Catalysts for portable, solid state hydrogen genration systems

Jason Robert Gabl, Purdue University

Abstract

Hydrogen and air powered proton exchange membrane fuel cells are a potential alternative to batteries. In portable power systems, the design requirements often focus on cost efficiency, energy density, storability, as well as safety. Ammonia borane (AB), a chemical hydride containing 19.6 wt. % hydrogen, has a high hydrogen capacity and is a stable and non-toxic candidate for storing hydrogen in portable systems. Throughout this work, Department of Energy guidelines for low power portable hydrogen power systems were used as a baseline and comparison with commercially available systems. In order to make this comparison, the system parameters of a system using AB hydrolysis were estimated by developing capacity and cost correlations from the commercial systems and applying them to this work. Supporting experiments were designed to evaluate a system that would use a premixed solid storage bed of AB and a catalyst. This configuration would only require a user input of water in order to initiate the hydrogen production. Using ammonia borane hydrolysis, the hydrogen yield is ∼9 wt. %, when all reactants are considered. In addition to the simplicity of initiating the reaction, hydrolysis of AB has the advantage of suppressing the production of some toxic borazines that are present when AB is thermally decomposed. However, ammonia gas will be formed and this problem must be addressed, as ammonia is damaging to PEM fuel cells. The catalyst focused on throughout this work was Amberlyst - 15; an ion exchange resin with an acid capacity of 4.7 eq/kg and ammonia adsorbent. At less than $0.30/g, this is a cost effective alternative to precious metal catalysts. The testing with this catalyst was compared to a traditional catalyst in literature, 20% platinum in carbon, costing more than $40/g. The Amberlyst catalyst was found to reduce the formation of ammonia in the gas products from ∼3.71 wt. % with the Pt/C catalyst to <0.01 wt. %. Since Amberlyst adsorbs ammonia, it acts as a "consumable" catalyst, requiring a minimum loading for complete reaction. A mass ratio of a least ∼10:1 Amberlyst to AB is needed for a complete reaction. Conversely, the Pt/C is a reusable catalyst and only used at 5 wt. % loading throughout testing. The activation energy of the Amberlyst catalyzed reaction were measured to be 11.6 kJ/mol, which improves upon the activation energy measured with the Pt/C catalyst of 49.3 kJ/mol, making it a more effective catalyst. However, from the results of an aging study, the proposed system configuration of storing the AB and Amberlyst together will need further development, as the theoretical hydrogen yield dropped from > 90 % to < 30 % over a 70 day aging study. This results implies the need for a systems scale solution, such as mechanical separation, or a material scale solution, such as coating the Amberlyst or AB in a water soluble coating. It was found that ammonia borane catalyzed hydrolysis, using Amberlyst - 15 as a catalyst, has potential to be a cost effective, energy dense, and safe option for generating hydrogen for a portable fuel cell system.

Degree

M.S.A.A.

Advisors

Pourpoint, Purdue University.

Subject Area

Aerospace engineering|Energy

COinS