Electrode reactions, fuel cells

A comparison of the E°s would lead us to predict that the reduction (it) would be favored over that of (i). This is certainly the case from a purely energetic standpoint, but as was mentioned in the section on fuel cells, electrode reactions involving 02 are notoriously slow (that is, they are kinetically hindered), so the anodic process here is under kinetic rather than thermodynamic control. The reduction of water (iv) is energetically favored over that of Na+ (iii), so the net result of the electrolysis of brine is the production of Cl2 and NaOH ( caustic ), both of which are of immense industrial importance ... 

The fuel cell electrode reactions are catalyzed by different materials in different temperature ranges. A classification of the fuel cells can be made on the basis of the electrolyte, which in turn determines the operating temperature and, with it, the catalysts to be applied in the electrodes. The electrode reactions that take place in the different types of fuel cells are summarized in Table 3, which also lists the electrolytes and operating temperatures [56]. 

Table 3. Different types of fuel cells, electrode reactions and operating temperatures. 

Study the kineties of fuel cell electrode reactions on well-characterized model eleetrodes and high surface area fuel eell electrocatalysts using modem eleetroanalytieal methods. Study the meehanisms of the reactions using state-of-the art in-situ spectroscopes. 

Although the overall reaction (3) is the same for both types of fuel cells, some differences can be observed in the fuel cell electrode reactions (1) and (2) ... 

Now we can apply the Tafel equation to fuel cell electrode reactions. As the overpotential at the anode is positive ( > 0), the first term in Equation 1.83 is negligible. Equation 1.83 can be simplified as... 

Electrode Kinetic and Mass Transfer for Fuel Cell Reactions For the reaction occurring inside a porous three-dimensional catalyst layer, a thin-film flooded agglomerate model has been developed [149, 150] to describe the potential-current behavior as a function of reaction kinetics and reactant diffusion. For simplicity, if the kinetic parameters, such as flie exchange current density and diffusion limiting current density, can be defined as apparent parameters, the corresponding Butler-Volmer and mass diffusion relationships can be obtained [134]. For an H2/air (O2) fuel cell, considering bofli the electrode kinetic and the mass transfer, the i-rj relationships of the fuel cell electrode reactions within flie catalyst layer can be expressed as Equations 1.130 and 1.131, respectively, based on Equation 1.122. The i-rj relationship of the catalyzed cathode reaction wifliin the catalyst layer is... 

Alkaline solutions are generally known to lead to better catal5Tic activities than acidic solutions for many relevant electrode reactions. However, owing to the paucity in the development of suitable electrolyte materials, such as alkaline membranes, there has been much less fundamental work in the area of fuel cell catalysis in alkaline media. Nevertheless, there are a few hopeful developments in new alkaline polymer membranes [Varcoe and Slade, 2005] that are currently stirring up interest in smdying fuel cell catalytic reactions in alkalme solution. 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

To use a real example, consider a hydrogen fuel cell. The reaction at electrode A is the hydrogen oxidation reaction,... 

Fuel cell electrodes are more complex structures than battery electrodes. They serve three functions (1) to ensure a stable interface between the reactant gas and the electrolyte, (2) to catalyze the electrode reactions, and (3) to conduct the electrons from or to the reaction sites. A significant problem is the control of the interface at the juncture of the reactant (gas) phase, the electrolyte medium, and the catalyzed conducting electrode, the so-called three-phase boundary , where the electrolyte, electrode, and gas all come together. A stable three-phase boundary is critical to good performance and long operation. Therefore, the porosity and the wetting behavior with electrolyte and the electrode surface must be precisely adjusted. 

One of the most daunting issues is the catalyst. The reactions at the electrodes tend to go slowly, and making or coating the electrode with a catalyst is essential to speed things up. Platinum is one of the most effective catalysts because it binds the reactants and holds them in place so that the reaction can proceed. But the problem is cost—platinum is a rare metal and not at all cheap. An ounce (28.6 g) of platinum costs about 1,200 as of May 2009. Compare that to gold, a precious metal that costs about 970 per ounce (28.6 g) as of May 2009. Fuel cell electrodes would be cheaper if they were made of gold (but gold is not an effective catalyst) ... 

The generation of heat always accompanies the operation of a fuel cell. The heat is due to inefficiencies in the basic fuel-cell electrochemical reaction, crossover (residual diffusion through the fuel-cell solid-electrolyte membrane) of fuel, and electrical heating of interconnection resistances. Spatial temperature variation can occur if any of these heat-generating processes occur preferentially in different parts of the fuel cell stack. For example, non-uniform distribution of fuel across the surfaces of electrodes, different resistances between the interconnections in a stack, and variations among... 

Although the Bonnemann method is very interesting by allowing to vary and to control easily the composition and the nanostructure of the catalyst and is adapted to the preparation of real fuel cell electrodes, it displays also some limitations. For example, bismuth-containing colloids could not be prepared with the Bonnemann method, and even in presence of platinum salts. Moreover, the presence of bismuth hinders the reduction of platinum salts [59], However, platinum-bismuth is a good catalyst for ethylene glycol electro-oxidation in alkaline medium [59-62], Moreover, colloid of tin alone could not be obtained, and the reaction was only possible by coreduction in the presence of a platinum salt. Then, other colloidal methods should be developed keeping in mind the necessity of a similar flexibility as that of the Bonnemann method. 

Anodic reactions at Pt have been claimed to be dependent upon the surface state of the platinum. The Kolbe reaction is perhaps the best known case (for a review, see Conway and Vijh, 1967) for which a change in the surface composition has been held responsible and indeed necessary for the reaction to occur. Thus, at a low potential, < 0-8 V, acetate in aqueous solution is completely oxidized to carbon dioxide and water on pure platinum sites (i.e. we have in effect a fuel cell electrode). On raising the potential, PtO and adsorbed oxygen begin to cover the surface and oxygen evolution takes place in the range between 1-2- 1-8 V. A further increase in the... 

A porous anode and cathode are attached to each surface of the membrane, forming a membrane-electrode assembly, similar to that employed in SPE fuel cells. Electrochemical reactions (electron transfer-l-hydrogenation) occur at the interfaces between the ion exchange membrane and electrochemically active layers of electrodes. Electrochemical reductive HDH occurred at the interfaces between the ion exchange membrane and the cathode catalyst layer when an electrical current is applied between the electrodes ... 

In the quest to improve fuel cell performance, the concept of fuel cell reactions requiring a three-phase interface was first proposed by Grove. In his initial experiment, he noticed that the reaction sped up when the three-phase area was large. In 1923, Schmid [7] developed the first gas diffusion electrode, which significantly increased the electrode active surface area and revolutionized fuel cell electrodes. The electrode contained a coarse-pore graphite gas-side layer and a fine porous platinum electrolyte layer. 

Figure 6.5. Impedance spectra for the oxygen reduction reaction at three different electrode potentials a 0.8 V b 0.7 V c 0.6 V. The microporous layer (loading 3.5 mg/cm2) of the electrode has varying PTFE content ( ) 10 ( ) 20 (A) 30 (+) 40 wt% [5], (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

In the real non-equilibrium conditions of a present-day MCFC with very successful electrode reform, the cell electrode reaction, voracious for fuel, consumes the reformer product and favourably influences the reform process. The latter turns out to operate well at 600 °C, compared with about 800 °C in a fired reformer coupled, say, to much less voracious hydrogen separation and storage. In the practical SOFC, 1000 °C at the anode promotes excessively vigorous electrode reform, which leads to a local electrode cold spot. There are also stability considerations (Gardiner, 1996). Hence the contemporary movement towards lower SOFC temperatures, via new ceria electrolytes, and interconnect change from ceramic to steel. A PEFC near Tq, must have a combustion-operated 800 °C reformer, since a Tq electrochemical reform process does not exist in practice. 

Although catalysis in electrochemical reactions was probably first specifically recognized by Frumkin at a conference in Leningrad in 1939, a first and perceptive definition of electrocatalysis seems to have been by Busing and Kauzmann in 1952 (72) in terms of the ability of various electrode surfaces to promote the velocity of the rate-determining step of the reaction. In this respect, their definition preceded the common use of this term in North America in the 1960s by some years, when it was applied to the activities of fuel-cell electrodes by Liebhafsky (7i). 

The apparent transfer coefficient of the cathodic reaction, ac, is a measure of the sensitivity of the transition state to the drop in electrostatic potential between electrolyte and metal [109,112]. According to Ref. 113, it is ac = 0.75 for the O2 reduction on Pt in aqueous acid electrolytes. In Ref. Ill the value ac = 1.0 was reported instead. Since the cathodic reaction is a complex multistep process, it might follow several reaction pathways, and the competition between them is affected by the operation conditions (rj, p, T). Therefore, different values of ac have been reported in different regimes of operation. Although in the simple reactions the transfer coefficient is a microscopic characteristic of the elementary act [112], for complex multistage reactions in fuel cell electrodes it is rather an empirical parameter of the model. The dependence of effective a for methanol oxidation on the catalyst layer preparation was recently studied [114]. 

By varying the particle size, shapes, separation, and support on planar model electrocatalysts, the influence of these properties on the electrocatal3dic reactions, e.g., on fuel cell electrodes, can be evaluated systematically. Some new challenges arise, such as the adhesion of the catalyst particles on new types of support materials (e.g., glassy carbon). However, most of the procedures and concepts of preparation and characterization are the same as in heterogeneous catalysis and photocatalysis. 

Carbon supported catalysts could be used in various chemical reactions, and more specifically as fuel cell electrodes, the carbon support being a good electrical conductor. 

Reaction 6 may involve an [H-H]+(ads) intermediate. The hydrogen electrode reactions are of interest from the standpoint of hydrogen-consuming fuel cells, competing reactions in various battery systems, the generation of hydrogen gas by water electrolysis, and the complementary cathodic reaction in metal corrosion in aqueous environments. The predominant pathway and rate-determining steps have been identified on a few metal electrode surfaces (30). 

The first pubhshed studies concerning the use of CNTs or CNFs in the field of heterogeneous catalysis date back to 1994 [18,19], and specific activities and selectivities of CNT or CNF-supported catalysts were reported. As a matter of fact, an industrial interest exists in the area of supported catalysts for fluid-phase reactions [20] or for fuel cell electrodes [21]. Since these first reports, numerous studies have brought better knowledge about these carbon nanostructmes and opened the way to their application in catalysis. Besides the study of the specific properties of these materials, special attention has been paid to the design of CNT/CNF supports, to the preparation of metal particles on CNTs or CNFs [22],... 

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