Gas-diffusion layer durability

C. Lee and W. Merida. Gas diffusion layer durability under steady-state and freezing conditions. Journal of Power Sources 164 (2007) 141-153. 

Proper water management in proton exchange membrane fuel cells (PEMFCs) is critical to PEMFC performance and durability. PEMFC performance is impaired if the membrane has insufficient water for proton conduction or if the open pore space of the gas diffusion layer (GDL) and catalyst layer (CL) or the gas flow channels becomes saturated with liquid water, there is a reduction in reactant flow to the active catalyst sites. PEMFC durability is reduced if water is left in the CL during freeze/thaw cycling which can result in CL or GDL separation from the membrane,1 and excess water in contact with the membrane can result in accelerated membrane thinning.2... 

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

The reliability/durability of these fuel cells is another major barrier hindering commercialization. Developing durable catalysts, membranes, gas diffusion layers, and bipolar plates are currently the major areas of concentration in the search for technical breakthroughs. 

In this section, operating conditions and modes that contribute to voltage decay or limit performance will be discussed. In section Materials Degradation and the Relation to Performance Loss and Shortening the PEMFC Lifetime, the durability issues related to the different components of the fuel ceU, that is, catalyst, the gas diffusion layers, membranes, bipolar plates, and seals will be presented in more detail. 

Abstract The polymer electrolyte fuel cell (PEFC) consists of disparate porous media microstructures, e.g. catalyst layer, microporous layer, gas diffusion layer, as the key components for achieving the desired performance attributes. The microstmcture-transport interactions are of paramount importance to the performance and durability of the PEFC. In this chapter, a systematic description of the stochastic micro structure reconstmction techniques along with the numerical methods to estimate effective transport properties and to study the influence of the porous structures on the underlying transport behavior is presented. 

Table 1 Surface energies (by the Owens-Wendt method) of two different SIGRACET gas diffusion layer (GDL) types before and after accelerated durability testing (different catalyst-coated membranes were used in the cells)...

In Part I, degradation phenomena of stack components, catalysts, membranes, gas-diffusion layers, membrane-electrode assemblies, bipolar plates, and sealings are discussed on the basis of their materials chemistry. Accelerating methods and recent progress in durability improvement are also reviewed by prominent authors in the field. 

Gas diffusion layer is of vital importance to the performance and durability of a PEM fuel cell. For portable systems like cell phones and laptops, flexible and more durable fabric GDLs are preferred over the paper GDLs. GDL-UNH is a fabric GDL that is robust, flexible and durable. Several commercial GDLs are also... 

To design the optimal diffusion layer for a specific fuel cell system, it is important to be able to measure and understand all the parameters and characteristics that have a direct influence on the performance of the diffusion layers. This section will discuss in detail some of the most important properties that affect the diffusion layers, such as thickness, hydrophobicity and hydrophilicity, porosity and permeability (for both gas and liquids), electrical and thermal conductivity, mechanical properties, durability, and flow... 

Once the durability testing of the fuel cells is finalized, the internal components are then characterized. For diffusion layers, some of these characterization techniques include SEM to visualize surface changes, porosimetry measurements to analyze any changes in porosity within the DL and MPL, IGC (inverse gas chromatography) to identify relative humidity effects on the hydrophobic properties of the DLs, contact angle measurements to observe any changes in the hydrophobic/hydrophilic coatings of the DL, etc. [254,255]. 

Wood, D. L. and Borup, R. L. 2009. Diuabihiy aspects of gas-diffusion and microporous layers. In Polymer Electrolyte Fuel Cell Durability, eds. F. N. Biichi, M. Inaba, and T. J. Schmidt, eds. New York, NY Springer. 

Flooding also has a negative effect on PEMFC durability. Too much water accelerates corrosion of the electrodes, the catalyst layers, the gas diffusion media, and the membrane (Pierre, 2000). Corrosion in turn increases ohmic losses. Dissolved catalyst particles and impurities can also be transported into the membrane, replacing H+ ions and reducing the proton conductivity over time, eventually leading to cell failure (Ge, 2007). 

FIGURE 11.10 Single fiber contact angles of Toray TGP-H materials (hydrophobized to 17 wt% fluorinated ethylene propylene, FEP) aged in accelerated fashion in different liquid-water environments. (With kind permission from Springer Science+Business Media Polymer Electrolyte Fuel Cell Durability, Durability aspects of gas-diffusion and microporous layers, 2009, pp. 159-195, Wood, D.L. and Borup R.L.)... 

PVD processes are traditionally used for the deposition of various coatings on tools and components and other metallic or ceramic substrates, for example, to improve surface hardness, wear resistance, and corrosion resistance. However, more and more plastic products are used today in fields where metals were previously predominant. In these cases, the end product should meet functional requirements (durability, scratch, corrosion, and chemical resistance) and decorative appearance. Another important application includes metallized and transparent inorganic gas-diffusion barrier layers on packaging films. 

---