Catalyst layers fuel cell

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. 

A fuel cell operating at a high current density consumes reactants and forms products quickly, and this leads to reactant depletion and product accumulation in the vicinity of the catalyst layer. Fuel cell performance is determined by the reactant and product concentrations within the catalyst layer and the reactant depletion and product accumulation affects fuel cell performance in two ways by Nemstian losses and reaction losses. Due to the electrochemical reaction at the catalyst layer, we have ... 

If a cell is flooded in the cathode catalyst layer, fuel cell polarization curve generated with helox (21% O2 and 79% He) as the cathode gas will he ... 

The fuel-cell sandwich describes the 1-D cross section of the fuel cell (see Figure 1) and is shown in Figure 5. For the single dimension, flow is taken to be normal to the various layers. Flow in the other directions is discussed in section 5. The fuel-cell sandwich contains the gas channels or flow fields, diffusion media, catalyst layers, and membrane. Additional layers are sometimes incorporated into the sandwich, such as separating the diffusion media into microporous and gas-diffusion layers. Fuel cells operate in the following manner. 

The excellent insulating and dielectric properties of BN combined with the high thermal conductivity make this material suitable for a huge variety of applications in the electronic industry [142]. BN is used as substrate for semiconductor parts, as windows in microwave apparatus, as insulator layers for MISFET semiconductors, for optical and magneto-optical recording media, and for optical disc memories. BN is often used as a boron dopant source for semiconductors. Electrochemical applications include the use as a carrier material for catalysts in fuel cells, electrodes in molten salt fuel cells, seals in batteries, and BN coated membranes in electrolysis cells for manufacture of rare earth metals [143-145]. 

Figure 3.52. Efficiency of a reversible PEM fuel cell as a function of the amount (at. % or mol %) of Ir in the form of IrOj relative to Pt in the positive electrode catalyst, for fuel cell electricity production (EC) or for water electrolysis (WE). Also the product of the two efficiencies relevant for storage cycles is shown. The catalyst is otherwise similar to that of Fig. 3.51, with PTFE and Nation channels. (From T. loroi, K. Ya-suda, Z. Siroma, N. Fujiwara, Y. Miyazaki (2002). Thin film electrocatalyst layer for unitized regenerative polymer electrolyte fuel cell. J. Power Sources 112, 583-587. Used by permission from Elsevier. See also loroi et al. (2004).

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

Electrochemical applications of a-BN include its use as carrier material for catalysts in fuel cells [297], as a constituent of electrodes in molten salt fuel cells [298, 299], as anticracking particles in the electrolyte for molten carbonate fuel cells [300, 301], and in seals for insulating terminals of Li/FeS batteries from the structural case [302], A BN-coated membrane is used in an electrolysis cell for the manufacture of high-purity rare earth metals from salt melts [381]. A porous boron nitride layer is applied to the upper outer surface of the electrolyte tube in sodium-sulfur batteries [303], and ceramic boron nitride separators are used in liquid fuel cells and batteries [304, 305]. Boron nitride powder may be included in the electrolyte of electrolytic capacitors for high-frequency utilization [306]. 

Optimization of the electrodes for these fuel cell systems has just started. Work has been done on the optimization of electrode structure for operation under hot, dry conditions, but less has been done to study catalysis under these conditions. Part of the reason for this is that as stated above there are no commercially available polymeric materials available for the development of new electrodes studies. It is hoped that until commercially available materials for this application become available that researchers offer to share their materials. This will, however, be insufficient as the ionomers developed for catalyst layers need different properties than ionomers developed to act as fuel cell membranes. The other major issue is that catalysts for fuel cells mn under conditions of water saturation have been developed using liquid phase electrochemical methods. It will be extremely important that new catalyst for fuel cells to be operated under hot, dry conditions be developed by solid-state electrochemistry. New methods must also be developed so that electrodes containing compatible ionomers can be tested. 

Fuel Starvation—Starvation of fuel can improve performance of a PEMFC exposed to CO [130]. Periodic fuel starvation causes the anode potential to increase and allows the oxidation and removal of the catalyst poisons from the anode catalysts, improving fuel cell performance. The preferred method has successive localized portions of the fuel cell anode momentarily and periodically fuel starved, while the remainder of the fuel cell anode remains elec-trochemically active and saturated with fuel so the fuel cell can continually generate power. However, when the cell is deprived of fuel, cell voltages can become negative as the anode is elevated to positive potentials and the carbon is consumed (carbon corrosion) instead of the absent fuel [131]. When this happens, the anodic current is generally provided by carbon corrosion to form carbon dioxide, resulting in permanent damage to the anode catalyst layer. 

Hayase et al. (2011) describe a nfini-fuel cell with monolithically fabricated electrodes. A porous Pt layer on an Si substrate was formed by immersing porous Si into a Pt plating bath containing HF. After formation of the porous catalyst layer, fuel channels were opened by applying dry etching from the opposite side of the Si wafer. Two such Si electrodes were hot-pressed on either side of a polymer electrolyte membrane. By feeding H2 and O2 to the corresponding electrodes, a peak power density of 145 mW/cm was obtained. 

Inoue H, Daiguji H, Hihara E. The structure of catalyst layers and cell performance in proton exchange membrane fuel cell. JSME Int J Ser B 2004 47-2 228-34. 

As mentioned above, besides alloys, in recent times core-shell nanoparticles attract increasing attention as possible candidate catalysts for fuel cell cathodes [16,17]. In these systems, a thin shell (monolayer to several atomic layers in thickness) of the active metal (usually Pt) surrounds a core of another metal or an alloy. In this way, the amount of costly noble metal... 

It was quite recently reported that La can be electrodeposited from chloroaluminate ionic liquids [25]. Whereas only AlLa alloys can be obtained from the pure liquid, the addition of excess LiCl and small quantities of thionyl chloride (SOCI2) to a LaCl3-sat-urated melt allows the deposition of elemental La, but the electrodissolution seems to be somewhat Idnetically hindered. This result could perhaps be interesting for coating purposes, as elemental La can normally only be deposited in high-temperature molten salts, which require much more difficult experimental or technical conditions. Furthermore, La and Ce electrodeposition would be important, as their oxides have interesting catalytic activity as, for instance, oxidation catalysts. A controlled deposition of thin metal layers followed by selective oxidation could perhaps produce cat-alytically active thin layers interesting for fuel cells or waste gas treatment. 

A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

Catalyst layer architecture As a consequence of the diminishing remrns from ever higher dispersion, the effort to increase the active catalyst surface area per unit mass of Pt has centered in recent years primarily on optimization of catalyst layer properties, aiming to maximize catalyst utilization in fuel cell electrodes based on Pt catalyst particle sizes of 2-5 nm. High catalyst utilization is conditioned on access to the largest possible percentage of the total catalyst surface area embedded in a catalyst... 

One of the critical issues with regard to low temperamre fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst layer under the harsh operating conditions, which mainly consist of two aspects electrochemical surface area (ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon support itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid fuel cells (PAECs) have shown that ECA loss is mainly caused by three mechanisms ... 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

The catalyst layer is the most expensive part of this fuel cell. It is made from a mixture of platinum, carbon powder, and PEM powder, bonded to a conductive carbon fiber cloth. We obtained ours from E-Tek Inc. The cost for an order of their ELAT catalyst cloth sheet includes a setup charge. So get together with others for a larger order if you want to keep costs down. We paid 360 for a piece of ELAT 15.2 centimeters by 15.2 centimeters [6 inches by 6 inches] including the 150 setup charge. This piece provides enough for about twelve disks. Each fuel cell requires two disks of ELAT and one larger disk of PEM to make the sandwich, so you can make six cells from this size... 

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