Mass transfer loss

In the search for cheaper materials, an problem often encountered is that the intrinsic activity is so low that it leads to much thicker electrodes, which in turn is counterproductive as it leads to high mass transfer losses. 

Other important properties of the ionomer include the permeability to both H2 and O2 gas. The PEM must not be too permeable to the reactive gases, as excessive gas crossover through the membrane would result in fuel efficiency loses. However, the ionomer in the electrodes must possess sufficient permeability to allow transport of the reactant without imposing any significant concentration gradients and/or mass transfer losses. 

Mass transfer loss (overpotential). Mass transfer (or concentration) overpotential ( ) is caused by the concentration gradients of the reactants. As... 

As shown in Fig. 1.4 of Chapter 1, under a load, PEM fuel cell performance is determined by four voltage losses the voltage loss caused by mixed potential and hydrogen crossover, which is related to the Pt catalyst status and the membrane properties the activation loss, which is related to the electrode kinetics the ohmic loss, which is determined by ohmic resistance and the voltage loss caused by mass transfer, which is affected by the characteristics of the gas diffusion layer and catalyst layer. The voltage loss caused by mixed potential and hydrogen crossover will be discussed in detail in Chapter 7. The activation loss, ohmic loss, and mass transfer loss can be calculated from the charge transfer resistance, ohmic resistance, and mass transfer resistance, which can be determined by EIS measurement and simulation. 

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... 

The variables Hact a and riacu are the activation overpotentials at the anode and cathode, respectively, nohm is the ohmic resistance losses in the fuel cell, a and Tin,t c are the mass transfer losses at the anode and cathode, respectively. These are discussed here. 

The anode and cathode mass transfer losses are given by Equations 5.141 and 5.142. 

In the previous sections, we looked into the losses in the fuel cell potential contributed by the resistance to the reaction kinetics at the cathode and anode (activation losses), resistance to ion or electron transport (ohmic losses), and the mass concentration variation near the electrode (mass transfer losses). In addition to these losses, fuel cells show significant potential losses as a result of a short circuit in the electrolyte and crossover of reactants through the electrolyte. 

In the previous section, each of the fuel cell losses, cathode and anode activation losses, ohmic losses, mass transfer losses, and losses owing to short circuit and reactant crossover was discussed, and expression for each loses overpotential or the polarizations were obtained. Now, we have net fuel cell overpotential from Equation 5.149... 

The kinetic losses, ohmic losses, mass transfer losses, and short circuit and crossover losses are illustrated in Figure 5.27. The dominant losses are typically activation losses and ohmic losses. At high current density, the mass transfer or concentration losses dominate. In the illustration, the internal losses are shown as constant however, the internal losses also depend on the current density. 

Contribution of activation losses, ohmic losses, and mass transfer losses and internal losses owing to short circuit and reactant crossover. 

As seen from Equation 5.155, the activation and mass transfer losses are similar in the anode and cathode though often the cathode losses dominate. The fuel cell polarization can be written in approximate form as... 

In this chapter, we will primarily focus on fluid flow, heat, and mass transport through gas flow channels and in solid porous electrodes, and its effect on the mass transfer loss. Solid-phase diffusion, charge transport in electrolyte membrane, and ohmic loss will be discussed in Chapter 7. Water transport will also be discussed in Chapter 7. 

Fluid flow and pressure variation in a fuel cell play a critical role in the distribution of reactant gas concentration at electrochemical reaction sites and, hence, in the distribution of local current densities and cause mass transfer loss. The governing equations for reactant gas flows in gas flow channels and in porous electrode-gas diffusion layers are given by conservation of mass and momentum equations. Solutions to these equations result in the distribution of pressure, P, and velocity field, which is also referred to as the bulk motion in the gas flow channels and porous electrode-gas diffusion layers. 

As we have mentioned earlier, one of the fuel cell voltage losses is the mass transfer loss or concentration loss caused by lower reactant gas concentration distribution at the reaction sites. Mass transport establishes reactant gas concentration distributions in gas supply channels and in the electrodes of a fuel cell, and hence in the distribution of local current densities. The gas supply rates to the anode-membrane and cathode-membrane interface must be sufficient enough to meet the gas consumption rate given by the electrochemical reaction rates. Any insufficient supply of gas to reaction sites may cause sluggishness in the reactions and cause mass transfer loss and reduction in fuel cell output voltage. 

Major factors that contribute to the mass transfer loss are as follows ... 

At higher current density where mass transfer loss is predominant, the second term that represents the reduction reaction or product becomes insignificant and the equation can be simplified by dropping this term as... 

Equation 6.141 represents the activation overpotential on the basis of the reactant gas concentration in bulk gas flow. The mass transfer loss can be estimated on the basis of the changes in activation overpotential owing to the variation in reactant gas concentration from the bulk flow to the reaction surface as follows ... 

Equation 6.143 represents the mass transfer loss owing to the activation reaction. 

Considering that the mass transfer loss is dominated by the oxygen concentration in cathode site, the limiting current density is estimated based on Equation 6.131a as... 

It can be noticed here that the expression is only valid for / <), and the mass transfer loss is very small or insignificant for very low current densities,/, ji- As the operating current density approaches the limiting current density value, the mass transfer loss sharply increases. 

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