Characterisation of Materials for Hydrogen Storage

Abstract

The present dissertation aimed at studying of materials for hydrogen storage. The option of sodium borohydride was found to be of interest, specially for portable and niche applications. A compreenhensive study of the kinetics and thermodynamics aspects of hydrogen generation from sodium borohydride was made.Three different types of sodium borohydride hydrolysis were considered, two of them self-hydrolysis, namely, water and methanolysis, and the other in presence of a stabilized solution and in the presence of a catalyst, referred to as catalyzed hydrolysis. For the catalyzed hydrolysis two types of catalyst were studied: a powder nickel based catalyst and a nickel foam supported catalyst.Sodium borohydride self-hydrolysis was studied under different temperatures and at different sodium borohydride concentrations. The pH variation during the reaction was also studied. It was found that increase on temperature and NaBH4 concentrations raise the hydrogen production rate. It was also observed that during the hydrolysis reaction occur an increase on pH, which slow down the kinetics of hydrolysis.The reaction of sodium borohydride with methanol (methanolysis) was also analyzed. It was found that the water/methanol rate, temperatures and sodium borohydride concentration exert considerable influence on the hydrogen produced rates. Methanolysis of sodium borohydride show to be feasible method for low-temperature hydrogen rate and the possibility of methanol regeneration can be used as potential high gravimetric density hydrogen storage system.For catalytic hydrolysis of sodium borohydride a nickel based powder catalyst was prepared by wet chemistry method. The catalyst show small particle size and large superficial area, which give them a high catalytic activity, hydrogen production rates of ~16 l min-1 g-1, the highest value find on literature for this type of catalyst. By the kinetics experiments made was possible estimated two different activation energy, one for high temperatures (31 kJmol-1) and another for low temperatures (68 kJmol-1) and a reaction order of 1.18 with respect to catalyst concentration. Powder catalysts show good durability without loosing of activity.The possibility of supporting the catalyst was also investigated using as support material nickel foam with large superficial area. Two types of deposition were used: "Doctor Blade" technique and Electrodeposition method. Both methods proved to have excellent catalytic activity; however on Doctor Blade supported catalyst some detachment of the powder was detected. Electrodeposited support catalyst shows no detachment of particle and no significant loss of activity with the various re-utilisations. Support catalyst allows the developing of reactors systems operated in dynamic mode with large autonomy.

Date of Award	31 Dec 2010
Original language	English
Awarding Institution	The University of Manchester
Supervisor	George Thompson (Supervisor)