Carbon-Free All-in-One Electrode Using Porous Ti Sheet for PEFCs
Authors: Kawachino Daiki , Yasutake Masahiro , Odoi Hirotoshi , Noda Zhiyun , Matsuda Junko , Hayashi Akari , Sasaki Kazunari 
Published: February 2020

Introduction

Currently, Pt-based electrocatalysts supported on carbon black are widely used for polymer electrolyte fuel cells (PEFCs). However, under the high potential during the PEFC start-stop cycles, carbon corrosion can occur leading to electrocatalyst degradation (1,2). Therefore, alternative catalyst support materials to the carbon black have been considered. Metal oxides, which are stable under cathode conditions, can be examined as alternative materials as the catalyst support (3-5). We have developed SnO2 and TiO2 supports dispersed on carbon fillers acting as the conductive backbone, exhibiting high durability (4,5). However, as far as we use carbon-based materials in the PEFC cathodes, carbon corrosion problem may continue to remain.

Here, we select titanium as both the catalyst support and the gas diffusion layer (GDL). Metallic titanium has high conductivity, and titanium support may also show high durability due to the formation of titanium oxide layer on the top surface of titanium. In addition, if the electrocatalyst layer has a sheet-like form, the membrane electrode assembly (MEA) can be prepared just by heat-pressing with the polymer membrane. In this study, we prepare a carbon-free
all-in-one electrode structure, consisting of Pt electrocatalyst, porous Ti-based sheet acting as both the catalyst support and the GDL, which can simplify the manufacturing process of the MEAs.

Experimental

Porous Ti sheets, shown in figure 1, were etched with 1M NaOH solution at 60°C for 1 hour. After that, the etched Ti sheet was heat-treated at 400°C in 5%H2-N2 for 30 minutes. Pt nanoparticles were decorated on the porous Ti sheet by the arc plasma deposition (APD). Pt loading on the Ti sheet was controlled by varying the number of APD pluses. The microstructure of the Pt/Ti sheet was observed by SEM, STEM, and TEM. Electrochemical measurements were made for the half cell and the single (full) cell to evaluate electrochemical surface area (ECSA) and I-V performance of the MEA using the Pt/Ti sheet.

Results and discussion

Figure 2 shows the FESEM image of the porous Ti sheet after the surface treatment. The surface of the porous Ti sheet was very rough on which nano-needles were grown with a diameter of ca. several nano-meters. Figure 3 shows ECSA after different APD pluses, measured by the half cell test. ECSA after APD 25 pluses was high approaching up to ca. 100 m2g-1. ECSA from APD 100 pluses to 2000 pluses was improved ca. twice due to the increase in surface area of the Ti sheet by the surface treatment. I-V performance using the Pt/Ti sheet as the cathode was still relatively lower than that using the commercial standard Pt/C catalysts. The surface layer having higher conductivity and higher surface area will be required to further improve I-V performance.

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Figure 1