Effectiveness factor of Pt utilization

The primary optimization target of CLs is the effectiveness factor of Pt utilization, Tcl- It includes a factor, that accounts for statistical limitations of catalyst utilization that arise on a hierarchy of scales, as specified in the following equafion. defermines the exchange current density ... 

Xia Z, Wang Q, Eikerling M, Liu Z (2008) Effectiveness factor of Pt utilization in cathode catalyst layer of polymer electrolyte fuel cells. Can J Chem 86 657-667... 

Incorporating statistical factors, which define Vnp and Vstat, as well as transport processes at all four scales in Figure 1.16 into model studies gives an estimate of the total effectiveness of Pt utilization. It lies in the range of 3%. This value is consistent with the result of an experimental assessment of the effectiveness factor of Pt utilization (Lee et al 2010). ... 

The overall effectiveness factor of Pt utilization of the CL, including transport... 

Equations 3.62 through 3.74 form a closed set of equations that can be solved for the functions < r,z),CH+ r,z),co2(r,z). Using these functions, the faradaic current density y>(z) can be obtained. This function can be employed to calculate the effectiveness factor of Pt utilization of the pore, defined as the total current produced by the pore, normalized by an ideal current that would be obtained, if reactant and potential distributions were completely uniform, with 4> (z) = , H+... 

In the hierarchical model, macroscale transport processes will be coupled to transport and reaction at the mesoscale, as illustrated in Figure 4.1 an explicit treatment of agglomerate effects is needed to properly assess the effectiveness factor of Pt utilization, which transpires as the key parameter in the structural optimization of CCLs. 

The reaction penetration depths. Id or la, are highly insightful parameters to evaluate catalyst layer designs in view of transport limitations, uniformity of reaction rate distributions, and the corresponding effectiveness factor of Pt utilization, as discussed in the sections Catalyst Layer Designs in Chapter 1 and Nonuniform Reaction Rate Distributions Effectiveness Factor in Chapter 3. Albeit, these parameters are not measurable. The differential resistances, Rd or Ra, can be determined experimentally either as the slope of the polarization curve or from electrochemical impedance spectra (Nyquist plots) as the low-frequency intercept of the CCL semicircle with the real axis. The expressions in Equation 4.33 thus relate the reaction penetration depths to parameters that can be measured. 

Average effectiveness factor of agglomerates (dimensionless) Effectiveness factor of Pt utilization (dimensionless) Surface-to-volume atom ratio of Pt nanoparticles (dimensionless) Reaction penetration depth of the catalyst layer due to ineffective mass transport (cm)... 

In order to make catalyst layers with high platinum utilization and better performance, we need to determine how various factors affect Pt utilization. Although this objective has been receiving more attention, we have not achieved a fundamental understanding of the relationships of composition, structure, effective properties, and fuel cell performance—a fact that may limit the optimal design and fabrication of CLs. 

As an example of how to use the insights conveyed in this chapter we provide an explicit comparison of overall effectiveness of Pt utilization (by atom number or catalyst weight) for conventional 3-phase composite and ultrathin CCLs, in Table 8.2. For conventional catalyst layers, the main detrimental factors arise at the nanoparticle scale and at the macroscopic scale due to triple-phase boundary requirements. For the nanostructured ultrathin CCLs it is assumed that a sputter-deposited continuous Pt layer is needed to provide electronic conductivity. It was suggested in [153] on the basis of cyclic voltammetry measurements that the irregular surface morphology of such catalysts corresponds to grain sizes of 10 nmwith... 

Prothrombin time PT is performed by adding thromboplastin (tissue) factor and calcium to citrate-anticoagulated plasma, recalcifying the plasma, and measuring the clotting time. The major utility of PT is to measure the activity of the vitamin K-dependent factors II, VII, and X. The PT is used in evaluation of liver disease, to monitor warfarin anticoagulant effect, and to assess vitamin K deficiency. 

Overall, including all the detrimental factors in catalyst utilization, it is quite likely that far less than 20% of the catalyst is effectively utilized for reactions. Ineffectiveness of catalyst utilization is a major downside of random three-phase composite layers, which are, nevertheless, the current focus in CCL development. Obviously, there are enormous reserves for improvement in these premises. The alternative could be to fabricate CCLs as extremely thin, two-phase composites 100-200 nm thick), in which electroactive Pt forms the electronically conducting phase, eventually deposited on a substrate. The remaining volume should be filled with liquid water, as the sole medium for proton and reactant transport. 

Numerous publications in fuel cell research dwell on the key issues that are related to Pt utilization in PEFCs. A brief survey of representative studies that focus on Pt utilization and effectiveness factors can be found in [56]. In the past, ambiguous definitions of catalyst utilization have been exploited and rather contradictory values have been reported. This could lead to a wrongful assessment as to how much the performance of fuel cells could be improved by advanced structural design of catalyst layers. As a matter of fact, the accurate distinction and determination of catalyst utilization and effectiveness factors has a major impact on defining priorities in catalyst layer research. 

At this point, it is important to realize that the ultimate optimization target of electrode design is not Pt utilization, which is a static statistical property of a catalyst layer, but more importantly the effectiveness factor, which includes as well the effects of non-uniform reaction rate distributions due to mass transport phenomena at finite current densities in the operating fuel cell. In simple ID... 

Evaluation of CL performance requires a number of parameters that define the ideal electrocatalyst performance, allowing deviations from ideal behavior to be rationalized and quantified. Ideal electrocatalyst performance is achieved when the total Pt surface area per unit volume, Stot, is utilized and when reaction conditions at the reaction plane (or Helmholtz layer) near the catalyst surface are uniform throughout the layer. These conditions would render each portion of the catalyst surface equally active. Deviations from ideal behavior arise due to statistical underutilization of catalyst atoms, as well as nonuniform distributions of reactants and reaction rates at the reaction plane that are caused by transport effects. This section introduces the effectiveness factor ofPt utilization and addresses the hierarchy of structural effects from atomistic to macroscopic scales that determine its value. 

As for the first assumption, the electrolyte phase must be treated as a mixed phase. It consists of a thin-film structure of ionomer at the surface of Pt/C agglomerates and of water in ionomer-free intra-agglomerate pores. The proton density is highest at the ionomer film (pH 1 or smaller), and it is much smaller in water-filled pores (pH > 3). However, the proton density distribution is not incorporated in the statistical utilization Tstat, but in an agglomerate effectiveness factor, defined in the section Hierarchical Model of CCL Operation. ... 

The catalyst layers evaluated in this model-based analysis are not intended to represent the best-in-class in terms of performance. Instead, the experimental studies were picked out from the literature because they provided porosimetry as well as performance data. Nevertheless, the low value of the CL effectiveness factor is a striking result of this analysis. The value of Fcl decreases from 4 % at jo < 0.4 A cm to 1 % aty o 1 A cm . This parameter incorporates statistical effects and transport phenomena across all scales in the CCL. The values found are consistent with an experimental evaluation of effectiveness factors by Lee et al. (2010) if the values found in that study are corrected with the atom utilization factor F p, the agreement is very good. The low value of Fcl suggests that tremendous improvements in fuel cell performance and Pt loading reduction could be achievable through advanced structural design of catalyst layers. 

The effectiveness factor (Spi) reflects how the utilized Pt is contributing to the overall current in PEMFC operating conditions, i.e., in the presence of ionic, electronic, or mass-transport limitations or combinations thereof. 

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