Numerical Analysis of Effects of Silica Coating for Use in Pt Catalyst on Cell Performance of Polymer Electrolyte Fuel Cells

Abstract

In order to improve the cell performance of polymer electrolyte fuel cells (PEFCs), it is essential to design the cathode catalyst layers (CLs) with the optimal morphology considering the mass transport such as electron, proton, and oxygen as well as an electrochemical reaction. As designing the cathode CLs, it is required to consider the fabrication method, compositions of catalyst ink, ionomer loading, resulting in effect of the cell performance. Particularly, ionomer as the pathway for proton conduction strongly influences proton transfer resistance and oxygen diffusion resistance. Too much ionomer included to CLs obstructs oxygen diffusion, and decreases the porosity and average void size in the CLs. Also, the thickness of the ionomer on Pt particles is increased, leading to an increase in the overvoltage. These results in performance loss. Thus, it is needed to reduce ionomer loading in the CLs. However, low ionomer loading reduces the ability of proton conductions, resulting in lower performance by an increase in proton transfer resistance. In our previous study, we introduced silica-coated Pt catalysts in order to control ionomer loading [1]. Silica-coated Pt catalysts, developed by Takenaka et al. have maintained a high activity for the oxygen reduction reaction during the durability tests because silica coating has merits of suppression of Pt particle agglomeration and diffusion of Pt cations in the catalyst, and easy control of surface characteristics like hydrophobicity or hydrophilicity [2]. In our previous study, we experimentally examined the effects of silica coating in the CLs on the catalyst ink, morphology of CLs, cell performance. As these results, catalyst ink for silica-coated Pt catalyst maintained good dispersion and high stability compared to that of non-coated Pt catalysts. In addition, the performance at 0.6 V for the silica-coated Pt catalysts with low ionomer loading showed higher than that of non-coated Pt catalysts at all the relatively humidity (RH). In particular, at low humidity conditions (20% RH), the silica-coated Pt catalysts showed significantly enhanced performance compared with non-coated Pt catalysts. These results suggest that the hydrophilic groups included in silica layers contribute to the improvement of proton conductivity.In this present study, we examined the numerical analysis of silica-coated Pt catalysts in order to understand the effect of silica coating on the performance in detail, such as current density distribution, overvoltage. In our research group, in order to understand the effect of nano and mesoscale structure of Pt/Carbon catalyst layer on cell performance and internal phenomena, various simulation models which included the effect of the structure of carbon aggregate, ionomer coverage, and formation of agglomerate, have already been developed with some experimental knowledge such as FIB-SEM observation, the actual pore size distribution of CLs, relative oxygen diffusion coefficient, agglomerate size distribution measurement in CL ink [3-5]. These simulation models were applied to the numerical analysis of the silica-coated Pt catalysts in the CLs. In our presentation, we will discuss the effect of silica coating on Pt catalysts in CLs on the cell performance analyzed numerically.AcknowledgmentThis work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.