Fuel cells half-electrochemical reactions

All the reactions listed in Table 4.1 are called half-electrochemical reactions. In reality, if there are no count half-electrochemical reactions to supply electrons, these reactions cannot occur. In an electrochemical cell, there are two half-electrochemical reactions, one is called the anode reaction, and the other is called the cathode reaction. For example, in a H2/O2 fuel cell, there are two half-electrochemical reactions ... 

In the previous section, we discussed fuel cell thermodynamics. However, in reality, fuel cell operation with an external load is much more practical than in a thermodynamic state. When a H2/air PEM fuel cell outputs power, the half-electrochemical reactions will proceed simultaneously on both the anode and the cathode. The anode electrochemical reaction expressed by Reaction (l.I) will proceed from H2 to protons and electrons, while the oxygen from the air will be reduced at the cathode to water, as expressed by electrochemical Reaction (l.II). For these two reactions, although the hydrogen oxidation reaction (HOR) is much faster than the oxygen reduction reaction (ORR), both have limited reaction rates. Therefore, the kinetics of both the HOR and the ORR must be discussed to achieve a better understanding of the processes occurring in a PEM fuel cell. 

The main elemental principle of a fuel cell is the direct electrochemical redox reaction that produces the electrical current. In the hydrogen/oxygen fuel cell, the redox reaction is composed of two electrochemical half equations - the hydrogen oxidation reaction (HOR) at the anode ... 

Fuel cells are electrochemical cells where the chemical energy of the fuel was converted into electricity for power generation with high efficiency [1,2]. Industrial purified hydrogen and air are often used in fuel cells to eliminate any pollution or emission, which is known as proton exchange membrane fuel cells (PEMFCs). In a typical PEMFC, a steam of hydrogen is deUvered to the anode side of the membrane electrode assembly (MEA) [3,4], At the anode, it is catalyzed by platinum (Pt) and split into protons and electrons. This oxidation half-cell reaction is represented as follows ... 

Methanol is one of the few alcohols that can be fed directly into a fuel cell and can be converted electrochemically at the anode. The DMFC can be fed with a gaseous or liquid fuel feed. The liquid DMFC generally uses a diluted methanol in water mixture (ty pically 1-2 molar) and only a fraction of the methanol is used at the anode (Collins, 2001). The DMFC, like an ordinary battery, provides DC electricity according to the following half reactions. 

A fuel cell is a device that converts the free energy change of a chemical reaction directly into electrical energy. This conversion occurs by two electrochemical half cell reactions. 

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

Figure 3.3.3 schematically depicts the basic structure of an electrochemical fuel cell device. Generally, in electrochemical cells the overall chemical redox reaction proceeds via two coupled, yet spatially separated half-cell redox reactions at two separate electrodes. 

Oxidation of CO is also important in fuel cell applications. By combining the half reaction for the CO2/CO couple, equation (a), with electrochemical reduction of O2, fuel cells may achieve a maximum open circuit potential given by IlSlfCOj/CO) - (02/H20) = 1.34 . In practice, electrocatalysts are required to lessen the normally high kinetic overpotentials for electrodic CO oxidation. An example of a CO/O2 fuel cell which operates at the relatively low T of 80°C by employing [Rh(CO)2Br2] as the electrocatalyst and the Rh couple to mediate CO oxidation is shown below . 

The best catalysts for the electrochemical oxidation and production of hydrogen are platinum metal and the hydrogenase enzymes. Both catalyze the reaction of two protons with two electrons to form H2, as shown in Equation 7.1. Because of its superior catalytic rates and overpotentials compared to other metals and because of its high stabihty compared to hydrogenase enzymes, platinum is currently used as the catalyst for both half reactions (the oxidation of H2 and the reduction of O2) in polymer electrolyte membrane (PEM) fuel cells, which have been proposed for automotive transportation [1]. However, the high cost of platinum provides a strong impetus for developing less expensive alternatives. 

A bioelectrochemical system (BES) is an electrochemical device used to convert electrical energy into chemical energy and vice versa. A BES consists of an anode and a cathode compartment, often separated by an ion-selective membrane. The anode is the site of the oxidation reaction which liberates electrons to the electrode and protons to the electrolyte the cathode is the site of the reduction reaction, which consumes the electrons to reduce a final electron acceptor. To maintain electroneutrality of the system, protons (or other cations) need to migrate to the cathode through the ion-selective membrane. Depending on the half-cell potentials of the electrodes, a BES can be operated either as a microbial fuel cell (MFC), in which electric energy is generated, or as a microbial... 

Thus, in a fuel cell the purely chemical combustion of neutral molecules (1.4) is spht up into two electrochemical reactions (1.1) and (1.2), which run with the participation of charged particles. Basically, any combustion reaction can be split up into a pair of electrochemical half-reactions and hence any fuel can be utilized in a fuel cell for direct conversion of AG into electric energy. 

The other member of the family of low-temperature fuel cells utilizing liquid methanol as a fuel and oxygen/air as oxidant is the direct methanol fuel cell (DMFC). The half-cell electrochemical reactions in DMFC are... 

Figure 6-1 depicts the operating configuration of the molten carbonate fuel cell. The half cell electrochemical reactions are... 

Fuel that has crossed over can react with O2 to produce a corresponding cathodic current density in the same order of magnitude, resulting in a depression of the cathode potential. It is believed that the H2 that has crossed over can form a local half-cell electrochemical reaction on the cathode, such as H2 2H -1- 2e , resulting in a mixed cathode potential, in a way similar to that of the half-cell reaction (Pt -I- H2O PtO + 2H -I- 2e ). 

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