Microporous layer

Fig. 4.5 Dissolution valence nv as a function of anodic current density for low doped p-type and strongly illuminated, low doped n-type samples (<1017cnT3, 2.5% HF, at RT). For current densities belowJPS the samples were measured with and without the microporous layer. This produces a minor difference in indicated by two data points.

Illumination of a microporous silicon layer during anodization changes the PL spectrum significantly, as discussed in Section 7.4, and may also be applied for structuring of microporous layers [As2, Dol]. 

QC in silicon structures requires dimensions of a few nanometers and is therefore proposed to be responsible for the formation of microporous films on Si electrodes, as discussed in Chapter 7. QC is independent of doping and is often found as a superposition to pore formation by SCR effects. Only for p-type silicon electrodes of doping densities of 1016-1017 cm-3 is no formation of SCR-related pores observed upon anodization in aqueous HF. This substrate doping regime is therefore best suited for formation of purely microporous layers. 

Detailed measurements of stress have revealed that microporous layers on bulk silicon are under compressive stresses that decrease with increasing porosity,... 

There are two main types of thin-film catalyst layers catalyst-coated gas diffusion electrode (CCGDL), in which the CL is directly coated on a gas diffusion layer or microporous layer, and catalyst-coated membrane, in which the CL is directly coated on the proton exchange membrane. In the following sections, these catalyst layers will be further classified according to their composition and structure. 

Louh, R. R, Ghang, A. C. C., Chen, V., and Wong, D. Design of electrophoretically deposited microporous layer/catalysts layer composite structure for power generation of fuel cells. International Journal of Hydrogen Energy 2008 33 5199-5204. 

Microporous Layers in Direct Liquid Fuel Cells..246... 

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

In this chapter, diffusion layers will be considered as fhe porous media that help the transport of fhe reactant fluids and producfs from one surface to another. In addition, the MPL will be defined as fhe additional layer or layers (made ouf of carbon black and water-repellenf particles) located between the CL and the DL. It is important to note that although "microporous layer" and "diffusion layer" are the common names for these components, as well as the ones used in this chapter, a number of different names can be found in the literature. 

Another important point regarding the fabrication process of MPLs is that, typically, when carbon fiber paper is used as the DL, the MPL is coated on only one surface of the CFP. However, when a carbon cloth DL is used, it is normally coated on both sides with MPLs. Section 4.3.S.4 will discuss these DLs with multiple microporous layers in more detail. 

Effect of Microporous Layer on Fuel Cell Performance... 

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. 

To shed some light on these issues and to be able to have a better understanding of the water transport when using MPLs, Atiyeh et al. [152] presented an experimental method designed to investigate the net water drag coefficient in order to have a better indication of the amount of water flowing from fhe cathode to the anode. They observed that the performance of fhe fuel cell improved when the anode, the cathode, or both had microporous layers. 

X. Wang, H. Zhang, J. Zhang, et al. A bifunctional microporous layer with composite carbon black for PEM fuel cells. Journal of Power Sources 162 (2006) 474 79. 

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. 

H. Nakajima, T. Konomi, and T. Kitahara. Direct water balance analysis on a polymer electrolyte fuel cell (PEFC) Effects of hydrophobic treatment and microporous layer addition to the gas diffusion layer of a PEFC on its performance during a simulated start-up operation. Journal of Power Sources 171 (2007) 457-463. 

A. Z. Weber and J. Newman. Effects of microporous layers in polymer electrolyte fuel cells. Journal of the Electrochemical Society 152 (2005) A677-A688. 

U. Pasaogullari and C. Y. Wang. Two-phase transport and the role of microporous layer in polymer electrolyte fuel cells. Electrochimica Acta 49 (2004) 4359-4369. 

H. K. Atiyeh, K. Karan, B. Peppley, et al. Experimental investigation of the role of a microporous layer on the water transport and performance of a PEM fuel cell. Journal of Power Sources 170 (2007) 111-121. 

J. Yu, M. N. Islam, T. Matsuura, et al. Improving the performance of a PEMFG with Ketjenblack EG-600JD carbon black as the material of the microporous layer. Electrochemical and Solid State Letters 8 (2005) A320-A323. 

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