Cathodes fuel cell, high electrical

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Polymer electrolyte fuel cells (PEFCs) have attracted great interest as a primary power source for electric vehicles or residential co-generation systems. However, both the anode and cathode of PEFCs usually require platinum or its alloys as the catalyst, which have high activity at low operating temperatures (<100 °C). For large-scale commercialization, it is very important to reduce the amount of Pt used in fuel cells for reasons of cost and limited supply. 

Provided that the required enzymes can be immobilized at, and electrically communicated with, the surface of an electrode, with retention of their high catalytic properties and there is no electrolysis of fuel at the cathode or oxidant at the anode, or a solution redox reaction between fuel and oxidant, the biocatalytic fuel cell then simply... 

In a fuel cell, the electrocatalysts generate electrical power by reducing the oxygen at the cathode and oxidizing the fuel at the anode [1], Pt and Pt alloys are the most commonly used electrocatalysts in PEFCs due to their high catalytic activity and chemical stability [99-103]. 

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 portion AQ = AH - AG = TAS of AH is transformed into heat. Ideal theoretical efficiencies % determined by the types and amounts of reactants and by the operating temperature. Fuel cells have an efficiency advantage over combustion engines because the latter are subdued to the Carnot limitation. High thermodynamic efficiencies are possible for typical fuel cell reactions (e.g., e,h = 0.83 (at 25°C) for H2 + I/2O2 -> H20(i)). The electrical potential difference between anode and cathode, = -AG/W(f, which is also called the electromotive force or open-circuit voltage, drives electrons through the external... 

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the US space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C. However, more-recent AFC designs operate at lower temperatures of roughly 23°C to 70°C. 

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminium oxide (LiAI02) matrix. Since they operate at extremely high temperatures of 650°C and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs. 

Cathode Recycle. Air from the cathode exhaust is recycled to the cathode inlet and increases the air temperature from the compressor by mixing. As with the previous case, this method requires certain compressor airflow to achieve the required A T across the fuel cell. Two methods for achieving cathode recycle are commonly considered. A high temperature blower powered by an electric motor can be used to recycle the flow. Also, an ejector uses the compressor as the primary flow to recycle the cathode air. These comparisons will assume a blower is used. 

Figure 16. Oxygen partial pressure dependence of the electrical conductivity of doped Ce02. The steep decrease is due to excess electrons, the flat behavior to oxygen vacancies. If we refer to typical oxygen partial pressures in an SOFC, viz. to 10" bar at the cathode and 0.2 bar at the anode, we see that the conductivity changes from ionic into n-type within a high temperature Ce02 based fuel cell. Reprinted from M. Godickemeier and L.J. Gauckler, J. Electrochem. Soc. 145 (1998) 414-421. Copyright 1998 with permission from The Electrochemical Society, Inc.

In practice, in all fuel cells that involve the utilization of 02 from air (Section 13.4.5), the oxygen reduction reaction [Eqs. (13.4) and (13.24)] is always rate determining for terrestrial applications. One can seejust how important it is to attempt to develop electrocatalysts for the cathodes of fuel cells on which the enhanced current density is high and Tafel slope is low and the efficiency of energy conversion, therefore, maximal. The direct relation of the mechanism of oxygen reduction and the associated Tafel parameters to the economics of electricity production and transportation is thus clearly seen. 

The carbonate ions are transported towards the anode electrode through the electrolyte, which is an eutectic carbonate melt The cathodic exhaust gas leaves the fuel cell and, due to its high temperature of about 500 °C, it can be used for subsequent steam generation, for additional production of electric energy via a micro turbine or for other purposes. 

---