PEMFC catalyst layers

Wu, J., Yi, B., Hou, M., Hou, Z., and Zhang, H. Influence of catalyst layer structure on the current distribution of PEMFCs. Electrochemical and Solid-State Letters 2004 7 A151-A154. 

Zhang, X., and Shi, P. Dual bonded catalyst layer structure cathode for PEMFC. Electrochemistry Communications 2006 8 1229-1234. 

Jung, U. H., Park, K. T., Park, E. H., and Kim, S. H. Improvement of low-humidity performance of PEMFC by addition of hydrophilic SiOj particles to catalyst layer. Journal of Power Sources 2006 159 529-532. 

Rajalakshmi, N., and Dhathathreyan, K. S. Catalyst layer in PEMFC electrodes— Fabrication, characterization and analysis. Chemical Engineering Journal 2007 129 31 0. 

Lee, D., and Hwang, S. Effect of loading and distributions of Nafion ionomer in the catalyst layer for PEMFCs. International Journal of Hydrogen Energy 2008 33 2790-2794. 

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... 

Polarization citrve for PEMFCs with two different cathode diffusion layers carbon fiber paper with one MPL and carbon fiber cloth with two MPLs. Operating conditions ceU temperature of 85°C, O2/H2 dewpoint temperatures of 90/100°C gas pressures of 2 atm. CFP DL was a TGP-H-090 with 20 wt% PTFE in the MPL. CCs were PWB-3 from Stackpole cathode CC had 15 wt% PTFE in the MPL near the CL and 30 wt% PTFE in the MPL near the flow field. The anode CC had 15 wt% PTFE in both MPLs carbon loading on the MPL was not specified. The catalyst Pt loading was 0.4 mg cm and the Nation loading was 1.1 mg cm for all catalyst layers the membrane was a Nation 115. (Modified from E. Antolini et al. Journal of Power Sources 163 (2006) 357-363. With permission from Elsevier.)... 

W. Sun, B. A. Peppley, and K. Karan. Modeling the influence of GDL and flow-field plate parameters on the reaction distribution in the PEMFC cathode catalyst layer. Journal of Power Sources 144 (2005) 42-53. 

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... 

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

Figure 1.6. PEM fuel cell catalyst layer structure [13]. (Reproduced from Journal of Power Sources, 102, Costamagna P, Srinivasan S, Quantum jumps in the PEMFC science and technology from the 1960s to the year 2000 Part II. Engineering, technology development and application aspects, 253-69 2001, with permission from Elsevier.)...

Figure 6.10. Comparison of Nyquist plots of fuel cells with different Nafion loadings in the catalyst layers of both the cathode and the anode [7], (Reproduced by permission of ECS—The Electrochemical Society, from Guo Q, Cayetano M, Tsou Y, De-Castro ES, White RE. Study of ionic conductivity profiles of the air cathode of a PEMFC by AC impedance spectroscopy.)...

In conclusion, it is possible to achieve high performance with PEMFCs under dry or low-humidity conditions by employing low equivalent weight protonconducting ionomers in the catalyst layer. 

This chapter has examined a variety of EIS applications in PEMFCs, including optimization of MEA structure, ionic conductivity studies of the catalyst layer, fuel cell contamination, fuel cell stacks, localized impedance, and EIS at high temperatures, and in DMFCs, including ex situ methanol oxidation, and in situ anode and cathode reactions. These materials therefore cover most aspects of PEMFCs and DMFCs. It is hoped that this chapter will provide a fundamental understanding of EIS applications in PEMFC and DMFC research, and will help fuel cell researchers to further understand PEMFC and DMFC processes. 

Figure 3.49. Slice of a PEM cell showing gas diffusion layer (A), catalyst layer (B) and membrane layer (C), at a magnification factor of 200 (a). Tunnelling electron microscope pictures of catalyst layer at a magnification factor of 500 (b), 18 400 (c) and in (d) 485 500. (From N. Siegel, M. EUis, D. Nelson, M.v.Spakovsky (2003). Single domain PEMFC model based on agglomerate catalyst geometry. J. Power Sources 115, 81-89. Used with permission from Elsevier.)...

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

SD is routinely used to deposit thin films and has proven benefits from economies of scale in the metallization of plastics. The technique has already been used to create enhanced and unique MEAs for H2 -air proton exchange membrane fuel cell (PEMFC) systems. In this project, JPL is pursuing the use of SD to create DMFC membrane electrode assembly structures with highly electro-active catalyst layers that will reduce the amount and cost of the Pt-alloy catalyst at the fuel cell anode. 

Pt-doped carbon aerogels have been used successfully in the preparation of cathode catalyst layers for oxygen reduction reaction (ORR) in PEMFC systems [83-86]. Thus, different Pt-doped carbon aerogels with a Pt content of around 20 wt% were prepared by impregnation [83]. Results obtained with these Pt catalysts were compared with others supported on carbon blacks Vulcan XC-72 and BP2000, which are commonly used as electrocatalysts. The accessibility of the electrolyte to Pt surface atoms was lower than expected for high-surface-area... 

In order to estimate the input of the cathode catalyst layer on the MEA cell performance, each cell before testing in methanol solution has been evaluated in PEMFC operation conditions, namely with hydrogen on the anode side and oxygen or air on the cathode. Fig. 6 shows the dependencies of cell voltage and cell voltage compensated for membrane resistance vs. current density. 

The membrane and ionomer humidification reqnire-ments are of paramount importance for PEMFC operation since the proton conductivity is a fundamental necessity in the membrane as well as in the electrode for the fnel cell to function. The operating conditions of cnrrent PEMFCs are dictated by the properties of the membranes/ionomers. At present, the most important membrane type (e.g., Naflon membranes from DuPont) is based on PFSA ionomers that are used in the membrane and the catalyst layers. Figure 21.3 shows the proton conductivity versus RH for three different... 

Modeling of PEMFC Catalyst Layer Performance and Degradation... 

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