Fuel cell performance PEMFC

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Figure 14.16 the shows fuel cell stack performance of a 1 kWe atmospheric PEMFC stack using PtRu anodes, operating on various gas compositions. As can be clearly seen, already small concentrations of CO lead to a large decrease of fuel cell performance. An air-bleed of 1.5% air in hydrogen is able to mitigate this ef-... 

As discussed previously, a number of different materials have been considered as potential candidates to be used as diffusion layers in PEMFCs and direct liquid fuel cells (DLFCs). The two materials used the most so far in fuel cell research and products are carbon fiber papers and carbon cloths, also known as carbon woven fabrics. Both materials are made from carbon fibers. Although these materials have been quite popular for fuel cells, they have a number of drawbacks—particularly with respect to their design and model complexity—that have led to the study of other possible materials. The following sections discuss in detail the main materials that have been used as diffusion layers, providing an insight into how these materials are fabricated and how they affect fuel cell performance. 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

In low-temperature fuel cells (AFC, PEMFC, DAFC, etc.), carbon materials are important since they are involved in the fabrication of BP, GDL, and CL. It appears that no other materials can replace carbon with the same properties (good electronic conductivity, good thermal and chemical stabilities, and low cost). But much work is needed to optimize carbon materials for fuel cell applications and to ensure that they meet the performance targets for conductivity, physical properties, and lifetime within operating stacks. 

Fuel cells, especially PEMFCs, can be used for various applications ranging from portable power supply for use in consumer electronic devices to stationary deployment for combined heat and power generation. Another potential application is transportation, in which fuel cell systems are developed for the propulsion of cars. The performance, operating conditions, costs, and durability requirements differ depending on the application. Transportation applications demand stringent requirements on fuel cell systems. Only the durability requirement in the transportation field is not as rigorous as the stationary application, although cyclic durability is necessary. 

The right choice of a carbon support greatly affects cell performance and durability. The purpose of this chapter is to analyze how structure and properties of carbon materials influence the performance of supported noble metal catalysts in the CLs of the PEMFCs. The review chapter is organized as follows. In Section 12.2 we give an overview of carbon materials utilized for the preparation of the catalytic layers of PEMFC. We describe traditional as well as novel carbon materials, in particular carbon nanotubes and nanofibers and mesoporous carbons. In Section 12.3 we analyze properties of carbon materials essential for fuel cell performance and how these are related to the structural and substructural characteristics of carbon materials. Sections 12.4 and 12.5 are devoted to the preparation and characterization of carbon-supported electrocatalysts and CLs. In Section 12.6 we analyze how carbon supports may influence fuel cell performance. Section 12.7 is devoted to the corrosion and stability of carbon materials and carbon-supported catalysts. In Section 12.8 we provide conclusions and an outlook. Due to obvious space constraints, it was not possible to give a comprehensive treatment of all published data, so rather, we present a selective review and provide references as to where an interested reader may find more detailed information. 

Franco AA (2012) PEMFC degradation modeling and analysis. In Hartnig C, Roth C (eds) Polymer electrolyte membrane and direct methanol fuel cell technology (PEMFCs and DMFCs). Volume 1 Fundamentals and performance. Woodhead, Cambridge, UK... 

Fig. 4.2 (a) Ideal thermodynamic efficitaicy of polymer electrolyte membrane fuel cells (PEMFCs) compared to that obtained in the Camot process, (b) Comparison of processes in a cogenerated heat engine with fuel cell performance (From [2])... 

Another way to consider the impact of membrane conductivity on fuel cell performance is shown in Fig. 17.5. Figure 17.5a shows the conductivity of a few different EW membranes as a functiOTi of temperature with the atmosphere inside the conductivity cell held at a fixed dew point of 80°C [17]. When the conductivity cell is at 80°C, the %RH is 100%. As the temperature of the cell increases, the %RH at a fixed dew point decreases, causing a decrease in the membrane conductivity. This is similar to the situation in some PEMFC applications where the cell temperature may rise while the humidity level of the incoming gases remains constant. The graph in Fig. 17.5b uses the same data. Here the conductivity is used to calculate the resistance of a 25 pm membrane, and using Ohms law, that resistance is used to calculate the voltage loss (ohmic loss) one would see in a fuel cell at a 0.6 A/cm current density [17]. This represents the fuel cell performance loss due to the loss of membrane conductivity (certainly not the only performance loss under these conditions ). 

In the literature, it is generally agreed that NO contamination is recoverable. When the contaminant source was shut off and pure air was turned on for a certain period of time, e.g., 24 hours, fuel cell performance could be totally recovered. Figure 6.8 shows the polarization curves of a PEMFC before NO contamination, after contamination, and after recovery. However, if the PEMFCs were exposed to NO continuously for a long period of time (e.g., 500 hours), performance could not be fully recovered [50]. This recoverability reduces the concern about the NO, contamination effect on PEMFC performance. 

The additive effect of multiple contaminants on fuel cell performance is found when the air contains a mixture of NO2 and SO2, and the H2 fuel contains H2S. Figure 6.12 shows the performanee of a PEMFC fed with air eontaining eaeh individual pollutant, and with the mixture. The effeet of the mixed eontaminants is the sum of their individual effects [21]. Recovery from the mixed contaminants was not reported, and more work is needed to gain a elear understanding. 

In addition, the presence of both CO and CO2 has a synergetic poisoning effect on cell performance, which is evident from the aggravated poisoning effect when CO2 is added to a PEMFC via a CO-containing fuel stream [5]. The presence of CO2 can also decrease H2 partial pressure, thus reducing fuel cell performance [5]. 

Although research has been limited, several studies conducted show the effects of cationic contamination on PEMFC performance. These studies, in general, have been of two types. In the first, researchers introduced cationic contaminant to the feed stream and measured fuel cell performance over time. In the second, they precontaminated the fuel cell to known levels to understand how contamination level directly relates to the percentage of protons displaced. A brief summary of the experiments and results from these studies follow below. 

Mathematical modeling is a useful tool for exploring the mechanisms of cationic contamination. One can use this new insight to help understand the effects of cationic contamination on fuel cell performance. Ultimately, the complexity of the needed model depends on its purpose. For many applications, simple models can qualitatively explain the mechanisms of cationic contamination on PEMFC systems. Models that are more complex help to quantify experimental data and make specific predictions. Understanding exactly how and to what extent cationic contamination... 

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. 

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