Platinum utilization

Using a carbon-supported Pt catalyst to replace Pt black can reduce the platinum loading by a factor of 10—from 4 to 0.4 mg/cm [74]. However, the platinum utilization in this PTFE-bound catalyst layer still remains low in the vicinity of 20% [75,76]. 

Numerous efforts have been made to develop in situ catalyst layer fabrication methods to lower Pt loading and increase platinum utilization without sacrificing electrode performance. 

The experimental optimization of Nafion ionomer loading within a catalyst layer has attracted widespread attention in the fuel cell community, mainly due to its critical role in dictating the reaction sites and mass transport of reactants and products [15,128-134]. Nafion ionomer is a key component in the CL, helping to increase the three-phase reaction sites and platinum utilization to retain moisture, as well as to prevent membrane dehydration, especially at low current densities. Optimal Nafion content in the electrode is necessary to achieve high performance. 

In order to make catalyst layers with high platinum utilization and better performance, we need to determine how various factors affect Pt utilization. Although this objective has been receiving more attention, we have not achieved a fundamental understanding of the relationships of composition, structure, effective properties, and fuel cell performance—a fact that may limit the optimal design and fabrication of CLs. 

Cheng, X., Yi, B., Han, M., Zhang, J., Qiao, Y., and Yu, J. Investigation of platinum utilization and morphology in catalyst layer of polymer electrolyte fuel cells. Journal of Power Sources 1999 79 75-81. 

J. Aragane, H. Urushiba, T. Murahashi, Platinum utilization in a phosphoric acid fuel cell. Denki Kagaku 1995, 63(7), 642-647. 

The hollow ATO-PANI spheres obtained via the Pickering approach were decorated by Pt nanoparticles using a standardized polyol process. The chosen synthesis protocol for Pt decoration used mild conditions, which hence did not impact the hollow sphere structure. The hollow spheres were then sprayed onto a commercial Nation membrane and applied at the cathode side of an MEA. Figure 7.14a depicts a cross-sectional SEM image of the oxidic hollow spheres. It can be clearly observed that the sphere structure is maintained after the polyol process for Pt decoration. Single cell tests of the novel electrode design showed a maximum power density of 58 mW cm and a platinum utilization... 

Taylor EJ, Anderson EB, Vilambi NRK (1992) Preparation of high-platinum-utilization gas diffusion electrodes for proton-exchange-membrane fuel cells. J Electrochem Soc 139 L45-L46... 

One additional function of catalyst support material is to enhance the platinum utilization in the electrode. This leads to an enlarged three phase reaction zone in the MEA and higher electrochemical active catalyst areas at the same catalyst loading of the electrode. For this reason, support materials with adequate surface area and porosity have to be chosen. Antolini [9] shows the benefits of using meso porous stractured carriers with pore sizes between 2 and 50 nm. Here, free pore volume is available for the electrolyte which enables an additional rise of three phase boundary zone and an increased interaction between catalyst and electrolyte [5, 6, 9-15]. 

Various metals/metallic oxides have been coated onto carbons with the aim to improve the platinum tolerance to poisons, increase the platinum utilization and to avoid carbon corrosion of the catalyst support. Sn, Ru, Ti, and Co metals as well as their oxides have been reported on in fair amounts as being transition metals capable of increasing the platinum utilization by removing hydroxide species that would be present on the platinum such that the platinum can further catalyze the ORR. Although these materials have been coated onto the carbon as a support, there are alloying characteristics that occur with the impregnated platinum which result and thus will not be touched up in this section. However, many of these oxides and metals have been used as stand-alone catalyst supports without the use of a carbon substrate, and are discussed further in Section 3.5.3. 

There are several advantages for the use of S-ZrOj as a catalyst support in PEMFC applications. Because of its hydrophilicity, it has been suggested that this type of fuel cell catalyst would be well suited for low-relative humidity conditions and possibly simplify fuel cell components to operate without the use of a humidifier. Due to the proton conductivity across the surface of the material, less Nafion iono-mer needs to be cast to form the TPBs. Platinum utilization increases as the S-ZrOj support acts as both the platinum and proton conductor and better gas diffusion to the catalyst site results from the decreased blockage of Nafion ionomer (Liu et al., 2006a,b). It is beheved that within porous carbon catalyst supports, platinum deposited within the pores may not have proton conductivity due to the perfluorosul-fonated ionomer unahle to penetrate into the pores. Thus, a TPB which is necessary for a catalyst active site will not be formed. Therefore, the S-ZrOj support has an additional benefit over porous carbon material supports in that by using the S-ZrOj as a support for platinum catalysts, the surface of the support can act as a proton conductor and platinum deposited anywhere on the surface of the support will provide immediate access to the electron and proton pathways thereby requiring less Nafion. Thus the use of S-ZrOj in fuel cell MEA components may potentially lower the cost of materials substantially, as the catalytic metals and membrane materials are among the most costly in a PEMFC. However, like most metallic oxides, the downside of their use stems from their relatively low electron conductivity and low surface areas that results in poor platinum dispersion. 

Silicon (Hayase et al., 2004 Yeom et al., 2005), conducting polymer (Lefebvre et al., 1999), and conductive diamond (Montilla et al, 2003 Bennett et al, 2005) have also been studied as catalyst support materials in PEM fuel cells. However, all of these materials result in lower platinum utilization than carbon support. In addition, the durability of silicon and conducting polymer in PEM fuel cell environment is also questionable. 

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