Electrochemical Reactions in Fuel Cells

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... 

Platinum also finds widespread application in other areas of electrochemistry where its catalytic properties are utilized. For example, the electrochemical reactions in fuel cells generally have high activation energies, and Pt-based electrodes are often employed [iii]. See also - Activation (of noble metal electrodes). 

Electrochemical reactions in fuel cells occurring on an electrode surface involve several steps. The electroactive species need to reach the electrode surface and adsorb on it, and then the electron transfer occurs at the electrode/electrolyte interface. The first step is mass transfer, and the second and third steps are electrode kinetics. If the mass transfer is fast, and the absorption and charge transfer are slow, the total reaction rate is determined by the electrochemical reaction kinetics. However, in the case of slow mass transfer and fast electrochemical kinetics, the mass transfer limits the whole reaction speed. In other words, the reactant that can reach the electrode surface will be consumed immediately, and the problem will be insufficient reactant on the electrode surface. 

Figure 12.19 Electrochemical reactions in fuels cells based on an oxide-ion electrolyte (case 1) and on a protonic conductor (case2).

Starting point for the emergence of electrocatalysis was the discovery that hydrocarbons could be oxidized at low temperatures (this fact had not been a part of the Ostwald scenario). Then it was discovered that synergistic effects were operative in the use of ruthenium-platinum catalysts for methanol oxidation, and that compounds such as platinum-free metalloporphyrins were useful catalysts for certain electrochemical reactions in fuel cells. Hopes were expressed that in the future expensive platinum catalyst could be replaced. Again, in the attempts of commercial realization of these discoveries considerable difficulties were encountered, which led to a period of disenchantment and pessimism in 1970s and 1980s. It had been demonstrated beyond doubt that, fundamentally, hydrocarbons could be oxidized at low temperatures, but practical rates that could be achieved were unrealistically low. It had also been demonstrated that fuel cells could be made to work without... 

The electrochemical reactions in fuel cells happen simultaneously on both sides of the membrane - the anode and the cathode. The basic fuel cell reactions are ... 

In this book, electrochemical ORR is our targeted system. This electrochemical ORR is one of the important chemical reactions of O2, which is also one of the necessary two reactions in fuel cells and metal—air batteries. 

Liu H, Manthiram A (2008) Tuning the electrocatalytic activity and durability of low cost Pd7oCo3o nanoalloy for oxygen reduction reaction in fuel cells. Electrochem Commun 10(5) 740-744... 

Li XG, Popov BN, Kawahara T, Yanagi H (2010) Recent advances in non-precious metal catalysts for oxygen reduction reaction in fuel cells. Electrochem Soc Trans 33 1769-1776... 

If hydrogen is supplied to the cell in excess of that required for the reaction stoichiometry, this excess will leave the fuel cell unused. In case of pure hydrogen, this excess may be recirculated back into the stack so it does not change the fuel cell efficiency (not accounting for the power needed for hydrogen recirculation pump), but if hydrogen is not pure (such as in reformate gas feed), unused hydrogen leaves the fuel cell and does not participate in the electrochemical reaction. The fuel cell efficiency is then ... 

The fuel cell represents another electrochemical reaction. The fuel cell requires a constant source of fuel, usually hydrogen, in order to continue to produce electricity. The electrochemical reaction involves the oxidation of hydrogen to produce water, representing the greenest energy opportunity (except that the production of hydrogen to power the fuel cell may not be as green as desired). 

Porous electrodes are commonly used in fuel cells to achieve hi surface area which significantly increases the number of reaction sites. A critical part of most fuel cells is often referred to as the triple phase boundary (TPB). Thrae mostly microscopic regions, in which the actual electrochemical reactions take place, are found where reactant gas, electrolyte and electrode meet each other. For a site or area to be active, it must be exposed to the rractant, be in electrical contact with the electrode, be in ionic contact with the electrolyte, and contain sufficient electro-catalyst for the reaction to proceed at a desired rate. The density of these regions and the microstmcture of these interfaces play a critical role in the electrochemical performance of the fuel cells [1]. 

Imagine an automobile thatmns in silence and without polluting emissions. Such an automobile, long a dream of the environmentally conscious, has recently become a reality. The power source is a fuel cell, an electrochemical cell that uses a combustion reaction to produce electricity. Hydrocarbons such as natural gas and propane can be used in fuel cells, but the cleanest fuel is molecular hydrogen. 

One way to illustrate the effect of the EDL is to compare in situ electrochemical reactions with their equivalent UHV counterparts. Due to their roles in fuel cells, the methanol oxidation reaction and the oxygen reduction reaction are two such reactions for which numerous in situ and UHV experiments have been performed. 

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