on the structure and properties of a GDL used in PEMFCs. Such cyclic compression is likely to occur due to efforts made to service PEMFC stacks to extend their operational lifetimes. In examining the impact of cyclic compression, this work has identified unique impacts of the same, which have not been reported in the literature that has largely focused on static compression studies. From the

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