Certain metals react with water to build a metal oxide. During the reaction the water molecule is split and hydrogen is released as by-product. Using this behavior metals in their elemental form can be applied as chemical hydrogen storage material. The hydrogen is produced on demand. Only metal and water have to be stored in separate vessels thus hydrogen is not present in its physical form during the storage time.
Also the separation of hydrogen in a gas mixture can be achieved via repeated oxidation (Eq. 1) and reduction (Eq. 2) of metals and metal oxides. Reductive species such as CO can be used for the reduction of the metal oxide to reach a higher yield of hydrogen (Eq.3).
Oxidation step: xMe + yH2O → MexOy + yH2 (Eq. 1)
Reduction step: MexOy + yH2 → xMe + yH2O (Eq. 2) MexOy + yCO → xMe + yCO2 (Eq. 3)
The product gas of the reaction of metals with steam is especially capable for polymer electrolyte fuel cells as the hydrogen is already humidified.
However, some of the metals react at room temperature, others need significantly higher temperatures. To evaluate the materials capable for the storage and separation of hydrogen thermodynamic and experimental validations of the different metals are conducted. Metals as hydrogen storage materials are compared with established energy storage systems and advantages and disadvantages are worked out. By means of thermogravimetric analysis selected metals are reduced with hydrogen and oxidized with steam to confirm the theoretical results. Kinetic parameters are derived from the measurements for system calculation.