Catalysts for Portable, Solid State Hydrogen Genration Systems

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$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 to90 % to \u3c 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