Fuel cell performance anode polarization

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

Several authors have shown the fuel cell performance with varying PTFE content [103, 106, 108, 109]. Figure 7.16 shows the polarizatirais curves of DMFC with different contents of PTFE on the anode and cathode GDL respectively [82]. An excess of PTFE has an adverse effect on the fuel cell polarization as observed in both graphs. The better performance is attained with different PTFE amounts at the anode and the cathode, indicating that an optimum amount has to be chosen according to the particular material and application. This is related to the fact that different amounts of water are present in the anode and the cathode. While an aqueous solution is fed to the anode, water reaches the cathode mainly by transport from the anode through the membrane, because dried O2 or air is typically supply to the cathode. 

Though water vapor plays a significant role in the fuel cell performance, the humidification of H2 and O2 is not necessary under fuel cell operating conditions. The water vapors produced at the cathode compartment are enough for the humidification of the membrane by the back-transport process from the cathode to the anode compartment of the cell (Fig. 21), thus affecting the ionic conductivity of the membrane electrolyte and, as shown below, on the polarization of the electrode/electrolyte interfaces. 

The influence of CO poisoning at the anode of an HT-PEFC was investigated by Bergmann et ul. [28]. The dynamic, nonisothermal model takes the catalyst layer as a two-dimensional plane between the membrane and gas diffusion layer into account. The effects of CO and hydrogen adsorption with respect to temperature and time are discussed in detail. The CO poisoning is analyzed with polarization curves for different CO concentrations and dynamic CO pulses. The analysis of fuel-cell performance under the influence of CO shows a nonlinear behavior. The presence of water at the anode is explicitly considered to take part in the electrooxidation of CO. The investigation of the current response to a CO pulse of 1.31% at the anode inlet showed a reversible recovery time of 20 min. 

Fig. 1 a RDE and b RRDE tests for PANI- and PPy-based catalysts. Fuel cell performance of PANI- and PPy-derived catalysts c polarization plots, d life tests. Cell temperature 80 C anode— 0.25 mg cm Pt on a woven-web GDL (E-TEK), 30 psig H2 cathode—catalyst loading 4 mg cm membrane— Nafion 1135. Reproduced with permission from Ref. [41], copyright (2009) The Electrochemical Society... 

Using a Pt anode electrode and a Pt-alloy cathode electrode, polarization tests were performed on the homopolymer 20H-PBI MEA (Fig. 13.13). The homopolymer produced a voltage of 0.69 V using a load of 0.2 A cm at 180°C and H2/air this is greater than the 0.663 V produced by p-PBI under the same conditions. The high acid doping level and the membrane chemistry significantly contribute to the excellent performance of the 20H-PBI membrane. Overall, the fuel cell performance of 20H-PBI is comparable to that of p-PBI. 

Work at the Jet Propulsion Laboratory (California Institute of Technology) and University of South California, Los Angeles, demonstrated for the first time the power output capability of a DMFC equipped with PEM [52]. From fliis work. Figure 4.2(a) shows flic improved polarization performance of flic PtRu/C anode with Nafion 117 electrolyte compared to 0.5 M H2SO4, whilst Figure 4.2(b) exemplifies the fuel cell performance with the catalyst coated membrane. 

This chapter mainly deals with the fundamentals of H2/air PEM fuel cells, including fuel cell reaction thermodynamics and kinetics, as well as a brief introduction to the single fuel cell and the fuel cell stack. The electrochemistry and reaction mechanisms of H2/air fuel cell reactions, including the anode HOR and the cathode ORR, are discussed in depth. Several concepts related to PEM fuel cell performance, such as fuel cell polarization curves, OCV, hydrogen crossover, and fuel cell efficiencies, are also introduced. With respect to fuel cell stmctures and components, the material properties and effects on fuel cell performance are also discussed. In addition, several important conditions for fuel cell operation, including temperature, pressure, RH, and gas stoichiometries and flow rates, and their effects on fuel cell operation, are also briefly presented. This chapter provides the requisite baseline knowledge for the remaining chapters. 

Fig. 18 Fuel cell performance for the p-PBI membranes from the sol-gel process. Polarization curves of fuel cells under H2/air (squares) and H2/O2 (circles)) without any feed gas humidification. The membrane PA doping level was approximately 32 mol PA/PRU. The catalyst loading in both electrodes was l.Omgcm" Pt, and the cell was operated at 160 °C at constant stoichiometry of 1.2 stoic and 2.5 stoic at the anode and the cathode, respectively...

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

Ultimately, the catalyst performance of a real fuel cell is of the greatest importance. The DEFC polarization curves for the two PtSn anode catalysts are tested and shown in Fig. 15.9. The characteristic data are summarized in Table 15.4. The PtSn-1 catalyst shows a strongly enhanced electron-oxidation reaction (EOR) activity and much better performance in both the activation-controlled region (low-current density region) and... 

The sources of polarization in PAFCs (with cathode and anode Pt loadings of 0.5 mg Pt/cm, 180°C, 1 atm, 100% H3PO4) have been discussed in Section 2 and were illustrated as half cell performances in Figure 2-3. From Figure 2-3, it is clear that the major polarization occurs at the cathode, and furthermore, the polarization is greater with air (560 mV at 300 mA/cm ) than with pure oxygen (480 mV at 300 mA/cm ) because of dilution of the reactant. The anode exhibits very low polarization (-4 mV/100 mA/cm ) on pure H2, and increases when CO is present in the fuel gas. The ohmic (iR) loss in PAFCs is also relatively small, amounting to about 12 m at 100 mA/cm. ... 

Figure 3.3.7 Theoretical (dashed dotted) and real (solid) cell voltage (V) - current density (I) performance characteristics of a fuel cell. Overpotentials are responsible for the difference between theoretical and real performance and cause efficiency losses. They split into (i) activation polarization overpotentials at anode and cathode due to slow chemical kinetics, (ii) ohmic polarization overpotential due to ohmic voltage losses along the circuit, and (iii) concentration polarization overpotentials due to mass-transport limitations. The activation overpotentials of the cathode are typically the largest contribution to the total overvoltage.

What, if any, relevance do such results have when predicting the influence of adsorbed bismuth on the CO of supported platinum nanoparticle catalysts In order to test the transferrability of results obtained on single crystals to practical fuel-cell anode catalysts, a series of experiments was performed [77] on a gas diffusion electrode of carbon-supported platinum (0.22 mg cm ) catalyst (Johnson Matthey). Figure 10 shows the results of polarization measurements for hydrogen oxidation at clean and bismuth-modified (0.65-ML) catalysts. In order to establish the CO tolerance of the electrodes, in addition to experiments involving pure H2,... 

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