[Small Fuel Cells] Motorola Lectures on 3D-structured Ultrasmall Fuel Cell

May 7, 2008 19:46 Kouji Kariatsumari, Nikkei Electronics

Motorola Inc made a lecture on the fabrication process of an ultrasmall fuel-cell system with a 3D structure for use in mobile phones. The lecture was titled "Fabrication of 3D Micro Fuel Cell for Cell Phone," at Small Fuel Cells 2008 (open from April 30 to May 2, 2008, in Atlanta).

Motorola is aiming at fabricating a fuel cell system composed of fuel cells, a fuel cartridge and a compact Li-ion rechargeable battery with a volume of 8mL, which is the size of the Li-ion rechargeable batteries in existing mobile phones.

When a volume of 2mL is allocated to the Li-ion rechargeable battery and 4mL to the fuel cartridge out of the total of 8mL, the cells can only have a volume of about 1mL.

Thus, the company intends to fabricate a 3D cell that can extend the reaction area of the cell, instead of the existing 2D (planar) cell. Specifically, the company formed a three-dimensional assembly of six cells in an area of 1cm² on a silicon wafer.

Each cell assembly is composed of roughly 100 ultra-small cylindrical cells. It measures approximately 100μm in both diameter and height.

The cell is composed of the fuel electrode, the electrolyte film and the air electrode arranged in this order from the inner circumference of the cylinder. Fuel is supplied through the holes formed on the lower side of the Si wafer, and air is supplied from the lateral side of the cylinder. The cell is fabricated by utilizing the semiconductor manufacturing process (Fig 1).

First, the photolithography process is repeated four times on the Si wafer to form required interconnects. Then, holes are made on the wafer by dry etching (DRIE) to form supply ports for the fuel (H2).

Next, in order to make electrodes in a three-dimensional manner, a bicylindrical structure with a height of about 100μm is formed by utilizing the pattern exposure technology. The structure is filled with a colloidal solution containing polystyrene (PS) particles, etc, and a deposit of PS particles is obtained by drying the structure.

A thin gold (Au) layer is formed in advance on the top surface of the Si wafer, and another Au layer is provided under the PS particle deposit. Thus, when an electric field is applied to the deposit, the surface of PS particles is plated with Au on the same principle as the electroplating.

After removing the original bicylindrical structure and the PS particles, an Au bicylindrical structure is prepared (Fig 2), which will serve as the (fuel and air) electrodes doubling the gas diffusion layer.

Next, a solvent of "Nafion," a fluorinated polymer material available from DuPont, is poured into the electrodes in the bicylindrical structure thus prepared to form the electrolyte film. Then, the upper portion of the fuel electrode and the electrolyte film is sealed to prevent the leakage of gas. The cell assembly is obtained after the final process to clean the surface of the air electrode.

According to Motorola, the performance of the fuel cell system will be announced shortly because the optimization of the final process is now underway.

24 Hours | 7 Days