Seiko Instruments Announces Micro Fuel Cell that uses Sodium Borohydride
Seiko Instruments (SII) released the development status of their micro fuel cell that uses Sodium Borohydride for hydrogen storage at the "Twelfth Fuel Cell Symposium" that was held in May of 2005. SII compared DMFCs and PEFCs, and while the energy density of methanol fuel is high, DMFCs have a high over-voltage, and output density and energy density are reduced by the voltage reduction caused by methanol crossover. They found that if an efficient way for storing hydrogen could be developed, it would be possible to obtain a higher output density using a PEFC.

The characteristics required of the hydrogen supply device for increasing energy density in a micro fuel cell are (1) speed of hydrogen generation (2) hydrogen storage density, and (3) ease of handling. The hydrogen storage material that SII focused on is Sodium Borohydride (SBH). If water is added to SBH, hydrogen will be generated (NaBH4 + 2H2O → NaBO2 + 4H2). In addition, this material rapidly generates hydrogen at room temperature and has the capability of up to 10 wt% hydrogen storage density. This material is one that is being focused on as a possible hydrogen storage material.
Sodium Borohydride (SBH) generates hydrogen through hydrolysis but for this prototype, an aqueous solution using an organic acid (malic acid) dissolved in water as a fuel was used to speed up the reaction. The hydrogen supply device consists of a fuel tank in which malic acid aqueous solution is placed, a check valve that controls the aqueous solution, and a reaction device where the solid SBH is placed. Hydrogen is generated when malic acid enters the reaction device where the SBH is stored. When the fuel cell starts generating power, the hydrogen begins to be consumed and the pressure inside the reaction device is reduced. As the pressure in the tank holding the malic acid aqueous solution is reduced, the pressure differential causes the check valve to open and the solution to be supplied to the reaction device. With increase in pressure in the reaction device caused by hydrogen generation, the check valve closes based on the pressure differential with the malic acid aqueous solution tank and stops supply of the aqueous solution. Because control of hydrogen supply is handled using the opening and closing of this mechanism in this manner, it does not require any power.
Figure, Configuration Diagram of SII's Hydrogen Supply Device
For this unit, output was fixed at a power generation of 1 W. Pressure fluctuation close to atmospheric pressure is associated with supply of hydrogen for this system (-5 kPag to + 10 kPag), but the affect of this pressure fluctuation on voltage was minimal and a uniform voltage output was provided.
SII's micro fuel cell is still at the fundamental research stage, but it appears that it will be able to achieve a power output density improvement of 2 to 3 times that of DMFCs. There are several outstanding issues including a requirement to further increase the speed of hydrogen generation, but this development should be closely watched. A package type device is envisioned for the hydrogen supply device. As the malic acid aqueous solution and the solid SBH are consumed, the company plans to recover the packages and fill them back up, forming a recycle system.