Cloth electrode assembly

In the single-chamber MFCs, an air cathode is commonly used with the anode at the opposite side of the chamber. Alternatively, the anode and cathode can be on the same side of the chamber if a membrane electrode assembly (MEA) or cloth electrode assembly (CEA) is used. Single-chamber MFCs can achieve much better performance than a two-chamber system due to the higher mass transfer rate and concentration of oxygen in air compared with water [3j. In a membrane-free single-chamber MFC, with the anode and cathode on opposite sides of the chamber, the biofihn developed on the cathode can function as a membrane to minimize oxygen diffusion into the anode chamber while allowing... 

The catalytic-electrocatalytic reactor consists of a membrane electrode assembly, such as Pt-black/Nafion/Pd/C sandwiched between sheets of porous carbon cloth, housed in a fuel cell assembly. 

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 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. 

The second electrolyser was a research unit assembled for SRNL by the University of South Carolina (USC). It was constructed with platinised carbon cloth electrodes, a Nafion 115 PEM electrolyte, carbon paper flow fields, solid graphite back plates, copper current collectors and stainless steel end plates. The USC electrolyser had an active cell area of 40 cm and a Pt catalyst loading of 0.5 mg/ cm (only one-eighth that of the commercial cell). The carbon-based configuration proved to be much more corrosive resistant than the commercial-type electrolyzer. A photograph of the two electrolyzer units is shown in Figure 3. 

The electrocatalytic layers are contacted by gas diffusion layers (GDLs), allowing the gases, H2 and O2, as reactants to be passed to the interface and liquid H2O as product removed fi om the interface. These GDLs consist of sheets of carbon cloth or paper with typically 100 pm thickness and 50 % porosity (Fig. 2). This arrangement of membrane, electrocatalytic layers, and gas diffusion layers is colloquially called membrane-electrode-assembly (MEA). 

We will show hydrogen pump impedance data from two 5-cm Nation 117 membrane electrode assemblies. Both S-cm cells were operated at 80°C in hydrogen pump mode with saturated water vapor and 2 atm hydrogen on both electrodes. They measured the AC impedance with a Parstat 2273 analyzer. The first membrane was prepared with 50% Na+ cation and had 0.2 mg/cm Pt on carbon electrodes with E-tek double-sided carbon cloth gas diffusion electrodes. The anode and cathode humidifier and cell temperatures were 94/92/80°C, respectively, and both H2 pressures were 30 psig and the flows were 200/100 seem (standard cubic centimeters per minute). Figure 8.22 shows the polarization curve. If fhe swept cathode voltage did not go much below -0.6 V, they obtained repeatable results. When extended below -0.9 V, they did observe temporary changes cell resistance. 

For fundamental research where the thermal parameters are not precisely known and to define unknown thermal transport parameters, we would like to directly measure the temperature profile within the electrolyte. To approach this problem, a thermocouple can be embedded directly in the diffusion media of a PEFC [32, 33]. However, the contact resistance between the diffusion media and the thermocouple becomes another unknown parameter. To circumvent these difficulties, Burford et al. invented a method to embed an array of microthermocouples directly between two 25-pm-thick Nafion electrolyte sheets of a membrane electrode assembly [34, 35]. Local temperature variation in PEFCs was determined to reach > 10°C at high current density for a thick diffusion media (>400 pm for woven cloth media). This proved that an isothermal assumption is typically not justified over a full range of performance and indicates phase change plays a role in water transport in PEFCs. An even smaller MEMs-based thermosensor array has been developed using vapor deposition [36] and has been embedded within a PEFC electrolyte, providing precise locational control of the sensor position. 

In principle, there are two ways of preparation of catalyst layer and its attachment to the ionomer membrane. Such a combination of membrane and catalyst layers is called the membrane electrode assembly or MEA. The first way of preparing an MEA is to deposit the catalyst layer to the porous substrate, the so-called gas diffusion layer, typically carbon fiber paper or carbon cloth, and then hot-press it to the membrane. The second... 

Assemble 5 mg (dry weight) of thermostated algae on the Clark electrode by a dialysis membrane and a special O-ring, which have the same dimensions of flow cell. A black cloth protected all. 

The features of the electrode used in this gas-phase electrocatalytic reduction of C02 are close to those used in PEM fuel cells [37, 40, 41] (e.g. a carbon cloth/Pt or Fe on carbon black/Nafion assembled electrode, GDE). The electrocatalysts are Pt or Fe nanoparticles supported on nanocarbon (doped carbon nanotubes), which is then deposited on a conductive carbon cloth to allow the electrical contact and the diffusion of gas phase C02 to the electrocatalyst. The metal nanoparticles are at the contact of Nation, through which protons diffuse. On the metal nanoparticles, the gas-phase C02 reacts with the electrons and protons to be reduced to longer-chain hydrocarbons and alcohols, the relative distributions of which depend on the reaction temperature and type of metal nanoparticles. Isopropanol forms selectively from the electrocatalytic reduction of C02 using a gas diffusion electrode based on an Fe/N carbon nanotube (Fe/N-CNT) [14, 39, 40]. Not only the nature of carbon is relevant, but also the presence of nanocavities, which could favor the consecutive conversion of intermediates with formation of C-C bonds. 

In Section 3, the slow rate of the ORR at the Pt/ionomer interface was described as a central performance limitation in PEFCs. The most effective solution to this limitation is to employ dispersed platinum catalysts and to maximize catalyst utilization by an effective design of the cathode catalyst layer and by the effective mode of incorporation of the catalyst layer between the polymeric membrane electrolyte and the gas distributor/current collector. The combination of catalyst layer and polymeric membrane has been referred to as the membrane/electrode (M E) assembly. However, in several recent modes of preparation of the catalyst layer in PEFCs, the catalyst layer is deposited onto the carbon cloth, or paper, in much the same way as in phosphoric acid fuel cell electrodes, and this catalyzed carbon paper is hot-pressed, in turn, to the polymeric membrane. Thus, two modes of application of the catalyst layer - to the polymeric membrane or to a carbon support - can be distinguished and the specific mode of preparation of the catalyst layer could further vary within these two general application approaches, as summarized in Table 4. 

On the other hand the De Nora electrolysis unit is well described in the literature (26. 27). The standard assembly was composed of 40 cells of the filter-press type (De Nora was already in the water electrolyser business). The diaphragm itself consisted of PVC cloth pressed between the ribs of graphite anodes and cathodes so as to separate the electrodes by a width of 2 mm. The life expectancy of such a diaphragm, under good operating conditions, was approximately three years. 

---