Investigation on Effect of Reducing Cathode Pt-Loading on Ohmic Overpotential in Catalyst Layer
Authors: Katayama Shota , Makino Sho , Matsuda Shinsuke , Arao Masazumi , Matsumoto Masashi , Imai Hideto , Asaoka Takahiko 
Published: December 2024

To reduce the use of precious metals in polymer electrolyte fuel cells (PEFCs), it is necessary not only to develop high activity catalysts but also to design optimal membrane-electrode-assembly (MEA) structure. In the past two decades, it becomes well known that oxygen transport resistance near the catalyst surface is a critical issue when reducing cathode catalyst loading due to concentration of oxygen flux on a small catalyst surface area [1-3]. On the other hand, as well as the case of oxygen flux, small catalyst surface area also increases proton current density near the catalyst particles, which can cause larger overpotential due to proton migration (ohmic overpotential). This is likely to be more important for recent-trend catalyst layer structure which avoids direct coating of ionomer on the catalyst to improve catalyst activity, e.g., employing porous carbon for the catalyst support [4-5]. Such a structure of catalyst layer can cause large resistance especially at a low humidity condition where no proton conduction path through water film exists. In this study, we investigated the effect of reducing catalyst loading on ohmic overpotential in catalyst layers to discuss the impact of local proton resistance quantitatively.
MEA samples were fabricated by a pulse spray method using commercial catalyst supported on porous carbon and solid carbon (TEC10E50E and TEC10V30E, TANAKA Kikinzoku Kogyo K.K.), respectively. Nafion D2020 was employed as an ionomer, and ionomer-carbon weight ratio (I/C) was varied. For each specification, MEAs with different Pt-loading on cathode were prepared with the value of ~0.05 mgPt/cm2 and ~0.2 mgPt/cm2, respectively. GORE-SELECT Membrane M788.12 was employed as a membrane. Humidity dependence of the cell performance was evaluated at a cell temperature of 80°C. To evaluate ohmic overpotential within the catalyst layer, the performance was evaluated varying oxygen partial pressure (p
O2). Structures of catalyst layers were analyzed using a scanning electron microscope and a transmission electron microscope.
Some of the evaluation results are shown below. Figure 1 shows the humidity dependence of the performance of the MEAs with different cathode catalyst loading at a high p
O2 condition of 100 kPa, where the oxygen concentration overpotential can be ignored in a wide current density range. The performance of the low Pt-loading MEA shows large humidity dependence. One of the reasons of this is humidity dependence of the catalyst activity reflecting the humidity dependence of the electrochemical surface area. Additionally, the apparent Tafel slope is larger at lower humidity, which indicates the contribution of the ohmic overpotential at such a low current density. Figure 2 shows the performance at various p
O2, where the current density is normalized by p
O2. Assuming a reaction order equal to 1 for the oxygen reduction reaction, this p
O2-normalized current should be identical at a certain electrode potential for the variation of p
O2 when the proton resistance is enough small and spatial reaction-distribution within the catalyst layer can be assumed homogeneous. For the high humidity performance of solid carbon MEA presented in Fig. 2 (c), the p
O2 normalized performance is almost identical for all the p
O2 conditions, which supports that ohmic overpotential is not significant in this current density range. On the other hand, at the relative humidity of 40%, the normalized performance shows large deviations, which reflect the proton resistance contribution, for all of the samples. The low Pt-loading samples show relatively larger deviations, especially for the porous carbon MEA, which indicates the effect of the local proton resistance. Origin of these proton resistance will be discussed in conjunction with the structures of the catalyst support and catalyst layer.
The results suggest that the local proton resistance in the vicinity of the catalyst particles causes overpotential large enough to suppress the cell performance. To investigate local proton transport character of developed catalyst, it should be useful to evaluate Pt-loading dependence of the ohmic overpotential.
References
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Acknowledgement
This presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1