Potentials in a fuel cell

However, if we draw current from the fuel cell, the reactions shift from the equilibrium state and the properties of the reaction environment immediately come into play. This is the case of fuel cell kinetics. Before proceeding to the discussion of kinetic relations it is advisable to consider the potentials in a fuel cell. 

Corrosion resistance is illustrated by the value of current density obtained at specific voltage potentials in a fuel cell simulating environment (e.g., HjSO (pH 3) or 1.0 M HjSO (accelerated)) (see Section 6.2.1). 

Potential difference between electrodes (cell potential) in a fuel cell [V]... 

Alternative approaches to nitric oxide formation include irradiation of air in a nuclear reactor (72) and the oxidation of ammonia to nitric oxide in a fuel cell generating energy (73). Both methods indicate some potential for commercial appHcation but require further study and development. 

For the reaction of hydrogen and oxygen to generate a current in a fuel cell, the anode needs to be polarized more positive than 0 V vs. NHE (Normal Hydrogen Electrode, the reference potential for all electrochemical reactions) for the oxidation of hydrogen, while the cathode needs to be polarized more negative than 1.229 V vs. NHE for the reduction of oxygen. 

The net result of current flow in a fuel cell is to increase the anode potential and to decrease the cathode potential, thereby reducing the cell voltage. Figure 2-3 illustrates the contribution to polarization of the two half cells for a PAFC. The reference point (zero polarization) is hydrogen. These shapes of the polarization curves are typical of other types of fuel cells. 

Figure 2-1 shows that the reversible cell potential for a fuel cell consuming H2 and O2 decreases by 0.27 mV/°C under standard conditions where the reaction product is water vapor. However, as is the case in PAFC s, an increase in temperature improves cell performance because activation polarization, mass transfer polarization, and ohmic losses are reduced. 

For many potential applications a fuel cell system must be capable of surviving and operating in extreme conditions. Presence of water in the membrane and fuel cell requires special attention to fuel cell stack and system design to allow system survival and start-up in extremely cold conditions. Most automotive systems have already demonstrated this capability. 

Let us suppose that the total current from all the fuel cells used in the electricitygenerating plant is I A. The number of seconds in a year is 3.1 x 107 and (as It is coulombs since t is the time in seconds), / = 3.1 x 107 = 1.8 x 1014 or 1 = 6.6 x 106 A. Now, all this current would be converted to electrical energy in the fuel cells at (say) about 0.7 V, which is a reasonable potential in the oxidation of methanol in a fuel cell with a good electrocatalyst. Hence, we should produce 4 x 106 W or about 4000 kW. 

In a fuel cell, the H2 and 02 pressures are not limited to 1 atm. Change in these pressures leads to deviation from the theoretical cell voltage. The relationships between the gas pressures, the anode and cathode potentials, and the cell voltage... 

The aim of this chapter is to show that the choice of a catalyst formulation leading to increase the activity and the selectivity of a given electrochemical reaction involved in a fuel cell can only be achieved when the mechanism of the electrocatalytic reaction is sufficiently understood. The elucidation of the mechanism caimot be obtained by using only electrochemical techniques (e.g. cyclic voltammetry, chronopotentiometry, chrono-amperometiy, coulo-metry, etc.), and usually needs a combination of such techniques with spectroscopic and analytical techniques. A detailed study of the reaction mechanism has thus to be carried out with spectroscopic and analytical techniques under electrochemical control. In short, the combination of electrochemical methods with other physicochemical methods cannot be disputed to determine some key reaction steps. For this purpose, it is then necessary to be able to identify the nature of adsorbed intermediates, the stractuie of adsorbed layers, the natirre of the reaction products and byproducts, etc., and to determine the amormt of these species, as a fimction of the electrode potential and experimental conditions. 

Penetration of the acid through the recast film all the way to the platinum surface was supported by voltammetric evidence and was apparently assisted by the potential multicycling routine employed to maintain an impurity-free platinum surface [1]. Such a study of a filmed platinum electrode immersed in aqueous acid solution is, therefore, less than perfect for providing good data on the rate of ORR at the interface between platinum and hydrated ionomer in a fuel cell cathode, where the only interfacial liquid is distilled water. 

---