Proton transport losses

This equation takes into account all the potential losses in the CCL the first term accounts for the ORR activation overpotential and the proton transport loss, while the second term represents the oxygen transport loss. 

This equation differs from Equation 4.248 by the absence of a quadratic in jo terms. Physically, this means that Equation 4.264 ignores the proton transport loss in the ACL. 

In the general case of mixed activation, oxygen and proton transport losses, the steady-state system of performance Equations (5.89 and 5.90) has to be solved first in order to determine the undisturbed profiles c (jc) and Note that in some cases, numerical solution to the steady-state version of the system (5.89) and (5.90) is difficult to obtain. To work around the problem, the equivalent system (4.156) and (4.55) can be solved. With c (x) and the system for perturbations (5.91) and (5.92) can be solved and the CL impedance can be calculated. 

FIGURE 5.19 (a) The impedance spectra of the CCL in the general case of mixed oxygen and proton transport losses. Indicated is the dimensionless cell current density Jq. The other parameters are listed in the last column of Table 5.6. (b) The oxygen concentration (solid lines) and the local overpotential (dashed lines) through the CCL for the same currents. 

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. 

A number of differenf approaches have been used to try to overcome some of these disadvantages of existing membranes. One such approach is to try to prevent water loss from the proton transport pathways, thus maintaining proton conductivity above the boiling point of wafer. Typically, this is attempted by adding hydrophilic inorganic species into the membrane. Furthermore, these particles in themselves may also be capable of proton conduction. 

The importance of the ionomer in the electrode for the performance of the PEMFC has been well known since the pioneering work of Raistrick et al. [37]. In the PEMFC, the electroosmotic drag of water due to the proton transport from the anode to the cathode leads to the membrane drying out from the anode side (back diffusion of water from cathode to anode compensates partly for the water loss from the anode side of the membrane). Therefore, the loss of conductivity of the ionomer at the anode is also an additional important issue related to the membrane topic, since the ionomer in the electrode needs to connect ionically and chemically to the membrane. In an investigation of the transverse water profile in Nafion in PEMFCs with a... 

The initial emphasis on evaluation and modeling of losses in the membrane electrolyte was required because this unique component of the PEFC is quite different from the electrolytes employed in other, low-temperature, fuel cell systems. One very important element which determines the performance of the PEFC is the water-content dependence of the protonic conductivity in the ionomeric membrane. The water profile established across and along [106]) the membrane at steady state is thus an important performance-determining element. The water profile in the membrane is determined, in turn, by the eombined effects of several flux elements presented schematically in Fig. 27. Under some conditions (typically, Pcath > Pan), an additional flux component due to hydraulic permeability has to be considered (see Eq. (16)). A mathematical description of water transport in the membrane requires knowledge of the detailed dependencies on water content of (1) the electroosmotic drag coefficient (water transport coupled to proton transport) and (2) the water diffusion coefficient. Experimental evaluation of these parameters is described in detail in Section 5.3.2. 

Electrocatalysts One of the positive features of the supported electrocatalyst is that stable particle sizes in PAFCs and PEMFCs of the order of 2-3 nm can be achieved. These particles are in contact with the electrolyte, and since mass transport of the reactants occurs by spherical diffusion of low concentrations of the fuel-cell reactants (hydrogen and oxygen) through the electrolyte to the ultrafine electrocatalyst particles, the problems connected with diffusional limiting currents are minimized. There has to be good contact between the electrocatalyst particles and the carbon support to minimize ohmic losses and between the supported electrocatalysts and the electrolyte for the proton transport to the electrocatalyst particles and for the subsequent oxygen reduction reaction. This electrolyte network, in contact with the supported electrocatalyst in the active layer of the electrodes, has to be continuous up to the interface of the active layer with the electrolyte layer to minimize ohmic losses. 

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. 

Consider the cathode channel (similar arguments are applicable to the anode channel of DMFC or hydrogen PEFC). For simplicity we assume that (1) the catalyst layer is thin enough, so that there are no voltage losses associated with proton transport across the layer and (2) the diffusion losses of oxygen in the backing and catalyst layers are negligible. 

Regardless of the microscopic phenomena, protonic conductivity is critically sensitive to the water content inside crystals and on their surface. Intrinsically nonconductive materials may apparently exhibit proton transport in wet environments due to adsorbed and/or condensed water. Consequently, numerous reports on the conductivity of compacted powders at 90-100% relative humidity, when vapor condensation in pores cannot be avoided, are excluded from consideration. Heating or cooling may cause H2O loss or uptake from the atmosphere, thus altering the conditions for proton transport in crystals. In such situations, the apparent found... 

In the oxygen depletion regime, jo I, only a thin sublayer with thickness L, adjaeent to the GDL, is active. The remaining sublayer with thickness L — 6eff) L, adjaeent to the membrane, is not used for reactions, due to the starvation in oxygen. For this situation with rather nonuniform reaction rate distribution, catalyst is used very ineffeetively. The inactive part causes overpotential losses due to proton transport in the polymer electrolyte, which could cause limiting current behavior, if the proton conductivity is low. 

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