Effectiveness of catalyst utilization

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. 

Pt utilization is a materials property. It is defined as the ratio of the eleetroeatalytieally aetive surfaee area, whieh is aecessible to eleetrons and protons, to the total surfaee area of Pt, 

Pt utilization has been eonsidered under different eonditions, viz. in the catalyst powder and in the membrane electrode assembly (MEA) of an operational fuel cell. The eleetroeatalytieally active surface area in the catalyst powder, 5p,, can 

4 r pt. For powders of carbon-supported Pt nanoparticles values of Wp, were reported to be 109% by Easton and Pickup [57], 125% by Shan and Pickup [58], and 100% by Schmidt et al. [59]. The fact that the estimated utilization is greater than 100% is usually attributed to contributions from background currents (e.g., including that for H2 evolution) to the measured charge. 

The purpose of studying Pt utilization in flie catalyst powder is to evaluate the electronic connectivity of carbon/Pt and maximum ionic accessibility of Pt nanoparticles. 100% of Pt utilization means that all of the Pt nanoparticles are coimected to the electronic conduction network and the entire Pt surface is accessible for protons. Close to 100% Pt utilization in the powder immersed in liquid electrolyte is possible, since only a negligible fraction of the Pt surface is covered by carbon particles. If Pt nanoparticles were supported by a polymer material such as poly(3,4-ethylenedioxythiophene)/poly(styrene-4-sulfonate), Pt utilization was reported to be only 43-62%, which is due to poor electronic contact of Pt nanoparticles with the polymer support and blocking of the Pt surface with the polymer [58]. 

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The motives for modeling of performance at the single agglomerate level are, thus, multifold. There are few open reports on the effect of composition inside agglomerates on mass transport, electrochemical reactions, and catalyst utilization. It is poorly understood whether pores inside agglomerates should be hydrophobic or hydrophilic. Agglomerates are of vital importance in determining the true active area and the effectiveness of catalyst utilization. 

The length scale i a 50 — 100 nm determines the effectiveness of catalyst utilization for spherical agglomerates. Analogous relations apply for ultrathin planar catalyst layers with similar thickness, L 100 — 200 nm. We consider layers that consist of Pt, water-filled pores and potentially an electronically conducting substrate. With these assumptions, we can put/(dfptc, XfXptc = 1 and g Sr) = 1. The volumetric exchange current density is, thus. 

As a major conclusion, primary pores inside agglomerates and ultrathin catalyst layers should be hydrophilic (maximum wetting). Under such conditions effectiveness of catalyst utilization can approach 100%. Moreover, the microscopic mechanism of the electrochemical reaction, represented by the transfer coefficient a, is essential for the effectiveness of catalyst utilization. 

Similar agglomerate approaches were adopted by Iczkowski and Cutlip [30] and by Bjbmbom [31], Those works already identified the doubling of the apparent Tafel slope as a universal signature of the interplay of mass transport limitations and interfacial electrochemical kinetics. Flooded agglomerate models have been employed since then to analyze sources of irreversible voltage losses, optimum electrode thickness, and effectiveness of catalyst utilization. Moreover, it was... 

As discussed above, f is a static materials property. It is the product of the specific electrocatalytic activity of the catalyst surface times statistical factors that arise at all scales due to the random morphology and distribution of the catalyst in the composite CL, as considered in Equation 8.2. The reaction penetration depth is a steady state property, which is mainly determined by the nonlinear coupling between transport of oxygen and protons and exchange current density. Together, both parameters, f and, determine the overall effectiveness of catalyst utilization. 

A calculation of the overall effectiveness of catalyst utilization in conventional CCLs has been recently performed in [56], including all of the aforementioned detrimental factors. This estimate suggests that less than 10% of the catalyst is... 

The exchange current density f is a static materials property defined in the section Catalyst Activity. The reaction penetration depth 8cl is a steady-state property determined by the interplay of transport properties of the layer and local electro-catalytic activity, embodied in f. Together, both parameters f and Scl determine the overall effectiveness of catalyst utilization. For illustration purposes, a simple scenario of this interplay for the case of severely limited oxygen diffusion will be considered below. 

Microlevel. The starting point in multiphase reactor selection is the determination of the best particle size (catalyst particles, bubbles, and droplets). The size of catalyst particles should be such that utilization of the catalyst is as high as possible. A measure of catalyst utilization is the effectiveness factor q (see Sections 3.4.1 and 5.4.3) that is inversely related to the Thiele modulus (Eqn. 5.4-78). Generally, the effectiveness factor for Thiele moduli less than 0.5 are sufficiently high, exceeding 0.9. For the reaction under consideration, the particles size should be so small that these limits are met. 

Cho et al. [140] examined the performance of PEM fuel cells fabricated using different catalyst loadings (20, 40, and 60 wt% on a carbon support). The best performance—742 mA/cm at a cell voltage of 0.6 V— was achieved using 40 wt% Pt/C in both anode and cathode. Antonie et al. [28] studied the effect of catalyst gradients on CL performances using both experimental and modeling approaches. Optimal catalyst utilization could also be achieved when a preferential location of Pt nanoparticles was close to the PEM side ... 

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 ... 

The bed effectiveness is an important factor for an optimal design of a PPR for a given application. A lower bed effectiveness means that more catalyst and a larger reactor is needed for the same process duty. Another important factor is pressure drop. As discussed above, a PPR with thin catalyst layers will have a high degree of catalyst utilization, so the amounts of catalyst and reactor space required are low. However, with thinner catalyst layers the construction cost of the PPR will be higher. 

Most of the studies of methane oxidation in the literature utilize simplified feedstreams and fresh catalyst samples, generally in the form of powders or pellets. A recent paper [22] used a laboratory simulated NG vehicle exhaust to study the removal of methane, NO and CO using a Pd-only monolith catalyst. They found that optimum conversion of all three constituents occurred slightly rich of stoichiometry. These results appear to have been obtained over fresh catalyst samples. The present work utilizes monolith catalysts and laboratory simulated NG vehicle exhaust to study the effect of catalyst loading and space velocity, Ce02 addition and variations in hydrocarbon composition. The effect of modulation amplitude and frequency aroxmd the stoichiometric point was also... 

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. 

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