Fuel cell performance concentration polarization

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. 

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.

As evident from the i-E response, the 1.0 M solution of methanol delivered better performance at low current densities compared with all concentrations of TMM studied at 90 C. However, at very high current densities (>750 mA/cm ) the 0.5 and 1.0 M solutions of TMM shows improved performance with respect to methanol. This type of behavior was observed at a number of different ceil operating temperatures. When the effect of TMM concentration upon cell performance was investigated, it was observed that at low current densities the solutions of low fuel concentration showed less polarization, whereas at higher current densities solutions of higher concentrations showed better performance. This trend in performance is due to fuel crossover effects which dominate at low... 

The simplest models analyzing the electrochemical performance are the polarization models [76]. These models are distinctively different in terms of complexity and applicability to fuel cell systems. In most cases these polarization models are developed for chemical systems consisting only of H2, H2O, and O2. The concentration losses in these models are calculated based on mixed diffusion model assuming equi-molar counter diffusion. However,... 

Methanol (MeOH) crossover from the anode to the cathode in the direct methanol fuel cell (DMFC) is responsible for significant depolarization of the Pt cathode catalyst. Compared to Pt-based catalysts, NPMCs are poor oxidation catalysts, of methanol oxidation in particular, which makes them highly methanol-tolerant. As shown in Fig. 8.25, the ORR activity of a PANI-Fe-C catalyst in a sulfuric acid solution is virtually independent of the methanol content, up to 5.0 M in MeOH concentration. A significant performance loss is only observed in 17 M MeOH solution ( 1 1 water-to-methanol molar ratio), a solution that can no longer be considered aqueous. The changes to oxygen solubility and diffusivity, as well as to the double-layer dielectric environment, are all likely to impact the ORR mechanism and kinetics, which may not be associated with the electrochemical oxidation of methanol at the catalyst surface. Based on the ORR polarization plots recorded at... 

This result may be explained by the observation that tar deposits from hydrocarbons are only removed by oxygen, not H2O, at SOFC operating temperatures [62]. Figure 7 shows the performance of a typical anode-supported cell [19]. Because of the relatively low hydrogen content in the reformed fuel, open-circuit voltages are 1.0 V and concentration polarization is pronounced, but power densities are only slightly lower than when the cells were run with pure hydrogen fuel. 

The fuel gas composition has a major effect on the cell voltage of SOFCs. The performance data (47) obtained from a 15-cell stack (1.7 cm active electrode area per cell) of the tubular configuration at 1,000 °C illustrates the effect of fuel gas composition. With air as the oxidant and fuels of composition 97 percent H2/3 percent H2O, 97 percent CO/3 percent H2O, and 1.5 percent H2/3 percent CO/75.5 percent CO2/2O percent H2O, the current densities achieved at 80 percent voltage efficiency were -220, -170, and -100 mA/cm, respectively. The reasonably close agreement in the current densities obtained with fuels of composition 97% H2/3% H2O and 97 percent CO/3 percent H2O indicates that CO is a useful fuel for SOFCs. However, with fuel gases that have only a low concentration of H2 and CO (i.e., 1.5 percent H2/3 percent CO/75.5 percent CO2/2O percent H2O), concentration polarization becomes significant and the performance is lower. 

Figure 4.38. Direct ethanol fuel cell polarization curves at 353 K. Ethanol concentration 2 M, Nation 117, O2 pressure 3 bar [183]. (Reproduced from Journal of Power Sources, 158(1), Rousseau S, Coutanceau C, Lamy C, Leger J-M, Direct ethanol fuel cell (DEFC) electrical performances and reaction products distribution under operating conditions with different platinum-based anodes, 18-24, 2006, with permission from Elsevier.)...

Figure 6.7. Polarization curves for different concentrations of NO . Cell temperature 60°C, H2/air (1.2/3.3) relative humidity H2 95% and air 0% backpressure 1.5 atm [49], (Reprinted from Electrochimica Acta, 51(19), Yang Daijun, Ma Jianxin, Xu Lin, Wu Minzhong and Wang Haijiang, The effect of nitrogen oxides in air on the performance of proton exchange membrane fuel cell, 4039-44, 2006, with permission from Elsevier.)...

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