Electrical conductivity PEMFC

For the support material of electro-catalysts in PEMFC, Vulcan XC72(Cabot) has been widely used. This carbon black has been successfully employed for the fuel cell applications for its good electric conductivity and high chemical/physical stability. But higher amount of active metals in the electro-catalysts, compared to the general purpose catalysts, make it difficult to control the metal size and the degree of distribution. This is mainly because of the restricted surface area of Vulcan XC72 carbon black. Thus complex and careM processes are necessary to get well dispersed fine active metal particles[4,5]. 

Similar metal sheets have also been used as DLs in the cathode of PEMFCs. Wilkinson et al. [37,38] presented the idea of using fluid distribution layers made out of metal meshes with electrically conductive fillers inside the holes of the meshes. A very similar idea was also presented by Fiamada and Nakato [39]. Eosfeld and Eleven [40] presented another example of fuel cells that use metal meshes as diffusion layers along with metal FF plates. 

In PEMFCs, it is critical to determine the appropriate amount of PTFE content on both the anode and cathode DLs because that can change the performance of the cell substantially. The most common loadings of PTFE and FEP are from 5 to 30 wt%. Bevers, Rogers, and von Bradke [100] studied the effect of PTFE content and sinter temperatures on the performance of a DL (Sigri PE 704 CFP). It was demonstrated that high PTFE content and high sinter temperatures led to high hydrophobicity as well as to low electrical conductivities, which decreased the performance of the fuel cell. 

The MEA (Figure 3.3.4) is the heart of every individual PEMFC. Operated on hydrogen and oxygen, the maximum individual cell voltage is approximately 1.23 V. In order to obtain higher cell voltages, individual MEAs are put in series in so-called PEMFC stacks as shown in Figure 3.3.6. In between each individual MEA, there are electrically conductive flow plates that provide flow paths for the fuel and oxidant. 

The materials for PEMFC electrodes should have good electrical conductivities and be stable in contact with electrolyte. Platinum-based electrodes have shown excellent electrochemical activities for PEMFC. 

Beside the MEA the bipolar plates are the key components in a PEMFC stack in terms of their contribution to weight, volume, and costs. Bipolar plates contain a fine mesh of gas channels called the flow-field, to ensure a uniform distribution of the process gasses (hydrogen and oxygen) of fuel and air across both sides of the MEA and the removal of the reaction products. Furthermore, the bipolar plates in PEM fuel cells separate the individual cells from each other and guarantee an electrical connection between them in series. Substantial requirements for the bipolar plates are a high electrical conductivity and corrosion resistance. 

From energy-density and volume-production points of view, the use of metallic bipolar plates in PEMFC stacks is important for the automobile industry. Metallic bipolar plates could provide thinner and hghter choices and improved thermal and bulk electrical conductivity. The use of SSs makes PEMFCs cost-effective. Whereas 316/316L SS may not be the optimum choice for the PEMFC bipolar plate... 

A PEMFC system typically comprises a fuel cell stack where MEAs are stacked between electrical conductive bipolar plates that have flow fields embedded in them, allowing the reactive gases to be supplied to the catalyst surface and allowing the reactant water to be carried away. The area of the MEA determines the amount of current that can be passed through a cell and the number of cells in the stack determines the voltage. Together, these define the power the stack is capable of providing. 

A well-qualified substrate layer should have excellent gas permeability, high electron conductivity, smooth surface, good mechanical strength, proper wettability, stable chemical and heat properties, as well as low cost. The most common SL materials used in PEMFCs are carbon-fiber-based products, such as non-woven carbon papers and woven carbon cloths. This is mainly due to their high porosity (> 70%) and good electrical conductivity. Besides these carbon materials, some metal substrates such as sintered porous titanium and stainless steel fiber felt have also been explored as SLs [12-14] because of their high mechanical strength, ductility, and low cost. 

Carbon nanotubes (CNTs) have been added to a polymeric matrix to improve their mechanical and other properties [69]. The use of CNTs in PEM must be carried out with caution because the well-known high electrical conductivity may cause short circuiting in proton exchange membrane fuel cells. The jt-n interaction between PBI and the side walls of CNT makes these two different materials compatible. Despite CNT-PBI composite membranes have shown enhancement in mechanical strength, the proton conductivity resulted in some cases compromised [70, 71]. Hence, different authors functionalized the CNTS in order to increase both the proton conductivity and the mechanical properties for hydrogen fed PBI-based HT-PEMFC [72, 73]. In this context,... 

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. 

Owing to the high electrical conductivity and the known stability of ITO, this material is particularly interesting for fuel cell catalyst supports. It has been proposed as an alternative material for the cathode support in PEMFCs that could avoid the corrosion problems experienced by conventional carbon supports. However, the application for PEMFC catalyst supports is novel and there are few reported works... 

Nanostructured TiOj has also been utilized as PEMFC catalyst supports with dimensions well below 100 nm. Like other TiOj structures, these materials also inherit high stability, high surface areas, and moderate electrical conductivity. Titanium oxide nanotubes are of particular interest for fuel cell catalyst supports due to their abihties to form porous networks, disperse catalytic metals upon the surface... 

A PEMFC consists of two electrodes and a solid polymer membrane, which acts as an electrolyte. The proton exchange membrane is sandwiched between two layers of platinum porous electrodes that are coated with a thin layer of proton conductive material. This assembly is placed between two gas diffusion layers such as carbon cloth, carbon paper enabling transfer of electrical current and humidified gas. Some single cell assemblies can be mechanically compressed across electrically conductive separators to fabricate electrochemical stacks. 

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