Characterization of Membrane Electrode Assembly

The MEA of the PEFC consists of proton conducting electrolyte membrane sandwiched between electrode catalyst layers. Some fuel cell MEAs 

The output of working electrode current density (current normalized by the active area of the working electrode) versus potential is used to determine the hydrogen crossover flux (mol/cm /s) from Faraday s law. 

Advanced instruments such as electron microscopy techniques (SEM and TEM) are used to characterize the microstructure of MEA components at various resolutions. Digital mapping of the morphology/catalyst clusters can be used to perform quantifafive analysis of the microstructure. 

We learned from the previous section that electrochemical active surface area can be determined from the CV method. The accurate measurement of surface area of the electrolyte membrane or electrodes is done by a technique known as the Brunauer-Emmett-Teller (BET) method (Brunauer et al., 1938). It is based on the physical adsorption of gas molecules on a solid surface. It is assumed that gas molecules physically adsorb on a solid in layers infinitely and there is no interaction between each adsorption layer. The BET equation is expressed as 

Equation 8.34 is an adsorption isotherm and can be plotted as a straight line with l/t Kpo/p) ] on the y-axis and (p/po) on the x-axis. This plot is called a BET plot, which is linear in the range of 0.05 p/p 0.35. The values of 

---

Figure 3.15. Schematic representation of the correlation between fuel cell impedance and polarization curve. (Modified from [23], with kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy, 32(8), 2002, 859-63, Wagner N. Figure 4.)...

Wagner N (2002) Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy. J Appl Electrochem 32(8) 859-63... 

N. Wagner [2002] Characterization of Membrane Electrode Assemblies in Polymer Electrolyte Fuel Cells using a.c. Impedance Spectroscopy, J. Appl. Electrochem., 32, 859-863. 

In general, PFSA membranes are characterized by excellent performance, electrochemical stability, suitable mechanical properties, and allow rapid startup. However, it appears necessary to ameliorate the PFSA membranes and ionomers to improve the operating efficiency of membrane-electrode assemblies of PEM electrolysers at practical current densities useful to reduce capital costs. PESA membranes used in electrochemical devices are essentially based on Nafion however, several alternative PSFA membranes with shorter pendant side chain have been developed by Dow, 3 M, Gore, Asahi Glass, Solvay Specialty Polymers, etc.. 

Unlike the RDE technique, which is quite popular for characterizing catalyst activities, the gas diffusion electrode (GDE) technique is not commonly used by fuel cell researchers in an electrochemical half-cell configuration. The fabrication of a house-made GDE is similar to the preparation of a membrane electrode assembly (MEA). In this fabrication, Nation membrane disks are first hot-washed successively in nitric acid, sulphuric acid, hydrogen peroxide, and ultra-pure water. The membranes are then coated with a very thin active layer and hot-pressed onto the gas diffusion layer (GDL) to obtain a Nation membrane assembly. The GDL (e.g., Toray paper) is very thin and porous, and thus the associated diffusion limitation is small enough to be ignored, which makes it possible to study the specific kinetic behaviour of the active layer [6],... 

Figure 9.8 (a) Schematic representation of oxidized to H. (b) Structure of the the structure and reactivity of the bio-inspired membrane-electrode assembly used for the H2-evolving nickel catalyst grafted on a carbon electrocatalytic characterization of the nanotube [51]. Electrons are exchanged Ni-functionalized CNTs under conditions... 

The gas diffusion layers are characterized mainly by their thickness and porosity (Figure 1.9). The hot-pressed assembly of the membrane and the gas-diffusion layer including the catalyst is called the Membrane-Electrode-Assembly (MEA). 

Mechanistic models can generally be characterized by the scope of the model. In many cases, modeling efforts focus on a specific part or parts of the fuel cell, like the cathode catalyst layer [39], the cathode electrode (gas diffusion layer plus catalyst layer) [40-42], or the membrane electrode assembly (MEA) [43, 44]. These models are very useful in that they... 

Fig. 7.4 The cell performance of Nafion 112 membrane electrode assembly (MEA) and Nafion-M02 sol-gel composite MEAs. 02 and H2 at 2.0 and 1.3 times stoichiometry flows, respectively, pressure at 1.0 atm, humidifier at 80C, and cell at HOC. Reprinted with permission from Ref. [33] N. H. Jalani, et al., Synthesis and characterization of Nafion -M02 (M=Zr, Si, li) nanocomposite membranes for higher temperature PEM fuel cells, Electrochim. Acta 51, 553-560 (2005). Copyright Elsevier...

Chapters 6 and 7 deal with the hydrocarbon polymers and composites targeted for high temperature PEM fuel cell applications. Specifically, chapter 6 deals with a series of high molecular weight, highly sulfonated poly(arylenethioethersulfone) (SPTES) polymers synthesized by polycondensation. They were characterized by different methods and tested for proton conductivity. Finally, membrane electrode assemblies (MFAs) were fabricated. 

Hung, Y., H. Tawfik and D. Mahajan. Durability and characterization studies of polymer electrolyte membrane fuel cell s coated aluminum bipolar plates and membrane electrode assembly. Journal of Power Sources 186 123-127, 2009. 

Song, M.A., Ha, S.I., Park, D.Y., Ryu, C.H., Kang, A.S., Moon, S.B., Chung, J.H., Development and characterization of covalently cross-linked SPEEK/Cs-TPA/CeOj composite membrane and membrane electrode assembly for water electrolysis, Int. J. Hydrogen Energy, 2013, 38,10502-10510. 

The catalytic (supported or unsupported) interface in the vast majority of direct liquid fuel cell studies is realized in practice either as a catalyst coated membrane (CCM) or catalyst coated diffusion layer (CCDL). Both configurations in essence are part of the electrode design category, which is referred to as a gas diffusion electrode, characterized by a macroporous gas diffusion and distribution zone (thickness 100-300 pm) and a mainly mesoporous, thin reaction layer (thickness 5-50 pm). The various layers are typically hot pressed, forming the gas diffusion electrode-membrane assembly. Extensive experimental and mathematical modeling research has been performed on the gas diffusion electrode-membrane assembly, especially with respect to the H2-O2 fuel cell. It has been established fliat the catalyst utilization efficiency (defined as the electrochemically available surface area vs. total catalyst surface area measured by BET) in a typieal gas diffusion electrode is only between 10-50%, hence, flie fuel utilization eflfieieney can be low in such electrodes. Furthermore, the low fuel utilization efficiency contributes to an increased crossover rate through the membrane, which deteriorates the cathode performance. 

Recently, Brzozowska et al. used NR and ex situ electrochemical techniques to characterize an innovative type of monolayer system intended to serve as a support for a bUayer lipid membrane on a gold electrode surface [51]. Zr ions were used to noncovalendy couple a phosphate-terminated self-assembled monolayer (SAM) formed on a gold surface to the carboxylate groups of negatively charged phos-phatidylserrne (PS). This tethered surface was then used for the formation of a PS hpid bilayer structure formed by vesicle fusion and spreading. NR studies revealed the presence of an aqueous environment associated with the tether layer which arises from nonstoichiometric water associated with the zirconium phosphate moieties [52]. 

---