Miniature Fuel Cell with Monolithically Fabricated Si Electrodes- Pd-Pt Catalyst by Upd-Slrr

Miniature fuel cells get a lot of attention for mobile devices. Recently, our group proposed a Si based fuel cell that has both fuel channel and catalyst layer on a Si substrate[1]. Recently, maximum power density reached 480 mW/cm² at 313 K with H₂-O₂ supply, and it is one of the best performances among MEMS fuel cells. However porous Pt was used as a catalyst and the Pt usage was quite high. Estimated usage was 3.6 mg/cm², while typical Pt usage in conventional fuel cells is only 0.1 mg/cm².

We have discussed Pd-Pt catalyst for reduction of the Pt usage in our miniature fuel cells, and reported about a novel porous Pd-Pt catalyst for our miniature fuel cells[2]. In the study, initially porous Si was formed by anodization in a HF solution, then the Si was replaced by Pd in a wet plating process. On the obtained porous Pd, Pt was deposited using UPD-SLRR technique[3-4] as shown in figure 1. Large reduction of Pt amount was expected, but evaluation of the Pd-Pt catalyst was very preliminary.

In this study, using the Pd-Pt catalyst formed by the UPD-SLRR technique, prototype fuel cells were made and the performance was evaluated.

Experimental

Miniature fuel cells were prepared using almost identical process to the previously reported one [1], in which after catalyst formation fuel channels were opened by plasma etching and monolithically fabricated Si electrodes were made. In the catalyst formation process, following process was used.

The silicon wafers used in this work were N-type (Resistivity 0.004-0.006 Ohm-cm), (100) oriented, double side mirror polished 100 um thick ones. Process condition for the porous Pd formation was reported elsewhere [2].Cu UPD was performed on the porous Pd chip in a solution of 0.1M H₂SO₄ and 0.1M CuSO₄. The electrolyte solution was deaerated for 30 minutes with N₂ gas and the whole vessel for the UPD was filled with N₂ during the experiment. In the UPD process, Cu wire was used as the reference electrode, and 50 mV versus Cu reference electrode was applied to the porous Pd electrode chip. Then, the porous Pd chip was moved to a Pt plating bath for the replacement of UPD Cu to Pt. Since a replacement reaction of Pd to Pt occurs automatically in a Pt plating bath, the electrode potential of porous Pd layer was shifted to cathodic side, and a specimen chip was immersed in the 2mM Pt plating bath for two minutes.

A thin gold film was deposited by sputtering on the edge of the catalyst layer to have electrical contact to the low resistive Si substrate, because direct electrical contact between the porous catalyst layer and Si substrate is poor. Then, two Si electrodes are hot-pressed onto either side of PEM sheets.

 Results

Figure 2 shows the typical EDS elemental analyses. A red line shows the spectrum after the Cu UPD, and blue line shows the spectrum after the SLRR reaction. The spectra were normalized with Pd maximum peaks. It was assumed that the expected reaction was realized. Pt/Pd weight ratio of the porous Pd-Pt catalyst was analyzed by ICP-AES and was estimated to be 0.41%, which means only 6 μg/cm² of Pt in the prototype cells. 

Catalyst activity was examined by using the porous layer as a cathode catalyst. Figure 3 shows the polarization curve of the prototype cells. The Pd  catalyst showed poor performance (black), while significant improvement was observed with the Pd-Pt catalyst (green). Compared to the Pt catalyst, potential drop in the low current zone was observed, and it may be due to partial Pt coverage on the Pd layer.

 References

[1] M. Hayase, T. Fujii, J. G. A. Brito-Neto, J. Electrochem. Soc., 158(4), B355-359 (2011)

[2] D. Ogura, T. Honjo, M. Hayase, 222th ECS Meeting abstract, No. 3331

[3] S.R. Brankovic, J.X.Wang，R.R.Adžić, Surface Science, vol.474, L179-179 (2001)

[4] D. Gokcen, S.E. Bae, S.R.Brankovic, J. Electrochem. Soc., 157(11), D582-587 (2010)