In polymer electrolyte fuel cells (PEFCs), the cathode catalyst layer (CL) needs a large amount of Pt because of the slow oxygen reduction reaction. Since electron, proton, and oxygen are necessary for the cathode reaction in PEFCs, achieving the optimum structure of cathode CL and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. Some researches reported that when Aquivion is used as the ionomer of the CL, the output voltage increases as compared with the CL using Nafion in the intermediate current density region (1). However, dominant factors improving the cell performance have not been elucidated quantitatively. In this study, the oxygen transport resistance at the Pt surface, which is thought to have a large contribution to the oxygen transport resistance inside the CL (2), was investigated using CLs fabricated with different ionomers.

In this study, catalyst coated membranes was fabricated with various cathode structure: 6 types of cathode side CLs using three kinds of Pt-supported carbons (TEC10E30E, TEC10E50E, TEC10E70TPM) having different Pt carrying densities and two types of ionomer, Nafion and Aquivion. All of these CLs were fabricated with I/C = 0.8.

First, to distinguish the oxygen transport resistance in the CL from the other resistance such channels, I-V measurement experiments were conducted with different total pressures (0.10MPa, 0.15MPa, 0.20MPa), and the limiting current density method (2) was applied. In the I-V measurement, the flow rate of cathode gas (oxygen and nitrogen mixture) was 2000 sccm with different oxygen concentration (1%, 2%, 3%) and the flow rate of anode hydrogen was 100 sccm with RH 80% (80°C). Next, the oxygen transport resistance at the Pt surface was evaluated. (The details will be described later). In this evaluation, the electrochemical surface area (ECSA [m²/g_Pt]) in the CL is needed. The ECSA was estimated by CV measurement. In the CV measurement, cathode was filled with N₂ gas with RH 100% (80°C). The flow rate of anode hydrogen was 100 sccm with RH100% (80°C).

Figure 1 shows I-V characteristics of the CL with Pt loadings of approximately 0.20 mg/cm². The output current density for the Aquivion based CL is higher than that of the Nafion based CL at a voltages around 0.5 V, whereas the Nafion based CL has superior performance in the operation at 0.1 V where oxygen supply is limited by various transport resistances. In this paper, we focus on the oxygen transport resistance at low voltage, in particular the resistance at the Pt surface, because the Pt surface resistance is considered to be dominant part of oxygen transport resistance under the low Pt loading CL.

Figure 2 shows the ECSA estimated from the CV measurement. The values of ECSA are similar with Nafion and Aquivion based CLs. As the Pt carrying density increases, the ECSA decreases. This is because the Pt particles aggregate on the carbon and the surface area becomes smaller. Using the simplified evaluation formula of oxygen transport resistance (2), the total oxygen transport resistance in the CL can be expressed as R_CL = R_Pt + R_diss + R_pore. The R_Pt means the resistance at the Pt surface, R_diss means the resistance of oxygen dissolves into ionomer, and R_pore means the oxygen diffusion resistance in the CL pores. The R_Pt is formulated as R_Pt = 1 / (A_Pt × γ × k_Pt) (2). Here, A_Pt is the reactive surface area of catalyst Pt per unit area of the CL, γ is ratio of oxygen concentration in the ionomer and CL pore, and k_Pt [m/s] is the oxygen transfer rate constant at the Pt surface. Figure 3 shows the result evaluating R_Pt with both ionomer based CLs. The ordinate represents R_CL and the abscissa represents 1/A_Pt. Using this result, k_Pt can be calculated as k_Pt = 1.5×10 ^-2 s/m with the Nafion based CL and k_Pt = 9.1×10 ^-3 s/m with the Aquivion based CL. As the oxygen transport resistance at the Pt surface is inversely proportional to the k_Pt, the resistance of Aquivion based CL is estimated to be larger. This indicates a possibility that Aquivion based CL is inferior due to its larger Pt surface resistance. The larger Pt surface resistance may be due to the characteristics of Aquivion: the shorter side chain and the hydrophilic property induce strongly condensed structure of Aquivion at the surface of Pt, and that results in inhibiting the transport of oxygen.

Reference

(1)C. Park, et al., J. Power Sources, 315,179 (2016).

(2)Tabe, et al., ECS Trans., 80(8), 205 (2017).