Miniature Fuel Cell with Monolithically Fabricated Si Electrodes- Application of a PDMS Flow Channel for Operando Observation -

Abstract

Miniature fuel cells have attracted attention as an ultimate portable power source, and we have developed monolithically fabricated Si electrodes for the miniature fuel cells [1,2]. In the fuel cells, two Si electrodes with porous Pt catalyst layer are pressed onto either side of a PEM (polymer electrolyte membrane) , and the prototype fuel cells showed promising performances of over 500mW/cm2 output at 313K with H2-O2. However, the high output was obtained with only small reaction area. Along with enhancement of the reaction area, the performance dropped. We assumed that the performance drop is caused by flooding due to increased water production, which was ignored because MEMS fuel cells are dimensionally small and the outputs are also small. In this study, we attempted to observe the behavior of produced water on the fuel flow channel using a novel transparent channel cover as shown in figure 1. Furthermore, it was assumed that the produced water stays in the concave portions on the catalyst layer, and stripe-shaped projections were formed on the channel cover to prompt drainage of the produced water from the fuel channels.The transparent fuel channel cover was fabricated with PDMS using soft lithography techniques. Initially, a mold for the cover was made with a dry film resist. Micro stripe pattern with 40μm width were made. Considering the PDMS shrinkage during polymerization, the mold pattern had to be formed a few percent larger than the desired shape. Figure 2 shows the whole view of the PDMS channel cover on a miniature fuel cell with Si electrodes. The reaction on the Si substrate was 1mm and 4mm long. Within the area, 100μm x 100μm sections were etched down to the backside catalyst layer. Magnified view on the reaction area is shown in figure 3. It was found that the stripes were finely aligned on the concave portions on the Si electrode.Figure 4 shows the polarization curves of the power generation tests. Results with the novel fuel channel cover with stripes showed higher output compared to the conventional simple cuboid straight shape cover. Figure 5 shows the microscopic view of the channel. While produce water droplets were observed on downstream area in the conventional cuboid shape channel, no droplets were observed with the novel channel cover. It was assumed that drainage of the produced water was successfully prompted and smooth fuel supply to the catalyst layer was realized by the novel fuel channel cover. Acknowledgement This work was partly performed under the Cooperative Research Program of “NJRC Mater. &Dev.” References [1] M. Kobayashi, T. Suzuki, M. Hayase, Journal of Power Sources, 267, 622-628 (2014).[2] R. Shirai, et al., Journal of The Electrochemical Society, 166(15), D834 (2019) Figure 1