Performance of PEFC Under Different Gas-Mixing Conditions

Abstract

Introduction For PEFCs’ grand-scale commercialization, their performance should be further improved. The performance of PEFC is the results of the distribution of the local current density and materials concentration, especially oxygen and water. In addition, the in-plane transport of gas in the gas diffusion layer (GDL), which is difficult to measure, affects the distribution of oxygen partial pressure and relative humidity (RH) by mixing the gas with different residence time. In this study, the distribution of local current density as well as oxygen partial pressure and RH was estimated by varying active area and gas velocity. Furthermore, two kinds of different gas channels, parallel and serpentine, were utilized to estimate the effect of gas mixing in the GDL. The concept of space time was used to discuss the effects of gas mixing in the perspective of chemical engineering. The results may provide a theory basis for cell design procedure. Theory The ORR kinetics can be expressed as Eq.1.-r SO = k gc p O (1)where -r SO is the oxygen consumption rate per active area [mol/(m2s)], p O is the local oxygen partial pressure [Pa] which was experimentally proved to be 1st-order to the reaction rate[1]. k gc represents the partial-pressure-based kinetics constant [mol/(Pa.m2s)] which is the function of cathode electromotive force and RH at a fixed temperature[2].PEFC has the gas-mixing behavior between plug flow reactor (PFR) and perfectly mixed flow reactor (CSTR), whose design equations can be written as Eqs.2 and 3.-r SO d(A/v 0) = C O0 Al dx O (2)-r SO (A/v 0) = C O0 Al x O (3)where A is the active area from the gas inlet [m2], v 0 is the inlet gas velocity [m/s], C O0 is the oxygen concentration in the inlet gas [mol/m3], Al is the cross-section area of a single gas channel [m2], and x O is the oxygen conversion. Experimental The membrane electrode assembly (MEA) with 10 mm thick catalyst layer and Nafion® ionomer and membrane (DuPont NR-212) was set up in a JARI-type cell. The polarization curves were measured with parallel and serpentine channels under different gas flow rates. The different areas of gaskets opening and GDLs were utilized to obtain different active areas, as shown in Fig. 1. The reactant gas was humidified in bubblers. The cell was operated at 80 °C and the pressure at the cell outlet was 1 atm. Results and Discussion Fig. 2 shows the experimental results of polarization curve. The integrated current per active area is higher in 4 cm2 active area comparing to 2 cm2 at high cell voltage in case of parallel channels, and vice versa at low cell voltage. Since the ORR-produced water humidifies the cell, which increases the activity of the catalyst, the current density near the outlet is higher than that near the inlet. When the current density is high enough, liquid water floods the cell, which causes the performance decrease. On the other hand, the polarization characteristics with different active areas are nearly the same in case of serpentine channels. Higher velocity in the gas channel promote the liquid water removal so that the flooding is not remarkable.Fig. 3 shows the relationship between oxygen conversion and area/velocity, which is proportional to the space time. The convex downward curve in case of the parallel channels indicates that the ORR in the cell behaves like an autocatalytic reaction in PFR. Whereas, the curve of the cell with serpentine channels is slightly convex upward, which infers the gas mixing in the cell is more remarkable, approaching to CSTR.As the oxygen conversion increasing, the water vapor partial pressure increases and the oxygen partial pressure decreases, so that a maximal reaction rate exists, as shown in Fig. 4. Therefore, a turning point of oxygen conversion exists, below which a gas channel working like CSTR e.g. serpentine channels should be chosen, and above which a gas channel working like PFR e.g. parallels channel should be chosen, as shown in Fig. 5. Conclusions The distribution of local current density and gas composition as well as gas mixing can be estimated by varying active area and gas velocity. The ORR in the cell with parallel channels can be regarded as an autocatalytic reaction in PFR. The gas mixing in the cell with serpentine channels is more remarkable than parallel channels. When the active area and gas flow rate are certain, an optimal gas channel which the gas-mixing character of is known can be selected in cell design. Acknowledgment This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1