Porous Metal Support for Gas Diffusion Electrode of PEFCs
Authors: Ikehara Teppei , Noda Zhiyun , Matsuda Junko , Nishihara Masamichi , Hayashi Akari , Sasaki Kazunari 
Published: December 2020

Introduction

Pt/C and PtCo/C are widely used as electrocatalysts for polymer electrolyte fuel cells (PEFCs). Carbon black support has high conductivity and high specific surface area for supporting highly-dispersed Pt catalyst nanoparticles. However, under high potential during the PEFC start-stop cycles, carbon black support is electrochemically oxidized, causing detachment and aggregation of Pt nanoparticles. In order to solve this issue, we have developed highly durable electrocatalysts using SnO2 and TiO2 supports, stable even under severe cathode conditions of PEFCs [1,2]. Here in this study, porous metallic Ti and Sn sheets are applied as the catalyst supports. Titanium support may exhibit high durability under cathode conditions of PEFCs due to the formation of stable TiO2 oxide layer on the outermost surface of Ti metal. Tin support may be prepared by reducing porous materials of SnO2. These porous metal sheets could also act as gas diffusion layers (GDL). Such structure, in which both the electrocatalyst layer and the GDL are integrated, has a potential to simplify the MEA structure and probably production processes. Here in this study, such gas diffusion electrodes integrating electrocatalyst and GDL are prepared and their electrochemical properties are evaluated.

Experimental

For preparing gas diffusion electrodes using porous Ti sheets, porous Ti sintered sheets with a thickness of about 45 μm and a porosity of around 40% were used. As titanium reacts with NaOH to form TiO2 nanosheets and subsequently TiO2 nanotubes [2,3], the porous Ti sintered sheets were chemically-etched in 1 M NaOH aqueous solution at 60°C for 1 h to increase surface area of the porous Ti sintered sheets, followed by heat treatment at 400°C for 30 min in 5%H2-N2 atmosphere. Pt catalyst nanoparticles were then decorated on the porous Ti sheets by the arc plasma deposition (APD). The Pt loading on the Ti sheets was controlled by varying the number of APD pluses. For preparing the porous Sn sheets, cellulose-based filter paper was used as a template. Commercial SnO2 sol was impregnated into the filter paper, and the cellulose-based template was then removed by oxidizing heat-treatment at 300°C for 1 h in air. Platinum acetylacetonate reagent dissolved in acetone was then impregnated, followed by heat-treatment to decorate Pt catalyst nanoparticles on the Sn-based porous support. The microstructure of porous Pt/Ti and Pt/Sn sheets was observed by field-emission scanning electron microscopy (FE-SEM). Electrochemical measurements were performed to evaluate their electrochemical surface area (ECSA).

Results and Discussion

Regarding the porous Ti sheets, Figure 1 shows an FE-SEM image of the porous Ti sintered sheet after the surface treatment by NaOH. It was confirmed that needle-like TiO2 nanostructures were formed with a length of about several hundred nm and a diameter of about 10 nm, on the surface of the Ti sheet [2]. In addition, the Pt/Ti sheets were prepared in which Pt loading was controlled by varying the number of deposition pulses in the APD method. The correlations between the Pt loading and the ECSA are shown in Figure 2. It was confirmed that electrochemical surface area per unit (apparent) electrode area (ECSA in m2 m-2) increased with increasing Pt loading. In contrast, it was confirmed that electrochemical surface area per unit Pt mass (ECSA in m2 g-1) decreased, which may be due to the morphological change of the Pt catalysts from isolated nanoparticles to dense thin films with increasing the number of APD pluses [2]. In the future, we will aim to further improve the power generation characteristics e.g. by further increasing the surface area of the Ti sheets and optimizing the Pt loading to achieve highly-dispersed loading of Pt catalysts. With regard to porous Sn sheets, Figure 3 shows STEM-EDS images of the porous Pt/Sn sheet prepared. In this sample, it was confirmed that Pt catalyst nanoparticles of about 2 to 3 nm in diameter were highly dispersed and supported on the Sn-based porous materials, still partially oxidized. Electrochemical properties of such unique electrodes will be reported and discussed.

References

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Figure 1