Current density, electrocatalyst

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

They have an exceedingly high specific activity per active site the turnover number y is as high as 10 to 10 s in certain enzyme reactions, while at ordinary electrocatalysts having a number of reaction sites on the order of 10 cm , yhas a value of about 1 s at a current density of lOmA/cm. Thus, the specific catalytic activity of tfie active sites of enzymes is many orders of magnitude fiigher tfian tfiat of all other known catalysts for electrochemical (and also chemical) processes. 

For (AGh 0), these two regimes are approximately balanced, and a maximum in the exchange current density is predicted. This is the optimal value of the descriptor, and it immediately suggests that a reasonable goal for a computational, combinatorial electrocatalyst search is to find alloys with AGh values close to zero. 

Figure 9.16 ORR activity of two mixed-metal monolayer electrocatalysts supported on Pd(l 11), expressed as the kinetic current density at 0.85 V as a function of the M Pt ratio in the Pd-supported Pt-M monolayer. (Reproduced with permission from Zhang et al. [2005b].)...

As discussed earlier, it is generally observed that reductant oxidation occurs under kinetic control at least over the potential range of interest to electroless deposition. This indicates that the kinetics, or more specifically, the equivalent partial current densities for this reaction, should be the same for any catalytically active feature. On the other hand, it is well established that the O2 electroreduction reaction may proceed under conditions of diffusion control at a few hundred millivolts potential cathodic of the EIX value for this reaction even for relatively smooth electrocatalysts. This is particularly true for the classic Pd initiation catalyst used for electroless deposition, and is probably also likely for freshly-electrolessly-deposited catalysts such as Ni-P, Co-P and Cu. Thus, when O2 reduction becomes diffusion controlled at a large feature, i.e., one whose dimensions exceed the O2 diffusion layer thickness, the transport of O2 occurs under planar diffusion conditions (except for feature edges). 

With the best electrocatalyst, that is, Pt-Sn (90 10)/XC72, the effect of temperature on the cell voltage E and power density P versus current density j characteristics is shovm in Figure 1.14. It appears clearly that increasing the temperature greatly increases the performance of the cell, from a maximum power density close to 5 mWcm at 50 ° C to 25 mW cm at 110 ° C, that is, five times higher. 

Figure 6.20. Experimental linear sweep voltammogram of carbon-supported high surface area nanoparticle electrocatalyst in oxygen-saturated perchloric acid electrolyte (room temperature). Solid curve pure Pt dashed curve Pt50Co50 alloy electrocatalyst. Inset a blow up of the kinetically controlled ORR regime. Inset b comparison of the specific (Pt surface area normalized) current density of the Pt and the Pt alloy catalyst for ORR at 0.9 V.

It must be stressed that there is still a fair chance of arriving at electrocatalysts that would achieve reasonable anode potentials (say +0.3 V vs RHE) at technically acceptable current densities (say 0.3 to 0.4 A/cm2). The situation for the next higher alcohol, ethanol, however, is almost hopeless. Any work aimed at developing catalysts for anodic oxidation of the much more inert hydrocarbons at low temperatures will certainly be frustrated. 

Since Ru02 and Ir02 are usually prepared by thermal decomposition of suitable precursors on an inert support, the morphology of the active layer is very like that of a compressed powder [486]. The surface area plays an important role since the roughness factor can be between 102 and 103. However, the low Tafel slope observed is a clear indication of electrocatalytic effects and the high surface area is the factor which extends the low Tafel slope to much higher current densities. Thus, the combination of these two factors renders these oxides very efficient electrocatalysts for H2 evolution. 

Fig. 11.11 Geometric current density vs. channel number for the chrono-amperometric screen in Fig. 11.10. Dots indicate the final chronoamperometric geometric current density of each channel. The grey coloring of the dots encodes the W content of the electrocatalysts (white - high, black - low).

In practice, in all fuel cells that involve the utilization of 02 from air (Section 13.4.5), the oxygen reduction reaction [Eqs. (13.4) and (13.24)] is always rate determining for terrestrial applications. One can seejust how important it is to attempt to develop electrocatalysts for the cathodes of fuel cells on which the enhanced current density is high and Tafel slope is low and the efficiency of energy conversion, therefore, maximal. The direct relation of the mechanism of oxygen reduction and the associated Tafel parameters to the economics of electricity production and transportation is thus clearly seen. 

It is often claimed that electrocatalysts in fuel cells are dependent on the exchange current density, i0 of the slowest reaction in the cell, (a) Make Tafel plots for i0 = 10 I,10 6, and 10-3 A cm-2 and bTaM = 0.12. (b) Then draw plots of the same type and the same i0, but with b values of 0.12, 0.05, 0.038, and 0.029 (T = 298 K). (c) Write out your conclusions concerning the interplay of /0 and b in the Tafel relation (B = RT/aF). (d) How does this relate to the choice of electrocatalytic surfaces for optimal fuel cell performance (Bockris)... 

Subsequent deployment of the new catalyst in the cathode layer of small-area MEAs first, then large-area MEAs, and finally fuel cell stacks represents the typical series of performance tests to check the practical viability of novel ORR electrocatalyst materials. Figure 3.3.15A shows the experimental cell voltage current density characteristics (compare to Figure 3.3.7) of three dealloyed Pt-M (M = Cu, Co, Ni) nanoparticle ORR cathode electrocatalysts compared to a state-of-the-art pure-Pt catalyst. At current densities above 0.25 A/cm2, the Co- and Ni-containing cathode catalysts perform comparably to the pure-Pt standard catalyst, even though the amount of noble metal inside the catalysts is lower than that of the pure-Pt catalyst by a factor of two to three. The dealloyed Pt-Cu catalyst is even superior to Pt at reduced metal loading. 

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