Fuel cell activation loss

The Figure III. 5 shows the proportions between the three types of losses in the fuel cell. Activation losses are by far the largest losses at any current density. 

Useful work (electrical energy) is obtained from a fuel cell only when a reasonable current is drawn, but the actual cell potential is decreased from its equilibrium potential because of irreversible losses as shown in Figure 2-2". Several sources contribute to irreversible losses in a practical fuel cell. The losses, which are often called polarization, overpotential, or overvoltage (ri), originate primarily from three sources (1) activation polarization (r act), (2) ohmic polarization (rjohm), and (3) concentration polarization (ricoiic)- These losses result in a cell voltage (V) for a fuel cell that is less than its ideal potential, E (V = E - Losses). 

Although hydrogen crossover and internal currents are equivalent, they physically have different effects in a fuel cell. The loss of electrons occurs after the electrochemical reaction has taken place and therefore the effect on both anode and cathode activation polarization would have the effect as depicted by Equation (III.33). Hydrogen that permeates through the membrane does not participate in the electrochemical reaction on the anode side, and in that case the total current resulting from the electrochemical reaction would be the same as the external current. However, hydrogen that permeates through the membrane to the cathode side may... 

The corrosion of different structural parts of fuel cells, leading to partial destruction and/or the formation of corrosion products that lower the activity of the electrodes, particularly in high-temperature fuel cells A loss of sealing of the cells for example, because of aging of packings, so that it becomes possible for reactants to reach the wrong electrode... 

Electrochemical impedance spectroscopy (EIS) has also been discussed in Chapter 3. EIS is generally used to diagnose the performance limitations of fuel cells. There are three fundamental sources of voltage loss in fuel cells kinetic losses (charge-transfer activation), ohmic losses (ion and electron transport), and mass transfer losses (concentration). EIS can be used to distinguish and... 

CV studies showed a progressive loss of active area of Pt when subjected to a harsh acid environment similar to an operating fuel cell. These losses may be explained by Pt migration and/or Pt dissolution-redeposition throughout the electrode. Particle redistribution after the CV study was confirmed by X-ray diffraction with the result of Pt agglomeration. 

Reduction of trichloroethene to ethane took place in a modified fuel cell to which was introduced, although the loss of catalytic activity with time could present a serious limitation (Ju et al. 2006). 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Carbon Deposition. The processing of hydrocarbons always has the potential to form coke (soot). If the fuel processor is not properly designed or operated, coking is likely to occur. Carbon deposition not only represents a loss of carbon for the reaction but more importantly also results in deactivation of catalysts in the processor and the fuel cell, due to deposition at the active sites. 

A final issue that faces this class of catalysts is stability in the fuel cell environment. Deactivation of materials in a fuel cell environment has been shown to be minimal in some studies,31,137 and severe in others.128,142 More active catalysts seem more susceptible to deactivation. Deactivation has been linked to the formation of peroxide and the loss of metal from the catalyst.128 On the other hand, demetallization has also been observed in pyrolyzed samples that did not lose activity with time.84 Another possible mode of deactivation could be due to the oxidation of the carbon surface. However, it seems reasonable that a complete understanding of the deactivation mechanism would first require a well-developed understanding of the active site. 

Lifetime performance degradation is a key performance parameter in a fuel cell system, but the causes of this degradation are not fully understood. The sources of voltage decay are kinetic or activation loss, ohmic or resistive loss, loss of mass transport, or loss of reformate tolerance (17). 

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. 

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