Hydrogen generation for fuel cell applications

Moiz Diwan, Purdue University

Abstract

As portable electronic devices are becoming more widespread and power-demanding, fuel cell systems show promise with higher specific energy (Wh/g) than batteries. Hydrogen fuel cells provide higher power density (W/L) and double the conversion efficiency as compared to direct methanol fuel cells but their deployment is hindered by the lack of effective methods for hydrogen storage. Compressed gas, carbon compounds and reversible metal hydrides do not provide sufficient H2 yield, while liquid hydrogen is not practical especially for portable applications. Chemical methods for hydrogen generation provide high specific energy at relatively easy storage conditions. Among such methods, fuel reforming can be applied in stationary systems. For portable and transportation applications, however, compounds such as sodium borohydride (SBH, NaBH4) and ammonia borane (AB, NH3BH3) are more practical hydrogen storage materials. They contain 10.7 and 19.6 wt% hydrogen, respectively. To release hydrogen from these compounds, thermolysis, catalytic hydrolysis, exothermic reactions using additional reactive mixtures have been suggested. All the current methods have disadvantages that decrease the efficiency of hydrogen storage systems. In this research, we report new approaches to release hydrogen from SBH or AB, and simultaneously from water, which do not require any catalyst and produce relatively high hydrogen yield and environmentally benign byproducts. One such approach involves the use of heterogeneous mixtures of SBH or AB with water and nanosize aluminum or micron size magnesium. Due to the highly exothermic metal-water reaction, such mixtures, upon ignition, exhibit self-sustained propagation of combustion wave with simultaneous release of hydrogen from the boron compounds and water. The second approach thermally activates AB hydrolysis in aqueous AB solutions and slurries under modest inert gas pressure. The investigations include digital video recording, pressure monitoring, thermocouple measurements, mass spectrometry, TGA/DSC, powder XRD analysis, NMR spectroscopy and isotopic (deuterium) labeling. The results show that the proposed methods provide H2 yield up to 10 wt% and are promising for hydrogen storage involving SBH or AB. The metal/water combustion methods could be used in compact power sources for portable electronic devices, while hydrothermolysis in aqueous AB solutions and slurries is attractive for transportation applications. In this work, we also model combustion wave propagation in heterogeneous solid metal - water mixtures. Production of a gaseous oxidizer (water vapor) in the combustion wave is an important feature of this model, which distinguishes it from previous developments for filtration combustion of powders and gas-generating systems. In the proposed model, the combustion wave structure includes a thin water-boiling front, a preheating zone with water vapor flowing through the porous medium, and a wide zone of reaction between the formed water vapor and the metal. This diffusion-limited model predicts the front velocity and thermal profile of the combustion wave for different metal particle sizes. A satisfactory agreement between the experimental and modeling results is demonstrated. The combined experimental and modeling approaches utilized in this research provide a fundamental understanding of new hydrogen generating systems for portable and transportation applications, and have the potential for commercialization.

Degree

Ph.D.

Advisors

Varma, Purdue University.

Subject Area

Chemical engineering

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