Polymer Electrolyte Fuel Cell (PEFC) is a promising energy technology for a future decarbonized society. PEFC is composed of porous electrode catalyst layers, which largely determines current-voltage (IV) performance. However, the optimal structure of porous catalyst layers is still unknown. Carbon supports are the main component to build up such porous structure. Mesoporous carbon (MC) supports are recently paid more attention to since they are used in Toyota’s fuel cell vehicles, MIRAI. We are also studying MC whose diameter is about 10 nm. Therefore, we are interested in a correlation between IV performance and the structure of catalyst layers based on MC supports. The aim of this study is to clarify the optimal structure of cathode catalyst layers for high IV performance. In this study, we synthesized two types of MC, MC bulk and fibers, and evaluated the IV performance and cathode structure of MC based cathode catalyst layers.
Experimental
MC bulk was prepared based on self-organization of Pluronic F127 and carbon precursors (resorcinol, formaldehyde, and triethyl orthoacetate). They were mixed in the acid solution, and then the supernatant was removed after centrifugation. The remaining portion was heated in the oven for polymerization of carbon precursors. The resulting material was heated by stepwise in the oven under the nitrogen atmosphere in order to decompose Pluronic F127 and carbonized precursors. MC fibers were made by electrospinning of the similar precursor solution mentioned in the above, but for electrospinning phloroglucinol instead of resorcinol was in use, and PVA was added. Then, after MC bulk and fibers were milled by hands or zirconia balls, Pt nanoparticles were deposited on MC supports by using Pt(acac)2 as a precursor. MEAs with 1cm2 electrodes were fabricated by spray-printing of catalyst dispersions on the Nafion film. Prepared MC based catalysts were used for the cathode, and 46.5% Pt/KB (TEC10E50E) was used for the anode.
Results and discussion
IV performance of MEAs using MC based cathodes was evaluated and compared to that of a standard MEA. MEA with MC bulk support showed slightly lower IV performance than that of standard MEA. By analyzing each overvoltage separately, ohmic overvoltage was found to be a biggest problem, and low concentration overvoltage was realized as an advantage. In order to find out the reason of low concentration overvoltage, the structure of cathode layers was evaluated using FIB-SEM. The 300 cross-sectional SEM images were obtained by slicing cathodes every 10 nm, and the cathode was reconstructed into a three dimensional image. As a result, the size of MC support was found to be much larger than that of KB. When pore diameter distribution was evaluated based on the reconstructed image, as shown in Figure 1, rather large pores expressed in red in the figure was observed for MC bulk based cathode probably owing to large MC particles. The low concentration most likely comes from the existence of such large pores. In this structural analysis, the analysis of mesopores is not included, and so in reality mesopores might have contributed to lowering concentration overvoltage. Similar analyses were done for MC fiber based cathodes. We have found that the IV performance and structure of cathodes is rather influenced by the milling processes, which will be discussed in detail.