Microporous layer water management

Microporous layers are now commonly used to improve the overall performance of a fuel cell. If is believed fhat they play a key role in overall water management within the fuel cell. However, if is still unclear exacfly how fhe MPL affecfs fhe wafer fransporf mechanism inside fhe DL and fhe MEA. Therefore, if is imporfanf to discuss briefly some of fhe differenf sfudies fhaf have fried to fackle fhis issue. 

S. Park, J. W. Lee, and B. N. Popov. Effect of PTFE content in microporous layer on water management in PEM fuel cells. Journal of Power Sources 177 (2008) 457-463. 

Adding a surface layer onto die SL has proven to be an effective way of improving the water/gas management of the whole GDL [15-18]. The microporous layer (MPL, also called a carbon base, in industry) consists of carbon or graphite particles mixed with a hydrophobic agent, usually polytetrafluoroethylcnc (PTFE),... 

There has been considerable effort expended for several years on water management characterization, modeling and the understanding of key transport mechanisms within the catalyst layer, microporous layer and gas diffusion substrate, water droplet removal from the surface of the substrate to the flowfield, and removal of droplets in the flowfield (Chen et al., 2007, Chen and Peng, 2008, Dai et al., 2008, Dai et al., 2009a, Hartnig et al., 2009, Kang and Ju, 2009, Lee et al., 2008, Li et al., 2008, Quick et al., 2009, Shah et al., 2006, Yu etal, 2009). 

Akey performance limitation in the polymer electrolyte fuel cell (PEFC) originates from the multiple, coupled and competing, transport interactions in the constituent porous components. The suboptimal transport behavior resulting from the underlying complex and multifunctional microstmctures in the catalyst layer (CL), gas diffusion layer (GDL) and microporous layer (MPL) leads to water and thermal management issues and undesirable performance loss. Therefore, it is imperative to understand the profoimd influence of the disparate porous microstmctures on the transport characteristics. In this chapter, we highhght the stochastic microstmcture reconstmction technique and direct transport simulation in the CL, GDL and MPL porous stmctmes in order to estimate the effective transport properties and imderstand the microstmctural impact on the imderlying transport behavior in the PEFC. 

Figure 6.7 Schematic of microporous layers used to enhance water management in PEFCs (a) slurry-based MPL consists of carbon particles, polymeric binder, and PTFE that is applied to the catalyst side of DM surface (6) porous polymer sheet bonded to outer surface of catalyst layer.

Capillary Pressure Management Use of a highly hydrophobic microporous layer on the cathode can be used to maintain a capillary pressure difference between the liquid-filled hydrophilic pores of the cathode and anode, and pump water (and crossover methanol) back toward the anode by capillary pressure forces. 

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