Water management layer

For example, if fhe DL is used on the side of fhe cell where fhe fuel or oxidant is in gas phase, then this part can be referred to as gas diffusion layer (GDL). When bofh fhe CL and the DL are mentioned as one component, then the name "diffusion electrode" is commonly used. Because the DL is of a porous nature, it has also been called "diffusion medium" (DM) or "porous transporf layer" (PTL). Sometimes the DL is also referred to as fhe component formed by an MPL and a backing layer. The MPL has also been called the "water management layer" (WML) because one of its main purposes is to improve the water removal inside the fuel cell. In this chapter, we will refer to these components as MPL and DL because these names are widely used in the fuel cell indusfry. 

T. Jian-hua, S. Zhao-yuan, S. Jin-song, and S. Zhong-qiang. Preparation of water management layer and effects of its composition on performance of PEMFCs. Energy Conversion and Management 49 (2008) 1500-1505. 

J. Shi, J. Tian, C. Zhang, and Z. Shan. A novel method for the preparation of a PEMFC water management layer. Journal of Power Sources 164 (2007) 284-286. 

An effective catalyst layer must serve multiple functions simultaneously electron and proton conduction, oxygen or hydrogen supply, and water management. The composition and structure of a CL can affecf all fhese functions... 

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. 

Passalacqua ef al. [143] were able to prove fhaf when an MPL is interposed befween fhe DL and fhe CL, fhe performance of fhe cell improves subsfan-tially. They concluded that the MPL reduced the size of fhe wafer droplefs, thus enhancing the oxygen diffusion. This layer also prevented fhe cafalysf particles from entering too far info fhe DL. Park ef al. [102] concluded fhaf wifh fhe addition of an MPL, bofh water management and electrical conductivity improved. 

Peled ef al. [177] also designed a novel MEA in order to improve the water back diffusion from fhe cathode to the anode side. They used a liquid-water barrier layer (LWBL), which consisted of a paste, made out of PTFE and carbon black particles, fhat was inserted in the pores of fhe CFP to form a layer inside fhe paper. Up to seven layers were necessary in order to achieve a uniform layer of 20-50 pm in thickness. Testing showed that the LWBL on the cathode DL creates a hydraulic pressure that forces (or pushes) the water back from fhe cafhode toward the anode, thus improving the cell s water management at different operating conditions. 

N. Holmstrom, J. Ihonen, A. Lundblad, and G. Lindbergh. The Influence of the gas diffusion layer on water management in polymer electrolyte fuel cells. Fuel Cells 7 (2007) 306-313. 

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. 

J. Ghen, T. Matsuura, and M. Hori. Novel gas diffusion layer with water management function for PEMFG. Journal of Power Sources 131 (2004) 155-161. 

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by... 

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... 

The GDL is located on the back of the CL in order to improve gas distribution and water management in the cell. This layer has to be porous to the reacting gases, must have good electronic conductivity, and has to be hydrophobic so that the liquid produced water does not saturate the electrode structure and reduce the permeability of gases. The GDL needs to be resilient and the material of choice for the PEMFC is usually carbon fiber, paper or cloth, with a typical thickness of 0.2-0.5mm [74,75], This macroporous support layer is coated with a thin layer of carbon black mixed with a dispersed hydrophobic polymer, such as P I LL, in order to make it hydrophobic. This latter compound can, however, reduce the electronic conductivity of the GDL, and limit the three-phase boundary access. 

The presence of water is critical for operation but in current PEMFCs proper water management is a delicate issue and poor control can greatly reduce the efficiency of the device. An excess of water can flood the catalyst and porous transport layers impeding the transport of reactants and eventually drowning the fuel cell. At low water content, the polymer electrolyte membrane can become a poor conductor and the reactivity at the electrodes is affected. Local hot spots arising due to the inefficient operation result in early degradation of the cell. ... 

Water management can be most simply achieved by providing the gas feed humidification level required to maintain the conductivity of the fuel-cell membrane and of the ionomers in the catalyst layers. Gas feed humidification has been achieved by a variety of methods including, for example, enthalpy exchangers [6] and porous bipolar plates [62]. The two latter approaches rely on utilization of stack-produced water, thereby eliminating the need of frequent water refill . The system in Fig. 29a, can use a condenser to... 

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