Hydrogen-fed fuel cells

The main electrochemical reactions for hydrogen-fed fuel cells are as follows ... 

At the end of twentieth century DMFC-driven low emission vehicles were visualized as a strategy for the future [1], although it was recognized that the main breakthrough needed for compete with hydrogen-fed fuel cells was the formulation of new membranes able to sustain high temperatures (150-160 °C)... 

In a hydrogen-fed fuel cell, water is produced at the rate of one mole for every two electrons. (Revisit Section 1.1 if you are not clear why.) So, we again adapt equation A2.2 to obtain... 

In the hydrogen-fed fuel cell, the rate of water production more or less has to be stoichiometric. However, if the fuel is a mixture of carbon monoxide with hydrogen, then the water production would be less - in proportion to the amount of carbon monoxide present in the mixture. If the fuel was a hydrocarbon that was internally reformed, then some of the product water would be used in the reformation process. We saw in Chapter 7, for example, that if methane is internally reformed, then half the product water is used in the reformation process, thus halving the rate of production. 

FIGURE 17.12 In a hydrogen-oxygen fuel cell, the two gases are fed in separately and are oxidized or reduced on the electrodes. A hot solution of potassium hydroxide between the electrodes completes the circuit, and the steam produced in the reaction evaporates from the cell continuously. 

A simple hydrogen-oxygen fuel cell differs in two major ways from a galvanic cell the electrodes are made of an inert material that doesn t react during the process, and hydrogen and oxygen gas are fed in continuously. 

Figure 2 shows the system schematic for a direct hydrogen reformer/fuel cell system. Compressed hydrogen is stored in a carbon-fiber wrapped composite vessel at pressures up to 340 atm. It is fed through a control valve to the fuel cell at 2 atm, where it reacts with oxygen from air to generate electricity and heat. Unreacted hydrogen is... 

Hissel, D., and Pera, M.C. (2010) Static and dynamic modeling of a diesel fed fuel cell power supply. Int.J. Hydrogen Energy, 35, 1377. 

As an example Fig. 4 shows the schematic of a hydrogen-oxygen fuel cell [20]. The electrodes are made of porous carbon and the electrolyte is a resin containing concentrated aqueous sodium hydroxide solution. Hydrogen gas is fed to the anode where it is oxidized, and oxygen is fed to the cathode where it is reduced according to the following reaction equations ... 

Activation losses In an H2/air-fed fuel cell, the activation losses are mainly at the cathode. The hydrogen oxidation rate is very fast on the standard Pt catalyst, indeed such that it is hard to measure it accurately. Estimates of the exchange current density are in the order of 0.24-0.60 A cmpt [45]. There is substantial evidence that the anode Pt loading can be as low as 0.05 mg cm with losses in the order of mV only [4], Durabihty issues as well as the presence of CO may require a higher loading this will be further discussed in the next chapters. 

Hydrogen-air fuel cells, which are used in transportation, backup power, and material handling applications, respond almost instantaneously, provided that they a fed adequate amounts fuel. Control schemes for hydrogen-based fuel cell systems for automotive applications are discussed by Pukrushpan et al. [64] in which the... 

The dominant type of PEMFC is the hydrogen/oxygen fuel cell, in which the anode is fed by hydrogen and the cathode is fed by oxygen or air. In this section, we will focus our attention on anode catalyst contamination caused by impurities in the hydrogen stream. 

---