Sodium borohydride solutions can provide the hydrogen storage necessary for fuel cells to be a viable source of power for electric vehicles.
Many people are looking to the hydrogen fuel cell to provide electrical power generation for personal vehicles. Unfortunately the problems associated with the centralized generation, shipping and on-board storage of hydrogen gas (H2) are holding up the wide spread introduction of this technology (Ashley, Scientific American, 2005). Alternatively hydrogen can be stored molecularly in an easily handled liquid solution and converted to hydrogen gas at the point of use. Systems using alcohols or other organic liquids are being developed, but all have the downside of producing CO2. Even if these organics are produced from biomass, they will be net contributors to the CO2 concentration in the atmosphere because of inefficiencies in the conversion process.
Sodium Borohydride
Sodium borohydride (NaBH4) has been proposed as a storage medium for hydrogen (Wakefield, Scientific American, 2002). It reacts with water to produce hydrogen (4H2) and sodium borate (NaBO2). Since NaBH4 is a solid (which complicates fueling) work has been done (Shang, Energy & Fuels, 2006) to develop a method to dissolve the borohydride in a water/caustic solution that impedes hydrogen formation. A metal catalyst, supported on carbon rods, can then be lowered into the solution to allow the hydrogen reaction to proceed. This also allows for a method of turning the hydrogen production off and on.
Cell Optimization
Trying to get the most economic hydrogen production out of a borohydride system requires optimizing a number of different factors. Caroline Cloutier (Transactions of the ASME, 2007) has done extensive experimental work to find the optimum conditions for this type cell. Her work has shown that, using a 10% hydroxide concentration, the optimum NaBH4 concentration is about 50%. The limiting factor appears to be the amount of borate that remains soluble in the solution as the borohydride is consumed.
Recycling Sodium Borate
Once the borohydride is consumed the solution no longer produces hydrogen. The resulting borate solution must be removed and replaced with fresh borohydride. Fortunately, work has been done on recycling the NaBO2 back to NaBH4 (Kojima, International Journal of Hydrogen Energy, 2003). The recycling process does consume more energy than is produced in the fuel cell, but that is expected in any energy storage system. If this recycling is done using off-peak energy production from a wind, solar, hydroelectric or even nuclear power generation facility, this system is completely carbon neutral.
Borohydride Infrastructure
To be a commercially viable power generation system for electric vehicles there would have to be a fueling infrastructure available to support those vehicles. A fueling station would consist of two sets of tanks; fuel tanks (50% NaBH4/10% Caustic solution) and recycle tanks (30% NaBO2/10% Caustic solution). Refueling would consist of emptying the vehicle's fuel tank into the station recycling tank and then re-filling tank with borohydride. Since there would be a contact hazard associated with the caustic solution both the recycling and refueling ports and lines would have some sort of positive connection devices to prevent spills. A fuel delivery truck would take the borate solution back for recycle.
A sodium borohydride fuel cell looks like a possible solution for providing electricity for electric cars. The borohydride fuel is clean, minimally hazardous, and recyclable, an obvious step above ethanol and Biodiesel combustion fuels. Refueling the vehicle would require removal of the spent fuel and would thus encourage recycling. No carbon emissions are associated with the fuel consumption, nor need be associated with its production. This is certainly a system worthy of further development.
