Hydrogen generation for fuel cell vehicle applications

Ahmad Yacoub Khalil Al-Kukhun, Purdue University

Abstract

The concerns over diminishing resources and the environmental impact of burning fossil fuels have focused attention on the development of alternative and sustainable energy sources for transportation applications. In this context, hydrogen is a potential clean and environmentally-friendly energy carrier. It has high energy density on a mass basis as compared to gasoline (120 MJ/kg for hydrogen vs. 44 MJ/kg for gasoline) but lower volumetric energy density (0.01 kJ/L for hydrogen at STP vs. 32 MJ/L for gasoline). A major obstacle for the development of hydrogen powered vehicles is the lack of safe, light weight, dense and energy-efficient means for hydrogen storage on-board vehicles. Current approaches for hydrogen storage either require low temperatures/high pressures or do not provide sufficiently high H2 yield to meet the 2015 US Department of Energy (DOE) requirements. Chemical hydrides offer the advantages of high hydrogen gravimetric capacity, along with relatively easier hydrogen release. Among the chemical hydrides, ammonia borane (NH3BH3, AB) has attracted considerable interest as a promising hydrogen storage candidate because of its high hydrogen content (19.6 wt %). In this research, we report novel approaches for hydrogen release from AB, which do not require any external catalyst and produce relatively high hydrogen yield near proton exchange membrane (PEM) fuel cell (FC) operating temperature (~85 °C) with rapid kinetics. One approach involves AB dehydrogenation in water (hydrothermolysis). In this approach, AB dehydrogenation is activated in water, resulting in simultaneous hydrogen generation from both AB and water. The results show that the proposed method provides H2 yield up to 14.3 wt% with rapid kinetics at 85°C—this is the highest H 2 yield reported in the literature. The second approach is AB thermal dehydrogenation (neat thermolysis) under heat management. In this method, the exothermicity of the first hydrogen equivalent release is utilized in initiating the second step of dehydrogenation, which allows faster dehydrogenation at lower temperature. The investigations include pressure monitoring, mass spectrometry, TGA/DSC, powder XRD analysis, FTIR, NMR spectroscopy and isotopic (deuterium) labeling. In all AB dehydrogenation methods, some ammonia is also generated along with hydrogen. The amount of ammonia formation from all proposed methods (methods developed by us and others) is also quantified in this work and, using both experiments and simulations, effective on-board methods to decrease ammonia to <1 ppm are developed. In this>work, we also elucidate the molecular reaction pathways of hydrogen release by DFT calculations along with TGA/MS and in-situ 11B and 1H NMR analyses. Two main reaction pathways are identified; the first proceeds by internal acidic (BH3) catalysis and subsequent dehydrogenation of AB to acyclic intermediates, while the other pathway involves the generation of cyclic intermediates which allows faster release of the second hydrogen equivalent at lower temperatures. The combined experimental and DFT approaches utilized in this research provide a fundamental understanding of new hydrogen generation methods. The last chapter in this thesis explores the potential use of magnesium borohydride, Mg(BH4)2, a newly synthesized metal complex hydride containing 14.9 wt% H2, as a hydrogen carrier. Several additives were tested to lower the hydrogen release temperature and to increase the hydrogen release kinetics. It was found that NbF5 is a very effective additive to improve the hydrogen release properties of Mg(BH4) 2. It is remarkable that Mg(BH4)2 with NbF5 begins to release hydrogen starting at ~ 75°C as compared to 270o°C for neat Mg(BH4)2. In this thesis, for both hydrogen storage materials AB and Mg(BH4)2, effective novel methods were devised to lower the dehydrogenation temperature to near PEM FC operating temperature (~85 °C).

Degree

Ph.D.

Advisors

Varma, Purdue University.

Subject Area

Chemical engineering

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