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]. 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 1 mm² reaction area. Along with enhancement of the reaction area, the performance dropped. We assumed that the performance drop is caused by poor adhesion between Si electrodes, because the porous Pt layer becomes depressed several micrometers from the substrate surface during the formation process using anodized porous Si and voids appears at interface between electrodes and PEMs as shown in figure 1(a).

In this study, we attempted to adapt PEM shape to the depression of the porous Pt catalyst layer as shown in figure 1(b). Generally, swelling of PEM caused serious damages to miniature fuel cells because of their fragile nature of porous-Pt and we have employed pore-filling membranes to avoid the swelling in our fuel cells. The pore-filling membrane has two components: a porous polymer substrate to provide mechanical strength and an electrolyte polymer in the pores for proton conductivity. In order to machine the PEM to fit the depression, we decided to apply O₂ plasma etching to the porous polymer substrate and fill electrolyte polymer into the substrate after the shaping, because damage to electrolyte with long O₂ plasma irradiation was a concern. To protect the central area of the porous polymer substrate from the plasma, circular copper plate was used as a mask and was put on the substrate in the process chamber. Plasma etching was applied on both side of the substrate. Figure 2 shows the porous polymer membrane after the plasma etching. As we expected, the substrate has circular thick area. Gas permeability of the porous polymer substrate did not change even after the several micrometers etching, and it was assumed that porous structure was maintained at plasma irradiated area. After the shaping, 2-acrylamido-2-methylpropan sulfonic acid (AMPS) was filled in to the substrate and the monomer was polymerized by heating [2]. Prototype cells with 4mm² reaction area with the proposed adapted shape PEM and conventional flat PEM were made and power generation tests were performed. With the adapted shape PEM prototypes, 780 mW/cm² at average and max. of 1050 mW/cm² outputs were obtained, which is the best among our results. While with conventional PEMs, average output was 320 mW/cm² and even maximum was 420 mW/cm².

References

[1] M. Kobayashi, T. Suzuki, M. Hayase, Journal of Power Sources, 267, 622-628 (2014).

[2] T. Yamaguchi et al., Journal of Membrane Science, 214, 283-292(2013).