Reaction kinetics PEMFC

The generated potential via exergonic reactions in direct fuel cells is partly used to promote electrode reactions (kinetic overpotential), while in indirect fuel cells, the fuel is first processed into simpler fuels (to reduce the kinetic overpotential) via conventional catalytic reactors, or CMRs, in which temperature is used as the key operating parameter to accomplish the desired kinetics and equilibrium conversions. Among the direct fuel cells, e.g., PEMFCs use hydrogen, direct methanol fuel cell (DMFC) uses methanol, while the SOFC can operate directly on natoral gas (Figiue 15.3). However, in indirect fuel cells, a complex fuel must be suitably reformed into simpler molecules such as H2 and CO before it can be used in a fuel ceU. 

Because the catalyst used for both the ORR and HOR is usually platinum or a platinum alloy, the CLs contribute a significant portion of the cost for a PEMFC [2]. Consequently, improving Pt utihzation is important for achieving cost reductions. The HOR and ORR take place at the triple-phase boundaries and in the areas within which active catalyst surface is usually large, providing an effective way to improve the CL reaction kinetics. This can be directly seen from the weU-known Butler-Vohner equation ... 

The dynamic model of a PEMFC can be realized in MATLAB and Simulink software for implementation in power systems [10]. Beginning with hydrogen flow, the three significant factors are input, output, and reaction flows during operahon [11]. The thermodynamic potential of the chemical energy that can be converted into electrical energy is derived from Nernst s law and is dependent on the partial pressures of the reactants and temperature. For reaction kinetic considerahon, overpotentials at both anode and cathode essentially constitute the energy required to drive a reaction beyond the state of thermodynamic reversibility. 

At a temperature of around 200 °C the fuel cell reaction should proceed much more quickly than it does at the lower temperatures typical for PEM fuel cells (PEMFCs). However, since both H3PO4 and H2P04 can adsorb onto the surface of the Pt catalyst, they significantly lower the reaction kinetics, especially for the oxygen reduction reaction (ORR). Consequently, a PAFC operated at about 200 °C proceeds much more slowly than a PEMFC operated at less than 90 °C. As shown in Figure 11.2, the performance of the state-of-flie-art PAFC operated at 160 °C is about 150 mV lower than that of a PEMFC operated at 70 °C. 

Pt is the most important catalyst material for PEMFCs, DMFCs, and PAFCs. Due to the relatively low operating temperatures of these fuel eells (from room temperature to about 200 °C), the reaction kinetics is slow, especially at the eathode for the ORR. Therefore, Pt and its alloys that have shown the highest eatalytic activity in these fuel cells have to be used as the catalysts. 

Understanding. As electrochemical reactors with gradients of species, temperature, and potential in all dimensions and over a broad range of scales from nanometers to meters, fuel cells are complex devices. Changing a single parameter (e.g., the gas humidity in a PEMFC) can result in effects on the scale of reaction kinetics and other parameters, all the way to temperature distribution in a fuel ceU stack. This multitude of effects, as well as their consequences as felt in important parameters such as efficiency or power density, is difficult or impossible to comprehend without modeling approaches. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

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