Hydrogen Generation from Chemical Hydrides

Abstract

Hydrogen is one of the most promising future clean energy carriers since it provides zero-emission energy. One barrier to the widespread usage of hydrogen as a transportation fuel is its safe, efficient, and inexpensive on-board storage, which is a combination currently lacking in conventional hydrogen storage technologies including cryogenic and high-pressure vessels. Chemical hydrides hold great promise in this respect due to their capacity to store hydrogen as a solid. Among the chemical hydrides, boron hydrides have the largest volumetric and mass hydrogen densities. The two major methods to produce hydrogen from a chemical hydride are hydrolysis and thermolysis. Since hydrolysis has very slow kinetics, catalytic hydrolysis can be used to achieve higher production rates. Previously reported work on hydrogen generation from chemical hydrides always used very small systems, and so they produced a limited amount of hydrogen. In contrast, in this paper the usage of novel catalysts and catalyst morphologies to yield the highest possible hydrogen generation kinetics is presented. An effective hydrolysis reaction occurs only when chemical hydrides are in contact with a certain catalyst. Ruthenium (Ru), platinum (Pt), nickel (Ni), palladium (Pd), cobalt (Co), Ni-B, Co-B, Co-P, Ni-Co-B, carbon nanotubes (CNT), and graphene are examples of these catalysts. Moreover, platinum supported on carbon (Pt/C), which is extensively utilized in proton-exchange membrane fuel cells (PEMFCs), is also appropriate for hydrogen gas generation. Precious metal catalysts are costly, whereas metal and alloy catalysts from iron, nickel, and cobalt are more inexpensive. Therefore, researchers have been trying to replace precious metal catalysts with inexpensive materials to make hydrogen generation less costly. The hydrogen gas generation rate is a very important performance metric for these catalytic systems. Interestingly, the hydrogen generation rate has been measured in several different ways. One of these ways is to measure the volume produced over the entire experiment and report as ml/min, L/h, L/day, m3/h, etc. The other is in situ measurement of hydrogen mass flow rates possibly by using a mass flow meter. However, for most of the studies, the reported hydrogen generation rate is normalized by the grams of catalyst and reported as L min-1 g-1 catalyst. However, this is not a robust way to express the catalytic activity since the catalytic activity is a function of the active surface area which is related to but not directly proportional to the amount of catalyst. In this work, the hydrogen generation rates and the rate equations reported in the literature are reinvestigated. The types of catalysts, novel catalyst morphologies, mechanisms for hydrogen generation, hydrogen generation models, and application areas for on-board hydrogen generation are discussed. Moreover, our group's current research investigating novel catalyst morphologies and the approach are discussed. © 2015 by Scrivener Publishing LLC. All rights reserved.