Fuel cell performance mass transport losses

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]. 

Most of the models show that fuel-cell performance is a balance among the various losses shown in Figure 3, in particular, ohmic losses and mass-transport limitations, which both increase with current. The reason for this is that the kinetic losses are hard to mitigate without significantly changing op-... 

Mass transport within the electrodes is of particular importance in determining the reflection of the porous media structure on the fuel cell performance. In fact, the main results of mass transport limitation is that the reactant concentrations (H2 and CO for the anode, and O2 for the cathode) at the reaction zone are lower than in the gas channel. When applying Equations (3.40) and (3.42), the result is that the lower the concentration of the reactants, the lower the calculated cell performance. The loss of voltage due to the mass transport of the gas within the electrodes is also referred to as concentration overpotential. Simplified approaches for determining concentration overpotential include the calculation of a limiting current, i.e. the maximum current obtainable due to mass transport limitation (cf. Appendix A3.2). 

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. 

Carbon supports strongly affect fuel cell performance. They may influence the intrinsic catalytic activity and catalyst utilization, but also affect mass transport and ohmic losses. This makes analyses of the role of carbon materials rather complicated. Although numerous studies have been devoted to the carbon support improvement, only a few have attempted to establish relationships between the substructural characteristics of carbon support materials and cell performance. The influence of carbon supports on the intrinsic catalytic activity is the subject of Section 12.6.1. In Section 12.6.2 we consider the influence of support on macrokinetic parameters such as the catalyst utilization, mass transport, and ohmic losses. In Section 12.6.3 we review briefly recent data obtained upon utilization of novel carbon materials as supports for fuel cell electrocatalysts. 

The performance of a fuel cell is characterized by its output voltage and current density, which is defined as the current per unit area of the cell. The fuel cell voltage drops at higher currents due to increasing catalytic activation losses, ionic and electronic resistances in the cell, and mass transport limitations. The cell efficiency is therefore proportional to the ratio of measured voltage to the ideal cell voltage (1.23 V and 1.21 V for hydrogen and methanol at 25 °C, respectively). 

The pores are for the transport of fuel cell reactants and product(s). Optimal porosity and pore size distribution can facilitate the mass transport process to minimize the fuel cell performance loss due to concentration overpotential. If some pores are more hydrophobic than others, what is the relative distribution Is the distribution of pore sizes and hydrophobicity within the allowable range ... 

Mass transport loss is defined as the loss in performance of the fuel cell due to limitations in mass transport processes. This performance loss is usually attributed to a reduction of the oxygen activity (associated to its partial pressure) at the electrode, in comparison to the oxygen partial pressure at the cell inlet. The accumulation of water in the transport pathways of the gaseous reactants can lead to increased mass transport losses and instability and can result in accelerated degradation. 

Loss of fuel cell performance due to kinetic, ionic, or mass transport losses. 

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. 

Mass transport, which governs the process of supplying reactants and removing products, is another important fuel cell process besides the electrochemical reaction process and the charge transport process as stated above. Poor mass transport leads to significant fuel cell performance losses due to reactant depletion and product accumulation in the catalyst layer. This type of loss is called a concentration loss and is most significant in the tail of the fuel cell j-V curve. 

Water flood and membrane dehydration are two main PEM performance limitations. Rama et al. [35] summarized live top events reflecting PEMFC performance degradation (1) activation losses, (2) mass transportation losses, (3) ohmic losses, (4) fuel efficiency losses, and (5) catastrophic cell failnre. Here, the activation, mass transportation, and ohmic losses will be discnssed in details since they are closely related to membrane failures. 

When fuel cell begins to operate and electrical power begins to output, the electrochemical reaction can lead to the depletion of reaction in catalyst layer. This depletion will affect the performance of fnel cell through mass transportation or concentration losses. The two major mass transportation effects considered in fuel cell modeling are (1) convective mass transfer, which occnrs in flow channels due... 

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). 

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