Hydrogen has been recognized as an environmentally friendly clean fuel of the future since it has various applications in power generation systems. For example, hydrogen can be used as a fuel for fuel cells, especially proton exchange membrane fuel cells, which use hydrogen and air to produce electricity, generating only water as a by-product. Fuel cells are being developed to replace traditional electricity generators because they produce clean, environmentally friendly energy. However, these fuel cells require external supply and storage devices for hydrogen. Extensive efforts have been made to develop a safe and efficient way to store hydrogen, especially in mobile applications. Conventional hydrogen storage technologies include liquid hydrogen, compressed gas cylinders, dehydrogenation of compounds, chemical adsorption into metal alloys and chemical storage as hydrides. However, each of these systems is either hazardous or bulky.
Another method of storing hydrogen has been proposed recently. This method uses a classical chemical hydride, such as NaBH4, as a hydrogen storage medium. The principle of this method is the reaction of the chemical hydride with water in the presence of a catalyst to generate hydrogen, as shown in the equation below:NaBH4+2H2O→4H2+NaBO2+HEAT
The borohydride, NaBH4, acts as both the hydrogen carrier and the storage medium. Ruthenium, Cobalt, Platinum or alloys thereof can be used as a catalyst in this reaction. It is to be noted that this reaction occurs without a catalyst in an acidic environment and only slightly under alkali conditions. This means the chemical hydride solution can be stored and has a shelf life under alkali conditions. This reaction is efficient on a weight basis since half of the hydrogen produced comes from NaBH4 and the other half comes from H2O. Borohydride is a relatively cheap material, usually used in wastewater processing, pharmaceutical synthesis, etc. Borohydride is also easier and safer to handle and transport than highly pressurized hydrogen or cryogenic hydrogen. As a result, there are some advantages to use borohydride as a method of storing hydrogen as a fuel for fuel cells.
There are several known examples of hydrogen generation systems that utilize chemical hydrides. One type of hydrogen generation system comprises a closed vessel for containing chemical hydride and a mechanical stirring mechanism disposed within the vessel for stirring the chemical hydride within the vessel. Water is injected into the vessel to react with chemical hydride and generated hydrogen is removed from the vessel through an outlet. The stirring mechanism means is used to ensure sufficient contact between the hydride and water while preventing the clumping of the hydride. Since the hydride is in solid phase in this system, the stirring mechanism is indispensable. However, in such systems the stirring mechanism consumes energy, increases the overall system weight and reduces system efficiency. Further, the noise generated in the stirring operation is undesirable. In addition, the reaction rate is low, making the fuel unresponsive, useless or very hard to control. The system also tends to be large and cumbersome.
Another type of hydrogen generation system employs a chemical hydride solution. In this system an aqueous chemical hydride solution is introduced to a catalyst bed to generate hydrogen. However, there are a number of problems associated with this liquid phased system. First, the by-product borate, in the above equation, NaBO2 is less soluble then the reactant borohydride, namely NaBH4. Specifically, NaBO2 is only approximately 20% soluble. This means that in order to generate hydrogen in a liquid phased system, and thereby reduce the problems associated with the aforementioned solid phased systems, the concentration of borohydride in the solution can only be about 20% which is much lower than borohydride's solubility in water. Therefore the achievable hydrogen density of the system is considerably limited.
A further deficiency of the aforementioned examples is that neither system is capable of responding in real time to the fuel (hydrogen) needs of the fuel cell. This ability is referred to as load following ability.