Membrane-electrode assembly catalyst layer

Su PH, Lin HL, Lin YP et al (2013) High temperature membrane electrode assembly catalyst layer preparation using various molecular weight polybenzimidazole binders. Int J Hydrogen Energy 38 13742-13753... 

Yasuda K, Taniguchi A, Akita T, loroi T, Siroma Z. 2006a. Characteristics of a platinum black catalyst layer with regard to platinum dissolution phenomena in a membrane electrode assembly. J Electrochem Soc 153 A1599-A1603. 

The function of the electrolyte membrane is to facilitate transport of protons from anode to cathode and to serve as an effective barrier to reactant crossover. The electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and pathways for reactant supply to the catalyst and removal of products from the catalyst [96], The GDL is a carbon paper of 0.2 0.5 mm thickness that provides rigidity and support to the membrane electrode assembly (MEA). It incorporates hydrophobic material that facilitates the product water drainage and prevents... 

Figure 2.1 shows a schematic structure of the fuel cell membrane electrode assembly (MEA), including both anode and cathode sides. Each side includes a catalyst layer and a gas diffusion layer. Between the two sides is a proton exchange membrane (PEM) conducting protons from the anode to the cathode. 

Schematic structure of a fuel ceU membrane electrode assembly (MEA), including both anode and cathode catalyst layers. (Based on Lister. S. and McLean, G. Journal of Power Sources 2004 130 61-76. With permission from Elsevier.)...

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... 

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

In the proton-emitting membrane or proton electrolyte membrane (PEM) design, the membrane electrode assembly consists of the anode and cathode, which are provided with a very thin layer of catalyst, bonded to either side of the proton exchange membrane. With the help of the catalyst, the H2 at the anode splits into a proton and an electron, while Oz enters at the cathode. On the inside of the porous anode is a thin platinum catalyst layer. When H2 reaches this layer, it separates into protons (H2 ions) and electrons. One of the reasons why the cost of fuel cells is still high is because the cost of the platinum catalyst is rising. One ounce of platinum cost 361 in 1999 and increased to 1,521 in 2007. 

A porous anode and cathode are attached to each surface of the membrane, forming a membrane-electrode assembly, similar to that employed in SPE fuel cells. Electrochemical reactions (electron transfer-l-hydrogenation) occur at the interfaces between the ion exchange membrane and electrochemically active layers of electrodes. Electrochemical reductive HDH occurred at the interfaces between the ion exchange membrane and the cathode catalyst layer when an electrical current is applied between the electrodes ... 

Apart of this traditional meaning, recently the term of membrane electrode (assembly) is used. It is defined as two electrodes (the anode and the cathode) with a very thin layer of catalyst, bonded to either side of an ion-exchange membrane. It is an element of polymeric membrane fuel cell. 

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. 

The electric circuit of membrane electrode assemblies is a combination of anode and cathode catalyst layers plus the membrane. In general, the anode catalyst layer is considered an electric circuit, the cathode catalyst layer is considered another electric circuit similar to that of the anode but with different RC values, and the membrane is treated as a resistance. These three electric circuits are connected in series to construct a whole-cell equivalent circuit. A typical impedance spectrum is shown in Chapter 1 as Figure 1.16. Since the anode reaction is significantly faster than the cathode, the RC electric circuit of the anode can be disregarded. 

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],... 

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

Figure 1. From the macroscale to the nanoscale a membrane electrode assembly has a polymer electrolyte membrane sandwiched between two catalyst layers and gas diffusion layers. The catalyst layer is composed of carbon particles impregnated with catalyst nanoparticles. Effective utilization of the catalyst particles depends on their local environment.

The heart of a fuel cell is the membrane electrode assembly (MEA). In the simplest form, the electrode component of the MEA would consist of a thin film containing a highly dispersed nanoparticle platinum catalyst. This catalyst layer is in good contact with the ionomeric membrane, which serves as the reactant gas separator and electrolyte in this cell. The membrane is about 25-100 p,m thick. The MEA then consists of an ionomeric membrane with thin catalyst layers bonded on each side. Porous and electrically conducting carbon paper/cloth current collectors act as gas distributors (Figure 27.1). Since ohmic losses occur within the ionomeric membrane, it is important to maximize the proton conductivity of the membrane, without sacrificing the mechanical and chemical stability. 

Figure 24 describes schematically the three recent modes of preparation of membrane/electrode assemblies based on commercially available dispersed platinum catalysts. Comparison of catalyst utilization obtained with the different PEFC catalyzation techniques is given in Fig. 25. The advantage in catalyst utilization of the thin-layer approach is clearly seen, increasing at the higher cell currents (lower cell voltage) thanks to minimized mass-transport limitations in the thin catalyst layer.

The combination of anode/electrolyte/cathode in proton exchange membrane fuel cell is usually referred to as the membrane electrode assembly (MEA).51 Usually the MEA was produced by attaching a catalyst layer (frequently Pt, Pt alloys, or other noble metals) on one side of porous gas diffusion electrodes. The catalysts... 

Membrane electrode assemblies (MEAs) are typically five-layer structures, as shown in Figure 10.1. The membrane is located in the center of the assembly and is sandwiched by two catalyst layers. The membrane thickness can be from 25 to 50 pm and, as mentioned in Chapter 10, made of perfluorosulfonic acid (Figure 11.3). The catalyst-coated membranes are platinum on a carbon matrix that is approximately 0.4 mg of platinum per square centimeter the catalyst layer can be as thick as 25 pm [12], The carbon/graphite gas diffusion layers are around 300 pm. Opportunities exist for chemists to improve the design of the gas diffusion layer (GDF) as well as the membrane materials. The gas diffusion layer s ability to control its hydrophobic and hydrophilic characteristics is controlled by chemically treating the material. Typically, these GDFs are made by paper processing techniques [12],... 

The catalyst can be bonded to the membrane surfaces by many different methods such as vacuum deposition (21). However, the performance of the membrane-electrode assembly produced by the vacuum deposition method is poor because a smooth metallic layer is formed while electrode material for an electrochemical cell should be rough (high surface area). 

The Jet Propulsion Laboratory (JPL) has researched the stated objectives by investigating sputter-deposition (SD) of designed anode and cathode nanostructures of Pt-alloys, and electronic structures and microstructures of sputter-deposited catalyst layers. JPL has used the information derived from these investigations to develop novel catalysts and membrane electrode assemblies (MEAs) that... 

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. 

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