Hydrogen generation using reactants that enable water dissociation

In this thesis, the hydrogen generation reactions of silicon and ferrosilicon 75, an industrial alloy formed primarily of silicon and iron disilicide, with aqueous sodium hydroxide solutions were investigated to assess their suitability for use in portable hydrogen generators. Hydrogen generation rates and yields and the induction period for hydrogen generation were determined by means of a water displacement method using an in-house developed data logging system. The materials were characterised before and after hydrogen generation using principally powder X-ray diffraction and X-ray photoelectron spectroscopy.

The volume of etchant solution in the reaction of silicon with water was found to have a substantial effect on the yield of hydrogen obtained. This study indicated that ~12 mL of water would be required per gram of silicon to achieve hydrogen yields of >80%, suggesting that at stoichiometry (2:1 molar ratio water:silicon), the hydrogen yield with respect to silicon would be <1 wt.%.

A simple pelleting process was developed to effectively eliminate the induction period in the reaction of pressed silicon powders with sodium hydroxide solution. Higher pelleting pressures led to lower induction periods (<30 s) and more rapid initial rates of reaction (up to ~1000 mL min-1 (g Si)-1)). The activation energy of the reaction was found to be 73 kJ mol-1 by means of an Arrhenius plot. The rate of hydrogen generation was found to be further enhanced by mixing sodium chloride and sodium polyacrylate with the silicon powder before pressing. This represents the first study of pelleting as an activation method for portable hydrogen generation.

For the first time, the process of hydrogen evolution from ferrosilicon 75 using sodium hydroxide has been investigated as a function of temperature using a combination of Xray photoelectron spectroscopy, X-ray diffraction and physical measurements.

Ferrosilicon 75 produces hydrogen by the action of sodium hydroxide on the silicon only, with the iron disilicide acting in the role of spectator/protector species for the silicon. A hydrogen yield of ~4.75 wt.% (with respect to the mass of ferrosilicon) was estimated by reaction of varying quantities of ferrosilicon with 5 mL of 40 wt.% sodium hydroxide solution. The reaction of ferrosilicon with aqueous sodium hydroxide solution to form hydrogen was found to have an activation energy of 90.5 kJ mol-1 by means of an Arrhenius plot.

Ferrosilicon was further activated toward hydrogen generation by processing using ball milling. An activation energy of 62 kJ mol-1 was obtained for the reaction of ball-milled ferrosilicon powder with sodium hydroxide solution. A series of composite powders were synthesised by ball milling ferrosilicon with various additives. Three different classes of additives were employed: salts, polymers and sugars. It was found that composites formed of ferrosilicon and sodium chloride, potassium chloride, sodium polyacrylate, sodium polystyrene sulfonate-co-maleic acid or fructose showed reduced induction times for hydrogen generation compared to that observed for ferrosilicon alone, and all but fructose also led to an increase in the maximum hydrogen generation rate.

Considering its low cost and toxicity and beneficial effects, sodium chloride is the most effective of these additives for activating ferrosilicon toward hydrogen generation.