Fully Monolithically Fabricated Si One-Chip Miniature Fuel Cell -Electrolyte Polymerization in Porous Si Layer

Abstract

A novel electrolyte layer formation for miniature fuel cells was proposed. Polymerization of electrolyte monomer was attempted in a porous Si layer. We have been developing a miniature fuel cell with MEMS techniques, and our prototype fuel cells with monolithically fabricated Si electrodes showed relatively good performance among miniature fuel cells[1]. However, adhesion between the Si electrodes and electrolyte membranes (PEM) was problematic, and poor adhesion causes lower performance. Besides, adhesion process of the electrodes and PEM is not suitable to MEMS process. Therefore, more monolithic structure is desired. Then, we have proposed a one-chip miniature fuel cell as shown in figure 1[2]. The one-chip fuel cell has a through porous Si layer, which is formed by anodization in HF solution, and the porous Si area is expected to work as a electrolyte layer. On both side of the through chip porous Si layer, catalyst layer is formed by wet plating technique. Some reports said porous Si showed ion conductivity, but the conductivity was too small for the fuel cell electrolyte layer, and electrolyte filling into the porous Si layer is needed. In some studies, polymer solution was immersed into the porous Si layer, but the performance of the obtained layer seems limited due to shrinkage of the electrolyte polymer. In this study, we attempted to fill electrolyte into the porous Si layer by electrolyte monomer polymerization in the porous Si. Viscosity of monomer solution was low, and easy penetration of the solution into the porous Si layer was expected. Then, the electrolyte monomer is polymerized and the packed polymer electrolyte layer is formed in the porous Si area. Experimental The through chip porous Si layer, which was formed by anodization in HF solution, showed independent straight pores with no connection to neighbor pores. The diameter of the pore was 40-70nm. After the porous layer formation, to raise hydrophilicity of porous Si surface for the easier penetration of the monomer solution, slight thermal oxidation (673 K for 10 min) was performed. 2-Acrylamido-2-methylpropane sulfonic acid (AMPS), which is a typical monomer containing sulfonic acid group, was chosen as the monomer. The AMPS solution was filled into the porous Si, and polymerization was performed by heating the chip. The monomer filling process was performed in nitrogen atmosphere at 275 K to prevent irregular polymerization. After 6 h immersion of the chip into the monomer solution, the chip was heated to 333 K in an oven for 6 h. Results After the polymerization, the chip was cleaved and the cross-section was analyzed as shown in figure 3. Uniform sulfur distribution, which is the component of the electrolyte, was observed in the porous Si region. A high magnification SEM image with low acceleration voltage in figure 4 showed that polymer was successfully formed in the pores. Crossover of hydrogen gas was also reduced to 1/600 by the electrolyte filling. But the crossover is still large, and further optimization will be performed.