Fuel Cells/Sodium Boro Hydride 01
From the Electric Vehicle Automobile Association meeting...
Sodium Boro-Hydride, Topping Off
Automotive Fuel Cells of the Future?
by Shirley Georgi
A new hydrogen fuel transporter, sodium boro-hydride, is now being considered to deliver hydrogen for powering fuel cell vehicles. To date, most prototype models have utilized methanol and gasoline with reformers to extract hydrogen or have stored direct hydrogen on board. But, in December at the EVAA (Electric Vehicle Association of America) Conference in Sacramento, Chrysler Corporation unveiled the Town & Country Natrium minivan which was fueled with sodium boro-hydride. The vehicle itself is appropriately named since Natrium is the Latin word for sodium. (See cover photo courtesty of DaimlerChrysler.)
In the Chrysler Town & Country Natrium’s fuel cell system, sodium boro-hydride is processed to extract hydrogen, which is combined with oxygen in the fuel cell stacks to produce electricity that drives the vehicle’s electric motor. The Natrium’s fuel cell system is produced by DaimlerChrysler’s fuel cell partner, Ballard/XCELLIS. The hydrogen being produced is formulated through Millennium Cell’s process known has Hydrogen On DemandTM which uses sodium borohydride, a derivative of borax, to generate and store hydrogen.
The ‘hydrogen’ is stored on board at ambient conditions in a liquid ‘fuel’ - an aqueous (water-based) solution of sodium borohydride NaBH4. Sodium borohydride is made from borax, a material that is found in substantial natural reserves globally. The process supplies pure hydrogen for energy applications without the need (and associated energy penalties) for compression or liquefaction.
The Hydrogen on DemandTM system releases the hydrogen stored in sodium borohydride solutions by passing the liquid through a chamber containing a proprietary catalyst. The hydrogen generation reaction can be written:
NaBH4 + 2H2O > 4H2 + NaBO2
The only other reaction product, sodium metaborate (analogous to borax), is water-soluble and environmentally benign. The reaction is exothermic; there is no need to supply external heat to access the hydrogen. The heat generated is sufficient to vaporize some of the water present, and as a result, the hydrogen is supplied at 100% relative humidity. This co-generated moisture in the H2 stream is an added benefit for fuel cells.
Preliminary Specifications for the Chrysler Town & Country Natrium minivan
Powertrain: front-wheel drive with regenerative braking
Motor: 35kW Siemens AC motor
Battery pack: 40 kW SAFT Lithium-ion
Fuel: Sodium Boro-Hydride, recyclable
Fuel Processor: Hydrogen on Demand system -Millennium Cell
Fuel Cell System: Ballard/XCELLIS
Fuel economy: 30 m.p.g. gasoline equivalent
0-60 m.p.h.: 16 seconds
Top speed: 80 m.p.h.
Range: 300 miles
Emissions: Zero emissions +
The reaction is totally inorganic (carbon and sulfur free), producing a high quality energy source without polluting emissions. It is said to be safe and easily controllable - hydrogen is only produced when the liquid fuel is in direct contract with the catalyst, thereby minimizing the amount of gaseous hydrogen on board at any given time. The fuel solution itself is nonflammable, nonexplosive and safe to transport.
Sodium boro-hydride - a good choice for fuel
“While sodium-boro-hydride will strike some as an unusual fuel for an automobile, it has the advantages of the other fuels that have been proposed for fuel cell vehicles, without the significant disadvantages,” said Thomas Moore, Vice President at DaimlerChrysler’s Liberty & Technical Affairs research and development group. Mr. Moore believes that the most important unresolved issue with the fuel cell vehicles is not the fuel cell, but the fuel itself. Sodium boro-hydride is safe; it can be handled in dry form. It is nontoxic and nonflammable. It is available in large supplies in the United States; in fact, the U.S. has the largest source of borax reserves in the world. Infrastructure issues are less challenging than with other fuels proposed for fuel cell vehicles. The weight-energy storage is almost equivalent to gasoline; this means it generates about the same amount of energy per gallon of fuel as gasoline. In addition, three environmental benefits are: 1) no hydrocarbons are contributing to greenhouse gas buildup; 2) no smog-forming compounds are produced, and 3) the fuel itself can be recycled.
In a typical system, a fuel pump directs fuel from a tank of sodium borohydride (NaBH4) solution into a catalyst chamber. Upon contacting the catalyst bed, the fuel solution generates hydrogen gas and sodium metaborate (in solution). The hydrogen and metaborate solution separate in a second chamber, and the borate is returned to a collection tank. The hydrogen gas optionally can be processed through a heat exchanger to achieve a specified level of humidity, and is then sent to the fuel cell for consumption. +
Optimal features of the Natrium minivan
When asked about special features of the Natrium minivan, Christian Mohrdieck, Senior Manager of Fuel Cell Systems at DaimlerChrysler, quickly notes that with this fuel cell system the vehicle has a range of 300 miles, longer than any fuel cell vehicle shown to date. And, in addition to the extended range, the Town & Country’s fuel cell system preserves the full utility of the Chrysler minivan. “The sodium boro-hydride fuel storage and processing system are completely packaged under the vehicle’s floor. There is no cabin intrusion at all, so the vehicle is fully usable.”
Additional interest in sodium boro-hydride
In December, Millennium Cell signed an agreement to sell fuel systems to PSA Peugeot Citeroën. The initial stage of the agreement calls for PSA Peugeot Citiroën to purchase the fuel systems, taking delivery early in 2002. The Hydrogen on DemandTM system will be integrated into a prototype based on PSA Peugeot Citeren’s recently announced Fuel Cell Cab.
As with other fuels, and hydrogen itself, sodium boro-hydride is still expensive to produce in small quantities and not enough is being produced to allow for economies of scale. Although the fuel is recyclable, the costs for this endeavor need to be studied and tested for economic feasibility. In reviewing the specifications for the fuel characteristics, the freezing point of NaBH4-30 solution is from -130F to -360 F; this temperature range will not allow the fuel to be used in climates such as Minnesota in the U.S. and other northern climate areas with cold severe winters. As with other fuels, additional issues related to safety, long term reliability, costs and infrastructure need to be studied, tested and proven for commercial potential. At the Detroit Auto Show this past January, Dieter Zestsche, President & CEO of Chrysler, said, “Getting to levels where we really talk about significant replacement of traditional drive lines by fuel cell vehicles in mass production, I would say realistically, we should rather think about a time span of 10 years and further.” 1
In spite of the challneges that lie ahead, this cell chemistry provides a creative approach to enhance the possibilities for commercializing fuel cell vehicles. Perhaps, today’s prime application for sodium borohydride as a bleaching agent in the paper industry will settle into a secondary position, as soldium boro-hydride becomes the safe fuel for tomorrow’s fuel cell vehicles.(02-02 BD71-2-3)
1 www.nbr.com/transcript, 01/09-02