Kinetics of Fuel Cell Reactions

Paik W, Springer TE, Srinivasan S. Kinetics of fuel cell reactions at the platinum/solid polymer electrolyte interface. J Electrochem Soc 1989 136 644-49. 

The section on basic principles contains background information on fuel cells, including fundamental principles such as electrochemistry, thermodynamics, and kinetics of fuel cell reactions as well as mass and heat transfer in fuel cells. The section on design explores important characteristics associated with various fuel cell components, electrodes, electrocatalysts, and electrolytes, while the section on analysis examines phenomena characterization and modeling both at the component and system levels. 

To better understand how a PEM fuel cell works, it is necessary to grasp the fundamentals of PEM fuel cells, including their cell structure and the thermodynamics and kinetics of fuel cell electrochemical reactions. In the following sections of this chapter, the fundamentals of H2/air PEM fuel cells will be discussed in detail. 

In this section, we derive a general expression to describe activation polarization losses at a given electrode, known as the Butler-Volmer (BV) kinetic model. The BV model is not the only (or necessarily the most appropriate) model to describe a particular electrochemical reaction process. Nevertheless, it is a classical treatment of electrode kinetics that is widely applied to study and model a majority of the electrode kinetics of fuel cells. The BV model describes an electrochemical process limited by the charge transfer of electrons, which is appropriate for the ORR, and in most cases the HOR with pure hydrogen. The fundamental assumption of the BV kinetic model is that the reaction is rate hmited by a single electron transfer step, which may not actually be true. Some reactions may have two or more intermediate charge transfer reactions that compete in parallel or another intermediate step such as reactant adsorption (Tafel reaction from Chapter 2) may limit the overall reaction rate. Nevertheless, the BV model of an electrochemical reaction is standard fare for a student of electrochemistry and can be used to reasonably fit most fuel cell reaction behavior. 

In this paper, we will discuss the thermodynamic principles involved in fuel cells as well as the kinetic aspects of their half cell reactions. In the kinetic considerations, we will also touch, briefly, on the fundamental problem of electrocatalysis. We will then proceed to describe different types of fuel cells and finally present the status of this new electrical generation device. 

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). 

The efficiency of fuel cells is largely limited by the kinetic barriers of the surface catalytic electrode reactions. In particular, the electroreduction of molecular oxygen at a PEMFC cathode severely limits high reaction rates and hence currents near the equilibrium cell voltage. 

MEA active area 4.4 cm. H2/air gases with 100% relative humidity [20]. (Adapted from Electrochimica Acta, 52, Song C, Tang Y, Zhang J, Zhang J, Wang H, Shen J, et al., PEM fuel cell reaction kinetics in the temperature range of 23-120°C, 2552-61. 2007, with permission from Elsevier.)... 

Chemists are studying the structure and kinetics of the photosynthetic reaction center both to understand the fundamentals of this important natural process and to design new materials that mimic nature s ability to harvest light energy at such high efficiency. Artificial photosynthesis may lead to carbon-based materials that will replace the silicon collectors in solar cells in the 21st century. This will help reduce human dependence on stored fossil fuels as energy sources in the future. 

This impressive list of attributes has provided the incentive for much of the research that has taken place on fuel cells in recent years. Naturally, the extent to which each of these characteristics is desirable depends on the application under consideration. Against these advantages should be set the many difficulties that have been encountered in the refinement of fuel cells towards practical power devices that are commercially viable, particularly when fuelled by natural gas, oil or coal. These primary fuels must first be reformed to hydrogen or methanol and then purified, which are tasks that present obstacles when chemical engineers seek to integrate the reformer to the fuel-cell stack and balance the characteristics of the two systems in terms of reaction kinetics and thermal management. 

More complicated are in-situ methods to analyse the electrochemical behaviour of fuel cell systems since the reactions analysed separately can differ significantly from those taking place in a real fuel cell system [106]. It is well known that the electrochemical kinetics are strongly dependent on the crystallographic orientation of the metal surface [107,108]. 

Therefore an increase in equUibrium ceU voltage with the logarithm of the partial pressures is expected thermodynamicaUy. However, the performance increase of fuel cells with pressure is normaUy much more pronounced because the kinetics of the reactions are strongly influenced by partial pressure increases (the reaction rate of the electrochemical reactions increases with higher concentrations). 

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