A study of hydrogen generation and storage in ammonia borane based systems

Sumit Basu, Purdue University

Abstract

Hydrogen (H2) is likely to be the fuel of the future because it can be produced from renewable energy sources. Safe and high-density on-board storage of hydrogen is a challenge. Ammonia borane (AB) is a promising candidate material for on-board hydrogen storage that can be dehydrogenated upon demand. An efficient route for its dehydrogenation should be identified and examined for its potential deployment in portable battery and vehicular applications. Two of the proposed dehydrogenation routes, hydrolysis and thermolysis, for AB were investigated in the present work. The reaction performances (rates) were analyzed, the reaction constants estimated, and their potential in meeting the US DOE targets of gravimetric and system hydrogen storage capacity discussed. The isothermal hydrolysis of ruthenium-catalyzed AB (1 wt.%) was characterized and the kinetic parameters, not reported in prior literature, estimated. The hydrogen release kinetics for the hydrolysis was measured. An appropriate kinetic model (Langmuir Hinshelwood) was adopted to describe the hydrolysis and interpret rate data. A new non-linear fitting algorithm, used in conjunction with the kinetic model, was developed for interpreting the intrinsic kinetic parameters of the reaction. The rate data, obtained using the developed algorithm, included both kinetic and diffusion-controlled regimes, where the latter was evaluated using the catalyst effectiveness approach. The ranges of investigated catalyst particle sizes and temperature were 20-181 μm and 26-56°C, respectively. The liquid phase effective mass diffusion coefficient for the hydrolysis was estimated through appropriate measurements and kinetic models. The hydrolysis was found to have an activation energy 60.4 kJ mol-1 , pre-exponential factor of 1.36x1010 mol (kg-cat) -1 s-1, adsorption energy -32.5 kJ mol -1, and effective mass diffusion coefficient 2x10-10 m2 s-1. These parameters, obtained under dilute AB conditions, were validated by comparing measurements with simulations of AB consumption rates during the hydrolysis of concentrated AB solutions (5-20 wt.%), and also with the axial temperature distribution in a 0.5 kW continuous-flow packed-bed reactor. Hydrolysis of AB was found to be much faster than that of sodium borohydride (SBH), and has the advantage of a lower basicity (pH ∼ 9.1 as opposed to 14). For similar reaction conditions and a lower pH, hydrogen generation from AB is 4 and 6 times faster than SBH at 25°C and 55°C respectively. The byproduct of AB thermolysis comprises B-N bonds that are easier to break than the B-O bonds in the hydrolysis byproduct. However, the H 2 yields obtained from AB thermolysis at ∼95°C is rather low (∼7 wt.%). The presence of an ionic solvent in the reaction enhances the release rates and yields. Hence, H2 release rates and yields from AB thermolysis, facilitated by the presence of an ionic solvent, 1-butyl-3-methylimidazolium chloride (bmimCl), were measured. A maximum yield of 5.4 wt.% is reported in a past thermolysis study for a 50 wt.% mixture of AB and bmimCl. The hydrogen yields obtained from the present study varied between 5.1-11.2 wt.%, for a mixture of 80 wt.% AB and 20 wt.% bmimCl, in a temperature range 85-120°C. The bmimCl caused an increase of ≤2 wt.% in the H2 yield when compared to the neat thermolysis case. It caused a significant increase in the molar H2 yield when compared to the neat thermolysis case, from above a temperature of 107°C. A maximum H2 storage capacity of 11.2 wt.% was achieved at 120°C. Neat thermolysis of AB yielded 9.9 wt.% of H2 at 120°C. The maximum increase for the aided thermolysis, over the neat case, was obtained at a temperature of 115°C with a storage capacity of 10.6%. The yields are sensitive to the purity and shelf-life of the bmimCl. The apparent kinetic parameters for neat AB thermolysis was determined using a multiple reaction model. The mean activation energy is 121 kJ mol -1 with a standard deviation of ±5 kJ mol-1 for a temperature range 100-105°C. Thus, AB dehydrogenation via its hydrolysis and thermolysis was investigated. The H2 release rates, yields, and the kinetic parameters and models describing the reactions are reported. The pros and cons of the two dehydrogenation routes are discussed. An overall comparison and selection of a dehydrogenation route for AB remains a continuing area of work although, thermolysis seems to have more advantages at this juncture. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Gore, Purdue University.

Subject Area

Chemical engineering|Mechanical engineering

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