Catalyst layer polarization curve

Polarization curves for Hj/Oj fuel cells at 50°C, 1 atm pressure. Curve A Nation impregnated (brush coated) PTFE-bound electrode (0.35 mg/cm Pt loading) curve B PTFE-bound catalyst layer (Pt loading 4 mg/cm ) curve C PTFE-bound electrode (Pt loading 0.35 mg/cm. (Based on Ticianelli, E. A. et al. Journal of the Electrochemical Society 1988 135 2209-2214. By permission of The Electrochemical Society.)... 

Polarization curves for a PEM fuel cell with different cathode catalyst layers. (Reproduced from Zhang, X. and Shi, P. Electrochemistry Communications 2006 8 1229-1234. With permission from Elsevier.)... 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

In the polarization curve, three parts can be observed kinetic, ohmic, and mass transfer. In the kinetic part, the cell voltage drop is due to the charge-transfer kinetics, i.e., the 02 reduction and H2 oxidation rate at the electrode surface, which is dominated by the kinetic I-rj equation (Equation 1.37). In the ohmic part, the cell voltage drop is mainly due to the internal resistance of the fuel cell, including electrolyte membrane resistance, catalyst layer resistance, and contact resistance. In the mass transfer part, the voltage drop is due to the transfer speed of H2 and 02 to the electrode surface. 

In Song et al. s same work [5], the effect that Nafion content in the catalyst layer had upon electrode performance was also investigated, following their work on the optimization of PTFE content in the gas diffusion layer. The optimization of Nafion content was done by comparing the performance of electrodes with different Nafion content in the catalyst layer while keeping other parameters of the electrode at their optimal values. Figures 6.8 and 6.9 show the polarization curves and impedance spectra of fuel cells with electrodes made of catalyst layers containing various amounts ofNafion . 

Figure 6.8. Polarization curves of fuel cells with electrodes made of catalyst layers containing various amounts of Nation ( ) 0.2 ( ) 0.8 (A) 2.0 mg/cm2 [5], (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

Fig. 43. Simultaneous fit to four polarization curves for a PEFC with 300 pm thick backing layer and 7.5 pm-thick catalyst layer. Different cathode feed stream compositions are used and a simultaneous fit is demanded using the same physical and transport parameters for the backing layer and the catalyst layer [100]. (Reprinted by permission of the Electrochemical Society).

Figure 11 (A) Stripping voltammetry (20 m Vs at 55 °C) of CO layers on humidified PEM fuel-cell anodes (1) platinum catalyst (2) platinum/molybdenum catalyst. Voltammetry in the absence of adsorbed CO on the platinum/molybdenum catalyst is shown in (3). Molybdenum-mediated electro-oxidation of adsorbed CO takes place on the alloy catalyst in the peak at 0.45 V and at lower overpotentials [79]. (B) Steady-state polarization curves of PEM fuel-cell anode at 85 °C for platinum (squares) and platinum/molybdenum catalysts in the presence of 100 ppm CO (filled points) and pure H2 (unfilled points). (From Ref 79.)... 

Figure 18 Polarization curves for the electrooxidation of H2 containing 0.1% CO on the PtMo—4 1 and 3 1 catalyst RTLEs normalized by the alloy-specific surface area in the layer and compared to the curves for the bulk alloy RDEs. T = 333 K.

Figure 4. Polarization curves (IR-free) of catalyzed SDC anode layer (sluiTy coated on YSZ) in humidified H2 (p[H2O] = 0.04 atm) at Yceii = 800°C. Dashed line is for SDC without metal catalyst, and solid line is the regression line for Ru-SDC. SDC particle size=1.6 pm, the amount of each metal catalyst 0.1 mg/cm (ca. 2.5 wt%). These metal catalysts were loaded on the SDC by impregnating 2 pL of metal salt solution containing each metal ion at 12.5 g/L, followed by thermal treatments. The particle size d estimated from XRD are shown in the table. Reproduced from Ref. 1.3. Copyright (1994), by permission from The Electrochemical Society.

Here, we derive accurate explicit approximations for the general implicit polarization curve of the catalyst layer Eq. (6.17) ... 

The polarization curves in Fig. 2 show that polyaniline is a more effective catalyst support than its composite with PSS, and that its effectiveness can be enhanced considerably by addition of a proton conductor (Nation) to the catalyst layer. The beneficial effect of Nation here is not surprising because polyaniline is not likely to be a good proton conductor. 

Figure 27 presents the polarization curves for oxygen reduction on carbon black promoted by different cobaltitesin alkaline solution. In these rotating disk-ring electrode experiments an extremely thin layer of a catalyst was applied to the pyrographite disk electrode. The analysis of the experimental data has shown that the O2 in the presence of the cobaltites is reduced mainly to water without the formation of hydrogen peroxide. 

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