Mass transport losses

Convection refers to fhe fransport of the reactant or product species by bulk fluid motion driven by natural or applied mechanical forces. The natural convection limitations are due to convective transport caused by differences in densities as a result of temperature or concentration. The species transport to the interface can also be limited by fhe fuel cell flow sfrucfures and fheir conditions. For example, in PEMFC, blockage of flow channels or pore structures in diffusion or elecfrode-cafalysf layers owing to the liquid phase can restrict the supply of fhe reactant to the interface. Accumulation of inert gases that do not participate in chemical reaction will limit the partial pressure of the reactant at the interface. This results to decreased reactions at the interface. The accumulation of chemical impurities at the reaction sites will prevent adsorption of desired reactant species. For example, in PEMFC, the presence of carbon monoxide degrades the platinum catalyst because the platinum preferentially adsorbs carbon monoxide, leaving few reaction sites for hydrogen adsorption and oxidation. This leads to high anodic overpotential. 

Consider a single-step electron transfer reaction af fhe interface of the electrode 

Assuming the charge transfer to be in equilibrium condition, that is, the net current density is zero, the Nernst equation can be used to relate the cell reversible potential to the concentration of reactant through the equation 

At equilibrium, the interface concentration is the same as the bulk concentration. Now, if we consider the potential of the electrode E at which the net current density is not zero, that is, j et = j 0, then the interfacial concentration is lower than the bulk concentration and the Nernst equation is written as 

The current density j 0 results in the departure of potential E from the equilibrium potential E. This potential E occurs owing to the difference between concentrations at the electrode surface and at the bulk concentration. Now, taking the difference between Equations 5.127 and 5.128, we have 

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In the current-voltage curve in Fig. 14.15, three different regions can be discerned. At low current densities, the performance is kinetically limited. In the linear part, ohmic losses are significant. At high current densities, mass transport losses dominate. 

The EOD coefficient, is the ratio of the water flux through the membrane to the proton flux in the absence of a water concentration gradient. As r/d,3g increases with increasing current density during PEMFC operation, the level of dehydration increases at the anode and normally exceeds the ability of the PEM to use back diffusion to the anode to achieve balanced water content in the membrane. In addition, accumulation of water at the cathode leads to flooding and concomitant mass transport losses in the PEMFC due to the reduced diffusion rate of O2 reaching the cathode. 

Similar observations were also presented by Songetal. [115] and Holmstrom et al. [97], especially when the fuel cell s performance af high currenf densities was investigated. In fact, it was shown that DLs without an MPL at the cathode side experienced major mass transport losses (and resistance) at... 

Prasarma et al. [185] were also able to observe an optimum thickness of DLs for fuel cells experimentally. They demonstrated that the thicker DLs experience severe flooding at intermediate current densities (i.e., ohmic region) due to low gas permeation and to possible condensation of water in the pores as the thickness of the DL increases. On the other hand, as the thickness of the DL decreases, the mass transport losses, contact resistance, and mechanical weakness increase significantly [113,185]. Through the use of mathematical modeling, different research groups have reported similar conclusions regarding the effect of DL thickness on fuel cell performance [186-189]. 

The relative rate of wall losses to mass transport losses is governed by... 

Besides the activation overpotential, mass transport losses is an important contributor to the overall overpotential loss, especially at high current density. By use of such high-surface-area electrocatalysts, activation overpotential is minimized. But since a three-dimensional reaction zone is essential for the consumption of the fuel-cell gaseous reactants, it is necessary to incorporate the supported electrocatalysts in the porous gas diffusion electrodes, with optimized structures, for aqueous electrolyte fuel-cell applications. The supported electrocatalysts and the structure and composition of the active layer play a significant role in minimizing the mass transport and ohmic limitations, particularly in respect to the former when air is the cathodic reactant. In general, mass transport limitations are predominant in the active layer of the electrode, while ohmic limitations are mainly due to resistance to ionic transport in the electrolyte. For the purposes of this chapter, the focus will be on the role of the supported electrocatalysts in inhibiting both mass transport and ohmic limitations within the porous gas diffusion electrodes, in acid electrolyte fuel cells. These may be summarized as follows ... 

Fig. 14 Phase diagram, which indicates the thickness that a catalyst layer of certain composition should have in order to perform in the intermediate regime (the region between the solid lines, which corresponds to minimal overvoltage losses), k is the factor of thickness rescaling relative to the reference thickness l (l = k ). In the kinetic regime voltage losses could be reduced by increasing l. In the oxygen depletion regime reduction of t will reduce mass transport losses and thereby improve performance.

Optimum thickness. At fixed composition a phase diagram of the catalyst layer can be generated, which establishes a relation between the optimum thickness interval of the catalyst layer and the target current density jo (or jo interval) of fuel cell operation. The optimum compromise between kinetic losses and mass transport losses is realized in the intermediate regime. The existence of an upper limit on the thickness beyond which the performance would start to deteriorate is due to the concerted impact of oxygen and proton transport limitations, whereas considered separately each of the effects would only serve to define a minimal thickness. 

GDE and the diffusivity of oxygen in air. It was found through the fitting in [4] that a much larger value of So was needed to explain experimental results. The results suggested that significant oxygen mass transport losses occur in the catalyst layer. For the purposes of this study, So is taken as a fitted parameter. 

In addition, it can be seen from Figure 3.19 and Figure 3.20 that the mass transport loss becomes significant when the fuel cell is operated at high current density. This is created by the concentration gradient due to the consumption of oxygen or fuel at the electrodes. 

When the activation voltage loss, the ohmic resistance loss, and the mass transport loss are all taken into consideration, the cell voltage will be... 

It is advantageous to make CLs as thin as possible in order to maximize the catalyst utilization and to reduce the iR loss and the mass transport loss. However, it becomes tricky regarding mass transport loss because if the CL is not severely flooded, it can reduce the mass transport loss, but if a proper water management situation is not achieved thin CL often can be severely or even completely flooded quickly because there is much less space to store water produced at the cathode (we can think the CL as a water reservoir, and a larger one takes longer time to be filled and has more time to send water out to the GDM, and thus it is less likely to be completely flooded). It can be seen from Table 2.11 that a water layer can become hundred times thicker than the CL in 1 second even at a current density as low as 0.1 A cm" if none of the water produced at the cathode is removed. Therefore, water management becomes even more crucial and difficult for thin CLs. 

In the limiting density region the significant mass transport loss is not due to consumption of fhe reacfant buf due to fhe flooding of the GDE. For example, a water layer as thin as 20 nm within the catalyst layer will cause the fuel cell not able to support a current density of 1.5 A cm . Minimizing flooding by keeping at least 15% porosity is necessary for a normal fuel cell operation. 

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