Cathod mass transport losses

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... 

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 ... 

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. 

Compared with oxygen reduction overpotential, both kinetic and mass-transport losses of the hydrogen electrode can be neglected. Therefore, the iR-free cell voltage, Ec(iR.jree), of a fuel cell operating on H2/O2 at low current densities (0.1 A/cnr) is controlled by voltage loss due to the O2 reduction kinetics, i.e., by the qact term in Equation 23.6. The cathode overpotential term, qact, for Pt catalysts at low current density (< 0.1 A/cm ) follows the Tafel equation ... 

Electrode materials play an important role in the performance (power output) and cost of bacterial fuel cells. This problem was the topic of two review papers. In a review by Rismani-Yazdi et al. (2008), some aspects of cathodic limitations (ohmic and mass transport losses, substrate crossover, etc.), are discussed. In a review by Zhou et al. (2011), recent progress in anode and cathode and filling materials as three-dimensional electrodes for microbial fuel cells (MFCs) has been reviewed systematically, resulting in comprehensive insights into the characteristics, options, modifications, and evaluations of the electrode materials and their effects on various actual wastewater treatments. Some existing problems of electrode materials in current MFCs are summarized, and the outlook for future development is also suggested. 

Cathode mass transport Higher mass-transport overpotentials in start/stop-cycled ceU Electrolyte fiooding due to carbon corrosion and resulting changes in hydrophobicity Significant increase of mass-transport overpotentials Severe corrosion of carbon results in loss of structural integrity of cathode catalyst layer and void volume loss... 

In region 111, for convenience, both dry anode and cathode cases are shown to peak at the same location, although this depends on the individual conditions and is not necessarily the case. Following the maximum local current, there is a downward trend resulting from local flooding or gas-phase mass transport losses at the electrode(s). This peak and downward trend will only occur if a reactant starvation condition (via flooding or high utilization) is reached. 

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