Carbon-supported membrane electrode assemblies

Fig. 1 COj concentration (solid symbols) and accumulative carbon weight loss (open symbols, integrated from the CO flux at the exit of the working electrode) as a function of time over a commercial conventional-carbon-supported membrane electrode assembly (MEA). The N -fed working electrode (50 cm min" ) was held at a potential of 1.2 V versus the H -fed counter/reference electrode (200 cm min" ) at 95°C and 80% inlet relative humidity (RHj i j). RHE reversible hydrogen electrode...

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... 

Modeling of Membrane-Electrode-Assembly Degradation in Proton-Exchange-Membrane Fuel Cells - Local H2 Starvation and Start-Stop Induced Carbon-Support Corrosion... 

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. 

In this work, three membrane electrode assemblies (MEAs) with the same composition of the anode Pt/Ru black layer and different cathode layers were prepared. The cathode layer of the first MEA ( 1) was made using a Pt-black catalyst with 3.6 mg/cm Pt loading on the cathode side. The cathode layer of the second MEA ( 2) consisted of two layers the external Pt-black layer in contact with the gas diffusion layer (2.8 mg/cm ) and the internal Pt/C layer (0.3 mg/cm ) in contact with the membrane. The cathode layer of the third MEA ( 3) was made using carbon-supported Pt/C catalyst from Tanaka Inc. (1.3 mg/cm ). The anode layers were made in a similar way using Pt/Ru-black with 5.0 O.lmg/cm catalyst loading and 10 wt % of Nafion. 

PadiUa-Serrano M N, Maldonado-Hodar F, Moreno-Castilla C (2005) Influence of Pt particle size on catalytic combustion of xylenes on carbon aerogel-supported Pt catalysts. Applied Catalysis B-Envinmmental 61 253-258 Kara H S, Smirnova A (2005) Method of f eparing Membrane Electrode Assemblies with Aerogel Supptnled Catalyst. WO/2005/086914... 

PEMFGs use a proton-conducting polymer membrane as electrolyte. The membrane is squeezed between two porous electrodes [catalyst layers (CLs)]. The electrodes consist of a network of carbon-supported catalyst for the electron transport (soHd matrix), partly filled with ionomer for the proton transport. This network, together with the reactants, forms a three-phase boundary where the reaction takes place. The unit of anode catalyst layer (ACL), membrane, and cathode catalyst layer (CCL) is called the membrane-electrode assembly (MEA). The MEA is sandwiched between porous, electrically conductive GDLs, typically made of carbon doth or carbon paper. The GDL provides a good lateral delivery of the reactants to the CL and removal of products towards the channel of the flow plates, which form the outer layers of a single cell. Single cells are connected in series to form a fuel-cell stack. The anode flow plate with structured channels is on one side and the cathode flow plate with structured channels is on the other side. This so-called bipolar plate... 

Typical results were obtained with a membrane-electrode assembly (MBA) consisting of a Nafion 117 membrane on which are pressed the anodic electrocatalysts (Pt or Pt-based catalysts dispersed on a high surface area carbon support) and the cathodic catalyst (usually Pt/C with metal loading from 40 % to 60 %). An example of the electrical characteristics of a DEFC with Pt/C, PtSn/C or PtSnRu/C anode catalysts is given in Fig. 2. 

As explained in a previous section, a membrane electrode assembly (MEA) consists of the polymer membrane that is sandwiched between an anode and a cathode electrode, respectively. The electrodes are composed of a conductive carbon network that supports a catalyst on a gas diffusion layer. An additive, such as polytetra-fluoroethylene (PTEE), helps bind the Pt/C catalyst to the gas diffusion layer. At the anode, the catalyst facihtates the oxidation of hydrogen into its constituent electrons and protons. As the protons are passed through the acid-doped membrane to the cathode, the electrons are passed through an external circuit, thereby creating electricity. Einally, the electrons and protons react with oxygen at the cathode electrode to form water as the final reaction product. 

Aquivion E87-12S short-side chain perfluorosulfonic acid (SSC-PFSA) membrane with equivalent weight (EW) of 870 g eq and 120 pm thickness produced by Solvay Specialty Polymers was tested in a polymer electrolyte membrane water electrolyser (PEMWE) and compared to a benchmark Nation N115 membrane (EW 1100 g eq ) of similar thickness [27]. Both membranes were tested in conjunction with in-house prepared unsupported Ir02 anode and carbon-supported Pt cathode electrocatalyst. The electrocatalysts consisted of nanosized Ir02 and Pt particles (particle size 2-4 nm). The electrochemical tests showed better water splitting performance for the Aquivion membrane and ionomer-based membrane-electrode assembly (MEA) as compared to Nafion (Fig. 2.21). Lower ohmic drop constraints and smaller polarization resistance were observed for the electrocatalyst-Aquivion ionomer interface indicating a better catalyst-electrolyte interface. A current density of 3.2 A cm for water... 

The membrane electrode assembly (MEA) is the heart of a fuel cell stack and most likely to ultimately dictate stack life. Recent studies have shown that a considerable part of the cell performance loss is due to the degradation of the catalyst layer, in addition to membrane degradation. The catalyst layer in PEMFCs typically contains platinum/platinum alloy nanoparticles distributed on a catalyst support to enhance the reaction rate, to reach a maximum utilization ratio and to decrease the cost of fuel cells. The carbon-supported Pt nanoparticle (Pt/C) catalysts are the most popular for PEMFCs. Catalyst support corrosion and Pt dissolution/aggregation are considered as the major contributions to the degradation... 

FIGURE 14.1 PEMFC materials imaged at different scales (a, b) a membrane-electrode assembly, (a, c) carbon-supported metal nanopartides. Courtesy of D. Michon/Artechnique—http //www.artechnique.fr/contacts.html is greatly appredated for image (a). Image (b) is reprinted from Reference [2] with permission of The Electrochemical Society. 

The membrane electrode assembly (MEA) is a delicate component in low-temperature fuel cells based on polymer electrolyte membranes. Its condition is affected by many factors (1) selection and preparation of MEA materials (catalysts, supporting carbon powder, membrane materials, binder for MEA hot pressing, etc.), (2) history of MEA usage, (3) fuel cell operation parameters, and so on. The resulting MEA condition exerts a strong influence on the fuel cell performance, which is also a function of running time. 

There is only one example in the literature of polyphosphazene performance in a proton-exchange membrane (PEM) hydrogen fuel ceU. Allcock and Lvov [45] tested a sulfonimide polyphosphazene membrane in a hy-drogen/oxygen fuel cell at room temperature and at 80 °C. The membrane-electrode-assembly (MEA) was fabricated from a 100 xm thick sulfonimide polyphosphazene membrane that was crosslinked with y-radiation (40 MRad). The polymer lEC was 0.99 mmol/g, with an equilibrium water swelling of 42%, and a proton conductivity of 0.058 S/cm. The anode and cathode were prepared from carbon-supported platinum (20% Pt on Vulcan XC-72R) at a Pt loading of 0.33 mg/cm. The electrodes were hot pressed onto the membrane at 65 °C and 400 psi for 30 s. As a reference, a Nafion 117 MEA was also prepared with the same electrode catalyst at a loading of 0.26 mg/cm for the anode and 0.48 mg/cm for the cathode. For Nafion, the electrodes were hot pressed at 125 °C and 1400 psi for 2 min. 

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