(592f) Design of Hydrogen-on-Demand System from a Sodium Borohydride Hydrolysis Reaction

Hydrogen has become one of the most promising future energy resources due to concerns about global warming and the depletion of fossil fuels. Hydrogen generation from the hydrolysis reaction of an alkaline sodium borohydride solution (NaBH₄) has drawn much attention due to its theoretically high hydrogen storage capacity (10.8 wt%). Because hydrogen can only be generated when an alkaline NaBH₄ solution contacts with a catalyst, it is easy to control the generation rate. Therefore, it is preferable to apply the hydrolysis reaction as a hydrogen-on-demand system for proton exchange membrane (PEM) fuel cell applications. Some research has been carried out, for example, Millennium Cell Inc. has developed a 1.2 kW hydrogen-on-demand^TM prototype using NaBH4 [1]. Italian National Agency for New Technologies (ENEA) has designed a 6 Wh prototype for cellular phones via NaBH₄ hydrolysis reaction [2]. Korean Fuel Cell Research Center has also developed a 2 W PEM fuel cell to power a cellular phone [3]. The objective of this work is to design a procedure to supply the optimum NaBH₄ feed flowrate needed to generate sufficient hydrogen in response to process demands. In chemical engineering nomenclature, this is equivalent to design a semi-batch reactor to meet the production demand.

Based on the first-order kinetics and isothermal condition, an analytical expression between the feed and effluent of a semibatch reactor can be derived. The relationship between the input NaBH4 flowrate and the output hydrogen generation is simply a first-order transfer function, where the time constant is the inverse of the reaction rate constant and the process gain follows the reaction stoichiometry, 4. Once the process characteristic of the hydrolysis reaction is determined, the NaBH₄ feed flowrate can be determined analytically as the combination of the impulse and step functions to obtain a constant hydrogen generation rate as shown in Fig. 1.

In the implementation phase, two scenarios are explored. One is to design a disposable hydrogen-on-demand system and the other is to design a hydrogen-on-demand system for variable power requirements. For the disposable design, a given amount of NaBH4 solution (which corresponds to the area of the impulse function) is initially placed in the reaction chamber. The reaction is initiated by adding the catalyst, followed by a constant NaBH4 feed flowrate (which corresponds to the step function). As for different power changes, the process inputs also adjusted to meet the demand. Figure 2 gives the response of the hydrogen generation rate according to this scenario. Finally, an experiment is setup to validate the design.

Keywords: sodium borohydride, hydrolysis reaction, hydrogen-on-demand