Catalyst layers high electrical conductivity

Electrode Materials The AFCs developed by F. Bacon utilized nonnoble sintered nickel metal catalysts. The high electrical conductivity of these porous electrodes permits use of current collection from monopolar stack plates [62]. The sintered metal is often applied in two separate layers, with large pores on the channel side, so that control of the hquid electrolyte-gas phase interface can be achieved by capillary and gas-phase pressure forces. 

Since only the Pt atoms on the Pt particle surface can participate in the catalysis process, one way to maximize Pt atom utilization is to deposit an extremely thin layer of Pt on a non-Pt particle to form a core-shell structure with the Pt layer as the shell and the non-Pt particle as the core. The electron conductivity of the core particles is not important because they are covered by a highly electrical conducting Pt shell. In addition, when a submonolayer to several monolayers of Pt are made on certain core metal particles, the catalytic ability of Pt improves due to electronic interactions between the shell Pt atoms and the interior core metal atoms that increase the Pt 5d orbital vacancies and thus increases the n electron donation from O2 to Pt atoms (electronic effect), and due to the decrease in the Pt-Pt atomic distance (geometric effect). This mechanism is similar to the improved catalytic ability of PtM and PtM Ny alloys, where M and N represent different metals and X and y their atomic contents in the alloy. For the Pt/core, PtM or PtM Ny, if there is some leaching out of the non-Pt metals, their corresponding cations can replace the protons of the PFSA either in the catalyst layer or in the membrane to reduce its proton conductivity as well as the catalyst-PFSA-reactant three-phase boimdaries, and thus decrease the fuel cell performance. The shape and crystalline facet of the Pt nanoparticles can also affect the catalytic activity. 

A PEFC consists of two electrodes in contact with an electrolyte membrane (Fig. 14.7). The membrane is designed as an electronic insulator material separating the reactants (H2 and 02/air) and allowing only the transport of protons towards the electrodes. The electrodes are constituted of a porous gas diffusion layer (GDL) and a catalyst (usually platinum supported on high surface area carbon) containing active layer. This assembly is sandwiched between two electrically conducting bipolar plates within which gas distribution channels are integrated [96]. 

Although the sputter deposition technique can provide a cheap and directly controlled deposition method, the performance of PEM fuel cells with sputtered CLs is still inferior to that of conventional ink-based fuel cells. In addition, other issues arise related to the physical properties of sputtered catalyst layers, such as low lateral electrical conductivity of the thin metallic films [96,108]. Furthermore, the smaller particle size of sputter-deposited Ft can hinder water transport because of the high resistance to water transport in a thick, dense, sputtered Ft layer [108]. Currently, the sputter deposition method is not considered an economically viable alternative for large-scale electrode fabrication [82] and further research is underway to improve methods. 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

The catalyst layer usually consists of carbon-supported catalyst or carbon black mixed with PIPE and/or proton-conducting ionomer (e.g.. Nation iono-mer). Because the sizes of the pores in a t) ical DL are in the range of 1-100 pm and the average pore size of the CL is just a few hundred nanometers, the risk of having low electrical contact between both layers is high [129]. Thus, the MPL is also used to block the catalyst particles and does not let them clog the pores within the diffusion layer [57,90,132,133]. 

A more favourable approach is the incorporation of the active species in an electrically conducting polymer layer which then acts as an (electrical) intermediate between the electrode surface and the catalyst. Polypyrrole is considered to be especially suitable because it is acceptably stable under ambient conditions (2), has a high conductivity and can be easily prepared electrochemically from a great variety of solvent systems, including aqueous solutions (3-5). The catalytic species that have been applied in such polypyrrole-based systems comprise metal particles (6-9), metal chelates (10-13) (with anionic side groups) and enzymes (14-18). 

It is well known that catalyst support plays an important role in the performance of the catalyst and the catalyst layer. The use of high surface area carbon materials, such as activated carbon, carbon nanofibres, and carbon nanotubes, as new electrode materials has received significant attention from fuel cell researchers. In particular, single-walled carbon nanotubes (SWCNTs) have unique electrical and electronic properties, wide electrochemical stability windows, and high surface areas. Using SWCNTs as support materials is expected to improve catalyst layer conductivity and charge transfer at the electrode surface for fuel cell oxidation and reduction reactions. Furthermore, these carbon nanotubes (CNTs) could also enhance electrocatalytic properties and reduce the necessary amount of precious metal catalysts, such as platinum. 

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. 

Carbon is generally used as catalyst support material because of its high electric and thermal conductivity, chemical stability, and porous structure [11]. The catalytic activity of the catalyst layer increases with increasing carbon surface area due to better platinum dispersion. High surface area carbon blacks such as Ketjenblack and Vulcan are therefore preferred in PEMFC application. However, carbon is thermodynamically unstable at normal cathode potentials between 0.5 and 1V. As shown in Figure 20.1a, carbon is oxidized to carbon dioxide (CO2) or carbon monoxide (CO) at high electrode potentials whereas it is reduced to methane (CH4) at low electrode potentials. The following reactions are relevant for fuel-cell operation ... 

In the electrodes for PAFC, the Vulcan XC-72 carbon black is most widely used catalyst support material [95]. The oxidation of Vulcan carbon black in the presence of phosphoric acid at 191 °C showed that the disordered central part of carbon particles was oxidized while the outer crystalline part remained intact [96]. Among the attempts to improve the oxidation resistance of Vulcan carbon black, the most widely used method is the heat treatment which increases the level of graphitization on the carbon surface [97]. The heat treatment of Vulcan carbon black at the temperature of 2200 °C which reduced the surface area of Vulcan from 240 to 80 m /g improved oxidation resistance more than twofold [98, 99]. Other highly graphitic carbon materials such as CNT [100] and graphene [101] have been used as support materials because of their high surface area and electrical conductivity. When selecting the carbonsupport material, the oxidation resistance is the critical property for carbon supports to enhance the durability of HT-PEMFC MEAs however, the surface area, shape, and size of support material should also be considered to achieve the desired dispersion of Pt particles as well as the pore structure within the catalyst layer. 

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